Imaging of the Pediatric Elbow

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CHAPTER 12 Imaging of the Pediatric Elbow

Kristen B. Thomas, Alan D. Hoffman, E. Richard Graviss

INTRODUCTION

Radiography is the primary imaging modality for evaluation of the elbow in children, as it is in adults. Although the radiographic views are the same, the pediatric patient is unique. Injury is the primary reason for evaluating the immature elbow. Children’s reactions to the process of imaging vary greatly, although they are usually related to the patient’s age and the nature of the injury sustained.

Modern radiographic equipment is a cornerstone for obtaining high-quality imaging studies. However, the most important component is a qualified radiologic technologist who understands the child’s anxieties and who has empathy for the child’s fears. Such a technologist is aware of patient and parent anxiety and that the minor motions of the elbow may cause pain. The assistance of an accompanying parent or guardian may be useful and, occasionally, is mandatory when there is insufficient technical help available for positioning. A gentle, friendly approach that is firm but reassuring will yield optimal radiographic examinations of the pediatric elbow.

The basic elbow study consists of anteroposterior and lateral views. The lateral view invariably is obtained first, because the child maintains an injured elbow in the flexed position. The patient is seated beside a radiographic table so that the arm can be elevated parallel to the level of the table top and a 90-degree flexed position can be maintained. The forearm should be supinated gently, with the thumb pointed up, positioning all three bones of the elbow in the lateral projection. The anteroposterior view then is obtained with the forearm positioned up and the elbow extended slowly as much as the injury allows. If necessary, the anteroposterior view can be divided into two segments: one with the humerus parallel to the radiographic film, and the other with the forearm parallel to the radiographic film. This provides better anatomic detail than does a single exposure with the elbow partially flexed and neither component parallel to the film.

Some unstable fractures and dislocations require splinting such that views obtained at right angles are usually sufficient for the initial diagnosis. The fracture or dislocation with obvious clinical deformity is usually less problematic than is the subtle fracture, which may go undetected. When the patient is examined for subtle fractures, the lateral view is extremely important, and positioning should be flawless. This view provides clues concerning the injured elbow, such as the anterior and posterior fat pad signs. It also allows for visual alignment of the distal humeral ossification segments with the shaft of the humerus and with the radius.

In certain instances, a fluoroscopic examination of the elbow may yield valuable information. The examiner can manipulate the elbow to obtain the precise obliquity required to best evaluate a subtle abnormality. Instead of repeating a radiograph multiple times, optimal positioning can be obtained while watching real-time fluoroscopy and then digital fluoroscopic spot radiographs are easily taken.

Tomography, using either a simple linear method or a complex motion system, can be used in the evaluation of growth plates that have closed prematurely following trauma. In most practices, computed tomography has completely replaced conventional tomography. Computed tomography examinations now take only seconds to perform, and sedation is usually not necessary, even in very young infants and children. Using current 64-slice multidetector computed tomography technology (MDCT), isovoxel images can be obtained in all three planes down to 0.6-mm collimation. This allows detailed imaging, with the bony trabecular pattern well seen. Examinations are obtained with the patient in the prone position, with the affected arm held above the head with about 90 degrees of flexion at the elbow. Sagittal and coronal two-dimensional reformatted images as well as three-dimensional reconstructions are then made from the raw data. MDCT is a sensitive (92%) and specific (79%) method of evaluating for radiographically occult elbow fractures.⁶ MDCT can also use automated tube current modulation to markedly decrease the radiation dose to the patient compared with fixed-tube current techniques. MDCT can also be performed with no image degradation through a cast.³ MDCT with reformatting can better delineate intra-articular fractures (Fig. 12-1). Three-dimensional imaging can also provide additional information and help define the joint relationships to aid surgical planning (Figs. 12-2 and 12-3). The resulting three-dimensional image can be rotated in all planes with computerized subtraction of the adjacent soft tissues and bones, if needed.

FIGURE 12-1 Computed tomography two-dimensional reformatted coronal image of intra-articular distal humeral intercondylar fracture in a 13-year-old boy who was injured while skateboarding.

FIGURE 12-2 Computed tomography three-dimensional reconstruction of a complex intra-articular distal humeral fracture from the posterior view in a 14-year-old boy injured while playing basketball.

