Application of an anisotropic bone-remodelling model based on a damage-repair theory to the analysis of the proximal femur before and after total hipreplacement

In this work, a new model for internal anisotropic bone remodelling is appl ied to the study of the remodelling behaviour of the proximal femur before and after total hip replacement (THR). This model considers bone remodellin g under the scope of a general damage-repair theory following the principle s of continuum damage mechanics. A "damage-repair" tensor is defined in ter ms of the apparent density and Cowin's "fabric tensor", respectively, assoc iated with porosity and directionality of the trabeculae. The different ele ments of a thermodynamically consistent damage theory are established, incl uding resorption and apposition criteria, evolution law and rate of remodel ling. All of these elements were introduced and discussed in detail in a pr evious paper (Garcia, J. M., Martinez, M. A., Doblare, M., 2001. An anisotr ophic internal-external bone adaptation model based on a combination of CAO and continuum damage mechanics technologies. Computer Methods in Biomechan ics and Biomedical Engineering 4(4), 355-378.), including the definition of the proposed mechanical stimulus and the qualitative properties of the mod el, In this paper, the fundamentals of the proposed model are briefly revie wed and the computational aspects of its implementation are discussed. This model is then applied to the analysis of the remodelling behaviour of the intact femur obtaining densities and mass principal values and directions v ery close to the experimental data. The second application involved the pro ximal femoral extremity after THR and the inclusion of an Exeter prosthesis . As a result of the simulation process, some well-known features previousl y detected in medical clinics were recovered, such as the stress yielding e ffect in the proximal part of the implant or the enlargement of the cortica l layer at the distal part of the implant. With respect to the anisotropic properties, bone microstructure and local stiffness are known to tend to al ign with the stress principal directions. This experimental fact is mathema tically proved in the framework of this remodelling model and clearly shown in the results corresponding to the intact femur. After THR the degree of anisotropy decreases tending, specifically in the proximal femur, to a more isotropic behaviour. (C) 2001 Elsevier Science Ltd. All rights reserved.