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Research Papers

Activity and Loading Influence the Predicted Bone Remodeling Around Cemented Hip Replacements

[+] Author and Article Information
Alexander S. Dickinson

Bioengineering Science Research Group,
University of Southampton,
Highfield,
Southampton SO17 1BJ, UK
e-mail: alex.dickinson@soton.ac.uk

Contributed by the Bioengineering Division of ASME for publication in the JOURNAL OF BIOMECHANICAL ENGINEERING. Manuscript received July 24, 2013; final manuscript received December 10, 2013; accepted manuscript posted December 16, 2013; published online March 24, 2014. Assoc. Editor: Tammy Donahue.

J Biomech Eng 136(4), 041008 (Mar 24, 2014) (10 pages) Paper No: BIO-13-1328; doi: 10.1115/1.4026256 History: Received July 24, 2013; Revised December 10, 2013; Accepted December 16, 2013

Periprosthetic bone remodeling is frequently observed after total hip replacement. Reduced bone density increases the implant and bone fracture risk, and a gross loss of bone density challenges fixation in subsequent revision surgery. Computational approaches allow bone remodeling to be predicted in agreement with the general clinical observations of proximal resorption and distal hypertrophy. However, these models do not reproduce other clinically observed bone density trends, including faster stabilizing mid-stem density losses, and loss-recovery trends around the distal stem. These may resemble trends in postoperative joint loading and activity, during recovery and rehabilitation, but the established remodeling prediction approach is often used with identical pre- and postoperative load and activity assumptions. Therefore, this study aimed to evaluate the influence of pre- to postoperative changes in activity and loading upon the predicted progression of remodeling. A strain-adaptive finite element model of a femur implanted with a cemented Charnley stem was generated, to predict 60 months of periprosthetic remodeling. A control set of model input data assumed identical pre- and postoperative loading and activity, and was compared to the results obtained from another set of inputs with three varying activity and load profiles. These represented activity changes during rehabilitation for weak, intermediate and strong recoveries, and pre- to postoperative joint force changes due to hip center translation and the use of walking aids. Predicted temporal bone density change trends were analyzed, and absolute bone density changes and the time to homeostasis were inspected, alongside virtual X-rays. The predicted periprosthetic bone density changes obtained using modified loading inputs demonstrated closer agreement with clinical measurements than the control. The modified inputs also predicted the clinically observed temporal density change trends, but still under-estimated density loss during the first three postoperative months. This suggests that other mechanobiological factors have an influence, including the repair of surgical micro-fractures, thermal damage and vascular interruption. This study demonstrates the importance of accounting for pre- to postoperative changes in joint loading and patient activity when predicting periprosthetic bone remodeling. The study's main weakness is the use of an individual patient model; computational expense is a limitation of all previously reported iterative remodeling analysis studies. However, this model showed sufficient computational efficiency for application in probabilistic analysis, and is an easily implemented modification of a well-established technique.

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Figures

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Fig. 3

Example profiles of the activity and load level adjustment factors Fact and Fload, used to scale the pre- and postoperative remodeling signals

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Fig. 2

Gruen zones (GZ) defined for virtual DXA scan, in terms of the stem shoulder-tip length (L) (a), and Sections defined for internal inspection of BMD

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Fig. 1

FE meshes and loads for intact (a) and implanted (b) models

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Fig. 8

Tornado plot of model sensitivity to activity, time and load parameter perturbations upon each Gruen zone's %BMD change 60 months postoperatively

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Fig. 4

Predicted percentage change in BMD per Gruen zone for the four control loading scenarios of varying threshold remodeling stimulus (Cases 1–4), shown in comparison to representative clinical results. Shaded region represents 95% confidence interval [3].

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Fig. 5

Virtual anterior-posterior X-rays showing changes in BMD throughout the bone for the four control loading scenarios with varying threshold remodeling stimulus (Cases 1–4). Shown immediately postoperatively, and after 24 and 60 months

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Fig. 6

Bone Mineral Density shown on bone sections for the four control loading scenarios with varying threshold remodeling stimulus (Cases 1–4). Shown immediately postoperatively, and after 24 and 60 months. Black represents 0 g/cc, and white represents greater than 1.75 g/cc.

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Fig. 7

Predicted percentage change in BMD per Gruen zone for the control and three activity and load profile scenarios (Cases 1, 5, 6, and 7), with 75% threshold remodeling stimulus, shown in comparison to representative clinical results. Shaded region represents 95% confidence interval [3].

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