Trauma

Trauma often necessitates MR imaging of the elbow in both the acute setting as well as the chronic situation such as the repetitive stresses experienced in sports activities, especially those involving a racquet.^46,47 Chronic trauma may also arise from a complication of an acute incident where the patient continues to be affected by symptoms long after the precipitating event. Patients with persistent elbow symptoms related to sports may develop scar tissue, tendinous degeneration, partial or complete tears of the tendons, or articular changes, all of which can preclude a rapid or complete recovery.^19,46

Those patients suffering from persistent elbow symptoms may be harbouring complications such as avascular necrosis or loose body formation resulting from an osteochondral injury. In such cases, MR imaging can determine the most appropriate clinical management of the patient, providing a wealth of information as to the size and position of the lesion and any associated fragment.^48–51

In the acute trauma setting, there are also situations that may require planar imaging such as MR; in particular, dislocations and complex fractures. Patients sustaining posterior dislocations of the elbow are at risk of associated ligament and capsular tears, avulsion fractures and instability. In the case of fractures, MR imaging is not always needed for purposes of identification, but can be vital in determining the management and prognosis.⁵² When a simple radial head fracture is identified on plain film, the treatment plan is usually straightforward: initial immobilization and early rehabilitation to avoid capsular restriction, particularly into extension. No advanced imaging is normally performed; however, the management and prognosis of comminuted fractures is often based on the associated injuries as opposed to the fracture itself. In these instances, cross-sectional imaging becomes indicated.

MR imaging of the elbow may also provide important information regarding the neural elements that pass by the articulation during their course into the forearm and hand. The radial, median and ulnar nerves follow complicated pathways and are prone to entrapment in multiple locations by a wide variety of causes such as injury, mass compression or developmental variants.^53–56 Surgery may also provoke scar tissue formation about the nerves, leading to adhesions, irritation, entrapment and, eventually, chronic denervation.^57,58 In such syndromes, the use of MR imaging can be useful in determining the extent of any neuropathy and associated denervation and evaluating the potential for recovery. Clinical examination, electromyography and nerve conduction tests are also essential parts of the work-up, allowing for both the structure and function of the involved neural structures.

Osseous anatomy (Figures 9.05–9.07, Box 9.02)

The elbow is a compound, hinge-type synovial joint (ginglymus) composed of the distal humerus, proximal ulna and proximal radius (Figures 9.01, 9.02).⁶⁶

Figure 9.05 • Axial imaging of the elbow using STIR sequences, demonstrating the full spectrum of the intricate anatomical structures revealed by MR imaging. The radial (3), ulnar (4) and median (5) nerves are all clearly visualized (E). A method for reviewing the anatomy is to select an individual anatomical structure and to trace it from proximal (A) to distal (F). Although time-consuming, this allows accuracy in identification of the structure and determination of any lesion affecting it.

Figure 9.06 • Coronal MR imaging of the elbow using STIR sequences from posterior (A) to anterior (D). Rather than be overwhelmed by the number of structures to identify, it is useful for the clinician to choose those structures that commonly present in practice with pathology, for example the common extensor tendon (structure 15) and appreciate the normal appearance in this set of images of a 24-year-old student.

Figure 9.07 • Sagittal T1-weighted MR imaging of the elbow sequences demonstrating the normal anatomical relationships about the articulation from medial (A) to lateral (F).

On the distal humerus, there are two prominent processes, the medial and lateral epicondyles. The distal articular surface of the humerus is wide and flat. The lateral third, the capitellum, articulates with the head of the radius, which has a disc-shaped articular surface. The medial third, the trochlea, articulates with the trochlear notch of the ulna.⁶⁷

This notch, which is large and C-shaped, is also sometimes termed the semilunar notch. The projection forming its upper border is called the olecranon process. It articulates with the posteriorly situated olecranon fossa of the humerus and may be palpated as the angular ‘point’ of the elbow. The projection that forms the inferior border of the trochlear notch, the coronoid process, enters the coronoid fossa of the humerus when the elbow is flexed. This fossa is separated from the cubital fossa anteriorly by only a thin layer of bone.⁶⁸

The lateral side of the coronoid process has a small, shallow notch or groove, called the radial notch. This articulates with the head of the radius, forming a synovial pivot (trochoid) joint, which allows supination and pronation of the forearm.⁶⁹ On the medial aspect of the coronoid process, approximately 2 cm posterior to the coronoid tip, is the sublime tubercle, the site of insertion for the ulnar collateral ligament.⁷⁰

Nerves

Nerves are distinguishable on MR imaging owing to their surrounding layer of fatty myelin. The median, radial and ulnar nerves are usually obvious on axial images of the elbow (Figure 9.05E); however, they can be difficult to identify owing to their size, variable anatomical relationships and low signal intensity. They have complex courses, running through multiple compartments.

