The power of copper in orthodontics
Some specific chemical elements when added to the nickel-titanium alloy can significantly alter its properties, and copper, in a specific way, brings transition temperature stabilization as one of these advantages. It was in the mid-90s that this technology was introduced to orthodontics.
When we talk about transition temperatures we have to remember the memory alloys, in the form nickel-titanium and copper-nickel-titanium, which can have two presentations: austenitic and martensitic (Figure 1), depending on the temperature at which they are found.
If the alloy is in austenitic form it will be more rigid and with an excellent capacity for elastic recovery of its shape, without significant deformation. If it is in the martensitic phase, it will be much more flexible, but liable to deformation and, often, unable to return to its initial shape. There are specific temperatures for each alloy, which determine in what phase its structure will be, according to the temperature of the environment.
For us orthodontists, the most important of these temperatures is the so-called final austenitic or Af, because above this, all grains of the metal alloy will be in their austenitic phase.
Figure 1. Cryptographic structure of a nickel-titanium alloy.
Transition temperatures are important because they determine, among other things, the tension needed to induce superelasticity * in shape memory alloys. For the superelasticity to be induced and the thread, consequently, to develop a constant force and recover its initial shape through the shape memory, it is necessary that there are crystals in the austenitic form. However, in orthodontics, it is ideal for all crystals to be in this phase since maximum elastic recovery is desired, as we want all teeth to be thoroughly aligned. Thus, it is necessary to work with these wires above their Af temperatures.
* We call superelasticity the property that a wire has to release an almost constant force during its deactivation, through the reverse change of structure (from martensitic to austenitic) when through tension, we previously induced the transformation from austenitic to martensitic phase.
In addition, if the ambient temperature is too far from Af, as is the case with superelastic nickel-titanium wires (Af of approx. 10 ºC), the wire becomes too rigid and a lot of tension is required to induce the transformation of early stage. (Figure 2) even if this is achieved, the forces will be too high and the brackets may be released. The interesting thing is that we work with a thread that has its Af temperature slightly below the temperature of the environment in which the thread will be used.
Figure 2. A. Alignment performed with a leveling wire in its austenitic phase.
B. (0.016 ”) that was transformed to its martensitic phase due to the great deflection.
Same case when replacing the round wire used by a rectangular one.
OBS. Even with exaggerated deflections, such as the one in the photo, there is the possibility that a superelastic nickel-titanium wire with very low temperature Af does not undergo enough phase transformation so that it becomes superelastic.
Because the orthodontic appliance operates at a temperature of 37ºC, (average body temperature of humans), it is necessary that the nickel-titanium or copper-nickel-titanium wire has an Af temperature around 35ºC. Thus, with a minimum of tension it is possible to induce superelasticity, and as a consequence, obtain an excellent elastic recovery as the thread is deactivated through its shape memory.
However, it is very difficult to be able to control and standardize Af at 35ºC during the manufacture of nickel-titanium wires without the addition of the copper element. Even when trying to modify the Af temperature of superelastic nickel-titanium wires through heat treatment, the result is not standardized. That is why we see “thermo-activated” threads on the market with the most varied Af and that do not work the way clinically desired.
In an ongoing study in partnership with the Federal University of Rio Grande do Norte (Table I), the final austenitic temperature of several copper-nickel-titanium alloys available on the market was measured using a technology called: scanning calorimetry differential. The preliminary results show that, among the brands tested, Orthometric's Flexy NiTi Copper wire showed the Af temperature closer to the 35ºC desired by orthodontists.
Final austenitic temperature of four commercial brands of copper-nickel-titanium wires measured by differential scanning calorimetry. (Preliminary results of Ariane Gonzaga's dissertation, under the guidance of Prof. Dr. Sergei Caldas, UFRN).
USE OF COPPER-NICKEL-TITANIUM ALLOY WIRES IN ALIGNMENT
A 0.016 ”wire (Flexy NiTi Copper, Orthometric, Marília) can be used easily in sapphire brackets for quick alignment without using cooling to engage the device.
Figure 3. 0.016 ”copper-nickel-titanium wire, with 35ºC Af (Flexy NiTi Copper, Orthometric, Marília), used for alignment with sapphire brackets (Iceram S, Orthometric, Marilia).
IMAGE A. Initial.
IMAGE B. Alignment obtained after 25 days, allowing the insertion of a 0.016 ”x 0.022” Flexy NiTi Copper wire.
This can be done because the same phase transformation that occurs with the use of wire cooling, occurs by applying voltage caused by wire deflection. With the Af temperature of 35ºC, even small deflections can induce superelasticity. Likewise, a sequence of copper-nickel-titanium arcs, with an Af temperature of 35ºC, can be used from leveling to the finalization stage in a treatment with large crowds. The increased size of self-ligating aesthetic brackets is offset by the superelastic properties of this metallic alloy.
0.016 ”copper-nickel-titanium wire, with Af of 35ºC (Flexy NiTi Copper, Orthometric, Marilia) being used for alignment with self-ligating ceramic brackets (Iceram SLB, Orthometric, Marilia). Same patient with 0.017 ”x 0.025” rectangular Flexy NiTi Copper wire inserted in the upper arch after 68 days of treatment. Same patient with 0.019 "x 0.025" Flexy NiTi and 0.017 "x 0.025" Flexy NiTi Copper wires inserted after 90 days in the upper and lower arches, respectively.
In order to obtain the maximum advantages in the use of a shape memory alloy wire in orthodontics, it is interesting that its Af temperature is approximately 35º C, with the addition of copper to the alloy, this can be easily achieved. However, not all alloys available on the market have this characteristic, with the Flexy NiTi Copper brand being the closest to 35º C.
Otuka, K & Wayman, CM Shape Memory Materials. Cambridge University Press, Cambridge. 284 p., 1998.
Tonner, RI & Waters, NE. The characteristics of superelastic NiTi wires in three point bending. Part I: The effect of temperature. Eur J Orthod., V.16, pp.409-419, 1994.
PROF. DR. RENATO PARSEKIAN MARTINS
- Master, Doctor and Post-Doctor in Orthodontics, UNESP - Araraquara
- Graduate Collaborating Professor, Orthodontics, UNESP - Araraquara
- Sandwich Doctorate at Baylor College of Dentistry in Dallas, Texas
- Associate Editor of Revista Clínica de Ortodontia Dental Press
- Member of the editorial board of several national and international magazines