ROENTGENOGRAPHIC EXAMINATION

The standard roentgenographic views of the cervical spine include anteroposterior, lateral, and both posterior oblique views, as well as an open-mouth anteroposterior projection of the odontoid process and the first two cervical vertebral segments (Fig. 16-2). The majority of cervical spine studies are performed for the evaluation of trauma. In the more severely injured individual, the most important views are a cross-table horizontal beam lateral image and an anteroposterior supine film. By keeping patient movement to a minimum, potentially fatal spinal cord injury is prevented.

Fig. 16-2 Normal roentgenographic study of the cervical spine. A, Anteroposterior view. B, Lateral view. C, Right oblique view. D, Odontoid view.

On the lateral exposure, C7 and occasionally C6 often are excluded, especially in heavy, short-necked individuals. In such instances, it becomes necessary to depress the shoulders by gentle but firm downward traction of the arms. If the cervicothoracic junction still has not been adequately visualized and severe injury to the neck has been excluded, a so-called swimmer’s view may be obtained. The patient lies in a prone-oblique position with the higher tube-side arm above the head and the lower table-side arm beside his or her body. The x-ray tube is angled 15 to 20 degrees toward the feet.

Other special projections can occasionally be of assistance. Flexion and extension lateral views often reveal minor degrees of subluxations resulting from damage to ligaments.

Dr. Don Weir of Saint Louis University developed the “pillar view” for better evaluation of the lateral articulating masses, the superior and inferior articulating facets, and their intervening joints. Moreover, this view brings into focus the anterior and posterior margins of the lamina (Fig. 16-3). The film is produced with the patient supine. Each side is done separately. The head is turned very slightly to one side, with the roentgenograph tube angled toward the feet 35 to 45 degrees and centered over the middle to lower vertebrae. The opposite side is then examined in a similar manner.

Fig. 16-3 Pillar views of the cervical spine. Right (A) and left (B) projections.

When available, CT with three-dimensional (3-D) reconstruction can eliminate plain film radiographs altogether for assessment of cervical spine trauma, and this can be performed quickly with minimal patient movement.

ROENTGENOGRAPHIC ANATOMY

A thorough understanding of the roentgenographic anatomy of the cervical spine is an absolute prerequisite to its proper evaluation. Each component of the individual vertebrae should be appraised separately on all views.

Odontoid View

Of the seven cervical vertebrae, the first two are anatomically distinct. The odontoid represents a superior extension of the body of C2 and is actually the vestigial body of C1. On the anteroposterior open-mouth film, the odontoid should be analyzed in terms of its position between the two lateral articulating masses of C1 (see Fig. 16-2, D). The spaces between the lateral edges of the odontoid and the medial borders of the C1 articulating masses should be equal. However, minor degrees of rotation can produce spurious inequality of these interval distances. How can this be determined? The alignment of the densities of the spinous processes of C1 and C2 can be seen. If the C2 spinous process is to one side or the other, then rotation is present.

Also on viewing the odontoid film, the transverse processes and lateral borders of the articulating masses of C1 and C2 should not be overlooked. The horizontal joints between the atlantoaxial articulating masses should be symmetric (see Fig. 16-2, D).

Confusing artifacts superimposed on the odontoid can be misleading and can result in misinterpretation of fractures of this structure. The inferior margin of the anterior or posterior arch of C1 can overlie the base of the odontoid and create a “Mach” effect, a radiolucent line produced by overlap of the edges of two bones. In a similar fashion, the space between the two incisor teeth may lay over the odontoid and yield an artifactual vertical cleft (Fig. 16-4).

Fig. 16-4 Artifacts over the odontoid process simulating fractures. A, Anterior arch of C1. B, Cleft of the incisor teeth.

It is not unusual for the odontoid process to be completely obscured by the base of the occipital bone if the head is held too far in extension. In such circumstances, have the view repeated with the patient’s head slightly more flexed.

TRAUMA

Recognition of an abnormal cervical spine should be a relatively simple task once normal roentgenographic anatomy has been mastered. After a traumatic lesion has been identified, the primary objective is the establishment of whether the condition is stable. The stability of a fracture is best determined by grouping the type of injury according to the mechanisms of trauma, which are outlined in Table 16-3. From this classification, a statement of the instability of the injury can be made (Table 16-4).

Table 16-3 Classification of Cervical Spine Injuries According to the Mechanisms of Injury*

* From Harris JH Jr: Acute injuries of the spine, Semin Roentgenol 13:53–68, 1978. Used by permission.

Table 16-4 Classification of Cervical Spine Injuries Based on Stability*

* From Harris JH Jr: Acute injuries of the spine, Semin Roentgenol 13:53–68, 1978. Used by permission.

Flexion Injuries

There are a variety of flexion injuries, which are described below.

Subluxation

The roentgenographic findings in this stable lesion may be minimal, often requiring flexion and extension lateral views for confirmation. The body and posterior elements remain intact; that is, show no fracture. However, there is major soft tissue involvement. The interspinous and posterior longitudinal ligaments, as well as the interfacetal joint capsules, are disrupted at the affected level. This allows the involved vertebral body to rotate anteriorly about its anterior and inferior corner, along with upward and forward displacement of its inferior articular facet on the lower adjacent vertebra’s superior articular facet. The interspinous distance widens, and the inferior intervertebral disc space narrows anteriorly and widens posteriorly. These alterations are accentuated on the flexion film (Fig. 16-5). In children younger than 8 years, with the head held in flexion, the second cervical vertebra is normally displaced anteriorly over C3. This malalignment must not be mistaken for subluxation.

Fig. 16-5 Subluxation injuries of the cervical spine. A, Mild subluxation of C6 and C7. B, Atlantoaxial subluxation. Note the increased width of space between the odontoid and the anterior arch of C1 in B.

Bilateral Interfacetal Dislocation

A more severe form of subluxation, bilateral interfacetal dislocation, represents a truly unstable situation. The inferior articular facets of the involved vertebra move not only up and forward but also over the superior articular facet of the distal vertebra and come to rest within the intervertebral foramina. Oblique projections will be necessary to establish the diagnosis, but these must be done with extreme caution. The changes on radiographic film are related to a force sufficient to rupture all of the surrounding supporting ligaments at the level of the displaced vertebra. Occasionally, there is an associated avulsion fracture.

Compression Fractures

Forceful flexion of the neck can result in an uncomplicated, simple wedge-shaped compression fracture of one and sometimes two or three vertebral bodies. Because they are stable, extension and flexion films are permissible.

Flexion Teardrop Fracture

Considered the most dangerous and unstable of all cervical spine injuries, flexion teardrop fractures are diagnosed on the cross-table lateral film only, with no indications or justification for acquiring further studies. The name of the lesion is derived from the fact that there is a fracture through the inferior and anterior corner of the body, but the major component is comminution of the body. The posterior fragments become displaced backward into the spinal cord, with consequential acute and severe neurologic deficits. The posterior neural arch is also frequently fractured.

Clay-Shoveler’s Fracture

This lesion is named for the injury acquired by people in occupations that require heavy lifting. This injury consists of a fracture through the spinous process of either C6 or C7 (Fig. 16-6). There is no significant ligamentous damage, and therefore a stable condition exists. Because C7 is sometimes difficult to project on the lateral film, the fracture can be missed. A clue might be apparent on the frontal film, where an extraspinous process fragment is sometimes evident as the result of inferior displacement. An unfused apophysis of a spinous process must not be misinterpreted as a fracture (Fig. 16-7).

Fig. 16-6 Clay-shoveler’s fracture of the cervical spine. A, A lateral view defines spinous process fractures of C6 and C7 (arrows). B, An AP projection demonstrates a “double” spinous process of C6, suggesting a fracture (arrow).

Fig. 16-7 An unfused apophysis of the spinous process of C7 simulates a fracture (arrow).

Flexion-Rotation Injuries

When the cervical spine is subjected to a rotational force in addition to flexion, a unilateral interfacetal dislocation may be observed (Fig. 16-8). An inferior articular facet of one body is displaced over the adjacent superior articular facet of the next inferior vertebra on one side only. The dislocated facet is more or less locked in place, but because of associated ligament damage, the lesion may be unstable; therefore, flexion and extension views are contraindicated. Oblique projections are most productive in identifying the displaced facet, but it may be suggested on the lateral film when the margins of the facets of a single vertebra do not superimpose on one another as the result of rotation. Additionally, a line drawn through the posterior borders of the bodies is offset at the level of the suspected injury.

Fig. 16-8 Unilateral interfacetal dislocation. A, Lateral view. B, Oblique view.

Extension Injuries

Various extension injuries are described as follows.

Posterior Neural Arch Fractures

The neural arch of one vertebra may be compressed between the posterior elements of the two adjoining vertebrae during maximal forced extension. A unilateral or bilateral fracture may be sustained. If bilateral, the fragment can be displaced posteriorly, yet there will be no encroachment on the neural canal; thus, the situation is a stable one (Fig. 16-9).

Fig. 16-9 Posterior neural arch fracture.

Extension Teardrop Fracture

Similar to the flexion variety, an extension teardrop fracture demonstrates a triangular-shaped fragment at the anterior-inferior margin of the body, most often involving C2, although other levels are affected (Fig. 16-10). However, the injury is not quite as severe, because there is no posterior involvement. When the neck is held in flexion, there is stability of the spine because the posterior ligament complex is intact. However, instability occurs during extension because the minor fragment remains attached to the anterior longitudinal ligament but not to the parent body.

Fig. 16-10 Extension teardrop fracture of C2 (arrow).

Hangman’s Fracture-Dislocation of C2

This lesion consists of vertical disruption of both pedicles of C2 and is created by flexion forces against the extended vertebrae. The body of C2 is thrust forward over C3, with concomitant rupture of both the anterior and the posterior longitudinal ligaments, giving rise to an unstable injury (Fig. 16-11).

Fig. 16-11 Hangman’s fracture-dislocation of the second cervical vertebra.

ROENTGENOGRAPHIC EXAMINATION

The thoracic spine is ordinarily examined by means of anteroposterior and lateral films. However, because of the superimposition of the shoulders, the upper three to four vertebrae are omitted from the lateral view, and if clinical symptoms point to this region, an oblique swimmer’s projection should be obtained. It is generally agreed that the above films will offer optimum information about the anatomy of the thoracic vertebrae. However, either or both oblique views of no more than 10 to 20 degrees can be obtained and provide a different aspect of a questionable or subtle change in the vertebral bodies or articulating facets.

Routine roentgenographic examination of the lumbar spine must be composed of anteroposterior, lateral, and both oblique projections as well as a cone-down lateral film of the lumbosacral joint.

Roentgenographic Anatomy

DEVELOPMENTAL VARIATIONS AND CONGENITAL ABNORMALITIES

Confusing anatomic variations are seen in the developing and mature spine. In infancy, the vertebral bodies are egg-shaped on lateral view. The upper and lower anterior corners are beveled until the apophyseal vertebral rings appear about each end plate. They fuse with the body by the fifteenth year, but occasionally they remain ununited and, except for the presence of a complete sclerotic border, often are confused with a corner fracture (Fig. 16-12). These are sometimes referred to as limbus vertebrae.

Fig. 16-12 An unfused ring apophysis of a lumbar vertebra as seen here can be confused with a fracture.

On the anteroposterior film, a vertical radiolucent cleft is seen superimposing on the vertebral bodies (Fig. 16-13). This represents the normal uncalcified portion of the posterior neural arch and spinous process and is not to be confused with spina bifida. At approximately 3 to 5 years of age, these will ossify and fuse. When they do fail to fuse completely, the cleft persists as a spina bifida occulta. This can occur anywhere in the spine, but is most common at L5. These are of no clinical importance.

Fig. 16-13 Normal thoracic spine of an infant. Note the vertical cleft of the neural arch.

True spina bifida is the result of a very wide defect in the posterior neural arch, usually involving multiple vertebrae and commonly found in the lumbar region. The widened spinal neural canal is evidenced by an increased interpedicular distance. These may be associated with a meningocele that produces a prominent posterior soft tissue density on the roentgenogram.

This wide gap of the dorsal arch and increased interpedicular width should also bring to mind the possibility of diastematomyelia, a condition in which the cord is divided by an intraspinal cartilaginous, fibrous, or bony spur. This has a predilection for the thoracolumbar junction. Syringomyelia can produce a similar appearance in the cervical spine where there is cystic dilation of the cord. The same type of appearance can also be seen with intraspinal tumors, such as lipomas or dermoids.

Vascular channels may give rise to confusing appearances. Blood vessels perforate the anterior cortical borders of each body and form a radiolucent line, termed the clefts of Hahn. A similar vascular defect that becomes more apparent in adulthood may exist along the posterior margin of the body.

Hypoplasia of the vertebrae, fusion or block vertebrae, and hemivertebrae constitute a few of the other possible spinal anomalies (Fig. 16-14).

Fig. 16-14 Anomalies of the spine. A, Hemivertebra of the lumbar spine. B, Fusion or block vertebrae.

TRAUMA

The focus of attention will now be shifted to some of the traumatic lesions involving the thoracolumbar spine. The majority of injuries disturb the muscular and ligamentous structures surrounding the spine. Just as in the cervical spine, the only visible roentgenographic findings may be scoliosis and/or straightening of the normal curvatures, which indicates muscle spasms. When a fracture is present, the usual appearance is a wedge-shaped compression deformity, the result of hyperflexion forces. Avulsion-type corner fractures of the anterosuperior body result from extension forces. They appear as a very sharp, nonsclerotic fragment as opposed to an unfused apophysis.

Quite frequently, the dilemma of determining the age of a fracture presents itself. In long-standing compression fractures, degenerative changes with hypertrophic spurs arise along the articular margins, and there may be an unpredictable degree of eburnation. Yet at times, these changes are absent, and the determination of age may be difficult, if not impossible. Radioisotopic bone scans that use ^99mTc phosphate compounds then become a valuable aid. Within 2 to 5 days of the injury, a recent collapse shows an increased uptake of the isotope, whereas older lesions show no activity. However, the abnormal uptake may remain detectable for 18 to 24 months or longer, depending in part on the extent of the fracture and its location.

Wedge-shaped deformities of the vertebral bodies necessitate differentiation from other causative processes such as Scheuermann disease. This represents a form of osteochondritis or aseptic necrosis that involves the end-plate ring apophyses and is seen in adolescents. Described as juvenile kyphosis, it predominates in the thoracic region. Schmorl’s nodes commonly accompany this spinal affliction. These are rounded-to-oval protrusions of the anterior margins of the end plates and develop from focal intrabody herniation of the nucleus pulposus.

Biconcave deformities of the bodies, or so-called fish vertebrae, have a predisposition for the thoracic and lumbar location. They represent a chronic progressive form of compression characteristically seen in postmenopausal patients with osteoporosis. Vertebral body compression fractures with a history of minimal or no trauma should alert the physician to the possibility of a metastatic or primary malignant process.

