Optimizing the concentration of 2,4,6-trimethylbenzoyldiphenylphosphine oxide initiator in composite resins in relation to monomer conversion

Residual TPO Content of Photopolymerized Additively Manufactured Dental Occlusal Splint Materials

Background/Objectives: Diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide (TPO) is widely used in the dental industry as a photoinitiator for resin-based materials, while its use may be further limited given its toxicological risks. The aim of this study was, therefore, to analyze the residual TPO content of 3D-printed resin-based dental splint materials. Methods: Six resin-based splint materials were analyzed: LuxaPrint Ortho Plus (DMG), FREEPRINT splint 2.0 (Detax), optiprint splint (Dentona), KeySplint Soft (KeyPrint), FREEPRINT ortho (Detax), V-Print splint comfort (Voco). Grid-shaped specimens were fabricated using the recommended workflow of each manufacturer (n = 18). TPO extraction was conducted using a maximum of eight extraction cycles of 72 h at a temperature of 37 °C until no more TPO eluates were detected by high-performance liquid chromatography (HPLC). The margin of safety (MoS) was calculated as the ratio between the Derived No-Effect Level (DNEL) and the estimated exposure based on the amount of TPO extracted. Results: The total amount of extracted TPO was the lowest for LuxaPrint Ortho Plus (Mean ± SD; 44.0 ± 17.1 ng/mL), followed by optiprint splint (80.6 ± 21.1 ng/mL), FREEPRINT splint 2.0 (127.4 ± 25.3 ng/mL), FREEPRINT ortho (2813.2 ± 348.0 ng/mL), V-Print splint comfort (33,424.6 ± 8357.9 ng/mL) and KeySplint Soft (42,083.5 ± 3175.2 ng/mL). For all tested materials, the calculated MoS was above the critical value of 1, demonstrating toxicological safety in the cured, clinically relevant state. Conclusions: Large differences in the residual TPO content were observed between the materials. Although the TPO content in the uncured state may exceed toxicological safety limits, appropriate curing of the investigated materials resulted in a significant reduction in TPO elution and, thus, in products with a very low toxicological risk for the patient.

Vat photopolymerization of multifunctional fresnel lenses for ocular management

In this study, multifunctional Fresnel lenses were explored as a potential solution for correcting vision in patients with color vision deficiency (CVD) and high myopia. Current studies have primarily focused on color vision correction through the 3D printing of glasses and contact lenses. However, the potential of 3D-printed multifunctional devices, such as Fresnel lenses, goes beyond addressing a single vision correction issue. For this study, computer-aided design (CAD) model of Fresnel lens with high diopter based on constant height configuration was developed. The CAD model was successfully fabricated using vat photopolymerization 3D printer, employing laboratory-prepared transparent HEMA resin. The resin was modified with two Atto dyes (565 nm and 488 nm), known for their ability to filter out problematic wavelengths (400-500 nm and 540-580 nm) to address color vision deficiency. The printed lenses were characterized by their chemical, physical, and optical properties using various characterization techniques. The focusing performance was evaluated using focal length measurements, and the results obtained were less than 2 mm deviation from the design value, having the potential to assist in higher myopic vision correction. The resulting optical spectra were compared with commercial glasses, revealing close agreement for CVD correction. These results expand the potential applications of multifunctional Fresnel lenses in ophthalmology, demonstrating their effectiveness as vision-correcting lenses and imaging systems.

Multi-peak Light-emitting Diode Curing Units and Polymerization of 3D-printed Crown and Bridge Resin

PURPOSE

This study aimed to evaluate the substitutive and additive efficacy of multi-peak light-emitting diode (LED) curing units for post-curing of a three-dimensional- (3D-) printed crown and bridge resin.

