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A Short Talk on a
Permanent Magnet
Yoshiro Iwama
Honorary Prof. of Nagoya University
A permanent magnet is defined as a magnetic material that can
memorize a magnetized state as it is. Such a function allows us to
substitute the permanent magnet for an electromagnet. Thus, the
practical application of the permanent magnet to loudspeaker has been
successfully developed and spreads throughout the world. This has
established the essential importance of the permanent magnet in the
electronics industry and facilitated the steady progress of the
magnetic materials and their applications. According to the
definition, it can be understood that magnetic recording media such
as magnetic tapes for audio or video and floppy disk also belong to a
family of the permanent magnet.
Using an example of Alnico 5 which is a trade name of an excellent
permanent magnet material produced by GE Co., we can illustrated how
the permanent magnet has been practically applied to loud speakers.
Based on a lot of experimental and theoretical researches, a
fundamental principle by which the permanent magnet can be made has
been established. According to it, if a ferromagnetic material like
iron is pulverized into micro-powder below a critical size, the
particles obtained become the row material of the permanent magnet.
They are called single domain particles and can be consolidated to
produce the permanent magnet. Furthermore, the progress in the
quality of permanent magnet is shown graphically from the 1920's to
date. It should be emphasized there that the advent of ferrite
magnets, later rare earth magnets led to great advances in wider
various fields of electronics.
As for magnetic recording media, they are also ferromagnetic iron
oxide particles. They made from ferromagnetic
Fe2O3 elongated fine particles. On looking back
to the remote past, the material that first allowed mankind to
recognize the magnetic phenomena was a natural magnetite which is
nothing but the natural permanent magnet. When we think about how the
magnetite is similar to the magnetic recording media widely used in
recent society, we remember that the wheel of fortune is ever in
motion.
Keywords : comment on permanent magnet, hard magnetic
materials, progress of magnets and their applications, rare earth
magnet
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Medical and Dental Applications of
Magnetism
Yohsuke Kinouchi
Department of Electrical and Electronic Engineering,
Faculty of Engineering, The University of Tokushima
Recently, magnetism has been applied in the field of medicine
and dentistry owing to the development of superior permanent magnets,
superconducting magnets, magnetic sensors and computer technologies.
Medical and dental applications of magnetism and related basic
research are reviewed. Applications for diagnosis include magnet
applied measurement, MRI, biomagnetic measurement and magnetic
stimulus. On the other hand, applications for treatment include
magnetic force retentions, magnetic guidance, magnetic hyperthermia
and magnetic effects for bone growth. Other applications and basic
research consist of magnetic cell separation, magnetic interaction,
biological effects of magnetic fields and magnetic biomaterials. As a
result, the future of dental applications of magnetism is discussed
briefly.
Keywords : magnetism, magnet, biomagnetism, magnetic
application, magnetic biomaterial
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Corrosion Resistance, Mechanical
Properties and Attractive Force of Pt-Fe-Nb Magnets
Osamu Okuno, Florentina Takako Iimuro*, Tsuyoshi Nakano*, Hitoshi Hamanaka*, Yohsuke
Kinouchi** and Yasuhiro
Matsui***
Department of Dental Materials Science, School of
Dentistry, Tohoku University
*Division of Metallurgy, Institute
for Medical and Dental Engineering, Tokyo Medical and Dental
University
**Department of Electrical and
Electronic Engineering, Faculty of Engineering, The University of
Tokushima
***Research and Development Group,
Nippon Mining Co., Ltd.
Pt-Fe-Nb magnets have excellent corrosion resistance and magnetic
properties. The maximum energy product is almost equal to that of
Sm-Co magnets. The purpose of this study was to investigate the
possibilities of Pt-Fe-Nb magnets as a functional dental material. In
oral environment Pt-Fe-Nb magnets were examined for corrosion
resistance, mechanical properties and attractive force for magnetic
stainless steel keeper of XM27. In a 1% lactic acid solution, the
anordic polarization curve of the magnet showed nearly equal
properties of pure platinum. After Pt-Fe-Nb magnet was immersed in 1%
lactic acid solution for seven days. The maximum dissolved ion was
1.6 µg/cm2 of Fe ion less than
that from 316L stainless steel, Pt-Fe-Nb magnets performed high
tensile strength of 758 MPa with a little ductility. Micro Vickers
hardness was 480 on average. 4×3×2 mm Pt-Fe-Nb magnet with
the sandwich type yoke of 4×3×0.82 mm XM27 stainless steel
achieved a retention force of 4. 9N (500 gf).
