How To Reduce Polymerization Shrinkage In Composite

J Appl Oral Sci. 2008 Aug; 16(4): 266–270.

INFLUENCE OF THE CURING METHOD ON THE POSTPOLYMERIZATION SHRINKAGE STRESS OF A Composite RESIN

Leonardo Gonçalves Cunha

¹DDS, MSc, PhD, Department of Prosthodontics, Academy of Taubaté, Taubaté, SP, Brazil

Roberta Caroline Bruschi Alonso

^twoDDS, MSc, PhD, Department of Restorative Dentistry, Dental Materials Division, Piracicaba Dental Schoolhouse, Land Academy of Campinas -UNICAMP, Piracicaba, SP, Brazil

Eduardo José Carvalho de Souza, Junior

^threeGraduate Pupil, Federal Academy of Bahia, Dental School, Salvador, BA, Brazil

Ana Christina Elias Claro Neves

^oneDDS, MSc, PhD, Department of Prosthodontics, Academy of Taubaté, Taubaté, SP, Brazil

Lourenço Correr, Sobrinho

^ivDDS, MSc, PhD, Full Professor, Department of Restorative Dentistry, Dental Materials Partitioning, Piracicaba Dental School, State University of Campinas -UNICAMP, Piracicaba, SP, Brazil

Mário Alexandre Coelho Sinhoreti

⁵DDS, MSc, PhD, Associate Professor, Department of Restorative Dentistry, Dental Materials Division, Piracicaba Dental Schoolhouse, Land University of Campinas -UNICAMP, Piracicaba, SP, Brazil

Received 2008 Jan 18; Revised 2008 Mar 12; Accepted 2008 Mar 29.

Abstract

The aim of this report was to evaluate the effect of different curing methods on the stress generated past the polymerization shrinkage of a restorative composite in two moments: immediately after calorie-free exposure and later on v min. Photoactivation was performed using 2 unlike low-cal sources: (ane) xenon plasma arc (PAC) light (1,500 mW/cm² – 3s) and (two) a quartz-tungsten-halogen (QTH) low-cal with three low-cal-curing regimens: continuous exposure (40 s at 800 mW/cmⁱⁱ – CL); soft-start (x s at 150 mW/cm² and 30 south at 800 mW/cm² - SS) and intermittent lite [cycles of 4 southward (ii s with light on at 600 mW/cm² and 2 s of light off), for 80s – IL]. The composite resin was applied between ii five-mm diameter metallic rods, mounted in a servohydraulic machine. The maximum stress was recorded immediately after lite exposure (FF) and afterward five min (5F). The results were submitted to ANOVA and Tukey's examination (five%). For each method, the results obtained in FF and 5F were, respectively: CL (3.58 and 4.46 MPa); SS (2.99 and iv.36 MPa); IL (3.11 and 4.32 MPa) and PAC (0.72 and 3.27 MPa). The stress generated by the polymerization shrinkage during light exposure can be associated with the photoactivation method used. A meaning increase in the stress level was observed during the mail service-curing catamenia upwards to 5 min, for all evaluated methods.

Keywords: Composite resins, Stress, Postal service-curing

INTRODUCTION

While blended restorations have become pop considering of their esthetic entreatment, some drawbacks inherent to the polymerization reaction even so need to exist overcome. As the cloth cures, an increment in stiffness accompanying volumetric changes that are confined past the cavity walls results in stresses that challenge the integrity of the bail betwixt the composite restoration and the tooth^one. Even in cases where bonding integrity is maintained, contraction stress is a potential source for problems, such as cuspal deflection¹².

The magnitude of such stress is dependent upon several factors related to cavity geometry (C-factor)⁶, material characteristics, such every bit monomer limerick, catalyst concentration and filler type and content, and restorative technique (placement technique and calorie-free curing method, for the photoactivated systems)². The clinician but has control over some of these factors, such as the irradiance and exposure fourth dimension²⁴.

In an try to reduce polymerization shrinkage stress, dissimilar curing methods have been suggested. Traditionally, quartz-tungsten-halogen (QTH) lights have been used in a continuous output style while emitting a adequately high irradiance^{half dozen} ^, ^vii. Still, radiations from this type of source tin also be practical in different manners. The "softstart" method employs an initial low irradiance for a specific duration followed past a high one equivalent in value to that of the continuous stage⁹. Previous studies have associated this method with better marginal integrity of composite restoratives¹³ ^, ¹⁵. Intermittent light method alternates periods of calorie-free on and calorie-free off. The light-off periods promote a reduction of the polymerization rate, which can be responsible for slower stress formation and college probability of bonding preservation^sixteen.

