Modelling the mechanical behaviour of the interface between prosthesis and bone

Abstract

The integrity of the cement-bone interface is vital to the long term stability of cemented arthroplasty. Although the factors affecting the strength of the cement-bone interface are well documented the behaviour and load transfer across the interface at the trabecular level has been largely neglected. In addition, modelling of the cement-bone interface has mostly been limited to evaluation at the continuum level. In the following study, two modelling approaches have been developed for evaluation of the microstructural behaviour of the cement-bone interface. The first technique used a unit cell as a simplification of the morphology of cancellous bone. Using this method, variations in volume fraction resulting from changes in trabecular thickness and porosity size were shown to influence the resulting apparent stiffness. When cement was added to the unit cell, the stiffness became significantly greater with increasing cement penetration. The second approach used high resolution computed tomography (CT) images of the microstructure of the interface to create micro finite element (?FE) models of the interface. A cancellous bone analogue was selected and smooth surface models were created. It was shown that correlation of the volume segmented from CT images to the actual volume was vital for accurate calculation of the apparent level stiffness. The cancellous bone analogue material was then used to create analogue specimens representative of the cement-bone interface. Two non-destructive techniques, micro CT imaging and acoustic emission, were used to monitor damage evolution in the interfacial region, with the aim of validating finite element models of the interface. Initiation and progression of damage through the cement and foam was isolated and characterised by analysis of the associated AE parameters, and correlated well with the CT data. Therefore, the ability of AE as a passive tool to provide early indication of failure in situ was demonstrated. When the cement-bone analogue interface was loaded in bending, damage initiated at stress concentrations formed by irregularities in the aluminium geometry, recesses and notches formed by flow of cement into the aluminium. ?FE models of the cement-bone analogue specimens were created. Linear elastic models showed regions of high stress at the failure loci. ?FE models of specimens with differing degrees of cement interdigitation were created and it was demonstrated that the local load transfer across the interface was different for different penetration depths. The combined use of experimental and computational methods has enabled evaluation of the behaviour of the cement-bone interface at the microstructural level. Further development of the models and the use of more clinically representative loading conditions will enhance the understanding of the role of interface morphology, trabecular architecture and properties on the resulting interface strength. In addition, these methods may be combined with macroscopic scale models of prosthesis/bone constructs to evaluate factors such as stem design on the interface conditions.

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ORCID for Martin Browne: orcid.org/0000-0001-5184-050X

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