Ricardo M. Carvalho

Relationship between surface area for adhesion and tensile bond strength--evaluation of a micro-tensile bond test.

OBJECTIVES The purpose of this study was to test the null hypothesis that there is no relationship between the bonded surface area of dentin and the tensile strength of adhesive materials. METHODS The enamel was removed from the occlusal surface of extracted human third molars, and the entire flat surface was covered with resin composite bonded to the dentin to form a flat resin composite crown. Twenty-four hours later, the bonded specimens were sectioned parallel to the long axis of the tooth into 10-20 thin sections whose upper part was composed of resin composite with the lower half being dentin. These small sections were trimmed using a high speed diamond bur into an hourglass shape with the narrowest portion at the bonded interface. Surface area was varied by altering the specimen thickness and width. Tensile bond strength was measured using custom-made grips in a universal testing machine. RESULTS Tensile bond strength was inversely related to bonded surface area. At surface areas below 0.4 mm2, the tensile bond strengths were about 55 MPa for Clearfil Liner Bond 2 (Kuraray Co., Ltd.), 38 MPa for Scotchbond MP (3M Dental Products), and 20 MPa for Vitremer (3M Dental Products). At these small surface areas all of the bond failures were adhesive in nature. SIGNIFICANCE This new method permits measurement of high bond strengths without cohesive failure of dentin. It also permits multiple measurements to be made within a single tooth.

Collagen Degradation by Host-derived Enzymes during Aging

Incompletely infiltrated collagen fibrils in acid-etched dentin are susceptible to degradation. We hypothesize that degradation can occur in the absence of bacteria. Partially demineralized collagen matrices (DCMs) prepared from human dentin were stored in artificial saliva. Control specimens were stored in artificial saliva containing proteolytic enzyme inhibitors, or pure mineral oil. We retrieved them at 24 hrs, 90 and 250 days to examine the extent of degradation of DCM. In the 24-hour experimental and 90- and 250-day control specimens, we observed 5- to 6-μm-thick layers of DCM containing banded collagen fibrils. DCMs were almost completely destroyed in the 250-day experimental specimens, but not when incubated with enzyme inhibitors or mineral oil. Functional enzyme analysis of dentin powder revealed low levels of collagenolytic activity that was inhibited by protease inhibitors or 0.2% chlorhexidine. We hypothesize that collagen degradation occurred over time, via host-derived matrix metalloproteinases that are released slowly over time.

Dentine permeability and dentine adhesion

OBJECTIVES The objectives of this paper are to review the structure of dentine as it pertains to adhesive bonding and to describe the importance of resin permeation into dentinal tubules and into spaces created between collagen fibrils by acid-etching during resin bonding. The advantages and disadvantages of separate acid-etching, priming and adhesive applications are discussed. DATA SOURCES Although not an exhaustive review, the concepts included in the review were obtained from the dentine bonding literature. STUDY SELECTION Attempts were made to critically evaluate what is known about dentine permeability and adhesion and what remains to be discovered. Speculations were made on a number of controversial issues that are not yet resolved. CONCLUSIONS Acid-etching of dentine produces profound changes in the chemical composition and physical properties of the matrix which can influence the quality of resin-dentine bonds, their strength and perhaps their durability.

Adhesion testing of dentin bonding agents: A review

Adhesion testing of dentin bonding agents was reviewed starting with the adhesion substrate, dentin, the variables involved in etching, priming and bonding, storage variables and testing variables. Several recent reports attempting to standardize many of these variables were discussed. Recent advances in the development of new bonding systems have resulted in bond strengths on the order of 20-30 MPa. At these high bond strengths, most of the bond failure modes have been cohesive in dentin. As this precludes measurement of interfacial bond strength, new testing methods must be developed. One such new method, a microtensile method, was described along with preliminary results that have been obtained. The last decade has produced major advances in dentin bonding. The next decade should prove to be even more exciting.