METHODS

A total of 792 disc- and 180 bar-shaped specimens were printed with a crown and bridge resin (NextDent C&B MFH) and post-cured using two LED curing units (VALO Cordless and Bluephase N G4) in fast and standard modes. Conventional post-curing (LC-3D Print Box, Group PC) was compared with LED-only curing (Groups V1, V2, B1, and B2) and LED-combined curing (Groups PV1, PV2, PB1, and PB2) in terms of microhardness, flexural strength, degree of conversion (DC), and CIE L*a*b* color and translucency parameters. Cytotoxicity of the resin eluates was evaluated using the WST-1 assay. Temperature increases on the resin surface were measured with infrared thermography. Data were statistically analyzed using ANOVA and Kruskal-Wallis tests (α=0.05).

RESULTS

The microhardness and flexural strength in Groups V1, V2, B1, and DC in all LED-only groups were lower than in other groups (p<0.05). Larger color disparities existed between Group PC and all LED-only groups than between Group PC and the others (p<0.05). There was no significant difference in cytotoxicity among the groups. The temperature increase was lowest in Groups V1 and PV1 during light curing (p<0.05).

CONCLUSIONS

Post-curing by multi-peak LED curing units was not as effective as the conventional post-curing device. Additional post-curing by LED curing units did not improve the material properties.

Effect of four different mono and multi-wave light-curing units on the Knoop hardness of veneer resin composites

OBJECTIVES

To evaluate the effect of mono and multi-wave light-curing units (LCUs) on the Knoop hardness of resin-based composites (RBC) that use different photoinitiators.

METHODS

Central incisor-shaped specimens 12 mm long, 9 mm wide, and 1.5 mm thick were made from 2 RBCs that use different photoinitiators: Tetric N-Ceram (Ivoclar Vivadent) - and Vittra APS (FGM), both A2E shade. They were light-cured with 4 different LCUs: two claimed to be multi-wave - VALO Grand (Ultradent) and Emitter Now Duo (Schuster); and two were monowave - Radii Xpert (SDI) and Elipar DeepCure-L (3 M Oral Care) using 2 different light exposure protocols: one 40 s exposure centered over the specimen; and two 20 s light exposures that delivered light from two positions to better cover the entire tooth. 16 groups with 10 specimens in each group were made. The Knoop hardness (KH, kg/mm2) was measured at the top and bottom of the specimen in the center and at the cervical, incisal, mesial, and distal peripheral regions. The active tip diameters (mm) and spectral radiant powers (mW/nm) of the LCUs were measured with and without the interposition of the RBC, as well as the radiant exposure beam profiles (J/cm²) delivered to the top of the RBCs. The data was analyzed using Three-way ANOVA and Tukey's tests (α = 0.05).

RESULTS

The VALO Grand (1029 mW) emitted twice the power of the Radii Xpert (500 mW). The KH values of VI and TN resin composite specimens were significantly affected by the LCU used (p < .001), the measurement location (p < .001), and the surface of the specimen (p < .001). LCUs with wider tip diameters produced greater Knoop hardness values at the peripheries of the 12 mm of long, 9 mm wide specimens. In general, the VALO Grand produced the highest KH values, followed by Elipar DeepCure-L, then by Radii Xpert. The Emitter Now Duo LCU produced the lowest values. Exposing the veneers from two locations reduced the differences between the LCUs and the effect of the measurement location. Only the VALO Grand could fully cover the composite veneer with light when the two locations were used.

SIGNIFICANCE

The light tip must cover the entire restoration to photocure the RBC beneath the light tip.

Influence of Mono- and Multiwave Light-curing Units on the Microhardness and Degree of Conversion of Light-cured Resin Cements

OBJECTIVES

This study evaluated the Knoop hardness (KH, N/mm2) and degree of conversion (DC, %) on the margins of light-cured resin cements with different photoinitiators using a single light-curing unit (LCU) with two heads (mono- and multiwave).

