Brazilian Journal of Oral Sciences Piracicaba Dental School - UNICAMP EISSN: 1677-3225 Vol. 3, Num. 11, 2004, pp. 609-614

Brazilian Journal of Oral Sciences, Vol. 3, No. 11, October-December 2004, pp. 609-614

Effect of photoactivation methods and base materials on the stress generated by the polymerization shrinkage of a resin composite

¹ Graduate Students – Department of Restorative Dentistry – Piracicaba Dental School – SP – Brazil
² DDS, MS, Associate Professor - Department of Restorative Dentistry – Piracicaba Dental School – SP – Brazil
³ DDS, MS, PhD Associate Professors -Department of Restorative Dentistry – Piracicaba Dental School – SP – Brazil

Correspondence to: Mário Alexandre Coelho Sinhoreti Av. Limeira, 901, Bairro Areião 13414-903 , Piracicaba – SP Phone: 55 (19) 3412-5374 Fax: 55 (19) 3412-5218 E-mail: sinhoret@fop.unicamp.br

Received for publication: June 01, 2004 Accepted: August 01, 2004

Code Number: os04032

Abstract

The aim of this study was to evaluate the effect of different photoactivation methods and base materials on the stress generated by the polymerization shrinkage of composites. The evaluated groups for the base material were: (G1) 1 coat of adhesive; (G2) 3 coats of adhesive, and (G3) flowable composite as a liner. The groups were divided in agreement with the photoactivation method: Continuous light (CL); Stepped Light (SL); Intermittent Light (IL); LED and Xenon Plasma Arc (XP). The generated stress was measured in a universal testing machine 5 minutes after the end of the photoactivation. The results were submitted to ANOVA and the averages values compared by Tukey test (5%). Inside of G1 group the mean values varied between 1.62 MPa (XP) to 2.22 MPa (CL), in which the XP method presented statistically inferior values to the other appraised methods. The values inside of G2 and G3 varied from 1.64 MPa (LED) to 2.15 MPa (CL) and 1.24 MPa (IL) to 1.92 MPa (SL), respectively, and the methods LED and IL presented statistically inferior values comparatively to the method CL inside of G2 and CL and SL inside of G3. The use of base materials was shown efficient in the reduction of the stress level generated by the polymerization shrinkage of restorative composites when LED or Intermittent light photoactivation methods are used.

Key Words: base materials, photoactivation methods, resin composite, shrinkage stress

Introduction

The polymerization shrinkage represents one of the main disadvantages of the resin composites because its occurrence generates stress, frequently associated with the postoperative sensibility, or with the breaking of the bonding, if the stress overcomes the bonding resistance of the dentalcomposite structures. Consequently, this situation promotes the formation of gaps, which allow the passage of fluids and bacteria, and they could lead the disease decay¹.

The intensity of the developed stress is associated with three main factors: (1) geometry of the cavity – C-Factor; (2) characteristic of the material and (3) restorative technique². As to the restorative technique, different photoactivation methods and low elastic module materials have been investigated for their influence on how stress is generated and distributed. The Stepped light photoactivation method represents one of the most used methods with this aim³, in which the initial photoactivation is promoted in low light intensity and a complementation is accomplished in the total intensity of the unit, for standardized periods. Some researches have associated this method with better marginal integrity of composite restorations^4-5.

Another modulated method is the Intermittent light, exposing the composite to cycles of 4 seconds (2 seconds light on followed by 2 seconds light off). The light-off periods promote a lower polymerization speed, responsible for slow stress formation and increasing the probability of the bonding preservation⁶. Besides the photoactivation method, the use of resilient liners with low elastic module materials between the restorative composite and the dental structure has been proposed. This technique can represent an effective procedure in the reduction of stress, because this intermediate area can act as an elastic zone, capable of absorbing part of the stress generated by the composite during the polymerization shrinkage^7-9.

The preservation of the tooth-composite bonding is one of the main factors involved in the longevity of the restoration. The preservation of this bonding is related to stress liberation of composite shrinkage or a modification in the polymerization kinetic, and this mechanism could be verified through the type of photoactivation method used, the restorative technique employed or both.

