Anatomy and Biomechanics of the Hand and Wrist

Skin

The skin of the hand is specialized to allow the many activities we take for granted. Complex dermatoglyphics, or “fingerprints,” are both unique to each individual and allow the fingers to grip of smooth objects. This is similar to the function of a ribbed sole on a running shoe, which allows grip to the smooth surface of a basketball court. Even the moisture on our fingertips is modulated for optimal grip.

The skin on the palmar surface of the hand is tethered to the underlying deeper tissue and ligamentous structures by specialized structures known as the septa of Legueu and Juvara. This arrangement minimizes the natural tendency of the skin to slide over underlying fat and fascia, thereby optimizing grip and creating a sturdy surface for tight grasping. This is in stark contrast to the skin of the dorsal hand ( Fig. 66.1A ), which is easily pulled and stretched (see Fig. 66.1B ). Several reproducible lines on the palms of our hands allow not only flexion of the fingers but also controlled “collapse” of the skin on the palms as well. The thenar, proximal palmar, and distal palmar creases are the visible lines that are evident on uninjured hands ( Fig. 66.2A ). The creases on the palmar side of the fingers—namely the palmar digital crease, proximal interphalangeal (PIP) flexion crease, and distal interphalangeal (DIP) flexion crease—also accommodate the specialized tethered skin on the palms and fingers while allowing full flexion and extension (see Fig. 66.2B ). The creases of the palm and digits also allow communication of anatomic locations on the hand. For example, “a 1-cm transverse laceration at the level of the proximal palmar crease in line with the middle finger ray” communicates the exact location of a hand laceration within the palm. Additionally, these creases often provide frames of reference for underlying structures, which aids in planning surgical incisions.

Nerves

Sensory nerves in the hand are the terminal branches of the radial, ulnar, and median nerves. The nerves of the fingertip pads that allow fine sensation are the terminal branches of the median and ulnar nerves. Although anatomic variability exists, the median nerve innervates the volar thumb and the index, middle, and radial half of the ring finger, whereas the ulnar half of the ring finger and the small finger are innervated by terminal branches of the ulnar nerve. The main nerve branches within the fingers are termed proper digital nerves and are further designated as radial or ulnar based on which half of the finger is being described ( Fig. 66.3 ). The radial nerve provides sensation to the dorsum of the hand and digits, but its terminal branches do not technically reach the tip of any digit. Dermatomal diagrams delineate the actual areas innervated by individual nerve roots (primarily C6-C8 in the hand), although some individual variation exists. It is the fine tactile sensation in the hand that allows us to “feel” surfaces, easily sensing the difference between the edge of a quarter and the edge of a nickel, for example. Normal two-point discrimination, our ability to sense two separate points of contact on our fingertips, is generally accepted as 5 mm ( Fig. 66.4A and B ).

Muscles and Tendons

The muscles and tendons of the hand are the “motors” and linkages that work the joints, allowing the flexion, extension, abduction, adduction and opposition that occur fluidly and seemingly effortlessly in our hands with nearly every task of daily life. The muscles are divided into those that are intrinsic to the hand, meaning that they originate and terminate within the hand, and the extrinsic muscles, which originate more proximally in the forearm but terminate in the hand. The intrinsic muscles are the lumbrical, dorsal, and palmar interossei and the thenar and hypothenar muscles. The thenar muscles are specifically named abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis. The hypothenar muscles, similarly, are named abductor digiti minimi, flexor digiti minimi, and opponens digiti minimi. Four dorsal interosseous muscles serve to abduct the digits, whereas the three palmar interosseous muscles cause adduction of the fingers. All of the interossei are innervated by the ulnar nerve. The lumbrical muscles have been given the moniker “workhorse of the extensor mechanism.” The lumbrical muscles course on the radial side of each metacarpal and then travel via the radial lateral bands to join in confluence with the extensor mechanism. The lumbrical muscles and their tendinous extension run palmar to the axis of rotation at the metacarpophalangeal (MCP) joint but dorsal to the axis of rotation at the PIP and DIP joints. Therefore when the lumbrical muscles contract, flexion occurs at the MCP joint and extension occurs at the PIP and DIP joints ( Fig. 66.5 A and B ). The lumbrical muscles originate on the flexor digitorum profundus (FDP) tendon just distal to the carpal tunnel and then insert via the radial lateral bands into the extensor mechanism. The actions of the FDP and lumbrical muscles are antagonistic; thus this unique arrangement allows the contraction of one muscle to maximally relax the other.

The extrinsic flexors in the hand are the flexor pollicis longus, which causes flexion of the interphalangeal joint of the thumb, and the flexor digitorum superficialis and FDP, which flex the proximal and DIP joints in the fingers, respectively ( Fig. 66.6A and B ). Both the flexor digitorum superficialis and FDP tendons run through the carpal tunnel, across the hand, and into the fingers. The extrinsic extensor muscles of the hand are discussed in the wrist section because all of these muscles cross the wrist joints but may act at the wrist, digits, or thumb.

Joints

The joints in the hand are primarily hinge, or ginglymus, joints, but they also allow some translational and rotational moments. This mechanism is especially true at the MCP joints; it enables the ability to spread the fingers, slightly rotate them, and fine-tune their grip on both large and small objects.

Distal Interphalangeal Joint

Like the PIP joint, the DIP joint acts as a hinge. It allows flexion and extension, but its bony and ligamentous anatomy effectively eliminates lateral motion and minimizes rotation. Digital DIP flexion is provided through contraction of the FDP, and DIP extension occurs via the contraction of the lateral bands, which coalesce into a terminal extensor tendon insertion at the distal phalanx dorsal epiphysis. Common sports-related injuries involving the DIP joint are “mallet finger,” which refers to an avulsion of the terminal extensor mechanism, and “jersey finger,” which refers to a disruption to the FDP tendon at its insertion ( Fig. 66.7A and B ).