Evaluation of the Effect of Different Types of Fluoride on Tensile Properties and Surface Roughness of Different Titanium-Based Archwires: An In vivo Study.

OBJECTIVES: The purpose of the study was to evaluate the effect of different types of fluoride on tensile properties and surface roughness of titanium based archwires clinically.
MATERIALS AND METHODS: Three groups of archwires, namely nickel-titanium (NiTi), heat activated NiTi, and TMA was evaluated clinically. Each group comprised four subgroups, namely as received group, nonfluoride control group, APF gel group (received one application of 1.23% APF gel), and sodium fluoride mouthwash group (patients rinsed twice daily with 0.2% sodium fluoride mouthwash). All the archwires were removed after 3 weeks of clinical use and evaluated for surface roughness with three dimensional optical profiler. Tensile strength and elongation rate was determined with Instron universal testing machine.
RESULTS: NiTi, heat activated NiTi, and TMA archwires with APF gel application showed the highest reduction in tensile strength and elongation rate and highest value for surface roughness, followed by sodium fluoride mouthwash group and finally wires without fluoride application. NiTi wires had the highest reduction in tensile strength with APF gel followed by heat activated NiTi and TMA.
CONCLUSION: APF gel with highest concentration of fluoride caused most degradation in the tensile properties and surface roughness of titanium based arch wires. Copyright:

INTRODUCTION

Archwire materials form a large part of our mechanotherapy. These archwire materials have to withstand mechanical, thermal, and chemical stresses in the patient's oral cavity.[1]

Intraoral exposure of nickel–titanium (NiTi) alters the topography and structure of the alloy surface through the surface attack in the form of corrosion fatigue or stress corrosion cracking attributed to nucleation of pitting or crevice corrosion sites acting as stress risers or crack starters.[2] The proposed sequela involves potential hypersensitivity reactions and biocompatibility concerns.

Implementation of preventive measures such as use of fluoride mouthrinses, fluoride-releasing adhesives, fluoride gels, varnishes, and dentifrices throughout the course of orthodontic treatment is required. Significant degradation in properties of NiTi and beta-titanium caused by hydrogen embrittlement occurs in fluoride solutions.[345] The effect of fluoride on the properties and surface roughness of titanium-based archwires in the clinical situation has not been evaluated.

Optical profiling (three-dimensional [3D] optical profiler) based on quasi-monochromatic and white light interferometric principles is a versatile measurement technology for examining surface topography with very high precision, offers fast, noncontact three-dimensional measurements. and does not require special sample preparation.[6]

Hence, this study was undertaken to clinically evaluate the effect of fluoride on tensile properties and surface characteristics of various titanium-based orthodontic wires during clinical use using a universal testing machine and 3D optical profiler so that better preventive measures can be implemented that would not affect the efficiency of the appliance.

Aims and objectives

To evaluate the tensile properties and surface roughness of

0.016”× 0.022” superelastic nitinol

0.016” × 0.022” heat-activated nitinol

0.016” × 0.022” Titanium Molybdenum alloy (TMA), after 3 weeks of clinical use following clinical application of acidulated phosphate fluoride (APF) gel (1.23%) for 3 min.

To evaluate the tensile properties and surface roughness of the above-mentioned wires following use of NaF mouthwash (0.2%) for 3 weeks

To compare the tensile properties and surface roughness of the above-mentioned wires after application of APF gel versus the use of NaF mouthwash, not exposed to fluoride (control) versus the fluoride groups, as-received wires with used wires not exposed to fluoride to determine the effect of oral environment on archwires.

MATERIALS AND METHODS

This study was conducted at the Department of Orthodontics, Meenakshi Ammal Dental College and Hospital. Three groups of titanium-based archwires were evaluated, each group comprised 40 samples of wires; hence, a total of 120 specimens were tested.

Materials used are

Each group [Table 1] comprised four subgroups with 10 samples each, namely Subgroup 1 – as received group – wires in as received condition; Subgroup 2 – nonfluoride control group – wires retrieved after 3 weeks from patients;

Table 1

Materials used in study

Arch wire	Alloy	Size (inch)	Sample size	Manufacturer
Group 1	Superelastic NiTi	0.016”×0.022”	40	3M Unitek
Group 2	Heat-activated NiTi	0.016”×0.022	40	3M Unitek
Group 3	TMA	0.016”×0.022	40	Ormco

NiTi: Nickel Titanium, TMA: Titanium Molybdenum alloy

Subgroup 3 – APF gel group – wires retrieved after 3 weeks from patients who received one application of APF (1.23%); and Subgroup 4 – sodium fluoride mouthwash group – wires retrieved after 3 weeks from patients who rinsed twice daily with sodium fluoride mouthrinse (0.2%).

