Prediction of micromotion initiation of an implanted femur under physiological loads and constraints using the finite element method U Andreaus* and M Colloca Department of Structural and Geotechnical Engineering, ‘Sapienza’ University of Rome, Rome, Italy The manuscript was received on 10 December 2008 and was accepted after revision for publication on 27 March 2009. DOI: 10.1243/09544119JEIM559 Abstract: In cementless total hip replacement surgery the conditions for micromotion initiation at the bone–stem interface and the role of stair climbing versus gait in promoting incipient slipping deserve attention. The goal of the present paper was to propose a finite element approach for analysing the structural behaviour of hip joint prostheses under physiological loadings and boundary conditions, which allows the prediction of micromotion initiation with low computational effort. In this paper, three-dimensional (3D) finite element analyses were performed of intact and implanted human femurs in order to address the above-mentioned problems. Accurate finite element models based on computed tomography images of a human femur were employed; tetrahedral elements were used to construct the models and the contact options of a full bond between the femoral bone and stem were also used. The shear strains at the contact between femoral bone and stem were evaluated. Two loading cases, namely walking and stair climbing, were applied to investigate the effect of different loading conditions on the shear strain patterns. Shear strains in the z direction can be reasonably considered a significant stimulus of slip initiation or fibrous tissue formation or both at the bone–stem interface, whereas shear strains in the x–y plane can be assumed to be a sensible measurement of the tendency to implant– bone micromotion under torsional loads. Comparisons with other studies are complicated by the difference in the methods and testing conditions used. If mobilization is to be initiated, rotational displacements at the interface should be sensible and significant parameters, i.e. the material, should be distorted to some extent. Thus, for a particular point on the bone–metal interface, the maximum shear strain in any direction within the interface plane will indicate the likelihood of slippage initiation at that point. The different femur states (intact and implanted) and loading conditions (walking and stair climbing) are compared. The stair-climbing loads resulted in the highest strains observed under any conditions, either intact or implanted. Keywords: implanted human femur, 3D finite element analysis, physiological loads, physiological constraints, micromotion 1 INTRODUCTION 1.1 Survey of the literature In total hip replacement, the damaged hip joint is replaced with a prosthesis; this procedure allows the patient to return to his or her normal daily activities and often provides relief from pain [1, 2]. Primary stability of femoral components has been estab- lished as a determinant of the clinical success of cementless total hip endoprostheses [3]. Sufficient initial mechanical stability at the bone prosthesis interface, with regard to post-operative partial weight bearing and rehabilitation, is a prerequisite for extensive biological integration of the stem and its long-term performance, the so-called secondary *Corresponding author: Facolta ` di Ingegneria, Sapienza Univer- sita ` di Roma, Via Eudossiana, 18, Roma, Lazio, 18 - 00184, Italy. email: ugo.andreaus@uniroma1.it 589 JEIM559 F IMechE 2009 Proc. IMechE Vol. 223 Part H: J. Engineering in Medicine