In accordance with another embodiment of the invention, an implant system includes an implantable hearing device according to one of the preceding embodiments and an external device including signal processing circuitry configured to transmit an implant communications signal to the implantable hearing device. The external device includes an innermost surface adapted to lie adjacent to the overlying skin and an external magnet or magnet assembly in the external device to be located over the overlying skin and magnetically configured to cooperate with the implant magnet of the implantable hearing device so as to hold the external device against the overlying skin.

The external magnet may be similar to the implant magnet, but may also be of a different design. In particular, since the external device can be taken off prior to an MRI procedure, the external magnet does not have to be specifically designed to cope with strong external MRI magnetic fields. Accordingly, the external magnet may be an arrangement of two magnets having magnetic dipole moments arranged perpendicular to the innermost surface of the external device, and hence to the skin, with their north and south poles arranged adjacent to the south and north poles of the implant magnet, respectively.

In some embodiments, the external magnet or magnet arrangement may have a north magnetic pole, a south magnetic pole, and may further as a whole have an overall magnetic dipole moment that is parallel to or at an angle of 30° or less with respect to the innermost surface of the external device. The external magnet may be of a similar design as disclosed in one of the embodiments with respect to the internal magnet above. The external magnet may have a north end portion including the north magnetic pole and a south end portion including the south magnetic pole, the north and south end portions each being formed from permanent magnetic material and each having an individual magnetic dipole moment that is inclined with respect to the overall magnetic dipole moment of the external magnet. The individual magnetic dipole moment in the north end portion may have a component pointing towards the innermost surface of the external device, and the individual magnetic dipole moment in the south end portion may have a component pointing away from the innermost surface of the external device.

In some embodiments, the external magnet is rotatable around a rotation axis that is perpendicular to the innermost surface of the external device or deviates from perpendicular by less than 30°, wherein in each available rotational position of the external magnet upon rotation around the rotation axis, the overall magnetic dipole moment is parallel to or at an angle of 30° or less with respect to the innermost surface, wherein the external magnet has a shape that is rotationally symmetric around its rotation axis. The external magnet may have a planar inner end surface facing the innermost surface of the external device.

In some embodiments, the external magnet may have an average diameter d_Ein a direction parallel to the overall magnetic dipole moment and an average thickness h_Ein a direction perpendicular to the innermost surface of the external device, wherein in one or both of the north and south end portions, the individual magnetic dipole moment is inclined with respect to the overall magnetic dipole moment by an angle α, wherein

arctan
⁡
(

h
E

/

(

d
E

/
2

)

)

-

15
⁢
°

≤
α
≤

arctan
⁡
(

h
E

/

(

d
E

/
2

)

)

+

7
⁢
°

,
preferably

arctan
⁡
(

h
E

/

(

d
I

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2

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)

-

10
⁢
°

≤
α
≤

arctan
⁡
(

h
E

/

(

d
E

/
2

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)

+

5
⁢

°
.

The advantages of these angular ranges are similar to those explained above with reference to the implant magnet. Similar to the case of the implant magnet, the angle α is measured in a plane that is perpendicular to the innermost surface of the external device, or in other words, in a plane that is at least approximately perpendicular to the skin.

In some embodiments, the north and south end portions of the external magnet are directly adjacent with each other, and in particular each form one of two halves of the external magnet. Alternatively, the north and south end portions of the external magnet are separated from each other by an intermediate portion having an individual magnetic dipole moment that is parallel to the overall magnetic dipole moment of the external magnet or deviates from parallel by less than 10°, preferably less than 5°.

In some embodiments, one or both of the north and south end portions of the implant magnet may have an inner section closer to the innermost surface of the external device and an outer section further away from the innermost surface of the external device, wherein an angle of inclination of the individual magnetic dipole moment with respect to the overall magnetic dipole moment in the inner section is less than in the outer section. The external magnet may have an inner end surface facing the innermost surface of the external device and an outer end surface facing away from the innermost surface of the external device. A middle plane is defined to be located at equal distance from the outer and inner end surfaces of the external device. The external magnet may fulfil one or both of the following criteria (i) and (ii):

- (i) at least 55%, preferably at least 65% of the total magnetic flux of a magnetic field generated outside of the external magnet when placed in isolation in air or a vacuum is located on a side of the middle plane at which the innermost surface of the external device is located in the assembled state,
- (ii) more than 50%, preferably more than 55% of the mass of the external magnet is located on a side of the middle plane at which the innermost surface is located, wherein in particular, the edges of the external magnet at the inner end surface are chamfered.

