| Abstract|| |
The purpose of this in-vitro study was to compare the amount of microleakage occurring in cervical restorations in groups of extracted teeth restored with four different types of resin composite materials, representing four basic categories widely used by restorative dentists today. This comparison was made, both with and without the application of cyclic lateral forces. A testing instrument developed specifically to reproduce cyclic lateral forces was utilized. The amount of microleakage that occurred was compared, both with and without the application of the forces. Within the limitations of the study, we found that the packable and the hybrid resin composite material exhibited significantly more leakage than either the flowable or the microfilled resin composite. This was observed in both the fatigued and the non-fatigued specimens. This finding supported the theory that the lower modulus of elasticity of flowable and microfilled resin composites allows more flexure of the restoration during the flexure of the tooth, that results from its reciprocal movements. The non-fatigued specimens restored with flowable and microfilled resin composite material had less microleakage than did those restored with the hybrid resin composite material. The findings in the study supported the possibility that lower modulus of elasticity of the microfilled and flowable resin composite materials may have some beneficial effects in reducing the stress placed on the adhesive bond at the resin tooth interface during the polymerization phase.
Keywords: Microleakage, microfilled resin composites, flowable resin composites, packable resin composite, hybrid resin composites
|How to cite this article:|
Alberto RF, de Ataide Id. Effect of cyclical lateral forces on microleakage of cervical resin composite restorations. J Conserv Dent 2007;10:5-13
|How to cite this URL:|
Alberto RF, de Ataide Id. Effect of cyclical lateral forces on microleakage of cervical resin composite restorations. J Conserv Dent [serial online] 2007 [cited 2019 Nov 19];10:5-13. Available from: http://www.jcd.org.in/text.asp?2007/10/1/5/42274
| Introduction|| |
Microleakage is defined as 'the clinically undetectable passage of bacteria and bacterial products, fluid molecules or ions from the oral environment along the various gaps present in the cavity restoration interface'. It has been established that a minimum of 1.0 pin space is definitely left at the tooth restoration interface even after employing the adhesive liners and bases  .
Many suggestions have been made as to factors that may be manipulated to reduce the occurrence of microleakage around the cavosurface margins of cervical resin composite restorations. Some studies suggest that an important factor in the occurrence of microleakage is the amount of stress created during the polymerization shrinkage of the resin composite material ,, . Suggestions for how this polymerization stress may be reduced include: alterations in the type, application and intensity of the curing light , ;the incremental layering of the resin during insertion , ; and the manipulation of the physical or chemical make up of the restorative material to alter the ability of the material to withstand plastic deformation during polymerization , .
Other studies point to stresses created by actual flexure of the tooth during natural function as a factor facilitating microleakage , . Some have suggested the utilization of a more flexible restorative material to help absorb the stresses caused by this tooth flexure thus reducing microleakage in cervical restorations. Lee & Eakle  in 1984 were the first to suggest that lateral occlusal forces on teeth could be a causative factor in the development of non-carious cervical lesions.
A photo elastic study by Kuroe et al  in 2000 found that an unrestored cervical lesion concentrated the stress of occlusal forces at the apex of the notch type lesion. The same study indicated that the stresses at the apex of the notch are reduced following the restoration of the lesion but a new concentration of force then develops, in the area of the occlusal and gingival margins of the restorations. These stresses can have an effect on the interface between the tooth and a restoration placed in the cervical region. The constant exertion of compressive and tensile stresses on the tooth restoration interface may lead to an increase in the amount of microleakage because of deterioration of marginal integrity or the actual dislodgement of the restorative material , .
When a bonded restoration is placed in the cervical area, it is critical that the materials used to restore the defect possess physical properties that can withstand the stresses created by tooth flexure. Studies in the past have suggested that restorative materials with a low Young's modulus of elasticity are more flexible. These materials with a low Young's modulus of elasticity are able to absorb some stresses induced by the tooth flexure, thereby decreasing the amount of microleakage or debonding that may be created by heavy lateral forces from occlusal interferences ,, .
| Materials and Methods|| |
Eighty human maxillary premolars, free of caries were selected for the study. The teeth selected were of patients in the age group of 12 to 18 years who underwent extraction for their orthodontic treatment. The extracted teeth were stored in 10% Formalin for a period of one month before they were used in this study.
