Dentin adhesion of bulk-fill composites and universal adhesives in class I-cavities with high C-factor.

OBJECTIVES: The aim of this study was to compare the dentin adhesion of bulk-fill composites in high C-factor class I-cavities before and after thermocycling to a control group using incremental layering technique. METHODS: A standardized class I-cavity was prepared into 195 human molars, then different universal adhesives were applied either in self-etch or etch & rinse mode, and the cavity was filled according to each materials application protocol. The material combinations used were a conventional layered composite as control, the respective bulk-fill product, two other bulk-fill composites made by different manufacturers, with one of them being tested using two different polymerization times. Furthermore, one thermoviscous bulk-fill composite and one self-adhesive restorative were examined of which the latter can only be applied in self-etch mode. In each group the dentin adhesion to the cavity bottom was measured using microtensile bond strength test initially (24 h water storage) and after thermocycling. All results were statistically analyzed using STATA 17.0. RESULTS: The statistical analysis showed significant differences between the control and the experimental groups (p < 0.001). The highest mean bond strength before (14.8 ± 10.7 MPa) and after aging (14.2 ± 11.5 MPa) was measured for the etch & rinse-control group. Among the bulk-fill groups, the etch & rinse technique consistently showed higher bond strengths. Bond strength of groups with shortened polymerization did not exceed 2.1 MPa. The bond strength of the self-adhesive restoration material was low before and after thermocycling (2.7 MPa/ 0.0 MPa). Groups with low bond strength values showed a high number of pre-testing-failures. CONCLUSIONS: Bulk-fill materials used in high C-factor class I-cavities showed lower bond strength during self-etch application. The same applies for a shortened polymerization regime, which cannot be recommended for high C-factor cavities. CLINICAL SIGNIFICANCE: Today, a large variety of materials and application techniques can be used when placing an adhesive restoration. Whether new instead of established procedures should be applied in high C-factor cavities has to be critically assessed, as they are a demanding scenario for adhesive restorations.

High C-Factor Cavities: How Do "Snowplow Technique", Adhesive Application Mode and Aging Influence the Microtensile Bond Strength to Dentin?

PURPOSE: To investigate the microtensile bond strength (µTBS) to dentin in class-I cavities using different layering techniques, adhesive application modes, and aging. MATERIALS AND METHODS: 150 caries-free human molars were randomly assigned to 8 experimental and 2 control groups (n=15 teeth/ group). For each tooth, a standardized class-I cavity was prepared (4x4x4 mm) and pretreated with a universal adhesive (self-etch or etch-and-rinse mode). Incrementally layered restorations served as the control. In the experimental groups, either lining with bulk-fill flowable composite and a layering technique, bulk filling, or the snowplow technique with one or two layers of viscous composite were applied. Four microsticks were obtained from each cavity. Half were tested initially and the other half after aging (thermocycling, 15,000 cycles, 5-55°C, n=30 sticks/group). Tobit regression was used for analyzing group differences, including analysis of interactions, Pearson's chi-squared test or Fishers's exact test for fracture analyses (significance level 0.05). RESULTS: Regression analysis showed significant differences in µTBS between groups initially and after aging. In both etching modes, lining with a bulk-fill flowable composite and layering technique achieved the highest µTBS both initially and after aging. In contrast to the etching mode (self-etch < etch-and-rinse), aging did not influence µTBS significantly. The predominant failure types were adhesive and mixed, with a significantly lower number of pre-test failures in the etch-and-rinse groups. CONCLUSION: The etch-and-rinse mode achieves higher µTBS in class-I cavities compared to the self-etch mode. The lining technique with bulk-fill flowable composite as well as the snowplow technique yielded the highest µTBS after aging, whereas bulk filling and its combination with the snowplow technique resulted in lower µTBS.

How "Universal" is Adhesion? Shear Bond Strength of Multi-mode Adhesives to Enamel and Dentin.

PURPOSE: The purpose of this in vitro study was to compare the shear bond strength of different universal adhesives to enamel and dentin with and without additional phosphoric acid etching before and after thermocycling (TC). MATERIALS AND METHODS: The mesial, distal, lingual, and buccal surfaces of the teeth were ground flat to expose two enamel areas and two dentin areas. Specimens were randomly divided into 36 groups (n = 10). Four adhesives were tested: Scotchbond Universal (SBU), Prime & Bond Elect (PBE), All-Bond Universal (ABU), and iBond Universal (IBU). The application was performed in the self-etch mode (SE) or in the etch-and-rinse mode (ER). Optibond FL (OFL) was used as an etch-and-rinse control. After composite buildups were made, half of the groups were tested before and the other half after TC. The failure mode was evaluated using a light microscope. RESULTS: In enamel, mean bond strengths ranged from 13.4 to 21.9 MPa in the SE mode before TC. When used as an ER adhesive, mean bond strengths exceeded 30 MPa. TC did not influence the mean bond strengths in either application mode. Regarding dentin, significant differences in the SE mode were adhesive dependent. Differences were only detected between SBU and IBU before TC and were not present after TC. When SBU, PBE, and ABU were used as ER adhesives, no differences were detected before TC. TC only affected IBU when compared to the other adhesives. CONCLUSION: Universal adhesives benefit from phosphoric acid etching, as bond strengths increased especially at enamel surfaces.

Fracture toughness versus micro-tensile bond strength testing of adhesive-dentin interfaces.

OBJECTIVE: To assess interfacial fracture toughness of different adhesive approaches and compare to a standard micro-tensile bond-strength (µTBS) test. METHODS: Chevron-notched beam fracture toughness (CNB) was measured following a modified ISO 24370 standard. Composite bars with dimensions of 3.0×4.0×25 mm were prepared, with the adhesive-dentin interface in the middle. At the adhesive-dentin interface, a chevron notch was prepared using a 0.15 mm thin diamond blade mounted in a water-cooled diamond saw. Each specimen was loaded until failure in a 4-point bend test setup and the fracture toughness was calculated according to the ISO specifications. Similarly, adhesive-dentin micro-specimens (1.0×1.0×8-10 mm) were stressed in tensile until failure to determine the µTBS. RESULTS: A positive correlation (r(2)=0.64) was observed between CNB and µTBS, which however was only nearly statistically significant, mainly due to the dissimilar outcome of Scotchbond Universal (3M ESPE). While few µTBS specimens failed at the adhesive-dentin interface, almost all CNB specimens failed interfacially at the notch tip. Weibull moduli for interfacial fracture toughness were much higher than for µTBS (3.8-11.5 versus 2.7-4.8, respectively), especially relevant with regard to early failures. SIGNIFICANCE: Although the ranking of the adhesives on their bonding effectiveness tested using CNB and µTBS corresponded well, the outcome of CNB appeared more reliable and less variable. Fracture toughness measurement is however more laborious and requires specific equipment. The µTBS nevertheless appeared to remain a valid method to assess bonding effectiveness in a versatile way.