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Physics, Use and
Design of Dental Magnets
A.P.Dias and B.W.Darvell
Rare-earth alloy magnets have been used to retain prostheses and to
move teeth for over twenty years. However, the literature on both
retention magnets and orthodontic magnet systems can be characterized
in general as lacking reference to physical principles: retention
expressed as force per unit mass or force per unit volume of magnet
assembly; the force-distance relationship said to be according to
various relationships; force said to vary with the speed of
separation; the stated need for a keeper of specified thickness and
the need for special high permeability alloys. Data are now presented
to demonstrate that the expected force-distance relationship
approaches an inverse fourth power law. The keeper may be a plane of
infinitesimal thickness; its relative permeability ceases to be
implant for µ/µ0 > ~200 in that the retentive
force could then be improved by no more than 1% at any given
distance; speed of separation has no bearing on the retention; the
development of special keeper alloys would be a waste of time and
effort. From the above it follows that: in retention system, even a
very small increase in the distance between magnet and keeper for
whatever reason would significantly reduce retention; the design of
orthodontic tooth movement systems should take the force-distance
relationship into consideration if a rapid rise or fall of force with
tooth movement is to be avoided. The development and design of dental
magnets and clinical procedures should take account of these factors
if the clinical outcome is to be satisfactory.
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The Basic and Clinical
Research of Z-1 Type Magnetic Attachment
ZHAO Yimin, LIU Baolin, HUANG Hui, CHENG Yong and ZHOU Tao
Application of magnetic force in retention of prostheses has been
tried more than half century. With the development of magnetic
materials and manufacturing technique, it has become a realization to
use magnetic force to retain oral and maxillofacial prostheses in
recent 20 years. As a mature retentive technique, magnetic attachment
has been applied in various prostheses more and more popularly now.
On the base of a lot of researchs on magnetic materials and
mechanics, we have developed Z-1 type magnetic attachment since 1987.
Z-1 type magnetic attachment is a type of closed-field magnetic
attachment, which consisted of a permanent magnet and a prefabricated
pin-cap keeper. The magnet is designed in sandwich form and made of
Nd-Fe-B permanent magnet; the keeper is made of Fe-Cr-Mo soft
magnetic alloy. It is of some obvious advantages besides the general
advantages of magnetic attachment, for example: Strong retentive
force, its average retentive force is 9.3N every attachment;
outstanding heat magnetic stability, it can be used under 110°C
and doesn't change its magnetic property; weak extra magnetic field,
its extra magnetic field strength is under 0.02T; sample operating
method etc. For above reasons, it has been applied in various
prostheses extensively in China and improved the retention of
prostheses effectively.
Some basic and clinical researches of Z-1 type magnetic attachment
are introduced in following.
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Use of the Magnet with Milled
Bar for Implant-supported Overdenture
Jau-Min Hong
The totally edentulous patient can benefit from fully implant
supported fix detachable bridge. When considering the anatomic
limitations of edentulous maxilla and mandible, the need for a
properly designed prosthesis that results in favorable biomechanical
stress distribution and fulfillment of patient needs, in regard to
esthetics, phonetics, comfort, and hygiene, an overdenture prosthesis
is usually the prosthesis of choice. According to the type of
supporting and retention implant overdenture prosthesis can be
classified into 3 categories : hybrid, implant supported, and implant
retained. Generally implant supported overdenture is the case of four
or more implants in the anterior region of upper or lower jaw, then
fabrication of an overdenture prosthesis supported and retained
anteriorly by implants and supported posteriorly the residual
alveolar ridge.
The purpose of this report describes the clinical application,
evaluation and laboratory procedures of the magnet attachment with
milled bar as retention for implant - supported overdenture.
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Evaluation of the
Retention Form of the Magfit Magnetic Attachment System
Y.Y.Shiau and C.C.Liu
Magnetic attachment has been successfully used on prosthetic
dentistry to improve the retention of the removable dentures. Its use
minimized the hazardous effect of the denture retention form on the
abutment, and its intra-denture retention from also minimized the
display of the clasp and rest thus improved the denture esthetics.
Besides, extensively destructed tooth or moderate periodontally
involved tooth which were previously regarded as insufficient can be
used as abutments when the concept of magnetic attachments is
applied.
Many types of magnetic attachments were developed since Gilling
(1981) proposed his concept of modern magnetic use. Development has
been based on searching a more powerful, smaller and less corrosive
magnetic assembly. Magfit system is one of the newly developed
magnetic systems having such characteristics. However, some possible
worries remained, among them is the retention of the magnet assembly
in the denture base of a removable partial denture.
In addition to the use of metal conditioner to enhance the
adhesion between the metal and the resin part, some modifications of
the magnet part were proposed to increase the retention ability of
the magnet in the denture base. However, modification caused either
increase of the size or the increase of the price. Since Magfit
magnetic attachment is the smallest system developed for the dental
use, the evaluation of the magnetic attachment retention can be on
this system only. The purpose of this study thererfore, was to
examine the potential risk of magnetic detachment of conventional
Magfit system and to evaluate the necessity of changing its shape to
obtain better retention.
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Study on The Abrasion
of Magnetic Attachment by Brushing
Naomasa Sawada, Akio Asano, Makoto Sakai, Mutsuo Yamauchi
and Tooru Nagasawa
Using different types of cleaning pastes and tooth-brushes of
different stiffness, we examined how magnetic attachments were
affected by brushing. The magnetic attachments were subjected to
100,000 times of brushing using brushing abrasion tester. Before and
after brushing test, measurement of the surface roughness, waviness
and attractive force was carried out.
The surface roughness of magnetic attachments underwent no
significant changes by brushing. The waviness of the magnetic
attachment was increased significantly by brushing with the exception
of the keeper brushed with soft tooth-brush. The attractive force
between the brushed keeper and unbrushed magnetic assembly underwent
no differences compared to the control (Attractive force between the
unbrushed keeper and magnetic assembly). However, the attractive
force between the brushed magnetic assembly and unbrushed keeper was
significantly decreased comparing to the control when brushed with
ordinary tooth-paste or denture cleaning paste using a stiff
tooth-brush.
These results suggest that the brushing at magnetic assembly with
a tooth-paste using a stiff tooth-brush is fraught with a risk of
decreasing the attractive force.
Keywords : magnetic attachment, brushing, hardness of
tooth-brush, tooth paste, waviness
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Experimental
Research on Retention of Magnetic Assemblies to Denture
Bases
Yoshinori Nakamura, Yoshinobu Tanaka, Takashi Ishida,
Yasuo Kishimoto, Takeshi Kanazawa, Yoshinori Desaki, Tohru Satou,
Kazuyuki Matsumoto, Akira Imaizumi and Yoshinobu Honkura
A magnetic assembly should firmly stay in a denture base. Generally
it is fixed into denture by use of selfcuring resin with metal
bonding agent. In this report, clinical factors influencing retention
of the magnetic assembly were analyzed through tensile strength test
on various conditioned models.
The results may be summarized as follows:
1) Sandblasting prior to fixation was very effective to improve
retention.
2) Metal bonding agent (Metalprimer II : GC Corp.) was still
effective even after one week of its application
3) Projections as mechanical undercut prepared on lateral walls of
the magnetic assembly showed better retention than the horizontal
grooves.
4) The most effective lateral contour of the magnetic assembly for
retention was found to be an inverse tapered face.
Keywords : magnetic attachment, magnetic assembly, metal
bonding agent, retention in a denture base, tensile strength
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Dentistry. All rights reserved.