title: On the fate of particles liberated from hydroxyapatite coatings in vivo.
abstract: OBJECTIVE
  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.
  
  
  RESULTS
  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.
authors.affiliation: UCD School of Mechanical and Materials Engineering, University College Dublin, Belfield, Dublin 4, Ireland.
authors.forename: C F
authors.initials: CF
authors.lastname: Dunne
authors.forename: J
authors.initials: J
authors.lastname: Gibbons
authors.forename: D P
authors.initials: DP
authors.lastname: FitzPatrick
authors.forename: K J
authors.initials: KJ
authors.lastname: Mulhall
authors.forename: K T
authors.initials: KT
authors.lastname: Stanton
mesh_terms: D020099:Coated Materials, Biocompatible / Q000009:adverse effects / Q000493:pharmacokinetics; D017886:Durapatite / Q000009:adverse effects / Q000493:pharmacokinetics; D006801:Humans; D007595:Joint Prosthesis / Q000009:adverse effects; D055595:Mechanical Phenomena; D010316:Particle Size; D052638:Particulate Matter / Q000009:adverse effects*
chemical_list: D020099:Coated Materials, Biocompatible; D052638:Particulate Matter; D017886:Durapatite