[Failure mechanisms of NiTi rotary instruments during clinical use: a microscopic study]

NiTi rotary instruments have been extensively used in the preparation of curved root canals, due to their superelastic quality. These instruments are less susceptible to unexpected fracture as compared to their stainless steel counterparts. However, unexpected fracture of NiTi instruments under complex stress within curved canals can lead to serious problems. The aim of this investigation was to identify and evaluate the various failure mechanisms which can cause fracture of NiTi rotary instruments under clinical conditions. The causes of failure of two brands of Hero and Profile rotary files have been studied. A total of 1133 instruments that failed during clinical use were collected from several dental clinics. After inspection under a stereomicroscope, all instruments were classified as either plastically deformed or fractured. The fracture surfaces of all broken instruments were examined following ultrasonic cleaning in ethanol and acetone, 30 minutes each. The fracture surfaces of more than 70 broken instruments were subjected to fractographic analysis using a scanning electron microscope. More than 68% of the 1133 specimens were fractured. Stereomicroscopic inspection showed three failure mechanisms for the studied instruments: fracture, plastic deformation due to bending and torsion and a combination of both mechanisms. Based on fractographic observations, microvoid coalescence and cyclic stable crack growth by striation formation were identified. According to these results, excessive plastic deformation and fracture may be considered as the two main mechanisms responsible for the failure of NiTi rotary instruments

effect of environmental pH on the setting of Mineral Trioxide Aggregate

One of the materials used for repairing root-end fillings is mineral trioxide aggregate [MTA]. MTA has been reported to remain soft when placed in perforations with a high degree of inflammation. The purpose of this study was to evaluate the effect of environmental pH on the setting and micro structure of MTA. In this interventional study, tooth-colored MTA [Pro Root, Dentsply, USA] was mixed according to the manufactures instructions. The mixture was placed into cylindrical polycarbonate tubes. The specimens were randomly divided into four groups of 9 and were subjected to a pressure of 3.22 Mpa - 6kg applied for 1 minute in a custom-made condenser device. Each of the 9 specimens were then stored at room temperature and saturated humidity in four plastic plates containing buffer solutions of butyric and propyonic acid with a pH of 4.4, 5.4, 6.4 and 7.4, for 4 days. The Vickers hardness test was performed on each specimen. The microstructures of the specimens were analyzed using a scanning electron microscope after sectioning the specimens with a scalpel. Data were subjected to one-way ANOVA. The lowest and highest pH values were 4.4 and 7.4, respectively. A statistically significant difference was observed between all four groups [p<0.05]. Unreacted MTA particles scattered in a non uniform matrix were seen in specimens stored in pH 4.4 solutions whereas cases kept in a pH 7.4 environment had distinctive crystalline structures embedded in a more uniform matrix. The results indicate that the acidic pH of inflamed tissues can adversely affect the setting of MTA