Kinematic Spectrum of Scapholunate Instability

Scapholunate instability (SLI) is a spectrum of wrist instability rather than a binary diagnosis. SLI can occur due to varying combinations of ligament injuries, resulting in variable clinical and radiological features, and, more importantly, representing a kinematic spectrum. It is necessary to assess and understand this kinematic spectrum to recreate normal wrist biomechanics during motion and loading. Dynamic CT has enabled the identification of objective changes that occur in the spectrum of SLI in vivo during wrist motion.

In dynamic SLI, where static radiographs appear normal, 4D CT shows that the kinematic pattern of the scaphoid changes. Specifically, the in-plane motion of the scaphoid becomes dominant, rather than out-of-plane motion. These kinematic changes are accentuated during radial deviation of the wrist. In radiographic static SLI, the scaphoid demonstrates radial translation, flexion, and internal rotation throughout the range of extension to flexion, as well as during radial deviation. The SLI lunate extends compared to normal but only during flexion and radial deviation. In SLI with ulnar translocation of the lunate, the lunate is translated ulnarly and volarly throughout wrist motion. These patients have been found to have a long radiolunate ligament injury. Additionally, in SLI with scaphoid dorsal dislocation, the maximum radioscaphoid joint proximity occurs at the dorsal lip of the radius, which returns to the normal radioscaphoid facet with wrist extension and ulnar deviation. Each of these kinematic problems is related to a different pathoanatomy, representing a unique pattern of ligament injuries requiring individualized management.