November, 2010

To Evaluate the Effects of Sterilization on the Tensile Strength and Surface Characteristics of Orthodontic Wires – An in vitro study

Dr. Madhvi Bhardwaj

Sr. Lecturer,Department of Orthodontics

Career P.G. Institute of Dental Sciences, Lucknow

Dr.(Mrs.) P.V. Hazarey,

Prof, & Head, Department of Orthodontics.

Sharad Pawar Dental College,Sawangi, Wardha(Maharashtra)

Abstract:

The study was being carried out to evaluate the effects of steam, dry heat, & cold  sterilization methods on the tensile strength of 0.016 inch Nickel-Titanium, Beta-Titanium and Stainless steel wires and  evaluate the surface topography of these wires before and after sterilization using these three types of sterilization methods.

Introduction:

Recent advances in orthodontic material technology have resulted a wide array of wires that exhibit a wide spectrum of properties1.

Knowing that the Ultimate tensile strength of an Orthodontic wire is of importance, any unfavorable change in it can alter the efficacy of Orthodontic treatment. Thus, it is necessary to know the effects of various sterilization procedures on tensile properties of Orthodontic wires.

Reusing of wires would be of some economic benefit if recycling would not affect the properties of the wires.

The present study was undertaken to evaluate the feasibility of recycled arch wires by determining whether the tensile strength and surface topography of arch wires is altered significantly by sterilization.

The tensile strength of the wires that were treated with artificial saliva was determined by using Instron testing machine and surface characteristics were determined by Scanning electron Microscope, both before and after sterilization.

If sterilization significantly alters these two properties of wires in an unfavorable direction, then use of recycled wire may cause the delivery of substandard care.

Review Of Literature:

Kusy R.P. and Greenberg A.R. (1981)2 concluded that Nitinol wires makes the most active “leveling” archwire, TMA is a superior intermediate archwire where flexibility is required and stainless steel and cobalt chrome are the wires of choice for finishing and where stability is required.

Drake S.R, Wayne D.M, Powers J.M, Asgar K.20 (1982)3 found in tension, the stainless steel wires had the least elastic strain or springback, whereas the Beta Titanium had the most.In bending and torsion, the stainless steel wires had the least stored energy at a fixed moment, whereas Nickel Titanium wires had the most. Spring rates in bending and torsion, however, were highest for stainless steel wires and lowest for Nickel Titanium wires.

Nicholson J.A. (1984)4 suggested that Orthodontists who attempted to reuse Nitinol wires faced a decrease in the expected mechanical properties of the wire.

Kapila S. et al (1991)5 It was concluded that recycling produced significant changes during loading of the arch wires but not during unloading. Representative scanning electron micrographs demonstrated increased pitting of wires after clinical exposure.

Evans T.J, Jones M.L., New Combe R.G. (1998)6 investigated the effect of surface roughness on the relative corrosion rates of wires of  four alloys- stainless steel, nickel titanium, cobalt chromium and beta titanium. They found that nickel titanium wires exhibited greatest corrosion resistant in the as received state. Polishing significantly reduced the corrosion rate of nickel titanium, and comparison between the four alloys in the polished state revealed no significant difference in their relative corrosion rate.

Yang W.S., Kin B. H., Kim T.W. (1999)7 showed that heat sterilization had no effect on the tensile properties of any of the Nickel – Titanium wires used in the experiment. Although SEM showed no changes in Ni –Ti and Optimalloy but surface changes were identified in sentalloy after heat sterilizations.

Eliades. T. et al (2000)8 studied the surface characterization of   Ni-Ti orthodontic wires that were used for 1 – 6 months intra orally. The wires were studied under the optical microscope which revealed islands of amorphous precipitants and accumulated microcrystalline particles.

Shin J.S. et al (2003)9 found corrosion products on the stainless steel wires increased whereas NiTi arch wires did not corroded.

Materials & Method:

Materials –

•                     Test Sample: Sample comprised of three types of straight length segments of Orthodontics wires i.e. 0.016” Stainless Steel wires, 0.016” Beta – Titanium (CAN), 0.016” Nickel – Titanium, which were designated as Group I, Group II and Group III respectively comprising of 40 samples each.

