Photoinitiator type and applicability of exposure reciprocity law in filled and unfilled photoactive resins

Objectives

To test the influence of photoinitiator type and filler particle inclusion on the validity of exposure reciprocity law.

Materials and methods

50/50 wt% Bis-GMA/TEGDMA resins were prepared with equimolar concentrations of camphorquinone/DMAEMA (0.20/0.80 mass%) (CQ) or Lucirin-TPO (0.42 mass%), and were used either unfilled or filled to 75 mass%. Specimens were cured with a halogen Swiss Master Light (EMS, Switzerland) using four different curing protocols: 400 mW/cm² for 45 s as reference protocol (18 J/cm²), 1500 mW/cm² for 12 s (18 J/cm²), 3000 mW/cm² for 6 s (18 J/cm²) and 3 s (9 J/cm²). Degree of conversion (DC) was measured in real time for 70 s by FT-NIRS and temperature rise using a thermocouple. Depth of cure was determined with a penetrometer technique.

Results

With respect to DC and depth of cure, exposure reciprocity law did not hold for any tested material, except for the depth of cure of filled CQ-based materials. At similar radiant exposure, DC was significantly higher (p < 0.05) for all unfilled and filled TPO-based materials compared with CQ-based materials. As exposure time was reduced and irradiance increased, TPO-based materials exhibited higher DC whilst an opposite trend was observed for CQ-based materials (p < 0.05). For similar curing regimes, depth of cure of CQ-based materials remained significantly greater than that of TPO-based materials. Adding fillers generally reduced DC, except at higher irradiance for CQ-based materials where a positive effect was observed (p < 0.05).

Significance

The validity of exposure reciprocity law was dependent on several factors, among which photoinitiator type and filler content were important. Lucirin-TPO is a highly reactive and efficient photoinitiator, which may allow the potential for a reduction in curing time of TPO-based photoactive materials in thin sections.     

Thermal shrinkage reveals the feasibility of pulse-delay photocuring technique

ObjectivesTo resolve the feasibility of the pulse-delay photocuring technique as a clinical strategy for reducing the detrimental polymerization stress induced in dental composites during the photocuring process.MethodsModel dental composites with high and low-filler contents were cured with the pulse-delay photocuring technique using different combinations of photocuring variables (irradiance, exposure time, and delay time). Irradiance used ranged from 0.1 W/cm² to 4 W/cm². The exposure time of the first pulse varied from 0.2 s to 27.2 s and the delay times ranged from 10 s to 120 s. The radiant exposure was varied from 4 J/cm² to 20 J/cm². A cantilever-beam based instrument (NIST Standards Reference Instrument 6005) was used to implement the photocuring technique for the measurement of the polymerization properties (the degree of monomer conversion, polymerization stress induced due to shrinkage, and temperature change due to the reaction exotherm and curing light absorbance) simultaneously in real-time. These properties were compared with those obtained using the conventional photocuring technique (i.e., using a constant irradiance for a fixed exposure time, a uniform exposure).ResultsThere exists a minimum radiant exposure, such that a reduction in the polymerization stress can be achieved without sacrificing the degree of monomer conversion by using the pulse-delay over the conventional photocuring technique. More specifically, stress reductions of up to 19% and 32% was observed with the pulse-delay when compared with the conventional photocuring technique at an irradiance of 0.5 W/cm² and 4 W/cm², respectively. The reduction occurred when the exposure time of the first pulse was greater than, but closer to, the gelation time (i.e., lower than the vitrification time) of the composite, regardless of the delay time used. Lower thermal shrinkage (contraction) during the post-curing time, rather than the stress relaxation during the delay time or lower degree of monomer conversion as claimed in the literature, is the cause of the reduction in the polymerization stress.SignificanceThe study clarifies a long-standing confusion and controversy on the applicability of the pulse-delay photocuring technique for reducing the polymerization stress and promotes its potential clinical success for dental restorative composites.     

High irradiance curing and anomalies of exposure reciprocity law in resin-based materials

Objectives

The aim of this work was to investigate the effect of high irradiance curing on resultant degree of conversion of ‘flowable’ resin composites and their counterpart higher viscosity paste materials.

