The biomechanical effects of extension metacarpal osteotomy on contact pressures in the trapeziometacarpal joint during lateral pinch were studied in 20 anatomic specimens using pressure-sensitive film. The gross appearance of joint surfaces was classified by severity of arthritic disease and correlated with specimen x-ray films taken before the osteotomy. Extension metacarpal osteotomy effectively unloaded the palmar contact area in nonarthritic and moderately arthritic specimens; primary contact areas and zones of peak pressure were shifted from the diseased palmar compartment to the normal dorsal compartment. In contrast, the pathologically congruent contact pattern seen in end-stage osteoarthritic joints was unaffected by osteotomy. The data demonstrate the efficacy of extension metacarpal osteotomy in unloading the palmar contact areas of normal and moderately arthritic joints but provide no biomechanical rationale for metacarpal osteotomy as originally described in treatment of advanced trapeziometacarpal osteoarthritis.