Extrinsic ligaments

The extrinsic ligaments are capsular ligaments that tie the radius and ulna to the carpal bones and traverse the radiocarpal joint, midcarpal joint, or both. The extrinsic ligaments may be divided into volar and dorsal components. The volar radiocarpal ligaments are stronger and thicker than the dorsal ligaments and are the major stabilizers of wrist motion. Significant variations have appeared in the descriptive anatomy and nomenclature of these ligaments [6-9], resulting in some confusion in the literature.

The volar extrinsic ligaments are subdivided into the radiocarpal and ulno-carpal ligaments. The most important extrinsic volar stabilizer of the wrist, the radioscaphocapitate ligament, is the most radial of the major volar ligaments. It connects the radius to the distal carpal row and plays a major role in preventing rotatory subluxation of the scaphoid. The ligament arises from the volar aspect of the styloid process of the radius, crosses the waist of the scaphoid, and inserts into the head of the capitate, coalescing ulnarly with the ulnocapitate ligament to form the arcuate or deltoid ligament. It should be noted, however, that some authors believe the deltoid ligament is an intrinsic ligament with a capitotriquetral (ulnar arm) and a capitoscaphoid (radial arm) component.

The radiolunotriquetral ligament is the second strongest volar ligament. It originates at the radial styloid process, adjacent to the radioscaphocapitate ligament, and extends ulnarly to attach to the volar aspect of the triquetrum. This ligament functions as a sling to the lunate bone. The radioscapholunate, or ligament of Testut, is a small volar ligament, which arises from the volar aspect of the distal portion of the radius and inserts into the proximal volar surfaces of the scapholunate interval. This structure plays less of a role in stabilizing the wrist, but its importance may be related to the neurovascular structures that lie within the ligament.

The volar ulnocarpal ligaments include the ulnolunate and ulnotriquetral ligaments extending from the ulnar styloid process and triangular fibrocartilage to the lunate and triquetrum, respectively. The volar extrinsic ligaments are typically hypointense on MR images, although they may display a striated appearance on three-dimensional images [10,11]. They may be best evaluated on transverse and oblique sagittal images [11] but are frequently seen, at least in part, on coronal images.

The dorsal extrinsic ligaments are deep to the extensor retinaculum and extensor tendons. These ligaments are not as important functionally as the volar ligaments. The dorsal radiocarpal ligament originates from the distal radius at Lister's tubercle, attaching distally to the dorsal horn of the lunate and dorsal triquetrum. The dorsal intercarpal ligament, the most prominent of the dorsal ligaments, arises in the triquetrum, coursing radially as it fans out to insert on the dorsal ridge of the scaphoid, the trapezium, and the trapezoid. The interval located distal to the dorsal radiocarpal ligament and proximal to the dorsal in-tercarpal ligament is used as a dorsal surgical approach.

The dorsal extrinsic ligaments may be consistently visualized using high-resolution thin-slice sequences, such as those obtained with three-dimensional Fourier transform and MR arthrography [10-13]. The ligaments may be seen on coronal images but are better assessed on transverse and sagittal images. Not infrequently, and in a similar fashion to the volar ligaments, the dorsal ligaments display a striated appearance that should not be mistaken for tears.

Intrinsic ligaments

The intrinsic or interosseous ligaments are intracapsular structures, which originate and insert on the carpal bones [6,7]. The two most important interosseous ligaments are the scapholunate and lunotriquetral ligaments (Fig. 2). They link the carpal bones in the proximal row, precluding communication of the radio-carpal with the midcarpal joints. The scapholunate and lunotriquetral ligaments are both C-shaped structures with dorsal and volar components and a central, thinner, intramembranous portion [8].

The scapholunate ligament is composed of thick dorsal collagen bundles, rendering its dorsal component the most important for carpal stability [7,8]. Volarly, the scapholunate ligament is looser, so it can accommodate the various articulating curvatures of the scaphoid and lunate [14].

Prestyloid Recess

Fig. 2. Normal anatomy—prestyloid recess and meniscus homologue. Oblique coronal MR-arthrographic image demonstrates a focal collection of contrast at the level of the prestyloid recess (asterisk) outlining the meniscus homologue (mh). The radial (arrow) and ulnar (arrowhead) insertions of the triangular fibrocartilage are also seen. SL, scapholunate ligament.

The lunotriquetral ligament links the lunate and triquetral bones. It is a smaller and more taut ligament than the scapholunate ligament. Its stronger and thicker volar component blends with the triangular fibrocartilage.

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