Lecture

Caries of enamel

Dental tissues

Enamel

Dentin

Cementum

Dental Pulp

In mammalian teeth both of enamel and dentine are highly mineralised tissue one providing wear resistance and the other provides fracture resistance, respectively.

Enamel structure Structurally, enamel is composed of millions of enamel

rods or prisms, which are the largest structural components, as well as rod sheaths and a cementing inter-rod substance in some areas. The rods are densely packed and intertwined in a wavy spiraling course, and each extends from the DEJ to the external surface of the tooth.

Enamel structure The structural components of the enamel rods or prism are

millions of small, elongated apatite crystallites that are variable in size and shape. The crystallites are tightly packed in a distinct pattern of orientation that gives strength and structural identity to the enamel prisms.

Enamel structure The long axis of the apatite crystallites within the central

region of the head (body) is aligned almost parallel to the rod long axis, and the crystallites incline with increasing angles (up to 65 degrees) to the prism axis in the tail region. The susceptibility of these crystallites to acid, either from an etching procedure or caries, appears to be correlated with their orientation.

Enamel morphology & histology Enamel covers the anatomic crown of the tooth and varies

in thickness in different areas.

The enamel is thicker at the incisal area (nearly 2 mm) and occlusal area (2.3-2.5 mm) of a tooth, and it becomes progressively thinner until it terminates at the cementoenamel junction.

Enamel is formed by cells called ameloblasts, which originate from the embryonic germ layer known as ectoderm.

Enamel morphology & histology Enamel is incapable of repairing itself once destroyed

because the ameloblast cell degenerates following formation of the enamel rod. The final act of the ameloblast cell is secretion of a membrane covering the end of the enamel rod. This layer is referred to as the Nasmyth membrane, or the primary enamel cuticle. This membrane covers the newly erupted tooth and is worn away by mastication and cleaning. The membrane is replaced by an organic deposit called a pellicle, which is a precipitate of salivary proteins. Microorganisms may invade the pellicle to form bacterial plaque, a potential precursor to dental disease.

Enamel chemical structure Chemically, enamel is a highly mineralized crystalline

structure containing from 95% to 98% inorganic matter by weight. The remaining constituents of tooth enamel are an organic content of about 1% to 2% and a water content of about 4% by weight.

Hydroxyapatite, in the form of a crystalline lattice, is the largest mineral constituent of enamel and is present 90% to 92% by volume. Other minerals and trace elements are contained in smaller amounts.

Enamel permeability Although enamel is a very hard, dense structure, it is

permeable to certain ions and molecules, permitting both partial and complete penetration. The route of passage appears to be through structural units that are hypomineralized and rich in organic content, such as rod sheaths, enamel cracks, and other defects.

Enamel solubility Enamel is soluble when exposed to an acid medium

(enamel demineralization occurs at pH of 5.5 or less), but the dissolution is not uniform. Solubility of enamel increases from the enamel surface to the DEJ. When f luorides are present during enamel formation or are topically applied to the enamel surface, the solubility of surface enamel is decreased.

Enamel solubility Fluoride concentration decreases toward the DEJ. Fluoride

additions can affect; 1-the chemical and 2- physical properties of the apatite mineral, it can influence the hardness, chemical reactivity, and stability of enamel while preserving the apatite structures.

Trace amounts of f luoride stabilize enamel by; 1- lowering acid solubility, 2-decreasing the rate of demineralization, 3- and enhancing the rate of remineralization. Evidence also shows that topical fluorides alter the oral bacterial f lora, thereby increasing resistance to dental caries.

Enamel caries

Clinical sites for caries initiation:

Pits & fissures of enamel:

It has the highest prevalence of all dental caries. The pits and fissures provide excellent mechanical shelter for organisms and harbor a community dominated by S. sanguis and other streptococci. The shape of the pits and fissures contributes to their high susceptibility to caries.

It expands as it penetrates into the enamel. Thus, the entry site may appear much smaller than the actual lesion, making clinical diagnosis difficult. The lesion is roughly parallel to the long axis of enamel rods.

