Thumb Radial Collateral Ligament Injuries Show Greater Rates of Metacarpophalangeal Subluxation Compared With Thumb Ulnar Collateral Ligament Injuries: A Comparative Imaging Analysis

Background: The ulnar and radial collateral ligaments (UCL and RCL) of the thumb are essential lateral stabilizers of the thumb metacarpophalangeal (MCP) joint. Despite anatomical and biomechanical findings suggesting a higher rate of MCP joint subluxation after RCL injury compared with that of UCL injury, this has not been clinically substantiated. This study examines and compares imaging of the thumb MCP joint in patients with operative RCL versus UCL injuries of the thumb. Methods: One hundred seventy-five operative thumb injuries were retrospectively reviewed, consisting of 131 UCL injuries, 40 RCL injuries, and 4 concomitant UCL and RCL injuries. Preoperative radiographs and magnetic resonance imaging (MRI) images were assessed for volar MCP joint subluxation, classified by severity based on percentage subluxation from grade I to IV. Results: On radiographs, the mean percent subluxation was 16.6 +/- 10.2% for UCL injuries compared with 35.5 +/- 16.2% for RCL injuries. On MRI, the mean percent subluxation was 30.4 +/- 16.5% for UCL injuries compared with 43.8 +/- 18.8% for RCL injuries. On radiographs, 0.8% of UCL injuries were classified as grade III or IV subluxation, while 22% of RCL injuries were classified as grade III or IV subluxation. On MRI, 11.6% of UCL injuries were classified as grade III or IV subluxation, while 23.5% of RCL injuries were classified as grade III or IV subluxation. Conclusions: Radial collateral ligament injuries show significantly higher volar subluxation of the MCP joint compared with UCL injuries on both plain radiographs and MRI. This has implications for operative indications of complete RCL injuries to help restore joint congruity.