Radiographic Analysis of Simulated First Dorsal Interosseous and Opponens Pollicis Loading Upon Thumb CMC Joint Subluxation: A Cadaver Study

Abstract

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.

Keywords: CMC; carpometacarpal; dynamic stability; first dorsal interosseous; opponens pollicis; selective strengthening; trapeziometacarpal.

Conflict of interest statement

Declaration of Conflicting Interests: The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Figures

Figure 1.

Experimental setup demonstrating cadaver preparation and muscle tagging.

Figure 2.

Fluoroscan demonstrating RS/AW ratio. An RS/AW ratio of 0 is representative of a healthy nondisplaced joint. Note. RS = radial subluxation; AW = articular width.

Figure 3.

Change in radial subluxation to articular width ratio (RS/AW) with increasing muscle loading. Note. The whiskers on the plot are the standard error as between-sample variability was large. Loading of the OP alone and OP + FDI produce significantly greater reduction of the joint than the FDI alone at larger levels of activation. *P < .0455 for comparisons within a loading level group (eg, at 100%, OP and FDI + OP are significantly greater than FDI alone). RS = radial subluxation; AW = articular width; OP = opponens pollicis; FDI = first dorsal interosseous.