First dorsal interosseous muscle contraction results in radiographic reduction of healthy thumb carpometacarpal joint

Introduction Hand therapists selectively strengthen the first dorsal interosseus (FDI) to stabilize arthritic joints yet the role of the FDI has not yet been radiographically validated. Purpose To determine if FDI contraction reduces radial subluxation (RS) of the thumb metacarpal (MC). Methods Fluoroscopy was used to obtain true anterior-posterior radiographs of non-arthritic CMC joints: 1) at rest, 2) while stressed and 3) while stressed with maximal FDI contraction. Maximal FDI strength during CMC stress and thumb MC RS and trapezial articular width were measured. The ratio of RS to the articular width was calculated. Results Seventeen participants (5 male, 12 female) participated. Subluxation of a stressed CMC significantly reduced and the subluxation to articular width ratio significantly improved after FDI activation. Conclusions Contraction of the FDI appears to radiographically reduce subluxation of the healthy thumb CMC joint. Further exploration on the FDI's reducibility and its carry-over effects in arthritic thumbs is needed. Level of evidence 4.