Anatomy of the hand.

The anatomy of the hand is efficiently organized to carry out a variety of complex tasks. These tasks require a combination of intricate movements and finely controlled force production. The shape of the bony anatomy in conjunction with the arrangement of soft tissues contributes to the complex kinesiology of the hand Injury to any of these structures can alter the overall function of the hand and therefore complicate the therapeutic management. The purpose of this article is to review the anatomy of the band with special emphasis on structures that relate to management of band injuries [Moran CA: Anatomy of the band Phys Ther 69:1007-1013,1989] Key Words: Anatomy; Hand; Hand injuries; Kinesiology/biomechanics, upper extremity, Upper extremity, band and wrist The hand is a complex anatomical system of dynamic and static structures. Very often the general clinician is wary of treating patients with hand injuries because of the hand's anatomical and kinesiological complexity. These injuries are further complicated by traumatic or surgical processes. The purpose of this article is to review the anatomy of the hand emphasizing important structures related to therapeutic management. Arthrokinematics Finger The carpometacarpal (CMC) joints of the fingers are classified as plane joints with one degree of freedom. However, the fifth CMC joint is often classified as a semisaddle joint with conjunctional rotation. The metacarpophalangeal (MCP) joints are classified as ellipsoidal or condylar joints with two degrees of freedom. Specifically, the metacarpal condylar surface is asymmetrical with a greater sloping configuration of the radial shoulder than the ulnar shoulder.3 This articular configuration plays a role in ligamentous orientation and subsequent kinesiology of the joint. Therefore, it is of functional importance in describing certain pathological conditions such as ulnar drift. The proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints are both bicondylar with subsequently greater congruency between bony surfaces. They have one degree of freedom. In addition, the axis of motion has an obliquity of 6 to 13 degrees in the coronal plane because of the unevenness of the condylar articulating surfaces at both PIP and DIP joints. This divergence of the distal aspect of these joints is accentuated by the pull of the flexor digitorum sublimus (FDS) tendons and allows for opposition with the thumb. This divergence of the interphalangeal (IP) joint is important clinically when restoring motion via joint mobilization techniques. Volar gliding of the middle phalanx should be accomplished with this coronal tilt added to the gliding motion, thus more accurately stressing the capsular tissues. Thumb The CMC joint of the thumb is a sellar joint exhibiting two degrees of freedom with reciprocally convex-concave surfaces allowing for the motions of flexion and extension (concaveconvex), abduction and adduction (convex-concave), and conjunctional rotation. The thumb MCP joint is similar to the finger MCP joints arthrokinematically. The thumb IP joint's articulating condyles also display an unevenness, resulting in an obliquity of the axis of motion of 5 to 10 degrees. The joint capsule is a fibrous structure composed of irregular, dense connective tissue that accepts stress and permits stretch in all directions of that joint's motion. Within the joint capsule is contained the synovial membrane from which synovial fluid is produced for these joints. The joint capsule of all the digital joints displays specific redundancies to accommodate for motion. Wise produced evidence for this configuration by demonstrating significant volar and dorsal redundancies in arthrograms of the digital joints. Bojsen-Moller identified a large, thin capsule surrounding the CMC joint of the thumb. Kuczynski noted that the joint surfaces could be distracted 3 mm when supporting capsular structures were removed. During periods when the hand is immobilized for surgical or traumatic reasons, the finger joint capsule will adaptively shorten in the immobilized position, preventing normal motion of the articular surfaces later. This configuration supports the clinical rationale to stress this structure during treatment for improved joint mobility. It would appear that maximal length is necessary for CMC joint rotation or opposition. Minami et al observed that the dorsal MCP joint capsule provides stability during long-axis distraction and rotatory motions." Specifically, the volar capsule of all finger joints is taut in extension, whereas the dorsal capsule is taut in flexion. Volar Plate The volar plate is a fibrocartilagous structure that serves as a volar articulating surface, an attachment for ligaments an additional confining structure for synovial fluid, and an inhibitor of dorsal dislocation during MCP joint extension. The volar-plate attachments at the MCP joint are capsular rather than bony, which permits hyperextension. A similar volar-plate attachment is noted at the DIP joint level. The plate attachments are different at the PIP joint level. Bowers et al identified a bony attachment of the PIP joint's volar plate that provides greater joint stability. In their analysis of joint ruptures, they observed that the static resistance to hyperextension is offered by the lateral insertion of the volar plate-collateral ligament at the margin of the phalangeal condyle. Ligaments There are important extracapsular and capsular ligaments that support and stabilize the hand. The most important extracapsular ligament is the transverse intermetacarpal ligament (TIML) (Fig. 1). It