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TECHNICAL PAPERS: Bone/Orthopedic

On the Importance of Considering Porosity When Simulating the Fatigue of Bone Cement

[+] Author and Article Information
Jonathan R. Jeffers, Martin Browne, Anne Roques

Bioengineering Sciences Research Group, School of Engineering Sciences,  University of Southampton, Southampton SO17 1BJ, United Kingdom

Mark Taylor1

Bioengineering Sciences Research Group, School of Engineering Sciences,  University of Southampton, Southampton SO17 1BJ, United Kingdomm.taylor@soton.ac.uk

1

To whom correspondence should be addressed.

J Biomech Eng 127(4), 563-570 (Jan 26, 2005) (8 pages) doi:10.1115/1.1934182 History: Received June 18, 2004; Revised January 26, 2005

Fatigue cracking in the cement mantle of total hip replacement has been identified as a possible cause of implant loosening. Retrieval studies and in vitro tests have found porosity in the cement may facilitate fatigue cracking of the mantle. The fatigue process has been simulated computationally using a finite element/continuum damage mechanics (FE/CDM) method and used as a preclinical testing tool, but has not considered the effects of porosity. In this study, experimental tensile and four-point bend fatigue tests were performed. The tensile fatigue S-N data were used to drive the computational simulation (FE/CDM) of fatigue in finite element models of the tensile and four-point bend specimens. Porosity was simulated in the finite element models according to the theory of elasticity and using Monte Carlo methods. The computational fatigue simulations generated variability in the fatigue life at any given stress level, due to each model having a unique porosity distribution. The fracture site also varied between specimens. Experimental validation was achieved for four-point bend loading, but only when porosity was included. This demonstrates that the computational simulation of fatigue, driven by uniaxial S-N data can be used to simulate nonuniaxial loadcases. Further simulations of bone cement fatigue should include porosity to better represent the realities of experimental models.

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Copyright © 2005 by American Society of Mechanical Engineers
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Figures

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Figure 1

Tensile test specimen, with a thickness of 3.5mm and a constant gauge section of 3.5mm×12mm×12mm

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Figure 2

Stress vs number of cycles to failure for uniaxial tension (a) without porosity and (b) with porosity simulated in the computational data. Regression lines are coincident and plotted through average fatigue life at each stress level

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Figure 3

The relationship between uniformly distributed random numbers (U) and those with a lognormal distribution (X)

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Figure 4

The number of pores simulated in the tensile and four-point bend FE models. The sample mean and standard deviation are given

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Figure 5

Stress vs number of cycles to failure for four-point bend loading (a) without porosity and (b) with porosity simulated in the computational data. Regression lines plotted through average fatigue life at each stress level

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Figure 6

Damage in three different four-point bend finite element models (with porosity) after failure (all tested at 35MPa). The damage accumulation failure scenario is apparent, with damage (dark regions) occurring at sites other than that of eventual failure. Element edges are not shown for the sake of clarity

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Figure 7

Experimental and computational probability of survival to one million load cycles (Weibull life) as a function of stress for the four-point bend test

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Figure 8

Damage in three different tensile test FE/CDM models (with porosity) after failure (all tested at 20MPa). The number of cycles to failure and fracture site is different for each model. Element edges are not shown for the sake of clarity

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