EFFECT OF THERMAL CYCLING AND FILLING TECHNIQUE ON LEAKAGE OF COMPOSITE RESIN RESTORATIONS

he objective of this study was to evaluate in vitro the effect on leakage of two incremental filling techniques and two composite resins with different elastic modulus and similar polymerization shrinkage. Eighty Class V cavities (4x4x2mm) were prepared in bovine incisors and were randomly restored with Z-250 (Z) or Durafill VS (D) + Single Bond in axial (a) or oblique (o) increments. The restorations were divided into two groups: Not Aged N (4-hour-storage in water at 37oC) and Aged A (1week storage in water at 37oC + 1000 x 5o-55oC / 1-min dwell time). The specimens were covered with 2 coats of nail varnish so that only the restoration margins were exposed to silver nitrate 50% (2h) and developed under fluorescent light (8h). After they were sectioned twice in buccal-lingual direction, the four exposed surfaces were digitized (Vidcap) and the silver nitrate penetration was measured (ImageLab) at the incisal and gingival walls. Data were analyzed by a 3-way ANOVA (Resin, Filling Technique and Aging) separately for incisal and gingival walls (α=0.05). Resin and Aging were statistically significant either for the incisal and the gingival walls. The microfill composite resin infiltrated more than the hybrid composite. The thermal cycling caused an overall increase in silver nitrate penetration. The filling technique affected leakage depending on the composite resin and aging regimen. Uniterms: Composite resins; Elasticity; Dental leakage.


INTRODUCTION
Composite resin restorations are subjected to polymerization shrinkage stress that may cause premature failure at the interface 3 .As aging tends to aggravate the restoration's original deficient seal 11 , minimizing polymerization shrinkage stress is the first step for a reliable long-term performance of composite resin restorations.
The restoration's C-factor is determinant in the polymerization shrinkage stress level: the higher bonded / unbonded area ratio, the higher the stress build-up 7 .Moreover, a stress relief mechanism by the resin's flow was also reported and considered as the key factor in reducing the adhesive interface breakdown 6 .Based on these findings, the incremental filling technique has been suggested to contribute to the C-factor reduction 1 .However, some authors, using finite element models, have contested this stress reduction capacity and have suggested different stress patterns according to the filling technique applied 21,26 .
The composite resin's shrinkage, elastic modulus and polymerization rate of various composite resins and adhesive systems have also been investigated.In general, high filler content materials as hybrid composites have exhibited low shrinkage and high elastic modulus, a positive and a negative feature, respectively, regarding polymerization shrinkage stress.In contrast, a microfilled composite have showed low shrinkage and low rigidity, a combination that might be potentially less damaging to the interface 15 .Although the relative contribution of each parameter remains unknown, they affect the amount of polymerization shrinkage stress developed.
Once the adhesive interface has survived the polymerization shrinkage stress, the long-term performance of the composite resin restorations is an extra source of concern, as other factors such as thermal changes are likely to challenge the adhesive interface.Thermal cycling is a commonly adopted in vitro aging procedure and, despite the variety of regimens described in a review of 130 studies, it generally increases marginal leakage 11 and was also reported to increase the gap width at the interface 20 .
Another issue of major concern is water sorption, which has been considered to cause the degradation of resin-dentin bonding 13 .Water sorption was also reported to decrease the marginal gap width, especially in microfilled composites 20 , and allow relaxation of polymerization shrinkage shear stress 8 .However, some caution must be taken in considering this potentially positive effect as the composite resin's selection depends on other features such as its mechanical properties, which could be compromised.
It is expected, for two composite resins with similar polymerization shrinkage, that the one with higher elastic modulus would generate more stress build-up at the interface with equivalent bond quality.It is still unknown whether this relationship remains with a variable adhesion substrate such as the dentin and the consequences after thermal aging as time dependent thermal stress σ (t) is also significantly dependent on the elasticity modulus (E) 23 .
The objective of this study was to evaluate in vitro the effect of two incremental filling techniques using composite resins of different elastic modulus and similar polymerization shrinkage on leakage of Class V restorations.The effect of thermal cycling was also assessed.
A 1%-chloramine solution was used as a disinfectant medium for the 80 bovine incisors used in this study.A standardized class V (4x4x2mm) cavity was prepared in the buccal surface of each tooth.The cavities were pumiced, acid etched (15s) and the adhesive (Single Bond -3M ESPE, St Paul, MN, USA) was applied according to the manufacturer's instructions.The teeth were restored in axial or oblique increments as represented in Figure 1  After the respective storage period, the specimens had their apical foramina blocked by sticky wax and cyanoacrylate (SuperBonder, Henckel Loctite, SP, Brazil) and covered with two coats of nail varnish except for 1mm beyond the restoration's exposed margins.This procedure was followed by immersion in a 50% AgNO 3 solution in a dark vial for 2h, washing in tap water and exposing to a developing solution under fluorescent light for 8h 17 .
Then, they were sectioned twice in buccal-lingual direction and the exposed surfaces (4) were observed at 40x magnification in a light microscope and the respective FIGURE 1-Expermental design images were digitized (Vidcap 32) so that the silver nitrate penetration (mm) could be measured with ImageLab software at the incisal and gingival walls of the cavities.Data were analyzed by a 3-way ANOVA at a confidence level of 95%.The factors considered were: Resin (2 levels), Filling Technique (2 levels) and Aging (2 levels) separately for the incisal and gingival wall data.

