Date of Award

5-2015

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Bioengineering

Advisor

Melinda Harman, Ph.D.

Committee Member

Robert Latour, Ph.D.

Committee Member

John DesJardins, Ph.D.

Abstract

For people with chronic hip pain due to arthritis or other conditions, a total hip replacement (THR) is a common procedure used to eliminate the pain. Due to the natural variation in patient anatomy, THR prostheses are becoming increasingly more modular to allow for a more customized initial replacement and for an easier revision if needed in the future. Currently, THR prostheses routinely have modular femoral heads to provide surgeons with intraoperative flexibility. This modularity allows the surgeons to maintain the proper leg length and restore hip biomechanics for the patient and insert the components more easily and precisely. Modular femoral heads attach to the neck cone on the femoral stem using a bore-cone taper junction. Different bore depths into the head alter the position of the head center relative to the end of the neck cone, effectively providing the surgeon a means of altering the head-neck length during surgery. This affects the moment arm at the bore-cone modular junction since the joint reaction force at the hip passes through the center of the femoral head. There are concerns that variations in this head-neck moment arm and other neck cone geometries can negatively impact the stability and surface of the modular bore-cone taper junction, leading to corrosion and other modes of surface damage.

The broad objective of this thesis was to use explanted THR prostheses and basic mathematical modeling to understand how variations in component geometry, specifically the head-neck moment arm (HMA) and neck cone geometry, may impact bore-cone taper junctions. The objective was accomplished by 1) characterizing surface damage on modular bore-cone tapers of explanted THR prostheses; 2) correlating surface damage with stem design features and patient factors, and 3) characterizing the stress distribution in different neck designs with varying taper geometry.

It was hypothesized that explanted THR prostheses would exhibit surface damage on the modular bore-cone tapers, that surface damage would vary with design type and taper geometry (specifically HMA and neck cone contact surface area), and that changing the taper geometry would affect the stress distribution in the neck.

The results from this thesis establish that explanted THR prostheses exhibit surface damage on modular bore-cone tapers, and that the corrosion was significantly correlated with neck diameter, neck stiffness, and patient body weight. HMA and the overall neck cone contact surface area did not have a significant correlation with corrosion. Changing neck cone geometry and HMA did not change the stress distribution in the neck in the simple computational models, but these changes did impact the magnitude of the stresses in the neck. There is still more information to learn about damage mechanisms existing at bore-cone modular junctions, but this thesis confirmed that severe corrosion can occur in modular bore-cone tapers during in vivo function. There remains a need to find the optimal design parameters for HMA and taper geometry in order to maintain the clinical benefits of varied neck lengths but minimize the potential design failures.

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