Title: Nickel titanium instruments after clinical use
Nickel titanium (NiTi) instruments using during root canal preparation, have undergone various changes over the past 2 decades. The traditional benefits of NiTi instruments were their resistance to corrosion, biocompatibility and a mechanical property known as superelasticity. Changes to NiTi instruments include modifications in instrument design, changes in NiTi metallurgy, and recently, different motions used to drive these files. Many studies have shown that NiTi rotary systems are able to prepare root canals with less canal transportation, and at a much faster rate than hand files. However, major concerns to the operator are safety and efficiency of mechanized NiTi files. Factors that contribute to instrument fracture include instrument design, rotational speed, root canal configuration, operator proficiency, number or uses/sterilization cycles, and method of use. This presentation will review the different instrument designs and changes in metallurgy of mechanized NiTi files that have evolved and evaluate how different instrument systems are stressed after clinical use. Two clinically observed stresses to NiTi instruments are file unwinding and breakage. The two mechanisms that can lead to NiTi instrument deformation and fracture, cyclic fatigue and torsional strain, will also be reviewed. Factors to be considered by the clinician to minimize the risk for mechanized NiTi instrument fracture will be discussed.