Purpose: Scaphoid nonunions are a common complication of scaphoid fractures and frequently progress to degenerative arthritis. This study evaluated the effect of scaphoid nonunion on the in vivo kinematics of the radioscaphocapitate articulation.
Methods: Computed tomography with a markerless registration technique was used to quantify motion of the scaphoid, lunate, and capitate in vivo, in 6 patients with unilateral scaphoid nonunion. The 3-dimensional helical axis of motion rotations of each bone were measured as a function of wrist flexion-extension and compared with those of the uninjured contralateral wrist. Mixed linear modeling was used to compare flexion-extension of the injured scaphoid fragments with those of the uninjured scaphoid, and the lunate of the injured wrist with the lunate of the uninjured wrist. Interfragmentary motion in the injured scaphoid was assessed by calculating rotation of the distal fragment relative to the proximal fragment, as well as the linear displacement between the 2 fragments at the fracture site.
Results: Flexion and extension of the distal scaphoid fragment was similar to that of the uninjured scaphoid. Extension of the proximal fragment was significantly decreased by 38%, compared with the uninjured scaphoid. Similarly, extension of the lunate in the injured wrist was significantly decreased, by 40%. Interfragmentary rotation was 33% of wrist motion in flexion and 35% of wrist motion in extension. Maximum interfragmentary displacement was on the order of 1 mm.
Conclusions: Scaphoid nonunions have a dramatic impact on carpal kinematics, partially uncoupling the proximal and distal carpal rows. Although the results of this in vivo study differ from past in vitro studies, the increase in lunocapitate motion we identified is consistent with the current theory that the scaphoid acts as a fundamental link between the proximal and distal carpal rows.