The effect of muscle loading on the simulation of bone remodelling in the proximal femur
Introduction
Bone resorption around hip implants is a common and well-documented reaction to total hip replacement operations. Numerous clinical studies based on Roentgenographic observations (e.g. Engh et al., 1993) or more accurate bone densitometry measurements (Smart et al., 1996, Kiratli et al (1992), Kiratli et al. (1996)) which examined and quantified the loss of bone stock have been published.
The phenomenon is usually explained as an adaptive remodelling response of bone tissue to a significant alteration of its stress environment (Huiskes, 1995): postoperatively the implant carries part of the load originally carried by bone alone and so the bone is ‘stress shielded’. Although metabolic or biochemical factors might have an effect on the process, bone remodelling is thought to be a predominantly mechanically regulated phenomenon (Frost, 2001).
Stress shielding is recognised as a factor that may limit the longevity of the joint reconstruction, since it reduces the support of the implant and therefore increases the risk of implant loosening (Huiskes, 1993). The reduced bone stock also undermines the probability of success of a possible revision surgery.
A considerable amount of research has been directed towards the development of theoretical models that describe and predict bone resorption patterns around implants (Huiskes et al., 1987, Levenston et al.,1992, Prendergast and Taylor, 1992; Weinans et al (1992), Weinans et al. (1994); van Rietbergen et al., 1993, Kerner et al., 1999). These models rely on some numerical method (usually finite element (FE) analysis) for calculating the spatial distribution of the remodelling stimulus. The majority of the published FE analyses of the proximal femur have used a simplified set of muscle and joint contact loads to represent the boundary conditions of the model, usually consisting of the hip joint contact force and the abductor muscle forces acting on the greater trochanter (e.g. Mann et al., 1997, Prendergast and Taylor, 1992; Huiskes and van Rietbergen, 1995).
However, the amount of data regarding muscle loading of the femur has increased considerably and consequently a more realistic representation of these forces in FE analysis is now possible (Duda et al., 1998; Stolk et al., 2001).
It is reasonable to expect that the application of different boundary conditions to the FE model will have an effect on the calculated distribution of the remodelling signal. Indeed, a recent study (Duda et al., 1998) showed significant differences in the femoral strain distribution as calculated from a complete and reduced sets of muscle loading applied to the same FE model.
It is not clear, however, how these different loading configurations can affect the outcome of the remodelling simulation. It has been argued (Weinans, 1991) that it is not important to represent all of the muscle actions, because the remodelling stimulus arises from the change in bone strains caused by the arthroplasty, rather than the actual local values.
The goal of this parametric study is to determine how different sets of boundary conditions affect the amount of bone resorption/deposition predicted in the proximal femur, following a hip replacement operation.
Section snippets
Materials and methods
For the purpose of this analysis, two finite element models were constructed, one representing the intact bone and the other the post-operative treated bone with the implant. The anatomy reconstructions were based on the male subject CT dataset of the Visible Human Project (VHP) of the National Institute of Health (NIH 1996). The dataset provides CT slices at 3 mm intervals with an in-plane resolution of 0.89 mm. In addition to the CT scans, a set of cryosection colour photographs of the same
Results
The course of periprosthetic bone remodelling over time can be viewed in a series of bar charts, which show the predicted change in bone mineral content (BMC) in several scanning windows for six remodelling increments and for each loading configuration applied (Fig. 4). Different bar charts are given for the lateral and medial sides of the femur and for each side three locations are examined: proximal, mid-stem and distal. The BMC values for each of these locations were calculated as the
Discussion
The objective of this study was to investigate the differences in the predicted periprosthetic adaptive response of bone caused by muscle loading configurations of varied realism applied to the same FE model. For this purpose two FE models (pre and post-operative) of the same bone were constructed, and strain adaptive remodelling theory was used to simulate the change of BMD over time around the prosthesis. Bone densitometry measurements were simulated and the BMC changes over time in 18
Acknowledgements
C. Bitsakos and J. Kerner were supported by the Arthritis Research Campaign, of Chesterfield, UK. I. Fisher was funded by Johnson and Johnson Orthopaedics Co. We also thank Prof. R. Huiskes and his colleagues at Nijmegen University for their generous help for Jan Kerner, whose visit was sponsored by the British Orthopaedics Research Society.
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