An In Vitro Study on Bond Strength Degradation of Low-Shrinkage Composite Resins to Demineralized Dentin After Thermal Cycling

Objectives: Variations in composite resin composition and aging time remain one of the main reasons for replacing esthetic restorations. This in vitro study aimed to evaluate the microtensile bond strength of a low-shrinkage composite resin on a demineralized dentin surface following adhesive interface degradation. Methods: Seventy-eight extracted human molars were prepared, and artificial caries lesions were induced. For microtensile bond strength (μTBS) testing, 60 teeth were randomly assigned to six experimental subgroups (n = 10 per subgroup) based on restorative system and thermal cycling condition. An additional 18 teeth were randomly assigned to six experimental subgroups (n = 3 each) for SEM analysis. Three restorative systems were evaluated, Z250 (conventional resin), K (Kalore resin), and P90 (Filtek P90 resin), each subjected to two thermal cycling conditions: without thermal cycling (NTC) and 12,000 thermal cycles (TC). Results: In the NTC groups, Z250 exhibited a significantly higher bond strength (25.29 ± 10.91 MPa) compared to K (9.69 ± 11.63 MPa) and P90 (9.81 ± 8.49 MPa) (p < 0.05). Following TC, a numerical decrease in bond strength was observed across all groups. Z250 (13.00 ± 10.76 MPa) maintained a significantly higher bond strength compared to K (4.30 ± 6.40 MPa) and P90 (0 ± 0 MPa) (p = 0.001). Notably, the P90 group showed a near-complete loss of bond strength after TC (0 ± 0 MPa), which was a statistically significant reduction compared to its NTC condition (p = 0.002). SEM analysis revealed a predominance of mixed failures in most experimental groups, while the P90 TC group showed a clear predominance of adhesive failures. Conclusions: This study demonstrates that the conventional Bis-GMA resin (Z250) consistently exhibited superior bond strength to demineralized dentin compared to the low-shrinkage resins (Kalore and Filtek P90) under both non-aged and aged conditions. While all materials experienced a reduction in bond strength after thermal cycling, the Filtek P90 system showed a catastrophic loss of adhesion after aging, indicating its particular susceptibility to degradation. These results emphasize the critical roles of resin chemistry and adhesive system selection in long-term bond durability in compromised dentin.