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Activation of the First Dorsal Interosseous Muscle Results in Radiographic Reduction of the Thumb CMC Joint: Implications for Arthritis Intervention
Corey McGee, OTR/L, CHT; Julie E. Adams, MD; Sara S. Van Nortwick, MD; Virginia H. O'Brien, OTD, OTR/L, CHT; Ann E. Van Heest, MD
University of Minnesota, Minneapolis, MN

Introduction: Hypermobility of the carpometacarpal (CMC) joint is a major etiological factor in the development of thumb arthritis. Stabilization of the CMC joint with reduction of joint subluxation theoretically reduces the risk of arthritis. The hypothesis of this study is that activation of the first dorsal interosseous (FDI) muscle will reduce CMC subluxation of the metacarpal as measured by fluoroscopy.

Methods: Subjects at least 18 years old were recruited. Exclusion criteria included a history of hand arthritis, positive grind test, pregnancy, and major conditions of ligamentous laxity. A certified hand therapist performed a grind test, measured grip and pinch strength, and maximal voluntary contraction of the FDI using the Rotterdam Intrinsic Hand Myometer. Fluoroscopy was used to obtain true AP radiographs of the CMC joint at 1) rest, 2) while stressed without activation of the FDI and 3) while stressed with activation of the FDI. Radial subluxation of the first metacarpal and metacarpal width were measured by 3 blinded surgeons as described by Wolf (2011).

Results: Seventeen subjects with 34 thumbs (5 male and 12 female) participated. Average age was 25.9(21-59). Thirteen right handed, one left handed, and 3 ambidextrous subjects were included. Two thumbs were excluded for a positive grind. Thirty-two thumbs were evaluated. Average maximal voluntary contraction of the FDI was 27N, lateral pinch 81N, and grip strength 347N. Twenty-seven thumbs demonstrated subluxation when stressed with reduction after activation of the FDI. Three thumbs were not subluxed at rest and did not sublux with stress or reduce with firing of the FDI, consistent with stiff CMC joints. Two thumbs were subluxed at rest but did not further sublux with stress. Inter-rater reliability of this categorization was high(ICC>.74). In the 27 thumbs that demonstrated increased subluxation with stress, subluxation while stressed averaged 0.6 cm(0.0-0.9) or 48%(29-75) of articular width. FDI activation reduced subluxation by an average of 0.5 cm(0.1-0.9) or 80%(20-120). The two thumbs with the same degree of subluxation at rest and with stress had subluxation of 0.5 cm and 0.7 cm corresponding to 43% and 63% of articular width, respectively. Reduction with FDI activation was by 0.3 cm and 0.2 cm or 67% and 28%, respectively. When the CMC joint was stressed and FDI activated, maximum FDI strength explained 32.3% of the variability in subluxation.

Conclusion: The FDI radiographically reduces subluxation of the thumb CMC joint. Strengthening the FDI may be an effective intervention in preventing arthritis.

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