The experimental wires were placed in patients undergoing treatment that involved aligning and leveling in 0.018” Roth mechanotherapy. All the patients were provided with a dentifrice (Colgate which contains 1000 ppm of fluoride) and were advised to brush twice daily. The patients were randomly selected with the following criteria:

Similar age range of 18–23 years

No medication or other intraorally administered substances

Similarity in gross malocclusion parameter that had no crowding and no rotations at the time of archwire placement in this study.

Archwire insertion and retrieval appointments were monitored by means of retrieval protocol.

Method of application of acidulated phosphate fluoride gel

APF gel (1.23%) (F-GEL-group pharmaceuticals) with a pH of 4.1 was used. 5 ml of the gel was uniformly dispensed on the foam tray with a 10 ml syringe and was applied to patients with experimental wires in Subgroup 3 with uniform pressure applied to all the areas of the foam tray for 3 min.

Fluoride mouthwash regimen

Patients in Subgroup 4 were supplied with 0.2% NaF mouthwash (pH of 6.25) with a measuring jar given by the manufacturer (S-FLO from Reddy's laboratory) and the patients were instructed to rinse 10 ml of solution twice daily. To ensure compliance, the subjects noted the timing of brushing and rinsing on a checklist, which the parent signed daily and was monitored by the operator.

Arch wire retrieval procedure

After 3 weeks, wires were removed by holding in the area distal to the central incisor bracket area. The collected archwires were rinsed with deionized water to detach any loosely bound precipitates. The wires were soaked for 5 min in acetone and then wiped with 70% isopropyl alcohol-soaked cotton gauze to remove surface debris and biofilm on the surface of the wires. Thereafter, the wires were handled only with forceps by holding the wire at the ends. The wires were placed in paper sterilization bags and stored in the incubator. All the tests were conducted within 7 days of sample collection. The specimens were evaluated for surface roughness and followed by the specimens were subjected to tensile testing.

Evaluation of surface roughness

Surface roughness was evaluated with a 3D optical profiler (Veeco metrology group – 3D optical profiler – model no WT1100). The method used by Lee et al. was used to evaluate the surface roughness. Three different regions of 0.295 mm × 0.225 mm area in each specimen 1 mm adjacent to the midline were evaluated and the intermediate value of average roughness was noted. The three-dimensional image analysis Zygo Corporation, Laurel Brook Road, Middlefield , CT 06455. USA, of optical profilers handles the profiling data. It gives the roughness average (Ra) value and root mean square roughness (Rq) value of the specimen.

Tensile testing

The tensile properties were measured with a universal testing machine (Model 4301, Instron Corp, Canton, Mass). A full-scale load of 1000 N was set in the machine. The machine was operated in tensile mode with a crosshead speed of 5 mm/min.[7] Each specimen was cut in the midline and placed in the jaws of the machine. The span of the wire between the crossheads was standardized as 30 mm. The load taken to break the wire was noted.

The tensile strength of specimen was calculated by the formula[7]

The extension values from the digital readout were noted and the elongation rate was calculated by the formula:

RESULTS

There was no statistically significant difference in the tensile properties between as received group and the nonfluoride control group for all the three groups. [Table 2].

Table 2

Comparison of mean values of tensile properties and surface roughness for as-received group (Subgroup 1) and nonfluoride control group (Subgroup 2)

Group	Parameter	Mean±SD		P*
		Subgroup - 1	Subgroup - 2
I (NiTi)	Tensile strength	1328.8±32.6	1301.8±40.4	0.12 (NS)
	Elongation rate	21.1±0.9	21.0±0.7	0.79 (NS)
	Ra	422.5±23.1	431.7±24.6	0.40 (NS)
	Rq	549.5±22.5	573.2±27.3	0.048 (significant)
II (heat activated NiTi)	Tensile strength	1152.2±35.5	1135.5±26.7	0.25 (NS)
	Elongation rate	22.3±1.3	21.9±1.5	0.51 (NS)
	Ra	411.0±35.7	456.8±34.7	0.009 (significant)
	Rq	549.1±39.0	594.3±40.2	0.02 (significant)
III (TMA)	Tensile strength	1108.7±26.6	1086.9±28.5	0.10 (NS)
	Elongation rate	5.1±1.2	5.0±1.0	0.72 (NS)
	Ra	427.8±40.9	464.7±35.5	0.045 (significant)
	Rq	559.2±61.3	609.3±42.8	0.048 (significant)

Ra: Roughness average, Rq: Root mean square roughness, SD: Standard deviation, NS: Not significant, NiTi: Nickel Titanium, TMA: Titanium Molybdenum alloy

There was a significant increase in surface roughness for the nonfluoride control group for heat-activated NiTi and TMA. There was a significant increase only in Rq value for NiTi.

