HAND AND WRIST (see also Anatomy of the Joints)
Bones and joints (Figure 20). The hand. The framework of the hand is formed by five
metacarpal bones; the digits (four fingers and a thumb) are made up of small bones called
phalanges. Each finger has three phalanges (proximal, middle, and distal); the thumb has
two (proximal and distal). The digits and their metacarpal rays are numbered from one to
five, starting with the thumb.
The metacarpals and phalanges are long bones. Each has a diaphysis or shaft, with
slightly flaring ends. The proximal end or base of each bone has an articular surface
where it forms a joint with the adjacent bone. The distal end or head of each bone (except
for the distal phalanges) also has an articular surface. In the distal phalanges, the
distal end is called the tuft; it does not have an articular surface, but provides
attachment for the soft tissue (pulp) of the digit tip.
The joints of the hand and wrist are named for the bones which they connect. Each
finger has two interphalangeal joints (IPs), distal (DIP) and proximal (PIP). The thumb
has only one IP joint. Between the proximal phalanges and the metacarpals are the knuckles
or metacarpophalangeal (MCP) joints. The IP and MCP joints are capable of flexion
(bending) and extension (straightening). In addition, the MCP joints are capable of
abduction (spreading of the fingers) and adduction (bringing the fingers together).
All of the IP and MCP joints have a similar arrangement of ligaments that bind the
joint together. On each side of the joint are the collateral ligaments (the radial
collateral ligament on the side toward the thumb; the ulnar collateral ligament on the
side toward the little finger). The palmar aspect of the joint is stabilized by a plate of
fibrocartilage called the volar plate.
The metacarpal bones articulate with the wrist to form five carpometacarpal (CMC)
joints. The first CMC joint has a saddle-shaped surface that allows it to move in all
directions. This gives the thumb its freedom of motion: extension and flexion (parallel to
the palm), abduction and adduction (at right angles to the palm), as well as some
rotation. In addition, the first metacarpal is set at an angle of 90 degrees from the
plane of the other four metacarpals, so that the thumb can easily come into contact with
each of the fingers. This opposability of the thumb makes it possible to pinch, grasp, and
manipulate objects efficiently.
In contrast to the first metacarpal, the other rays of the hand have limited mobility.
The second and third metacarpals are sometimes referred to as the "stable rays",
because their CMC joints have virtually no movement. The fourth and fifth metacarpals are
referred to as the "mobile rays" because their CMC joints have some movement,
rotating slightly to allow cupping of the hand.
The wrist is not a single joint, but consists of multiple joints. It contains eight
small, irregularly-shaped carpal bones, arranged in two rows, proximal and distal. (The
eight bones together are called the carpus.) There is a midcarpal joint between the two
rows, as well as intercarpal joints between individual bones in each row. The distal row
articulates with the five metacarpal bones (at the CMC joints, discussed above). The
proximal row articulates with the two bones of the forearm, the radius and ulna, forming
the radiocarpal and ulnocarpal joints. (The radius is on the thumb side of the wrist, the
ulna on the little finger side.) In addition, the distal radius and ulna articulate with
each other, forming the distal radioulnar (DRU) joint. The DRU joint allows the radius and
ulna to rotate around each other, so that the forearm can be pronated (rotated palm
downward) and supinated (rotated palm upward). The movements of the other wrist joints are
complex; together they produce the wrist movements of flexion (bending toward the palm),
extension (bending backward, also called dorsiflexion), radial deviation (bending toward
the radial side), and ulnar deviation (bending toward the ulnar side.
The distal end of the radius has a pointed projection or styloid process, which forms
part of the radiocarpal joint surface. The distal end (head) of the ulna also has a
styloid process; but unlike the radius, the ulna does not articulate directly with the
carpus. Interposed between the ulnar head and the carpus is a disc of
which is attached to the ulnar styloid. The radiocarpal and ulnocarpal joints together
form a single continuous joint cavity, and are sometimes considered as one (radiocarpal)
joint. The DRU joint has its own separate joint cavity.
The joints of the wrist are enclosed by a fibrous joint capsule, and are further bound
together by multiple ligaments which blend with the capsule. On the radial and ulnar sides
of the wrist, respectively, are the radial and ulnar collateral ligaments. Supporting the
palmar aspect of the wrist is a complex web of ligaments collectively known as the volar
radiocarpal ligaments. Supporting the dorsum (back) of the wrist are the dorsal
radiocarpal ligaments. The collateral, volar, and dorsal ligaments are sometimes referred
to as extrinsic ligaments, because they are outside the joint capsule. The intercarpal
ligaments (connecting the individual carpal bones) are called intrinsic ligaments, because
they are inside the capsule.
The ulnar side of the wrist is bound together by a group of structures known as the
triangular fibrocartilage complex (TFCC). This consists of the ulnocarpal disc, along with
two sets of ligaments that are attached to it: the ulnocarpal and the radioulnar
ligaments. The stability of the DRU joint depends on the integrity of the TFCC,
particularly the radioulnar ligaments.
