This study examines the effect of metacarpophalangeal (MCP) joint position on force distribution at the trapeziometacarpal (TMC) joint. Twenty fresh-frozen cadaveric specimens were tested with Fugi film in four different positions: unrestrained, neutral, 300 of flexion, and maximal extension. In all specimens, MCP joint flexion moved the center of pressure of the TMC joint to a more dorsal position. Hyperextension of the MCP resulted in the center of pressure at the TMC joint to be most volar. The position of the center of pressure at the TMC joint was most variable and dependent upon the MCP joint angle in the severely osteoarthritic specimens.
Osteoarthritis of the TMC joint has been shown to begin on the volar most portion of the TMC joint with progressive cartilage wear proceeding in a dorsoradial direction. MCP joint hyperextension is felt to be a secondary response to progressive subluxation of the TMC joint and adduction of the first ray. This study shows that MCP hyperextension concentrates forces in the palmar portion of the TMC joint where early cartilage wear is most evident. However, it remains unclear if MCP joint laxity is causative or secondary. The authors suggest that patients with hyperextension laxity of the MCP joint may be predisposed to TMC osteoarthritis and suggest that symptomatic patients with MCP hyperextension may benefit from splinting or surgical stabilization of the MCP joint to alleviate TMC joint symptoms by moving TMC joint forces to the less involved dorsal surface. However, MCP stabilization will increase total TMC force and further studies are necessary before this can be considered a viable treatment alternative in basilar thumb arthritis.
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