The effects of ion implantation on rate of tooth movement: An in vitro model

Rosemary Ryan, DDS, a Graham Walker, PhD, b Katherine Freeman, Dr PH, c and George J, Cisneros, DMD, MMSc d Bronx, N.Y.

Recently, the ion-implantation process has been applied to orthodontic wires. By altering the surface composition of a wire, the ion-implantation process supposedly decreases the frictional forces produced during tooth movement. The purpose of this study was to compare the amount of tooth movement produced by different orthodontic wire compositions, under identical conditions, by using an in vitro model. The wires tested were stainless steel, nickel-titanium (control and ion implanted), and beta-titanium (control and ion implanted). The amount of tooth movement was measured and compared. Results demonstrate that, stainless steel produced the least frictional force during in vitro tooth movement, followed by ion-implanted nickel-titanium, ion-implanted beta- titanium, untreated nickel-titanium, and finally, untreated beta-titanium. A Wilcoxon rank sum test showed statistically significant differences in the amount of movement seen with the ion-implanted wires when compared with their untreated counterparts. (Am J Orthod Dentofac Orthop 1997;112: 64-8.)

T o produce tooth movement, the force generated from an orthodontic appliance must first overcome static frictional forces. To continue this movement, orthodontic forces must be greater than the kinetic frictional forces produced from the movement itself as well as the resistance caused by the periodontium. 1 In orthodontics, the appliance system one selects may effect the proficiency of tooth movement. 2

A review of the literature shows arch wire com- position is among the variables that has an impact on the frictional forces produced in tooth move- ment? -3 Various compositions may create greater frictional forces during tooth movement. As a result, they have an effect on the proficiency of tooth movement. 4 Hence, it is important to understand which arch wire composition may increase or de- crease the rate of tooth movement. Of equal impor- tance is the investigation of processes that may help improve arch wire material.

Recently, the ion-implantation process, often used in the mechanical engineering field, has been

aFormer Resident, Department of Dentistry, Division of Orthodontics, Montefiore Medical Center-Albert Einstein College of Medicine. Cur- rently in private practice in New York City. bDepartment of Mechanical Engineering, Manhattan College. °Division of Biostatistics, Montefiore Medical Center-Albert Einstein College of Medicine. aDepartment of Dentistry, Division of Orthodontics, Montefiore Medical Center-Albert Einstein College of Medicine. Reprint requests to: Dr. George J. Cisneros, Montefiore Medical Center, 111 E. 210th St., Bronx, NY 10064. Copyright © 1997 by the American Association of Orthodontists. 0889-5406/97/$5.00 + 0 8/1/74766

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applied to orthodontic wires. This process alters the surface composition of a wire. It has been proposed that the ion-implantation process decreases the fric- tional forces produced during tooth movement? If frictional forces were decreased during the sliding mechanics of orthodontic movement, the efficiency of tooth movement may be improved.

The purpose of this project was to compare the amount of tooth movement produced by different orthodontic wire compositions, under identical con- ditions, by using an in vitro model. The primary hypothesis was that various wire compositions pro- duce different frictional forces. Those wires that produce the least frictional force would produce the greatest movement, thus effecting the proficiency of tooth movement. The arch wires examined were those that are commonly used by practitioners. Included in the study were wires that were treated with the ion-implantation process.

MATERIALS AND METHODS

A test apparatus was developed to simulate the distal movement of a maxillary canine, fitted with one type of metal bracket and five different wires. The design w a s

modeled after the apparatus used by Tanne, Matsubara, Shibaguchi, and Sakuda (Fig. 1). 5

The arch wires tested were untreated nickel-titanium (Neo SentaUoy, GAC), treated nickel-titanium (Neo Sen- talloy ion-Guard, ion implantation of nitrogen into nickel- titanium wire, GAC), untreated beta-titanium (TMA, Ormco), treated beta-titanium (TMA low friction, ion implantation of nitrogen into beta-titanium, Ormco), and

American Journal of Orthodontics and Dentofacial Orthopedics Ryan et al. 65 Volume 112, No. 1

Fig. 1. Test apparatus: Prosthetic tooth embedded in wax, wire is engaged in brackets with spring activated. Note: Measurements were taken from posterior post to mesial edge of bracket.

stainless steel (Tru-Crome, Rocky Mountain). The wire size was 0.016 × 0.022 inch. The bracket used was a standard edgewise bracket with 0 ° tip and torque and a slot size of 0.022 × 0.028 inch. Five identical maxillary canines were banded along the gingival third of the crown.

