Cusp Deformation of Bonded Class II RBC Determined by DIC

Objective: Cusp deformation of Class II RBC restorations during light-curing is a well-known problem. Because of the curvature of cusps, the use of strain gauges often generates misleading deformation data. The objective of this study was to explore the use of Digital Image Correlation (DIC) to determine cusp-deformations during and after light-curing of Class II RBC restorations.

Method: MOD cavities were cut in extracted intact premolars mounted in stone. A PVS impression material was applied on the cavity walls to keep them from being contaminated during spraying of the contrasting coating. The coating comprised of one layer of flat white Valspar spray paint, followed by a speckle pattern applied using flat black Valspar spray paint. After removal of the cavity protecting PVS, the specimen was attached to an optical bench with two digitizing cameras aiming to generate stereoscopic images of the specimen. Baseline data was generated to assure excellent dot imaging before applying the RBC materials.  Internal walls were then treated with an all-in-one self-etching adhesive (Bond Force) and cured, and the cavity incrementally filled with Estelite Omega (A1). Using a software (Vic-snap 2007), each increment was captured while being light-cured as well as after 72-h storage in a humidor. Recorded images were processed using VIC-3D 2009, which determined differences in dot spatial positions during the entire restorative process.  

Result: The DIC approach revealed that the cusp-tips were displaced 15-20 μm in a direction towards them during curing. After 72-h in humidor, it was found that the cusp-tips displacement had increased another 3-4 μm, suggesting a certain level of post-cure shrinkage. 

Conclusion: These preliminary data demonstrate that DIC has the potential of becoming a useful tool to determine how deformations of the tooth may occur during dental treatments such as the curing process of dental composites in cavity preparations.