Effect of increasing tip distances of polywave light curing unit on microhardness profiles of bulk-fill composites containing different photoinitiator systems

Introduction
Resin composites offer aesthetic and adhesive advantages, but polymerization efficiency can be affected by tip distance and photoinitiator compatibility. This study evaluated the effect of increasing curing distances of a polywave light curing unit (LCU) on microhardness of bulk-fill composites with different photoinitiator systems. This in vitro study aimed to evaluate and compare the sorption and solubility of a contemporary light-cured resin luting cement with those of resin-modified glass ionomer and self-adhesive resin luting cements, when immersed in distilled water and artificial saliva.

Material and methods
Standardized cylindrical specimens (4 mm thick × 4 mm in diameter) were prepared and light-polymerized using a polywave LCU at 0 mm, 2 mm, 4 mm, and 6 mm distances. Samples (n = 80) were divided randomly into two equal groups (n = 40 per group), depending on the type of photoinitiator present in the composite: camphoroquinone (CQ)-containing composite, SDR Plus Bulk Fill Flowable (n = 40), and composite containing a combination of CQ and Ivocerin, i.e., Tetric N-Ceram (n = 40). Vickers microhardness (VHN) was measured on top and bottom surfaces after 24 hours of dark storage. Data were analyzed using ANOVA and independent t-test (p < 0.05).

Results
Tetric N-Ceram showed significantly higher microhardness values than SDR at all distances. Increasing tip distance led to a reduction in VHN for both materials, more markedly in SDR. The difference in microhardness between top and bottom surfaces was non-significant for Tetric N-Ceram at all distances tested. However, for SDR, the difference in microhardness between top and bottom surface was significant at tip distances of 4 mm and 6 mm.

Conclusions
Curing tip distance negatively affects microhardness in bulk-fill composites. Tetric N-Ceram outperformed SDR when cured with polywave LCU.