Mechanical behaviour of a prosthesized human femur: a comparative analysis between walking and stair climbing by using the finite element method.

The aim of this study is to perform finite element investigation of the mechanical behaviour of a prosthesized human femur during walking and stair climbing. Such activities are in fact encountered with the highest frequencies during daily living. In order to numerically analyze the stress shielding of the femoral bone with an artificial hip replacement, the strain and stress distributions both in the femur and in the stem were evaluated by using the finite element method. From a set of CT images the geometry of the femur was recovered and meshed. An operation of virtual surgery allowed to insert the metal stem (constructed by a CAD code) in the medullary canal. Numerical simulations showed evidence that, when comparing walking with stair climbing for stresses and strain energy density, maxima and minima values change dramatically, even if the localizations remain the same ones. Furthermore, for each type of daily activity, the unloading of bone tissue is confirmed in the case of prosthesized femur with respect to the physiological one, and hence the well-know phenomenon of stress shielding is exhibited. The development of a computational model allowed to deal with the complexity of the biomechanical problem and to describe quantitatively the mechanical behaviour of bone tissue in contact with the metal stem. The calculated results motivate the performed analyses because they are sufficient to activate the mechanisms of deposition and resorption in the bone tissue at contact with the artificial biomaterial. Thus, stair climbing confirms to be a critical task for primary stability of the prosthesized femur with respect to the activity of walking.