Mechanical behaviors of dental class V restorations under occlusal loading

Objectives: The occlusal loading causes the deformations of class V composite restorations and lead to debonding on tooth-composite interface. The purpose of this study was to investigate the effects of restorative materials on the deformation and failure probability of class V composite restorations under mechanical loading by the digital image correlation (DIC) and acoustic emission (AE) methods.
Methods: Twenty five human premolars were collected. Box-cavity, 2.5mm in depth and 4mm in height, was prepared at the cervical area. These teeth were divided into five groups to be restored by different materials: group 1: a nanocomposite Filtek-Z350 (3M-ESPE); group 2: a packable composite Filtek-P60; group 3: Filtek-Z350 filling with a lining; group 4: Filtek-P60 with a lining; group 5: Filtek-Bulkfill filling. For groups 3 and 4, Filtek-Z350flowable was applied on all cavity walls at 1 mm thickness as the lining. The restored premolars were mechanically loaded at five stages: 0-250 N, holding at 250 N for 10 sec, 250-500N, holding at 500N for 10 sec, 500-1000 N. The deformations were continuously recorded by a CCD camera, and then analyzed by DIC. A sensor was attached underneath the tooth to monitor the AE events.
Results: Under 250N, unlined groups 1 and 2 showed the lowest shear strain (0.033%, 0.034%) at tooth-composite margins, followed by lined groups 3 and 4 (0.059%, 0.066%), and group 5 (0.1%). All groups show minimal events (4-16) during 0-250N loading. The lined groups 3 and 4 show more events (44, 38) under high load (>500N) compared to unlined groups 1 and 2 (24, 21) (p<0.05). Groups 5 showed significantly more total events (221) than the other group (p<0.05).
Conclusions: Using flowable composite lining increased the interfacial deformation, and affected the composite-tooth bonding under high loads. Combining DIC and AE provides comprehensive examinations of the restorations under loading.