Apatite, biological crystal structure

Ideally, hydroxyapatite has the formula mentioned above. The synthetic material usually contains fewer than 10 Ca-ions and more than 2 OH-ions per crystal unit. Important differences in crystal structure, composition and specific surface exist between synthetic and biologic apatite. These differences result from the processing method of the raw materials and the synthetic method used. 

In their publication, LeGeros and LeGeros (1993) oudine the different apatites, ranging from natural apatite (minerals) to biological (human dentin, enamel, and bone) and synthetic (chemically synthesized) apatite. This publication clearly shows that apatite is a group of crystalline compounds. The most important of these compounds is calcium hydroxyapatite. All the related crystal structures, such as fluoroapatite, chloroapatite, and carbonate apatite are derived from it. 

Biological apatite crystals are too small to allow direct measurement of their density. However, the calculated density from estimated unit cell contents (see later section on atomic structure of biological apatites) is about 5% less than HAP (Elliott 1997). 

Zapanta-Le Geros et al. (1970) have recently reviewed the subject of infrared spectra of carbonate-containing synthetic and biological apatites. Klein et al. (1970) have used the polarized infrared specular-reflectance technique to study single crystals of apatites. They found this technique to be a more powerful method for the analysis of the vibration spectra of crystalline structures than the powder absorption techniques. 

Substitutions in the HA structure are possible. Substitutions for Ca, PO4, and OH groups result in changes in the lattice parameter as well as changes in some of the properties of the crystal, such as solubility. If the OH" groups in HA are replaced by F" the anions are closer to the neighboring Ca " ions. This substitution helps to further stabilize the structure and is proposed as one of the reasons that fluoridation helps reduce tooth decay as shown by the study of the incorporation of F into HA and its effect on solubility. Biological apatites, which are the mineral phases of bone, enamel, and dentin, are usually referred to as HA. Actually, they differ from pme HA in stoichiometry, composition, and crystallinity, as well as in other physical and mechanical properties, as shown in Table 35.7. Biological apatites are usually Ca deficient and are always carbonate substituted (COs) " for (P04). For... 

Mention has already been made of substitution within the crystal lattice. A foreign ion, provided that it is similar in size to the ion which it replaces, may exchange for a normal hydroxyapatite constituent. This process is called heteroionic exchange, whereas the exchange of like ion for like is called isoionic exchange. There are, thus, two ways in which ions other than calcium, phosphate or hydroxyl may become part of the structure of biological apatite. Firstly,... 

In spite of considerable research no particular chemical structures have been found to have a clear-cut epitactic function for the nucleation of biological apatite crystals. A bone morphogenetic factor has been postulated but, although active extracts have been prepared from cultures of mineralizing bone, no specific substance has yet been isolated. 

The final result is a pattern of biological apatite crystals which is appropriate to the mechanical functions of the particular tissue. While the same general principles apply, the details of the process will vary between different mineralized tissues producing their characteristic structures. 

The other ceranfic widely used are phosphate salts of calcium, with the chosen phase usually being hydroxyapatite. This material is conventionally prepared by thermal methods at temperatures well in excess of 1000°C. As a result of their preparation at high temperatures, the salts are carbonate free and are made up of much larger and more perfect crystals than those found in biological apatite minerals including bone. The imperfect crystalline structure of bone mineral leads to the natural material being soluble and reactive with respect to body fluids. In contrast, the synthetic materials are much less reactive than those found in living tissue and problems with biocompatibility can arise. 

Figure 13.7 Schematic model of apatite evolution via the conglutination of HA nanocrystallites. Under the control of biological components such as Gly and Glu, the HA subunits could be reorganized. The modifiers could determine the different evolutionary forms, for example, onedimensional linear assemblies or two-dimensional plates. The crystallized HA were cemented by the amorphous phase with the flexible structure. The AGP could transform...

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