Abstract: BACKGROUND: The assessment of fracture risk still depends on bone mineral density testing. However, it ignores other mechanisms that affect fractures, including three-dimensional bone structures and material properties on multiple scales. Although bone mineral density provides useful information, its value in predicting fracture risk is limited.  
OBJECTIVE: A three-dimensional finite element model of proximal femur with different cancellous bone volume fractions was constructed to explore the effect of bone volume fraction on the apparent mechanical response of proximal femur.
METHODS:  The CT data of a volunteer’s proximal femur were collected and imported into Mimics to reconstruct the three-dimensional model in DICOM format. The osteoporotic cortical bone and cancellous bone were given corresponding material parameters. Then, the model was imported into Abaqus and the finite element model with 35%, 30%, 25%, 20% and 15% cancellous bone volume fractions was constructed by uniform deletion of the script. A reference point was established above the femoral head and a concentrated load was applied to the area of contact with the acetabulum above the femoral head to analyze the difference of mechanical response of the proximal femur under orthostatic stress.  
RESULTS AND CONCLUSION: (1) Under the condition of standing load, the tensile stress on the upper and outer side of the femoral neck was always greater than the compressive stress on the medial and inferior side of the neck. With the decrease of cancellous bone volume fraction, the tensile stress and compressive stress of proximal femur increased gradually, and the maximum tensile stress and maximum compressive stress of 15% model were 1.91 times and 1.42 times of 35% model, respectively. The maximum principal strain increased by 4.76 times, and the overall stiffness of the femur decreased by 58%. (2) Under the condition of standing on one foot, the cortical bone of the femur bore more stress than the cancellous bone, and the cancellous bone played an indispensable role in the overall elastic response of the femur. (3) With the decrease of the volume fraction of cancellous bone, the tensile stress of the lateral superior side of the femoral neck increased greatly. The region of the lateral superior side of the neck is the most significant area of bone mass loss, and it is also in the area of stress concentration under the fall load. This suggests whether the relevant parameters of the lateral superior side of the neck (such as bone mineral density and volume fraction) may become a more sensitive index for predicting brittle femoral neck fracture, which is worth further exploring.

Key words: bone volume fraction, cancellous bone, femur, osteoporosis, single foot erect attitude, finite element method, biomechanics, strength theory