Dentine properties

The properties described above have important consequences for the way in which these skeletal tissues are subsequently preserved, and hence their usefulness or otherwise as recorders of dietary signals. Several points from the discussion above are relevant here. It is useful to ask what are the most important mechanisms or routes for change in buried bones and teeth One could divide these processes into those with simple addition of new non-apatitic material (various minerals such as pyrites, silicates and simple carbonates) in pores and spaces (Hassan and Ortner 1977), and those related to change within the apatite crystals, usually in the form of recrystallization and crystal growth. The first kind of process has severe implications for alteration of bone and dentine, partly because they are porous materials with high surface area initially and because the approximately 20-30% by volume occupied by collagen is subsequently lost by hydrolysis and/or consumption by bacteria and the void filled by new minerals. Enamel is much denser and contains no pores or Haversian canals and there is very, little organic material to lose and replace with extraneous material. Cracks are the only interstices available for deposition of 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 . 

The zinc polycarboxylate cement sets within a few minutes of mixing and hardens rapidly. Strength is substantially developed within an hour. However, even when fully hardened the cement exhibits marked plastic behaviour. Its most important property is its ability to bond permanently to untreated dentine and enamel. 

The glass polyalkenoate cement has the important property of adhering to untreated enamel and dentine as many workers have shown (Wilson McLean, 1988 Lacefield, Reindl Retief, 1985). It also appears to adhere to bone and base metals (Hotz et al., 1977). 

Cements based on phytic add set more quickly than their glass polyalkenoate or dental silicate cement cormterparts, but have similar mechanical properties (Table 8.2). They are unique among add-base cements in being impervious to acid attack at pH = 2-7. Unfortunately, they share with the dental silicate cement the disadvantage of not adhering to dentine. They do bond to enamel but this is by micromechanical attachment - the cement etches enamel - and not by molecular bonding. Lack of adhesive property is a grave weakness in a modern dental or bone... 

Bone and teeth in mammals and bony fishes all rely on calcium phosphates in the form of hydroxyapatite [Ca5(P04)30H]2, usually associated with around 5% carbonate (and referred to as carbonated apatite). The bones of the endoskeleton and the dentin and enamel of teeth have a high mineral content of carbonated apatite, and represent an extraordinary variety of structures with physical and mechanical properties exquisitely adapted to their particular function in the tissue where they are produced. We begin by discussing the formation of bone and then examine the biomineralization process leading to the hardest mineralized tissue known, the enamel of mammalian teeth. 

Armstrong WG (1964) Modifications of the properties and compositon of the dentin matrix caused by dental caries. Adv Oral Biol 1, 309-332. 

Crosslinked polymers are widely used as dental materials (1-31. Perhaps the most challenging application is in the restoration of teeth (4). The monomers must be non-toxic and capable of rapid polymerization in the presence of oxygen and water. The products should have properties comparable to tooth enamel and dentin and a service life of more than a few years. In current restorative materials such properties are sought using so-called "dental composites" which contain high volume fractions of particulate Inorganic fillers (5-71. However in the present article attention is concentrated on one commonly used crosslinked polymeric component, and on the way in which some of its properties are influenced by low volume fractions of fillers. 

Fluorine is an essential element involved in several enzymatic reactions in various organs, it is present as a trace element in bone mineral, dentine and tooth enamel and is considered as one of the most efficient elements for the prophylaxis and treatment of dental caries. In addition to their direct effect on cell biology, fluoride ions can also modify the physico-chemical properties of materials (solubility, structure and microstructure, surface properties), resulting in indirect biological effects. The biological and physico-chemical roles of fluoride ions are the main reasons for their incorporation in biomaterials, with a pre-eminence for the biological role and often both in conjunction. This chapter focuses on fluoridated bioceramics and related materials, including cements. The specific role of fluorinated polymers and molecules will not be reviewed here. 

The demand for aesthetic dental restorative materials continues to increase and may be the most important criterion for the promising future of the aesthetic polymeric composite resins. As the physical, mechanical, and wear properties of these materials improve, their use in dentistry will expand. The acid-etching of dental enamel [20] and dentin bonding procedures [21] will allow for conservative cavity preparation and the preservation of healthy tooth structure. 

Table 1.35 Composition and Selected Properties of Inorganic Phases in Adult Human Enamel, Dentine, and Bone...

Table 5.14 Selected Mechanical Properties of Human Dentin and Enamel...

Mechanical Properties of Candidate Materials. The mechanical properties of enamel and dentin were presented earlier in Table 5.14. We will use these values as the basis for our material selection process. Of these properties, compressive strength is the most important. The candidate material should have a compressive strength at least that of enamel, which is about 384 MPa. 

Zimmerman, S. Physio chemical properties of Enamel and Dentine. In Dental biochemistry, pp. 112. Lazarri, E. P. (ed.). Philadelphia Lea Febiger 1976... 

The three mineralized hard parts are the teeth of certain mollusks called chitons, the dentin component of vertebrate teeth, and the skeleton of the larvae of the sea urchin. They all possess very different material properties. Taxonomically they are also very different, with each being formed by organisms that belong to different phyla the Mollusca, the Chordata and the Echinodermata respectively. 

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],... 

The isotropic microhardness properties of dentin contrast markedly with the anisotropic microhardness properties of various types of bone [64]. Microhardness is strongly dependent on crystal orientation, and less dependent on fibril orientation. Wang and Weiner [33] therefore proposed that the isotropy arises from a combination of structural features related to crystal orientation. The crystal layers in adjacent fibrils within the same bundle, are not aligned... 

Another unusual property of dentin is that its hardness properties vary continuously in three dimensions [33]. This is clearly under cellular control, and it was inferred that this design feature is part of the mechanism for distributing the applied compressive stress in such a way as to avoid cracks from developing. Unlike bone, teeth are generally not replaced (except once at puberty or... 

Summary of Mechanical Properties of Bone, Dentine [3], Hydroxyapatite [3], and CBPCs. 

Property Natural Biomaterials Cortical bone Dentine Sintered hydroxyapatite Dense CBPCs... 

Bone is the most common simulant of ivory. In smaU items or as inlay it can be difBcult to tell which material has been used as both bone and ivory appear much the same colour and have many similar properties. However, bone contains none of the structural patterns of ivories, for example the engine turned pattern of elephant and mammoth ivory or the tapioca pattern of the secondary dentine in walrus ivory. Instead it has the black dots or lines of the Haversian canals (nutrient bearing canals) (Figs 4.2 and 4.3). 

Veis and Schlueter (1963) have studied the solubility and swelling properties of dentine collagen after decalcification by ethylenediamine-tetraacetic acid (EDTA). The dentine colleen behaved as if it were more extensively cross-linked than corium collagen. Having noted that the decalcified dentine collagen still contained some phosphate that was not in combination with calcium and could not be removed by EDTA,... 

Whilst the use of enamel and dentine as test substrates is widespread, they are complex materials to work with due to the natural variability both within and between specimens. A number of authors have examined alternative materials, which have similar mechanical properties to enamel and dentine, to use as test substrates. Acrylic [19, 20] and synthetic hydroxyapatite [21] have been proposed as suitable materials for abrasion testing, where mechanical effects dominate. These materials have several advantages since they are available as relatively large, smooth samples and exhibit better intra- and inter-sample reproducibility than their natural counterparts. This may, therefore, give better discrimination between test products for formulation development. However, the use of natural enamel and dentine is preferred, particularly for studies that aim to understand interactions between toothpaste products and tooth hard tissues. Other methods for assessing toothpaste abrasivity to hard tissues include gravimetry [22], scanning electron microscopy [23] and laser reflection [24]. 

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