Comparison of forces generated during root canal shaping and residual stresses of three nickel-titanium rotary files by using a three-dimensional finite-element analysis.

The study was aimed to compare the stress distribution during simulated root canal shaping and to estimate the residual stress thereafter for some nickel-titanium rotary instruments. Three brands of instruments (ProFile, ProTaper, and ProTaper Universal; Dentsply Maillefer) were scanned with micro-computed tomography to produce a real-size, 3-dimensional model for each. The stresses on the instrument during simulated shaping of a root canal were analyzed numerically by using a 3-dimensional finite-element package, taking into account the nonlinear mechanical behavior of the nickel-titanium material. From the simulation, the original ProTaper design showed the greatest pull in the apical direction and the highest reaction torque from the root canal wall, whereas ProFile showed the least. In ProTaper, stresses were concentrated at the cutting edge, and the residual stress reached a level close to the critical stress for phase transformation of the material. The residual stress was highest in ProTaper followed by ProTaper Universal and ProFile.

Effect of irradiation mode on polymerization of dental composite resins.

Many researchers have sought to reduce the polymerization shrinkage in composite resins. Controlling the irradiation light seems practical because the polymerization process initiates with light by activating the photoinitiators. The present study evaluated the effect of irradiation mode on microhardness and polymerization shrinkage of composite resins. Three different irradiation modes, STD, EXP, and MED, were taken under 40-s exposure condition. As results, the irradiation mode significantly affected the microhardness difference. However, in all products, the microhardness difference between the STD and MED modes was less than 10%. In all irradiation modes, microhardness was linearly correlated with the filler content of the tested specimens. The effect of mode on the difference of the polymerization shrinkage was not uniform even though the shrinkage values decreased following the order of the STD > EXP > MED mode. An inverse correlation was found between filler content and polymerization shrinkage in all irradiation modes and specimen thicknesses. In spite of the statistical significance of the irradiation modes on the difference of the measured values, their numeric difference was not great. Especially, compared with the STD and MED mode, the exponentially increasing light (EXP mode) produced no noticeable difference except for a delayed shrinkage process.