Tooth material

In the late 1940s a reaction against this idea of a dental material took place. Increasing attention was paid to problems of compatibility between the restoration and the tooth. We now believe that a restorative should be at one with the tooth material in all respects. It should possess identical properties. Its thermal characteristics should be the same as those of the tooth and its appearance should match that of the enamel. It should provide some therapeutic action. In fact, a restorative material should no longer be regarded as a filling but as an enamel or dentine substitute . 

To achieve such compatibility the primary requisite is that the restorative adheres to tooth material. This concept of adhesion is hardly to be foimd in the literature of the 1920s and 1930s. For that reason we find no attempt at developing tooth adhesives in that period. Adhesion was, apparently, only recognized as a desirable property in the 1950s. It seems for some reason to be associated with the introduction of simple resins as dental restorative materials. Although they were not a great success, attempts were made to bond them to tooth material. 

There are many obstacles to permanent adhesion under oral conditions. The substrate is a biological tissue and subject to change, and the presence of moisture represents the worst kind of situation for adhesion. Water is the great barrier to adhesion. It competes for the polar surface of tooth material against any potential polymer adhesive. It also tends to hydrolyse any adhesive bond formed. These twin obstacles gave rise to considerable doubt as to whether materials adhesive to tooth material could be developed at all (Cornell, 1961). 

Another cause of inflammation is leakage of bacteria from the mouth at the interface between the cement and tooth material. Adhesion at the interface reduces this effect. 

Glass polyalkenoate cement has a unique combination of properties. It adheres to tooth material and base metals. It releases fluoride over a long period and is a cariostat. In addition it is translucent and so can be colour-matched to enamel. New clinical techniques have been devised to exploit the unique characteristics of the material. 

The glass polyalkenoate cement uniquely combines translucency with the ability to bond to untreated tooth material and bone. Indeed, the only other cement to possess translucency is the dental silicate cement, while the zinc polycarboxylate cement is the only other adhesive cement. It is also an agent for the sustained release of fluoride. For these reasons the glass polyalkenoate cement has many applications in dentistry as well as being a candidate bone cement. Its translucency makes it a favoured material both for the restoration of front teeth and to cement translucent porcelain teeth and veneers. Its adhesive quality reduces and sometimes eliminates the need for the use of the dental drill. The release of fluoride from this cement protects neighbouring tooth material from the ravages of dental decay. New clinical techniques have been devised to exploit the unique characteristics of the material (McLean Wilson, 1977a,b,c Wilson McLean, 1988 Mount, 1990). 

The glass-ionomer cement is the most versatile of all the dental cements and has been developed for a variety of applications (McLean Wilson, 1974, 1977a,b,c Swift, 1988b van de Voorde, 1988 Wilson McLean, 1988 Mount, 1990). Many of its applications depend on its adhesive quality which means that, unlike the non-adhesive traditional filling materials, it does not require the preparation of mechanical undercuts for retention and the consequent loss of sound tooth material. 

The glass polyalkenoate cement was originally intended as a substitute for dental silicate cements for the aesthetic restoration of front (anterior) teeth (Wilson Kent, 1972 Knibbs, Plant Pearson, 1986a Osborne Berry, 1986 Wilson McLean, 1988). It is suitable for restoring anterior cavities in low-stress situations, that is when the restoration is completely supported by surrounding tooth material. These cavities occur on the adjacent surfaces of neighbouring teeth (class III cavities) and at the gum line (class V cavities). 

Laswell et al., 1971 Arato, 1974). All were prone to excessive dissolution and only one had adequate strength and film thickness. Their working characteristics were found to be unduly sensitive to changes in temperature and humidity (Simmons, D Anton Hudson, 1968). All were inferior to conventional zinc phosphate cements. No further development of these cements has taken place, nor is it likely that interest in them will be revived. The modem water-activated glass-ionomer cement has filled this niche and has vastly superior properties including adhesion to tooth material. 

