Retentive force and magnetic flux leakage of magnetic attachment in various keeper and magnetic assembly combinations

The Journal of Prosthetic Dentistry

267April 2011

Hasegawa et alHasegawa et al

Clinical ImplicationsRetentive force depends on keeper size rather than the potential mag-netic strength of cup-yoke-type magnetic assemblies. For effective use, the size of the magnet-keeper combination should be the same, and there should be a complete junction between the keeper and yoke.

Statement of problem. Magnetic attachments are commonly used for overdentures. However, it can be difficult to identify and provide the same type and size of magnetic assembly and keeper if a repair becomes necessary. Therefore, the size and type may not match.

Purpose. This study evaluated the retentive force and magnetic flux strength and leakage of magnetic attachments in different combinations of keepers and magnetic assemblies.

Material and methods. For 6 magnet-keeper combinations using 4 sizes of magnets (GIGAUSS D400, D600, D800, and D1000) (n=5), retentive force was measured 5 times at a crosshead speed of 5 mm/min in a universal testing machine. Magnetic flux strength was measured using a Hall Effect Gaussmeter. Data were statistically analyzed using a 1-way ANOVA, and between-group differences were analyzed with Tukey’s HSD post hoc test (α=.05).

Results. The mean retentive force of the same-size magnet-keeper combinations was 3.2 N for GIGAUSS D400 and 5.1 N for GIGAUSS D600, but was significantly reduced when using larger magnets (P<.05). Magnetic flux leakage was significantly lower for corresponding size combinations.

Conclusions. Size differences influence the retentive force and magnetic flux strength of magnetic attachments. Re-tentive force decreased due to the closed field structure becoming incomplete and due to magnetic field leakage. (J Prosthet Dent 2011;105:266-271)

Retentive force and magnetic flux leakage of magnetic attachment in various keeper and magnetic assembly combinations

Mikage Hasegawa, DDS,a Yoshitada Umekawa, DDS, PhD,b Eiich Nagai, DDS, PhD,c and Tomohiko Ishigami, DDS, PhDd

Nihon University School of Dentistry, Tokyo, Japan

Supported by a Grant-in-Aid for Scientific Research (21592475) from JSPP and by the Sato Fund, Nihon University School of Dentistry.

aPostgraduate student, Department of Partial Denture Prosthodontics.bAssistant Professor, Department of Partial Denture Prosthodontics.cAssistant Professor, Department of Partial Denture Prosthodontics.dAssistant Professor, Department of Partial Denture Prosthodontics.

Dental magnetic attachments of various types and sizes that have satis-factory retentive force and stability are now commercially available. An over-denture with a magnetic attachment is a useful choice for an abutment tooth with chronic periodontal disease, be-cause the magnetic attachment dissi-pates the lateral stress component on the abutment teeth and improves poor clinical crown-to-root ratios. Magnet-

ic attachments are useful in not only prosthodontics but also maxillofacial prosthetics. Their use has been de-scribed in several publications.1-8

From among the various sizes and types of magnetic attachments available, magnetic attachment size is usually selected according to the cross-section of the retained root.4-8 Most commercially available mag-netic attachments consist of 2 com-

ponents, a keeper, which is generally made from stainless steel, and a cor-responding magnetic assembly, which is composed of a magnet and yoke made from ferromagnetic material1-8

(Fig. 1).Conventional overdenture place-

ment involves embedding the mag-netic assembly in the denture base and inserting its corresponding keep-er into the abutment root. The mag-

netic assembly holds the keeper with a retentive force.4,7 However, space is required for the magnetic assembly, which may result in thin denture base areas surrounding the assembly. The magnetic assembly works as a fulcrum or supporting point and, therefore, long-term use of a magnetic overden-ture often requires repair of the mag-netic assembly or that a replacement be inserted due to fracture.9-11 When this occurs, it can be difficult to iden-tify the type and size of the existing magnetic assembly and keeper, which makes it possible to insert the correct system.

Recently, concerns have been ex-pressed about the potential health risks of exposure to magnetic fields.12 In 2009, the International Commission on Non-Ionizing Radiation Protection (ICNIRP) revised the guidelines for ex-posure to static magnetic fields.12,13 At present, while no deleterious effects on human health due to exposure to magnetic flux have been substanti-ated, there is a need to understand the extent of magnetic field exposure in the oral area resulting from the use of magnetic attachments.12-16

The method for measuring mag-netic flux distribution involves the placement of a suitable hall probe sensor with a gauss meter on or near the portion of the specimen that is ex-pected to generate the greatest mag-netic flux density. Accordingly, this study sought to evaluate the influence of differences in size of the keeper and magnetic assembly on magnetic field

exposure and retentive force. The null hypothesis tested was that size differ-ences between the keeper and mag-netic assembly do not influence the magnetic field exposure or retentive force of the magnetic attachment.

