Orthodontic Tooth Movement in Rats Using Ni Free Ti Based Shape Memory Alloy Wire

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Orthodontic Tooth Movement in Rats Using Ni-Free

Ti-Based Shape Memory Alloy Wire

Hiroyasu Kanetaka

1

, Yoshinaka Shimizu

2

, Hideki Hosoda

3

, Ryo Tomizuka

1

, Akihiro Suzuki

1

,

Shachiko Urayama

*

, Tomonari Inamura

3

, Shuichi Miyazaki

4

and Teruko Takano-Yamamoto

1 1Division of Orthodontics and Dentofacial Orthopedics, Tohoku University Graduate School of Dentistry, Sendai 980-8575, Japan 2Division of Oral and Craniofacial Anatomy, Tohoku University Graduate School of Dentistry, Sendai 980-8575, Japan

3Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan 4Institute of Materials Science, University of Tsukuba, Tsukuba 305-8573, Japan

The purpose of this study was to evaluate the performance and the usefulness of a newly developed Ni-free Ti-based shape memory alloy (SMA) wire in orthodontic tooth movement by comparing with a nickel-titanium (Ni-Ti) alloy wire. A Titanium-niobium-aluminum (Ti-24Nb-3Al) SMA wire, which was considered to be biocompatible because it contained no nickel, was newly developed and mechanical property of this new alloy was improved by severe cold rolling reduction. Twenty-one male Wistar-strain rats (age; 6 weeks) were used in the animal experiment. A Ti-Nb-Al alloy wire and an orthodontic superelastic wire (Ni-Ti alloy wire) were set in the oral cavities of rats, and orthodontic palatal movement of maxillary first molars was performed with an initial load of 15 gf. The amount of tooth movement was measured and periodontal structures were histologically examined. The Ti-Nb-Al alloy wire was effective for palatal tooth movement without any adverse reaction in rats. There was no significant difference in the amount of tooth movement between the Ti-Nb-Al group and the Ni-Ti group. Histological observation of the periodontal tissues revealed no differences between the two groups. These results indicate that Ti-Nb-Al alloy wire has excellent mechanical properties suitable for orthodontic tooth movement, suggesting that Ti-Nb-Al wire may be used as a practical nickel-free shape memory and superelastic alloy wire for orthodontic treatment as a substitute for Ni-Ti alloy wire.

[doi:10.2320/matertrans.48.367]

(Received October 6, 2006; Accepted December 5, 2006; Published February 25, 2007)

Keywords: nickel-free shape memory alloy, biocompatibility, cold rolling, tooth movement, orthodontics, rat

1. Introduction

The unique properties of shape memory and superelasticity have made shape memory alloys (SMAs) very useful biomaterials.1–6) Superelasticity is a phenomenon in which the stress value remains fairly constant up to a certain point of wire deformation. At the same time, when the deformed wire rebounds, the stress value remains fairly constant. The property is very advantageous for orthodontic tooth move-ment, and the SMA wires have thus been widely used in orthodontic treatment.5,6)

At present the nickel-titanium (Ni-Ti) alloy is the only practical SMA in medical use. However, biocompatibility of Ni-Ti alloy has been questioned because it contains a large amount of nickel.7,8) Ni-Ti alloy may thus induce nickel hypersensitivity,9–11)and it is recomended to avoid use of this wire in nickel sensitive patients. Moreover, recent numerous animal studies evaluating the carcinogenicity of nickel have supported the view that nickel is a potent carcinogen.12–14)

For these reasons, practitioners in various medical fields have been looking forward to the development of Ni-free SMA with high biocompatibility. Ni-free Ti-based shape memory alloys composed of non-toxic elements have been systematically examined15–17) and it was demonstrated that titanium-niobium-aluminium (Ti-Nb-Al) alloy exhibited the best mechanical performance among these Ni-free shape memory and superelastic alloys.16,17)

This new alloy (Ti-Nb-Al) must be examined to exhibit stable mechanical properties in oral cavity under the complicated biochemical and physical environment. The

aim of this study was to evaluate the performance of a newly developed Ni-free Ti-based SMA wire as an orthodontic wire by comparing it with a Ni-Ti alloy wire in animal experiment.

