Effects of Load Cycling on the Microleakage of Beveled and Nonbeveled Margins in Class V Resin-Based Composite Restorations

Abstract

This study evaluated the influence of mechanical loading and thermocycling on microleakage of class V resin-based composite restorations with and without enamel bevel. Sixty class V cavity preparations measuring 3.0 mm wide (mesio-gingivally) x 2.0 mm high (occluso-gingivally) x 1.5 mm deep with the occlusal margin in enamel and the gingival margin in cementum were prepared on the buccal surfaces of human premolars using a #12 diamond round bur (Drendel and Zweiling Diamant GmbH, Lemgo, Germany) in a high-speed, water-cooled handpiece. The specimens were then divided into two groups of 30 specimens each, based on the type of enamel cavosurface margin configuration as beveled or nonbeveled (butt joint). After restoring the preparations with a flowable resin-based composite (Tetric Flow, Ivoclar Vivadent-AG, Schaan, Liechtenstein) and finishing and polishing with sequential discs (Sof-Lex Pop-on, 3M-ESPE, St. Paul, MN, USA), the teeth were stored at 37° and 100 percent humidity. Twenty-four hours later, half of the specimens in each group (nonbeveled "N" or beveled "B") were exposed to a cycling loading for 250,000 cycles to simulate occlusal loading and assigned to two subgroups (NL+ or BL+), while the remainder of the specimens in each group were only maintained in a 100-percent-humidity environment, without any cyclical loading, until tested (NL+ or BL+). The specimens were sealed with sticky wax (Kemdent, Associated Dental Products, Swindon, UK) and nail polish. The apical foramen of each tooth was sealed with sticky wax and the rest of the tooth was covered with nail varnish, except for an area within 1.0 mm around the composite restoration. To detect marginal leakage, all of the samples were stored in a 0.5 percent basic fuchsine solution for 24 hours. The specimens were then sectioned longitudinally using a low-speed diamond blade (IsoMet, Buehler Ltd., Lake Bluff, IL, USA), machined, and evaluated under 25X magnification using a stereomicroscope (M9, Wild Heerbrugg, Switzerland). The specimens were scored on a scale from 1 to 4 on the degree of dye penetration. The qualitative data were analyzed by the Mann-Whitney U test at a 5 percent significance level (p<0.05). The null hypothesis of this study was that there is no difference in microleakage between beveled and nonbeveled class V buccal preparations in premolar teeth restored with resin-based composite and subjected to simulated occlusal loading and thermocycling. In each group the gingival margin showed significantly more microleakage than the enamel margin (p<0.05). Load cycling did not result in an increase in microleakage in nonbeveled (p=0.259) or in beveled (p=0.053) occlusal margins. However, the gingival margins showed a statistically significant difference in microleakage after load cycling whether in cavities with enamel occlusal bevel (p=0.004) or in groups without a bevel. This means the enamel margin configuration of the enamel occlusal margin had no effect on decreasing microleakage in the gingival aspect of class V composite restorations. In general, the nonbeveled preparations in this study had significantly less microleakage than the bevel specimens whether they were loaded occlusally or not (p=0.001). Within the limitations of this in vitro study, no benefit was derived from placing an enamel cavosurface bevel on the occlusal margin of a standardized class V composite restoration located at the cementoenamel junction. The most important consideration is to prevent microleakage along the gingival margin regardless of whether the occlusal enamel margin is beveled.