Another important traumatic lesion, particularly of the lumbar spine, that can be easily overlooked is the transverse process fracture (Fig. 16-15). The importance of its recognition rests with the fact of frequently related renal injury. It is important not to mistakenly call the unfused apophysis of the transverse process a fracture.

Fig. 16-15 Transverse process fractures of the lumbar spine in two different patients. A, Fracture of L4 on the left on plain film (arrow). B, CT in another trauma patient demonstrating a fracture of the transverse process (arrow).

A seldom-seen injury to the lumbar vertebrae is the Chance fracture (Fig. 16-16). An extreme flexion force, often related to seat belt injuries, causes a fracture through the spinous process, across the neural arch or lamina, and into the posterosuperior aspect of the vertebral body. An isolated fracture of the spinous process is uncommon in the thoracic and lumbar regions.

Fig. 16-16 Chance fracture of the lumbar spine that extends through the body into the posterior neural elements and can be seen in individuals wearing seat belts. A, Anteroposterior view demonstrating lateral displacement of the pedicles (arrows). B, Lateral view showing a fracture extending through the body (curved arrow) and involvement of the neural arch (straight arrow).

One more pitfall to watch for in the assessment of spinal trauma is the unfused ossification center of the articular facets, which can resemble a fracture (Fig. 16-17). In evaluating the posterior elements, it is important to keep in mind the entities of spondylolysis and spondylolisthesis (see Chapter 8).

Fig. 16-17 Unfused apophysis of the superior articular facet of L5 (arrow).

TUMORS

Except for metastatic neoplasms to the spine, tumors—both benign and primary malignant lesions—are very infrequent. The vertebrae are the most commonly affected portion of the skeleton for metastasis and account for approximately 40% of all secondary tumors of bone. A few of the common and not-so-common tumors are discussed.

Hemangioma

Hemangiomas of the vertebral body may be the most common benign tumor, although few have been documented pathologically. Typically, their appearance is that of increased vertical trabeculations producing a somewhat striated pattern (Fig. 16-18). They should not be confused with Paget’s disease or osteoblastic metastasis.

Fig. 16-18 Hemangioma of a lumbar vertebra. A, Plain film. B, CT image. Note incidental finding of a herniated disc (arrow).

INFECTION

For obvious reasons, infections involving the spine have decreased in frequency over the years. Nevertheless, this must be considered in the differential diagnosis of a painful back and abnormal roentgenographic findings. Acute bacterial infections or pyogenic spondylitis can involve either the disc space, the vertebral body, or both. The former condition, termed pyogenic discitis, is more common in the younger population, apparently on the basis of a healthy vascularization of the cartilage and, therefore, easy access by blood-borne bacteria. This results in a closed-space infection that can secondarily extend into the bodies.

On the other hand, infection of the vertebral body, particularly its anterior two thirds, is more commonly identified in the adult. Originating in the substance of the body, the infection may then secondarily involve the interspace by extension.

An important principle in pathophysiology should be reemphasized at this point. Cartilage serves as no barrier to the extension of infection and thereby is vulnerable to destruction. Conversely, the disc cartilage is very resistant to malignant cellular infiltration, so tumors tend to remain confined to the body.

In both pyogenic discitis and vertebral osteomyelitis, the earliest roentgenographic findings may be joint space narrowing. Depending on the aggressiveness of the offending bacteria and the time of institution of therapy, the surrounding bone shows varying degrees of demineralization brought on by hyperemia. The end plates then demonstrate progressive loss of continuity with irregular destruction and focal areas of subchondral reactive sclerosis. Extension into the posterior third of the body, then into the dorsal appendages, as well as into the adjoining vertebrae, may occur. The body may eventually collapse. Some degree of surrounding soft tissue swelling is invariably present and detectable on roentgenograms and corroborated on MRI (Fig. 16-19).

Fig. 16-19 Pyogenic discitis. A, Anteroposterior and B, lateral views of the thoracic spine demonstrating pyogenic discitis (large arrows). The disc space has been destroyed with extension of the infection into the vertebral bodies with trabecular compression and reactive sclerosis. An associated paravertebral soft tissue mass (open arrows) indicates expansion of the inflammatory process. Note aorta (small arrows). C, Corresponding sagittal T-I post-gadolinium MRI images exhibit the infection extending through the posterior neural elements (pedicles and lamina) into the epidural space and producing cord compression.

Tuberculosis of the spine, or Pott’s disease, is an extreme rarity among today’s abnormal spine studies. The vertebral column remains the most common skeletal site for this chronic infection. The midthoracic and thoracolumbar junctions constitute the most prevalent sites of involvement.

Because of its insidious and chronic nature, the spinal lesions are usually advanced when first examined roentgenographically, although this is not always the situation. An occasionally prominent feature is a large paravertebral soft tissue mass that constitutes the abscess. In later phases, there may be extensive calcification within this mass (Fig. 16-20). The bony structures show a decrease in density, and there is narrowing of one or more of the disc spaces. The margins of the end plates manifest irregular destruction, and a mottled sclerotic and lytic appearance extends into the bodies, which display varying degrees of compression.

Fig. 16-20 Pott’s disease of the spine. The chronic tuberculous process has resulted in calcified paravertebral psoas abscesses.

PAGET’S DISEASE

One of the distinctive roentgenographic characteristics of spinal Paget’s disease is that the disorder involves the entire vertebra—the body and all of the posterior elements, including the transverse and spinous processes (Fig. 16-22). Generally, there is an increase in density produced by a thickened trabeculae. A picture frame appearance can be imparted to the body, the result of a dense peripheral margin of thickened cortex. Furthermore, there is an actual increase in volume of the entire vertebra. This finding, along with total vertebral involvement, distinguishes Paget’s disease from an osteoblastic metastasis.

Fig. 16-22 Spinal Paget’s disease and osteoblastic metastasis. A, Paget’s disease demonstrates increased trabeculations, a bone-within-bone appearance, and increased volume including the pedicles. B, Osteoblastic metastasis.

ROENTGENOGRAPHIC EXAMINATION

In the majority of cases, a complete and optimal roentgenographic study of the shoulder girdle needs only include upright—standing or sitting—anteroposterior views with the humerus in internal and external rotation. These two projections usually permit adequate evaluation of the bony structures constituting the shoulder. In the uncooperative child or severely traumatized patient, these films may be obtained while the patient is in the supine position. On these roentgenograms, the complete clavicle should be visualized in addition to the entirety of the scapula and at least the proximal third of the humerus. Special attention should be given to the alignments of the glenohumeral, acromioclavicular, coracoclavicular, and sternoclavicular joints. The tuberosities of the humeral head and its articular surface should be specifically scrutinized. The acromium and coracoid processes, in addition to the borders and flat surfaces of the scapula, require attention.

The standard examination, of course, may require modification, because there are certain situations when special views must be obtained for better delineation of specific structures. In an acutely injured shoulder in which immobilization of the shoulder is required, the humerus ordinarily is held in internal rotation. To obtain roentgenograms of the humerus in external rotation without moving the arm, patients can be positioned by rotating them 40 degrees posteriorly with the affected side toward the film. Because of pain, this requires that patients be sitting or standing.

Upright exposures are also beneficial in demonstrating fat-fluid levels. This occurs when a fracture has extended into the joint through the articular cortex; this most often happens with fractures of the greater tuberosity. Such information is useful since articular cartilage damage is certain, and the patient should be informed that degenerative osteoarthritis in time is a possibility.

When there is suspicion of a fracture involving the shaft of the humerus below the neck, in addition to an anteroposterior view, a transthoracic lateral projection is necessary for evaluation of alignment (Fig. 16-23). The glenohumeral relationship can also be accessed with proper exposure, but because of superimposed ribs, fractures above the neck are difficult to see. Instead, a transscapular study is more helpful (Fig. 16-24). This view is obtained by having the patient face the film at an angle of 45 degrees with the affected side toward the film holder. The resultant picture forms a Y where the acromium, coracoid, and scapular body intersect. The humeral head will be superimposed on the Y. This is useful not only for fractures but also for posterior dislocations. This transscapular examination is always used when studying the scapula, in addition to the routine anteroposterior roentgenogram. CT or MRI imaging, particularly the latter, can provide additional valuable information especially regarding soft tissue involvement. The multiplanar capabilities of these imaging modalities augments visualization of gross pathologic findings.

Fig. 16-23 Transthoracic view of the shoulder.

Fig. 16-24 The scapular Y view. The humeral head should be centered at the intersection of the coracoid, spine, and body. This view also can visualize any bony abnormalities of the underneath surface of the acromion which could cause impingement.

Posterior shoulder dislocations are notorious for their ability to avoid detection on the standard anteroposterior views; they are frequently associated with seizure disorders. The tangential projection is an excellent means for detecting the abnormality. This is performed by angling the roentgen tube approximately 30 degrees so that the central ray passes tangentially across the glenoid articular surface. In this manner, the glenohumeral relationship is much better defined than on the conventional straight anteroposterior view.

The axillary film also provides another perspective of the shoulder anatomy (Fig. 16-25). The acromium, coracoid, glenohumeral joint, and humeral head are viewed in a different plane. By holding the film against the top of the shoulder, the picture is produced by abducting the arm and centering the roentgen tube through the axilla. The study requires a special “grid” cassette for the film. It is helpful in determining the presence of a humeral head displacement, but it is not recommended when a dislocation has just been reduced, because the required abduction may reluxate the joint.

Fig. 16-25 Axillary view of the shoulder. Note the glenohumeral relationship, the hooklike coracoid process, and the acromium behind the head of the humerus.

The clavicle itself requires two projections in the anteroposterior plane to evaluate it properly. One film should be made perpendicular to the plane of the body, whereas the other exposure is made with a 15- to 20-degree cephalad angulation.

The acromioclavicular joints are best inspected with an anteroposterior film and tilting the roentgen tube 15 degrees toward the head. Comparative films are necessary to determine the presence of a separation, and this should be performed with and without weight bearing.

A more difficult area to examine roentgenographically is the sternoclavicular joint. Because of bony superimposition, CT may be required, but because this is not universally available, both oblique and lateral views may suffice. However, if these attempts are unsuccessful, the patient can be positioned prone. The roentgen tube is then centered through the sternoclavicular joints and angled toward the head at a 35-degree tilt.

ROENTGENOGRAPHIC ANATOMY AND DEVELOPMENTAL VARIATIONS

A considerable degree of confusion can be created by the ossification centers of the shoulder. The proximal humeral epiphysis does not become visible, as a rule, until the fourth to eighth month of life. There are two, occasionally three, separate centers. By the age of 20, the proximal epiphysis fuses to the shaft of the humerus. Before complete closure of the epiphyseal suture, the lucent line has a peculiar angulated appearance (Fig. 16-26). Overlap of the suture line occurs no matter what projection is used and consequently produces an image simulating a fracture.

Fig. 16-26 Normal proximal humeral epiphysis in a child.

A deceptive appearance of the proximal humeral shaft is the deltoid tuberosity, which, incidentally, receives the tendinous attachment of the deltoid muscle. The thickened cortex in this area may bring to mind the periosteal reaction of infection or tumor.

On a congenital basis, there may be a complete absence of the clavicle or at least partial underdevelopment of its lateral end. This represents hereditary cleidocranial dysplasia, a disease that also affects the skull, pelvis, hips, and other skeletal regions.

Normally, the outer end of the clavicle appears less dense than the middle and sternal aspects, the result of a lesser thickness of overlying soft tissue. Soft tissues are also responsible for a discrete line density paralleling the superior border of the clavicle that measures no more than 4 mm in width and is described as the “accompanying shadow.” This shadow may be thickened in cases of subtle fractures.

The rhomboid fossa is a familiar anatomic variation of the clavicle and is found along the inferior border at its medial aspect as a notch-like depression. This represents the location of the insertion of the costoclavicular (rhomboid) ligament, which secures the first rib to the clavicle. Its irregular appearance has been misinterpreted as a destructive process.

TRAUMA

Of all the joints in the body, the shoulder is the most frequent site of dislocations. More than 97% are anterior and can be described as subglenoid or subcoracoid, depending on the location of the humeral head. Invariably, this form of dislocation can be visualized by a single anteroposterior roentgenogram (Fig. 16-27). Whenever a dislocation occurs, it is not unusual to have an associated fracture; this should be searched for on the film. With an anterior dislocation, a fracture of the greater tuberosity may exist; the lesser tuberosity is vulnerable in posterior dislocations. The lower glenoid margin is also susceptible in either type of dislocation. An infraction of the articular cartilage without a visible fracture is also possible in dislocations and may require MRI to demonstrate.

Fig. 16-27 Anterior dislocation of the shoulder, AP.

Not infrequently with intraarticular extension of a fracture in the absence of true dislocation, an intact joint capsule may become progressively distended with blood, and if enough blood accumulates, there will be inferior displacement of the humeral head away from the glenoid. This condition is termed pseudosubluxation, or hanging shoulder.

With chronic recurring anterior dislocations, a defect becomes apparent along the superolateral aspect of the humeral head. This cortical infraction, present because of impaction against the anteroinferior rim of the glenoid, is commonly referred to as Hill–Sachs deformity. This abnormality is usually best demonstrated on an internally rotated anteroposterior view (Fig. 16-28).

Fig. 16-28 Hill–Sachs deformity in chronic recurring anterior shoulder dislocation.

The less frequent posterior dislocation is much more difficult to visualize roentgenographically. The problem arises because the humeral head on the standard anteroposterior projections shows no apparent separation from the glenoid fossa when in fact it is separated. The posterolateral orientation of the glenoid articular surface accounts for this to some extent. Ordinarily, in the normal shoulder the head of the humerus overlaps about three fourths of the glenoid fossa. When it becomes posteriorly dislocated because of lateral displacement of the head, this overlap is less, but the change may be very subtle. This perplexing situation can usually be solved by using the 30-degree tangential view, the axillary projection, or the transscapular film (Fig. 16-29). With recurrent posterior dislocations of the shoulder, a defect of the inferoposterior cartilagenous labrum may not be visible by stand-ard radiography. This is termed a Bankart lesion, which can be diagnosed by MRI or CT arthography.

Fig. 16-29 Posterior shoulder dislocation. A, Anteroposterior view showing some discrepancy in the glenohumeral joint. B, A posterior fracture-dislocation.

A very rare form of dislocation of the shoulder joint is luxatio erecta, a condition where the humeral head is located under the glenoid rim and the humeral shaft is directed above the head in fixed abduction (Fig. 16-30). A lateral fall on an elevated arm produces such an abnormality. The acromium process of the scapula acts as a fulcrum pushing the head of the humerus down and out of the glenohumeral joint. The dynamics of the applied forces can result in a fracture of the acromium, the lower lip of the glenoid, or the greater tuberosity of the humerus. Often, there is an associated tear of the rotator cuff.