2. Materials and Methods

2.1 Ti-Nb-Al alloy

Nickel-free Ti-based SMAs composed of non-toxic ele-ments have been systematically examined and 73 mol% Ti-24 mol% Nb-3 mol% Al alloy showed the best mechanical performance among them. The alloy was well developed with deformation and recrystallized textures and mechanical property of the alloy was improved by severe cold rolling leading to over 99% in thickness reduction.

Figure 1 shows SEM-EBSP images of the solution-treated Ti-Nb-Al alloy obtained after cold rolling. It is obvious that a

f112gh110i recrystallized texture was strongly developed

in the alloy with 99% thickness reduction, and that the grain size became smaller with increasing reduction rate.18) The

f112gh110itexture is suitable for large tensile superelastic

strain. And also, the reduction of grain size shown in 99% cold rolled material would improve mechanical properties, resulting in good superelastisity.19)

Figure 2 shows the superelastic behaviour of Ti-Nb-Al alloy at room temperature with different cold rolling reductions.16,20,21) The tensile direction of both specimens was parallel to the rolling direction. It is seen that superelastic shape recovery, which does not include elastic recovery, of 95% cold rolled alloy is 1% whilst that of 99% cold rolled alloy is 1.5%. The improvement of superelasticity by severe cold rolling was explained by the strong development of

f112gh110i recrystallized texture due to 99% cold rolling *Graduate Student, Tohoku University Graduate School of Dentistry

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as shown in Fig. 1.21)Above the reasons, Ti-Nb-Al alloy was chosen for the evaluation of orthodontic tooth movement. About 20 g of Ti-Nb-Al alloy ingot was made by Ar-arc melting in Ar-1%H2 atmosphere with a non-consumable W electrode using high purity Ti, Nb and Al (99.99% grade). No chemical analysis was made because of small weight change during melting. The ingot was homogenized at 1273 K for 2 h in a vacuum environment. Then, a wire with 0.3 mm in diameter was made of the ingot by cold extrusion without intermediate annealing at Nippon Cross Rolling Co. Ltd. The wires were cut into 6 cm in length, and annealed at 850C for 20 min in vacuum followed by quenching into water.

Figure 3 shows the cyclic loading-unloading stress-strain curve of a Ti-Nb-Al alloy wire with a diameter of 0.3 mm tested at room temperature, with a constant strain increment

of 1% per cycle. It is emphasized that, as in cold-rolled sheet materials, superelasticity appears in the wire material. The superelastic strain (which does not include elastic recovery) was 1% and the residual strain was 0.5% after 3% of total deformation. However, the flow stress of the wire reached 500 MPa at 3% strain, which is higher than 250 MPa seen in the rolled materials in Fig. 2. By comparing Fig. 3 and Fig. 2(a), it is also seen that the stress-strain loops are narrower in the wire material than in the sheet. Such differences in deformation behaviour must be due to variations in microstructure and texture developed in the wire fabrication process and the heat treatment. The superelastic wire would be advantageous as an orthodontic wire by providing large orthodontic force due to large superelastic stress.

ND=112β RD=101β TD=111β

IQmap ND RD TD

Rolling rate 95%

Rolling rate 99%

ND

RD TD

(rolling direction) RD

TD(transverse direction) (normal direction)

ND

245µm 245µm 245µm

245µm

245µm

245µm

245µm

245µm

mean 150φ µm

mean φ 100µm

Fig. 1 SEM-EBSP images of solution-treated Ti-24 mol%Nb-3 mol%Al alloy obtained after cold rolling resulting in 95% and 99% thickness reductions. IQ map, ND, TD, RD stand for image quality map, normal direction, rolling direction and transverse direction, respectively.

0 50 100 150 200 250 300

0 1 2 3

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/ MPa

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0 50 100 150 200 250 300

0 1 2 3

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/ MPa

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2.2 Animal experiment

Ti-Nb-Al round wires (73 mol%Ti, 24 mol%Nb and 3 mol%Al) of 0.3 mm (0.012-inch) diameter were used for the Ti-Nb-Al alloy standardized spring and Ni-Ti round wires of 0.3 mm (0.012-inch) diameter (Nitinol, 3M Unitek, Monrovia, CA, USA) were used for Ni-Ti alloy standardized spring [Fig. 4(a)].

