Nickel Titanium Wires in Orthodontics: Quick Revision

Nickel Titanium (NiTi) wires have revolutionised orthodontics, offering unique properties like shape memory and superelasticity that significantly enhance treatment outcomes. Since their introduction in the 1970s, NiTi wires have become an essential tool in every orthodontist’s arsenal. This blog explores their material science, classifications, and clinical relevance in contemporary orthodontic practice.

The Birth of NiTi: A Naval Invention with a Dental Destiny

The origins of NiTi trace back to the late 1960s when the US Naval Ordnance Laboratory explored shape memory alloys for military use. Buehler and Wiley (1962) first used this alloy in dentistry, later named Nitinol (Nickel Titanium Naval Ordnance Laboratory). Its unique metallurgical behaviour quickly attracted the attention of orthodontists, leading to widespread adoption in the 1970s.

Composition and Metallurgical Phases

The typical NiTi wire composition includes 52% nickel, 43% titanium, and around 3% cobalt (Kusy, 1997). The alloy exhibits two fundamental metallurgical phases:

• Austenitic Phase: Stable at higher temperatures, characterised by a hexagonal crystal structure.

• Martensitic Phase: Dominant at lower temperatures, with a cubic structure.

An intermediate R-phase with a rhombohedral structure can appear during the transformation between these two phases (Khier et al., 1991; Bradley et al., 1996).

Classification of NiTi Alloys

Kusy (1997, 2000) identified three primary types of NiTi archwires:

1. Martensitic Stabilised Alloy (e.g. Unitek’s Original Nitinol):

   • Properties: Low stiffness, no superelasticity, high springback.

   • Clinical Use: Suitable for mild crowding and in cases where low continuous force is adequate.

2. Austenitic Active Alloy (e.g. Superelastic Nitinol):

   • Exhibits pseudoelasticity, a stress-induced transformation from austenitic to martensitic phase.

   • Key Feature: Constant force during deflection, ideal for effective alignment.

   • Requires a minimum 2 mm deflection to activate plateau behaviour (Tonner & Waters, 1994).

   • Copper NiTi is a modified version with added copper and chromium, offering reduced hysteresis and improved control of the temperature transformation range (TTR).

3. Martensitic Active Alloy (e.g. Neo-Sentalloy):

   • Demonstrates thermoelasticity, a temperature-dependent phase transformation.

   • Wire is ductile at room temperature and becomes active at oral temperature (Bishara et al., 1995).

   • Benefits include easier ligation and a narrow TTR, producing gentle forces at mouth temperature.

Superelasticity and Hysteresis

Superelasticity is a defining property of certain NiTi wires, where stress-induced transformation yields a flat load-deflection curve—providing constant force over a wide range of tooth movements. However, this behaviour only occurs when both austenitic and martensitic phases coexist.

Hysteresis, another key trait, describes the difference between the force required to activate the wire (loading) and the force delivered (unloading). This enables continuous low-force delivery even in severely misaligned arches—ideal for patient comfort and biologically safe tooth movement.

Temperature Transformation Ranges (TTR) and Clinical Application

NiTi wires are available with varying TTRs to accommodate patient-specific needs:

• 27°C NiTi: Superelastic in the mouth; suitable for average patients needing rapid alignment.

• 35°C NiTi: Delivers gentler forces for low pain threshold or compromised periodontal health.

• 40°C NiTi: Recommended for very sensitive patients; used as initial rectangular wires.

Graded Thermoelastic NiTi offers different force levels across arch segments—lower anteriorly and higher posteriorly—enhancing biomechanical control.

Other Titanium-Based Alloys

1. Beta Titanium (TMA):

   • Composition: 79% Ti, 11% Mo, 6% Zn, 4% Sr.

   • Offers excellent torque control and weldability but higher friction.

2. Titanium-Niobium:

   • More flexible than TMA (60% stiffness), ideal as finishing wires.

3. Multistrand Stainless Steel Wire:

   • Flexible and strong; however, its clinical performance is inferior to NiTi in range and springback (Kusy & Dilley, 1984; Tidy, 1989).

Clinical Evidence and Limitations

Despite the technological advancement of NiTi wires, clinical trials offer mixed evidence. A Cochrane review (Wang, 2013) and studies by Abdalrahman et al. (2015) and O’Brien et al. (1990) found no significant differences in tooth alignment efficiency or pain perception among various NiTi types during initial alignment. Additionally, superelasticity only initiates after significant deflection, which may paradoxically alter the wire’s TTR—rendering it non-superelastic in some cases.

Conclusion

Nickel Titanium wires have transformed modern orthodontics by enabling light, continuous forces with predictable biomechanical responses. Understanding their structure, phase behaviour, and clinical implications helps orthodontists tailor treatment plans to each patient’s needs. While innovations continue to evolve—such as dual-dimension wires and ion-implanted coatings—NiTi remains a cornerstone of effective, efficient, and comfortable orthodontic treatment.