Objective: To employ a laser triangulation method for measuring the dynamic shrinkage of composite resins during polymerization, and to investigate the maximum shrinkage rate (S(max)) and the time at maximum shrinkage rate (t(max)).
Methods: A novel experimental set-up based on laser triangulation was utilized which was capable of real-time shrinkage strain measurements. Uncured composite was condensed into a PTFE mould and irradiated for 40 s. The laser signals obtained from lateral composite surfaces were monitored using a laser triangulation measuring system and were converted into the displacement of the tested surface. The volumetric shrinkage derived from the linear shrinkage was calculated by multiplying 3. Total volumetric shrinkage (%S) of the five restorative materials (AP-X, Charmfill, Charisma, Durafill VS, and Herculite Precis) was measured using three methods (laser triangulation method, Acuvol, and buoyancy method). S(max) and t(max) were measured using the laser triangulation method. Statistical analyses were done using the two-way ANOVA (P<0.05) and post hoc test.
Results: The highest shrinkage value was measured by Acuvol, followed by laser method, and the lowest was by buoyancy method. All the three methods of measuring %S generated the same, statistically secured ranking for the five light-cured restorative materials: DurafillVS < AP-X < Herculite Precis < Charisma < Charmfill. %S measured by laser triangulation method varied between 2.06% and 3.37%. S(max) varied between 4.39 μm/s and 29.25 μm/s. t(max) varied between 0.77 s and 1.59 s. Significant differences in S(max) (F=734.87,P<0.01) and t(max) (F=53.24,P< 0.01) for five composite resins were found.
Conclusion: Laser triangulation method offers several advantages over the conventional methods of measuring polymerization shrinkage. It is simple, compact, non-invasive and suitable for measuring the dynamic polymerization shrinkage in real time without delay. Therefore, it can be used to characterize the shrinkage kinetics in a wide range of visible-light-cure materials.
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