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Journal of Orthodontics, Vol. 35, No. 4, 249-254, December 2008 doi:10.1179/14653120722752
© 2008 British Orthodontic Society
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Scientific Section

The effects of a static magnetic field on orthodontic tooth movement

Minako Sakata, Yoshitake Yamamoto, Nobuyoshi Imamura, Shunsuke Nakata and Akihiko Nakasima

Facully of Dental Sciences, Kyushu University, Japan

Address for correspondence: M. Sakata Email: min0105{at}dent.kyushu-u.ac.jp


   Introduction
Top
 Introduction
Materials and methods
 Results
 Discussion
 Conclusion
 References
 
Orthodontic tooth movement, which requires formation and resorption of alveolar bone may be influenced by a magnetic field. The promotional effects of pulsed electromagnetic fields (PEMF) on bone metabolism have been well-demonstrated, i.e. enhancement of osteoblastic proliferation1Go and differentiation,2Go effects on alkaline phosphatase production,3Go and net flux and uptake of calcium.4Go PEMFs have been successfully used in the treatment of bone fractures, bone grafts, osteotomies, osteonecrosis, and osteoporosis.5Go,6Go As for the effects of PEMF on tooth movement, it has been suggested that PEMF could increase bone deposition and the rate of tooth movement.7Go,8Go Although PEMFs yield both a magnetic field and electric current, no definite conclusion can be drawn as to which factor is more responsible for bone formation.

Rare-earth magnets, which generate a static magnetic field (SMF), have also been used for many years as a ‘force source’ in orthodontic treatment such as space closure,9Go molar distalization,10Go–12Go intrusion,13Go and traction of impacted teeth,14Go–16Go and palatal expansion.17Go Effective tooth movement may be induced by an attractive magnetic force which increases as the distance between the magnets decreases.18Go Some studies have suggested that SMF may:

However, controversy remains as to whether or not the biological effects of SMF contribute to tooth movement. To date, there have been no studies examining whether whole-body exposure to SMF influences tooth movement instead of using fine magnets incorporated into orthodontic appliances.

Accordingly, the purpose of this study was to determine whether the application of SMF could influence the pattern of tooth movement and changes to periodontal tissue during experimental orthodontic tooth movement in rats.


   Materials and methods
Top
 Introduction
 Materials and methods
Results
 Discussion
 Conclusion
 References
 
Animals
Thirty-four, six-week-old male Wistar rats were used in this study. All animals were purchased from Japan SLC, Inc., for reasons of availability, cost and genetic homogeneity. This also provided an advantage for comparison with other work. Rats were numbered 1 to 34; animals numbered with an odd number served as the controls. The rats were randomly divided into two groups: control and experimental (17 animals in each group). Fifteen animals in each group served for measurements of tooth displacement, and two animals in each group served for the histological examination. A sample size calculation for the number of animals necessary to achieve 80% power and a significance level of P<0.05 was carried out using a two-sided continuity corrected chi-squared test.21Go A sample size of 15 animals per group was calculated as sufficient to detect a difference in tooth displacement of 0.10 mm between two groups.22Go–24Go The rats were fed a diet of pellets with water ad libitum. The animals in the experimental group were exposed to a static magnetic field during the experimental period. To assess whole body effects, body weight was recorded prior to each procedure. Ethical clearance was granted by the animal experiments’ ethics committee of Kyushu University. All rats remained healthy throughout the experimental period, and no rats died or lost body weight.

Static magnetic field – exposure system
In the present study, SMF was applied using the experimental magnetic unit (X-5046, NEOMAX Co., Osaka, Japan). SMFs were produced by built-in Neodymium-iron-boron magnets (NEOMAX47, NEOMAX Co., Osaka, Japan) at the top and bottom of the unit. The magnetic flux density was monitored with a Gauss/Tesla meter (SERIES 6010, F.W.BELL, Orlando, FL, USA). The distribution of magnetic flux density inside the unit is shown in Figure 1Go. The flux density in the central area was 460 mT. The rats in the experimental group were kept in an acrylic cage placed in the unit during the experimental period.


