INTRODUCTION

At the beginning of the 1970's Andreasen^1,2 introduced the first nickel-titanium (Ni-Ti) wire in orthodontics. This can be attributed to William F Buehler^3,4 who discovered this alloy. He named this wire NITINOL, as acronym of the two most important elements composing the alloy: nickel (Ni), titanium (Ti) and NOL for Naval Ordnances Laboratory in Silver Springs, Maryland, USA where the alloy was discovered.

According to Miura^5,6,7,8 Ni-Ti alloys possess two main characteristics that render them unique in orthodontics: shape memory and super-elasticity. Shape memory refers to the ability of the material to return to its original shape through temperature transformation phases. Super-elasticity means the possibility of generating constant forces for a long period of time; tension being the force to cause this property.

Transformation phases

The unique characteristics exhibited by Ni-Ti alloys are mainly due to the transformation phases they undergo. The first one is the high temperature phase, also called austenitic phase, the other low temperature phase is called martensitic phase (Figures 1 A-B), these phases do not appear at a given temperature, rather, they possess different temperatures where these changes gradually appear. Ni-Ti alterations apparent in austenitic and martensitic phases are basically at three temperatures: initial, peak and final. Depending on the final application for which the product manufactured with this alloy is intended, it will be important to determine transformation temperature. This can be clearly observed in the graphs known as thermograms which are generated during the differential scanning calorimetry test (Figure 2). Austenite final (AF) is the most important temperature to determine from the clinical and manufacturer's point of view. It is considered the most important since, at this phase, the alloy is stable and exhibits the final, work-suitable shape.

Figures 1 A-B A) Austenitic phase: shape of orthorhombic crystal, angles exhibit 90° angle, B) Martensitic phase: shape of monoclinic crystal, angles displace braking their original 90°. 

Figure 2 Transformation phases of NiTi alloys in heating and cooling curves represented in the thermograms. 

Ni-Ti alloys classification

Kusy⁹ made up a classification by dividing these orthodontic alloys in three main groups: the first one is passive (passive martensitic) the other two are active (austenitic and martensitic). All of them are practically composed of 50% Ni and 50% Ti (with small composition variants). They possess the unique characteristic of returning to their original shape (shape memory). Differences between them mainly lie in two aspects: the first one is that active forces generate constant load during the phases, and the second difference, also found between active forces lies in the temperature at which this transformation phase is generated.

According to Kusi, the first alloy to be used in orthodontics was martensitic passive. This alloy possessed the shape memory property only in name. This was due to the fact that errors incurred upon during manufacturing process when stretching the material to shape it caused incapacity to generate continuous forces, only preserving the ability to return to its original shape. Nevertheless, forces generated by these alloys when compared to other archwires such as stainless steel, were lesser by one fifth. In 1971 this alloy appeared for the first time in Orthodontics with the commercial name of NITINOL (UNITEK Co) (Figure 3).

Figure 3 Martensitic passive: Archwires return to original shape but exhibit linear behavior. Force generated when compared with other traditional archwires such as stainless steel and NiCrCo is of one third. 

With time, a second generation of nickel titanium was generated, under the name of active austenitic. When describing this material, Kusi mentioned as its main characteristic the fact that, differing from passive martensitic, the wire did not only generate mild force, it also presented the unique characteristic of generating continuous forces at both the activation and deactivation phases., and this was mainly due to the performance they exhibited at the austenitic phase (high temperature phase) and the martensitic phase (low temperature phase) in the process of mechanical transformation between both phases.

In this austenitic-active alloy both phases can be clearly observed: the beginning is with a lineal force three times greater than the force generated by a passive martensitic archwire. This linear force disappears and brings about a curve where the archwire generates a continuous force for a long period of time; this is known as the activation phase (Figure 4 A-B). The archwire experiences a transformation from martensitic phase to austenitic phase; once this latter is completed, the archwire once more experiences a linear fall for a short period, to then generate a continuous force for a long period which is expressed in the curve of the graph. This phase, encompassing from point C to D, and from D to F is considered the most important from the orthodontic clinical standpoint (Figure 4), since it is in this deactivation phase where dental movement is generated. (B-C-D-E). Nevertheless, this austenitic-active alloy suffers from a great disadvantage, since the temperature at which the archwire experiences this transformation (AF austenitic final) is far removed from body temperature.

Figure 4 Active Austenitic and Martensitic. Just like the passive form, they return to original shape but force generated in the activation phase (B-C) and deactivation phase (C-D) present a vale which generates continuous forces for a long period of time. Difference between both lies in the temperature at which these changes take place. 

Finally, active-martensitic alloys, whose most important characteristic is the fact that, besides preserving activation and de-activation phases of active austenitic alloys, their transformation phase is mechanically generated within the mouth since it is close to body temperature; therefore, they generate a continuous force for long periods, favoring thus ideal cell activity for dental movement in Orthodontic treatments. Miura introduced this type of alloys in Orthodontics.¹⁰

MATERIAL AND METHODS

Eleven archwires from different commercial brands were used for the present study (Table I), cuts were executed at the areas corresponding to premolars, since this area is the area presenting lesser tension during arch manufacturing. Approximate length of all samples was 4 mm (Figure 5); to this effect a low speed diamond disk was used; the area was cooled in order to avoid overheating of material and results alteration. Finally, all samples were weighed in order to achieve the greatest possible similarities in the size of all samples (Figure 6). Only in the case of Bioforce, the sample was divided in two sections, since this archwire, differing from all others, generates a mild force for anterior teeth, a moderate force for premolar zone, and heavy force in the posterior section, in this case the sample was not taken in the premolar area.