MATERIAL AND METHODS

The magnetic attachments used in this study are presented in Table I. Two sizes of cylindrical keepers, D400 and D600 and 4 sizes of cylindrical magnetic assemblies, D400, D600, D800, and D1000 were assessed. The structure of the magnetic attachment is shown in Figure 1. This magnetic as-sembly is a cup-yoke type of magnetic assembly, which is surrounded by a thin layer of yoke made of super ferrit-ic stainless steel (UNS S44627).2 The D400 keeper was tested in combina-tion with 3 different sizes of magnetic assemblies, namely, D400, D600, and D800, and the D600 keeper was also tested in combination with 3 different sizes of magnetic assemblies, namely, D600, D800, and D1000. The com-binations tested and the groups as-signed are shown in Table II. Hereafter, the combinations tested are designat-ed by abbreviating the keeper to ‘K’ and magnetic assembly size to ‘M’; for example, the K4M4 combination represents the D400 keeper and D400 magnetic assembly. The combination of D400 keeper and D1000 magnetic assembly was not tested because the D400 keeper is too small to mistake it for the D1000 magnetic assembly in

clinical practice. The combination of the D600 keeper and D400 magnetic assembly was also excluded because the paired fitting of the larger keeper with the smaller magnet would not af-fect retentive force and magnetic flux leakage, because the closed magnetic circuit between keeper and magnet is completed.

Retentive force was measured in a universal testing machine (EZ-Test; Shimadzu Co, Kyoto, Japan) using a retentive force testing jig (K797-01; Tokyo Giken, Inc, Tokyo, Japan); for 6 types of magnet-keeper combina-tions at a crosshead speed of 5 mm/min. The vertical retentive force test-ing jig consisted of a linear ball slide (LSP 1390; THK America, Inc, Scha-umburg, Ill) and a universal joint to regulate traction in the perpendicular direction (Fig. 2). The jig was installed in a universal testing machine. Rect-angular parallelepiped acrylic resin blocks (15 x 15 x 20 mm) were set in the bottom and traction side of the jig. The keeper and magnetic assembly were attached with cyanoacrylate ad-hesive (Aron Alpha; Toagosei Co, To-kyo, Japan) in the center of the acrylic resin block. Without allowing for any space between the keeper and mag-net assembly, the magnetic assembly was held in place by its force of at-traction. The magnetic retentive force of each attachment was measured by attaching the magnetic assembly to the keeper and then dislodging it. Each magnet-keeper combination in a group was tested 5 times, and the mean values were compared.

Magnetic flux leakage was mea-sured using a Gaussmeter (F.W. Bell 5180; Sypris Test and Measurement, Orlando, FL) and dedicated measur-ing probe (STB1X-0201; Sypris Test and Measurement, Orlando, Fla) Tektronix Services Solutions. Thirty measurements (6 groups x 5 mea-surements) (Table II), were made at 3 points: P1, beside the keeper; P2, at the bottom of the keeper; and P3, at the outside interface of the keeper and magnetic assembly (Fig. 3). The probe has an active area that is lo-

1 Structure of magnetic attachment (GIGAUSS D) used.


267April 2011


Clinical ImplicationsRetentive force depends on keeper size rather than the potential mag-netic strength of cup-yoke-type magnetic assemblies. For effective use, the size of the magnet-keeper combination should be the same, and there should be a complete junction between the keeper and yoke.

Statement of problem. Magnetic attachments are commonly used for overdentures. However, it can be difficult to identify and provide the same type and size of magnetic assembly and keeper if a repair becomes necessary. Therefore, the size and type may not match.

Purpose. This study evaluated the retentive force and magnetic flux strength and leakage of magnetic attachments in different combinations of keepers and magnetic assemblies.

Material and methods. For 6 magnet-keeper combinations using 4 sizes of magnets (GIGAUSS D400, D600, D800, and D1000) (n=5), retentive force was measured 5 times at a crosshead speed of 5 mm/min in a universal testing machine. Magnetic flux strength was measured using a Hall Effect Gaussmeter. Data were statistically analyzed using a 1-way ANOVA, and between-group differences were analyzed with Tukey’s HSD post hoc test (α=.05).

Results. The mean retentive force of the same-size magnet-keeper combinations was 3.2 N for GIGAUSS D400 and 5.1 N for GIGAUSS D600, but was significantly reduced when using larger magnets (P<.05). Magnetic flux leakage was significantly lower for corresponding size combinations.