Figure 1
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  		Figure 1 Distribution of magnetic flux density inside the unit. The minimum density was 200 mT, while the maximum density was 460 mT at the central area

 
Induction of tooth movement
An orthodontic appliance was inserted on the right maxillary first molar and a mesially directed force of 40 g was applied (Figure 2Go). The force level was verified using a dynamometer. To prevent slippage of the appliance, notches were made on the lateral sides of the incisors and molar. A stretched stainless steel closed-coil spring (0.006x0.020 inch, 3MUnitek, USA) was suspended between the maxillary right first molar and two maxillary central incisors. Insertion of the orthodontic appliance was performed under general anaesthesia via intraperitoneal injection of sodium pentobarbital (40 mg/kg; Abbot Laboratories, IL, USA) following the administration of inhalant anaesthesia with diethyl ether.


Figure 2
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  		Figure 2 Orthodontic appliance used in the experiment

 
Measurement of tooth displacement
On days 0, 1, 3, 5, 7 and 14 of tooth movement, silicone impressions (STANDOUT, Kerr, MI, USA) of the right maxillary molars were taken under anaesthesia with inhaled diethyl ether, and models were cast with dental stone (New Plastone, GC Co. Ltd, Tokyo, Japan). The relative separation between the first and second molar was measured using digital calipers (DIGIMATIC, Mitutoyo, Kanagawa, Japan) with sharpened tips (accurate to 0.01 mm) in order to ascertain the distance between the distobuccal cusps of these molars. Measurements of tooth displacement at each time were repeated 5 times for each animal and the mean value was taken as the tooth displacement (mm). The rates of tooth displacement per day (mm/day) were worked out from the distance of tooth displacement. The same investigator carried out all the measurements blinded. The margins were calculated for intra-investigator reproducibility. They were compared by analysis of variance (ANOVA) followed by Scheffe’s post-hoc test. No significant differences were found among the five measurements (P>0.05). A CONSORT diagram showing the flow of animals through each stage of the study is shown in Figure 3Go.


Figure 3
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  		Figure 3 A CONSORT diagram showing the progress of the study

 
Statistical analysis
The group means and standard deviations were calculated for body weight, the amount of tooth displacement and the rate of tooth displacement. The statistical process was performed using Stat View version 5.0 (SAS Institute Inc., Cary, NC, USA). These data were compared between groups by analysis of variance (ANOVA) followed by Scheffe’s post-hoc test. A level of P<0.05 was considered to indicate a significant difference.

Histological examination
Two animals in each group were killed on days 7 and 14 of tooth movement for the histological examination. Under general anesthesia the animals were perfused at a constant pressure via the left ventricle with 0.1M phosphate-buffered saline (PBS), followed by perfusion fixation with 8% paraformaldehyde in PBS. The maxillae were excised and immersed in the same fixative solution as that used for perfusion at 4°C overnight, and then decalcified in 10% ethylenediamine tetraacetic acid solution at 4°C for 12 days. Samples were cut in half along the sagittal plane, and embedded with pre-cooled O. C. T. compound (FineTek, SAKURA Co. Ltd, Tokyo, Japan) using conventional methods. Five-micrometer thick serial sections of the roots of the first and second molar were sectioned (in cross-section) with the surrounding tissues with a microtome. The mesial sides of the distobuccal root at the coronal one-third of the maxillary first molars were selected for observation. They were stained with haematoxylin and eosin.


   Results
Top
 Introduction
 Materials and methods
 Results
Discussion
 Conclusion
 References
 
Weight changes of animals
The average weight of the rats increased continuously in both the experimental and the control groups. No significant difference in weight was found between the two groups (Figure 4Go). Food and water consumption appeared to be unaffected by the orthodontic appliance or exposure to SMF.


Figure 4
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  		Figure 4 Body weight (mean ± SE). Changes in body weight. The weights of the rats increased continuously in both groups with no significant difference in weight between the two groups

 
Tooth displacement
Orthodontic tooth movement was evidenced by a gradual increase in the inter-dental space between the first and second molars. No space was observed inter-dentally between the second and the third molars, thus indicating little or no mesial movement of the second molar. The cumulative tooth displacement gradually increased through 14 days in both the experimental group and the control group (Figure 5Go). At day 1 and day 3, there were no significant differences in tooth displacement between the two groups. In the following periods, the displacement in the experimental group was significantly greater than that in the control group at days 5, 7 and 14 (P<0.05).


