Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Engineering Science

Program

Mechanical and Materials Engineering

Collaborative Specialization

Musculoskeletal Health Research

Supervisor

Langohr, Daniel G.

Abstract

Uncemented humeral stems cause stress shielding which result in bone resorption when used in total shoulder arthroplasty (TSA). Shorter length stems show a decrease in stress shielding, however the effect of humeral short stem positioning and humeral head contact and positioning on bone stress is currently not known, hence CT-based tools and Finite Element (FE) methods are used to quantify the effects of the mentioned variables on bone stresses after TSA.

Eight male cadaveric humeri were virtually constructed from computed tomography (CT) data, with a generic short humeral short stem as the implant.

The results of this work show that central stem positioning is preferred, and that for humeral head contact, full contact with the humeral resection results in the smallest changes in bone stress and bone volume with resorption potential. For humeral head position, tradeoffs in terms of cortical and trabecular bone loading were observed for both positions investigated.

Summary for Lay Audience

Osteoarthritis of the shoulder joint can lead to severe pain, discomfort and a reduced range of motion. Total shoulder arthroplasty (TSA) is an effective treatment to alleviate pain and restore shoulder function in the case of shoulder arthritis. Number of cases patients who go through TSA has significantly increased in the recent years. In the TSA surgery, the surface known as the “articular surface” is removed. This surface is the in-between surface in the shoulder joint, which is a ball-and-socket type of joint. After the removal of the articular surface, the canal of the humerus bone, the long bone of the upper arm, is prepared to accept the stem of the implant. Bone is a dynamic structure, meaning that it is adapts itself according to loads exerted on it. After TSA, when bone is replaced by a metallic implant, the stress distribution changes at the upper end of the humerus (proximal humerus). This means that some parts of the proximal humerus are shielded from stress, and hence do not experience the same loads as before the TSA. As time goes by these locations are prone to bone loss, a phenomenon known as bone resorption, which can lead to implant loosening, which requires revision surgery.

Humeral implants consist of two parts, the head and the stem. Since both components are responsible to transfer the load from the head to the rest of the bone; therefore, their positioning and contact with the resection plane are important factors.

For the position of the stem and the head, we looked for locations where the stress pattern of the implanted shoulder and healthy shoulder are as similar as possible, and bone loss is minimized. We fo1und that for stem position, the optimal positioning is the it is when its axis is aligned with the axis of the bone shaft and head contact is best when there is full contact between the resected plane and the backside of the head. We also studied the optimal position for the head and found no significance difference for when head is situated closer to the body or away.

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