Technique-dependent differences in internal adaptation, porosity and degree of conversion of short fibre-reinforced composite restorations in deep MOD cavities

OBJECTIVES: Evaluate and compare the internal gap (IG), closed porosity (CP), and degree of conversion (DC) of short fibre-reinforced composite (SFRC) restorations applied using various techniques in an in vitro model. MATERIALS AND METHODS: Five restorative protocols were tested (n = 20/group): (1) flowable bulk-fill SFRC; (2) flowable liner + flowable bulk-fill SFRC; (3) polyethylene fibre mesh + flowable liner + flowable bulk-fill SFRC; (4) flowable bulk-fill SFRC + high-viscosity SFRC (snowplow technique); and (5) conventional layered resin-based composite (RBC). IG (gap volume/interface volume) and CP (void volume/restoration volume) were assessed by micro-CT. DC was measured at three depths using micro-Raman spectroscopy. Data were analyzed using ANOVA, General Linear Model, and correlation tests (α = 0.05). RESULTS: Significant differences were found among groups for IG, CP, and DC (p < 0.001). Groups 1, 2, and 4 showed significantly lower IG (0.46-0.53%) than Group 3 (2.78%) and Group 5 (1.09%). Group 1 demonstrated the lowest CP (0.11%), while Group 4 exhibited the highest (0.5%). DC ranged from 74.8 to 88.4%, with significant depth-related variations. Group 2 and Group 5 did not differ in average DC (p = 0.99). CONCLUSIONS: The flowable bulk-fill SFRC techniques demonstrated favorable internal adaptation, low CP, and acceptable DC, supporting their potential use in deep cavities. In contrast, polyethylene fibre mesh reinforcement increased IG formation, whereas the snowplow technique achieved high DC at the expense of increased CP. CLINICAL RELEVANCE: Flowable bulk-fill SFRC may improve internal adaptation, reduce porosity, and achieve acceptable DC in deep cavities. Polyethylene fibre mesh and snowplow techniques exhibit technique-dependent limitations.