Elsevier

Bone

Volume 45, Issue 2, August 2009, Pages 226-231
Bone

Prediction of proximal femur strength using a CT-based nonlinear finite element method: Differences in predicted fracture load and site with changing load and boundary conditions

https://doi.org/10.1016/j.bone.2009.04.241Get rights and content

Abstract

The annual occurrence of hip fracture due to osteoporosis as of 2002 had reached 120,000 in Japan. The increase has been very rapid. From a biomechanical perspective, hip fractures are thought to be caused in real settings by different directions of loading. Thus, clarification of the loading directions under which the proximal femur is most vulnerable to fracture would be helpful for elucidating fracture mechanics and establishing preventive interventions. The purpose of the current study was to clarify the influence of loading direction on strength and fracture site of the proximal femur using the CT-based nonlinear FE method to determine loading directions under which the proximal femur is most vulnerable to fracture. Contralateral femora were analyzed in 42 women with hip fracture (mean age, 82.4 years), comprising 20 neck fractures and 22 trochanteric fractures. Within 1 week after fracture, quantitative CT of the contralateral femur was performed in each patient and 3-dimensional FE models were created. One stance loading configuration (SC) and four different fall loading configurations (FC) were assigned. Nonlinear FE analysis was performed. Differences in fracture loads depending on differences in loading direction were analyzed and correlations among fracture loads in different loading directions were assessed. Next, fracture sites were also analyzed. Mean predicted fracture load in the SC was 3150 N. Mean fracture loads were 2270 N in FC1, 1060 N in FC2, 980 N in FC3, and 710 N in FC4. The correlation between predicted fracture loads in SC and those in each FC was significant with a correlation coefficient of 0.467–0.631. Predicted fracture sites in the SC appeared at the subcapital region in all patients and were categorized as neck fracture. However, trochanteric fractures occurred in all fall configurations except FC1. In FC1, a significant correlation was seen between real fracture type and predicted type. The current investigation could contribute to the acquisition of useful knowledge allowing the establishment of more efficacious means of preventing hip fractures.

Introduction

The annual occurrence of hip fracture due to osteoporosis as of 2002 had reached 120,000 in Japan. In Japan, the increase has been very rapid, with a 1.7-fold increase from 1987 to 1992 and a 2.2-fold increase from 1987 to 2002 [1]. More than 90% of fractures were reportedly caused by falls from standing height [2], [3]. However, some cases display no clear evidence of falls or trauma [3], [4]. From a biomechanical perspective, hip fractures are thought to be caused in real settings by different directions of loading. Thus, clarification of the loading directions under which the proximal femur is most vulnerable to fracture would be helpful for elucidating fracture mechanics and establishing preventive interventions.

Pinilla et al. [5] and Fujii [6] investigated the influence of loading direction on fracture load of the proximal femur. The results of these studies were derived by conducting compression testing of proximal femoral specimens obtained from excised cadaveric femora. One limitation of these studies was that only one load direction could be tested on one specimen and no other direction could be tested using the same specimen. To address this limitation, Ford et al. [7] and Keyak et al. [8] reported simulation studies on the influence of load direction using a computed tomography (CT)-based finite element (FE) method. However, those studies investigated strength of the excised femora and the analytical method was limited to a linear FE method. In addition, none of these studies examined fracture sites in detail. We have established our own CT-based nonlinear FE method to accurately predict fracture load and site on the proximal femur [9].

The purpose of the current study was to clarify the influence of loading direction on strength and fracture site of the proximal femur using the CT-based nonlinear FE method to determine loading directions under which the proximal femur is most vulnerable to fracture. In this study, validity of the FE method was also evaluated by analyzing strength and fracture site of the contralateral femur in patients with hip fracture, through which we examined whether our FE method could create fracture in the contralateral femur identical to the real fracture in the patient.

Section snippets

Patients and methods

Contralateral femora were analyzed in 42 women with hip fracture (mean age, 82.4 years; range, 70–92 years; mean height: 146 cm; mean weight: 44 kg), comprising 20 neck fractures (mean age, 81.9 years; mean height: 147 cm; mean weight: 45 kg) and 22 trochanteric fractures (mean age, 82.9 years; mean height: 146 cm; mean weight: 44 kg). No significant differences were seen in mean age (p = 0.60), height (p = 0.68) or weight (p = 0.72). Fifty-three female patients with hip fracture were admitted to the

Results

Mean (± standard deviation (SD)) predicted fracture load in the SC was 3150 ± 611 N. Mean fracture loads were 2270 ± 600 N in FC1, 1060 ± 248 N in FC2, 980 ± 229 N in FC3, and 710 ± 174 N in FC4 (Fig. 4). Mean predicted fracture load was significantly larger than in the SC than in all FCs except FC1 (p < 0.001). To compare mean fracture loads among FCs, the load in FC1 was significantly larger than those of FC2, FC3 and FC4 (p < 0.01, p < 0.001, p < 0.001, respectively). Load was significantly larger in FC2 than

Discussion

From the results of our study, the predicted strength under the loading condition simulating a fall in the posterolateral direction was smaller than that by a fall in the lateral direction. These results were consistent with those of a previous study that conducted mechanical testing using cadaveric specimens [6] and predicted strength using a CT/FEM [7], [8].

The magnitude of strength of the proximal femur under a certain loading condition differs depending on the bone density distribution and

Acknowledgments

This work was funded by a grant in aid for Scientific Research received from the Japan Society for the Promotion of Science (14657356).

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