DENTURE ATTACHMENT

Abstract: A denture attachment is planted in a denture base in such a manner as to oppose a keeper (103) made of a soft magnetic substance, and clamped by the keeper (103) through magnetic attraction. The attachment comprises at least plate-like three yokes made of a soft magnetic substance and at least two plate-like magnets the N and S poles are oriented in the direction of the thickness of the attachment. In the drawing, the attachment comprises a center yoke (10), a first magnet (1), a second magnet (2), a first side yoke (11) and a second side yoke (12). The magnetic poles of the first and second magnet (1, 2) on both sides of the center yoke (10) are characteristically opposed to one another. Therefore, an independent magnetic loop is formed for each magnet. Because of this novel structure, the magnetic force is about twice stronger than conventional while the denture has a compactness required generally. Spacers (21 and 22) made of a non-magnetic substance may cover the surfaces of both magnets (1 and 2) in contact with the keeper (103), and a casing (3) made of a non-magnetic substance may cover the entire portion other than the surfaces in contact with the keeper so as to improve both corrosion resistance and wear resistance.

Description: Technical field This invention relates to dental attachments which utilize magnetic attractive force to retain a denture on tooth roots, specifically to the ones which have a thin disk shape with smaller cross section than that of a tooth root. 2. Background Art As shown in figure 23, an existing method to attach a denture on the gingiva by a dental magnetic attachment is as follows. Root cap 102 is set on the tooth root 101 in the gingiva to form an abutment tooth for the denture 110. A keeper 103 made of corrosion-resisting soft magnetic alloy is embedded in the center of top surface of root cap 102. A dental attachment 111 is embedded in the center of bottom face of the denture so as to face the keeper 103. The attachment 111 attracts the keeper 103 magnetically, thus the denture 110 is fixed on the root surface 102. Japanese patent application Laid- Open (Kokai) No. 1-303145, 2-295557, 2-295558 disclose a dental attachment 111 of which an exploded view is shown in figure 24, sectional view in figure 25. This existing attachment consists of a block of magnet 112, a spacer 113 made of corrosion resisting non-magnetic alloy to cover one face of the magnet, a pair of yokes 114, 115 that made of corrosion resisting soft magnetic alloy to sandwich the magnet 112 and the spacer 113 in between, and a cap 116 made of corrosion resisting non-magnetic alloy to cover whole other parts. This dental attachment has a magnet 112 with the N-S direction parallel to the keeper 113. A magnetic circuit is formed in which the magnetic flux generated at the magnet 112, passes through the yoke 114, keeper 103, then the other yoke 105, finally returns to the magnet 112. This closed magnetic circuit gives relatively strong attractive force. Japanese patent application Laid- Open (Kokai) No. 57-164054 disclose a dental attachment 121 of which magnet arrangement is shown in figure 26. The attachment in the denture is shown in figure 27. This dental attachment consists of plurality of pairs of permanent magnet 122 and soft magnetic part 123. The permanent magnets 122 and the soft magnetic parts 123 are arranged alternatively so that the like magnet poles face each other. When a soft magnetic body, so called attachment keeper, is placed in contact with the attachment 121, almost all magnetic flux pass along the keeper so that it is attracted to the attachment 121 quite firmly. Disclosure of the invention There is considerable demand for dental magnetic attachments to have an acceptable combination of stronger attractive force (more than 4,9035 N (500 gram force)) and small height (below 1.5 mm). The demand requires advanced technique in magnetics that improves the magnetic attractive force per unit volume F/V, that is, reduce the height without increasing the sectional area of the dental magnetic attachment. Since F/V in a given magnetic structure is determined by the characteristics of the material, it is difficult to improve F/V. This invention is offered to solve the above mentioned problem, and offers an attachment with a combination of small height and strong attractive force, in other words, offers the means to increase the attractive force per unit volume F/V drastically. First the structure of the invented attachment is described. Here 'height' is defined as a length taken along the direction of attraction, and 'thickness' is defined as a length taken along the direction normal to attraction. The invented attachment consists of at least three yokes and at least two pieces of magnet. The yokes are made of soft magnetic material, and a face of the yokes attaches to said keeper by magnetic attraction. The magnets are sandwiched by the yokes. Their magnetization direction are parallel to the thickness-direction and the poles of each magnet which contact to the central yoke has like polarity. Each magnet forms a mutually independent magnetic circuit in which the magnetic flux generated at one pole of the magnet pass through one of two yokes in contact with the magnet, said keeper, and the other of said yokes and returns into the other pole of the magnet. The ratio of H/S 1/2 , in which H denotes the height of the magnet and S denotes the area of attracting face S, is not more than 0.55 with the attractive force of not less than 4,9035 N (500gf). It is preferable that H/S 1/2 is not more than 0.50. The ratio of TH/S 1/2 , in which TH denotes the total height of the magnet and keeper, is not more than 0.75, preferably not more than 0.70. The invented attachment is used in a denture in similar manner to the existing ones which is embedded in the denture base so that it faces to the keeper embedded on the tooth root and act attractive force on it. The present invention described above can have more than two pieces of magnet, although the structures with two or three pieces of magnet are preferable for the practical application, and more than three pieces of magnet are not required except for some particular cases. Keeping practical use in mind, typical structure with two pieces of magnet will be described below in detail. The invented attachment with the typical structure consists of two pieces of magnets, the central yoke, and two outer yokes. The yokes are made of soft magnetic material. The magnets having their magnetization direction parallel to the thickness-direction are sandwiched by the central yoke and one of the outer yokes. Two magnets are arranged to sandwich the central yoke while the like poles of the both magnets face each other. The whole assembly of the magnets and the central yoke are sandwiched by the outer yokes. As a consequence two magnetic circuits are formed in the attachment. One of the magnetic circuits is formed in which the magnetic flux generated by the inner pole of the magnet passes through said central yoke, said keeper and said outer yoke, and returns to another pole of the magnet. Another magnetic circuit is formed in the other half of the attachment in a similar manner except that it has the counter direction of flux rotation. The height of the keeper is preferably nearly equal to the thickness of the outer yoke. A keeper with reduced height does not create sufficient magnetic attracting force, on the other hand a keeper with increased height makes the total height of the attachment large so that it can not meet the demand of small height for the attachment. The magnetic characteristics of the material have great influence on the performance of the invented attachment. It is preferable to use the following materials. First the magnet should have the maximum energy product of not less than 20 MGOe, for example rare earth magnets. It is further preferable that the magnet has good corrosion resistance. Second, soft magnetic material for the yokes should have a corrosion resistance with the saturation magnetization of not less than 13000 G and magnetic permeability of not less than 3000. In the case that corrosive magnets are used, it is preferable