The anatomy of the hand is complex, intricate, and fascinating. Its integrity is absolutely essential for our everyday functional living. Our hands may be affected by many disorders, most commonly traumatic injury. For any physician or therapist treating hand problems, the mastery of such anatomy is fundamental in order to provide the best quality of care.
A total of 19 bones constitute the basic skeleton of the hand. These are grouped into metacarpals and phalanges.
The hand contains 5 metacarpal bones.
Each metacarpal is characterized as having a base, a shaft, a neck, and a head. The first metacarpal bone (thumb) is the shortest and most mobile. It articulates proximally with the trapezium. The other 4 metacarpals articulate with the trapezoid, capitate, and hamate at the base. Each metacarpal head articulates distally with the proximal phalanges of each digit.
The hand contains 14 phalanges.
Each digit contains 3 phalanges (proximal, middle, and distal), except for the thumb, which only has 2 phalanges. To avoid confusion, each digit is referred to by its name (thumb, index, long, ring, and small) rather than by number.
There are many joints within the hand. These can be placed into two categories: metacarpophalangeal (those between the metacarpals and the phalanges) and interphalangeal (those in between the numerous phalanges).
At the metacarpophalangeal joints, lateral motion is limited by the collateral ligaments, which are actually lateral oblique in position, rather than true lateral. This arrangement and the shape of the metacarpal head allow the ligaments to be tight when the joint is flexed and loose when extended (ie, cam effect). The volar plate is part of the joint capsule that attaches only to the proximal phalanx, allowing hyperextension. The volar plate is the site of insertion for the intermetacarpal ligaments. These ligaments restrict the separation of the metacarpal heads.
At the interphalangeal joints, extension is limited by the volar plate, which attaches to the phalanges at each side of the joint. Radial and ulnar motion is restricted by collateral ligaments, which remain tight through their whole range of motion. Knowledge of these configurations is of great importance when splinting a hand in order to avoid joint contractures.
Muscles and Tendons
The muscles of the hand are divided into intrinsic and extrinsic groups. The intrinsic muscles are located within the hand itself, whereas the extrinsic muscles are located proximally in the forearm and insert to the hand skeleton by long tendons.
The extensor muscles are all extrinsic, except for the interosseous-lumbrical complex, which is involved in interphalangeal joint extension. All of the extrinsic extensor muscles are innervated by the radial nerve. This group of muscles consists of 3 wrist extensors and a larger group of thumb and digit extensors.
The extensor carpi radialis brevis (ECRB) is the main extensor of the wrist, along with the extensor carpi radialis longus (ECRL) and extensor carpi ulnaris (ECU), which also deviate the wrist radially and ulnarly, respectively. The ECRB inserts at the base of the third metacarpal, while the ECRL and ECU insert at the base of the second and fifth metacarpal, respectively.
The extensor digitorum communis, extensor indicis proprius, and extensor digiti minimi extend the digits. They insert to the base of the middle phalanges as central slips and to the base of the distal phalanges as lateral bands. The abductor pollicis longus, extensor pollicis brevis, and extensor pollicis longus extend the thumb. They insert at the base of the thumb metacarpal, proximal phalanx, and distal phalanx, respectively.
The extensor retinaculum prevents bowstringing of tendons at the wrist level and separates the tendons into 6 compartments. The extensor digitorum communis is a series of tendons to each digit with a common muscle belly and with intertendinous bridges between them. The index and small finger each have independent extension function through the extensor indicis proprius and extensor digiti minimi.
The extrinsic flexors consist of 3 wrist flexors and a larger group of thumb and digit flexors. They are innervated by the median nerve, except for the flexor carpi ulnaris (FCU) and the flexor digitorum profundus to the small and ring finger, which are innervated by the ulnar nerve.
The flexor carpi radialis is the main flexor of the wrist, along with the flexor carpi ulnaris and the palmaris longus, which is absent in 15% of the population. They insert at the base of the third metacarpal, the base of the fifth metacarpal, and the palmar fascia, respectively. The FCU is primarily an ulnar deviator. The 8 digital flexors are divided in superficial and deep groups. Along with the flexor pollicis longus, which inserts at the thumb distal phalanx, they pass through the carpal tunnel to provide flexion at the interphalangeal joints.
At the palm, the flexor digitorum superficialis tendon lies volar to the profundus tendon. It then splits at the level of the proximal phalanx and reunites dorsal to the profundus tendon to insert in the middle phalanx. The flexor digitorum profundus perforates the superficialis tendon to insert at the distal phalanx. The relationship of flexor tendons to the wrist joint, metacarpophalangeal joint, and interphalangeal joint is maintained by a retinacular or pulley system that prevents the bowstringing effect.
