Since wear debris is recognised as a major cause of hip prosthesis loosening in the last decade, research was aimed to lower the coefficient of friction. Polyethylene revealed to be the principal source of debris so new bearing materials were introduced in the effort to increase implant longevity.

In 1965 Mc Keee Farrar introduced metal on metal bearings, in 1970 Boutin ceramic on ceramic. Both of them demonstrated the capability to reduce wear debris more than 97% as compared to PE-metal coupling but have possible risks. Ceramic heads are scratch resistant but they can fracture, then they offer limited options as lenght and diameter. Metal on metal bearing couples give all the options but can determine high blood metal ions level, potentially dangerous. To avoid these risks new coupling materials were introduced: oxinium and cross-linked polyethylene.

Cross-linked ultra-high molecular weight polyethylene was first employed for liners more than 30 years ago with excellent results. On the other hand oxinium is a metal compound of 97,5% zirconium and 2,5% niobium, whose surface is transformed into a smooth ceramic with a coefficient of friction that is half that of cobalt-chrome, and same wettability and hardness of ceramic. Oxinium-cross-linked polyethylene produce nearly undetectable wear even if subjected to abrasive conditions.

Oxinium femoral heads offer the option of different neck lenght and head diameter. Laboratory testing demonstrated that rough oxinium heads provided 61% less polyethylene wear than did the rough metal heads. Clinical results have a too short follow-up to be reported but are promising.

The purpose of this study was to determine the biological effects of the elastic modulus of the femoral stem in canine hip arthroplasty. Cementless total hip arthroplasty was performed in 12 dogs, six had a low elastic modulus polyacetal resin stem and six had a high modulus stainless steel stem. The components were otherwise similar. At six and 12 months after operation, radiographic and histomorphometric analysis showed that those with steel implants had more cortical porosity than did the other group (p less than 0.01). We suggest that the elastic modulus of the implant is an important factor in controlling cortical bone resorption. A low modulus femoral prosthesis can significantly decrease bone resorption which might otherwise eventually lead to implant failure.