ENGS 88 Honors Thesis (AB Students)

Degree Program

A.B.

Year of Graduation

2019

Faculty Advisor

Professor Douglas Van Citters

Document Type

Thesis (Senior Honors)

Publication Date

Spring 6-5-2019

Abstract

Oxidation of the Ultra-High Molecular Weight Polyethylene (UHMWPE) tibial inserts of total knee arthroplasty devices is a major factor underlying multiple modes of failure for these devices, including delamination, wear, and fracture. Previous research has demonstrated that oxidation of UHMWPE is driven by a high concentration of free radicals in the polyethylene. However, even new devices created with undetectable amounts of free radicals are oxidizing in vivo. One theory is that, in the absence of residual free radicals, oxidation is facilitated by absorbed species (e.g. lipids, ROS) delivered or exacerbated by contact stress. However, no method exists to comprehensively measure the oxidation or the dimensional change (a manifestation of articulation, load, and stress) of the articular surface of tibial inserts. In the present work, two methodologies were developed to meet these needs and proof-of-concept results were generated from surgically-retrieved tibial inserts.

To measure dimensional change, a coordinate measuring machine was used to probe the surfaces of “control” inserts (negligible in vivo duration and assumed unaltered dimensions) and “test” inserts (extended in vivo duration). The surfaces were then reconstructed and aligned using traditional quality control software to determine dimensional change in the test device as compared to the control. Oxidation measurement was computed by measuring multiple cross-section slices of tibial inserts with a Fourier Transform Infrared Spectroscopy Microscope to generate a number of rectangular oxidation scans along the articular surfaces. A 2D map of articular surface oxidation patterns was constructed via a MATLAB program by condensing each rectangle into an anteroposterior line of maximum oxidation values.

Proof-of-concept results included dimensional change plots with values similar to those measured using a traditional analogue metrology technique. Oxidation patterns appeared to be highest in regions of greatest dimensional change, but the small sample size precluded strong conclusions. This work represents the first set of computer programs and approaches for quantitatively assessing tibial insert dimensional change throughout the full xy plane and for mapping insert oxidation in multiple dimensions. As the proof-of-concept results demonstrate, such tools can inform future research into the relationship between stress and oxidation in tibial inserts.

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