Variations in cyclic fatigue resistance among ProTaper Gold, ProTaper Next and ProTaper Universal instruments at different levels.

AIM: To compare the cyclic fatigue resistance of ProTaper Gold (PTG, Dentsply Tulsa Dental Specialities, Tulsa, OK, USA), ProTaper Next (PTN, Dentsply Tulsa Dental Specialities) and ProTaper Universal (PTU, Dentsply Tulsa Dental Specialities) instruments at different levels. METHODOLOGY: A total of 72 files were used to evaluate the cyclic fatigue of PTU (F2), PTN (X2) and PTG (F2) at 5 mm (n = 12) and 8 mm (n = 12) from the tip in 3-mm-radius steel canals with a 60° angle of curvature. The time to fracture was recorded. The Kolmogorov-Smirnov test was used to assess the normality of the samples distribution, and the statistical analysis was performed using the independent sample t-test (P < 0.01). RESULTS: Significant differences were found amongst the instruments 5 mm from the tip (P < 0.01). The PTG files had the highest CF resistance, and the PTN files displayed greater CF resistance than the PTU files. No significant differences were found between the PTG and PTN files 8 mm from the tip (P > 0.01). The PTG and PTN files demonstrated greater CF resistance than the PTU files (P < 0.01). CONCLUSIONS: The PTG instruments were the most resistant 5 and 8 mm from the tip; however, at 8 mm, there was no difference between the PTG and PTN instruments. The PTU files had the lowest CF resistance at all levels.

[Biophysics of nerve excitation].

The studies testifying to the presence of the interrelation between the physiological functions of the organism and physical and chemical processes in nerves are discussed. Changes in some physical and chemical parameters observed both upon elicited rhythmic exaltation of nerves and during the spontaneous rhythmic activity of neurons are analyzed. Upon rhythmic exaltation, a complex of physical and chemical processes is triggered, and reversible structural and metabolic rearrangements at the subcellular and molecular levels occur that do not take place during the generation of a single action potential. Thus, only in conditions of rhythmic exaltation of a nerve, it is possible to reveal those processes that provide exaltation of nerves in the organism. The future possibilities of the investigations combining the biophysical and physiological approaches are substantiated. Characteristic changes in physicochemical parameters are observed in nerves during the generation of a series of action potentials of different frequency and duration ("frequency dependence") under normal physiological conditions, as well as in extreme situations and in nerve pathology. The structural and metabolic rearrangements are directly related to the mode of rhythmic exaltation and proceed both in the course of rhythmic exaltation and after its termination. Participation and the basic components of the nervous fulcrum (an axon, Shwan cell, myelin, subcellular organelles) in the realization of rhythmic exaltation is shown. In the coordination of all processes involved in rhythmic exaltation, the main role is played by the systems of redistribution and transport of intercellular and endocellular calcium. The idea is put forward that myelin of nerve fibers is not only an isolator, but also an "intercellular depot" of calcium and participates in the redistribution of different ions. Thus, the rhythmic excitation is of great importance in the realization of some physiological functions, the adaptation to changing conditions, the liquidation of consequences of paralogical processes, the formation of mechanisms of "memory", etc.