[High incidence of total hip arthroplasty aseptic loosening with ion-coated titanium femoral heads].

PURPOSE OF THE STUDY: The purpose of this study was to determine the frequency of aseptic loosening among a series of total hip arthroplasties evaluated at 84 months and to search for the cause. Two types of acetabular cups had been implanted. It was hypothesized that the ion coating of the titanium head could be involved in the deterioration of titanium/polyethylene implants.

MATERIAL AND METHODS: Two non-cemented acetabular cups differing only by the presence or not of a hypoxyapatite coating were studied. Different types of femoral heads (stainless steal, chromium-cobalt, alumina, zincrona, nitrurated titanium, ion-coated titanium) and femoral stems (with or without cement) were implanted. Sixty-two ion-coated titanium heads were implanted and 47 patients with 52 heads were reviewed. Clinical outcome was assessed with the Postel-Merle-d'Aubigné score and the Livermoore method was used for radiological assessment of the bone-implant interface and polyethylene wear. The physico-chemical properties of one titanium head explanted after aseptic loosening were also studied.

RESULTS: At 84 months follow-up, the mean clinical score was 15.8/18 points. Mean polyethylene wear was 0.18 mm/year. There were 13 revisions for aseptic loosening: two bipolar, nine acetabular and two femoral. Mean wear for the explanted implants was 0.34 mm/year. Metallosis was observed in eight cases. Arthroplasties with the same types of femoral stem and acetabular implants but with other types of heads (stainless steal, chromium-cobalt, alumina, zincrona, nitrurated titanium) led to only one case of aseptic loosening among 118 implantations. Electron microscopy demonstrated the presence of scratch lines, disappearance of the nitrogen ion layer, decreased hardness, and increased roughness of the titanium head.

DISCUSSION: The poor friction properties of titanium are well known. To improve performance, ion coating has been proposed. This technique consists in projecting nitrogen ions onto the surface of the head to form a surface coating measuring about one micron. The high incidence of aseptic loosening, polyethylene wear, metallosis, and modifications of the head surface (disappearance of the nitrogen ion layer, scratch marks, etc.) suggest ion-coated titanium heads could be the cause of these aseptic loosenings.

CONCLUSION: Ion-coating has not provided good protection of the titanium head. Patients with this type of head should be followed carefully in order to detect aseptic loosening or metallosis early.