European Society of Biomechanics S.M. Perren Award 2014: Safety factor of the proximal femur during gait: a population-based finite element study.

It has been suggested that the mechanical competence of the proximal femur is preserved with respect to physiological loading conditions rather than accidental overloading, but the consequences of this adaptation for fracture risk in the elderly remain unclear. The goal of the present study was to analyse the safety factor of the human femur in the two most frequent daily activities, level walking and stair climbing, and to understand the dependence, if any, of this safety factor on age, volumetric bone mineral density (vBMD), and gender. To this aim, a finite element study was performed on 200 subjects (116 women and 84 men), spanning a large range of age (23-84 years) and vBMD levels (T-score from 0 to -3.59). For the first time, finite element models that included a subject-specific description of the anatomy and mineral density distribution of each bone were coupled with a personalisation of the loads acting on the proximal femur during movement, including the action of the muscles and their variability across the population. The results demonstrate that the human proximal femur is characterised by a high safety factor (on average five, never reaching fracture threshold), even in the presence of advanced age and low mineral content. These results corroborate the hypothesis that the relationship between loading and mechanical competence is generally preserved in the elderly population for the most frequent motor activities, walking and stair climbing. Interestingly, a decrease of the safety factor was observed with increasing lifespan and reduced mineral content in women but not in men.