Evaluation of cuspal deflection and microleakage of an incrementally packed versus a low shrinkage bulk-fill nanohybrid resin composite: an in-vitro study

Background Despite the numerous advantages of using resin-based composite restorations, polymerization shrinkage and the resulting stresses remain significant concerns. These stresses are associated with two critical clinical outcomes: cuspal deflection and microleakage. This study aimed to evaluate the cuspal deflection and microleakage of an incrementally packed nanohybrid and a low-shrinkage bulk-fill resin composite. Methods To assess cuspal deflection, 40 maxillary premolars were randomly divided into two groups ( n = 20). Standardized mesio-occluso-distal (MOD) cavities were prepared, and one group was restored with incremental Filtek Z250XT and the other with the low-shrinkage bulk-fill resin composite Grandio ® SO x-tra. Cuspal deflection was measured after cavity preparation and five minutes post-restoration using a universal horizontal metroscope. For evaluation of microleakage, each group was further subdivided into non-thermocycled and thermocycled subgroups. Microleakage was assessed using a 2% methylene blue dye solution at 37 °C for 24 h, followed by sectioning and examination under magnification. Results The highest mean cuspal deflection was recorded in the incremental group (12.80 ± 2.57 μm) compared to the bulk-fill group (9.60 ± 2.41 μm). A statistically significant difference in cuspal deflection was observed between the two groups ( p = 0.010). No significant difference was found in microleakage scores between the incrementally packed and bulk-fill groups ( p = 0.305). The incrementally packed group exhibited the highest mean microleakage score (259.66 ± 20.53 μm), while the bulk-fill group had the lowest mean score (251.10 ± 15.36 μm). Regarding the effect of thermocycling on microleakage, regardless of the type of resin composite used, a statistically significant difference was noted between the non-thermocycled subgroup (241.12 ± 10.17 μm) and the thermocycled subgroup (269.63 ± 11.97 μm) ( p < 0.001). Conclusion Low shrinkage bulk-fill composite reduced cuspal deflection in comparison to the incrementally packed materials. Bulk-fill and incrementally packed resin composites both showed comparable vulnerability to microleakage. The thermocycling procedure exerted a deteriorating effect on marginal integrity.