Dr. Leon Laub
Director, Product Development, RMO, Inc. Adjunct Clinical Professor, Dept. of Orthodontics, University of Illinois, Chicago, IL Professor Emeritus, Loyola University School of Dentistry, Chicago, IL
[This article was written for Rocky Mountain Orthodontics. The Orthodontic CYBERjournal editors felt that it merited publication for general information.]
When I was in college, one of my Professors showed me a wire that could be bent into complex shapes, then after dipping into warm water it sprang back to its original form. It was fun to play with, but had no applications at that time. Many years later, this same material has revolutionized the ability to move teeth. The material was developed for entirely other reasons at the U.S. Naval Ordnance Laboratory (NOL). It is an alloy of 55% Nickel (Ni) and 45% Titanium (Ti) and became known as NITINOL. The internal structure of NiTi wires consists of an arrangement of atoms in patterns that differ at low and high temperature.
A TRANSITION TEMPERATURE separates the low and high temperature structure. The internal structure, or configuration of atoms in the wire, changes when the temperature of the wire goes through the transition temperature. A structure that is distinct and homogeneous is known as a PHASE. The low temperature phase in NiTi wires is called MARTENSITE. The high temperature phase is called AUSTENITE. Martensite and austenite have atomic structures that differ from each other. Properties of the wire are related to its structure. Heating or cooling a wire through its transition temperature occurs with a change in properties. Therefore the properties of martensite and austenite differ. A thermal wire is delivered to the clinician in its martensite phase. Wire having the martensite phase feels soft and bends easily outside the mouth. In the mouth, the wire warms up to body temperature and its phase changes from mar tensite to austenite. The wire “remembers” its arch shape before the clinician bent it and attempts to return to this shape. This property is known as SHAPE MEMORY. At body temperature, the wire has an austenite phase and its properties are then called SUPERELASTIC. The superelastic wire feels stiff and springy; when the clinician attempts to bend it, it immediately recovers to its original shape without taking a permanent bend.
ALL NiTi wires used in orthodontics have both SHAPE MEMORY and SUPERELASTIC properties. If you understand this fact, then under standing the properties and uses of the wires is easy. A transition temperature distinguishes shape memory from superelastic properties. Control of processing and heat treatment during production establishes the transition temperature. Below the transition temperature, the wire has shape memory properties. Above the transition temperature, the wire has superelastic properties. A NiTi wire can be processed during production to go through a transition at any temperature between 0˚C – 100˚C. In orthodontics, wires are mainly used at two temperatures: Room Temperature (20˚C) and Mouth Temperature (35˚C). Manufacturers supply wires at room temperature that either have shape memory properties or are superelastic. Superelastic wires, as-received by the clinician, are already above their transition temperature. As soon as a force is removed from bending the wire, the wire returns to its original shape. The wire is not permanently bent.
NITI THERMAL OR HEAT-ACTIVATED WIRES
A wire having its transition temperature below, but close to 35˚C, is called a thermal wire. The manufacturer supplies a wire to the clinician that is bendable and remains bent below its transition temperature. As the clinician engages the wire in the mouth, the force that he exerts is absorbed by the wire. When the wire equilibrates in the mouth, it passes through its transition temperature and becomes superelastic. Part of the energy absorbed by the wire then exerts a force on the teeth, as the wire attempts to return to its original arch shape, which leads to remodeling of the teeth. The wire’s return to its original arch shape after passing through its transition temperature is the shape memory property. Small diameter round NiTi thermal wire is often the wire chosen for initial alignment of teeth. The wire bends easily when the clinician engages it into the arch slots. When in the mouth, the patient’s body temperature provides the energy to drive it through its transition temperature with the resultant change in phase. Outside the mouth, the wire is fully martensitic and soft; once inside the mouth it becomes fully austenitic and springy.
RMO’s heat activated wire for the SWLF System is called Thermal NiTi. Recently, smaller size round wires were added to the product line in the Natural Arch Shape. A 0.013” round wire is used as the initial alignment wire for .018” arch slot brackets for teeth that are highly irregular. A 0.015” round wire is used for .022” arch slot brackets. These smaller heat activated wires lead to faster unscrambling of teeth. Brands of thermal wires are not all the same. The purity of the raw materials used in the alloy, optimizing composition by adjusting the minor ingredients in the formula, whether the alloy is specifically formulated to be only a thermal wire or used to produce both thermal and superelastic wires, and processing parameters all account for differences among brands.