State of the art etch-and-rinse adhesives

OBJECTIVES The aim of this study was to explore the therapeutic opportunities of each step of 3-step etch-and-rinse adhesives. METHODS Etch-and-rinse adhesive systems are the oldest of the multi-generation evolution of resin bonding systems. In the 3-step version, they involve acid-etching, priming and application of a separate adhesive. Each step can accomplish multiple goals. Acid-etching, using 32-37% phosphoric acid (pH 0.1-0.4) not only simultaneously etches enamel and dentin, but the low pH kills many residual bacteria. RESULTS Some etchants include anti-microbial compounds such as benzalkonium chloride that also inhibits matrix metalloproteinases (MMPs) in dentin. Primers are usually water and HEMA-rich solutions that ensure complete expansion of the collagen fibril meshwork and wet the collagen with hydrophilic monomers. However, water alone can re-expand dried dentin and can also serve as a vehicle for protease inhibitors or protein cross-linking agents that may increase the durability of resin-dentin bonds. In the future, ethanol or other water-free solvents may serve as dehydrating primers that may also contain antibacterial quaternary ammonium methacrylates to inhibit dentin MMPs and increase the durability of resin-dentin bonds. The complete evaporation of solvents is nearly impossible. SIGNIFICANCE Manufacturers may need to optimize solvent concentrations. Solvent-free adhesives can seal resin-dentin interfaces with hydrophobic resins that may also contain fluoride and antimicrobial compounds. Etch-and-rinse adhesives produce higher resin-dentin bonds that are more durable than most 1 and 2-step adhesives. Incorporation of protease inhibitors in etchants and/or cross-linking agents in primers may increase the durability of resin-dentin bonds. The therapeutic potential of etch-and-rinse adhesives has yet to be fully exploited.

In vivo Preservation of the Hybrid Layer by Chlorhexidine

Host-derived proteases have been reported to degrade the collagen matrix of incompletely-resin-infiltrated dentin. This study tested the hypothesis that interfacial degradation of resin-dentin bonds may be prevented or delayed by the application of chlorhexidine (CHX), a matrix metalloproteinase inhibitor, to dentin after phosphoric acid-etching. Contralateral pairs of resin-bonded Class I restorations in non-carious third molars were kept under intra-oral function for 14 months. Preservation of resin-dentin bonds was assessed by microtensile bond strength tests and TEM examination. In vivo bond strength remained stable in the CHX-treated specimens, while bond strength decreased significantly in control teeth. Resin-infiltrated dentin in CHX-treated specimens exhibited normal structural integrity of the collagen network. Conversely, progressive disintegration of the fibrillar network was identified in control specimens. Auto-degradation of collagen matrices can occur in resin-infiltrated dentin, but may be prevented by the application of a synthetic protease inhibitor, such as chlorhexidine.

Chlorhexidine Preserves Dentin Bond in vitro

Loss of hybrid layer integrity compromises resin-dentin bond stability. Matrix metalloproteinases (MMPs) may be partially responsible for hybrid layer degradation. Since chlorhexidine inhibits MMPs, we hypothesized that chlorhexidine would decelerate the loss of resin-dentin bonds. Class I preparations in extracted third molars were sectioned into two halves. One half was customarily restored (etch-and-rinse adhesive/resin composite), and the other was treated with 2% chlorhexidine after being acid-etched before restoration. Specimens were stored in artificial saliva with/without protease inhibitors. Microtensile bond strengths and failure mode distribution under SEM were analyzed immediately after specimens’ preparation and 6 months later. With chlorhexidine, significantly better preservation of bond strength was observed after 6 months; protease inhibitors in the storage medium had no effect. Failure analysis showed significantly less failure in the hybrid layer with chlorhexidine, compared with controls after 6 months. In conclusion, this in vitro study suggests that chlorhexidine might be useful for the preservation of dentin bond strength.