Group Wire No. of Samples
Group I Stainless Steel 40
Group II Beta Titanium 40
Group III Nickel Titanium 40

Each group was then divided into four subgroups:

•         Subgroup A – Control Group

•         Subgroup A1 – samples sterilized by Cold Sterilization

•         Subgroup A2 – samples sterilized by Dry Heat

•         Subgroup A3 – samples sterilized by Autoclaving

Group I

(Stainless Steel)

Group II

(Beta Titanium)

Group III

(Nickel Titanium)

Sub group Sub group Sub group
A A1 A2 A3 A A1 A2 A3 A A1 A2 A3
No. 10 10 10 10 10 10 10 10 10 10 10 10

Artificial Saliva prepared with following composition-0.4gm NaCl

0.4gm KCl

0.8gm CaCl2.2H2O

0.01gm NaS.5H2O

1gm CO (NH2) (Urea)

1 liter Distilled Water.

Equipments :

•          Dry heat sterilizer

•         Autoclave

•         Cold Sterilization with 2 % Glutaraldehyde (Bioclenz-G)

•         Incubator BTI – Biotech™ (S No. 02885)

•         Instron machine – 4467 (Instron Corporate Series IX Automated Materials Testing System)

•         Scanning Electron Microscope (Japan Electron Optical Lab. JSM-6380A)

PROCEDURE:

1 cm segment of each sample (Control Group as well as Experimental Group) were placed on a Brass Stub which was previously coated with a doubled sided carbon tape. Brass stub was then placed into the sample holder.

Sample holder was placed inside specimen chamber and then magnification of sample was adjusted and photographs at 100 X were recorded.

The recycling of the samples in respective experimental group was carried out as follows:

–        Group I Subgroup A1, Group II Subgroup A1 and Group III Subgroup A1 were recycled using Cold sterilization with 2% Glutaraldehyde for 8 -10 hrs

–        Group I Subgroup A2, Group II Subgroup A2 and Group III Subgroup A2 were recycled using Dry heat sterilization at 3200 F (1600 C)for 60 mins

–        Group I Subgroup A3, Group II Subgroup A3 and Group III Subgroup A3 were recycled using Autoclave sterilization at 2500 F (1210 C) for 15 mins under 15 psi pressure.

After recycling of experimental group, samples were again studied under Scanning Electron Microscope at 100 X and photographs were recorded.

Tensile Strength Testing:

Each Sample was standardized to 7 inches length and tensile strength was tested using Instron machine. Test was carried out for both, Control Group as well as Experimental Group.

The ends of the wire were attached to the fixtures of the Instron Machine out of which one was movable while the other was fixed. A 30 kN load – cell was attached to the machine and was set with a cross head speed of 1 mm/min.

The wire was thus stretched vertically till it was fractured at which point the readings were noted. Same procedure was repeated for each sample.

The tensile strength recorded was the maximum stress value in Mega Pascal (MPa) just prior to fracture of the test wires.

OBSERVATION & RESULT:

After evaluating, the observations were subjected to paired t test, one way ANOVA, and multiple comparison (Bonferroni) tests and following conclusions were drawn:

  • Dry heat sterilization produced statistically significant increase in the ultimate tensile strength of Beta – Titanium and Nickel Titanium wires following three cycles.
  • Autoclave sterilization significantly increased the tensile strength of Beta – Titanium and Nickel Titanium wires.
  • Cold sterilization showed no statistically significant change in the tensile strength of Beta – Titanium and Nickel Titanium wires.
  • Tensile strength property of stainless steel wire was not significantly affected by all the three methods of sterilization.
  • SEM photographs suggested that there was gross increase in pitting roughness of the surface topography of all the three types of wires after incubation in artificial saliva. However, thereafter, there was no considerable difference in the surface topography of wires after three sterilization cycles. This shows that this change in surface topography may be due to incubation in artificial saliva i.e. simulated oral environment and not due to sterilization method.