Mounting and Preparation of the specimen:
Notch shaped preparations were made in the buccal surface of each tooth using an air rotor hand piece with coarse grit, flame shaped diamond abrasive point. The occlusal cavosurface margin was prepared in enamel and the gingival cavosurface margin was prepared 1.0 mm apical to the cementoenamel junction. The occluso-gingival height of the preparation was 3.0 mm and the axial depth was 1.5 mm at the deepest point of the wedge-shaped preparation.
The roots of the selected teeth were notched horizontally with a carbide bur. Each tooth was oriented so that the long axis of the tooth was parallel to the tabletop and placed into a Poly Vinyl Chloride ring. Auto polymerizing resin was then used to anchor it in place. The acrylic resin surrounded the root up to a level 1.0 mm below the gingival cavosurface margin of the resin- composite restoration. The specimens were allowed to set in water at 24°C for 24 hours.
Following the completion of the cervical preparation, the 80 teeth were randomly divided into four groups of 20 each in number. Each of these groups received a different restorative material. The four types of resin composites were 3M Filtek P60 (Packable), 3M Filtek Z250 (Hybrid), 3M ESPE Filtek Flow (Flowable) and 3M Filtek A110 (Microfilled). Each of these materials was used according to the manufacturer's instructions.
The prepared cavities were etched with 3M Scotchbond Etching Gel (3M ESPE) containing 35% phosphoric acid for 15 seconds and then rinsed for 10 seconds. Using a fully saturated brush tip, a coat of 3M Single Bond Adhesive system (3M ESPE) was applied to the etched enamel and dentin. After gently drying for 2-5 seconds it was light cured for 10 seconds. A second coat of the adhesive was applied and light cured immediately for another 10 seconds. The resin composites were placed into the preparations in three increments. Each increment was thoroughly light cured prior to placement of the next increment. Curing time for each material was as per the recommendations of the manufacture. Excess restorative material was removed using finishing burs. The restorations were then polished with Sof-Lex abrasive disks (3M ESPE).
The wedge-shaped defects were restored with 3M Filtek P60, which is a visible, light activated, radiopaque, restorative composite resin designed to be used in posterior restorations. The filler in Filtek P60 is zirconia/silica. The inorganic filler loading is 61% by volume. The inorganic filler loading is 61% by volume (without silane treatment) with a particle size of 0.19 to 3.3 microns. It contains BIS-GMA, UDMA and BIS-EMA resins. The modulus of elasticity is 11 GPa(Giga Pascals)
The wedge shaped defects were restored with 3M Filtek Z250, which is a visible, light activated, radiopaque, restorative composite designed to be used in both anterior and posterior restorations. The filler in Filtek Z250 is zirconia/silica. The inorganic filler loading is 60% by volume (without silane treatment) with a particle size of 0.Olto 3.5 microns. It contains BIS-GMA, UDMAand BIS-EMAresins. The modulus of elasticity is 11000 MPa.
The wedge shaped defects were restored with 3M ESPE Filtek Flow restorative, which is a low viscosity, visible light activated, radiopaque flowable composite resin. The filler in Filtek Flow restorative is zirconia/silica. The inorganic filler loading is 47% by volume with a particle size of 0.01 to 6 microns. The average particle size for the filler is 1.5 microns. It contains Bis-GMA, and TEGDMA resins. The modulus of elasticity is 6500 MPa.
The wedge shaped defects were restored with 3M Filtek A110 anterior restorative, which is a visiblelight activated, microfilled restorative designed for use in non-stress bearing restorations where esthetics are of prime concern. Restorative material contains 40% colloidal silica by volume with an average particle size of 0.04 microns (the particle size range is 0.01-0.09 microns). The resin system consists of BIS-GMA and TEGDMA. The modulus of elasticity is 5700 MPa.
Cyclic Fatigue testing:
Each group of 20 teeth was then divided into groups of ten each. Ten of each group were not subjected to cyclic fatigue stresses (non fatigued control specimens and the other 10 were subjected to fatigue stresses (fatigued specimens).