Twenty-one male Wistar-strain rats (age; 6 weeks) were used in this study. Rats were divided into a control group (no springs were fitted in 7 rats) and 2 experimental groups: 7 rats were fitted with Ti-Nb-Al alloy springs (Ti-Nb-Al group); and 7 rats were fitted with Ni-Ti alloy springs (Ni-Ti group). Ti-Nb-Al alloy and Ni-Ti alloy wires were bent into standardized shape [Fig. 4(a)] and set in the oral cavities of the rats, and orthodontic palatal movement of maxillary first molars was performed with an initial load of 15 gf. [Fig. 4(b)].

2.3 Medical observation and dental impression

All rats were housed in cages (3 rats/cage) in an

air-conditioned and lighted environment, according to the guidelines for Animal Research of Tohoku University. Prior to appliance placement, rats were acclimatized for a week and were fed a diet of ground pellets with water. Body weight was recorded and oral and systemic conditions were checked during the acclimatization and experimental periods.

After treatment, maxillary impressions of 6 rats in each group were taken on days 1, 3, 7, 10, 14, 17, and 21 using silicone rubber impression paste (Flexicon Regular Type, GC, Tokyo, Japan) under anesthesia with inhaled diethyl ether (diethyl ether, Wako Pure Chemical Industries, Ltd., Osaka, Japan). Dental impressions were immediately sunk into the water and precise plaster (New Fujirock, GC, Tokyo, Japan) were poured into them after about one hour. Water powder ratio (W/P) of the precise plaster was 0.20, and the poured plaster was kept at room temperature away from direct sunlight for about one hour and then removed from the casts.

2.4 Measurements of tooth movement

The precise plaster models were scanned using a flat bed scanner (CanoScan 5200F, Canon, Tokyo, Japan), and were magnified to 1000% using digital imaging software (Photo-shop, Adobe systems, San Jose, CA). Digitized images were printed with a laser printer (microline5200, Oki Electric Industry Co., Ltd., Tokyo, Japan) and were traced. The second and third molars in the maxilla were used as references for superimposition of the tracings. Displacement of the mesiobuccal cusp of maxillary first molar was measured with micrometer calipers based on superimposi-tions [Fig. 5].

The error of the measurements was found to be 0.0080 mm when 20 randomly selected samples were measured in a blind test by a single investigator. Errors were calculated as Error¼pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffid2=2n (d: difference between two measure-ments,n: number of samples).

2.5 Histological procedure

On days 7 and 21, rats were sacrificed with an overdose of

0 100 200 300 400 500 600

0 1 2 3

Stress,

σ

/ MPa

Strain (%)

Fig. 3 Cyclic loading-unloading stress-strain curve of Ti-24 mol%Nb-3 mol%Al alloy wire with a diameter of 0.mol%Nb-3 mm tested at room temperature with a constant strain increment of 1% per cycle.

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(b)

6.0mm

5.0mm

M

1

M

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M

3

M

1

M

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pentobarbital sodium salt and perfused with 4% paraformal-dehyde through the ascending aorta for 15 minutes. The upper jaws containing the first molars were excised as samples. The samples were decalcified in 10% EDTA solution for 45 days at room temperature. They were then dehydrated in a graded series of ethanol and embedded in paraffin. Periodontal tissues of the distopalatal root of the upper first molar were examined with a light microscope in serial 5-mmcross-sections of the region 1.0 mm beneath the root furcation. Sections were stained with hematoxylin and eosin.

2.6 Statistical analysis

Data were subjected to two-way analysis of variance (ANOVA) followed by Tukey’s multiple comparison test.

3. Results

3.1 Systemic and oral condition of rats

The animal weight in each group showed a gradual increase within normal limits. There were no significant differences between the control and the two experimental groups in body weight [Fig. 6]. Systemic examination revealed no pathological findings except for slight gingival inflammation caused by the physical presence of the springs in the experimental groups.

3.2 Amount of tooth movement

Ti-Nb-Al and Ni-Ti alloy springs resulted in palatal tooth movement of the rat first molars. The time-courses of tooth movement in Ti-Nb-Al and Ni-Ti groups are shown in Fig. 7. There was no significant difference in the amount of movement between the two groups.