Keywords : Pt-Fe-Nb magnet, attachment, corrosion resistance,
mechanical properties, retention
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The Development of the
Sandwiched-type Dental Magnetic Attachment
Yoshinobu Tanaka, Yoshinobu Honkura*, Kazuo Arai*, Aki Watarai*, Kenji Hiranuma and Yoshirou Iwama**
The First Department of Prosthodontics, School of
Dentistry, Aichi- Gakuin University
*The Electronics Department, Aichi
Steel Works Ltd.
**Professor emeritus, The
Department of Engineering, Nagoya University
A dental magnetic attachment, denture retainer using rare earth
magnets, needs to have small size, high attractive force and
protection for the magnet against corrosion in the oral cavity.
Although several magnetic attachments with enough attractive force to
retain the denture have been developed so far, they have problems
concerning its size and the corrosion of the magnet. In order to
obtain high attractive force with small size, an advanced
sandwiched-type magnetic attachment, in which a rare-earth magnet is
sandwiched by a pare of yokes, has been developed through three
dimensional magnetic field analysis. Furthermore, the magnet is
covered with a stainless steel spacer and cap whose seams are
laser-welded. As a result, the attachment is small and it produces
high attractive force and perfect protection of the magnet from
corrosion.
Keywords : rare earth magnet, magnetic attachment, three
dimensional magnetic field analysis, sandwiched-type
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Analysis of the Retention Capacity
of Magnetic Attachments
Yohsuke Kinouchi and Osamu Okuno*
Department of Electrical and Electronic Engineering,
Faculty of Engineering, The University of Tokushima,
*Institute for Medical and Dental
Engineering, Tokyo Medical and Dental University
Recently, superior magnetic attachments have been developed and put
to clinical use. The purpose of this study is to analyze the capacity
(upper bound) of magnetic retention for closed- field type of
magnetic attachments. A cup yoke type of attachment is used as a
model for the analysis.
Retention capacity increases with saturation magnetization Js of
yoke and keeper materials. If material having Js=1.8T is used, which
is the maximum value for dental stainless steel, the retention
capacity is 63 and 76 gw/mm2 for an
attachment using Sm-Co and Nd-Fe-B magnets respectively.
To realize such retention will be a problem in the further
development of magnetic attachments.
Keywords : magnetic attachment, cup yoke, rare earth magnet,
magnetic force, finite element method
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A Comparative Study of Currently
Available Dental Magnetic Attachments
Yoshinobu Tanaka, Kazumoto Hoshiai, Kazuhiko Nakano, Kenji
Hiranuma, Yoshinobu Honkura*, Kazuo
Arai*, Tsutomu Ishijima** and Toshihiro Hirai**
The first Department of Prosthodontics, School of
Dentistry, Aichi- Gakuin University
*Research & Development, Aichi
Steel Works, LTD.
**Department of Removable
Prosthodontics, School of Dentistry,
Higashi-Nippon-Gakuen-University.
Several types of dental magnetic attachments are commercially
available. Two new magnetic attachments have been put on the market
in Japan. In this study, we have compared eleven types of attachments
including three developed by ourselves. The purpose of this study is
to compare characteristics such as outer size, inner structure,
attractive force, anti -corrosive seals, magnetic leakage around the
attachments, and accessories, from a clinical point of view. Although
each attachment has advantages and disadvantages, as the
synthetically evaluation based on our investigations, the attachments
of Igloo, Shiner and MAGFIT600 were found to provide high performance
and stability.
