A motorized device was developed to simulate movements of light curing unit.
Different cements, LCUs and exposure protocols can lead to different in vitro results.
Delivering intermittent light, the properties of the resin cements is decreased.
Increasing the exposure times increases the properties of the resin cements.
A single-peak LCU can activate a resin that uses Ivocerin™ as the photoinitiator.
To evaluate the effect of exposure time and moving the light-curing unit (LCU) on the degree of conversion (DC) and Knoop microhardness (KH) of two resin cements that were light-cured through ceramic.
Two resin cements: AllCem Veneer APS (FGM) and Variolink Esthetic LC (Ivoclar Vivadent) were placed into a 0.3 mm thick matrix in 6 locations representing the canine to canine. The resins were covered with 0.5 mm thick lithium disilicate glass-ceramic (IPS e.max CAD, Ivoclar Vivadent). A motorized device moved the LCUs over the ceramic when the LCU was on. Two single-peak LCUs: Elipar DeepCure-L (3M Oral Care) and Emitter C (Schuster), and one multi-peak: Bluephase G2 (Ivoclar Vivadent) were used with 3 different exposure protocols: a localized exposure centered over each tooth for 10 or 40 s; moving the tip across the 6 teeth for a total exposure time of 10 or 40 s; and moving the tip across the 6 teeth resins for a total exposure time of 60 or 240 s. After 24 h, the DC and KH were measured on the top surfaces and the data was analyzed using three-way ANOVA and Tukey’s tests ( α = 0.05).
Interposition of 0.5 mm of ceramic reduced the irradiance received by the resin by approximately 50%. The 40 s localized exposure over each tooth always produced significantly higher DC and KH values. Moving the LCUs with a total exposure time of 10 s resulted in the lowest DC and KH. There was no beneficial effect on the DC or KH when the multi-peak (violet-blue) LCU (Elipar DeepCure-L or Bluephase G2), but the lower light output from a small tip LCU reduced the DC and KH values (Emitter C).
Moving the LCUs when photo-curing light-cured resin cements is not recommended. This study showed that a single-peak LCU could activate a resin cement that uses Ivocerin™ as well as the multi-peak LCU.
Bonded ceramic veneers that could be between 0.3 to 1 mm thick are more conservative than full ceramic crowns [ ]. These ceramic veneers are used to improve the color, change the tooth morphology, close diastemas, restore anterior guidance, and for extensive anterior rehabilitations [ , ].
Most of the resin cements used to bond ceramic veneers to teeth are light-cured resin cements. These cements provide a high bond strength to enamel [ ], a long clinical working time, improved color stability, and the opportunity to use try-in systems [ ]. Currently, most resin luting cements use the Norrish type II camphorquinone (CQ) [ ] photoinitiator. However, some now include the new germanium-based Norrish type I photoinitiator, commercially known as Ivocerin™, that should reduce yellowing of the resin over time [ , ]. The Type I photoinitiators have an absorption peak that is located at a lower wavelength compared to CQ, whose maximum absorption occurs at approximately 470 nm. In contrast, the maximum absorption peak for Ivocerin™ is in the violet region at 412 nm. This might suggest that using a single-peak LCU could negatively affect the polymerization of resin-based materials that use Ivocerin™. However, Ivocerin™ is also activated by light up to 460 nm [ , ].
LCUs exhibit different degrees of irradiance uniformity, power output, light tip area [ , ], and the shorter wavelengths (violet light) have a decreased penetration through materials compared to the longer blue wavelengths of light [ ]. Also, if a single peak and a multi-peak LCU both have the same power output, the multi-peak LCU will emit less blue light than a single peak LCU, and a LCU that delivers light between 430 to 460 nm may be able to adequately activate Ivocerin™.
