Effects of conditioning of dentin
The principal effects of conditioning of dentin may be classified as
a) Physical changes
b) Chemical changes
Physical changes
Increases or decreases in the thickness and morphology of smear layer changes in the shape of dentinal tubules.
Chemical changes
a) Modification of the fraction of organic matter
b) Decalcification of the inorganic portion
Conditioning of dentin may be done by several means
1) Chemical
a) Acids
b) Calcium chelators
2) Thermal
a) Lasers
3) Mechanical
a) Abrasion
When dentin is cut for cavity preparation, the wrenched cutting debris of the dentin forms a thin smear layer on the surface. It is also driven into the dentinal tubule apertures displacing the odontoblast process and forming a smear plug at a depth of less than 10-micron meter. Etching dissolves the smear layer and part of the peritubular dentin, leaving tapered cylindrical holes of that depth.
In an experiment on monkey, dentin wall demineralized with a phosphoric acid jelly etchant for 60 sec was completely re-mineralized after 4 months. This results indicates that etching did not result in deleterious effect upon either the collagen fibers or the odontoblast processes, because the presence of collagen fibers maintaining their proper cross bonded structure as a base for apatite crystals to attach to and of the vital odontoblast processes to supply the calcium phosphate from the pulp is essential for remineralization of dentin.
A.J.Gwinnett and M.D.Jendresen (1978) have concluded from their experiments and observations that the surface of acid conditioned eroded dentin is significantly different from that of acid conditioned normal dentin. They further observed the depth of penetration of resin is also less in acid treated eroded dentin where many tubules remain partially occluded by intratubular insoluble deposits.
Ruse and Smith (1991) found when common conditioning agents were used, it has been found by X-ray photo electron microscopy that the outermost surface contains only 10% or less of the calcium and phosphorus initially present. They concluded that the treatment of dentin with acidic conditioners leaves the surface so depleted of calcium and enriched by organic residues that subsequently placed bonding systems should be based upon agents able to interact with organic components of dentin. Bonding agents that rely on chelation to calcium are unlikely to be successful when applied to acid etched dentin unless they penetrate into the demineralized matrix to reach normal, mineralized dentin.
Acid etching of dentin is not harmless but represents one more source of acute irritation to the pulpodentin complex in addition to the vibratory, thermal, mechanical and evaporative stimuli that accompany cavity a preparation. However, it is not as irritating as has been previously thought.
Nakabayashi (1982) introduced the concept of hybridization. The technique consists of applying an acid, ranging in concentration from 10% to 30% to the surface of dentin. Within 15 minutes the acid selectively dissolves away the inorganic component of the dentin to a depth of 5 to 10 microns. It then flows in the dentinal tubule for up to 100 microns at which point it diffuses laterally into the peri-tubular dentin for up to 10 microns.
As in the previous case the calcium component is selectively eliminated. Then these spaces are replaced by an insoluble resin component that completely encapsulates all exposed collagenous fiber.
He also reported that dentin conditioning by citric acid containing ferric chloride followed by a dentin bonding agent containing 4 META (methacryloxyethyl trimellitate anhydride) was effective method of dentinal bonding.
Concerning the bonding mechanism, he proposed that diffusion and impregnation of monomers into the subsurface of pretreated dentinal substrate and their polymerization, creating a hybrid layer of resin reinforced dentin. This newly formed hybrid layer may be thought of as an admixture of polymer and dentinal components, creating a resin dentin composite. This technique not only enhances the shear bond strength of the resin to the dentin but also increase the potential against micro leakage and postoperative sensitivity.
Nakabayashi (1985) suggested that the acidic treatment partially demineralized a zone of the dentin near the surface, facilitating an infiltration process of compatible monomers. The polymerized resin forms a reinforced zone of dentin on which a resin based restorative material can be bonded. The bond strength is not dependent upon interlocking at the dentinal tubules.
Kurosaki et al (1987) found that etching of dentin of the clinical cavity floor allows the chemically adhesive composite resin to produce resin tags of tapered, cylindrical or tubular form as well as impregnated dentinal layers. These changes will considerably improve the bond strength as well as the tubule aperture seal.
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