The Cyclic Loading Apparatus was designed like the one suggested by Fruits et al (2002)26. The tooth specimen was mounted so that the force from the bumper was applied to the occlusal half of the clinical crown. With the specimen in a fixed position the drums rotated and contacted either the buccal or lingual surface of the specimen thus cyclically loading the specimen in the buccal and lingual directions. The specimen recovered under no load and was then loaded from the opposite direction. The disc rotated at 35 rpm and allowed a recovery time of 0.28 seconds between each impact. Each specimen was subjected approximately 8,400 cycles and then removed.
The tooth specimen was mounted so that the force from the bumper was applied to the occlusal half of the clinical crown. With the specimen in a fixed position the drums rotated and contacted either the buccal or lingual surface of the specimen thus cyclically loading the specimen in the buccal and lingual directions. The specimen recovered under no load and was then loaded from the opposite direction. The disc rotated at 35 rpm and allowed a recovery time of 0.28 seconds between each impact. Each specimen was subjected approximately 8,400 cycles and then removed.
Both, the 10 non-fatigued and the 10 fatigued specimens of each group were then stained and microscopically examined. Prior to staining, the surfaces of all teeth in each group were stained with finger nail polish to prevent extraneous leakage of the dye into the tooth structure. The juncture of the acrylic resin and the root surface was also sealed to prevent leakage between the acrylic resin and the tooth. All specimens were stored in 0.5 % Basic Fuchsin dye for 24 hours at 24°C. The specimens were then stored in water at 24°C until sectioning was accomplished. Each specimen was sagittally sectioned with diamond-bladed circular saw mounted on a micromotor handpiece.
| Examination|| |
The sectioned halves were examined under 4x magnification with a stereomicroscope for leakage of the dye between the resin-tooth interface. The degree of marginal leakage was evaluated with the use of the following rating criteria, adapted from a system used by Willer et al  .
0 = no evidence of penetration at the interface of the restoration and the tooth.
1 = penetration less than or equal to one sixth of the interface of the restoration and the tooth.
2 = penetration less than or equal to one third of the interface of the restoration and the tooth.
3 = penetration less than or equal to one half of the interface of the restoration and the tooth.
4 = penetration less than or equal to two thirds of the interface of the restoration and the tooth.
5 = penetration less than or equal to five sixths of the interface of the restoration and the tooth.
6 = penetration less than or equal to the entire interface of the restoration and the tooth.
| Discussions|| |
Microleakage - the passage of bacteria, fluids, chemical substances, molecules and ions between the tooth and its restoration-is an intrinsic problem of direct filling gold, amalgam, resin and cast metal restorations and is clinically undetectable. Microleakage is used as a measure by which clinicians and researchers can predict the performance of the restorative materials in the oral environment. The importance placed on this measure is based on the fact that no available restorative material is perfectly adaptive or adhesive to the tooth  .
The most notable factors that contribute to marginal leakage of composites are
1. Composite resin restorations are very much technique sensitive. Any step that is deviation from the actual procedure leads to failure, including an increase in marginal leakage  .
2. Marginal gaps formed at the tooth restoration interface primarily result from dimensional changes like polymerization shrinkage of the setting resin. After the resin has set, masticatory forces, thermal changes and water absorption further affect the size and shape of the gaps  .
| Observations & Results|| |
a) All composite restorative resins shrink during polymerization. The volumetric polymerization shrinkage usually is in the range of 1.67-5.68%; the lesser being for the light activated ones. In case of bonding agents being used to bind the restorative resin to the tooth structure, shrinkage results in the development of tensile and/or shear stresses at the tooth restoration interface. Within certain limits, the adhesive bond is able to withstand these stresses. Once the stresses exceed the bond strength and elastic deformation of the combined system, a separation at the tooth restoration interface may occur leading to microleakage. It is usually the bond with dentin that is compromised during shrinkage, whereas the bond with enamel is generally strong to withstand the same amount of forces.
b) Functional stress incurred on restorations by cyclic mastication is another factor in inducing micro leakage of resin restorations. Occlusal stresses enhance leakage because of repeated plastic or elastic deformation of the restoration.
c) The marked difference in the coefficient of thermal expansion of restorative resins and tooth structure also has a detrimental effect on adhesion. The composites have a higher coefficient of thermal expansion compared to that of the tooth.
