This is a cadaveric study performed on seven elbows. The authors applied a unilateral (lateral) hinged external fixator to seven cadaveric elbows. They tested cantilever lateral bending at three flexion angles in both varus and valgus directions. The stiffness was measured at each flexion angle of 0 degrees, 45 degrees, and 90 degrees. The authors then made a 3 mm gap between the humerus and the ulna, consistent with an interposition arthroplasty. In the second series, they did not permit a gap between the bones, simulating ligamentous repair or reconstruction. In the third, the authors applied a 4 kg axial load to the ulna to simulate passive muscle tension across the joint. The study was then repeated.
The results indicated that, 1) motion was noted primarily at the connections of the pins to the external fixator frame; 2) there was marked decrease in the stiffness with increasing the flexion angles in both varus and valgus stress. The decrease was more evident in varus. The authors found that the presence of a 3 mm gap resulted in a decrease in loading. In essence, the stiffness of the construct decreased as the joint was distracted. Finally, the authors concluded that the stiffness of the construct was more apparent in varus than in valgus testing. The stiffness in valgus was approximately one-fourth of that in varus. The significance of this is that the lateral application of the fixator, as is typical in a clinical scenario, results in significantly less stability in valgus with a valgus load than with a varus load.
Journal of Shoulder and Elbow