METHODS AND MATERIALS

Three types of resin cements were used with different photoinitiators: Megalink Esthetic (Odontomega, São Paulo, Brazil) with a camphorquinone photoinitiator; Allcem Veneer (FGM, Joinville, Brazil) with the Advanced Polymerization system (APS), and Variolink Esthetic LC (Ivoclar Vivadent, Schaan, Liechtenstein). Thirty samples were collected and divided into six groups (n=5 each). The resin cement samples were made into the shape of a maxillary right central incisor and photoactivated under a 0.5-mm-thick ceramic sheet. A single LCU (Radii Xpert, SDI) with two heads (mono- and multiwave) was used. The tip of the LCU was positioned at the center of the sample in a standardized manner. Raman spectroscopy was performed to evaluate the DC, and KH was evaluated through the Knoop microhardness test. Five regions were evaluated: cervical, mesial, buccal (center), distal, and incisal.

RESULTS

There was a significant difference in the DC only for the type of cement (p<0.001), indicating that the cement with the APS photoinitiator presented excellent results. There were significant differences in the type of cement (p<0.001), type of light (p<0.001), region (p<0.001), and the interaction between the type of cement and type of light (p<0.001). The resin cement with the APS photoinitiator cured with monowave light showed the highest KH values. The beam profiles of all groups, with and without the interposition of ceramic and resin cement, were examined by light transmission.

CONCLUSIONS

The cement with the APS photoinitiator presented the best results with respect to the DC and KH. In comparison with mono- and multiwaves, the LCU may not be a determining factor for the properties of light-cured resin cements. The buccal region showed the best results for DC and KH, indicating the need for a greater amount of light-curing at the cementation margins.

Characterizing light engine uniformity and its influence on liquid crystal display based vat photopolymerization printing

Vat photopolymerization (VP) is a rapidly growing category of additive manufacturing. As VP methods mature the expectation is that the quality of printed parts will be highly reproducible. At present, detailed characterization of the light engines used in liquid crystal display (LCD)-based VP systems is lacking and so it is unclear if they are built to sufficiently tight tolerances to meet the current and/or future needs of additive manufacturing. Herein, we map the irradiance, spectral characteristics, and optical divergence of a nominally 405 nm LCD-based VP light engine. We find that there is notable variation in all of these properties as a function of position on the light engine that cause changes in extent of polymerization and surface texture. We further demonstrate through a derived photon absorption figure of merit and through printed test parts that the spatial heterogeneity observed in the light engine is significant enough to affect part fidelity. These findings help to explain several possible causes of variable part quality and also highlight the need for improved optical performance on LCD-based VP printers.

Can TPO as Photoinitiator Replace "Golden Mean" Camphorquinone and Tertiary Amines in Dental Composites? Testing Experimental Composites Containing Different Concentration of Diphenyl(2,4,6-trimethylbenzoyl)phosphine Oxide

The aim of this research was to compare the biomechanical properties of experimental composites containing a classic photoinitiating system (camphorquinone and 2-(dimethylami-no)ethyl methacrylate) or diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) as a photoinitiator. The produced light-cured composites consisted of an organic matrix-Bis-GMA (60 wt.%), TEGDMA (40 wt.%) and silanized silica filler (45 wt.%). Composites contained 0.27; 0.5; 0.75 or 1 wt.% TPO. Vickers hardness, microhardness (in the nanoindentation test), diametral tensile strength, resistance to three-point bending and the CIE L* a* b* colorimetric analysis was performed with each composite produced. The highest average Vickers hardness values were obtained for the composite containing 1 wt.% TPO (43.18 ± 1.7HV). The diametral tensile strength remains on regardless of the type and amount of photoinitiator statistically the same level, except for the composite containing 0.5 wt.% TPO for which DTS = 22.70 ± 4.7 MPa and is the lowest recorded value. The highest average diametral tensile strength was obtained for the composite containing 0.75 wt.% TPO (29.73 ± 4.8 MPa). The highest modulus of elasticity characterized the composite containing 0.75 wt.% TPO (5383.33 ± 1067.1 MPa). Composite containing 0.75 wt.% TPO has optimal results in terms of Vickers hardness, diametral tensile strength, flexural strength and modulus of elasticity. Moreover, these results are better than the parameters characterizing the composite containing the CQ/DMAEMA system. In terms of an aesthetic composite containing 0.75 wt.%. TPO is less yellow in color than the composite containing CQ/DMAEMA. This conclusion was objectively confirmed by test CIE L* a* b*.