In this way, the objective of this study was to evaluate the effect of different photoactivation methods on the stress generated by the polymerization shrinkage of a resin composite with the application of low elastic module base materials.

This experiment used cylindrical metallic devices having, in one of the extremities, a screw formation so the device could be screwed to the universal testing machine (Instron, model 4411) used in this study, and a flat and plane circular area on the other extremity where the different base materials were applied. Before each test, the flat area was submitted to polishing with aluminum oxide sandpapers granulation 600 and pressure blowing with aluminum oxide particles 50 µm. In a next step, one coat of Scotchprime Ceramic Primer silane agent (3M Dental Products) was applied and left for a period of 60s of air-drying. The polishing, pressure blowing and silanization procedures were accomplished in all the devices used, which were separated in three groups: (group 1) one coat of adhesive agent; (group 2) three coats of adhesive agent, and (group 3) one coat of adhesive agent and lining with low elastic module composite Protect Liner F (Kuraray Co.), with a thickness of 0.7 mm.

The composite samples were prepared in transparent plastic conduits 5 mm in diameter and 5 mm height for Groups 1 and 2; and 4.3 mm in height for the Group 3. The Filtek Z250 composite (3M Dental Products) was inserted in the conduit until its total completion. The standardization of the composite volume used in the different samples was performed in the universal testing machine (Instron, model 4411), previously to the test, controlling the size of the sample (5 mm to groups 1 and 2 and 4.3 mm to group 3).

In the inferior area of the universal testing machine (Figure, F) was fixed a metallic mold with a central hole of 8 mm in diameter and cone format (Figure, D). This region was filled with the same composite and photoactivated previously to the accomplishment of the test. This place was the bond region of the sample to the inferior area of the universal testing machine.

After the assemblage of the system, in superior area, with the adaptation of the metallic device already with the base material to be evaluated, and inferior area, with the fixation of the metallic mold filled with the composite previously polymerized, the conduit filled with the unpolymerized composite was placed between the two extremities of the machine, respecting the distance of 5 mm for the groups 1 and 2 and 4.3 mm for the group 3 (Figure, E).

The photoactivation used simultaneously two units in opposed sides of the sample (Figure, C). The evaluated photoactivation methods were (Table 1): (1) Continuous light (800 mW/cm² for 40s); (2) Stepped light (150 mW/cm² for 10s following for 800 mW/cm² during 30s); (3) Intermittent light (cycles of 4s, 2s light on and 2s with light off for 80s at 600 mW/cm²); (4) LED (170 mW/cm² for 40s) and (5) Xenon plasma arc (1500 mW/cm² for 3s).

After the photoactivation period, an additional time of 5 minutes was awaited for each sample, and the stress value generated after that period was recorded. During the period of additional time, the abrupt fall of the stress values was associated with the occurrence of partial fracture of the sample. This was related to the located adhesive or cohesive rupture, causing stress liberation and fall in these values. The number of samples in this situation was registered for each group. The stress values generated in kgf were converted to MPa with the formula: stress value (kgf) / area of the sample (cm²) x 0.098. To each new sample a new cylindrical metallic device was used in the superior area and a new composite layer was prepared in the inferior area.

Ten samples were prepared for each appraised group, in a total of 15 groups, in agreement with the photoactivation method and base material used, in a total of 150 samples. The stress values obtained for each base material, in relation to the different photoactivation methods, were submitted to ANOVA (One-way) and the averages compared in the Tukey test (5%) to verify differences among the photoactivation methods.

After the test was finished, five cylinders of each photoactivation method were chosen at random for the Knoop hardness measurement, in order to determine, indirectly, the cure degree of the composite submitted to the different photoactivation methods.

The representative samples of each group were put in lateral position on a glass plate and polystyrene resin was poured (Resapol T208) to keep them fixed. Each group was flattened with carbide sandpaper of decreasing grit (320, 400, 600 and 1200) on an automated polisher APL-4 (Arotec Ind. Com) to obtain standardized surfaces. The cured resin was ground and polished until there was a wear of about half of the sample, exposing their whole extension, followed by the polishing with diamond paste containing particles of 1 µm and 0.25 µm.

