Hand
The human hand is the athlete’s tactile connection to his or her sport. Our hands are virtually linked to every sport via a handle, a stick, a glove, or even our bare fingers. Athletes may be said to have good hand-eye coordination, good ball-handling skills, or even quick hands, but in each case the relationship between the sport and the athlete’s hands is obvious. Hand anatomy and biomechanics form the framework for understanding the human ability to grip, let go, cup, spread, flex, extend, and even strike, using the most complex anatomic tool of our bodies. The wrist, forming the linkage between the forearm and the hand, is discussed later in this chapter because it allows our hands to be placed in a multitude of positions and is integral in the interactions that lead to throwing, catching, twisting, or any of the innumerable actions that our hands can perform.
Skin
The skin of the hand is specialized to allow the complex activities we take for granted. Complex dermatoglyphics, our “fingerprints,” each unique to an individual, allow optimal grip of smooth objects, like a baseball, much like the ribbed sole of a running shoe allows grip to a smooth basketball court. The skin on the palmar surface of the hand is tethered by septae to the underlying tissues to minimize the natural tendency of our skin to slide over underlying fat and fascia. This tethering of the palmar skin improves grip as well, creating a sturdy surface for tight grasping. Even the moisture on our fingertips is modulated to allow optimal grip. Compare the skin on the dorsum of the hand ( Fig. 70-1, A ) which is easily pulled and stretched, to that on the palm of our hands ( Fig. 70-1, B ), which is firmly tethered to the deeper structures. The septae of Legueu and Juvara form the specialized tethers to the deeper tissue and ligamentous structures in the palm of the hand that limit the skin mobility on the palmar surface, thereby optimizing grip. Several reproducible lines on the palms of our hands allow flexion not just of the fingers but controlled “collapse” of the skin on the palm as well. The thenar, proximal palmar, and distal palmar creases are the visible lines that are evident on uninjured hands ( Fig. 70-2, A ). 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 our palms and fingers while allowing full flexion and extension ( Fig. 70-2, B ). The creases of the palm and the 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, index, middle, and radial half of the ring finger, whereas the ulnar half of the ring finger and the small finger are innervated by ulnar nerve terminal branches. 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. 70-3 ). 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 to 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. 70-4, A 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, and opposition that occur fluidly and seemingly effortlessly in our hands with nearly every task of everyday life. The muscles are divided into those that are intrinsic to the hand, meaning 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, the 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, 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. 70-5, A and B ). Interestingly, 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 action of the FDP and lumbrical muscles are antagonistic, and 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 distal interphalangeal joints in the fingers, respectively ( Fig. 70-6, A 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 will be discussed in the wrist section, because all of these muscles cross the wrist joints but may act either at the wrist, digits, or thumb.
Joints
The joints in the hand are primarily hinge, or ginglymus, joints but also allow some translational and rotational moments. This mechanism is especially true at the MCP joints, which gives us an ability to spread our fingers, slightly rotate them, and fine-tune our grip of both large and small objects.
Metacarpophalangeal Joint of the Fingers
The bony architecture of the MCP joint allows for significant motion, including hyperextension and flexion in the sagittal plane, adduction/abduction in the frontal plane, and rotatory motion of the base of the proximal phalanx (P-1) on the metacarpal head. The cartilaginous surface of the metacarpal head has a trapezoidal shape, being broader on the palmar surface. MCP joint stability is dependent on surrounding collateral and accessory collateral ligaments, volar plate, capsule, and extrinsic flexor and extensor tendons. The collateral and accessory collateral ligaments provide lateral static stability. The collateral ligaments originate dorsal to the metacarpal head axis of motion and insert into tubercles on the sides of the P-1. Accessory collateral ligaments have their origin palmar to the proper collateral ligaments and insert into the palmar base of P-1 and the volar plate. Because of the dorsal metacarpal origin of the collateral ligaments and the cam shape of the metacarpal head in the sagittal plane, the ligaments have laxity in extension but are taut in flexion. This characteristic is the basis for the recommendation that most hand injuries be splinted with the MCP joints in full flexion, or the so-called “safe position.” The safe position for the interphalangeal joints is in extension because of disparate anatomy at the PIP and DIP joints. Accessory collateral ligaments, along with interosseous and lumbrical tendons, provide additional lateral (adduction/abduction) stability. The volar plate provides a block to hyperextension and constitutes the third side of the anatomic box that provides static MCP stability. The volar plate has a broad, firm distal attachment to P-1 and a membranous loose origin from the metacarpal neck. The laxity of the collateral ligaments in extension places the volar plate at risk of rupture (usually proximally) with excessive or forceful MCP hyperextension. The dorsal capsule is relatively loose, and the extrinsic extensor tendons extend the MCP joint through the sagittal bands’ attachment to the base of P-1 and the volar plate.
Proximal Interphalangeal Joint
The PIP joint is a highly congruous hinge joint, with stability provided by the matching articular surfaces of the phalanges and the combination of a thick volar plate and stout collateral ligaments (see Fig. 70-4 ). The tight fit of the opposing articular contours increases stability, especially when the PIP is under axial load. The collateral ligaments are thick and composed of proper and accessory components. The proper ligaments insert into the base of the middle phalanx (P-2) and the volar plate, whereas the accessory collateral ligaments insert only into the volar plate. The volar plate is very thick distally where it inserts into the volar lip of P-2, whereas proximally it thins out and has two proximal projections that attach to P-1, called the check rein ligaments. This arrangement allows the PIP joint to flex more than 110 degrees. The condyles of the head of P-1 are not cam shaped, and the PIP joint does not get permanently stiff when immobilized in full extension. On the contrary, the PIP joint has a propensity to develop flexion contracture, with shortening of the volar plate, when immobilized or held in a flexed position as the result of an injury. PIP flexion contractures are most pronounced after trauma to the ring and small digits. Eaton described the soft tissue constraints of the PIP joint as three sides of a box; instability occurs when at least two sides of this box are disrupted. The central slip of the extensor apparatus attaches at the dorsal epiphysis of P-2 and is frequently avulsed in volar PIP dislocations from a hyperflexion mechanism of injury. The PIP joint is notorious for the challenges posed by intraarticular injury and the consequences of poor management, often leading to recalcitrant stiffness.
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. 70-7, A and B ). These entities will be further discussed in Chapter 77 .