Methods

Five commercial flowable materials (Venus; Heraeus Kulzer, Synergy D6; Coltene, Premise; Kerr, Grandio; Voco and Gradia; GC Corp) and their counterpart higher viscosity restorative versions were tested. Specimens were cured with a halogen Swiss Master Light (EMS, Switzerland) using five different curing protocols with similar radiant exposure (18 J/cm²): 400 mW/cm² for 45 s, 900 mW/cm² for 20 s, 1500 mW/cm² for 12 s, 2000 mW/cm² for 9 s and 3000 mW/cm² for 6 s. Degree of conversion (DC) was measured in real time by Fourier transform near infrared spectroscopy (FT-NIRS).

Results

Three- and subsequent two way ANOVA testing revealed significant differences (p ≤ 0.02) with respect to “composite type” and “cure protocol” for DC for all 5 product comparisons. Supplementary one-way ANOVA also revealed significant differences between curing protocols (p < 0.05). The majority of higher viscosity resin composite paste materials exhibited similar DC regardless of curing protocol. However, a significant decrease in DC for specimens cured at 3000 mW/cm² for 6 s compared with 400 mW/cm² for 45 s was observed for the flowable materials, Grandio (41 ± 0.36 and 62 ± 1.15%, respectively) and Venus (44 ± 0.44 and 67 ± 0.44%, respectively). Conversely, other flowable materials exhibited little or no significant differences between curing modes. Generally, a higher degree of conversion was observed for flowables compared with their more viscous counterpart, except at high irradiance for those materials where a reciprocal relationship with exposure time was not observed.

Conclusions

The validity of exposure reciprocity law and final degree of conversion depends on several factors, amongst which resin viscosity and filler content were important. Practitioners should be aware of the importance of resin composite constituents and irradiation protocols. Information on material composition and appropriate radiation sources by manufacturers may assist practitioners with the selection of appropriate curing protocols for specific material/light curing unit combinations with the aim of reducing the incidence of under-cured restorations and the clinical impact thereof.     

A simple method for the measurement of polymerization shrinkage in dental composites

OBJECTIVE: In this study a simple non-contact method was developed to measure the polymerization shrinkage of dental composites. METHODS: A gas pycnometer was used to determine the volumes of specimens prior to and after photopolymerization and from which the total volumetric shrinkage could be determined. RESULTS: Four commercial composites were studied and were found to have polymerization shrinkages varying from 1.6 to 2.5%. The method was found to be labour efficient and produced reproducible results with a standard deviation of approximately 10%. SIGNIFICANCE: This method is appropriate for shrinkage measurements where only the total amount shrinkage is required and in particular for the measurement of shrinkage of photocured materials which are sensitive to water absorption.     

Efficient prediction of the packing density of inorganic fillers in dental resin composites for excellent properties

ObjectiveThe purpose of this study is to develop a mathematical model for efficient prediction of the packing density of different filler formulations in dental resin composites (DRCs), and to study properties of DRCs at the maximum filler loading (MFL), thereby providing an effective guidance for the design of filler formulations in DRCs to obtain excellent properties.MethodsThe packing density data generated by discrete element model (DEM) simulation were used to re-derive the parameters of 3-parameter model. The modifier effect was also induced to modify the 3-parameter model. DRCs with 10 filler formulations were selected to test properties at the MFL. The packing densities of binary and ternary mixes in DRCs were calculated by 3-parameter model to explore the regularity of composite packing.ResultsThe predicted packing density was validated by simulation and experimental results, and the prediction error is within 1.40 vol%. The optimization of filler compositions to obtain a higher packing density is beneficial to enhancing the mechanical properties and reducing the polymerization shrinkage of DRCs. In binary mixes, the maximum packing density occurs when the volume fraction of small fillers is 0.35−0.45, and becomes higher with the reduction of particle size ratio. In ternary mixes, the packing density can reach the maximum value when the volume fractions of large and small fillers are in the 0.5−0.75 and 0.15−0.4 ranges, respectively.SignificanceThe modified 3-parameter model can provide an effective method to design the multi-level filler formulations of DRCs, thereby improving the performance of the materials.