Bone Mineral Density (BMD) is widely used indicator for fragile hip fracture due to osteoporosis. However, osteoporosis alone is an insufficient indicator as BMD is affected by age and gender. The maximum BMD in hip is achieved at the age of 25 to 29 and it deteriorates with age \[1\] causing 18% tendency of hip fracture for women and 6% for men in their entire lifetime \[2\].  In reality, hip fracture is also affected by bone morphology and largely occurred due to the impact of load and its direction \[3,4\]. Therefore, hip fracture analysis considering only one factor is insufficient. In this work, we aim at analyzing the influence of loading directions – mimicking different falling postures, and bone density distributions in predicting the hip fracture risk.

QCT images of both left and right femurs of 20 anonymous adults were considered in this work. Medical image processing software – Mimics® 16.0 was used to generate the 3D geometry of each femur from its DICOM image dataset. The femurs were meshed with 4-node tetrahedral element with 6 degrees of freedom. The local bone density – expressed in Hounsfield Unit (HU) – correlated with CT voxel results in an inhomogeneous density distribution. The bone materials properties were mapped on the elements based on the HU and CT gray values associated with empirical relations \[5\].

Element-wise material distribution was implemented using Bonematv3.2 \[6\]. To simulate sideways fall, displacement-based boundary conditions were applied such that the distal end of the femur was fixed from all direction and greater trochanter was fixed on the loading direction. The fall load equivalent to  P=8.25w(h/170)\^0.5, where *w* and *h* denote weight and height of a body, respectively \[7\], was applied in the femoral head from 90^0^ to 160^0^ measured from sagittal plane.  The FEA results show that loading direction has considerable impact on the stress variations in femur. The maximum stress does not occur when the loading direction is perpendicular to the sagittal plane. Instead the stress in femoral neck (FN) region increases with increase in angle measured from sagittal plane and maximum stress occurs ranging from 120^0^ to 160^0^. The stress based fracture risk at the femoral neck is calculated using Fracture Risk Index (FRI) defined as . The results indicate that hip fracture may occur not only from the sideways impact force, but also due to the impact force direction. Additionally, FRI increases with decrease in bone density as determined from the CT data and distributed via Bonemat.

The cumulative effect of loading direction and bone density distributions on hip fracture shows that maximum stress occurs at FN region where the bone density is low and loading direction deviates from 90^0^ when measured from sagittal plane.

References

\[1\] Szulc, P et al., *Bone*, 26(2), 123–129, 2000;

\[2\] Gillespie, W. J. et al., BMJ, 322, 968–975, 2001;

\[3\] Fajar, J. K. et al., *J. Orthop. Transl.*, 13, 1–6, 2018;

\[4\] Pinilla, T. P. et al., Calcified Tissue Int., 58, 231-235, 1996;

\[5\] Helgason, B et al., Clin. Biomech., 23(2), 135-146, 2008;

\[6\] Taddei, F et al., Med. Eng. Phys., 29(9),973-979,2007;

\[7\] Kheirollahi, H. et al, BioMed Res. Int ,2015,1-15,2015;


Effect of Loading Direction and Bone Density Distribution in Assessing Hip Fracture via Finite Element Analysis

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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