The Influence of Various Photoinitiators on the Properties of Commercial Dental Composites

The aim of this article was to compare the biomechanical properties of commercial composites containing different photoinitiators: Filtek Ultimate (3M ESPE) containing camphorquinone (CQ); Estelite Σ Quick (Tokuyama Dental) with CQ in RAP Technology^®; Tetric EvoCeram Bleach BLXL (Ivoclar Vivadent AG) with CQ and Lucirin TPO; and Tetric Evoceram Powerfill IVB (Ivoclar Vivadent AG) with CQ and Ivocerin TPO. All samples were cured with a polywave Valo Lamp (Ultradent Products Inc.) with 1450 mW/cm². The microhardness, hardness by Vicker's method, diametral tensile strength, flexural strength and contraction stress with photoelastic analysis were tested. The highest hardness and microhardness were observed for Filtek Ultimate (93.82 ± 17.44 HV), but other composites also displayed sufficient values (from 52 ± 3.92 to 58,82 ± 7.33 HV). Filtek Ultimate not only demonstrated the highest DTS (48.03 ± 5.97 MPa) and FS (87.32 ± 19.03 MPa) but also the highest contraction stress (13.7 ± 0.4 MPa) during polymerization. The TetricEvoCeram Powerfill has optimal microhardness (54.27 ± 4.1 HV), DTS (32.5 ± 5.29 MPa) and FS (79.3 ± 14.37 MPa) and the lowest contraction stress (7.4 ± 1 MPa) during photopolymerization. To summarize, Filtek Ultimate demonstrated the highest microhardness, FS and DTS values; however, composites with additional photoinitiators such as Lucirin TPO and Ivocerin have the lowest polymerization shrinkage. These composites also have higher FS and DTS and microhardness than material containing CQ in Rap Technology.

Buffering capacity and antibacterial properties among bioactive glass-containing orthodontic adhesives

This study was to evaluate the acid-buffering capacity and antibacterial properties of orthodontic adhesives containing bioactive glasses (BAGs) (45S5, 45S5F, S53P4), Hydroxyapatite, beta-tricalcium phosphate, and Canasite. Fillers comprising 15 wt% bioactive glasses, HAp, β-TCP, and Canasite incorporated with 55 wt% silanated glass were added to a mixture of UDMA/TEGDMA. Acid-buffering capacity was tested by exposing disc-shaped samples of each adhesive to medium of bacteria-produced acids, and pH changes were recorded at 24 and 48 h. Antibacterial properties were assessed by indirect testing by exposing polymerized adhesive samples to a medium and direct testing by immersing the specimens in solutions containing S. mutans and S. sanguinis. A significant buffering capacity was shown by the 45S5, 45S5F and S53P4 BAG adhesives. The antibacterial properties were not significant in all experimental adhesives. Therefore, the experimental orthodontic adhesives containing BAGs demonstrated a significant buffering capacity but did not show significant antibacterial properties against S. mutans and S. sanguinis.

The Photoinitiators Used in Resin Based Dental Composite-A Review and Future Perspectives

