1

Introduction

Contemporary bonding procedures used in the placement of resin–composite restorations can cause exposure and activation of endogenous dentin proteases . This process causes gradual destruction of poorly infiltrated collagen fibrils within the hybrid layers of adhesive bonds to dentin . Degradation of collagen within the hybrid layer can compromise the durability of adhesive bonds and facilitate a reduction in bond strength over time.

Manufacturers have simplified etch-and-rinse adhesives from three-step (involving etchant, primer and adhesive) to two-step systems (etchant and adhesive) by combining hydrophilic primers with hydrophobic adhesives with various solvents. But with this change the durability of resin–dentin bonds has decreased . More recently, phosphoric acid etchants have been replaced by incorporating acidic monomers into a solvated adhesive to create single-step, self-etching adhesives. Application of both etch-and-rinse and self-etch adhesives causes activation of matrix metalloproteinases (MMPs) and cysteine cathepsins . These are host-derived proteolytic enzymes that are bound to the dentin collagen matrix. When uncovered by etching, the MMPs slowly solubilize the collagen fibrils and remain active even after resin-infiltration. Apparently, the time-dependent degradation of the hybrid layer is most evident in the use of etch-and-rinse adhesives .

Several strategies are being explored for preventing enzymatic degradation of the dentin collagen and to address the concerns related to the poor durability of resin-bonded dentin interfaces. Tjäderhane et al. , Montagner et al. and Sabatini and Pashley have recently reviewed the current understanding of collagen degradation and have discussed the relative merits/drawbacks of the techniques under exploration. One of the foremost approaches for inactivation of the dentin proteases involves using cross-linking agents. Covalent cross-links produced with exogenous cross-linkers (e.g. glutaraldehyde, grape seed extract and carbodiimides) inactivate the active sites of dentin proteases by reducing the molecular mobility of the active site or by changing negatively charged ionized carboxyl groups into positively charged amides . Of the current crosslinkers, carbodiimide, has some attractive qualities, including very low cytotoxicity, and an ability to preserve dentin bond strength within clinically acceptable treatment times .

Mazzoni et al. recently reported promising results on the use of an EDC conditioning treatment in stabilizing dentin bonds. They evaluated the microtensile strength of dentin bonds for two different etch-and-rinse systems (Optibond FL and Adper Scotchbond Multi-Purpose) and assessed the degradation over a 12-month period. The EDC treatment consisted of exposing the etched dentin to a 0.3 M EDC solution for 1 min prior to bonding. In comparison to the control groups, the EDC treated samples exhibited between 25 and 35% higher bond strengths after 12 months storage. A related study by the group using zymography showed that the EDC treatment was successful in inactivating dentin gelatinases, thereby preventing degradation of the collagen.

While bond strength is an important metric of performance, resin–dentin bonded interfaces are subjected to cyclic loading and thus may undergo failure by fatigue and/or fatigue crack growth. Fatigue failures are considered of substantial importance to the success of restoratives . Yet, studies in this area are scant and the contribution of fatigue to resin–dentin bond failures has received rather limited attention overall . If flaws are located within either the resin adhesive or the hybrid layer, e.g. as a result of hydrolysis or incomplete resin infiltration , then the interface durability depends on its resistance to the “propagation” of these defects via cyclic crack extension. However, progressive failure of the resin–dentin bonded interface by either cyclic or slow crack growth has not been addressed by the dental materials community.

Soappman et al. proposed an approach for evaluating the fatigue crack growth resistance of resin–dentin bonds, but it has not been applied to assess the effectiveness of an EDC treatment on dentin bond durability. Therefore, the primary objective of this study was to evaluate the effect of an experimental EDC conditioning treatment applied during dentin bonding, on the fatigue crack growth resistance of the adhesive interface. The null hypotheses to be tested were that an EDC treatment (consisting of 0.5 M and 1 min exposure) applied during dentin bonding (1) does not change the immediate fatigue crack growth resistance of the resin/dentin interface, and (2) has no effect on the fatigue crack growth resistance up to a 6 month period of storage.

2

Materials and methods

The specimens utilized for this approach involve sections of coronal dentin that were obtained from caries-free human third molars and obtained with informed signed consent. The teeth were obtained from participating clinics in Maryland with record of age (18 ≤ age ≤ 30 yrs) according to an approved protocol (#Y04DA23151). Each tooth was sectioned using a slicer/grinder (Chevalier Smart-H818II, Chevalier Machinery, Santa Fe Springs, CA, USA) with diamond abrasive slicing wheels (#320 mesh abrasives) and copious water coolant. The sections were obtained from the mid coronal region ( Fig. 1 a) as necessary for the specimen geometry. The remaining materials used in the development of the specimens included a three-step etch-and-rinse adhesive (Scotchbond Multipurpose, SBMP, 3M ESPE) and compatible resin composite (Z100, 3M ESPE).

