Objective: To estimate the effect of artificial saliva on load deflection characteristic of commercially available pre-formed nickel titanium orthodontic wires. Methods:This study included 15 wires of 3 kinds of commercially available round wire (3M, AO and ORMCO) NiTi wires of 0.016 dimensions. In an attempt to provide baseline information, five specimens of each wire were examined in the as-received condition (TO). The remaining wires were divided into two groups of five. Half of them were kept inside artificial saliva for the duration of one month (T1) and rest were kept in the air (T2). Three-point bending test was performed after one month (30 days) on a universal testing machine. The data collected from the generated graphs was further utilized for statistical analysis. Results:The greater force was shown by 3M and AO as compared to ORMCO. No significant difference was observed after exposing the load values of 3M and AO for one month to artificial saliva (T1) whereas ORMCO was affected significantly. Conclusion: The load of ORMCO was significantly decreased by the saliva. Though it failed to impart statically significant effect on 3M and AO wires.
Key words: Artificial Saliva, NiTi Wires, Universal Testing Machine, Three-point Bending.
Objetivo: Estimar el efecto de la saliva artificial sobre la característica de deflexión de carga de los alambres de ortodoncia preformados de níquel-titanio disponibles comercialmente. Métodos: Este estudio incluyó 15 alambres de 3 tipos de alambres redondos disponibles comercialmente (3M, AO y ORMCO) alambres NiTi de 0.016 dimensiones. En un intento de proporcionar información de referencia, se examinaron cinco muestras de cada cable en el estado de recepción (TO). Los cables restantes se dividieron en dos grupos de cinco. La mitad de ellos se mantuvo dentro de saliva artificial durante un mes (T1) y el resto se mantuvo en el aire (T2). La prueba de flexión de tres puntos se realizó después de un mes (30 días) en una máquina de prueba universal. Los datos recopilados de los gráficos generados se utilizaron además para el análisis estadístico. Resultados: 3M y AO mostraron la mayor fuerza en comparación con ORMCO. No se observaron diferencias significativas después de exponer los valores de carga de 3M y AO durante un mes a saliva artificial (T1) mientras que ORMCO se vio afectado significativamente. Conclusión: La carga de ORMCO fue disminuida significativamente por la saliva. Aunque no logró impartir un efecto estadísticamente significativo en los cables 3M y AO.
Palabras clave: saliva artificial, alambres de NiTi, máquina de ensayo universal, flexión de tres puntos.
Nickel-Titanium (NiTi) orthodontic archwires were first introduced to the orthodontics by Andreasen and Hilleman in 19711. Nitinol, an acronym derived from Ni-Nickel, Ti-titanium, Nol-Naval ordinance laboratory, was developed by William F Buehler, a research metallurgist at Naval Ordinance Laboratory, also known as Naval Surface Weapons Center in Silver Springs, Maryland,the USA for space programme in 1960. NiTi orthodontic wires are known to exhibit mechanical properties like low stiffness, shape memory, high springback as well as super-elasticity. These wires are widely used in orthodontic treatment as initial alignment wires.
Three major types of Ni-Ti alloy includes: the original first type having stabilized martensitic structureknown as martensitic alloys produced from the process of cold working. It consists of Ni-55% and Titanium-45%. Due to its cold-working process, the effect of thermo-dynamical memory gets suppressed which abolishes the possibility of altering the structure by varying temperature1. The second type includes superelastic nickel-titanium alloy, known as austenitic active alloy which is the perfected kind of stabilized alloy. Its qualitative mechanical feature is its super-elasticity which is attributed by the load-deflection curve, in a specific manner where on this curve during the unloading process, an area of plateau is observed. This implies that the equal pressure is applied on the teeth by the wire for most part of the deflection. The thermoelastic alloy—Martensiticactive alloysare the third type of alloy that possesses the effect of shape memory depending on the temperature.
A perfect archwire should employ light and constant forces having potential to produce an adequate biological response in the periodontal ligament to obtain physiological bone remodelling2. Super-elasticity is characterized by the load-deflection plot during unloading with a horizontal region which implies that a constant force may be exerted through a particular range of tooth movement2. This alloy possesses the capacity to change the crystalline form with changes in the temperature, and the temperature at which this change occurs is known as “temperature transition range” or TTR. The alloys are said to exhibit shape memory if they could plastically deformed in their martensitic phase and return to an austenitic phase, recovering their original shape while heated through certain temperature transition range (TTR). Load Deflection Rate (LDR) is the external loading required for unit deformation which is signified by the unit length deformation by generated force in the field of orthodontics.
