INCREASED RISK FOR REVISION OF ACETABULAR CUPS COATED WITH HYDROXYAPATITE: A REGISTER STUDY ON 8,043 HIPS

Abstract

Background: Hydroxyapatite (HA) is the main inorganic component of bone, and HA coating is widely used on acetabular cups in hip arthroplasty. It has been suggested that this surface finish improves cup survival, but there is little evidence to support this.

Patients and methods: All patients registered in the Swedish Hip Arthroplasty Register between 1992 and 2007 with an uncemented acetabular implant that was available with or without HA coating were identified. A study population of 8,043 hips with the most common cup types manufactured either with or without HA coating (Harris-Galante, Romanus and Trilogy) was investigated. Kaplan-Meier survival analysis and a Cox regression model including type of coating, age, sex, primary diagnosis, cup type, and type of stem fixation were used to calculate adjusted risk ratios (RR) of the risk for revision.

Results: Kaplan-Meier analysis of all patients indicated a non-significant trend towards inferior performance of the HA-coated cups (p=0.78). When stratified for age, Kaplan-Meier analysis revealed inferior survival of HA-coated cups when compared to non-coated cups in the age group < 50 years (p=0.031). A Cox regression model showed that HA coating was a significant risk factor for cup revision due to aseptic loosening (adjusted RR 1.645; 95% CI 1.315–2.058). Age at primary arthroplasty < 50 years, a diagnosis of paediatric hip disease, a cemented stem, and the Romanus and Harris-Galante cup types were also associated with significantly increased risk for cup revision due to aseptic loosening. There was no difference in the hazard patterns when the risk for revision for any reason was chosen as the endpoint of the analysis. The risk for revision due to infection was not influenced by the type of coating.

Discussion: Our results derived from register data on 8,043 hips indicate that HA coating does not enhance survival of cups when using aseptic loosening as an endpoint. On the contrary, hydroxyapatite coating is a significant risk factor for cup revision due to aseptic loosening when adjusted for other covariates such as age, sex, cup design and primary diagnosis. HA coating cannot be generally recommended as a surface treatment of acetabular cups in younger patients. This conclusion is medically and economically relevant, as many young patients today receive HA-coated cups, and because HA-coated implants are more expensive.