Fig. 16-30 Luxatio erecta of the shoulder. Note the superior orientation of the humeral shaft and the head of the bone locked under the glenoid.

Fractures involving the proximal portion of the humerus are classified according to involvement of the head, either tuberosity, or the surgical or anatomic neck. Additionally, a fracture-displacement of the yet unfused epiphysis may occur.

For descriptive purposes, anatomists describe the surgical neck as that portion of the humerus just inferior to the tuberosities where there is a normal narrowing. The constriction or shallow groove at the articular margin of the head of the humerus is referred to as the anatomic neck.

Fractures of the proximal portion of the humerus must be evaluated by at least two views at right angles to one another. An anteroposterior view alone with a transthoracic lateral projection fulfills this criterion, but obliquely directed x-ray films may be necessary to best demonstrate displacement and/or angular deformity.

Transverse subcapital fractures of the surgical neck along with avulsions of the greater tuberosity are the most commonly encountered injuries to the shoulder. A fracture-displacement of the ununited epiphysis creates a more difficult diagnostic challenge. The normal epiphyseal line itself, as already mentioned, makes interpretation troublesome. The normal epiphysis has a uniform width and dense margins, whereas a fracture demonstrates varying thickness with a sharp, nonsclerotic edge. Often, there is impaction or distraction of the fragments causing some degree of overlapping, which makes the diagnosis somewhat simpler. Of course, if the presence of an abnormality is uncertain or indeterminate, an accurate comparison with the opposite uninjured shoulder should be performed.

The infraglenoid area of the scapula is the most common location of fractures of this bone. The normal lucent nutrient canal in this region with its dense margins should not offer much concern. Instead of a discrete fracture line, only a zone of increased density offers any clue to the injury. The clinician should also search for underlying rib fractures.

A complete fracture of the clavicle in most circumstances will exhibit overriding of the fragments when they involve its midportion. Because of the forces applied by the pectoralis minor muscle, as well as the weight of the arm, the lateral fragment is displaced inferiorly in most cases. An incomplete or nondisplaced fracture may be so subtle as to avoid detection initially, but the two standard views, including straight anteroposterior and cranially angulated projections, will in most instances reveal the fracture.

TUMOR

The proximal aspect of the humerus is a relatively common location for a benign solitary cyst, which may appear as either simple or multiloculated (Fig. 16-31). Such cysts are found within the metaphysis and exhibit destruction of the medullary spongiosa with more or less expansion of the bone. Thinning of the cortex can attain paper thickness, and pathologic fractures are a very common event, even in the absence of significant trauma. When initially discovered, the cysts extend to the epiphyseal line but do not involve the growth center itself. When followed serially until fusion of the epiphysis, the cyst appears to “migrate” toward the middle of the shaft as growth of the bone ends progresses.

Fig. 16-31 Benign cysts of the humerus. A, A multiloculated cyst in the metaphysis shows a fresh pathologic fracture. B, A simple solitary cyst in the midshaft demonstrates a healing fracture.

Primary malignant tumors of the shoulder are a rarity and are often of the sarcomatous variety. Osteogenic sarcoma as well as the round-cell tumors, Ewing’s sarcoma, and reticulum cell sarcoma have already been described. Secondary metastatic disease is much more prevalent than primary lesions, particularly disease of the proximal aspect of the humerus. As with cysts, pathologic fractures frequently accompany these malignant changes.

ARTHRITIS

The roentgenographic alterations of osteoarthritis of the shoulder are similar to those seen in the knee and are described in the section about the knee. However, the process more often has a posttraumatic relationship. Rheumatoid arthritis of the shoulder, as in other joints, demonstrates articular erosions without much bone production or osteophyte formation as is seen in osteoarthritis. However, unique to this area, erosions of the lateral ends of the clavicles may be present. This pathologic change, nevertheless, is not specific for rheumatoid arthritis and may be seen in hyperparathyroidism and other less common diseases.

CALCIFIC TENDINITIS

Calcific tendinitis represents a disease entity that is particularly prone to develop in the shoulder. The majority of roentgenographic examinations performed for this disease are negative for abnormal changes. The radiographic diagnosis is made by the identification of calcifications involving the tendinous insertions of the rotator cuff muscles into the head of the humerus. This most commonly affects the supraspinatus tendon as it crosses over the superior articular surface of the humeral head and inserts into the greater tuberosity. Pathogenically, the presence of calcium infers organization of a focal hematoma resulting from tearing of some of the tendon’s fibers. The subacromial bursa overlies and is in contact with the supraspinatus tendon and, because of this close relationship, may become secondarily inflamed. In fact, rupture of the puttylike calcifications into the bursa heralds the onset of subacromial bursitis. The density of the calcium can be seen to extend along the lateral aspect of the humeral head as it finds its way into the subdeltoid extension of the subacromial bursa.

ROENTGENOGRAPHIC EXAMINATION

Standard examination of the elbow includes a straight anteroposterior film with the forearm fully extended and supinated, as well as a true lateral view with the joint flexed 90 degrees and the forearm supinated. Not uncommonly, particularly with trauma, the patient is unable or unwilling to extend or flex the elbow. To delineate the distal humeral articulating surface when the elbow is held in flexion, the anteroposterior view can be obtained by orienting the central beam of the x-ray tube perpendicular to the distal humeral shaft. In the same way, the proximal radial and ulnar details and relationships can be observed by pointing the tube perpendicular to their shafts. Medial and lateral oblique exposures may become necessary to better analyze the radial head and shaft, the humeral condyles, or the coronoid process of the ulna.

ROENTGENOGRAPHIC ANATOMY

Before considering specific elbow lesions, pertinent roentgenographic anatomy of the elbow must be considered. Evaluating the different bony landmarks and their relationships can prove extremely useful. In children, a great deal of confusion arises because of the ossification sequence of the growth centers. The capitellum, trochlea, and the medial and lateral epicondyles compose the centers of the distal portion of the humerus. In addition, the elbow contains the olecranon apophysis and the radial head epiphysis (Fig. 16-32).

Fig. 16-32 Normal elbow of a 9-year-old child. A, Lateral view. B, Anteroposterior view.

The first of the distal humeral epiphyses to appear is the capitellum, which develops by the age of 2 years. At about 6 years of age, the medial epicondylar center becomes visible. Next to become evident is the trochlea at 10 years of age, and finally, the lateral epicondyle appears near the twelfth year of life.

The radial head epiphysis calcifies at about the same time as the medial epicondyle, in the vicinity of the sixth year. Before the age of 10 years, the olecranon ossification center is not visible.

The ossification centers normally fuse between the ages of 14 and 16 years, although union of the medial epicondyle may not occur until the eighteenth year of life.

Following their union, the medial and lateral epicondyles form the flared segment of the distal third of the humerus. Between the condyles on both the ventral and dorsal surfaces are indentations identified as the coronoid and olecranon fossae, respectively, as they relate to the anatomic segments of the ulna. This area may be very thin and appear as a zone of rounded rarefaction on the anteroposterior film. In fact, an actual opening, termed the supratrochlear foramen, is occasionally present.

Contained within these recesses are the fat pads, fairly sensitive indicators of the presence of a distended joint capsule. In the normal elbow, assuming a true lateral flexion film is obtained, the posterior fat pad is not visible. The anterior fat pad forms a slim, triangular lucency adjacent to the anterior humeral cortex. With the presence of joint fluid, such as blood resulting from an intraarticular fracture, one or both of these fat pads will be elevated. The so-called fat-pad sign is not specific and may be seen in conditions other than trauma, such as pyarthrosis or rheumatoid arthritis. In cases of injury, a positive fat-pad sign should be searched for from the beginning and, if found, should initiate further investigation for an occult fracture, particularly of the radial head (Fig. 16-33). Oblique views may be required, and if necessary, a repeat examination may be done in 7 to 10 days, at which time a fracture should be apparent. On occasion, no fracture will be found, and the distended joint may be on the basis of a cartilage infraction or capsular tear. In adults, an intraarticular fracture may be present in the absence of fat-pad elevation, but in children it is a more reliable indicator, being found in approximately 90% of elbow fractures (Rogers, 1978).

Fig. 16-33 Radial head fracture with a positive fat-pad sign. A, Lateral view showing elevation of both anterior and posterior fat pads (arrows). B, A fracture line through the radial head (arrow) is well delineated on the anteroposterior projection.

Two other helpful relationships are of assistance when evaluating normal elbow anatomy. The first is the anterior humeral line. On the lateral film, a line is drawn along the anterior margin of the humeral shaft and extended through the joint. If the capitellum is divided into equal thirds, this line normally passes through the middle third. It is a simple and useful index in analyzing the normal 140-degree angle that the articular structures form with the shaft of the humerus. This easy maneuver often alerts the physician to a subtle transcondylar fracture in children: the line extends through or anterior to the anterior third division of the capitellum (Fig. 16-34).

Fig. 16-34 Transcondylar fracture of the humerus. A, lateral view revealing anterior and posterior fat-pad elevation. An obvious fracture is identified, but note how the anterior humeral line extends anterior to the capitellum. B, Anteroposterior view clearly outlining the fracture (arrows).

The second practical indicator is the radiocapitellar line, denoted by a line drawn through the longitudinal axis of the radius. This line always passes through the capitellum, regardless of which projection is being viewed. When the radial head is dislocated, this relationship no longer exists. Whenever there is a fracture of the ulnar shaft, this procedure should be used because the radial head is often dislocated in what constitutes a Monteggia fracture.

TRAUMA

Sixty percent of all elbow fractures in children are supracondylar. The lateral epicondyle is involved about 15% of the time, and the medial epicondyle accounts for 10% of all fractures. The radial and olecranon ossification centers are infrequently traumatized. In adults, the radial head or neck marks the most common elbow area for fractures. Unlike the pediatric age group, adults seldom suffer fractures of the distal third of the humerus.

A supracondylar fracture should more aptly be called a transcondylar fracture because it extends across the condyles and is seen through the coronoid and olecranon fossae (see Fig. 16-34). The anterior humeral line indicates posterior displacement of the distal fragment when the fracture is complete, which is seen in 75% of cases. However, the remainder are incomplete, and visualizing the fracture line may be impossible. Almost always the posterior fat pad is elevated.

When the lateral epicondylar epiphysis is traumatized, it usually contains a fragment of metaphyseal bone. Because it serves for the attachment of forearm extensor tendons, the fragment becomes displaced posteriorly and inferiorly.

Little Leaguer’s elbow, discussed in Chapter 6, describes an injury in which the medial epicondyle is separated. Ordinarily, the avulsed center is best identified on the anteroposterior film, often lying adjacent to the capitellum. In rare instances, the fragment may become displaced into the medial joint space, where it can simulate the trochlear ossification center before its actual appearance.

The coronoid process is frequently avulsed in posterior dislocations of the elbow and is impacted against the trochlea. Whenever an elbow dislocation occurs, the radius and ulna are displaced lateral and posterior to the humerus in almost all instances. In children, when the radius and ulna are medial to the humerus, there is not a dislocation but rather a fracture through the entire distal humeral epiphysis. This is extremely important to recognize, because treatment of the two are entirely different.

ROENTGENOGRAPHIC EXAMINATION

Routine roentgenographic study of the wrist consists of posteroanterior, oblique, and lateral projections. The oblique film is achieved by orienting the wrist at a 45-degree angle to the plane of the film with the ulnar side down. There are several special wrist views that are necessary for the evaluation of specific anatomic features. One of the more frequently used studies is that for the carpal navicular bone. A fracture of this bone may easily escape detection. When clinical symptoms point to a navicular injury yet the routine study appears normal, this view becomes mandatory. The palm is placed against the film holder with the thumb and index finger spread apart. The tube is angled 45 degrees toward the elbow. This results in an elongated appearance of the navicular, but the distortion is minimal, and it is projected free of superimposition from the other carpal bones. The midportion of the navicular, where the majority of fractures occur, is clearly outlined.

Roentgenographic examination of the phalanges and metacarpals is performed with posteroanterior and oblique films. When the fingers are evaluated, the individual digits should also be examined with true lateral views.

ROENTGENOGRAPHIC ANATOMY, DEVELOPMENTAL VARIATIONS, AND PATHOLOGY OF THE INDIVIDUAL BONES

In this section on the hand and wrist, anatomy, variations, and abnormalities (especially those of the individual carpal bones) are considered together.

The carpus is composed of a proximal and distal row of four bones each. To remember their names, the infamous mnemonic of “Tilly’s pants” still applies, but a picture is sometimes more than words can describe (Fig. 16-36).

Fig. 16-36 Normal carpal bones: 1, navicular; 2, lunate; 3, triquetrum; 4, pisiform; 5, greater multangular (trapezium); 6, lesser multangular (trapezoid); 7, capitate; 8, hamate.

The proximal row is concave in alignment toward the hand, whereas the distal row is more or less convex in the same direction as seen on the anteroposterior film. The navicular, lunate, triquetral, and pisiform bones constitute the proximal column; the distal row is made up by the greater multangular (trapezium), lesser multangular (trapezoid), capitate, and hamate bones. For proper interpretation, each bone should be identified and scrutinized separately.

Throughout the skeleton, constitutional disease such as congenital hypothyroidism (cretinism) and other endocrine and metabolic diseases affect and alter the time of appearance and growth rate of ossification centers, but the wrist and hand together provide one of the most sensitive indicators of growth retardation of bone age. A handbook for the normal growth and development of the hand and wrist is found in most radiology departments (Greulich and Pyle).

A few infrequently described congenital abnormalities affect the carpus. Agenesis or hypoplasia of one or more of the bones may be found. Fusion anomalies or synostoses also are occasionally seen.

Anatomic variations occur with such frequency that some of the more common ones are described to permit differentiation from pathologic conditions.

Accessory bones do appear about the carpal bones but are very infrequent. They are mentioned merely to make one aware of their existence, and they need to be recalled when considering the possibility of an avulsion fragment.

Small, well-defined, round to oval areas of increased density are often identified in any of the carpal bones, metacarpals, and phalanges. They represent clinically insignificant bone islands (Fig. 16-35).

Fig. 16-35 Angulated navicular view. An incidental finding is a dense bone island within the body of the carpal navicular bone.

Tiny, rounded lucencies with well-delineated sclerotic margins coincide with vascular channels, but cystlike areas are also frequently encountered within or along the cortical borders of any of the hand or wrist bones, especially the carpal bones. These may be the result of medullary fibrosis or hemorrhagic cysts. When related to the articular cortex, these cysts are the result of synovial herniation in osteoarthritis when the other classic signs of this disease are present. Erosive cysts at the justaarticular margins are diagnostic of rheumatoid arthritis.

In the forthcoming discussion, all of the individual wrist and hand bones are considered separately.

