| Carpal bones | |
|---|---|
 | |
 Labelled image showing the eight carpal bones. | |
| Details | |
| Identifiers | |
| Latin | os carpale pl. ossa carpi |
| MeSH | D002348 |
| TA98 | A02.4.08.001 |
| TA2 | 1249 |
| FMA | 23889 |
| Anatomical terms of bone | |
Thecarpal bones are the eight smallbones that make up thewrist (carpus) that connects thehand to theforearm. The terms "carpus" and "carpal" are derived from theLatincarpus and theGreekκαρπός (karpós), meaning "wrist". Inhuman anatomy, the main role of the carpal bones is toarticulate with theradial andulnar heads to form a highly mobilecondyloid joint (i.e.wrist joint),[1] to provide attachments forthenar andhypothenar muscles, and to form part of the rigidcarpal tunnel which allows themedian nerve andtendons of theanterior forearm muscles to be transmitted to the hand andfingers.
Intetrapods, thecarpus is the sole cluster of bones in the wrist between theradius andulna and themetacarpus. The bones of the carpus do not belong to individualfingers (or toes in quadrupeds), whereas those of the metacarpus do. The corresponding part of thefoot is thetarsus. The carpal bones allow the wrist to move and rotate vertically.[1]
The eight carpal bones may be conceptually organized as either two transverse rows, or three longitudinal columns.
When considered as paired rows, each row forms an arch which is convex proximally and concave distally. On the palmar side, the carpus is concave and forms thecarpal tunnel, which is covered by theflexor retinaculum.[2] The proximal row comprises thescaphoid,lunate,triquetral, andpisiform 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 themetacarpals.[3]
Biomechanically and clinically, the carpal bones are better conceptualized as three longitudinal columns:[4]
In this context the pisiform is regarded as asesamoid bone embedded in the tendon of theflexor carpi ulnaris.[4] The ulnar column leaves a gap between the ulna and the triquetrum, and therefore, only the radial or scaphoid and central or capitate columns articulate with the radius. The wrist is more stable in flexion than in extension, mainly because of the strength of various capsules and ligaments than the interlocking parts of the skeleton.[3]
Almost all carpals (except the pisiform) have six surfaces. Of these thepalmar oranterior and thedorsal orposterior surfaces are rough, for ligamentous attachment; the dorsal surfaces being the broader, except in the lunate.
Thesuperior orproximal, andinferior ordistal surfaces are articular, the superior generally convex, the inferior concave; themedial andlateral surfaces are also articular where they are in contact with contiguous bones, otherwise they are rough and tuberculated.
The structure in all is similar:cancellous tissue enclosed in a layer ofcompact bone.
| Name | Proximal/radial articulations | Lateral/medial articulations | Distal/metacarpal articulations |
|---|---|---|---|
| Proximal row | |||
| Scaphoid | radius | capitate, lunate | trapezium, trapezoid |
| Lunate | radius, articular disk | scaphoid, triquetral | capitate, hamate (sometimes) |
| Triquetrum | articular disk | lunate, pisiform | hamate |
| Pisiform | triquetral | ||
| Distal row | |||
| Trapezium | scaphoid | trapezoid | first andsecond metacarpal |
| Trapezoid | scaphoid | trapezium, capitate | second metacarpal |
| Capitate | scaphoid, lunate | trapezoid, hamate | third, partly second andfourth metacarpal |
| Hamate | triquetral, lunate | capitate | fourth andfifth |
Occasionallyaccessory 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.[2]
| Bone | Average | Variation[6] | Variation[7] |
|---|---|---|---|
| Capitate | 2.5 months | 1–6 months | 1–5 months |
| Hamate | 4–5.5 months | 1–7 months | 1–12 months |
| Triquetrum | 2 years | 5 months to 3 years | 9 months to 4 years and 2 months |
| Lunate | 5 years | 2–5.5 years | 18 months to 4 years and 3 months |
| Trapezium | 6 years | 4–8 years | |
| Trapezoid | 6 years | 4–8 years | |
| Scaphoid | 6 years | 4–7 years | |
| Pisiform | 12 years | 8–12 years |