The light-curing protocol used is a crucial step when luting ceramic veneers. The exposure time and the % of light transmitted through the ceramic will affect the amount of energy delivered. This will influence the degree of conversion (DC) of the monomers [ , ], and microhardness of the cement [ ]. If the resin-based cement does not achieve its optimum DC, this can increase the microleakage, decrease the color stability, reduce its physical properties, and reduce the bond strength [ ]. The microhardness is also related to the polymerization quality of the resin cement [ , ]. Although longer exposure times will likely produce higher DC values [ , ] and improved mechanical properties of resin cements [ ], some manufacturers have recommended that exposure time can be reduced from 40 to 10 s, depending on the irradiance delivered by the LCU. Clinicians are often guided by clinical speakers who sometimes provide little scientific background for the protocols they use. For example, some promote that the curing light tip can be swept across the ceramic veneers in a constant to and fro movement and produce the same amount of polymerization in a shorter overall exposure time. Since the LCU is continuously moving over the teeth, the light exposure received by each tooth is intermittent. While this may produce short term clinical success, this reduced exposure time [ ] and the intermittent light technique caused by moving the light tip [ ] may produce lower KH and DC values of the resin-based cement, which may then compromise the longevity of the veneer.
Therefore, the objective of this study was to evaluate the effect of continuously moving both single-peak and multi-peak LCUs for different exposure times on the degree of conversion and Knoop (KH) microhardness of light-cured resin cements that used different photoinitiators. The null hypothesis was that DC and KH values of resin cements that use different photoinitiators would not be affected by the light-curing protocol, exposure time, or the type of LCU.
Materials and methods
Two light-cured resin cements with different photoinitiators were used: AllCem Veneer (FGM, Joinville, SC, Brazil), and Variolink Esthetic LC (Ivoclar Vivadent, Schaan, Liechtenstein). The translucent shade was used for both cements. The information provided by the manufacturer about their luting materials is summarized in Table 1 . Three commercial light-curing units (LCUs) were used in this study: a multi-peak Bluephase G2 (Ivoclar Vivadent, Schaan, Liechtenstein); and two single-peak, Elipar DeepCure-L (3M Oral Care, St. Paul, MN, USA); and Emitter C (Schuster, Santa Maria, RS, Brazil). The information about these LCUs is summarized in Table 2 . Three light-curing protocols were tested, and a mechanical device was used to reproducibly sweep the LCUs over the six maxillary anterior teeth. The degree of conversion (DC) and Knoop microhardness (KH) at the top surface of the cements were measured 24 h after light exposure in a blinded procedure regarding the tested resin cement and light-curing protocol.
|Luting materials||Shade||Exposure time (s)||Composition||Filler (wt%)||Photoinitiator||Manufacture|
|Allcem Veneer APS||Trans||40||Methacrylic monomers, Barium aluminosilicate glasses, silicon dioxide||63.0||Camphorquinone, APS – 2- (Dimethylamino) ethyl Benzoate (co-initiator), other proprietary Type I and II initiators, and other special proprietary co-initiators.||FGM, Joinville, SC, Brazil|
|Variolink Esthetic LC||Neutral||10||Bis-GMA, UDMA, TEGDMA, ytterbium trifuoride, boroaluminofuorosilicate glass, spheroidal mixed oxide, benzoylperoxide, stabilizers, pigments||60−68||Ivocerin™||Ivoclar Vivadent, Schaan, Liechtenstein|
|Light Curing Units/LCU||Serial number||LCU type/ wavelength emission||Battery/mains||Tip/light conductor||Manufacturer|
|Bluephase G2||236679||LED/multi-peak||Mains||Optical fiber/black||Ivoclar Vivadent, Schaan, Liechtenstein|
|Elipar DeepCure-L||932125030659||LED/single-peak||Battery||Optical fiber/black||3M, St Paul, MN, USA|
|Emitter C||L1351617C||LED/single-peak||Battery||Optical fiber/translucent||Schuster, Santa Maria, RS, Brazil|
Characterization of Light Curing Units (LCUs)
The irradiance (mW/cm 2 ), emission spectrum (mW/cm²/nm), emission peak wavelength (nm), and radiant exposure (J/cm 2 ) delivered by each LCU in standard mode was measured using the MARC resin calibrator (BlueLight Analytics, Halifax, NS, Canada). The LCU tips were positioned touching the surface of the upper sensor of MARC, and the values were measured during a 20 s exposure. Then, one piece of a 0.05 mm thick polyester strip (TDV, Pomorode, SC, Brazil) was placed over the sensor and covered with the 0.5 mm thick lithium disilicate glass-ceramic IPS e.max CAD (Ivoclar Vivadent). The LCU tip was positioned 1 mm above the surface of ceramic, and the measurement process was repeated.
