The Impact of Scaphoid Malunion on Carpal Motion: An In-Vitro Analysis

Background: The clinical significance of scaphoid malunion is debated and its effect on wrist kinematics is poorly understood. Alterations to scaphoid morphology in other pathologies result in significant sequelae and arthrosis. By understanding the impact of scaphoid malunion on carpal kinematics, better insight can be garnered to inform models of wrist motion and clinical treatment of this injury. Methods: Seven cadaveric upper extremities underwent active wrist flexion and extension in a custom motion wrist simulator with scaphoid kinematics being captured with respect to the distal radius. A three-stage protocol of progressive simulated malunion severity was performed [intact, 10° malunion (Mal 10°), 20° malunion (Mal 20°)] with data analyzed from 45° wrist extension to 45° wrist flexion. Scaphoid malunions were modelled by creating successive volar wedge osteotomies and reducing the resultant scaphoid fragments with two 0.062" Kirshner wires. Motion of the scaphoid, lunate, trapezium-trapezoid was recorded using optical trackers. Results: Increasing scaphoid malunion severity did not significantly affect scaphoid or trapezium-trapezoid motion (p > 0.05); however, it did significantly alter lunate motion (p < 0.001). Increasing malunion severity resulted in progressive lunate extension across wrist motion (Intact - Mal 10°: mean difference (mean dif.) = 7.1° ± 1.6, p < 0.05; Intact - Mal 20°: mean dif. = 10.2° ± 2.0, p < 0.05). Conclusions: In this in-vitro model, increasing scaphoid malunion severity was associated with progressive extension of the lunate in all wrist positions. The clinical significance of this motion change is yet to be elucidated, but this model serves as a basis for understanding the kinematic consequences of scaphoid malunion deformities.