Effect of Thickness/Light Distance on Flexural Strength of Bulk-Fill Resin-Composites

Objectives: This study evaluated the flexural strength (FS) of bulk-fill resin composites (BF) as a function of specimen thickness and light exposure distance.
Methods: Specimens (n=96) were prepared from two BF materials and divided into 8 groups (n=12/group) according to the three experimental factors investigated, namely material-M (X-tra fil-XTF and Admira Fusion x-tra-AFT, Universal shade, VOCO, Germany), specimen thickness-T (mm) (2, 4) and light exposure distance-D (mm) (2, 8). All specimens had standard width (2 mm) and length (12 mm). Specimens at each thickness were prepared in separate silicon mold, covered with polyester strip, and photo-polymerized at the respective distance from top of the specimen surface. Photo-polymerization was performed at 2 overlapping cycles (20 s each). Three-point bending test (0.5 mm/min) was performed and data were statistically analyzed (3-Way ANOVA, Tukey`s, t-test) (alpha=0.05). Two-parameter Weibull distribution values including the Weibull modulus, scale (m) and shape (0), values were calculated.
Results: Material type (p=0.000), specimen thickness (p=0.019) and light exposure distance (p=0.000), significantly affected the FS, while interaction terms were significant only for thickness vs distance (p=0.000). The mean FS (MPa) values for XTF showed significantly higher values for 4mm(T)x8mm(D) (180.9±13.7) than those for 2mm(T)x2mm(D) (162±15.2), 2mm(T)x8mm(D) (158±8.7) and 4mm(T)x2mm(D) (163.9±11.7). For AFT, 4mm(T)x8mm(D) (108.1±11.6) presented significantly higher values than those of other thickness-distance combinations (88.5±5.4 - 78.4±6.7). XTF demonstrated significantly higher FS values compared to AFT within each thickness and distance parameter (p=0.000). While for AFT, Weibull distribution presented higher scale (m) for 4mm(T)-2mm (D) (18.7), for XTF 2mm(T)-10mm (D) (22.7) was the highest, compared to other combinations (AFT: 10.1-15.7; XTF: 12-18.2).
Conclusions: Flexural strength of bulk-fill composites tested varied depending on the material type. Thickness of the bulk and the corresponding light exposure distance should be considered for optimum strength.