Applied Oral Physiology. Robin Wilding
Enamel: Clinical Aspects
Response of the Pulp–Dentin to Caries
Abstract
This chapter covers a wide range of topics in order to link the structure of a tooth with its vulnerability to bacteria and their products. The hard tissues of teeth are not merely passive participants in their destruction. There is a constant ebb and flow of demineralization which is dependent on macroscopic conditions, such as the diet and level of oral hygiene, but equally on the microscopic environment of the tooth such as the presence of pits and fissures in enamel. Some demineralization of dentin is prevented by the defense barriers set up by living cells of the pulp–dentin. A knowledge of the natural defense capacity of enamel and dentin to resist destruction by bacterial products allows us to rethink orthodox restorative practices. The use of low-intervention clinical procedures has arisen through awareness of the capacity of the dental hard tissues to resist destruction and repair. The search for minimally invasive procedures must continue while too many teeth are not saved by dentistry but are caught up in a downward spiral of treatment and retreatment until they are eventually beyond further repair.
Keywords: enamel structure, ameloblasts, hydroxyapatite, enamel prisms, etching enamel, early enamel caries, arrested caries
2.1 Enamel: Clinical Aspects
2.1.1 Enamel Minerals
Enamel is about 95% mineral by weight but 87% by volume. Most of this mineral is hydroxyapatite, but there are other apatites and other minerals (magnesium and carbonates), so the mineral phase of enamel is not pure hydroxyapatite. The less mineralized the enamel is, the whiter and more opaque it is; the more mineralized, the more translucent enamel allows the yellow color of the underlying dentin to show through. Examples of white, poorly mineralized enamel are the enamel of deciduous teeth and chalky patches of fluorosis and early carious lesions in adult teeth. Enamel crystals are about 10 times thicker and much longer than the apatite crystals of bone or dentin; the volume is about 1,000 times greater. The surface area is therefore relatively low, and this factor makes enamel crystal less reactive and soluble to acids than are apatite crystals of bone or dentin. The crystal also contains very little of the more soluble carbonates and magnesium salts found in dentin and bone. The enamel crystals are highly orientated and tightly packed. These factors account in part for the greater hardness of enamel in comparison to dentin and also for its resistance to acid demineralization. In addition, enamel has a lower percentage of organic material than dentin (see Appendix B Table B.1).
Enamel crystals are packed into larger units, the enamel prism, which is the structure made by each ameloblast as it lays down enamel during development. The prisms are about 100 crystals wide but are very long and may run uninterrupted from the dentin to the tooth surface. Both crystals and prisms have their long axis parallel to each other and are directed toward the surface of the tooth. However, at the prism boundaries, the crystals tend to turn outward and face their neighboring crystals. In this interprismatic zone, the enamel crystal orientation is slightly less ordered, and there is a little more space between them (▶ Fig. 2.1). This makes the interprismatic regions slightly weaker and more easily disrupted by tooth wear. During wear, whole prisms break off leaving a sharp edge to the enamel which is needed to shred food (▶ Fig. 2.2). During preparation of a cavity for a composite or amalgam restoration, the margins of enamel must be prepared so that all the enamel prisms are supported. This prevents their chipping off later and causing a failure at the margins of the restoration (▶ Fig. 2.3) (see Appendix B.1 Physical Properties of Enamel and Dentin).
2.1.2 Enamel: Non-Minerals
The non-mineral phase of enamel consists of proteins and water. This phase is not mixed randomly within the mineral but surrounds the large mineral crystals which are packed tightly together. The minute spaces in between the enamel crystals account for the slight porosity of enamel. During tooth development, the first formed enamel is very poorly mineralized. It appears chalk like and is dissolved readily by acids. As the newly formed enamel matures, proteins are removed and the enamel becomes more resistant to dissolution by acids (see Appendix B.2 Enamel Proteins).
2.1.3 Enamel Etching
The disorientation of crystals in the interprismatic zone makes it more susceptible to acid demineralization than the well-orientated crystals of prismatic enamel. It is the interprismatic zone which first demineralizes due to early caries. However, the dissolution pattern varies when organic acids are used to etch enamel. Etching enamel is a routine procedure when using restorative resins, because the resin bonds to a roughened surface better than to a smooth one. The two common etching patterns are etching of the prism center and etching of the interprismatic substance (▶ Fig. 2.4).
Fig. 2.1 A diagrammatic representation of the formation of enamel prisms during tooth development. Ameloblasts secrete enamel prisms parallel to each other with the enamel crystals orientated in the same direction. In the interprismatic zone, the crystals turn outward. The enamel matrix is secreted and mineralized by the ameloblast (red arrow), and the protein then resorbed (blue arrow).
Fig. 2.2 A magnified view of a fractured enamel surface reveals parallel prisms packed together. Scanning electron microscope (SEM) image (magnification × 300) of an enamel surface, prepared by fracture in the direction of the enamel prisms. The fractured surface reveals enamel prisms stacked like the grain in a log of wood. The surface of the tooth was etched to reveal the ends of the enamel prisms.
2.1.4 Early Enamel Caries
Plaque is an aggregate of bacteria in a sticky matrix which forms on the surface of enamel. Acid which has been formed as a product of bacterial metabolism may dissolve the surface enamel. If the surface breaks down, it may form a visible defect in the enamel which under normal light appears as a white patch. Early enamel caries seen under polarized light reveals four distinct zones of mineralization (▶ Fig. 2.5). The outer surface zone is well mineralized by replacement ions from plaque and saliva. However, the body of the lesion is poorly mineralized. Deeper to the body of the lesion, a darker zone represents some remineralization, while the deepest zone is yet again demineralized. These zones of demineralization and remineralization illustrate the dynamic series of events which are occurring in the early lesion. Caries is not simply a process of continued demineralization.
2.1.5 Arrested Enamel Caries
The early lesion in enamel may be reversed