Reviews
Karol Nosalik
1, A, B, D, F, Maciej Kawala
2, A, B, FContemporary NiTi Archwires – Mechanical Properties
Współczesne łuki NiTi – właściwości mechaniczne
1 Orthodontics Clinic, 5th Military Hospital with polyclinic in Cracow, Poland 2 Division of Dental Prothetics, wroclaw Medical University, Poland
A – koncepcja i projekt badania; B – gromadzenie i/lub zestawianie danych; C – opracowanie statystyczne; D – interpretacja danych; E – przygotowanie tekstu; F – zebranie piśmiennictwa
Abstract
Nowadays, in times of strong commercialization of the medical sector, the conscious selection of orthodontic arch-wires based on scientific evidence has a crucial meaning. The unique mechanical properties of NiTi alloy make it one of the most commonly used materials for orthodontic archwires. This article defines terms specific to a group of NiTi archwires, such as thermoelastic martensitic transformation, temperature transitional range (TTR) of mar-tensitic transformation, shape memory effect, superelasticity and the associated termoelasticity and pseudoelastic-ity. This article aims to systematize the knowledge of the mechanics of NiTi archwires available today based on the literature review (Dent. Med. Probl. 2012, 49, 3, 433–437).
Key words: NiTi archwires, superelasticity, mechanical properties.
Streszczenie
Obecnie, w czasach silnej komercjalizacji sektora medycznego, świadomy dobór leczniczych łuków ortodontycz-nych na podstawie dowodów naukowych nabiera zasadniczego znaczenia. Niezastąpione właściwości mechaniczne stopu NiTi spowodowały, że jest to jeden z najczęściej stosowanych materiałów do produkcji łuków ortodontycz-nych. w pracy zdefiniowano terminy swoiste dla grupy łuków NiTi, takie jak: termosprężysta przemiana mar-tenzytyczna, zakres temperatur przemiany martenzytycznej (TTR), zjawisko pamięci kształtu, superelastyczność oraz związana z nią termoelastyczność i pseudoelastyczność. Celem pracy jest usystematyzowanie wiedzy z zakresu mechaniki dostępnych współcześnie łuków NiTi na podstawie przeglądu piśmiennictwa (Dent. Med. Probl. 2012,
49, 3, 433–437).
Słowa kluczowe: łuki NiTi, superelastyczność, właściwości mechaniczne.
Dent. Med. Probl. 2012, 49, 3, 433–437
issN 1644-387X © Copyright by wroclaw Medical University and Polish Dental society
Nowadays, particular emphasis is placed on evidence-based medicine; nevertheless, conscious selection of orthodontic archwires based on clin-ical trials is of paramount importance. However, the intensive commercialization of the medical sector, manifested by persistent marketing of com-panies which offer archwires, very often stands in opposition to that trend. sellers’ assurances of the ‘magical’ effects of the improved archwires are not always supported by specific information regard-ing temperature transitional ranges of martensite transformation (TTR) of NiTi archwires, mechan-ical properties or conditions in which the trials were performed. Frequently, doctors are misled by
the name of the material which suggests the pres-ence of superelastic properties. For the assessment of clinical applications of NiTi archwires, two fun-damental attributes are of great significance: tem-perature transitional range (TTR), which ought to be similar to the temperature of the oral cav-ity and low deactivation force affecting the peri-odontium [1, 2].
Contemporary NiTi archwires comprise a big group of materials varying in properties. They are extensively used in the orthodontic practice, par-ticularly at the initial stage, due to their high elas-ticity and wide working range. Proffit presents a general division of NiTi archwires into two
ma-jor categories: M-NiTi (stabilized martensitic al-loys) and A-NiTi (austenitic alal-loys). M-NiTi wires are much springier than other orthodontic alloys (stainless steel, cobalt-chromium, beta-titanium) and are quite strong, but they have poor formabil-ity and are not described as having shape mem-ory effect or superelastic behaviour. The A-NiTi group includes archwires utilizing the shape mem-ory effect and superelasticity that is manifestetd by very large reversible deformations [3]. Discrepan-cies between the mechanical properties of the two groups in question resulted in their different ap-plications. Thin, round A-NiTi archwires are used when a wide range of activation, generating small, constant force is required. Thicker, and very often more rectangular M-NiTi archwires are employed in later stages of treatment when some springy but stiffer material is desired [3].