The presented paper concerns current knowledge of commercial and alternative photoinitiator systems used in dentistry. It discusses alternative and commercial photoinitiators and focuses on mechanisms of polymerization process, in vitro measurement methods and factors influencing the degree of conversion and hardness of dental resins. PubMed, Academia.edu, Google Scholar, Elsevier, ResearchGate and Mendeley, analysis from 1985 to 2020 were searched electronically with appropriate keywords. Over 60 articles were chosen based on relevance to this review. Dental light-cured composites are the most common filling used in dentistry, but every photoinitiator system requires proper light-curing system with suitable spectrum of light. Alternation of photoinitiator might cause changing the values of biomechanical properties such as: degree of conversion, hardness, biocompatibility. This review contains comparison of biomechanical properties of dental composites including different photosensitizers among other: camphorquinone, phenanthrenequinone, benzophenone and 1-phenyl-1,2 propanedione, trimethylbenzoyl-diphenylphosphine oxide, benzoyl peroxide. The major aim of this article was to point out alternative photoinitiators which would compensate the disadvantages of camphorquinone such as: yellow staining or poor biocompatibility and also would have mechanical properties as satisfactory as camphorquinone. Research showed there is not an adequate photoinitiator which can be as sufficient as camphorquinone (CQ), but alternative photosensitizers like: benzoyl germanium or novel acylphosphine oxide photoinitiators used synergistically with CQ are able to improve aesthetic properties and degree of conversion of dental resin.

Use of Artificial Neural Network in Determination of Shade, Light Curing Unit, and Composite Parameters' Effect on Bottom/Top Vickers Hardness Ratio of Composites

Objective

To assess the influence of light emitting diode (LED) and quartz tungsten halogen (QTH) light curing unit (LCU) on the bottom/top (B/T) Vickers Hardness Number (VHN) ratio of different composites with different shades and determination of the most significant effect on B/T VHN ratio of composites by shade, light curing unit, and composite parameters using artificial neural network.

Method

Three composite resin materials [Clearfil Majesty Esthetic (CME), Tetric N Ceram (TNC), and Tetric Evo Ceram (TEC)] in different shades (HO, A2, B2, Bleach L, Bleach M) were used. The composites were polymerized with three different LED LCUs (Elipar S10, Bluephase 20i, Valo) and halogen LCU (Hilux). Vickers hardness measurements were made at a load of 100 g for 10 sec on the top and bottom surfaces and B/T VHN ratio calculated. The data were statistically analyzed with three-way ANOVA and Tukey test at a significance level of 0.05. The obtained measurements and data were then fed to a neural network to establish the correlation between the inputs and outputs.

Results

There were no significant differences between the B/T VHN ratio of LCUs for the HO and B shades of CME (p>0.05), but there were significant differences between the B/T VHN ratio of LCUs for shade A2 (p<0.05). No significant difference was determined between the B/T VHN ratio of LCUs for all shades of TNC (p>0.05). For TEC, there was no significant difference between the B/T VHN ratio of halogen and LED LCUs (p>0.05), but a significant difference was determined among the LED LCUs (p<0.05). The artificial neural network results showed that a combination of the curing light and composite parameter had the most significant effect on the B/T VHN ratio of composites. Shade has the lowest effect on the B/T VHN ratio of composites.

Conclusion

The B/T VHN ratio values of different resin-based composite materials may vary depending on the light curing device. In addition, the artificial neural network results showed that the LCU and composite parameter had the most significant effect on the B/T VHN ratio of the composites. Shade has the lowest effect on the B/T VHN ratio of composites.

Evaluation of phenyl-propanedione on yellowing and chemical-mechanical properties of experimental dental resin-based materials

Objective

To evaluate the influence of phenyl-propanedione on yellowing and chemical-mechanical properties of experimental resin-based materials photoactivated using different light curing units (LCUs).

Material and Methods

Experimental resin-based materials with the same organic matrix (60:40 wt% BisGMA:TEGDMA) were mechanically blended using a centrifugal mixing device. To this blend, different photoinitiator systems were added in equimolar concentrations with aliphatic amine doubled by wt%: 0.4 wt% CQ; 0.38 wt% PPD; or 0.2 wt% CQ and 0.19 wt% PPD. The degree of conversion (DC), flexural strength (FS), Young's modulus (YM), Knoop hardness (KNH), crosslinking density (CLD), and yellowing (Y) were evaluated (n=10). All samples were light cured with the following LCUs: a halogen lamp (XL 2500), a monowave LED (Radii), or a polywave LED (Valo) with 16 J/cm2. The results were analysed by two-way ANOVA and Tukey's test (α=0.05).