Navicular

The navicular bone becomes visible somewhat late, usually by the fifth to sixth year. This bone may be entirely absent (agenesis), or it may become assimilated (fused) with the radial epiphysis. A small hypoplastic navicular may result in a malformed radial–carpal joint. Quite often, a tubercle arises from the distal and lateral corner of the bone. A partial division of the navicular resulting from incomplete fusion of two ossification centers can simulate a fracture. This condition requires follow-up studies to determine healing, but the same may be found in the contralateral wrist.

Serial examinations of a fractured carpal navicular bone at about 3- to 4-week intervals are imperative in view of the possible complications. These include malunion, nonunion, and avascular necrosis. The latter presents as a progressively increasing sclerosis of the proximal fragment as a result of vascular interruption to this segment.

Transverse fractures of the navicular most often involve the midportion, but any segment can be involved, including avulsions of the lateral tubercle. Dislocations of the navicular are discussed in the following section with the lunate bone. In recent years, these posttraumatic findings of navicular fractures and complications have been readily assessed by means of MRI (Fig. 16-37).

Fig. 16-37 A, A standard linear roentgenographic tomogram of the wrist shows the faint sclerotic line of a healing fracture. B, MRI of both wrists in the same patient confirms the clinically suspected avascular necrosis of the proximal carpal navicular fragment (arrow).

Lunate

Around the fourth to fifth year of life, the lunate bone becomes visible by ossification. Like the navicular, it may develop from two separate centers; if these centers fail to fuse, complete or partial fracturelike lines result.

One of the most notable pathologic changes affecting the carpal lunate is Kienböck’s aseptic necrosis. This form of osteochondritis is described in more detail in Chapter 7. The carpal lunate is infrequently fractured but is involved in one of the more important traumatic lesions of the wrist—dislocations. Three important types of wrist dislocations have been described. The first is a transnavicular perilunate dislocation (Fig. 16-38). In this condition, there is a fracture at the midnavicular. The proximal pole fragment and the lunate maintain their normal relationship with the radial articulation, but the distal pole segment of the navicular bone and the remaining carpal bones become displaced posteriorly.

Fig. 16-38 Transnavicular perilunate dislocation. A, Posteroanterior view. Note the obvious displaced fracture of the navicular and the overlap of the proximal pole fragment and the capitate, which should alert one to the presence of more than just a simple fracture. B, Lateral view showing typical posterior displacement, particularly of the capitate, with the lunate maintaining its normal position.

In a perilunate dislocation, the navicular is intact, and it along with the carpus becomes dorsally dislocated. The lunate remains in normal position.

The third type of carpal displacement is a pure lunate dislocation (Fig. 16-39). The articular relationship between the lunate and the capitate is disrupted. The lunate rotates anteriorly, which is best appreciated on the lateral view. On the anteroposterior film, the lunate takes on a somewhat triangular appearance, which should alert the clinician to this abnormality.

Fig. 16-39 Pure lunate dislocation. A, The lateral view reveals the semilunar articular margin of the lunate faces forward. B, On the frontal view, note the abnormal relation of the lunate to the navicular, capitate, and triquetrum.

Metacarpals and Phalanges

When describing the individual fingers, the term ray is used. Each of the five rays is composed of a metacarpal and its three associated phalanges (proximal, middle, and distal).

A number of congenital abnormalities exist at birth. Included are fusion of two or more of the digital rays, a condition referred to as syndactyly. Duplicative anomalies, or polydactyly, can involve any of the rays. Arachnodactyly of the fingers occurs in the generalized skeletal disorder of Marfan’s syndrome; here, the digits are elongated and very thin.

When assessing the metacarpal bones, there are several important anatomic features to be considered (Fig. 16-40). The epiphysis of the first metacarpal bone (the thumb) is located proximally, whereas the growth center occupies the distal aspect of the remaining four metacarpals. This property is consequential in terms of an examination made for assessing the presence of fractures in the growing patient. But, as is usual, there is always some variation to confuse the issue. On occasion, the clinician may see what appear to be epiphyses involving the distal first metacarpal or the proximal second metacarpal; these are appropriately termed pseudoepiphyses.

Fig. 16-40 Normal hand bones of a 9-year-old child. Note the position of the epiphyses of the metacarpal bones.

Sesamoid bones about the hand and wrist deserve mention because of their frequency and occasional mistaken identity as fractures. They typically overlap the heads of the metacarpal bones. A fracture of a sesamoid is very rare, being more commonly found in the foot.

The appearance of the phalanges, particularly the terminal ones, is extremely variable, and the variants, for all practical purposes, should be considered normal. The ungual tuberosity or tuft of the distal phalanx can have many shapes and sizes, yet a number of disease processes may alter them considerably. Deformity and erosive changes of the tufts are seen in scleroderma, sarcoidosis, psoriasis, and leprosy.

Cystlike or erosive changes in the tufts can be ascribed to glomus tumors, which produce pressure erosion of bone and are accompanied by severe pain. Enchondromas can involve any of the phalanges and are prone to pathologic fracture.

Of all of the fractures involving the fingers, those of the tuft are probably the most common and may be avulsion types or comminuted. When analyzing the injured finger, the importance of obtaining a true lateral view, in addition to anteroposterior and oblique projections, cannot be stressed too much (Fig. 16-41). This becomes particularly apparent in cases of volar plate injuries. A fragment of bone is avulsed from the palmar aspect at the base of the middle phalanx and involves the proximal interphalangeal joint. The result of hyperextension forces, the fragment may be obscured on all but the lateral film.

Fig. 16-41 Volar plate fracture of the finger. A, Posteroanterior view. B, Oblique view. C, Avulsion fracture of the volar plate (arrow).

Degenerative osteoarthritis is a relatively common disorder of the terminal interphalangeal joints of the elderly. The deformity is characterized by joint space narrowing, irregularity of the articular margins, subarticular sclerosis, and hypertrophic spurs. Clinically, palpable Heberden’s nodes are a distinctive, characteristic finding of this disease.

Rheumatoid arthritis produces distinctive transformations in the joints of the hand and wrist (Fig. 16-42). The earlier roentgenographic finding may be periarticular soft tissue swelling. Later, demineralization about the joint with slight widening of the joint space will be noted—the result of inflammatory hyperemia with intraarticular fluid and synovial thickening. This progresses to juxtaarticular cortical erosions, which are related to synovial hypertrophy and pannus formation. Eventually, the joint space becomes narrowed, and classic ulnar subluxations occur. Unlike degenerative osteoarthritis, there is no reactive spur production in rheumatoid arthritis.

Fig. 16-42 Typical radiographic findings of rheumatoid arthritis of the hands.

Hyperparathyroidism produces a pathognomatic change in the hands. Typically, there is subperiosteal resorption along the radial aspect of the middle phalanges.

ROENTGENOGRAPHIC EXAMINATION

The anteroposterior view is the only standard projection required for roentgenographic examination of the pelvis. This film should include the entirety of the body pelvis, from iliac crests to the ischial tuberosities, which inherently encompasses the sacrococcyx and sacroiliac joint. Furthermore, both hips are imaged on the film, and this should include the femur to the subtrochanteric region (below the lesser trochanter) whenever possible. There are special projections that aid the analyses of certain pelvic segments and clarify suspected regions. Films performed in the anteroposterior direction with the x-ray tube angled 30 degrees toward the head (cephalad) and 30 degrees toward the feet (caudad) are practical under certain circumstances. These give a different perspective of the sacroiliac joints, sacrococcyx, the iliac wings, and the anterior pelvic arch (the ischiopubic rami). Nondisplaced fractures of the rami may go undetected initially on the straight anteroposterior exposure but can be clearly delineated on these angled projections.

The anteroposterior views may suffice for complete preliminary pelvic assessment in cases of trauma. Minimal motion of the patient is the best policy to prevent possible compromise to already injured soft tissues, for example, blood vessels and urinary bladder. However, oblique films can be helpful adjuncts on follow-up studies. The supine patient is first rotated 45 degrees with the right hip down against the film (right posterior oblique) and then to the left 45 degrees (left posterior oblique). Because of the outward orientation of the sacroiliac joints, oblique films allow the viewer to look straight down the joints without confusing overlap of the free edges. Moreover, the ischiopubic rami and the margins of the obturator foramen are visual-ized in a different manner. The posterior margin of the acetabulum can also lend itself to more direct inspection. Acetabular fractures, a topic discussed later in this chapter, require not only oblique roentgenograms but also lateral films for proper evaluation. However, CT has become essential for the evaluation of complex acetabular fractures identified on plain film, and with the new spiral technology, 3-D reconstruction images can be performed.

When the focus of attention is the hip, it is useful to include both sides on a single film for comparative reasons. The straight anteroposterior roentgenogram of the pelvis and hips is an appropriate examination, but it is a must to view the hip from the lateral aspect when at all possible. This can be achieved in one of two ways: (1) a frog-leg position with the femur maximally rotated externally, or (2) a horizontally directed roentgenogram with the tube placed along the inner aspect of the thigh and directed through the hip to a grid film placed alongside the hip. This latter method is generally the desired technique in cases of fracture, because the patient usually does not tolerate rotation of the leg and hip, and little or no motion of the injured part is preferred.

ROENTGENOGRAPHIC ANATOMY AND DEVELOPMENTAL VARIATIONS

The pelvis is formed by two innominate bones, each consisting of an ilium, an ischium, and the pubis, which are distinct entities in youth but fuse to form a singular solid structure in the adult. The sacrum serves as a posterior bridge between the two by way of the essentially nonmobile sacroiliac joints. The epiphyses and apophyses of the pelvis and hips ordinarily do not unite until the second decade of life. The apophyses of the iliac crest normally make their appearance by the twelfth to fifteenth year and fuse by 21 to 25 years of age. They are separated from the body of the ilium by no more than 2 to 3 mm, often have an irregular rippled appearance, and may show segmentation into two or more parts.

Small centers of ossification arise from the anterior inferior iliac spines by the thirteenth year and fuse 2 to 3 years later. Athletes are prone to avulsion of these centers, and this should be looked for when there is localized pain in a sports-related injury (see Chapter 15). Oblique films are most useful in such situations, and a view of the opposite side is almost always needed to make the diagnosis.

Cheerleader’s “splits” can create a similar avulsion of the ischial tuberosity apophysis on one or both sides (Fig. 16-43). The time of appearance and fusion of these ossification centers parallels that of the iliac crest.

Fig. 16-43 Avulsion fracture (arrow) of the ischial tuberosity on the left side.

Until the tenth to eleventh year of life, a radiolucent cartilage separates the ischium and pubis along the inferior ramus (Fig. 16-44). This area of normal development is frequently misjudged as a fracture. During the process of union, this region appears more dense and expanded so as to give the impression of callus formation or tumor. The bilateral appearances are often asymmetric.

Fig. 16-44 Normal pelvis and hips of a 10-year-old child. A, Anteroposterior view with the hips in the neutral position. B, Anteroposterior view with the hips in the froglike lateral position. Note the triradiate cartilage and the bulbous appearance of the ischiopubic synchondroses of the inferior rami.

The triradiate cartilage forms a Y-shaped configuration at the acetabulum and constitutes the junctures of the pubis, ischium, and ilium (see Fig. 16-44). This becomes completely filled in with bone at about the time of puberty. There have been many occasions when this, too, has been called a pelvic fracture.

In evaluating the symphysis pubis, it is more important for the inferior margins to align, whereas the superior borders are frequently and normally offset. Ordinarily, the width of the symphysis pubis joint measures no more than 8 mm in adults and 10 mm in children. Widening of the joint is characteristic of late pregnancy.

The posterior margin of the acetabulum, somewhat obscured by the femoral head and requiring an oblique view to see adequately, may arise from a separate ossification center and easily simulate a fracture because of its linear appearance. This variation is often a bilateral finding. A variable-sized ununited center, the os acetabuli, may persist throughout life. It is located along the superolateral margin of the acetabulum (Fig. 16-45).

Fig. 16-45 Os acetabuli. The right side demonstrates this normal variant (arrow). None is seen on the left.

The developmental features are of utmost importance in consideration of the anatomy of the hip. The femoral capital epiphysis appears during the first year. Synostosis of the head with the femoral neck is completed by the eighteenth year, but a cartilaginous fissure may persist. A central indentation along the articular margin of the femoral head corresponds to the fovea centralis, where the ligamentum teres is embedded.

The greater trochanter apophysis becomes visible by the fifth year and unites at the same time as the femoral capital epiphysis, namely, 18 years. The line of fusion may also persist for a long time and result in confusion.

In addition to the bones themselves, there are certain soft tissue densities about the hips and pelvis that demand attention. The shadows of the obturator internus, the iliopsoas, and gluteus medius muscles are ordinarily outlined by radiolucent adipose tissue. Because of their close approximation to the joint capsule, blood or pus that distends the joint is reflected on the roentgenogram by displacement of these fat stripes.

TRAUMA

A rather significant degree of correlation exists between the presence of an extracapsular subtrochanteric hip fracture and a pathologic process. In other words, do not take for granted that such a fracture is related purely to trauma, because this is a favorable site for metastasis. Hip dislocations are discussed in Chapter 10. From a radiologist’s point of view, it is useful to comment on some of the roentgenographic changes seen in this type of injury. First of all, hip dislocations are classified as anterior, posterior, and central. In the most common posterior form of dislocation, the injury may not always be readily apparent on the anteroposterior film. There may be a slight difference in the size of the femoral heads as a result of slight rotation of the displaced hip. Shenton’s line may be askew. This is a continuous, smooth line formed along the sweep of the inner margin of the femoral neck, and it normally follows the inferior boundary of the arched contour of the superior ischiopubic ramus. Any disruption of this line would indicate a dislocation. The posterior lip of the acetabulum may be fractured, but as previously noted, a persistent unfused apophysis is sometimes located here.

When an anterior dislocation is present, the femoral head may lie medial and below the acetabulum, sometimes superimposed on the obturator foramen. However, the head infrequently may overlie the acetabular roof and simulate a posterior dislocation. This requires a horizontal groin lateral roentgenogram for differentiation.

When either a posterior or an anterior dislocation has been reduced, postreduction roentgenograms should be inspected for associated fractures. Also, the width of the hip joint space requires measurement. A difference of more than 2 mm should make one suspect the possibility of interposed tissue, such as a portion of the torn capsule, which necessitates surgical removal. Short of surgical exploration, the diagnosis may need CT, MRI, or both.

A central dislocation of the hip is always associated with an acetabular fracture; hence, the condition is termed a central fracture-dislocation where the femoral head intrudes into the pelvis. However, a central acetabular fracture may exist without a dislocated hip. There are four basic types of acetabular fractures, but the central form constitutes the most common.

