Thermographic analysis of the effect of composite type, layering method, and curing light on the temperature rise of photo-cured composites in tooth cavities

Abstract

Objectives

The purpose of this study was to investigate temperature rise in the composite and dentin of a class I cavity in extracted human molars under different restoration conditions, including the use of different composite types, layering methods, and curing lights.

Methods

Open occlusal cavities were prepared on 28 extracted human molars. A conventional (Filtek Z250) and a bulk-fill (Filtek Bulk Fill Posterior; BFP) composite were used to restore the preparations. BFP was incrementally layered or bulk-filled. Bulk-filled BFP was cured with two different lights, the Elipar S10 and the BeLite. Each layer was illuminated for 20 s, while thermograms of the specimens were recorded for 100 s using an infrared thermal camera. Temperature changes on the composite and dentin surfaces were obtained at points of interest (POI) pertaining to successive incremental distances of 0.75 mm from the top of the cavity to the pulp. The polymerization kinetics of each composite was determined using photo-differential scanning calorimetry.

Results

The greatest temperature rise was observed 0.75 mm apical from the top of the cavity. All groups showed over 6 °C maximum temperature rise (ΔT max ) at the pulpal side of the dentin. Upon curing, Z250 reached ΔT = 5 °C faster than BFP; however, ΔT max of the two composites were comparable at any POI. Bulk filling showed greater ΔT max than incremental filling at 0.75 mm apical from the top and in the middle of the cavity. The Elipar S10 light generated faster temperature changes in the curing composite at all recorded positions throughout the depth of the cavity and greater ΔT max in all POIs compared to BeLite.

Significance

Real-time thermographic analysis demonstrated that the composite type and layering method did not influence the temperature rise at the pulpal side of dentin during composite restoration of an occlusal preparation in a tooth. The amount and initial rate of temperature increase was most affected by the radiant exposure of the light curing unit. Within the limitations of this in vitro study, when irradiation time is constant, a curing light with higher radiant power can induce relatively high thermal transfer, thereby increasing the risk of pulpal damage.

Introduction

Heat can be generated and transferred within a tooth during dental treatment with various dental materials and instruments. High-speed handpieces produce frictional heat that induces an intrapulpal temperature rise . Photo-polymerization of dental composites involves both an exothermic chemical reaction within the material and the introduction of radiant heat from the curing light . The temperature rise of composites during polymerization is influenced by several factors such as, the type, intensity, and exposure time of the curing light ; the composition, thermal transmission properties, and heat capacity of the composite ; and the distance from the cavity floor .

Thermal damage to the pulp can be induced by excessive temperature rise during restorative treatment . It was previously reported that intrapulpal temperature increases of 5.5 °C, 11.1 °C, and above 11.1 °C in monkeys led to 15%, 60% and almost 100% irreversible pulpal damage to teeth, respectively . However, in a study with human teeth, an increase of 11.2 °C produced no pulpal damage .

In many previous in vitro studies, thermistors , thermocouples , differential scanning calorimetry (DSC) , and differential thermal analysis (DTA) have been used to measure temperature rise during the photo-curing of composites. However, thermistors and thermocouples require invasive procedures to create direct contact with tooth structure and materials and can measure temperature change only within localized area. DSC and DTA have mostly been used to analyze the polymerization heat of composites, and they are not suitable for measuring the temperature in a specific region on or within the tooth and restorative material.

On the other hand, an infrared thermal camera can measure surface temperatures without contact, and provides two-dimensional thermal distribution images with a sensitivity of 0.1 °C. Thus, thermal imaging analysis has an advantage over previous methods because it provides simultaneous measurements over an extensive area .

Previous studies that measured temperatures using a thermal camera during light curing of composites have mostly been performed as in vitro studies using artificial molds. The greatest maximum temperature was detected in lighter colored and thinner samples . When different resin-containing materials were compared, the maximum temperature rises were measured in descending order of flowable composite, conventional composite, resin modified glass ionomer, compomer, and packable composite. The type and content of the filler likely contributed to the results. In addition, an overlying dentin disk serving as a thermal insulator limited temperature rise . During photo-curing of a composite, the temperature in the middle of the cavity was higher than that on the surface or at adjacent areas of the cavity walls . Several studies showed that LED curing lights caused less temperature change in the pulp chamber compared to QTH curing lights . However, the irradiance (mW/cm 2 ) of the curing light was reported to influence the temperature rise: LED curing lights with higher irradiance generated higher temperature rise compared to QTH curing lights with lower irradiance . In addition, the light curing mode, such as standard vs. ramp, of LED curing lights affected temperature increases . When a composite was used to restore human anterior teeth in vivo , an average temperature rise of 5.4 ± 2.5 °C was observed, but the temperature could rise to 12 °C depending on the curing tip angle and quantity of the composite .

