<i>In Vitro</i> Resorbability of Three Different Processed Hydroxyapatite

Abstract

Hydroxyapatite has been used as bone substitutes in many applications due to its biocompatibility and osteoconductivity. Generally, it is considered to be biostable and shows limited resorption in the body. In some circumstances, resorption of bone substitutes is more desirable since it could accelerate the bone healing process. It is known that processing route is one of the crucial parameters that could affect the properties of materials. Three different processes were employed in this study to fabricate hydroxyapatite samples including low temperature transformation of three-dimensionally printed calcium sulfate (HA1), high temperature sintering of three-dimensionally printed hydroxyapatite (HA2) and high temperature sintering of mold pressed hydroxyapatite (HA3). HA1 was found to contain high porosity and low crystallinity whereas HA2 had high porosity and high crystallinity. HA3 had low porosity, but high crystallinity. In vitro resorbability of these samples was studied by submerging all the samples in simulated body fluid (SBF) for 1, 7, 14 and 28 days and determining their phase composition, density change, liquid absorption, ions release and microstructure. It was found that HA1 showed the greatest density loss and liquid absorption followed by HA2 and HA3 respectively. Calcium and phosphorus ions in SBF were observed to decrease with submerging times for HA1 and HA2, but remained constant for HA3. SEM studies showed that new calcium phosphate crystals were found to form on the surface of the HA1 and HA2 samples whereas none was found on HA3. These results suggested that HA1 had the greatest resorbability and calcium phosphate crystals forming ability on its surface followed by HA2 and HA3 respectively. Therefore, porosity and crystallinity of the samples resulting from different processing routes are important factors for in vitro resorbability of hydroxyapatite.