Orthodontic archwires having high LDR apply excessive force on teeth. Their strength apparently decreases quickly with tooth movement3. It is measured in terms of force per unit and LDR is expressed in N/mm (g/mm). Most of the published results are limited to the wires in as received condition as there is only little information about the effect of saliva on the mechanical characteristics of NiTi wires. The aim of the current study is to evaluate the effect on load deflection characteristics of nickel-titanium pre-formed orthodontic arch wore in as received condition and after one month of storage in artificial saliva.
In this experimental study, the load deflection characteristics of three kinds of 0.016 Nickel-titanium wires were studied: AO, 3M, ORMCO as shown in Image 1, 2&3. In an attempt to conduct the study involving load deflection of the wires, three-point bending test using Universal Testing Machine (as shown in Image 4,5,6 & 8) was conducted. Other materials included were Arch from template, Artificial Saliva-Modified Fusayama (Image 7) consisted of NaCl (400 mg/L, KCl (400 mg/L), CaCl2.2H2O (795 mg/L), NaH2PO4.H20 (690 mg/L), KSCN (300 mg/L), Na2S.9H20 (5 mg/L) and urea (1000 mg/L).
The experimental study was conducted in the Department of Orthodontics and Dentofacial Orthopaedics, Yenepoya Dental College and Hospital, Deralakatte, Mangaluru, India. Ethical Clearance Number YEC2/2018/38 was obtained from the ethical committee of Yenepoya University. To assess the load deflection of the wires viz. 3M, AO, ORMCO, three-point bending test was performed using universal testing machine. For this purpose, fifteen wires of each kind were divided into group of three including T0: as-received condition; T1: kept inside artificial Saliva; T2: kept in the air. Five of them were tested immediately to provide baseline information on the as-received wire (T0). Rest of them were further divided into two groups of five. In order to evaluate the effect of artificial saliva, half of them were placed inside the artificial saliva (T1) and the remaining half were kept in the air for the duration of one full month. Modified Fusayama artificial saliva was used, whose pH was adjudged to 7 by using sodium hydroxide. The study design was an In Vitro Study design. The Inclusion Criteria were as follows: Nicke-titanium wires only; Nickel-titanium wires of 0.016 x 0.022” dimensions. The Exclusion Criteria were as follows: Wires other than NiTi; Deformed wires. After one month, the three-point bending test was performed on all the samples.
To perform the test, the apparatus of universal testing machine (Zwick-Roell Z020, Germany) with a 20 kN compression load cell calibrated after the pre-load of 0.2 N was used. The testing machine was operated at a crosshead speed of 0.5 mm per minute and 1 mm per minute for all the groups (Figure 1-12). Then by means of MacLab/4 analogue- digital converter analogue output signal was passed to a computer. The wires of the control group were also included under three-point bending test so that they could be as an indicator for comparing the results of the tests.
All statistical analyses were conducted using SPSS 11.5 software (IBM SPSS Statistics for Windows, version 23.0 Armonk, NY: IBM Corp). One-way ANOVA followed by Post Hoc Test in order to compare the load deflection when stored in artificial saliva was used. Therefore, ANOVA tests, Post HOC tests and normality distribution curve were used to carry out statistical analyses. The mean and standard deviations of shear bond strength of individual groups were also compared. P<0.05 denoted statistically significant.
Post Hoc Tests for multiple comparisons:
Comparison of artificial saliva between the companies for weight W2:
Post Hoc Tests for multiple comparisons:
Comparison between the weights in each company:
One-way ANOVA followed by Post HOC test is used to compare the load deflection when stored in an artificial saliva between the companies when weight is W1. A significant difference in the load deflection between the groups with p<0.001 was observed here.
Table 1 shows the comparison of NiTi wires in artificial saliva between the companies for weight (W1). The results obtained from one-way ANOVA with post HOC test showed a statistically significant difference in load deflection between groups with p<0.001. The most obvious differences were observed between ORMCO and the wire types viz. 3M and AO. Generally, the maximum load deflection among the three 0.016 inch wires was exerted by ORMCO (1156.500±65.803 at W1 & 1126.00±110.136 at W2) and the minimum load deflection was exerted by 3M (445.80±94.608 at W1 & 402.60±57.657 at W2).
