The purpose of this study was to validate a new method to investigate the polymerization shrinkage vectors of composite during light curing and to evaluate the overall utility and significance of the technique.
An optical instrument was developed to measure the location and direction of the polymerization shrinkage strain vectors of dental composite during light curing using a particle tracking method with computer vision. The measurement system consisted of a CCD color camera, a lens and a filter, and software for multi-particle tracking. A universal hybrid composite (Z250, 3M ESPE, St. Paul MN, USA) was molded into thin disk-shaped specimens (un-bonded and bonded) or filled into a cavity within a tooth slab (bonded). The composite surface was coated with fluorescent particles prior to light curing. The images of the fluorescent particles were stored at 2 frames/s for 10 min, and the movements of the particles on the composite surface were tracked with computer vision during curing. The polymerization shrinkage strain vectors as a function of time and location were analyzed. The volume shrinkage of the composite was also measured for comparison.
The linear and volume shrinkage of the composite at 10 min were 0.75 (0.12)% and 2.26 (0.18)%, respectively. The polymerization shrinkage vectors were directed toward the center of the specimen and were isotropic in all directions when the composite was allowed to shrinkage freely without bonding. In contrast, the shrinkage vectors were directed toward the bonding surface and were anisotropic when the composite was bonded to a fixed wall. The regional displacement vectors of composite in a tooth cavity were dependent on the location, depth and time.
The new instrument was able to measure the regional linear shrinkage strain vectors over an entire surface of a composite specimen as a function of time and location. Therefore, this instrument can be used to characterize the shrinkage behaviors for a wide range of commercial and experimental visible-light-cure materials in relation to the composition, boundary condition and cavity geometry.
Current dental composites are inevitably linked with polymerization shrinkage that can compromise the success and longevity of the restoration. The polymerization stress may produce interfacial de-bonding, substrate micro-cracking, and cuspal flexure, potentially resulting in post-operative hypersensitivity, micro-leakage, secondary caries, and ultimately restoration failure .
Several test methods have been used to measure polymerization shrinkage in dental composites. Dilatometers and buoyancy (density in water) measurements have been used to measure volumetric shrinkage, while the bonded disk and strain gage method have been used to determine the axial and post-gel linear shrinkage, respectively. All these methods focus on the measurement of the gross polymerization shrinkage value for the composite.
In the clinical application of the composite to the cavity preparation, the direction of shrinkage can be affected by the cavity geometry, the direction of the curing light, and certain boundary conditions, such as adhesion to the cavity walls . Therefore, determination of localized shrinkage strain vectors is likely more meaningful than simply assessing gross shrinkage to understand the biomechanical phenomenon caused by polymerization contraction.
Finite element analysis (FEA) was adopted to assess the effect on the shrinkage strain vectors of the curing light direction and bonding to the cavity walls . Digital image correlation (DIC) is another method used to measure the full-field polymerization shrinkage strain of dental composites . Imaging of the full-field shrinkage strain demonstrated that the localized shrinkage strain is related to the location within the specimen depth and the sampling time. However, the DIC technique uses a white powder or black spray paint to produce sufficient contrast for tracking individual spots on the specimen surface, resulting in lack of initial data during light curing due to the intense curing light saturating the camera.
Recently, the 3D micro-CT imaging technique has been used to evaluate the amount and direction of regional polymerization shrinkage in dental composites. Marker fillers added to the composite allowed visualizing the 3D displacement vectors of the fillers generated by polymerization shrinkage. However, this method is limited to comparing pre- and post-curing images without providing temporal information during light curing.
No study to date has provided detailed information about the actual localized flow of composite as a function of time during the initial photo-polymerization period. Lee et al. developed a new optical method of particle tracking with computer vision to measure the shrinkage kinetics of light cured composites. The measurement system was able to track the movement of a marker connected to a composite specimen, which allowed the instrument to measure the true linear shrinkage of the composite during light curing. A limitation of the study was that the regional shrinkage strain vectors could not be observed directly on the composite surface. Therefore, we developed an updated instrument that solves the limitation of the previous method and that can determine the spatial-time domain shrinkage strains on the whole composite surface directly during light curing.
The purpose of this paper is to report on the new optical method to investigate the polymerization shrinkage strain vectors of composites during light curing and to evaluate the overall utility and significance of the technique.