The effect of the orientation of the noncircular radial head on elbow kinematics

Objective. The objective of this study was to identify the effect of radial head shape and orientation on elbow kinematics in the otherwise intact elbow. Design. Biomechanical study, analyzing simulated active motion of cadaveric arms. Background. A discrepancy exists between the noncircular anatomy of the radial head and radial head prostheses. The effect of radial head shape is unknown. Methods. Kinematic effects of radial head shape were tested in six fresh-frozen upper extremities. A custom-made native radial head prosthesis was used to simulate altered shape conditions, by rotating the radial head 90°. Three-dimensional spatial orientation of the ulna was recorded, during simulated active motion. A three factor anova was used to compare (a) nominal and 90° oriented conditions, (b) throughout the flexion arc (c) in three forearm positions (P<0.05). Post-hoc Tukey tests were done to assess significance. Results. No significant effect of altering radial head shape was found on total ulnohumeral laxity and angulation during gravity valgus stress. We did find a significant effect on total ulnar axial rotation and rotation during gravity valgus stress. Conclusion. The outer shape of the radial head seems to change rotation of the ulna during flexion-extension in an otherwise intact elbow. Relevance The shape of the radial head effects intact elbow kinematics. Clinical importance of this finding is clear. If a sub-optimally placed radial head prosthesis were to be used in an otherwise intact elbow, the elbow could be at risk for early ulnohumeral arthritis.