Tooth

Tooth is another calcified phase in human and other vertebrates. Unlike bone, teeth consist of at least two different biominerals, the enamel (crown, the part above the gum line) and dentin (roots, the part below the gum line) [41], 

The nanosized HA building block in dentin is smaller than that of enamel, being about 25 nm wide, 4nm thick and 35 nm long. Dentin is similar to bone in many respects, including a similar composition and hierarchical structure up to the level of the lamellae bone hence, most of the statements made above relating to bone are equally applicable to dentin. Although the chemical composition of mineral in 

Figu re 13.1 Schematic illustration (not drawn to scale) of the hierarchical assembly of enamel structure, from the millimeter to the nanometer scale. The ruler below the diagram demonstrates the typical scale distribution of each assembly level. IP, interprism L, 

prisms appear as bands of approximately cross-sectioned Y, prisms are relatively longitudinally arrayed. Reprinted from Ref [47]. 

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Tooth decay Tooth enamel Toothp aste... 

A number of salts of the monofluoro- and hexafluorophosphoric acids are known and some are commercially important. The salts of difluorophosphoric acid are typically less stable toward hydrolysis and are less well characterized. Sodium monofluorophosphate [7631-97-2] the most widely used dentifrice additive for the reduction of tooth decay, is best known (see Dentifrices). Several hexafluorophosphates can be prepared by neutralization of the appropriate base using hexafluorophosphoric acid. The monofluorophosphates are usually prepared by other methods (57) because neutralization of the acid usually results in extensive hydrolysis. 

Sodium monofluorophosphate is used ia most dentifrices at a concentration of 0.76 wt % which produces the desired fluoride level of 1000 ppm although one extra strength dentifrice has 1.14 wt % and 1500 ppm F. Although the mechanism of its efficacy ia reducing dental decay is not completely understood (75), it almost certainly reacts with the apatite of the tooth converting it to fluoroapatite which is less soluble ia mouth acids (see Dentifrices). 

Dental x-rays provide valuable information on the health of teeth which cannot be obtained by any other medical imaging modaUty. Dental x-ray procedures use a piece of film placed in the mouth between the tongue and the teeth. A 60 to 70 keV source of x-rays, located outside the mouth, is directed at the film. Metal fillings attenuate x-rays striking the film and therefore appear white in a projection image. Tooth decay appears dark as it attenuates x-rays less than normal tooth enamel. 

Medicine. The polymethacrylates have been used for many years in the manufacture of dentures, teeth, denture bases, and filling materials (116,117) (see Dental materials). In the orthodontics market, methacrylates have found acceptance as sealants, or pit and fissure resin sealants which are painted over teeth and act as a barrier to tooth decay. The dimensional behavior of curing bone-cement masses has been reported (118), as has the characterization of the microstmcture of a cold-cured acryUc resin (119). Polymethacrylates are used to prepare both soft and hard contact lenses (120,121). Hydrogels based on 2-hydroxyethyl methacrylate are used in soft contact lenses and other biomedical appHcations (122,123) (see Contactlenses). 

Article of Manufacture. An article of manufacture is an iavention such as a two-headed tooth bmsh, an iatravenous fluid bag, or an optical fiber "made" by a machine. One example of an iavention which could be considered an article of manufacture is U.S. Patent No. 5,241,990 tided "Irrigation/Aspiration Valve and Probe for Laparoscopy" (2). 

For many years, there has been concern by medical professionals and nutritionists over the effects of dietary sugar on human health. Sucrose has been imphcated as a cause of juvenile hyperactivity, tooth decay, diabetes meUitus, obesity, atherosclerosis, hypoglycemia, and nutrient deficiencies. 

Anticariogenicity. Sugar alcohols are not fermented to release acids that may cause tooth decay by the oral bacteria which metabolize sugars and starches (208). As a result, use of sugar alcohols in sugar-free chewing gum, pressed mints, confections, and toothpaste has been widely accepted. 

The tetracyclines are strong chelating agents. Both the A-ring and 11,12 P-diketone systems are active sites for chelation (16). This abiUty to chelate to metals, such as calcium, results in tooth discoloration when tetracycline is administered to children (17). 

The gravity take-up must be used when handling hot materials in order to maintain chain and sprocket tooth engagement, as the chain length changes with thermal expansion. 

The precerammed billets, already having appropriate tooth color, are loaded into a dedicated high temperature press and transfer-molded at 1100°C into phosphate-bonded molds. 

When freshly mixed, the carboxyHc acid groups convert to carboxjiates, which seems to signify chemical adhesion mainly via the calcium of the hydroxyapatite phase of tooth stmcture (32,34—39). The adhesion to dentin is reduced because there is less mineral available in this substrate, but bonding can be enhanced by the use of minerali2ing solutions (35—38). Polycarboxylate cement also adheres to stainless steel and clean alloys based on multivalent metals, but not to dental porcelain, resin-based materials, or gold alloys (28,40). It has been shown that basic calcium phosphate powders, eg, tetracalcium phosphate [1306-01-0], Ca4(P0 20, can be substituted for 2inc oxide to form strong, hydrolytically stable cements from aqueous solution of polyacids (41,42). 