Conclusions. Size differences influence the retentive force and magnetic flux strength of magnetic attachments. Re-tentive force decreased due to the closed field structure becoming incomplete and due to magnetic field leakage. (J Prosthet Dent 2011;105:266-271)

Retentive force and magnetic flux leakage of magnetic attachment in various keeper and magnetic assembly combinations

Mikage Hasegawa, DDS,a Yoshitada Umekawa, DDS, PhD,b Eiich Nagai, DDS, PhD,c and Tomohiko Ishigami, DDS, PhDd

Nihon University School of Dentistry, Tokyo, Japan

Supported by a Grant-in-Aid for Scientific Research (21592475) from JSPP and by the Sato Fund, Nihon University School of Dentistry.

aPostgraduate student, Department of Partial Denture Prosthodontics.bAssistant Professor, Department of Partial Denture Prosthodontics.cAssistant Professor, Department of Partial Denture Prosthodontics.dAssistant Professor, Department of Partial Denture Prosthodontics.

Dental magnetic attachments of various types and sizes that have satis-factory retentive force and stability are now commercially available. An over-denture with a magnetic attachment is a useful choice for an abutment tooth with chronic periodontal disease, be-cause the magnetic attachment dissi-pates the lateral stress component on the abutment teeth and improves poor clinical crown-to-root ratios. Magnet-

ic attachments are useful in not only prosthodontics but also maxillofacial prosthetics. Their use has been de-scribed in several publications.1-8

From among the various sizes and types of magnetic attachments available, magnetic attachment size is usually selected according to the cross-section of the retained root.4-8 Most commercially available mag-netic attachments consist of 2 com-

ponents, a keeper, which is generally made from stainless steel, and a cor-responding magnetic assembly, which is composed of a magnet and yoke made from ferromagnetic material1-8

(Fig. 1).Conventional overdenture place-

ment involves embedding the mag-netic assembly in the denture base and inserting its corresponding keep-er into the abutment root. The mag-

netic assembly holds the keeper with a retentive force.4,7 However, space is required for the magnetic assembly, which may result in thin denture base areas surrounding the assembly. The magnetic assembly works as a fulcrum or supporting point and, therefore, long-term use of a magnetic overden-ture often requires repair of the mag-netic assembly or that a replacement be inserted due to fracture.9-11 When this occurs, it can be difficult to iden-tify the type and size of the existing magnetic assembly and keeper, which makes it possible to insert the correct system.

Recently, concerns have been ex-pressed about the potential health risks of exposure to magnetic fields.12 In 2009, the International Commission on Non-Ionizing Radiation Protection (ICNIRP) revised the guidelines for ex-posure to static magnetic fields.12,13 At present, while no deleterious effects on human health due to exposure to magnetic flux have been substanti-ated, there is a need to understand the extent of magnetic field exposure in the oral area resulting from the use of magnetic attachments.12-16

The method for measuring mag-netic flux distribution involves the placement of a suitable hall probe sensor with a gauss meter on or near the portion of the specimen that is ex-pected to generate the greatest mag-netic flux density. Accordingly, this study sought to evaluate the influence of differences in size of the keeper and magnetic assembly on magnetic field

exposure and retentive force. The null hypothesis tested was that size differ-ences between the keeper and mag-netic assembly do not influence the magnetic field exposure or retentive force of the magnetic attachment.

MATERIAL AND METHODS

The magnetic attachments used in this study are presented in Table I. Two sizes of cylindrical keepers, D400 and D600 and 4 sizes of cylindrical magnetic assemblies, D400, D600, D800, and D1000 were assessed. The structure of the magnetic attachment is shown in Figure 1. This magnetic as-sembly is a cup-yoke type of magnetic assembly, which is surrounded by a thin layer of yoke made of super ferrit-ic stainless steel (UNS S44627).2 The D400 keeper was tested in combina-tion with 3 different sizes of magnetic assemblies, namely, D400, D600, and D800, and the D600 keeper was also tested in combination with 3 different sizes of magnetic assemblies, namely, D600, D800, and D1000. The com-binations tested and the groups as-signed are shown in Table II. Hereafter, the combinations tested are designat-ed by abbreviating the keeper to ‘K’ and magnetic assembly size to ‘M’; for example, the K4M4 combination represents the D400 keeper and D400 magnetic assembly. The combination of D400 keeper and D1000 magnetic assembly was not tested because the D400 keeper is too small to mistake it for the D1000 magnetic assembly in

clinical practice. The combination of the D600 keeper and D400 magnetic assembly was also excluded because the paired fitting of the larger keeper with the smaller magnet would not af-fect retentive force and magnetic flux leakage, because the closed magnetic circuit between keeper and magnet is completed.

