THE EFFECT OF ARTICULAR CONFORMITY AND THE SIZE OF THE HUMERAL HEAD COMPONENT ON LAXITY AND MOTION AFTER GLENOHUMERAL ARTHROPLASTY - A STUDY IN CADAVERA

We used a cadaveric model to examine the mechanical effects of changes in the conformity of the articular surfaces and the size of the humer al head component in glenohumeral arthroplasty. The experimental syste m permitted a manual clinical examination of the glenohumeral joint wh ile sensors monitored the humeroscapular position and orientation as w ell as the forces and torques applied by the examiner. Four preparatio ns were compared: an anatomical humeroscapular preparation and three g lenohumeral arthroplasty preparations (one with anatomically sized com ponents and a radius of curvature of the glenoid that was four millime ters larger than that of the humeral head, one with anatomically sized components and a radius of curvature of the glenoid that was equal to that of the humeral head, and one with a nonanatomical, large humeral head component and a radius of curvature of the glenoid that was equa l to that of the humeral head). All motions, including flexion, extern al and internal rotation, and maximum elevation, were diminished with use of the non-anatomical, large humeral head component. Laxity of the joint on drawer and sulcus tests was not affected by the conformity o f the articular surfaces but was decreased significantly by implantati on of the large humeral head component. The kinematics of the glenohum eral joint were not markedly altered by reduction of the uniformity be tween the articular surfaces of the prosthetic components. In all prep arations, obligate displacement of the humeral head associated with a passive range of motion occurred at smaller angles with the large hume ral head component. CLINICAL RELEVANCE: Our results indicate the impor tance of selecting appropriately sized prosthetic components for the s pace available and point to the variables of shoulder arthroplasty tha t a surgeon must master to compensate for loss or resection of bone an d to complement soft-tissue releases currently used in clinical practi ce.