Tendon Force Changes after 1st Carpometacarpal Arthroplasty with Differing Degrees of Metacarpophalangeal Hyperextension: A Cadaveric Study.
Peter R Henning, MD, Ryan E Harold, MD, Brandon S Smetana, MD and Gregory A Merrell, MD, Indiana Hand to Shoulder Center, Indianapolis, IN
Intro: Previous studies have evaluated tendon forces acting upon the native thumb carpometacarpal (CMC) joint, but there is a lack of literature describing these forces after first CMC arthroplasty. We evaluated the change in CMC and pinch force of intrinsic and extrinsic muscles acting on the thumb and if these forces were affected by differing metacarpophalangeal (MP) position in a cadaveric model.
Material and Methods: In a cadaver model, after simulated first CMC arthroplasty, baseline loads were applied to flexor pollicis longus (FPL), abductor pollicis longus (APL), extensor pollicis longus (EPL), extensor pollicis brevis (EPB), abductor pollicis brevis (APB), flexor pollicis brevis (FPB), and adductor pollicis (ADD). An additional one kilogram load was applied to each tendon individually and changes in CMC and pinch force were measured. This process was repeated for seven different MP positions.
Results: In lateral pinch, FPL, APB, and ADD all result in similar increases in force regardless of MP position. Loading of the EPL, EPB, and APL all resulted in stable decreases that are also independent of MP position. The FPB demonstrated the largest contribution, which increased with higher degrees of MP hyperextension. Forces at the CMC joint were increased with each tendon tested and were not influenced by MP position. The ADD did demonstrate the smallest contribution to increased CMC force across MP positions. Forces of each tendon tested were more efficiently transmitted to the CMC joint than to lateral pinch force.
Conclusions: Selective loading of the FPB resulted in the largest increase to lateral pinch force and the second largest increase in CMC force. In contrast, loading of the ADD resulted in 10% less pinch force, but nearly 20% less CMC force. These effects were generally independent of MP position. These data provide some of the first biomechanical data of the thumb after first CMC arthroplasty, but care should be taken when applying these results to in vivo biomechanics.
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