In add-on to the QTH lite, xenon plasma arc (PAC) is claimed every bit an alternative lite source, associated with loftier irradiance (upward to ane,300 mW/cm^two)²¹ levels during a short exposure, from three to nine s¹⁴ ^, ^twenty. According to the manufacturers, three s using PAC light source is equivalent to 30-45s of irradiation using a conventional QTH source with irradiance level = 500mW/cm²,^x. Short curing times with high irradiance in composite resin are desirable past many dentists, however, information technology can produce inadequate polymerization and worse marginal accommodation, increasing the microleakage along the dentin margins¹⁸ ^, ²⁰.

In improver, the stress generated from the polymerization shrinkage of restorative composites is non limited to the period of calorie-free exposure. Previous studies accept shown a continuous increase of stress for dissimilar periods of fourth dimension afterwards the end of photoactivation^xiii ^, ²⁶ ^– ²⁷. The increment of the stress level after the terminate of calorie-free exposure is associated with an additional polymerization and with the occurrence of thermal shrinkage of the restorative blended¹⁵ ^, ²⁴ ^– ²⁶. Therefore, the aim of this written report was to evaluate the issue of unlike curing methods on the stress generated by the polymerization shrinkage of a composite resin immediately afterward lite exposure period and five min after the end the photoactivation. All curing methods were hypothesized to promote a significant increase of the stress later the end of light exposure, simply showing dissimilar patterns and levels of stress generation.

Textile AND METHODS

Filtek Z250 blended resin (3M/ESPE, St. Paul, MN, USA) was used for fabrication of the specimens submitted to the different curing methods.

A cylindrical metallic device (Figure 1B), continued to the actuator, was attached to the upper portion of a universal testing car (Instron, model 4411, Canton, MA, USA). I of the ends of the metal device had a apartment and plane circular area. Earlier the test, the apartment surface area of each metallic device was polished with 600-dust aluminum oxide sandpapers and force per unit area blowing with a 50-μm aluminum oxide particle stream.

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Polymerization stress testing configuration. (A) upper portion of the organization, connected to the load cell; (B) cylindrical metallic device; (C) photoactivation units in position; (D) lower metallic mould with cardinal hole; (E) blended specimen; (F) lower portion of the system

Afterward these 2 steps, one glaze of Scothbond MultiPurpose adhesive organization (3M/ESPE) was applied in each circular expanse of the metallic device. The agglutinative layer was cured prior to the examination for 10 s. In the junior area of the testing machine (Effigy 1F), a metallic mould with a coneshaped 8-mm-diameter central hole was stock-still (Figure 1D). This region was filled completely with the same composite and a i kg load was applied to obtain a standardized surface and remove composite excesses. This increment of blended was cured prior to the accomplishment of the test. Information technology represented the bond region of the composite specimen to the inferior area of the testing machine. The composite specimens were prepared in transparent plastic conduits (5 mm in diameter and 5 mm pinnacle), which were filled with the material until its complete filling. Standardization of the composite book used in the different specimens was performed in the universal testing machine, prior to testing, by controlling specimen size.

Thereafter, the conduit filled with the uncured composite was placed between the two extremities of the machine, maintaining a distance of 5 mm (Effigy 1E). Two units were simultaneously used for curing in opposed sides of the specimen (Figure 1C). The evaluated curing methods are described in Tabular array 1. The stress value showed in the test system was recorded immediately later on photoactivation. An additional time of 5 min was waited for each specimen, and the stress value generated later this period was recorded again. The stress values recorded in kgf were converted to MPa with the equation: stress value (kgf) / area of the specimen (cm²) x 0.098.

TABLE 1

Low-cal-curing methods with their outputs and respective manufacturers

Curing Method	Curing Protocol	Equipment	Manufacturer
Continuous QTH lite	800 mW/cm² for 40 s	40 2500	3M, St. Paul, Us
Stepped QTH low-cal	x south - 150mW/cm² 30 due south - 800mW/cmⁱⁱ	Forty 2500	3M, St. Paul, U.s.
Intermittent QTH light	600 mW/cm² in cycles of 4s, 2s lite on and 2s light off for fourscore s	Optilux 150 (adapted)	Demetron Res Corp, Danbury, The states
Xenon plasma arc (PAC)	ane,500 mW/cm² for iii s	Apollo 95E	DMD, Westlake Village, USA

For every new specimen, a new cylindrical metallic device was used in the superior area and a new composite layer was prepared in the inferior area. X specimens were prepared per group.

The stress information obtained for the unlike curing methods were submitted to ane-way ANOVA and Tukey's test. Significance level was set up at 5%.