For multiple comparison, Post HOC test was performed. On comparing the 3M with AO, a less statistically significant mean difference of -135±44.466 at W1 and -159±48.352 at W2 were observed whereas on comparison with ORMCO, a highly significant mean difference of -710.200±44.466 at W1 and -723.400±W2 was observed (Table 2). Likewise, on comparing AO with 3M and ORMCO at weight W1 and W2, a mean difference of 135±44.466 at W1 and -564.400±48.352 at W2 (3M) was observed whereas a statistically significant mean difference of 575.200±44.466 at W1 and -564.400±48.352 at W2 (ORMCO) was observed (Table 4). Comparison of ORMCO with 3M and AO showed a mean difference of 710.200±44.466 at W1 and 723.400±48.352 at W2 (3M) and a mean difference of 575.200±44.466 at W1 and 564.400±48.352 at W2 (AO) (Table 2&4). From this, it can be concluded that a statistically significant mean difference is observed on comparing ORMCO with 3M and AO at W1 and W2.
Comparison of the weights (W1 and W2) on exposure to artificial saliva (T1) in each company showed the mean difference of ORMCO as 1126 and 1156 with that of 3M as 445.80 and 402.60 and AO as 580.80 and 561.60 (Table 5). To compare between the weights, independent test was used. No significant difference between the weights with p>0.05 were observed. Therefore, it can be concluded that no statistically significant difference falls in the values between the weights. There was no considerable difference seen between the groups on comparing the load values of T0 and T2 specimens. One-month immersion of wires in artificial saliva notably affected the load values of ORMCO and AO where T1 specimens of these two wires were generally associated with significantly lower forces compared with control (T0) specimens. It had been observed that the load values of T1 for 3M were not significantly different from those of T0 specimens.
The results of this study revealed that the load deflection values of ORMCO after one month of storage showed a significant decrease whereas no significant affect observed in 3M and AO. Thus, this fact showed that the stability of the force produced by all types of NiTi wires is almost similar and least affected by the action of saliva. Largely due to their extra-ordinary properties of super-elasticity and shape memory, super-elastic nickel-titanium arch-wires have gained wide acceptance as initial alignment arch-wires by the Orthodontists since last 10-15 years. The information regarding behaviour of these wires is totally based on the mechanical laboratory testing with an emphasis on 3-point bending tests to study the load deflection characteristics without simulating the various variables encountered during clinical situations.
Since the results of this in-vitro study are in agreement with Oltjen’s study, thus it concludes that the wires engaged in brackets during bending test show more stiffness than those that were set on two pillars without any attachment. The Oltjen’s study too stated that the ligation method of the wire to brackets is another essential factor that increases the fore of the wire. Kasuya showed that the ligating the NiTi wires to the brackets increases the force of the wire and brings about a change along the load-deflection curve.
Thus, our present study is in accordance with the concept of Kasuya. The lowest level of force belonged to self-ligating brackets and after that ligation by stainless steel ligature. The elastomeric ligature also caused elimination of the super-elasticity of NiTi wires and the load deflection curve of the NiTi wires became linear similar to steel wires. Our results are also in accordance to the results of Tonner and Waters, whose load values of as-received ORMCO wire is 0.02 N. Corrosion behaviour and surface characteristics of nickel-titanium Orthodontic arch wires had been evaluated by various studies.
Also, the effect of saliva on their chemical properties is nearly clear. However, the effect of saliva on the mechanical properties of NiTi wires is still under debate. The study has several limitations. The results derived from the study showed differences due to inter-batch variations during the manufacturing of wires. There is a questionable validity in extrapolating numerical load values to clinical situations despite the attempt to resemble the clinical conditions. Also, three-point bending test failed to represent the masticatory forces which indicated that their status as the standard wire test might not be fully appropriate. Cyclic loading from the masticatory forces may contribute to bind and release function of the wire, so the force generated beneath this may cause a permanent set in the wire. Oral cavity boasts a unique environment that cannot be fully simulated by the artificial saliva.
Despite the limitations of the study, it has been concluded that the load deflection characteristic of ORMCO pre-formed NiTi wires in as received condition was found to be statistically significant in comparison to AO and 3M pre-formed NiTi wires. On exposure to natural environment for one month, the effect on load deflection characteristics of ORMCO pre-formed NiTi wires was greater as compared to AO and 3M pre-formed NiTi wires. The load deflection of ORMCO was decreased by artificial saliva but it failed to impart any effect on 3M and AO. After one full month storage of in as-received wires in saliva as well as exposure to natural environment, the load deflection values of 3M were significantly lower as compared to ORMCO and AO. Also, a significant difference in the values between the weights (W1, W2) was observed. Thus, the present study concluded with the fact that the saliva decreased the load of ORMCO wires significantly, and failed to impart a statistically significant impact on 3M and AO wires.
No potential conflict of interest relevant to this article was reported.
This research was supported by the Department of Orthodontics and Dentofacial Orthopaedics, Yenepoya Dental College and Hospital, Deralakatte, Mangaluru, India.