Biomechanical effect of increasing or decreasing degrees of freedom for surgery of trapeziometacarpal joint arthritis: a simulation study.

Osteoarthritis of the trapeziometacarpal (TMC) joint can be treated by arthrodesis and arthroplasty, which potentially decreases or increases the degrees of freedom (DoF) of the joint, respectively. The aim of our study was to bring novel biomechanical insights into these joint surgery procedures by investigating the influence of DoF at the TMC joint on muscle and joint forces in the thumb. A musculoskeletal model of the thumb was developed to equilibrate a 1 N external force in various directions while the thumb assumed key and pulp pinch postures. Muscle and joint forces were computed with an optimization method. In comparison to that of the 2-DoF (intact joint) condition, muscle forces slightly decreased in the 0-DoF (arthrodesis) condition, but drastically increased in the 3-DoF (arthroplasty) condition. TMC joint forces in the 3-DoF condition were 12 times larger than the 2-DoF joint. This study contributes to a further understanding of the biomechanics of the intact and surgically repaired TMC joint and addresses the biomechanical consequences of changing a joint's DoF by surgery.