The mechanical behavior and durability of resin-dentin bonds are crucial to the longevity of resin composite restorations and the success of contemporary restorative dentistry. While degradation of the organic protein matrix has been considered one of the primary challenges to the longevity of resin composite restorations, there is still limited understanding of the influence from host-derived proteolytic enzymes on the durability of resin-dentin bonds.

The primary objective of this dissertation is to characterize the fatigue crack growth (FCG) resistance of adhesive bonds to dentin obtained using two different commercial adhesive resins (Single Bond Plus (SB) and Scotch Bond Multi-Purpose (SBMP), both products of 3M) and evaluate the effect of collagen degradation on the durability of these bonded structures. The effect of a pretreatment with ethyl-3-(3- dimethylaminopropyl) carbodiimide (EDC) to resist collagen degradation was also evaluated, as well as its ability to stabilize the resin-dentin bond durability. The FCG resistance was evaluated using a bonded interface compact tension (CT) specimen configuration. The fracture surfaces were examined using scanning electron microscopy (SEM) to understand mechanism of fatigue crack propagation and the importance of the EDC pretreatment. Furthermore, a novel method for characterizing the resin-dentin bonded interface using scanning-based nanoscopic dynamic mechanical analysis (NanoDMA) was adopted to analyze degradation of resin-dentin bonds.

Results showed that the SB adhesive system has superior resistance to the initiation of FCG (ΔK_th=0.6 ± 0.05 vs 0.5 ± 0.05, in MPa·m^0.5;​ Z = 5.1626, p < 0.001), while the rate of propagation, as distinguished by the Paris Law exponent, is not significantly different (m of 17.5 ± 1.1 vs 16.1 ± 0.6). Both resin adhesives exhibit poorer resistance than those of dentin and resin composite (ΔK_th=0.8 ± 0.1 and 0.7 ± 0.02, respectively, in MPa·m^0.5). SEM analysis of the fracture surface showed that the crack initiates and propagates predominately along the hybrid layer of the bonded interface. Cohesive failure of resin tags in the SB system was more predominant than in SBMP system, while the latter has more adhesive failure of resin tags.

With aging there is gradual degradation in FCG resistance of both SBMP (fresh:​ ΔK_th=0.5 ± 0.05 and 6 months:​ =0.45 ± 0.02, respectively, in MPa·m^0.5) and SB (fresh:​ ΔK_th=0.6 ± 0.05 and 6 months:​ =0.51 ± 0.02, respectively, in MPa·m^0.5). Also, the crack propagation rate increases for the SB system (fresh:​ m=17.5 ± 1.1 and 6 months:​ =19.5 ± 2.5) and for SBMP system (fresh:​ 16.0 ± 0.6 and 6 months:​ 21.5 ± 2.8). Results of EDC pretreatment showed that for both resin adhesive systems the FCG performance in the initial stage (without aging) is not significantly different than the control. However, after 6 months of storage the EDC pretreatment stabilized the fatigue crack growth resistance in terms of the stress intensity threshold and the fatigue crack growth exponent (up to nearly 35% difference at 6 months’ storage for SBMP). Observation of the fracture surfaces using SEM showed a decreased presence of adhesive failure of the resin tags with EDC pretreatment after 6 months of storage.

Results from the NanoDMA tests of the bonded interface showed that the EDC treatment had no influence on the dynamic mechanical behavior of the hybrid layer immediately after bonding. After aging for 3 and 6 months there was no reduction in the apparent stiffness of the hybrid layer as defined by the complex modulus and storage modulus. However, based on changes in the loss modulus and tanδ values, there was a significant reduction in the viscoelastic behavior of the hybrid layers with aging for both adhesive systems. The degradation with storage period was greatest for the SB adhesive;​ without EDC treatment, the reduction in tanδ of the hybrid layer exceeded 80% in 6 months. These results will support the development of new dental materials for improving resin-dentin bonds and facilitate substantial improvements in oral health.