Almost all movements of the upper extremities during routine daily life and many athletic activities require a painless and stable elbow joint. Studying the elbow is a diagnostic challenge because of its complex anatomy. MR imaging is an extremely important tool in the evaluation of common elbow disorders, the spectrum of which ranges from obvious acute lesions to chronic overuse injuries whose imaging manifestations can be subtle.
Key points
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The elbow is a complex anatomic region difficult to assess even by experienced examiners.
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Precise imaging diagnosis helps the management of acute lesions and chronic overuse injuries of the elbow.
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MR imaging is of great importance for patients on the way to regaining a normal elbow joint that permits a functional range of painless motion.
Imaging protocols
MR images are acquired using T1-weighted and T2-weighted and proton density (PD) images, with and without fat suppression. Current protocols differ between specialists, according to their preferences. MR imaging of the elbow is directly dependent on the use of surface coils specialized for the imaging of extremities; moreover, the field of view should be limited to the minimal area necessary to encompass all anatomic structures.
The elbow must be placed close to or at the scanner’s isocenter to provide the field homogeneity required for off-center fat suppression. Typically, patients are placed in a supine position with their arm to the side, or in a lateral decubitus position with their back leaning against the scanner. The patient also can be positioned prone with their arm extended overhead (the Superman position), a position that, albeit less well tolerated and more prone to introducing motion degradation artifacts, can be useful with larger patients. In certain situations, such as with biceps insertion pathology, the tendon can be imaged better with the elbow flexed, the shoulder abducted, and the forearm supinated: the so-called FABS (flexed abducted supinated) position.
Imaging protocols
MR images are acquired using T1-weighted and T2-weighted and proton density (PD) images, with and without fat suppression. Current protocols differ between specialists, according to their preferences. MR imaging of the elbow is directly dependent on the use of surface coils specialized for the imaging of extremities; moreover, the field of view should be limited to the minimal area necessary to encompass all anatomic structures.
The elbow must be placed close to or at the scanner’s isocenter to provide the field homogeneity required for off-center fat suppression. Typically, patients are placed in a supine position with their arm to the side, or in a lateral decubitus position with their back leaning against the scanner. The patient also can be positioned prone with their arm extended overhead (the Superman position), a position that, albeit less well tolerated and more prone to introducing motion degradation artifacts, can be useful with larger patients. In certain situations, such as with biceps insertion pathology, the tendon can be imaged better with the elbow flexed, the shoulder abducted, and the forearm supinated: the so-called FABS (flexed abducted supinated) position.
Anatomy
The elbow is a complex joint formed by 3 distinct articulations: the ulnohumeral, radiohumeral, and radioulnar articulations. The first 2 articulations function as a hinge, permitting flexion and extension between the arm and forearm; the last 2 accomplish the pivot motion of pronation and supination and are functionally linked to the distal radioulnar joint and the wrist. These joints have a common articular cavity and share a multitude of enveloping structures, including the synovium, capsule, and ligaments, all of which render the elbow a single anatomic unit.
The osseous anatomy of 3 bones: the humerus, radius, and ulna; and, although examining the elbow can appear deceptively simple, knowledge of a few potential MR imaging pitfalls ( Box 1 ) can greatly reduce the risk of misdiagnoses, including osteochondral defects ( Figs. 1 and 2 ).
Pseudodefect of the capitellum
Pseudolesion of the trochlear notch
The anterior bundle of the UCL and the LUCL frequently have striated appearance
Bifid DBT
Anconeus epitrochlearis
Ulnar nerve high signal intensity on fluid-sensitive sequences
Plicae size less than 3 mm
The capsule of the elbow is reinforced by strong collateral ligaments, but is relatively weak and loose anteriorly and posteriorly, permitting a large range of flexion through extension. The ulnar collateral ligament (UCL) consists of 3 ligamentous bundles ( Fig. 3 , Table 1 ). The anterior bundle is the major medial stabilizer of the elbow and, fortunately, is the easiest to image (see Box 1 , Figs. 2 and 7 ). It arises from the medial epicondyle and attaches to the coronoid process (sublime tubercle). This bundle itself has functional anterior and posterior components, with the former component more important in extension and the latter more important in flexion; however, they are not seen as separate structures. The posterior bundle extends from below the medial epicondyle to the medial olecranon, forming the floor of the cubital tunnel. It becomes a secondary stabilizer of the elbow when the joint is flexed beyond 90°. The transverse bundle consists of fibers that extend from the anterior and posterior bundles and does not contribute to elbow stability.