Retentive force was measured in a universal testing machine (EZ-Test; Shimadzu Co, Kyoto, Japan) using a retentive force testing jig (K797-01; Tokyo Giken, Inc, Tokyo, Japan); for 6 types of magnet-keeper combina-tions at a crosshead speed of 5 mm/min. The vertical retentive force test-ing jig consisted of a linear ball slide (LSP 1390; THK America, Inc, Scha-umburg, Ill) and a universal joint to regulate traction in the perpendicular direction (Fig. 2). The jig was installed in a universal testing machine. Rect-angular parallelepiped acrylic resin blocks (15 x 15 x 20 mm) were set in the bottom and traction side of the jig. The keeper and magnetic assembly were attached with cyanoacrylate ad-hesive (Aron Alpha; Toagosei Co, To-kyo, Japan) in the center of the acrylic resin block. Without allowing for any space between the keeper and mag-net assembly, the magnetic assembly was held in place by its force of at-traction. The magnetic retentive force of each attachment was measured by attaching the magnetic assembly to the keeper and then dislodging it. Each magnet-keeper combination in a group was tested 5 times, and the mean values were compared.

Magnetic flux leakage was mea-sured using a Gaussmeter (F.W. Bell 5180; Sypris Test and Measurement, Orlando, FL) and dedicated measur-ing probe (STB1X-0201; Sypris Test and Measurement, Orlando, Fla) Tektronix Services Solutions. Thirty measurements (6 groups x 5 mea-surements) (Table II), were made at 3 points: P1, beside the keeper; P2, at the bottom of the keeper; and P3, at the outside interface of the keeper and magnetic assembly (Fig. 3). The probe has an active area that is lo-

1 Structure of magnetic attachment (GIGAUSS D) used.

268 Volume 105 Issue 4


269April 2011

Hasegawa et al Hasegawa et al

2 Retentive force testing jig in universal testing machine. A, Universal joint. B, Linear ball slide. C, Universal testing machine. D, Acrylic resin block with keeper. E, Acrylic resin block with magnetic assembly.

Table I. Magnetic attachment investigated

Table II. Groups for combinations and dimensions of keeper and magnetic assembly (n=5)

Magneticassembly

Keeper

Nd-B-Fe magnet　

Ferritic stainless steel(UNS S44627)


Material

Nd, B, Co, Fe,Dy, V, Nb, Al, K

Fe, Cr, Mo, Si, Mn

Fe, Cr, Mo, Si, Mn

GIGAUSS D400 GIGAUSS D600

GIGAUSS D800GIGAUSS D1000

GIGAUSS D400 GIGAUSS D600GIGAUSS D800

GIGAUSS D1000

Composition ManufacturerBrand Name and Lot

802081804141

804141804141

802081804141804141804141

Number

Magnet

YokeShield disk

Component

K4M4

K4M6

K4M8

K6M6

K6M8

K6M10

3.0

3.0

3.0

3.6

3.6

3.6

(mm)Diameter

3.0

3.6

4.2

3.6

4.2

4.9

(mm)Diameter

0.6

0.6

0.6

0.7

0.7

0.7

(mm)Thickness

1.3

1.3

1.3

1.3

1.3

1.3

(mm)Thickness

D400

D600

D800

D600

D800

D1000

AssemblyMagnetic

D400

D400

D400

D600

D600

D600

KeeperGroups

cated 0.3 mm from the tip surface, and the measurement was performed when the probe was in contact with the specimen (Fig. 4).

Statistical analysis was performed using statistical software (SPSS v12 for Windows; SPSS, Inc, Chicago, Ill). Data were analyzed with a 1-way analysis of variance (ANOVA), and differences between the groups were analyzed with Tukey’s Honestly Signif-icant Difference (HSD) post hoc test (α=.05). The relationship between retentive force and maximum mag-netic flux leakage (P1) was assessed using Pearson’s correlation coefficient (α=.05).

RESULTS

The mean force was 5.1 ±0.17 N for K6M8, 3.61 ±0.20 N for K6M8, and 3.51 ±0.10 N for K6M10 (Fig. 5). As the difference in size between the keeper and magnetic assembly in-creased, the retentive force decreased, although there was no significant dif-ference between K6M8 and K6M10 (P<.05).

