Bone mineral: update on chemical composition and structure

The structure of the Ca–P solid phase in bone was first identified by deJong in 1926 as a crystalline calcium phosphate similar to geological apatite by chemical analyses and, most importantly, by X-ray diffraction [1]. The X-ray diffraction data was confirmed a few years later [2]. These findings initiated a flurry of research on a more detailed chemical composition and crystal structure of both geological and synthetic apatites and of bone mineral, initially carried out principally by geologists, crystallographers, and chemists, but later by biochemists and physiologists because of the clear potential of this new information to shed light on the biological and physiological functions of bonemineral and as indicators of disorders of the skeletal system. It soon became clear that there were significant structural and chemical compositional differences between the many different geological hydroxyapatites, synthetic hydroxyapatites, and the apatite crystals found in bone and related skeletal tissues in addition to the very large size of the geological and many of the synthetic apatite crystals, compared with the extremely small particle size of bone mineral. Further studies were directed in roughly three avenues: continued more careful and complete analytical compositional data of bone mineral, from which it was clearly established that the chemical composition of bone crystals in many ways did not correspond to the chemical compositions of stoichiometric hydroxyapatite. Indeed, the bone crystals were found to contain significant and varying amounts of carbonate and HPO4 ions. Much later, it was discovered by a variety of techniques, including solid-state NMR [3], Raman spectroscopy [4], and inelastic neutron scattering [5], that the biological bone apatites contain only a very small percentage of the total number of hydroxyl groups present in highly purified synthetic calcium hydroxyapatites. Other studies clearly pointed out that a substantial fraction of the phosphate ions are situated on the surfaces of the bone mineral crystals which are mainly protonated and in a disordered environment [6], in contrast to the phosphate ions in lattice positions, which are unprotonated. Also, other uniquely protonated phosphate ions were identified in bone mineral by phosphorus 31 NMR spectroscopy, which are not present in synthetic calcium phosphate apatites [7]. Structural studies were first carried out to determine crystal size by measuring the extent of X-ray diffraction peak broadening [8], which yielded crystal sizes varying from 31 to 290Å. More detailed structural data were obtained by the then recently introduced field of electron microscopy and electron diffraction [9–11], which revealed that the bone crystals were thin plates, approximately 500Å long, 250Å wide, and 100Å thick. However, calculations from low-angle X-ray diffraction scattering studies [12–14] were more consistent with the conclusions that the bone crystals were very much smaller than those observed by Osteoporos Int (2009) 20:1013–1021 DOI 10.1007/s00198-009-0860-y