Evaluation and Comparison of The Effect of Artificial saliva and Mouthwash Solution on Force Degradation of Different Types of Orthodontic Traction Aids ( Comparative in Vitro Study )

Aims: To determine the amount of force degradation of three types of orthodontic traction aid polyurethane elastomeric chain (PEC), nickel titanium closed coil spring (NiTi CCS) and stainless steel closed coil spring (SS CCS). Material and methods: Samples were extended to a specific distance which was 18 mm in case of PEC and 20 mm in case of NiTi CCS and SS CCS. Each extended sample will then incubated in dry condition, artificial saliva, and mouthwash solution, for total incubation period of three weeks. Results: showed that samples in dry condition always showed minimum force degradation percent than in wet conditions. It also showed that the increase of incubation time leads to increase in force degradation percent with major degradation percent located within first twenty four hours. Conclusions: in dry and two wet conditions, NiTi CCS reflects the lowest percent of force degradation, PEC showed highest percent, while SS CCS showed intermediate one.


INTRODUCTION
Orthodontic traction aids are essential component of fixed orthodontic appliance, one of the major drawbacks of current traction aid types are their ability to lose traction force with time during action, a phenomenon called "force degradation".
Elastomeric chain are economical and easy to use though they absorb water and saliva, permanently stain and suffer of breakdown of internal bonds, that lead to permanent deformation. Also, elastomeric chain undergoes stress relaxation (degradation), which is a decrease in the magnitude of force transmitted while held at a fix strain. This relaxation is in part due to Evaluation and Comparison of The Effect of Artificial saliva and Mouthwash Solution on Force Degradation of Different Types of Orthodontic Traction Aids (Comparative in Vitro Study) rearrangement within the polymer structure (1) . In addition to elastomeric chain, closed coil springs of various types nickel titanium, stainless steel, and cobalt chromium also show force loss after activation but in an amount lower than that shown by synthetic elastomeric chain (2,3) .
The aims of this study are 1. Detection and comparison the magnitude of force production and force degradation among three types of traction aids (polyurethane elastomeric chain, SS closed coil spring, and NiTi closed coil spring). 2. Determining and comparing the amount of force degradation for each type of incubated stretched traction aids at five intervals of whole incubation period (0 hour, 1 hour, 24 hours, 1 week, and at end of incubation at 3 weeks). 3. Comparison of the amount of force degradation among three incubation media for each type of traction aid.

Samples
The total number of samples consisted of 90 pieces of traction aids (30 pieces of PEC, 30 pieces of NiT CCS and 30 pieces of SS CCS), that divided into: A-Polyurethane Elastomeric Chain (Continuous, Clear) Each piece of PC consisted of 6 loops. The total length of each piece was 17.0mm (Dentarum, Germany).

B -Nickel Titanium Closed Coil Spring
The total length of spring is 12.0mm. The spring has the following dimensions: an inner diameter of 0.030 inch and a wire size of 0.010 inch (Dentaurum, Germany).

C -Stainless Steel Closed Coil Spring
The total length of SS CCS was 17.5mm. The spring has the following dimensions: an inner diameter of 0.028 inch and a wire size of 0.009 inch (Orthomatrix, USA).

Preparation of Artificial Saliva Solution
The formula of preparation of artificial saliva solution was described by Bar-rett et al., (9)  The components of artificial saliva were measured and added to the (500)ml deionized water and 500ml distilled water. The pH was adjusted to 6.75 ± 0.15 with sodium hydroxide and/or with HCl.

Mouthwash Solution
Mouthwash solution of ZAK type which contains (Chlorhexidine Digluconate 0.12% and Sodium fluoride 0.05%) is ready made and commercially available, it present with a pH of 5.1.

Preparation of Acrylic Blocks
Acrylic blocks were prepared according to Ferriter et al., and Kim et al., (5,10) . The acrylic block was contains 10 stainless steel pins in above surface, 1cm apart from each other, each two blocks are joined together by two SS screws, so the space between pins (inter pins distance) can be adjusted for each type of sample. In case of PC, screws adjusted so the pins are separated for distance of 18mm, in case of NiTi CCS and SS CCS the pins were separated for a distance of 20mm.