RESULTS

The mean stress values generated by blended shrinkage after the end of the curing flow and later the additional fourth dimension of five min are presented on Table 2.

Tabular array 2

Stress means in MPa (SD) generated by the photoactivation methods in the both time points

Curing Method	Flow
Curing Method	Immediately after lite exposure	5 min after light exposure
Continuous QTH light	3.58 a, A (0.54)	iv.46 a, B (one.09)
Stepped QTH lite	ii.99 a, A (0.30)	4.36 a, B (0.40)
Intermittent QTH light	3.11 a, A (0.31)	iv.32 a, B (0.49)
Xenon plasma arc	0.72 b, A (0.14)	3.27 b, B (0.58)

All curing methods presented significantly college mean stress values (p<0.05) later the additional time of 5 min, in comparison to the results obtained immediately later on the end of light exposure.

PAC curing method presented the highest stress increase - from 0.72 MPa (end of light exposure) to three.27 MPa (after 5 min) -, corresponding to an increase of78%. Continuous QTH curing method presented the to the lowest degree stress increase - from 3.58 MPa (end of light exposure) to iv.46 MPa (after five min), respective to a percent increase of nineteen.seven%. Intermittent QTH curing method showed a stress increment from three.11 MPa (terminate of light exposure) to 4.32 MPa (after 5 min), respective to an increase of 25.7%. For soft-start QTH curing method, the stress values ranged from two.99 MPa (end of calorie-free exposure) to 4.36 MPa (after 5 min), a percentage increment of 31%.

At the stop of the light exposure and after the 5-minute menstruation, no statistically pregnant differences were observed (p>0.05) amidst the curing methods regarding the generated stress values, except for PAC, which produced significantly lower values (p<0.05) than the other methods.

Discussion

Polymerization shrinkage still represents the main disadvantage of blended resins. Shrinkage is associated with the decrease of the spaces amongst the monomers during the formation of the polymeric chains of the organic matrix^xix. When shrinkage takes place under weather condition of confinement, stresses are generated, produced by bonding to cavity walls³.

Sakaguchi and Ferracane²³ (1998) observed a loftier correlation betwixt shrinkage stress and radiant exposure (r^two=0.97). However, based on the results of the present study, stress formation is not express to the low-cal-exposure time because a pregnant increase in stress was observed after the cease of the photoactivation. The 5-minute post-lite exposure menstruum was responsible for a mean additional stress formation from nineteen.7 to 78%. Stress generated after the end of the lite exposure is associated with a continuity of the polymerization reaction in the absence of light (boosted polymerization) and with the occurrence of a temperature decrease in the composite mass (thermal shrinkage)²³ ^, ²⁶.

According to Halvorson¹³ (2002), the additional polymerization from v min to 24 h represents, depending on the material, as much every bit 19-26% of the final conversion. Approximately 37 and 62% of this process occurs inside the first 5 and 60 min, respectively. During light exposure, the rate of gratuitous radicals from acrylates and metacrylates follows a feature blueprint throughout the course of the reaction due to diffusion limitations on the reacting species. This pattern is manifested early in the reaction by an increase in the radical concentration. As a outcome, the charge per unit of polymerization accelerates (autoacceleration) to a maximum in spite of the subtract in monomer concentration. After calorie-free emission, the dimethacrylate molecules are probably able to diffuse and react with reactive ends even after their viscosity has increased¹³. Later on having passed through this maximum, the rate of radical propagation becomes diffusion express and the polymerization rate decelerates, often towards a limited conversion in the presence of unreacted monomer and pregnant population of radicals¹³.

Curing methods associated with longer lite-exposure times were related to a reduced post-polymerization shrinkage stress. To such methods, a higher evolution of the caste of conversion, and, consequently, of stresses occurred during photoactivation. Therefore, a less amount of conversion and stresses were observed afterwards the terminate of the light exposure, considering the loftier correlation between conversion and stress²⁵. In addition, such situation could be related to the mobility of reactive molecules in the polymer network. During polymerization, costless radicals located at functional groups of unreacted molecules are trapped inside the polymer network due to the rapid increase in viscosity¹⁷ ^, ²⁶. For postpolymerization, unreacted molecules with free radicals demand to exist sufficiently mobile in guild to make contact with other reactive molecules in the polymer network. The composites with college degree of conversion are considered to grade a more than rigid network than specimens with lower conversion. Therefore, in the initially highly polymerized composites the chance for the residue unpolymerized monomers to exist involved in farther reaction is lower. Consequently, the inverse is also truthful, with a higher amount of post-polymerization when the reaction is advancing slowly²⁶.