Results

No statistical differences were found between the different photoinitiator systems to KNH, CLS, FS, and YM properties (p≥0.05). PPD/CQ association showed the higher DC values compared with CQ and PPD isolated systems when photoactivated by a polywave LED (p≤0.05). Y values were highest for the CQ compared with the PPD systems (p≤0.05).

Conclusion

PPD isolated system promoted similar chemical and mechanical properties and less yellowing compared with the CQ isolated system, regardless of the LCU used.

Monomer elution in relation to degree of conversion for different types of composite

OBJECTIVES

The aim of this study was to determine the degree of conversion (DC) and the monomer release of three composite types when employed following a layer- and bulk-filling technique.

METHODS

The release of monomers from a 'conventional paste-like' (Filtek Z250), a 'conventional flowable' (Filtek Supreme XTE Flowable) and a 'bulk-fill' flowable composite (Filtek Bulk Fill Flowable) from the same manufacturer (3M ESPE, Seefeld, Germany) was determined. Ten cylindrical specimens per composite were built, either in two 2-mm layers or in one 4-mm bulk. DC was measured at the specimen top and bottom surface using micro-Raman spectroscopy, after which the specimens were immersed in 2 ml absolute ethanol for 24 h at 37 °C. This solution was refreshed weekly during six weeks and the concentration of BisGMA, BisEMA(6), BisPMA, UDMA, TEGDMA and BPA was determined by liquid chromatography/mass spectroscopy.

RESULTS

DC at the specimen top and bottom was similar except for the bulk-fill technique, which resulted in significantly lower DC at the specimen bottom. The release of BisGMA and TEGDMA was initially very high, but rapidly dropped in the second week. In contrast, the release of BisPMA and UDMA increased initially, but then declined towards the sixth week. BisEMA(6) release was relatively steady over time. All composites released small amounts of BPA. The total monomer release was significantly lower for the layer- than the bulk-filling technique.

CONCLUSIONS

The slightly reduced degree of conversion at 4-mm depth resulted in a higher monomer elution when the composite was applied following a bulk-fill application method.

CLINICAL SIGNIFICANCE

Applying a flowable and a bulk-fill composite following a bulk-fill application method resulted in a significantly reduced degree of conversion at the bottom of polymerized composite specimens when compared to a layer-application method. This reduced polymerization degree was reflected in significantly increased monomer release.

Evaluation of cell responses toward adhesives with different photoinitiating systems

OBJECTIVES

The photoinitiator diphenyl-(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) is more reactive than a camphorquinone/amine (CQ) system, and TPO-based adhesives obtained a higher degree of conversion (DC) with fewer leached monomers. The hypothesis tested here is that a TPO-based adhesive is less toxic than a CQ-based adhesive.

METHODS

A CQ-based adhesive (SBU-CQ) (Scotchbond Universal, 3M ESPE) and its experimental counterpart with TPO (SBU-TPO) were tested for cytotoxicity in human pulp-derived cells (tHPC). Oxidative stress was analyzed by the generation of reactive oxygen species (ROS) and by the expression of antioxidant enzymes. A dentin barrier test (DBT) was used to evaluate cell viability in simulated clinical circumstances.

RESULTS

Unpolymerized SBU-TPO was significantly more toxic than SBU-CQ after a 24h exposure, and TPO alone (EC50=0.06mM) was more cytotoxic than CQ (EC50=0.88mM), EDMAB (EC50=0.68mM) or CQ/EDMAB (EC50=0.50mM). Cultures preincubated with BSO (l-buthionine sulfoximine), an inhibitor of glutathione synthesis, indicated a minor role of glutathione in cytotoxic responses toward the adhesives. Although the generation of ROS was not detected, a differential expression of enzymatic antioxidants revealed that cells exposed to unpolymerized SBU-TPO or SBU-CQ are subject to oxidative stress. Polymerized SBU-TPO was more cytotoxic than SBU-CQ under specific experimental conditions only, but no cytotoxicity was detected in a DBT with a 200μm dentin barrier.