Figure 5
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  		Figure 5 Cumulative tooth displacement (mean ± SE). The tooth movement gradually increased throughout the 14 days. At days 1 and 3, there were no significant differences in tooth displacement between the two groups. In the subsequent period, the displacement in the experimental group was significantly greater than that in the control group (*P<0.05)

 
Rate of tooth displacement
Changes in the rate of tooth movement are shown in Figure 6Go. An initial rapid movement caused by compression of the periodontal ligament was seen in both groups. The rate of tooth displacement from day 1 to day 5 in the experimental group was greater than that in the control group (P<0.05).


Figure 6
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  		Figure 6 Rate of tooth displacement (mean ± SE). The rate of displacement from day 1 to day 5 in the experimental group was greater than that in the control group (*P<0.05)

 
Changes to the periodontium
At day 7 in the control group, a limited area of hyalinization and bone resorption were evident (Figure 7aGo). At day 14 in the control group, an area of hyalinization and root resorption in the cementum were observed in some regions (Figure 7bGo). At days 7 and 14 in the experimental group, neither hyalinized tissue nor severe root resorption was seen (Figure 7c,dGo).


Figure 7
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  		Figure 7 Mesial side of the distobuccal root in the right maxillary first molar (H&E stain). (a) At day 7 in the control group, a limited area of hyalinization (Hy) and bone resorption were evident. (b) At day 14 in the control group, a hyalinized area and root resorption in the cementum were observed in some regions. (c,d) At days 7 and 14 in the experimental group, neither hyalinized tissue nor severe root resorption was seen

 

   Discussion
Top
 Introduction
 Materials and methods
 Results
 Discussion
Conclusion
 References
 
In each group, tooth displacement was on average 0.1 mm at day 1. Several authors have reported a periodontal ligament (PDL) of 0.1 mm in width in normally occluded rats.25Go,26Go Therefore tooth displacement of 0.1 mm at day 1 could perhaps be explained by the visco-elastic modification of the PDL and the strain on the alveolar bone. Tooth displacement gradually increased throughout the 14 days. Although there were no significant differences in cumulative tooth displacement between the two groups up until day 3, in the subsequent period, the displacement in the experimental group was significantly greater than that in the control group. The difference in cumulative tooth displacement from day 5 to day 14 between the two groups is considered to reflect the difference in the rate of tooth displacement from day 1 to day 5. The initial rapid tooth movement is attributed to compression of the PDL which is followed by formation of hyalinized tissue at subsequent experimental periods, indicated by cessation of tooth movement at the ‘lag’ phase. Recent reports have suggested that the application of PEMF in conjunction with orthodontic forces can increase the rate of orthodontic tooth movement of incisors in guinea pigs, accompanied by the absence of a ‘lag’ phase.7Go,8Go In the current investigation, the rate of tooth displacement decreased after the initial movement, although decrease in the rate of tooth displacement in the experimental group was smaller than that in the control group. Histological examination showed hyalinized tissue at days 7 and 14 in the control group, although no hyalinized tissue or root resorption in the cementum was observed in the experimental group. The suggestion that SMFs induce earlier formation and removal of hyalinized tissue27Go was supported in this study. This may represent either a suppression of clastic inflammatory activity or conversely, facilitated early repair of tissue. The suggestion that SMFs shorten the recruitment and initiation phase of osteoclast development8Go may explain why the modulatory effect of SMF appeared significant during the early period of tooth movement.

A weakness of the study was that the force applied to move the rat molar was heavy from the viewpoint of the histological change of periodontal tissue. It should be pointed out that an initial orthodontic force of 40 g could be converted into 160 g/cm2 using the root area of the rat molar.28Go This is approximate to the force magnitude recommended by Jarabak29Go and has been applied in many studies dealing with experimental tooth movements in rats.30Go,31Go On the other hand, some studies have demonstrated that light continuous forces produce effective tooth movement with minimum tissue damage such as hyalinized or necrotic change in periodontal tissue and root resorption.32Go,33Go Kohno et al. have reported that tooth movement under light forces (less than 10 g) was constant and did not show a ‘lag’ phase seen under heavy forces.34Go Further studies on the effects of SMF using light orthodontic force are required.

