Post-traumatic carpal instability.

A classification of the subtle patterns of carpal instability, exclusive of major carpal fractures, dislocations and fracture dislocations is proposed. This classification is based on a modification of Navarro's concept of the carpus; it is composed of 3 vertical longitudinal columns: lateral (scaphoid); central (lunate and distal carpal row); medial (triquetrum). Carpal dissociations may occur between the lateral and central columns (lateral instability), within the central column (central instability), between the central column and the triquetrum (medial instability), and between the entire carpus and the distal radioulnar articular surface (proximal instability). Lateral carpal instabilities are further subdivided according to the different components of the central column that articulate with the scaphoid. Therefore, 3 main lateral patterns may be identified: scaphoid-trapezium-trapezoid subluxation; scaphoid-capitate diastasis; scaphoid-lunate dissociation. An example of central instability is presented to illustrate this particular type of carpal dissociation. Medial, or lunate-triquetrum instability, is believed responsible for the volar-flexed intercalated segment instability pattern, in which the lunate collapses into a volar-flexed position and there is longitudinal "crumpling" of the radiocarpal link. Proximal carpal or radiocarpal instability may occur in an ulnar (ulnar translocation), dorsal (dorsal subluxation), or volar direction (volar subluxation). It is usually associated with loss of the anatomic alignment of the distal radius.