Analysis of Geometry and Bending Angulation of Nickel -Titanium Instrument

Objectives:With the advances in material technology, nickel titanium (Ni-Ti) files are introduced to facilitate the instrumentation of curved canals due to their superior flexibility. Nevertheless, separation of Ni-Ti files is still a subject of debate. The purpose of this study was to compare the mechanical behaviors of different geometries of Ni-Ti files via simulating variable curvatures of root canals during shaping. ProTaper® F1, F2, and F3 were selected due to different geometries of design and their prevalence in Taiwan.

Methods:Each group contained 10 files progressing static bending test in different restricted points experimentally by universal testing machine.

Results:When evaluated by Schneider Technique, our results showed as the restricted points move backward from 3 mm to 9 mm, the bending angle of maximal crystal transformation will be minimized, meaning the crystal transforming from Austenite to Martensite abundantly. However, considering among different files and restricted points, the critical angle of elasticity is always close to 45°;When evaluated by Pruett Technique, the bending angle of maximal crystal transformation of ProTaper® F2 and F3 tend to reduce gradually, but F1 do not show obvious difference when the restricted point move backward from 3 mm to 9 mm. ProTaper® F1, F2, and F3 have approximate critical angle of elasticity, whatever the bending angle of maximal crystal transformation. These experimental results correlated well with FEM analysis.

Conclusions:If the root canals have severe curvatures and long working lengths, they are more unfavorable to shaping. Our present study may provide information about instruments under cyclic tensile and compressive stresses, and about the limitations of using rotary Ni-Ti instruments. The modification of geometry and materials should be further investigated