For the D400 keeper combina-tions, the mean force of K4M4 was the highest (3.2 ±0.05 N), and was significantly different (F=63, df=2, P<.05) from the K4M6 and K4M8 groups. K4M6 (2.65 ±0.10 N) also demonstrated a significantly higher

retentive force than K4M8 (2.7 ±0.10 N) (P<.001, P<.05). For the D600 keeper combinations, K6M6 showed significantly higher retentive force (F=148.81, df=2, P<.05) than the oth-er groups (P<.001, P<.05).

The results for magnetic flux leak-age are shown in Figure 6. All groups showed significantly higher magnetic flux leakage at P1 than at P2 and P3 (F=799, df=2, P<.05) (P=4.2 x 10-7, P<.05). Significantly reduced mag-netic flux leakage was observed for the same-size keeper and magnetic assembly combinations of D400 and D600, whereas it was significantly increased for the other different-size combinations (P<.05).

3 Measurement points for magnetic flux strength.

5 Mean retentive force results for each group. Bars indicate SDs and bars with same low-ercase letter are not significantly different from the mean values of each group (P>.05).

4 Details of locations measured by Gaussmeter probe, with active area 0.3 mm from tip surface. B represents magnetic flux density.

Magneticassembly

Keeper P3

P2P1

Probe lateral view

Probe top view

Active area

Active area

0.76 mm0.064 mm

0.30 mm

1.27 mm

+B

6

Rel

ativ

e Fo

rce

(N)

Groups

5

4

3

2

1

0K4M4

3.23

K4M6

2.65

K4M8

2.38

K6M6

5.10

K6M8

3.61

K6M10

3.51a a



269April 2011

Hasegawa et al Hasegawa et al

2 Retentive force testing jig in universal testing machine. A, Universal joint. B, Linear ball slide. C, Universal testing machine. D, Acrylic resin block with keeper. E, Acrylic resin block with magnetic assembly.

Table I. Magnetic attachment investigated

Table II. Groups for combinations and dimensions of keeper and magnetic assembly (n=5)

Magneticassembly

Keeper

Nd-B-Fe magnet　



Material

Nd, B, Co, Fe,Dy, V, Nb, Al, K

Fe, Cr, Mo, Si, Mn

Fe, Cr, Mo, Si, Mn

GIGAUSS D400 GIGAUSS D600

GIGAUSS D800GIGAUSS D1000

GIGAUSS D400 GIGAUSS D600GIGAUSS D800

GIGAUSS D1000

Composition ManufacturerBrand Name and Lot

802081804141

804141804141

802081804141804141804141

Number

Magnet

YokeShield disk

Component

K4M4

K4M6

K4M8

K6M6

K6M8

K6M10

3.0

3.0

3.0

3.6

3.6

3.6

(mm)Diameter

3.0

3.6

4.2

3.6

4.2

4.9

(mm)Diameter

0.6

0.6

0.6

0.7

0.7

0.7

(mm)Thickness

1.3

1.3

1.3

1.3

1.3

1.3

(mm)Thickness

D400

D600

D800

D600

D800

D1000

AssemblyMagnetic

D400

D400

D400

D600

D600

D600

KeeperGroups

cated 0.3 mm from the tip surface, and the measurement was performed when the probe was in contact with the specimen (Fig. 4).

Statistical analysis was performed using statistical software (SPSS v12 for Windows; SPSS, Inc, Chicago, Ill). Data were analyzed with a 1-way analysis of variance (ANOVA), and differences between the groups were analyzed with Tukey’s Honestly Signif-icant Difference (HSD) post hoc test (α=.05). The relationship between retentive force and maximum mag-netic flux leakage (P1) was assessed using Pearson’s correlation coefficient (α=.05).

RESULTS

The mean force was 5.1 ±0.17 N for K6M8, 3.61 ±0.20 N for K6M8, and 3.51 ±0.10 N for K6M10 (Fig. 5). As the difference in size between the keeper and magnetic assembly in-creased, the retentive force decreased, although there was no significant dif-ference between K6M8 and K6M10 (P<.05).

For the D400 keeper combina-tions, the mean force of K4M4 was the highest (3.2 ±0.05 N), and was significantly different (F=63, df=2, P<.05) from the K4M6 and K4M8 groups. K4M6 (2.65 ±0.10 N) also demonstrated a significantly higher

retentive force than K4M8 (2.7 ±0.10 N) (P<.001, P<.05). For the D600 keeper combinations, K6M6 showed significantly higher retentive force (F=148.81, df=2, P<.05) than the oth-er groups (P<.001, P<.05).

The results for magnetic flux leak-age are shown in Figure 6. All groups showed significantly higher magnetic flux leakage at P1 than at P2 and P3 (F=799, df=2, P<.05) (P=4.2 x 10-7, P<.05). Significantly reduced mag-netic flux leakage was observed for the same-size keeper and magnetic assembly combinations of D400 and D600, whereas it was significantly increased for the other different-size combinations (P<.05).

