Some useful tidbits of information from Summit's Fundementals of Operative Dentistry

Enamel is as hard as steel (But even steel products can wear). Cavity outline forms should be designed so that the margins avoid regions of heavy occlusal force.

A deep fissure is formed by incomplete fusion of lobes of cuspal enamel in the developing tooth. The resulting narrow clefts provide a protected niche for acidogenic bacteria and the nutrients they require. It is estimated that caries lesions are five times more likely to occur in occlusal fissures and two and a half times more likely to occur in buccal and lingual fissures than in proximal smooth surfaces.

Although craze lines in the surface enamel are of little consequence, pronounced cracks that extend from developmental grooves across marginal ridges to axial surfaces, or from the margins of large restorations, may portend coronal or cuspal fracture. A crack defect is especially critical when the crack, viewed within a cavity preparation, extends through dentin or when the patient has pain while chewing. A cracked tooth that is symptomatic or involves dentin requires a restoration that provides complete coronal coverage or at least adhesive splinting. It should be noted, however, that even if a crack is identified early in patients with a diagnosis of reversible pulpitis and a crown is placed, subsequent root canal treatment may still be necessary in about 20% of the cases.

The initial effect of acid contact in etching enamel for bonding to restorative materials is to remove about 10 μm of surface enamel, which typically contains no rod structure. Then, with rod and interrod structure exposed, the differential dissolution of enamel rod and interrod structure forms a three-dimensional macroporosity. The acid-treated enamel surface has a high surface energy so that resin monomer flows into, intimately adapts to, and polymerizes within the pores to form retentive resin tags that are up to 20 μm deep. At the same time, the internal cores of all the exposed individual crystals are solubilized to create a multitude of microporosities. It is these countless numbers of minitags, formed within the individual crystal cores, that contribute most to the enamel-resin bond. Because there are 30,000 to 40,000 enamel rods per square millimeter of a surface of cut enamel, and the etch penetration increases the bondable surface area 10- to 20-fold, the attachment of resin adhesives to enamel through micromechanical interlocking is extremely strong.