The proximal and distal radioulnar joints form a unique articular arrangement between the radius and ulna, allowing pivot motion of the forearm and positioning the hand in space. Typically imaged in conjunction with the elbow, radiographs, computed tomography (CT), and MR imaging of the proximal radioulnar joint contribute unique diagnostic information. Because dysfunction of the distal radioulnar joint is often a result of instability, dynamic CT protocols stressing the joint in addition to anatomic imaging with radiographs and MR imaging is valuable. Detailed knowledge of the patient’s clinical condition and careful selection of imaging protocols will maximize the benefits.
Key points
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The proximal radioulnar joint (PRUJ) and distal radioulnar joint (DRUJ) are complex joints; diagnosis of pathologic abnormality requires an understanding of not only anatomy but also the relevant biomechanics.
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Radiography forms the initial evaluation of both joints.
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Computed tomography (CT) is useful for the evaluation of bony structures and can provide 3-dimensional modeling of complex abnormalities, such as fractures.
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MR imaging should be obtained when soft tissue such as ligamentous, muscular, or tendinous pathologic abnormality is suspected.
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Particularly in the evaluation of the DRUJ, knowledge of the patient’s operative history and findings on physical examination are important to tailor protocols such as dynamic CT to extract maximal benefit.
Rotational motion of the forearm allows 3-dimensional positioning of the hand in space
The PRUJ and DRUJ form the proximal and distal articulations between the radius and the ulna, a unique skeletal arrangement, which, along with the interosseous membrane, allows for the 150° pivot motion of the forearm from pronation to supination, positioning the hand in space, and enabling a wide variety of tasks.
Derangements of the DRUJ and PRUJ may occur secondary to trauma to the hand, wrist, forearm, or elbow, in the setting of degenerative or overuse injuries or within the context of inflammatory or crystalline arthropathies.
Evaluation of these dynamic joints is challenging for hand surgeons and radiologists alike due not only to the structural relationships but also to the functional interaction between the various bones, muscles, and ligaments.
Rotational motion of the forearm allows 3-dimensional positioning of the hand in space
The PRUJ and DRUJ form the proximal and distal articulations between the radius and the ulna, a unique skeletal arrangement, which, along with the interosseous membrane, allows for the 150° pivot motion of the forearm from pronation to supination, positioning the hand in space, and enabling a wide variety of tasks.
Derangements of the DRUJ and PRUJ may occur secondary to trauma to the hand, wrist, forearm, or elbow, in the setting of degenerative or overuse injuries or within the context of inflammatory or crystalline arthropathies.
Evaluation of these dynamic joints is challenging for hand surgeons and radiologists alike due not only to the structural relationships but also to the functional interaction between the various bones, muscles, and ligaments.
Anatomy and biomechanics
Proximal Radioulnar Joint
The PRUJ is a synovial pivot joint formed by the interface between the cylindrical radial head and the radial notch of the ulna ( Fig. 1 ). Motion at the joint consists of rotation of the radial head. Proximally, pronation is facilitated by the pronator teres, while supination is a result of contraction of the biceps brachii and the supinator muscles. Radial head translation is limited by the annular ligament, which spans the anterior and posterior margins of the radial notch of the ulna, effectively trapping the radial head within. The quadrate ligament is a focal thickening of the inferior annular ligament, which attaches to the radial neck, further anchoring the radius. Support of the annual ligament is provided by attachments to the radial collateral ligament and the joint capsule, which is shared with that of the elbow joint.
Interosseous Membrane
The interosseous membrane, or middle radioulnar joint, is a thin band of fibrous tissue extending from the radius proximally to the ulna distally. This syndesmosis results in attachment of the 2 bones, allowing distribution of forces during loading.
Distal Radioulnar Joint
Another synovial pivot joint, the DRUJ, consists of the relationship between the sigmoid notch of the distal radius and the ulnar head ( Fig. 2 ). The sigmoid notch of the radius is a hemicylindrical, longitudinally oriented concavity on the ulnar aspect of the distal radius. The adjacent ulnar head is similarly hemicylindrical, of a smaller radius than the sigmoid notch, however, allowing for anterior and posterior translation as well as rotation of the joint. The head of the ulna subluxes dorsally in pronation and volarly in supination. A third degree of freedom exists, allowing longitudinal translation of the ulna with respect to the radius. The multiple possible axes of translation and rotation introduce an inherent instability of the DRUJ, constrained primarily by tendinous and ligamentous structures.
The ligamentous supporting structures of the DRUJ consist of the dorsal and volar radioulnar ligaments, the ulnocarpal (ulnolunate and ulnotriquetral) ligaments, and the triangular fibrocartilage complex (TFCC), which is anchored to ulnar styloid and the base of the styloid (fovea). The structure, function, and pathology associated with the TFCC are discussed in greater detail in the article Triangular Fibrocartilage Complex by Drs Cody, Nakamura, Small, and Yoshioka.
Additional soft tissue stabilizers are the deep head of the pronator quadratus and the extensor carpi ulnaris. The pronator quadratus provides active approximation of the DRUJ in pronation and passive support in supination. Because of its position overlying the dorsal distal ulna and investment by the extensor retinaculum, the extensor carpi ulnaris subsheath partially stabilizes the DRUJ. Finally, a thin joint capsule provides minimal support.
Imaging algorithms
Proximal Radioulnar Joint
An imaging algorithm for evaluation of the PRUJ is provided in Fig. 3 .
Distal Radioulnar Joint
An imaging algorithm for the DRUJ is provided in Fig. 4 . Note that preimaging knowledge of the wrist examination (stable or unstable) is helpful.
Imaging of the proximal radioulnar joint
Radiography
The PRUJ is typically completely included in standard anteroposterior and lateral views of the elbow, which may demonstrate abnormal alignment of the radial head or fractures of the radial head and the proximal ulna. Dislocation of the radial head is best assessed by comparing the position of the radial head relative to the capitellum rather than the proximal ulna. The radiocapitellar line, a line drawn along the shaft of the proximal radius through the capitellum, should intersect the center of the radial head on both views ( Fig. 5 ). If there is displacement of the radial head with respect to this line, dislocation should be considered. Exaggeration of the normally visualized anterior fat pad or the presence of a posterior fat pad on the lateral view indicates an elbow effusion. In the setting of known or suspected trauma, the presence of an elbow effusion without evidence of fracture on the standard radiographic views should prompt additional oblique radiographs or advanced imaging such as CT to exclude an occult fracture.
