Mode of failure in the dentin-adhesive resin–resin composite bonded joint as determined by strength-based (μTBS) and fracture-based (CNSB) mechanical testing

Objective: To determine the failure mode between dentin-adhesive resin–resin composite bonded joint produced with a chevron-notch short-bar (CNSB) and microtensile test methods. Methods: Forty teeth were randomly selected for microtensile and forty for CNSB specimen fabrication and stored in 0.5% chloramine T at 37°C until respective static load to failure testing at 30 and 180 days. Failure modes were categorized by SEM and tested with Fisherʹs exact test. Within respective mechanical testing methods the probability of failure curve distributions being significantly different were analyzed by the Wald chi-square statistic. Results: The characteristic fracture toughness at 30- and 180-day storage was 0.82 and 0.87 MPa m1/2, while the Weibull Modulus (m) for the failure distributions, was 4.60 and 4.56, respectively. No significant difference was demonstrated in the failure distributions between these groups (p=0.45). The characteristic tensile strength (μTBSo) at 30- and 180-day storage was 52.53 and 14.71 MPa with an m of 3.04 and 1.56, respectively. Failure distributions for μTBS groups were significantly different (p<0.001). KIvM failure modes, regardless of storage time, were within the adhesive joint with 30-day debonds primarily through the top region of the hybrid layer (THL) and after 180-days involving the bottom of the hybrid layer (BHL). The 30-day μTBS group demonstrated a propensity to debond in dentin or resin composite substrates but after 180-days storage debonds again involved the BHL. Significance: The weak links in the dentin-adhesive resin–resin composite bonded joint may be the interphase regions between the THL and the adhesive resin and the BHL and dentin.