The Effect of Thumb Metacarpophalangeal Hyperextension on Thumb Axial Load and Lateral Pinch Force in a Cadaver Model of Thumb Trapeziectomy and Flexor Carpi Radialis Suspensionplasty

Purpose

The purpose of our study was to investigate, in a cadaver model, the effect of increasing thumb metacarpophalangeal (MCP) joint hyperextension on thumb axial load and key pinch force after thumb trapeziectomy and flexor carpi radialis suspensionplasty. We developed a cadaveric model to test whether thumb MCP joint hyperextension after trapeziectomy would have a negative effect on key pinch force and increase loads across a reconstructed thumb carpometacarpal (CMC) joint.

Methods

We created a cadaveric biomechanical model that varied thumb MCP joint hyperextension while measuring thumb CMC axial and key pinch force under standardized loads. Direct observations were made of how key pinch and axial thumb CMC force change with increasing thumb MCP joint hyperextension. We measured the thumb key pinch force and axial thumb CMC joint load with the thumb MCP joint in 0°, 10°, 20°, 30°, 40°, 50°, and 60° of hyperextension.

Results

There was a 0.88 N (2.4%) increase in axial force across the thumb CMC per every 10° of increasing thumb MCP joint hyperextension. We found a 0.53 N (4.4%) reduction in key pinch force for every 10° of increasing thumb MCP joint hyperextension. Therefore, at 60° of thumb MCP joint hyperextension, the axial force across the thumb CMC increased by 5.3 N (14.6%) and the key pinch force was weakened by 3.2 N (26.6%).

Conclusions

With progressive thumb MCP joint hyperextension after thumb CMC arthroplasty, we found a decrease in key pinch force and an increase in axial thumb CMC joint force. The decrease in key pinch force was larger than the relatively small increase in thumb CMC force.

Clinical relevance

This study helps elucidate the biomechanics of the thumb CMC joint after resection arthroplasty with thumb MCP joint hyperextension and helps understand the interplay between these 2 conditions.