To date, few studies have used human teeth to analyze temperature changes within a cavity preparation and at the surrounding dentin during composite curing. In a recent study using thermocouples, a flowable bulk-fill composite showed greater temperature increase than conventional composite in an extracted human tooth . The flowable bulk-fill composites have lower mechanical properties and wear resistance, thus requiring an additional capping layer with a conventional composite . In contrast, certain high-viscosity bulk-fill composites are designed to be used alone, without a capping layer. For placement of 4 mm of high-viscosity bulk-fill composites, moderate irradiance with exposure time of 20 s was required to maintain the Vicker’s hardness and indentation modulus . Generally, an adequate depth of cure in bulk-fill composites is achieved by increasing the translucency of the material. One composite, EvoCeram Bulk Fill, claims to have incorporated an additional photo-initiator (Ivocerin), besides the camphoroquinone/amine initiator system, for producing enhanced depth of cure . These bulk-fill composites have the advantage of reducing chair time due to their more simplified procedures; however, concerns about polymerization shrinkage stress have been raised. A recently developed posterior bulk-fill composite, Filtek Bulk Fill Posterior, claims to incorporate high-molecular weight dimethacrylate-based monomers in order to reduce polymerization shrinkage stress. Due to the high-molecular weight monomers that require fewer carbon-carbon double bond reactions to achieve cure, this type of bulk fill resin may produce less heat during the polymerization process. However, no study has conducted a thermal analysis of newly developed high-viscosity bulk-fill composites during restoration of a tooth cavity.

The purpose of this study was to investigate the temperature rise in the composite and dentin of class I cavities in extracted human molars under different restoration conditions, including the use of different composite types, layering methods, and curing lights.

Materials and methods

Composites and light curing units

Two composites, Filtek Z250 (Z250, 3M ESPE, St. Paul, MN, USA) and Filtek Bulk Fill Posterior composite (BFP, 3M ESPE), were compared ( Table 1 ). According to the manufacturers’ instructions, BFP allows up to a 5 mm depth of cure. Two LED light curing units, Elipar S10 (3M ESPE) and BeLite (B&L Biotech, Ansan, Korea), were used for photo-curing of the composites. Based on the manufacturer’s information, the Elipar S10 LED curing light emits energy over a wavelength range of 430–480 nm with a single peak at 455 nm ± 10 nm. The irradiance of this curing unit is presented as 1200 mW/cm 2 , and the diameter of guide tip was 9.8 mm. BeLite is an LED curing light that provides several modes of curing. The BeLite emits energy over a wavelength of 430–490 nm with a single peak at 460 nm. The general composite filling mode (Norm mode, 800 mW/cm 2 ) was used in this study. The tip diameter of BeLite was 8.8 mm. The radiant power of the Elipar S10 and BeLite curing lights were 0.76 W and 0.69 W, respectively, when measured using a power detector (UP55N-300F-H12, Gentec-EO, Quebec, Canada), confirming that the output of Elipar S10 was greater than that of the BeLite.

Table 1
Composites used in this study.
Composite (code) Type Composition Filler Shade Manufacturer
Filtek™ Z250 (Z250) Micro-hybrid BisGMA 82 wt% A2 3M ESPE, St. Paul, MN, USA
Conventional Universal BisEMA (60 vol%)
UDMA Zirconia/silica
(0.01–3.5 μm, average 0.6 μm)
Filtek™ Bulk Fill Posterior (BFP) Nano-hybrid AUDMA 76.5 wt% A2 3M ESPE
Bulk Fill Posterior AFM (58.4 vol%)
UDMA Non-agglomerated/non-aggregated zirconia (4–11 nm)/silica (20 nm)
DDDMA Aggregated zirconia/silica cluster filler
YbF 3 filler (agglomerated, 100 nm)
Bis-GMA, bisphenol-A-diglycidyl ether dimethacrylate; Bis-EMA, bisphenol-A-polyethylene glycol diether dimethacrylate; UDMA, urethane dimethacrylate; AUDMA, aromatic urethane dimethacrylate; AFM, addition-fragmentation monomer; DDDMA, 1, 12-dodecanediol dimethacrylate; YbF 3 , ytterbium trifluoride.

