|Year : 2015 | Volume
| Issue : 2 | Page : 154-158
|Comparison of variation in the light curing cycle with a time gap and its effect on polymerization shrinkage, degree of conversion and microhardness of a nanohybrid composite
Arunajatesan Subbiya, Newbegin Selvakumar Gold Pearlin Mary, Malathi Suresh, Paramasivam Vivekanandhan, Malarvizhi Dhakshinamoorthy, Vridhachalam Ganapathy Sukumaran
Department of Conservative Dentistry and Endodontics, Sree Balaji Dental College and Hospital, Bharath University, Narayanapuram, Pallikaranai, Chennai, Tamil Nadu, India
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|Date of Submission||09-Nov-2014|
|Date of Decision||27-Dec-2014|
|Date of Acceptance||15-Jan-2015|
|Date of Web Publication||12-Mar-2015|
| Abstract|| |
Background: Giving a time gap and distance during curing can decrease the polymerization shrinkage.
Aim: To evaluate the effect of time gap and distance between the curing tip and restoration on the polymerization shrinkage, degree of monomer conversion (DOC), and microhardness of a nanohybrid composite.
Materials and Methods: A total of 50 standardized cylindrical specimens (Z350, 3M ESPE) were fabricated using a brass mould. The curing was done in contact with the sample surface for 20 seconds in the control group. In the four experimental groups, curing was initiated at 1-cm distance, followed by variation in the time gap and the curing cycle. The polymerization shrinkage, DOC, microhardness was calculated.
Statistical Analysis: One-way analysis of variance (ANOVA) and post hoc-Dunnett test were used to analyze the data.
Results: Curing at 1-cm distance for 10 seconds with a gap of 10 seconds and finishing the curing cycle with 20 seconds at 0 cm proved to be an appropriate technique to reduce the polymerization shrinkage without significantly affecting the DOC and microhardness.
Conclusion: A simple innovative modification of varying the distance of curing and a time gap in the curing cycle can decrease the polymerization shrinkage without affecting the DOC and microhardness.
Keywords: Curing distance; polymerization shrinkage; time gap
|How to cite this article:|
Subbiya A, Pearlin Mary NG, Suresh M, Vivekanandhan P, Dhakshinamoorthy M, Sukumaran VG. Comparison of variation in the light curing cycle with a time gap and its effect on polymerization shrinkage, degree of conversion and microhardness of a nanohybrid composite. J Conserv Dent 2015;18:154-8
|How to cite this URL:|
Subbiya A, Pearlin Mary NG, Suresh M, Vivekanandhan P, Dhakshinamoorthy M, Sukumaran VG. Comparison of variation in the light curing cycle with a time gap and its effect on polymerization shrinkage, degree of conversion and microhardness of a nanohybrid composite. J Conserv Dent [serial online] 2015 [cited 2019 Dec 11];18:154-8. Available from: http://www.jcd.org.in/text.asp?2015/18/2/154/153055
| Introduction|| |
Composite resins have become the most popular direct restorative material despite its disadvantages regarding polymerization shrinkage, post-operative sensitivity, and estrogenic potential.  The polymerization of composite resins occurs in two phases: Pre-gel phase and post-gel phase. During the pre-gel phase, the organic matrix is able to flow and undergo molecular rearrangement, compensating the shrinkage forces. During this phase, there is a predominance of linear polymer chains, following which the resin reaches a viscous state, establishing its gel point. , The period after the gel point where the resin loses its flowing ability is referred to as the post-gel phase. During this phase, the polymerization shrinkage stress is transferred to the tooth restoration interface.  Various techniques carried out to reduce of polymerization shrinkage and its effects include incremental layering, oblique layering, increase in filler load, change in organic matrix, different polymerization modes, etc.  Polymerization by different modes such as ramp curing, pulse curing, and stage curing uses the pre-gel phase to overcome the effect of polymerization shrinkage by altering the intensity of light as intensity of light decreases with increase in distance between the composite resin and the curing tip. 
