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

Abstract

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.

Introduction

Hydroxyapatite (HA) is used clinically as a bioactive coating on a range of important medical implants [1] having been successfully applied to cobalt chrome [2], nitinol [3], stainless steel [4] and various alloys of titanium [5], [6]. In such applications, it has been clearly shown to reduce bone loss [7], improve osteoblast proliferation [8], enhance bone formation [9] and generally improve the osseointegration of the implants [10]. Its widespread use in dental [11], orthopaedic [12] and maxillofacial [13] applications has indicated excellent long term clinical outcomes [14], [15]. The most commonly employed method for producing commercial grade HA coatings is via high temperature plasma spraying [16], [17]. Despite the widespread use of this technology, there are significant on-going concerns with the integrity of the bonding that takes place between the HA layer and the implant surface. The effects that the high temperatures used in the process have on HA crystallinity, thereby leading to creation of amorphous phases are generally implicated in this regard [17], [18]. Since highly crystalline coatings have been shown to produce lower dissolution rates and enhanced fixation in vivo [19], development of alternative HA coating processes has been a long standing development goal [8], [10], [20]. However, most of the alternative processes suggested involve the use of complex chemistry and/or vacuum technology. Hence, none of the techniques developed to date have had the correct cost-benefit ratio to displace plasma spraying as the coating technology of choice.

One of the approaches used to try to directly enhance the osseointegration of metallic implants, with or without the use of a bioactive agent, has been to abrade the surface with an appropriate media within a grit blasting environment. For example, Ishikawa et al. [21] developed a coating process in which HA was blasted at a surface using standard sand blasting equipment. This was found to produce a HA coating which showed promising results in vivo [22], but was not utilised commercially. This type of blasting with sintered apatite grit, termed microblasting when the grit particles are in the micron size range, has been shown to enhance bone formation in vivo, even though the methods employed do not produce a complete surface coating [23]. The general value of the method has, however, been recognised and apatitic grit blasting is well established in the orthopaedic industry. It is predominantly used as a pre-treatment processing step prior to other forms of surface modification [24], indicating that it has potential to be commercially competitive. Further developments by Gbureck et al. [25] involved coating abrasive alumina particles (Al₂O₃) with an outer layer of HA and then abrasively blasting a substrate surface with this composite powder. The presence of the alumina was found to produce a significant improvement in HA adhesion through tribochemical bonding without contaminating the surface with alumina. However, since this method requires significant chemical modification of the precursor powder abrasive is has not been employed commercially.

More recently, O'Donoghue and Haverty [26] have developed a novel alternative to conventional blasting technology which seeks to overcome the major limitations of the earlier grit blast and related technologies. This new deposition technique, which is termed CoBlast, uses an advanced version of micro-blasting in which both an abrasive and a dopant are applied to the substrate surface simultaneously without the need for any form of pre-surface treatment. This technique produces a modified surface condition that can be tailored with regard to its composition and structure by altering the dopant properties. The use of alumina and HA as the components in the CoBlast process is applied commercially under the trade name OsteoZip+™. In this study, the surfaces produced by both direct use of HA as the abrasive and from the CoBlast approach, have been characterised and assessed with regard to their in vitro response to osteoblast-like cells and their ability to engender bone growth in an in vivo animal model. The resulting data are used to test the hypothesis that the simultaneous use of an abrasive (alumina) and dopant (HA) in the micro-blasting process produces an integral, well adhered HA layer capable of engendering an osteoconductive response from an implant surface.