In the NiTi group, APF gel group has the lowest mean value for both tensile strength and elongation rate. Further, there was a statistically significant difference between the APF gel group and the sodium fluoride mouthwash group [Table 3].

Table 3

Comparison of mean values for tensile properties and surface roughness for Group I (NiTi)

Parameter	Subgroup	Mean±SD	P**	Significant groups at 5% level#
Tensile strength	Subgroup - 2	1301.8±40.4	<0.0001 (significant)	Subgroup 2 versus 3, 4
	Subgroup - 3	1128.8±51.4		Subgroup 4 versus 3
	Subgroup - 4	1234.5±35.0
Elongation rate	Subgroup - 2	21.0±0.7	<0.0001 (significant)	Subgroup 2 versus 3, 4
	Subgroup - 3	12.1±1.3		Subgroup 4 versus 3
	Subgroup - 4	13.9±1.3
Ra	Subgroup - 2	431.7±24.6	<0.0001 (significant)	Subgroup 2 versus 3, 4
	Subgroup - 3	569.3±48.5
	Subgroup - 4	537.4±38.1
Rq	Subgroup - 2	573.4±27.3	<0.0001 (significant)	Subgroup 2 versus 3, 4
	Subgroup - 3	789.8±121.5
	Subgroup - 4	717.7±58.2

Ra: Roughness average, Rq: Root mean square roughness, SD: Standard deviation, NiTi: Nickel Titanium, ** Highly significant

APF gel group has the highest mean value for both the parameters of surface roughness and no statistically significant difference between the APF gel group and the sodium fluoride mouthwash group.

In heat activated NiTi group, APF gel group has the lowest mean value for both tensile strength and elongation rate and no statistically significant difference between the APF gel group and the sodium fluoride mouthwash group [Table 4].

Table 4

Comparison of mean values for tensile properties of Group II (heat activated NiTi)

Parameter	Subgroup	Mean±SD	P**	Significant groups at 5% level#
Tensile strength	Subgroup - 2	1135.5±26.7	<0.0001 (significant)	Subgroup 2 versus 3, 4
	Subgroup - 3	1045.8±30.9
	Subgroup - 4	1070.5±23.1
Elongation rate	Subgroup - 2	21.9±1.5	<0.0001 (significant)	Subgroup 2 versus 3, 4
	Subgroup - 3	12.2±1.8
	Subgroup - 4	13.5±1.6
Ra	Subgroup - 2	456.8±34.7	<0.0001 (significant)	Subgroup 2 versus 3, 4
	Subgroup - 3	637.0±50.8		Subgroup 4 versus 3
	Subgroup - 4	586.1±40.9
Rq	Subgroup - 2	594.3±40.2	<0.0001 (significant)	Subgroup 2 versus 3, 4
	Subgroup - 3	797.7±63.1
	Subgroup - 4	750.3±54.7

Ra: Roughness average, Rq: Root mean square roughness, SD: Standard deviation, NiTi: Nickel Titanium, ** Highly significant

In TMA group, APF gel group has the lowest mean value for both tensile strength and elongation rate and no statistically significant difference between the APF gel group and the sodium fluoride mouthwash group [Table 5].

Table 5

Comparison of mean values for tensile properties and surface roughness of Group III (TMA)

Parameter	Subgroup	Mean±SD	P**	Significant groups at 5% level#
Tensile Strength	Subgroup - 2	1086.9±28.5	<0.0001 (significant)	Subgroup 2 versus 3, 4
	Subgroup - 3	997.8±40.3
	Subgroup - 4	1020.0±31.8
Elongation rate	Subgroup - 2	5.0±1.0	<0.0001 (significant)	Subgroup 2 versus 3, 4
	Subgroup - 3	2.4±0.5
	Subgroup - 4	3.0±1.0
Ra	Subgroup - 2	464.7±35.5	<0.0001 (significant)	Subgroup 2 versus 3, 4
	Subgroup - 3	537.7±30.3
	Subgroup - 4	507.7±23.5
Rq	Subgroup - 2	609.3±42.8	<0.0016 (significant)	Subgroup 2 versus 3, 4
	Subgroup - 3	667.4±29.3
	Subgroup - 4	649.9±23.5

Ra: Roughness average, Rq: Root mean square roughness, SD: Standard deviation, TMA: Titanium Molybdenum alloy, ** Highly significant

APF gel group has the lowest mean value for both tensile strength and elongation rate and no statistically significant difference between the APF gel group and the sodium fluoride mouthwash group.