The Knoop Hardness readings were performed in an indenter (HMV-2000) under load of 50 grams for 15 seconds. Twentyone indentations per sample were carried out, totaling 105 indentations per group, regarding each photoactivation method.

The hardness data were submitted to the ANOVA (one-way) and the averages compared in the Tukey test (5%).

The results regarding the stress values generated by the composite shrinkage in the different appraised groups are shown on Table 2.

When one coat of adhesive was applied on the metallic device surface, statistical difference (p<0.05) was observed with the xenon plasma arc method, which showed smaller stress value, statistically different from the others.

With the application of three adhesive coats, the continuous light method presented the largest stress mean values, with statistical difference (p<0.05) of the Intermittent and LED methods, that did not present statistical differences (p>0.05).

The Stepped light and Xenon plasma arc methods presented intermediate results and did not differ from the other appraised methods (p>0.05).

In the groups that a liner was prepared with low elastic module composite the Stepped and Continuous light methods presented the largest mean values and they differed statistically (p<0.05) from the Intermittent and LED methods. The Xenon plasma arc method presented intermediate results and did not differ from the other methods (p>0.05).

Table 3 shows the mean values of Knoop hardness for the different photoactivation methods. The Stepped and Continuous light photoactivation methods promoted the largest Knoop hardness mean values, showing no statistical difference (p>0.05). The Intermittent light, LED, and Xenon plasma arc methods did not show statistical differences when compared (p>0.05). However, they presented inferior results and differed statistically (p <0.05) from the Continuous and Stepped light methods.

The polymerization shrinkage of the resin composites still represents the main disadvantage of this material. This shrinkage is associated with the shortening space among the monomers during the formation of the polymeric chains of the organic matrix, resulting in stress formation in the tooth-restoration interface¹⁰.

The C-factor of a cavity, as previously related, has great importance in the stress intensity developed. In this study, the C-factor could be calculated through the dimensions of the samples, equal to 0.5. Studies demonstrated that in restorations with C-factor < 1,0, the polymeric chains rearrangement of the composite and the consequent liberation of part of the generated stress is enough to preserve the bonding^11-12. In this study, in any sample the generated stress was enough to promote total fracture of the bonding. However, partial ruptures were verified in certain evaluated groups.

The Continuous light photoactivation method was representative of the occurrence of this situation. The process of partial fracture was verified in nine of the thirty samples prepared for this group (five for the group with one adhesive coat and four for the group with three adhesive coat). Such situation can be related to the speed of stress development. The exhibition to the high light intensity (800 mW/cm²) since the initial moment, promotes a fast stress formation, reducing the probability of bonding preservation. The largest stress averages verified by the Continuous light method can be related to the largest conversion degree conferred to the composite by this method, verified in indirect way through the Knoop hardness test, in which this method presented hardness values statistically superior to the others, except for the Stepped light method, that followed the same pattern observed in the evaluation of the stress generated by the shrinkage. Silikas et al.³ found high correlation (r²>0.99) between conversion degree and generated stress, results also verified by Davidson and Feilzer⁸.

The modulation of the photoactivation process has been associated with low damages in the bonding interface, without causing damages to the physical and mechanical properties of the material⁸. The Stepped light represents one of the most studied modulated methods. The use of reduced light intensity initially promotes a smaller number of free radicals, reducing or limiting the number of monomer groups that are converted to polymer. The consequence of this process is that the polymerization reaction will be processed more slowly, allowing the stress relief and a slow stress formation. When the high light intensity is applied to complement the polymerization reaction, a significant part of the reaction will already have occurred, increasing the probability of bonding preservation¹³.

