610 the journal of prosthetic dentistry volume 81 number 5
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Microleakage of Class V resin modified g
TOLEDANO ET AL
THE JOURNAL OF PROSTHETIC DENTISTRY MAY 1999 613 However, this bonding is not so strong and does not produce an adequate marginal sealing. 20,21 The increase in leakage of Dyract resin composite also could be attributed to thermal expansion mismatch with tooth substance, which is reported to be signifi- cantly higher than that of conventional cements and also less than that of composites, 9,10 perhaps due to their different chemical composition. Leakage of com- posite resin restorations may be attributed to a con- traction gap produced by polymerization shrinkage and expansion and contraction with temperature changes, because the coefficient of thermal expansion of com- posites is different from that of the dental hard tissues. Glass ionomer cements exhibit limited shrinkage dur- ing setting and their coefficient of thermal expansion is similar to that of dentin. 4 Mitra and Conway 9 reported that Fuji II LC and Vitremer materials had coefficients of thermal expansion of 31.5 and 11.5 ppm/°C, respectively, and Silux Plus microfilled composite 56.6 ppm/°C 7 days after curing. Dyract has a composition closely related to the microfilled composites and has a coefficient of thermal expansion of 40.52 ppm/°C (P Hammesfahr, verbal communication, 1998). This may explain why Dyract resin composite is more sus- ceptible to thermal stresses than the other materials. Also, because the resin component of the material adheres poorly to the cervical dentin than to enamel, this justifies, in part, that the Dyract resin composite revealed more leakage at the gingival margin than at the enamel margin. Although Vitremer glass ionomer cement displayed microleakage values between those of Fuji II LC and Dyract materials, there was no statistically significant difference among the 3 materials. Some authors have pointed out that significant dimensional changes and surface hardening can occur after initial light curing of the resin component of resin-modified glass ionomers, and further contraction continues for the first 24 hours as the material matures. 10,11 Because both Vitremer and Fuji II LC glass ionomer cements contain approx- imately the same percentage of resin, which is less than that for Dyract composite, it could be thought that this is another reason to explain the different microleakage patterns. 10,11 Uno et al 16 considered that the differ- ences observed between Vitremer and Fuji II LC glass ionomer cements might be due to differences in matu- ration of setting reactions. Although the results obtained from this study may not be directly extrapolated to the clinical situation, they provide some information regarding the perfor- mance of the restorative system evaluated. Independent long-term clinical data are still required. Download 62.61 Kb. Do'stlaringiz bilan baham: |
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