Edge strength of resin-composite margins

Objectives Marginal integrity is a major clinical problem in restorative dentistry. The aim of this study was to evaluate the applicability of an edge strength measurement device in an in vitro test to determine the force required to fracture flakes of material by a Vickers indentation at progressively increasing distances from an interface edge of bulk material. Methods Five representative resin-composites were investigated. Fourteen disks of specimens (12 mm diameter × 2.5 mm thick) were prepared for each material. These were divided into seven sub-groups corresponding to different edge-distances (0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0 mm). An edge strength measurement device (CK10) (Engineering Systems, Nottingham, UK) was used. The mode of the failure of each specimen was examined under the integral microscope of the CK10. Results The force (N)-to-fracture at a distance of 0.5 mm from the edge was defined as the edge strength. The highest failure force (edge strength) was observed for Tetric Ceram (174.2 N) and the lowest for Filtek Supreme (enamel) (87.0 N). Correlations between the failure-forces to fracture materials with edge-distance were regression analyzed giving coefficients (r) ranging from 0.94 (p = 0.02) to 0.99 (p = 0.01). Two modes of failure were observed: chipping and – generally at greater distances – cracking. Significance Edge strength is a definable and potentially useful parameter to characterize this aspect of clinically related behavior. A standardized distance of 0.5 mm from the specimenʹs edge, when chipping failure prevails, is suitable and convenient as a reference point.