The main line of development now lies with its successor, the glass-ionomer cement, which uses a similar glass, but in which phosphoric acid is replaced by poly(acrylic acid) this cement is more resistant to acid erosion and staining and has the great advantage of adhesion to tooth material. 

Aluminium ions released from the dental silicate cement are also absorbed by hydroxyapatite and have a similar beneficial effect to that of fluoride (Halse Hals, 1976 Putt Kleber, 1985). Thus, the dental silicate cement confers protection against caries (dental decay) on surrounding tooth material. 

Another biological disadvantage is that dental silicate cement does not bond to tooth material, and harmful substances and bacteria can percolate between it and the tooth, giving rise to secondary caries and pulpal irritation (Going, Massler Dute, 1960). These effects are magnified when dissolution of the cement occurs. 

For certain AB cements, used in dentistry, optical properties are important for their overall acceptability as materials. The two particular properties of interest have been colour and translucency, both of which need to match natural tooth material as closely as possible if good aesthetics are to be developed (Wilson McLean, 1988). Of the AB cements currently used in dentistry, the glass-ionomer cement has the best aesthetics, since it has a... 

Evaluation of these optical properties may be done by simple observation this approach is useful clinically (Knibbs, Plant Pearson, 1986), since acceptability of the colour match to the surrounding tooth material can be readily seen without the need for instrumental measurement. On the other hand, for quantitative evaluation of optical properties, some kind of instrumental measurement is necessary, and the property usually evaluated is opacity. 

Colour and opacity have been foimd to be connected for glass-ionomer cements (Crisp et al., 1979 Asmussen, 1983), with darker shades giving increased opacity. However, this is merely a consequence of the underlying physical relationships, and is not thought to be a clinical problem (Wilson McLean, 1988), mainly because the stained tooth material for which the darker shades are necessary for colour match is itself of reduced translucency. 

For many years, the process of caries was thought to be irreversible and to result in permanent loss of tooth material. This process eventually leads to the development of a cavity, and a considerable part of the dental professions time... 

Another example is LIBS application for real-time identification of carious teeth (Samek et al. 2003). In the dental practice, usually more healthy tissue is removed than ultimately necessary. Carious and healthy tooth material can be identified through the decrease of matrix elements Ca and P in hydroxyapatite and/or the increase of non-matrix elements, typically Li, Sr, Ba, Na, Mg, Zn and C, using pattern recognition algorithms. A fiber-based LIBS assembly was successfully used for this task. As for the case of phosphate ores evaluation, the efforts aimed at normalizing the spectrum collection conditions and procedures, so that the spectra are sufficiently reproducible for precise quantitative... 

The properties exhibited by polyelectrolytes make them nearly-ideal candidates for dental material formulations. Dental polyelectrolytes are generally considered to be nontoxic and are able to adsorb chemically to the hydrophilic surface of tooth material through ionic interactions. Ionic cross-linking of the polyelectrolyte with multivalent cations (Zn2+, Mg2+, Al3+, Ca2+) results in the formation of a rigid and insoluble cement matrix. The stability and strength of the cement is attributed to the fact that, if a bond is broken, it can be reformed as long as the other bonds are maintained. Even today, polyelectrolytes are the only materials which are known with certainty to form a bond, which is stable with time, to tooth material [120]. In addition to long-term stability, many polyelectrolytes are translucent and possess cariostatic properties [121]. 

A variety of organic adhesives which are capable of forming strong bonds between a polymeric (acrylate) restoration and the hydrophilic tooth material have recently been developed. A number of these monomers, which possess a pendent ionizable group, are polymerized in the mouth to form an adhesive layer. Alginates, which are used as impression materials, are formed by the reaction of the sodium salt of anhydro-beta-d-mannuronic acid with calcium sulfate. Calcium ions crosslink the linear polymer to form a gel. This reaction is carried out inside the mouth, and the gel formed retains the shape of the oral cavity. 