Coefficient of thermal expansion of composites
=20-25 x 10 -6 /°C
Coefficient of thermal expansion of enamel
= 11.4 x 10 -6 /°C
Coefficient ofthermal expansion of dentin
= 8.3 x 10 -6 /° C
Combined thermal and occlusal stresses have shown to induce more microleakage as compared to leakage induced by the same stresses individually.
d) Composite resin restorations have a tendency to absorb water from the environment, causing the restoration to expand. Thus the property of water sorption is able to counteract polymerization shrinkage to a little extent. In contrast to the polymerization contraction stresses, which are generated at a rapid rate, relief of stresses by hygroscopic expansion proceeds more slowly. The resins with the largest quantity of filler have the least water sorption (water uptake is the property of the resin component of the matrix). It should be stressed here that though water sorption may improve marginal adaptation of composite resin, it impairs its mechanical properties.
The pathology of cervical lesions of teeth is various and multifactorial. The chemical erosion theory explains how various types of acids decalcify the enamel. Chemical erosions are generally caused by acids from dietary sources, the environment and the stomach. According to Jones (1989) regurgitation can introduce endogenic acids into the mouth and it has been suggested by Bevenius et al (1988) that impaired salivary action may exacerbate the severity of the lesion. Acid erosions show tooth structure loss over a wide area with no sharp line angles while idiopathic cervical erosions are generally wedge shaped and limited to the cervical area of teeth  .
A second theory by Bergstrom et al (1988) is based on abrasive wear induced by tooth brushing and dentifrice. In this process the particle size and hardness of the abrasive particles in the toothpaste, the pressure exerted and the frequency of brushing are of primary concern  .
A third theory proposed by Lee and Eakle (1984)  suggested the possible etiologic factor in cervical erosion is the tensile stress caused by mastication and malocclusion. The local causes play a secondary role in dissolution of the tooth structure to create the lesion. Lateral forces can create tensile stresses that disrupt hydroxyapatite crystals in the enamel, allowing small molecules, such as those of water, to penetrate and render these crystals more susceptible to chemical attack and further deterioration.
Lateral forces create cervical regions of tension and compression, as indicated by the arrows. The magnified section depicts disruption of chemical bonds between enamel rods. Small molecules enter between hydroxyapatite crystals and prevent reestablishment of bonds to make crystals more susceptible to breakage and chemical dissolution.
The first two theories are well documented but the stress theory by Lee and Eakle (1984)  remains controversial. According to the latter, a lesion created as a result of tensile stresses should possess certain characteristics:
1 . The lesion should be at or near the cemento-enamel junction, which is the fulcrum.
2. The region of greatest stress would be a wedge-shaped area at the fulcrum, which is the typical morphology of cervical erosive lesions.
3. The directions of the lateral force that generates the tensile stress would determine the location of the lesion. For example, if there were two directions of lateral forces that are acting on the same tooth, the created lesion would be a combination of two lesions generated by each of the two forces.
4. The size of the lesion would be directly related to the magnitude and frequency of application of the tensile force.
The morphology of the occlusal surfaces of the teeth (buccal and lingual inclined planes and fossae) is such that most forces generated during functional and parafunctional jaw movements are directed along the long axis of teeth or in a bucco-lingual direction. The presence of adjacent teeth can help to direct forces bucco-lingually. When third molars are missing, second molars do not develop erosions on the distal surfaces because the major forces are directed axially or bucco-lingually; and the adjacent first molar prohibits mesial bending of the tooth, which would be necessary to generate tensile stresses on the distal surface.
The recovery time was reported in the study, in an effort to be as descriptive as we could of our method. The presence of a recovery time, however, would simulate a natural functional occlusal cycle. The teeth are normally engaged in one or the other direction and then separated before the next loading force is applied during mastication.