In some embodiments, the north and south end portions of the external magnet are formed from anisotropic magnet elements each having a preferred magnetization direction, wherein the anisotropic magnet elements are joined with each other or with an intermediate portion arranged in between, wherein the preferred magnetization directions are arranged at an angle with respect to the overall dipole moment of the external magnet as a whole. The external magnet may be a rare earth magnet, e.g., a rare earth magnet including neodymium, such as neodymium-ion-boron magnets, and/or including samarium, such as samarium-cobalt magnets, and/or including terbium, dysprosium, holmium or combinations thereof.

Embodiments of the present invention also include a hearing implant system containing a magnet arrangement according to any of the foregoing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows portions of a typical cochlear implant system and the magnetic interaction between the implant magnet and the external implant magnet.

FIG. 2 illustrates the force interactions that can occur between an implant magnet and the applied external magnetic field for an MRI system.

FIG. 3A shows an example of a cylindrical implant magnet with a V-shaped magnetic angle according to an embodiment of the present invention.

FIG. 3B shows an example of a cochlear implant device with two implant magnets of the type shown in FIG. 3A.

FIGS. 4A and 4B show how the magnetic fields of the implant magnets align to cooperate with an external holding magnet.

FIGS. 5A and 5B show how the magnetic fields of the implant magnets align in the presence of an MRI magnetic field.

FIGS. 6A and 6B show an example of an embodiment with a diametrically magnetized supplemental cylindrical magnet located between the cylindrical magnets.

FIGS. 7A and 7B show an example of an embodiment with soft magnetic material located between the cylindrical magnets.

FIG. 8 shows an embodiment of cylindrical implant magnets that are configured to be limitedly rotatable through a limited rotation range according to an embodiment of the present invention.

FIGS. 9A-9C show orientations of the cylindrical implant magnets for different orientations of an external MRI magnetic field.

FIGS. 10A-10C show orientations of the cylindrical implant magnets for reverse magnetization m a reduced rotatability embodiment of the present invention.

FIG. 11 shows a further example of a cochlear implant device using an implant magnet with north and south end portions having individual magnetic dipole moments that are inclined with respect to the overall magnetic at the moment of the implant magnet.

FIG. 12 is a perspective view of an implant magnet of the type used in the device of FIG. 11.

FIG. 13 is a side view of the device of FIG. 11.

FIG. 14 is a schematic sectional view showing an implant device and an external device with corresponding magnets.

FIG. 15 is a schematic sectional view showing magnets of an implant and an external device.

FIG. 16 illustrates the direction of the magnetic flux generated by an ordinary implant magnet that is homogeneously magnetized.

FIG. 17 illustrates the direction of the magnetic flux generated by an implant magnet having north and south end portions having individual magnetic dipole moments that are inclined with respect to the overall magnetic dipole moment of the implant magnet according to an embodiment of the present invention.

FIG. 18 illustrates the flux density of the ordinary implant magnet of FIG. 16.

FIG. 19 illustrates the flux density of the implant magnet of FIG. 17 according to an embodiment of the present invention.

FIG. 20 shows the magnetic flux density generated upon the interaction of a homogeneously magnetized external magnet and the ordinary implant magnet of FIG. 16.

FIG. 21 shows the magnetic flux density generated upon the interaction of homogeneously magnetized external magnet and the implant magnet of FIG. 17 according to an embodiment of the present invention.

FIG. 22 is a schematic sectional view of an implant magnet and an external magnet according to a further embodiment of the invention.

FIG. 23 is a schematic view illustrating the torque applied by an external magnetic field to a conventional implant magnet and an implant magnet according to an embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

A larger distance between the two magnetic poles has the advantage that the attractive magnetic force for an external magnet does not decrease so steeply with increasing distance between the magnets. Embodiments of the present invention are directed to an improved implant magnet arrangement that uses two cylindrical implant magnets with magnetization direction having a V-shaped magnetic angle. These magnets are mounted in the implant device such that a “strong” side (i.e. the side with high magnetic flux) faces at least partly towards the overlying skin. Both magnets are mounted inside a magnet case in which they can turn also around the rotation axis of the case.