Muscles and bursae

Ligaments and capsule

The articular capsule of the elbow extends from the humerus, covering all the articular surfaces down to the radial neck and inferior aspect of the coronoid process of the ulna. It is weaker on its anterior and posterior aspects, the medial and lateral aspects being strengthened by the medial and lateral collateral ligament complexes.⁶⁸ The anterior and posterior fat pads may be seen on lateral radiographs of the elbow joint as well as on MR imaging. Normally, with the elbow in a flexed position, the anterior fat pad is visible on radiographs as a relatively linear lucency in contact with the anterior aspect of the distal humerus. The posterior fat pad is not normally visible as it is hidden deep within the olecranon fossa. Articular oedema from effusion, haemarthrosis or synovitis will cause these fat pads to be displaced away from the humerus, resulting in radiographic visualization of the posterior fat pad and anterosuperior elevation of the anterior fat pad which adopts a curvilinear configuration.⁷⁸ The fat pads are visible on MR imaging, particularly on T1-weighted or fat-sensitive sagittal sequences (Figure 9.08).

Figure 9.08 • Sagittal T1-weighted MR imaging of the elbow demonstrating the normal appearance of the posterior (a) and anterior (b) fat pads which surround the articular capsule of the elbow. Note that the anterior fat pad is seen as being flush with the anterior humerus, the normal appearance which is reproduced on the lateral radiograph as a thin radiolucent line.

The ulnar collateral ligament (UCL) complex contributes to medial elbow stability (Figure 9.01A). It comprises three distinct bundles: the anterior, posterior and transverse portions.⁶⁹ The anterior bundle extends from the medial epicondyle to the coronoid process and is best seen on coronal images (Figure 9.09).^65,68 Its proximal aspect is flared and tapers distally; it is the principal restraint against valgus stress.^79,80 The posterior fibres extend from the medial epicondyle to the olecranon. The transverse (or oblique) portion extends from the olecranon to the coronoid process and does not really contribute to stability of the joint. The posterior and transverse portions are not always present, but this is rarely an issue from a biomechanical or clinical standpoint.⁶⁸ They are difficult to see on MR images; however, their presence is inferred as they form the floor of the cubital tunnel.⁸¹

Figure 9.09 • Coronal T2-weighted MR imaging sequence of the elbow with an exquisite depiction of the lateral or radial collateral ligament (structure 24). This ligament should appear as a low signal intensity band coursing its way in an arc-like fashion laterally, as demonstrated in this image. The medial or ulnar collateral ligament can also be clearly seen in the proximal aspect (28).

The radial collateral ligament complex includes the lateral collateral ligament (LCL), the annular and accessory collateral ligaments and the LUCL (Figure 9.01B).⁶⁸ It contributes to the lateral stability of the elbow and resists varus stress.^82,83

The LCL extends from the anterior aspect of the lateral epicondyle and inserts into the annular ligament and supinator fascia. The posterior bundle, the LUCL, extends from the lateral epicondyle and inserts into the ulna (hence the name) at the crista supinatoris.⁶⁸ The LUCL is the prime restraint against varus stress and its disruption will severely diminish the stability, leading to posterolateral rotatory instability of the elbow.^83,84 This ligament is best seen on coronal images. The better-known annular ligament is the prime stabilizer for the proximal radioulnar joint⁶⁹ and is best assessed on axial slices (Figure 9.05).⁶⁵

Osseous variants

Most commonly asymptomatic, the supracondylar spur (or process) and its associated ligament are found in approximately 1% of the population. The spur, which is located on the anteromedial surface of the humerus, 5–6 cm above the epicondyle, can be rudimentary and tubercle-like or quite large.⁸⁵ The ligament of Struthers connects the bony outgrowth to the medial epicondyle, forming the supracondylar foramen or arcade of Struthers. The median nerve and branches of the brachial artery run through the foramen (Figure 9.10). In some cases, the arcade can be the site of an anomalous origin of the pronator teres muscle.^86–88 These close anatomical relationships can account for the symptoms that may be experienced by some patients and will be addressed along with the other causes of median nerve entrapment.

Figure 9.10 • The anomalous ligament of Struthers runs from the supracondylar spur, an anatomical variant, to the medial epicondyle. It forms the arcade of Struthers, which contains the brachial artery and the median nerve and can be a site of entrapment.

This variant is easy to visualize on plain film radiographs, especially if the arm is in a slightly oblique position. The spur will point towards the elbow joint, as opposed to its principal differential diagnosis, the osteochondroma, which points away from the joint. Although location and appearance are key to distinguishing the two, MR imaging demonstrates the cartilaginous cap in a developing osteochondroma.⁸⁶

Two accessory ossicles are known to occur about the elbow joint: the os supratrochleare and the patella cubiti. The os supratrochleare is a developmental ossicle believed to originate from an accessory ossification centre of the olecranon. When present, it is located within the joint capsule in the olecranon fossa. It may restrict extension or can be asymptomatic. Because of its intra-articular position, it may benefit from synovial nutrition and increase in size. Its shape may be altered by the battering received from the adjacent olecranon. It is most common in the dominant arm of males.^89,90

The patella cubiti is a rare sesamoid bone found in the tendon of the triceps, mimicking the patella in the quadriceps tendon. It may also be asymptomatic or cause restriction in the extension range of motion.⁹¹

Muscular variants

The other major elbow variants consist of accessory muscles or additional muscular bundles. Again, they may be incidental findings, although their size may cause impingement on neurovascular bundles. Clinically, this would present as any other proximal impingement syndrome with nerve-specific muscle weakness and paraesthesia in the forearm and hand.

Many types of aberrant muscles have been described, such as additional origins that may be fused or partially fused with neighbouring muscles. Variations in the location of the insertions may also be present. These variants are not at all rare and have been described for many years; it is therefore somewhat surprising that their existence is not generally appreciated by clinicians and often overlooked by them when formulating a differential diagnosis.

