CHARACTERIZATION OF SURFACE DAMAGE TO ALUMINA BEARINGS IN TOTAL HIP EPLACEMENT

Abstract

Alumina bearings are an available option in total hip replacements in the United States. Alumina has high strength and low friction and wear as an articulating surface. To determine the potential damage that can occur to alumina surfaces during implantation and dislocation of such bearings, we examined several explanted bearins at a microscopic and microchemical level.

Alumina femoral heads and acetabular liners that-were rejected during surgery because of metal staining, or removed from patients after known times of implantation were examined in the scanning electron microscope to observe the extent of surface degradation. 13 samples were examined. Four samples consisted of a femoral heads that had come in to contact with the metal acetabular shell during implantation, and therefore had metal staining on the surface. Nine femoral heads and a cetabular linerpairs were removed from patients with a history of recurrent dislocations of the prosthetic hip. Of these 5 pairs had grossly visible surface damage, and four did not. These latter four pairs, and the four femoral heads with surface staining were each coated with carbon and observed in a scanning electron microscope fitted with an energy dispersive X-ray analysis (EDAX) attachment for microchemical analysis.

Alumina heads that had inadvertently rubbed against the metalacetabular shell during surgery demonstrated dark metallic staining on the surface. EDAX analysis indicated that the stain had a composition very close to a Ti-6Al-4V alloy. Closer examination in the SEM revealed that the metal stain has a particulate structure, with the size of the particles ranging from sub-micron to several tens of microns. These titanium stains could be removed by a benign chemical etch, leaving no corrosion of the a lumina bearing surface. In contrast, alumina heads and liners removed after various times of implantation show significant surface deterioration. A wide range of features were observed, including uneven wear, cracks, embedded particles, deep groves and pits.

Considerable surface staining of aluminafemoral heads can occur during surgery if they come into contact with metal. This staining occurs due to surface deposition of metal particles which can be easily removed by wiping the surface with a benign etchant. If not removed, the presence of metal reduces the smoothness of the alumina, and may contribute to three-body wear. In contrast, alumina bearings removed from patients with multiple hip dislocations show significant, non-uniform-surface degradation. The observed features, such as embedded particles, cracks, and pits can contribute significantly to surface wear, which was grossly evident in 5 alumina head/liner pairs retrieved for this study. These data suggest that multiple dislocations in total hips with alumina bearings may need earlier surgical intervention because of the surface damage to the bearings, and potential for catastrophic wear.