[The influence of hygroscopic expansion on nanohybrid composite restorations]

Hygroscopic expansion of resin materials is a common phenomena due to water adsorption into the resin matrix. This expansion is dependent upon the chemical structure of the material. Water sorption of resin dental material may contribute to stress release immediately after polymerization shrinkage has occurred. The aim of this study was to evaluate the influence of water storage on nanohybrid composite by assessment of marginal enamel sealing of restorations.

30 human teeth extracted for periodontal or orthodontic reasons were used in these study. Cavities were prepared and restored using a nanohybrid composite (Ceramix Mono, Dentsply DeTrey) and a total-etch adhesive system (Prime&Bond NT, Dentsply DeTrey). The teeth were then randomly divided in three groups which were stored for various times in deionized water (one hour, 24 hours and 7 days). After the storage period, the teeth were immersed in methylen blue 1% for 24 h. Then specimens were axially cut and the dye penetration within the occlusal enamel interface was evaluated using optic microscope.

Kruskal-Wallis test showed significant differences of dye penetration between the long-term hydrated groups and the group hydrated for only one hour (p < 0.05). The mean value of dye penetration decreased from 0.04 mm to 0.1 mm after water storage for both 24 hours and 7 days. However, good sealing in enamel margins of restorations was observed even without hydration.

Our results confirm that nanohybrid composites used with total etch adhesives systems have good ability to seal the enamel margins and water sorption can improve this sealing.