Five identical aluminum rectangular trays were fabri- cated. Ten metal posts were glued at opposite ends of each tray.

Brackets, as previously stated, were welded on both the teeth and the posts. The brackets were first welded on to bands, then the bands were contoured to fit the teeth and the posts. Horizontally, the brackets were positioned to sit along the superior edge of the band. Vertical position was established by following the long axis of the tooth (the long axis of the tooth was established with survey equipment). Care was taken to position the brack- ets on the posts along the same horizontal plane.

An initial batch of wax was made (50% base plate and 50% utility wax) and used for the entire experiment. Wax was melted and placed into the apparatus (between each run wax was removed and replaced with fresh wax). A tooth was then positioned in the melted wax with a positioning jig. The apparatus was then allowed to cool overnight (at least 12 hours) at room temperature. Before each run, the tooth was examined for wax and debris that may have accumulated within the bracket slot. If debris was noted, then the tooth and wax were removed from the apparatus. The tooth and tray were cleaned with a flame and this process was repeated. After the wax was cooled to room temperature, the bath was allowed to equilibrate for 1 hour (44 ° C).

While the temperature was equilibrating, the wires were placed into the anterior and posterior posts and through the bracket on the tooth. The wire was engaged in the brackets on the posts with ligature wire and on the tooth with an

elastomeric ring (all from the same batch). The tooth was engaged with an elastomeric ring to ensure that a constant force was applied from run to run. Finally, stops were placed on the two most distal ends to stabilize the wire.

A 150 gm closed coil, nickel-titanium spring (GAC) of a fixed length was then placed on the bracket of the tooth and extended to the posterior bracket. With each run, the spring was extended a total of 14 mm inclusive of its original length (measured with a vernier caliper). All springs were tested with a Cortex force gauge to assure that a 150 gm force was delivered. Springs were replaced after three runs, because it was found in the pilot study that the spring decayed by the seventh run.

The apparatus was then placed into the water bath at 44 ° C (-+ 1.5 ° C) and observed. The total experiment time in the water bath was 20 minutes. During that time, the temperature was monitored every 3 minutes to check for fluctuations.

Measurements were taken with a vernier caliper from the posterior post to the mesial edge of the bracket. The initial and final lengths were recorded and subtracted from each other to obtain the total movement. Finally, for each run of the experiment a new piece of sample wire was used. The teeth were randomly placed for each run. The experimenter was blind as to which wire was being studied. Finally, the wire was rotated systematically from tray to tray.

The systematic rotation was designed as follows: In the first run, wire no. 1 was used in tray no. 1; wire no. 2 in tray no. 2 and this was true for all five sets of wire. Next, in the second run of the experiment, wire no. 2 was used in tray no. 1, wire no. 3 was used in tray no. 2, for all five sets.

An initial pilot study was performed to determine the feasibility of this research method. In this part of the study, stainless steel was tested by using the same tray 10 different times.

66 Ryan et al. American Journal of Orthodontics and Dentofacial Orthopedics July 1997

Table I. Descr ip t ive statist ics: Eva lua t i on of too th m o v e m e n t for the five different wi re types

Treated Stainless steel* NiTi[" NiTi

(ram) (ram) (ram)

Mean tooth movement 3.75 1.19 2.50 Standard deviation 1.49 0.59 0.73 Variance 2.24 0.35 0.54 Range 5.4 2.2 2.8

TMA t (ram)

Treated TMA (mm)

0.92 1.35 0.59 0.46 0.35 0.21 2.4 1.6

*Stainless steel was significantly different from each of the other wire types (p < 0.05). ?Untreated wires were significantly different from their treated counterparts.

After the initial pilot study was completed, a final pilot study was performed primarily to assure all the trays produce consistent results. In addition, this study was performed to obtain estimates of the means and standard deviations of the outcome variables so the sample size could be calculated. Here, stainless steel wires were tested in all trays simultaneously, this was repeated twice to get a total of 10 runs. The results were compared and reported.