The biceps brachii provides a good example of this. Although the name of the muscle suggests the unvarying presence of two heads, the biceps can in fact possess anywhere between one and five heads. A third head is found in some 10%–20% of the population,^92,93 so is not in any way a rarity. As long ago as 1875, the anatomist Macalister published a review of 48 different variations for the morphology of the biceps,⁹⁴ and more have been reported since.^95–99 Another example of relatively frequent variation is the brachioradialis muscle, which may be conjoined with the supinator. Alternatively, its tendon may be split into different slips and cause entrapment if the radial nerve passes between them.⁷⁵

Although accessory muscles or muscle bundles may not cause overt symptoms in the patient, they can be quite puzzling to the practitioner examining the MR images. The goal is not to be familiar with each and every variation (unless one is in search of a new life-long hobby), but to acknowledge that they are not rare occurrences and to be sufficiently familiar with the ‘normal’ muscles to identify a variant.

Paediatric considerations

The elbow region undergoes a significant transformation during the process of bony maturation. The multiple apophyses and epiphyses ossify at different times, making the evaluation of images of the growing child quite challenging and potentially confusing.

Until recently, it was customary to obtain radiographs of the uninvolved arm; however, the increased exposure to ionizing radiation of developing tissue is no longer considered acceptable on a routine basis. On plain film, it is difficult on occasion to differentiate between a newly ossified centre and a fracture fragment. Fortunately, the ossification process follows a predictable order, for which the acronym CRITOE or CRITOL may be used in order to remember its sequence (Table 9.03).

Table 9.03 Ossification centres for the elbow in order of appearance: the CRITOE acronym

The capitellum is the first such centre to become ossified and therefore radiologically visible; this happens towards the end of the first year of life. This is followed at approximately 2-yearly intervals by the radial head, medial (or, for acronymic purposes, internal) epicondyle, trochlea, olecranon process and, lastly, the lateral (external) epicondyle. These centres are usually completely fused by 16 years of age.^100,101

This knowledge aids the practitioner quite considerably in their diagnosis: if, for example, an osseous fragment is noted in the region of the lateral epicondyle and the olecranon process centre is not yet visible, it would be appropriate to suspect a fracture. Understanding the process of maturation may, therefore, eliminate the need for cross-sectional imaging. Nonetheless, there are cases in which MR may be required to assess any cartilaginous injuries that may have been sustained; these will be discussed later. Ossification centres may also be multicentric and appear fragmented. On MR images, normal developing centres will demonstrate high signal intensity on T1-weighted images, corresponding to their higher fatty marrow content.

Osteochondral injuries

These lesions, formerly referred to as osteochondritis dissecans, are classically seen in the femur and talus; nevertheless, the elbow may be also affected, especially at the capitellum.^102–105 Rare reports of trochlea, radial head and olecranon process lesions have also been published.^102,106,107 The condition is defined as a subchondral reaction in the bone that may lead to osteochondral fragment formation and articular deformity as the overlying cartilage collapses in to the area of osseous destruction.¹⁰⁸

Adolescent males are most commonly affected, shortly before the point when the epiphyseal growth plate fuses.^109,110 A history of athletic activity suggesting acute trauma or chronic overuse may be present; yet, in some cases, the history may be unremarkable.^109,111 Athletes who participate in throwing events or gymnasts should be monitored closely when developing elbow pain as the constant valgus stress created by impaction accentuates the stress on the lateral aspect of the elbow.^107,112 Patients may present with pain, tenderness and swelling, particularly in the lateral soft tissues surrounding the joint. Catching or locking sensations are suggestive of the presence of a loose fragment.^109,113

It is important to clarify the terminology before moving on. There is much confusion between the terms ‘osteochondritis dissecans’, ‘Panner’s disease’ and ‘osteochondrosis of the capitellum’. Some authors utilize the terms interchangeably; some others establish them as different entities, although the evolution and pathophysiology of the processes are not well understood. Regardless of these semantic differences, most authors agree on this: the term ‘osteochondritis’ is undesirable owing to the inconsistency of inflammatory markers in this condition.^109,113,114 When considered as separate entities, Panner’s disease is described as a condition affecting younger children (aged 5 to 11 years) which does not lead to loose body formation or residual deformity. For others, the conditions are linked and part of the continuum of osteochondrosis, wherein Panner’s disease would represent an early phase of the spectrum.^115–118 Here, the terms ‘osteochondrosis’ (to identify the condition) and ‘osteochondral defect’ (to define the sequelae) will be used.