FIGURE 12-3 Magnetic resonance imaging of a 13-year-old boy with elbow pain, coronal (A) and sagittal (B) views. T1-weighted images show a defect in the capitellum. No loose body is seen. The clinical diagnosis was Panner’s disease, and symptoms resolved in a few months without specific therapy.

Magnetic resonance imaging (MRI) and ultrasonography are increasingly being used to evaluate the elbow. MRI can evaluate cartilage, bone marrow, and soft tissue structures (Fig. 12-4).⁸ Radiographs do not show bone bruising, or cartilaginous or soft tissue injury and can underestimate physeal injury. MRI is also occasionally used to better define elbow fractures.² Owing to the length of the MRI examination (at least 20 minutes), children younger than 5 years old will usually need sedation so that optimal MRI images can be obtained. In children with elbow trauma, MRI reveals a broad spectrum of bone and soft tissue injury, including ligamentous injury, beyond that recognized by radiographs. However, the additional information afforded by MRI usually does not change treatment or clinical outcome in acute elbow trauma.⁹ MRI can be very useful in the evaluation of osteochondritis dissecans (OCD) of the capitellum. MRI provides information about the size, location and stability of the OCD lesion. All of these factors are important when deciding treatment options (see Chapter 20 for more discussion). Unstable OCD lesions in the capitellum have a peripheral rim of high signal or an underlying fluid-filled cyst on T2-weighted images (Fig. 12-5). Stable OCD lesions have no peripheral signal abnormality.¹² Loose bodies in the elbow joint can be visualized by MRI or MDCT, but smaller detached bone fragments are usually better visualized using MDCT (Fig 12-6).

FIGURE 12-4 Computed tomography three-dimensional reconstruction of an 11-year-old boy with the clinical diagnosis of Panner’s disease. Several small loose bodies are seen adjacent to the capitellum.

FIGURE 12-5 Magnetic resonance imaging (T2 sagittal image) of a 12-year-old boy demonstrates osteochondritis dessicans (OCD) of the capitellum with increased signal extending to the articular surface consistent with full-thickness cartilage loss. This indicates a potentially unstable OCD bone fragment that has not yet detached. A moderate elbow joint effusion is also present.

FIGURE 12-6 Computed tomography axial image of a 12-year-old female gymnast demonstrates a small intra-articular loose body within the posterior elbow joint (arrow) secondary to osteochondritis dissecans of the capitellum. The tiny bone fragment was not visible on radiographs.

Ultrasonography has the ability to dynamically delineate soft tissues and cartilage in detail.¹³ Soft tissue swelling, a mass (including vascular masses investigated with duplex Doppler and color flow Doppler), joint effusion, and fractures, particularly in infants and young children with unossified or minimally ossified epiphyses, are studied with this modality.^1,⁷ Ultrasound can detect early changes of medial epicondylar fragmentation and OCD of the capitellum, even in the asymptomatic stage in selected populations such as young baseball players.¹⁰

As with other portions of the appendicular and axial skeletons, side-to-side comparison may be helpful when one is presented with an unfamiliar or a rare variant. Comparison views need to be obtained only in selected cases,^14,¹⁵ such as when consultation with the standard text of normal cases is not helpful.^5,^11,¹⁷

NORMAL DEVELOPMENT

The maturation sequence at the elbow is more variable than that of the hand and wrist. Nonetheless, an appreciation of the normal sequence and timing of the appearance of ossification centers and maturation patterns is important for an understanding of the radiographic appearances of the elbow in children (Fig. 12-7). Several mnemonics have been suggested to help remember the time of appearance of the ossification of these centers. We find that the cross-connecting ossification centers (see Fig. 12-7B) are particularly helpful in remembering at least the order of ossification of these centers. An atlas entitled Radiology of the Pediatric Elbow⁵ shows standards for elbow maturation in children. To consistently evaluate the developing elbow, one must analyze each of the secondary centers of ossification, accounting for its appearance, configuration during development, and associated changes as it matures and eventually fuses with the humeral shaft. The descriptions that follow are brief, but they outline the major points of development and maturation of the centers.