As seen in Figure 16-46, a central acetabular fracture may be transverse or oblique. The transverse type extends from the anterior acetabular margin backward through the ischial spine, whereas the oblique form is directed more superiorly to the greater sacrosciatic notch. Both actually divide the innominate bone into superior and inferior segments.

Fig. 16-46 Types of acetabular fractures. This drawing of a lateral view of the pelvis describes the basic fractures that can involve the acetabulum: A-A1, posterior rim fracture; B-B1, posterior column fracture (ilioischial); C-C1, central transverse fracture; C-C2, central oblique fracture; D-D1, anterior column fracture (iliopubic).

From Thaggard A, Harle TS, Carlson V: Fractures and dislocations of bony pelvis and hip, Semin Roentgenol 13:117–133, 1978. Used by permission.

The second variety of acetabular fracture, as discussed previously, involves the posterior rim. This most often is produced by a posteriorly dislocated hip.

Two other categories of acetabular fractures are depicted in the schematic drawing of Figure 16-46: anterior (iliopubic) and posterior (ilioischial) column fractures. Oblique films, or better, CT, are required for their proper interpretation.

In addition to acetabular fractures, the remainder of the pelvis can be fractured in various ways. It is best to classify these as either stable or unstable.

Stable fractures can be categorized into avulsions, ischiopubic rami fractures, iliac wing fractures, and fractures of the sacrococcyx. Avulsions of the anterior superior and inferior iliac spines and the ischial tuberosities have already been discussed.

The most frequent pelvic fractures are those involving the ischiopubic rami. Occasionally, these may be stress-type infractions. Their visualization may call for cephalic and caudal tilt films.

A fracture of the iliac wing often results from a direct lateral blow to the pelvis. These are best depicted with an oblique roentgenogram.

Anteroposterior and lateral exposures of the sacrococcyx are required for the assessment of fractures, but it is often useful to include an anteroposterior cephalic (upward) tilt projection. Overlying intestinal content can obscure a fracture in this area, and CT may be required.

A variety of fractures and/or dislocations result in unstable conditions of the pelvis. Among the more common types that might be encountered is the straddle fracture (Fig. 16-47, A). This situation exists when there are vertical fractures involving the superior and inferior ischiopubic rami on both sides. Less frequently, the fractures are unilateral but with separation of the symphysis pubis. Approximately 30% of such injuries are associated with urethral or bladder trauma.

Fig. 16-47 Types of unstable pelvic fractures. A, Straddle fracture. B, Malgaigne’s fracture with an ipsilateral double vertical fracture. C, Malgaigne’s fracture with dislocation of the sacroiliac joint. D, Sprung pelvis. E, Bucket-handle fracture.

From Dunn AW, Morris HD: Fractures and dislocations of the pelvis, J Bone Joint Surg Am 50:1639–1648, 1968. Used by permission.

More serious forms of unstable fractures are classified as double vertical fracture-dislocations (see Fig. 16-47). There are three such types, and all have in common a double component involving the pelvic ring, anterior and posterior to the acetabulum.

When there are unilateral vertical fractures of the ischiopubic rami or a dislocation of the symphysis pubis in combination with a fracture about the ipsilateral sacroiliac joint or a dislocation of that joint, the condition is termed Malgaigne’s fracture (Fig. 16-47, B and C). The hemipelvis on the involved side may become displaced up or down and create a true unstable situation.

A “sprung pelvis” is another form of an unstable double vertical injury. Here, there is separation of one or both sacroiliac joints and a disjunction of the symphysis pubis (Fig. 16-47, D). Careful inspection of the sacroiliac joints should be made in any patient with displacement of the pubis.

The third type of double vertical pelvic injury is the so-called bucket-handle fracture (Fig. 16-47, E). There are fractures through the upper and lower rami of the anterior pelvic ring. The opposite or contralateral sacroiliac joint is separated or demonstrates a juxtaarticular fracture.

CT has revolutionized the radiologist’s ability to evaluate fractures of the pelvis. The transverse images provide greater anatomic detail, which allows one to distinguish important relationships and significant fragment displacements not readily apparent on the standard x-ray films. Furthermore, alterations in the soft tissues can be better determined, such as the development of associated hematoma formation. Totally unsuspected fractures that are not apparent on routine films often are visualized by CT. Furthermore, the sacroiliac joints are much better depicted (Fig. 16-48).

Fig. 16-48 These CT images of the pelvis dramatically disclose the extent of fractures not apparent on a routine x-ray film. Note also the unsuspected displaced fragment at S1 that extends into the central neural canal. Additionally, a fracture of the right transverse process of L5 was documented on the CT films that was not appreciated on the standard roentgenographic films (not shown). A, Routine pelvic roentgenographic film showing an obvious fracture of the left hemipelvis (arrow). B, Comminuted fracture of the acetabulum. C, Unsuspected fracture of S1 (arrowheads). Note the large hematoma (curved arrow).

TUMOR

Primary tumors of the pelvis are seldom seen and usually present no diagnostic dilemmas. Most of these are similar to those described in the discussion of the spine. On the other hand, metastatic disease is quite common in the pelvis and constitutes approximately 12% of all bone metastatic sites. Difficulty in distinguishing minimal involvement may be related to confusing overlying intestinal gas and fecal material. In such cases, the sensitivity of nuclear bone scanning helps to solve these difficult situations.

Not infrequently, a pelvic roentgenogram is seen that shows a marked increase in density. The majority of these cases represent either diffuse osteoblastic metastasis or Paget’s disease. The differentiation, as already observed, can often be made using basic characteristics of each (Fig. 16-49). Notably, Paget’s disease discloses increased volume of bone and a greatly thickened trabecular pattern. Moreover, it may be confined to one side of the pelvis, a less likely occurrence in metastasis. In a small percentage of individuals (fewer than 1%), Paget’s disease may transform to osteogenic sarcoma.

Fig. 16-49 Increased bone density in the pelvis. A, Paget’s disease. B, Osteoblastic metastasis.

INFECTION

Infections involving the hips and pelvis, such as acute pyogenic arthritis and tuberculous osteomyelitis, are extremely rare today. Secondary seeding of infection, either directly from pelvic inflammation or through the bloodstream, can take place and result in either an infected joint or osteomyelitis.

ROENTGENOGRAPHIC EXAMINATION

The basic minimum examination of the knee consists of supine anteroposterior and flexed lateral views. The latter film should be obtained with the knee bent at least 45 degrees with the affected knee flat on the table and the patient on his or her side, although this may be impossible in an acutely injured, fluid-filled joint or in a severely osteoarthritic knee. Certain situations call for other views. Internal and external oblique films may help visualize otherwise nonvisible subtle fractures of the tibial plateaus or femoral condyles. These features, as well as the intercondylar space and the tibial spine (intercondylar crest), can also be evaluated with a tunnel film (Fig. 16-51). The tangential, or “sunrise,” projection affords another perspective of the patella and the femoropatellar joint (Fig. 16-52). A fat-blood level may be detected on an across-the-table horizontal beam roentgenogram and indicate an intraarticular fracture in which marrow fat and blood have been extruded into the joint.

Fig. 16-51 “Tunnel” view of the knee.

Fig. 16-52 “Sunrise” view of the knee.

Stress films, including valgus and varus manual forces, can bring out ligamentous injuries or laxity. Standing anteroposterior weight-bearing studies give information related to subtle joint space narrowing in degenerative osteoarthritis. Occasionally, CT and MRI can elucidate abnormalities not identifiable on standard films.

ROENTGENOGRAPHIC ANATOMY AND DEVELOPMENTAL VARIATIONS

The distal femoral epiphysis normally appears in the ninth month of gestation and therefore is used in determining fetal maturity. At birth, it measures approximately 5 mm. The distal femoral epiphysis normally fuses at 20 years of age. As in other areas of the skeleton, the epiphyseal line may persist as a faint track. The intercondylar fossa, best evaluated on the tunnel film, forms a smooth arch. It serves primarily as a compartment for the proximal origins of the anterior and posterior cruciate ligaments.

The joint space, divided into medial and lateral compartments, normally measures 3 to 5 mm in height. It represents the thickness of the articular cartilages of the femur and tibia as well as the medial and lateral semilunar cartilages.

The proximal tibial epiphysis becomes visible by ossification in the last 2 months of fetal life. Like the femoral epiphysis, it fuses to the shaft by the twentieth year. Between the ages of 7 and 15 years, the tongue-shaped anterior and inferior extension of the epiphysis is visible and forms the anterior tibial tuberosity or spine (Fig. 16-53, A). The tuberosity is an extremely variable structure that develops in an irregular fashion and often has a fragmented appearance. This should not be confused with a fracture or Osgood–Schlatter disease.Its features on the anteroposterior view can be particularly bewildering (Fig. 16-53, B). On the frontal view, a radiolucent cleft often appears over the proximal tibial shaft.

Fig. 16-53 Normal appearance of the anterior tibial tuberosity in a 12-year-old. A, Lateral view. B, Anteroposterior view. Note the radiolucent line (arrow) superimposing the tibial metaphysis.

The articular surfaces of the proximal third of the tibia present a varied number of appearances, and this is especially true of the intercondylar eminence of the tibial spine. There are two major crests, lateral and medial. Occasionally, the clinician may identify an anteromedial tubercle to which the anterior cruciate ligament attaches. A similar but less frequently found posterolateral tubercle marks the insertion of the posterior cruciate ligament. The “tunnel” view of the knee usually demonstrates this anatomy to the best advantage. Small avulsion fragments arising from the crests and presenting as loose intraarticular bodies are best projected on this radiographic film view.

The patella is considered a large sesamoid bone embedded in the quadriceps tendon. It should always be examined with a tangential “sunrise” film in addition to the lateral and frontal roentgenograms. On the anteroposterior projection, the details of the patella may become completely lost in the shadow of the distal portion of the femur, and hence pathologic changes may be overlooked, thereby requiring tangential, lateral, and sometimes oblique views.

Ossification of the patella is irregular in nature and arises from multiple foci. In the young, it may be divided into several segments. It often has a granular appearance with irregular borders. Before complete fusion of the patellar ossification centers, there can be confusion with fractures, or its irregular outline may suggest osteochondritis (Fig. 16-54).

Fig. 16-54 Normal appearance of the patella in an 8-year-old. Note the irregular, fragmented, and sclerotic character simulating osteochondritis.

One of the more difficult problems associated with roentgenographic appraisal of the patella is the commonly observed anomaly of patella partita. This condition is usually bilateral but must be differentiated from fractures. A bipartite status is the most prevalent form, but multipartite patellae also exist, and as many as six different segments have been reported. In the usual instance, or bipartite state, a radiolucent line separates a smaller segment that almost always occupies the upper-outer quadrant, although many other rare variations are found (Fig. 16-55). There have been cases described for patellar partition into anterior and posterior portions.

Fig. 16-55 Bipartite patella. A, Anteroposterior. B, Lateral. C, Sunrise view. Note the upper outer position of the separate bone, the sclerosis of the line, and its curvilinear course, whereas a fracture would be sharp and straight. This is an unusual case of unilateral involvement.

A somewhat distracting osseous shadow, the fabella, is seen in 10% to 20% of knees along the posterior aspect (Fig. 16-56). A small sesamoid bone of varying size and shape, it lies within the lateral head of the gastrocnemius muscle and is best seen on the lateral view. Many times, it overlies the border of the lateral femoral condyle and appears as an avulsion fragment on the anteroposterior film. The bone is most often a bilateral finding and is seen more often in males. It must be differentiated from a fracture, loose joint body, foreign body, and phlebolith.

Fig. 16-56 The fabella (arrows). A, Lateral view. B,Anteroposterior view.

The soft tissues about the knee deserve particular attention and must not be overlooked on the knee roentgenogram. The fundamental observation in the presence of joint effusions is an anterior displacement of the patella and an elongated oval area of increased density above the patella that represents fluid in the suprapatellar bursa. A small amount of fluid may not produce these changes.

Not uncommonly, short slivers of ossification immediately above and below the patella and usually attached to it represent the tendons of the quadriceps muscle and the patellar ligament, respectively. They can be misleading shadows to the unwary and probably represent the end result of tendinitis, much like that seen in the rotator cuff tendons of the shoulder.

Usually on the basis of degenerative processes, the menisci may become calcified. These must be differentiated from calcifications occurring in the articular cartilage, a process seen in degenerative osteoarthritis but also in pseudogout (chondrocalcinosis), a rare disorder in which calcium pyrophosphate is deposited in the cartilage.

It is not too surprising to visualize a radiolucent slit within the knee joint and, usually more frequently, within the hip and shoulder joints of children. This phenomenon is linked to a vacuum effect and is caused by pulling on the extremity or by producing forced adduction or abduction. It is important that this not be interpreted as abnormal. Nevertheless, in the elderly population, it is a pathologic observation and may be seen in severe arthritis with degeneration of the cartilage. The finding is a potential perception in a vertebral spine with advanced degenerative disc disease.

TRAUMA

The majority of roentgenograms done because of trauma to the knee will be normal. At times, a joint effusion will be demonstrated with soft tissue fullness of the suprapatellar bursa, but visualized fractures are uncommon. With fractures extending into the joint, a fluid–fluid level can be demonstrated on cross-table lateral views resulting from blood layering with marrow fat (lipohemarthrosis; Fig. 16-57). Fractures can involve one or both femoral condyles, the tibial tuberosity, or the proximal third of the fibula as well as the patella. As many views as necessary should be utilized to evaluate these injuries. Stress fractures about the knee, especially the proximal tibia, are not uncommon and occasionally show a dense line of compacted trabeculae but may require a radionuclide bone scan or MRI to document.

Fig. 16-57 Cross-table lateral view of knee demonstrating a fluid–fluid level of a lipohemarthrosis indicating an intraarticular fracture.

Femorotibial dislocations are rare. These can be anterior or posterior, and are often associated with a fracture. The major concerns in such situations are tearing of the collateral and cruciate ligaments and, more acutely important, severing of the popliteal artery, which will require angiography for proper assessment.

TUMOR

Of the many benign tumors involving the knee region, osteochondroma is probably one of the more frequent, with the exception of benign cortical defects (Fig. 16-58). The latter are peripherally located, round to oval radiolucencies that often measure no more than 2 cm. They have a sharply defined sclerotic margin and are common in the metaphyseal areas of the distal portion of the femur and the proximal and distal thirds of the tibia. They are asymptomatic and discovered incidentally on roentgenograms performed for other reasons. With advancing age, they tend to disappear. The osteochondroma is seen as a bony protuberance of variable size, contiguous with the femoral or tibial metaphyseal or diaphyseal bone marrow and extending away from the joint in the direction of the tendons and muscles (Fig. 16-59). Osteochondromas have an invisible cap of cartilage. Solitary osteochondromas are less frequently demonstrated about other joints of the body. When these exotoses are multiple, they constitute a hereditary bone dysplasia designated as diaphyseal aclasis.

