6. Evaluation of Leakage Flux out of Foreign Dental Magnetic Attachments

M. Nishida¹, Y. Tegawa², M. Akutagawa³, and Y. Kinouchi³

¹ Graduate School of Advanced Technology and Science, The University of Tokushima,

² Institute of Health Sciences, Graduate School, The University of Tokushima,

³ Institute of Technology and Science, Graduate School, The University of Tokushima

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Introduction

The tissues in the mouth may be exposed to the magnetic flux leaking out of the magnetic attachments for a long time. Therefore, it is important to discuss biological effects of them.

Objective

Objective of this study is to evaluate the effects to the human body of the leakage flux out of foreign dental magnetic attachments.

Materials and Methods

Table.1 shows analyzed seven types of foreign dental magnetic attachments. Dyna Magnet, Titanmagnetics, Microplant and Multipurpose Magnet system are open magnetic circuit type of magnetic attachments. On the other hand, Maxi Magnet, Magnedisc 800 and Smart Kit are closed magnetic circuit type of magnetic attachments. Many of them used for the implant overdentures.

Fig.1 shows the analysis models of foreign magnetic attachments. These models were made by reference to internal structures that was reported in the 17^th Conference on Magnetic Application in Dentistry¹⁾.

Titanmagnetics, Microplant and Multipurpose Magnet system of open type magnetic attachments have the magnet in the Keeper, and attracted each other by both magnetic forces. Magnedisc 800 has a magnetic circuit of a cup type. Maxi Magnet and Smart Kit are sandwich type of magnetic attachments. However, the fig.1 (5) and (7) show their magnetic circuits difference in several ways from the Japanese sandwich type.

The construction materials used a rare-earth magnet and a soft magnetic stainless steel with high corrosion resistance for the keeper and yoke. Properties of each material and software used in the analysis are given below.

• Magnet : Rare-earth magnet (Nd-Fe-B), Magnetization J = 1.3 T
• Soft Magnetic material : Stainless steel, Bs =1.6 T, μr=5000
• Software : Modeling (FEMAP, NST, Co., LTD., Japan),
       Finite Element Method (μ-MF, μ-TEC Co., LTD., Japan)

The leakage magnetic flux is evaluated by the comparison of the FEM analysis and the guideline that is proposed by ICNIRP and WHO. According to the guideline, static magnetic field strength of general public exposure is up to 40mT²⁾.

Results and Discussions

1. Magnetic attraction

The magnetic attachment may have a small gap between a magnetic assembly and a keeper in the long term use. Fig.2 shows the Magnetic attraction with the gap varied from 0mm to 0.2mm.

In case of the 0mm gap, Magnedisc 800 has the largest magnetic attraction. However, it decreases rapidly when the gap length becomes large. Multipurpose Magnet System shows the attractive force more than 830[gf]. The other open magnetic circuit types are small magnetic attraction at the 0mm gap. However, Titanmagnetics, Microplant and Multipurpose Magnet system retain more than 90% attractive force of the maximum value when the gap length is 0.1mm. The magnetic attractions of Maxi Magnet and Smart Kit reduce to 80% and 10% of the maximum value at the 0.1mm gap respectively.

2. The leakage magnetic flux density

Fig.3 shows the leakage magnetic flux of foreign magnetic attachments at the 0mm gap, and shows the length to the WHO's guideline value from the surface of the magnetic attachment. It shows that the leakage magnetic flux of open magnetic attachment is spreading out of the magnetic attachment.

3. The leakage magnetic flux at the tissue in the mouth

In Fig.4 (a), it is supposed that the distance from the magnetic attachment to the tissue of alveolar ridge is far 1.0mm for horizontal and vertical direction. Fig.4 (b) shows the leakage magnetic flux density for its position. The leakage magnetic flux of The Japanese magnetic attachments (Cup type and Sandwich type)³⁾ for the comparison.

As a result, the leakage magnetic flux of open magnetic circuit types exceed the WHO's guideline, i.e. 40mT. It is particularly large in Multipurpose Magnet System because it has the magnet in the keeper and the large diameter of the magnet. Three types of closed magnetic circuit and Japanese magnetic attachments do not exceed the 40mT at the tissue in the mouth.

It is shown that the leakage magnetic flux against the air gap in Fig.5. It is that the leakage magnetic flux of all the open types are more than 40mT, however the closed types of them are less than 40mT at the gap length is 0.2mm.

4. The leakage magnetic flux of the Keeper

It is necessary to investigate the leakage magnetic flux out of the keeper because Multiporpuse Mgnaet System, Titanmagnetics and Mircoplant have the magnet in the keeper.

Fig.6 shows the leakage magnetic flux of the keeper having a magnet, red point is the tissue place　as shown in Fig.4 (a) and the length to the WHO's guideline value from the surface of the keeper. As a results, their leakage magnetic flux are less than the results of Fig.3. However, the leakage magnetic flux at the tissue in the mouth exceed the 40mT. Multipurpose Magnet system is 110mT, Titanmagnetics is 105mT, and Microplant is 42.3mT.

Conclusions

Under the analytical conditions presented here, the results of this paper indicate that the leakage magnetic flux of all the open types of foreign magnetic attachments exceed the WHO's guideline. However, the closed types of them satisfy the WHO's guideline at the position of the tissue in the mouth.

References

1. Takada Y, Okuno O: Internal structures and materials, J J Mag Dent 17 (2):92 - 94, 2008.

2. Environmental Health Criteria; 232 STATIC FIELDS: 8.1.2 Static magnetic field. WHO 2006.

3. Nishida M, Tegawa Y, Kinouchi Y: Evaluation of Leakage Flux out of a Sandwich type Magnetic Attachment, J J Mag Dent 17 (2):39 - 43, 2008.

Discussion Board