New concept in durability improvement of hip total joint endoprostheses

Starting from the well-known fact that the rolling movement always has a lower friction compared to sliding friction, the authors have conceived and realized a pivoting movement joint on a "layer of balls" with "compensation space", placed between the acetabular cup and the femoral head. This technical solution allows free self-directed migration of the balls, depending on the resistance opposed, with successive occupation of the "compensation space". As a concept, the proposed technical solution excludes the existence of a cage for maintaining the relative positions of the spheres. It can be observed that the smallest values of the force and of the friction coefficient are obtained for the prostheses with balls and self-directed movement (approximately 5 times smaller than the values obtained for a classical prosthesis). For all the couples tested, the friction force grows with the growth of the normal load and of the oscillation speed. Changing the contact mechanism for the artificial hip joint from one sliding contact between two large surfaces, to a multitude of rolling contacts, could lead to some problems regarding functionality and durability of the active prosthesis elements. The key to an accurate evaluation of damaging mechanisms acting on THP with self directed rolling balls is a clear and complete picture of the load transfer mechanism.