3.3 Histological findings

Photographs of the pressure side of the distopalatal root of the maxillary first molars of rats in control group, Ti-Nb-Al group, and Ni-Ti group on days 7 and 21 are shown in Fig. 8. In the control group, the periodontal tissue showed fibrous tissue and vessels. After days 7 and 21, the root and alveolar bone facing the periodontal space showed a smooth surface. A few osteoclasts were present on the surface of bone facing the periodontal space. In the experimental groups, the periodontal space narrowed and hyalinized tissue was seen on day 7. The surface of alveolar bone showed some resorption lacunae with or without osteoclasts. On day 21, the periodontal space expanded. The alveolar bone surface appeared rough with numerous round osteoblastic cells. Histological findings revealed no distinct difference between the two experimental groups.

4. Discussion

4.1 Biocompatibility of Ti-Nb-Al alloy

Ti-24Nb-3Al alloy is expected to be composed of a single

1

M

2

M

3

M

d

Fig. 5 Measurement of the tooth movement. Displacement (d) of the mesiobuccal cusp of maxillary first molar was measured with micrometer calipers based on superimpositions.

Fig. 6 Changes in body weight with time in the control group, Ti-Nb-Al group, and Ni-Ti group.

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-Ti phase within the usual temperature range of the oral cavity. A single phase alloy usually has resistance against galvanic corrosion compared with a multi-phase alloy, because it does not cause any difference in potential.22,23) Even under stress, the stress induced martensite still contains the same chemical composition to the parent phase. In this case, galvanic corrosion between parent and martensite phases must be small due to the chemical similarity.

Moreover, each element composing the Ti-Nb-Al alloy shows high biocompatibility. Titanium, which makes up 73 mol% of this alloy, exhibits good mechanical properties, high corrosion resistance, and excellent biocompatibility and is used for many dental and medical applications and instruments.24) Niobium has been reported to have good cytocompatibility in vitro and good biocompatibility in animals.25–27) Regarding aluminum, some previous reports mentioned that aluminum might play a role in the patho-genesis of Alzheimer’s disease,28,29)but recent investigations have denied the association.30–33)It is noted that addition of Al to the Ti-Nb alloy is effective to prevent!-embrittlement and to reduceMs, resulting in improvement of reliability and superelastic strain.

4.2 Application to tooth movement

In orthodontic treatment, observation of the tooth move-ment pattern and histological changes in periodontal tissues are important for efficient tooth movement without discom-fort to the patient or ensuing tissue damage.34,35)

The time course of tooth movement showed a stepwise displacement pattern with the lag phase in both groups. The periodontal tissue reveals specific histological changes according to magnitude or type of application force.36–38)

Many investigations have described the formation of a hyalinized area in the lag phase in initial stages.39,40)Bone resorption on the compressive side starts with removal of the hyalinized tissue41)and recruitment of osteoclasts.34,35)In this animal study, histological findings of the newly developed alloy wire application showed similar changes, and proved to be equal to existing Ni-Ti alloy wire application in histological effects.

These results might be explained by the excellent mechanical properties of Ti-Nb-Al alloy equivalent to Ni-Ti alloy. It was previously reported that this kind of Ni-free shape memory titanium alloys exhibit superior cold work-ability. Severe cold rolling resulting in over 99% thickness reduction can be done without intermediate annealing. Moreover, superelastic behavior can be improved by appro-priate thermo-mechanical treatments.

It is suggested that Ti-Nb-Al alloy wire may be used as a practical nickel-free shape memory and superelastic alloy wire for orthodontic treatment as a substitute for Ni-Ti alloy wire.

4.3 Clinical advantages

The environments in oral cavity or human body are complicated and severe. Moreover, metal devices are expected to exert force magnitude and type suitable for the reaction of cell or tissues in medical use.

In present orthodontic treatment, wires with shape memory and superelastic alloy are indispensable materials and have been used in the oral cavity for about two years. It was reported that the surface of Ni-Ti alloy was resistant because of a titanium oxide coating.42–44) However, a recent study revealed that fluoride ion accelerates corrosion of Ni-Ti

7 days

21 days

Control

NiTi

TiNbAl

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alloy.45–47)For these reasons, orthodontic shape memory and superelastic alloy wires should be composed of biocompat-ible elements.

