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Review
. 2011 Aug;90(8):953-68.
doi: 10.1177/0022034510391799. Epub 2011 Jan 10.

Limitations in bonding to dentin and experimental strategies to prevent bond degradation

Y Liu  1 , L Tjäderhane, L Breschi, A Mazzoni, N Li, J Mao, D H Pashley, F R Tay
Affiliations
Review

Limitations in bonding to dentin and experimental strategies to prevent bond degradation

Y Liu et al. J Dent Res. 2011 Aug.

Abstract

The limited durability of resin-dentin bonds severely compromises the lifetime of tooth-colored restorations. Bond degradation occurs via hydrolysis of suboptimally polymerized hydrophilic resin components and degradation of water-rich, resin-sparse collagen matrices by matrix metalloproteinases (MMPs) and cysteine cathepsins. This review examined data generated over the past three years on five experimental strategies developed by different research groups for extending the longevity of resin-dentin bonds. They include: (1) increasing the degree of conversion and esterase resistance of hydrophilic adhesives; (2) the use of broad-spectrum inhibitors of collagenolytic enzymes, including novel inhibitor functional groups grafted to methacrylate resins monomers to produce anti-MMP adhesives; (3) the use of cross-linking agents for silencing the activities of MMP and cathepsins that irreversibly alter the 3-D structures of their catalytic/allosteric domains; (4) ethanol wet-bonding with hydrophobic resins to completely replace water from the extrafibrillar and intrafibrillar collagen compartments and immobilize the collagenolytic enzymes; and (5) biomimetic remineralization of the water-filled collagen matrix using analogs of matrix proteins to progressively replace water with intrafibrillar and extrafibrillar apatites to exclude exogenous collagenolytic enzymes and fossilize endogenous collagenolytic enzymes. A combination of several of these strategies should result in overcoming the critical barriers to progress currently encountered in dentin bonding.

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Figures

Figure 1.
Figure 1.
Schematics on the use of experimental adhesives or MMP inhibitors to prevent the degradation of resin-dentin bonds. (A) A schematic depicting the use of experimental adhesives with increased degree of conversion and esterase resistance for bonding to acid-etched dentin. However, such adhesives may not completely infiltrate the entire depth of the hybrid layer. Left side depicts denuded collagen fibrils being degraded by matrix metalloproteinases (MMPs) and cathepsins. Right side depicts cleaved, degraded collagen fragments prevented from dissociating from the collagen molecule via intermolecular and intramolecular cross-links. Because of these cross-links, measuring the amount of hydroxyproline from a degraded dentin collagen matrix is likely to underestimate the extent of collagen degradation. M, MMP; K, cathepsin K. (B) A schematic depicting the use of MMP inhibitors or MMP-inhibitor-conjugated adhesives for bonding to acid-etched dentin. Left side: Unlike MMP-8, MMP-2 is thought to function by unwinding the triple collagen helix prior to scission of the tropocollagen molecules. Right side: The catalytic domain of MMPs is blocked in the presence of a broad-spectrum MMP inhibitor. M, MMP; K, cathepsin K.
Figure 2.
Figure 2.
Schematics on the use of cross-linking agents or ethanol wet-bonding to prevent the degradation of resin-dentin bonds. (A) A schematic depicting the use of cross-linking agents for silencing the collagenolytic activities in bonded acid-etched dentin. Cross-linking may cause conformational changes in the active site, catalytic domain, and/or binding site of MMPs, eliminating the enzymes’ ability to degrade the substrate. Left side depicts hypothetical MMP via its catalytic domain. Right side depicts allosteric inhibition of MMPs via their other non-catalytic domains. M, MMP; K, cathepsin K. (B) A schematic depicting the use of the ethanol wet-bonding technique for bonding hydrophobic adhesives to acid-etched dentin. Both apatite-depleted extrafibrillar and intrafibrillar spaces are infiltrated by hydrophobic adhesive without nanophase separation. Left side depicts progressive water replacement of collagen matrix by ethanol, with the shrunken fibrils suspended in ethanol. Right side depicts immobilization of MMP by resin that is analogous to “molecular printing” but without removal of the enzyme. M, matrix metalloproteinase; K, cathepsin K.
Figure 3.
Figure 3.
Stained transmission electron micrographs comparing the thickness of hybrid layers (low magnification, between open arrows) and dimensions of the collagen fibrils and interfibrillar spaces (high magnification, open arrowheads). (A,B) Resin-dentin interface bonded with a hydrophilic adhesive by the water wet-bonding technique. (C,D) Resin-dentin interface bonded with an experimental hydrophobic adhesive by the ethanol wet-bonding technique. D, laboratory-demineralized intact dentin.
Figure 4.
Figure 4.
Biomimetic remineralization of resin-dentin bonds as a potential means of preventing the degradation of resin-dentin bonds. (A) A schematic depicting the use of a biomimetic remineralization mechanism for refilling water-rich, resin-sparse regions of the hybrid layer with apatite for exclusion of exogenous collagenolytic enzymes and fossilization of endogenous collagenolytic enzymes. Left side depicts the action of a sequestration analog such as polyaspartic acid or polyacrylic acid in stabilizing amorphous calcium phosphate nanoprecursors, and the use of a templating analog such as sodium trimetaphosphate to initiate the nucleation and growth of apatite within the intrafibrillar spaces of a collagen fibril. Right side depicts apatite crystallites fossilizing the collagen molecules, protecting them from exogenous and endogenous collagen-degrading enzymes. M, MMP; K, cathepsin K. (B) Stained transmission electron micrograph showing the degradation of collagen fibrils within the hybrid layer (asterisk) after aging. SH, stained, intact hybrid layer; A, dentin adhesive; D, laboratory-demineralized intact dentin. (C) Unstained transmission electron micrograph of the resin-dentin interface that has undergone 3 months of biomimetic remineralization with a remineralizing composite and in the presence of sequestration and templating biomimetic analogs present in the incubation medium. Note the striking similarity between the remineralized part of the hybrid layer (R) and the degraded part of the hybrid layer shown in Fig. 4B. UH, unstained, non-degraded part of the hybrid layer; A, adhesive; M, mineralized intact dentin. (D) Nanoscopic dynamic mechanical analysis showing the distribution of complex modulus (i.e., loss modulus/storage modulus) across the resin-dentin interface after 3 months of immersion in a control medium. A, adhesive; H, hybrid layer; D, intact dentin. Note the complete lack of high modulus (i.e., light-colored material within the hybrid layer or resin tag). (E) Nanoscopic dynamic mechanical analysis showing the distribution of complex modulus (i.e., loss modulus/storage modulus) across the resin-dentin interface after 3 months of biomimetic remineralization. A, adhesive; H, hybrid layer; D, intact dentin. Note the presence of high-modulus material throughout the hybrid layer and resin tag.
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