attaches to and courses between volar plates at the level of the metacarpal heads along the entire width of the hand. These structures, the TIML and the metacarpal heads, comprise the distal transverse arch.13 Full extensibility of this ligament is necessary for grasping activities and prehensile activities. Zancolli noted that the extensibility of the TIML between the third and fourth MCP joints and between the fourth and fifth MCP joints is particularly important because the fourth and fifth rays descend 5 to 7 mm with respect to the third ray when making a full fiSt.14 Therefore, if this mobility is restricted because of traumatic scarring or "immobilization tissue shortening," function is greatly hampered. The capsular collateral ligaments of all finger and thumb joints provide important joint stability. The MCP joint collateral ligaments have been investigated in light of their probable involvement in the pathomechanics of ulnar drift. The radial and ulnar MCP joint collateral ligaments are eccentrically placed, which allows for selective tautness and slackness throughout their range of movement (Fig. 2). This placement also promotes the arthokinematic motions of roll and slide. The change in ligament length is due to the prominent condylar shoulder that the collateral ligaments must pass over. In addition, Hakstian and Tubiana observed that the radial collateral ligament is consistently longer than the ulnar collateral ligament. This finding was reported as part of their anatomical investigation of the etiology of ulnar drift. They noted that the tendency toward an ulnar-deviated position was due to ligament length and the slope differences of the radial and ulnar condylar shoulders. In addition, they observed that the greatest stability at the MCP joint was provided by the ligaments. They resected all supporting structures sequentially to produce this observation. This stability was also observed by Minami et al. All of these data confirm the rationale of maintaining ligament length by splinting the MCP joint in flexion. The MCP joint ligaments have dual attachments-bony and glenoid (Fig. 3). Thus, the classical distinction is noted: The fan or glenoid portion arises from the metacarpal head and attaches to the volar plate, and the cord or collateral portion arises from the metacarpal head and attaches to the base of the phalanx. In contrast, the PIP and DIP joint collateral ligament attachments are completely bony (Fig. 4). Kuczynski observed only bony attachments of the PIP joint collateral ligaments. Shrewsbury and johnson noted that the two components of the DIP joint collateral ligaments overlap by as much as one third.11, The collateral ligaments of the PIP and DIP joints are concentrically placed and are of equal length. Thus, the ligaments are maximally taut throughout the range of motion. In addition, the phalangeal articular surfaces are square and fairly regular in condylar prominence, adding to the consistent ligament tautness. Kuczynski notes that ligament length can be maintained in extension, contradicting the clinical notion of splinting IP joints in slight flexion to maintain length. The thumb CMC joint displays, a unique configuration of ligaments: three capsular, one extracapsular. They are the anterior oblique ligament, the dorsoradial ligament, the posterior oblique ligament, and the intermetacarpal ligament (Fig. 5). The capsular ligaments are oblique in orientation, suggesting the maximal support provided by these structures occurs during thumb rotation and pinch-prehensile activities. Pieron methodically recorded the positions that caused maximal span of these oblique ligaments.6 He observed that the dorsoradial position caused maximal span in 31/2 of the 4 ligaments (Fig. 6). Therefore, according to his findings, that joint position would be the optimal splinting position to maintain ligament length. Eaton and Littler observed that the anterior oblique ligament provided stabilizing function by stabilizing the volar beak of the first metacarpal. The fourth ligament, which is extracapsular, is the intermetacarpal ligament, more commonly known as the Y" ligament.6,11 This ligament arises from the volar aspect of the trapezium and courses distally between the first and second metacarpals. It then bifurcates and attaches to the bases of those metacarpals. The Y ligament has a tethering function of keeping the first metacarpal on the trapezium.8 This lack of congruency most often happens during thumb abduction and forceful key-pinch activities, and changes in this ligament have notably affected thumb function, particularly in the rheumatoid thumb. Herve-Muscle Arrangement Movements of the hand are accomplished by a combination of intrinsic and extrinsic musculature. Specific kinesiological reviews of muscle function are beyond the scope of this article. The reader is urged to consult other references for this information. The three peripheral nerves of the upper extremity (radial, median, and ulnar) innervate the intrinsic and extrinsic muscles of the hand. The forearm musculature are presented in Gardner et al as distinct layers (Fig. 7).22 This approach aids the clinician in palpation and muscle testing and will be used in this review. At the volar wrist level, the tendons assume a specific anatomical arrangement (Fig. 8). All volar tendons pass through the carpal tunnel with the exception of the flexor carpi ulnaris tendon as it attaches to the pisiform and the palmaris longus tendon, which passes superficial to the volar carpal ligament. Usually, the palmaris longus tendon lies superficial to the median nerve and, therefore, can be