Surface enamel layer

Dentin constitutes the bulk material of all vertebrate teeth. The outer working surface of a vertebrate tooth is composed of a much stiffer material called enamel, or in the case of fish, enameloid [6]. The two materials work together during mastication to provide the tooth with its functional properties [33]. In general the softer dentin functions in distributing and absorbing the compressive stress that is transmitted through the outer enamel layer [26],... 

Adhesion of fired enamel to the metal base is assessed practically by impact tests (e.g. by a falling steel ball). If the adhesion is poor, tfie enamel will peel off on impact, showing a clean metal surface. More precise determinations are based on tensile tests of metal specimens joined with an enamel layer. Very good adhesion is then indicated by failure in the enamel and not in the contact surface. 

Finally, the ameloblasts secrete an unusual basal lamina, which partially mineralizes into the surface enamel crystals, the enamel cuticle. By this time, the enamel organ has shrunk so that its outer layers have merged with the ameloblast layer at the completed enamel surface postmaturation stage). This reduced enamel epithelium is rubbed off as the teeth erupt, but the enamel cuticle is abraded more slowly and gradually replaced by proteins from saliva, the acquired pellicle (Sect. 12.1.3). 

Fluorine compounds added to toothpaste and public drinking-water supplies have greatly reduced the incidence of cavities. Fluoride protects teeth in two ways. As teeth form, fluoride from food and drink is incorporated into the enamel layer. The fluoride makes the enamel stronger and more resistant to decay. Once teeth are present in the mouth, fluoride in saliva bonds to teeth and strengthens the surface enamel. This surface fluoride attracts calcium, which helps to fill in areas where decay has begun. 

The simplest technological process of manufacturing gel-based inhibited films is co-extrusion of a polymer blend (PE) with Cl or its solution in a plasticizer, which was realized in the USSR in the 1980s. This, however, worsened properties of the Cl due to joint processing with PE melt and irrational consumption since Cl isolated from both sides of the film. The latter feature was partially eliminated by application of a lacquer or enamel layer on the hose film surface [50]. 

Rubber cup prophylaxis with pumice paste removes a small amount of surface enamel, typically corresponding 0.6. 0pm of the outer layer [139]. The actual amount removed depends on the speed of the hand-piece, the abrasivity of the paste and the time spent cleaning the teeth [139,140]. The layer removed includes the fluoride-rich layer formed as a result of exposure to fluoridated toothpastes. This level of fluoride can be replaced by applying further fluoride topically after the polishing step [141]. 

Initially, the front end of the tungsten microrod is dipped into an enamel layer of a uniform thickness to get the desired insulation length. A stainless steel sheet having a central hole of a 2.5-mm diameter, with top and bottom surfaces insulated by enamel, was used as the cathode. A front end insulated microtool was used as the anode, positioned at the center of the hole, and immersed in the electrolyte. The microtool was machined by EMM with different machining parameters and then insulation was removed by dipping the front end of the microtool in acetone. 

The well-known fluoride gradient decreases very sharply with depth from the enamel tooth surface towards the amelo-dentinal junction [318,350,251]. Minor variations in the thickness of outermost enamel layers can profoundly affect analysis unless proper care is exercised to avoid extraneous contamination. Biopsy techniques are thus difficult since sampling alters teeth surfaces and two identical surface areas do not exist [318]. Abrasion with silicon carbide-glycerol slurries for less than 1 min has proved quite satisfactory for this purpose. Samples of about 40 /ig, and equivalent to about 0.3 jum enamel, are either collected in plastic [351] or rubber cups [318], dissolved in 0.5 M perchloric acid and analysed directly with the fluoride electrode. Most teeth except those posteriorly located can be analysed by the abrasion technique. 

Thus any biopsy technique must control depth and total enamel removed since a thick layer will be invariably lower in fluoride than a thin layer from the same enamel. A major criticism of this biopsy technique is that the pressure, area and depths of the enamel regions examined are not carefully standardised and an electrically driven abrasive device is considered to facilitate greater control of quantitative removal of surface enamels [320,321]. 

The flow of enamel on vertical surfaces of vitreous enamelware while it is being fired the fault is visible as roughly horizontal lines or waves and is caused by the enamel being too fluid, by overfiring, or by the enamel layer being too thick. 

Phosphoric acid has traditionally been used as a conditioner of enamel and dentin for adhesive restorative treatments, as desalbed in section Polymers in Dental Adhesion . 37 % phosphoric acid has been shown to decalcify enamel and dentin in a desirable pattern, thus facilitating impregnation of adhesive monomers required for placement of composite resin restorations in the tissue matrix. Despite the known efficiency of 37 % phosphoric add gels in acid-etching enamel and dentin, it has been shown that its effects in increasing the surface porosity of the pseudo-intact surface layer of enamel lesions was not sufficient [117]. To overcome this limitation, 15 % hydrochloric acid (generally for 2 min) has been tested and shown to remove about 30 pm of surface enamel, thus allowing for much improved penetration of viscous resins in white spot lesions [116]. 

In the glass-bottle industry the bottles can be cooled in a dilute SOj/SO, atmosphere to increase chemical resistance. A similar effect has been noted with vitreous enamel. It has been postulated that a thin layer of —OH groups or — OH—HjO (hydronium) ions is adsorbed on the surface of a fired enamel. These ions are transformed into — OSO, or —OSO3 in the presence of oxides of sulphur which are more resistant to further acid attack. It is known that the acid resistance of a recently fired enamel improves on ageing, probably due to the enamel reaction with SOj/SO, in the atmosphere and it is quite common for the grading to improve from Class A to Class AA (BS 1344). 

The bond strength to enamel (2-6 to 9-9 MPa) is greater than that to dentine (1-5 to 4-5 MPa) (Wilson McLean, 1988). Bond strength develops rapidly and is complete within 15 minutes according to van Zeghbroeck (1989). The cement must penetrate the acquired pellicle (a thin mucous deposit adherent to all surfaces of the tooth) and also bond to debris of calciferous tooth and the smear layer present after drilling. Whatever the exact mode of bonding to tooth stmcture, the adhesion is permanent. The principles and mechanism of adhesion have already been discussed in Section 5.2. 

Washing machines currently on the market are constructed almost exclusively with drums and laundry tubs of corrosion-resistant stainless steel or with an enameled finish that is inert to alkaline wash liquors. Nevertheless, various machine components are made of less detergent resistant metals or alloys. To prevent corrosion of these parts, modern detergents contain corrosion inhibitors in the form of sodium silicate. The colloidal silicate that is present, deposits as a thin, inert layer on metallic surfaces, thereby protecting them from attack by alkali. 

In coronal caries, the enamel of the tooth crown is affected. With lasting caries, the lesion deepens and acquires a conical shape. In polarized light microscopy, zones with different mineral densities can be distinguished, such as the lesion body and the mineralized surface layer... 

---