Abstract
IntroductionGeometrical designs of nickel-titanium (NiTi) rotary instruments have a considerable influence on their mechanical performance, and thereby have the tendency to fracture.AimTo evaluate the effect of cross-sectional geometry, pitch, taper, and off-center cross-section on the stress distribution in NiTi instruments under bending and torsion conditions using finite element analysis (FEA).Materials and methodsEleven theoretical three-dimensional finite element (FE) models of rotary instruments were constructed and divided according to the geometric design tested into; group 1 (cross-section geometry): four FE models with four different cross-section designs (triangle, convex triangular, parallelogram, and rectangle). Group 2 (pitch): three FE models with different pitches (5, 10, and 15 threads). Group 3 (taper): two FEA models with two different tapers (0.04 and 0.06). Group 4 (centering): two FEA models (off-centered and centered cross section). The behavior of the instrument under bending and torsional conditions was analyzed mathematically in SolidWorks software package.ResultsThe pattern of stress distribution was varied by altering cross-sectional geometry, pitch, taper, and off-center cross section.ConclusionNo single geometrical design could be beneficial for all stress conditions. To decrease the stress accumulation during bending of rotary files in curved root canals, the rotary NiTi instruments should be designed with rectangle cross-section configuration, low pitch, reduced taper, and with centered cross section. This design improved the flexibility of rotary NiTi instruments. However, to improve the torsional resistance during root canal preparation of narrow canals, the file should be designed with parallelogram cross-sectional configuration, low pitch, increased taper, and eccentric cross-section design.
Highlights
Geometrical designs of nickel-titanium (NiTi) rotary instruments have a considerable influence on their mechanical performance, and thereby have the tendency to fracture
The pattern of stress distribution was varied by altering cross-sectional geometry, pitch, taper, and offcenter cross section
To decrease the stress accumulation during bending of rotary files in curved root canals, the rotary NiTi instruments should be designed with rectangle cross-section configuration, low pitch, reduced taper, and with centered cross section. This design improved the flexibility of rotary NiTi instruments
Summary
Geometrical designs of nickel-titanium (NiTi) rotary instruments have a considerable influence on their mechanical performance, and thereby have the tendency to fracture. The fracture of rotary endodontic instruments during endodontic treatment of curved root canals is often considered an enigma for endodontic practitioner. They arise on exceeding torsional or bending stresses. Fracture of the NiTi rotary endodontic instrument may occur due to repeated bending stresses (compressive and tensile) accumulated at the point of maximum stress of the file in a curved root canal. Torsional fracture arises when the tip of the file becomes locked in the root canal, while the shank of the rotary instrument continues to rotate (Kim et al 2012)
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