Carpus

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.

In human anatomy, the main role of the carpus is to facilitate effective positioning of the hand and powerful use of the extensors and flexors of the forearm, but the mobility of individual carpal bones increase the freedom of movements at the wrist.

Etymology

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

As a whole

In human anatomy, the carpal bones can be classified as belonging to two transverse rows or three longitudinal columns.

The pair of rows together form an arch which is convex proximally and concave distally. On the palmar side, the carpus is concave, forming the carpal tunnel which is covered by the flexor retinaculum. Because the proximal row is simultaneously related to the articular surfaces of the radius and the distal row, it adapts constantly to these mobile surfaces. The bones of this row - scaphoid, lunate, and triquetrum - have their individual movements. The scaphoid contributes to the stability of the midcarpus as it articulates distally with the trapezium and the trapezoid. The distal row is more rigid as its transverse arch moves with the metacarpals.

Biomechanically and clinically, the carpal bones are better understood as arranged in three longitudinal columns: # A radial scaphoid column consisting of the scaphoideum, trapezium, and trapezoideum # A lunate column consisting of the lunate and capitate # A ulnar triquetral column consisting of the triquetrum and hamatum. In this context the pisiform is regarded as a sesamoid bone embedded in the tendon of the flexor carpi ulnaris. # 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. # 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, # 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 # The ligaments of the intermetacarpal articulations which unite the metacarpal bones: the dorsal, interosseous, and palmar metacarpal ligaments

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 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 and adduction (movement towards the ulna, so called ulnar deviation or adduction) of 40 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 degrees.

Radial abduction/ulnar adduction

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.

Individual bones

Almost all carpals (except the pisiform) have six surfaces. Of these the palmar or anterior and the dorsal or posterior surfaces are rough, for ligamentous attachment; the dorsal surfaces being the broader, except in the lunate.

The superior or proximal, and inferior or distal surfaces are articular, the superior generally convex, the inferior concave; the medial and lateral 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 of compact bone.

Created from the initial letter of each of the eight carpal bones, in the order most commonly referenced, the sentence "some lovers try positions that they can't handle" is used as a mnemonic device. Another mnemonic is "she looks too pretty; try to catch her."

Accessory bones

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. no of carpal bones 27 sesamoid bone -pisiform(flexor carpi ulnaris) first bone to appear is capitate. |- ! Bone !! Average !! Variation |- | Capitate || 2.5 months || 1–6 months |- | Hamate || 4-5.5 months || 1–7 months |- | Triquetrum || 2 years || 5 months to 3 years |- | Lunate || 5 years || 2-5.5 years |- | 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 are ossified endochondrally (from within the cartilage) and the ossific centers appear only after birth.

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

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

See also

Bone terminology

Terms for anatomical location

Carpal tunnel

Carpal tunnel syndrome

Notes

References

External links

- "Palm of the Hand: Carpal bones"

Category:Short bones Category:Wrist