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American Association for Hand Surgery

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Effect of MP Joint Hyperextension After CMC Arthroplasty on Pinch Force and CMC Joint Load
Ryan E Harold, MD, Peter R Henning, MD, Brandon S Smetana, MD and Gregory A Merrell, MD, Indiana Hand to Shoulder Center, Indianapolis, IN

Introduction:
Thumb carpometacarpal (CMC) joint osteoarthritis (OA) is a common and often debilitating condition that is frequently associated with metacarpophalangeal joint (MPJ) hyperextension deformity. The indications to treat thumb MPJ deformity are not well defined in the literature and the treatment options are numerous. We developed a cadaveric model to test if MPJ hyperextension after trapeziectomy would negatively impact lateral pinch force and increase loads across a reconstructed CMC joint.
Materials and Methods:
Our goal was to specifically evaluate how thumb biomechanics change with increasing amounts of MPJ hyperextension after trapeziectomy and suspensioplasty for thumb CMC OA. To do this we created a cadaveric biomechanical model that allowed us to vary the MPJ hyperextension, while concomitantly measuring CMC force and lateral pinch force under standardized loads (see Figure 1 & 2 below). We performed a trapeziectomy and ligament reconstruction using the FCR tendon on 8 cadaveric specimens. Weights were hung off of tendons to simulate 1.0 kg of lateral pinch and kept constant during testing. We measured lateral pinch force and CMC force as the MPJ was held in 7 different positions with an external fixator: 0°, 10°, 20°, 30°, 40°, 50°, and 60° of hyperextension. This allowed us to make direct observations on how lateral pinch force and CMC force change with increasing amounts of MPJ hyperextension.
Results:
We found a 2.3% increase in CMC force per every 10 degrees of increasing MPJ hyperextension (see Table 1). We also found a 4.7% reduction in lateral pinch force per every 10 degrees of increasing MPJ hyperextension (see Table 2). Therefore, at 60 degrees of MPJ hyperextension, CMC force increased by 14% and lateral pinch force was weakened by 28%.

Conclusion:
With progressive thumb MPJ hyperextension after CMC arthroplasty, we found a decrease in lateral pinch force, and an increase in CMC joint force. Therefore, unaddressed MPJ hyperextension may lead to weaker hand function and more stress on the reconstrcted CMC joint. Additional studies could help define the clinical implications of these new findings.


Figure 1: Testing setup with weights and pulleys

Figure 2: Testing jig, adjustable external fixator, and sensors




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