Fatigue limits for composite restorations with and without glass ionomer cement liners
By Claudine Pereira Assis, DDS
Marcos Ribeiro Moyses, DDS, MSc, PhD
Hercilia Marburg Teixeira, DDS
Jose Carlos Rabelo Ribeiro, DDS, MSc, PhD
Joao Gustavo Rabelo Ribeiro, DDS, MSc, PhD
Sergio Candido Dias, DDS, MSc, PhD
Featured in General Dentistry, September/October 2009
Pg. 485-489
Posted on Friday, September 04, 2009
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This laboratory study compared the flexural endurance limits of clinical combinations of dental composite with and without glass ionomer cement (GIC) liners. Using only composite (Filtek Z350), specimens (10 mm long x 2 mm wide x 2 mm thick) in the control group were produced. Two GICs (Vitremer and Vitrebond) were used with the composite to prepare the test groups. Flexural strength and flexural fatigue limit (FFL) tests were performed. The FFL was determined using the staircase method. Data were analyzed by one-way ANOVA and Tukey’s test.
There was a significant difference in flexural strength values between the composite-only specimens and those produced by composite and GIC (p < 0.05). No statistical difference was observed in the flexural strength values between composite with Vitremer and composite with Vitrebond (p < 0.05). No statistically significant differences were detected in FFL values between composite with Vitremer and composite with Vitrebond; in addition, the mean value of the composite-only specimens differed statistically from those of both composite with Vitremer and composite with Vitrebond (p < 0.05). The FFL was lower than the flexural strength, indicating a decrease in flexural strength of 45–50%. Using GICs with composite decreased the mechanical properties (FFL and flexural strength) of the composite.
Received: November 12, 2008
Accepted: March 11, 2009
Since the very first dental composites were developed, many efforts have been made to improve their clinical performance.1 However, dental composites still have some characteristics that diminish their clinical performance, including polymerization shrinkage and marginal leakage.2 Polymerization shrinkage of dental composite leads to mechanical stresses on enamel and dentin by the adhesive, especially in posterior teeth because of their geometric configuration.3
It has been demonstrated that different resin material components can be released in an adjacent aqueous phase.4 Therefore, when applied to a wet surface (such as dentin), uncured free monomers released from resin-based materials may diffuse across dentinal tubules to reach the pulpal space.5 Released monomers cause chemical damage to cultured cells.6 In addition, uncured resin components that reach the pulpal space have caused noticeable chronic inflammatory response and inner dentinal resorption.7 Paul et al recommended applying biocompatible liners to the pulpal floor of deep cavities before placing adhesive restorations, to prevent any damage to the pulp tissue.7
A particular category of bioactive materials, represented by glass ionomer cements (GICs), has been recommended for a wide range of clinical applications.8 GICs present several important properties that are expected from an ideal restorative material, such as fluoride release, a coefficient of thermal expansion and modulus of elasticity similar to those of dentin, the ability to bond to both enamel and dentin, and biocompatibility.9-12 However, GIC has relatively poor mechanical properties; in areas of high stress, GICs must be protected by dental composite.13 To minimize these problems, Sachdeo et al have recommended combining GIC with dental composite (a process commonly known as sandwich restorations).14
Strength (tensile, shear, compressive, or flexural) values often are relied on as indicators of structural performance for brittle dental materials, including composites.15 Flexural strength is the mechanical property selected by the International Standards Organization for screening resin-based filling materials.16
Since restorative dental composites are subjected to masticatory loading over time, fatigue resistance (especially in stress-bearing areas) is considered an important preclinical screening.17,18 Fatigue in dental restorations is influenced by cyclic masticatory forces and the corrosive attack of oral fluids.19 The flexural fatigue limit (FFL) is more useful than flexural strength for determining clinical events.20
This laboratory study sought to mimic clinical combinations of dental composite with and without GIC liners and to compare their flexural endurance limits as a better means for predicting clinical success.