Six bovine incisors of similar color were selected. They were cleaned and their roots were removed with a high-speed water-cooled diamond disc (American Burrs, Palhoça, SC, Brazil). The coronal parts of the teeth were reshaped using a high-speed diamond bur (No. 4138, KG Sorensen, Cotia, São Paulo, Brazil) to approximately the size of the human canine, lateral and central incisor maxillary teeth [ , ]. To try to simulate the light reflection so that it would be closer to clinical conditions, the teeth were embedded in a translucent epoxy resin (Buehler, Lake Bluff, IL, USA) while maintaining the size, proportion, and spacing between the six anterior maxillary teeth [ , ]. The facial surfaces of the embedded teeth were finished with 600, 800, 1000, 1200-grit SiC sandpaper (3M, Sumaré, SP, Brazil) and then polished with 6 μm, 3 μm, 1 μm, and ¼ μm-grit diamond polishing pastes (Arotec, Cotia, SP, Brazil) with respective polishing cloths (Arotec, Cotia, SP, Brazil) for 2 min at the same speed ( Fig. 1 A).
Ceramic veneer fabrication
One lithium disilicate glass-ceramic IPS e.max CAD (Ivoclar Vivadent, Schaan, Liechtenstein) HT/A1/C14 ceramic block was cut into four 0.5 mm thick samples each simulating a ceramic veneer using a precision saw (Isomet 1000, Buehler, Lake Bluff, IL, USA). The cut samples were then crystallized and glazed in a dental furnace (Programat EP 3010, Ivoclar Vivadent) according to the manufacturer’s instructions.
Photo-activation device and sample fabrication
A device ( Fig. 1 D and E) was developed (Odeme Dental Research, Luzerna, SC, Brazil) to reproducibly sweep the LCUs over the teeth. The device has an adjustable speed stepper motor attached to a metal base that moved the LCU from the left to right canine tooth ( Fig. 1 F–H). The LCU was attached to the device, and the movement speed was set to 5 s (10 mm/s) from the right to the left canine tooth position (see Video S1 – supplement file).
Two light-cured resin cements that used different photoinitiators were used. AllCem Veneer APS (FGM, Joinville, SC, Brazil) that uses camphorquinone plus a proprietary Advanced Photoinitiator System; and Variolink Esthetic LC (Ivoclar Vivadent, Schaan, Liechtenstein) that uses only Ivocerin™. The luting materials were placed into the six holes over the six teeth that each had a 4.0 mm diameter in a 0.3 mm thick acetate sheet. Then, 0.05 mm thick polyester strips (TDV), were placed over the teeth. The matrix was placed over the six anterior teeth with the holes located at the center of each tooth ( Fig. 1 B). After the holes had been filled with cement, the 0.05 mm thick polyester strips (TDV) were placed over the resin cement. The 0.5 mm thick samples of the ceramic were placed over the polyester strips, checking that the junction between any two ceramic samples was not over the sample of resin cement ( Fig. 1 C). The LCU tip was then fixed 1.0 mm above the ceramic sample ( Fig. 1 I).