Temperature measurements using thermal image analysis during photo-curing of composites

Preparation of tooth specimens

Twenty-eight extracted, caries-free human molars stored in 0.5% chloramine-T solution were used for the study. After flattening the occlusal surface of each tooth to standardize the cavity preparation, class I cavities (bucco-lingual 4 mm, occluso-gingival 3 mm, and mesio-distal 6 mm) were prepared using a flat-end cylindrical diamond and a high-speed handpiece. To expose the mesial side of the cavity and tooth, each tooth was sectioned bucco-lingually along the long axis at the center of the cavity, and cut horizontally at 3 mm below the CEJ at the mesial aspect ( Fig. 1 ). The prepared specimens were embedded in plastic cylinders (15 mm in diameter and 15 mm in height) at 5 mm below the CEJ and stored at 100% relative humidity. Photographs of the sectioned surfaces of all specimens were taken with a digital camera to clarify the cavity outlines of the thermal images.

Fig. 1
Schematic diagram of the prepared tooth cavity.

Thermal imaging apparatus

At a room temperature of 30 ± 0.5 °C with 40% relative humidity, an infrared thermal camera (VarioCamhr head 700, InfraTec GmbH, Dresden, Germany) was placed on an optical plate, and each tooth specimen was fixed 8 cm from the lens with the sectioned surface facing the camera. The object emissivity was set at 1.0, the default value of the program. The tip of the curing unit was positioned 2 mm above the occlusal surface of the cavity. To restrict direct light exposure to the mesial surface of the specimen, an opaque paper, which was large enough to cover the bucco-lingual length of the tooth, was positioned at 45° in contact with the mesial line angle of the cavity ( Fig. 2 ). The infrared imaging camera was equipped with a microbolometer focal plane array detector, which offers a high-resolution display (640 × 480 pixel, pixel size 0.25 mm) with a spectral range of 7.5–14 μm and a resolution of 0.03 °C at 30 °C.

Fig. 2
Experimental set-up for the temperature measurement with a thermal imaging camera. (A) Thermal imaging camera, (B) prepared tooth specimen, (C) radiation shield, and (D) curing light.

Measurement of temperature changes during photo-curing of composites

The specimens were divided into four groups (n = 7 per group) with regard to the type of composite, layering method, and light curing unit ( Table 2 ). In Groups 1 and 2, cavities were filled using two incremental layers (1.5 mm) of Z250 or BFP, respectively, followed by photo-curing with the Elipar S10. In Groups 3 and 4, a bulk fill of BFP was photo-cured with Elipar S10 or BeLite, respectively.

Table 2
Four experimental groups with varying composites, layering methods, and curing lights.
Group Composite Layering method Curing light
1 (Z-I-E) Z250 Incremental (1.5 mm × 2 layers, 24 mm 3 each) Elipar S10
2 (B-I-E) BFP Incremental Elipar S10
3 (B-B-E) BFP Bulk (3.0 mm, 48 mm 3 ) Elipar S10
4 (B-B-B) BFP Bulk BeLite
Z-I-E, Z250-Incremental-Elipar S10; B-I-E, BFP-Incremental-Elipar S10; B-B-E, BFP-Bulk-Elipar S10; B-B-B, BFP-Bulk-BeLite.

After applying Single Bond Universal adhesive (3M ESPE) to the cavity with a rubbing motion (20 s) and photo-curing (10 s) with the curing light for each group, the specimen was placed in front of the thermal camera. Each cavity was filled with the designated composite by the incremental or bulk filling method by sculpting flat at the open mesial cavity surface, and each layer was cured for 20 s. From the start of curing, thermal images were recorded for 100 s at a rate of 2 frames/s using the IRBIS-3 professional analysis program (InfraTec, GmbH).

The saved images were analyzed in the range of 20.0–50.0 °C. Six points-of-interest (POIs), labeled P1–P6 (P1, top center of the cavity; P2, 0.75 mm apical from P1; P3, middle center of the cavity (1.5 mm apical from P1); P4, 0.75 mm coronal from the cavity base; P5, center of the cavity floor; P6, 1 mm apical from P5), were selected on the images ( Fig. 3 ). The temperature change at each POI was obtained over time. The maximum temperature (T max ), maximum temperature rise (ΔT max ), and time to reach ΔT = 5 °C were measured for each group.

