Dynamic CT features of scapholunate instability during the wrist extension to flexion-An in vivo study.

Purpose: This biomechanical study aimed to assess the change in the radioscaphoid and the radiolunate angles during wrist extension to flexion in scapholunate instability compared to the healthy wrist.

Methods: Dynamic CT scans of 19 participants with no history of wrist pathology and 19 patients with scapholunate instability without degenerative changes were selected. Motion sequence studied was wrist extension to flexion. Image segmentation followed by 3D registration techniques were used to calculate the displacement field between scaphoid and lunate models in each time point. The radiocarpal angles and centroid positions were computed in the neutral wrist and throughout wrist motion. Comparisons were made between the normal and the scapholunate instability wrists.

Results: The scapholunate instability wrist had less range of extension to flexion compared to the normal wrist. The scapholunate instability scaphoid was flexed and internally rotated in the neutral wrist position and throughout the range of motion. The scapholunate instability scaphoid centroid remained radial and distal in the extended wrist. The scapholunate instability lunate was significantly extended from wrist 20° extension to 50° flexion. The scapholunate instability lunate centroid was more ulnar from 70° wrist extension to 50° flexion.

Conclusions: The scapholunate instability scaphoid demonstrated rotational abnormalities in two planes throughout the wrist motion and abnormal centroid positions in the extended wrist. The lunate extension in scapholunate instability was significant mostly during wrist flexion. Radioscaphoid instability appear to be the primary pathology in SLI and can be used to differentiate the SLI wrist from the normal throughout wrist extension to flexion. Lunate extension is not different between the normal and scapholunate instability in the extended wrist.

Level of evidence: Level III.