Elsevier

Journal of Biomechanics

Volume 32, Issue 11, November 1999, Pages 1251-1254
Journal of Biomechanics

Brief communication
Mechanical strength of the cement–bone interface is greater in shear than in tension

https://doi.org/10.1016/S0021-9290(99)00107-4Get rights and content

Abstract

The objective of this study was to determine the relative mechanical properties of the cement–bone interface due to tensile or shear loading. Mechanical tests were performed on cement–bone specimens in tensile (n=51) or shear (n=55) test jigs under the displacement control at 1 mm/min until complete failure. Before testing, the quantity of bone interdigitated with the cement was determined and served as a covariate in the study. The apparent strength of the cement–bone interface was significantly higher (p<0.0001) for the interface when loaded in shear (2.25 MPa) when compared to tensile loading (1.35 MPa). Significantly higher energies to failure (p<0.0001) and displacement before failure (p<0.01) were also determined for the shear specimens. The post-yield softening response was not different for the two test directions. The data obtained herein suggests that cement–bone interfaces with equal amounts of tensile and shear stress would be more likely to fail under tensile loading.

Introduction

Excessive shear stresses at the cement–bone interface have been cited as a mechanism by which the cement column subsides within the proximal femur (Ling, 1992; Miles et al., 1990). However, finite element analysis of cemented femoral hip components has shown that substantial tensile stresses can develop across the cement–bone interface (Chang et al., 1998). This suggests that it could be possible to initiate failure of the interface with tensile loading. To determine the mechanism of interfacial failure, be it due to tensile, shear, or a combination of tensile or shear loading requires determination of the mechanical properties of the cement–bone interface under both tensile and shear loading.

A direct comparison between the tensile and shear strength properties of the cement–bone interface is difficult due to confounding variables including the amount of trabecular bone left in the femoral canal after broaching, quality of the bone in the canal and infiltration of cement into the trabecular spaces. An approach has recently been developed to quantify these variables and includes the use of quantitative computed tomography of the bone and direct measurement of the amount of interdigitation between the cement and bone (Mann et al., 1997a). These quantities can then be used to develop a relationship between the strength of the interface and the amount of bone interdigitated with cement.

The purpose of this study was to determine if the cement–bone interface would be more likely to fail under tensile or shear loading. Mechanical test results from a previously reported tensile test study (Mann et al., 1997a) are compared to new data from shear tests.

Section snippets

Materials and methods

Cement–bone test specimens were fabricated from six fresh-frozen human proximal femurs (mean age 58, range 25–77) that had been broached, cleaned with a saline lavage, and cemented using contemporary cementing techniques. Institutional Review Board approval at the University of Alabama at Birmingham was received for the use of human cadaveric material. Details of the specimen preparation technique are presented elsewhere (Mann et al., 1997a). Quantitative CT scan (mg/cc K2HPO4) density (Lotz et

Results

The load versus displacement response for the cement–bone interface was similar in nature for the tensile and shear loading conditions (Fig. 2). Both experienced a linear elastic response followed by yielding and an exponential type decay to complete failure. There was a large amount of variability in the mechanical response for both tensile and shear loading conditions (Table 1).

The apparent strength was 66% larger (p<0.0001) for loading in shear when compared to loading in tension. Failure

Discussion

The results from this study showed that the shear strength of the cement–bone interface is significantly greater than the tensile strength. In addition, the total energy required to cause complete failure in shear loading was significantly greater than needed for tensile loading. The results also showed that the amount of bone interdigitated with the cement could be used to explain at least 50% of the variability in the strength of the interface, based on the regression results.

The method used

Acknowledgements

This research was supported by a grant from the National Institutes of Health (AR 42017).

References (8)

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    Tensile strength of the cement–bone interface depends on the amount of bone interdigitated with PMMA cement

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  • P.B. Chang et al.

    The effects of proximal bonding, stem geometry, and neck length on cemented femoral stem performance

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  • R.S.M. Ling

    The use of a collar and precoating on cemented femoral stems is unnecessary and detrimental

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  • J.C. Lotz et al.

    Mechanical properties of the trabecular bone from the proximal femur: a quantitative CT study

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There are more references available in the full text version of this article.

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