Effect of keeper after MRI irradiation on attractive force
Shin-ichi Masumi1, Eri Makihara1, Hiroshi Ikeda2, Hiroshi Shimizu2, Masafumi Oda3,
Yasuhiro Morimoto3, Takafumi Watanabe1, Shogo Tsuda1,
Mayumi Yagi1, Masahiro Arita1, Takumi Masumi1 and Akira Kikuchi4
1.
2.
3.
4.

Division of Occlusion & Maxillofacial Reconstruction, Department of Oral Function, Kyushu
Dental University
Division of Biomaterials, Department of Oral Functions, Kyushu Dental University
Division of Oral and Maxillofacial Radiology, Department of Science of Physical Function, Kyushu
Dental University
NEOMAX Engineering---------------------------------------------------------------------------------------------------------Abstract

The purpose of this study was to evaluate the attractive force of the dental magnetic
attachment before and after MRI irradiation against the keeper.
Duplicate the lower non-magnetic table of the ISO tensile test jig with an immediate
polymerization resin, and then attaching the magnetic assembly to the upper
non-magnetic table of the jig and each keeper to the lower duplicated resin table. The
attractive force before irradiation was measured with a universal tensile tester. Next, aresin-made table with a keeper was placed in a mold that assumed the upper and lower
incisors, premolars, and molars, and the attractive force was measured after MRI
irradiation for about 16 minutes.
There was no significant difference in the suction force before and after irradiation,and it was found that there was no problem with the attractive force even if MRI
irradiation was performed for the patient with keeper.
---------------------------------------------------------------------------------------------------------Introduction
There are some reports that patients with dental magnetic attachments have decreased
attractive force after MRI irradiation. In this study, MRI was irradiated to acommercially available magnet keeper, and verification was performed by measuring
the attractive force of the dental magnetic attachment before and after irradiation against
the keeper.
Materials and Methods
1． Duplication of lower non-magnetic table
Prior to the experiment, 6 of the lower non-magnetic table of the ISO tensile test jig

were duplicated with an immediate polymerization resin (Orthocrystal, Nissin), themagnetic assembly (Hyper Slim 4813 magnet structure, NEOMAX) was placed on theupper non-magnetic table (Fig.1), and the duplicated lower non-magnetic table wasmounted. A keeper (DB keeper 4813, NEOMAX) was adhered to the lower
non-magnetic table (Fig.2).

Fig.1 Upper non-magnetic table withmagnetic assembly.

Fig.2 duplicated lower non-magnetictables with keeper.
UA: upper anterior, UP: upper premolar
UM: upper molar, LA: lower anterior
LP: lower premolar, LM: lower molar

2． Tensile test
An ISO tensile test jig was attached to a universal tensile tester (Autograph AGS-H,Shimadzu Corp.), and the attractive force before irradiation was measured at acrosshead speed of 5 mm /min (Fig.3).

Fig.3 Universal tensile tester with ISO
tensile test jig.

3． Fabrication of mold for MR imaging
Next, an acrylic mold that assumed six upper and lower anterior teeth, premolars, andmolars was fabricated (Fig.4). The positions of the upper and lower central incisors,
first premolars, and second molars were determined with reference to the report byTakigami et al.1). The distance from the occiput to the distal end of the second molar
was calculated from MR images stored at the Department of Dental Radiology, Kyushu
Dental University Hospital.

10mm

15mm

15mm

12mm

20mm18mm

100mm

100mm

Upper

Lower

Fig.4 Fabricated mold for MR imaging

After fixing the upper and lower molds on the acrylic plate, a socket was provided to fix
each lower non-magnetic tables using silicone putty. The distance between the upperand lower mold was 40 mm (Fig.5,6).

Fig.5 Upper and lower molds fixed
on the acrylic plate4.