Fig. 16-58 Benign cortical defect of the distal femur (arrow).

Fig. 16-59 Osteochondroma of the knee.

Aneurysmal bone cysts are found in the neural arches of the vertebrae, in the ends of long bones, and most frequently in the distal third of the femur. They become very expansive but are confined by a thin layer of cortical bone and contain many septa of bone.

A giant-cell tumor characteristically involves the distal portion of the femur. It is considered a benign vascular lesion, but occasionally it transforms into a locally invasive malignant process. There is a gradual eccentric expansion of a thinned cortex, but not to the same extent as an aneurysmal bone cyst. A “soap-bubble” appearance is characteristic. The lesion involves the closed epiphysis and metaphysis and extends to the subarticular cortex but not into the joint.

Although not ordinarily considered under the topic of tumors, bone infarcts, frequently located in the distal third of the femur, can create confusing roentgenographic changes. They occur in caisson disease, pancreatitis, and various vascular disorders. Any segment of bone, epiphysis, metaphysis, or diaphysis can be involved, but the alterations are confined to the medullary cavity. They appear as irregular sclerotic longitudinal streaks, occasionally with fine cystic patterns resembling a corkscrew (Fig. 16-60).

Fig. 16-60 Bone infarct of the distal third of the femur. A, X-ray. B, Radionuclide bone scan. C, Coronal T-1 weighted spin-echo magnetic resonance image.

The knee has the dubious distinction of being the most common site for the development of osteosarcoma. The metaphysis of the distal portion of the femur accounts for approximately 75% of all osteogenic sarcomas. This primary bone malignancy tends to occur during puberty, with boys being more commonly affected than girls. The roentgenographic changes take many forms, either being purely osteolytic or showing a mixed destructive and osteosclerotic pattern (Fig. 16-61). Extensive soft tissue involvement is the rule. Periosteal reaction can be exuberant, eventually forming the typical spiculated “sunburst” appearance, although this is not always present (Fig. 16-62).

Fig. 16-61 Osteosarcoma of the knee. A, Anteroposterior view; B, Lateral view. Note the mottled bone destruction with interrupted periosteal reaction and soft tissue extension.

Fig. 16-62 Osteosarcoma. A, Lateral roentgenogram of the proximal third of the tibia. B, Coronal MRI of the same area. The medullary extent of the disease (arrows) is easily identified by MRI.

Under the heading of round-cell tumors, Ewing’s sarcoma is manifested as a solitary lesion arising in the diaphysis and metaphysis of long bones, particularly the femur; however, it has been observed with relative frequency in the humerus and ulna as well as the pelvis. Originating in the bone marrow, it produces a characteristic layered periosteal thickening resembling an “onion skin.” A variable patchy dissolution without significant expansion is imparted to the osseous architecture. Unfortunately, the symptoms of fever and pain and the sarcoma’s roentgenographic manifestations can resemble those of osteomyelitis in the child and adolescent, and indeed the distinction may be extremely difficult to make.

ARTHRITIS

Osteoarthritis is a familiar affliction of the knee (Fig. 16-63). Degenerative wear and tear produce the changes with progressing age, but secondary posttraumatic arthrosis is also a leading offender. Minimal joint space narrowing is one of the earliest roentgenographic changes reflecting wearing and thinning of the articular cartilage. Standing weight-bearing films might be required to demonstrate this finding. The decreased height of the joint space may be accompanied by increasing sclerosis of the subarticular region of the tibia, quite often the medial condyle. The femoropatellar joint undergoes similar alterations of narrowing and associated sclerosis of the articular surface of the patella. Eventually, the process leads to bony spurs arising from the articular margins of the femur, tibia, and patella. These osteophytes may become significantly large so that function is impaired.

Fig. 16-63 Osteoarthritis of the knee. A, Anteroposterior view; B, Lateral view.

Subchondral cysts, which are not always evident, are a unique feature of osteoarthritis, even in joints other than the knee. They are well-defined round to oval lucencies, one or more in number. They measure anywhere from a few millimeters to 3 to 4 cm. One proposed theory for their evolution states that, because they are lined with synovium, they represent protrusions of the membrane through a defect in the articular cartilage. A small channel forms a direct communication between the cysts and the joint space, and this has been proved on pathologic dissections. A change in the intraarticular fluid and pressure dynamics of the disorganized joint is thought to be the mechanism for their formation. The actual size of the intercondylar fossa, as seen on the tunnel view, may enlarge somewhat in osteoarthritis. However, this feature is more pronounced in rheumatoid arthritis and the arthrosis of hemophilia with repeated intraarticular hemorrhages.

OSTEOCHONDRITIS

There are several disease entities classified as osteochondritis or aseptic necrosis involving the knee. The abnormalities of osteochondritis dissecans have already been described. Osgood–Schlatter disease is another form involving the anterior tibial tubercle. As mentioned previously, there is a significant variation in the roentgenographic appearance of the normal tuberosity, and the diagnosis is primarily a clinical one. Even though there may be sclerosis and/or fragmentation, this does not necessarily constitute Osgood–Schlatter disease. The only roentgenographic abnormality, almost universally present in the acute phase, is overlying soft tissue swelling. The principal purpose for obtaining the films is to exclude some other lesion, such as tumor or infection.

One other type of osteochondritis that might be encountered in the knee is Blount’s disease, or tibia vara. For some unknown reason, possibly stress, a localized growth disturbance occurs along the medial-posterior aspect of the tibial metaphysis. The changes may appear between the ages of 1 through 12 years and culminate in outward bowing of one or both legs.

Roentgenographically, the medial tibial metaphysis is widened and forms a broad spur that extends both medially and posteriorly. The medial surface of the tibial epiphysis becomes flattened and produces a slope of concavity where this growth center is normally convex.

Without correction, the fully developed knee may exhibit a persistent downward slope of the medial tibial articular surface. The medial femoral condyle hypertrophies to compensate for the tibial deformity.

ROENTGENOGRAPHIC EXAMINATION

Three views are mandatory for proper evaluation of the ankle, and three projections are necessary for the foot examination. As elsewhere in the skeleton, modified and special films can clarify suspicious areas. For the ankle, the three films include anteroposterior, lateral, and oblique projections. The oblique study, or mortise view, is obtained by rotating the foot internally 10 to 15 degrees (Fig. 16-64). Even though the calcaneus, tarsal bones, and bases of the metatarsal bones are not considered anatomically a part of the ankle, they should be included, because associated or isolated injuries to these structures may be found when symptoms point only to the ankle. A good example of this is a fracture of the base of the fifth metatarsal bone.

Fig. 16-64 Normal ankle films. A, Anteroposterior view; B, Oblique (mortise) view.

An external oblique view might be requested when there is still a question of an abnormality in the absence of findings on the three standard roentgenographic films. A subtle fracture of either malleoli may be brought out in this way.

In the presence of soft tissue swelling after trauma but without an associated fracture, ligamentous damage must be considered. Stress anteroposterior filming using manual abduction and adduction of the heel may be indicated. Widening of the ankle mortise strongly suggests a tear in either or both the medial deltoid ligament or lateral collateral ligaments. Because of pain, maneuvering the ankle for stress views may require local anesthesia. However, the orthopedic surgeon may wish an MRI study of the ankle for defining the complex array of ligaments, tendons, and muscles, as well as assessing for traumatic bone bruises, fractures and abnormal fluid collections (edema and hematomas).

The standard roentgenographic study of the foot should include anteroposterior, internally rotated oblique, and lateral projections. Because of superimposition, the metatarsal and phalangeal bones cannot be evaluated to any extent on the lateral film, but the talus, calcaneus, and tarsal bones are relatively clearly outlined. Additionally, the talocalcaneal, talonavicular, and calcaneocuboid joints are well depicted. On the anteroposterior film, the cuboid and lateral cuneiform bones are superimposed, and the bases of the metatarsals tend to be obscured by overlap. These bones are projected into profile with internal oblique films. An externally rotated oblique view will delineate the first metatarsal and medial cuneiform bones.

The toes should be examined with anteroposterior and internal and external oblique films. An individual toe that causes concern requires a lateral view. This may be performed by having the patient hold the toe in extension with a pencil while the other toes are held in flexion, a maneuver that also is utilized for finger roentgenograms.

The heel is best demonstrated with lateral and axial (tangential) films. However, as noted later in this chapter, oblique exposures sometimes aid in the study of this bone.

ROENTGENOGRAPHIC ANATOMY, VARIANTS, AND PATHOLOGY

Ankle

The ankle mortise is formed by the distal third of the fibula (lateral malleolus), styloid process of the tibia (medial malleolus), the horizontal articular plate of the tibia (plafond), and the dome-shaped articular surface of the talus (tenon).

On the lateral view, the width of the joint space between the tibia and talus narrows from anterior to posterior in the normal subject. Because of this, most ankle dislocations are anterior, except where there is a disruptive fracture of the mortise.

As mentioned previously regarding other growth centers, a linear sclerotic zone of bone condensation or a lucent line of incomplete union may persist in the distal portion of the tibia where the epiphyseal line existed. Additionally, just as in the distal thirds of the radius or femur, a variable number of regular transverse bands of increased bone density may be observed in the tibial metaphysis. These so-called growth lines are normal; they tend to disappear with age and should not be confused with a pathologic process such as the lines of lead poisoning.

Not infrequently, the tip of the medial or the lateral malleoli arises from separate ossification centers that fail to fuse. The resultant os subtibiale and os subfibulare can be variable in size, but like all accessory bones, they have a well-defined thin cortical margin throughout their circumference. This will differentiate them from recent fractures.

Because of the high frequency of accessory bones in the ankle and foot, comparative views are recommended, because the findings are usually, but not always, bilateral. If the distinction between fracture and an accessory ossicle is difficult, the clinician should consider one of several textbooks dealing with normal variants (Birkner, 1978; Keats, 2007; Kohler and Zimmer, 1993).

One of the best-known accessory skeletal elements of the ankle in addition to the os subtibiale and os subfibiale is the os trigonum (Fig. 16-65). Situated behind the talus near the posterior aspect of the talocalcaneal joint, it is best viewed on the lateral roentgenogram. Its shape and size are variable, and it may measure 1 cm or more. Despite its frequency, it still is commonly misinterpreted as a fracture.

Fig. 16-65 Os trigonum (arrow).

Rarely, the os trigonum may be mimicked by a fracture of the posterior tubercle of the talus. This may occur when the posterior talar process becomes wedged between the posterior articular rim of the tibia and the calcaneus with severe forced plantar flexion.

On the anteroposterior projection of the ankle, the Achilles tendon is seen as a thick band of slightly increased density behind the tibia. The resultant shadow at times can create a bewildering roentgenogram. On the mortise film, the clinician may see a horizontal V-shaped lucency through the medial margin of the talus. This corresponds to the inner margin of the talus, which corresponds to the inner margin of the talocalcaneal joint.

Overlap of the cortical margins of the fibula and tibia occurs in most views of the ankle, except a correctly positioned mortise film. This results in an apparent radiolucent defect termed a Mach effect and often leads to fracture misinterpretation.

On the lateral view of the ankle, distension of the joint capsule with blood can be discerned in either or both the anterior or posterior soft tissue compartments. Such a finding is often indicative of a fracture of the distal portion of the tibia or talus but not the lateral malleolus, because the synovial membrane of the capsule invests only the talus and tibia but not the distal third of the fibula.

Benign cortical defects are a relatively frequent finding in the distal portion of the tibia in children and are just as common in the femur and tibia about the knee. Their characteristics have been described in the section of the knee. They are easily distinguishable on roentgenograms from more serious lesions (Fig. 16-66).

Fig. 16-66 Benign cortical defect (arrow) of distal tibia.

Osteochondritis dissecans, discussed elsewhere, is most commonly found involving the knee. This form of avascular necrosis, which is thought to be related to trauma, occurs in other areas as well, including the elbow and ankle. An osteochondral fragment of varying size, but usually small (less than 5 mm), can retain its postion in relation to the articular margin or become displaced into the joint as a loose body. Rarely, the abnormality may not be identified on standard radiographs. CT or MRI may be required to detect the defect in the clinical setting of persistent pain and disability and a normal radiographic study (Fig. 16-67).

Fig. 16-67 Osteochondritis dissecans of the talus. A, Axial and B, coronal CT images of the ankles readily demonstrate the osteochondral bone nidus of the talus on the right side (arrows).

Foot

Overlap of the individual bony elements of the foot tends to make evaluation of the roentgenographic anatomy somewhat difficult. Anatomically, the bones of the foot include the phalanges, metatarsals, cuboid, the three cuneiforms, navicular, and the calcaneus, as well as the talus, although the latter is also considered a component of the ankle.

Of the tarsal bones, the talus is second only to the calcaneus as the most frequent bone to be fractured. The majority of these injuries are chip and avulsion types. They may occur along the anteroposterior surface of the neck, and therefore a lateral film is required. Such fractures have also been described along the medial, lateral, and posterior processes. A fragment of the posterior eminence may simulate the os trigonum. Less commonly, the talus is fractured through the neck.

The talus is more susceptible to dislocation than the other tarsal bones because it is the only bone in the lower extremity that does not have direct muscle attachments. Furthermore, because of its tenuous vascular supply, posttraumatic aseptic necrosis of the talus may eventually take place.

The calcaneus anatomically consists of a body and a large posterior tuberosity. The sustentaculum tali forms a platform of bone along the inner superior surface of the body that provides support to the anterior portion of the talus. The posterior facet, behind the sustentaculum tali, is that portion of the talocalcaneal (subtalar) joint that slopes downward, as seen on the lateral reoentgenogram, and into which the lateral triangular process of the talus projects (Fig. 16-68).

Fig. 16-68 Axial view of the os calcis. Note the sustentaculum tali (arrow).

The apophysis of the posterior calcaneal tuberosity appears early in life and fuses in or about the seventeenth year. The margins of the apophysis and the posterior tuberosity can look quite irregular and ragged. The apophysis itself may exhibit fragmentation and varying degrees of sclerosis. Sever’s disease, a form of osteochondritis or aseptic necrosis (discussed in a previous section), has been ascribed to this ossification center, but much like Osgood–Schlatter’s disease of the knee, the diagnosis is principally clinical, because the normal roentgenographic film appearance is extremely inconstant. Once fused, the area may show multiple irregular striations and some irregularity of the margins. It is of interest that in rheumatoid spondylitis the posterior surface of the tuberosity may display typical “whiskering” or fraying of its margins much like that seen in the ischial tuberosities of the pelvis.

