Your wrist bones harden in a spiral order that can reveal your age
A newborn's wrist contains no true bone, only cartilage. The eight small carpal bones start hardening after birth in a roughly spiral sequence, beginning with the capitate and hamate in the first year and finishing with the pea-shaped pisiform after more than a decade. Forensic experts use that timetable to estimate a person's age.
The name comes from Greek karpos, wrist, whose root is linked to plucking, a motion the wrist performs. Anatomists group the eight bones into two rows. The upper row, nearest the forearm, holds the scaphoid, lunate, triquetral and pisiform; each can shift slightly on its own, adapting to the moving surfaces around it. The lower row is stiffer, moving together with the long bones of the palm. On the palm side the bones form a hollow roofed by a band of tissue, creating the carpal tunnel through which the median nerve and forearm tendons pass into the hand.
Each carpal, apart from the pisiform, has six faces, some smooth for joints and some roughened where ligaments anchor. Inside, spongy bone sits within a shell of compact bone. More than 20 extra wrist bones have been described in some people, but only four are accepted as genuine. The pisiform is actually a sesamoid bone growing inside a tendon, like a kneecap.
Movement is generous. From a straight position, with the middle finger aligned over the capitate, the hand can bend about 15 degrees toward the thumb side and 40 degrees toward the little finger side, and flexion plus extension span a total of 170 degrees. The wrist is steadier when bent forward than back, thanks mainly to ligaments rather than interlocking bone. Combined with elbow and shoulder, its motions come close to those of a ball-and-socket joint.
Evolution has trimmed the set. Early fossil amphibians such as Eryops had three rows totalling twelve wrist bones. Most mammals and reptiles lost the fifth lower carpal, and humans also lost a bone called the centrale. A modern bird's wing keeps just two carpals, one a fusion of four originals. In crabs and lobsters, carpus names part of the claw-bearing leg.
Source: Carpal bones
