Resin-Dentin Bonding Interface: Mechanisms of Degradation and Strategies for Stabilization of the Hybrid Layer

Abstract

Several studies have shown that the dentin-resin interface is unstable due to poor infiltration of resin monomers into the demineralized dentin matrix. This phenomenon is related to the incomplete infiltration of the adhesive system into the network of exposed collagen fibrils, mainly due to the difficulty of displacement and subsequent replacement of trapped water between interfibrillar spaces, avoiding adequate hybridization within the network of collagen fibrils. Thus, unprotected fibrils are exposed to undergo denaturation and are susceptible to cyclic fatigue rupture after being subjected to repetitive loads during function. The aqueous inclusions within the hybrid layer serve as a functional medium for the hydrolysis of the resin matrix, giving rise to the activity of esterases and collagenolytic enzymes, such as matrix metalloproteinases, which play a fundamental role in the degradation process of the hybrid layer. Achieving better interdiffusion of the adhesive system in the network of collagen fibrils and the substrate stability in the hybrid layer through different strategies are key events for the interfacial microstructure to adequately function. Hence, it is important to review the factors related to the mechanisms of degradation and stabilization of the hybrid layer to support the implementation of new materials and techniques in the future. The enzymatic degradation of collagen matrix, together with resin leaching, has led to seeking strategies that inhibit the endogenous proteases, cross-linking the denudated collagen fibrils and improving the adhesive penetration removing water from the interface. Some of dentin treatments have yielded promising results and require more research to be validated. A longer durability of adhesive restorations could resolve a variety of clinical problems, such as microleakage, recurrent caries, postoperative sensitivity, and restoration integrity.

Analysis of laboratory adhesion studies in eroded enamel and dentin: a scoping review

“…Collapsed demineralized fibrils and excess water content are also observable, leading to a deficient hybrid layer and impairing the penetration and in situ polymerization of the adhesive, ultimately affecting bond strength [ 17 , 47 ]. Since eroded dentin leads to an increase of water present in the matrix, nanoleakage becomes a bigger threat to hybrid layers [ 42 ].Consequently, this water is partially responsible for accelerated activation of endogenous proteases, the so-called matrix metallo-proteinases (MMPs), capable of hydrolyzing the organic matrix [ 63 , 64 ]. In fact, this lengthened contact between water and monomers found in the adhesive can ultimately lead to an accelerated hybrid layer degradation and failure of the restoration [ 58 ].…”

“…As eroded dentin is more vulnerable to hybrid layer breakdown, it is important to stabilize the interface. In fact, MMPs can be activated by exposure to low pH and are able to progress dental erosion [ 63 , 67 ]. The successive pH demineralization-remineralization cycles are not sufficient to inhibit MMP activity, although their optimum pH to function is around neutral conditions [ 67 , 68 ].…”

Effect of chitosan on resin-dentin interface durability: A 2 year in-vitro study

“…[19][20][21] Cross-linking of collagen was done by using different cross-linking agents as formaldehyde, grape seed extract, glutaraldehyde, carbodiimide, transglutaminase, proanthocyanidin, genepin, and riboflavin. [22][23][24][25][26][27] Chitosan is a natural biocompatible polymer. It is capable of forming a fibrillar network, with superior mechanical properties, at micro and/or nano-scale.…”

“…31,32 Due to its outstanding properties, modification of demineralized dentin using chitosan could be of great importance in dentin bonding. Although previous studies [22][23][24][25][26][27][28][29][30] have proved the importance of collagen network strengthening in providing a more stable and durable bonded interface, the effect of chitosan on dentinal collagen has not been fully investigated yet.…”

Dentin Bonding and SEM Analysis of a New Experimental Universal Adhesive System Containing a Dendrimer

“…Dentin bonding is obtained by mechanical retention of the adhesive monomers within the hybrid layer (HL) and the dentin tubules. The HL results from the infiltration and polymerization of the adhesive monomers between the collagen fibers exposed due to partial demineralization of dentin, sometimes enhanced by chemical adhesion [5,6]. The stability of the HL is critical for bonding efficacy; however, the main reason for failure has been attributed to the presence of water and acidic components that enhance enzymatic and hydrolytic degradation of newly formed HL [6].…”