A Systematic review on aligning efficiency of superelastic NITI: in comparison with conventional NITI and multistranded stainless steel archwires

Background: The initial leveling and aligning phase of orthodontic treatment involve the use of nickel titanium wires. The nickel titanium wires exhibit unique properties like shape memory and superelasticity. In the beginning, conventional nitinol wires were used, which lacked the superelastic property. Nowadays, superelastic nickel titanium wires are being used commonly. This systematic review aims to evaluate the efficiency of Superelastic NITi in the initial alignment of orthodontic treatment in comparison with conventional NiTi and multistranded stainless steel archwires by measuring the amount of decrowding and the time taken for decrowding. Method: A systematic literature search was performed on Pubmed, Cochrane, Google scholar & Lilacs. Result: Thirty four studies were identified by the search and ten studies satisfied the inclusion criteria and one study was excluded after abstract reading. Finally nine articles were included for quality assessment. Conclusion: This systematic review concluded that there was no significant differences in the aligning efficiency of superelastic NiTi in comparison with conventional NiTi and multistranded stainless steel wires.


Introduction
Levelling can be defined as the first active phase of orthodontic treatment involving correction of individually malpositioned teeth, bracket slot leveling, correction of tooth arch discrepancy, initial correction of the curve of Spee, and torque adjustment (Evans et al. 1998). The ideal aligning archwire should have excellent strength, excellent springiness, and light continuous force of about 50g (Proffit et al. 1995). The common and broadly used materials are stainless steel and nickel titanium. In 1970, with the efforts of Dr.GeorgeAndreasen (Andreasen et al. 1971), the first commercially available NiTi alloy, Nitnol was invented. The first discovered nitinol was a martensitic stabilised alloy. Ironically the shape memory property was present in the composition only because this property was suppressed by cold working of the alloy during manufacturing. Hence it was a passive alloy with low force, low stiffness and good springiness but lacked formability. It was considered superior to stainless steel wires in that period. Later, Chinese and Japanese NiTi with super elastic properties were introduced by Burstone (Burstone et al. 1985) andMiura (Miura et al. 1986) respectively. Proffit (Proffit et al. 1999) has classified NiTi archwires into two major categories: M-NiTi (stabilised martensitic alloys) and A-NiTi (austenitic alloys). M-NiTi wires are much springy, lack formability, shape memory and superelasticity. The load deflection plot for M-NiTi wire is somewhat similar to other alloys (steel, TMA). The A-NiTi wires present superelasticity. It has an activation/deactivation plateau. On loading, the A-NiTi starts with a slope (stiffness) which produces three times the activation force of the conventional M-NiTi wires. Then the stiffness becomes comparable to M-NiTi. There is a stress-induced transformation of austenite to martensite. Upon unloading, reverse occurs, the linear region associated with martensitic phase gives way to the second plateau at a lower force. The martensite is transformed into austenite. When this transformation is complete, the initial high slope of austenite is revisited. This is called as the springback. The second plateau region in which martensite changes to austenite and changes shape is called pseudoelasticity. Pseudoelasticity occurs locally in place of large deformations due to the formation of stress induced martensite (SIM). In simpler words, austenitic active alloys are more flexible and have good spring back at room temperature and if a certain force is applied, small areas of stress induced martensite will be formed, which makes them less stiff in those areas consequently making it easier to fit into a slot. The force deflection of superelastic NiTi or austenitic active NiTi and the multistranded stainless steel wires are less than the conventional stainless steel wires making them favourable for correction of crowding (Quintao et al. 2009). The degree of decrowding or tooth alignment is usually assessed using Little's irregularity index (Little et al. 1975). This index is calculated using a three-dimensional digital model or directly on the model using a vernier calliper (Setchell et al. 1984). There are numerous in-vitro studies available to evaluate the superiority of one archwire over the other. There is a scarcity of in vivo studies assessing the efficiency of initial archwires in the literature (Evans et al. 1998). Hence this systematic review has been carried out to find out the efficiency of superelastic NiTi over conventional NiTi and multistranded stainless steel wires.

Selection criteria
Randomised controlled trials (RCT), prospective controlled trials, clinical trials, Studies comparing the Superelastic NiTiarchwire with conventional NiTi archwire and superelastic Niti with multistranded Stainless steel archwire, Studies which used round initial wires, Upper or lower full arch fixed appliances were included. In vitro studies, Case reports, case series, review articles, abstract papers, Trials which have studied pain perception and discomfort level with initial archwires, Lingual appliances and studies which have used rectangular NiTi wires or square NiTi wires for initial alignment were excluded.

Primary objectives
The variables of interest of this systematic review are: The amount of decrowding or theTooth alignment reported as LII/ DII/ ITA/ tooth movement (LII; Little's irregularity index, DII; Dental irregularity index, ITA; Index of tooth alignment). The Time taken for decrowding.

Risk of Bias in included studies
The quality assessments of the studies were screened by using (Higgins and Green. Cochrane reviewer's hand book 2009). The study was assessed to have a "High risk" of bias if it did not record a "Yes" in three or more of the four main category, "Moderate Risk "if two out of four categories did not record a "Yes," and "Low Risk" if all the four categories recorded: randomization assessor, Blinding and Completeness of follow-up were considered adequate. In case of non randomized and clinical trials without control group, it is recorded as not applicable. (TABLE-1&2), (GRAPHS-1&2).