Keywords : magnetic attachment, retentive force, corrosion
resistance
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Effects of Dental Metals on MRI
Shin-ichi Masumi, Masahiro Arita, Osamu Okuno*, Hirofumi Kido, Katsuaki Takeya, Ryuji
Chigusa, Shigeki Miyake, Masao Morikawa and Shizuo Toyoda
Department of Removable Prosthodontics, Kyushu Dental
College
*Division of Metallurgy, Institute for Medical
and Dental Engineering, Tokyo Medical and Dental University
The aim of this research was to examine the effects of metals used in
dentistry upon magnetic resonance imaging (MRI). Thirteen disk-shaped
metals were used in this study. Each sample was set into a volunteer
who had produced a normal MR image when tested. The subjects were
imaged with a 0.1T MRI device and the results were examined for MR
defects. Au, Ag, Au-Ag-Pd and Am produced no defect. Ni-Cr, Co-Cr,,
Ti, SUS304 and Inconel produced small amounts of MR defects. SUS405,
17-4PH, Pd-Co-Ni and Sm-Co produced large defects. Those metals which
create MR image degradation must not be used for fixed prosthesis
which could easily be made from other metals. If the MR image
degrading metals are to be used in dentistry, their application
should be restricted to removable prosthesis.
Keywords : dental metal, MRI, degradation
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The Influence of Denture Cleansers
on a Dental Magnetic Attachment, MAGFIT 600
Yuuji Furushima, Yoshinobu Tanaka, Yoshinobu
Honkura*, Kazuo Arai*, Naoki Nagamachi, Hironobu Kiba, Akihiro
Kureyama and Kenji Hiranuma
The First Department of Prosthodontics, School of
Dentistry, Aichi-Gakuin University
*The Electronics Department, Aichi
Steel Works Ltd.
The popularity of denture cleansers to eliminate stains and fungus
from removable denture bases has increased in Japan. Several types of
denture cleansers are available. Since dentures are repeatedly
cleaned with these solutions for a comparatively long time, some
negative effects to the denture materials were suspected.
In this paper we will describe the study of the effects of denture
cleansers on the dental magnetic attachments which were recently
developed by our university.
35 samples, made of magnetic stainless steel, were repeatedly
immersed in solutions made from six different types of denture
cleansers. These samples were then examined through electric
microscopics and EDS analysis. They were also examined for changes in
weight.
No corrosion was found on the sample surfaces, but some solutions
caused deposits which could possibly decrease the attractive force of
the magnetic attachment.
Keywords : denture cleanser, dental magnetic attachment, EDS
analysis
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Influences of the Dental Magnetic
Keeper Area and the Setting Location on Attractive Force
Hiroshi Mizutani, Noritake Kadoyama, Minoru Ai, Hikari
Ozaki, Shin Ishikawa and Kazuo Nakamura
The First Department of Prosthodontics, Faculty of
Dentistry, Tokyo Medical and Dental University
The attractive force of a magnetic attachment with different keeper
areas or setting locations was investigated in this study. The
attachment which formed a closed circuit, consisted of HICOREX MD and
KEEPER MK (Hitachi Metals, Ltd.). The results were as follows;
l. Contrary to expectations, the attractive force but increased if
the cut width of the keeper was within the limit of l.5mm, compared
with the original one. When the width was l.0mm, the force was the
highest at about 370gf, but declined abruptly when it was over 2.0mm.
2. The attractive force increased when the horizontal distance
between the magnetic part and the keeper was within l.3mm, When the
distance was 0.5mm, the maximum force was nearly 370gf.
3. The reason why the attractive force was increased was that the
saturation of the magnetic flux density of the yoke was higher than
that calculated. It reached the saturation value when the keeper was
slightly cut or horizontally moved.
4. The attractive force of the keeper, covered with the dental
palladium alloy, showed a lower numerical value than that of the
bared keeper, since the accuracy of fit between the magnetic part and
the keeper was lost due to casting.
Keywords : magnetic attachment, attractive force, saturated
magnetic flux density
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Removable Dental Magnetic Keepers
and the Potential MRI Hazard
Kazuo Nakamura, Shin Ishikawa, Minoru Ai, Hiroshi
Mizutani, Fumiko Doi and Osamu Okuno*
The First Department of Prosthodontics, Faculty of
Dentistry, Tokyo Medical and Dental University
*Division of Metallurgy, Institute
for Medical and Dental Engineering, Tokyo Medical and Dental
University
The object of this study was to consider how to cope with the
potential hazard during magnetic resonance imaging (MRI) by use of
magnetic keepers.
Three methods for removing the keeper are discussed.
l. A preformed keeper, which is cemented on the root cap and can
be removed by use of an ultrasonic scaler.