In this study, the final stress reached by Stepped light method was similar to the Continuous light method. In the same way, it could also be verified, by Knoop hardness test, that the conversion degree was similar between the two methods, once again associating such results with the high correlation (r²>0,99) between conversion degree and generated stress³. These results are in agreement with those found in the studies of Cunha et al.¹, Price et al.¹⁴, and Davidson-Kaban et al.¹⁵, stating that Stepped light method, when compared with the Continuous light method, does not reduce the conversion degree of the material. However, in spite of no statistical difference between the stress values between the two appraised methods, differences were verified in the total of partially fractured samples between the two groups. While in a total of nine samples such process was verified for the Continuous light method, for the Stepped light method in only two samples the same situation was verified, associating the initial photoactivation period in low intensity with the efficiency in the preservation of the bonding, modifying the formation process and distribution of the generated stress. The other modulation method of the halogen light used in this study was the Intermittent light. Theoretically, this method brings as advantage slow polymerization of the composite, due to the occurrence of the light-off periods, promoting a slow formation and accommodation of the polymeric chains in the initial phase of polymerization. For the stress generation, the use of this method proved to be highly satisfactory. The stress values were significantly inferior when compared with the Stepped and Continuous light methods, except for the group with one adhesive coat, in which no statistical differences was verified (p>0.05). On the other hand, when the conversion degree was indirectly verified, the Intermittent photoactivation method presented results statistically inferior to the Stepped and Continuous light methods. The inferior results related to the Intermittent light method can be associated with the difference in the energy density values among the three appraised methods, with values of 31.2 J/cm² for the Intermittent light method and 64 and 51 J/cm² for the Continuous and Stepped light methods, respectively. The smaller shrinkage stress generated with this method can be associated with the smaller degree of conversion of the samples.

In the Xenon plasma arc photoactivation method, the group in which one coat of adhesive was applied, a statistically inferior mean stress value could be verified when compared with the other methods. The smallest generation of stress, in this case, can be associated with the smallest conversion degree promoted by this method. The energy density in this case was only 9 J/cm², about six times inferior to the values found for the Continuous light (64 J/cm²) and Stepped light methods (51 J/cm²). The smallest conversion degree could be confirmed by the Knoop hardness test, in which the Xenon plasma arc method presented results statistically inferior to the Stepped and Continuous light methods (p<0.05). The same results were reported by Cunha et al.¹ and Sharkey et al.¹⁶ . The fact of having no statistical difference among these methods in the groups with low elastic module materials application can be associated with the polymerization speed. The high light intensity (1500 mW/ cm²), which the composite is exposed to promotes a highspeed polymerization, with fast stress formation. The accentuated formation of stress soon at the beginning of the polymerization process turns little efficient the absorption of these stress for the low elastic module material layer (application of three coats of adhesive and flowable composite). In so being, this method was shown little effective in the stress reduction of the composite, even if associated with a smaller conversion degree.

Finally, the LED photoactivation method (“Light Emitting Diodes”) uses a different light source from the other methods. The blue LED, that is used for the composite photoactivation, presents a strait light spectrum, with wavelengths between 438 and 501 nm, with pick intensity in 465 nm^17-18, which is coincident with the absorption pick of the camphorquinone, between 465 and 470 nm¹⁹. This characteristic turns this light source effective for the polymerization of composites and financially favorable, due to its long life time (superior to 10.000 hours) and no need of traditional components as wavelengths filter selectors and ventilation systems.

In this study a smaller generation of stress was verified in the samples photoactivated by LED. Significantly inferior results were verified for the LED photoactivation compared with the Continuous and Stepped light methods, except for the group in which one adhesive coat was applied. The slow polymerization speed of this method makes possible that the high resilient areas could absorb part of the stress generated by the composite shrinkage. However, through the Knoop hardness test, a smaller degree of conversion was also verified for this method, when compared with the Continuous and Stepped light methods. This way, maybe the smallest conversion degree was the responsible for the smallest stress generated in the composite polymerization. The same results were also verified by Jandt et al.¹⁷, and Stahl et al.¹⁸. In this specific case, the smallest conversion degree can be associated with the small number of LEDs present in the unit used in this study (7 LEDs), what checks to the unit light intensity of 170 mW/cm².