Materials and methods
This study utilized one dental composite (Filtek Z350, 3M ESPE) and two resin-modified GICs (Vitremer and Vitrebond, both 3M ESPE). Table 1 lists the materials used and their type and composition.
Flexural strength and FFL were determined according to Braem et al.21 The FFL was studied as a percentage of the clamped fracture strength following a staircase method.21,22 To evaluate flexural strength, 10 rectangular specimens of each material were cast in steel molds with internal dimensions of 10 mm x 2 mm x 2 mm (Fig. 1).20,23-25
An opposing matrix (0.6 mm wide) made of silicone was built and placed at the base of the steel mold and a layer of composite (1.4–1.6 thick) was placed in one increment. The composite layer was covered with a polyester strip and a glass slide, then placed under a 1 kg load for 10 seconds to flatten the surface.26 At that point, the load and the glass slide were removed. The composite was photocured (Optilux 401, Kerr Demetron) for 40 seconds (from the top side only); an analogical radiometer (Gnatus) was used to monitor the light intensity between 620 mW/cm2 and 650 mW/cm2.
After curing, the opposing matrix was removed and the remaining space in the mold was filled with 0.4–0.6 mm of GIC. The GIC layer was covered with a polyester strip and a glass strip, then placed under a 1 kg load for 10 seconds to flatten the surface; the load and the glass slide then were removed. As with the composite, the GIC was photocured for 40 seconds from the top side only using the Optilux 401; the analogical radiometer monitored that the light intensity remained between 620 mW/cm2 and 650 mW/cm2. The bar edges were lightly polished to remove stress risers. The specimens for flexural strength tests were stored in distilled water in an incubator (at 37°C) for 24 hours.
The flexural strength tests were performed in an MTS 810 mechanical test machine (MTS Systems Corporation) at a cross-head speed of 1.0 mm/minute, with a loading force of 5 kN. The flexural strength was calculated by the equation flexural strength (in MPa) = 3 x L x D/2 x w x h2, where L is the failure load (in Newtons), D is the distance between the supports (in mm), w is the width of the specimen (in mm), and h is the height of the specimen (in mm). Data were analyzed by one-way ANOVA. Tukey’s test was used for multiple comparisons, with the global significance level set at 5%.
The specimens used to test the FFL were prepared in the same way as those used for the flexural strength test. The FFL was determined by 10,000 cycles under equivalent test conditions at a frequency of 5 Hz. The staircase approach was used to determine fatigue.27,28 If the specimen did not fail within 10,000 cycles, the stress level of the next specimen was increased by 4% of the clamped fracture strength. If the specimen failed, the stress level was decreased by 4%.19 This percentage was established after pilot testing in accordance with the 2005 study by Abe et al.20
The first specimen was tested at approximately 60% of the initial flexural strength value. As the data are concentrated around the mean stress, this method requires fewer specimens than other methods.27,28 The FFL was determined for 20 specimens of composite and 20 specimens of composite with Vitrebond; however, 25 specimens of composite with Vitremer were used due to increased specimen failure.20 Using the MTS 810 mechanical testing machine, tests were conducted under water spray (at 37°C).
The mean FFL was determined using the equation FFL = X0 + d(Σini/Σini + 0.5). The standard deviation (SD) was measured using the equation SD = 1.62d{[Σini Σi2ni – (Σ ini )2/(Σ ni )2] + 0.029}, with X0 indicating the lowest stress level considered in the analysis and d the fixed stress increment. To determine the FFL, the data analysis was based on the least frequent event (failures versus nonfailures). For the FFL equation, a negative sign was used when the analysis was based on failures. The lowest stress level considered was designated as i = 0, the next as i = 1, and so on, and ni was the number of failures or non-failures at the given stress level. The fatigue data were analyzed using ANOVA, while Tukey’s test was used for multiple comparisons, with the global significance level set at 5%.