to cover the whole attachment with the corrosion-resistant coating to assure corrosion resistance of the invented dental attachment. Plated or sputtered coating of corrosion-resistant metal, resin coatings are the examples of the coating. The thickness of the coating is preferably 5 - 200 micro-meter thick. It is more desirable to cover exclusively the exposed magnet surface with the corrosion resistant coating except the face in contact with the yokes. In order to prevent the magnet from corrosion perfectly, it is necessary to cover the magnets and yokes with the cap made of corrosion-resistant non-magnetic material except the attracting face of the attachment to the keeper, while covering the contact face, which is the one facing to the keeper, of the magnets with the spacer made of corrosion resistant non-magnetic material. The permeability of the material is preferably under 1.2. The cap and the spacers must be put tougher on the yokes by electron beam welding or laser welding to seal the magnet perfectly. According to the present invention, it is similarly preferable to cover the lateral faces, which faces are not in contact with the yokes, of the magnets with the corrosion resistant non-magnetic annular case. Now the design parameters to reduce the volume of the attachment will be described. The square edge on the top of the outer yokes can be chamfered. It is because the chamfering makes the magnetic flux flow smoother and enhances the attractive force. Similarly, the top face of the central yoke can have a V-shaped groove for the same reason. In the case that the attachment is covered by corrosion-resistant cap, it should have a similar shape to the yokes. In addition a bridge attached on the top of the central yoke over the V-shaped groove may enhance fixation of the attachment to the denture. Two magnetic circuits in the invented attachment bring a major merit of reducing the height of the attachment to a half of that of existing ones with a single magnetic circuit. Furthermore the circuits enhance the attractive force and reduce the central yoke thickness. The merits may be due to the following mechanism. First the major merit concerning the attachment height will be described. The attractive force of the attachment is proportional to B 2 S, here B denotes the magnetic flux density at the attracting face, S denotes the cross sectional area of the attracting face. In the case of a given cross sectional area S and a given kind of materials, B is determined by the pole area of the magnets as long as the magnetic flux leakage is negligible. A single magnet with a given pole area is equivalent to two magnets with a half pole area. This means that two magnets with a half certain height can replace a single magnet with a certain height, so that the two magnets give the same attractive force as that of the attachments with a single magnet. In other words the height of attachment is reduced to a half with a given attractive force. Next other merits brought by the two magnetic circuits will be described. Generally a magnetic circuit consists of a permanent magnet which generates magnetic flux, soft magnetic material in which magnetic flux flows, and non-magnetic material which plays a role of barrier to the magnetic flux. In spite of the existence of the non-magnetic barrier, magnetic flux that flows in a magnetic circuit tend to leak to a certain extent. The leakage makes the effective flux density decrease and causes deterioration in attractive force. Two magnetic circuits are formed when the invented attachment and the keeper 103 are set in contact as seen in figure 1. The magnetic flux which is generated at the south pole of the left-hand magnet 1 passes through the left-hand half of the central yoke 10, the left-hand half of the keeper 103, and the outer yoke 11 and returns to the north pole. Thus the left-hand magnetic circuit is completed. In the same manner the right-hand magnetic circuit is formed. The magnetic flux which is generated at the south pole of the right-hand magnet 2 passes through the other half of the central yoke 10, the the other half of the keeper 103, and the right-hand outer yoke 12 and returns to the north pole of the magnet 2. At the center plane of the central yoke these two magnetic fluxes which are come from magnets on both sides run into collision and repel each other. The repulsion by a magnetic circuit could form a stronger barrier than a non-magnetic barrier to the other magnetic circuit. It suppress the leakage from the central yoke for the other magnetic circuit and compresses the flux to a higher density. As a consequence it brings a merit of stronger attractive force and the other merit that the central yoke requires less thickness than the total thickness of the outer yokes. After the collision the magnetic fluxes inside the central yoke flow into the keeper and divide into two fluxes and run into both outer yokes and return to the outer side pole of the magnets. The formation of these complementary magnetic circuits give a compact attachment with strong magnetic attractive force. Further, the formation of two magnetic circuits enable to reduce the height of the keeper to a half of existing ones, because the height of the keeper equals to the thickness of outer yokes, of which the thickness is a half of that in existing attachment with single magnetic circuit. Thus the total height of the dental attachment and the keeper can be reduced. For the invented attachment the value of H/S 1/2 , in which S denotes the area of the apparent attracting face and H denotes the magnet height measured from the attracting face, becomes not more than 0.55 with the attractive force of not less than 4,9035 N (500 gram force). It means that the attachment can take a thin disc shape. So far the reason that the invented attachment has a desirable combination of small height H and strong attractive force has been described. The principle is applicable to the attachment with magnets of three pieces or more. The height of the magnets is reduced to less than 1/3 when three magnets or more are used and three or more magnetic circuits are formed. Similarly the thickness of the keeper for the invented attachment is less than 1/3 of those used in existing magnetic attachments. The total area of the poles of the magnets used in the present invention is nearly equal to that of a single magnet used in the existing attachment shown in figure 25. Therefore the total flux in the present invention in which three magnets or more are used is equal to the flux in the existing attachment with a single large magnet. Because of this the attractive force of the presently invented attachment is nearly equal to that of the existing attachment shown in figure 25. That is, the presently invented attachment has not less attractive force than that of the existing ones in spite of its small height. For the practical application of this invention, two or three pieces of magnet are sufficient, and more than three pieces of magnet are not required except for some particular cases. However, it is possible to reduce the height further, for example the value of H/S 1/2 to be not more than 0.1, by increasing the number of magnet from 10 to 20. In those cases the attractive forces are also nearly equal to the strong attractive force of existing dental attachments. The previous inventions described before with figure 26, 27, which is disclosed in Japanese patent application Laid- Open (Kokai) No. 57-164054, has a similar arrangement of magnets and yokes to the present invention. However the objective of the previous invention is to cover a wide range of dentition and it does not improve the attractive force per unit volume of the attachment. In other words the height of the attachment is not reduced. On the contrary in the present invention the height of the attachment is inverse-proportional to the number of magnets so that the attractive force per unit volume of the attachment is improved proportionally to the number of magnets, which is the essential point of the present invention. There are desirable dimensions of the present invented attachment to apply to teeth. It is preferable for the present invented dental attachment