Application of screw displacement axes to quantify elbow instability

Objectives. To determine if screw displacement axis patterns describing elbow joint motion: (1) change after ligament transection in vitro; (2) can reflect subtle changes in stability as a function of forearm position; (3) can reflect dynamic stabilization of the ligament insufficient elbow provided by muscle activity. Design. An in vitro kinematic study of eighteen cadaveric specimens tested in a joint simulator. Background. In the elbow joint, screw displacement axes have been employed for proper positioning and design of endoprostheses. The effect of instability on screw displacement axes has not been previously reported. Methods. Passive and simulated active flexion, with the forearm maintained in both pronation and supination, was performed on eighteen intact and ligament insufficient elbows. Instability was produced by transection of the medial collateral or lateral collateral ligament complexes. Kinematics were recorded using an electromagnetic tracking device and analyzed with a repeated measures design. Results. During passive motion, division of either ligament caused deviation of screw displacement axes compared to the intact state (P<0.05). Transection of the medial/lateral collateral ligament generated greater instability with the forearm maintained in pronation/supination compared to supination/pronation (P<0.05). Muscle activation increased stability similar to the intact state (P>0.05). Conclusions. These results are consistent with observations determined using traditional kinematic descriptors. Screw displacement axes can readily detect changes in stability due to ligament sectioning.