The fourth carpometacarpal joint.

The base of the fourth metacarpal and the corresponding hamate/capitate articulation were the areas of most significant variation in 142 cadaveric wrists that were dissected to assess the variation of the shapes of the second through the fifth carpometacarpal joints. Five different shapes of the fourth metacarpal base were identified. The base of the fourth metacarpal was generally either flat (85.9%) or conical (14.1%). There was a fourth metacarpal/capitate articulation present in 81.7% of the specimens. The presence or absence of a fourth metacarpal/capitate articulation and whether or not the fourth metacarpal base was flat or conical were easily identifiable on radiographs. Specific types of fourth metacarpal bases could not, however, be identified by radiography.