3 Measurement points for magnetic flux strength.

5 Mean retentive force results for each group. Bars indicate SDs and bars with same low-ercase letter are not significantly different from the mean values of each group (P>.05).

4 Details of locations measured by Gaussmeter probe, with active area 0.3 mm from tip surface. B represents magnetic flux density.

Magneticassembly

Keeper P3

P2P1

Probe lateral view

Probe top view

Active area

Active area

0.76 mm0.064 mm

0.30 mm

1.27 mm

+B

6

Rel

ativ

e Fo

rce

(N)

Groups

5

4

3

2

1

0K4M4

3.23

K4M6

2.65

K4M8

2.38

K6M6

5.10

K6M8

3.61

K6M10

3.51a a



271April 2011


For each group of the D400 and D600 combinations, retentive force and maximum magnetic flux leakage showed a strong negative correla-tion (γ=-0.97 and 0.93, respectively; P<.001 for both).

DISCUSSION

The results show that the magnet-ic field exposure increased and reten-tive force decreased as the difference in size between the keeper and mag-netic assembly increased; therefore, the null hypothesis was rejected.

The 2009 ICNIRP guidelines for exposure to static magnetic fields state that the acute exposure limit for the general public, for any part of the body, is 400 millitesla (mT). Further-more, practical policies need to be implemented to prevent inadvertent harmful exposure of people with im-planted electronic medical devices or implants containing ferromagnetic materials, as these can result in much lower restriction levels, such as 0.5 mT. This lower level assumes whole body exposure that occurs with the use of magnetic resonance systems,

since the protection of medical de-vices is consistent with protection of the body against flying metal objects, because of substantial mechanical forces in a static field. 12,13

A static field does not vary over time. In contrast, time-varying elec-tromagnetic fields are produced by alternating currents. Exposure to elec-tromagnetic fields has been steadily increasing as society’s demands for electrical items grow, and ever-advanc-ing technologies and changes in social behavior have created more and more artificial sources. Even in the absence of an external electric field, tiny elec-trical currents exist in the human body due to chemical reactions that occur as part of normal bodily functions.

The possible impacts of electro-magnetic fields on human health have been reported.12 Several stud-ies have been conducted in an effort to determine biological responses to static magnetic fields ranging in flux densities from millitesla to several tesla, and these studies have been re-viewed comprehensively by a number of authors and organizations.12,14,16,17 Overall, there is little convincing evi-

dence from cellular and cell-free mod-els of any biologically harmful effects of exposure to magnetic fields with flux densities up to several tesla.13-16 However, despite a lack of evidence of any such deleterious effects on mar-ginal tissues, there is a need to clarify how the long-term use of magnetic attachments in the oral area might af-fect patients, because compared with the Earth’s magnetic field, which is in the range of 0.024 to 0.066 mT, the strength of the magnetic flux leakage is more than 10 mT.

The present study tested closed-field systems of magnetic assemblies of the cup-yoke type in which the yoke was made of the ferritic stain-less steel.4,8 A closed magnetic circuit is formed between the magnetic as-sembly and the keeper by ensuring a complete interface between the keeper and the yoke surrounding the magnet. Since the yoke consists of ferromag-netic material and offers the least re-sistance, it becomes the magnetic flux path, and, therefore, the magnetic flux is shunted through the yoke to reach the keeper efficiently. This not only eliminates much of the external

6 Magnetic flux strength results for keeper-magnetic assembly combinations at measurement points. Bars indicate SDs, and asterisks (*) indicate mean values differ significantly (P>.05).

Magnetic Flux Strength (millitesla)20 40

D400 keepercombinations at P1





* * *

* * *

* * *

* * *

* * *

* * *


60 80 100 120 140 160 180

D400 assembly

D600 assembly

D800 assembly

D1000 assembly

magnetic flux field, it also makes the attachment more effective (Fig. 3).

The results indicate that the mag-netic flux strength decreased in pro-portion to the square of the distance. As the magnetic attachment is not at-tached directly to oral tissue because the keeper is attached to a structure composed of metal dental materials, there is sufficient distance from the magnetic attachment to the marginal gingival tissue. In this study, when the keeper and magnetic assembly were of the same size, magnetic field leak-age at 0.3 mm from the magnetic as-sembly was not above 40 mT, and, therefore, its recommended usage should not adversely affect the human body. However, magnetic flux leakage was increased and retentive force was significantly decreased as the size dif-ference between the keeper and mag-net increased. These results indicate that the retentive force decreased due to magnetic field leakage and to the closed-field structure becom-ing incomplete. Taken together, the retentive force of the cup-yoke type of magnetic attachment depends on the size of the keeper rather than on the potential magnetic strength of the magnetic assembly. Thus, for effective use of magnetic attachments, the size of the magnet assembly and keeper should be the same or correspond as closely as possible, and there should be a complete interface between the keeper and yoke.

