Studies on bone-derived biological apatite : structural and compositional investigations, assessments and simulations

Abstract

Biological apatite (BAp) is the primary inorganic component of biological calcified tissues such as bones and teeth. Due to the similarity in structure and chemical composition, xenogenic bone graft materials composed of BAp have been widely used in orthopaedics, oral and maxillofacial surgery, and implant dentistry. However, a comprehensive understanding of BAp’s physicochemical properties (such as morphology, composition and solubility) has not yet been obtained because of the variety of its sources and the effects of extraction methods, as well as the drawbacks and limitations of examination methods. Therefore, the aim of this work was to investigate the structure and chemical composition of vertebrate bone-derived BAp, to assess the influence of thermal treatment on its physicochemical properties, and also to examine the effect of heating temperature and magnesium on its solubility by using synthetic hydroxyapatite (HAp) and carbonated HAp (CHAp) as simulation models.

Bone-derived BAp from several vertebrates was extracted by both low-power plasma ashing and thermal treatment. It was found that each consisted of platelet-like CHAp crystals with low crystallinity, which became rod-like or irregular particles, of larger size and higher crystallinity but lower carbonate (〖CO3〗^(2-)) content, on high-temperature treatment.

The solubility of HAp (as a model for BAp) in potassium chloride-deuterium oxide (KCl-D2O), a potential medium for the investigation of HAp’s solubility behaviour, was close to that in potassium chloride-protium oxide (KCl-H2O), indicating that there was no significant effect of D2O on the equilibration of HAp with its solution.

The effect of heating temperature on CHAp, as a simulation of BAp, was found to be temperature-dependent with respect to its physicochemical properties. Crystal growth, aggregation and fusion were observed on heating at 800 °C and above. With increasing temperature, the carbonate content decreased while the crystallinity increased. The solubility of the as-prepared material and those sintered at 600 and 700 °C was higher than the reference curve for HAp, while that of the material treated at higher temperatures was indistinguishable from the reference.

The effect of magnesium on HAp’s solubility was concentration-dependent: a general, quasi-logarithmic increase in solubility was found with increasing content of magnesium in the background solution. Whether this is due to nucleation inhibition, or solution complexation, is yet to be ascertained. However, magnesium deficiency would appear not to be the direct chemical cause of osteoporosis.

It is concluded that platelet-like BAp is the common inorganic component of vertebrate bones. BAp crystals derived from cortical and trabecular bone may have differing thermal sensitivities. BAp’s in vivo structure and composition as well as solubility are vulnerable to alteration during extraction involving thermal treatment. Magnesium has evident influence on BAp’s solubility although the mechanism is as yet unknown. D2O, which appears not to affect the solubility behaviour of HAp, may provide a means of preparing deuterated material suitable for detailed crystallographic analysis, such as by neutron diffraction, with a view to resolve several anomalous aspects of the solubility of calcium phosphates in general, and HAp in particular.