Cyclic fatigue testing of nickel-titanium endodontic instruments

Area of Science:

• Endodontics
• Materials Science
• Biomedical Engineering

Background:

• Nickel-titanium (NiTi) rotary instruments are widely used in endodontics.
• Understanding cyclic fatigue is crucial for preventing instrument separation during root canal procedures.
• Previous studies have focused on various factors affecting NiTi instrument failure.

Purpose of the Study:

• To investigate the effect of canal curvature (angle and radius) and operating speed on the cyclic fatigue life of Lightspeed NiTi engine-driven instruments.
• To introduce a novel method for evaluating canal curvature, considering both angle and abruptness.

Main Methods:

• Simulated root canal curvatures using stainless-steel guide tubes (30-60° angles, 2-5 mm radii).
• Tested Lightspeed instruments (#30 and #40) under a simulated load (10 g-cm) at varying speeds (750-2000 rpm).
• Determined cycles to failure and analyzed fracture modes using scanning electron microscopy.

Main Results:

• Operating speed did not significantly affect cycles to failure.
• Instrument separation occurred at the point of maximum flexure within the simulated curvature.
• Larger diameter instruments (#40) exhibited significantly fewer cycles to failure than smaller ones (#30).
• Cycles to failure decreased significantly with smaller radii (2 mm vs. 5 mm) and increased curvature angles (>30°).
• Ductile fracture was identified as the fatigue failure mode.

Conclusions:

• Canal curvature (radius and angle) and instrument size are more critical predictors of NiTi rotary instrument separation than operating speed.
• Standardized fatigue testing should incorporate dynamic operation in a flexed state.
• The radius of curvature should be considered as an independent variable in evaluating root canal instrumentation studies.