The recovery time in the study could be controlled by two factors:
1. The space between the contact bumpers on the revolving wheel of the apparatus
2. The speed at which the revolving wheel (with the contact bumpers on it) turned.
Since the wheel had a fixed diameter, the space between the bumpers could not be easily changed. The amount of recovery time was controlled by adjusting the rpm of the wheel itself. It was adjusted until it was felt that there was adequate time for recovery between impacts. This was strictly based on the judgment. There is no evidence that this is the ideal time to replicate the masticatory process accurately. The time between impacts in the mouth would obviously not be this short. However, in the lab this process was speeded up to allow more specimens to be subjected to the fatigue test in a shorter time. Currently, there is some controversy concerning the benefits of utilizing a resin composite restorative material with a low modulus of elasticity for cervical lesions. Some studies have indicated that the use of a material with a low modulus of elasticity will reduce the formation of cervical gaps and marginal leakage. Other studies have indicated that there may be little if any significant difference in the amount of micro leakage in relation to the modulus of elasticity. The present study was designed to specifically investigate the effects of tooth flexure created by lateral cyclic loading on the amount of micro leakage observed in cervical lesions restored with materials of varying rigidity.
Almost all of the specimens from both the control groups and the fatigued groups showed some signs of staining caused by leakage. This may have been the result of contraction shrinkage during curing, which caused the resin to pull away from the cavosurface margin. These restorations were placed in extracted teeth. The manufacturer's suggested instructions for "wet technique" bonding for in vitro testing were followed, but bench top studies usually cannot duplicate the environment on the surface of a vital tooth.
A restorative material with a low modulus of elasticity is usually considered to be more flexible and thus may be able to absorb some of the stress induced by the tooth flexure. This absorption could, theoretically, decrease the amount of micro leakage ordebonding created by tensile forces. The flowable resin composite and a microfilled resin composite material have a much lower modulus of elasticity and ought to be a more flexible material.
Hybrid resin composite and packable resin composite material exhibited significantly more leakage than either the flowable or the microfilled resin composite. This was observed in both the fatigued and the non-fatigued specimens. This finding supports the theory that the lower modulus of elasticity of the flowable or the microfilled resin composites allows more flexure of the restoration during the flexure of the tooth that results from its reciprocal movements. The decreased rigidity of these resin composites may also decrease both the stress created by tooth flexure and the polymerization contraction stress placed on the adhesive bond at the resin-tooth interface.
The statistical analysis also revealed that the amount of microleakage was significantly different among the four groups. The cyclically loaded specimens restored with microfilled and flowable composites did exhibit a lower amount of microleakage than those restored with the hybrid resin composite and packable composite; however the difference was not statistically significant.
The control specimens restored with the flowable resin composite material had less microleakage than did those restored with hybrid resin composite material. This finding supports the possibility that the lower modulus of elasticity of the microfilled and flowable resin composite material may have some beneficial effects in reducing stress placed on the adhesive bond at the resin-tooth interface during polymerization phase.
A study by Condon and Ferracane (2000)  suggested that composites with a composition of lower levels of filler particles are less likely to produce high levels of polymerization stress during placement. There has been some discussion that a longer duration of the 'gel' phase during the polymerization of certain resin composites may allow a release of contraction stress , . Whether this may be the cause of the difference observed in the polymerization stress, is beyond the scope of the present study.
| Conclusion|| |
Within the limitations of the study the following conclusions can be drawn:
1. Among the specimens that were subjected to cyclic fatigue stresses, those restored with packable resin composites and hybrid resin composites exhibited significantly more leakage at the restoration tooth interface than the ones restored with flowable resin composites and microfilled resin composites.
2. Among the specimens that were not subjected to cyclic fatigue stresses, those restored with packable resin composites and hybrid resin composites exhibited significantly more leakage at the restoration tooth interface than did those restored with either flowable resin composites or microfilled resin composites.
The occurrence of microleakage is used to evaluate the success of restorative materials and procedures. While restorative materials and techniques have improved, manipulation of the materials continues to affect the success of the restorations. Microleakage as a yardstick can provide much useful information regarding the performance of the restorative materials and procedures, but as a clinical occurrence it remains a primary source of restorative failure.
[Figure 1],[Figure 2],[Figrue 3],[Figure 4],[Figure 5],[Figure 6],[Figure 7],[Figure 8],[Figure 9],[Figure 10],[Table 1],[Table 2].
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Ryan Francisco Alberto
Department of Conservative Dentistry, Goa Dental College & Hospital, Bambolim, Goa 403202
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figrue 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10]
[Table 1], [Table 2]