used as a landmark for palpation of the nerve. The volar carpal ligament attaches to the tubercle of the scaphoid and the tubercle of the trapezium radially and to the pisiform and hook of the hamate ulnarly to form the roof of the carpal tunnel. Within the carpal tunnel, passing laterally to medially, are the following structures: flexor carpi radialis (FCR) tendon, flexor pollicis longus FPL) tendon, median nerve, FDS tendons, and flexor digitorum profundus FDP) tendons. The arrangement of the finger flexor tendons as shown in Figure 8 is important to remember when evaluating partial wrist lacerations. In this type of injury, the FCR tendon, median nerve, and some or all of the FDS tendons can be lacerated, leaving the FPL and FDP tendons intact because these tendons lie deep within the carpal tunnel. Clinically, the patient could display inconsistent thumb and finger flexor motions. The intrinsic muscles are categorically presented as the thenar muscle group, the hypothenar muscle group, and the interosseous muscles. The thenar muscle group is composed of the abductor pollicis brevis, flexor pollicis brevis, opponens pollicis, and adductor pollicis muscles. These muscles are primarily innervated by the motor branch of the median nerve with the exception of the adductor pollicis muscle, which is supplied by the deep motor branch of the ulnar nerve. The hypothenar muscle group is composed of the abductor digiti minimi, flexor digiti minimi, and opponens digiti minimi muscles. The hypothenar muscle group, the three volar interosseous muscles and the four dorsal interosseous muscles, are supplied by the ulnar nerve. The lumbrical muscles, which arise from the FDP tendon of each digit to insert laterally on the extensor hood, are innervated by both median and ulnar nerves. Specifically, the two lateral muscles are innervated by the median nerve, and the two medial muscles are innervated by the ulnar nerve. The dorsal forearm muscles are innervated by the radial nerve (posterior interosseous nerve). These muscles are also arranged within two layers in the forearm. The extensor tendons are contained within six compartments at the level of the wrist Fig. 9). The first compartment contains the extensor pollicis brevis and abductor pollicis longus tendons. When these tendons develop tenosynovitis from overuse or disease pathology, it is usually described as deQuervain's syndrome. Compartment two contains the extensor carpi radialis longus and extensor carpi radialis brevis tendons. Compartment three contains the extensor pollicis longus tendon. This tendon takes a 45-degree turn at Lister's tubercle to travel distally to the thumb. The extensor digitorum communis and extensor indicis tendons are found within the fourth compartment. Compartment five holds the extensor digiti minimi tendon, and the extensor carpi ulnaris tendon is contained in compartment six. The median nerve enters the hand through the carpal tunnel. Motor and sensory nerve fibers pass deep to the volar carpal ligament, and occasionally variation is observed in the routing of the motor branch of the median nerve. This branch, which innervates the thenar muscle groups, may pass deep to or superficial to the volar carpal ligament. The ulnar nerve enters the hand through the tunnel of Guvon, or Guyon's canal, which is a space formed under the pisohamate ligament. The nerve is subject to variation at this level because it can pass through the tunnel with both sensory and motor components or as the motor branch only. This anatomical variation of the ulnar nerve results in variation of clinical presentation following trauma or transient compression. Dorsally, the radial nerve is present in the hand via its sensory branch, the superficial radial nerve. The motor branch, the posterior interosseous nerve, innervates the dorsal extrinsic musculature only and does not innervate the intrinsic musculature. Skin The most superficial structure of the hand is the skin. it provides a durable covering, which is highly innervated volarly for efficient tactile gnosis. The volar surface is endowed with fixed fit pads in addition to numerous eccrine glands, which aid in nonslip grasping. The various lines or creases of the skin follow the normal stresses imposed by the movements of the hand. Extensibility and innervation of the skin are extremely important to ultimate function of the hand. The hand is innervated volarly by the median and ulnar nerves; dorsally, it receives innervation from all three nerves. On the volar surface, the thumb and the index and long fingers are innervated by the median nerve. The ulnar nerve supplies sensation to the ring and little fingers. Because the crossover area of the median and ulnar nerves can include shared innervation of the long and ring fingers, the autonomous zones for the median nerve and the ulnar nerve innervation are the index and little fingers, respectively. Dorsally, the superficial radial nerve supplies the area from the thumb to the long finger from the level of the wrist distal to the PIP joints of the index and long fingers. The area from the PIP joint to the tip is innervated by the median nerve. The ulnar nerve innervates the area from the wrist to the tip of the ring and little fingers on the medial aspect of the dorsal hand. Summary This article presented a functional review of hand anatomy to assist the general clinician in gaining a better understanding of the hand. Although complex, the numerous structures are arranged to maximize function. Capsular and extracapsular structures were discussed, and an overview of the muscle and nerve arrangements was presented.