In vitro stability of a highly crystalline hydroxylapatite coating in a saturated citric acid solution

A novel pressurized hydrothermal post-plasma-spray process has been develop ed to convert the crystalline non-HA and amorphous components of plasma-spr ayed hydroxylapatite coatings back into crystalline HA. The process, known commercially as MP-1(TM), was used to produce coatings comprising approxima tely 96% crystalline HA. The in vitro solubility of the coating in saturate d citric acid solution has been measured to simulate the effect of implant detoxification procedures, which use citric acid as a cleaning medium. The MP-1 coating solubility in saturated citric acid solution (pH = 1) was comp ared to that of coatings with crystalline HA contents ranging from 37.5-82% . All coatings showed an initial sharp rise in coating dissolution, which c orrelated with crystalline Hi content, followed by a steady state dissoluti on rate. After 60 s at 25 degrees C, the MP-1 coating showed a 65% decrease in solubility compared to a highly amorphous coating (AM-2). All coatings showed very similar steady state dissolution rates, except for AM-2, which was significantly higher. SEM analysis showed that the AM-2 coating surface was degraded substantially more than the other coatings, resulting in part ial coating exfoliation. A mechanism of coating dissolution is proposed, in which the initial rapid leaching of soluble phases from the coating leaves behind a porous layer of highly crystalline HA at the coating surface. The stability of this porous crystalline layer leads to steady state, diffusio n-limited dissolution of the remainder of the coating. The observed two-reg ime dissolution profile can be accurately represented by a 2-parameter mode l, which predicts the initial sharp rise in coating dissolution followed by a slower, steady state loss in coating mass. Model parameters were determi ned from experimental solubility data, and were shown to correlate with the percentage of crystalline EW in the coatings. The present data suggest tha t the treated coating is significantly more resistant to degradation from a ggressive detoxification procedures such as citric acid burnishing. (C) 199 9 John Wiley Br Sons, Inc.