FIG. 3A shows an example of a cylindrical implant magnet 300 with a V-shaped magnetic angle α according to an embodiment of the present invention, and FIG. 3B shows an example of a cochlear implant device 305 with two implant magnets 300 of the type shown in FIG. 3A. The cochlear implant device 305 contains signal processing circuitry (not shown) configured for receiving an implant communications signal transmitted from an external transmitting coil through overlying skin of an implanted patient, and an outermost surface 308 adapted to lie between the overlying skin and underlying skull bone of the implanted patient.

There is a magnet case 306 within the implant device 305 with a case rotation axis 307 that is perpendicular to the outermost surface 308 of the implant device 305. The magnet case 306 is configured to be rotatable about the case rotation axis 307. Typically, the magnet case 306 is surrounded by a receiver coil of the implant device 305. The magnet case 306 may be metallic (e.g. made of titanium), or it may be made of a biocompatible non-metallic material (e.g. PEEK, FEP, PTFE, PSU, etc.) and may be coated (e.g. with Parylene). The magnet case 306 may be adapted to facilitate long-term hermetic encapsulation, and/or it may be adapted to be surgically removable for minimized susceptibility to MRI artefacts.

An implant magnet arrangement includes one or a plurality of cylindrical magnets 300 located within the magnet case 306 and configured to cooperate with a corresponding external holding magnet in an external device located over the overlying skin to magnetically hold the external device against the overlying skin. Each cylindrical implant magnet 300 has a center cylinder axis 301 that is perpendicular to the case rotation axis 307, and each cylindrical magnet 300 is configured to be rotatable about its center cylinder axis 301.

Each cylindrical magnet 300 has an outer cylindrical surface 302 with a north magnetic pole and a south magnetic pole. In the most general sense, a “cylindrical surface” is a surface consisting of all the points on all the lines which are parallel to a reference line and which pass through a fixed plane curve in a plane not parallel to the given line. In the present disclosure, the cylinder is a so-called right circular cylinder, in which the “fixed plane curve” is a circle, and the reference line is a line that is perpendicular to circle plane, for example the center cylinder axis 301. A north magnetic direction 303 is defined by a radial vector extending from the center cylinder axis 301 to the north magnetic pole. And a similar south magnetic direction 304 is defined by a radial vector extending from the south magnetic pole to the center cylinder axis 301. The north magnetic pole and the south magnetic pole are arranged with respect to each other so as not to lie on a common diameter through the center cylinder axis 301 such that the north magnetic direction 303 and the south magnetic direction 304 form a “magnetic angle” a that is less than 180 degrees with a vertex at the center cylinder axis 301. For example, the magnetic angle α may specifically be between 90° and 140° (or some other defined range). Such magnetic angle α can be established for example by forming the cylindrical magnet 300 from two preformed portions 309 and 310 which are magnetized according to the above-mentioned north and south magnetic directions 303, 304 in a manner that will be explained with reference to a further embodiment in more detail below. In the embodiment shown in FIG. 3, the preformed portions 309, 310 each correspond to a longitudinal half of the full cylindrical magnet 300 which are attached to each other along an interface indicated by the dashed line 311 in FIG. 3A.

FIGS. 4A and 4B show how the magnetic fields of the cylindrical magnets 300 align to cooperate with one or more external holding magnets 403 in an external device 402 where the cylindrical magnets 300 magnetically align with respect to each other to create a common line of magnetic flux through the magnets, the magnet case, and the overlying skin to cooperate with the external holding magnet 403. Since the two cylindrical magnets 300 are arranged closely together (e.g., less than 2 mm between their outer cylindrical surfaces 302), the two adjacent magnetic poles form an attractive magnetic connection with a common magnetic direction that is parallel to the outer surface 308 of the implant device 305 and the overlying skin 401. Due to the V-shaped magnetic angle, the magnetic direction of the non-adjacent halves of the two cylindrical magnets 300 guides the magnetic flux towards the outer surface 308 of the implant device 305 and the overlying skin 401, thereby allowing a strong magnetic attraction with the external holding magnet 403 that is almost as strong as with older-type axially magnetized implant magnets that have a magnetization direction that is also normal to the skin 401.