Preparation Of Sample Attachments (Metallic Hooks)
Each attachment consisted of metallic ruler; the ruler was cut in L shape. A stainless steel hook was soldered to the middle of the L shaped ruler in a horizontal direction. This ruler was attached in a upper clamp in the universal tensile testing machine. A similar ruler was prepared in the same manner and attached in the lower clamp in the machine. The purpose of these hooks was for sample attachment in the testing machine.

Method of Samples Extension Lengths Determination
The lengths of extensions of traction aids samples are chosen so that each sample will produce a (200 ±5)gm force at that extension, this force used by study of Bousquet et al., (11) , The method of determining the sample extension length in a relation to a specific force value produced by that extension, was used by Ferriter et al., and Angolkar et al., (5,2) .
To determine samples extension lengths that produce 200±5gm; an experi-ment was conducted by taking a five pieces of each sample type, each piece of each sample type was attached to the universal testing machine and extended to a limit that produce 200gm force, and the length of the extended sample was measured to determine the extension that produce 200 ±5gm. After measurement, the results show the following: 1.The extensions of PC samples that produce 200 ± 5gm were 18 mm. 2. The extensions of SS CCS and NiTi CCS sample that produce 200 ± 5gm were 20 mm.

Incubation Methods
Ten pieces of each traction aids type used attached to three acrylic blocks; each block contains 10 samples of each material of either PC, NiTi CCS or SS CCS which extended to their specific extensions.
In case of dry condition, the three loaded blocks were stored in incubator at 37°C for the whole incubation period which was three weeks.
In case of artificial saliva, another three loaded blocks immersed inside artificial saliva solution and stored inside incubator at 37°C for whole incubation period which was three weeks.
While in case of mouthwash solution, another three loaded blocks were incubated initially inside artificial saliva solution at 37°C inside incubator, after that the three blocks removed from saliva solution and immersed inside mouthwash solution which present inside glass containers and stored inside the same incubator at 37°C. The immersion period inside mouthwash solution was for two minutes, twice daily for three weeks. After the end of two minutes immersion period; the loaded blocks were returned to artificial saliva container at 37°C until next mouth wash immersion period. The whole incubation period was three weeks.

Measurement Methods
Measurement of force generated by each of the samples was performed at five time periods, which were: 1. Initially(0 hour, just before incubation) the measurement at 0 h used to determine initial force (IF). 2. One hour period.
The term initial force was reserved for the force values recorded during the first stretch of the material when taken from the manufacturer's envelopes, i.e., the material was not manipulated before being stretched (12) . From the initial force (IF) value the percentage of force degradation (FD %) can be obtained according to the following equation (13) : The extended sample was attached between the two ends of sample holder (which was an instrument used to carry sample without relaxation, and a similar instrument was used by Josell et al., (14) and transferred to the machine without any relaxation in sample. In the machine, the sample was attached to pins between upper and lower fixtures attached to upper and lower clamps. After measurement, the sample reattached to two opposite pins in block, and stored in incubator. The next measurements performed in the same manner.
Therefore, in case of dry condition, the following testes were resulted in relation to material-time combinations, and their force degradation was measured, these tests were:

RESULTS
The descriptive statistics of the results of force production and percent of force degradation of PC samples in dry condition, artificial saliva and mouthwash solution are listed in Table (1). While, the descriptive statistics of the results of force production and force degradation of NiTi CCS samples in dry condition, artificial saliva and mouthwash solution are listed in Table (2). The descriptive statistics of the results of force production and force degradation of SS CCS samples incubated in air, artificial saliva and mouthwash solution are listed in Table (3). The analysis of variance (ANO-VA) of the FP for PC, NiTi CCS and SS CCS in all five immersion periods in dry condition shows significance difference (p< 0.001) among them as in Table (4).    The results of Duncan multiple range test Table (7) showed that test PCN3w had the lowest amount of FP in all times in artificial saliva of normal pH (highest FD), with a significance difference (p≤ 0.05) from other tests.
While the tests of initial force (PCN0h, NiTiN0h and SSN0h) shows the highest amount of FP. The remaining tests distributed on statistical levels of significance between the upper and lower level.
The analysis of variance (ANOVA) of the FP for PC, NiTi CCS and SS CCS in five immersion periods in mouthwash solution shows significance difference (p< 0.001) among them as in Table (8).

