BY411: The Hand & Wrist

I would like to touch on a few general topics relating to my specialty dissection of the hand.

The hand is considered to be the manual region of the upper limb distal to the forearm. Twenty-seven bones constitute the hand and wrist and are considered as carpals, metacarpals, and phalanges. There are eight carpal bones which are arranged in a proximal row and a distal row. The proximal row includes the scaphoid, lunate, triquetrum, and pisiform bones while the distal rowincludes the trapezium, trapezoid, capitate, and hamate bones. The distal row of the carpal bones articulate distally with the proximal surfaces of the metacarpals, also called the bases of the metacarpals. There are five metacarpals, corresponding to digit 1 to digit 5, and articulating distally with the proximal phalanges, creating the knuckles of the hand. There are fourteen phalanges total, which include the proximal, middle, and distal phalanges of digit 2 through digit 5. Digit 1, the pollex, is made up of two phalanges, a proximal phalanx and a distal phalanx, rather than three. These bones of the wrist and hand serve as anatomical landmarks as well as attachment points for the muscles that move the hand.

Many of the muscles of the forearm have tendons that insert on the hand including the: flexor carpi radialis, palmaris longus, flexor carpi ulnaris, flexor digitorum superficialis, flexor digitorum profundis, flexor pollicis longus, extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), extensor digitorum, extensor digiti minimi (EDM), extensor carpi ulnaris, abductor pollicis longus (APL), extensor pollicis brevis (EPB), extensor pollicis longus (EPL), extensor indicis. These muscles perform actions such as flexion, extension, adduction, and abduction of the hand as well as flexion and extension of the digits. The tendons of these muscles pass under the extensor retinaculum and the flexor retinaculum, wrapped in synovial tendon sheaths, preventing the tendons from bowing and holding them in place on their osseofibrous tunnels. On the dorsal side of the hand there are six synovial tendon sheaths and each lies in a different osseofibrous tunnel, so there are also six osseofibrous tunnels, and through these pass twelve tendons of nine extensor muscles. The first tendon sheath includes the tendons for APL and EPB; the second tendon sheath includes the tendons of the ECRL and ECRB; the third tendon sheath includes the tendon of EPL; the fourth tendon sheath includes the tendon for extensor indicis as well as the four tendons of extensor digitorum; the fifth tendon sheath includes the tendon of EDM; the sixth tendon sheath includes the tendon of extensor carpi ulnaris. In contrast to the numerous tendon sheaths on the dorsal side of the hand, on the palmar side there are only three: a common sheath containing the tendons of flexor digitorum superficialis and flexor digitorum profundus; a sheath containing the tendon of flexor pollicis longus; a sheath containing flexor carpi radialis.

Adduction and abduction of the digits is primarily performed by the interosseous muscles of the hand, except for in the case of digit 1, the pollex. The interosseous muscles insert on the extensor expansions which are flattened areas of four tendons found on the dorsal surface of the metacarpals as well as along the phalanges of digits 2-5. The extensor expansions function to hold the extensor tendon in a central position along each of digits 2-5. After passing through the extensor expansion, the extensor tendon divides into a median band and two lateral bands which continue to the base of the middle phalanx and to either side of the distal phalanx base. The lateral bands provide insertion points for tendons of both the interosseous muscles and the lumbrical muscles of the hand. To even further stabilize the extensor tendons, the extensor expansions are attached to the palmar ligament on both sides.

On the palmar surface of the hand is an insertion point of the plamaris longus muscle, the palmar aponeurosis. When the palmaris longus muscle is present, which is not always the case, the palmar aponeurosis is the expanded tendon of the muscle. The palmar aponeurosis is the deep, and very thick, portion of the palmar fascia that distally extends to form four bands that attach to the bases of the proximal phalanges. In contrast to the thickness of the palmar aponeurosis, there are two areas of the palmar fascia that are much thinner and these areas are known as the thenar eminence and the hypothenar eminence. The thenar fascia covers the thenar eminence at the base of the pollex, whereas the hypothenar fascia covers the hypothenar eminence at the base of digit 5. Somewhat between these two eminences is where the palmar aponeurosis can be found.

The muscles that make up the thenar and hypothenar eminences are intrinsic muscles, meaning that their proximal attachment point is on the hand. The muscles of the thenar eminence include the abductor pollicis brevis, flexor pollicis brevis, and the opponens pollicis, whereas the muscles of the hypothernar eminence include the abductor digiti minimi, flexor digiti minimi, and the opponens digiti minimi. The thenar and hypothenar groups of muscles are often considered the thenar and hypothenar compartments, respectively. When considering the compartments of intrinsic muscles of the hand, there are also three other compartments in addition to the thenar and hypothenar: the adductor compartment, the central compartment, and the interosseous compartment. In terms of the muscular contents of these other three compartments, the adductor compartment is made up of the adductor pollicis muscle, the central compartment includes the lumbrical muscles, and the interossei muscles are found in separate interosseous compartments. Although only the muscular contents of the compartments are listed, the contents of the compartments do not consist of only muscles.

The thenar and hypothenar compartments are two of the more easily found landmarks of the surface anatomy of the hand. The thenar eminence serves as a way to identify the location of the superficial and deep palmar arches. The superficial palmar arch is found in the center of the palm, terminating on the thenar eminence, and the deep palmar arch is located about 1cm more proximal than the superficial palmar arch. Another anatomical landmark on the hand is the knuckle formed between the head of the metacarpal and the proximal phalanx. The knuckles are important landmarks when finger amputations are performed. While the aforementioned landmarks were not discussed in class for various reasons, the anatomical snuff box was an area that was heavily discussed. The anatomical snuff box is an area which is medially bordered by the tendon of the extensor pollicis longus, and laterally bordered by the tendons of the extensor pollicis brevis and the tendon of the abductor pollicis longus. This is an area used to palpate the scaphoid bone when it is thought to have possibly been fractured following a fall that was caught by bracing one's self with the hand.
Diagram of hand surface