White spot lesions

Fig. 9.13. Quantitative analysis of the contrast from a white spot lesion in human tooth enamel (i) micrograph, 370 MHz (ii) V(z) curves of selected points along the line in the micrograph (iii) Rayleigh velocity and attenuation calculated from V(z) measured at each of the points on the line in (i) (Peck et al. 1989).

The way the V(z) curves change over enamel containing a white spot lesion is illustrated in Fig. 9.13 (Peck et al. 1989). At each of the points labelled by letters along the line across the centre of the picture in Fig. 9.13(i), measurements of V(z) were made. Four of these are presented in Fig. 9.13(ii) the letters correspond to the points in Fig. 9.13(i). Curve (a) comes from the intact surface zone. There are three well pronounced maxima apart from the focal maximum, with a fourth trying to appear. The period of the oscillations is about 13 pm. Curve (f) comes from the core of the lesion. The attenuation is much more severe here, and so the heights of the maxima fall off much more rapidly away from focus. The period is also smaller it is reduced to about... 

White spot lesions are the earliest macroscopic evidence of enamel caries [37], The lesions are caused by acids formed by bacterial fermentation of dietary sugars. This leads to a fall in plaque pH and dissolution of the mineral component of the tooth enamel. Under normal conditions, the demineralization process is balanced by remineralization due to diffusion of ions (Ca, P and hydroxyl) from saliva into the enamel when plaque pH returns to neutrality. However, if demineralization extent exceeds that of remineralization, then an incipient lesion is formed. 

The unusual and important characteristic of a white spot lesion is that the majority of the demineralization occurs in the subsurface region. Typical lesions possess a surface layer which appears relatively unaffected by the acid attack. If lesion progression is left unchecked, then eventually the surface layer collapses to yield a cavity. 

White spot lesions vary from person to person, from tooth to tooth and from surface to surface, as well as with age of the lesion. Their nonstandard nature makes analysis of the lesion incredibly difficult. In order to increase the level of control over experiments with lesions, a standard method of producing in vitro lesions has been used in this study. Many of the features of white spot lesions can be mimicked by in vitro lesions, though no method completely replicates the development of the natural lesion. The lesions produced are usually referred to as caries-like lesions indicating that they are not a natural white spot lesion, but an artificially produced lesion for experimental analysis. These artificial lesions provide invaluable information about the formation, the processes of demineralization and remineralization, and the composition of the lesion at different stages, in addition to being the basis for the understanding of possible treatments of the lesion. 

To make the white spot lesions, human premolars were coated with nail polish except for a 5-mm square on their buccal surfaces. This ensured that demineralization always occurred at a specific location. The samples were then immersed in 200 ml methyl cellulose gel with 200 ml lactic acid solution (pH 4.6) poured on top, but separated by a sheet of filter paper. The teeth were stored in this solution at 3 7°C for 14 days to allow the lesions to develop. After removal from the gel, the teeth were washed and sectioned through the center of the lesions to enable the cross-section of each lesion to be viewed. The two cross-sections created for each lesion were then mounted in a low temperature cure epoxy and polished to an optical finish using 1/4-micron grit (diamond paste). 

Fig. 7. Mechanical property maps of a cross-sectioned white spot lesion (a) is the hardness and (b) the reduced elastic modulus. The regions marked A-C are described in the main text. 

Commonly, treatments of enamel white spot lesions have either been preventive (noninvasive), with a combination of fluoride-based remineralization [110] and readaptation of the patient s diet, or restorative (invasive), where the lesion is drilled and treated with the polymeric restorative materials and strategies described in sections Polymers in Dental Adhesion and Polymers in restorative composites resins . Recently, monomers that are commonly utilized for adhesive restorative treatments, or combinations thereof, were modified to enable impregnation of white spot lesions with photocrosslinkable materials of low viscosity (Fig. 9.3) [111]. The rational behind this strategy stems from the idea that the infiltrant occludes the lesion porosity and blocks further diffusion pathways for cariogenic acids [111]. Moreover, polymeric resins are much more resistant to acid degradation than enamel apatite is resistant to acidic dissolution, hence further cavitation is prevented after infiltration and photopolymerization (Fig. 9.2). 

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]. 

Huang, T.T., He, L.H., DarendelQer, M.A., Swain, M.V. Correlation of mineral density and elastic modulus of natural enamel white spot lesions using X-ray microtomography and nanoindentation. Acta Biomater. 6, 4553-4559 (2010)... 

A clinical problem with this technique is that brackets bonded by composite resins in this way have a tendency to develop regions of deminerali2ation around them. This leads to the formation of white spots on the tooth surface. Typically where white spot lesions develop, they affect several teeth of a patient. 

To overcome the problems of white spot lesions, and also to exploit the lower inherent bond strength, glass-ionomer cements are increasingly being used for this application. Their ability to release fluoride ensures that there are no sites of deminerali2ation, hence no white spot formation. There is also much less damage when the brackets are removed (Charles 1998). 

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