Table I  : Ultimate Tensile strength of  Group I (Stainless Steel)

Experimental Groups F-Value

(A1,A2 & A3)

A2  Vs A 3 A1  Vs A 2 A1  Vs A 3
Sub group A1 Sub group A2 Sub group A3 0.34 1.74 3.31 5.05
2032.81 ± 19.27 2029.50 ± 10.65 2027.75 ± 9.63
(Control group)

Sub group A

2030.07 ± 7.22
t-value 0.43 0.13 0.73
p-value 0.67 0.89 0.48 0.74 1.00 1.00 1.00
Significance NS

p>0.05

NS

p>0.05

NS

p>0.05

NS

p>0.05

NS

p>0.05

NS

p>0.05

NS

p>0.05


Graph I : Mean Ultimate Tensile strength of Group I (Stainless Steel)

Table IA: Paired Samples Test (Group I)

Pair Paired Differences T Degrees of freedom p-value
Mean Std. Deviation Std. Error Mean 95% Confidence Interval of the Difference
Lower Upper
Pair 1 :

Sub group A –A1

-2.74 20.19 6.38 -17.19 11.70 0.43 9 0.678

NS

p>0.05

Pair 2 :

Sub group A –A2

0.57 13.40 4.24 -9.01 10.16 0.13 9 0.895

NS

p>0.05

Pair 3 :

Sub group A –A3

2.31 9.96 3.15 -4.81 9.44 0.73 9 0.481

NS

p>0.05

Table IB: One way ANOVA (Group I)

Source of variation Sum of Squares Degrees of freedom Mean Square F-value p-value
Between Groups 132.10 2 66.05 0.34 0.71

NS

p>0.05

Within Groups 5202.96 27 192.70
Total 5335.06 29

Table IC: Multiple Comparisons : Bonferroni (Group I)

Sub Group Mean Difference

(I-J)

Std. Error p-value 95% Confidence Interval
Lower Bound Upper Bound
A1 A2 3.31 6.20 1.00

NS

p>0.05

-12.52 19.16
A1 A3 5.05 6.20 1.00

NS

p>0.05

-10.78 20.90
A2 A3 1.74 6.20 1.00

NS

p>0.05

-14.10 17.58

Table II  : Ultimate Tensile strength of  Group II (Beta Titanium)

Experimental groups F-Value

(A1,A2 & A3)

A2  Vs A3 A1  Vs A2 A1  Vs A3

Sub group A1

Sub group A2 Sub group A3 2072.74 188.74 -202.78 -14.04

1241.80 ± 3.55

1444.58 ± 6.56 1255.84 ± 11.39
(Control group)

Sub group A

1242.44 ± 8.21

t-value 0.20 51.54 2.66
p-value 0.840 0.000 0.026 0.000 0.000 0.000 0.001
Significance NS

p>0.05

S

P<0.05

S

P<0.05

S

p<0.05

S

p<0.05

S

p<0.05

S

p<0.05

Graph II : Mean Ultimate Tensile strength of Group II (Beta Titanium)

Table IIA: Paired Samples Test (Group II)

Pair Paired Differences t Degrees of freedom p-value
Mean Std. Deviation Std. Error Mean 95% Confidence Interval of the Difference
Lower Upper
Pair 1 :

Sub group A –A1

0.64 9.75 3.08 -6.33 7.62 0.20 9 0.840

NS

p>0.05

Pair 2 :

Sub group A –A2

-202.14 12.40 3.92 -211.01 -193.27 51.54 9 0.000

S

p<0.05

Pair 3 :

Sub group A –A3

-13.40 15.91 5.03 -24.78 -2.01 2.66 9 0.026

S

p<0.05

Table IIB: One way ANOVA (Group II)

Source of variation Sum of Squares Degrees of freedom Mean Square F-value p-value
Between Groups 256482.84 2 128241.42 2072.74 0.000

S

p<0.05

Within Groups 1670.50 27 61.87
Total 258153.34 29

Table IIC: Multiple Comparisons : Bonferroni (Group II)

Sub Group Mean Difference

(I-J)

Std. Error p-value 95% Confidence Interval
Lower Bound Upper Bound
A1 A2 -202.78 3.51 0.000

S

p<0.05

-211.76 -193.80
A1 A3 -14.04 3.51 0.001

S

p<0.05

-23.02 -5.06
A2 A3 188.74 3.51 0.000

S

p<0.05

179.76 197.72

Table III  : Ultimate Tensile strength of  Group III (Nickel Titanium)

Experimental Group F-Value

(A1,A2 & A3)

A2  Vs A3 A1  Vs A2 A1  Vs A3

Sub group A1

Sub group A2 Sub group A3 124.32 -3.54 -57.5 -61.0

1350.90 ± 6.76

1408.45 ± 11.41 1411.90 ± 10.36
(Control group)

