Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Collaborative Specialization

Musculoskeletal Health Research

Supervisor

Ferreira, Louis M

Abstract

Research involving joint mechanics is typically conducted at the macroscopic level. However, joints and joint replacements often fail because load transfer at the microscopic level is not well understood. This gap in knowledge reduces our ability to preoperatively predict patient outcomes and assess irreversible failure modes for a variety of surgical interventions prior to clinical adoption. The present work aims to advance full-field experimental measurement techniques applied to better understand the internal load transfer of the human shoulder joint by simultaneously combining mechanical testing protocols, microCT imaging, and digital volume correlation (DVC) methods.

A CT-compatible loading apparatus was fabricated to allow for mechanical loading of cadaveric shoulder specimens within a cone beam microCT scanner. DVC was used to measure full-field displacements and strains throughout the internal structure of bone under controlled loading scenarios. Initially, the full-field experimental data was used to assess predictions generated by corresponding continuum-level finite element models (FEMs). Varying assumptions (e.g., boundary conditions, material mapping equation used, etc.) required to generate the simulations were assessed. The results of the validation efforts demonstrated that continuum-level FEMs of the shoulder can predict the experimental full-field displacements with high accuracy if the boundary conditions are replicated correctly. Good agreement was found between the strains predicted and the experimental measurements obtained by DVC with the highest predictive errors found in locations that experienced the highest magnitude of experimental strain.

The full-field experimental methods were further applied to evaluate the magnitude of full-field strain that trabecular bone within the shoulder can withstand prior to fracture. An experimental workflow which involved stepwise compressive loading with microCT images captured at each loading step was performed until macroscopic failure occurred. Internal strains throughout the trabecular structure were resolved using DVC. Bone density measurements and trabecular morphometric parameters were compared to outcome measures such as apparent strength and the local strain measured by DVC. The experimental data collected provides fundamental knowledge for future studies implementing DVC and lays the foundation for future validation studies that utilize full-field experimental measures to assess the predictive accuracy of FEMs of the musculoskeletal system.

Summary for Lay Audience

To ensure optimal surgical outcomes for clinical interventions involving bone, it is useful to understand how bone will react under external mechanical forces. However, bone is a complex living material and is constantly undergoing structural changes based on the requirements of daily life. Therefore, understanding and predicting the mechanical behaviour of bone before an orthopaedic surgery is challenging.

Within this dissertation, simulated joint loads were applied to cadaveric human shoulders within a microCT scanner. The purpose was to capture 3D images of bone in an undeformed and deformed state to better understand the structural response of bone under controlled loading scenarios. First, the experimental data was applied to assess computer simulations which are commonly used in the design of joint replacement components. For the first time, the experimental validation data shed light on potential best practices that should be considered when generating computer simulations of the human shoulder. In addition, mechanical testing was performed on trabecular bone that was directly retrieved from patients undergoing total shoulder arthroplasty due to end stage osteoarthritis. Collectively, this knowledge provides fundamental knowledge for the biomechanics research field seeking to improve patient outcomes for shoulder joint replacement surgeries.

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