There are few studies in literature comparing the physical properties of nanohybrid composites with varying distances of light curing intensity, and there is less research evaluating the effect of a short gap within the advocated curing time. Therefore, the aim of the present study was to evaluate the effect of varying the distance and giving a short time gap during the curing cycle on the polymerization shrinkage, degree of conversion and the hardness of a nanohybrid composite.
| Materials and methods|| |
Nanohybrid composite resin (Filtek Z350 Universal Restorative, 3M/ESPE, St. Paul, MN, USA) of A2 shade was used in this study. Cylindrical specimens were fabricated using a brass mould of 5-mm diameter and 2-mm depth, for the standardization of the samples.  This mould was constructed as two parts with a split in between for easy disassembly of the cured specimen. The whole assembly was made to rest in the brass block, with four corner slots for additional support. A brass jig was fabricated to maintain 1-cm and 2-cm distance from the curing tip to the composite [Figure 1]. The inner surface of the jig was polished to prevent adhesion of composite resin to the mould. A pilot study was done with 1-cm and 2-cm curing distance. Since 2-cm distance did not show any favourable results within the selected parameters, the final study was done with 1-cm curing distance.
|Figure 1: Schematic representation of the jig during fabrication and curing of samples|
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The experiment was broadly divided into four experimental groups and one control group with 10 samples in each group. Uncured composite resin was packed as a single increment and well-adapted into the brass mould. Mylar matrix strip was placed on the upper surface of the samples that aided in the formation of a smooth surface and digital pressure was applied to prevent voids. 
Light polymerization was performed with quartz-tungsten-halogen light curing unit (Elipar 2500- 3M ESPE, St. Paul, MN, USA), at 600 mW/cm 2 and periodical monitoring of the curing light intensity was done with radiometer (Optilux Model 100, SDS Kerr; Donbury, CT, USA).
GROUP C (control): Curing was done in contact with the sample surface (0-cm distance) for 20 seconds according to manufacturer's instructions.
Group 1: Curing was initiated for 10 seconds at 1-cm distance between the light curing tip and the composite surface, immediately, followed by 20 seconds curing in contact with the sample surface.
Group 2: Curing was initiated for 10 seconds at 1-cm distance, a time gap of 10 seconds was given, followed by 20 seconds curing in contact with the sample surface.
Group 3: After curing for 10 seconds from 1-cm distance; a gap of 10 seconds was given, followed by curing for 10 seconds in contact with the sample surface. This was again followed by a gap of 10 seconds and completing it with 10 seconds curing in contact with the sample surface.
Group 4: Curing was done at 1 cm for 10 seconds, followed by curing in contact with the sample surface for 10 seconds. Then a gap of 10 seconds was given and final curing was completed with 10 seconds at 0-cm distance.
The polymerization shrinkage of the cured specimens was measured using stereomicroscope (HZ CS02-700X, Olympus, Japan), 24 hours after curing.  The readings were taken at eight different places and the average was calculated. Shrinkage was calculated by measuring the difference between the standard diameter of the mould and the diameter of the specimen at 24 hours after polymerization. The percentage of polymerization shrinkage was calculated using the formula:
Degree of monomer conversion
The DOC was analyzed using Fourier Transformation Infrared Raman (FTIR) spectrometer (Thermo Scientific Inc., Waltham, USA). After 24 hours of curing, the samples were placed in the spectrometer and the post irradiation spectra were obtained [Figure 2]. Spectra were collected in the mid infrared region between 1550 and 1680/cm with 128 scans, at the range of 4000/cm.  The conversion of the unreacted c = c (aliphatic) to the c-c (aromatic) was measured for all the experimental groups using the formula:
Vickers hardness test was performed with Vickers hardness tester (Model no: HM 210B, Mitutoyo, Japan). The surface was polished before collection of data to avoid false results and to protect the diamond tip from damage.  Five indentations were made on the irradiated surface and the mean was calculated as the Vickers hardness for the sample. 
Statistical analysis was done using statistical package of social sciences (SPSS) version 10.0 and the data were analysed using one way analysis of variance (ANOVA) and the multiple comparisons between the experimental groups were done with post hoc-Dunnett test. A probability value less than 0.05 was considered to be statistically significant (P < 0.05).
| Results|| |
The mean values for the polymerization shrinkage, degree of conversion, and microhardness are tabulated [Table 1]. The results showed that there was no statistically significant difference between the control and group I for polymerization shrinkage. But there was a reduction in the polymerization shrinkage in groups 2, 3, and 4 in comparison with the control group [Figure 3]. This reduction in polymerization shrinkage was statistically significant (P < 0.05).
Regarding the DOC, the mean difference between the control and the experimental groups was not more than 2.83% and is clinically acceptable. 