Regional Measurement of Resin-Dentin Bonding as an Array

During the development of the microtensile bond-testing method, large variations in bond strengths were noted among serial sections. The reason for these variations is unknown. The purpose of this work was to determine the consistency of resin-dentin bond strengths across the occlusal surface of coronal dentin by dividing composite resin buildups into an array of 1 x 1 mm beams, the top half consisting of composite resin, and the bottom half consisting of dentin. Extracted human third molars had the occlusal enamel removed as a single section by means of a diamond saw. Resin composite buildups were made after the dentin was bonded with either One-Step or MacBond. After being stored in 37°C water for 1 day, the teeth were vertically sectioned at 1-mm increments into slabs of bonded teeth. Each slab was further subdivided by vertical sections into 1 x 1 x 8 mm beams. Each beam was assigned an x-y coordinate and tested for tensile bond strength. Two different clinicians (A and B) performed the same procedures using One-Step in a parallel study. Using One-Step, clinician A obtained a large number of zero bonds in superficial dentin but fewer in deep dentin. This resulted in a very large standard deviation in bond strengths (mean ± SD of 22 ± 20 MPa in superficial dentin and 27 ± 14 MPa in deep dentin). Clinician B obtained much higher (p < 0.001) and more uniform bond strengths with One-Step (56 ± 13 MPa in superficial dentin and 57 ± 12 MPa in deep dentin). With MacBond, there were no zero bonds and hence less variation, with a mean of 41 ± 13 MPa in superficial dentin and 27 ± 12 MPa (x ± SD) in deep dentin. When pairs of Z100 resin composite cylinders were bonded together with One-Step and then sectioned into an array, there was little variation in regional bond strength (37 ± 1 MPa). Dividing bonded resin composite buildups into an array of 20 to 30 1 x 1 x 8 mm beams allows for the evaluation of uniformity of resin-dentin bonds. The method used in this study detected local regional differences in resin-dentin bond strengths. The largest differences were shown to be related to technique rather than to material. The results indicate that resin-dentin bonds may not be as homogenous as was previously thought.

Optimizing dentin bond durability: control of collagen degradation by matrix metalloproteinases and cysteine cathepsins

OBJECTIVES Contemporary adhesives lose their bond strength to dentin regardless of the bonding system used. This loss relates to the hydrolysis of collagen matrix of the hybrid layers. The preservation of the collagen matrix integrity is a key issue in the attempts to improve the dentin bonding durability. METHODS Dentin contains collagenolytic enzymes, matrix metalloproteinases (MMPs) and cysteine cathepsins, which are responsible for the hydrolytic degradation of collagen matrix in the bonded interface. RESULTS The identities, roles and function of collagenolytic enzymes in mineralized dentin has been gathered only within last 15 years, but they have already been demonstrated to have an important role in dental hard tissue pathologies, including the degradation of the hybrid layer. Identifying responsible enzymes facilitates the development of new, more efficient methods to improve the stability of dentin-adhesive bond and durability of bond strength. SIGNIFICANCE Understanding the nature and role of proteolytic degradation of dentin-adhesive interfaces has improved immensely and has practically grown to a scientific field of its own within only 10 years, holding excellent promise that stable resin-dentin bonds will be routinely available in a daily clinical setting already in a near future.

Bonding of self-etch and total-etch adhesives to carious dentin

Carious dentin is partially demineralized and contains mineral crystals in the tubules. This may permit the deeper etching of intertubular dentin but prevent resin tag formation during bonding. We hypothesize that resin adhesives will produce lower bond strengths to caries-infected and caries-affected dentin compared with normal dentin. We tested this by measuring the microtensile bond strength of a total-etch adhesive and an experimental self-etching adhesive (ABF) to caries-infected, caries-affected, and sound dentin and by correlating those results with ultrastructural observations. The bond strengths of both adhesives to sound dentin were significantly (p < 0.05) higher than those to caries-affected dentin, which, in turn were significantly (p < 0.05) higher than those to caries-infected dentin. For both adhesives, hybrid layers in caries-affected dentin were thicker but more porous than those in sound dentin. The lower bond strengths may be due to the lower tensile strength of caries-affected dentin. Clinically, this may not be a problem, since such lesions are normally surrounded by normal dentin or enamel.