in order to assess the clinical applicability of orthodontic archwires three main properties are essential: strength, stiffness or springiness and working range. in practice, higher springiness al-lows using higher activation of the archwire [3–8]. All of the properties may be defined by referring to the load-deflection plot [2]. The diagram is a graphical representation of the wire’s behaviour while being subjected to bending.
For M-NiTi archwires the load-deflection plot bears some similarity to other orthodontic al-loys (steel, TMA) [9]. initially, the curve runs in a straight line. its inclination in relation to the Y-Axis represents the stiffness of the wire. The small-er the inclination the more springiness thsmall-ere is in the archwire [3, 9, 10].
The load-deflection plot for A-NiTi archwires differs considerably (Fig. 1). its characteristic
fea-ture is so-called activation/deactivation plateau. The load-deflection curve, after exceeding a cer-tain force value becomes a horizontal line (activa-tion plateau) and the archwire may absorb more load, generating a deactivation force (deactiva-tion plateau), on the periodontal fibres, the value of which is constant and lower than the value of the activation force. The phenomenon of the dis-crepancy between the activation and deactivation force is referred to as the elastic hysteresis [2]. The hysteresis loop for the superelastic Chinese NiTi is characterized by being flat while loading and un-loading [11]. Putting it into practice, the superelas-tic NiTi archwire exerts a smaller force upon the periodontal fibres than that which was used to put the archwire into the bracket slot (Fig. 2). Low-er deactivation forces exLow-erted by the supLow-erelastic nickel-titanium archwires correspond to the phys-iological response of the bone and minimize the negative undermining resorption [12].
After Begg’s introduction of the low force tech-nique, a search was initiated for a material with lower stiffness and increased elasticity to replace the known and highly valued stainless steel [9]. The groundbreaking, though accidental, discov-ery was made by Buhler et al. in 1963. They ob-served that an object treated with heat which was earlier deformed, returns to its original shape in a particular temperature range. The return to the shape before the deformation is referred to as the shape Memory effect (sMe) [4, 9, 13–15]. Thanks to Andreasen’s commitment it was introduced to orthodontics in 1972. The first orthodontic NiTi was a martensitic stabilized alloy and did not show the shape memory effect in the intraoral temper-ature range. it was characterized by a linear load-
Fig. 1. Diagram representing the load-deflection curve
for stainless steel, TMA and NiTi alloy
Ryc. 1. schemat przedstawiający wykres zależności
między naprężeniem a odkształceniem dla stali nierdzewnej, stopu TMA i stopu niti
Fig. 2. Diagram representing the load-deflection curve
for superelastic NiTi alloys
Ryc. 2. schemat przedstawiający wykres zależności
między naprężeniem a odkształceniem dla superelasty-cznych stopów niti
-deflection curve and was much more elastic than steel. A main drawback of the first NiTi was the lack of clinically applicable formability (impossi-ble to bend) [4, 9]. Trials were continued, which resulted in the introduction two new superelas-tic alloys – Chinese and Japanese NiTi in the nid – 1980 s. Further research led to selecting active NiTi alloys with an active martensite structure (thermoelastic archwires) and the active austenite (pseudoelastic archwires) [9, 16, 17].
