Radius/Carpus/Distal Radioulnar Joint: Bones and Ligaments

HISTORICAL BACKGROUND

The bones that make up the carpal wrist have names with ancient origins. The name of the scaphoid is derived from the Greek word scaphe, meaning boat or trough. The lunate was previously named the semilunar owing to its resemblance to the moon: the translation of lunate is “crescent-shaped.” The capitate is also known as the os magnum because it is the largest bone in the carpus. Finally, the pisiform translates to “pea,” which matches its size and shape.

Much of the modern original work in the anatomy of the wrist was performed by Landsmeer, whose painstaking dissection was the basis for the names of many wrist ligaments. Landsmeer published an atlas of hand anatomy and co-authored a landmark paper with Berger describing the extrinsic ligament anatomy of the carpus.

OSSEOUS ANATOMY

The wrist is the link between the forearm and the hand. The carpus is composed of 15 bones excluding the sesamoid and supernumerary bones ( Fig. 1-1 ). These include the distal radius and ulna, the two rows of the carpus, and the bases of the five metacarpals. The carpal rows are divided into the proximal and distal carpal rows. In the proximal carpal row are found the scaphoid, lunate, triquetrum, and pisiform. The pisiform articulates only with the triquetrum, since it lies volar to it and is enveloped by the flexor carpi ulnaris tendon except on its deep surface ( Fig. 1-2 ). The pisiform is considered by many to be a sesamoid bone, but it provides an important lever arm for the flexor carpi ulnaris tendon. The trapezium, trapezoid, capitate, and hamate make up the distal carpal row from radial to ulnar in the wrist. The hook of the hamate extends volarly 1 to 2 cm radial and distal to the pisiform and serves as the attachment site for several ligaments. The five metacarpal bases articulate with this row of carpal bones.

The carpus includes three specific joint types: radiocarpal, midcarpal, and carpometacarpal. The radiocarpal joint is composed of the distal articular surface of the radius in addition to the triangular fibrocartilage complex (TFCC). The distal radius has two articular facets, named after the carpal bone that they contact—the scaphoid and lunate facets, respectively. The facets on the radius are divided by the interfossal ridge, which is often a raised portion of the radius, but can also have a fibrous ridge. The distal radius and TFCC articulate as a whole with the proximal carpal row, which forms a convex articular facet.

The midcarpal joint contains three articulations. On the radial side is the scaphotrapeziotrapezoid joint; centrally, the scaphoid and lunate articulate with the capitate; and ulnarly, the hamate and triquetrum form a helicoid-shaped joint.

The carpometacarpal joints vary widely in their degree of stability. The base of the first metacarpal and trapezium make up a unique “saddle” joint, which has a remarkable degree of motion because of the anatomy of these bones. In comparison, the index and middle carpometacarpal joints are tightly interlocked with little motion because of their tight capsular and ligamentous structure. The ring and small carpometacarpal articulation is less restrained, with a greater degree of motion.

Finally, the distal radioulnar joint forms a separate and distinct joint through the sigmoid notch of the radius into which fits the distal ulna. The radius rotates around the stable ulna with supination and pronation.

LIGAMENTOUS ANATOMY

All the ligaments of the wrist are intracapsular with the exception of three ligaments: the transverse carpal ligament, or flexor retinaculum, the pisohamate ligament, and the pisometacarpal ligament. Nearly all the wrist ligaments are contained within capsular sheaths of loose connective tissue and fat. This often makes it difficult to visualize individual ligaments when approaching the carpal joints surgically. From within the joints, the ligaments can be viewed as distinct structures, best seen during wrist arthroscopy or visualizing the volar ligaments via a dorsal approach between the carpal bones.

There are two general categories of ligaments: intrinsic and extrinsic. Intrinsic ligaments have their origin and insertions within the carpus, with a large area of insertion onto cartilage rather than bone and many fewer elastic fibers compared with extrinsic ligaments. These ligaments tend to avulse from insertion or origin rather than rupture in mid-substance. The extrinsic ligaments of the wrist form connections between the forearm and carpus. These ligaments are stiffer with lower ultimate yield than intrinsic ligaments.

Extrinsic Carpal Ligaments

The extrinsic ligaments of the carpus provide support and form the linkage between the long bones of the forearm and the eight carpal bones. The volar extrinsic ligaments form an inverted V-shaped configuration and are best visualized during wrist arthroscopy. In fact, visualization of the ligaments from outside the wrist joint is nearly impossible, since the capsule of the wrist blends the extrinsic ligaments together. These ligaments include the radioscaphocapitate, long radiolunate, radioscapholunate, and short radiolunate on the radial carpus. The ulnolunate, ulnolunocapitate, and ulnotriquetrocapitate ligaments form the volar ulnar ligamentous complex ( Fig. 1-3 ).

On the radial side of the wrist, the radioscaphocapitate ligament originates in a broad sheet from the tip of the radial styloid to the midscaphoid fossa and inserts on the distal scaphoid pole, acting as a support to the waist of the scaphoid. The next most ulnar ligament is the long radiolunate ligament, which originates from the remaining portion of the scaphoid fossa of the radius and attaches to the radial volar aspect of the lunate. Abutting the long radiolunate ligament is the radioscapholunate ligament (or ligament of Testut), which is not actually a true ligament but a capsular tissue through which course blood vessels including terminal branches of the anterior interosseous artery. This structure goes through the volar capsule and attaches dorsally into the scapholunate interosseous ligament. Finally, the short radiolunate ligament has its origin from the lunate fossa and inserts onto the radial side of the lunate ( Fig. 1-4 ).

The ulnocarpal complex includes the most superficially located ulnocapitate ligament, which originates from the ulnar fovea, passing volarly to support the lunotriqetral interosseous ligament, and inserts partially onto the capitate and mostly blends into the radioscaphocapitate ligament. The ulnolunate and ulnotriquetral ligaments are interdigitated proximally as they originate from the palmar radioulnar ligament and then split to attach distally to the lunate and triquetrum, respectively.

Based on the definition of extrinsic ligaments connecting to bones outside the carpus is a single dorsal extrinsic ligament, the dorsal radiocarpal, or dorsal radiotriquetral, ligament. The dorsal intercarpal ligament, which lies at the same level but connects the scaphoid, trapezium, and trapezoid to the triquetrum, is often classified as an extrinsic ligament. This ligament complex, which forms a lateral V shape, has been shown to indirectly stabilize the scaphoid dorsally during wrist range of motion ( Fig. 1-5 ).