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A study of a novel aesthetic archwire on its frictional properties and its applications to orthodontics
This article was originally published by Wolters Kluwer and was migrated to Scientific Scholar after the change of Publisher.
Abstract
Objectives
To compare sliding friction of prototype 0.018-inch fiber-reinforced polymer composite (FRPC) archwire with 0.018-inch nickel titanium archwire using various bracket-arch wire combinations.
Materials and Methods
Two wires were tested against four different brackets (3M Gemini Twin bracket; 3M Clarity metal-reinforced ceramic bracket; Ormco Inspire ICE ceramic bracket; and 3M SmartClip) using the Universal testing machine to study and compare frictional characteristics.
Results
There was no significant difference noted for the frictional wear generated between the various archwire and bracket groups (P = 0.542). No statistical significance was detected within individual archwire-bracket groups. A multiple comparison of groups showed significant difference in frictional wear. Least significance difference multiple comparison revealed statistical significance ( P< 0.05) when comparing Gemini-FRPC with ICE-FRPC group. No other groups showed any significant difference.
Conclusion
FRPC and NiTi wire show comparable frictional wear when used with ICE, Gemini, Clarity, and SmartClip brackets.
Keywords
Aesthetics
arch wire-bracket Fiber-reinforced polymer composite (FRPC) wires
sliding friction
INTRODUCTION
Composites have traditionally been used in dentistry as aesthetic tooth-colored restorative materials. Their aesthetic and mechanical properties in the oral cavity are well established. The composite material’s brittle nature is also recognized as a stumbling block in manufacturing of archwires from composites. Through composite technology [Figure 5], an optimal wire in terms of aesthetics and mechanical properties has been fabricated from continuous fibers and polymer matrix, giving rise to a prototype Fiber-reinforced Polymer Composite (FRPC) archwire, potentially suitable for use in orthodontics.[1]
- Sample wires of 0.018-inch FRPC (a) and NiTi (b) archwires tested. Both the test wires were supplied in straight form in order to avoid the trifocal eclipse of the preformed archwires
- Brackets tested in the study. (a) Gemini True-twin Stainless Steel (3M Unitek, St. Paul, Minnesota) (b) SmartClip True-twin Self-ligating Stainless Steel (3M Unitek, St Paul, Minnesota) (c) Clarity True-twin Polycrystalline metal-reinforced Ceramic (3M Unitek, St Paul, Minnesota) (d) Inspire ICE True-twin Monocrystalline Ceramic (Ormco, Orange, California)
- Close-up image showing customized jig. Customized jig allowed the bracket to traverse a specified distance over a specific period of time
- Image showing experimental setup of the Universal testing machine with bracket and archwire mounted on a customized jig
- Schematic representation of pultrusion process for the fabrication of composite archwires
Springback, stiffness, formability, resilience modulus, biocompatibility, and low friction are desirable characteristics[2] of an archwire for optimum mechanical performance during orthodontic treatment. The FRPC archwire prototype has been shown in a previous study[1] to be comparable in terms of its mechanical properties to clinical nickel titanium wire when tensile and three-point bending tests were carried out. The translucent[14] nature of the polymer matrix confers the aesthetic property to the FRPC archwire. The fiber content gives the material flexibility, overcoming the inherent problem of brittleness. At various stages of the orthodontic treatment, the mechanical properties required of the archwire changes. Different fiber material, fiber content, and fiber arrangement can modify the mechanical properties of the wire to suit the mechanical requirements of the archwire at various stages of treatment.
When metallic wires are used in conjunction with ceramic brackets, although the brackets are aesthetic, the arch wire is still visible. Coated aesthetic archwires have been introduced, but such archwires have higher friction and the aesthetic coating tends to dehisce[3-6] Also, these wires are opaque; there is no transmission of light and therefore, no transmission of the tooth color through the wires. The FRPC archwire, being translucent in nature, will allow for transmission of the color of teeth, thereby improving aesthetics in cases where ceramic brackets are used.
Allergic reactions to nickel,[7-9] a metallic ion found commonly in contemporary metallic archwire, may be averted with the FRPC archwire. In addition, studies have been conducted on the mechanical properties of the FRPC wire. Preliminary typodont studies[1] have shown the wire’s ability to align teeth. These studies collectively suggest a strong viability for the wire’s clinical usefulness.
These FRPC wires do show promise in providing an aesthetic archwire for orthodontic use. Specifics such as formability, weldability, and frictional coefficients haven’t been ascertained as yet, as very little research has been done on FRPC wires. Friction is a critical mechanical consideration throughout the course of fixed mechanotherapy.[10-13] The success or failure of fixed mechanotherapy may be greatly influenced by the frictional properties of the materials used and how friction is controlled and used in the mechanics of treatment.