| Bundle | Location | Function | View |
|---|---|---|---|
| Anterior | From epicondyle to coronoid process | Major medial stabilizer | Coronal |
| Posterior | Cubital tunnel floor | Secondary stabilizer | Axial |
| Transverse | Between anterior and posterior bundles | No contribution to stability | — |
The radial collateral ligament (RCL) is a complex structure ( Fig. 4 , Table 2 ). The annular ligament is the primary stabilizer of the proximal radioulnar joint (PRUJ) and is best seen on axial images wrapping around the side of the radial head from one side of the radial notch to the other. The RCL arises from the anterior margin of the lateral epicondyle, inserts onto the annular ligament and fascia of the supinator muscle, and is best seen on coronal images. The lateral ulnar collateral ligament (LUCL) is the major posterolateral stabilizer and can be seen on coronal or sagittal images, arising more posteriorly and superficially from the lateral epicondyle, and inserting onto the supinator crest.
| Ligament | Location | Function | View |
|---|---|---|---|
| Annular | Around radial head | Primary stabilizer PRUJ | Axial Sagittal |
| Radial | From epicondyle to annular ligament | Annular ligament stabilizer | Coronal |
| Lateral ulnar | From epicondyle to supinator crest | Major posterolateral stabilizer | Coronal Sagittal |
Numerous muscles originate at or insert into the osseous surfaces of the elbow. They can be divided into anterior, posterior, medial, and lateral compartments ( Table 3 ).
| Compartment | Muscles |
|---|---|
| Lateral | Extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum communis, extensor carpi ulnaris, extensor digiti minimi, anconeus, supinator |
| Medial | Pronator teres, flexor carpi radialis, palmaris longus, flexor carpi ulnaris, flexor digitorum superficialis, flexor digitorum profundus, flexor pollicis longus, brachioradialis |
| Anterior | Brachialis, biceps |
| Posterior | Triceps |
The distal biceps tendon (DBT) consists of 2 unique anatomic and functional subunits, even though it usually is seen, on imaging, inserting as a single, flat paratenon-lined extrasynovial structure with no tendon sheath ( Fig. 5 ). The superficial fibers of the biceps tendon form a broad aponeurotic sheet, termed the lacertus fibrosus (LF), which sweeps across the antecubital fossa, covering and protecting the median nerve and brachial artery. It also blends with the antebrachial fascia that covers the superficial forearm flexors, thereby assisting with forearm supination and elbow flexion. An intact LF can also prevent retraction of a ruptured biceps tendon, facilitating repair of chronic ruptures.
The triceps is the only major extensor of the elbow. It is a tripinnate muscle composed of long, lateral, and medial heads. As a result of this laminar anatomy, each component may be injured separately. It primarily inserts onto the olecranon and posterior joint capsule.
The 3 major nerves in the elbow are the radial, median, and ulnar nerve. The ulnar nerve is the most frequently injured because it is relatively exposed while coursing through the cubital tunnel; however, ulnar neuropathy may also arise from nontraumatic causes (see Box 1 , Fig. 2 ).
The bursae about the elbow joint include the olecranon, bicipitoradial, and interosseous bursae. Bursitis can arise from different causes, with the olecranon bursa the most commonly affected ( Fig. 6 ). Several synovial plicae have been described, with the posterolateral plicae being the most frequently identified (see Box 1 , Fig. 6 ).
Elbow instability
The elbow is one of the most stable articulations in the human skeleton. Because the forces that cross the elbow joint are principally valgus in nature, the joint is not often subjected to varus stress, nor does its articular anatomy predispose it to true varus instability ( Box 2 ).
- 1.
The most important static soft tissue constraints are the LUCL and the anterior bundle of the UCL.
- 2.
Caution should be exercised when making the diagnosis of partial UCL detachment: remember the striated appearance of the ligament.
- 3.
UCL midsubstance disruptions account for most traumatic injuries.
- 4.
Conventional MR imaging shows sensitivity, specificity, and accuracy that approach 100% for full-thickness UCL tears.
- 5.
The anterior bundle of the UCL is often intact even when the radial head and coronoid process are both fractured.
- 6.
Posttraumatic laxity, disruption, or avulsion of the LUCL is considered to be the main contributing factor in PLRI.
- 7.
The most common injury to the LUCL is soft tissue avulsion at the proximal humeral attachment, often with concomitant injury to the common origin of the RCL.
- 8.
Elbow dislocation is most commonly associated with a torn capsule, but the capsule may be intact if the coronoid process is fractured.
- 9.
An abnormal posterolateral plicae usually reflects incompetence of the UCL.
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