As this was an in vitro study, the results obtained cannot be directly extrapolated to other dental magnet-ic attachments in clinical use. Further studies are required to investigate other similar magnetic attachments

and the behavior of the keeper-mag-netic assembly under simulated in vivo conditions.

CONCLUSIONS

Within the limitations of this study, the following conclusions were drawn:

1. Retentive force was highest when the same sized magnetic assem-bly and keeper were used. The reten-tive force of the magnetic attachment depends on the keeper size. The larger the size difference between the keeper and magnetic assembly, the greater the decrease in retentive force.

2. Magnetic flux strength was lowest when the same sized magnetic attachment and keeper were used. Magnetic flux leakage increased as the size difference between the keeper and magnetic assembly increased.

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5. van Waas MA, Kalk W, van Zetten BL, van Os JH. Treatment results with immediate overdentures: an evaluation of 4.5 years. J Prosthet Dent 1996;76:153-7.

6. Obatake RM, Collard SM, Martin J, Ladd GD. The effects of sodium fluoride and stannous fluoride on the surface roughness of intraoral magnet systems. J Prosthet Dent 1991;66:553-8.

7. Boeckler AF, Morton D, Ehring C, Setz JM. Mechanical properties of magnetic attach-ments for removable prostheses on teeth and implants. J Prosthodont 2008;17:608-15.

8. Riley MA, Walmsley AD, Harris IR. Magnets in prosthetic dentistry. J Prosthet Dent 2001;86:137-42.

9. Gonda T, Ikebe K, Dong J, Nokubi T. Effect of reinforcement on overdenture strain. J Dent Res 2007;86:667-71.

10.Kokubo Y, Fukushima S. Magnetic attach-ment for esthetic management of an over-denture. J Prosthet Dent 2002;88:354-5.

11.Ohashi N, Koizumi H, Ishikawa Y, Furuchi M, Matsumura H, Tanoue N. Relation between attractive force and keeper surface characteristics of iron-neodymium-boron magnetic attachment systems. Dent Mater J 2007;26:393-400.

12.International Commission on Non-Ionizing Radiation Protection (ICNIRP). Guidelines on limits of exposure to static magnetic fields. Health Phys 2009;94:540-14.


14.National Radiological Protection Board (NRPB). Review of the scientific evidence for limiting exposure to electromagnetic fields (0-300 GHz). Doc NRPB 2004;15:1-215.

15.Miyakoshi J. Effects of static magnetic fields at the cellular level. Prog Biophys Mol Biol 2005;87:213-23.

16.Noble D, McKinlay A, Repacholi M. Effects of static magnetic fields relevant to human health. Prog Biophys Mol Biol 2005;87:171-372.

17.World Health Organization. Environmental health criteria monograph no. 232. Static fields. Geneva: World Health Organization; 2006. p. 112-23. Available at: http://www.who.int/peh-emf/publications/reports/ehc-static/en/index.html

Corresponding author:Dr Mikage HasegawaDepartment of Partial Denture ProsthodonticsNihon University School of Dentistry1-8-13 Kanda Surugadai, Chiyoda-kuTokyo 101-8310JAPANFax: +81-3-3219-8350E-mail: [email protected]

Copyright © 2011 by the Editorial Council for The Journal of Prosthetic Dentistry.



271April 2011


For each group of the D400 and D600 combinations, retentive force and maximum magnetic flux leakage showed a strong negative correla-tion (γ=-0.97 and 0.93, respectively; P<.001 for both).

DISCUSSION

The results show that the magnet-ic field exposure increased and reten-tive force decreased as the difference in size between the keeper and mag-netic assembly increased; therefore, the null hypothesis was rejected.

The 2009 ICNIRP guidelines for exposure to static magnetic fields state that the acute exposure limit for the general public, for any part of the body, is 400 millitesla (mT). Further-more, practical policies need to be implemented to prevent inadvertent harmful exposure of people with im-planted electronic medical devices or implants containing ferromagnetic materials, as these can result in much lower restriction levels, such as 0.5 mT. This lower level assumes whole body exposure that occurs with the use of magnetic resonance systems,

since the protection of medical de-vices is consistent with protection of the body against flying metal objects, because of substantial mechanical forces in a static field. 12,13

A static field does not vary over time. In contrast, time-varying elec-tromagnetic fields are produced by alternating currents. Exposure to elec-tromagnetic fields has been steadily increasing as society’s demands for electrical items grow, and ever-advanc-ing technologies and changes in social behavior have created more and more artificial sources. Even in the absence of an external electric field, tiny elec-trical currents exist in the human body due to chemical reactions that occur as part of normal bodily functions.

