Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Biomedical Engineering

Supervisor

Dr. Cynthia E. Dunning

Abstract

Mechanical loosening is a common mode of joint replacement failure. For cemented implants, loosening at the implant-cement interface may be affected by stem surface design. Altering the surface topography facilitates the infiltration of bone cement onto the stem, creating a mechanical interlock, improving interface stability. However, few in-vitro studies have investigated this. Therefore, the purpose of this thesis was to investigate the effect of stem surface treatments and substrate materials on stem-cement interface stability in-vitro.

Four separate studies were performed to assess the stability of various stem surface treatments, with two substrate materials, under three loading modes. Titanium and cobalt chrome implant stems were custom machined and treated with one of four surfaces: smooth, sintered beads, plasma spray, and circumferential grooves. Sintered bead and plasma sprayed stems were tested in independent torsion, compression and bending; circumferential groove designs were compared in torsion and then compression. All stems were potted in aluminum tubes using PMMA, and loaded cyclically using a materials testing machine. A custom optical tracking system (resolution under 5 μm) was validated for use, and subsequently employed to measure stem-cement interface motion during loading. Overall, results showed surface treatments improved stability, but this was affected by substrate material. Across all loading modes, beaded treatments applied to titanium stems, and plasma spray treatments applied to cobalt chrome stems, improved interface stability and strength when large surface treatment areas were employed. Additionally, the machining of circumferential grooves onto the stem surface improved interface strength in compression, with no influence in torsion.

A final study was performed using μ-CT imaging to observe stem and cement motion under bending loads. A custom-built loading device applied static loads to smooth titanium stems, while acquiring CT images of the stem-cement interface. Interface motion was quantified by comparing scans before and after the stem underwent cyclic loading. Results indicated the stem and the surrounding cement had displaced following loading, yet the stems remained relatively stable.

These studies offer valuable information regarding the effect of stem surface treatments on stem-cement interface mechanics under various loading modes and will be used in the development of future implant systems.


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