Dynamic monitoring of optical properties during photopolymerization of dental resins

Objectives: The change in optical characteristics through the bulk of curing photopolymers is not fully understood. This study demonstrates simultaneous group refractive index (RI) change and physical thickness throughout curing of experimental dental resins. Methods: Four BisGMA/TEGDMA resins (60/40wt%) containing 0.2% CQ and 0.3% DMAEMA initiators and various concentrations of BHT (0.0, 0.1, 0.5, 1.0%) inhibitor were prepared. Each sample (n=3) was cured through a 4mm diameter aperture in a disc-shaped mould (12mm diameter, 2mm thick) for 60s with a QTH curing lamp (~700mW/cm²). Group RI and shrinkage were calculated prior to, during and following irradiation using spectral domain low coherence interferometry with a super-luminescent diode source (λ_max=845nm, bandwidth=49nm). Data was analysed by ANOVA and post-hoc Tukey tests (P<0.05). Results: The interface between layers with an abrupt RI change was located with an accuracy of <0.1µm. Prior to curing a mean RI of 1.519±0.003 of each resin was recorded. Throughout irradiation there was no significant difference between mean time to reach 90% peak RI for 0.0 and 0.1% BHT resins (15.7±0.2 and 15.4±0.3s, respectively; P=0.09), although a significant increase for 0.5 and 1.0% BHT (20.3±0.3 and 48.1±0.4s, respectively; P<0.01) was observed. Following 90s post-irradiation RI continued to rise, although a significantly higher RI value for 0.0% (1.5546) compared with 0.5 and 1.0% BHT (1.5511 and 1.5473, respectively) was recorded (P<0.05). Each resin exhibited a strong negative linear relationship between RI change and shrinkage (r²=0.985). Conclusions: The relationship between the optical and physical density demonstrates the potential of this novel technique as a non-invasive tool for measurement of the curing characteristics of photopolymerisation.

[This research is funded by EPSRC grant EP/E026257/1]