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Thesis Format

Integrated Article

Degree

Master of Engineering Science

Program

Mechanical and Materials Engineering

Collaborative Specialization

Musculoskeletal Health Research

Supervisor

Johnson, James

Abstract

One of the main drawbacks of hemiarthroplasty is that it involves the articulation of a foreign material against the native cartilage, and hence the native cartilage experiences accelerated wear. It is hypothesized that lowering the stiffness of hemiarthroplasty implants will decrease contact pressure and increase contact area, thus decrease wear. Lowering the stiffness was done using finite element analysis to lower the stiffness of the implant materials, and then to change the internal structure of the implants to increase their compliance. Structural changes produced no noticeable or favourable results, whereas material produced improved results for stiffnesses below a Young’s modulus of 300 MPa. A cadaveric study was done to compare a high, mid, and ultra-low stiffness material to the native state. It was found that only the ultra- low stiffness material improved contact mechanics. Future hemiarthroplasty implant designs should focus on using materials with a Young’s modulus below the aforementioned threshold.

Summary for Lay Audience

When bones fracture at the joint and cannot be put back together with screws or plates, or when the cartilage in a joint is damaged (causing pain), a joint replacement is often used to fix the problem. Hemiarthroplasty is the replacement of one side of a joint. In hemiarthroplasties, the native cartilage and the implant are in contact. They are often preferred for their lower invasiveness, and preservation of native tissue; however, their main drawback is that the native cartilage experiences accelerated wear. The cause of this is thought to be because the implants are much stiffer than the bone and cartilage of the opposing side. It is thought that lowering the stiffness of hemiarthroplasty implants will decrease contact pressure and increase contact area of the joint.

Lowering contact pressure and increasing contact area has been shown to decrease cartilage wear. Lowering the stiffness of an implant can be done by using a material that is less stiff or by changing the design of the implant so that it can deform more easily. This work studied both methods using computational analyses on computers called finite element analysis, followed by a cadaveric study. It was found that structural changes were not effective, but the material changes were effective once the stiffness of the material was reduced below a certain point. This is relevant because it gives guidance for future implant design and how to create hemiarthroplasty implants that will preserve the cartilage they articulate against.

Future hemiarthroplasty implants should be made of the lowest stiffness materials that are possible without compromising the integrity of the implant and its ability to perform.

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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