The carpal bones areossified endochondrally (from within the cartilage) and the ossific centers appear only after birth.[5]The formation of these centers roughly follows a chronological spiral pattern starting in the capitate and hamate during the first year of life. The ulnar bones are then ossified before the radial bones, while the sesamoid pisiform arises in the tendon of theflexor carpi ulnaris after more than ten years.[6] The commencement of ossification for each bone occurs over period like other bones. This is useful in forensic age estimation.[7]
There are four groups of ligaments in the region of the wrist:[8]
The hand is said to be instraight 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 themidposition of the hand which corresponds to an ulnar deviation of 12 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 degrees andadduction (movement towards the ulna, so called ulnar deviation or adduction) of 40 degrees when the arm is in strictsupination and slightly greater in strictpronation.[9]Flexion (tilting towards the palm, so called palmar flexion) andextension (tilting towards the back of the hand, so called dorsiflexion) is possible with a total range of 170 degrees.[10]
Duringradial 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.[9]
Radial abduction is produced by (in order of importance)extensor carpi radialis longus,abductor pollicis longus,extensor pollicis longus,flexor carpi radialis, andflexor pollicis longus.[11]
Ulnar adduction causes a tilting or dorsal shifting of the proximal row of carpal bones.[9]It is produced byextensor carpi ulnaris,flexor carpi ulnaris,extensor digitorum, andextensor digiti minimi.[11]
Both radial abduction and ulnar adduction occurs around a dorsopalmar axis running through the head of the capitate bone.[9]
Duringpalmar flexion the proximal carpal bones are displaced towards thedorsal side and towards thepalmar side duringdorsiflexion. While flexion and extension consist of movements around a pair of transverse axes — passing through the lunate bone for the proximal row and through the capitate bone for the distal row — palmar flexion occurs mainly in theradiocarpal joint and dorsiflexion in themidcarpal joint.[10]
Dorsiflexion is produced by (in order of importance)extensor digitorum,extensor carpi radialis longus,extensor carpi radialis brevis,extensor indicis,extensor pollicis longus, andextensor digiti minimi. Palmar flexion is produced by (in order of importance)flexor digitorum superficialis,flexor digitorum profundus,flexor carpi ulnaris,flexor pollicis longus,flexor carpi radialis, andabductor pollicis longus.[11]
Combined with movements in both the elbow and shoulder joints,intermediate orcombined movements in the wrist approximate those of aball-and-socket joint with some necessary restrictions, such as maximum palmar flexion blocking abduction.[10]
Anteroposterior gliding movements between adjacent carpal bones or along the midcarpal joint can be achieved by stabilizing individual bones while moving another (i.e. gripping the bone between the thumb and index finger).[12]
The structure of the carpus varies widely between different groups oftetrapods, even among those that retain the full set of five digits. In primitive fossilamphibians, such asEryops, 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 theradiale,intermedium, andulnare, after their proximal articulations, and arehomologous with the scaphoid, lunate, and triquetral bones respectively. The remaining bones are simply numbered, as the first to fourthcentralia (singular:centrale), and the first to fifthdistal carpals. Primitively, each of the distal bones appears to have articulated with a single metacarpal.
However, the vast majority of latervertebrates, including modernamphibians, have undergone varying degrees of loss and fusion of these primitive bones, resulting in a smaller number of carpals. Almost allmammals andreptiles, 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 modernbird, 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.[13]
The carpus and tarsus are both described as podial elements or (clusters of) podial bones.[14]
In somemacropods, the scaphoid and lunar bones are fused into the scapholunar bone.[15]
Incrustaceans, "carpus" is the scientific term for the claws or "pincers" present on some legs. (SeeDecapod anatomy)
The Latin word "carpus" is derived from Greekκαρπὁς meaning "wrist". The root "carp-" translates to "pluck", an action performed by the wrist.[16]