The characterization of NiTi archwires re-quires employing notions specific to this type of materials. They include: thermoelastic martensite transformation, temperature transitional range, shape memory effect, superelasticity and related thermoelasticity and pseudoelasticity. Contem-porary NiTi archwires can be found in two crys-tal structures: low-temperature form – marten-site and high-temperature austenitic form. Going from one phase to the other is called thermoelas-tic martensite transformation, which is a revers-ible phenomenon and is dependent on the changes of the temperature of the environment or the ap-plied force. The transformation is characterized by the occurrence of the transitory phase R between martensite and austenite and the specific temper-ature range for this transformation (TTR), the be-ginning and end of phase forming: Ms, Mf, As and Af. The martensite transformation is of great im-portance for the clinical use of NiTi archwires as it is responsible for the shape Memory effect and the pseudoelastic properties (siM) [1, 15, 18].
in the NiTi alloys group the temperature tran-sitional range is broad and it ranges from –200oC
to +200oC. A subtle change of chemical
composi-tion or thermo-mechanical processing has a great influence upon the TTR value [15, 19]. The addi-tion of copper in CuNiTi alloys results in the nar-rowing of the TTR e.g. for Thermo-Active
Cop-perNiTi40® the transformation from martensite to
austenite starts at 24.5oC and ends at 40oC [30]. The
forces generated by this archwire are low; there-fore, it is recommended for patient experiencing periodontal problems. in practical terms, TTR is of vital importance when assessing the quality of the archwire, since it is responsible for the pres-ence of superelastic properties. TTR trials indi-cated that the Af temperature of half of the arch-wires in question varied from 28oC to 38oC, which
proves that in the intraoral environment they are austenitic archwires. Applying the archwires, the Af of which is significantly higher than the body temperature (Nitinol® – 62oC), does not result
in any force or exerting intermittent or irregu-lar force on the periodontal fibres [1, 11, 20, 30]. From the clinical point of view, TTR should oscil-late between slightly below or close to the
temper-ature in the oral cavity i.e. from 35oC to 37oC, so
that the archwire can present phase R, the transi-tion between martensite to austenite. As a general rule, it must be acknowledged that the superelas-tic archwire in the austenite phase is stiffer than in the martensite phase, however both these phas-es are stiffer than the same archwire in the transi-tory phase R [1].
Thermoelastic archwires producers claim that between 37–40oC, archwires are fully active,
as-suming 37oC is the dominant intraoral
tempera-ture. The range of temperature in the oral cavi-ty, however, varies from one area to another and depends on the food eaten and the patient’s be-haviour – habitual breathing through the mouth, smoking cigarettes, and is on average 35.5oC.
Thermoactive archwires, the Af of which is close
to 40oC (40°C Thermo-Active Copper NiTi,
Neo-sentalloy® 200 g) should not be used at the initial
stages with the patients breathing through their mouths, whose intraoral temperature ranges 30– 33oC [21, 30].
As far as the biomechanics is concerned, an important phenomenon occurring within the Ni-Ti archwires group is the regaining of shape af-ter deformation. The shape memory effect occurs as a result of thermoelastic martensite transfor-mation, i.e. the transfiguration of crystallograph-ic structure between the martensite and austen-ite within a specific TTR [23]. in the active alloys (thermoelastic and pseudoelastic) the effect of re-gaining the shape is a consequence of a mechani-cal action (force reduction) or temperature action (heating) [4, 19, 22]. in terms of the clinical use, if TTR is too similar to the temperature in the oral cavity, the shape memory effect may occur whilst placing the archwire and consequently hamper a precise adjustment of the archwire in the brack-et slot. when TTR is above the intraoral tempera-ture, achieving the shape memory effect is possible by rinsing the oral cavity with warm liquids [19].
NiTi archwires can easily transform their crys-talline structure from martensite into austenite when subjected to the temperature change – ther-moelasticity, or from austenite to martensite as a result of exerting force – pseudoelasticity [34]. The presence of these two properties proves the superelasticity of the archwire [12]. The result of the phenomenon of thermoelasticity is the regain-ing of the shape – sMe. in practical terms, a pa-tient may activate or deactivate the archwire by fol-lowing the doctor’s instructions and washing his mouth with either cool or warm liquids. The sci-entific base enabling the use of thermoelastic arch-wires are various accounts arguing that bone re-models itself more effectively if subjected to a dy-namic load rather than a static one [26, 36, 38].