As a result of both pilot studies, a probability distri- bution showed that a standard deviation of 0.5 mm would be appropriate for multiple runs of a particular wire type. On the basis of this value for the standard deviation, to detect a difference of 1.0 ram, 18 replicates of the exper- iment were needed (a total of 18 wires for the five wire types). This sample size was determined to yield 80% power, by using a type I error of 0.05 in a two- tailed test.

RESULTS

When all five wires were compared, there was a significant difference between each of the wires and the average amount of tooth movement. Stainless steel produced the most movement, followed by treated nickel-titanium, treated beta-titanium, un- treated nickel-titanium, and finally, untreated beta- titanium (Table I). The average amount of tooth movement with stainless steel was 3.75 ram, fol- lowed by treated nickel-titanium at 2.50 mm, treated beta-titanium at 1.35 ram, next was untreated nickel- titanium at 1.19 ram, and finally, untreated beta- titanium had the least movement at 0.92 mm.

A two-way analysis of variance showed a signif- icant difference among the wire types (p < 0.001) and failed to show a significant difference among the test runs for each wire type. The differences among wire types was confirmed by a Kruskal-Wallis test. The Kruskal-Wallis test (chi-square approximation) was used in addition to the analysis of variance because it was not entirely clear that all of the assumptions for the analysis of variance were met.

The Dunnett 's test also showed that stainless steel was significantly different from the rest of the wires.

Wilcoxon rank sum tests showed significant dif- ferences between the treated and untreated nickel- and beta-titanium wires, (p < 0.05). This test con- firmed significant differences in the amount of tooth movement seen with the treated wires when com- pared with their untreated counterparts.

Descriptive statistics showed stainless steel pro- duced the most average movement, followed by treated nickel-titanium, treated beta-titanium, un- treated nickel-titanium, and finally, untreated beta- titanium. In addition, stainless steel produced the greatest variation in movement, whereas treated beta-titanium wire demonstrated the least.

In observing the descriptive statistics, a post-hoc analysis was performed to determine whether the variance for stainless steel differed significantly from the other wire types. An F-test indicated stainless steel has significantly greater variance (p < 0.05).

DISCUSSION

The aim of this research project was to deter- mine how differences in arch wire composition effect the proficiency of tooth movement, in particular, those wires whose surfaces received ion implanta- tion of nitrogen.

The results clearly demonstrated that the ion- implantation process does reduce the frictional forces produced during tooth movement. Consistently and significantly the treated wire, when compared with the corresponding untreated wire, produced a greater mean tooth movement. The ion-implantation pro- cess tends to increase stress fatigue, hardness, and wear regardless of the composition of the material. 6 The hardness of a material is generally defined as resistance to scratching or wear. 7 Because the coef- ficient of friction is inversely proportional to the "hardness," by increasing the hardness of a material the friction is decreased. 7 Perhaps this explains why

American Journal of Orthodontics and Dentofacial Orthopedics Ryan et al. 67 Volume 112, No. 1

a reduction in friction was seen in the two different wire types, regardless of the composition.

Past studies comparing stainless steel, untreated nickel-titanium, and beta-titanium have shown that stainless steel produces less frictional force with tooth movement, followed by nickel-titanium, then beta-titanium. 2,8 The order observed in our study was essentially the same regardless of whether or not the wires were ion-implanted.

Interestingly, stainless steel wire had the great- est variability of movement. This large variability was probably due to irregularities in the surface characteristics of the wire. These irregularities may not be present on the other wires because of the difference in the manufacturing process itself or because of better quality control. Both the nickel- and beta-titanium wires used in the study were processed "in house" by the same company that sells them. Stainless steel, however, is manu- factured by only a few major distributors and distributed to the individual retail companies. Because there is only one manufacturer of NiTi- ionGuard, i.e., GAC, there may be better quality control regulation for this wire. This result may be expressed clinically as less variation in frictional forces produced when sliding a tooth along an arch wire. Dental tipping during sliding mechanics also impacts on the creation of frictional force. We attempted to examine the degree of tipping in our study design. However, the results were not useful for critical (statistical) analysis. To measure the degree of tipping, a fixed length of color coded wire was placed in the vertical slot of each bracket and photographed before and after each run of the study. However, comparisons, before versus after, could not be made because measurements could not be reliably reproduced. Future studies should investigate this factor by photographing each individual tray rather than together, as we had attempted. By doing so, reliable data on tipping should be able to be generated.

