Nitinol is a unique shape memory alloy, characterized by its ability to automatically restore its original shape after undergoing plastic deformation at a specific temperature. It possesses remarkable properties such as excellent plasticity, wear resistance, corrosion resistance, high damping, and superelasticity.
(1) Phase Transition and Characteristics of Alloys of Nickel and Titanium:
Nitinol, as its name suggests, is a binary alloy composed of nickel and titanium. It exhibits two different crystal structure phases known as austenite and martensite, which undergo phase transformation based on temperature and mechanical pressure. The cooling sequence of NiTi alloy involves the parent phase, R phase, and martensite phase. The austenite phase, cubic and stable at higher temperatures or when unloaded, contrasts with the rhombohedral R phase, while the martensite phase, hexagonal and ductile, is present at lower temperatures or under load.
(2) Unique Properties of Nickel-Titanium Alloys:
Shape memory characteristics: Nitinol demonstrates shape memory behavior, where the material returns to its original shape when heated above the Af temperature after being deformed in the martensitic phase below the Mf temperature.
Superelasticity: Nitinol exhibits superelasticity, which allows it to withstand strains beyond its elastic limit when subjected to external forces. It can automatically recover its original shape upon unloading.
Sensitivity to intraoral temperature changes: In the field of orthodontics, stainless steel and CoCr alloy wires are minimally affected by temperature changes within the mouth. However, nickel-titanium wires are more sensitive to these temperature variations.
Corrosion resistance: Research suggests that the corrosion resistance of nickel-titanium wires is comparable to that of stainless steel wires.
Non-toxicity: Due to its atomic composition, including nickel and titanium, Nitinol shape memory alloy is considered non-toxic. Although nickel is known to have carcinogenic effects, its concentration in Nitinol is significantly lower.
Gentle orthodontic force: Various orthodontic wires, such as austenitic stainless steel, cobalt-chromium-nickel alloy, nickel-chromium alloy, Australian alloy, gold alloy, and titanium alloy wires, are commercially available. Each wire type applies different levels of force for orthodontic treatment.
Excellent shock absorption characteristics: Increased archwire vibrations resulting from chewing or nocturnal grinding can lead to detrimental effects on dental roots and periodontal tissues.
nitinol
titanium alloy wire
nickel-titanium wire from TopTiTech
(3) Classification of Nickel-Titanium Alloy Wires (Evans and Durning Classification Method):
Early classifications included gold archwire, cobalt-chromium alloy wire, and stainless steel round wire (1940s).
Martensitic stabilized alloys, predominantly martensitic nickel-titanium alloys, were developed in the 1960s. These archwires possess lower stiffness and generate lighter correction forces.
Austenite-activated alloys, such as Chinese and Japanese nickel-titanium alloys introduced in the 1980s, exhibit austenitic behavior regardless of their placement inside or outside the oral cavity.
Martensite-activated nickel-titanium alloys emerged in the 1990s. These alloys exhibit multiple states at room temperature, easily undergo deformation, and have a transformation temperature range (TTR) lower than oral temperature or close to it.
Graded thermodynamic nickel-titanium alloys, with a TTR higher than oral temperature (around 40℃), remain in multiple states when placed in the oral cavity, resulting in a relatively soft archwire.
(4) Clinical Applications of Nickel-Titanium Alloy Wires:
Initial alignment and leveling: Nickel-titanium alloy archwires, with their superelastic and shape memory properties, are commonly used in the initial stages of orthodontic treatment due to their lower stress-strain curve.
Nickel-titanium springs: Used in orthodontics, these springs utilize the superelasticity of nickel-titanium to create tension or compression, aiding in tooth movement and gap opening.
L-H archwire: Developed by Dr. Soma in Japan, the "LH" archwire, named after "Low Hysteresis," exhibits minimal stress differences between activation and tooth movement, allowing it to return to its original position slowly.
