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Sideways fall has been identified as the most critical situation for the elderly to develop hip fractures. The impact force onto the greater trochanter is the key factor for predicting fracture risk. For the elderly, the impact force can only be determined by dynamics simulations, and the dynamics model must be first validated by experiments before it can be applied in clinic. In this study, subject-specific whole-body dynamics models constructed from dual energy X-ray absorptiometry (DXA) images of the subjects were validated by controlled and protected fall tests using young volunteers. The validation results suggested that subject-specific dynamics model is much more accurate in predicting impact force induced in sideways fall than conventional non-subject-specific dynamics model. Therefore, subject-specific dynamics model can be applied in clinic to improve the accuracy of assessing hip fracture risk.

The impact force applied to the greater trochanter during sideways fall is a critical factor for determining whether or not a hip fracture would occur. However, the impact force is subject-dependent as it is related to the subject's anthropometric parameters and the kinematic variables in fall. It cannot be accurately predicted by the currently available dynamics models. We developed and validated a method for constructing subject-specific dynamics models to more accurately predict the impact force. The anthropometric parameters required in the model were obtained from the subject's whole body DXA (dual energy X-ray absorptiometry) image. The subject-specific dynamics models were then validated by protected fall tests using young volunteers. The effects of anthropometric parameters on the impact force were investigated using 90 clinical DXA images obtained from a local osteoporosis clinic center. The impact forces predicted by subject-specific dynamics models had much better agreement with the experimental data, compared with those predicted by the existing empirical functions. The parametric study results indicated that although body weight and height are the dominant parameters affecting the impact force, other parameters such as the hip vertical velocity before impact also have considerable effects. This finding suggests that the existing empirical functions that only consider body weight and height may not be able to accurately predict the impact force. As whole body DXA images are readily available in osteoporosis clinic centers, the proposed method may have potential applications in the clinic to improve the assessment of fallinduced hip fracture risk.

The analysis of the gathered data revealed that there is a need to develop modified biomechanical models for more accurate prediction of the impact force and appropriately adopt them in hip fracture risk assessment tools in order to achieve a better precision in identifying high-risk patients. Graphical abstract Impact force to the hip induced in sideways falls is affected by many parameters and may remarkably vary from subject to subject.

In the reported research, a subject-specific multibody dynamics model was proposed to predict impact force induced in lateral fall of the elderly. Parameters such as anthropometric dimensions, segment masses, mass center, and mass moment of inertia that are required for constructing the dynamics model were extracted or calculated from a whole body DXA image of the subject. Governing equations of the fall process were established and computer codes were developed for solving the equations. The dynamics model was then validated by a controlled fall test using young volunteer. Good agreements between predicted and experimental results were observed, indicating that the proposed dynamics model has the capability to predict subject-specific impact force induced in fall.

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Fall-induced hip fracture is a major worldwide health problem among the elderly population. Nowadays, hip replacement surgery represents a big part of the orthopedic surgeons' workload and has associated remarkable clinical and social cost implications. Hip fractures have several complications including medical and surgical treatment. A significant number of biomechanical models have been introduced to study hip fracture risk. The purpose of proposing the biomechanical models for predicting the hip fracture risk is to introduce prevention and protection activities that may reduce the number of hip fractures. For accurate prediction of hip fracture risk, the fracture procedure and the parameters that affect the risk of hip fracture should be well studied. The objective of this study is to investigate in-depth the hip fracture anatomy, causes, and consequences.