To properly evaluate the heel, especially in cases of trauma, in addition to the lateral film a tangential (axial) projection and both oblique views are required. To obtain the axial exposure, the patient is seated on the table with the heel against the film. A towel or long piece of gauze is placed around the ball and toes of the foot, and the individual is instructed to flex the foot by pulling on the cloth. The tube is angled 40 degrees toward the head and centered over the heel. Multiple axial films from angles of 20 to 40 degrees may be necessary to properly demonstrate the location and extent of a fracture. Here again, however, CT assessment reveals much more detail and not only diagnoses a fractured calcaneus (or other tarsal bone) but reveals the degree of separation and displacement to determine the appropriate surgical approach for fixation.

Fractures of the os calcis are often the result of a fall from a height feet-first; crushing injuries with compression and comminution are produced. In 10% of such cases, the fractures are bilateral, and in a similar percentage there will be an associated injury of the lumbar or thoracic spine caused by the vertical compression forces dispersed into the back.

The lateral convex and medial concave surfaces of the calcaneus should be well delineated on the axial view. If the film is accurately exposed, the sustentaculum tali along the medial aspect will also be identified.

When viewing the os calcis on the lateral examination, it is mandatory that Boehler’s angle be measured. This is formed by the intersection of (1) a line drawn from the dome of the os calcis at the talocalcaneal joint to the anterior process of this same bone, and (2) a line extending from the posterior tubercle to the dome of the calcaneus (Fig. 16-69). Normally, this measures between 20 and 40 degrees. With compression fractures, this angle is reduced. Subtle compression fractures may be overlooked without measuring this angle.

Fig. 16-69 This lateral film of an ankle demonstrates Boehler’s angle. Normal measurement is 20 to 40 degrees.

Stress fractures of the os calcis are not manifested immediately after injury, similar to other bones of the body. It may take 10 or more days before a line of sclerotic endosteal new bone is detected that parallels the posterior border of the heel.

Exostoses or spurs are a common finding in the heel. They can arise from the posterior or inferior margin of the tuberosity. The posterior bony outgrowths extend in the direction of the Achilles tendon. Inferior or plantar spurs grow toward the sole of the foot within the plantar fascia. These excrescences may lead to local irritation and subsequent pain.

A somewhat rounded to triangular area of lucency that is relatively well circumscribed is occasionally observed in the body of the calcaneus on the lateral roentgenogram (Fig. 16-70). This finding can create interpretive difficulties, because it simulates a cyst. This has been proved anatomically to represent a normal area of thinned, deficient trabeculae. This is usually an incidental finding and can be very worrisome when seen. True cysts, however, rarely occur.

Fig. 16-70 Normal lucency within the os calcis simulates a cyst.

When considering the tarsal navicular bone roentgenographically, there are several lesions that deserve discussion because of their relatively common occurrence. Traumatic injuries to the tarsal navicular are uncommon. Avulsion fractures along the dorsal surface may be found near the talonavicular joint and require differentiation from an os supranaviculare. The medial tuberosity that serves for the insertion of the posterior tibial tendon may be subjected to forces resulting in fracture. Importantly, such a fracture may be associated with a fracture of the cuboid bone, often with dorsal subluxation of the navicular. An accessory bone, the os tibiale externum, located behind the tuberosity, is more frequent than tuberosity fractures and may be confused with one (Fig. 16-71).

Fig. 16-71 Os tibiale externum (arrow).

Of all of the tarsal bones, the cuboid initially has the most striking features because of its multifragmented appearance. After a short interval, the fragments become united into one solid structure.

Rarely encountered, cuboid anomalies are synostoses to the calcaneus, talus, navicular, or metatarsals. Accessory bones closely related to the cuboid are the relatively common os peroneum and less frequent os vesalianum. Their distinction from avulsion fractures is not always simple.

It is a rare occasion for the cuboid to exhibit an isolated fracture. Ordinarily, there are associated tarsal injuries, usually the tuberosity of the navicular or anterior process of the calcaneus.

The three cuneiform bones are anatomically labeled and numbered as follows: (1) medial or internal, (2) middle, and (3) lateral or external. Like the navicular and cuboid bones, the internal cuneiform may demonstrate multicentric ossification. A not so infrequent finding is division of the first cuneiform into dorsal and plantar segments representing a true bipartite condition. Isolated fractures of any of the three cuneiforms are definitely an uncommon situation.

With the correct roentgenographic projection on the internal oblique film, the joint space between the first and second cuneiform bones can appear quite wide and might lead the clinician to a false impression of a separation. The middle and external cuneiforms often elude appropriate inspection because of their inconspicuous positions within the framework of the bony arch.

In the growing foot, the epiphyseal centers of the metatarsal bones are essentially similar in location to those of the hand. They appear in the third year and normally fuse about 15 years of age. Each of the metatarsals contains one epiphysis, but the fifth metatarsal also possesses an apophysis at its proximal end. The epiphyses of the second through fifth metatarsals are distal, whereas that of the great toe is proximal. On rare instances, the clinician may encounter pseudoepiphyses at the bases of the lateral four metatarsals, particularly the third and also the head or distal aspect of the first metatarsal. Occasionally, a cleft divides the epiphysis of the great toe metatarsal bone and should not be misconstrued as a fracture.

Several interpretive challenges are presented by the appearance of the base of the fifth metatarsal bone. The longitudinally oriented shell-like apophysis can be roentgenographically dissimilar in the feet of the same individual and vary in size and shape. Furthermore, it may persist unfused throughout life, but this is an uncommon event. Ordinarily, the distinction between the apophysis and a fracture is relatively simple, because the growth line is oriented to the axis of the shaft, whereas a fracture is transverse in almost all instances (Fig. 16-72). An accessory bone, the os vesalianum, alluded to previously, is located near the junction of the proximal metatarsal tuberosity and the cuboid. This fact should be remembered when evaluating trauma to this area.

Fig. 16-72 Transverse fracture (large arrows) through the base of the fifth metatarsal bone. Note the normal shell-like apophysis (small arrow) that will increase in size before fusion.

A moderate degree of overlap of the bases of the metatarsal bones is noted to a greater or lesser degree on all views, but more so on anteroposterior projections. With the Mach effect in mind, this can lead and has led to the diagnosis of many erroneous fractures. The internal oblique film tends to reduce this problem to some extent.

When viewing the foot, the tarsometatarsal joints form a somewhat curvilinear line convex toward the toes. This is disrupted only by the recessed base of the second metatarsal bone, which results from a relatively short middle cuneiform. The base of this metatarsal is therefore wedged between the first and third cuneiform bones. Furthermore, the medial margins of the base of the second metatarsal and the middle cuneiform are always in line.

In the proximal space between the bases of the first and second metatarsal bones may be found the os intermetatarseum. It is located along the dorsal surface and, like most accessory ossicles, can assume variable sizes and shapes. Radiopacities in the form of arteriosclerotic vascular plaques can also be seen in this same interdigital space. Both of these can simulate avulsion fractures.

One traumatic lesion involving the metatarsal area merits special attention but, fortunately, is infrequent. Lisfranc’s fracture-dislocation involves the tarsometatarsal junction and consists essentially of dorsal displacement of the metatarsal bases. Two basic forms exist: homolateral and divergent. In the homolateral type, the lateral four metatarsals are dislocated posteriorly and laterally, often with associated fractures at the bases (see Chapter 13). The divergent type exists when the first metatarsal is displaced medially and the others are dislocated laterally. The disfigurations may be subtle, and the normal straight alignment of the second metatarsal and middle cuneiform bones should be used (Fig. 16-73).

Fig. 16-73 A, Normal alignment of the second metatarsal and middle cuneiform bones (arrows). B, Lateral dislocation of the three middle metatarsal bones (a divergent form of a Lisfranc type of injury).

Osteochondritis dissecans, discussed elsewhere, is most commonly found involving the knee. This form of avascular necrosis, which is thought to be related to trauma, occurs in other areas as well, including the elbow and ankle. An osteochondral fragment of varying size, but usually small (less than 5 mm), can retain its position in relation to the articular margin or become displaced into the joint as a loose body. Rarely, the abnormality may not be identified on standard radiographs. CT or MRI may be required to detect the defect in the clinical setting of persistent pain and disability and a normal roentgenographic study (see Fig. 16-73).

MRI OF THE SPINE

The vertebral bodies and their posterior neural arch components, as well as the surrounding muscles, can be well demonstrated with MRI. Additionally, the intervertebral discs are optimally visualized. MRI also depicts the contents of the central neural canal, including the cord, nerve roots, dura, and ligamentum flavum. The neural foramina and associated exiting nerve roots can be identified on sagittal and axial views. Figure 16-84 illustrates the normal midline sagittal appearance of the cervical, thoracic, and lumbosacral spine. Standard MRI protocol consists of 3- to 5-mm contiguous sagittal images from the neural foramina from one side to the other (Fig. 16-85). Selected axial images through the intervertebral disc spaces are also performed (Fig. 16-86). Occasionally, coronal views are obtained that simulate roentgenographic contrast myelography.

Fig. 16-84 Normal midline sagittal MRI of the entire spine. A, Cervical spine. B, Thoracic spine. C, Lumbar spine. Note in detail the cord (closed arrows), the conus medullaris (open arrow), and the bones and intervertebral discs. Also note the vascular channels in the posterior aspects of the vertebral bodies (vertical arrow).

Fig. 16-85 Off-midline sagittal MRI of the lumbar spine demonstrates the neural foramina and their exiting nerve roots (arrow).

Fig. 16-86 Selected axial image of the L5, S1 disc space (see the sagittal inset with the cursor). Note the dural sac and two descending nerve roots (black) surrounded by the epidural fat (white).

Congenital Malformations

MRI of congenital malformations of the spine in some instances may be the only means of delineating the abnormality. Sagittal cervical spine images clearly depict the Arnold–Chiari malformation with herniation of the cerebellar tonsils through the foramen magnum. There may be an associated syringomyelia dissecting the cord, and this can be extremely well seen on MRI studies (Fig. 16-87). MRI provides a noninvasive way to evaluate menin-goceles and meningomyeloceles. With these entities, a tethered cord and associated lipoma are well appreciated below the conus medularis.

Fig. 16-87 A and B, Syringomyelia of the cervical cord (arrows) without cerebellar herniation on T1- and T2-weighted spin-echo images.

Disc Disease

The role of MRI in the assessment of disc herniation, degenerative disc disease, and spondylitis with osteophyte formation compared with CT and contrast myelography currently is undergoing considerable scrutiny in clinical practice. A few studies agree that the accuracy of the three modalities in these disease states is relatively equal in lumbar spine evaluations. However, with increased experience and further technical advancements, MRI is taking the lead. The diagnostic sensitivity in the cervical and thoracic regions favors the use of MRI. CT at these spinal levels is very limited unless intrathecal contrast is used, because the cord and dural sac are not well demonstrated. MRI shows these structures well.

The clinical diagnosis of a herniated intervertebral nucleus pulposus can be very dramatic but on occasion can be very subtle. After following a conservative clinical approach for a period of time without improvement of symptoms, the clinician may elect to evaluate the patient by some type of imaging procedure to diagnose and document the presence or absence of a herniated disc. Graphic visualization of a herniated disc can be an imaging challenge by any method. In the cervical region, bulging, protruding, and herniated discs lend themselves readily to MRI evaluation (Fig. 16-88), although the distinction between bulging and herniation may at times be impossible. Lumbar spine disc herniation is best visualized by MRI, essentially replacing CT (Fig. 16-89). But the choice of procedure depends on availability, economic considerations, and to some extent, patient selection. An equivocal CT study may require MRI to best delineate the lesion, but less often the opposite is true.

Fig. 16-88 Herniated disc of the cervical spine at the C5-C6 level (arrows). A, Sagittal, and B, axial views. Note on the lateral image an appearance simulating paste being squeezed from a tube. Also note deformity of the dural sac.

Fig. 16-89 Herniated disc of the lumbar spine at the L4-L5 level (arrows). A, Sagittal, and B, axial MRI images. Note the deformity of the dural sac and the enchroachment on the left neural foramen. C, CT image of the same disc.

One area of difficult clinical management is the postoperative spine. Contrast myelography or CT seldom can distinguish between postsurgical scar tissue and a recurrent herniated disc. With the introduction of an intravenously administered “contrast” agent, MRI is capable of making the distinction between the two. Gadolinium is a stable paramagnetic metal ion that is combined with diethylenetriamine pentaacetic acid (DTPA). When injected, the substance can enhance tissues with adequate vascularity by changing their magnetic field. Disc material, being essentially nonvascular, will not enhance, whereas scar tissue, which possesses a vascular supply, will (Fig. 16-90).

Fig. 16-90 Distinguishing between postoperative scar and recurrent herniated disc by MRI using gadolinium. A and B, Pre- and postcontrast images demonstrating enhancing scar (brighter) at L5, S1 (arrows). C and D, Pre- and postcontrastfilms depicting a herniated disc (arrows) with no enhancement at L4, L5 in the same patient.

Spondylitis

Early degeneration of the intervertebral disc material can be identified by MRI even before changes occur on roentgenographic films and CT; these changes include disc space narrowing and spur formation. A change in signal intensity (brightness) of the disc on the MRI sagittal images heralds the beginning of the pathologic process, a result of dehydration of the nucleus pulposis (Fig. 16-91). As the disease progresses with disc space compression and production of osteophytes, MRI aids in defining the degree of spinal stenosis that can occur on the central neural canal and the neural foramina in both the axial and sagittal planes. The encroachment of degenerative spurs on the dural sac, their displacement of nerve roots, and their effect on the cord are all adequately assessed by MRI (Fig. 16-92).

Fig. 16-91 Degenerative disc disease of the lumbar spine at the L4-L5 level. Narrowing of the interspace is evident, and osteophyte formation is noted (open arrows). Also note the decreased signal intensity (darkness), greatest at L4-L5 but involving all discs; this indicates the decreased water content of the degenerating nucleus pulposus. Posterior osteophyte (closed arrow) resulting in some degree of spinal stenosis.

Fig. 16-92 Degenerative disc disease of the cervical spine at the C5-C6 level. A, Sagittal, and B, Axial images. A large posterior spur (arrows) deforms the dural sac extending to the right and is creating stenosis of the neural foramen and subarticular recess.

Infection

MRI of infectious processes in and about the spine can be very rewarding. Figure 16-93 demonstrates an unsuspected epidural abscess of the lumbar spine in an individual presenting with a relatively sudden onset of lower extremity neurologic symptoms including back pain, fever, and elevated white blood cell count. A lung infection was thought to be the source for this hematogenously introduced infection.

Fig. 16-93 A and B, Sagittal lumbar spine images at two different planes, and C, axial image at the same L5, S1 level. An irregular dark (low signal intensity) area (arrows) signifies an epidural abscess.

More common but still infrequent are spinal infections occurring after surgery to the spine. Osteomyelitis of the bony vertebrae with extension into the discs and eventual advancement into the epidural space can be easily pictured by MRI. The use of gadolinium can even further improve the ability to detect infections of the spine.

