Abstract
Objectives
Cavity lining with flowable composites have been proposed to improve initial marginal adaptation and minimize shrinkage stresses. The purpose of this study was to evaluate if prewarming of composites would influence the flow and enhance marginal adaptation thus the effect of pre-warming different types of composites on their properties are reported.
Methods
Six different composites were used in this study including a flowable and a polyacid modified composite. Uncured composites were pressed between two glass plates with a known load and the film thickness was measured to determine flow. Polymerization shrinkage was measured by means of a one-dimension contacting transducer. Flexural strength was determined using a three-point bend test. Microleakage was determined in human lower third molars on both enamel and dentin restoration interfaces. Cytocompatibility was analyzed with an Alamar Blue redox cell proliferation assay. The flow properties, linear shrinkage, flexural strength, microleakage and cytocompatibility were evaluated at 22 °C and 60 °C.
Results
The results indicated that the film thickness for each of the materials was significantly lower at 60 °C and the linear shrinkage was greater as a result of the higher degree of polymerization. The flexural strength of Spectrum TPH and Wave were found to be statistically significantly higher with pre-warming, however the other composites did not exhibit any differences. Microleakage studies showed that pre-warming had no significant bearing on the results and alamarBlue ® results showed that the pre-heating did not have an effect on the cytotoxicity however the levels of cytotoxicity varied between the composites that can be attributed to the composition.
Significance
Pre-warming of the composites studied enhanced flow as observed by measuring film thickness and did not significantly affect other properties.
1
Introduction
Esthetics have been an important guiding concept in dental materials and as a consequence there is an increasing use of dental composite resins in dentistry. The success of dental composites in restorative dentistry stems from their good esthetic properties and adequate durability and consequently materials are constantly evolving to improve on existing properties. Composite restorations comprise of photopolymerizable dimethacrylate based resins with inert fillers that undergo an addition-cured polymerization to yield a hardened restorative material . The extent of polymerization of dental composite resins ranges between 50 and 75% conversion, which has a direct bearing on the physical and mechanical properties and thus the durability of the restoration. Incomplete polymerization can lead to the presence of unreacted monomer within the restoration that may leach into saliva resulting into increased diffusion of oral fluids. Both oral fluids and unreacted monomer act as plasticizers, decreasing mechanical strength, dimensional stability and allow bacterial growth due to the ingress of oral fluids. Unreacted monomers can also cause allergic and sensitivity reactions.
Variants of composite resins have included materials with different flow characteristics such as packables and flowables although there is little clinical evidence to indicate their clear benefits. The low viscosity resin composites or the flowable composites have been shown to have a better marginal seal in Class V restorations in comparison to the traditional dental composites . Large Class I restorations with minimal dentin support have been also reported to show reduced marginal enamel fractures when lined with a low viscosity resin composite . One of the problems of placing resin composites has been achieving a good marginal contact of the restorative material and minimizing the gap between the tooth and the restoration, thus flowable composites with their greater ease of flow is expected to enhance marginal adaptation . However, the low filler content of the flowable composite may also cause greater net shrinkage with lower elastic modulus than the traditional composites.
Recent literature suggests that there are benefits in increasing the flowability of composite resins by raising the temperature of the composite before placement and thus obtain a better adaptation in the cavity . A commercial device was introduced ( Fig. 1 : Calset , AdDent Inc., Danbury, CT, USA) that pre-heats dental materials to 37, 54 or 60 °C prior to placement in the cavity preparation with other similar devices now being marketed. A moderate increase in temperature of a resin composite is expected to exhibit enhanced flow due to the higher thermal energy that allows molecular motion. Enhanced flow can be advantageous in placement of composites, especially in the case of stiffer materials and better adaptation to the cavity and recent research also indicates that there is a higher degree of conversion of the composite resins when cured at slightly raised temperatures.
The objective of this study was to evaluate the effect of pre-heating dental composite resins by measuring the uncured film thickness (flow), flexural strength, polymerization shrinkage, microleakage and cytocompatibility of six dental composite resins at both 22 °C (room temperature) and 60 °C.
2
Materials and methods
2.1
Materials
Six dental composite materials were evaluated in this study and their specifications are given in Table 1 . They consisted of a universal composite (Spectrum), three posterior composites (Herculite, Heliomolar, Filtek P60), a flowable composite (Wave), and a compomer (F2000). One group of specimens for each trial were fabricated under ambient laboratory conditions (21 °C ± 1), while the other group was pre-heated to 60 °C in a Calset compule heating unit).
