The distal radius is the only forearm bone that articulates directly with the bones of the carpus (scaphoid and lunate). The distal radius has three articular surfaces: radiocarpal, distal radioulnar, and the triangular fibrocartilage. The radiocarpal surface of the radius is concave and tilted in two planes. The radius has an ulnar inclination or tilt of 15 to 25 degrees in the frontal plane and a volar tilt of 10 to 15 degrees in the sagittal plane. The ulna is separated from the carpal bones by a cartilage complex on its distal end, the triangular fibrocartilage complex (TFCC). This cartilage complex forms a smooth continuous ulnarly directed extension of the distal radial surface and supports the lunate and triquetrum, on the distal ulna. The triangular fibrocartilage complex is also the main stabilizer of the distal radioulnar joint. The distal radius has a concave sigmoid notch on its ulnar aspect that articulates with the curvature of the ulnar head permitting rotation of the wrist. The distal radioulnar joint is also supported by dorsal and volar radioulnar ligaments that merge with the triangular fibrocartilage complex.
The eight carpal bones are arranged in two rows (Fig...262-.1.A Eig 2.62.z1S). The distal row (trapezium, trapezoid, capitate, and hamate) is quite stable. These carpal bones are joined tightly together and to their adjoining metacarpals. The distal carpal row and metacarpals move together as a unit forming a relatively rigid stable arch. The proximal row (scaphoid, lunate, triquetrum, and pisiform) is also arranged in an arch in the frontal plane located between the distal radius and distal carpal row. The proximal carpal row functions as a mobile middle link or "intercalated segment" in this arrangement. By virtue of its position, the proximal carpal row is potentially unstable. The scaphoid holds a unique position, acting as a stabilizing strut and linking the proximal and distal carpal rows on the radial aspect of the wrist. This explains the scaphoid's greater propensity for injury.
FIG. 262-1. A. Key elements on a normal PA view. (1) The carpal bones form three smooth arcs; (2) carpal bones are separated by a 1- to 2-mm space; (3) scaphoid has an elongated shape; (4) radius has an ulnar inclination of 15 to 25 degrees; (5) radial styloid projects 8 to 18 mm (average 13 mm). Half the lunate articulates with the radius, and the ulna and adjacent radial surface are equal in length (neutral ulnar variance). B. Bony anatomy. (With permission from Chin HW: Injuries of the wrist, in Hart RG, Rittenberry JJ, Uehara DT (eds): Handbook of Orthopaedic Emergencies. Philadelphia, Lippincott-Raven, 1998.)
The forearm muscles that insert onto the base of the metacarpals produce wrist motion. There are no direct tendon insertions on the carpal bones (except for the pisiform that is a sesamoid bone of the flexor carpi ulnaris). The carpal bones move passively in response to hand position. Although the radiocarpal joint is often referred to as the "wrist joint," it is important to realize that wrist motion is nearly equally divided between the radiocarpal and midcarpal joint. This is best appreciated when viewing carpal movement from the sagittal view. During flexion and extension each row moves in the same direction and with similar degrees of angulation.
The carpal bones are stabilized to one another by intrinsic ligaments and to the bones of the forearm by the extrinsic ligaments. The key extrinsic ligaments are arranged in three arcades; two are volar and one dorsal. The two volar ligaments are arranged in two inverted V-shaped arches. The apex of one arch inserts on the lunate supporting the proximal carpal row, while the other arch reaches out to the distal carpal row with its apex inserting onto the capitate. There is an inherently weak area (space of Poirier) between these two palmar arches. This area lies at the junction of the lunate and capitate and also widens with dorsiflexion of the wrist. Forceful dorsiflexion of the wrist could tear the capsule here and produce a perilunate or lunate dislocation. There is a single dorsal arcade that acts as a sling across the back of the wrist. These arcades have origins on the rim and styloid of the radius on one side, and distal ulna and triangular fibrocartilage complex on the other side; injuries to either insertion point are therefore potentially unstable.
The intrinsic ligaments are largely responsible for holding the carpal bones together as a kinematic unit in their respective carpal rows. Because the proximal carpal row is a mobile middle link in this three-link system, the intrinsic ligaments within this segment are particularly important because of their greater propensity for injury. The intrinsic ligaments of the proximal carpal row are named after the respective carpal bones they connect: the scapholunate and triquetrolunate. The proximal carpal row is also under important internal dynamic forces. The palmar flexed posture of the scaphoid produces a flexion torque on the lunate that is counterbalanced by an extension torque from the triquetrum. Unfortunately, this delicate balance is lost if either ligament is disrupted, and produces a volar or dorsal tilt of the proximal carpal row and carpal instability.
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