Alomari, Nusair, Ali IntroducIno One major drawback of resin composite re- storative materials is the polymerization shrink- age caused by the dimensional rearrangement of monomers into polymer chains during polym- erization reaction.1 Clinically, the polymerization shrinkage is restrained by the developing bond of the restorative material to the cavity walls. This restriction induces polymerization shrinkage stress, which counteracts the developing resin- tooth bond by pulling the setting resin composite material away from the cavity walls. Failure of the bond at the tooth-resin interface will result in mi- croleakage and postoperative sensitivity.2,3 In ad- dition, it was shown that polymerization shrinkage stress tremendously weakens the performance and longevity of dental composites.4 Numerous approaches have been proposed to minimize the shrinkage stress by manipulation of curing proto- cols and placement techniques.5 Among these ap- proaches, soft-start curing (which can be either in a step or a ramp mode) and pulse curing have at- tracted extensive investigations.6-11 The hypothesis for advocating these curing protocols is that the initial low-light intensity could facilitate a certain degree of shrinkage stress relaxation before the system reaches the vitrification stage and while in the pre-gel phase. 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 re- lief advantage of soft-start curing might be in fact a result of reduction in the degree of conversion of the resin.11-13 Previous studies have shown that polymeriza- tion stress magnitude is influenced by the charac- teristics of the cavity to be restored.14-17 The cavity configuration or C-factor is defined as the ratio of the bonded to the unbonded surface area.17 Dur- ing light-induced polymerization of resin compos- ite, the shrinkage forces in high C-factor cavities cannot be relieved by resin flow, resulting in the debonding of one or more walls.14-19 Using differ- ent adhesive systems, it has been found that the C-factor of the cavity negatively affect the micro- tensile bond strength to dentin.20 In addition to conventional halogen-based light activation units, light-emitting diode (LED) has October 2011 - Vol.5 been introduced for the polymerization of resin composite restorations. LEDs hold several advan- tages over halogen-based units, including hav- ing 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 com- posite restorations. Therefore, the objective of this study was to compare the amounts of microleak- age observed when using two different modes of LED curing light (fast and soft-start) for two de- signs 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, disinfect- ed 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 tu- bules, 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 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 car- bide 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. The bur was replaced every five cavities. The cavi- ties were prepared in enamel and dentin at the cemento-enamel junction (CEJ). The deepest part of the cavities was at the CEJ (2 mm deep). The coronal slope was in enamel and ended at a 2 mm distance from the CEJ. The apical slope was in dentin and ended at a 2 mm distance from the 401 European Journal of Dentistry