Bone and Joints

The radius and ulna are the bones of the forearm; they connect the hand to the arm. Proximally, the main articulation is between the ulna and the distal humerus, forming the hinge of the elbow. The radius functions primarily to allow pronation and supination. At the wrist, the roles are reversed. Here, the distal radius provides the primary articulation with the carpal bones of the hand, with the ulna participating mainly in pronation and supination. The radius accepts approximately 80% of the weight transfer from the hand across the wrist. Distal to the radius and the ulna at the wrist are the carpal bones, which are organized into two rows. The proximal row consists of the scaphoid, the lunate, the triquetrum, and the pisiform. The distal row includes the trapezium, the trapezoid, the capitate, and the hamate.

The radial and the ulnar styloids are pal- pable subcutaneous landmarks at the wrist. The distal pole of the scaphoid is palpable on the palmar aspect of the wrist. The scaphoid is also palpable in the “anatomic snuff box,” the space between the extensor pollicis brevis and longus tendons to the thumb. The pisiform and the hook of the hamate are pal- pable on the volar ulnar surface of the palm.

Each finger of the hand is composed of a metacarpal bone and three phalanges, with the exception of the thumb (Fig. 13).

Figure 13
The bones of the wrist and hand. A more detailed description of the carpal bones can be found in many anatomy textbooks

The thumb has a proximal and a distal phalanx articulating at a single interphalangeal joint, while the remaining fingers have proximal, middle, and distal phalanges that form a proximal interphalangeal (PIP) joint and a distal interphalangeal (DIP) joint.

Ligaments and Soft Tissues

The shaft of the radius is attached to the shaft of the ulna by an interosseous membrane. This membrane, technically a ligament, helps transfer forces from the radius (which supports the hand) to the ulna, which has the primary articulation with the humerus. Distally, the radius is attached to the proximal row of carpal bones by strong volar ligaments. The radius is attached to the ulna by the dorsal and volar radioulnar ligaments of the triangular fibrocartilage complex. The proximal and the distal rows of carpal bones are connected via a joint capsule that allows for both flexion/extension and radial/ulnar deviation of the hand. The metacarpophalangeal joints as well as the interphalangeal joints are stabilized by joint capsules and col- lateral ligaments. The ulnar collateral ligament of the thumb is especially susceptible to sprains (causing a skier’s or gamekeeper’s thumb).

There are two important transverse ligaments at the wrist. On the dorsal surface, there is an extensor retinaculum. This pulley- like structure tethers the extensor tendons close to the bones of the wrist, even as the muscle contracts. The extensor retinaculum houses six separate synovial sheaths, creating six extensor compartments. There is also a transverse covering to the volar surface, the main component of which is the trans- verse carpal ligament. This is functionally a “flexor retinaculum” and forms the roof of the carpal canal. This canal (or tunnel) houses nine flexor tendons and the median nerve as they pass into the hand from the forearm (Fig. 14).

Figure 14
The carpal tunnel, shown here in cross section, is formed by the transverse carpal ligament above and the carpal bones below. The carpal tunnel con- tains the median nerve and nine flexor tendons.

Compression of the median nerve at the wrist causes carpal tunnel syndrome.

Muscles

The muscles of the hand can be categorized into two groups: intrinsics and extrinsics. Intrinsic muscles reside exclusively in the hand itself. The extrinsic muscles have tendons that attach in the hand but their muscle bel- lies reside in the forearm. The thenar group of intrinsics powers the thumb. This group includes the abductor pollicis brevis, the op- ponens pollicis, the flexor pollicis brevis, and the adductor pollicis. The first three are sup- plied by the median nerve in the hand after it passes under the transverse carpal ligament and are responsible for opposition. A similar (but functionally less important) group of muscles, the hypothenar muscles, attach to the little finger.

The final constituents of the intrinsic group are the lumbricals and the in- terosseous muscles. The dorsal and volar (palmar) interosseous muscles originate from the metacarpals and insert on the proximal phalanges. The dorsal interosseous muscles abduct the fingers; the volar group adducts the fingers. The intrinsics also have a key role in finger flexion and extension. To un- derstand this role, the anatomy of the extrin- sics, the flexor and extensor tendons of the long fingers, must first be understood (Fig. 15)

Figure 15
Insertion of the two flexor and common extensor tendons and the course of the lumbrical muscle. Note that the lumbrical originates on the volar sur- face and inserts onto the extensor tendon; it thus flexes the metacarpophalangeal joint and extends the interphalangeal joints. The interosseous mus- cles that abduct and adduct the digits are not shown in this view.

The tendons of the finger flexor muscles pass into the wrist via the carpal tunnel. They then travel to the fingers bound by fibrous digital sheaths, an intricate pulley system that both nourishes the tendons and prevents “bowstringing” when the muscles contract.

The superficialis attaches to the middle pha- lanx, and the profundus attaches to the distal phalanx. The superficialis is, as its name im- plies, superficial; thus, to allow the profun- dus (ie, deep) flexor to reach the distal pha- lanx, it divides into two slips, and the profundus tendon passes between them (Fig 11, right). The superficialis attaches to the middle phalanx and flexes the PIP joint. The profundus, attaching to the distal phalanx, flexes the DIP joint.

Note that there is no extrinsic flexor tendon that attaches directly to the proximal phalanx. Flexion of the metacarpophalangeal joint, rather, is powered by the intrinsics: the interosseous muscles, which attach to the proximal phalanx; and the lumbricals, which blend into the common extensor tendon on the dorsal surface. The lumbricals originate from the flexor digitorum profundus tendon, cross the finger on its radial border, and at- tach dorsally into the common extensor ten- don. The lumbricals, thus, power metacarpophalangeal flexion as well as extension of the interphalangeal joints—especially when the metacarpophalangeal joint is flexed. The combined function of the intrinsics and the extrinsic flexor-extensor tendons gives a smooth and coordinated composite motion to the digits.

Nerves and Blood Vessels

The three major nerves of the hand are the radial, median, and ulnar nerves. The radial nerve, through its posterior interosseus branch, provides innervation to extensors of the fingers. The radial nerve also provides sensation on the radial aspect of the dorsum of the hand. The median nerve provides sensation to the thumb and index and long fingers, as well as to the radial half of the ring finger. It provides the motor innervation of all of the forearm flexor muscles, with the exception of the flexor carpi ulnaris and the ulnar aspect of the flexor digitorum profundus (that part of the muscle going to the ring and little fingers), which are supplied by the ulnar nerve. The ulnar nerve most importantly provides all of the motor innervation of most of the intrinsic muscles of the hand. The ulnar nerve also provides sensation to the ulnar aspect of the palm, to the little finger, and to the ulnar aspect of the ring finger. The lumbricals to the index and long finger and the thenar muscles, with the exception of the adductor pollicis, are supplied by the median nerve.

The radial artery enters the hand dorsally (although the pulse is more easily palpated on the volar side) and terminates in the deep palmar arch. The ulnar artery enters the hand through Guyon’s canal, at which point it is very close to the ulnar nerve. It terminates in the superficial palmar arch farther distal in the hand. In most people, these two arches communicate. This allows a patient with an injury to one of the two main arteries to maintain viability of the hand and fingers. The palmar arches supply the common digital arteries that further bifurcate into proper digital arteries. Each finger has a radial and ulnar artery that run in the volar aspect of the finger adjacent to the flexor sheath with the digital nerves.