Schematic diagrams describing (a) the location and orientation of coronal dentin sections obtained for the bonded interface specimens (outlined by the dashed line) and (b) the inset Compact Tension (CT) specimen geometry for characterizing the fatigue crack growth resistance. The diagram highlights the dentin (D) and resin composite (C), and orientation of the opening mode load (P). Adhesive bonding was performed to the occlusal aspect of the dentin sections, with the pulp side facing away from the bonded interface.

Bonded interface Compact Tension (CT) specimens were prepared from the dentin sections using a special molding technique that has been described in detail in previous studies . Briefly, the dentin sections represented half of the completed CT specimen geometry ( Fig. 1 b). Adhesive bonding was performed to the occlusal aspect of the dentin sections, with the pulp side facing away from the bonded interface. The occlusal edge was etched for 15 s (SB 37% phosphoric etchant) and rinsed with water in preparation for bonding. Then the SBMP primer and adhesive were applied to the etched surface according to the manufacturer’s recommendations. Thereafter, these sections were placed in a specially designed mold that enabled incremental application of the resin composite as necessary to complete the CT geometry. A thin Mylar sheet was placed at one end of the interface to introduce a molded notch as evident in Fig. 1 b. The composite was cured on both sides for 40 s using a quartz–tungsten–halogen light-curing unit (Demetron VCL 401, Demetron, CA, USA) with output intensity of 600 mW/cm ² and with tip diameter wider than 10 mm. Power emission of the curing light was measured and validated using a PM10 thermopile (Coherent, Santa Clara, CA) attached to a Fieldmate meter (Coherent) calibrated to NIST standards. The bonded sections were released from the mold and two holes were introduced using a miniature milling machine to facilitate the opening mode loading. A detailed description of the procedures used for specimen preparation has been presented previously .

The durability of the bonded interfaces was evaluated with and without an experimental treatment formulated to inactivate endogenous dentin proteases. For the treated specimens, the application of primer and adhesive was preceded by conditioning the demineralized collagen using an experimental solution of 0.5 M ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) for 60 s. The specimens were then rinsed with water (15 s) and then lightly blotted. The remainder of the specimen preparation process was identical to that for those specimens without EDC. Following the aforementioned procedures, the specimens were placed within a phosphate-buffered artificial saliva at 37 °C until further evaluation. A total of 36 specimens were prepared overall and consisted of an equal number of non-cross-linked control and cross-linked specimens (that received EDC treatment). The fatigue crack growth resistance of the specimens was evaluated after a storage period of 0, 3 or 6 months, resulting in six specimens in each cell (treatment and time). Those specimens evaluated at 0 months (i.e. without storage) are considered to represent the “immediate” fatigue crack growth resistance and were tested after a period of at least 48 h from the date of preparation.

The CT specimens were subjected to cyclic Mode I loading using a universal testing system (EnduraTEC Model ELF 3200, Minnetonka, MN, USA) with load capacity and sensitivity of 225 N and ±0.01 N, respectively. All experiments were performed within a bath of Hanks Balanced Salt Solution (HBSS) at room temperature (pH 7.4). The loading was applied under load control actuation, a frequency of 5 Hz and stress ratio ( R ) of 0.1. Measurement of the crack length was accomplished using an imaging system that consisted of a microscope (Optem zoom 70xl 391940, QIOPTIQ, Luxembourg) and CCD camera. Sequential measurements of the crack length were used to estimate the incremental crack extension (Δ a ) as a function of the loading cycles in the increment Δ N . In general, the increment of cyclic loading ranged from 5 ≤ Δ N ≤ 30k cycles and the increment of crack growth extended from 0.02 ≤ Δ a ≤ 0.15 mm. Cyclic loading was continued until the specimen underwent complete fracture. These procedures have been used previously for evaluating the resin–dentin bonded interface and the fatigue crack growth resistance of dentin .

The fatigue crack growth experiments provided measurements of crack length as a function of the cyclic loading history for each of the specimens evaluated. The data was used to determine the incremental crack growth rate (Δ a /Δ N ) and the corresponding stress intensity range (Δ K ) over the total length of crack extension achieved. For the resin/dentin inset CT specimens the stress intensity range was estimated as a function of crack length according to

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