Specially prepared biphasic calcium phosphate (BCP) macroporous ceramics consisting of an intimate association of beta tricalcium phosphate (beta-TCP) and hydroxyapatite (HA) with beta-TCP/HA weight ratios of 15/85, 35/65, and 85/15 were implanted in surgically created periodontal osseous defects in dogs and recovered after 6 months. A decrease in average size of crystals in BCP ceramics and an increase in the size of microporosities in the surface and at the core of the ceramic after implantation were observed, indicating that in vivo dissolution has taken place. The resorbability (reflecting in vivo dissolution) of BCP ceramics depended on their beta-TCP/HA ratios, the higher the ratio, the greater the resorbability. The formation of microcrystals with crystallographic properties and Ca/P ratio similar to those of bone apatite crystals were also observed. The abundance of these crystals were directly related to the beta-TCP/HA ratio of the BCP ceramic before implantation. The formation of the bone apatite-like crystals may be due to the precipitation of calcium and phosphate ions released from the dissolving ceramic crystals (the beta-TCP component dissolving preferentially to the HA component). Results from this study suggested that one of the means of controlling resorbability (in vivo dissolution) of BCP ceramic is by varying its beta-TCP/HA ratio.

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High resolution electron microscopy has provided precise measurements of human enamel crystallites from the first stages of enamel development to the last stage of maturation, through counting of lattice planes and statistical analysis. Thickness and width measurements of human enamel crystallities in their mature stage are 263 A (21.9 standard error) and 683 A (134 standard error), respectively. The growth curve of enamel crystallites was drawn, the increase in the number of lattice planes in the course of enamel development was demonstrated. Fusions and dislocations related to the maturation process were visualized and discussed. Difficulties linked to the use of ultrastructural techniques were mentioned. The question of the organic stroma of the crystals remained unsettled.

This study correlated ultrastructural observations on the presence of beta-tricalcium phosphate (beta-TCP) in arrested dentin caries with physico-chemical observations on the in vitro formation of Mg-substituted beta-TCP. The ultrastructural studies were made using high-resolution transmission electron microscopy (TEM) with the capability of microdiffraction and microanalysis on sites less than 10 nm in diameter. Mg-substituted beta-TCP was obtained, by a precipitation method, from solutions with Mg/Ca molar ratios of 5/95 and higher. Such correlations led to the postulation of a possible chronological sequence of physico-chemical events occurring at the crystal level during the progress and arrest of caries in human dentin. It is suggested that the initial mechanism for the observed occurrence of large crystals of Mg-substituted beta-TCP and of apatite in the tubule lumen is due to the dissolution of the dentin mineral (a CO3- and Mg-rich calcium OH-apatite) and reprecipitation of Mg-substituted beta-TCP, followed by that of CO3- and Mg-poor apatite.

Identification and indexing of lattice planes through high-resolution transmission electron microscopy provided precise measurements of intertubular crystals of arrested caries in dentine compared to sound dentine. In arrested caries, mean crystal width was 310 Å (SD 44) with a range from 200 to 400 Å. Mean thickness was 86 A (SD 19.3) with a range from 57 to 130 Å. In sound dentine, mean width was not very different at 296 Å (SD 39) with a similar range from 200 to 400Å. Mean thickness was lower: 31.6 Å (SD 5.4) with a range from 24 to 41 Å. A growing process due to thickening of the crystallites was pointed out. The coexistence of whitlockite and apatite within the large crystals of intratubular dentine in arrested caries was shown through high-resolution electron microscopy and micro-electron diffraction. Two stages of arrested caries could be found at the crystal level: firstly, a precipitate of the material dissolved at the very beginning of the carious process, and secondly, remineralization of decalcified dentine by growth of residual crystals (intertubular dentine) and by a maturating process of whitlockite towards apatite (intratubular crystals).