There are some strengths associated with this study. For example, we adopted whole-body exposure to SMF in order to exclude the influence of attraction or repulsion or corrosion of the magnet. Many researchers have investigated the effects of SMF on orthodontic tooth movement by using fine magnets incorporated into orthodontic appliances. Secondly, the magnetic unit we used in the current study produced SMF of at least 200 mT at the area farthest from the center of the unit, which was considered sufficient to induce tissue reaction. In many studies of magnetic fields, the flux density was up to 100 mT. In the study using rat calvaria cell culture,35Go SMF of 160 mT stimulated bone formation by promoting osteoblastic differentiation and/or activation. Tengku et al. reported that incorporation of SMF of 10–17 mT into an orthodontic appliance did not enhance tooth movement, despite the increase in tartrate-resistant acid phosphatase activity.27Go The pattern of tooth movement and the changes to periodontal tissue under such strong SMF have not been discussed.

The results of this study indicate that incorporation of SMF into an orthodontic appliance may have the potential to produce effective tooth movement and shorten a treatment time.

However, further investigations into the long-term effects of SMF on tissue reaction are also necessary when the clinical application of SMF is considered.


   Conclusion
Top
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusion
References
 
In conclusion, the present findings suggest that the application of SMF can accelerate orthodontic tooth movement in rats. SMF was able to increase the rate of tooth movement during the early period of its application.


   References
Top
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusion
 References
 
1 De Mattei M, Caruso A, Traina GC, Pezzetti F, Baroni T, Sollazzo V. Correlation between pulsed electromagnetic fields exposure time and cell proliferation increase in human osteosarcoma cell lines and human normal osteoblast cells in vitro. Bioelectromagnetics 1999; 20: 177–82.[CrossRef][Medline]

2 Landry PS, Sadasivan KK, Marino AA, Albright JA. Electromagnetic fields can affect osteogenesis by increasing the rate of differentiation. Clin Orthop 1997; 338: 262–70.[CrossRef][Medline]

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19 Kotani H, Kawaguchi H, Shimoaka T, Iwasaka M, Ueno S, Ozawa H, Nakamura K, Hoshi K. Strong static magnetic field stimulates bone formation to a definite orientation in vitro and in vivo. J Bone Miner Res 2002; 17: 1814–21.[CrossRef][Medline]

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22 Liu L, Igarashi K, Haruyama N, Saeki S, Shinoda H, Mitani H. Effects of local administration of clodronate on orthodontic tooth movement and root resorption in rats. Eur J Orthod 2004; 26: 469–73.[Abstract/Free Full Text]

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26 Lasfargues JJ, Saffar JL. Inhibition of prostanoid synthesis depresses alveolar bone resorption but enhances root resorption in the rat. Anat Rec 1993; 237: 458–65.[CrossRef][Medline]

27 Tengku BS, Joseph BK, Harbrow D, Taverne AAR, Symons AL. Effect of a static magnetic field on orthodontic tooth movement in the rat. Eur J Orthod 2000; 22: 475–87.[Abstract/Free Full Text]

28 Noda K, Yoshii T, Nakamura Y, Kuwahara Y. The assessment of optimal orthodontic force in various tooth movements—Comparisons of tooth movement, root resorption, and degenerating tissue in tipping movement. J Jpn Orthod Soc 2000; 59: 329–41.

29 Jarabak JR. Development of a treatment plan in the light of one’s concept of treatment objectives. Am J Orthod Dentofac Orthop 1960; 46: 48

30 Konoo T, Kim YJ, King GJ. Intermittent force in orthodontic tooth movement. J Dent Res 2001; 80: 457–60.[Abstract/Free Full Text]

31 King GJ, Archer L, Zhou D. Later orthodontic appliance reactivation stimulates immediate appearance of osteoclasts and linear tooth movement. Am J Orthod Dentofac Orthop 1998; 114: 692–7.[CrossRef][Medline]

32 Kawarizadeh A, Bourauel C, Zhang D, Gotz W. Correlation of stress and strain profiles and the distribution of osteoclastic cells induced by orthodontic loading in rat. Eur J Oral Sci 2004; 112: 140–7.[CrossRef][Medline]

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34 Kohno T, Matsumoto Y, Kanno Z, Warita H, Soma K. Experimental tooth movement under light orthodontic forces: rates of tooth movement and changes of the periodontium. J Orthod 2002; 29: 129–35.[Abstract/Free Full Text]

35 Yamamoto Y, Ohsaki Y, Goto T, Nakasima A, Iijima T. Effects of Static magnetic fields on bone formation in rat osteoblast cultures. J Dent Res 2003; 82: 962–6.[Abstract/Free Full Text]





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