Incisal Wall
The factors, Resin and Aging, were significant (p<0.05):Durafill VS infiltrated more than Z-250 -the mean silver nitrate penetrations were 0.27±0.12mmand 0.19±0.12mm,respectively.The thermal cycles increased leakage.No significant difference was detected between the axial and the oblique filling techniques (p>0.05).
The Resin x Technique interaction was significant (p<0.05) and the Da group infiltrated more than all the other combinations (Table 1).The Resin x Aging interaction was also significant (p<0.05); the ZN group presented the lowest silver nitrate penetration and the DA group the highest.Intermediate values were exhibited by the groups DN and ZA, which were not statistically significant different (p>0.05).

Gingival Wall
Similar results were found at the gingival wall considering only the main factors Resin and Aging, which were significant (p<0.05): the mean silver nitrate penetration was 0.68±0.28mmand 0.55±0.19mmfor Durafill VS and Z-250 respectively.Silver nitrate penetration increased from a mean value of 0.49±0.13mm to 0.74±0.27mmwith thermal cycles.The filling technique was not significant (p>0.05).
The second order interaction was significant (p<0.05).Za and Do groups were not affected by thermal cycles as no differences were observed between Not Aged and Aged groups.In contrast, Zo and Da groups were significantly affected by the thermal cycles (p<0.05).

