Association between lunate morphology and carpal collapse in cases of scapholunate dissociation.

PURPOSE: Type II lunate morphology has recently been shown to decrease the incidence of dorsal intercalated segment instability (DISI) deformity in patients with scaphoid nonunions. A similar association has been suggested for scapholunate dissociation, but a formal comparison has yet to be performed. The purpose of this study was to determine if an association exists between lunate morphology and DISI in cases of scapholunate dissociation.

METHODS: A retrospective review was performed on 58 patients with the diagnosis of scapholunate dissociation as determined by use of radiographs, magnetic resonance imaging, arthrotomy, and arthroscopy. Posteroanterior radiographs were used to assess the presence of a medial facet on the lunate and to determine the distance between the capitate and the triquetrum. A DISI deformity was defined as a radiolunate angle >15 degrees, and scapholunate instability was defined as a scapholunate angle >60 degrees using the tangential method. Statistical analysis was performed with a chi-squared test and kappa test.

RESULTS: Twenty-five patients had a type I lunate, and 33 patients had a type II lunate. A total of 15 patients had DISI deformity on preoperative radiographs; of these, 10 patients with a type I lunate and 5 patients with a type II lunate had DISI deformity. This difference was found to be significant.

CONCLUSIONS: In cases of scapholunate dissociation, type II lunates were associated with a significantly lower incidence of DISI despite having radiographic or arthroscopic evidence of a complete scapholunate interosseous ligament tear. Osseous morphology may play a role in the development of a radiographic DISI deformity. Further research is required to assess the clinical importance of this finding and the biomechanical cause of this phenomenon.

TYPE OF STUDY/LEVEL OF EVIDENCE: Prognostic II.