Fig.6 Lower non-magnetic tables set
into the sockets

MR imaging

The mold with the lower non-magnetic table with keeper was installed in the gantry ofMRI (1.5-T, full-body, MR system (EXCELART VantageTM Powered by Atlas; Toshiba,Tokyo, Japan)). It was irradiated for about 16 minutes to image the head and neck tumor.
The attractive force of the keeper after irradiation was measured by the same method as
before irradiation and the attractive force before and after irradiation was compared andexamined.

Results
Table 1 shows the results of evaluation of the attractive force of the magnetic
attachment before and after MRI irradiation on the keeper.
Table 1 Results of evaluation of the attractive force of the magnetic
attachment before and after MR irradiation on the keeper.

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The attractive force of Hyper Slim 4813 is displayed as 9.6N. The attractive force ofthe magnetic attachment using the keeper before MRI irradiation has a large variation of
4.42N to 8.96N on average, which is considered to be a technical error when bonding
the keeper to the lower non-magnetic table. Similarly, the attractive force afterirradiation has 4.97 N to 9.54 N on average and the attracive force tended to increase
after MRI irradiation.
The attractive force before and after MRI irradiation at each position was examined,
and no significant difference was observed at all positions. It was found that there wasno problem with the attractive force even when the keeper was irradiated with MRI.
Discussion
Tsuchibashi et al.2) described that when an MRI examination was performed with adenture with a magnetic attachment, significant artifacts were generated, and thespontaneous magnetization of the magnet portion was reduced due to the magnetic fieldof the MRI, which reduced the attractive force. It is reported that MRI must be
performed after removing the denture, which seems to be performed at most facilities inJapan.
On the other hand, there have been reports of keeper dropout during MRI
examinations2), and cases where the attractive force of the magnetic attachment
decreases after MRI examinations 3). In this regard, Yamamoto et al.4) reported that in astatic magnetic field of MRI, a torque was generated so that the keeper surface becameparallel to the magnetic field, and the keeper was displaced very slightly, and thecondition of contact with the magnet assembly changed. In addition, in an object withanisotropic shape such as a keeper, the magnetization induced inside the object varies
depending on the direction of the object in the magnetic field, and as a result, the torquein which the long axis direction of the object points in the direction of the magnetic
field. In other words, the long axis of the object tends to be oriented in the direction of
the magnetic field.
In this experiment, there was no significant difference in the attractive force before andafter MRI irradiation to the keeper in all six positions. Therefore, there is no problem in
the attractive force even if MR imaging is performed while the keeper is worn in theoral cavity. In addition, a tensile test using a keeper after MRI irradiation showed atendency for the attractive force to increase at any positions. It is possible that thekeeper itself is slightly magnetized by MRI irradiation, but we will discuss the details in
the future.
Conclusion
In a case report that a patient with a dental magnetic attachment had reduced theattractive force after MR imaging, the authors verified the case by irradiating acommercially available magnetic keeper with MRI and measuring the attractive force ofthe dental magnetic attachment before and after irradiation. It was found that there wasno problem with the attractive force even if MRI irradiation was performed for thepatient with keeper.
Conflict of interest
Regarding this report, there is no conflict of interest among all authors.
References
1) Takigami, Y., Yamaguchi, Y. and Yoshitake, K.: Dentoskeletal changes caused by a plaster model inadults with normal occlusion. J. Jpn. Stomatol. Soc. 2001; 50: 293-298.
2) Tsuchibashi, T., Fujita, I., Maki, T., Kitagawa, M. and Suzuki, T.: Influence of dental magnetic
attachment on MR imaging. Jpn. J. Rad. Tech. 1998; 54: 517-520.
3) Yamamoto T. Issues and expectations of metallic materials in MRI. Mateia Japan 2010; 49: 157-160．
4) Yamamoto T., Nagayama Y. and Tamura M.: A blood-oxygenation-dependent increase in bloodviscosity due to a static magnetic field. Phys. Med. Biol. 2004; 49: 3267-3277.