Fracture Strength of Bulk vs. Incrementally Filled Resin-Based Composites

Objectives: In the ISO 4049 standard to determine the fracture strength, the resin-based composite (RBC) is filled into a 25 mm long and light-cured in bulk from the top and bottom. Currently, most RBC restorations are not light-cured in bulk and are instead incrementally placed and light cured. However, knit lines can form between each increment of light-cured RBC. This study determined the fracture strength of incrementally cured RBCs to examine the effect of knit lines on their strength.
Methods: Thirty beams (25 mm x 2 mm x 2 mm per ISO 4049) of conventional (Tetric Evoceram, shade A2) and another thirty for bulk-fill RBCs (Filtek One Bulk Fill, shade A2) were made using either the incremental or the bulk-fill technique, respectively. For the incremental technique, the RBC was applied in increments of 2 mm, while the bulk-fill consisted of a single application to create the beam using an elastomeric mold. The beams were subjected to four-point bending test using a custom-made, fully articulating four-point flexural fixture in the ElectroForce 3200 (Bose) mechanical testing machine. The failure load was recorded, and the strength (in MPa) was calculated according to the ISO 6872 standards. Data were analyzed using maximum likelihood estimation (ISO 20501) to determine Weibull modulus and characteristic strength. Each fractured beam was examined under optical microscopy (Keyence).
Results: The Weibull modulus and characteristic strength of the tested groups, as well as the 95% confidence interval, are reported in Table 1. The bulk-fill/bulk -cure method resulted in failures primarily from the voids within the beams of RBC near the tensile surface (68%) and corner defects (32%). In contrast, beams produced using the incremental method predominately failed at the knit lines between each increment (63%), followed by corner defects (37%).
Conclusions: This study highlights the effect of knit lines on the fracture strength of incrementally filled and incrementally light-cured RBC restorations. The bulk-fill method resulted in greater strength and less variability in strength compared to the incremental method.