The Migration and Wear of Reverse Total Shoulder Arthroplasty

Reverse total shoulder arthroplasty (RTSA) inverts the ball and socket geometry of the shoulder. Though projected to become the most common shoulder replacement in the next decade, RTSA suffers from a high complication and revision rate, with implant loosening requiring revision. As the number of indications and demand from younger patients for RTSA continues to grow, there is the need to identify implant fixation techniques that promote longevity. Radiostereometric analysis (RSA) is the current standard for measuring implant migration, which, if continuous in the first year postoperatively is highly predictive of later loosening and failure. RSA has also been used to measure polyethylene wear, known to contribute to implant loosening through periprosthetic bone resorption. The objectives of this thesis were to compare early implant migration between different RTSA fixation techniques, and to assess the in vivo polyethylene wear rate of RTSA at mid-to-long-term follow-up. To accomplish these objectives, the use of RSA for RTSA was first validated using a phantom setup. Subsequently, patients were prospectively randomized to compare cemented to press-fit humeral stems, and bone graft to porous metal-augmented glenosphere baseplates. Imaging was acquired postoperatively through one year. Separately, patients with an implant term-of-service greater than five years were recruited and imaged at a single timepoint. All migration analyses were performed in model-based RSA, with the addition of an in-house software for wear analysis. Significantly greater migration was observed with press-fit compared to cemented stems six months and one year postoperatively, though both groups demonstrated stability from six months onward. There were no differences at any time point between glenosphere lateralization groups. Polyethylene wear was measurable and multidirectional, with values comparable to simulation studies. The primary contribution of this work is the first-ever clinical RSA for RTSA study, the results of which provide the best possible evidence on the predicted longevity of cemented vs. press-fit humeral fixation, and bony vs. porous metal glenosphere lateralization. The secondary contribution is the first evaluation of in vivo RTSA polyethylene wear; the results from both studies influencing clinical care and the design of next-generation shoulder implants