The pilot studies were performed to establish a reproducible/reliable design method and to derive an estimate of the variability within a wire group. In the research design, care was taken to improve the reliability of measurements made with this testing apparatus. The prosthetic teeth were weighed and measured several different ways to assure they were identical. The arch wires were the same dimensions and tested in the same environment under the identical conditions. One batch of wax was used for the entire experiment and changed between each run. The same posi-

tioning jig was used for each of the five trays for each run, therefore the initial position of each of each tooth was consistent. The water bath was monitored for each run of the experiment to avoid temperature fluctuations. Each time the coil spring was used, it was measured to assure it was extended the same amount for each run and the same amount of force was delivered. Finally, the wires were rotated from tray to tray, the teeth were randomly selected and the experimenter was blind to which wire was used. The wires were numbered from one to five by an unrelated third party who did not reveal which wire corresponded to which number until the experiment was com- pleted. The investigator could determine the dif- ference between stainless steel and nickel-tita- nium, but was blinded as to which wire was treated or untreated, thus observer bias was eliminated.

The ion-implantation process may have many applications in the orthodontic field. As shown, the implantation process tends tO decrease the amount of frictional forces produced during slid- ing mechanics. One may consider coating the inside of those brackets which correspond to the teeth one wants to slide along an arch wire. 3 For example, the anterior brackets may have the ion-implantation process, whereas the posterior brackets do not. This would help keep the poste- rior teeth as anchors while facilitating movement of the anterior teeth along the arch wire. Another consideration may be t he implantation of other beneficial materials into arch wires or brackets. For example, the implantation of fluoride into orthodontic wires.

Clearly, the ion-implantation process does have a n effect on the amount of tooth movement seen in an in vitro model. Thus it may be inferred that the ion implantation of nitrogen onto the surface of orthodontic wires does decrease the frictional forces produced during tooth movement.

CONCLUSIONS

1. Stainless steel produces the least frictional- force during in vitro tooth movement, followed by treated nickel-titanium, treated beta-titanium, un- treated nickel-titanium, and finally, untreated beta- titanium.

2. There were statistically significant differences in the amount of movement seen with the ion-im- planted wires when compared with their untreated counterparts. Thus it can be inferred the ion- implantation process produces less frictional forces during tooth movement.

3. Stainless steel wire has the greatest variability in

68 Ryan et al. American Journal of Orthodontics and Dentofacial Orthopedics July 1997

movement when compared with treated and un- treated nickel and beta titanium. This range may be due to the variations in the manufacturing process.

REFERENCES

1. Frank CA, Nikolai RJ. A comparative study of frictional resistances between orthodontic bracket and arch wire. Am J Orthod 1980;78:593-609.

2. Garner JL, Allai WW, Moore BK. A comparison of frictional forces during simulated canine retraction of a continuous edgewise arch wire. Am J Orthod Dentofac Orthop 1986;90:190-203.

3. Kusy RP, Tobin EJ, Whitley SP. Frictional coefficients of ion-implanted alumina against ion-implanted beta-titanium in the low load, low velocity, single press regime. Dent Mater 1992;8:167-72.

4. Smith R, Burstone C. Mechanics of tooth movement. Am J Orthod Dentofac Orthop 1984;85:294-307.

5. Tanne K, Matsubara S, Shibaguchi T. Wire friction from ceramic brackets during simulated canine retraction. Angle Orthod 1991;61:285-90.

6. Ryssel H, Glawischnig H. Ion implantation: equipment and techniques. New York: Springer-Verlag, 1983:332-55.

7. Glaeser WA. An engineer's guide to friction. Columbus (OH): Defense Metal Information Center, Battelle Memorial Institute, 1970:1-11.

8. Kapila S, Angolkar PV, Duncanson MG, Nanda RS. Evaluation of friction between edgewise stainless steel brackets and orthodontic wires of four alloys. Am J Orthod Dentofac Orthop 1990;98:117-26.

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