Nickel-Titanium Alloys in Endodontics: A Narrative Review

The introduction of nickel-titanium (NiTi) rotary instruments has profoundly transformed modern endodontics, enabling safer and more conservative shaping of complex root canal systems. Despite technological advances, unexpected instrument fracture remains a relevant clinical concern, highlighting the importance of understanding the metallurgical and biomechanical mechanisms governing instrument behavior. This review aims to critically analyze the metallurgical evolution, biomechanical performance, and clinical implications of NiTi instruments, with particular emphasis on heat-treated alloys, phase transformations, shaping kinematics, and stress management during canal preparation. A non-systematic narrative review methodology was adopted. Relevant scientific literature addressing NiTi metallurgy, cyclic fatigue resistance, torsional stress, corrosion phenomena, glide path preparation, and rotary versus reciprocating kinematics was analyzed, integrating experimental, metallurgical, and clinical studies selected for their conceptual relevance to contemporary endodontic practice. The clinical performance of NiTi instruments results from the interaction between alloy metallurgy, geometric design, and operative protocols. Thermomechanical treatments increase martensitic phase presence at clinical temperatures, enhancing flexibility and cyclic fatigue resistance. However, surface defects, corrosive environments, and stress accumulation remain critical factors in instrument failure mechanisms, indicating that material properties alone do not determine clinical safety. Contemporary root canal shaping should be interpreted as a biomechanical stress management process rather than simple canal enlargement. Integration of metallurgical knowledge with operative strategies represents a fundamental prerequisite for safer and biologically conservative endodontic treatment.