SWLF Thermal NiTi starts with a formula that is optimized for heat activated wires; the same alloy is not used to also produce superelastic wires. A NiTi alloy that is used to produce both thermal and superelastic wires is similar to a multi-function tool. The tool works satisfactorily for several purposes, but doesn’t excel at any one. However, the tool that is specifically designed only for one purpose excels at that application. This is one reason why SWLF Thermal NiTi is the market leader. One way to evaluate consistent performance of a wire brand is to test round and rectangular wires from the same brand. If the manufacturer gave proper attention to the starting alloy and processing conditions, properties such as resiliency and transition temperature should be the same. Brands of thermal wires were evaluated for resiliency and transition temperature. Results are given below. A standard industry test for wire stiffness is named after the manufacturer of the testing machine: Tinius Olsen Stiffness Test. The method is to bend a length of straight wire from 0 to 90 degrees (right angle) while simultaneously measuring the force and bend angle. Release the load after the wire is bent 90 degrees and record the set angle (angle that the wire is permanently deformed) after the load returns to 0. A stiffness test in progress is shown in Figure 1.
Stiffness test results are compared for three types of materials in Figure 2: SWLF Stainless Steel, SWLF Beta III Titanium (Ti-Mo), and RMO Orthonol (Ni-Ti).
The Stiffness Test measures the force (load) required to bend (deflect) a wire. Loading represents a Doctor applying a force on a wire when first engaging it into the bracket arch slots. Deflection is the angle that the wire is bent during placement. The loading part of the force-deflection curve looks similar to a load-elongation, or stress-strain diagram. However, actual values for mechanical properties (e.g., modulus of elasticity, proportional limit, and ultimate tensile strength) can only be read from the stress-strain diagram; not from a stiffness test graph. Although, the slope of the linear part of the force-deflection curve does describe the relative stiffness of the wires tested. The steeper the line, or the larger its slope, the greater is the stiffness. Comparing the initial slopes of the three materials in Figure 2, the order from stiffest to most flexible wire is SWLF Stainless Steel, SWLF Beta III Titanium (Ti-Mo), and RMO Orthonol (Ni-Ti). The clinical meaning of this result will be discussed below in the section on Ti-Mo wires.
Stiffness test results for SWLF Thermal NiTi 0.016” Round wires at 100˚F are shown in Figure 3.
The loading part (forward curve) of the test, from 0 to 90 degrees deflection, is directly related to the force that a doctor puts on a wire during placement. After the wire is engaged, the force is removed. The energy absorbed by the wire exerts a force on the brackets, which leads to remodeling teeth. The unloading portion of the curve, from 90 to 0 degrees, indicates the magnitude of the force that the wire exerts on the tooth and how constant that force will be. If the wire is entirely resilient, the force returns to 0; i.e., there is no permanent wire deformation. In clinical usage, wires are not usually bent more than 30 degrees. It is clinically meaningful to focus on the unloading portion of the curve between 0 and 30 degrees deflection. According to Dr. Robert Ricketts, constant low forces are considered ideal to move teeth. The NiTi wire is resilient; it is always active. As a result, the wire always provides a small continuous force to move teeth.
For some wires, after the force applied to the wire by the Doctor is removed, the wire is permanently bent and stops working to move teeth. This is the case for Stainless Steel and Beta III Titanium. See Figure 2. For these wire types, the force to move teeth diminishes rapidly and the wire is permanently bent. The angle it is bent is called the set angle; its value is the angle when the force returns to 0.
To evaluate differences among brands of thermal NiTi wires, testing was performed for stiffness, resiliency and transition temperatures. Brands included: SWLF Thermal NiTi (RMO), CuNiTi 35˚C (Ormco), Sentaloy/Neo Sentaloy (GAC), and Thermal Activated NiTi (3M Unitek). In-house test results are described below.
Stiffness results, using a Tinius Olsen Stiffness Tester at 100˚F, for both round and rectangular thermal NiTi wires, show that SWLF Thermal NiTi wires require a lower force to engage the wire during placement and have the lowest, continuous force during unloading (after the applied force is removed) to move teeth. See Figure 3.
Resiliency among brands of thermal NiTi wires was compared by bending wires back and forth multiple times and measuring the number of degrees of permanent deformation after testing. The wire having the smallest permanent deformation is the most resilient. Results show that SWLF Thermal NiTi has the lowest permanent deformation; the same low numerical value was obtained for both round and rectangular wires. All other brands had larger amounts of permanent deformation. For other brands, the round and rectangular wire results differed which indicates that the wires do not have consistent properties.