FIGS. 5A and 5B show how the cylindrical magnets 300 align in the presence of an MRI magnetic field 501. As can be seen, the two cylindrical magnets 300 immediately align relative to the external magnetic field 501 so that the torque generated by the respective magnetizations of the different halves of each magnet 300 interacting with the external magnetic field 501 cancels out. In FIG. 5B to 7B, the magnetization is schematically represented by the hatched lines.

FIGS. 6A and 6B show an example of an embodiment with a diametrically magnetized supplemental cylindrical magnet 601 that is located between the cylindrical magnets 300. The supplemental cylindrical magnet 601 is configured to couple the common line of magnetic flux between the cylindrical magnets 300. This provides an increased distance between the two local maxima of the magnetic flux through the skin 401 thus improving the magnetic attraction to the external magnet 403.

FIGS. 7A and 7B show an example of an embodiment with soft magnetic material 701 that is located between the cylindrical magnets 300 to couple the common line of magnetic flux between the cylindrical magnets 300.

In the embodiments described above, the cylindrical magnets 300 are configured to be fully rotatable about the center cylinder axis 301 through a complete rotation range of 360 degrees, and so that the magnet case 306 containing the cylindrical magnets 300 can turn around its case axis 307. Otherwise, when the center cylinder axis 301 were fixed and a strong external magnetic field 501 is oriented anti-parallel to the cylindrical magnets 300, the magnets would flip by 180° and the magnetically-strong side of the magnets would then face towards the underlying skull in a medial direction instead of towards the skin in lateral direction. Note that the orientation of the external magnetic field 501 in a given MRI scanner is different when an implant user is scanned with the head first versus with legs first. Accordingly, it is advantageous if the magnet arrangement can handle both of these orientations of the external magnetic field 501. Moreover, there is no general convention of the orientation of the external magnetic field 501 in MRI scanners, and in some cases where two MRI scanners are arranged next to each other in the same facility, the orientations of the respective external magnetic fields 501 are even deliberately chosen to be of opposite orientation.

Still, a further design variant with two cylindrical magnets in V-shaped magnetization also works when the magnets have only one degree of freedom and have a rotation angle limited to about 90° only. FIG. 8 shows an embodiment of cylindrical implant magnets 800 that are configured to be limitedly rotatable through a limited rotation range 801 of less than 180 degrees according to another embodiment of the present invention. For example, the cylindrical implant magnets may have a V-shaped magnetization with a magnetic angle between 100° and 140°, and the limited rotation range 801 may be 90°.

FIGS. 9A-9C show orientations of the limited rotation range cylindrical implant magnets 800 for different orientations of an external MRI magnetic field 501. As long as the strong external magnetic field 501 has a component oriented parallel to the overall magnetization of the cylindrical implant magnets 800, the magnets behave the same as with the other embodiments described above.

FIGS. 10A-10C show orientation of the limited range cylindrical implant magnets for reverse magnetization m a reduced rotatability embodiment of the present invention. When a strong external magnetic field is oriented antiparallel to the overall magnetization of the cylindrical implant magnets 800, they cannot turn by roughly 180° to align to be parallel with the external magnetic field. Instead, each implant magnet 800 reverses its magnet polarity.

In the embodiments described above, despite some individual parts of the implant magnet being oriented perpendicular to the skin surface, the magnets do not weaken in an MRI environment because they immediately turn into a safe orientation relative to the strong static magnetic field of the MRI scanner. Each individual magnet always has a component parallel to the strong static magnetic field of the MRI scanner. And so each individual magnet always aligns such that there is no torque to the outside. The magnetic flux is directed to the skin side and reduced in medial direction. Therefore, the MRI artefact reaches less into the medial direction and is more oriented towards the skin side.

Shown in FIG. 11 is an example of a further implant device, in the specific embodiment a cochlear implant device 905 that is generally similar to the cochlear implant device 305 of FIG. 3B. The cochlear implant device 905 contains signal processing circuitry (not shown) configured for receiving an implant communications signal transmitted from an external device, such as an external device as shown under reference sign 402 in FIGS. 4B and 6A through overlying skin 401 of an implanted patient.

The implant device 905 includes an outermost surface 908 adapted to lie between the overlying skin 401 and underlying skull bone and is at least approximately parallel to the skin 401 of the implanted patient. The implant device 905 further comprises an implant magnet 900 configured to cooperate with an external holding magnet in an external device 402 to be located over the overlying skin 401 to magnetically hold the external device against the overlying skin 401.