The formulation of calcium chelate materials is based upon the formation of a low-solubiUty chelate between calcium hydroxide and a sahcylate. Dycal utilizes the reaction product of a polyhydric compound and sahcyhc acid. Other sahcyhc acid esters can be similarly used. Vehicles used to carry the calcium hydroxide, extenders, and fillers may include mineral oil, A/-ethyl- -toluenesulfonamide [80-39-7] and polymeric fluids. The filler additions may include titanium dioxide [13463-67-7] zinc oxide, sihca [7631-86-9], calcium sulfate, and barium sulfate [7727-43-7]. Zinc oxide and barium sulfate are useflil as x-ray opacifying agents to ensure a density greater than that of normal tooth stmcture. Resins, rosin, limed rosins, and modified rosins may serve as modifiers of the physical characteristics in both the unset and set states. 

The calcium chelate cements are limited to the use of a cavity liner. They may be placed directly over an exposed tooth pulp to protect the pulp and stimulate the growth of secondary dentin, or used as a therapeutic insulating base under permanent restorations. The high alkalinity and high solubihty of these materials prohibits use in close proximity to soft tissues or in contact with oral fluids. 

The chelated calcium cementing materials are suppHed as two-part paste products. In use, equal parts of the two pastes are thoroughly mixed together to give a fluid mass that can be appHed without pressure over an exposed tooth pulp or in a deep-seated cavity. Under the influence of the oral temperature and humidity, the fluid mass sets to a hard, strong, therapeutic protective seal. 

Resin cements have excellent aesthetic quahties and are essentially insoluble in mouth fluids. Compressive strength is low, but can be increased by the addition of fillers. They have no inherent adhesion to the tooth. Retention is dependent on mechanical locking when the cement flows into irregularities on the surfaces of the substances being cemented. 

Type IV dental stones are used to make casts for a single tooth, for crown or inlay work, and for a complete dental arch. 

The linear thermal expansion of base-plate waxes should not exceed 0.8% at 25—40°C. It is desirable to invest any waxed-up case as soon after waxing is completed as possible. This minimises changes in articulation owing to tooth shift, and changes in palatal thickness owing to lifting of the palatal section by wax shrinkage caused by variations in room temperature or by the release of stress. 

Impression Waxes. Impression waxes include those waxes used to obtain a negative cast of the mouth stmcture (impression waxes), waxes used to estabhsh tooth articulation (bite-registration waxes), and waxes used to detect tooth interference and high spots or improper fit of denture bases (disclosing waxes). They must be plastic and moldable at mouth temperatures, and chill to a stiU nonplastic mass upon cooling within a few degrees below mouth temperature. 

Study waxes ie, waxes for carving, are useflil in the study and modeling of tooth forms and the teaching of anatomical detail. Carving wax compositions include paraffin, ceresin, ozokerite, camauba wax, montan waxes, and Acrawax C. Fillers and pigments may be added. 

The phases and their proportions present ia hardened amalgam are controlled by many factors. The composition of the alloy the size, shape, and size distribution of the particles the thermal history of the cast ingot and the comminuted alloy and the surface treatment of the particles are some of the factors for which the manufacturer is responsible. The tooth cavity preparation and the mixing, compacting, and finishing techniques of the dentist can make the difference between satisfactory and unsatisfactory restorations, even with the best of alloys. A minimal amount of residual mercury and porosity are needed to obtain the most serviceable restorations (138). 

Amalgam restorations are prepared by mixing a powdered alloy with mercury to form a plastic moldable mass that is packed or condensed iato the prepared cavity. The cavity is designed to provide mechanical retention, maximum marginal mass, support to absorb the functional stresses transmitted through the restoration, and maximum protection to the remaining tooth stmcture. The restoration reestabUshes the normal tooth anatomical form and function. 

Next, the amalgam must be condensed properly ia the prepared, clean, and dry cavity. In general, the maximum effective load tolerated by the patient should be used on a condenser of optimum size, to remove as much mercury from the mass as possible. The amalgam must be carved to the contour of the lost tooth tissue it is replaciag. 

The stiffness of pure titanium can be increased slightly by alloying alloys such as Ti—6A1—4V may be specified for partial dentures requiring additional rigidity. Titanium appHances do not tarnish or corrode in the mouth, have no metallic taste, and are easy to clean because plaque and calculus do not adhere to them. The relatively low thermal conductivity of titanium (relatively close to that of tooth enamel) gives these appHances a seemingly natural feel in the mouth and minimises thermal sensitivity (175). 

Radiopaque materials are used to determine the location of aspirated dentures and fragments (205,206). Opacifying additives include barium sulfate, barium fluoride, barium or bismuth glasses, and brominated organic monomers and polymers. The incorporation of these additives into the resin base or tooth can adversely affect physical properties. Radiopaque materials meeting the requirement for ANSI/ADA specifications for denture-base polymer have been described (207). 

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