|Year : 2018 | Volume
| Issue : 1 | Page : 13-16
Comparative evaluation of the effect of variation in light-curing cycle with a time gap and its effect on polymerization shrinkage and microhardness of conventional hydrophobic sealants and moisture-tolerant resin-based sealants: An in vitro study
Department of Pedodontics and Preventive Dentistry, Tamil Nadu, India
|Date of Web Publication||3-Jul-2018|
Dr. Packialakshmi Arumugam
No. 3/1105, 2nd Street, Sulaiman Nagar, Metukuppam, Thoraipakkam, Chennai - 600 097, Tamil Nadu
Source of Support: None, Conflict of Interest: None
Aim: This study aimed to evaluate the effect of light-curing modes (standard mode and modified mode) on the polymerization shrinkage and microhardness of a conventional hydrophobic resin sealant – Helioseal F – and a moisture-tolerant resin sealant – Embrace™ WetBond™.
Subjects and Methods: A total of forty glass ring molds (8.5 mm in inner diameter and 2 mm in height) were prepared, and etching of the internal surface of the molds was done for 5 min with hydrofluoric acid. The materials (n = 20/group) were placed into the molds and, in Group I, curing was done in contact with the sample surface for 20 s. In Group II, curing was initiated for 10 s at 1-cm distance; a time gap of 10 s was given, followed by 20 s curing in contact with the sample surface. The volumetric polymerization shrinkage and microhardness were calculated.
Statistical Analysis Used: All data were analyzed statistically using unpaired t-test at P < 0.05.
Results: Curing cycle did not significantly affect the polymerization shrinkage and microhardness of both conventional and moisture tolerant resin-based sealants. The composition of the sealant had a direct influence on polymerization shrinkage and microhardness values.
Conclusion: In the clinical scenario where isolation is highly critical, one may consider the use of moisture-tolerant resin-based sealants. This could be augmented with soft-start polymerization which would result in lower degree of polymerization shrinkage without affecting the physical properties, thereby yielding enhanced clinical performance of the pit-and-fissure sealants.
Keywords: Moisture-tolerant sealant; polymerization shrinkage; time gap
|How to cite this article:|
Arumugam P. Comparative evaluation of the effect of variation in light-curing cycle with a time gap and its effect on polymerization shrinkage and microhardness of conventional hydrophobic sealants and moisture-tolerant resin-based sealants: An in vitro study. Indian J Multidiscip Dent 2018;8:13-6
|How to cite this URL:|
Arumugam P. Comparative evaluation of the effect of variation in light-curing cycle with a time gap and its effect on polymerization shrinkage and microhardness of conventional hydrophobic sealants and moisture-tolerant resin-based sealants: An in vitro study. Indian J Multidiscip Dent [serial online] 2018 [cited 2018 Nov 17];8:13-6. Available from: http://www.ijmdent.com/text.asp?2018/8/1/13/235730
| Introduction|| |
There is a remarkable decrease in the caries incidence and prevalence rate in the past few decades as suggested by the current research. Occlusal surfaces remain highly susceptible to the initiation of caries due to the complex morphology of pits and fissures. In the war against decay in pits and fissures, there has not been a stronger combatant than the pit-and-fissure sealant which provides a physical barrier and halts the incipient lesions and prevents cavitation. Conventional pit-and-fissure sealants are extremely sensitive to moisture due to their hydrophobic nature and thus, require a clean, dry, and etched enamel surface during placement.
A recent, significant development is the development of moisture-tolerant chemistry that is activated in the presence of moisture and recommended for slightly moist tooth surfaces. Polymerization is the chemical reaction by which small molecules are transformed into large polymer chains or networks. The main problem associated with composite dental material curing process is polymerization shrinkage and the consequent development of shrinkage stress.
