Ossify

hold. watch your wrist arrive.

hold to age a hand X-ray — the wrist fills in last

A newborn hand X-ray shows fingers and almost no wrist. The eight carpal bones are still cartilage, which the beam passes straight through, and they ossify one at a time: capitate and hamate in the first months, the triquetral at about two or three years, the pisiform not until roughly nine to twelve. The growth plates at the ends of the finger bones show up as separate caps in early childhood and close in the mid-to-late teens, which is why a paediatrician can read a child’s bone age off a single film of one hand — and why, once the last plate has closed, the same film can no longer tell you an age at all. A baby carries roughly 270 to 300 bones and an adult about 206, not because any are lost, but because so many of them fuse.

Concepts: anatomy, growth.

An interactive you play with one thumb, on Wundr.