Adhesion to Impaired DentinCaries Affected and Caries Infected DentinThere are few studies that investigate the bonding characteristics of impaired dentin. Most caries excavation methods tend to leave decayed and/or caries-infected dentin as a bonding substrate for adhesives. Adhesion strength is influenced by the method of caries removal and the type of adhesive. The bond strength will be 20-50% lower in caries-affected dentin than the bond strength with healthy dentin, and even lower in caries-infected dentin. The decrease in bond strength is due to the fact that caries-affected and infected dentin is more porous, contains more water and a thicker hybrid layer, which is not necessarily well infiltrated, despite the bonding strategy . This lower bond strength that occurs in caries-affected dentin is due to structural changes caused by caries progression. Caries will reduce the mineral content and crystallinity of hydroxyapatite and alter the secondary structure of collagen. Additionally, decreased distribution of healthy collagen fibrils and proteoglycans. All of these structural changes will result in a substrate with inferior mechanical properties, which will have a significant impact on adhesion strength. There is evidence that adhesives will polymerize poorly in caries-affected dentin. Studies have been conducted and in each study, etch and rinse adhesives have higher bond strength than self-etch adhesives when bonded to caries-altered dentin. The binding strength necessary for clinical success remains to be determined. Non-carious sclerotic lesions These lesions have unique characteristics; they have a hypermineralized top layer of different thickness that contains bacterial inclusions that sit on top of denatured collagen fibrils... middle of paper ...... resistance. Other ways to improve bond strength include wet ethanol bonding which preserves the hybrid layer and provides long term bond strength. Bonding to root dentin It is considered a great challenge to overcome in adhesive dentistry. Root dentin differs from coronal dentin, so the bonding will be different. Achieving good infiltration and micromechanical retention remains a challenge due to limited access and vision, regional variation in bond strength, and the presence of a thick smear layer. The bond strength differs along the root canal and is lower than that of coronal dentin. Dynamic characteristics keep changing due to aging, intrinsic and extrinsic factors. Resin-dentin bonds are even less predictable. Recent research attempts to improve binding with collagen cross-linkers and inhibit enzymatic degradation.