Year of Publication

2007

Document Type

Dissertation

College

Graduate School

Department

Biomedical Engineering

First Advisor

David Pienkowski

Abstract

Acrylic bone cement is widely used as a structural material in orthopaedics, dentistry, and orofacial surgery. Although bone cement celebrates four decades of success, it remains susceptible to fatigue fracture. This type of failure can directly lead to implant loosening, revision surgery, and increased healthcare expenditures. The mechanism of fatigue failure is divided into three stages: 1) fatigue crack initiation, 2) fatigue crack propagation, and 3) fast, brittle fracture. Adding reinforcing fibers and particles to bone cement is a proposed solution for improving fatigue performance. The mechanical performance of these reinforced bone cements is limited by fiber ductility, fibermatrix de-bonding, elevated viscosity, and mismatch of fiber size and scale of fatigue induced damage. In this dissertation, I report that adding small amounts (0% - 10% by weight) of multiwall carbon nanotubes (MWNTs) enhances the strength and fatigue performance of single phase bone cement. MWNTs (diameters of 10-9 10-8 m; lengths of 10-6 10-3 m) are a recently discovered nanomaterial with high surface area to volume ratios (conferring MWNT bone cement composites with large interfaces for stress transfer) that are capable of directly addressing sub-microscale, fatigue induced damage. MWNTs (2wt%) significantly increased the flexural strength of single phase bone cement by a modest 12%; whereas, similar additions of MWNTs dramatically enhanced fatigue performance by 340% and 592% in ambient and physiologically relevant conditions, respectively. Comparing the fatigue crack propagation behaviors of reinforced and unreinforced single phase bone cements revealed that the reinforcing mechanisms of MWNTs are strongly dependent on stress intensity factor, K, a numerical parameter that accounts for the combinatorial effect of the applied load and the crack size. As the crack grows the apparent stress at the crack tip intensified and the MWNTs lost their reinforcing capabilities. For that reason, it is likely that the predominant role of the MWNTs is to reinforce the bone cement matrix prior to crack initiation and during the early stages of crack propagation. Therefore, MWNTs are an excellent candidate for improving the clinical performance of bone cement, thereby improving implant longevity and reducing patient risk and healthcare costs.

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