The effect of fiber aspect ratio and volume loading on the flexural properties of flowable dental composite

To evaluate the efficacy on flexural properties of flowable dental resin composite reinforced with short glass fiber of different aspect ratios (ARs) and volume percent loadings. It is hypothesized that with the addition of randomly oriented fibers it is possible to significantly improve flexural strength and modulus while maintaining flowability.

Ten groups of samples with varying glass fiber volume loads (0, 5%, 10%, 20%, 40% and 60%) and three different ARs (5.2, 68 and 640) were tested in three point bending to fracture according to ISO 4049. A flowable resin composite was used as the control and also as the filled resin composite that was subsequently reinforced with fibers. Load deflection results were used to calculate flexural strength and flexural modulus. SEM images were used to determine the mode(s) of failure, to describe surface features of reinforcement and were correlated with force displacement graphs. All results were statistically analysed using ANOVA followed by post hoc Tukey's test. Level of significance was set at 0.05.

When compared to the "sculptable" control (68.6 vol% filler loaded) results for flexural strength varied from a mean reduction of 42% (p>0.05) for the low AR group to an increase of 77% (p<0.001) for the high AR samples. Flexural modulus results varied from a low of 6.6 [0.67] GPa for the non reinforced spatulated control to 20.3 [1.31] GPa (p<0.001) for the 60% loaded low AR group. The low fiber loaded mid AR group was still flowable with 49% total loading (5% fiber/44% filler) but gave strength values (181.2 [33.5] MPa) 30% higher than the "sculptable" control (p>0.05) and comparable modulus.

This study shows that short and very short glass fibers can significantly reinforce flowable dental composite. The fiber's aspect ratio was shown to be more important than volume loading for flexural strength. It appears possible to produce a light cured short glass fiber reinforced flowable material with superior flexural properties compared to conventional universal composites.