SURFACE MODIFICATION ISSUES FOR ORTHOPAEDIC IMPLANT BEARING SURFACES

In total joint replacement, generally a polished metal surface articulates against an ultra high molecular weight polyethylene (UHMWPE) counter bearing surface. Metals used include 316L stainless steel, Co-Cr-Mo alloy, and Ti-6AI-4V alloy (particularly with a hardened N⁺ ion implanted surface). Minimizing friction and UHMWPE wear is of prime concern for long-term performance. Additionally, it is desirable to minimize metal ion release which results from constant removal and reformation of passive surface oxides and oxyhydroxides during articulation. Long term effects from the presence of potentially toxic or carcinogenic ions of Cr, Co, Mo, Ni, V, and Al are not well known. Inert ceramic bearing surfaces eliminate this issue and are also resistant to potential three-body wear from bone cement debris or potential stray porous metal coating material. However ceramic (Al₂O₃ and ZrO₂) materials are only available for total hip replacement. For total knee replacement, ft is too difficult and expensive to manufacture a monolithic ceramic knee surface, thus various surface coating methods are being investigated. These methods include plasma sprayed Al₂O₃ and ZrO₂, TiN and amorphous diamond like coatings via PVD/CVD methods, and in-sKu oxidation. In other cases, the existing metal surfaces are simply hardened using methods such as N⁺ ion implantation and oxygen or nitrogen diffusion hardening. This paper reviews the limitations of existing total joint systems and the effectiveness of various surface modification methods of orthopaedic implant bearing surfaces on friction, abrasion, UHMWPE wear, and metal ion release.