Although radiographs remain the initial evaluation method of choice for osteochondrosis, the use of MR imaging is increasing owing to its sensitivity and the aid that it provides in determining the prognosis and, thereby, patient management. MR imaging is definitely superior to plain film radiography in detecting early osteochondral defects (Figure 9.11).^{64,104,119,120}

Figure 9.11 • Fat-suppressed T2-weighted MR imaging of the elbow in the sagittal (A) and coronal (B) planes together with T1-weighted imaging in the coronal plane (C). A region of increased signal intensity is noted in the subarticular area of the capitellum, most notably anteriorly (arrow in A). A mild joint effusion is also present with a collection of fluid posterior to the distal humerus. In figure B, a small region of increased signal is again noted in the region of the capitellum (arrow) with the suggestion of slight flattening of the subchondral bone. Note the small, round high signal region adjacent to the medial aspect of the elbow on the skin (arrowhead), which is a vitamin E tablet placed to indicate where the patient is symptomatic to assist the interpreter in localizing the region of complaint. In figure C, a focal defect is suggested in the capitellum. The imaging features are consistent with an osteochondral injury with an associated joint effusion, with no evidence of displacement of a fragment. These multiple images demonstrate changes consistent with early stages of osteochondrosis/osteochondral injury. Heterogeneous signal changes are noted in the anterior aspect of the capitellum on the sagittal (A) and coronal (B) fat-suppressed images. Subtle changes are also noted on the coronal T1-weighted image (C), which was obtained slightly more posteriorly than that of image B.

In the early stages, plain film radiology may appear normal or show only subtle density changes. Flattening of the articular surface and fragmentation of the subchondral bone may be observed as the condition progresses. Free fragments may be observed in the later stages, often lodged in the coronoid or olecranon fossae.¹⁰⁹

MR imaging can demonstrate significant abnormalities, even when radiographs are normal. Signal changes can be observed on most sequences. Fat-suppressed T2-weighted images will demonstrate a focal increase in signal intensity in the subchondral region of the capitellum. On the T1-weighted sequences, focal areas of either low signal intensity surrounding a more intermediate signal area or of homogeneous low signal intensity correspond to the involved area (Figure 9.12). If fragments are present, these will be of the same intensity as the native bone, and are easier to locate if surrounded by joint effusion. MR imaging is also superior in assessing the size, location, displacement, stability and viability of any fragment, all of which are factors in treatment and prognosis determination.^63,64

Figure 9.12 • Sagittal T2-weighted MR imaging sequence of the elbow demonstrating focal high signal in the trochlea adjacent to the articular surface (oval). This represents bone marrow contusion/infarction of the bone secondary to avulsion of the medial collateral ligament (not visualized in current study).

A high intensity zone surrounding the fragment on both T2-weighted and post-contrast T1-weighted images is a sign of instability of the fragment, suggesting that surgical intervention may be required. Imaging enhancement of the fragment with contrast is considered a positive sign suggesting adequate blood supply and viability. Timely detection may therefore determine the most appropriate management protocol, providing a better outcome for the patient.^64,110,121

Early or stable lesions respond well to conservative treatment, mainly the discontinuation of the aggravating activity. The cessation or serious limitation of activity is imperative for a successful outcome. The prognosis for the condition also depends on the age of the patient and the severity of the original lesion. Unstable lesions, where an isolated fragment is present, usually necessitate surgical management. The principal long-term complication in osteochondrosis is degenerative joint disease.¹²²

A normal variant may mimic the presentation of osteochondral defects in the elbow. Occasionally, the junction of smooth articular cartilage of the capitellum and the rough, non-articular cartilage on the posterior aspect of the lateral epicondyle may form a groove and simulate a bony defect. This appearance is referred to as the ‘pseudodefect of the capitellum’. The location on the posterior side is the key to recognition along with recognition of the continuity of the cartilage interface and lack of underlying osseous change.^110,123

Bone injuries

MR imaging is not usually necessary to initially detect the majority of elbow fractures; however, MR imaging is superior to radiography in the evaluation of occult fractures, stress injuries and bone contusions, which may significantly affect both prognosis and management.^78,123 Fat-suppressed T2-weighted images are considered the most sensitive but least specific images for their detection of these often subtle osseous conditions.⁶⁴

Fractures are demonstrated as a linear pattern of increased T2-weighted and decreased T1-weighted signal intensities. Signal changes associated with bone bruises are typically more diffuse, visible as a large region of increased signal intensity on fluid-sensitive sequences, affecting the subchondral bone and bony marrow about the site of the injury (Figure 9.12).⁶⁴

MR imaging is also the best modality to evaluate the inevitable soft tissue injuries associated with fractures and determine their effect on both the clinical management and the prognosis of the trauma. For example, radial head fractures are often associated with disruption of the lateral and medial collateral ligaments. The damage to the soft tissues and the small fragments cannot be seen on plain film radiographs.^124,125

Assessing damage to the growth plate and developing articular cartilage can also be performed using MR imaging. This makes the modality particularly useful for assessing traumatic injuries in the paediatric patient, notwithstanding the difficulties in imaging younger patients discussed above. The prognosis and management of growth-plate (Salter–Harris) injuries is greatly influenced by the location of the damage, making an accurate description of the injury a high priority. MR imaging offers an important advantage since the ossification centres of the distal humerus are not well seen on plain film radiographs. However, this benefit has to be clinically weighed against the risks of sedation in this population, often necessary to ensure image quality.^64,110,123

Certain sites within the elbow are more prone to fractures than others; the most common location of fracture in adults is the radial head.⁷⁸ The mechanism of injury always involves some form of axial overloading, such as falling on an outstretched hand (FOOSH).¹²⁶ The ‘FOOSH’ mechanism may also be responsible for injuries throughout the upper extremity: fractures at the distal radius (Colles’ fracture), capitellum or the scaphoid as well as dislocations of the wrist, elbow or even glenohumeral joint. It is also a frequent cause of ligament ruptures, especially of the ulnar collateral ligament.^78,127

When examining a patient with a suspected radial head fracture, it is therefore of the utmost importance to assess the rest of the upper extremity to explore the possibility of comorbid damage.³ With conventional radiographic evaluation of radial head fractures, it is imperative to look for the anterior fat pad ‘sail sign’ or the elevated capsular fat plane posteriorly, both of which indicate capsular oedema and are very closely associated with radial head fractures, even in the absence of overt findings.¹²⁸ If the fracture is radiographically occult on plain films, MR imaging can confirm the presence of a fracture by demonstrating effusion, marrow oedema and the actual fracture line.

