Enamel adhesives hydroxyapatite

The precise nature of the adhesion of the polyelectrolyte cements to untreated dental enamel and dentine has yet to be established. The earliest theory was due to Smith (1968) who speculated that the polyacrylate chains of the cement formed a chelate with calcium ions contained in the hydroxyapatite-like mineral in enamel and dentine. Beech (1973) considered this unhkely since it involved the formation of an eight-membered ring. Beech studied the interaction between PAA and hydroxyapatite, identified the formation of polyacrylate and so considered that adsorption was due to ionic attraction. 

Acid etching pretreatment of dentin prior to application of a resin restorative leaves the collagenous component of this substrate substantially untouched. The collagen at the dentin surface interferes with the mechanical adhesion of the hydroxyapatite, (the inorganic constituent of dentin) to the adhesive. Thus, in contrast to its successful use with enamel, etching pretreatment produces a bond between dentin and restorative resin that is too weak to withstand the forces encountered at the interface under clinical conditions. 

In the realm of dentistry, restoration and protection of tooth enamel are of great importance in operative and conservative dentistry. Yamamoto et al. (2013) used PLD to create a freestanding flexible double-layered sheet composed of a 4 pm thin hydroxyapatite (HAp) layer coated with a 0.5 pm thin TCP layer. The adhesive strength between the HAp/TCP sheet and enamel was 5.7 MPa, decidedly higher than that between the monolayered HAp sheet and enamel (1.9 MPa). Electron microscopical observation revealed that the HAp/TCP sheet was largely fused with the enamel. Therefore, the double-layered HAp/TCP sheet can be used as a material to promote the repair of tooth eruption and to maintain healthy dentine. 

Another application of dental composites is orthodontic archwires. One example is a unidirectional pultruded S2-glass-reinforced dimethacrylate thermoset resin. Depending on the yam of glass fiber used, the fiber volume fraction varied from 32 to 74 percent The strength and modulus were comparable with those of titanium wires. Orthodontic brackets were also made from composites with a polyethylene matrix reinforced with ceramic hydroxyapatite particles, resulting in isotropic properties and good adhesion to enamel. 

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