SHEAR BOND STRENGTH AND INTERFACIAL MICROSCOPIC EXAMINATION OF TWO TYPES OF FLOWABLE RESIN-COMPOSITE TO ENAMEL SURFACE OF DECIDUOUS MOLARS

Aim: Evaluation of shear bond strength of a self-adhering flowable resin-composite versus total-etch one to enamel surface of deciduous molars. Interfaces between these restorations and primary enamel were also microscopically investigated.
Materials and Methods: Twenty freshly extracted human deciduous molars were used. For the shear bond strength, twelve molars were embedded in acrylic blocks, such that their buccal surfaces were aligned with the acrylic. The enamel surfaces were subjected to minimal grinding. The teeth were randomly divided into two groups, Group I: Self-adhering flowable resin-composite (Dyad™-flow, Kerr, USA); Group II: Total-etch flowable resin-composite necessitate etching and bonding (Filtek™Z350-XT, 3M-ESPE, USA). A specially designed holed-split Teflon mold was used for constructing resin-composite cylinders (3x3mm) over the buccal surfaces of the mounted teeth. For group I, resin-composite was applied directly on teeth surfaces using the mold and light-cured for 20seconds. For group II, using the mold the following steps were performed: acid etching (15 seconds), 2) bonding agent (light-curing 20 seconds) and 3) resin-composite (light-curing 20 seconds). The teeth were stored in 37°C distillate water for 24 hours. The shear bond strength was recorded and statistically analyzed. Modes of failure were studied using digital microscope. For interfacial examination, cavities (class V) were prepared in buccal surface of eight teeth, filled by the two flowable composites as previous (n=4/group) and scanned using scanning electron microscope. For each group, two teeth were examined from buccal aspect, while the other two were sectioned and inspected.
Results: Mean bond strength values for groups I and II were 5 and 21.6 MPa respectively with highly significant difference P=0.005 (P value ≤ 0.01). Modes of failure for groups I and II were [100%adhesive] and [16.7%cohesive within tooth + 83.33%mixed] respectively. SEM micrographs of group I revealed a gap at enamel-restoration interface, while group II showed cohesive failure within enamel at the margins.
Conclusions: The bonding performance of the self-adhering resin-composite “Dyad™-flow” still needs further enhancement. Modifications may be required to prevent marginal enamel cracks with the use of Total-etch “Filtek™Z350-XT” flowable resin-composite.