In adults, the anterolateral aspect of the radial head is especially vulnerable to injury because it lacks subchondral bone.¹²⁹ By contrast, in children, fracture of the radial neck is more common, though the morphological appearance is similar.¹³⁰ The degree of comminution is important to assess in all fractures of the proximal radius as it is one of the most important factors in the long-term prognosis. When the fracture is not well visualized on plain film radiography, MR imaging may be indicated.¹³¹

Fractures of the coronoid process of the ulna are distraction injuries caused by the avulsion of the brachialis muscle from dislocation or hyperextension. They can occur in conjunction with or independently from fractures of the radial head. If seen together, they are likely the result of a posterior dislocation caused by a FOOSH.¹³² When associated with a sprain of the ulnar collateral ligament, they form the ‘terrible triad of the elbow’, invariably challenging from a management standpoint.^133,134

Fractures of the medial and lateral epicondyles are both most commonly avulsion injuries caused by contraction of the flexor or extensor/supinator muscle groups respectively. Direct impact, creating varus or valgus stress, may also be a cause. MR imaging is helpful once again to determine the degree of comminution and damage to the adjacent articular cartilage. Complex fractures will most likely be managed surgically.^135,136

Dislocations

The elbow joint is the second most commonly dislocated joint in adults (following the shoulder). In children, it is the commonest site of such injuries and the mechanism of injury is usually traction.^78,123,137 This is referred to technically as radial head subluxation, and colloquially as ‘pulled elbow’ or ‘nursemaid’s elbow’ referring to the frequent presentation whereby a parent (or nursemaid) has jerked or pulled a small child’s arm whilst holding hands; the injury is usually inadvertent. It may be reduced successfully in most cases without any requirement for imaging. In adults, a FOOSH or forced hyperextension is the typical cause of this injury and the situation is usually rather more complex.^138–141

The ulna and radius can dislocate together or individually. Because of the requirement for pronation and supination, they are closely approximated in all movements. This nature of this closed kinematic chain means that when significant trauma is detected in one area of the chain, the remaining components should be carefully scrutinized to eliminate collateral injury.^29,142 Classical examples of this phenomenon include the Monteggia fracture-dislocation (radial head dislocation associated with an ulnar fracture) and the Essex-Lopresti injury (a comminuted, displaced fracture of the radial head with dislocation of the distal radioulnar joint).⁷⁸

These injuries are usually quite evident both clinically and with conventional radiography. The utility of MR imaging mainly resides in the assessment of the adjacent soft tissue damage, underlying osseous complications and surgical planning (Figures 9.13–9.16). In particular, small fractures and capsular disruptions missed on plain film may be linked to residual instability.¹²⁵ Other injuries associated with dislocation are contusions at the anterior coronoid process, radial head and posterior capitellum; musculotendinous strains involving the brachialis muscle; ligamentous and capsule sprains; and neurovascular damage.¹⁴³

Figure 9.13 • Sagittal, fat-suppressed, T2-weighted MR imaging sequences of the elbow. A dramatic example of a posterior dislocation of the ulna and radius demonstrating the complete loss of articular apposition and also the extent of joint effusion and disruption of the soft tissue anatomy with massive oedema (arrows). Such information may be important prior to surgical intervention to guide the approach used. As well as the joint effusion, regions of low signal intensity also are noted within, which are likely to correspond to areas of intra-articular haemorrhage and/or cartilage lesions resulting from the trauma (circle).

Figure 9.14 • Axial, fat-saturated, T2-weighted MR imaging sequences of the elbow performed through the distal humerus (A) and proximal forearm (B) in a patient with an acute posterior dislocation of the elbow. The images show massive soft tissue oedema (arrows) with fluid present in the olecranon and radial head fossa. There is also complete loss of apposition of the articular surface of humerus and ulna. In such a case, MR imaging may be performed to determine the extent of soft tissue involvement including the neurovascular damage. Also note the extent of the posterior subcutaneous oedema.

Figure 9.15 • Coronal T1-weighted (A) and fat-suppressed T2-weighted (B) MR imaging sequences of the elbow. The images demonstrate the complete loss of articular apposition between the distal humerus and the radius/ulna in a case of an acute traumatic posterior elbow dislocation. Note the extent of the synovial effusion with total disruption of normal anatomic integrity in both images.

Figure 9.16 • Sagittal, fat-suppressed, T2-weighted MR imaging (A) of the elbow following severe trauma showing joint distension with posterior dislocation of the ulna. Surprisingly, the triceps insertion is still intact despite the degree of force necessary to dislocate the articulation. The same protocols in the coronal plane (B) also demonstrate the extensive joint effusion as well as the subcutaneous oedema. On closer inspection of the osseous components, note how the proximal radius, in particular the head and neck, demonstrate high signal owing to the bone marrow oedema associated with a fracture. Lateral dislocation of the radial head and an intra-articular fragment are also noted.