DISCUSSION
The excellent bond to enamel has been shown to survive long water storage and protect the resin-dentin bond against degradation 4 .The comparatively weaker bond to dentin has been reported to result in more severe leakage at the gingival than at the incisal wall of the restorations 5,9,14 .
The present study supports these previous observations, as the mean incisal values of silver nitrate penetration were approximately one third of those found at the gingival wall.Thermal cycling was also reported to be an effective aging process 11 and was included to determine whether the adhesive interface's rupture occurs immediately after the restoration, which could be related to polymerization shrinkage stresses, or as a result of the aging process.
Resin and Aging affected leakage at the incisal and gingival walls.For both substrates, Durafill VS showed higher silver nitrate infiltration than Z-250.These results were not expected in the Not Aged group, because the higher elastic modulus of Z-250 should have produced more stress buildup at the interface than Durafill VS.A possible explanation might be related to composition differences between the composite resins that could have favored chemical compatibility between the resin and adhesive system from the same manufacturer (Z-250 and Single Bond, both manufactured by 3M).
Incompatibility between simplified-step adhesives and chemically-cured or dual-cured composites was reported in some studies 12,19 , but cannot be considered as a general rule 10,16 .Furthermore, incompatibility was suggested to be influenced not only by the chemical composition of the adhesive, but also by that of the composite 12 .A partial incompatibility might be a possible explanation for the higher leakage of Durafill VS, although it was demonstrated that it does not affect light cured composites' bond strength even after thermal cycling 2 .
Thermal stress (σ) was reported to be directly proportional to the elastic modulus (E) and to the thermal expansion coefficient (α).In a restoration, the resultant interfacial stress under thermal variation (σ) is expected to be high if the product of these two properties (E x α) leads to a severe mismatch between the tooth and the composite 23 .Therefore, it is possible that a partial incompatibility remained undetected in previous studies because such mismatch's effect on stress did not exceed the bond strength.
Durafill VS higher leakage is unlikely to be related to this mismatch: a 13GPa 23 elastic modulus was attributed for dentin and 18.5GPa 18 and 6.48GPa 15 for Z-250 and Durafill VS, respectively.Moreover, Z-250's α is expected to be approximately half that of Durafill VS' as it was previously reported for Z-100 (22.5±1.4 x 10 -6 / o C) and Silux Plus (41.6±1.5 x 10 -6 / o C) 22 , composite resins of similar type 25 .As a result, Z-250's thermal stress (σ) would be expected to be about 1.5 higher than Durafill VS' if their bond strength were similar.Therefore, a reduction in Durafill VS' bond strength related to a decreased chemical compatibility with the adhesive might have caused its higher leakage.
Although Technique was not significant either for the incisal or for the gingival wall, the filling technique adversely affected leakage depending on the composite resin and aging, as indicated by the significant interactions.The most severe leakage was observed for Da group while Z-250 restorations were not affected by the filling technique (Table 1).Likewise, the second order interaction at the gingival wall was significant (p<0.05) and thermal cycles adversely affected only the Zo and Da groups.
In previous finite elements studies, the oblique technique was considered to lead to the highest tensile stress buildup to the adhesive interface, while the bulk filling, to the lowest.The gingival-occlusal filling technique (similar to the axial technique in this study) reached an intermediate tensile stress level 21,26 .Therefore, the influence of the filling technique observed in the second order interaction in this study can be related to differences in the tensile stress buildup suggested by finite element models and the possible partial adhesive-resin incompatibility.
For Z-250, the oblique filling technique resulted in lower silver nitrate infiltration when the Not Aged group was considered.However, the high stress level generated during the cure of the composite could have led to more residual stress that caused the interface's rupture after the thermal cycles.For Durafill VS, the oblique technique caused premature failure for the Not Aged without further infiltration detected for the aged groups.
In contrast, for Durafill VS, the axial technique with relatively lower tensile stress might have favored the interface's survival, minimizing the possible partial incompatibility effect in the Not Aged group.However, because the coefficient of thermal expansion of this resin is relatively high 22 , a significant increase in leakage was detected after the thermal cycles.
An additional group with a period of storage in water similar to that of the Aged group without thermal cycles would be helpful to determine the isolated effect of water or temperature on leakage.However, this was beyond the scope of this study, which focused the polymerization shrinkage effect (Not Aged group -4h) and the thermal cycling effect (Aged group -1 week), both dependent on the elastic modulus (E) and the corresponding dimensional changes involved.
As a variety of composite resins and adhesive systems are currently available, no general theoretical models can be used in the selection of a restorative procedure.Further studies are needed to clarify the relative contribution of the factors that rule the amount of polymerization shrinkage stress generated such as the elastic modulus, polymerization rate and shrinkage.
In this study, the microfill composite resin infiltrated more than the hybrid composite independently of its lower elastic modulus and similar polymerization shrinkage.Thus, another factor, such as incompatibility with the adhesive, might have produced its bond strength reduction.The filling technique was observed to adversely affect leakage depending on the composite resin and the aging regimen applied.As aging was found to increase leakage significantly, even at the enamel interface, future studies should investigate the basis for the development of more durable composite resin restorations.
using a light-curing unit (Variable Intensity Polymerizer, Bisco, Schaumburg, IL, USA) at a constant intensity of 600mW/ cm 2 (80s).The restorations were finished with Sof-Lex Pop-On disks (3M ESPE, St. Paul, MN, USA) and randomly divided into two groups: Not Aged (4-hour-storage in water at 37 o C) and Aged (1-week storage in water at 37 o C + 1000 thermal cycles with baths of 5 o and 55 o C with 1-min dwell time).

TABLE 1 -
Silver Nitrate Leakage at the Incisal and Gilgival wall