TOTAL SHOULDER ARTHROPLASTY BIOMECHANICS - A STUDY OF THE FORCES AND STRAINS AT THE GLENOID COMPONENT

The objective of this study was to examine how changes in glenohumeral joint conformity and loading patterns affected the forces and strains developed at the glenoid. After removal of soft tissue (muscles, liga ments, and labrum), force-displacement data were collected for bath na tural and prosthetically reconstructed joints. Joints were shown to de velop higher forces for a given translation as joint conformity increa sed. A rigid body model of joint contact forces was used to determined the so-called effective radial mismatch of each joint. For the purpos es of this study the effective radial mismatch is defined as the misma tch required for a rigid body joint to have the same force-displacemen t relationship as the joint in question, This parameter is an indicati on of the deformation at the articular surface. The effective radial m ismatch dramatically increased with increasing medial loads, indicatin g that under physiological loads, the effective radial mismatch of a j oint is much greater than its measured mismatch at no load. This incre ase in effective mismatch as medial loads were increased was found to be threefold greater in cartilaginous joints than in reconstructed joi nts, Rosette strain gages positioned at the midlevel of the glenoid ke el in the reconstructed joints revealed that anterior/posterior compon ent loading lends to fully reversible cyclic keel strains. The highest compressive strains occurred with the head centered in the glenoid, a nd were larger for nonconforming joints (epsilon = 0.23 percent), Thes e strains became tensile just before rim loading and were greater for conforming joints (epsilon = 0.15 percent). Although recorded peak str ains are below the yield point for polyethylene, the fully reversed cy clic loading of the component in this fashion may ultimately lead to c omponent toggling and implant failure.