DISSCUSSION
As a general view, the NiTi CCS samples lose less force than SS CCS and PC samples, this finding agree with Angolkar et al., (2) who found that NiTi CCS showed less FD than SS CCS and PC. In dry condition and at end of immersion period, NiTi CCS showed less FD percent than SS CCS which intern showed less FD percent than PC, as shown in Figure (1).The superelasticity, increased elastic limit and springback, lower load deflection rate, and less stiffness of Ni-Ti CCS than SS CCS who show linear load deflection curve and no phase transition might be the cause for this difference in FD percent (15,2,16,17,18) . The largest FD percent noticed in PC samples might be due to the mechanism of permanent deformation and rearrangement within the polymer structure (4,7,19,20) . The force loss in SS CCS samples in dry condition may be due to plastic deformation which takes place by slipping, twining (movement that divides the lattice into two symmetric parts) or a combination of the two (19) .
In the remaining two wet media, all samples types showed more FD percent than in those percents in dry condition. In artificial saliva, NiTi CCS samples showed less FD percent than SS CCS which intern showed less FD percent than PC, as shown  Figure (2). This agrees with Han et al., (21) who mentioned that NiTi springs are more resistance to degradation of mechanical properties in a simulated oral environments over SS springs and polyurethane elastics.
Harris et al., (22) , Eliades et al., (23) , Eliades et al., (20) , Kaneko et al., (24) and Kao et al., (25) mentioned that nickel, chromium and iron are the most corrosive elements in artificial saliva and titanium was the most corrosion resistance element. The titanium oxide layer formed on the NiTi wires is more stable than its chromium counterpart present in SS wires, particularly in environments containing chloride anion, lead to inferior corrosion resistance of SS relative to titanium alloys. Stainless steel is not considered resistant to chloride ions at any temperature or concentration, chloride can penetrate and destroy the passivity that is responsible for corrosion resistance of SS, in addition to chlorides, other aggressive substance is the sulfurated compounds found in saliva (26,27,19) . The effect of water immersion on PC may take various forms: solubility with leaching out of constituents, swelling, and environmental stress cracking (28) .
In mouthwash solution, all sample types showed increased in FD percent, where the least FD percent noticed in NiTi CCS, fol-lowed by SS CCS, and then PC, as shown in Figure (3).
Mouthwash solution contain fluoride ions that accelerate corrosion and hydrogen embrittlement in NiTi and SS CCS lead to more FD percent, and acidity of the mouthwash accelerate this processes. This agree with Reclaru et al., (29) , Watanabe et al., (30) , Kaneko et al., (24) , Walker et al., (31) and Walker et al., (32) . Polyurethane chain also affected by mouthwash solution lead to crease FD percent, this might be due to fect of fluoride ions and chlorhexidin that lead to more deteriorative effect (1,8) .

CONCLUSIONS
Nickel titanium CCS is more appreciate device for space closure than PC and SS CCS. If other factors had been constant, the FD percent of all samples in dry condition showed smaller value than those in wet conditions. If other factors had been constant, an increase of incubation time lead to increase in FD percent, with highest rate of degradation located within first twenty four hours. If other factor had been constant, Ni-Ti CCS showed lowest FD percent than PC which intern showed highest FD percent, while SS CCS showed the intermediate one.
In wet conditions and at the end of three weeks immersion period; PC and NiTi CCS samples incubated in mouthwash solution showed higher force degradation percent than samples incubated in artificial saliva, while SS CCS samples incubated in artificial saliva of normal pH and mouthwash solution showed non significant difference in force degradation percent at end of incubation period.