Ligamentous trauma

The ulnar and radial collateral ligament complexes, as well as the capsule, can all be well evaluated with MR imaging. Damage to the collateral ligaments is a common, sports-related, chronic stress injury. It may, however, also occur following acute events such as dislocation.^136,143

On plain film radiographs, subtle soft tissue densities corresponding to avulsed fragments adjacent to the involved epicondyle and gapping of the joint may be the only sign of injury. Any gapping may be accentuated on stress radiographs.⁷⁸ By contrast, MR images may demonstrate signs of acute or chronic ligamentous sprain: increased signal in or surrounding the ligament on T2-weighted images, representing oedema or haemorrhage; thickening or thinning of the ligament and waviness, laxity or discontinuity of the fibres (Figure 9.17). Ligamentous ruptures will be demonstrated by discontinuous fibres and abnormalities of the surrounding signal.^143–145

Figure 9.17 • Axial, T2-weighted, fat-suppressed MR imaging of the elbow (row A) and coronal, T2-weighted, fat-suppressed images (row B) demonstrating a partial lateral collateral ligament avulsion injury. In row A, a region of low signal intensity surrounded by high signal is noted about the radial head consistent with oedema about the lateral collateral ligament (oval). In row B, the region of abnormality is noted about the lateral aspect of the distal humerus, with an area of decreased signal intensity and enlargement of the lateral collateral ligament (oval) as well as a slightly wavy appearance. A mild joint effusion is also noted on both coronal images, which may be associated with such injuries.

Radial collateral ligament

Injuries to the radial collateral ligament may occur in acute traumatic events such as a FOOSH, causing hyperextension along with a varus stress, or as a result of chronic epicondylitis, causing lateral epicondylar soft tissue degeneration and tearing of the common extensor tendon.⁴⁸ Rotational instability may be present but difficult to elicit if the patient is guarding.⁸² On MR images, as with other ligament tears, there will be discontinuous fibres with adjacent signal irregularities (Figure 9.18). Posterolateral rotatory instability, as seen with lateral ulnar collateral ligament (LUCL) incompetence, will result in a rotary subluxation of the humeroulnar joint and subluxation or dislocation of the radiohumeral joint. This pattern of instability may also be secondary to a surgical lateral extensor release.¹⁴⁹

Figure 9.18 • Coronal STIR MR imaging of the elbow demonstrating the ruptured radial collateral ligament which is continuous, ‘bunched-up’ and disorganized (oval). However, remember that it would be necessary to review all slices in the planes available to confirm the presence and full extent of the lesion.

Tendinous injuries

Although uncommon injuries, biceps and triceps tendon tears can be assessed with MR imaging, which can confirm the rupture and distinguish between complete and partial tears as well as excluding differential diagnoses.^123,149 Rupture of the biceps tendon typically occurs acutely in middle-aged men. If it occurs distally, it will arise near or at the insertion site on the radial tuberosity.^144,150,151 On MR imaging, the low signal intensity of the tendon will be absent on axial images.^123,143 The T2-weighted images will show high signal intensity corresponding to oedema in the antecubital fossa (Figure 9.19). Retraction of the tendon fibres may be observed on the sagittal images. The amount of retraction is usually linked to a concomitant injury to the bicipital aponeurosis (lacertus fibrosus).

Figure 9.19 • Sagittal STIR MR imaging of the elbow demonstrating a region of diffuse increase in signal intensity in the anterior compartment of the elbow, running along the biceps muscle and tendon (oval). This represents disruption of the muscle/tendinous fibres due to a partial rupture of the lower portion of the biceps tendon.

Tendinopathy and partial tears are also common. In both chronic and acute injury, thinning of the tendon may be seen; however, on occasion, the tendon may also appear thickened. In the acute scenario, this is due to the oedema associated with the intrasubstance damage. By contrast, the thickening seen in chronic injury is caused by accumulated fibrosis, which will have a less intense signal on the fluid-sensitive images. Partial ruptures are also characterized by the presence of high T2-weighted signal intensity, indicating bone marrow oedema in the radial tuberosity; and presence of fluid (high T2 signal intensity) in the interosseous fossa, corresponding to associated bursitis.^64,152 Low signal intensity on both T1-weighted and T2-weighted images represents osseous proliferation at the radial tuberosity, which may result from such an injury.¹⁵¹ Differentiation of partial tears from bicipital tendinopathy is difficult as one may predispose to the other. It is important to consider and correlate the clinical presentation to inform the differential diagnosis.

