# Impact of Downward Load and Rotational Kinematics on Root Canal Instrumentation with a Heat-Treated Nickel–Titanium Rotary Instrument

## TL;DR

This study examines how downward load and rotational motion affect root canal shaping using heat-treated NiTi instruments in curved canals.

## Contribution

The study introduces a comparison of continuous rotation and torque-sensitive reciprocation under varying downward loads for curved canal instrumentation.

## Key findings

- OTR-3N specimens showed significantly lower upward force compared to OTR-2N specimens.
- CR-2N specimens had a larger canal-centering ratio than OTR-2N specimens near the apex.
- Using OTR with 2–3 N downward load improved canal-centering and reduced screw-in force compared to CR.

## Abstract

This study analyzed how different downward loads and rotational kinematics influence NiTi rotary instrumentation outcomes. Heat-treated NiTi instruments were used to prepare extracted human single-rooted premolars with a moderate canal curvature. Instrumentation was performed using an automated endodontic instrumentation device with controlled downward loading and torque/force sensing, under different downward load settings (1, 2, and 3 N), employing either continuous rotation (CR) or optimum torque reverse (OTR) motion, which is a torque-sensitive reciprocation. Instrumentation was completed without instrument fracture or ledge formation in all six groups. OTR-3N specimens displayed a significantly lower upward force (i.e., screw-in force) than OTR-2N specimens (p < 0.05). OTR-1N specimens required a significantly longer instrumentation time than CR-1N specimens and the other OTR specimens (p < 0.05). At 1 mm from the apex, CR-2N specimens showed a significantly larger canal-centering ratio (i.e., larger deviation) than OTR-2N specimens (p < 0.05). Overall, applying a downward load of 2–3 N in OTR mode provided shaping efficiency similar to CR, but with a reduced screw-in force and enhanced canal-centering in the apical region, supporting the use of OTR as a promising alternative to CR for curved canal preparation using heat-treated NiTi instruments.