Biological responses to hydroxyapatite surfaces deposited via a co-incident microblasting technique

Hydroxyapatite (HA) is routinely used as a coating on a range of press-fit (cementless) orthopaedic implants to enhance their osseointegration. The standard plasma spraying method used to deposit a HA surface layer on such implants often contains unwanted crystal phases that can lead to coating delamination in vivo. Consequently, there has been a continuous drive to develop alternate surface modification technologies that can eliminate the problems caused by a non-optimal coating process. In this study two methods for creating a HA layer on metal alloys that employ micro-blasting have been evaluated to determine if the inclusion of an abrasive agent can enhance the in vitro and in vivo performance of the modified surface. The first method employs direct micro-blasting using HA as the abrasive media, while the second employs a simultaneous blasting with an alumina abrasive and coincident blasting with HA as a dopant. Whereas, both methods were found to produce a surface which was enriched with HA, the respective microstructures created were significantly different. Detailed surface characterisation revealed that the use of the abrasive produced disruption of the metal surface without producing detectable incorporation of alumina particles. Roughening of the metal surface in this way breached the passivating oxide layer and created sites which subsequently provided for impregnation, mechanical interlocking and chemical bonding of HA. The co-incident use of an alumina abrasive and a HA dopant resulted in a stable surface that demonstrated enhanced in vitro osteoblast attachment and viability as compared to the response to the surface produced using HA alone or the metal substrate control. Implantation of the surface produced by co-incident blasting with alumina and HA in a rabbit model confirmed that this surface promoted the in vivo formation of early stage lamellar bone growth.