Fabrication of Low-Crystalline Hydroxyapatite from Gypsum Based on Phosphate Treatment

Objectives: Hydroxyapatite (HAP) has been widely used as a bone substitute material. Most of the HAP products are prepared by sintering at high temperature. However, the original shape is difficult to retain due to shrinkage brought about by the sintering process. Furthermore, the sintered HAP cannot be resorbed by the osteoclasts, therefore the sintered HAP cannot be replaced with the newly-formed bone. In this study, we tried to fabricate a low-crystalline HAP with various shapes from gypsum block at lower temperature without sintering. Subsequently, we also investigated the biocompatibility of the fabricated material in vivo. Methods: Calcium sulfate hemihydrate was mixed with distilled water and set gypsum was prepared. Gypsum blocks were immersed in 1mol/L trisodium phosphate aqueous solution at 100 deg C for 24hours. The specimens were characterized by powder X-ray diffraction and FT-IR after the reaction. Biocompatibility of the fabricated material was investigated through an in vivo evaluation using rats. The material was filled into the artificial bone defect created in the rat tibia. As a control, sintered HAP was also filled in the same manner. After 2 and 4weeks, 1micrometer-thick sections of the implant sites were observed under light microscopy. Results: By compositional analysis, we confirmed that low-crystalline HAP was fabricated with its original shape retained. Histologically, as early as 2weeks of implantation, the fabricated low-crystalline HAP was almost circumscribed by newly-formed bone. Resorption by the osteoclasts could also be observed on the fabricated HAP. On the other hand, sintered HAP was surrounded by connective tissue. Conclusions: Low-crystalline HAP monolith was successfully prepared from gypsum through phosphate treatment at lower temperature and it showed much better biocompatibility and osteoconductivity than sintered HAP. Therefore it has a potential to be an ideal bone substitute material wherein new bone deposition and resorption took place.