HRTEM STUDY OF BIOLOGICAL CRYSTAL-GROWTH MECHANISMS IN THE VICINITY OF IMPLANTED SYNTHETIC HYDROXYAPATITE CRYSTALS

Calcium phosphates are widely used as biomaterials. Ultrastructural as sessments are of the utmost importance in our understanding of interfa cial phenomena. The aim of this study was to learn more about the newl y formed crystal growth mechanisms. The interfaces between implanted s ynthetic hydroxyapatite crystals (HAS) and newly formed crystallites w ere thoroughly examined on a molecular level. The bone-grafting materi al (HAS) was implanted into two adult patients, and small biopsies wer e recovered 6 months after implantation. The raw biomaterial was analy zed by x-ray diffraction and high-resolution transmission electron mic roscopy (HRTEM). Six months after their implantation, the HAS aggregat es were surrounded by a mineralized bone matrix. Tiny crystallites als o filled the spaces between the HAS crystals within the aggregates. Th ese newly formed crystallites growing at the surfaces of the implanted HAS crystals appeared to be apatitic. The crystallographic investigat ions of the nucleation and growing mechanisms of the newly formed crys tallites were performed by HRTEM in association with computer simulati on and mathematical processing of digitized images. A relationship was noted between the orientation axes of crystallites growing nearby and the zone axes of the implanted HAS, thus strongly suggesting a guidin g or substratum role of the HAS particles.