The distal radius has three articular facets: the scaphoid facet, the lunate facet, and the sigmoid notch, which articulates with the distal ulna. The normal distal radius is characterized by several measurements: radius inclination (22°), lateral tilt (11° volar), radius height (12 mm of styloid height relative to the ulnar corner of the lunate facet), and length relative to the ulna (normal is neutral ulnar variance; Figure 24.1). Change from these normal values is assessed following fracture; this, along with articular displacement, often dictates treatment. There is a prominent transverse ridge 2 mm proximal to the volar rim of the lunate facet and 12 mm proximal to the scaphoid facet (Figure 24.2). Volar plate placement on the fractured distal radius is preferably done proximal to this ridge so that this bony prominence is the most volar surface contacting the flexor tendons.

The carpal bones are arranged in two rows: the proximal (scaphoid, lunate, triquetrum, pisiform) and distal (trapezium, trapezoid, capitate, hamate) rows. These bones interact in a complex but coordinated fashion to produce wrist motion as described subsequently in the Kinematics section. Notable from a surgical standpoint, the scaphoid derives a retrograde vascular supply from a dorsal branch of the radial artery entering the dorsal ridge of the scaphoid. Thus, the most extensive dissection for scaphoid fracture is accomplished from the volar wrist to avoid disrupting the vascular supply. Second, the lunate is wider volarly than dorsally, making it more susceptible to palmar dislocation. This contributes in part to the preponderance of volar lunate dislocations compared to dorsal dislocations as the final step in perilunate instability.

The primary volar ligamentous stabilizers of the carpus include the radioscaphocapitate and long and short radiolunate ligaments (Figure 24.3). These ligaments originate from the volar articular margin of the distal radius and must be preserved during volar approach to the distal radius for fracture fixation. Violation of these volar ligaments can result in iatrogenic ulnar carpal translation. Preservation of the radioscaphocapitate ligament origin is the limiting factor for volar bony excision (4 mm) during radial styloidectomy.

The dorsal intercarpal and dorsal radiocarpal ligaments are the key spanning dorsal ligaments (Figure 24.3). The dorsal radiocarpal ligament originates from the distal radius to insert on the triquetrum, and the dorsal intercarpal ligament originates on the triquetrum to insert on the scaphoid, trapezium, and trapezoid. These ligaments can be identified and incised in line with their fibers to allow a ligament-sparing approach to the dorsal aspect of the distal radius and carpus. The primary intrinsic carpal ligaments of the proximal carpal row are the scapholunate and lunotriquetral. The scapholunate interosseous ligament is strongest dorsally, while the lunotriquetral interosseous ligament is most substantial volarly. Care should
be taken during a dorsal exposure of the wrist to prevent iatrogenic injury to the scapholunate ligament, which is routinely located slightly ulnar to the longitudinal axis of the radius passing through Lister’s tubercle.

On the ulnar aspect of the wrist, the triangular fibrocartilage complex (TFCC) is the confluence of an articular disc, ulnocarpal meniscus homolog, dorsal and volar radioulnar ligaments, the floor of the extensor carpi ulnaris (ECU) tendon sheath, and the volar ulnocarpal ligaments (Figure 24.4). The TFCC runs from the ulnar fovea to the radius and triquetrum. It is biconcave in shape. The dorsal and volar radioulnar ligaments are the primary stabilizers of the distal radioulnar joint (DRUJ). The dorsal radioulnar ligament is preserved when exposing the dorsal wrist by carefully elevating the fourth extensor compartment without dissecting deep to the fifth extensor compartment overlying the DRUJ. The fact that only the peripheral 10% to 40% of the TFCC is vascularized dictates routine repair of peripheral TFCC tears versus débridement of central tears of the articular disc.

The extensor tendons run in six separate fibro-osseous sheaths under the extensor retinaculum at the level of the wrist. From radial to ulnar, the six compartments are: (1) abductor pollicis longus/extensor pollicis brevis (APL/EPB), (2) extensor carpi radialis longus and brevis (ECRL/ECRB), (3) extensor pollicis longus (EPL), (4) extensor digitorum communis/extensor indicis proprius (EDC/EIP), (5) extensor digiti minimi (EDM), and (6) extensor carpi ulnaris (ECU). The muscle bellies of the first compartment cross dorsal to the second compartment tendons 6 cm to 7 cm proximal to the wrist crease. Within the first dorsal compartment, the APL commonly consists of multiple tendinous slips, and the EPB may be contained within a separate subsheath prior to its insertion
on the proximal phalanx of the thumb. The EPL angles 45° radially from its origin on the ulna as it passes around the ulnar aspect of Lister’s tubercle to insert on the distal phalanx of the thumb, where it extends and retropulses the thumb. During the utilitarian dorsal approach to the wrist between the third and fourth extensor compartments, care must be taken to identify and protect the EPL, as it is the only extensor tendon deviating substantially from a longitudinal course. Thus, most dorsal exposures of the wrist—whether for arthrodesis, arthroplasty, or traumatic reconstruction—are performed by opening the extensor retinaculum into the third extensor compartment to first identify and then protect the EPL tendon before elevating surrounding extensor compartments off the radius (Figure 24.5). The fourth extensor compartment contains the posterior interosseous nerve in its radial-sided floor. The posterior interosseous nerve is commonly excised during dorsal wrist exposures to provide pain relief as a partial wrist denervation—although some argue for preservation, as the nerve provides proprioceptive information for the carpus. Also, within the fourth extensor compartment and commonly used for tendon transfers, the EIP can be differentiated from the EDC to the index finger by its more distal muscle belly, a lack of tendon juncturae, and its location typically ulnar and deep to the EDC tendon. Clinically, presence of the EIP is confirmed by independent active extension of the index metacarpal-phalangeal joint while all other fingers remain in a fist. Within the fifth compartment, the EDM tendon typically has two slips that course directly over the distal radial-ulnar joint. The EDM is the landmark for operative exposure of the DRUJ and TFCC. The sixth compartment containing the single ECU tendon is typically exposed to débride tendonitis or to stabilize the ECU tendon. Importantly, the extensor retinaculum is expansive and anchored to the more volar-ulnar surface of the ulna, with ECU stability dependent on a competent ECU subsheath.

The median nerve enters the hand through the carpal tunnel, running under the transverse carpal ligament, which spans from the scaphoid tubercle and trapezial ridge to the pisiform and hook of the hamate. The carpal tunnel is, on
average, 22 mm wide and 26 mm long. The carpal canal contains the median nerve, which is the most superficial structure, four tendons of the flexor digitorum superficialis (FDS), four tendons of the flexor digitorum profundus (FDP), and the flexor pollicis longus (FPL) tendon (Figure 24.6). At the carpal tunnel, the median nerve is 94% sensory and 6% motor. The palmar cutaneous branch of the median nerve arises 5 cm to 7 cm proximal to the wrist crease and travels within the median nerve epineurium for 16 mm to 25 mm before passing superficial to the transverse carpal ligament to supply sensation to the thenar eminence. Injury to the palmar cutaneous branch of the median nerve produces a small area of paresthesia, but, more important, often produces substantial pain and hypersensitivity over the course of the nerve.