The effect of preheating on microhardness and flexural strength of bulk-fill resin composites: an in-vitro study

The objective of this study was to assess the impact of preheating on the microhardness and flexural strength of bulk-fill resin composites.

In this in vitro study, forty-two specimens were prepared of each composite, X-tra fil and Opus Bulk Fill, resulting in 84 disk-shaped specimens for microhardness testing and 84 bar-shaped specimens for flexural strength analysis. The specimens were divided into four groups as follows: Group 1: X-tra fil composite with preheating (at 68°C for 15 minutes), group 2: X-tra fil composite without preheating (at room temperature), group 3: Opus Bulk Fill composite with the same preheating method, group 4: Opus Bulk Fill composite without preheating. Microhardness was assessed using the Vickers test with a diamond indenter, and flexural strength was measured using a 3-point flexural test. Statistical comparisons were performed on the calculated results.

In the preheated groups, both X-tra fil and Opus Bulk Fill composites exhibited significantly higher mean flexural strength compared to the non-preheated groups (p<0.001). However, there was no significant difference in the mean microhardness between the two groups for either type of composite (p=0.719). Additionally, the mean flexural strength and microhardness of X-tra fil composite, in both preheated and non-preheated conditions, were higher than those of the Opus Bulk Fill composite (p<0.001).

Preheating bulk-fill composites to 68°C has no detrimental effect on their microhardness and increases the flexural strength of these materials. Furthermore, the degree of microhardness and flexural strength in bulk-fill composites varies between brands and is influenced by their chemical compositions.