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Resin-based composite for Class IV restoration is a conservative alternative for maxillary incisor fracture. Little is known about the effect of lingual margin configurations on the longevity of these restorations. This in vitro experiment compared the mean fracture strength among four lingual margin configurations (butt joint, 45˚ bevel, 60˚ bevel and chamfer) for Class IV resin-based composite restorations. A total sample size of n=100 human extracted lower incisors were selected, then the teeth were randomly assigned to one of the four lingual margin configuration groups (n=25) and restored with resin-based composite. After thermocycling (5000 cycles, 5°C-55°C with 30 seconds dwell time), they were subjected to inter-incisal static load (135˚ angulation) until failure (N). Failure mode was determined. In vitro fracture strength was compared among the four groups using oneway ANOVA at alpha=0.05. Mean standard deviation of fracture strength and frequency distribution of failure modes were reported. Results revealed no significant effect on the fracture strength for the type of lingual margin configurations (F(3,96)=0.13; p=0.9435). The data showed that 71% of failure modes resulted in complete tooth fracture (intact restoration), 11% in total adhesive failure, 7% in adhesive only facial, 6% in total cohesive, 4% in cohesive only facial, 1% in avulsion. Fischer's exact test revealed no statistically significant association (p>0.05) between the margin configurations and failure modes. Within the limitations of this study, it can be concluded that any of the four lingual margin configurations are acceptable in Class IV preparation in terms of fracture strength under static load. v

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