| Spectrum TPH Dentsply, Milford, DE (hybrid) | Bis-GMA, Bis-EMA, TEDGMA | Barium aluminumborosilicate glass/silicon dioxide | <1, 0.04/57% | 20 | 509002350 |
| Herculite Unidose XRV Kerr, Orange, CA (hybrid) | Bis-GMA, EBADMA, TEGDMA | Barium aluminoborosilicate | 0.6/59% | 40 | 706766 |
| Heliomolar, Ivoclar (microfine) | Bis-GMA, UDMA | Dispersed silica – silanized copolymer, ytterbium trifluoride | 0.04–0.2/64% | 40 | H17737 |
| Filtek P60, 3M ESPE St. Paul, MN (posterior) | Bis-GMA, Bis-EMA, UDMA | Zirconia/silica | 0.01–3.5/61% | 20 | 5TP |
| F2000, 3M ESPE St. Paul, MN (compomer) | CDMA, GDMA | Fluroaluminosilicate (FAS) glass/silica | 3–10/67% | 40 | 19971008 |
| Wave SDI, Southern Dental Industries (flowable) | UDMA | Strontium glass | /44% | 20 |
2.2
Flow studies using film thickness
Each of the restorative materials were packed into 5 mm diameter × 3 mm brass rings placed on glass slides to obtain uniform film thicknesses of the uncured materials. The ring was removed with care and a glass slide (75 mm × 38 mm × 1 mm) placed gently over the uncured material. Subsequently, a 10 N weight was applied for 120 s and the flattened composite discs were the light cured as per manufacturers’ specifications, and their diameters measured to an accuracy of 0.01 mm using a micrometer. Three repeats ( n = 3) of each material at both temperatures were carried out. When testing the preheated composites, the brass ring, spatula, glass slides, and weight were heated to 60 °C and maintained before each measurement.
2.3
Polymerization shrinkage
Polymerization shrinkage of all six materials was determined by means of a one-dimension contacting transducer. The apparatus used has been previously described by Deb et al. which in turn was based on an instrument used by Watts and Cash . The set up of the apparatus is shown in Fig. 2 . In brief, a 7.5 mm diameter × 1 mm thick brass-ring mold was placed onto a 1 mm thick rigid glass slide, filled with the test composite, and covered with a 0.1 mm flexible glass microscope cover slip. A LVDT displacement transducer was placed above this, connected to an S7M transducer amplifier (RDP Electronics, Wolverhampton, UK). Output was recorded in DC voltage on an Altai M3800 Multimeter (Altai Group Ltd., Merseyside. UK). Light curing was carried out from below the specimen through the glass slide. Readings were taken after 10 s, 20 s, 30 s, 40 s, 60 s, 120 s, 300 s, and 480 s of light exposure. Three repeats ( n = 3) of each material at both temperatures were carried out.
2.4
Flexural strength
Rectangular bar specimens (25 mm × 3 mm × 1.5 mm) were fabricated using a customized mold. The materials were packed into the mold, covered by an acrylate strip and smoothed with a glass slide to achieve a uniform surface finish. Three overlapping sections of the composite were light cured with a conventional halogen curing-light (Optilux, Demetron Research Corp., Danbury, CT, USA) with irradiation times as per manufacturer’s specification. After irradiation, the specimens were placed into deionized water at 37 °C. Eight specimens ( n = 8) of each material at both ambient and preheated polymerization temperatures were fabricated and tested after 2 weeks. The maximum load ( P ) exerted on the specimen prior to fracture was recorded, and the flexural strength was calculated 3 Pa / bd 2 where P is the maximum load, ‘ a ’ is the distance between the supports (20 mm), ‘ b ’ is the specimen width and ‘ d ’ is the specimen thickness.
2.5
Microleakage
Sixteen freshly extracted caries-free lower third molar teeth were randomly divided into eight subgroups and stored in saline in a refrigerator prior to preparation. Two wedge-shaped cavities were prepared for each tooth ( Fig. 3 ) one on the mesial surface and the other on the distal surface, using a tungsten carbide bur in a high-speed air turbine handpiece (Toplight 895) under a water spray. The inciso-apical width of the cavity was approximately 2 mm, and the depth was 3 mm, as measured by a rotary file with 1 mm markings. The cavities were placed 1 mm above the dentino-enamel junction (DEJ) and extended into the dentin below the DEJ. The prepared cavity was etched with 3 M Scotchbond (St. Paul, MN, USA) for 15 s, rinsed with distilled H 2 O and blotted dry. Two consecutive coats of adhesive were then applied thinly on the cavity floor and dried for approximately 3 s, after which it was light cured for 10 s. The composite materials were packed into the prepared cavity in two increments, each time irradiated as per manufacturer’s specifications. Adper Single Bond Adhesive (Lot 7AU, 3M ESPE, St. Paul, MN, USA) was used for Heliomolar, Wave, and F2000; and Prime & Bond NT (Lot 0306001369, Dentsply, York, PA, USA) was used for Spectrum.