Subgroup A

1353.78 ± 8.90
t-value 0.65 14.62 10.45
p-value 0.53 0.000 0.000 0.000 1.000 0.000 0.00
Significance NS

p>0.05

S

P<0.05

S

P<0.05

S

p<0.05

NS

p>0.05

S

p<0.05

S

p<0.05

Graph III : Mean Ultimate Tensile strength of Group III (Nickel Titanium)


Table IIIA: Paired Samples Test (Group III)

Pair Paired Differences t Degrees of freedom p-value
Mean Std. Deviation Std. Error Mean 95% Confidence Interval of the Difference
Lower Upper
Pair 1 :

Sub group A –A1

2.88 13.96 4.41 -7.10 12.87 0.65 9 0.530

NS

p>0.05

Pair 2 :

Sub group A –A2

-54.67 11.82 3.73 -63.12 -46.12 14.62 9 0.000

S

p<0.05

Pair 3 :

Sub group A –A3

-58.12 17.57 5.55 -70.69 -45.55 10.45 9 0.000

S

p<0.05

Table IIIB: One way ANOVA (Group III)

Source of variation Sum of Squares Degrees of freedom Mean Square F-value p-value
Between Groups 23486.74 2 11743.37 124.32 0.000

S

p<0.05

Within Groups 2550.42 27 94.46
Total 26037.17 29

Table IIIC: Multiple Comparisons : Bonferroni (Group III)

Sub Group

Mean Difference

(I-J)

Std. Error p-value 95% Confidence Interval
Lower Bound Upper Bound
A1 A2 -57.55 4.34 0.000

S

p<0.05

-68.64 -46.45
A1 A3 -61.00 4.34 0.000

S

p<0.05

-72.10 -49.91
A2 A3 -3.45 4.34 1.000

NS

p>0.05

-14.54 7.63

DISCUSSION:

The mean values of ultimate tensile strength for Group I sub group A, Group I sub group A1, Group I sub group A2 and Group I sub group A3 were found to be 2030.07 ± 7.22, 2032.81 ± 19.27, 2029.50 ± 10.65 and 2027.75 ± 9.63 respectively.(Table I and Graph I)

The mean values for Group II sub group A, Group II sub group A1, Group II sub group A2 and Group II sub group A3 were 1242.44 ± 8.21, 1241.80 ± 3.55, 1444.58 ± 6.56 and 1255.84 ± 11.39 respectively. (Table II and Graph II)

For Group III sub group A, Group III sub group A1, Group III sub group A2 and Group III sub group A3 mean values of ultimate tensile strength were 1353.78 ± 8.90, 1350.90 ± 6.76, 1408.45 ± 11.41 and 1411.90 ± 10.36 respectively. (Table III and Graph III)

Results of the present study on Group I arch wires when compared with those carried out by Smith G.A. et al. (1992)10, Staggers J. A. and Margeson D. (1993)11, Pernier C. et al (2005)12 are in accordance with them.

Staggers J. A. and Margeson D. (1993)11, Pernier C. et al (2005)12 in their study found no significant alteration in surface characteristics of Group II wire before and after sterilization which was in accordance with the present study.

Smith G.A. et al. (1992)10 compared effect of clinical use and sterilization on 0.016 inch Beta-titanium wire (TMA, Ormco Corp., Glendora, Calif.).In their study tensile tests showed no clinically significant difference between before and after disinfected/sterilized wires. However, results of the present study suggest that there is statistically significant increase in the ultimate tensile strength of the Group II wires following Dry heat sterilization and Autoclaving.

The results of the present study were in accordance to the study carried out by Buckthal and Kusy (1988)13 i.e. Cold sterilization with 2% glutaraldehyde did not significantly alter the tensile strength property of Group III arch wire.

Also, the study showed that there was no significant increase in pitting on the wire surface following cold sterilization with 2% glutaraldehyde, which is in accordance with the findings of Buckthal and Kusy (1988)13

Result of the present study when compared for Group III wire are in accordance with those found in study conducted by Mayhew M. J. and Kusy R. P. (1988)14, Staggers J. A. and Margeson D. (1993)11, Crotty O.P. Davis E.H. & Jones S. P.(1996)15, Lee S.H. et al (2001)16, Pernier C. et al (2005)11 .

