On the fate of particles liberated from hydroxyapatite coatings in vivo.

PURPOSE: Hydroxyapatite (HA) has been used as a coating for orthopaedic implants for over 30 years to help promote the fixation of orthopaedic implants into the surrounding bone. However, concerns exist about the fate of the hydroxyapatite coating and hydroxyapatite particles in vivo, especially in the wake of recent concerns about particulates from metal-on-metal bearings. METHODS: Here, we assess the mechanisms of particle detachment from coated orthopaedic devices as well as the safety and performance concerns and biomedical implications arising from the liberation of the particles by review of the literature. FINDINGS: The mechanisms that can result in the detachment of the HA coating from the implant can be mechanical or biochemical, or both. Mechanical mechanisms include implant insertion, abrasion, fatigue and micro-motion. Biochemical mechanisms that contribute to the liberation of HA particles include dissolution into extra-cellular fluid, cell-mediated processes and crystallisation of amorphous phases. The form the particles take once liberated is influenced by a number of factors such as coating method, the raw powder morphology, processing parameters, coating thickness and coating structure. CONCLUSIONS: This review summarises and discusses each of these factors and concludes that HA is a safe biomimetic material to use as a coating and does not cause any problems in particulate form if liberated as debris from an orthopaedic implant.

Crystallization modifies osteoconductivity in an apatite-mullite glass-ceramic.

The response to implantation of novel apatite glass-ceramics was evaluated using a weight bearing in vivo bone implant model. Five novel glasses with varying calcium to phosphate ratios were cast as short rods and heat-treated to crystallize principally apatite. One glass ceramic had an apatite stoichiometry (Ca : P=1.67); three were phosphate-rich and one calcium-rich. One of the phosphate-rich glasses was also tested in its glassy state to determine the effect of crystallization on the biological response. Rods were implanted into the midshaft of rat femurs and left for 28 days. The femurs were then harvested and processed for scanning electron microscopy, energy dispersive X-ray microanalysis and conventional histology as ground and polished sections. Four of the materials exhibited evidence of osseointegration and osteoconduction. However, there was a marked inflammatory response to one of the phosphate-rich glass-ceramics, and to the non-crystallized glass. Crystallization of the latter significantly improved the bone tissue response. The glass-ceramic with an apatite stoichiometry elicited the most favorable response and merited further study as an osteoconductive bone substitute in maxillofacial and orthopedic surgery.