Systematic approach to identifying fourth and fifth carpometacarpal joint dislocations.

Dislocations of the carpometacarpal (CMC) joints are rare injuries which often have subtle radiographic findings that may be overlooked. A systematic approach for evaluation of the carpometacarpal joints is developed from analyzing the radiographic findings in 17 cases of CMC dislocations at the fourth and fifth CMC joints. Principles of parallelism, symmetry, overlapping articular surfaces, indistinct cortical rim, and parallel M lines are used in this evaluation. Ten patients had combined fourth and fifth CMC joint dislocations and seven had solitary dislocations at the fifth CMC joint. Most of these CMC dislocations are dorsal (71%). In combined fourth and fifth CMC joint dislocations, loss of parallelism and symmetry is observed at the fourth CMC joint with overlap identified primarily at the fifth CMC joint. Fracture of the fourth metacarpal base is often an associated injury. In solitary fifth CMC dislocations, slight ulnar offset of the fifth metacarpal on the hamate is the most consistent finding in association with loss of parallelism, symmetry, and overlap. Fracture of the radial margin of the fifth metacarpal base is often observed with solitary fifth CMC joint dislocations. A systematic approach and establishing the diagnosis from standard radiographic views is emphasized.