Effect of Light Distance/Specimen Length on Flexural Strength of Resin-Composite

Objectives: To evaluate the flexural strength (FS) of nano-hybrid resin composite (NRC) as a function of light exposure distance and specimen length.
Methods: Specimens (n=160) were prepared from NRC (Grandio, A2, VOCO, Germany) and divided into 16 groups (n=10/group) according to the two experimental factors investigated, namely light exposure distance (mm) (0, 2, 4, 8) and specimen length (mm) (10, 12, 15, 25). All specimens had standard width (2 mm) and thickness (2 mm). Specimens at each length were prepared in separate silicon mould and photo-polymerized from varying distances from top of the specimen surface. Depending on the specimen's length, photo-polymerization was performed from 1 to 5 overlapping cycles (20 s each). Three-point bending test (0.5 mm/min) was performed and data were statistically analysed (Two-Way ANOVA, Tukey`s) (alpha=0.05). Two-parameter Weibull distribution values including the Weibull modulus, scale (m) and shape (0), values were calculated.
Results: Light exposure distance (p=0.000) and specimen length (p=0.000) significantly affected the FS results, while interaction terms were not significant (p=0.200). The mean FS (MPa) values did not show significant difference for 0mm (132.6±14) and 2mm (129.6±14.7) (p>0.05) being significantly higher than for 4mm (118.7±13.7) and 8mm (119.1±13.5) (p<0.05). The mean FS values as a function of length were in descending order as follows: 10mm (132.4±13.6) > 12mm (124.6±15.6) > 15mm (116.8±14.8) (p<0.05). There was no significant difference between 10mm (132.4±13.6) and 25mm (126.3±12.8) and between 25mm (126.3±12.8) and 12mm (124.6±15.6) (p>0.05). Weibull distribution presented higher scale (m) for 10mm length-8mm distance combination (24.65) compared to other combinations (9.1-17.6).
Conclusions: Increasing the light exposure distance from 2mm up to 8mm decreases the FS of the tested nano-hybrid resin composite. Increasing the overlapping polymerization cycles may compensate for the decrease in FS, especially for 25mm length resins.