Afnan O Al-Zain
Statement of the Problem: Fracture failure of resin-matrix composite (RMC) increased in the past decade from 29.5%- 39.1% due to multiple factors that may include non-uniform polymerization across the RMC surface. Investigating polymerization patterns within the bulk of a RMC can contribute to greater understanding of fracture etiology. Purpose: The purpose of this study was to investigate the relationship of an irradiance-beam-profile area from different lightemitting- diode (LED) curing-units on the degree of conversion (DC) and Knoop microhardness (KH) and cross-link density (CLD) uniformity within a RMC at two clinically relevant distances and explore the correlation among them. Methodology: Irradiance-beam-profiles were generated from six light-curing units (LCUs); one quartz-tungsten-halogen, two single and three multiple-emission-peak LED units and combined with the corresponding power measurements. The radiant exposure was maintained, and a mapping approach was used to investigate DC (micro-Raman spectroscopy), KH (hardness tester) and %KH reduction as an indicator of CLD (ethanol-softening method) within a nano-hybrid RMC increment at various depths cured at two light-tip distances. The localized irradiance correlation with the corresponding DC, KH and %KH reduction was explored. Findings: Non-uniform DC, KH and %KH reduction was observed within the specimens and localized polymerization discrepancies were significant at specific depths and points, which did not follow a specific pattern regardless of the LCU or curing distance. A mapping approach within the specimens provided detailed polymerization characterization. Localized irradiance was weakly correlated with the corresponding DC, KH and %KH reduction on the top RMC surfaces at both distances. Polymerization of the RMC investigated did not reflect the LCU irradiance pattern at the area assessed, and no LCU demonstrated uniform polymerization at all points for the measurements investigated at both distances. Therefore, the LCUs explored do not result in uniform polymerization, which may potentially increase the risk of RMC fracture.