Failure of an ACCIS metal-on-metal hip resurfacing prosthesis: A case report

The introduction of coatings on joint replacements was intended to reduce wear volumes, prevent corrosion and reduce metal ion release. However, retrieval analysis to confirm their in vivo performance has been limited. The aim of study was to examine the coating on a retrieved hip prosthesis to determine whether substantial damage or wear had occurred. A single advanced ceramic-coated implant systems titanium niobium nitride-coated hip resurfacing prosthesis was retrieved for examination. Wear volumes and surface roughness were measured to quantify the damage to the articulating surfaces. The coating had been completely removed from substantial parts of both the femoral head and the acetabular cup, corresponding to areas of wear as measured by a coordinate measuring machine. The total wear rate (61 mm3/year) was high and corresponded with the high metal ion levels recorded pre-operatively (83 ppb chromium and 110 ppb cobalt). Roughness data indicated that the coating has a lower roughness than the substrate and substantially lower than the boundary between the coating and the substrate. The wear rate is very high and it is likely that damage to the coating resulted in accelerated wear of the prosthesis. No sudden change between the boundary of the substrate and the coating was observed that would indicate delamination or failure of the coating substrate interface layer. While coatings may offer some theoretical benefits, they may also result in catastrophic failure of the prosthesis.