The carpal bones are the eight small bones that make up the wrist (carpus) that connects the hand to the forearm. The terms "carpus" and "carpal" are derived from the Latin carpus and the Greek καρπός (karpós), meaning "wrist". In human anatomy, the main role of the carpal bones is to articulate with the radial and ulnar heads to form a highly mobile condyloid joint (i.e. wrist joint), to provide attachments for thenar and hypothenar muscles, and to form part of the rigid carpal tunnel which allows the median nerve and tendons of the anterior forearm muscles to be transmitted to the hand and fingers.

In tetrapods, the carpus is the sole cluster of bones in the wrist between the radius and ulna and the metacarpus. The bones of the carpus do not belong to individual fingers (or toes in quadrupeds), whereas those of the metacarpus do. The corresponding part of the foot is the tarsus. The carpal bones allow the wrist to move and rotate vertically. The proximal row comprises the scaphoid, lunate, triquetral, and pisiform bones which articulate with the surfaces of the radius and distal carpal row, and thus constantly adapts to these mobile surfaces. Within the proximal row, each carpal bone has slight independent mobility. For example, the scaphoid contributes to midcarpal stability by articulating distally with the trapezium and the trapezoid. In contrast, the distal row is more rigid as its transverse arch moves with the metacarpals.

Biomechanically and clinically, the carpal bones are better conceptualized as three longitudinal columns:

  1. Radial scaphoid column: scaphoid, trapezium, and trapezoid
  2. Lunate column: lunate and capitate
  3. Ulnar triquetral column: triquetrum and hamate

In this context the pisiform is regarded as a sesamoid bone embedded in the tendon of the flexor carpi ulnaris.

! Name !! Proximal/radial<br />articulations !! Lateral/medial<br />articulations !! Distal/metacarpal<br />articulations

|-

! colspan="4" | Proximal row

|-

| Scaphoid || radius || capitate, lunate || trapezium, trapezoid

|-

| Lunate || radius, articular disk || scaphoid, triquetral || capitate, hamate (sometimes)

|-

| Triquetrum || articular disk || lunate, pisiform || hamate

|-

| Pisiform || &nbsp; || triquetral || &nbsp;

|-

! colspan="4" | Distal row

|-

| Trapezium || scaphoid || trapezoid || first and second metacarpal

|-

| Trapezoid || scaphoid || trapezium, capitate || second metacarpal

|-

| Capitate || scaphoid, lunate || trapezoid, hamate || third, partly second<br />and fourth metacarpal

|-

| Hamate || triquetral, lunate || capitate || fourth and fifth

|}

Accessory bones

thumb|Location of the accessory ossicles of the carpals

Occasionally accessory bones are found in the carpus, but of more than 20 such described bones, only four (the central, styloid, secondary trapezoid, and secondary pisiform bones) are considered to be proven accessory bones. Sometimes the scaphoid, triquetrum, and pisiform bones are divided into two.

|-

! Bone !! Average !! Variation

  1. The ligaments of the wrist proper which unite the ulna and radius with the carpus: the ulnar and radial collateral ligaments; the palmar and dorsal radiocarpal ligaments; and the palmar ulnocarpal ligament. (Shown in blue in the figure.)
  2. The ligaments of the intercarpal articulations which unite the carpal bones with one another: the radiate carpal ligament; the dorsal, palmar, and interosseous intercarpal ligaments; and the pisohamate ligament. (Shown in red in the figure.)
  3. The ligaments of the carpometacarpal articulations which unite the carpal bones with the metacarpal bones: the pisometacarpal ligament and the palmar and dorsal carpometacarpal ligaments. (Shown in green in the figure.)
  4. The ligaments of the intermetacarpal articulations which unite the metacarpal bones: the dorsal, interosseous, and palmar metacarpal ligaments. (Shown in yellow in the figure.)

Movements

The hand is said to be in straight position when the third finger runs over the capitate bone and is in a straight line with the forearm. This should not be confused with the midposition of the hand which corresponds to an ulnar deviation of 12&nbsp;degrees. From the straight position two pairs of movements of the hand are possible: abduction (movement towards the radius, so called radial deviation or abduction) of 15&nbsp;degrees and adduction (movement towards the ulna, so called ulnar deviation or adduction) of 40&nbsp;degrees when the arm is in strict supination and slightly greater in strict pronation.

Flexion (tilting towards the palm, so called palmar flexion) and extension (tilting towards the back of the hand, so called dorsiflexion) is possible with a total range of 170&nbsp;degrees.

Radial abduction/ulnar adduction

thumb|Left: Ulnar adduction<br />Right: Radial abduction

thumb|Left: Dorsiflexion<br />Right: Palmar flexion

During radial abduction the scaphoid is tilted towards the palmar side which allows the trapezium and trapezoid to approach the radius. Because the trapezoid is rigidly attached to the second metacarpal bone to which also the flexor carpi radialis and extensor carpi radialis are attached, radial abduction effectively pulls this combined structure towards the radius. During radial abduction the pisiform traverses the greatest path of all carpal bones.

Ulnar adduction causes a tilting or dorsal shifting of the proximal row of carpal bones.

Other animals

The structure of the carpus varies widely between different groups of tetrapods, even among those that retain the full set of five digits. In primitive fossil amphibians, such as Eryops, the carpus consists of three rows of bones; a proximal row of three carpals, a second row of four bones, and a distal row of five bones. The proximal carpals are referred to as the radiale, intermedium, and ulnare, after their proximal articulations, and are homologous with the scaphoid, lunate, and triquetral bones respectively. The remaining bones are simply numbered, as the first to fourth centralia (singular: centrale), and the first to fifth distal carpals. Primitively, each of the distal bones appears to have articulated with a single metacarpal.

However, the vast majority of later vertebrates, including modern amphibians, have undergone varying degrees of loss and fusion of these primitive bones, resulting in a smaller number of carpals. Almost all mammals and reptiles, for example, have lost the fifth distal carpal, and have only a single centrale - and even this is missing in humans. The pisiform bone is somewhat unusual, in that it first appears in primitive reptiles, and is never found in amphibians.

Because many tetrapods have fewer than five digits on the forelimb, even greater degrees of fusion are common, and a huge array of different possible combinations are found. The wing of a modern bird, for example, has only two remaining carpals; the radiale (the scaphoid of mammals) and a bone formed from the fusion of four of the distal carpals.

The carpus and tarsus are both described as podial elements or (clusters of) podial bones.

In some macropods, the scaphoid and lunar bones are fused into the scapholunar bone.

In crustaceans, "carpus" is the scientific term for the claws or "pincers" present on some legs. (See Decapod anatomy)

Etymology

The Latin word "carpus" is derived from Greek meaning "wrist". The root "carp-" translates to "pluck", an action performed by the wrist.

See also

  • Carpal tunnel
  • Carpal tunnel syndrome
  • List of mnemonics

Notes

References

  • —"Palm of the Hand: Carpal bones"