ABSTRACT
Objective:
The purpose of this study was to determine the effect of common methods of sterilization on the tensile strength of Beta titanium, Stainless steel, Australian Stainless steel, Copper Nickel-Titanium, and Nickel-Titanium wires. It also aimed to evaluate the changes in tensile strength values caused by repeated cycles of sterilization.
Materials and Methods:
A sample of 225 orthodontic wires, i.e., beta-titanium, stainless steel, Australian stainless steel, copper nickel-titanium, and nickel-titanium wires, were collected from different manufacturers. These wires were divided into three groups, which consists of Groups 1, 2, and 3. Four methods of sterilization used in this study were as follows: (i) autoclave (250°F for 20 min), (ii) dry heat sterilization (375°F for 20 min), (iii) ethylene oxide sterilization (54°C for 4 hrs), and (iv) 2.45% acidic glutaraldehyde (10 hrs).
Results:
The results of this study showed that there was increase in tensile strength of beta-titanium and nickel-titanium wires using autoclave and dry heat sterilization. No statistically significant difference in tensile strength of stainless steel and Australian stainless steel archwires. The tensile strength of copper nickel-titanium wires decreased following 0, 1, and 5 cycles of sterilization.
Conclusion:
The lack of statistically significant differences established in the study of new and sterilized orthodontic archwires gives us reason to conclude that the orthodontic arch wires can be sterilized because the sterilizing processes do not affect their tensile strength and the orthodontists could thus ensure the maximum safety of their patients.
KEYWORDS: Orthodontic arch wires, recycling, sterilization, tensile strength
INTRODUCTION
Sterilization is the process, which destroys all the forms of microorganisms including viruses, bacterial, and mycotic spores.[1] Given the magnitude of the threat that infectious agents pose to us, as well as their proclivity to develop in real time, it is critical that all healthcare communities, including our own, adopt, and follow successful infection control protocols.[2]
Gold orthodontic wires were only ones usable until the 1930s. In 1929, Austenitic stainless steel was introduced as an orthodontic wire, and it quickly surpassed gold in popularity.[3] Later, Australian wire, a unique form of steel used in a variety of procedures and treatment philosophies, was incorporated into orthodontics.[4]
Andreasen and Hilleman, in 1971, pioneered the use of nickel-titanium (Ni-Ti) alloys in orthodontics.[5] In terms of physical properties, newer titanium alloys have many benefits over precious metal and stainless steel.[6] Nickel-titanium wires with copper added became available in mid-1990s. These wires have better defined thermal properties than Niti super-elastic wires due to the inclusion of copper.[7]
The use of newer materials is influenced by cost, as titanium alloys are 5 to 40 times more expansive than stainless steel.[6] Because of the price disparity between these wires, it has been suggested that wires made of these alloys may be sterilized and reused.[8] If recycling do not have a direct effect on the properties of devices in question, reusing materials would be advantageous economically.[6]
When a wire’s tensile strength is reduced, it becomes vulnerable to breakage, posing as problem for both the patient and the orthodontist.[8]
This study aimed to understand the effect of different methods of sterilization on orthodontic arch wires and to examine whether there will be any changes in tensile strength of wires after repeated cycles of sterilization.
MATERIALS AND METHODS
Source of data
A sample of 225 orthodontic wires were collected from different manufacturers.
Armamentarium
Orthodontic wires
Totally, 225 orthodontic wires of dimension 0.016” in diameter and 7” in length were used in this study. Wires used in the study were as follows:
0.016” Beta-Titanium (ORMCO)
0.016” Stainless Steel (RABBIT FORCE)
0.016” Australian Stainless Steel (A J WILCOCK)
0.016” Copper Nickel-Titanium (ORMCO)
0.016” Nickel-Titanium (RABBIT FORCE)
Sterilization methods
Sterilization methods used in this study were:
Autoclave (GNATUS)
Dry heat sterilization (INDIAMART)
Ethylene oxide sterilization (PCI)
2.45% acidic glutaraldehyde (GLUTYHYDE®2%)
Universal testing machine
A Universal Testing Machine (UTT1205 PSI Sales, India) was used to test tensile strength of various types of wires.
Custom-made assembly
A custom-made assembly consisted of upper and lower member, which were attached to the Universal Testing Machine.
METHODOLOGY
Samples
225 wires were divided into three groups, which consists of:
Group 1- Control group
Group 2- One cycle of sterilization
Group 3- Five cycles of sterilization
Groups 2 and 3 were further subdivided into subgroups according to various sterilization techniques used in this study.
The sterilization technique
The wires were put into three different cycles of sterilization which being 0, 1, and 5. The four methods of sterilization used in this study were as follows:
1. Autoclave (GNATUS) the wires were sterilized at 250°F for 20 minutes and 100 minutes, respectively.
2. Dry Heat Sterilization (INDIAMART) wires were sterilized at 375°F for 20 min and 100 minutes, respectively, in hot air oven.
3. Ethylene Oxide Sterilization (PCI) Wires were sterilized in ethylene oxide chamber at 54°C for 4 hrs and 20 hrs, respectively.
4. 2.45% Acidic Glutaraldehyde (GLUTIHYDE® 2%) Groups 2 and 3 were cold sterilized in 2.45% acidic glutaraldehyde for 10 hrs and 50 hrs, respectively.
Testing method
An Instron Universal Testing machine (UTT1205 PSI Sales, India) was used to measure the ultimate tensile strength of wires. The ultimate tensile strength of the wire is the peak of the curve in the plastic range. It is the maximum stress of force a material can withstand.
