Trapeziometacarpal (TMC) joint arthritis is a common and debilitating condition of the hand. We defined a radiographic measure of trapezial inclination (trapezial tilt) and found a positive correlation between an increased trapezial tilt and severity of TMC joint arthritis. Radiographs (Robert's views) were obtained from 50 pairs of normal hands to evaluate the trapezial tilt to assess radial inclination of the trapezium with respect to the second metacarpal. The trapezial tilt was also measured in 65 hands from 43 patients with various stages of TMC joint arthritis and compared with the normal value. The trapezial tilt for hands without arthritis was 42 degrees +/- 4 degrees, Eaton stages I and II was 42 degrees +/- 4 degrees, and Eaton stages III and IV was 50 degrees +/- 4 degrees. Trapezial tilt angles from the Eaton III and IV group were significantly greater than those of the normal and Eaton I and II groups. Advanced TMC joint arthritis (Eaton III and IV) is associated with an increased trapezial tilt. Mild TMC joint arthritis with an increased trapezial tilt may be treated surgically. We speculate that a trapezio-trapezoid and trapezio-II metacarpal arthrodesis, or an opening wedge osteotomy of the trapezium might arrest the progression of TMC joint arthritis by resetting the slope of the trapezium and decreasing the shear stress within the TMC joint.