Results
Thirty four studies were identified by the search and ten studies satisfied the inclusion criteria and one study was excluded after abstract reading. Finally, nine studies were included for quality assessment based on the inclusion criteria (Fig-1) The amount of decrowding and the time taken for decrowding of superelasticNi-Ti, conventional NiTi and multistranded stainless steels were recorded as the variable of interest Comparisons of two groups (Table 4&5) were made in this review. 1) First group consisting of superelastic NiTi and conventional NiTi with two studies. 2) Second group consisting of superelastic NiTi and multistranded stainless steelwith seven studies In Group I (Table 4) there was no statistically significant difference in the tooth alignment and the time taken for alignment between the wires.
In Group II (Table 5) Ramya et al and West et al have reported statistically significant differences in the time taken for alignment between superelastic NiTi and multistranded stainless steel wires. West et al has reported statistically significant differences between superelastic NiTi and multistranded stainless steel wire. The superelastic NiTi exhibits superelasticity and has better mechanical properties like good springback, low stiffness, high resilience, which is also present in multistranded stainless steel wires ( Kapila et al.1989). The load deflection rate of superelasticNiTi is also less than conventional NiTi and multistranded stainless steel wires (Quintao et al. 2009).Although superelastic NiTi was found to be superior to multistranded stainless steel andconventional NiTi in bench studies (Tonner 1994, Kusy 1997, it has to be noted that multi-stranded wires be it braided, twisted or coaxial have similar properties and great potential for use in the early stages of orthodontic treatment (Quintao et al. 2009). However, their low elastic limit makes them susceptible to plastic deformation because of external forces, such as chewing (Kapila et al. 1989). Superelastic NiTi is the only one archwire which produces the same level of force at any range of deflection and hence biologically more acceptable (Tonner et al. 1994). No clinically significant evidence therefore could be drawn from this systematic review which can be attributed to the following variables which could have been a probable source of bias.

Intervention
Although all studies used round initial wires, dimensions varied.

Degree of baseline crowding
Marco et al has included patients with a degree of crowding, which allowed deflection of archwire to 2mm and has checked for the homogeneity within the sample. Cobb et al has mentioned the inclusion criteria of LII>5mm. Ramya et al has mentioned the inclusion criteria of LII -7to 9 mm. Satpal et al has reported the inclusion criteria as moderate crowding 4 to 6mm and severe crowding 6 to 9 mm. Reem et al has reported the homogeneity of baseline data but has not mentioned about the degree of crowding before treatment. Jones et al has not reported the degree of crowding before treatment.

Period of follow up
Ranged from 5 to 8 weeks. No standardization for period of observation

Method of evaluation
Both 3D measurements and manual measurements using vernier callipers were used in these studies. The studies also used various different software for measuring digitally. Kevin et al used contact

Discussion
A total number of nine controlled clinical trials or randomised control trial were included in the current systematic review based on the selection criteria. These trials were aimed at testing whether there was a difference in the alignment efficiency of superelastic NiTi in comparison with conventional NiTi or multistranded stainless steel. The nine studies were divided as two groups. Group I (  (Table 5) which compared superelastic niti with multistranded SS Marco AbdoGravina et al. 2013 has arrived at a mean irregularity score of superelastic NiTi at 178.1and multistranded stainless steel at 180.9. He has also compared the differences in labial proclination of the lower incisors and curve of spee correction caused by superelastic NiTi & multistranded stainless steel, which, in fact, showed no difference. Both the wires invariably caused proclination of the lower incisors. There were no differences between the wires in correction of curve of spee, LII & IMPA. Catia et al. 2005 (Table 3&5) considered the alignment in the whole upper arch, whole lower arch, upper anterior area and lower anterior area and has given the mean irregularity score for SE Niti and multistranded SS wires in all four segments. This is the only study which has included the whole upper and lower arch and also the upper anterior and lower anterior area. The explanation given by the author was that the interbracket distance affects the wire properties and therefore, in order to assess the clinical performance of wires, it is imperative that all the segments of the arches should be studied. Catia has concluded that no statistically significant differences were found between the SE NiTi and multistranded stainless steel wires under the study conditions. Jones et al. 1990 had found that the mean tooth movement for SE Niti was 1.82 mm and muktistranded stainless steel was 1.53 mm which was not statistically significant. Sandu et al. 2012 gave the mean tooth movement as the mean difference between the pre treatment and post treatment crowding.
He has compared SE niti and multistranded stainless steel wires in PAE brackets and Begg brackets. He has concluded that there was no statistically significant difference between SE Niti and multistrandedniti in their efficiency in PAE brackets. There was no difference in the time taken for decrowding between SE Niti and conventional Niti, as reported by one study Rheem et al, which was not statistically significant. Time taken for decrowding as reported by Ramya et al was less for superelastic NiTi when compared with multistranded stainless steel, and it was statistically significant. No data could be collected for overall tooth movement and time taken for decrowding from Cobb et al. 1998 andWest et al. 1995 Cobb et al has given a distribution of the data, and West et al has used geometric mean ratios of the alignment scores.

Conclusion
It has to be admitted that all studies had a high risk of bias out of which one was a randomized control trial with Level 1 evidence. Two studies Ramya et al and West et al have reported significant differences between superelasticNiTi and multistranded stainless steel. Neither of the two studies which compared superelasticNiTi, and Conventional NiTi reported any significant differences between them. The lack of statistically significant evidence from this systematic review has been set out in the discussion part. This systematic review as such has to conclude that no reliable evidence could be synthesized from the studies selected.