The intrinsic muscles are situated totally within the hand. They are divided into 4 groups: the thenar, hypothenar, lumbrical, and interossei muscles.
The thenar group consists of the abductor pollicis brevis, flexor pollicis brevis, opponens pollicis, and adductor pollicis muscles. All are innervated by the median nerve, except for the adductor pollicis and deep head of the flexor pollicis brevis, which are innervated by the ulnar nerve. They originate from the flexor retinaculum and carpal bones and insert at the thumb’s proximal phalanx.
The hypothenar group consists of the palmaris brevis, abductor digiti minimi, flexor digiti minimi, and opponens digiti minimi. They are all innervated by the ulnar nerve. This group of muscles originates at the flexor retinaculum and carpal bones and inserts at the base of the proximal phalanx of the small finger.
The lumbrical muscles contribute to the flexion of the metacarpophalangeal joints and to the extension of the interphalangeal joints. They originate from the flexor digitorum profundus tendons at the palm and insert on the radial aspect of the extensor tendons at the digits. The index and long finger lumbricals are innervated by the median nerve, and the small and ring finger lumbricals are innervated by the ulnar nerve.
The interossei group consists of 3 volar and 4 dorsal muscles, which are all innervated by the ulnar nerve. They originate at the metacarpals and form the lateral bands with the lumbricals. The dorsal interossei abduct the fingers, whereas the volar interossei adduct the fingers to the hand axis.
*Above shows the innervation of the nerves through the muscles of the wrist and hand, as well as the the tendons, ligaments, and arteries that run through.
As previously mentioned, the hand is innervated by 3 nerves: the median, ulnar, and radial. Each has sensory and motor components. Variations from the classic nerve distribution are so common that they are the rule rather than the exception. The skin of the forearm is innervated medially by the medial antebrachial cutaneous nerve and laterally by the lateral antebrachial cutaneous nerve.
The median nerve is responsible for innervating the muscles involved in the fine precision and pinch function of the hand. It originates from the lateral and medial cords of the brachial plexus (C5-T1). In the forearm, the motor branches supply the pronator teres, flexor carpi radialis, palmaris longus, and flexor digitorum superficialis muscles. The anterior interosseus branch innervates the flexor pollicis longus, flexor digitorum profundus (index and long finger), and pronator quadratus muscles.
Proximal to the wrist, the palmar cutaneous branch provides sensation at the thenar eminence. As the median nerve passes through the carpal tunnel, the recurrent motor branch innervates the thenar muscles (abductor pollicis brevis, opponens pollicis, and superficial head of flexor pollicis brevis). It also innervates the index and long finger lumbrical muscles. Sensory digital branches provide sensation to the thumb, index, long, and radial side of the ring finger.
The ulnar nerve is responsible for innervating the muscles involved in the power grasping function of the hand. It originates at the medial cord of the brachial plexus (C8-T1). Motor branches innervate the flexor carpi ulnaris and flexor digitorum profundus muscles to the ring and small fingers. Proximal to the wrist, the palmar cutaneous branch provides sensation at the hypothenar eminence. The dorsal branch, which branches from the main trunk at the distal forearm, provides sensation to the ulnar portion of the dorsum of the hand and small finger, and part of the ring finger.
At the hand, the superficial branch forms the digital nerves, which provide sensation at the small finger and ulnar aspect of the ring finger. The deep motor branch passes through the Guyon canal in company with the ulnar artery. It innervates the hypothenar muscles (abductor digiti minimi, opponens digiti minimi, flexor digiti minimi, and palmaris brevis), all interossei, the 2 ulnar lumbricals, the adductor pollicis, and the deep head of the flexor pollicis brevis.
The radial nerve is responsible for innervating the wrist extensors, which control the position of the hand and stabilize the fixed unit. It originates from the posterior cord of the brachial plexus (C6-8). At the elbow, motor branches innervate the brachioradialis and extensor carpi radialis longus muscles.
At the proximal forearm, the radial nerve divides into the superficial and deep branches. The deep posterior interosseous branch innervates all the muscles in the extensor compartment: supinator, extensor carpi radialis brevis, extensor digitorum communis, extensor digiti minimi, extensor carpi ulnaris, extensor indicis proprius, extensor pollicis longus, extensor pollicis brevis, and abductor pollicis longus.
The superficial branch provides sensation at the radial aspect of the dorsum of the hand, the dorsum of the thumb, and the dorsum of the index finger, long finger, and radial half of the ring finger proximal to the distal interphalangeal joints.