Numerous methods such as incremental layering, oblique layering, increase in the percentage of filler, change in organic matrix, and various modes of polymerization have been carried out to reduce polymerization shrinkage and its effects. Polymerization by various modes such as stage curing, ramp curing, and pulse curing overcomes the effect of polymerization shrinkage by altering the intensity of light, as the intensity of light decreases with increase in distance between the composite resin and the curing tip.
Under the light of all expressions above, the present study was performed to evaluate the effect of varying the distance and giving a time gap during the curing cycle on the polymerization shrinkage and the hardness of a conventional resin-based sealant and moisture-tolerant resin-based sealant.
| Subjects and Methods|| |
A total of forty glass molds (8.5-mm inner diameter and 2-mm height) were prepared using a low-speed IsoMet ® saw. The internal surfaces of the glass rings were roughened and etched for 5 min with porcelain hydrofluoric acid-etching gel. The glass molds were then weighed in air and in water with a Shimadzu AY220 electronic balance (Shimadzu Corp., Kyoto, Japan) to determine their density and volume.
The two pit-and-fissure sealants, namely Helioseal F and Embrace™ WetBond™, were used in the study. For each material, two experimental groups with ten samples were defined according to the method of curing.
Group I (control): standard mode of curing
Curing was done in contact with the sample surface (0-cm distance) for 20 s according to manufacturer's instructions.
Group II (study group): modified mode of curing
Curing was initiated for 10 s at 1-cm distance; a time gap of 10 s was given, followed by 20 s curing in contact with the sample surface.
A brass jig was fabricated to maintain 1-cm distance from the curing tip to the composite. A total of forty samples of each fissure sealant were placed in the glass molds, and the molds were placed between two glass slides to ensure that the sealant was well distributed within the mold. Light polymerization was performed using a LED-curing unit with a 10-mm diameter light tip. The outputs of the light tips were checked using a Demetron digital curing radiometer (Kerr Corporation) at 850 mW/cm 2, and the light output of the light-curing unit was measured for every 10 samples. After polymerization, the samples were stored at 37°C for 24 h under dark dry conditions after light curing and were weighed in air and in water to determine their density and volume.
Volumetric shrinkage calculation
Volumetric polymerization shrinkage for each system (n = 40) was measured through the specific density method modified by Puckett and Smith.
V0= (πD2 h)/4(1a)
V1= 103 (W0 − W1) ρT(1b)
V2 = 103 (W2 − W3) ρT(1c)
V3 = V2 − V1 and(1d)
ΔV = V0 − V3(2)
V0 – Volume of the cylindrical hole of the glass ring (mm 3),
D – Inner diameter of the glass ring (mm), h is the glass ring's height (mm),
V1 – Volume of the glass ring (mm 3),
W0 – Weight of the glass ring in air (g),
W1 – Weight of the glass ring in water (g),
ρT – Density of water at temperature T (g/mm 3),
V2 – Volume of the glass ring + composite sample (mm 3),
W2 – Weight of the ring + composite sample in air (g),
V3 – Volume of adhesive paste sample after polymerization (mm 3),
W3 – Weight of the ring + composite sample in water (g),
ΔV – Volumetric shrinkage (mm 3)
Using the relations (1)-(2), it was found that
% shrinkage = 102 (ΔV/V0).
Measurement of microhardness
Vickers hardness test, which is the most reliable microhardness measurement test, was performed with Vickers hardness tester. The surface was polished before calculating microhardness to avoid false results and to protect the diamond tip from damage. Three indentations were made on the irradiated surface and the mean was calculated as the Vickers hardness for the sample. The data obtained from both the experimental and control groups were subjected to statistical analysis. An unpaired t-test was constructed (P = 0.05) for comparisons of mean values.