Figure 5 Archwire cuts in premolar areas. 

Figure 6 Weight of each of the samples. 

Differential scanning calorimetry was the method used to assess temperature phases of Ni-Ti alloys. The appliance used for this study was manufactured by Seiko SII-DSC6220, Seiko Instruments, Japan (Figure 7). All eleven samples were subjected to three measurements.

Figure 7 Appliance used for Differential Scanning test (Seiko Sil-DSC6220). 

Samples were placed in sealed containers and were subjected to a controlled temperature cycle; changes in phases of materials were recorded according to energy absorption or production. Samples’ temperature was recorded in a range of -100 °C to 100 °C; liquid nitrogen was used as coolant (Figure 8) at a 10 °C per minute speed, this was used to acquire both heating and cooling curves. All samples exhibited a thermogram which was duly printed for later assessment.

Figure 8 Nitrogen-based cooling system used in the test. 

RESULTS

Five different thermogram patterns were observed. They mainly indicated transformation phases of each sample, In some cases they presented ill defined peaks and intermediate phases (phase R) between austenitic and martensitic phases. In the case of CuNiTi archwires, all of them presented one peak in both the heat and cold curve (Figure 9A); NeoSentalloy 80, 160, Bioforce Anterior and Nitinol HA exhibited one cold peak and two heat peaks (Figure 9B); Nitinol SE presented two heat peaks and one cold peak (Figure 9C); Neosentalloy 240 and Bioforce posterior presented two heat peaks and two cold peaks (Figure 9D), finally LH Titan exhibited three heat peaks and three cold peaks (Figure 9E).

Figure 9A CuNiTi 35 Pattern. 

Figure 9B NeoSentalloy 80 pattern. 

Figure 9C Nitinol SE pattern. 

Figure 9D 4 Bioforce post pattern. 

Figure 9E 5 LH titan pattern. 

Results of mean and standard deviation of austenitic final (AF) temperature of all samples can be seen in Table II.

In the present study several aspects were taken into account in order to determine oral temperature. We know as a rule that temperature in the mouth changes constantly due to intake of hot or cold foods. For the present study 35 °C was taken as base oral temperature, since it was considered as ideal for this type of tests¹⁴; 25 °C was taken as environmental temperature (room temperature). Figure 10 indicates the values of samples and shows how these approach or move away from the 35 °C value. Only two samples out of the total sample number exhibited values above 35 °C: LH Titan (38.2 °C) and Nitinol SE (60.4 °C).

Figure 10 Values of all samples taking as base a temperature of 35°C. 

AF point of Bioforce Posterior, CuNiTi 27° and Nitinol archwires exhibited values below environmental temperature. Bioforce Anterior, NeoSentalloy 80 g, 160 g, 240 g, LH Titan and CuNiTi 40° and 35° archwires exhibited AF very similar to oral temperature, finally, Nitinol SE archwires presented AF well above oral temperature.

DISCUSSION

Nickel Titanium alloys possess unique characteristics in their transformation phases; they have the consequence of generating light and constant forces when passing from one phase to the other. Nevertheless, from the clinical point of view, it is of the utmost importance that this transformation take place at a temperature very similar to oral temperature, so as to be able to obtain maximum benefits when using these alloys in Orthodontics treatments.

In the manufacturing of different appliances using NiTi alloys, the active range of NiTi alloys is determined by the AF point; thus we have, as an example, in the catalogue of metal companies specialized in the production of these products, that point AF range can vary from 0 ¿C to -20 ¿C (Group N) up to 95 ¿C to 115° (Group H) (Figure 11). In this particular case, all final products which are going to be used in the human body must be manufactured with alloys whose point AF active range is found in Group B (20 ¿C to 40 ¿C).

Figure 11 Table of AF point of all materials depending on the final application of NiTi alloy. 

These materials are later subjected to a new process to be suitably formed and shaped. To this effect they are subjected to a new thermal treatment, where AF point of each new product is altered. Depending on the quality control and manufacture of the same product this point will be closer or farther from body temperature, and thus, its transformation phase might exhibit the following variables in the mouth with their corresponding secondary effects:

Special care must be taken when using CuNiTi40 archwires. Manufacturer's publicity mentions that they generate intermittent forces which are ideal for cases with periodontal problems, but since these archwires exhibit 38 ¿C AF, at this temperature their behavior indicates that transformation phase is closer to that of an active martensitic wire. This is possibly due to the fact that incorporation of hysteresis-reducing elements such as Copper and other elements such as Chrome to stabilize transformation temperature, hinder control of these materials¿ homogeneity, and, when compared to results of other studies¹⁵ they show great variability in AF temperature values.

Moreover, another low hysteresis arch such as LH Titan exhibits 38 °C AF. This will render it, on the one hand, malleable and easy to manipulate to be introduced in the mouth, since it presents a transformation phase slightly above body temperature, this arch is also suitable for patients with periodontal problems.^16,17,18

CONCLUSIONS

In the present study of eleven archwires, we can estimate that seven were found to be in a range close to oral temperature. For this reason we can expect them to exhibit Shape Memory property, and generate continuous and light forces for a period of time ideal for orthodontic treatment. Nevertheless, in our study we discovered that some archwires did not possess the Shape Memory that would enable them to be clinically used in the mouth as advertised by their manufacturers. Further research will be necessary so as to determine possible causes for these discrepancies.