to have an attracting area of not more than 20.0 mm 2 , more preferably not more than 14.0 mm 2 , still more preferably 11.4 mm 2 . When the attracting face has rectangular shape, it is preferable to have its longer side to be not more than 5.0 mm and its shorter side to be not more than 4.0 mm, more preferably not more than 4.2 mm and not more than 3.4 mm, respectively, still more preferably 3.8 mm and 3.0 mm, respectively. As described above, the present invented dental attachment has strong attractive force with small height, in other words, increased F/V, attractive force per unit volume, to two times by adopting the structure with formation of two magnetic circuits. It should be noted that the present invention is no different from existing ones in the fixation with the denture, corrosion resistance or wear resistance. When three magnets are used in this invention, the number of the magnetic circuits becomes three and F/V is further increased. Brief Description of the Figures Figure 1 shows a cross sectional view of the dental attachment and the keeper disclosed in embodiment 1. Figure 2 shows a horizontal sectional view of the dental attachment disclosed in embodiment 1. Figure 3 shows a cross sectional view of the dental attachment of the present invention and the keeper disclosed in embodiment 4. Figure 4 shows a horizontal sectional view of the dental attachment disclosed in embodiment 4. Figure 5 shows an exploded view of the dental attachment disclosed in embodiment 4. Figure 6 shows the structure of the case in embodiment 4. Figure 7 shows a cross sectional view of the dental attachment and the keeper disclosed as a modification of the embodiment 4. Figure 8 shows a cross sectional view of the dental attachment disclosed as another modification of the embodiment 4. Figure 9 shows a cross sectional view of the dental attachment and the keeper disclosed as yet another modification of the embodiment 4. Figure 10 shows a cross sectional view of the dental attachment and the keeper disclosed yet another modification of the embodiment 4. Figure 11 shows a horizontal sectional view of the dental attachment disclosed as a modification of the embodiment 4. Figure 12 shows a cross sectional view of the dental attachment disclosed in embodiment 5. Figure 13 shows a cross sectional view of the dental attachment disclosed in embodiment 6. Figure 14 shows an example of horizontal sectional view of the dental attachment. Figure 15 shows another example of horizontal sectional view of the dental attachment. Figure 16 is a graph showing the comparison of the performance of the dental attachments. Figure 17 is a graph showing the relation between the thickness of spacer and magnetic attractive force to the unit volume. Figure 18 is a graph showing the relation between the reduced volume at the shoulder part of the side yoke and magnetic attractive force to the unit volume. Figure 19 shows a cross sectional view of the dental attachment and the keeper disclosed in embodiment 10. Figure 20 shows a horizontal sectional view of the dental attachment disclosed in embodiment 10. Figure 21 shows a cross sectional view of the dental attachment and the keeper disclosed in embodiment 11. Figure 22 shows a cross sectional view of the dental attachment and the keeper disclosed in comparative sample 1. Figure 23 shows a cross sectional view of the denture retention by an existing dental attachment. Figure 24 shows an exploded view of another existing dental attachment. Figure 25 shows a cross sectional view of the existing dental attachment of Figure 24. Figure 26 is a schematic drawing showing the magnetic circuit and the combination of permanent magnet and soft magnetic body in yet another existing dental attachment. Figure 27 shows the bottom view of the existing dental attachment of figure 26 used in the denture. Description of Numbered Items 1 magnet 2 magnet 3 cap 4 bridge 5 case 10 central yoke 109 central attracting face 11 outer yoke 119 first attracting face 12 outer yoke 129 second attracting face 21, 22 spacer 51, 52 case part 101 tooth root 102 root surface 103 keeper 110 denture 111 dental attachment 112 magnet in the existing attachment 113 spacer in the existing attachment 114,115 yokes in the existing attachment 116 cap in the existing attachment T direction of thickness The Best Mode of the Invention Embodiments of the dental attachment will be described below with figures. (Embodiment 1) A cross sectional view of the dental attachment in embodiment 1 is shown in figure 1, and a horizontal sectional view in figure 2. This attachment has a central yoke 10 in which one of the faces parallel to the direction of thickness T is the magnetic attracting face 109 to the keeper 103. The magnet 1 having its magnetization direction parallel to the direction of the thickness T is placed in contact with the central yoke 10. The magnet 2 having its magnetization direction parallel to the direction of the thickness T is placed in contact with another face of tne central yoke 10 so that the like poles of the magnet 1 and the magnet 2 face each other. The outer yoke 11 is placed in contact with another face of the magnet 1, which is the opposite face to the one in contact with the central yoke 10, and one of its faces parallel to the direction of the thickness T forms the attracting face 119 to the keeper 103. The outer yoke 12 is also placed in contact with another face of the magnet 2, which is the opposite face to the one in contact with the central yoke 10, and one of its faces parallel to the direction of the thickness T forms the attracting face 129 to the keeper 103. The central yoke 10, the magnet 1, the magnet 2, the outer yoke 11, another outer yoke 12 are enclosed in the cap 3 made of non-magnetic alloy. The face to the keeper 103, namely, central attracting face 109, the attracting face 119, and the attracting face 129 are exposed at the opening of the cap 3. The attracting faces of the magnet 1 and the magnet 2 with the keeper 103 are covered with spacer 21, 22 made of non-magnetic alloy. The boundary of the attracting face on the cap 3, spacer 21, 22, central yoke 10, the outer yoke 11, and another outer yoke 12 are welded hermetically by electron beam welding or laser welding so that saliva does not come inside the cap 3 and corrode the magnet. (Table 1) In the present embodiment 1, one of the magnetic circuits is formed in the magnet 1, the central yoke 10, the keeper 103, and the outer yoke 11. Similarly the other of the magnetic circuits is formed in the magnet 2, the central yoke 10, the keeper 103, and the outer yoke 12. In order to pass the two magnetic fluxes in parallel, the central yoke 10 has 1.57 times larger thickness of that of outer yoke 11, 12. These two magnetic circuits give strong attractive force twice as much as the force of existing dental attachments. The obtained attractive force is 607 gf, larger than 500 gf necessary for dental attachment. H/S 1/2 , in which S denotes the area of attracting face and H denotes the height from the attracting face, equals to 0.45. It is below the value of 0.55 stipulated in the former section as a characteristic of the present invention. TH/S 1/2 , in which T denotes the total height of the keeper and the attachment, equals to 0.69, below the value of 0.75 stipulated in the former section as a characteristic of the present invention. (Embodiment 2) The attachment in the embodiment 2 has the same structure with the same material as in embodiment 1 as shown in table 2, however the dimensions of the parts are modified. By the modification the attractive force has increased to 618 gf although the cross sectional area also increased a little. H/S 1/2 and TH/S 1/2 of the embodiment 2 are equal to 0.42 and 0.47, respectively. (Table 2) (Embodiment 3) The attachment in the embodiment 3 has the same structure with the same material however the thickness is increased in order to decrease the height to 1.3 mm for the magnet 1 and the magnet 2 as shown in table 3. By the modification the attractive force showed 564 gf. H/S 1/2 and TH/S 1/2 of the embodiment 3 are equal to 0.37 and 0.58, respectively. (Table 3) (Embodiment 4) The attachment according to the present