Laboratory research showed that soft-start polymerization partially relieves shrinkage stress and achieves improved marginal integrity of the restoration.10 On the other hand; some researchers have showed that the stress relief advantage of soft-start curing might be in fact a unlikely result of reduction in the degree of conversion of the resin.11�C13 Previous studies have shown that polymerization stress magnitude is influenced by the characteristics of the cavity to be restored.14�C17 The cavity configuration or C-factor is defined as the ratio of the bonded to the unbonded surface area.17 During light-induced polymerization of resin composite, the shrinkage forces in high C-factor cavities cannot be relieved by resin flow, resulting in the debonding of one or more walls.

14�C19 Using different adhesive systems, it has been found that the C-factor of the cavity negatively affect the microtensile bond strength to dentin.20 In addition to conventional halogen-based light activation units, light-emitting diode (LED) has been introduced for the polymerization of resin composite restorations. LEDs hold several advantages over halogen-based units, including having extended lifetimes, more light efficiency, not requiring filters, and having higher resistance to shock and vibration.9,21 No studies have yet examined the combined effects of C-factor and soft-start mode of LED curing light on microleakage of class V resin composite restorations. Therefore, the objective of this study was to compare the amounts of microleakage observed when using two different modes of LED curing light (fast and soft-start) for two designs of class V cavity preparation: a V-shaped cavity design and a box-shaped cavity design.

The hypothesis tested was that combining the effect of fast curing mode and high C-factor cavity will not affect the degree of microleakage around class V resin composite restorations. MATERIALS AND METHODS Eighty freshly extracted non-carious human maxillary premolar teeth were used in the study. After extraction, the teeth were cleaned, disinfected in 10% formalin for two weeks and then stored in physiological saline. Only undamaged teeth were used. To ensure that no dye would reach the cavities via the pulp chamber and the dentinal tubules, the apical foramina were sealed as follows.

The root apices were cut off; then, a cavity was prepared at the cut root apices with an inverted cone carbide bur (Meisinger, Hager & Meisinger GmbH, Germany) and restored with glass ionomer restorative material (Fuji II Cilengitide LC, GC Corporation, Tokyo, Japan). The teeth were randomly divided into four equal groups (Table 1). In groups I and II, V-shaped class V cavities were prepared on the vestibular surfaces of the teeth, using straight fissure carbide bur, size 57 (American Numbering system) (DENTSPLY International, 221 W. Philadelphia Street, York, PA 17405-0872; USA; Lot # 401809) at high speed handpiece and with water-cooling.