Enamel mineral phase

Rey, C., Renugopalakrishnan, V., Shimizu, M., Collins, B. and Glimcher, M.J. 1991 A resolution-enhanced Fourier transform spectroscopic study of the environment of the COj ion in the mineral phase of enamel during its formation and maturation. Calcified Tissue International 49 259-268. 

In addition, the presence of low concentrations of fluoride in saliva also has the effect of preventing dissolution of hydroxyapatite from enamel at low pH, an effect that has been shown to apply at values of pH as a low as 4.2 [54,55], Thus, it is the fluoride in solution that has the effect of reducing solubility, rather than the fluoride in the mineral phase [51], This effect requires extremely small amounts of fluoride, typically in the sub-ppm range [51,56], and has the effect of shifting the balance between demineralisation and remineralisation so that loss of the hard tissue is inhibited. 

Fluoride also brings about a change in composition in natural hydroxypatite, since it not only undergoes a simple exchange with hydroxyl ions but also promotes the formation of a phase containing less carbonate than the initial hydroxyapatite [65]. Fluoride is taken up more readily by demineralised enamel than by sound enamel [66], which means its availability causes a self-healing effect in the mineral phase of the hard tissue. 

The nature of mineral phases present in bone, dentin, enamel and other phosphatic tissues, and their mode of formation have been subjects of lively discussions among health scientists and crystallographers. Bioscientists most commonly accept the viewpoint that the inorganic phase of bones or teeth is principally hydroxyapatite, Caio(P04)6(OH)2, and deviation in Ca/P ratio from common hydroxyapatite (Ca/P = 1.667) observed in mineralized tissues is explained by the presence of amorphous phosphates. In contrast, many crystallographers favor the idea of carbonate apatite, i.e. dahllite, as the major crystalline phase in biophosphates and they doubt the existence of amorphous phases. The topic has been reviewed14,15,22,28, 37,44,47,348-358) no common consent has yet been reached. In the following an attempt is made to at least coordinate the controversial findings. 

In enamel and hypermineralized dentin more than 100 repeat periods were counted across one single crystal. However, in normal bone and dentin, individual crystals show fewer repeats, commonly less than 10 and as little as 4 this would correspond to a cross-sectional width of about 40 to 90 A360. The length of the crystals is in the range of a few 100 A. In summary, at least two morphologically distinct types of crystallites are present in bones and teeth, and the smaller-sized fraction represents the bulk of the mineral phase. 

It is true, of course, that in vitro experiments will produce other mineral phases, but since these systems are not at equlibrium within a physiological system, there is no reason for accepting them as valid histochemical similitudes. Never has a diffraction pattern of normal bone (or dental enamel) been shown to contain interference maxima (lines) of any other... 

This value is higher than those cited earlier, possibly because of our use of relatively newly erupted enamel, which is less crystalline, and hence more soluble than enamel which has matured [63], However, it is very close to one value quoted for dissolution of bulk human enamel [64] and very close indeed to the value calculated by Zhang et al. [46], who suggested that demineralisation kinetics were controlled in part by a mineral phase with an apparent Ken of 2 X 10-58 mol9l 9. Given the divergence of reported values for both human and bovine enamel, it seems unlikely that comparison of Ken values alone is useful when comparing the behaviour of enamel types, except in cases where experimental conditions are very similar. 

Another recent, nontraditional investigation involved the use a of pump-probe technique to study the laser-ablation of tooth enamel [80] and the influence of an optically thick water layer applied onto the tooth surface [81]. Laser ablation was performed in the presence and absence of water, and synchrotron FT-IRM used to probe the chemical composition of the enamel in the thus-formed crater. FT-IRM revealed the formation of new mineral phases deposited along the crater walls after repetitive laser pulses, while the nonapatitic phases reduced the efflciency of... 

The hard tissue of the tooth substance consists of a protective outer coat of enamel and an underl nng dentin phase. The latter, in turn, connects to the inner core of soft tissue pulp), which is interpenetrated by nerve strands and blood vessels. The enamel, which covers essentially the visible part of the tooth and indeed represents the hardest tissue in the body, is composed almost entirely (97% by weight) of mineral-type hydroxyapatite (a crystalline calcium phosphate) in addition to a few percent of water and organic, mostly proteinaceous, matter. Dentin, constituting the major proportion of tooth substance, contains less mineralized phase (69% hydroxyapatite) but a comparatively large proportion of organic matter and water. Compositional and physical property data for enamel and dentin [3] are summarized in Tables 1 and 2. 

Similar effects can occur in patients who suffer from oesophageal reflux as part of a wider range of medical problems, including hiatus hernia [114], Once again, the low pH of the reflux causes erosion of the mineral phase of the tooth enamel, with specific inner surfaces of the maxillary teeth being the most affected. 

To understand some of these challenges, it is necessary to consider the anatomy of the tooth. In particular, the composition and stracture of the two main tissues, enamel and dentine, need to be examined in order to understand how they influence adhesive bonds. Details of the composition of these tissues are shown in Table 5.1, from which it can be seen that the enamel comprises a much greater amount of mineral phase than the dentine. Consequently it is harder and stronger, and is also more brittle [4]. 

The success of acid-etching is due in part to the structure and composition of enamel [4]. As can be seen in Table 5.1, enamel contains almost no protein phase or water. Hence a fraction of the mineral phase can be removed without causing the enamel structnre to collapse. This procedure produces a chalky appearance in the enamel, and once this develops, the etching process is considered to be complete and the enamel is ready to receive the liquid resin component. A scanning electron miCTo-scope (SEM) image of etched human dentine is shown in Fig. 5.1. 

Properties of this material have been reported. It has been found to bond to enamel and dentine, and to do so reliably and with good durability [107]. Results from X-ray photoelectron spectroscopy (XPS) and fourier transform infrared spectroscopy (FTIR) show that this bonding is the same as for conventional glass-ionomers, and involves the formation of chemical bonds to calcium in the mineral phase of the tooth. There is also evidence of micromechanical adhesion in this material [107]. 

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... 

The tooth consists essentially of two types of tissue, the enamel and the dentin. Their compositions are shown in Table 56.2. From this it can be seen that the mineral phase of the enamel far exceeds that of the dentin, resulting in the enamel being much the harder and more brittle of the two tissues. 

In order to carry out the various processes involved in dental repair, the tooth generally has to be cut with some sort of instrument. This is usually a rotary bur of the so-called dental drill. The cutting process results in the formation of a layer known as the smear layer on the surface of the tooth (O Fig. 56.1). The smear layer is thin, typically 1-2 pm deep, and is very tenacious. It consists of mineral phase embedded in denatured collagen, and is essentially material with a less ordered structure than uncut enamel or dentin. 

Body fluids have a very high supersaturation with respect to hydroxyapatite, which cannot be explained by the small particle size of bone mineral. In fact, they behave as agueous solutions which are in metastable eguilibrium with DOHA. However, the minerals in bone, dentin, dental enamel and dental calculus contain considerable amounts of Na, Mg and CO3, in addition to calcium and phosphate which are the major components. Therefore, the phases mentioned above which all show a solubility comparable to that of DOHA, all come into consideration as components of these minerals. 

The use of high-concentration gels and varnishes has been practised clinically for many years by dentists and dental hygienists [180]. When originally formulated, they were designed to be used in application procedures based on the concept that fluoride becomes incorporated into the crystalline phase of the enamel and leads to the development of a more acid-resistant form of apatite. They were not expected to make any difference to the levels of fluoride in saliva, or to influence the demineralisation/dissolution phase of the behaviour of tooth mineral. 

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