The nature and type of initiation scheme plays an important role in the performance of the adhesive [194,202-204]. Stresses due to polymerization shrinkage lead to the creation of a gap between the adhesive and tooth material. In the case of bulk chemical initiation, shrinkage stresses tend to create gaps at all interfaces, drawing material inward isotropically. With a photoinitiation scheme, polymerization begins at the free surface and pulls the material away from the dentin towards the free surface [194]. Thus the gap is created at the... 

Lack of adhesion of a dental restoration to tooth structure results in microleakage at tooth-restoration interface. This occurrence can result in discoloration at the margin of the restoration, or in the formation of caries. Occlusal forces on the restoration and differences between the coeffidents of thermal expansion of the cement and tooth material can lead to leakage. In addition, oral fluids and moisture may affect the adhesion. Microleakage of composite resin restorations has been reviewed by Ben-Amar [233]. Microleakage is not as serious a problem with glass-ionomer cements as it is with resin-based restorative materials, due to reduced polymerization shrinkage [234]. 

The reason to extend the experiments to tooth material was the idea that the matrix would have a less porous structure compared to human haversian bone and be less exposed to diagenetic alteration. While the porosity in human bone is mainly determined by a complicated network between the Haversian system and the Volk-mann canals that are perpendicular to it, especially enamel is a far denser material than human bone and its organic content is significantly less (2% of organic material only). But in contrast to the enamel, dentine has a similar composition of the organic and the inorganic matrix compared to bone, and it has a high microporosity due to nerve canals that start from the pulpa and stop close to the enamel-dentine junction (edj). However, these nerve canals have a smaller diameter than a haversian pore (70 pm) and the canals are orientated parallel and are not connected with each other. So a fluorine ion cannot percolate from one pore to another, as it is the case in a human bone, but it has to overcome the distance from one canal to the next one by diffusion. So the permeability is low and this results in a smaller diffusion rate D. 

A familiar use of some fluoride compounds is in toothpastes. Studies show that small amounts of fluorides can help reduce tooth decay. Fluorides are deposited as new tooth material is formed, making it strong and resistant to decay. 

Teeth are a good indication of past exposure to metals because of their physical stability. The tooth material is digested in a Teflon vessel using bomb combustion at elevated temperature and pressure followed by diluting in deionised water to a known volume and analysed against a standard calibration curve for metals of interest. 

Fig. 3. H SPI images of human teeth a. shows a three-dimensional maximum intensity image of a 64 data set, TR, 100 ms b. is a slice from the same tooth, taken just below the cusp c. shows another tooth with an amalgam-filled caries. The bright areas are caries under the amalgam filling, the grey areas are healthy tooth material, and the black area the filling. Note that there is no apparent image distortion caused by the metal amalgam filling." ...

Early non-cavitated carious lesions only may be repaired by remineraUzation processes. However partly because of the uncertainty in outcome, and partly because results are more reliable, the method of choice for the repair of a tooth damaged by caries is surgical removal of the carious region, followed by repair with some sort of synthetic material. The act of cutting out damaged tooth material (enamel and dentine) is known to compromise the mechanical properties of the tooth [43], as shown by the results in Table 1.1. In this study, a set of 10 non-carious teeth was used per experimental set. They were mounted in dental stone and tested in compression, with the load at failure in kilograms recorded. 

Dental implants and prosthetics alone account for a substantial proportion of the dental industry. It is therefore of no surprise that researchers focus heavily on this avenue. In contrast to other industries, dental industry nanocoatings do not perpetually involve the deposition of thin nanolayers onto a substrate. Often they can refer to the incorporation of nanostructured materials or particles into coatings on contact surfaces. For instance, a recent piece of work examined the usage of nanostructured hydroxyapatite (HA) as a filler material for root canal. HA (a commonly used material in coating implants to aid cell proliferation) particles sized at approximately 26 mn were incorporated into root canal sealer at variable ratios. At high concentrations, there was little difference in film thickness (implying they would meet ISO standards for root canal sealers). The observed improvements suggested that nanostructured HA could be used to formulate more stable tooth material interfaces [40]. 

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