The 1653 pounds load cell was used by machine having crosshead speed of 0.5 mm/minute for all the sample wires. The tensile strength recorded was the maximum stress value in pounds per square inch just prior to fracture of the test wire, i.e., the ultimate tensile strength was recorded.
RESULTS
This study was conducted on a sample of 225 wires, divided into three groups: Groups 2 and 3 were further subdivided into subgroups according to various sterilization techniques used in this study [Table 1].
Table 1.
One and five cycles of sterilization
| Sterilization techniques | No. of wires |
|---|---|
| Autoclave | 5 |
| Dry Heat Sterilization | 5 |
| Ethylene Oxide Sterilization | 5 |
| 2.45% Acidic Glutaraldehyde | 5 |
Descriptive statistical analysis including mean and standard deviation was calculated for each of the test groups. The results of ANOVA test evaluating the beta-titanium (TMA) wires sterilized by dry heat sterilization revealed that there was increase in tensile strength after one cycle of sterilization. However, there were no significant differences in the control group and the wires sterilized 5 times. The beta-titanium (TMA) wires sterilized by autoclave, ethylene oxide, and 2.45% acidic glutaraldehyde revealed no statistically significant difference in tensile strength of wires following 0, 1, and 5 cycles of sterilization.
ANOVA evaluation of stainless steel (SS) and Australian stainless steel (AS) wires revealed that there was no statistically significant difference in tensile strength.
The results of copper nickel-titanium (Cu-Ni-Ti) wires showed that there was a significant decrease in tensile strength of wires following 0, 1, and 5 cycles of sterilization using any of sterilization methods.
Autoclave and dry heat sterilization of nickel-titanium (NiTi) wires revealed that there was an increase in tensile strength of wires after 0, 1, and 5 cycle of sterilization. But the mean tensile strength after 5 cycles was not significantly different than 1 cycle.
DISCUSSION
Precious metal arch wires were once widely used in orthodontics, but advancements in metallurgy and wire manufacturing technology have enabled alternative alloy systems to be adopted.[6] However, with the advent of pre-adjusted edgewise systems, the use of wires that do not need bending has grown in popularity.[9]
If sterilization changes the mechanical properties of the wire in a significant and/or unpredictable way, using recycled wires may result in substandard treatment, which would be unethical. If the results of sterilization are predictable or negligible, the decision on arch wire reuse can be made.[6]
The present study was performed to determine the effects of various sterilization techniques on the tensile strength of orthodontic wires. The wires selected for the purpose of this study were round 0.016” as this is the most commonly used archwire for initial levelling and alignment.
The study revealed that the sterilization and reuse of orthodontic wires do not alter the tensile strength as expected. Yet, these differences were not significant. A similar study conducted by Kannan et al. (2012)[10] evaluated the effects of sterilization on the mechanical properties of wires and revealed that no detrimental effects on properties of nitinol and beta-titanium were seen following dry heat, autoclave, and 2% glutaraldehyde solution. The changes observed in nickel-titanium wires may be due to change in structural phases of martensitic stabilized nitinol.
In case of beta-titanium (TMA), titanium has a hexagonal close-packed (HCP) crystal form structure below 1625°F. At a higher temperature, titanium wire arranges into a body-centered cubic (BCC) lattice, referred to as the β-phase. With addition of molybdenum, titanium-based alloys maintained their β-structure when cooled to room temperature. However, there are chances of HCP formation at isolated sites. This may probably result in slight increase of tensile strength and yield strength.
In case of stainless steel (SS) and Australian stainless steel (AS) wires, there was no statistically significant difference in tensile strength after 0, 1, and 5 cycles of sterilization using dry heat, autoclave, ethylene oxide, and 2.45% glutaraldehyde.
Sterilization of copper-nickel-titanium (Cu-Ni-Ti) wire resulted in decrease in the tensile strength, but study done by Sreekanth, Laxman and Muralidhar (2011)[11] showed no detrimental changes observed in tensile properties of copper NiTi wire.
Controversial findings may be due to the adjustments of testing machines and manufacturing procedures. Different manufacturers formulate their own standards for their products. One wire lot can differ from the next lot from the same manufacturer. Information about the processing of the wires is proprietary to each manufacturer, and the processing during the production of orthodontic wires has an important influence on their mechanical properties. The difference could also be related to the cross section of wires, machine adjustments, and manufacturing procedures (Oshagh et al. 2012).[9]
Thus, wire recycling may be 1 method of reducing orthodontic practice overhead; however, not every wire may be recycled. Beta-titanium, stainless steel, and Australian stainless steel wires with bends are not candidates for recycling since same bends will rarely fit more than one patient. Nickel-titanium wires are usually placed without orthodontic bends and most brands of nickel-titanium wires were incapable of retaining a bend, which makes this type of wire ideal for sterilization and reuse (Staggers and Margeson 1993).[12]
CONCLUSION
However, the results of this study suggested that orthodontist who choose to recycle beta-titanium (TMA), stainless steel (SS), Australian stainless steel (AS), and nickel-titanium (Ni-Ti) archwires need not to be concerned about wire’s ultimate tensile strengths after sterilization. The lack of statistically significant differences noticed in this study of new and sterilized orthodontic archwires gives us reason to conclude that the orthodontic arch wires can be sterilized and orthodontists thus can ensure the maximum safety of their patients.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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