# A Literature Review of Adhesive Systems in Density: Key Components and Their Clinical Applications
## 2 Enamel and Dentin--An Overview  
A main objective of restorative dentistry is to bond restorative materials to enamel, dentin, or both [33]. Chemically, enamel structure is a highly mineralized crystalline substance comprising 95 to 98% inorganic matter by weight. The inorganic content of enamel relies on HAp, the largest mineral component, which is present at concentrations of 90-92% by volume, and its proper arrangement as enamel prisms to maintain its structure [42]. However, in the free spaces between the HAp crystals, a network of organic materials exists. A resilient mineral, formed by the interaction between minerals, collagen I fibrils, and other proteins, is present within dentin and bone. Despite this, enamel is much tougher due to being more mineralized than bone. Also, enamel does not contain collagen; though, when it matures, a small quantity of specialized matrix proteins is produced inside the enamel [43]. The remaining enamel constituents include an organic phase, which reports for about 1 to 2%, and water (3 to 4%), which accounts for about 4% by weight [44]. The rods of enamel are tightly packed and interlaced in a wavy pattern that extends from the dentinoanamel junction (DEJ) to the tooth's exterior surface. Aprismatic enamel has an optic axis approximately parallel to the enamel's surface, with an acid-resistant surface that protects against enamel degradation in the oral setting [45]. The DEJ, or transition zone between dentin and enamel, together with the enamel tufts made of hypomineralized crack-like defects, plays a crucial part in tooth resilience and preventing the cracks propagation [46]. Typically, enamel rods are positioned perpendicular to both the DEJ and the tooth surface. After the enamel has been treated with an acidic conditioner, the \"ends of the enamel rods\" onto the surfaces are exposed for the bonding agent [44, 45]. However, the rods can be placed parallel to the interior surface of a tooth preparation, when the \"sides of the enamel rods\" are revealed following acid conditioning [44]. Dentin is essential in the clinical use of adhesive restorations. Dentin is a complex tissue, and although known to be highly mineralized, it has a lower mineral level (70 wt% mineral by weight and 45% by volume) and a higher organic composition (20 wt% organic phase by weight and 33% by volume), with water making up the remaining fraction. Despite being less mineralized than enamel, dentin has a higher mineral content than bone or cementum [45]. The morphology of dentin is very complex. Its microstructure is naturally well-oriented, containing several dentinal tubules occupied by odontoblast processes or their residues, with a density ranging between (19-45) \(\times\) 1000/mm\({}^{2}\) and an average diameter of 0.8-2.5 \(\upmu\)m [47]. Detinal tubules spread through the whole dentin thickness, starting from the DEJ or the cementum to the pulp [21; 48; 49]. These tubules are bounded by two dissimilar dental phases: the intertubular dentin (ITD) and the peritubular dentin (PTD) [50]. The larger part of the dentin volume is surrounded by ITD, a composite containing type I collagen fibrils irregularly fortified with nanoplatelets of carbonated HAp [45]. This configuration makes the dentin tougher and stronger. The PTD contains no collagen and is more mineralized, harder, and stiffer than the bordering ITD [51; 52]. An increase in the diameter of the dentinal tubules inside the deep dentin, along with the convergence point of the tubules toward the pulp chamber, results in an exponential surge in the dentin permeability when the dentin is prepared deeper [53]. Thus, in the superficial dentin, bonding is primarily intertubular, while in the deep dentin, bonding is mainly intratubular [54] (Figure 1). allows cutting the teeth when making the specimen. Amelogenin, enamel, and perlecan are three proteins found in enamel. The latter is found in the dental papilla and follicular intercellular gaps.