Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Johnson, James A.

2nd Supervisor

Athwal, George S.

Affiliation

The Roth McFarlane Hand and Upper Limb Centre, St. Joseph's Hospital

Co-Supervisor

Abstract

There is a scarcity of literature on the performance of the stemless humeral components. This present work describes the development of a novel loading simulator for the quantification of implant performance, as well as its use in the evaluation of implant parametric design and surgical protocol decision variables.

Interface fixation of humeral components was first evaluated using computational methods to determine the optimal metric to quantify implant fixation. Distractive micromotion was found to be the defining micromotion direction in two different designs of humeral implants.

A loading simulator capable of replicating 3D physiological loads, and comprehensive loading and digital image acquisition program were successfully developed. High resolution digital tracking methods were commissioned to quantify implant fixation.

The novel apparatus was used to compare the use of bone specimens to polyurethane bone surrogate materials using the clinically relevant variable of the degree of press-fit. It was found that the polyurethane analogue materials commonly utilized for the evaluation of implant performance did not accurately replicate the results as were collected in the biological specimens, and that fixation does not linearly increase with press-fit. Moving forward, it was concluded that only bone would be employed for the fixation studies herein.

One of the most important clinical variables with respect to the fixation of stemless implants is neck shaft angle. This was evaluated in terms of primary fixation in a clinically available stemless reversed implant. This was also assessed using a computational framework for a larger range of inclinations. The results of both works were in agreement; finding that decreasing neck-shaft angle resulted in decreased fixation of the implant evaluated.

This present work represents an advancement of knowledge regarding the performance of shoulder arthroplasty humeral components and provides a more thorough evaluation methodology than has been previously utilized during studies of the same nature. Decreasing neck shaft angle to increase range-of-motion comes at the cost of implant primary fixation, and over-increasing implant press-fit may compromise the fixation of stemless implants. Moreover, the relevance of focusing on normal micromotion due to its prominence for the stemless implant designs was shown to be a key outcome.

Summary for Lay Audience

Shoulder replacement surgeries are a common medical intervention for patients with damaged or diseased shoulders. This surgical intervention is designed to replace the anatomic shoulder joint with an artificial substitute and has been proven to help patients regain their pre-injury independence and quality of life. Recently, there have been advancements to the design of shoulder implants that have not been thoroughly investigated. This dissertation is directed towards the evaluation of the novel stemless reversed shoulder implant variant, with focus on the fixation properties of the bone-implant interface. This work evaluated implant fixation using both computational and experimental methodologies.

Initially, implant fixation phenomena were investigated using computer simulation approaches in order to identify the best method to quantify implant stability. It was found that bone-implant gapping motion was the most effective value to measure for the implant geometry under investigation. Second, a novel implant evaluation apparatus was created to replicate the forces that an implant would realistically experience after surgery. A high-resolution camera system was used to measure bone-implant gapping during loading. This loading system was then used in an experimental study to investigate how using foam surrogate materials in the place of bone specimens influences the results of those analyses. It was found that foam surrogates do not replicate the results obtained with bone specimens. Subsequently, this work evaluated the effect of the surgical implantation variable of “neck shaft angle” both computationally and experimentally. These two studies both showed that increasing neck shaft angle resulted in increased fixation, and the latter work additionally revealed that increasing neck shaft angle also favourably increased implant survivorship during loading.

Overall, this work developed an effective strategy to thoroughly evaluate the fixation behaviour of stemless implants after surgery by replicating realistic load magnitudes that implants might experience during use. This is important, as there are a limited number of methods for implant evaluation that exhibit acceptable mimicry of the real implant loading environment.

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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