SIGNIFICANCE

Not only DC and monomer-release determine the biocompatibility of adhesives, but also the cytotoxicity of the (photo-)initiator should be taken into account. Addition of TPO rendered a universal adhesive more toxic compared to CQ; however, this effect could be annulled by a thin dentin barrier.

Ultra-fast light-curing resin composite with increased conversion and reduced monomer elution

OBJECTIVES

To test the null hypotheses that photoactive resin composites containing a Type I photoinitiator would exhibit reduced DC or increased monomer elution at substantially short curing times compared with materials based on a Type 2 ketone/amine system.

METHODS

Two experimental resin composites were prepared, using either Lucirin-TPO or camphorquinone/DMAEMA. Specimens were light-cured using appropriate spectral emission that coincided with the absorption properties of each initiator using different irradiation protocols (0.5, 1, 3, 9s at 500, 1000 and 2000mW/cm(2) for Lucirin-TPO based composites and 20 or 40s at 1000mW/cm(2) for Lucirin-TPO and camphorquinone-based composites). Degree of conversion (DC) was measured by Raman spectroscopy, propagating radical concentrations were collected by means of electron paramagnetic resonance (EPR) and monomer leaching was characterized using high-performance liquid chromatography (HPLC).

RESULTS

The null hypotheses were rejected, except for a single irradiation protocol (0.5s @ 500mW/cm(2)). Lucirin-TPO-based composites could cure 20 times faster and release at least 4 times less monomers in comparison to camphorquinone-based composites. At 1000mW/cm(2), and 1s irradiation time for curing times of 1s, Lucirin-TPO based composites displayed 10% higher DC. The difference in polymerization efficiency of Lucirin-TPO compared with camphorquinone-based resin composites were explained using EPR; the former showing a significantly greater yield of radicals which varied logarithmically with radiant exposure.

SIGNIFICANCE

Lucirin-TPO is substantially more efficient at absorbing and converting photon energy when using a curing-light with an appropriate spectral emission and otherwise a limitation noted in several previous publications. At concentrations of 0.0134mol/L, Lucirin-TPO-based composites require a minimum light intensity of 1000mW/cm(2) and an exposure time of 1s to provide significantly improved DC and minimal elution compared with a conventional photoinitiator system. The use of a wide range of curing protocols in the current experiment has realized the significant potential of Lucirin-TPO and its impact for clinical applications, in replacement to materials using camphorquinone.

Reduced polymerization stress of MAPO-containing resin composites with increased curing speed, degree of conversion and mechanical properties

OBJECTIVES

The degree and rate of photopolymerization in resin-based dental composites will significantly affect polymer network formation and resultant material properties that may determine their clinical success. This study investigates the mechanical properties, the generation of stress from polymerization, tooth cusp deflection and marginal integrity of experimental resin composites that contain different photoinitiators.

METHODS

Experimental light-activated resin composites (60vol% particulate filled in 50/50mass% bis-GMA/TEGDMA) were formulated using a monoacylphosphine oxide (MAPO) photoinitiator and compared with a conventional camphoroquinone (CQ)-based system. Similar radiant exposure was used (18Jcm(-2)) for polymerization of each material although the curing protocol was varied (400mWcm(-2) for 45s, 1500mWcm(-2) for 12s and 3000mWcm(-2) for 6s). Degree and rate of polymerization was calculated in real-time by near infrared spectroscopy and the generation of stress throughout polymerization measured using a cantilever beam method. Flexural strength and modulus were acquired by three-point bend tests. Standardized cavities in extract pre-molar teeth were restored with each material, the total cuspal deflection measured and post-placement marginal integrity between the tooth and restoration recorded.