2. A keeper with a screw hole.
The keeper is cemented on the root cap and can be removed by
screwing the driver in it.
3. A custom-made keeper with a male screw around it and a nut made
of non-magnetic dental casting alloy.
The surface of the keeper removed by ultrasonic scaler was damaged
and could not be used again.
The screw hole in the keeper was worn out after being removed a
few times.
The screw-type keeper and the nut were easy to fix and remove, but
the diameter of the nut was larger than the original preformed
keeper.
It was concluded that the screw-type keeper and the nut were most
effective, if the area of the root surface was sufficient.
Keywords : magnetic attachment, MRI, removable keeper
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A New Method for Orthodontic
Tooth Movement making use of a Magnetic Attachment.
Masahiko Tsuruta and Yoshinobu Tanaka*
Department of Orthodontics, Tsurumi University, School of
Dental Medicine,
*First Department of
Prosthodontics, Aichi-Gakuin University, School of Dentistry
The purpose of this report was to demonstrate a new magnetic device
that could be useful for orthodontic tooth movement. Studies have
shown that a magnet can depress teeth as well as move teeth
mesio-distally. Recently the authors have developed a new dental
magnetic assembly (DMA) using a samarium-cobalt magnet. In the
present study we tried to move a canine tooth with this magnetic
device. A DMA device was put into a plastic tooth in the position of
the extracted premolar while a stainless keeper was bonded to the
distal surface of the canine. The maximum attractive force between
magnet and keeper was approximately 680g. If the space between the
keeper and DMA device was adjusted to about 0.5mm, the initial
potential force was calculated to be 70g. The plastic tooth with the
magnet was incorporated into the multi-lingual bracket system. Every
two weeks, adjustment was needed to provide space for the tooth
movement. The results show that this DMA device is useful for
movement of the tooth as well as improving the aesthetic appearance
of the orthodontic appliance.
Keywords : rare earth magnets, dental magnetic attachments,
orthodontic appliances, tooth movement
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Report of a Case : Salivary Gland
Tumor of the Palate using MR Examination for the Diagnosis
Takashi Kaneda, Kaoru Ozawa, Manabu Okada, Mitsuhisa
Ozawa, Hidekuni Tanaka*, Kasumi
Kimura* and Hirotsugu
Yamamoto*
Departments of Radiology and *Pathology
Nihon University School of Dentistry at Matsudo
Recently, MRI has been used in the field of the temporomandibular
joint, but rarely for the oral legions. In this paper, we reported a
case of adenocystic carcinoma of the palate and the role of MRI in
its diagnosis.
The patient was a 38-year-old man with the chief complaint of
swelling of the palate. MR examinations were performed using a 0. 2
Tesla permanent magnet system and a head coil.
MR showed the intermediate T1 and inhomogeneously high T2 weighted
signal intensities in the palate.
MR was superior to conventional radiography and plain CT in the
ability to demonstrate solid component and extent of the tumors.
Although, destruction of the bony cortex is diagnosed more
effectively by tomography and CT.
MR examination is useful to make diagnosis of the palate.
Keywords : salivary gland tumor, MRI, diagnostic imaging
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Clinical Applications Making the
Best use of the Properties of the Dental Magnet
Nobuo Ishihata, Hiroshi Mizutani, Kazuo Nakamura, Shin
Ishikawa, Yuji Suzuki and Minoru Ai
The First Department of Prosthodontics, Tokyo Medical and
Dental University Faculty of Dentistry
Since 1980 we have been fundamentally and practically investigating
how a Sm-Co magnet can be applied to a dental magnetic attachment as
a retainer. We have observed the progresses of dozens of the magnetic
attachments' cases. We have found that the magnetic attachment had
the characteristic properties as was shown below which the other
retainers didn't have;
l) The abutment teeth don't receive a strong pulling force when
the denture is inserted and removed.
2) The abutment teeth don't receive a harmful lateral force or a
harmful rotating force when the denture functions.
3) The magnetic attachment doesn't have a fixed direction to
remove or insert, which the other retainers have. In this study, we
will explain our experimental data regarding this magnetic
attachment, along with clinical cases.
Keywords : magnetic attachment, clinical application
Copyright (c) The Japanese Society of Magnetic Applications in
Dentistry. All rights reserved.
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