invention in embodiment 4 has a different structure from embodiment 1-3 described above. As shown by cross sectional view in figure 3, horizontal sectional view in figure 4, and exploded view in figure 5, the lateral face of the magnet 1, namely the face not in contact with the central yoke 10 nor the outer yoke 11, is covered by an annular case 5 made of corrosion resistant non-magnetic material. Similarly the lateral face of the magnet 2, namely the face not in contact with the central yoke 10 nor the outer yoke 12, is covered by a similar annular case 5 made of corrosion resistant non-magnetic material. The attachment does not have a case covering the magnet 1, magnet 2, and yokes 10, 11, 12, neither does it have spacers covering the attracting face of the magnets. The contacting part of case 5 and yokes 10, 11, 12 are welded hermetically by electron beam welding or laser welding so that saliva does not come inside and corrode the magnet. The case 5 consists of a U-shaped part 51 that encloses the magnet 1 or magnet 2 and a flat plate part 52 that covers the top face of the magnets as shown in figure 6. They are welded together hermetically by electron beam welding or laser welding. The structure of the case 5 is not restricted to the above mentioned one. Two U-shaped parts can be butt-welded together, or four flat plates can be welded at the ends to form the case, or one plate can be bent annularly and both ends are butt-welded together, or annular case can be formed as one body and the magnet is inserted in the opening. In any structure the case 5 should be made of as low permeability material as possible and thickness should be as thin as corrosion resistance and weldability permit in order to obtain a strong attractive force of magnetic attachment. The dimensions and magnetic characteristics and kinds of material of the parts used in the embodiment 4 are shown in table 4. There is no cap nor spacer as in the former embodiments, but case 5 is added. It showed 590 gf of attractive force in spite of decreased cross sectional area of 9.8 mm 2 , although the height is increased a little to 1.6 mm. H/S 1/2 and TH/S 1/2 of the embodiment 4 equal to 0.51 and 0.73, respectively. (Table 4) As a modification of embodiment 4, the outer upper side of the outer yoke 11 and the outer yoke 12 can be chamfered as in the cross sectional view shown in figure 7. Also as shown in figure 8 the top face of the central yoke 10 can have a V-shaped groove. The chamfering makes the magnetic flux flow smoother and enhances the attractive force. In addition the outer yoke 11 and the outer yoke 12 can have a groove on their sides as shown in figure 9 and figure 10. This groove plays the role of an anchor for the fixation of the attachment to the denture. Dental resin fills the groove and the attachment is fixed firmly on the denture. Stripes on the surface of the attachment are also effective for the fixation. Further, the four corners of the outer yoke 11 and the outer yoke 12 can be chamfered as shown in the horizontal view in figure 11. (Embodiments 5) The attachment in the embodiment 5 has the same structure and the same material with the same magnetic characteristics as in embodiment 1, however the outer yokes 11 and 12 are chamfered at the outer top side as shown in figure 12. By the modification the volume of the attachment is reduced, and fitting to the denture is improved. It has an attractive force of 603 gf, which compares favorably with that of 607 gf obtained by the embodiment 1. H/S 1/2 and TH/S 1/2 of the embodiment 3 are equal to 0.45 and 0.66, respectively. (Table 5) (Embodiment 6) The attachment in the embodiment 6 has the same structure and the same material with the same magnetic characteristics as embodiment 1. The specifications are shown in table 6. The central yoke 10 is chamfered at the top face and a bridge 4 is placed to enhance the fixation with the denture as shown in figure 13. The bridge can be a plurality of stick-shaped bridges or a plate with a plurality of punched holes. The central yoke is so chamfered that it makes the magnetic flux flow smoother. The attractive force showed 601 gf. H/S 1/2 and TH/S 1/2 of the embodiment 6 are equal to 0.45 and 0.66, respectively. (Table 6) (Embodiment 7) The attachment in the embodiment 7 uses different materials for the magnet 1, the magnet 2, spacers 21, 22 and cap 3. It showed a maximum attractive force of 666 gf. H/S 1/2 and TH/S 1/2 of the embodiment 7 are equal to 0.45 and 0.69, respectively. (Table 7) (Embodiment 8) The attachment in the embodiment 8 has the same structure and dimensions as in embodiment 1, however iron-cobalt alloy (a typical commercial alloy is Permendur) with the saturation magnetization of 24000 G is used for the material of the yokes 10, 11 and 12 as shown in table 8. By the modification the attractive force increased drastically to 1017 gf, the strongest in the embodiments disclosed in this application. H/S 1/2 and TH/S 1/2 of the embodiment 3 are equal to 0.45 and 0.63, respectively. In this embodiment the attracting face of the attachment is coated with high chromium-content alloy layer with the thickness of 20 micro-meters to protect the alloy from corrosion. (Table 8) (Embodiment 9) The attachment in the embodiment 9 has the same structure and dimensions as in embodiment 1, however pure iron with the saturation magnetization of 22000 G and permeability of 10000 is used for the material of the yokes 10, 11, 12 and the keeper 103 as shown in table 9. By the modification the attractive force showed 818 gf, the next strongest in the embodiments disclosed in this application. H/S 1/2 and TH/S 1/2 of the embodiment 3 are equal to 0.45 and 0.63, respectively. In this embodiment the attracting face of the attachment is coated with high chromium content alloy layer with the thickness of 20 micro-meters to protect the alloy from corrosion. (Table 9) The embodiments described above have rectangular horizontal cross section as shown in figure 2. However the design can be modified so that it can fit to the denture. For example chamfering the corner as shown in figure 14, round shape as shown in figure 15, or polygon, ellipse, pseudo-circle can be adopted for the cross sectional shape of the attachment. Similarly the three-dimensional shape can be modified. For example the corner of the both side yokes 11, 12 are cut away to have the cross sectional shape of a triangle. Trapezoid or tapered shape of the attachment is also acceptable. (Embodiment 10) A cross sectional view of the dental attachment in embodiment 10 is shown in figure 19, and a horizontal sectional view in figure 20. This attachment has two central yokes 10 in which one of the faces parallel to the direction of thickness T is the magnetic attracting face 109 to the keeper 103. The central magnet 100 having its magnetization direction parallel to the direction of the thickness T is sandwiched by two central yokes 10 from both faces. The magnet 1 having its magnetization direction parallel to the direction of the thickness T is placed in contact with the other face of central yoke 10 so that the like poles of the magnet 1 and the central magnet 100 face each other. The magnet 2 having its magnetization direction parallel to the direction of the thickness T is placed in contact with another central yoke 10 so that the like poles of the magnet 2 and the central magnet 100 face each other. The outer yoke 11 is placed in contact with another face of the magnet 1, which is the opposite face to the one in contact with the central yoke 10, and one of its faces parallel to the direction of the thickness T forms the attracting face 119 to the keeper 103. The outer yoke 12 is also placed in contact with another face of the magnet 2, which is the opposite face to the one in contact with the central yoke 10, and one of its faces parallel to the direction of the thickness T forms the attracting face 129 to the keeper 103. The central yokes 10, the magnet 1, the magnet 2, the outer yoke 11, and the outer yoke 12 are enclosed in the cap 3 made of non-magnetic alloy. The face to the keeper 103, namely, central attracting face 109, the attracting face 119, and the attracting face 129 are exposed at the