Experimental rationale for the selection of rotary NiTi instruments for mechanical preparation of calcified root canals

INTRODUCTION. Mechanical preparation of calcified and partially obliterated root canals is associated with an increased risk of locking and fracture of rotary NiTi instruments. The resistance of NiTi instruments to mechanical loading is determined by the combined influence of the NiTi alloy type and the geometric features of the working part.

PURPOSE. To provide an experimental rationale for the selection criteria of rotary NiTi instruments for mechanical preparation of calcified root canals based on a comparative laboratory assessment of torsional resistance and the design parameters of the working part of rotary file systems.

MATERIALS AND METHODS. The study comprised two stages. At the first stage, torsional testing was performed on 45 instruments divided into three groups of 15 specimens each: E-Flex Path Blue instruments manufactured from thermally treated Blue NiTi alloy (group IA), PathFile instruments manufactured from conventional NiTi alloy (group IB), and E-Flex Path Gold instruments manufactured from thermally treated Gold NiTi alloy (group IC). Maximum torque and angle of rotation at fracture were determined. At the second stage, scanning electron microscopy and morphometric analysis were used to evaluate the apical portion of the working part, cross-sectional geometry, and the inner-to-outer diameter ratio (K_D) of the PathFile (group IIA), SoCo Plus (group IIB), and Scout RaCe (group IIC) instrument systems at the D₀ and D₃ levels.

RESULTS. The maximum torque of the group IB specimens was 2.50±0.12 N⋅cm and was 2.08 and 2.45 times higher than the corresponding values in groups IC and IA, respectively. The greatest angle of rotation at fracture was recorded for the group IA instruments (540±45°), followed by group IC (455±40°), whereas the lowest value was observed in group IB (390±35°; p < 0.001). The instruments differed in the geometry of the apical working part and cross-sectional shape. The highest K_D values were recorded for the group IIB specimens: 0.51 at the D₀ level and 0.74 at the D₃ level.

CONCLUSIONS. The findings support the use of maximum torque, angle of rotation at fracture, and the geometric characteristics of the apical portion as criteria for the differentiated selection of rotary NiTi instruments for the preparation of calcified root canals, taking into account the risk of torsional locking and the characteristics of the mineralized obstruction.

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