Tears of the triceps tendon, whether complete or partial, are even rarer injuries. Tears may be seen after acute incidents (such as a FOOSH injury) or from chronic, repetitive events related to sports. The MR imaging findings will be similar to those described in the biceps tendon, but will be present in or surrounding the olecranon process, with associated bone marrow changes observed on both axial and sagittal images. Small avulsion fractures may be visualized at the olecranon on MR imaging and also on the lateral plain film views.^63,141

Nerve entrapment

All three major nerves of the upper extremity can be trapped as they pass through the elbow and present clinically as neuropathies that need to be differentiated from more proximal and distal entrapment syndromes. Consideration will also need to be made as to the possibility of ‘double crush’ injuries; positive findings in the elbow do not remove the requirement for a full analysis of the kinematic chain as a whole.^54,85,160

Compression of the ulnar nerve in the cubital tunnel (cubital tunnel syndrome) is the second most common neuropathy in the adult population (after carpal tunnel syndrome with median nerve neuropathy).^28,52,54,161 Pronator (median nerve) and radial tunnel syndrome are also quite regular clinical presentations.^54,55,141 The aetiology is often varied but is mostly associated with chronic overuse processes.^4,120 The symptoms are variable and may be similar to those of tendinous injuries, particularly in patients with radial entrapment, which may present as a lateral epicondylitis that has been unresponsive to treatment. Long-term medial epicondylitis may also be associated with median nerve entrapments.^54,141

The median, radial and especially the ulnar nerve are surrounded by a layer of fat, which permits good visualization of each nerve on axial sequences.¹⁴³ Inflammatory changes will be shown as heterogeneous or homogeneous increased signal intensity on T2-weighted sequences within the contours of the nerve. The diameter of the nerve may be increased around the compression site and there may be blurring of the individual fascicles (Figures 9.20–9.22). ^63,142

Figure 9.20 • Axial T2-weighted MR imaging of the elbow with fat suppression, demonstrating high signal intensity in the region of the ulnar nerve, which would ordinarily be of intermediate signal, in a case of ulnar neuropathy (oval).

Figure 9.21 • Axial T1-weighted MR imaging of the elbow in the same patient illustrated in Figure 9.20 demonstrating thickening, heterogeneous signal intensity and irregularity of the ulnar nerve (oval) due to ulnar neuropathy.

Figure 9.22 • Coronal T2-weighted MR imaging of the elbow in the same patient illustrated in Figures 9.20 and 9.21, with fat suppression, demonstrating high (bright) signal in the ulnar nerve with associated thickening (oval) due to ulnar neuropathy. P = proximal, D = distal.

The surrounding muscular structures may demonstrate changes as well. It is important to assess the surrounding muscles for the presence of associated morphological changes such as muscle denervation or oedema (increased T2-weighted, fat-saturated T2-weighted and inversion recovery sequences), muscle atrophy and fatty degeneration (reduction in overall muscle belly size and increased T1-weighted signal). Early diagnosis and treatment offer the best prognosis, as these trophic changes may be irreversible after 6 months.^63,141,142

With neuropathies, the location and aetiology of compression may be determined at the same time. The radiologist and clinician need to identify and clinically correlate normal variations; muscular hypertrophy; soft tissue masses relating to tumours or infection; trauma-induced neuropathies; or scar tissue formation. The presence of any of these findings does not automatically constitute a diagnosis unless there is evidence of entrapment that explains the patient’s clinical presentation; no clinician wants to treat a radiological finding that was, in fact, benign!

Cubital tunnel syndrome or neuropathy is compression of the ulnar nerve in the area of the cubital tunnel retinaculum.^69,156 A wide variety of causes can be responsible: developmental causes associated with an accessory muscle like the anconeus epitrochlearis^72,157,158 or an aberrant triceps insertion¹⁵⁹; a shallow cubital tunnel¹⁶⁰; post-traumatic causes associated with malunion or non-union of fractures^53,161; muscle hypertrophy¹⁶²; repeated valgus stress injuries^48,148; and medial epicondylitis.^26,163 The clinical presentation can vary with the degree of entrapment from mild paraesthesia to severe deformation and muscle atrophy.¹⁵⁶

The symptoms will manifest in the fifth digit and the medial aspect of the fourth digit, along the ulnar nerve distribution … exactly the same symptoms as you may have experienced if you have every hit your ‘funny bone’. It is important to elicit the presence of any ulnar nerve signs and symptoms proximal to the wrist, such as paraesthesia or weakness in the flexor carpi ulnaris muscle, as this can differentiate between ‘Tunnel of Guyon’ syndrome and more proximal entrapment of the nerve.^156,164

Radial neuropathy is not as frequently seen as ulnar nerve entrapment.^28,54 Depending on the compression site, it may present clinically with either sensory and motor dysfunction or a motor deficit alone (posterior interosseous nerve syndrome). There will be findings of denervation in the muscles supplied by the radial nerve and sensory disturbances in the dorsal and radial aspects of the wrist and hand.^156,164 If the entrapment occurs distally, in the arcade of Frohse, there will only be muscular problems, as the radial nerve has by that time divided into a main sensory branch and the posterior interosseous branch, which supplies the supinator muscle and all the extensor muscles of the forearm with the exception of the extensor carpi radialis longus, which is supplied by the radial nerve before its bifurcation.^68,75,164

In the general population, the median nerve is most often entrapped in the carpal tunnel of the wrist.⁸⁸ It may, however, be compressed in the elbow region, most commonly in athletes.^54,55 This occurs at the level of the pronator muscle and is termed pronator syndrome. It is also possible for the anterior interosseous branch only to be affected.¹⁶⁵ The median nerve gives off this major branch after passing between the two origins of the pronator teres, approximately 5 cm inferior to the lateral epicondyle. The anterior interosseous nerve is a strictly motor branch supplying the pronator quadratus, the flexor pollicis longus and the radial half of the flexor digitorum profundus.^74,75