The restored teeth were stored in airtight glass vials at 37 °C and 100% humidity. This was achieved by suspending the individual teeth in a holder with blue tack (Bostick ® ) and placing a moist bed of cotton. The tooth was not allowed to come in contact with the water bed but maintained in an 100% humid environment. The teeth were stored for 3 weeks and then subjected to dye penetration studies. The roots of the teeth were sealed and the entire tooth was then covered in nail varnish with the exception of the restoration and a 1 mm window surrounding it, and submersed in Rhodamine B dye.
After 24 h, the teeth were removed from the dye solution, sectioned longitudinally through the restorations using a slow speed (135 rpm) diamond saw (360 μm thick, Labcut 1010, Agar Scientific Ltd., UK). Tooth sections were then wet polished using three grades of silicon carbide paper (grit size: p1000, p2000 and p4000 Struers, Glasgow).
For microscopic examination, the tooth sections were mounted horizontally flat in Plasticine™ (Harbutts Plasticine ® , UK) on a glass slide. Both halves were examined. Immersion oil was placed on the sample before and after placing a cover slip (170 μm thick). Oil immersion objectives of magnification 20×/0.8 NA/oil, 63×/1.4 NA/oil and 100×/1.4 NA/oil plus 10× eyepiece were used to examine the specimens. The interfaces were examined using a tandem scanning type of confocal microscope (TSM, Noran Instruments, USA). Images from the confocal microscope were recorded using a 35 mm camera (Kodak Elite Chrome 400). The extent of dye penetration at the incisal enamel and apical dentin margins was scored according to the following criteria, as used in a previous study by Kubo, et al. :
Incisal enamel margins:
- •
0 = No evidence of dye penetration.
- •
1 = Dye penetration up to 1/2 of the enamel depth.
- •
2 = Dye penetration greater than 1/2 of the enamel depth, but not beyond the dentino-enamel junction.
- •
3 = Dye penetration beyond the dentino-enamel junction.
Apical dentin margins:
- •
0 = No evidence of dye penetration.
- •
1 = Dye penetration just at the cavity margin (less than 0.1 mm).
- •
2 = Dye penetration up to 1/5 (approximately 0.5 mm) of the cavity depth.
- •
3 = Dye penetration greater than 1/5 of the cavity depth.
2.6
Biocompatibility
Disc shaped specimens (2 mm diameter × 1 mm thick, n = 4) of each of the composites were light cured for alamarBlue ® tests and one specimen ( n = 1) for MTT (8 mm diameter × 1 mm thick) by curing at both temperatures. A human fibroblast cell-line was used for the tests. alamarBlue ® for cell proliferation : Cells were cultured in Dulbecco’s modified Eagle’s medium (DMEM) (Sigma–Aldrich, Poole, UK), supplemented with 15% fetal calf serum (FCS), 0.02 m l -glutamine, penicillin and streptomycin. Tissue culture plastic was used as the negative (nontoxic) control and complete Eagle’s medium containing 10% alcohol was used as the positive control. For the assessment of proliferation, fibroblast cells were seeded at a density of 1 × 10 4 cells/μL on both test materials and controls (4 replicates for test materials, 8 for controls) on a 96-well plate. The cells were cultured at 37 °C in a humidified atmosphere with 5% CO 2 . After 48 h the medium was removed and substituted with 100 μL of 10% alamarBlue ® diluted in phenol-red free medium, and the plate was incubated under the same conditions for a further 4 h. The absorbance was read at test wavelength 570, and reference 630 nm.
2
Materials and methods
2.1
Materials
Six dental composite materials were evaluated in this study and their specifications are given in Table 1 . They consisted of a universal composite (Spectrum), three posterior composites (Herculite, Heliomolar, Filtek P60), a flowable composite (Wave), and a compomer (F2000). One group of specimens for each trial were fabricated under ambient laboratory conditions (21 °C ± 1), while the other group was pre-heated to 60 °C in a Calset compule heating unit).
| Spectrum TPH Dentsply, Milford, DE (hybrid) | Bis-GMA, Bis-EMA, TEDGMA | Barium aluminumborosilicate glass/silicon dioxide | <1, 0.04/57% | 20 | 509002350 |
| Herculite Unidose XRV Kerr, Orange, CA (hybrid) | Bis-GMA, EBADMA, TEGDMA | Barium aluminoborosilicate | 0.6/59% | 40 | 706766 |
| Heliomolar, Ivoclar (microfine) | Bis-GMA, UDMA | Dispersed silica – silanized copolymer, ytterbium trifluoride | 0.04–0.2/64% | 40 | H17737 |
| Filtek P60, 3M ESPE St. Paul, MN (posterior) | Bis-GMA, Bis-EMA, UDMA | Zirconia/silica | 0.01–3.5/61% | 20 | 5TP |
| F2000, 3M ESPE St. Paul, MN (compomer) | CDMA, GDMA | Fluroaluminosilicate (FAS) glass/silica | 3–10/67% | 40 | 19971008 |
| Wave SDI, Southern Dental Industries (flowable) | UDMA | Strontium glass | /44% | 20 |
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