As indicated in FIG. 11 to 13, the implant magnet 900 has a north magnetic pole, a south magnetic pole, and as a whole has an overall magnetic dipole moment m that is parallel to the outermost surface 908 and hence parallel to the skin 401 (not shown in FIG. 11 to 13). The overall magnetic dipole moment m need not be precisely parallel to the outermost surface 908 (the skin 401), but should be at least approximately parallel to it, for example form an angle of 30° or less, preferably 20° or less with respect to said outermost surface 908.

The implant magnet 900 is rotatable around a rotation axis 907 that in the embodiment shown is perpendicular to said outermost surface 908 so that in each available rotational position of said implant magnet 900 upon rotation around its rotation axis 907, the overall magnetic dipole moment m is parallel to said outermost surface 908.

As is seen in FIGS. 12 and 13, the implant magnet 900 has an outer end surface 912 facing said outermost surface 908 of said implant device 905 (i.e. towards the skin 401 in the implanted state), and an inner end surface 913 facing away from said outermost surface 908 (i.e. towards the inside of a patient head in the implanted state). Both of the outer and inner end faces 912, 913 are planar surfaces and are perpendicular to said rotation axis 907. For example, the implant magnet 900 may have a cylindrical disc shape, in which the side face is formed by a cylindrical surface, as shown in FIG. 12. However, the side face need not be precisely cylindrical, but could have a slightly conical shape, as shown in FIG. 13. It is however preferred that said implant magnet 900 has a shape that is rotationally symmetric around said rotation axis 907.

Moreover, the implant magnet 900 has a north end portion 914 including said north magnetic pole and a south end portion 915 including said south magnetic pole. Both of said north and south end portions 914, 915 are formed from permanent magnetic material and each have an individual magnetic dipole moment 916, 917 that is inclined with respect to said overall magnetic dipole moment m, as will be explained in more detail with reference to FIG. 14.

FIG. 14 again schematically shows an implant device 905 in a sectional view including an implant magnet 900, as well as an external device 955 comprising an external magnet 950. By magnetic interaction between the internal and external magnets 905, 950, the external device 955 may be attached to the skin 401 of a patient. The internal magnet 900 is comprised of two halves, one being formed by the north end portion 914 and the other by the south end portion 915. The hatched lines with arrowheads indicate the local magnetization M. It is seen that the local magnetizations in the north and south end portions 914, 915 have deviating directions, thereby leading to what was referred to as the “magnetic angle” with reference to the first embodiment of the invention above. Each of the north and south end portions 914, 915 has an individual magnetic dipole moment 916, 917, which corresponds to the space integral over the magnetization M in the respective portion. It is then seen that although the overall dipole moment m of the implant magnet 900 as a whole is directed parallel to the outermost surface 908 of the implant device 905, each of the individual magnetic dipole moments 916, 917 are inclined with respect to the overall dipole moment m.

More precisely, it is seen that the magnetic dipole moment 916 in said north end portion 914 is inclined in a plane perpendicular to the outermost surface 908/skin 401, to have a component pointing towards said outermost surface 908, and said individual magnetic dipole moment in said south end portion 915 is inclined to have a component pointing away from said outermost surface. This leads to a situation where the bigger part of the magnetic flux B generated by said implant magnet 901 is located to the outside, where it is needed for generating a holding force holding the external magnet 950.

In particular, in preferred embodiments, when simply regarding the internal magnet 900 in isolation, i.e. without presence of the external magnet 950 and when placed in air or vacuum, at least 55%, preferably at least 65% or even 70% or more of the total magnetic flux B of a magnetic field generated outside of the implant magnet 900 is located “outside” of a middle plane arranged at equal distances from the outer and inner end surfaces 912, 913. Herein, “outside of the middle plane” means the side at which said outermost surface 908 is located in the assembled state. This way, the holding force can be significantly increased over and implant magnet of the same size and material which would be homogeneously magnetized parallel to the skin 401.