APF gel group has the highest mean value for both the parameters of surface roughness. Further, there was no statistically significant difference between the APF gel group and the sodium fluoride mouthwash group.

DISCUSSION

Nakagawa et al.[8] reported that fluoride-containing rinses or gels might be harmful to titanium devices if the pH of these prophylactic materials was below neutral. Further, they showed that there was an increase in the corrosion behavior of titanium in fluoride solutions with different fluoride concentrations and pH. Kononen et al.[9] and Kaneko et al.[5] reported that titanium alloys are more susceptible to localized or generalized attack with degradation of properties and increase in surface roughness due to reduction in corrosion resistance when placed in solutions containing fluoride.

Hence, this study was undertaken to clinically evaluate the effect of fluoride on the tensile properties and surface roughness of three commonly used titanium-based archwires, namely NiTi, heat-activated NiTi, and TMA.

Topical fluorides in the form of gels and rinses containing 1000–10000 ppm of fluoride with pH between neutral and 3.5 are commonly used during orthodontic treatment.[10] Zachrisson[11] reported that APF gel application is recognized as a convenient professional method of fluoride application in orthodontic patients who are uncooperative and highly susceptible to caries lesions. Øgaard et al.[10] reported that daily mouthrinsing with a neutral 0.2% sodium fluoride solution gave virtually complete caries protection as compared with nonrinsing group.

To obtain a more objective numerical value for surface roughness, a 3D optical profiler was used to calculate Ra and Rq values as recommended by Lee and Chang[6] and Prososki et al.[12] Ra is used to express surface roughness as determined by mechanical means, it can be easily and consistently determined. Rq describes the optical finishing status of the surface and is an important piece of statistical data because it expresses the standard deviation of the sample.

Uniaxial tensile testing was performed for all the 120 specimens after evaluation of surface roughness. Miura et al.[13] and Asgharnia and Brantley[14] reported that tensile testing is the most acceptable method to clearly demonstrate the comparative mechanical properties of archwires.

The results of this study comparing the as-received wires and the wires used in patients who received no fluoride application [Table 3] showed that there was a statistically significant increase in surface roughness values for all the three groups. However, there was no significant degradation in tensile properties. The main cause has been considered to be surface defects generated during wire manufacture, fatigue, or corrosion-related phenomenon.

The results of the study comparing the NiTi wires with and without fluoride application indicated a statistically significant increase in surface roughness and decrease in tensile strength and elongation rate (P < 0.05). One of the reasons for this is that the corrosion resistance of titanium alloys decreases in solutions containing fluoride. In acid fluoride solutions, the protective titanium film reacts with hydrofluoric acid to form sodium titanium fluoride resulting in breakdown of protective film because NiTi alloys absorb hydrogen due to the high affinity of titanium to hydrogen as reported by Huang.[15] The breakdown of film leads to a decrease in corrosion resistance as reported by Nakagawa et al.[16]

The highest reduction in tensile strength and elongation rate was in the APF gel group, followed by the sodium fluoride mouthwash group with a mean of and control group. The as-received wires had the highest mean value. One of the reasons for a decrease in tensile strength and elongation rate is due to hydrogen embrittlement in the presence of fluoride in the oral cavity.[3]

The mechanism proposed for hydrogen embrittlement is stress-induced hydride formation and cleavage mechanism was reported by Kononen et al.[9]

Further, there was a statistically significant difference in tensile properties between the patients who received APF gel and neutral sodium fluoride mouthwash (P < 0.05). This is due to the difference in fluoride concentration and pH of fluoride solutions.

The results of the study comparing with and without fluoride application for heat-activated NiTi and TMA wires were also similar to NiTi wires. The possible mechanism for hydrogen embrittlement for TMA is hydrogen-induced decohesion mechanism.[17]

The highest reduction in tensile strength was for NiTi wires under the APF gel group. This was followed by the heat-activated NiTi wires and TMA. Surface roughness was highest for heat-activated NiTi wires under the APF gel group followed by NiTi and TMA.

Clinical implications of the present results suggest that orthodontists should avoid using fluoride solution with low pH and high concentrations when using archwires with titanium protective film. Hence, a better preventive regimen should be designed without significantly reducing the efficiency of the appliance.

SUMMARY AND CONCLUSION

The effect of use of APF gel with high fluoride concentration and low pH on titanium-based archwires causes most degradation in tensile properties and increased surface roughness. Hence, fluoride applications with low concentrations and neutral pH might be used or fluoride applications should be devised to prevent the reduction in efficiency of the appliance.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.