The ideal transition temperature for a thermal NiTi wire is open mouth temperature, 35˚C. When the transition temperature is close to 35˚C, the Doctor has a longer working time for placement than at lower temperatures. SWLF Thermal NiTi wires have a transition temperature of 32˚C for both round and rectangular wires. All other brands have transition temperatures in the range of 23˚ to 29˚C. For other brands, the transition temperatures for round and rectangular wires differed, indicating inconsistencies in the alloys used. More importantly, several brands had transition temperatures close to room temperature, so that the transition from thermal to superelastic properties was mostly completed before the wire was engaged in the mouth. Then, shape memory properties are nearly non-existent; it is more difficult for the Doctor to engage the wire; and it is more uncomfortable for the patient. For the low transition temperature brands, the amount that the wire can be bent at room temperature is reduced, and the wire exerts a greater force on the teeth during placement.
CONCLUSIONS BASED ON TEST RESULTS
SWLF Thermal NiTi Wires are superior to other brands of thermal wires based on test results for:
- Stiffness Testing (loading curve): lower force needed to place wire – greater patient comfort
- Stiffness Testing (unloading curve): lowest and most consistent force is exerted by wire to move teeth – wire is always working to move teeth; ideal force to move teeth
- Resiliency: most resilient wire among brands tested – completely recovers in the mouth without taking a permanent bend
- Transition Temperature: very close at open mouth temperature, and much higher than other brands – longer working time for Doctor and greater patient comfort
- Transition Temperature: temperature is the same for round and rectangular wires – gives consistent and predictable forces to move teeth
- Transition Temperature: wire has a large recoverable strain during placement – smallest possibility to over bend wire during placement by Doctor
NITI SUPERELASTIC WIRES
Properties of superelastic wires differ from thermal activated wires. It has been thought that temporary or permanent bends could not be made in superelastic wires. This is not true. A bend can be temporarily made to a superelastic wire in the mouth whenever needed. To do this, cool the wire through its transition temperature. The structure is converted from austenite to the low temperature phase, martensite, and the bend is made. Products that can cool the wire, such as a coolant spray dabbed on the region of the wire to be bent, change the structure at the point of application. As soon as that section of wire warms again to body temperature, the phase changes again from martensite to austenite. The wire exerts a force to move teeth, as it attempts to revert to its original arch shape. Permanent bends can be made to a superelastic wire by heating the region to red hot with a lighter, remove the flame, and bend to the shape needed. The wire becomes dead soft and will not return to its original shape.
RMO’s superelastic wires are called Orthonol and Bio-Lastic. Orthonol is available in the Natural and Ideal Arch Shapes. Bio-Lastic is available in the Ricketts Penta-Morphic shapes, with a dimple in the anterior section. These wires are entirely austenite at room temperature (20˚C). These wires, as-received by the clinician, have already gone through their transition temperature.
Titanium-Molybdenum wires were developed to have properties between stainless steel and Ni-Ti. The composition is: 79% Ti (Titanium), 11% Mo (Molybdenum), 6% Zr (Zirconium), and 4% Sn (Tin). Most Ti-Mo brands do not contain Nickel (Ni). Nickel is associated with allergic responses in some people, and Ni-containing alloys are contraindicated for them. People have become sensitized to Ni from wearing objects such as earrings or buttons that are nickel plated. The first Ti-Mo brand that was patented and commercially developed is called TMA (Ormco). The patent expired a few years ago and now there are several brands of Ti-Mo wires on the market, each having different properties.
In the SWLF System, the Ti-Mo wire that is used for finishing cases in adults and for periodontal patients is called Beta III Titanium. The name Beta III refers to a unique phase in the Ti-Mo System. Other names that are used for the same type of alloy are: Beta Titanium and Bendaloy (RMO). Beta III Titanium has excellent formability, good corrosion resistance, and a smooth polished surface.
Compared to Stainless Steel, Beta III Titanium has moderate stiffness (modulus of elasticity is lower); it is more resilient, and can be deflected more without permanent deformation. In Figure 2, the initial slopes for Stainless Steel and Beta III Titanium show the relative stiffness between the two wires. A greater slope for the initial straight line portion of the curves indicates that Stainless Steel is stiffer. Beta III Titanium wires are approximately 40-45% less stiff. Appliances made from Ti-Mo can be activated without permanent deformation compared to stainless steel.
Properties of Beta III Titanium wires make them an excellent choice for many clinical applications. Moderate stiffness permits root correction. Excellent formability allows intricate loop designs to be made. Bends can be formed for overcorrection. Tie-backs can be placed due to the wire’s ductility.
Titanium-based alloys have changed the way orthodontists treat cases. Clinicians are just beginning to learn how to use these wires efficiently and their performance limitations. Among alloy types, brands are not all the same. Property testing and clinical evaluations distinguish among brands. RMO has taken the leadership role among orthodontic companies to provide superior, high quality Titanium-based wires at very competitive pricing.