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Trapeziometacarpal Joint Stability: Lateral Pinch or Power Grip
Jorge Luis Orbay, MD1; Anthony R Martin, BS2; Lauren Vernon, PhD1; Michael Mijares, MD3
1The Miami Hand and Upper Extremity Institute, Miami, FL, 2University of Pennsylvania, Philadelphia, PA, 3University of Miami/Jackson Memorial Hospital, Miami, FL

Introduction: Trapeziometacarpal (TM) joint arthritis is a common but poorly understood condition that often results in significant impairment. All cases present clinically with some degree of dorsal subluxation. Therefore, a dorsally directed unbalanced force component may have a role in disease development. The primary objective of this study was to understand forces inducing dorsal subluxation and determine if they result from key pinch or power grip.
Materials & Methods: Five non-arthritic mid-humerus cadaveric specimens were prepared by stabilizing the wrist, fixing the 2nd through 5th metacarpal-phalangeal joints in 60 flexion, and mounting the forearm onto a frame. Extrinsic and intrinsic tendons were prepared for weight suspension. Specimens were loaded into power grip and key pinch configurations, and all TM ligaments were resected in order to record their effect on joint stability. Stability was evaluated statically and dynamically in power grip and key pinch.
Results: The TM joint was statically and dynamically stable in the key pinch configuration in all 5 specimens after resection of all supporting ligaments (Figure 1). On the other hand, the TM joint was unstable in the power grip configuration in all 5 specimens after resection of all supporting ligaments. In this configuration, 3 specimens dislocated and 2 subluxed prior to a dynamic challenge (statically), and all 5 specimens dislocated when dynamically challenged.

Conclusion: The forces induced on the TM joint during pinch do not generate a shearing component requiring ligament restraint to maintain stability; the forces induced during grip do. Therefore, dorsal subluxation and resulting arthrosis may be the result of a dorsal shearing force generated during gripping in the presence of incompetent TM ligaments. Power grip generates shear forces that may promote or exacerbate TM osteoarthritis, while pinch does not. This may have relevance in conceiving rehabilitation protocols, surgical techniques and designing prosthetic joints.


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