Rosenstein, Benjamin E.

Background: Therapy programs to treat thumb carpometacarpal (CMC) arthritis may engage selective activation and reeducation of thenar muscles, particularly the first dorsal interosseous (FDI) and opponens pollicis (OP) to reduce subluxation of the joint. We describe the effect of simulated selective activation of the FDI and OP muscles upon radiographic subluxation of the thumb CMC joint. Methods: In a cadaver model of CMC subluxation, loads were applied to the FDI, the OP, and then concomitantly at 0%, 25%, 50%, 75%, and 100% maximal loads and radial subluxation of the joint and reduction in subluxation was measured. Results: Selective activation of the OP, alone, improved the subluxation ratio (SR) in a dose-dependent manner. Selective activation of FDI, alone, demonstrated minimal effects on SR. Concomitant activation of OP and FDI improved the SR across all loading states, and activation of 75% and greater, when compared with FDI activation alone, resulted in a statistically significant improvement in SR to within 10% of the presubluxed joint. Conclusions: Concomitant activation of the FDI and OP acts to reduce subluxation of the thumb CMC joint in a dose-dependent fashion. The OP is likely the predominant reducing force. Hand therapy programs that focus on selective strengthening programs likely function in part to encourage patients to activate the easily palpable and easily understood FDI. Concomitant coactivation of the OP may be the major reducing force to elicit clinical and radiographic reduction of subluxation, improved thumb positioning, and reduction of pain and arthritic symptoms.

Objective: Symptomatic thumb carpometacarpal (CMC) joint osteoarthritis is common. Early data support selective strengthening of the first dorsal interosseous (FDI) and opponens pollicis (OP) muscles for conservative management with improvement in pain and functional outcomes scores. There are limited biomechanical studies to support such strengthening programs. The purpose of these studies was to quantify the kinematics and joint kinetics at the thumb CMC joint during simulated pinch with selective activation of the FDI and OP muscles. The first study was completed with all ligamentous attachments of the CMC joint intact. The second study was completed with all CMC ligaments released. Hypothesis: Our hypothesis was that increased loads in the FDI and OP result in effects to the joint kinematics and reduce subluxation. Results: Our results from both studies show the OP and FDI act synergistically to center the first metacarpal on the trapezium and reduce subluxation during simulated pinch. The OP is the predominant corrective force but tends to overcorrect if unopposed. The FDI tempers the action of OP and contributes to improved CMC congruency. Conclusion: Our conclusion is these biomechanical data support selective thenar muscle re-education and strengthening programs for conservative management of thumb CMC joint osteoarthritis and pain.