opening of the cap 3. The attracting faces of the magnet 1 and magnet 2 with the keeper 103 are covered with spacer 21, 22 made of non-magnetic alloy. The boundary of the attracting face of the cap 3, spacer 21, 22, central yoke 10, the outer yoke 11, and the outer yoke 12 are welded hermetically by electron beam welding or laser welding so that saliva does not come inside the cap 3 and corrode the magnet. The shape, magnetic properties and materials for the parts of the dental attachment in embodiment 10, namely the central magnet 100, the magnets 1, 2, the outer yokes 11, 12, the central yoke 10, 10, spacer 21, 22, 23, the cap 3, and keeper 103 are shown in table 17. (Table 10) In the present embodiment 10, the first magnetic circuit is formed in the magnet 1, one of the central yoke 10, the keeper 103, and the outer yoke 11. Similarly the second magnetic circuit is formed in the central magnet 100, one of the central yoke 10, the keeper 103, and the other central yoke 10. Similarly the third magnetic circuit is formed in the magnet 2, one of the central yoke 10, the keeper 103, and the outer yoke 12. In order to pass the two magnetic fluxes in parallel, the central yoke 10, 10 have 1.66 times larger thickness of that of the outer yoke 11 or outer yoke 12. These three magnetic circuits give a strong attractive force and require less height for the attachment. The obtained attractive force is 515 gf, larger than 500 gf necessary for dental attachment, with the height of only 1.03 mm. H/S 1/2 , in which S denotes the area of attracting face and H denotes the height from the attracting face, equals to 0.31. It is below the value of 0.55 stipulated in the former section as a characteristic of the present invention. TH/S 1/2 , in which T denotes the total height of the keeper and the attachment, equals to 0.44, and below the value of 0.75 stipulated in former section as a characteristic of the present invention. (Embodiment 11) The attachment in embodiment 11 has a similar structure to embodiment 4. It consists of the magnet 1, the magnet 2, the central yoke 10, the outer yoke 11, and the outer yoke 12 as shown in the cross sectional view in figure 21, without the cap and spacers covering the attracting face of the magnets. It uses Nd14Fe77B8, a magnet with very high maximum energy product, as the magnet material similar to embodiment 7 in order to reduce the volume. In other words; the present embodiment applies a combination of the structure of the embodiment 4 and the magnet material of the embodiment 7. Because of the structure and the material, the obtained attractive force is 620 gf, larger than 500 gf necessary for dental attachment, without increasing the number of the magnet more than 3. It also achieves the height of only 1.2 mm. H/S 1/2 equals to 0.39, below the value of 0.55 stipulated in former section as a characteristic of the present invention. TH/S 1/2 equals to 0.58, below the value of 0.75 stipulated in former section as a characteristic of the present invention. (Table 11) Next, one comparative example and four existing examples are described to show the superiority of the present invention. (Comparative example 1) The cross sectional view of the dental attachment in comparative example 1 is shown in figure 22. It is prepared to compare the dental attachment in embodiment 11 with two magnets to the existing dental attachment with one magnet. The comparative example is designed to have the attractive force of 620 gf, same as the one in the embodiment 11. Dimensions of the parts of the comparative example 1 are shown in table 12. The height of the attachment is 2.4 mm and the thickness of the keeper is 1.2 mm as shown in table 12. H/S 1/2 equals to 0.78, larger than the value of 0.55 below which is a characteristic of the present invention. TH/S 1/2 is 1.16, larger than the value of 0.75 below which is another characteristic of the present invention. As seen from comparison for the dental attachment in comparative example 1 and the embodiment 11, the present invention gives much smaller height and smaller total height indicated by H/S 1/2 and TH/S 1/2 for a given attractive force. (Existing example 1) This example is the one found in the market currently. Hereafter it is referred to as attachment A. This existing attachment consists of a block of magnet 112, a spacer 113 made of corrosion resisting non-magnetic alloy and covering one surface of the magnet, a pair of yokes 114, 115 that are made of corrosion resisting soft magnetic alloy and sandwich the magnet 112 and the spacer 113 in between them, and a cap 116 made of corrosion resisting non-magnetic alloy which covers all other parts. It has a structure of a magnet sandwiched by two yokes. The dimension and material used in each part is shown in table 13. It has a height of 2.5 mm, a cross sectional area of 12.0 mm 2 . Its height is much larger compared to the average height of 1.5 mm for the present embodiments although the attractive force of 605 gf is similar to them. The height limits the application of the attachment. Its H/S 1/2 and TH/S 1/2 equal to 0.72 and 1.01 respectively and considered to be rather large. (Table 13) (Existing example 2) This example is another one found in the market currently. Hereafter it is referred as attachment B. This existing attachment has a similar structure to existing example 1 except reduced height of 1.5 mm as shown in table 14. Because of the reduced height, the attractive force is decreased to 400 gf, which is not sufficient as a dental attachment. (Table 14) (Existing example 3) This example is yet another one found in the market currently. Hereafter it is referred as attachment C. The dimension and material used in each part is shown in table 15. This example has a particular structure in which a cylindrical magnet is placed so that one pole is facing to the keeper and the magnetic flux generated at the other pole is lead by a cup-shaped yoke to the keeper. It shows only 352 gf of attractive force in spite of a rather large cross sectional area of 12.6 mm 2 . It is inferior even to the existing example 2 described before. (Table 15) (Existing example 4) The dental attachment as existing example 4 has a structure of a magnet sandwiched by two yokes. As dimension and material used in each part is shown in table 16, it has the same cross sectional area and the same material for the parts of the present embodiment 1. The magnet has a thickness equal to the total of the magnet 1 and the magnet 2 in the embodiment 1. Thus it gives a comparative example for the present invention utilizing multiple pieces of magnet. The example has only 335 gf of attractive force, far below that obtained by the present invention. It is because the example has only one magnetic circuit, while the present invention has two or more magnetic circuits. (Table 16) In table 17 the attractive force (gf), attractive force per unit volume F/V (gf/mm 2 ), the height (mm), and cross sectional area (mm 2 ) of the attachment are shown for all of the eleven embodiments, one comparative example and four existing examples. (Table 17) The embodiment 8 is the most superior one in both the attractive force and the attractive force per unit volume. The embodiment 9 has the second best performance following to the embodiment 8 in spite of using inexpensive pure iron without using the special alloy such as iron-cobalt-vanadium. The embodiment 3 has showed that the height can be reduced to 1.3 mm, the embodiment 4 has showed that the cross sectional area can be reduced to 9.8 mm 2 to obtain the attractive force of more than 500 gf, a requirement for a dental attachment. The embodiment 10 has showed that the height can be even reduced by using three pieces of magnet. In the embodiment 11 with very small height, a value of H/S 1/2 of 0.39 and of TH/S 1/2 of 0.58 with the attractive force of 620 gf is obtained. For each embodiment, the value of H/S 1/2 is below 0.51, satisfying the stipulation for the present invention to be below H/S 1/2 of 0.55. The attractive force per unit volume for the embodiments of the present invention are shown in figure 16 in comparison with the existing examples. Evidently the present invention is quite superior over