Aim of the study
A prospective study was carried out with the aim to compare sliding friction for a prototype FRPC archwire with that for a Nickel-titanium archwire using various bracket-archwire combinations using the Universal testing machine (INSTRON 5848 Micro Tester; Instron Corporation; Norwood, Massachusetts, U.S.A).
MATERIALS AND METHODS
Selection of archwires and brackets
The 0.018-inch FRPC was compared against a 0.018-inch Nitinol (NiTi) archwire. The two wires were tested against four different brackets: 3M Gemini 0.022 inch slot Twin bracket; 3M Clarity metal-reinforced 0.022 inch slot ceramic bracket; Ormco Inspire ICE 0.022 inch slot ceramic bracket; and the 3M SmartClip, MBT (McLaughlin, Bennett and Trevisi) prescription 0.022 inch slot self-ligating bracket. All the different brackets had similar torque and angulation (−7° torque, 0° angulation). For purpose of standardization, the upper right first premolar brackets were used for all tests. A frictional study of the archwire-bracket interface using an Universal testing machine was carried out for each bracket and archwire combination. With two archwires and four different brackets, there were a total of eight possible archwire and bracket combinations. [Figures 1 and 2] [Table 1].
| Brackets Brand | Manufacturer | Configuration | Structure |
|---|---|---|---|
| Gemini | 3M Unitek, St. Paul, Minnesota | True-twin | Stainless steel |
| Clarity | 3M Unitek, St Paul, Minnesota | True-twin | Polycrystalline metal-reinforced Ceramic |
| Inspire Ice | Ormco, Orange, California | True-twin | Monocrystalline Ceramic |
| SmartClip | 3M Unitek, St Paul, Minnesota | True-twin | Self-ligating stainless steel |
Friction testing
To simulate wear of the archwire, the universal testing machine was used to carry out sliding friction tests with each archwire bracket combination. For each test, a new archwire and bracket was used. Test wires used were cut from a straight portion of a preformed archwire if no straight wires were available. Each wire was measured precisely with a pair of Vernier calipers (Mitutoyo Corporation, Kanagawa, Japan) and cut to produce test wires of length 10 cm. A customized jig was engineered to hold the test wire stationary and straight. The wire-jig assembly was adjustable vertically and horizontally to allow for correction of wire position. The jig was welded onto a table which was secured via screws to the landing of the universal testing machine.
The test bracket was also held in place by another customized jig [Figure 3]. This bracket jig held the bracket while at the same time was attached to the cross head of the universal testing machine via an aluminum rod [Figure 4]. The cross head is the moveable part of the universal testing machine. Hence, in this experimental set-up, it is the bracket that moves along the wire, instead of the wire being pulled through the bracket. The bracket was localized onto the bracket-jig assembly using 2 parallel blocks and a straight length of 0.021 inch by 0.025 inch stainless steel wire. This was done to ensure that the bracket slot would be parallel to the motion of sliding when the test is conducted. After localization, the wire was secured within the bracket using a stainless steel ligature wire. The ligature was twisted till the test wire was firmly secured in the bracket slot and then it was untwisted 3 turns so that it does not cause binding of the wire to the bracket. This was carried out for all test wire and bracket combinations except for those which involved the 3M Smartclip self-ligating bracket, in which case no ligature wires were used to secure the test wire.
When the test wire and bracket were placed in their respective jigs and assembled in the universal testing machine, it gave a test area exactly at the middle of the test wire. Localizing the test area was crucial as the wires were preserved for further testing of the areas of the wires that have undergone frictional wear in the subsequent parts of the study.
The cross head of the universal testing machine was set at a speed of 0.5 mm per minute. It moved the brackets upward for 2 minutes and then downward for 2 minutes, making one cycle lasting 4 minutes. A total of 10 cycles was done for each test wire and bracket combination. Each test therefore lasted 40 minutes. The area of wear that was created was 1mm in length on the test wire and this was the area that was studied in the following 2 parts of this study. This was done in the hope that the wear created will approximate the normal wear an archwire experiences when used clinically. Results of frictional wear of each archwire-bracket test were captured from the universal testing machine on the dedicated Merlin software. The Merlin software controlled and monitored the velocity at which the crossheads moved and recorded the frictional wear as represented by the load in Newtons, as the bracket was pulled along the wire.
STATISTICAL ANALYSIS AND RESULTS
Statistical analyses were carried out using the Statistical Package for Social Sciences (SPSS version 14; SPSS Inc, Chicago IL) software. The mean and standard deviation of the frictional wear was represented by the load in Newtons as the bracket was pulled along the wire with the universal testing machine. A one-way ANOVA was carried out to compare between groups and also within groups of the friction wear testing with the universal testing machine. Method error calculation could not be carried out as the tested wires could not be subjected to a repeated frictional test.