With regard to microhardness, the mean of the control and the four experimental groups were 84.45, 83.25, 82.66, 81.62, and 83.06, respectively, which was statistically significant (P < 0.05) and clinically acceptable  [Table 2] and [Figure 4].
|Table 2: Statistical analysis — Post hoc test, Dunnett test, multiple comparison between the control and the experimental groups (P value)|
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| Discussion|| |
The control group showed the highest polymerization shrinkage (4.25%) which is comparable to other studies which is typically in the range of 4-5% and results in the development of internal stresses. ,, This is because of the continuous light with highest intensity resulting a significant rise of temperature. ,, Experimental groups 2, 3, and 4 showed a statistically significant reduced polymerization shrinkage compared to the control group. The least polymerization shrinkage (1.65%) was observed in group 4.
The two parameters discussed in this study that compensates for the polymerization shrinkage - Curing distance and time gap during the polymerization cycle. In the pulse delay curing mode used in experimental groups 2, 3, 4, the short initial exposure of 10 seconds will activate only a minor part of the camphorquinone molecules and hence give rise to relatively few growth centres. Consequently, the propagation of polymerization will predominantly add one molecule of monomer after the other to the growing polymer chain. The final cure will activate a large part of the remaining camphorquinone giving rise to multitude of growth centres which will favour the formation of a relatively linear polymer structure.  The waiting time of 10 seconds before the final cure results in softer polymer matrix that increases the flow, providing sufficient time for stress relief, thereby effectively compensating for the polymerization shrinkage. The mechanism of reduced polymerization shrinkage seems to be partly attributable to the prolonged gel point which allows the composite to flow for a longer time period. ,
Degree of monomer conversion
The degree of polymerization has a potentially large role in determining the ultimate physical and mechanical properties of the material. The maximum DOC was observed in the control group (84.45%), followed by group 1 (83.25%) and group 4 (83.06%) with a statistically significant difference between the control group and the experimental groups. The DOC in all the experimental groups is within the clinically acceptable as the DOC of most of the composites are in the range of 60-80%. , The fact that all the specimens in the experimental groups were cured for 20 seconds at 0-cm distance albeit periodical gaps has ensured optimal DOC.
During the pulse delay curing mode in the first part of the polymerization cycle, the resinous material is not fully polymerized but contains a small amount of free residual monomer and the polymer structure harbors considerable quantities of pendant double bonds. During the final curing cycle, the quantity of the remaining double bonds increases that result in the same DOC as in a rapid curing cycle. The slow curing gives rise to a slightly different polymer structure mainly with a little change in the cross linking density.  Decrease in viscosity and increase in free radical mobility occurs with rise in temperature. Such rise in temperature during the final stage of the curing cycle, results in additional polymerization with improved conversion. 
Hardness of the material is directly proportional to the amount of polymer cross linking and depth of cure of the material. Vickers hardness tester was used in this study as it is highly reliable, reproducible, and sensitive to small surface changes. , The mean value the Vickers hardness value for the control group was 87.44, which was the highest among the groups. This was statistically significant from the experimental groups. The microhardness in the experimental groups is within the acceptable clinical values as the microhardness of most composites is between 60-80. 
The normal pulse delay technique incorporates a waiting period between exposures. Curing commences at low intensity - 150 mW/cm, 2 and is then paused for a given period ranging from few seconds to few minutes, light is then applied at high intensity (800-1200 mW/cm 2 ) in one or more pulses. This technique has been shown to achieve a constant DOC, reduced polymerization shrinkage, fewer gaps in the polymerized resin matrix with a better resin-tooth interface compared to the conventional curing cycle.  According to Yoshikawa et al.,  the greatest reduction in polymerization shrinkage, of up to 34%, is achieved with a waiting period of 3-5 min. The slow curing of the nanohybrid composite Z350 in the pulse delay curing mode produces reduced polymerization contraction in cavities due to stress relief. The short flash of light during the first part of the curing cycle, followed by a time gap before the final cure has resulted in significant reduction polymerization shrinkage (up to 38%) without affecting the mechanical properties. The result of this study has shown that a conventional QTH curing light can achieve a far better result, comparable to a variable intensity curing unit, by a simple modification of curing cycle. Further studies on the effect on the technique on stress induced during polymerization on natural tooth needs to be investigated.
| Conclusion|| |
In the light of the results of this investigation, the following conclusions can be derived:
- There is a significant reduction in the polymerization shrinkage of the nanohybrid composite due to variation in the curing distance and the time gap in the curing cycle.
- Among the experimental groups, group 2, where curing was done at 1-cm distance for 10 seconds with a gap of 10 seconds and finishing the curing cycle with 20 seconds at 0 cm proves to be an appropriate technique to reduce the polymerization shrinkage without significantly affecting the DOC and microhardness of the nanohybrid composite.
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Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2]
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