In the embodiment of FIG. 14, the implant magnet 900 has a diameter d_Iin a direction parallel to the overall magnetic dipole moment and a thickness h_Iin a direction perpendicular to said outermost surface. In view of this geometry, in both of said north and south end portions 914, 915, said individual magnetic dipole moment 916, 917 is inclined with respect to said overall magnetic dipole moment m by an angle α=arctan (h_I/(d_I/2)). Herein, the angle α is measured with respect to a plane that is parallel to the outermost surface 908. This choice for the angle α is large enough for providing a significant improvement in attraction force with the external magnet 950 over a prior art implant magnet of same size and material that would be homogeneously magnetized parallel to the skin 401. A significantly larger angle α has been found to be less favorable, for two reasons. First, for larger angles α, the distance between the north and south poles would decrease, which in turn would lead to an excessive decrease in holding force with increasing distance from the implant magnet 900. Secondly, the angle α of inclination between each individual magnetic dipole moment 916, 917 in said north and south end portions 914 and 915 with respect to the overall magnetic dipole moment m should generally be no more than 60°, and with a certain safety margin, preferably no more than 50°, such as to avoid a situation in which the implant magnet 900 could be inadvertently weakened or demagnetized in a strong external MRI field in case the patient does not hold his or her head straight during the MRI procedure, but for example tilted to one side by e.g. up to 30°. Preferred ranges for the angle α are arctan (h_I/(d_I/2))−15°≤α≤arctan (h_I/(d_I/2))+7°, more preferably arctan (h_I/(d_I/2))−10°≤α≤arctan (h_I/(d_I/2))+5°, provided that in each case, preferably α≤50°.

Note that the external magnet 950 in the external device 955 of the embodiment of FIG. 14 has a similar structure than the internal magnet 905. That is to say, the external magnet 950 has an overall magnetic dipole moment m that is parallel to the innermost surface 958 of the external device 955 that lies adjacent to the skin 401. The external magnet 950 likewise has a north end portion 964 and a south end portion 965 in which the respective individual magnetic dipole moment 966, 967 is inclined with respect to the overall dipole moment m, such that the individual magnetic dipole moment 966 in said north end portion 964 has a component pointing towards the innermost surface 958 of the external device 955, and such that the individual magnetic dipole moment 967 in said south end portion 965 has a component pointing away from said innermost surface 958 of the external device 955. However, while the implant magnet 905 has been specifically devised for compatibility with an MRI external magnetic field, the external device 955 can be taken off the cochlear implant user prior to the MRI procedure, such that it does not need to employ a similar design. In particular, the external magnet 950 need not be rotatable, and it is also not necessary that its overall magnetic dipole moment m is parallel to the innermost so face 958 (skin 401). Nevertheless, in preferred embodiments, the design of the external magnet 950 is similar to that of the internal magnet 905, incorporating some or all of the features that were described in the above summary of embodiments of the invention.

FIG. 15 shows an embodiment which is similar to that of FIG. 14, with the main difference that the edges of the implant magnet 905 at the inner end surface 913 are chamfered. This allows for an improved magnetic flux, and further allows for concentrating more than half of the magnet mass towards the outside of the middle plane, i.e. on a side of said middle plane at which said outermost surface 908 is located.

FIG. 16 illustrates the direction of the magnetic flux generated by an ordinary implant magnet that is homogeneously magnetized. In contrast to this, FIG. 17 illustrates the direction of the magnetic flux generated by an implant magnet 900 of the type shown in FIG. 11 to FIG. 14 having north and south end portions 914, 915 with individual magnetic dipole moments 916, 917 that are inclined with respect to the overall magnetic dipole moment of the implant magnet 900.

FIG. 18 illustrates the flux density of the ordinary implant magnet of FIG. 16. In other words, while FIG. 16 only shows the direction of the magnetic flux, FIG. 18 indicates the flux density, which is represented by the size of the arrows displayed in the diagram. It is seen that both, the magnetic flux direction and the magnetic flux density are mirror symmetric with respect to the aforementioned middle plane of the magnet, which is a consequence of the homogeneous magnetization. FIG. 19 shows the flux density of the implant magnet 900 according to an embodiment of the invention of FIG. 17. It is seen that in the implant magnet 900 of this embodiment, the poles are shifted “upwards” in the representation of FIG. 19, such that they are located above the middle plane. Indeed, as the skilled person will appreciate, the poles correspond to the regions where the flux density at the surface of the implant magnet 900 is the highest, and these locations are found at the upper left and right corners in the representation of FIG. 19, which is hence consistent with what is schematically shown in FIGS. 14 and 15. Moreover, it is seen that of the total magnetic flux of the magnetic field generated outside the implant magnet 900 when taken in isolation, the bigger part is located above the middle plane. Accordingly, the magnetic field generated by the implant magnet