Muscles and tendons (Figure 21). Movement of the fingers is carried out by several
groups of muscles. The muscles that flex the fingers, primarily flexor digitorum
superficialis and flexor digitorum profundus, are located in the palmar aspect of the
forearm. These muscles each give rise to four long tendons that pass through the palmar
side of the wrist and hand. The superficialis tendons insert on the bases of the middle
phalanges; the profundus tendons insert on the bases of the distal phalanges. Both sets of
tendons act together to flex the wrist, MCP and PIP joints; however, only the profundus
tendons flex the DIP joints. (There are also flexor tendons which insert on the metacarpal
bones, and flex only the wrist.)
The muscles that extend the fingers, primarily the extensor digitorum, are located in
the dorsal aspect of the forearm, and have long tendons that pass through the dorsum of
the wrist and hand. In each finger, the extensor tendon gives rise to a fibrous expansion
which inserts on the middle and distal phalanges. These tendons extend the wrist,
PIP, and DIP joints. (Again, there are extensor tendons which insert on the metacarpal
bones, and extend only the wrist.)
In addition to the flexors and extensors, the hand has two groups of intrinsic muscles
(so called because they are located in the hand itself rather than the forearm). The
interosseous muscles originate on the metacarpal bones and insert on the phalanges; these
muscles abduct and adduct the fingers. The lumbrical muscles originate on the flexor
digitorum profundus tendons, and insert into the extensor expansions; their function is to
make fine adjustments in the positions of the fingers.
Movements of the thumb are carried out by eight different muscles. These include two
flexors (flexor pollicis brevis and flexor pollicis longus); two extensors (extensor
pollicis brevis and longus); two abductors (abductor pollicis brevis and longus); one
adductor (adductor pollicis); and one muscle that opposes the thumb by rotating the CMC
joint (opponens pollicis). Three of these muscles (flexor brevis, abductor
opponens pollicis) form the thenar eminence, the fleshy area of the palm at the base of
the thumb. Adductor pollicis is located in the palm adjacent to the thenar eminence. The
other four muscles are located in the forearm, and have long tendons that pass through the
wrist to the thumb. The adductor and the three brevis muscles insert on the base of the
proximal phalanx, and are responsible for movements of the MCP joint. Flexor and extensor
longus insert on the distal phalanx, and move both the MCP and IP joints. The remaining
two muscles (abductor longus and opponens) insert on the first metacarpal, and move the
CMC joint of the thumb.
On the palmar side of the wrist, the nine long flexor tendons (two for each finger and
one for the thumb) pass together through a space called the carpal tunnel (Figure 22). The
floor of the carpal tunnel is formed by the carpal bones; the roof of the tunnel is formed
by the flexor retinaculum, a band of fibrous tissue which stretches across the
After passing through the carpal tunnel, the tendons diverge in the palm on their way to
the digits. In each finger the two flexor tendons, traveling together, enter another
tunnel consisting of a fibrous tendon sheath lined with synovium. (In the thumb, the
arrangement is similar, except that there is only one tendon inside the sheath.) These
digital tendon sheaths act as pulleys, holding the tendons in proper position for
mechanical efficiency. The synovial linings provide lubrication, permitting the tendons to
glide smoothly inside the sheaths during hand movements. (The flexor tendon sheaths of the
hand are shown in Figure 23.)
On the dorsal side of the wrist, the extensor tendons of the fingers and thumb (as well
as the long abductor tendon of the thumb) pass underneath a similar band of fibrous tissue
called the extensor retinaculum. However, instead of going through a common tunnel, the
tendons are separated into different compartments by septa (walls) of connective tissue.
After passing the wrist, the tendons diverge on their way to the digits. In each digit,
the extensor tendon gives rise to a fibrous expansion that inserts on the phalanges.
(Unlike the flexor tendons, the extensor tendons do not have synovial sheaths in the
digits. However, all the tendons are surrounded by synovium at the wrist.)
Two tendons of the thumb, the extensor pollicis brevis and abductor pollicis
are of particular interest. Both of these tendons arise from muscles in the dorsal
forearm. On their way to the thumb, they cross the radial side of the wrist in the region
of the radial styloid, and pass together underneath the extensor retinaculum. They usually
occupy the same compartment; however, in some individuals, the compartment may be
subdivided by additional septa. Furthermore, in many individuals these tendons are
reduplicated, so that there may be two or more of each.
Nerves. There are three major nerves that supply the hand. The median nerve enters the
palm by passing through the carpal tunnel, along with the nine flexor tendons. The ulnar
nerve also enters the palm, by passing through a separate compartment on the ulnar side of
the wrist, called Guyon's canal. The radial nerve crosses over the radial styloid (on the
radial side of the wrist) to reach the dorsum of the hand.
The median nerve supplies sensation to the palmar surface of the hand, including the
thumb and fingers, except for the little finger and the ulnar half of the ring finger,
which are supplied by the ulnar nerve. The radial nerve supplies most of the dorsal
surface of the hand, again, except for the little finger and ulnar half of the ring
finger, which are supplied by the ulnar nerve. In addition, the dorsal surfaces of the
fingertips (with the same exceptions) are supplied by the median nerve. (For a map of
these sensory areas, see Figure 36 in Anatomy of the Nervous System.)
The motor branches of the median nerve supply the three thenar muscles, as well as the
lumbrical muscles of the index and middle fingers. All the other muscles of the hand are
supplied by the ulnar nerve. The radial nerve has no motor branches in the hand.