The findings of the present report confirmed these considerations. From the results of a previous written report⁸, testing the same curing methods, continuous QTH calorie-free curing method was associated with the highest caste of conversion. In the present study, this curing method was related to the smallest percentage of post-polymerization stress, with an increase of19.7%. On the other manus, PAC, associated with a low degree of conversion during the calorie-free exposure in the previous study⁸, was related to the highest level of postpolymerization stress, showing an increment of 78% in our experiment. In addition, comparing both curing methods, a previous report observed a lower mean hardness value associated with PAC when compared to continuous QTH low-cal curing²⁸. The hardness measurement can be used to exam, indirectly, the caste of conversion of a composite. Therefore, the significantly lower maximum stress generated by PAC later five min (3.27 MPa), when compared to continuous QTH low-cal (4.46 MPa), may be related to a lower caste of conversion associated with the PAC method.

For the modulated curing methods, soft-start and intermittent QTH low-cal, a delay of the reaction contributed to tiresome down the development of shrinkage stress to its ultimate value. In add-on, the fourth dimension catamenia for viscous period was extended¹¹. Soft-outset curing method is an example of this situation. The period of initial cure in reduced irradiance (150 mW/cm²) was responsible for a slower stress generation, associating this method with the second smallest hateful of stress generated during the light exposure flow (ii.99 MPa). However, after 5 min, the stress value (4.36 MPa) was similar to that of continuous QTH calorie-free method. The percent increase in the post-polymerization for soft-showtime QTH light method was 31.five%. Cunha, et al.⁸ (2003) observed no statistically meaning departure in the hardness values between continuous and soft-start QTH light-curing methods. The hardness test tin be used to appraise, indirectly, the degree of conversion of a composite. Therefore, the equivalent values of stress generated subsequently five min with these ii methods are associated with a loftier correlation (r²>0.99) betwixt the degree of conversion and generated stress, equally ended by Silikas, et al.²⁵(2000).

The aforementioned state of affairs tin exist observed with intermittent QTH calorie-free method. Light exposure in cycles was responsible for a slower polymerization, with depression polymerization rate. Therefore, at the end of calorie-free exposure, the stress value (3.11 MPa) was lower to that observed for continuous QTH light method. This situation is acquired by a delay of the polymerization reaction, which will contribute to slow down the evolution of shrinkage stress to its ultimate value. The percent increase in the post-polymerization period for intermittent QTH light method was 25.seven%.

An interesting finding of this written report was that modulated curing methods tend to take lower stress generation during light exposure. Immediately after the end of low-cal emission, a reduction of 17% and thirteen% in the total stress generated, for Stepped QTH calorie-free and Intermittent QTH light, respectively, was observed, both compared to continuous QTH light. For the latter, the reaction might take evolved too fast, well-nigh eliminating the opportunity for pasty flow, leading to a dramatic increase in stiffness after a relatively low degree of conversion³. Equally a upshot, stress develops almost immediately after polymerization is triggered; so, most part of the monomer- to-polymer conversion and hence the stress, occurs later the polymer matrix has reached a significant level of rigidity³. However, for the modulated methods, the initial depression irradiance or light exposure in cycles led to a decrease in the initial polymerization, reflected as a reduction in the stress rate, thereby modifying the generation and distribution of stresses, equally reported elsewhere^six. In improver, the reduction in the stress rate has been associated with higher bond strength of composite restorative materials^seven.

Stress generation afterwards the stop of the low-cal exposure period may likewise exist dependent on thermal shrinkage due to heat loss⁴. The heat supplied during the light exposure menstruum promotes the expansion of the composite mass due to the increase of its internal temperature. At the end of this menses, the loss of internal oestrus causes volumetric contraction, with consequent stress generation^iv ^, ⁵ ^, ²³, as observed in this study. Therefore, curing methods associated with high light-curing temperatures, such as continuous QTH lite, were related to a more significant occurrence of thermal shrinkage due to a higher supply of oestrus during the low-cal exposure, with consequent higher shrinkage of the composite mass at the end of this flow.

CONCLUSION

The first hypothesis tested in this written report, referring to the beingness of a pregnant increase in the stress generated after the end of light exposure was validated by the results. The percentage increase in the shrinkage stress ranged from xix.7 to 78%. The 2nd hypothesis tested in this study was also validated, because each method was related to a different blueprint of stress generation. In both periods, during light exposure and after lite exposure, these patterns were related to the degree of conversion and to the thermal shrinkage level promoted by each method.

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Articles from Journal of Applied Oral Science are provided here courtesy of Bauru School of Dentistry

Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4327535/