An anatomic and radiographic study of the osteoarthritic trapeziometacarpal joint was conducted on 27 surgical specimens harvested during basal joint arthroplasty. Surface wear patterns were analyzed by hyaline cartilage staining, and relative involvement of trapezium and metacarpal was calculated as the trapezium/metacarpal eburnation ratio. Eburnation occurred only on facing trapeziometacarpal surfaces in contact areas of the palmar compartment. Metacarpal degeneration began at the palmar joint margin adjacent to the beak ligament and extended dorsally, while trapezial degeneration originated on the central palmar slope and spread centrifugally with more advanced disease. Eburnation consistently involved a greater surface area on the trapezium than on the metacarpal in a ratio of nearly 3:1. A decreasing trapezium/metacarpal eburnation ratio correlated closely with worsening radiographic stage. These observations suggest translation of metacarpal on trapezium in the production of arthritic surface lesions and support a hypothesis of pathologic joint instability as the cause of trapeziometacarpal osteoarthritis.