[Numerical variants and congenital fusions of carpal bones]

The number of carpal bones may be increased or decreased by the fact of anatomical variants or true congenital anomalies. Numerical increment arises from additional or from split bones. Over twenty accessory carpal bones have been described but the commonest are the os centrale carpi, the os radiale externum, the triangular bone and the styloideum bone. Additional carpal bones usually result from a failure of fusion of their ossification centers. A congenital origin is not clearly established for all these ossicles. The scaphoid and lunate may split into two or three bones and several cases of bipartite hamulus of the hamatum have been reported. A carpus with only seven bones results from the congenital absence of a normal bone, which mainly affects the scaphoid, lunate and triquetrum, or from a synostosis between two carpal bones, usually the lunate and triquetrum. Congenital fusions originate from an absence of joint cavitation into the embryo and chondrification of the joint interzone. Numerical carpal variants are uncommon as independent entities but occur with a relative high frequency in association with complex malformations of the hand. These anomalies are detectable on plain radiographs of the wrist, but CT-scan and MR-Imaging are useful to differentiate bipartite and accessory bones from carpal fractures or posttraumatic injuries, carpal fusions having to be distinguished from bony ankylosis.