Enamel-dentine junction

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. 

Fig. 11. Fluorine profiles in two tooth samples (Seeberg, Switzerland, 3750 bc, top, and Radfeld, Austria, 1200-750 bc). The clearest fluorine profiles arise from the enamel-dentine junctions (edj) and from the nerve canals (in human tooth only). The diffusion constants of these naturally grown profiles are similar in both tooth samples, although their provenance and age is very different. But, the values differ significantly from those measured in human bone compacta (Fig. 9).

There has been some interest in recent years in the phenomenon described as hidden caries , and studies have been conducted to model processes occurring at the enamel-dentine junction (EDJ). ten Cate et al. [11] studied the effect of slightly elevated levels of fluoride, such as the typical salivary background concentration resulting from water fluoridation or use of fluoridated toothpastes, on the de- and remineralisation of enamel and dentine lesions. They concluded that slightly elevated fluoride levels may be considerably less effective in inhibiting lesion progression in dentine than in enamel, i.e. that differences in intrinsic solubility are important. 

Outside a carious cavity, S. mutans accounts for less than 1% of the microbiota on teeth surfaces, and lactobacilli for less than 0.1%. Within a developing cavity, S. mutans often accounts for 5% or more of the microbiota. Once a cavity is established, the local pH often falls below 4.0, which stops the growth of S. mutans and all other oral bacteria except L. casei. The L. casei grows on the monosaccharide residues of proteoglycans at the enamel-dentinal junction and in the non-mineralized portions of dentinal sheaths and tubules. S. mutans therefore initiates enamel dissolution and lactobacilli dissolve dentin. The lactobacilli excrete lactic acid which dissolves dentinal hydroxyapatite, releasing collagen. Asaccharolytic bacteria follow the lactobacilli, infect the pulp and spread to the bone at the apex of the infected tooth. 

X-ray diffraction patterns from enamel sections cut in the longitudinal direction of a tooth show that the c-axes (the hexagonal axes) of the apatite crystals (also their long direction) are highly oriented in the direction of the enamel rods (sometimes called prisms) (Miles 1967). The rods run from the enamel surface (to which they are nearly normal) to the enamel-dentin junction and are about 4-7 pm in diameter. 

The composition of dentin is similar to that of bone, with crystal sizes of about 50 x 25 x 2 nm3, much smaller than those in enamel. Dentine connective tissue comprises a network formed by randomly intertwined mineralised collagen fibrils permeated by tubules that radiate from the pulp cavity towards the dentin-enamel junction (Figure 3.4c). 

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. 

Its topography reveals the unique structure consisting of aligned prisms or rods with 5pm diameter that extent approximately perpendicular from the dentin-enamel junction towards the tooth surface. Each rod consists of tightly packed carbonated hydroxyapatite crystals with very high aspect ratio. Nano-indentation studies revealed a pronounced anisotropy as the stiffness differs parallel and perpendicular to the rod extension. Even so, different fibre orientation on a micro level as shown in Figure 3.4b account for a quasi-isotropic behaviour. 

Raman photoluminescence piezospectroscopy of bone, teeth and artificial joint materials has been reviewed by Pezzotti (2005) with emphasis placed on confocal microprobe techniques. Characteristic Raman spectra were presented and quantitative assessments of their phase structure and stress dependence shown. Vibrational spectroscopy was used to study the microscopic stress response of cortical bone to external stress (with or without internal damages), to define microscopic stresses across the dentine - enamel junction of teeth under increasing external compressive masticatory load and to characterise the interactions between prosthetic implants and biological environment. Confocal spectroscopy allows acquisition of spatially resolved spectra and stress imaging with high spatial resolution (Green etal., 2003 Pezzotti, 2005 Munisso etal., 2008). 

Anatomically, the dentine has a tubular strucTure, with microscopic tubules radiating outwards from the c entral pulp cravity. These tubules range in diameter between 0.9 and 2.5 pm, with the diameter being widest near the pulp and becoming reduced as the tubule extends towards the dentino-enamel juncTion [2]. These tubules are filled with odontoblast prtKesses. 

Fibrils of this latter substance within the dentine act as a scaffold for the mineral crystalhtes. These crystallites reinforce the dentine matrix and the whole structure acts as a support for the enamel. The mineralized dentine has the important biomechanical function of preventing cracks propagating from the enamel, which is very brittle, through the dentino-enamel junction into the dentine [34]. This prevents the enamel crown from fracturing when loaded. 

Non-carious cervical lesions of the tooth are typically wedge-shaped and show loss of tooth tissue mainly on the buccal surfaces of the tooth close to the cemento-enamel junction. This is the case, regardless of the tooth affected [72]. When the tooth is loaded asymmetrically, there are typically flexing stresses, and these produce tension on one side of the tooth and compression on the other. Both types of force are located close to the cemento-enamel junction [86], The result is that tooth mineral fractures in this region, and falls away, causing a non-carious lesion to develop. These lesions typically involve exposure of the dentine [72]. 

V. Imbeni, J.L. Kruzic, G.W. Marshall, S.J. Marshall, R.O. Ritchie, The dentin-enamel junction and the fracture of human teeth. Nature Mater. 4 (2005) 229-232. 

The two tissues, enamel and dentine, are connected by the dentino-enamel junction, which has distinctive characteristics of its own. It unites the thin and brittle enamel layer to the thicker, tougher underlying structure of dentine. Its mechanical properties make it ideal for the function of uniting two materials with such dissimilar properties, and one of its most important functions is to prevent cracks from passing through from the enamel to the dentine [7]. This feature protects the entire tooth from mechanical failure and is important in maintaining the tooth in service for long periods of time. 

C.P. Lin, W.H. Douglas, Structure-property relations and crack resistance at the bovine dentin-enamel junction, J. Dent. Res. 73 (1994) 1072-1078. 

Utilized a miniature punch shear apparatus to determine shear strength and toughness perpendicular to the direction of dentinal tubules. Dentin harvested from the cemento-enamel junction to one-third the distance to the root apex. Strengths novel measurements, precise measurements, defined specimen location, defined orientation of testing. Limitations tooth type not defined for constrained tests, teeth stored in mineral oil prior to testing. 

Xu, C. et al. (2009) Chemical/molecular structure of the dentin-enamel junction is dependent on the intratooth location. Calcif. Tissue Int, 84 (3), 221-228. 

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