existing examples. In some embodiments above spacers 21, 22 are applied to protect the magnet from corrosion and wear. The inventors have investigated the effect of the spacer thickness on the attractive force by experiment. The results are shown in figure 17. The thickness should be reduced to obtain a strong attractive force. Also an experiment is carried out to investigate the effect of chamfering at the shoulder of the side yokes 11, 12 on the attractive force. As shown in figure 18, it is found out that the best result is obtained by a chamfered volume ratio of 0.1. The attractive force per unit volume increases at the beginning of chamfering. It may be due to the smoother flow of magnetic flux. The invented attachment offers a strong attractive force in the severe limit of volume, especially in the limit of the total height together with keeper, by applying a new structure of multiple magnetic circuits. It is shown numerically by comparison with existing example 1 (A) and the embodiment 8 that the height of the attachment is reduced from 2.5 mm to 1.5 mm, the thickness of the keeper is reduced from 1.0 mm to 0.8 mm, and the total height of the attachment and the keeper is reduced from 3.5 mm to 2.3 mm, less than 2/3. Furthermore, the cross sectional area is reduced from 12.0 mm 2 to 11.0 mm 2 , and the attractive force is increased to the double, from 605 g to 1017 g. For the attractive force per unit volume the performance is improved by three times. Thus it is shown that the present invention is far superior to the former inventions. Embodiment 1 Part dimension(mm) magnetic property material magnetic attachment (omit keeper) height H cross sectional area S - - 1.5 11.0(mm 2 ) H/S 1/2 =0.45 the first magnet height width thickness maximum energy product rare earth magnet the second magnet 1.2 3.0 0.45 30MGOe Sm2Co17 central yoke height width thickness saturation induction permeability soft magnetic alloy 1.4 3.0 1.1 16000G 4000 19Cr-2Mo steel first side yoke height width thickness saturation induction permeability soft magnetic alloy second side yoke 1.4 3.0 0.7 16000G 4000 19Cr-2Mo steel spacer height width thickness permeability non-magnetic alloy 0.2 3.0 0.45 1.02 17Cr-12Cr-2Mo steel cap cap thickness of cap permeability non-magnetic alloy 0.1 1.02 17Cr-12Cr-2Mo steel keeper thickness saturation induction permeability soft magnetic alloy 0.8 16000G 4000 19Cr-2Mo steel Embodiment 2 Part dimension(mm) magnetic property material magnetic attachment (omit keeper) height H cross sectional area S - - 1.5 12.8(mm 2 ) H/S 1/2 =0.42 the first magnet height width thickness maximum energy product rare earth magnet the second magnet 1.2 3.0 0.3 30MGOe Sm2Co17 central yoke height width thickness saturation induction permeability soft magnetic alloy 1.4 3.0 1.5 16000G 4000 19Cr-2Mo steel first side yoke height width thickness saturation induction permeability soft magnetic alloy second side yoke 1.4 3.0 0.95 16000G 4000 19Cr-2Mo steel spacer height width thickness permeability non-magnetic alloy 0.2 3.0 0.30 1.02 17Cr-12Cr-2Mo steel cap thickness of cap permeability non-magnetic alloy 0.1 1.02 17Cr-12Cr-2Mo steel keeper thickness saturation induction permeability soft magnetic alloy 0.9 16000G 4000 19Cr-2Mo steel Embodiment 3 Part dimension(mm) magnetic property material magnetic attachment (omit keeper) height H cross sectional area S - - 1.3 12.4(mm 2 ) H/S 1/2 =0.37 the first magnet height width thickness maximum energy product rare earth magnet the second magnet 1.2 3.0 0.65 30MGOe Sm2Co17 central yoke height width thickness saturation induction permeability soft magnetic alloy 1.2 3.0 1.2 16000G 4000 19Cr-2Mo steel first side yoke height width thickness saturation induction permeability soft magnetic alloy second side yoke 1.2 3.0 0.75 16000G 4000 19Cr-2Mo steel spacer height width thickness permeability non-magnetic alloy 0.2 3.0 0.65 1.02 17Cr-12Cr-2Mo steel cap thickness of cap permeability non-magnetic alloy 0.1 1.02 17Cr-12Cr-2Mo steel keeper thickness saturation induction permeability soft magnetic alloy 0.75 16000G 4000 19Cr-2Mo steel Embodiment 4 Part dimension(mm) magnetic property material magnetic attachment (omit keeper) height H cross sectional area S - - 1.6 9.8(mm 2 ) H/S 1/2 =0.51 the first magnet height width thickness maximum energy product rare earth magnet the second magnet 1.2 3.0 0.45 30MGOe Sm2Co17 central yoke height width thickness saturation induction permeability soft magnetic alloy 1.6 3.0 1.1 16000G 4000 19Cr-2Mo steel first side yoke height width thickness saturation induction permeability soft magnetic alloy second side yoke 1.6 3.0 0.7 16000G 4000 19Cr-2Mo steel case thickness of case width permeability non-magnetic alloy 0.2 3.0 1.02 17Cr-12Cr-2Mo steel keeper thickness saturation induction permeability soft magnetic alloy 0.7 16000G 4000 19Cr-2Mo steel Embodiment 5 Part dimension(mm) magnetic property material magnetic attachment (omit keeper) height H cross sectional area S - - 1.5 11.0(mm 2 ) H/S 1/2 =0.45 the first magnet height width thickness maximum energy product rare earth magnet the second magnet 1.2 3.0 0.45 30MGOe Sm2Co17 central yoke height width thickness saturation induction permeability soft magnetic alloy 1.4 3.0 1.1 16000G 4000 19Cr-2Mo steel first side yoke height width thickness saturation induction permeability soft magnetic alloy second side yoke 1.4 3.0 0.7 16000G 4000 19Cr-2Mo steel chamfered shoulder (figure 5) spacer height width thickness permeability non-magnetic alloy 0.2 3.0 0.45 1.02 17Cr-12Cr-2Mo steel cap thickness of cap permeability non-magnetic alloy 0.1 1.02 17Cr-12Cr-2Mo steel keeper thickness saturation induction permeability soft magnetic alloy 0.7 16000G 4000 19Cr-2Mo steel Embodiment 6 Part dimension(mm) magnetic property material magnetic attachment (omit keeper) height H cross sectional area S - - 1.5 11.0(mm 2 ) H/S 1/2 =0.45 the first magnet height width thickness maximum energy product rare earth magnet the second magnet 1.2 3.0 0.45 30MGOe Sm2Co17 central yoke height width thickness saturation induction permeability soft magnetic alloy 1.4 3.0 1.1 16000G 4000 19Cr-2Mo steel V-shaped groove on top, bridge-like part (figure 6) first side yoke height width thickness saturation induction permeability soft magnetic alloy second side yoke 1.4 3.0 0.7 16000G 4000 19Cr-2Mo steel spacer height width thickness permeability non-magnetic alloy 0.2 3.0 0.45 1.02 17Cr-12Cr-2Mo steel cap thickness of cap permeability non-magnetic alloy 0.1 1.02 17Cr-12Cr-2Mo steel keeper thickness saturation induction permeability soft magnetic alloy 0.7 16000G 4000 19Cr-2Mo steel Embodiment 7 Part dimension(mm) magnetic property material magnetic attachment (omit keeper) height H cross sectional area S - - 1.5 11.0(mm 2 ) H/S 1/2 =0.45 the first magnet height width thickness maximum energy product rare earth magnet the second magnet 1.2 3.0 0.45 35MGOe Nd14Fe77B8 central yoke height width thickness saturation induction permeability soft magnetic alloy 1.4 3.0 1.1 16000G 4000 19Cr-2Mo steel first side yoke height width thickness saturation induction permeability soft magnetic alloy second side yoke 1.4 3.0 0.7 16000G 4000 19Cr-2Mo steel spacer height width thickness permeability non-magnetic alloy 0.2 3.0 0.45 1.02 Ti cap thickness of cap permeability non-magnetic alloy 0.1 1.02 Ti keeper thickness saturation induction permeability soft magnetic alloy 0.8 16000G 4000 19Cr-2Mo steel Embodiment 8 Part dimension(mm) magnetic property material magnetic attachment (omit keeper) height H cross sectional area S - - 1.5 11.0(mm 2 ) H/S 1/2 =0.45 Cr diffusion layer of 20 µm thick at attracting face the first magnet height width thickness maximum energy product rare earth magnet the second magnet 1.2 3.0 0.45 30MGOe Sm2Co17 central yoke height width thickness saturation induction permeability soft magnetic alloy 1.4 3.0 1.1 24000G 9000 Fe-49Co-2V first side yoke height width thickness saturation induction permeability soft magnetic alloy second side yoke 1.4 3.0 0.7 24000G 9000 Fe-49Co-2V spacer height width thickness permeability non-magnetic alloy 0.2 3.0 0.45 1.02 17Cr-12Cr-2Mo steel cap thickness of cap permeability non-magnetic alloy 0.1 1.02 17Cr-12Cr-2Mo steel keeper