Smith G.A. et al. (1992)10 compared the effect of clinical use and sterilization on three types of 0.016 inch Group III (Align, “A” Company, San Diego, Calif.; Nitinol, Unitek/3 Corp., Monrovia, Calif.; OSE, Gaithersburg, Md.) Tensile tests showed no clinically significant difference between before and after disinfected/sterilized wires. However, results of the present study suggest that there is statistically significant increase in the ultimate tensile strength of the Group III wires following Dry heat sterilization and Autoclaving.

CONCLUSION:

This study have demonstrated no appreciable loss in properties of the wires after repeated cycles of sterilization.  A common reason given for not recycling the wires is fear of reducing the wire’s strength. The results of the present study suggest that sterilization and reuse of wires does not unfavorably alter their tensile strength.

To conclude, whether or not recycling is a practical method of reducing Orthodontic treatment cost, it should be left for each practitioner to decide.

Plate I

(Beta Titanium)

SEM of Control group wire

SEM of Experimental group wire before Sterilization

SEM after Cold Sterilization

SEM after Dry Heat Sterilization

SEM after Autoclaving

Plate II

(Nickel Titanium)

SEM of Control group wire

SEM of Experimental group wire before Sterilization

SEM after Cold Sterilization

SEM after Dry Heat Sterilization

SEM after Autoclaving

Plate III

(Stainless Steel)

SEM of Control group wire

SEM of Experimental group wire before Sterilization

SEM after Cold Sterilization

SEM after Dry Heat Sterilization

SEM after Autoclaving

References:

1. Wilkinson J.V.: Some metallurgical aspects of orthodontic stainless steel. Angle Orthod 1962; 48: 192-206.

2. Kusy RP, Greenberg AR.: Effects of composition and crosssection on elastic properties of orthodontic wires. Angle Orthod 1981; 51: 325-41.

3. Drake S.R, Wayne D.M, Powers J.M, Asgar K.: Mechanical properties of orthodontic wires in tension, bending and torsion. Am J Orthod Dentofacial Orthop 1982; 82: 206-210.

4. Nicholson J.A.: An analysis of nitinol in a simulated oral environment. Am J Orthod 1984; 85: 453.

5. Kapilla S, Reichhold GW, Anderson RS, Watanabe LG.: Effects of clinical recycling on mechanical properties of nickel-titanium alloy wires. Am J Orthod Dentofac Orthop 1991; 100: 428-35.

6. Evans T.J, Jones M.L, New Combe R.G.: Clinical comparison and performance of three aligning archwires. Am J Orthod 1998; 114: 32-39.

7. Shin J.S. Oh K.T. Hwang C.J.: In-vitro surface corrosion of stainless steel and NiTi Orthodontic appliances Aust Orthod J 2003; 19: 13-18.

8. Yang W.S., Kin B. H., Kim T.W.: Surface and mechanical changes in  nickel titanium wires after heat sterilization. (abstract) Eur J Orthod 1999; 21: 630.

9. Eliades. T. Eliades G. Athansiou A.E. and Bradley T.G.: Surface characterization of retrieved NiTi wires. Eur J Orthod 2000; 22: 317-326.

10. Smith G.A. von Fraunhofer J.A. Casey G.R.: The effect of clinical use and sterilization on selected orthodontic arch wires. Am J Orthod 1992; 102: 153-159.

11. Staggers J. A. and Margeson D.: The effects of sterilization on the tensile strength of orthodontic wires. Angle Orthod 1993; 63: 141-144.

12. Pernier C. Grosgogeat B. Ponsonnet L. Benay G. Lissac M.: Influece of autoclave sterilization on the surface parameters and mechanical properties of six orthodontic wires. Eur J Orthod 2005; 27: 72-81.

13. Buckthal JE and Kusy RP.: Effects of cold disinfectants on the mechanical properties and surface topography of nickel-titanium arch wires. Am J Orthod Dentofac Orthop 1988; 94: 117-22.

14. Mayhew MJ and Kusy RP.: Effects of sterilization on the mechanical properties and surface topography of nickel-titanium arch wires. Am J Orthod Dentofac Orthop 1988; 93: 232-6.

15. Crotty O.P. Davis E.H. Jones S.P.: The effect of cross-infection control procedures on the tensile and flexural properties of superelastic Nickel Titanium wires. Br J Orthod 1996; 23: 37- 41.

16. Lee S.H. Chang Y.I.: Effects of recycling on the mechanical properties and surface topography of nickel titanium alloy wires. Am J Orthod Dentofacial Orthop 2001; 120: 654-663.

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