Enamel caries Pits & fissures of enamel:

A lesion originating in a pit or fissure affects a greater area of the DEJ than does a comparable smooth surface lesion. In cross-section, the gross appearance of a pit-and-fissure lesion is an inverted V with a narrow entrance and a progressively wider area of involvement closer to the DEJ.

Enamel caries Smooth enamel surface:

The smooth enamel surfaces of the teeth present a less favorable site for plaque attachment. Plaque usually develops only on those smooth surfaces that are near the gingiva or are under proximal contacts. Lesions starting on smooth enamel surfaces have a broad area of origin and a conical, or pointed, extension toward the DEJ.

Enamel caries Smooth enamel surface:

The path of ingress of the lesion is roughly parallel to the long axes of the enamel rods in the region. A cross-section of the enamel portion of a smooth surface lesion shows a V shape with a wide area of origin and the apex of the V directed toward the DEJ.

Enamel caries Root surface:

The root surface is rougher than enamel and readily allows plaque formation in the absence of good oral hygiene. The cementum covering the root surface is extremely thin and provides little resistance to caries attack.

Root caries lesions have less well defined margins, tend to be U-shaped in cross-section, and progress more rapidly because of the lack of protection from an enamel covering.

Assessment of pathological and traumatic conditions whether it needs to be treated 1- Color changes associated with demineralization:

enamel is mostly gray and semitranslucent, the color of a tooth depends upon the color of the underlying dentin, thickness of the enamel, and amount of stain in the enamel. The amount of translucency of enamel is related to variations in the degree of calcification and homogeneity. Abnormal conditions of enamel usually result in aberrant color.

Assessment of pathological and traumatic conditions whether it needs to be treated 1- Color changes associated with demineralization:

Enamel becomes temporarily whiter within minutes when a tooth is isolated from the moist oral environment by a rubber dam or absorbents. Thus the shade must be determined before isolation and preparation of a tooth for a tooth-colored restoration. This change in color is explained by the temporary loss of loosely bound (or exchangeable) water (less than 1% by weight).

Assessment of pathological and traumatic conditions whether it needs to be treated 1- Color changes associated with demineralization:

On clean, dry teeth, the earliest evidence of caries on the smooth enamel surface of a crown is a white spot. These lesions are usually observed on the facial and lingual surfaces of the teeth. White spots are chalky white, opaque areas that are revealed only when the tooth surface is desiccated, and are termed incipient caries. These areas of enamel lose their translucency because of the extensive subsurface porosity caused by demineralization.

Assessment of pathological and traumatic conditions whether it needs to be treated 1- Color changes associated with demineralization:

Care must be exercised to distinguish white spots of incipient caries from developmental white spot hypocalcifications of enamel. Incipient caries will partially or totally disappear visually when the enamel is hydrated (wet), while hypocalcified enamel is relatively unaffected by drying and wetting. Hypocalcified enamel does not represent a clinical problem except when its appearance is objectionable esthetically.

Assessment of pathological and traumatic conditions whether it needs to be treated 1- Color changes associated with demineralization:

The surface texture of an incipient lesion is unaltered and is undetectable by tactile examination with an explorer. A more advanced lesion develops a rough surface that is softer than the unaffected, normal enamel. Softened chalky enamel that can be chipped away with an explorer is a sign of active caries.

Assessment of pathological and traumatic conditions whether it needs to be treated 1- Color changes associated with demineralization:

Similar incipient lesions occur on the proximal smooth surfaces but usually are undetectable by visual or judicious tactile (explorer) examination. Incipient lesions sometimes can be seen on radiographs as a faint radiolucency, limited to the superficial enamel. When a proximal lesion is clearly visible radiographically, the lesion may have advanced significantly and histologic alteration of the underlying dentin probably has already occurred.

Assessment of pathological and traumatic conditions whether it needs to be treated 1- Color changes associated with demineralization:

It has been shown experimentally and clinically that incipient caries of enamel can remineralize.

The etched crystallites serve as nucleating agents for remineralization. Calcium and phosphate ions from saliva can then penetrate the enamel surface and precipitate on the highly reactive crystalline surfaces in the enamel lesion. The supersaturation of the saliva with calcium and phosphate ions serves as the driving force for the remineralization process.

