Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Engineering Science

Program

Mechanical and Materials Engineering

Collaborative Specialization

Musculoskeletal Health Research

Supervisor

Johnson, James

Affiliation

St. Josephs Hospital - Hand and Upper Limb Center

2nd Supervisor

King, Graham

Affiliation

St. Josephs Hospital - Hand and Upper Limb Center

Co-Supervisor

Abstract

Hemiarthroplasty, where one side of a joint is replaced, is a minimally invasive procedure. It allows for the preservation of native tissue, though a significant ramification is accelerated cartilage wear when articulating with high stiffness materials that do not mimic the mechanical stiffness of the native tissue. An implant that employs a lattice design can significantly lower the stiffness of a solid structure whilst maintaining strength. This study was conducted to investigate the effect of implementing a porous internal lattice structure with a thin outer shell on the articular contact mechanics, using a radial head hemiarthroplasty. It was hypothesized that a porous internal lattice structure would reduce the effective stiffness of the implant, thus increasing hemiarthroplasty contact area and reducing contact stress relative to a solid implant. A BCC lattice was used to create the porous core of the implant surrounded by a 0.5 mm solid outer shell and was fabricated using polyamide PA2200 as the surrogate material. The lattice porosity of the radial head constructs was modified by increasing the size of the internal strut diameter (i.e., 0.4, 0.6, and 0.9 mm). A cadaveric study was performed to compare the contact mechanics of a native radial head, mid-modulus solid implant, and 65, 74, and 80%, porous implants under uniform compression over a 6-minute testing period. Contact area and stress were quantified using a Tekscan sensor interposed at the articulation. It was found that an internal lattice design can reduce articular stresses of a solid implant by approximately 40 – 65%. Future studies should further investigate the efficacy of a porous internal lattice structure using varying lattice designs, implant materials, and loading conditions to validate the effects on articular contact mechanics of hemiarthroplasty implants and ability to withstand physiologic loading conditions.

Summary for Lay Audience

Joint replacement is necessary when there is severe damage to the bones or cartilage. Hemiarthroplasty – where only one side of a joint is replaced – has become the preferred treatment method over total arthroplasty as it is cost-effective, requires less recovery time, and is minimally invasive preserving more natural tissue. A significant challenge with hemiarthroplasty is accelerated wear of cartilage when in contact with a foreign implant. Hemiarthroplasty implants are commonly made using materials much stiffer than bone or cartilage which is thought to be the reason for the rapid occurrence of cartilage damage. Lowering implant stiffness is thought to preserve cartilage health by increasing contact area and decreasing contact pressure. A lattice structure is a mesh-like structure comprised of solid beams or struts between which is empty space (pores). These structures have been shown to effectively reduce the stiffness of a solid structure while maintaining sufficient strength. Reducing the stiffness allows the implant to deform more easily and a lattice is especially advantageous in this regard as the struts can bend and absorb some of the applied force taking pressure off the cartilage on the opposing surface.

The cadaveric study presented explored changes in contact area and pressure found by using a structural lattice design – with varying strut sizes – as the internal core of a radial head hemiarthroplasty implant surrounded by a solid outer shell as compared to a solid implant and the native radial head. It was found that using a lattice structure as an implant core was effective in reducing contact stresses which could therefore reduce cartilage wear. Future studies should study the use of lattice structures in hemiarthroplasty implants by using alternative lattice designs, implant materials, and loading conditions to strengthen the validity of these findings and determine the ability to withstand a typical range of applied forces experienced during common daily activities.

Available for download on Sunday, December 31, 2023

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