Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Engineering Science

Program

Biomedical Engineering

Supervisor

Willing, Ryan

2nd Supervisor

Holdsworth, David

Co-Supervisor

Abstract

Treatment of infected orthopaedic implants often require two separate surgeries to clear the infection and install a replacement implant. Current antimicrobial drug delivery methods are inefficient, and each additional surgery increases the physiologic burden on patients, with an increased risk of infection, decreased range of motion and longer recovery times. Porous orthopaedic implants, with adequate strength for permanent implantation and the ability to house and elude more efficient antibiotic delivery systems, could result in a single surgery to treat orthopaedic infections. Therefore, finite element models of porous gyroid structures at different porosities were developed and compared with experimental results. Discrepancies in predicted and actual apparent elastic moduli and fatigue lives were found due to anomalies caused from the 3D-printing of the specimens, however, similar relationships across porosities were observed. A case study of a porous gyroid humeral stem was created to evaluate the mechanical capabilities of the porous stems as well as their impact on humerus mechanobiology. The porous stems were found to exhibit adequate strength in three different bone densities during four different arm motions. Some evidence of improved bone remodelling behaviours was observed when comparing the effects of the porous stems versus a traditional solid stem.

Summary for Lay Audience

Infection in orthopaedic implants often requires intervention through a process known as two-staged revision surgeries. One surgery is performed to remove the infected implant and replace it with a temporary implant loaded with antibiotics. About 90% of these antibiotics remain trapped within the temporary implant and do not get delivered to the body. The other 10% of the antibiotics are delivered at an unideal rate, with an initial burst of antibiotics followed by a rapid decline of antibiotic delivery. This process of drug delivery increases the body’s resistance to antibiotics. Once the infection has been cleared, a second surgery is performed to replace the temporary implant with a new permanent implant. A porous implant that has adequate strength for permanent implantation could be used to house a more ideal antibiotic delivery system, which delivers a larger percentage of drugs at a more efficient rate. This could allow for just a single surgery to be performed to treat an infected orthopaedic implant, as opposed to two surgeries. In addition to improving drug delivery, a secondary benefit of a porous implant is the ability to better mimic the natural structure of bone. This imitation provides the potential for mitigating stress shielding, which is a known complication of orthopaedic implants that causes a loss of bone density.

A porous structure, known as the gyroid, was analyzed at five different porosities using computer models and physical experimental tests. A computer model was developed that was able to demonstrate accurate relationships between cylindrical gyroid porosities for static compression and fatigue tests. Shoulder implants were then created using these gyroid structures and implemented into computer models to predict their mechanical behaviours. These models showed that the gyroid-based shoulder implants, at porosities of 60%, 70%, and 80%, should be strong enough to withstand typical arm motions. It was also determined that compared to traditional solid implants, these porous implants should not have any further negative impacts on the bone remodelling of humeri of three different bone qualities. Future work should explore variations of these gyroid-based shoulder implants, such as the use of porosity gradients, to attempt to further improve the resulting bone effects.

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