Assessment of pathological and traumatic conditions whether it needs to be treated 1- Color changes associated with demineralization:

The presence of trace amounts of fluoride ions during this remineralization process greatly enhances the precipitation of calcium and phosphate, resulting in the remineralized enamel becoming more resistant to subsequent caries attack because of the incorporation of more acid-resistant fluorapatite. Remineralized (arrested) lesions can be observed clinically as intact, but discolored, usually brown or black spots.

Assessment of pathological and traumatic conditions whether it needs to be treated 1- Color changes associated with demineralization:

The change in color is presumably due to trapped organic debris and metallic ions within the enamel. These discolored, remineralized, arrested caries areas are intact and are more resistant to subsequent caries attack than the adjacent unaffected enamel. They should not be restored unless they are esthetically objectionable.

Assessment of pathological and traumatic conditions whether it needs to be treated 2- Cavitation:

Increasing demineralization of the body of the enamel lesion results in the weakening and eventual collapse of the surface covering. The resulting cavitation provides an even more protective and retentive habitat for the cariogenic plaque, thus accelerating the progression of the lesion.

The DEJ provides less resistance to the carious process than either the enamel or the dentin. The resultant lateral spread of the lesion at the DEJ produces the characteristic second cone of caries activity in the dentin (cavitation).

Assessment of pathological and traumatic conditions whether it needs to be treated 3- Wear: it is 4 types.

Abrasion may result from (1) improper brushing techniques, (2) holding a pipe stem by the teeth, (3) tobacco chewing, (4) vigorous use of toothpicks between adjacent teeth. It is usually seen as a sharp, V-shaped notch in the gingival portion of the facial aspect of a tooth.

Erosion may result from chemical wear or loss of tooth surface from chemicals such as lemons.

Assessment of pathological and traumatic conditions whether it needs to be treated 3- Wear: it is 4 types.

Attrition may result from memechanical wear of the incisal or occlusal surface as a result of functional or parafunctional movements of the mandible as in bruxism malocclusion.

Iidiopathic erosion or abfraction may result from (heavy) eccentric occlusal force.

Enamel may be completely lost, so that dentin is exposed.

Assessment of pathological and traumatic conditions whether it needs to be treated 4- faults & fissures: For example, a tooth surface that normally is smooth occasionally have a pit in the enamel. pit faults in

enamel are restored best with use of a modified preparation. Adequate retention is obtained by etching the enamel

Assessment of pathological and traumatic conditions whether it needs to be treated 5- Cracks:

Transillumination can be used to check for cracks or caries in the tooth, as well as for tooth color changes that may indicate loss of vitality.

Assessment of pathological and traumatic conditions whether it needs to be treated 5- Cracks:

Often small "craze lines" are seen in the enamel that indicate minute fractures of that structure. These craze lines usually are not clinically significant unless associated with cracks in the underlying dentin, or the patient has pain during chewing. So, the tooth requires restoration by complete coverage of the cusp.

Cracks that extend from developmental grooves across marginal ridges to axial walls or from the margins of large restorations may cause cuspal fracture.

Enamel bonding An established operative technique involves acid etching of

the enamel surface for the micromechanical "bonding" of composite restorative materials or pit and fissure sealants directly to the etched surface.

Enamel prism (rod)

Cross-sectional views of micromechanical retention of enamel bonding systems. Schematic view of macrotags and microtags.

Enamel bonding The etchant usually is a 35% to 50% solution of

phosphoric acid. This etching produces an irregular and pitted surface with numerous microscopic undercuts by an un-even dissolution of enamel rod heads and tails. Composite or pit-and-fissure sealant is bonded to the enamel surface by resin tags formed in the acid-etched enamel rod structures.

A, Enamel rods unetched. B, Enamel rods etched, creating numerous microundercuts. C, Resin bonding agent engaging microundercuts, creating "resin tags" for mechanical bond

to tooth. D, Composite material chemically bonded to resin bonding agent.