Elsevier

Journal of Biomechanics

Volume 45, Issue 11, 26 July 2012, Pages 1959-1964
Journal of Biomechanics

Fracture risk in the femoral hip region: A finite element analysis supported experimental approach

https://doi.org/10.1016/j.jbiomech.2012.05.011Get rights and content

Abstract

The decrease of bone mineral density (BMD) is a multifactorial bone pathology, commonly referred to as osteoporosis. The subsequent decline of the bone's micro-structural characteristics renders the human skeletal system, and especially the hip, susceptible to fragility fractures. This study represents a systematic attempt to correlate BMD spectrums to the mechanical strength characteristics of the femoral neck and determine a fracture risk indicator based on non-invasive imaging techniques. The BMD of 30 patients' femurs was measured in vivo by Dual-energy X-ray absorptiometry (DXA). As these patients were subjected to total hip replacement, the mechanical strength properties of their femurs' were determined ex-vivo using uniaxial compression experiments. FEA simulations facilitated the correlation of the DXA measurements to the apparent fracture risk, indicating critical strain values during complex loading scenarios.

Introduction

Osteoporosis is a multifactorial bone disease concerning roughly 4% of the human population (Melton et al., 1992). As an asymptomatic condition, osteoporosis fails to exhibit noticeable symptoms, particularly at early stages and thus is usually underdiagnosed. Untreated however, this clinically silent disease is likely to increase the risk of fragility fractures (Ettinger, 2008, Rockwood et al., 1990, Cooper et al., 1992). Due to its high morbidity and global nature, osteoporosis is considered a pathology with a significant socioeconomic impact (Ray et al., 1997).

The affected patients' bone mineral density (BMD) is drastically reduced, deteriorating the bones' micostructural characteristics as a result of excessive bone resorption followed by insufficient bone formation during remodelling (Frost and Thomas, 1963, Raisz, 2005). The pathogenesis has been associated to dietary aspects (Hackett et al., 2009), immobilisation (Minaire, 1989), hyper-parathyroidism (Dupree and Dobs, 2004), vitamin D deficiency (Holick, 2004), alteration of biochemical markers like hormones (Parfitt et al., 1995, Black et al., 2003) and aging (Newton-John and Morgan, 1970). Regardless of aetiology, decreased bone mineral density renders the skeletal system susceptible to fracture, predominantly occurring at the hip (Bohr and Schaadt, 1985), the vertebral column (Old and Calvert, 2004) and wrist (Dempster, 2011).

According to the World Health Organisation, osteopenia and osteoporosis are defined by the patient's bone mass deviation, when compared to that of an average, young and healthy adult (World Health Organisation, 1994) when measured by DXA.

Even though DXA can accurately determine the minerals and lean soft tissue of the examined area, the overall accuracy of the measurement is impaired by the subtraction of the indirectly calculated fat mass (St-Onge et al., 2004). Furthermore, DXA results are represented as mass per area, thus not considering the anisotropy of the bone tissue and are hence a qualitative and not quantitative index of the bone structure (Lochmuller et al., 2000).

Several other methods have been recently introduced to determine bone mineral density (Genant et al., 1996, Braun et al., 1998); DXA nevertheless is still widely considered as the method of choice. Other techniques like peripheral quantitative computed tomography (pQCT) may be accurate in measuring BMD at peripheral skeletal sites; however they exhibit restrictions that prohibit measurements at the proximal femur (Augat et al., 1996, Augat et al., 1998).

The aim of this investigation is to determine the correlation of the bone mineral density in the femoral neck, as measured by DXA, to experimentally determined strength characteristics of the bone. This, followed by the introduced FEA simulation, will facilitate the use of DXA as an indicator of fragility fracture risk in the hip region, as there is a consensus throughout literature that hip fractures involve the most severe consequences of osteoporotic bone loss.

Section snippets

Materials and methods

This study was conducted on femoral neck samples, harvested from patients undergoing total hip replacement due to osteoarthritis. Written consent was obtained in all cases. In order to determine the samples' structural integrity, standard X-rays (anterior–posterior) of the pelvis were taken preoperatively in all cases. Patients with a short femoral neck, large cysts in neck region or previous surgeries in proximal femur were excluded from the study.

Overall 30 patients (27 female and 3 male)

Results

A correlation of characteristic and mean values (BMD and T-score) determined by DXA measurements to the corresponding mechanical properties (yield stress and elastic modulus) of the examined specimens is reflected in Table 1. The Young modulus values (E) are in good coherence with previously presented data covering similar spectrums 12.643–28.536 GPa (in our study), 10.4–19.3 GPa (Keller et al., 1990) and a mean value of 18.2 GPa according to Reilly and Burstein (1997). The determined yield stress

FEA simulation

In order to associate the ultimate compression strength of the samples to fragility fracture risks of the femoral neck, a model of a human femur was developed, to simulate a gait type loading scenario considering combined multiaxial forces (Jacobs et al., 1997).

Computed Tomography (CT) was employed to reconsider all anatomical aspects in a 3D model. Even though both CT and Magnetic Resonance Imaging (MRI) demonstrate high inherent image contrast between bone and soft tissue, CT is slightly more

Discussion

DXA scans in the hip region are conventionally performed in the trochanter, the Ward's triangle and the femoral neck (in an orthogonal area of 6 by 10 mm). The aim of our study was to correlate the BMD obtained from DXA to the mechanical strength characteristics of the examined area, so as to provide surgeons with a DXA based risk assessment concerning fragility fractures.

The introduced experimental investigation affirmed the reliability of BMD in predicting the mechanical properties of the

Conclusions

Bone mineral density measured by DXA, regardless of limitations associated to the technique's ability to capture bone quality, is a strong predictor of bone strength in the femoral neck region. Supported by an adequate FEA simulation, DXA may be regarded as a valuable tool during the prediction of BMD spectrums which present a significant risk of fragility fractures.

Conflict of interest

The authors would like to state that this work is not subject to any conflict of interest. In these terms, there do not exist any financial or personal relationships with other scientists/people and/or organisations that could have inappropriately influenced our work.

Acknowledgements

The authors would like to thank BETA CAE Systems SA for providing them with the CAE pre-processor ANSA, used during surface and volume generation and meshing of the introduced model.

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