The results herein suggest that the restorative technique, with the use of low elastic module materials as liners, as the photoactivation method, can influence the stress generation process in the moment of polymerization. The modification in the polymerization kinetics, through the use of reduced light intensity during the photoactivation of the composite, can be associated with an increase of the probability of bonding preservation propitiated by the slow stress formation, as verified by the Intermittent and LED photoactivation methods. However, this situation should be promoted without compromising the physical-mechanical properties of the restorative material used. In that sense, the Stepped light photoactivation method somehow showed to be efficient in the bond preservation when compared with the Continuous light method. The use of low elastic module materials together with photoactivation methods in restorations with composite showed to be a satisfactory technique in the absorption process of part of the stress generated by the shrinkage. This characteristic is related to the high resilience of this area. One should consider that the device used in this experiment represents a condition hardly found in a clinical situation. In general, due to the complex format of cavities, the configuration factor (C-Factor) will be responsible for a stress generation more intense than verified in this study. In this way, the results can serve as a protocol of what can happen clinically, in which different photoactivation methods and base materials are used separately or not, contributing to the bonding preservation in an adhesive restorative procedure.

1. Cunha LG, Sinhoreti MAC, Consani S, Sobrinho LC. Effect of different photoactivation methods on the polymerization depth of a light-activated composite. Oper Dent 2003; 28: 155-9.
2. Unterbrink GL, Liebenberg WH. Flowable composites as “filled adhesives”: Literature review and clinical recommendations. Quintessence Int 1999; 30: 249-57.
3. Silikas N, Eliades G, Watts DC. Light intensity effects on resincomposite degree of conversion and shrinkage strain. Dent Mater 2000; 16: 292-6.
4. Koran P, Kürschner R. Effect of sequential versus continuous irradiation of a light-cured resin composite on shrinkage, viscosity, adhesion, and degree of polymerization. Am J Dent 1998; 11: 17-22.
5. Uno S, Asmussen E. Marginal adaptation of restorative resin polymerized at reduced rate Scan J Dent Res 1991; 99: 440-4.
6. Obici AC, Sinhoreti MAC, de Goes MF, Consani S, Correr Sobrinho L. Effect of the photo-activation method on polymerization shrinkage of restorative composites. Oper Dent 2002; 27: 192-8.
7. Ikemi T, Nemoto K. Effects of lining materials on the composite resins shrinkage stresses. Dent Mater J 1994; 13: 1-8.
8. Davidson CL, Feilzer AJ. Polymerization shrinkage and polymerization shrinkage stress in polymer-based restoratives. J Dent 1997; 25: 435-40.
9. Feilzer AJ, Doren LH, de Gee AJ, Davidson CL. Influence of light intensity on polymerization shrinkage and integrity of restoration-cavity interface. Eur J Oral Sci 1995; 103: 322-6.
10. Peutzfeldt A. Resin composites in dentistry: the monomer system. Eur J Oral Sci 1997; 105: 97-116.
11. Feilzer AJ, de Gee AJ, Davidson CL. Setting stress in composite resin in relation to configuration of the restoration. J Dent Res 1987; 66: 1636-9.
12. Feilzer AJ, de Gee AJ, Davidson CL. Quantitative determination of stress reduction by flow in composite restorations. Dent Mater 1990; 6: 167-71.
13. Ernst CP, Kürschner R, Rippin G, Willershausen B. Stress reduction in resin-based composites cured with a two-step lightcuring unit. Am J Dent 2000; 13: 69-72.
14. Price RB, Rizkalla AS, Hall GC. Effect of stepped light exposure on the volumetric polymerization shrinkage and bulk modulus of dental composites and an unfilled resin. Am J Dent 2000; 13: 176-80.
15. Davidson-Kaban SS, Davidson CL, Feilzer AJ, de Gee AJ, Erdilek N. The effect of curing light variations on bulk curing and wallto-wall quality of two types and various shades of resin composites. Dent Mater 1997; 13: 344-52.
16. Sharkey S, Burke F, Ziada H, Hannigan A. Surface hardness of light-activated resin composites cured by two different visiblelight sources: an in vitro study. Quintessence Int 2001; 23: 401-5.
17. Jandt KD, Mills RW, Blackwell GB, Ashworth SH. Depth of cure and compressive strength of dental composites cured with blue light emitting diodes (LEDs). Dent Mater 2000; 16:41-7.
18. Stahl F, Ashworth SH, Jandt KD, Mills RW. Light-emitting diode (LED) polymerization of dental composites: flexural properties and polymerization potential. Biomater 2000; 21: 1379-85.
19. Anusavice KJ. Phillips materiais dentários 10.ed. Rio de Janeiro: Guanabara Koogan; 1998.

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