| Results|| |
The mean values for the polymerization shrinkage of Helioseal F and Embrace™ WetBond™ using Group I and Group II are tabulated in [Table 1]. The intergroup comparison of polymerization shrinkage (%) of Group I and Group II using the unpaired sample t-test showed statistically insignificant results (P > 0.05) [Table 1]. The intragroup comparison of polymerization shrinkage (%) of Group I and Group II using the unpaired sample t-test showed statistically significant results (P < 0.05) [Table 2]. The mean microhardness values of Helioseal F and Embrace™ WetBond™ of Group I and Group II using the unpaired sample t-test are summarized in [Table 3]. The intragroup comparison of microhardness values of Group I and Group II using the unpaired sample t-test was found to be statistically significant (P < 0.05) [Table 4].
|Table 1: Intergroup comparison of polymerization shrinkage (%) of Group I and Group II using the unpaired sample t-test|
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|Table 2: Intragroup comparison of polymerization shrinkage (%) of Group I and Group II using the unpaired sample t-test|
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|Table 3: The mean microhardness values of Helioseal F and Embrace™ WetBond™ of Group I and Group II using the unpaired sample t-test|
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|Table 4: Intragroup comparison of microhardness of Group I and Group II using the unpaired sample t-test|
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| Discussion|| |
Pit-and-fissure sealants is an effective tool of comprehensive approach to caries prevention on an individual basis or as a public health measure for at-risk populations. Despite intense research efforts, polymerization shrinkage is an inherent disadvantage of resin-based sealants, which causes minimal gap formation between the tooth and material interfaces and eventually leads to microleakage. Various methods such as modifications of the formulation of the material or the curing scheme have been developed in an attempt to reduce the shrinkage stress as a result of polymerization shrinkage.
In modified curing mode used in the present study, the short initial exposure of 10 s activates only a minor part of the camphorquinone molecules and hence gives rise to relatively few growth centers. A waiting period of 10 s followed by final cure increases the flow, providing sufficient time for stress relief, thereby effectively compensating for the polymerization shrinkage.
In the current study, specific density method, modified by Puckett and Smith, was used which is a simple, applicable, and well-established test method. This technique involves calculation of specific density by weighing the specimen before and after polymerization prior to determining volumetric shrinkage. The results of the present study revealed that both modified mode of curing (soft start) and standard mode of curing techniques resulted in similar polymerization shrinkage. Although the differences were not significant, less contraction was produced in soft-start mode than standard modes. Yap et al. observed no influence of soft-start polymerization on the effectiveness of cure and postgel shrinkage soft-start curing modes. For both the materials (Helioseal F and Embrace™ WetBond™) used in the present study, the curing mode did not have a significant effect on microhardness scores. This was in accordance with the study conducted by Bayindir et al.
As the components of various composites react and cross-link in different manners, their polymerization shrinkage also differ significantly. Among the multiple factors that intervene on composite polymerization shrinkage, material composition appears to be the most important factor.
Triethyleneglycol dimethacrylate, which is present in Helioseal F, exhibits high volumetric shrinkage and hence high contraction stress  which can be attributed to the higher shrinkage scores for Helioseal F. The Embrace™ WetBond™ sealants consist of highly crosslinked polymers with increasing monomer (di, tri, and multifunctional methacrylate) rank and increased pendant group size, which provide high structural stability, increased resistance to solvents, improved mechanical stability, and lower volumetric shrinkage during polymerization. Inclusion of highly acidic resin monomers, monomethacrylates, and multifunctional methacrylates also increases the adhesive properties. In addition, reinforcement of resin with glass fibers tends to produce better mechanical flexural properties than filler reinforcement.
Recently, concerns have been raised about the possibility of esterogenicity of chemicals, especially bisphenol-A (BPA) and BPA-A-dimethacrylate, leaching out of sealants.
| Conclusion|| |
In the clinical scenario where isolation is highly critical, one may consider the use of moisture-tolerant resin-based sealants. This could be augmented with soft-start polymerization which would result in lower degree of polymerization shrinkage without affecting the physical properties, thereby yielding enhanced clinical performance of the pit-and-fissure sealants.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3], [Table 4]