Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Engineering Science

Program

Biomedical Engineering

Supervisor

Langohr, G Daniel G

2nd Supervisor

Medley, John B

Affiliation

University of Waterloo

Co-Supervisor

Abstract

The reverse total shoulder arthroplasty (RTSA) has quickly grown to become the most commonly used shoulder arthroplasty design; however, reports have shown evidence of RTSA failures related to polyethylene wear and damage. Therefore, the present work investigated the wear of crosslinked polyethylene (XLPE) in environments similar to that of an in vivo RTSA. Additionally, a computational model was developed based on a previous study of the shoulder motions obtained from a selection of typical patients with RTSA. This model quantified the amount of glenohumeral motion that an RTSA may be subjected to in vivo and provided an approximate value for the number of cycles that the RTSA-bearing shoulder may see annually. The in vitro RTSA wear simulation detected a significant decrease in polyethylene wear for XLPE in humeral cup liners compared with an earlier very similar study using non-XLPE. The computational model based on in vivo data suggested that smaller neck shaft angles of te implant might reduce polyethylene damage and also suggested that 1.25 million cycles in our joint wear simulator provided a good representation of 1 year in vivo. It is likely that the use of XLPE in the RTSA may reduce the number of failures related to wear.

Summary for Lay Audience

The reverse total shoulder arthroplasty (RTSA) has become the most popular shoulder replacement design used today. RTSA is used to treat a variety of conditions including, but not limited to, shoulder arthropathy, shoulder arthritis, shoulder fracture, and failure of previous shoulder implants. There have been reports showing evidence of RTSA failures related to wear of the humeral cup that warrants an investigation into the materials being used. In hip implants, a very wear resistant crosslinked polyethylene (XLPE) is widely used. However, in RTSA, a non-crosslinked polyethylene material is the most common material used to line the humeral cup. When wear-related implant failures were seen in total hip arthroplasty, they were essentially eliminated by using XLPE. This material has now been described as the gold standard liner material in total hip replacements (and is gaining popularity in the knee). It is logical to believe using XLPE in RTSA may yield similar benefits to its use in the hip. Therefore, the objective of the present work was to investigate XLPE wear in environments similar to that of an in vivo RTSA. Wear simulations were conducted to measure the total volume of XLPE wear that may occur in the RTSA. Additionally, a computer model was developed based on a previous in vivo study of shoulder motion in RTSA patients. This model predicted the amount of relative motion that the RTSA may see annually. The computer model also simulated 4 different RTSA designs to investigate the effects that changing specific design parameters has on relative motion and risk of scapular notching (another common RTSA complication). The results of the simulations suggested that XLPE was significantly more wear resistant than non-XLPE in the RTSA. The computer model predicted that roughly 1.25 million cycles in our shoulder wear simulator represented 1 year in vivo. The model indicated that the risk of scapular notching was reduced by using implants having the specific geometrical feature of a lower neck shaft angle. The results of this work will help influence RTSA testing protocol and design and will afford surgeons the ability to make more informed clinical predictions.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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