ReviewDentin bonding systems: From dentin collagen structure to bond preservation and clinical applications
Introduction
Adhesive systems can be considered revolutionary in many aspects of conservative dentistry, making possible previously inconceivable clinical maneuvers. Current adhesive systems allow clinicians to bond to tooth structure without the need of a retentive cavity since they provide immediate bond strength.
Two different strategies can presently be employed in resin bonding procedures: the etch-and-rinse technique (E&R) and the self-etch (SE) or etch-and-dry technique. Regardless of the strategy used, dentin bonding relies on the formation of the “hybrid layer” (HL), a structure composed of demineralized collagen fibrils reinforced by the resin matrix [1], [2].
The goal of adhesive procedures is to form and maintain a tight adhesive-dentin interface that is stable for a number of years, providing retentive strength, marginal seal, and clinical durability [3]. However, regardless of the advances in dental materials, the HL created on the variable and dynamic organic dentin phase is not perfect, and may fail over time, inducing marginal discolorations, marginal leakage and subsequent loss of retention of the composite restoration [3], [4], [5], [6], [7], [8].
The aim of this review is to analyze and critically assess the available research on the factors that influence the stability of the resin–dentin bonds and the strategies for preservation of the adhesive interface over time.
Section snippets
Adhesive systems and adhesion strategies to dentin
Since resin monomers themselves cannot infiltrate mineralized tissues, traditionally, adhesive bonding systems consist of an acid, primer and adhesive. Acid is used for the removal of mineral crystals and exposure of the collagen fibrils. Primer is a hydrophilic solution of resinous monomers, which allows the infiltration of the resinous monomers, especially in demineralized dentin. The adhesive itself contains mixtures of monomers that penetrate the surfaces treated with the primer, creating a
Dentin collagen structure
In order to better understand the resin infiltration processes underlying collagen impregnation during HL formation and degradation over time, it is necessary to understand the basic structure and composition of dentin, with special attention to the organic matrix, and the changes that occur in the structure during adhesive procedures.
Dentin is a mineralized collagen matrix that contains approx. 30–50 vol% organic material and approx. 20 vol% of water [25]. The composition of dentin can vary in
Resin degradation
Two general patterns of degradation of the HL have been described: the disorganization and solubilization of collagen fibrils and the hydrolysis and leaching of the adhesive resin from the interfibrillar spaces [53], [54].
Hydrolytic degradation occurs only in the presence of water and is a chemical reaction capable of breaking covalent bonds between polymers causing loss of the resin mass [6], [55]. Hydrolysis is considered the primary reason for resin degradation within the hybrid layer [56].
Degradation of the collagen scaffold/fibrils
The collagenolytic activity in dentin was first reported by Dayan et al. [76], whereas Tjäderhane et al. [77] provided further clarification and attributed this activity to the matrix metalloproteinases (MMPs). Pashley et al. [78] demonstrated that collagen can degrade over time in aseptic conditions, indicating that it could be caused by intrinsic matrix proteases. Interestingly, the intrinsic dentin gelatinolytic and collagenolytic activity reported was significantly lower in specimens
Strategies to reduce HL degradation
Endogenous proteases are actually hydrolases, since they require unbound water to cleave collagen peptides. During dentin bonding procedures, it is very difficult, if not impossible, to fully envelope the deepest portion of the demineralized collagen fibrils within the HL with resin. The gaps between the exposed collagen fibrils are filled with water, enabling the activation of the endogenous enzymes and collagen degradation, leading to plasticization of the adhesive resin, and mechanical
Conclusions
Although there are many more hurdles to be overcome in the field of adhesive dentistry, impressive progress in the understanding of the processes underlying HL degradation, as well as in the development of strategies for the preservation of the adhesive interface have been achieved. The removal of the unbound water from the hybrid layer and the silencing of the endogenous enzymatic activity, have been achieved using chemical agents and physical approaches, which are increasing in number,
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