Light exposure variables
As the resin cements use two different types of photoinitiators that absorb light at different wavelengths (Ivocerin™, violet-blue spectrum; and Camphorquinone, blue spectrum), three LCUs were tested on this study: a multi-peak LCU, Bluephase G2 (Ivoclar Vivadent); and two single-peak LCUs, Elipar DeepCure-L (3 M Oral Care, St Paul, MN, USA), and the Emitter C (Schuster, Santa Maria, RS, Brazil). Since the exposure time recommended by the two resin cement manufacturers are different ( Table 1 ), two different exposure times were used for both resin cements in combination with three light-curing protocols, namely: (1) a localized exposure centered over each tooth for 10 or 40 s; (2) moving the tip across the 6 teeth for a total exposure time that was within the time recommended by the manufacturer to photoactivate just one tooth, namely a total of 10 s or 40 s; and (3) moving the tip across the 6 teeth so that each tooth received a total time of 6 × 10 s = 60 s, or 6 × 40 s = 240 s. After light-curing the resin cement samples, the 0.3 mm thick acetate sheets were removed from the teeth. The specimens were stored dry for 24 h at room temperature (25 °C) in dark envelopes that blocked the ambient light.
Degree of conversion (DC)
The degree of conversion (DC) at the top of each specimen of resin cement (n = 5 specimens per tooth location, and a total of 30 resin samples) was measured 24 h after light-curing the resin cements. The DC was determined using attenuated total reflectance/Fourier transform infrared spectroscopy (ATR/FTIR, Vertex 70, Bruker, Ettlingen, Baden-Württemberg, Germany). A baseline measurement of each uncured resin cement was recorded by scanning the specimens 32 times over a range from 400 cm −1 to 4000 cm −1 at a resolution of 4 cm −1 . The post-curing spectrum was acquired after the specimens had been stored dry at 25 °C for 24 h. All the data were collected at 25 °C ± 1 °C and 60 ± 5% humidity conditions, shielded from ambient and room light. The DC was calculated from the aliphatic (1638 cm −1 ) and aromatic (1608 cm −1 ) ratios of cured (C) and uncured (U) resin specimens. The formula used to calculate the degree of conversion was DC (%) = (1 – C/U) × 100 [ ].
Knoop microhardness (KH)
The Knoop microhardness (KH, N/mm²) at the top of the resin cement samples was measured from Knoop indentations made as soon as possible after the 24 h DC measurement. The resin cement matrices were positioned on the microhardness tester (MicroMet 5104, Buehler, Lake Bluff, IL, USA) and 5 measurements were made in five different positions (one for each quadrant and one at the center) at the top surface of each 4 mm diameter resin cement sample by applying a load of 50 gf for 15 s [ , ], for a total of 5 Knoop indents per sample, 30 indents per matrix and 180 indents per group (n = 5).
DC and KH data were analyzed for normal distribution and homoscedasticity using the Shapiro-Wilk and Levene’s tests. The effect of the location of KH measurement was checked by using two-way repeated measurement for the light-curing protocol (3), LCU (3), and the measurement positions (5). Since the location of the measurement had no significant effect, the means of 5 values were used for the analyses. Three-way ANOVA was used to compare the main factors: light curing protocol (3), LCU (3), and light-cured resin cement (2). Multiple comparisons were made using Tukey’s post hoc test. To analyze the effect of the light-curing protocol on the 6 teeth positions, one-way ANOVA was used for all the groups. All tests used a significance level of α = 0.05, and all analyses were performed using Sigma Plot 12.5 (Systat Software Inc, San Jose, CA, USA).
Light curing unit output
Fig. 2 and Table 3 report the characteristics of the LCUs. In 10 s, the Elipar DeepCure-L delivered 8.6 J/cm² through the ceramic, the Bluephase G2 delivered 6.4 J/cm² and the Emitter C delivered the lowest radiant exposure of 6.0 J/cm². The emission peaks from the Bluephase G2 are located at 412 and 457 nm, from the Elipar DeepCure-L at 451 nm, and at 454 nm from the Emitter C. The irradiance (mW/cm²/nm) when 0.5 mm ceramic was interposed between sensor and LCUs tips was reduced by 53.6 % for Elipar DeepCure-L, 49.9 % for Emitter C, and by 51.9 % for the violet wavelength peak and by 45.6 % for the blue wavelength peak for Bluephase G2, compared to the irradiance delivered by the LCUs without the ceramic interposed.