RESUMO
The multitude of total hip replacement systems cannot mask the fact that it is after all a matter of friction between two surfaces. The wear and distorsion resulting from this friction are still to be precisely evaluated for each marketed hip system. These two parameters have indeed to be evaluated when new devices are used. The "in vivo" behavior of a hip system is best approached when studied on a model reproducing the hip movements and the cyclic loads endured by the hip throughout walking. Data obtained using this type of model allow us to approach the behavior of metal-polyethylene interfaces: there is always a grinding period whose duration varies from 300,000 to 2,000,000 cycles. The friction factors during this initial grinding period may be two to three times greater than the final ones. Data obtained upon wear over 5 millions cycles correspond to clinical findings. Concerning the high density polyethylene-metal interface, variations of molecular size or supply of supplementary molecules may increase the friction factors up to three times the initial values. But wear may vary much more.