FIGURE 12-7 A, Normal left elbow showing the secondary centers: capitellum (c); medial epicondyle (m); radial head (r); trochlea (t); olecranon (o); and lateral epicondyle (l). B, The approximate age at time of appearance of these centers is indicated in years. The cross connecting the secondary centers of the distal humerus serves as a reminder of the order of ossification of these centers.

(Modified from Brodeur, A. E., Silberstein, M. J., Graviss, E. R., and Luisiri, A.: The basic tenets for appropriate evaluation of the elbow in pediatrics. Curr. Probl. Diagn. Radiol. 12:1, 1983.)

CAPITELLUM

The capitellum, the first of the elbow’s six centers to ossify, generally becomes radiographically visible during the first and second years of life. Initially spherical, it flattens posteriorly to conform to the adjacent distal end of the humerus. The physis is broader posteriorly than anteriorly, giving the capitellum the appearance of a downward tilt; however, this appearance gradually disappears during the first decade (Fig. 12-8). During maturation, the capitellum fuses with the trochlea and the lateral epicondyle before it unites with the humeral shaft (Fig. 12-9).

FIGURE 12-8 Lateral elbow radiograph of a 2.5-year-old girl. A line along the anterior humeral shaft normally intersects the posterior half of the middle third of the capitellum. The continuation of the curved coronoid line just touches the anterior edge of the ossified capitellum. The angle formed by the coronoid line and humeral shaft line should contain the majority of the ossified capitellum.

FIGURE 12-9 A 13-year-old girl in whom the capitellum has joined with the lateral epicondyle and trochlea before fusion with the humeral shaft. Note the normal sclerotic radial epiphysis that is wider than the radial neck.

The orientation of the capitellum with the humerus can be evaluated with a true lateral projection. The anterior surface of the humerus is gently bowed posteriorly, from the insertion of the deltoid muscle to the superior aspect of the coronoid fossa. A line drawn along the anterior surface of the humerus, from the deltoid insertion to the top of the coronoid fossa, should pass through the middle third of the capitellum. For practical reasons, most lateral examinations of the elbow do not include the deltoid insertion; therefore, one must use the most proximal portion of the humerus included on the radiograph. These two points determine the anterohumeral line, which passes precisely through the posterior half of the middle third of the capitellum. The capitellum is oriented anteriorly to the distal humerus. One also may draw a curvilinear line along the coronoid fossa. The extension of that line inferiorly should touch the anterior portion of the capitellum.

These two lines permit the detection of subtle supracondylar fractures, particularly Salter-Harris type I supracondylar fractures, with minimal posterior displacementof the distal humeral epiphysis with the capitellar ossification center.

RADIOCAPITELLAR LINE

The radiocapitellar line is a line drawn through the long axis of the proximal radial shaft that should, in the absence of dislocation, pass through the middle of the capitellum ossification center. This is generally true in anteroposterior, lateral, or any oblique projection. In early development, however, the radial metaphysis is wedged so that on the anteroposterior projection a normal radial shaft line may appear to extend laterally to the capitellum. However, on the lateral projection, the normal radiocapitellar line can be appreciated (Fig. 12-10). In older patients, although it may appear that the radiocapitellar line is normal in one projection in a patient with a radial head dislocation, it invariably will be abnormal in the projection taken at right angles, generally the lateral projection.¹⁸

FIGURE 12-10 A, Normal 7-month-old girl with apparent abnormal radiocapitellar line on the anteroposterior radiograph because of wedging of the metaphysis. B, The relationship between the radial shaft and capitellum is normal on the lateral radiograph.

MEDIAL EPICONDYLE

The medial epicondyle is the second elbow ossification center to appear in the normal sequence, usually at about 4 years. Lying posteromedially, it is often best appreciated on the lateral projection (Fig. 12-11). Frequently, it develops from more than one ossific nucleus. Although it is the second humeral ossification center to appear, its development is slow, and it is usually the last center to unite with the humeral shaft in the normal child, sometimes as late as 15 or 16 years of age.²⁰ This center may fuse with the trochlea before uniting with the humeral shaft. Injuries involving the nonunited medial epicondyle are relatively common and are among the most difficult to evaluate. Consequently, to avoid errors, Rodgers suggests making a habit of identifying the presence and the position of the medial epicondyle ossification center in each case.¹⁶ A classic example of the importance of appreciating the sequence of humeral ossification center appearance is avulsion and displacement of the medial epicondyle ossification center. This frequently results in the displacement of the medial epicondyle into the normal position of the trochlear ossification center. In a child between 4 and 8 years of age, at the time of appearance of the medial epicondyle and the trochlear ossification centers, a radiograph suggesting a trochlear ossification center, without visualization of a medial epicondyle center, should suggest that fracture and dislocation of the medial epicondyle have in fact occurred.¹³

