Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Engineering Science

Program

Biomedical Engineering

Supervisor

Willing, Ryan T.

Abstract

The incidence of total shoulder arthroplasty procedures (TSA) to treat osteoarthritis has experienced the most rapid growth among all human joint replacements. However, stress shielding of proximal bone following its reconstruction is a complication of TSA triggering unfavorable adaptive bone remodeling, especially for osteoporotic patients.

A better understanding of how the shape and density of the shoulder vary among members of a population can help design more effective population-based orthopedic implants. Therefore, finite element models representing healthy, osteopenic, and osteoporotic bone qualities in a population were developed using our statistical shape and density model. Bones were reconstructed with hollow- and solid-stemmed implants and resulting changes in bone stresses were calculated. We concluded that the use of more compliant stems, such as hollow stems, could marginally mitigate the effect of stress shielding at the proximal humerus. Further increasing the compliance of stems by making them porous could improve bone-implant mechanics.

Summary for Lay Audience

Osteoarthritis of the shoulder is a joint disease that can result in severe pain and stiffness. Total shoulder arthroplasty (TSA) is a clinically successful surgery to relieve pain and restore the natural range of motion to the arthritic shoulder joint. The number of patients who undergo TSA has experienced rapid growth, more than any other joint in the human body over the past decades and is continuing to grow. A seven-fold increase in its incidence is predicted for the next decade. During TSA, an implant is inserted into the humerus bone, the bone of the upper arm, to reconstruct the shoulder joint. However, due to altered loading transmission following implantation, the proximal (near the upper end) humerus will be shielded from experiencing stress. Bone is a self-optimizing structure, which means that it adapts its structure according to the exerted loads. Therefore, the reduction of stress in the proximal humerus can lead to bone loss, implant loosening and, finally, a need for revision surgery.

Humeral implants are comprised of two sections, namely, the stem component and the head component. The design of humeral implants and specifically their stem component has a significant influence on the overall implant success, as the stem is responsible for load transfer from the head component of the implant to the surrounding bone.

We found that the use of more compliant shoulder implants with hollow stems could marginally mitigate the effect of stress shielding and consequently reduce the need for revision surgeries of the shoulder. Also, an exacerbation of stress shielding was found for patients suffering from osteoporosis, a bone disease in which deterioration of bone tissue occurs. Our study suggests that further increasing the compliance of implant stems by making them also porous could increase the bone stresses at the proximal humerus and, therefore, further limit the stress shielding.

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