On the other hand, mechanical property of the wire is very important for tooth movement. Light continuous force is desirable for efficient tooth movement. It was confirmed that newly developed Ti-Nb-Al alloy had excellent mechanical properties equivalent to Ni-Ti alloy and could generate optimal force for orthodontic tooth movement.

From these view points, Ti-24Nb-3Al alloy wire has clinical advantages for orthodontic treatment. Moreover, these Ni-free SMA could serve as a useful biomaterial with wide applications in the medical field.

5. Conclusions

It was considered that newly developed Ti-Nb-Al alloy without Ni had high biocompatibility, not inducing hyper-sensitivity or carcinogenicity, and exhibited excellent me-chanical performance in oral cavity with complex environ-ment. The present results suggest that the Ti-Nb-Al alloy can be used as a Ni-free shape memory and superelastic alloy wire for orthodontic treatment, and serve as a substitute for Ni-Ti alloy. We have been investigating the long-term biocompatibility and mechanical stability of this material in vivo for the development of low invasive treatment.

Acknowledgements

We are grateful to Mr. Toshihiro Onodera (Tohoku University Graduate School of Dentistry) for his technical assistance. Grand-in-Aid for Fundamental Scientific Re-search Kiban B (No. 173604, 205–2007), Wakate B (No. 18760522, 2006–2007) and Scientific Research of Priority Areas 438 Next-Generation Actuators Leading Breakthroughs (No. 17040013, 2005–2006) are greatly acknowledged for their financial support.

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Figure

Fig. 1SEM-EBSP images of solution-treated Ti-24 mol%Nb-3 mol%Al alloy obtained after cold rolling resulting in 95% and 99%thickness reductions
Fig. 1SEM-EBSP images of solution-treated Ti-24 mol%Nb-3 mol%Al alloy obtained after cold rolling resulting in 95% and 99%thickness reductions p.2
Fig. 2Cyclic loading-unloading stress-strain curves of (a) 95% cold rolled and (b) 99% cold rolled Ti-24 mol%Nb-3 mol%Al alloy testedat room temperature with a constant strain increment of 1% per cycle.
Fig. 2Cyclic loading-unloading stress-strain curves of (a) 95% cold rolled and (b) 99% cold rolled Ti-24 mol%Nb-3 mol%Al alloy testedat room temperature with a constant strain increment of 1% per cycle. p.2
Fig. 3Cyclic loading-unloading stress-strain curve of Ti-24 mol%Nb-3 mol%Al alloy wire with a diameter of 0.3 mm tested at room temperaturewith a constant strain increment of 1% per cycle.
Fig. 3Cyclic loading-unloading stress-strain curve of Ti-24 mol%Nb-3 mol%Al alloy wire with a diameter of 0.3 mm tested at room temperaturewith a constant strain increment of 1% per cycle. p.3
Fig. 4Schematic view of a standardized spring (a) and applied spring in rat maxilla (b).
Fig. 4Schematic view of a standardized spring (a) and applied spring in rat maxilla (b). p.3
Fig. 5Measurement of the tooth movement. Displacement (d) of themesiobuccal cusp of maxillary first molar was measured with micrometercalipers based on superimpositions.
Fig. 5Measurement of the tooth movement. Displacement (d) of themesiobuccal cusp of maxillary first molar was measured with micrometercalipers based on superimpositions. p.4
Fig. 6Changes in body weight with time in the control group, Ti-Nb-Algroup, and Ni-Ti group.
Fig. 6Changes in body weight with time in the control group, Ti-Nb-Algroup, and Ni-Ti group. p.4
Fig. 7Time course of tooth movement in the Ti-Nb-Al group and Ni-Tigroup.
Fig. 7Time course of tooth movement in the Ti-Nb-Al group and Ni-Tigroup. p.4
Fig. 8Photographs of the pressure side of the distopalatal root of the maxillary first molars of rats in the control group, Ti-Nb-Al group,and Ni-Ti group on days 7 and 21.
Fig. 8Photographs of the pressure side of the distopalatal root of the maxillary first molars of rats in the control group, Ti-Nb-Al group,and Ni-Ti group on days 7 and 21. p.5