FIGURE 12-11 A, A 10-year old boy with a normal posteromedially lying ossification center for the medial epicondyle (arrows) seen posterior to the humeral shaft on the lateral projection. B, Another 10-year-old boy who sustained trauma resulting in avulsion of the medial epicondyle, which is displaced anteriorly (arrows) on the lateral projection, and displaced medially (C), and rotated on the anteroposterior projection.

RADIAL HEAD EPIPHYSIS

The initial ossification of this epiphysis is fairly predictable and usually occurs in the fifth year (see Fig. 12-7B). Although usually beginning as a sphere, the radial head epiphysis often matures as one or more flat sclerotic centers. This pattern may be mistakenly interpreted as a fracture. With maturation, the physis on the anteroposterior radiograph is wider laterally than medially, and this appearance, combined with the medial angulation of the radius at the junction of its shaft and neck, may suggest dislocation on anteroposterior views. Lateral projection of the elbow will not confirm a suspected dislocation. With further maturation of ossification of the proximal radial ossification center, the normal relationship of the radius and capitellum can be seen on anteroposterior radiographs. Notches or clefts of the metaphysis of the proximal radius often are seen as normal variations of ossification during maturation.^11,¹⁷

Because fractures of the radial neck are extracapsular, they are not associated with hemarthrosis and abnormalities of the humeral fat pads.¹⁹

TROCHLEAR EPIPHYSIS

Ossification of the trochlea appears at about 8 years and often is initially multicentric (Figs. 12-12 and 12-13B). The trochlea frequently maintains an irregular contour during its development and should not be confused with abnormal processes such as trauma or avascular necrosis (Fig. 12-14). The trochlea will fuse with the capitellum before fusion with the distal humeral shaft. It is seldom fractured, except when associated with the vertical component of a supracondylar fracture or when its lateral edge is involved with a lateral condylar fracture.

FIGURE 12-12 Multiple ossification nuclei of developing trochlea (arrow) in a 9-year-old boy.

FIGURE 12-13 A, Lateral radiograph with lucent region in the proximal radial shaft (arrows). B, Anteroposterior view shows prominent but normal radial tuberosity (arrow). Residual changes from previous transcondylar fracture of the humerus are seen.

FIGURE 12-14 A 9-year-old boy with beginning ossification of the lateral epicondyle (arrow) from a thin sliver widely separated from the metaphysis. Note the irregular outline of the developing ossification center of the trochlea.

OLECRANON

The ossification center of the olecranon usually develops at 9 years of age, shortly after the trochlea and just before the lateral epicondylar epiphysis. The proximal end of the ulna flattens and becomes sclerotic just before the olecranon physis ossifies. Two ossification centers most often develop, and there is great variability in the configuration of the epiphysis. This results in an occasional misdiagnosis of acute fracture. The posterior ossification center is usually bigger than the anterior ossification center (Fig. 12-15), and these separate centers generally unite before fusion with the proximal humerus. This process usually begins at about 14 years of age.

FIGURE 12-15 A 13-year-old boy with double ossification center of the olecranon. The anterior nucleus is smaller.

The pattern of closure of the olecranon physis is distinct, with fusion occurring first along the joint line and then extending posteriorly. Frequently, fractures are wedged in the opposite direction.²¹

The olecranon physis has prominent sclerotic margins just before closure. Fusion proceeds posteriorly from the joint side or the anterior surface (Fig. 12-16). During its development, the physeal line remains relatively perpendicular to the ulnar shaft. As a result of differential growth, often with maturation, the olecranon growth plate, which initially is proximal to the elbow joint, comes to lie at a midelbow joint level by the time of fusion. This “wandering physeal line of the olecranon” does not occur in all individuals.⁴