Quantitative analysis of friction study
The raw data from the frictional wear testing was corrected to take into account the change in the direction of the crossheads as each test cycle was completed. This was done by a positive value indicating crossheads moving upward, and a negative value indicating crossheads moving downward. The magnitudes of the readings were considered and the direction, i.e., the positive or negative was discounted. The first 10 seconds and last 10 seconds of readings of each 2-minute cycle were also eliminated to factor out inertia as the crossheads change direction. The difference between the final mean frictional values and the initial mean frictional values was evaluated to determine frictional wear caused by the testing. The initial and final frictional values were the average of the first 2 minutes and last 2 minutes of each 40-minute cycle test. This value was taken to be the frictional wear generated from the testing.
From descriptive statistics of frictional wear [Table 2], it can be seen that the frictional wear generated showed a range of values among the different archwire and bracket combination. The highest frictional wear generated was with the Gemini-FRPC group. The results also indicated that some archwire and bracket combinations showed less friction at the end of testing, indicating that the coefficient of friction was possibly reduced after frictional wear.
| Friction | N | Mean | Std. Deviation | Std. Error | 95% Confidence Interval for Mean | |
|---|---|---|---|---|---|---|
| Lower Bound | Upper Bound | |||||
| Clarity-FRPC | 5 | −27.88 | 33.59 | 15.02 | −69.59 | 13.83 |
| Clarity-NiTi | 5 | −22.83 | 34.74 | 15.53 | −65.96 | 20.31 |
| Gemini-FRPC | 5 | 313.10 | 802.59 | 358.93 | −683.45 | 1309.64 |
| Gemini-NiTi | 5 | 75.41 | 159.06 | 71.13 | −122.09 | 272.91 |
| ICE-FRPC | 5 | −71.13 | 17.08 | 7.67 | −92.33 | −49.92 |
| ICE-NITi | 5 | −28.48 | 21.44 | 9.59 | −55.10 | −1.86 |
| SmartClip-FRPC | 5 | 27.48 | 72.85 | 32.58 | −62.97 | 117.93 |
| SmartClip-NiTi | 5 | −6.94 | 48.27 | 21.59 | −66.88 | 53.00 |
N, number;
There was no significant difference noted for the frictional wear generated between the various archwire and bracket groups (P = 0.542). No statistical significance was detected within individual archwire-bracket groups.
A multiple comparison was done to see which of the groups showed significant difference in frictional wear. Least significant difference (LSD) multiple comparisons revealed statistical significance (P < 0.05) when comparing the Gemini-FRPC with the ICE-FRPC group. No other groups showed any significant difference.
DISCUSSION
The FRPC wires that were tested in this study have been designed and manufactured with the intention to function as wires to be used in the initial alignment phase of orthodontic treatment. The use of equivalent diameter contemporary nickel-titanium archwires for comparison of friction and surface roughness is therefore appropriate.[15-19] Different brackets and the two archwire materials were combined to give different bracket-archwire interface combinations to quantify the level of sliding friction using a universal testing machine. Findings from this study were interpreted in context to provide clinical relevance. Results from the friction studies showed no significance difference between groups. From the LSD multiple comparison, it was seen only the ICE-FRPC group showed statistical significance when compared to the Gemini FRPC group. No other groups showed any significant difference. If however, a post-hoc Bonferroni adjustment were done for the multiple comparisons, then the Gemini-FRPC group would no longer be statistically significant. All other FRPC combination groups did not show any statistical significance in the frictional testing. This would indicate that the FRPC wire actually performed as well as NiTi wire during friction testing. This result has to be taken with caution due to the small sample size. The sample size was small as running of the test was very time consuming and the availability of FRPC archwire, being a prototype, was also limited.
FRPC archwires are still in its infancy in development. Future directions in experimentation would be to include a fluid medium like saliva to compare wet and dry states of frictional testing.[20,21] Experiments with the introduction of second order bends should also be considered. No experiments involving third-order bends can be carried out since a rectangular cross-sectional FRPC wire has not been manufactured successfully as yet. It would also be interesting to study the amount of polymer loss due to wear. This could be extrapolated as the amount that a patient could be potentially exposed to and the effects it may have on the patient’s health. A more in-depth look at surface roughness using a scanning electron microscope and atomic force microscopy may give us more insights to this FRPC, which could be planned for future experimentation.
CONCLUSION
FRPC and NiTi wires show statistically comparable frictional wear when used with ICE, Gemini, Clarity, and SmartClip brackets. However, validity of this result needs to be taken with caution since fiber-reinforced wires are experimental and not universally commercially available and also the sample size was small.
Source of Support:
Nil.
Conflict of Interest:
None declared.
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