RESULTS

Generally, MAPO- exhibited a significantly higher degree of conversion (72±0.8 to 82±0.5%) compared with CQ-based materials (39±0.7 to 65±1.6%) regardless of curing protocol (p<0.05) and MAPO-based materials exhibited less difference in conversion between curing protocols. CQ-based materials exhibited between ∼85 and 95% of the maximum rate of polymerization at <15% conversion, whereas MAPO-based RBCs did not approach the maximum rate until >50% conversion. Higher irradiance polymerization had a significant deleterious effect on the mechanical properties of CQ-based materials (p<0.05) whereas MAPO-based materials exhibited increased strength and modulus and were less affected by the curing method. Total cuspal deflection in restored extracted teeth was higher for CQ- compared with MAPO-based materials cured at the lowest irradiance curing protocol (12.9±4.0 and 8.3±1.5μm) and similar at 3000mWcm(-1) for 6s (10.1±3.5 and 9.0±1.5μm). A significant decrease in marginal integrity was observed for CQ-based RBCs cured at high irradiance for short exposure time compared with that of the MAPO-based RBC cured using a similar protocol (p=0.037).

SIGNIFICANCE

Polymer network formation dictates the final properties of the set composite and the use MAPO photoinitiators may provide an effective restorative material that exhibits higher curing speeds, increased degree of conversion, strength and modulus without compromise in terms of polymerization stress and marginal integrity between tooth and restoration.

Photoinitiators in dentistry: a review

Polymerization of Resin Based Composites (RBCs) initiated by a light curing unit activating photoinitiators. Different RBCs require different light energy levels for proper curing. Manufacturers are now producing RBCs with more than one initiator and not all of these will be properly polymerised with blue LED lights. An added problem is that manufacturers do not always indicate the type of photoinitiators in their materials. This review discusses the importance of matching the spectral output of LCUs to the absorption spectra of RBCs and the consequences of spectral mismatch. Resin based composites (RBCs) were first introduced in the 1960s and with development of effective and reliable dentine bonding systems2, have been used routinely as a filling material for both anterior and posterior teeth. The early RBCs were either chemically cured two component materials or photo-initiated materials that used UV initiators in the beginning and then transitioned to visible light initiators such as camphorquinine which was introduced in 1978.3 The first report of a light curing material was of an ultraviolet (UV) cured fissure sealant. However, due to the limited penetration depth of the UV light and the potential health hazards, this system was quickly abandoned. The advancement of science yielded light curing materials which contributed to a significant clinical progress over the UV and chemically cured RBCs. Additional advancements to direct RBC restoration materials included luting agents for ceramic restorations, pit and fissure sealants and resin modified glass ionomers. Polymerization in an RBC is initiated by a light curing unit (LCU); this technology is based on the use of photoreactive systems that absorb light irradiation from the LCUs at appropriate wavelength. Then the photoinitiators contained in the RBCs, absorb the incoming photons from the LCU and the monomers in the molecular structure become excited and in that active state, there is a change from monomers into a polymer network. The success of this technology hinges on matching the spectral emission of the LCU with the requirements of the photoinitiator system to convert the monomers into a polymer network. The amount of activated photo initiator depends on the concentration of photoinitiator in the material, the number of photons to which the material is exposed and the energy of the photons (wavelength), the latter depending on the curing light.The most common photoinitiator in dental materials today is camphorquinone, which has a peak activity around 470 nanometres. The factors affecting polymerization include filler type, size and loading, the thickness and shade of the restorative material, the effectiveness of light transmission (eg. light guide tips being free from debris and scratches), exposure time, distance of the light source from the restorative material and light intensity. It is important to note that the photoinitiator activation occurs at specific wavelengths, in other words, the optimum efficiency is obtained when the peak absorptivity of the photoinitiator corresponds with the spectral emission from the LCU. Commercially available curing units have different light intensities and light sources, with energy levels in QTH, LED and other LCUs ranging from 300 to more than 2000 mW/cm.