Dorsoradial Instability of the Thumb Metacarpophalangeal Joint: A Biomechanical Investigation

Abstract

Purpose To define the role of the dorsal capsule and associated dorsal fibrocartilage (DFC) and their interactions with the radial collateral ligament (RCL) as a thumb metacarpophalangeal (MCP) joint stabilizer. Methods Eight cadaveric thumbs were mounted onto a custom jig with 20 N of muscle load applied. The thumb position in space was digitized to measure ulnar-radial, pronation-supination, and volar-dorsal laxity at 0 degrees, 30 degrees, and 60 degrees MCP joint flexion. Serial sectioning was performed and measurements were repeated for the intact state, proper RCL insufficiency, proper and accessory (complete) RCL insufficiency, complete RCL with 50% DFC (radial) insufficiency, and complete RCL with complete DFC insufficiency. Results Ulnar-radial deviation, pronation-supination, and volar-dorsal translation significantly changed at 30 degrees and 60 degrees MCP joint flexion when comparing complete RCL insufficiency with complete RCL with 50% DFC insufficiency. At 30 degrees flexion, significant increases were found in ulnar deviation, pronation, and volar translation, and there was a decrease in supination. At 60 degrees flexion, ulnar deviation, pronation, and volar translation increased and radial deviation decreased significantly. At 30 degrees flexion, the resting position significantly pronated and translated volarly. At 60 degrees flexion, the resting position significantly shifted ulnarly, pronated, and translated volarly. Conclusions The DFC acts as a secondary stabilizer of the thumb MCP joint, working in tandem with the RCL. It acts by stabilizing the MCP joint dorsoradially when external forces are applied across the joint. This cadaveric study shows that RCL insufficiency with a concomitant DFC injury is less likely to be stable than RCL injuries alone, and that this effect is more pronounced with MCP joint flexion. Clinical relevance Increasing incompetence of the secondary stabilizers of the RCL, such as the DFC, will likely result in increased clinical instability upon physical examination. The results of this study also suggest the need to consider repair of the DFC at the time of RCL repair. Copyright (C) 2017 by the American Society for Surgery of the Hand. All rights reserved.