Kinematics of the ulna during pronation and supination in a cadaver study: implications for elbow arthroplasty

Objective. Aim of this study was to exactly describe and quantify kinematics of the ulna during pro- and supination. Design. Biomechanical study in fresh frozen cadavers. Background. A previous MRI study revealed a varus/valgus motion of the ulna averaging 7.1° during pro-/supination. Axial rotation, however, could not be quantified. Methods. Sixteen arms were examined in a new apparatus that fixed the humerus on a template and allowed forearm rotation. Motion of a Kirschner wire placed in the ulna was recorded in steps of 30° by two perpendicularly arranged charge coupled device cameras during pro- and supination. Results. From supination to pronation the ulna showed a semi-lunar evasive motion in the coronal and transverse plane with an initial varus shift, then a dorsal and finally a valgus shift. Motion in the coronal plane averaged 14.14° (SD 4.78). Valgus angles of the ulna in 30°, 60° and 90° pronation were significant (P<0.05) to each other and the neutral position. Varus angles of the ulna in 30°, 60° and 90° supination (P<0.01) were significant to each other and the neutral position. A maximum ulnar axial pronation rotation of 3.2° (SD 2°) was noted. Axial rotation angles of 90° and 60° of pronation were significant to each other and to the neutral position (P<0.05), respectively. Conclusions. To prevent increased stress on the bone–cement interface in elbow arthroplasty, a mean axial rotation of at least 3.2° should be possible.