Anterior interosseous nerve syndrome is demonstrated clinically as weakness of the thumb and index finger in conjunction with a complaint of proximal forearm pain. Patients with pronator syndrome generally experience an aching pain in the anterior portion of the proximal forearm and intermittent paraesthesia. Weakness or frank sensory loss is unusual.^156,164 Sources of entrapment leading to a pronator syndrome are multiple, including the ligament of Struthers,^55,84 muscle hypertrophy,¹⁶⁶ aberrant muscle origins,¹⁶⁷ and soft tissue masses.¹⁶⁸ If there are concomitant findings in the median and radial nerve distribution, one should consider cervical radiculopathy or brachial plexitis as differential diagnoses.¹⁶⁴

Bursitis

The olecranon and bicipitoradial bursae are not typically seen on MR imaging unless engorged with fluid. Patients will present with an antecubital mass or swelling at the posterior aspect of the elbow, depending on the affected bursa. Inflammatory conditions of the bursa can be imaged with MR and generally produce a high fluid signal intensity in the olecranon or bicipitoradial bursae on T2-weighted images and low signal intensity on T1-weighted images. The appearance of bicipitoradial bursitis, which can be insidious or present after athletic activity, is similar on MR imaging.^169,170

MR is not necessary to manage bursitis in most cases; nonetheless, one must keep in mind that bursitis can be more than a reactive process at a site of high frictional stress. Inflammation of the olecranon bursa, for example, is classically seen after trauma, yet may also be present in inflammatory conditions such as rheumatoid arthritis and crystal deposition disorders such as gout.^171,172 Acute osteomyelitis in children and chronic osteomyelitis in adults is often associated with infectious bursitis. Performed usually only on cases that are refractory, MR imaging shows hyperintense marrow oedema on T1-weighted and T2-weighted images and cortical destruction.^173,174

Articular conditions

Synovial folds or plicae represent a thickened fold of synovial tissue, situated at the lateral aspect of the radiocapitellar joint. They are also referred to as a radiohumeral meniscus or labrum.^175,176 Plicae are most commonly seen in the knee joint and are associated with chondromalacia on the adjacent articular surface.^177,178 The only plain film finding in the elbow may be sclerotic changes in the radial head and capitellum. As with their main differential diagnosis (loose bodies), they can be asymptomatic, or may cause ‘snapping’ of the elbow, locking and pain in the lateral aspect of the joint line.

Conditions typically associated with synovial articulations, such as inflammation, degeneration or proliferation, can affect the elbow and can be visualized with MR imaging (Figure 9.23A,B).

Figure 9.23 • (A) Anteroposterior and lateral conventional radiographs of the elbow demonstrating numerous intra-articular densities associated with a joint effusion. The likely diagnosis based on these findings is that of synovio-osteochondrometaplasia although there is a differential diagnosis depending on the clinical history, based on synovial-based lesions. (B) MR imaging of the elbow performed in the sagittal plane in the same patient, T1-weighted with intra-articular contrast, demonstrates a number of low signal intensity zones (oval) in the anterior and posterior aspect of the elbow joint associated with the articular distension, consistent with synovio-osteochondrometaplasia.

Rheumatoid arthritis (RA) is a common, systemic arthritic condition; involvement of the elbow joint is seen in 20% to 50% of rheumatoid patients.^144,179,180 The diagnosis is not usually dependent solely on imaging, but rather based on clinical criteria, which also include joint stiffness, presence of nodules and serological markers.^113,181 Nevertheless, elbow involvement may cause debilitating pain and joint destruction in this condition, which begins as a proliferative synovitis and rapidly progresses to destruction of the articular cartilage and subchondral bone.¹⁸² On plain film, this is demonstrated by a loss of joint space and bony erosions.¹⁸¹ On MR images, thickening of the synovial membrane and associated pannus formation occurs and intra-articular fluid accumulates. An increased signal intensity from the oedematous synovium is seen, with hyperintense joint effusion. In a patient exhibiting clinical signs of rheumatoid arthritis, these are imaging signs that best confirm the diagnosis.¹⁴⁴

Erosions, which exhibit decreased signal intensity, are radiologically visible and the pathognomonic nodules show as mixed signal intensity masses in the skin, synovium and tendons. Erosions in the cartilage and subchondral bone as well as any associated bursitis are also well visualized.¹⁸³ If intravascular contrast is used, the synovium will demonstrate enhancement, especially prominent on fat-saturation sequences. The destruction of both soft tissue structures and bone can lead to severe elbow instability. In late-stage RA, when the joint space is almost completely destroyed, MR may show a decrease in the synovitis.¹⁸³ Although other imaging modalities such as ultrasound may be used to follow the course of RA, MR imaging is able to evaluate better the subtle alterations to the bone as well as to the tendons that may not be detectable using ultrasound.

Osteoarthritis is seen less commonly in the elbow than in weight-bearing joints, and commonly there is a predisposing factor that has resulted in the degeneration of articular cartilage, such as trauma or arthropathy. Findings are typically evident on plain films.¹⁸¹ MR imaging may be used to evaluate the integrity of the cartilage, identify focal defects or chondromalacia, and further qualify the findings seen on radiography. Osteoarthritis is a long-term complication of instability and impingement.^173,184