Pseudoelasticity occurs locally in a place of large deformation, forming a phase of the stress induct-ed martensite – siM, in the archwire whose struc-ture is almost entirely austenitic. so as to take ad-vantage of the pseudoelastic properties, the defor-mation of the NiTi archwire above 50–70 degrees is necessary. Only with such a bend, the superelas-tic plateau of activation can be achieved. From the clinical perspective, this situation can be encoun-tered most frequently in the vicinity of the incisors of the lower jaw, where teeth are crowded and the distance between brackets are reduced. The trials confirm a higher efficiency of superelastic multi-stranded arches than superelastic single-core in the phase of levelling [24, 31]. The use of rectan-gular superelastic archwires (e.g. CuNiTi) in the initial phase of levelling is supported by the idea that they generate low, but constant force retaining the three-dimensional control of the tooth’s posi-tion. Nevertheless, as some studies prove, the tor-sion of superelastic rectangular archwire does not generate superelastic behaviour [30, 33]. There are no significant differences in functioning between CuNiTi archwires and superelastic NiTi archwires in the phase of levelling [32]. some of the archwires exhibit superelastic properties, but only if the bend is larger than 2 mm [30]. The size of the superelas-tic archwire is of no great influence over the gen-erated force. For NiTi of 0,018” diameter as well as for 0,020”, the plateau of deactivation was dis-played on the same level [35]. Considerably higher efficiency of superelastic archwires is present only in the application employing Begg’s slot. Their ef-fectiveness compounded with the use of the edge-wise-type appliance is comparable to the multi-stranded steel archwires [37]. if the crowding is moderate, superelastic archwires need not to be used. in that case, martensitic stabilized archwires (nitinol) of small diameter or multistranded steel wire are sufficient. For achieving optimal forces, it
is essential to select an appropriate diameter and to eliminate rectangular archwires from the levelling phase [2]. Trials evaluating superelastic and con-ventional NiTi archwires with regard to the move-ment speed of a tooth as well as the pain sensation have not validated the differences in their proper-ties [2]. The superelastic A-NiTi significantly re-duces the need for replacing thin archwires with the thicker for levelling and alignment [19].
There are many classifications of NiTi arch-wires. Kusy categorizes them taking the crystalline structure into consideration as follows: conven-tional nitinol-stabilized martensite, Pseudoelas-tic-active austenite and ThermoelasPseudoelas-tic-active mar-tensite [4]. waters provides a different classifica-tion, based on the dissimilarities in TTR: i – active martensite alloys the TTR of which ranges from the room temperature to the body temperature, ii – austenite alloys whose TTR is below the room temperature, iii – alloys characterized by narrow TTR close to the body temperature [27]. There is also a dichotomy splitting them into superelastic and tempered NiTi [25]. evans and Durning troduced a classification of orthodontic alloys in-to five categories: 1) stainless steel, gold; 2) first stabilized NiTi; 3) superelastic wires – active aus-tenites TTR of which is below the intraoral tem-perature (pseudoelastic); 4) thermodynamic wires – active martensite whose TTR is close to the in-traoral temperature; 5) a group of thermodynam-ic wires featuring diverse TTR (e.g. cuNiTi 35oC,
37oC, 40oC) [28, 29].
To conclude, NiTi archwires must be regard-ed as an indispensible tool in the orthodontist’s inventory. The selection of NiTi archwires ought to be based on scientific evidence rather than ad-vertisement. specific information regarding their mechanical properties is vital while making the choice of the archwire, as it translates into their clinical action.
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Address for correspondence:
Karol Nosalik ul. sienkiewicza 9/6 30-033 Kraków Poland Tel.: 605 367 633 e-mail: knosalik@wp.pl Received: 23.06.2012 Revised: 6.07.2012 Accepted: 16.07.2012
Praca wpłynęła do Redakcji: 23.06.2012 r. Po recenzji: 6.07.2012 r.