thickness saturation induction permeability soft magnetic alloy 0.6 16000G 4000 19Cr-2Mo steel Embodiment 9 Part dimension(mm) magnetic property material magnetic attachment (omit keeper) height H cross sectional area S - - 1.5 11.0(mm 2 ) H/S 1/2 =0.45 Cr diffusion layer of 20 µm thick at attracting face the first magnet height width thickness maximum energy product rare earth magnet the second magnet 1.2 3.0 0.45 30MGOe Sm2Co17 central yoke height width thickness saturation induction permeability soft magnetic alloy 1.4 3.0 1.1 22000G 10000 pure iron first side yoke height width thickness saturation induction permeability soft magnetic alloy second side yoke 1.4 3.0 0.7 22000G 10000 pure iron spacer height width thickness permeability non-magnetic alloy 0.2 3.0 0.45 1.02 17Cr-12Cr-2Mo steel cap thickness of cap permeability non-magnetic alloy 0.1 1.02 17Cr-12Cr-2Mo steel keeper thickness saturation induction permeability soft magnetic alloy 0.6 22000G 10000 pure iron Embodiment 10 Part dimension(mm) magnetic property material magnetic attachment (omit keeper) height H cross sectional area S - - 1.03 11.0(mm 2 ) H/S 1/2 =0.31 the first magnet height width thickness maximum energy product rare earth magnet the second magnet 0.8 3.0 0.3 30MGOe Sm2Co17 central yoke height width thickness saturation induction permeability soft magnetic alloy 0.93 3.0 0.78 16000G 4000 19Cr-2Mo steel first side yoke height width thickness saturation induction permeability soft magnetic alloy second side yoke 0.93 3.0 0.47 16000G 4000 19Cr-2Mo steel spacer height width thickness permeability non-magnetic alloy 0.13 3.0 0.3 1.02 17Cr-12Cr-2Mo steel cap thickness of cap permeability non-magnetic alloy 0.1 1.02 17Cr-12Cr-2Mo steel keeper thickness saturation induction permeability soft magnetic alloy 0.45 16000G 4000 19Cr-2Mo steel Embodiment 11 Part dimension(mm) magnetic property material magnetic attachment (omit keeper) height H cross sectional area S - - 1.2 9.6(mm 2 ) H/S 1/2 =0.39 the first magnet height width thickness maximum energy product rare earth magnet the second magnet 1.0 2.6 0.6 35MGOe Nd14Fe77B8 central yoke height width thickness saturation induction permeability soft magnetic alloy 1.2 2.8 1.2 16000G 4000 19Cr-2Mo steel first side yoke height width thickness saturation induction permeability soft magnetic alloy second side yoke 1.2 2.8 0.6 16000G 4000 19Cr-2Mo steel annular case thickness of case width permeability non-magnetic alloy 0.1 2.8 1.02 17Cr-12Cr-2Mo steel keeper thickness saturation induction permeability soft magnetic alloy 0.6 16000G 4000 19Cr-2Mo steel Embodiment 12 Part dimension(mm) magnetic property material magnetic attachment (omit keeper) height H cross sectional area S - - 2.4 9.6(mm 2 ) H/S 1/2 =0.78 magnet height width thickness maximum energy product rare earth magnet 2.0 2.6 1.2 35MGOe Nd14Fe77B8 first side yoke height width thickness saturation induction permeability soft magnetic alloy second side yoke 2.4 2.8 1.2 16000G 4000 19Cr-2Mo steel annular case thickness of case width permeability non-magnetic alloy 0.2 2.8 1.02 17Cr-12Cr-2Mo steel keeper thickness saturation induction permeability soft magnetic alloy 1.2 16000G 4000 19Cr-2Mo steel Exsisting example 1 (A) Part dimension(mm) magnetic property material magnetic attachment (omit keeper) height H cross sectional area S - - 1.5 12.0(mm 2 ) H/S 1/2 =0.72 magnet height width thickness maximum energy product rare earth magnet 2.1 4.0 1.0 30MGOe Sm2Co17 first side yoke height width thickness saturation induction permeability soft magnetic alloy second side yoke 2.4 4.0 1.0 16000G 4000 19Cr-2Mo steel spacer height width thickness permeability non-magnetic alloy 0.3 4.0 1.0 1.02 17Cr-12Cr-2Mo steel cap thickness of cap permeability non-magnetic alloy 0.1 1.02 17Cr-12Cr-2Mo steel keeper thickness saturation induction permeability soft magnetic alloy 1.0 16000G 4000 19Cr-2Mo steel Exsisting example 2 (B) Part dimension(mm) magnetic property material magnetic attachment (omit keeper) height H cross sectional area S - - 1.5 12.0(mm 2 ) H/S 1/2 =0.43 magnet height width thickness maximum energy product rare earth magnet 1.1 4.0 1.0 30MGOe Sm2Co17 first side yoke height width thickness saturation induction permeability soft magnetic alloy second side yoke 1.4 4.0 1.0 16000G 4000 19Cr-2Mo steel spacer height width thickness permeability non-magnetic alloy 0.3 4.0 1.0 1.02 17Cr-12Cr-2Mo steel cap thickness of cap permeability non-magnetic alloy 0.1 1.02 17Cr-12Cr-2Mo steel keeper thickness saturation induction permeability soft magnetic alloy 0.8 16000G 4000 19Cr-2Mo steel Exsisting example 3 (C) Part dimension(mm) magnetic property material magnetic attachment (omit keeper) height H cross sectional area S - - 1.5 12.6(mm 2 ) H/S 1/2 =0.42 magnet height diameter maximum energy product rare earth magnet 1.0 3.0 20MGOe SmCo5 cup yoke height diameter saturation induction permeability soft magnetic alloy 1.5 0.5 11000G 4000 30Cr-2Mo steel disk yoke height diameter saturation induction permeability non-magnetic alloy 0.2 2.8 11000G 4000 17Cr-12Cr-2Mo steel shield ring height outer diameter inner diameter permeability non-magnetic alloy 0.2 3.0 2.6 1.02 17Cr-13Cr-2Mo steel keeper thickness saturation induction permeability soft magnetic alloy 1.0 11000G 4000 30Cr-2Mo steel Exsisting example 4 Part dimension(mm) magnetic property material magnetic attachment (omit keeper) height H cross sectional area S - - 1.5 11.0(mm 2 ) H/S 1/2 =0.45 magnet height width thickness maximum energy product rare earth magnet 1.2 3.0 0.9 30MGOe Sm2Co17 first side yoke height width thickness saturation induction permeability soft magnetic alloy second side yoke 1.4 3.0 0.7 16000G 4000 19Cr-2Mo steel spacer height width thickness permeability non-magnetic alloy 0.2 3.0 0.9 1.02 17Cr-12Cr-2Mo steel cap thickness of cap permeability non-magnetic alloy 0.1 1.02 17Cr-12Cr-2Mo steel keeper thickness saturation induction permeability soft magnetic alloy 0.8 16000G 4000 19Cr-2Mo steel Dimensions and Performances for the embodiments, the comparative example, the existing examples attractive force (gf) F/V (gf/mm 3 ) dental attachment height H (mm) keeper height (mm) total height (mm) cross sectional area (mm 2 ) H/S 1/2 TH/S 1/2 Embodiment 1 607 36.9 1.5 0.8 2.3 11.0 0.45 0.69 Embodiment 2 618 32.3 1.5 0.9 2.4 12.8 0.42 0.67 Embodiment 3 564 35.1 1.3 0.75 2.05 12.4 0.37 0.58 Embodiment 4 590 40.1 1.6 0.7 2.3 9.8 0.51 0.73 Embodiment 5 603 36.5 1.5 0.7 2.2 11.0 0.45 0.66 Embodiment 6 601 36.4 1.5 0.7 2.2 11.0 0.45 0.66 Embodiment 7 666 40.4 1.5 0.8 2.3 11.0 0.45 0.69 Embodiment 8 1017 61.6 1.5 0.6 2.1 11.0 0.45 0.63 Embodiment 9 818 49.6 1.5 0.6 2.1 11.0 0.45 0.63 Embodiment 10 515 53.3 1.03 0.45 1.48 11.2 0.31 0.44 Embodiment 11 620 53.8 1.2 0.6 1.8 9.6 0.39 0.58 Comparative Example 620 26.9 2.4 1.2 3.6 9.6 0.78 1.16 Exisiting example 1 605 20.2 2.5 1.0 3.5 12.0 0.72 1.01 Exisiting example 2 400 22.2 1.5 0.8 2.3 12.0 0.43 0.66 Exisiting example 3 352 18.6 1.5 1.0 2.5 12.6 0.42 0.7 Exisiting example 4 335 20.3 1.5 0.8 2.3 11.0 0.45 0.69

