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Radiographic Analysis Of Simulated First Dorsal Interosseous And Opponens Pollicis Activation Upon Thumb CMC Joint Subluxation: A Cadaver Study
Julie E. Adams, MD1; Virginia H. O'Brien, OTD, OTR/L, CHT1; Erik Magnusson, MD2; Benjamin Rosenstein, MA3; David Nuckley, PhD4
1Department of Orthopaedics, University of Minnesota, Minneapolis, MN; 2Department of Orthopaedic Surgery, University of Washington, Seattle, WA; 3Medical School, University of Minnesota, Minneapolis, MN; 4Zimmer Spine, Minneapolis, MN

Introduction: Therapy to treat thumb CMC arthritis includes selective strengthening of thenar muscles, particularly the 1st dorsal interosseous (FDI) and opponens pollicis (OP) to reduce subluxation of the joint, improve pain and decrease arthritic symptoms. Although there are now favorable clinical series, there are little biomechanical data available to support these techniques. This investigation describes the effect of selective activation of the FDI and OP muscles upon radiographic subluxation of the thumb CMC joint.

Methods: 11 fresh frozen cadavers were thawed and dissected to expose the FDI and OP and sutures placed for application of loads. The thumb CMC joint was dissected and capsulotomy performed such that the joint could freely subluxate with application of a 5 kg load to the metacarpal. Loads were applied to the FDI, the OP, and then concomitantly to both muscles at 0%, 25%, 50%, 75% and 100% maximal loads of 30 and 40 N respectively; at each point AP radiographs were obtained and assessed for subluxation of the joint (radial subluxation (RS) / articular width (AW) ratio).

ANOVA testing was used to compare mean RS/AW ratio across all loaded states. Chi-squared testing was used to evaluate reduction >50% v.s. reduction <50%.

Results: 7 complete and 4 partial sets of measurements were obtained as the OP were irreparably torn during testing in some cases. Selective activation of the OP, alone, improved the subluxation ratio across all loaded states. Increasing activation of the OP resulted in improved subluxation ratio in a dose dependent manner. Selective activation of FDI alone demonstrated minimal effects on RS/AW.
Concomitant activation of OP and DI lessened subluxation across all loading states. Subluxation ratios improved in a dose dependent manner with increasing activation with the lowest ratio at 100% of 0.10. Selective activation at 25% of DI and OP demonstrated the only ratio superior to loading of OP alone (0.341 vs 0.407). All other states demonstrated less improvement compared to OP alone, suggesting that the optimal combination of forces are 25% of maximal force of both FDI+OP. ANOVA testing for the test model demonstrated significance (p=0.001).

Conclusions: These data suggest that concomitant activation of the FDI and OP reduce subluxation of the thumb basilar joint. Therapy for strengthening programs likely function in part to encourage patients to activate the easily palpable FDI. Concomitant co-activation of the OP may be the major reducing force to elicit clinical and radiographic reduction of subluxation.

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