Fig. 3
Points of interest (POIs): P1, top center of the cavity; P2, 0.75 mm apical from P1; P3, middle center of the cavity (1.5 mm apical from P1); P4, 0.75 mm coronal from the cavity base; P5, center of the cavity floor; P6, 1 mm apical from P5.

Measurement of thermodynamic polymerization kinetics of composites

To investigate the effect of the heat of composite cure on the temperature change in the tooth cavity, thermodynamic polymerization kinetics of the composites during photo-curing were measured using photo-DSC (DSC Q-200, TA Instruments, New Castle, DE, USA). Samples were distributed into four groups according to composite material (Z250 vs. BFP) and curing light (Elipar S10 vs. BeLite). Thirty mg of composite was loaded into a pre-weighed aluminum sample pan, which was then placed in the calorimeter cell with the cover open. The pan was exposed to the curing light, which was positioned 8 mm above the surface of the sample, for 20 s. An empty pre-weighed aluminum pan was used as a reference. To determine the heat produced from the curing light alone, the cured composite sample was re-exposed to the light two more times at intervals of 1.5 min . The DSC thermogram was obtained for 6 min. The heat of cure was calculated as the difference between the overall heat produced during the first light exposure and the average heat produced when the cured sample was re-exposed to the light. Maximum heat flow and peak heat flow time were also obtained from the DSC thermogram (n = 3).

Statistical analysis

The results obtained from the thermal images were analyzed according to the differences in material, layering method, and curing light using independent Student’s t-tests. In each group, the values for each POI were analyzed by one-way ANOVA and Tukey’s post hoc comparisons (SPSS version 23.0, SPSS Inc., Chicago, IL, USA) ( α = 0.05). Data from the photo-DSC measurements were analyzed using two-way ANOVA, followed by Tukey’s post hoc test.

Materials and methods

Composites and light curing units

Two composites, Filtek Z250 (Z250, 3M ESPE, St. Paul, MN, USA) and Filtek Bulk Fill Posterior composite (BFP, 3M ESPE), were compared ( Table 1 ). According to the manufacturers’ instructions, BFP allows up to a 5 mm depth of cure. Two LED light curing units, Elipar S10 (3M ESPE) and BeLite (B&L Biotech, Ansan, Korea), were used for photo-curing of the composites. Based on the manufacturer’s information, the Elipar S10 LED curing light emits energy over a wavelength range of 430–480 nm with a single peak at 455 nm ± 10 nm. The irradiance of this curing unit is presented as 1200 mW/cm 2 , and the diameter of guide tip was 9.8 mm. BeLite is an LED curing light that provides several modes of curing. The BeLite emits energy over a wavelength of 430–490 nm with a single peak at 460 nm. The general composite filling mode (Norm mode, 800 mW/cm 2 ) was used in this study. The tip diameter of BeLite was 8.8 mm. The radiant power of the Elipar S10 and BeLite curing lights were 0.76 W and 0.69 W, respectively, when measured using a power detector (UP55N-300F-H12, Gentec-EO, Quebec, Canada), confirming that the output of Elipar S10 was greater than that of the BeLite.

Table 1
Composites used in this study.
Composite (code) Type Composition Filler Shade Manufacturer
Filtek™ Z250 (Z250) Micro-hybrid BisGMA 82 wt% A2 3M ESPE, St. Paul, MN, USA
Conventional Universal BisEMA (60 vol%)
UDMA Zirconia/silica
(0.01–3.5 μm, average 0.6 μm)
Filtek™ Bulk Fill Posterior (BFP) Nano-hybrid AUDMA 76.5 wt% A2 3M ESPE
Bulk Fill Posterior AFM (58.4 vol%)
UDMA Non-agglomerated/non-aggregated zirconia (4–11 nm)/silica (20 nm)
DDDMA Aggregated zirconia/silica cluster filler
YbF 3 filler (agglomerated, 100 nm)
Bis-GMA, bisphenol-A-diglycidyl ether dimethacrylate; Bis-EMA, bisphenol-A-polyethylene glycol diether dimethacrylate; UDMA, urethane dimethacrylate; AUDMA, aromatic urethane dimethacrylate; AFM, addition-fragmentation monomer; DDDMA, 1, 12-dodecanediol dimethacrylate; YbF 3 , ytterbium trifluoride.
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Nov 22, 2017 | Posted by in Dental Materials | Comments Off on Thermographic analysis of the effect of composite type, layering method, and curing light on the temperature rise of photo-cured composites in tooth cavities
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