Objectives: Dental resin composite restoratives exhibit inadequate curing depths and the change in optical properties throughout curing of photoactive resins is not fully understood. This study investigates the effect of various photoinitiators on light transmission (LT), refractive index (RI) and thickness change of curing dental resins.
Method: Experimental resin formulations were prepared using a 1:1 BisGMA, TEGDMA mixture containing either low (0.0134 mol/dm3), intermediate (0.0405 mol/dm3) or high (0.0678 mol/dm3) concentrations of photoinitiator (PI) Camphoroquinone, Lucrin TPO or Phenyl propanedione. Disc specimens (n=3) of each resin (4x2mm thick) were cured for 90s. LT was measured in real time using a modular spectrometer. RI and thickness change was measured simultaneously by a novel technique using low coherence interferometry.
Results: Resins with no photoinitiator exhibited a similar decrease in LT during irradiation compared with the LCU alone. Maximum rate of LT increase was greater (P<0.05) for CQ than PPD. From high to low concentrations, means (S.D's) for CQ were 31.2 (1.3), 19.4 (1.0) and 5.0 (0.7) and for PPD -1.5 (0.4), -2.2 (0.6) and -6.7 (0.9) respectively. TPO showed no significant differences in decreasing rate of LT between the three concentrations respectively (P=0.40), although TPO exhibited a significantly lower rate of LT change than CQ and PPD (P<0.05).
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| LT RATE (max) (mWcm-2s-1) | REFRACTIVE INDEX | THICKNESS (mm) | ||
RESIN | PI | Pre Cure | Post Cure | Pre Cure | Post Cure | |
CQ | High | 31.2 (1.3) | 1.5037 (0.0021) | 1.5530 (0.0047) | 2.04 (0.01) | 1.95 (0.03) |
Intermediate | 19.4 (1.0) | 1.5039 (0.0020) | 1.5521 (0.0022) | 2.07 (0.03) | 1.96 (0.03) | |
Low | 5.0 (0.7) | 1.5026 (0.0036) | 1.5448 (0.0086) | 2.04 (0.02) | 1.94 (0.01) | |
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TPO | High | -11.0 (0.8) | 1.5029 (0.0016) | 1.5512 (0.0032) | 2.07 (0.05) | 1.98 (0.01) |
Intermediate | -10.5 (0.7) | 1.5030 (0.0089) | 1.5453 (0.0038) | 2.10 (0.08) | 2.01 (0.02) | |
Low | -10.2 (0.6) | 1.5020 (0.0014) | 1.5475 (0.0014) | 2.11 (0.00) | 2.02 (0.03) | |
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PPD | High | -1.5 (0.4) | 1.5005 (0.0018) | 1.5343 (0.0021) | 2.10 (0.00) | 2.00 (0.00) |
Intermediate | -2.2 (0.6) | 1.5046 (0.0034) | 1.5301 (0.0047) | 2.10 (0.01) | 2.01 (0.00) | |
Low | -6.7 (0.9) | 1.5011 (0.0060) | 1.5202 (0.0047) | 2.07 (0.02) | 2.02 (0.03) | |
* a minus (-) sign is indicative of a net decrease in LT
Conclusion: The absorption by a photoinitiator and photo-bleaching significantly affected LT and RI change throughout photopolymerisation. A greater understanding of PI absorption on light attenuation and curing extent is critical for improving curing depths of dental resin based composites.
Funded by EPSRC grant # EP/E026257/1