INITIAL BONE-MATRIX FORMATION AT THE HYDROXYAPATITE INTERFACE IN-VIVO

Dense, sintered, slip-cast hydroxyapatite rods were implanted transfem orally in young adult rats. The femora were excised after 2 and 4 week s and, following fixation, either embedded in methyl methacrylate for light microscopy, decalcified and prepared for transmission electron m icroscopy, or freeze fractured in liquid nitrogen for scanning electro n microscopic analysis. The latter was performed on the two tissue fra gments that remained after freeze fracturing, from which the first con tained the implants and the second comprised tissue that had been imme diately adjacent to the hydroxyapatite rods. Undecalcified light micro scopic sections revealed extensive bone tissue formation around and in contact with the hydroxyapatite rods. The initial bone matrix apposed to the implant surface, as demonstrated with scanning electron micros copy, was either composed of globular deposits or an organized network of collagen fibers. The deposits, which ranged in size from 0.1-1.1 m u m, fused to form a cement-like matrix to which collagen fibers were attached. Degradation of the hydroxyapatite surface resulted in the pr esence of unidirectionally aligned crystallites, with which the newly formed bone matrix was closely associated. Ultrastructural analysis of the bone-hydroxyapatite interface with transmission electron microsco py revealed a 50-600-nm-wide collagen-free granular zone, comprising o ne or more 40-100-nm-thick electron-dense layer(s). These structural a rrangements most probably partially represent the globular deposits an d proteinaceous material adsorbed onto and partially in the degrading hydroxyapatite surface. Although the latter change in surface topograp hy may have enhanced bonding of the cement-like matrix to the hydroxya patite, the cause for this change in topography and the type of bond f ormed are, at present, unknown. (C) 1995 John Wiley and Sons, Inc.