10 years have passed since the first insertion of a magnetic attachment in prosthesis by Japanese dental clinicians. During those 10 years no particular problem occurred and the magnetic attachment has gradually become accepted by most general clinicians.
The aim of this study is to investigate the long-term usefulness for prognosis of the magnetic attachment "Magfit" that can be used in the metal plate partial dentures. The evaluation was done "Magfit" in 853 plates between 1993 and 2003. The metal plate dentures were introduced in 103 plates and were used in 240 abutment teeth.
The results were as follows.
1. The average use number per plate were about 1.5 for the acrylic resin plate and about 2.3 for the metal plate, with a difference of about one.
2. About 1.5 times the number of mandible abutments was used on tooth classification working condition of the maxilla abutments, and the maxillary canine was mainly used.
3. 42% of them used only a magnetic attachment as retainer and the remaining 58% used other retainers jointly.
4. The survival rate of the abutment tooth of the metal plate case was 95% after 5 years and 88% after 10 years.
The above results show that among retainers of partial denture, the magnetic attachment proved to be very useful.
Magnetic attachment is a new clinical tool used to stabilize removable dentures. We developed a very useful magnetic attachment named "MAGFIT SYSTEM" and introduced it as the first clinically practical magnetic attachment in 1992. As a standard technique in the Magfit system, a cast-on technique is generally proposed. Although it is a very simple method, it could cause roughness on the keeper surface, which would bring about deterioration of the retentive force of the attachment. Therefore, we have recently introduced a new Keeper-Cementing technique with a prefabricated plastic, because titanium has a higher melting point.
Recently, the CAD/CAM system has been introduced into dentistry. Although it presently works very well in the production of simple crowns and in the fabrication of specific prostheses, such as post crowns and telescopic crowns, a number of unsolved problems remain.
We will introduce a new program for dental CAD/CAM systems. The keeper metal coping can now be produced using only CAD/CAM systems, which will solve the clinical problems mentioned above.
Applying a ready-made dental magnetic attachment to a removable crown and bridge presents various problems. As a solution, a removable crown bridge system assembled with the Fe-Pt magnet alloy outer cap and the soft magnetic stainless steel inner cap has already been investigated1-4）. In this paper, using the integral element method, the 3-dimentional magnetic field of the disk model was analyzed to determine the optimal method for the design and magnetization of removable crown and bridge system. The attractive force, the magnetic leakage flux density and the residual flux density were examined by the 3-dimentional magnetic field analysis. As a result, sectoral four pole magnetizing was better magnetization method for the Fe-Pt magnet in four kinds of magnetization investigated in this paper.
Optimal design and magnetization of the removable crown and bridge system made with the Fe-Pt magnet and SUS447J1 magnetic stainless steel were investigated by the integral element magnetic field analysis using the experimental magnetic properties.
As a result, when the thickness of the outer cap top is 1.0mm or more, an enough attractive force for retaining the crown was obtained. The magnetic flux in the vicinity of the attracted side of the inner cap model was saturated well. And the magnetic leakage flux for all the calculated models has satisfied the safety standard of the magnetic field.