Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Biomedical Engineering

Collaborative Specialization

Musculoskeletal Health Research

Supervisor

Teeter, Matthew G.

Abstract

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.

Summary for Lay Audience

One in five Canadians suffers from arthritis, a progressive joint disease. With no cure, many patients opt for joint replacement, whereby the ends of the damaged bones are replaced with metal implants and separated with a plastic liner. Over time, artificial joints can become loose, causing pain and reduced function. Loosening can result from initially poor attachment between the implant and bone, and made worse by wear of the plastic liner. It is best to remove and replace loose implants, though this procedure is expensive and patient satisfaction decreases each time it is performed. For this reason, it is important to identify implant designs and materials with strong initial attachment. It is known that implant movement in the first year after surgery is predictive of later loosening requiring reoperation. By identifying implants that move more than others, they can be removed from the orthopedic market prior to widespread use. The objectives of this thesis were to use a three-dimensional x-ray technique to compare the early movement of implant components in reverse total shoulder replacement (RTSR), as well as wear of the plastic liner. Patients were recruited and randomized into implant groups comparing the use of bone cement to no bone cement with the metallic stem inserted into the upper arm bone, and the use of either bone graft or porous metal structural enhancement with the metallic hemisphere attached to the shoulder blade. A separate group of patients, with at least five years of use of their joint replacement, were recruited to investigate mid-to-long-term wear of the plastic liner. Results show that stems with bone cement had less movement than stems without, though neither group moved appreciably after six months, suggesting long-term stability. There were no differences in metal hemisphere movement using either bone graft or porous metal structural enhancement. The observed liner wear was measurable and comparable to estimates from simulation studies. Overall, this work is the first to compare different implant-bone attachment techniques in RTSR, and first to measure wear of the plastic liner inside the body. Results from this work will influence future implant design and clinical care.

Creative Commons License

Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License

Available for download on Tuesday, March 16, 2021

Share

COinS