Tumor

MRI has been found to be highly sensitive for the detection and characterization of tumors involving the spine. Gadolinium plays an important role in the evaluation of these lesions.

Secondary or metastatic neoplasms are the most commonly encountered tumors that involve the spine (Fig. 16-94). These may be hematogenous, such as from lung or breast cancer; lymphatic, such as that from the prostate; or by direct extension from a contiguous lesion. Better definition of the margins of these tumors is afforded by the use of gadolinium, which distinguishes the tumor from surrounding edema and thereby better establishes radiation ports.

Fig. 16-94 Metastasis (arrows) to the sacrum from an occult lung cancer. A, Sagittal, and B, Coronal MRI. C, CT image. D, Nuclear bone scan of the pelvis. All three modalities show sensitivity to the process, but MRI provides more anatomic information.

Primary spinal tumors are divided into intradural intramedullary and intradural extramedullary as well as extradural lesions. Intramedullary neoplasms may not be optimally visualized on unenhanced MRI because of extensive infiltration, associated edema, or intratumor necrosis or hemorrhage. Ependymomas and astrocytomas constitute the most common intramedullary tumors. A less common lesion is the hemangioblastoma. All three are best evaluated on MRI after gadolinium injection.

Intradural extramedullary tumors are commonly found to be meningiomas, neurofibromas, or schwannomas (Fig. 16-95). On conventional unenhanced MRI studies, these may be difficult or impossible to identify, especially, if they are less than 5 mm. Larger lesions can displace the dural contents and are more easily detected by these indirect signs.

Fig. 16-95 Meningioma of the cervical spine (arrow) displays high signal intensity (bright).

Metastases constitute the majority of extradural tumors and, as noted, are best evaluated by magnetic imaging. However, one area where MRI has an obvious advantage over any other imaging procedure is in the presence of diffuse leptomeningeal spread of tumor. There is a marked increase in signal arising from the thickened leptomeninges after gadolinium administration.

MUSCULOSKELETAL MRI

Diagnostic orthopedic applications of MRI for disorders of the bones, joints, and soft tissues of the extremities are rapidly gaining acceptance. Although MRI will not totally replace the use of CT, nuclear imaging, or even conventional roentgenographic films in evaluation of the musculoskeletal system, there are certain areas and specific abnormalities that are best examined by the magnet. Because of its excellent inherent contrast resolution, MRI can superbly delineate the separate soft tissue components of the extremities (Fig. 16-96). Muscles, tendons, ligaments, fat, cartilage, and fibrous tissue are all excellently displayed. Although the calcium of cortical and trabecular bone possesses no magnetic signal, it can be defined by the adjoining marrow and surrounding muscles and tendons. However, this property of magnetism prevents MRI from defining cortical stress fractures. Tumoral calcifications and ossifications, as well as early cortical destruction and endosteal and periosteal reaction, which are adequately seen on CT, cannot be optimally identified by MRI, which gives it a distinct disadvantage in such situations. Nevertheless, despite these limitations, MRI has become the diagnostic modality of choice for the evaluation of bone and soft tissue trauma such as cartilage, ligament, tendon, or muscular damage.

Fig. 16-96 A, Coronal MRI of the femur and thigh. B, Axial image of the tibia and fibula and associated soft tissue detail.

With the development of newer software, the role of MRI has expanded in the area of vascular anatomy. MRI angiography is able to depict brachiocephalic and cerebral vessels and is showing promise in peripheral angiography as well. Without the use of contrast material, arterial and venous structures can be displayed using appropriate phase sequences. As technologic research in this area progresses, it is hoped that MRI angiography may someday replace conventional invasive angiographic procedures.

Tumor

The surgical approach for the treatment of malignant bone and soft tissue tumors today is a conservative one. The objective is limb salvage and restricting resections to a limited degree, thereby providing as much functional capability as possible. The multiplanar display of MRI and its excellent contrast resolution aid greatly in defining the size, location, and extent of a tumor. It can adequately assess marrow involvement, skip lesions, and invasion of adjoining muscles, compartments, blood vessels, and nerves (Fig. 16-97). Of great importance is establishment of the integrity of adjacent joints and articular surfaces.

Fig. 16-97 Chondrosarcoma. A, Anteroposterior roentgenogram of the distal third of the tibia. B, Sagittal MRI of the same area.

MRI cannot, unfortunately, differentiate between benign or malignant tissue characteristics in the majority of cases. There are a few exceptions; giant cell tumor of bone, for example, will give rise to characteristic fluid–fluid levels within a multi-septated lesion, a unique feature for these neoplasms (Fig. 16-98). Certain types of tissues such as fat or the vessels of a hemangioma have distinctive appearances (Fig. 16-99). Fluid-filled cystic lesions can be separated from solid masses with MRI, and this can aid in appropriate assessment such as needle aspiration or biopsy.

Fig. 16-98 Giant cell tumor of the tibia. A, Coronal T-1 spin echo and B, axial T-2 weighted images demonstrate a sharply demarcated multiseptated lesion. Note fluid?fluid levels (arrows) on the axial film, a finding that can be seen in benign tumors.

Fig. 16-99 Benign lipoma. Axial MRI of the thighs demonstrates a sharply marginated lesion (T) with high signal intensity.

Trauma

The primary diagnostic modality for the study of skeletal injuries does not include magnetic imaging. These are reserved, of course, for standard roentgenologic procedures. However, an evaluation of soft tissue injury, especially deep tissues, becomes more difficult with conventional tests. MRI, because of its multiplanar ability and its exquisite soft tissue contrast and resolution, is gaining greater acceptance for elucidating extraosseous injuries.

Contusions of soft tissue that result in hemorrhage, edema, and inflammation, as well as hematoma formation, can be depicted by MRI. The location and size of the process and, importantly, the involvement of adjacent muscles, tendons, ligaments, blood vessels, and nerves are easily assessed. This can serve as a guide to excision and evacuation procedures when required.

As a result of direct trauma, muscles, ligaments, and tendons can be torn from their insertion sites. Such findings, especially in the shoulder, knee, and ankle but also in other locations, are directly visualized by magnetic scanning, something that until now was difficult to achieve even with arthrography and might be surmised only clinically or by direct surgical exposure.

Evaluation of the knee joint is presently approached by direct visualization via arthroscopy, which has essentially supplanted arthrography. However, orthopedic surgeons are beginning to rely more frequently on the noninvasive accurate diagnostic images provided by MRI. The menisci, cruciate and collateral ligaments, articular cartilage, synovial capsule, and surrounding muscles are extremely well depicted on MRI scans (Fig. 16-100). Recent studies have shown MRI to have an accuracy of 90% for meniscal tears and 95% for cruciate ligament tears (Fig. 16-101). Magnetic imaging can define early degeneration of menisci and cartilage and even intrameniscal tears not visible by endoscopy. Loose intraarticular osteochondral bodies are easily detected by MRI.

Fig. 16-100 Normal knee anatomy. A, Coronal MRI shows the intercondylar notch (open arrow), the medial meniscus (M), and the lateral meniscus (closed arrow). B, Sagittal MRI shows the normal anterior (A) and posterior horns of the medial meniscus (arrow). C, Sagittal MRI demonstrates the normal anterior (A) and posterior horns of the lateral meniscus (arrow).

Fig. 16-101 A, Tear in the lateral meniscus of a sagittal image of the knee. B, Disruption of the anterior cruciate ligament (arrow). C, Normal-appearing posterior cruciate ligament, albeit slightly kinked because of the tear in the anterior cruciate ligament (arrows).

The diagnostic capabilities of MRI in examining the shoulder have only recently been realized because of the development of better designed, dedicated surface coils and updated computer software programs. Although arthrography is still being used and in some instances combined with CT, MRI appears to be taking the same direction as it did in knee evaluations and is becoming more frequently used. Coronal oblique, sagittal oblique, and axial planes of imaging of the shoulder offer a tremendous advantage over other modalities in addition to the use of variable-pulse sequencing protocols, which can be modified to bring out or accentuate subtle findings. Degenerative changes as well as partial or complete tears of the rotator cuff are sensitive to MRI detection (Fig. 16-102).

Fig. 16-102 Coronal oblique MRI of the shoulder demonstrates a rotator cuff tear (arrow).

Impingement of the supraspinatus tendon as caused by an abnormal acromium process or osteophyte formation of the acromioclavicular joint can lead to early degenerative changes and makes the tendon more susceptible to tears. Tendinitis, bursitis, rupture of the biceps tendon, or determination of the integrity of the cartilagenous glenoid labrum also lend themselves to identification by MRI.

MRI is also helpful for assessing injuries to other regions of the musculoskeletal system. As will be mentioned later, aseptic or avascular necrosis of the navicular bone of the wrist is receptive to MRI detection. Several studies have indicated the value of MRI in assessing carpal tunnel syndrome. In this condition, magnetic imaging can define thickening of the tendon sheaths and distortion in the outline of the median nerve.

The elbow and ankle can be appropriately analyzed for traumatic abnormalities by MRI (Fig. 16-103). Tears of the Achilles tendon are a prime example, and the severity and extent of the lesion can be properly determined by the magnet. Loose bone or cartilagenous bodies on occasion can be appreciated within joints.

Fig. 16-103 A and B, Normal sagittal images of the ankle. Note the Achilles tendon (arrows).

Examination of the temporomandibular joints is facilitated with the use of MRI. Tears and dislocations of the meniscus and whether it reduces with opening or closing of the jaw are discerned.

Acute trauma, chronic stress tenosyovitis, or bursitis often leads to the production of synovial fluid as a result of inflammation. Traumatic hemorrhage into joints or bursae can also occur. These abnormal fluid collections are well demonstrated by MRI.

Osteonecrosis

There are numerous causes of aseptic necrosis or avascular osteonecrosis of bone, the most common being trauma that results in disruption of the regional vascular supply. Nontraumatic etiologies include vascular thrombosis secondary to hemaglobinopathies such as sickle cell anemia. A commonly encountered reason for aseptic necrosis is exogenous steroids. Less frequently found diseases accounting for osteonecrosis are barotrauma, irradiation, collagen vascular diseases (vasculitis), lymphoproliferative diseases (Hodgkin’s lymphoma), pancreatitis, and even some cases of gout.

The common denominator in the pathogenesis of both traumatic and nontraumatic aseptic necrosis is vascular compromise. The resultant marrow ischemia, which most often occurs in the growth centers or metaphyseal regions of long bones, provokes an inflammatory response with vascular congestion and edema of the bone marrow.

This pathophysiologic response can be detected by conventional radiographs on a delayed basis or early by the sensitive but nonspecific and poorly resolved images of a nuclear bone scan. CT can also detect the changes of avascular necrosis relatively early, but the findings are also nonspecific, and CT of the distal parts of extremities has been found to be very limited.

MRI of osteonecrosis has been determined to be very sensitive and specific early in the disease process. In the femoral head (Fig. 16-104), the images delineate subartic-ular areas of decreased signal intensity (dark). Injuries to the carpal navicular bone, as has been pointed out previously, can defy detection by standard imaging. Appreciation of the process of aseptic necrosis of the proximal navicular fracture fragment requires early detection for best results, and MRI provides a very accurate means to determine this abnormality. Other regions vulnerable to osteonecrosis, such as the humeral head (Fig. 16-105), knee (Fig. 16-106), mandibular condyle, and spine, are easily examined by magnetic imaging.

Fig. 16-104 A, Normal coronal MRI of the hips. B, A coronal view of the hips demonstrates avascular necrosis on the left (arrow).

Fig. 16-105 Coronal oblique MRI shoulder image demonstrates osteonecrosis of the humeral head in a steroid user (arrows).

Fig. 16-106 A coronal MRI image of the knee delineates the typical defect of osteochondritis dissecans (arrow), which was also well demonstrated on the sagittal views (not shown).

DUAL ENERGY X-RAY ABSORPTIOMETRY AND OSTEOPOROSIS

Osteoporosis now has and will continue to have a significant socioeconomic impact on health care. The morbidity and mortality related to osteoporotic fractures in the postmenopausal patient have not received appropriate attention until the last few decades. A satisfactory and low-cost method of calculating bone mineral density (BMD) had to be developed that would adequately diagnose and precisely quantify and predict those individuals who are at increased risk for insufficiency fractures of the spine, hips, and other skeletal locations. Accurate BMD measurements could also be used for therapeutic response.

A number of techniques have been developed over the past few years, the majority of them linked in some way to the computation of the attenuation of ionizing radiation through bone. The first commercially available dual energy x-ray absorptiometer (DEXA) scanner was introduced in 1987, and has since undergone considerable refinement and improvement.

BMD values are expressed as gm/cm². Measurements are made over the upper four lumbar vertebrae and the left hip (Fig. 16-107). The results are then given as standard deviations or percentile scores. A Z-score is a measure of the difference of the patient’s BMD and the mean BMD for individuals matched by age and sex. A T-score measures the difference in standard deviations of the patient’s BMD and the mean BMD of normal young persons. A fracture risk profile then can be established and appropriate therapeutic measures instituted.

Fig. 16-107 Standard DEXA scan charts for the evaluation of osteoporosis. A, Lumbar spine and B, the hip.

Conditions and factors that can lead to imprecise calculations of BMD include severe degenerative disease of the spine or hip resulting in osteophytes and eburnation of the bone. Compression fractures involving vertebrae also result in erroneous values secondary to the sclerosis that occurs with healing. Other considerations that cause inaccurate readings are Paget disease, diffuse osteoblastic metastases, extensive calcified plaque of the abdominal aorta overlying the spine, and scoliotic curvatures. Metal artifacts related to the field of data acquisition of measurements, including x-ray contrast media (barium) and orthopedic devices such as Harrington rods must be considered. Recent radionuclide studies can even interfere and give spurious results.

A number of indications have been established for acquiring BMD measurements. Probably the most important is the estrogen-deficient woman. In addition to the postmenopausal female, this can be seen in malabsorption syndromes (celiac disease), poor diet and anorexia, disorders of menstruation, and premature ovarian failure. Illnesses such as Cushing disease, hyperparathyroidism, renal osteodystrophy, and rheumatoid arthritis can lead to osteoporosis. Decreased BMD is also associated with certain drugs such as corticosteroids, anticoagulants, and antineoplastic medications. Also, those with radiographic evidence suggesting osteoporosis (namely, osteopenia with or without spine and hip fractures) are candidates for BMD assessment. Finally, any person being monitored who is on preventive and therapeutic prescription (such as estrogen, alendronate, and raloxifene or calcitonin) meets the criteria for DEXA evaluation of BMD.

DEXA techniques have also been used in the evaluation of the postoperative hip, especially with the newer prosthetic materials. After placement of an uncemented prosthesis, the degree of bone remodeling and healing can be assessed. A decrease in BMD can serve as a predictor for prosthetic failure.