Although there are many surgical options to treat thumb carpometacarpal (CMC) arthritis, a precise etiology for this common disorder remains obscure. To better understand the physiology of the thumb CMC joint and treat pathology, it is helpful to examine the biomechanics, hormonal influences, and available surgical treatment options, along with the evolutionary roots of the thumb; its form and function, its functional demands; and the role of supporting ligaments based on their location, stability, and ultrastructure. It is important to appreciate the micromotion of a saddle joint and the role that sex, age, and reproductive hormones play in influencing laxity and joint disease. Minimally invasive surgery is now challenging prevailing treatment principles of ligament reconstruction and plays a role in thumb CMC joint procedures.
The thumb carpometacarpal (CMC) joint is often affected by osteoarthritis--a mechanically mediated disease. Pathomechanics of the CMC joint, however, are not thoroughly understood due to a paucity of in vivo data. We documented normal, in vivo CMC joint kinematics during isometric functional tasks. We hypothesized there would be motion of the CMC joint during these tasks and that this motion would differ with sex and age group. We also sought to determine whether the rotations at the CMC joint were coupled and whether the trapezium moved with respect to the third metacarpal. Forty-six asymptomatic subjects were CT-scanned in a neutral position and during three functional tasks (key pinch, jar grasp, jar twist), in an unloaded and a loaded position. Kinematics of the first metacarpal, third metacarpal, and the trapezium were then computed. Significant motion was identified in the CMC joint during all tasks. Sex did not have an effect on CMC joint kinematics. Motion patterns differed with age group, but these differences were not systematic across the tasks. Rotation at the CMC joint was generally coupled and posture of the trapezium relative to the third metacarpal changed significantly with thumb position. The healthy CMC joint is relatively stable during key pinch, jar grasp, and jar twist tasks, despite sex and age group. Our findings indicate that directionally coupled motion patterns in the CMC joint, which lead to a specific loading profile, are similar in men and women. These patterns, in addition to other, nonkinematic influences, especially in the female population, may contribute to the pathomechanics of the osteoarthritic joint.