Claims: A dental magnet attachment, which can be embedded in the denture base so as to face a soft magnetic keeper (103) which can be embedded in the top of the root surface, and attract the keeper by magnetic force, comprising: a plate-shape central yoke (10) which is made of soft magnetic material, and has a central magnetic attracting face to the keeper (103) being formed by one face parallel to its thickness direction; a plate-shape first magnet (1) having its magnetization direction parallel to its thickness direction and placed in contact with one side surface of the central yoke; a plate-shape second magnet (2) having its magnetization direction parallel to its thickness direction and placed in contact with the other side face of the central yoke (10), the same poles of the first magnet (1) and the second magnet (2) facing each other relative to the central yoke (10); a first side yoke (11) made of soft magnetic material and placed in contact with the other side face of the first magnet (1) which is opposite to the one side face in contact with the central yoke (10), and one face parallel to its thickness direction forming the attracting face to the keeper (103); and a second side yoke (12) made of soft magnetic material and placed in contact with the other side face of the second magnet (2) which is opposite to the one side face in contact with the central yoke (10), and one face parallel to the its thickness direction forming the attracting face to the keeper (103), wherein lateral faces of the first magnet (1) and second magnet (2) which are not in contact with the central yoke (10), first side yoke (11) and the second side yoke (12) are covered with first annular case (5) and second annular case (5) respectively made of corrosion resistant non-magnetic material, the first annular case (5) and second annular case (5) are connected with the central yoke (10), the first side yoke (11) and the second side yoke (12), respectively, the first magnetic circuit passing through the first magnet (1), the central yoke (10), the keeper (103) and first side yoke (11) is formed, and the second magnetic circuit passing through the second magnet (2), the central yoke (10), the keeper (103) and the second side yoke (12) is formed, the thickness of the central yoke (10) is larger than the thickness of the first and second side yokes (11, 12), and the ratio of H/S 1/2 defined by the height H of the first magnet (1) and the second magnet (2), and the area S of attracting face S is not more than 0.55, and the attractive force is not less than 4,9035 N (500 gf). A dental magnetic attachment as set forth in claim 1, wherein portions of the first side yoke (11) and the second side yoke (12) remote from the keeper (103) has smaller thickness than portions of the first side yoke (11) and the second yoke near (12) to the keeper. A dental magnetic attachment as set forth in claim 1, wherein the cross sectional shape parallel to the attracting face is polygon, ellipse or pseudo-circle. A dental magnetic attachment a set forth in claim 1, wherein a portion of the central yoke (10) remote from the keeper (103) is provided with a V-shaped groove in its center. A dental magnetic attachment as set forth in claim 1, wherein, the first and second side yokes (11, 12) have a groove on their outer faces as an anchor for the fixation of the attachment to the denture. A dental magnetic attachment as set forth in claim 1, wherein the area of the attracting face is not more than 20,0 mm 2 . A dental magnet attachment, which can be embedded in the denature base so as to face a soft magnetic keeper (103) which can be embedded in the top of the root surface, and attract the keeper by magnetic force, comprising: plate-shape central first yoke (10) and central second yoke (10) which are made of soft magnetic material, having respectively a central first and second magnetic attracting face to the keeper (103) being formed by one face parallel to the thickness direction of the central first yoke (10) and the central second yoke (10); a central magnet (100) placed between the central first yoke (10) and the central second yoke (10), and having its magnetization direction parallel to its thickness direction; a plate-shape first side magnet (1) having its magnetization direction parallel to its thickness direction and placed in contact with one side surface of the central first yoke (10); a first side yoke (11) made of soft magnetic material and placed in contact with the other face of the first magnet (1) which is opposite to the one side face in contact with the central first yoke (10), and one face parallel to its thickness direction forming the attracting face to the keeper (103); a plate-shape second side magnet (2) having its magnetization direction parallel to its thickness direction and placed in contact with the other side face of the central second yoke (10); a second side yoke (12) made of soft magnetic material and placed in contact with the other side face of the second side magnet (2) which is opposite to the one side face in contact with the central second yoke (10), and one face parallel to its thickness direction forming the attracting face to the keeper (103), wherein the lateral faces of the central magnet (100), the first side magnet (1) and second side magnet (2) which are not in contact with the central first yoke (10) and the central second yoke (10), first side yoke (11) and the second side yoke (12) are covered with respectively annular central case, annular first side case and annular second side case made of corrosion resistant non-magnetic material, the central case, first side case and second side case are connected with the central first yoke (10) and the central second yoke (10), the first side yoke (11) and the second side yoke (12), the first magnetic circuit passing through the central magnet (100), central first yoke (10), keeper (103) and central second yoke (10) being formed, the second magnetic circuit passing through the first side magnet (1), the central first yoke (10), the keeper (103) and first side yoke (11) being formed, and the third magnetic circuit passing the second side magnet (2), the central second yoke (10), the keeper (103) and second side yoke (12) is formed, the thickness of the central first and second yokes (10) is larger than the thickness of the first and second side yokes, and the ratio of H/S 1/2 defined by the height H of the central magnet (100), first side magnet (1) and second side magnet (2) and the area S of attracting face S is not more than 0,55, and the attractive force is not less than 4,9035 N (500 gf). A dental magnetic attachment as set forth in claim 7, wherein the central case is welded to the central first yoke (10) and the central second yoke (10), the first side case is welded to the first side yoke (11) and the central first yoke (10), the second side case is welded to the second side yoke (12) and the central second yoke (10). A dental magnetic attachment as set forth in claim 7, wherein the first and the second side yokes (11, 12) have a groove on each outer side as an anchor for the fixation of attachment to the denture. A dental magnetic attachment as set forth in claim 7, wherein the area of the attracting face is not more than 20.0 mm 2 . A dental magnet attachment, which can be embedded in the denture base so as to face a soft magnetic keeper (103) which can be embedded in the top of the root surface, and attract the keeper by magnetic force, comprising: at least three plate-shape yokes (10, 11, 12) made of soft magnetic material, and attracting surfaces to the keeper being formed by faces parallel to the thickness direction of the yokes; at least two plate-shape magnets (1, 2, 100) having a magnetization direction parallel to their thickness direction and placed in contact with the adjacent yokes, opposing poles of the magnets sandwiching the yoke having same polarity, wherein the lateral face of each of the magnets (1, 2, 100) not contacting with the yokes is covered by an annular case (5) made of anti-resistant non-magnetic material, each of the annular cases (5) is connected to the yokes, each of the magnets forming a magnet circuit passing through the magnet, one yoke contacted with the magnet, the keeper and the other yoke contacted with the magnet, and the thickness of a central yoke sandwiched by the magnets is larger than the thickness of the side yokes contacted with one side face of the magnet. A dental magnetic attachment as set forth in claim 11, where the case is welded with the central yoke (10) and first and second side yokes (11, 12). A dental magnetic attachment as set forth in claim 11, wherein the first and second side yokes (11, 12) have a groove on each outer side as an anchor for the fixation of the attachment to the denture. A dental magnetic attachment as set forth in claim 11, wherein the area of the attracting face is not more than 20.0 mm 2 .

Inventors: FUJII HIDEKI, HONKURA YOSHINOBU, TANAKA YOSHINOBU, YOKOYAMA TAKASHI, FUJII, HIDEKI, HONKURA, YOSHINOBU, TANAKA, YOSHINOBU, YOKOYAMA, TAKASHI, FUJII, Hideki Aichi Steel Works, Ltd., HONKURA, Yoshinobu Aichi Steel Works, Ltd., TANAKA, Yoshinobu Aichi Steel Works, Ltd., YOKOYAMA, Takashi Aichi Steel Works, Ltd.
Applicants: AICHI STEEL WORKS LTD, AICHI STEEL WORKS, LTD.
Application year: 1995
Main classification: A61C13
IPC codes: A61C13, A61C8