Introduction
Nickel-titanium (NiTi) rotary instruments are engine-driven files that shape the canal once a glide path exists. Their flexibility keeps preparations centred in curved canals with fewer procedural errors than stainless steel, but they can fracture inside the canal. Hand instruments and ISO sizes: topics 42 and 39.
The nickel-titanium alloy
Endodontic NiTi contains about 55–56 % nickel and 44–45 % titanium by weight. It has two crystal phases: austenite (high-temperature, stiffer) and martensite (low-temperature, soft, easily deformed), with an intermediate R-phase. Temperature or stress switches between them.
- Superelasticity: under stress, austenite turns into martensite; when the stress is removed it turns back and the file regains its shape. Recoverable strain is up to about 8 %, against less than 1 % for stainless steel (Figure 1A).
- Shape memory: deformed martensite recovers its original shape when heated above its transformation temperature.
- Low modulus of elasticity (about one-quarter to one-fifth of steel): the file follows the curve instead of straightening it.
- Cuts less efficiently than steel; surface machining defects can start cracks.
Thermomechanical treatment
Heat treatment raises the transformation temperatures, leaving more martensite or R-phase at body temperature: more flexibility and cyclic-fatigue resistance.
- M-Wire: mixed phases; more flexible than conventional NiTi (ProTaper Next, WaveOne).
- CM-Wire (controlled memory): mainly martensitic, no spring-back; can be pre-bent and recovers on autoclaving (HyFlex CM).
- Gold and Blue treatment (after grinding; coloured oxide layer): very flexible, highly fatigue resistant (ProTaper Gold, WaveOne Gold, Reciproc Blue).