Results
Tables 2 and 3 present the mean flexural strength and FFL values (in MPa ± SD) of the restorative materials. There was a significant difference in flexural strength values between the composite-only samples and both the composite with Vitremer and composite with Vitrebond samples. No statistical difference was observed for the flexural strength values between composite with Vitremer and composite with Vitrebond. No statistically significant differences in FFL values were detected between composite with Vitremer and composite with Vitrebond, while the composite-only mean value differed statistically from that of composite with Vitremer and composite with Vitrebond.
The relationship between flexural strength and FFL is demonstrated in Table 4. The FFL was lower than the flexural strength for all materials studied, indicating a decrease of 45–50%.
Discussion
The materials tested in the present study varied widely in terms of flexural strength and FFL. The flexural strength reflects the maximum limit of force to which a material is subjected, while the FFL refers to the material’s resistance to cyclic loading, with a force well below the ultimate fracture strength of the material.19
Clinically, dental composite can be subjected to considerable flexural stresses in both anterior and posterior teeth; moreover, these stresses may cause patients with some parafunctional habits (bruxism and clenching) to show a more severe flexural stress.28 The FFL values in the present study are still higher than the stresses generated during normal clinical function (that is, 5–20 MPa) or hard biting.21 Occasionally, higher peak stress may occur (for example, during maximal clenching); in such instances, the stresses may rise to 100 MPa or more and the materials will be stressed beyond their FFL more frequently, resulting in localized or generalized types of fatigue.21
Materials with high flexural strength did not obviously demonstrate the best FFL, as noted by Lohbauer et al.29 The FFL of all materials proved to be approximately 50% of their flexural strength. This decrease in FFL also is reported in the literature.17,19,20,29
Differences between the composition of the composite specimens and the combined composite and GIC specimens affected the results of flexural strength and FFL, indicating that the mechanical properties are highly dependent on the concentration and particle size of the filler.30 In a 1989 article, Drummond stated that crack propagation in composite occurs mainly around or through the second phase particles (inter- or intracrystalline), depending on filler content and interparticle distance.31 When composite and glass ionomer are evaluated separately, composite shows better physical and mechanical characteristics.19,32 The results of the present study demonstrated that GIC combined with composite did not improve flexural strength, while the FFL resulting from this combination was approximately 60% that of composite alone. The reduction of flexural strength and FFL is almost certainly due to an effective reduction in the thickness of the composite in the dual-material (composite and GIC) stick.
Although there are clinical advantages to combining composite and GIC in direct posterior teeth restorations, the present study showed that flexural strength and FFL decreased when compared with the values obtained for composite alone. However, the flexural strength and FFL values still were higher than the stresses generated during clinical functioning.
To correlate mechanical response under the influence of water with the chemical nature of the test materials under investigation, additional research is necessary to assess the FFL of composite used in combination with GIC as a function of storage time in a simulated oral environment.
Conclusion
Within the limits of the present investigation and the method used in this study, the association of glass ionomer with composite decreased the mechanical properties (FFL and flexural strength) of the composite. There were statistically significant differences in both flexural strength and FFL values when GICs and composite were combined, compared to composite used alone.
Acknowledgements
This in vitro study was supported in part by PROSUP CAPES/Vale do Rio Verde University—UNINCOR No. 32021011.
Disclaimer
The authors have no relationship with any of the manufacturers listed in this article.
Author information
Drs. Assis and Teixeira are graduate students, Department of Restorative Dentistry, Dental School of Tres Coracoes, Vale do Rio Verde University, UNINCOR, MG, Brazil, where Drs. Moyses, Jose Ribeiro, Joao Ribeiro, and Dias are professors.
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Manufacturers
Gnatus, Ribeirao Preto, SP, Brazil; 55.16.2102.5000, gnatus.com.br
Kerr Demetron, Orange, CA; 800.537.7123, www.kerrdental.com
MTS Systems Corporation, Eden Prairie, MN; 800.328.2255, www.mts.com
3M ESPE, St. Paul, MN; 888.364.3577, www.3mespe.com
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General Dentistry, September/October 2009 , Volume 57 , Issue 5
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