Gap Formation, Flexural Strength and Conversion: Pre-heating and Photoactivation-method Influences

Objectives: to verify pre-heating and photoactivation-method influence on gap formation, degree of conversion (DC) and flexural strength (FS) of a nanofilled composite (Filtek Z350, 3M/ESPE). Methods: two continuous photoactivation methods (600mW/cm2 for 20s or 40s) and one two-step soft-start method (200mW/cm2 for 20s followed by 500mW/cm2 for 40s) were tested. Photoactivation was performed in pre-heated samples (68ºC – Calset, Addent) or without any thermal treatment. Marginal adaptation was evaluated by SEM (n=10). Replicas of Class V fragments had the gaps at tooth-composite interface measured (200X magnification). DC test (n=3) was performed by FT-Raman spectroscopy. Cylindrical specimens of 5x2mm (diameter x height) were dry stored for 24h at 37°C. DC of both top and bottom surfaces were measured. FS (n=10) was obtained by three-point flexural test in specimens with 10x2x1mm (length x width x height) after 24h water-storage at 37°C. Statistics of marginal adaptation data was performed by analysis of gap frequency, Friedman and Kruskal-Wallis tests. Three-way ANOVA and Tukey's test (p<0.05) were used to DC and FS. Results: Axial walls of restorations showed higher gap frequency (85% of walls with gaps) compared to lateral walls (59%). Pre-heated groups showed lower gap frequency (49%), compared to groups without thermal treatment (58%). In DC test only the main factor surface was statistically significant (%±SD)=63.1±2.6 for top and 61.3±2.5 for bottom surfaces; p<0.05). In FS test only the main factor photoactivation method was statistically significant (p<0.01). Two-step technique presented lower FS. Mean FS values (MPa±SD)= 179.3±13.9 for 600mW/cm2 during 40s; 174.3±17.0 for 600mw/cm2 during 20s; 152.9±19.0 for 200mW/cm2 + 500mW/cm2 technique. Conclusions: Preheating showed a better interface integrity no affecting DC or FS. Photoactivation methods with low inicial irradiance may cause reduction of FS values, in spite of promoting similar DC.