The possible impacts of electro-magnetic fields on human health have been reported.12 Several stud-ies have been conducted in an effort to determine biological responses to static magnetic fields ranging in flux densities from millitesla to several tesla, and these studies have been re-viewed comprehensively by a number of authors and organizations.12,14,16,17 Overall, there is little convincing evi-

dence from cellular and cell-free mod-els of any biologically harmful effects of exposure to magnetic fields with flux densities up to several tesla.13-16 However, despite a lack of evidence of any such deleterious effects on mar-ginal tissues, there is a need to clarify how the long-term use of magnetic attachments in the oral area might af-fect patients, because compared with the Earth’s magnetic field, which is in the range of 0.024 to 0.066 mT, the strength of the magnetic flux leakage is more than 10 mT.

The present study tested closed-field systems of magnetic assemblies of the cup-yoke type in which the yoke was made of the ferritic stain-less steel.4,8 A closed magnetic circuit is formed between the magnetic as-sembly and the keeper by ensuring a complete interface between the keeper and the yoke surrounding the magnet. Since the yoke consists of ferromag-netic material and offers the least re-sistance, it becomes the magnetic flux path, and, therefore, the magnetic flux is shunted through the yoke to reach the keeper efficiently. This not only eliminates much of the external

6 Magnetic flux strength results for keeper-magnetic assembly combinations at measurement points. Bars indicate SDs, and asterisks (*) indicate mean values differ significantly (P>.05).

Magnetic Flux Strength (millitesla)20 40






* * *

* * *

* * *

* * *

* * *

* * *


60 80 100 120 140 160 180

D400 assembly

D600 assembly

D800 assembly

D1000 assembly

magnetic flux field, it also makes the attachment more effective (Fig. 3).

The results indicate that the mag-netic flux strength decreased in pro-portion to the square of the distance. As the magnetic attachment is not at-tached directly to oral tissue because the keeper is attached to a structure composed of metal dental materials, there is sufficient distance from the magnetic attachment to the marginal gingival tissue. In this study, when the keeper and magnetic assembly were of the same size, magnetic field leak-age at 0.3 mm from the magnetic as-sembly was not above 40 mT, and, therefore, its recommended usage should not adversely affect the human body. However, magnetic flux leakage was increased and retentive force was significantly decreased as the size dif-ference between the keeper and mag-net increased. These results indicate that the retentive force decreased due to magnetic field leakage and to the closed-field structure becom-ing incomplete. Taken together, the retentive force of the cup-yoke type of magnetic attachment depends on the size of the keeper rather than on the potential magnetic strength of the magnetic assembly. Thus, for effective use of magnetic attachments, the size of the magnet assembly and keeper should be the same or correspond as closely as possible, and there should be a complete interface between the keeper and yoke.

As this was an in vitro study, the results obtained cannot be directly extrapolated to other dental magnet-ic attachments in clinical use. Further studies are required to investigate other similar magnetic attachments

and the behavior of the keeper-mag-netic assembly under simulated in vivo conditions.

CONCLUSIONS

Within the limitations of this study, the following conclusions were drawn:

1. Retentive force was highest when the same sized magnetic assem-bly and keeper were used. The reten-tive force of the magnetic attachment depends on the keeper size. The larger the size difference between the keeper and magnetic assembly, the greater the decrease in retentive force.

2. Magnetic flux strength was lowest when the same sized magnetic attachment and keeper were used. Magnetic flux leakage increased as the size difference between the keeper and magnetic assembly increased.

REFERENCES 1. Sasaki H, Kinouchi Y, Tsutsui H, Yoshida

Y, Karv M, Ushita T. Sectional prostheses connected by samarium-cobalt magnets. J Prosthet Dent 1984;52:556-8.

2. Akaltan F, Can G. Retentive characteristics of different dental magnetic systems. J Pros-thet Dent 1995;74:422-7.

3. Matsumura H, Kawasaki K. Magnetically connected removable sectional denture for a maxillary defect with severe undercut: a clini-cal report. J Prosthet Dent 2000;84:22-6.

4. Maeda Y, Nakao K, Yagi K, Matsuda S. Composite resin root coping with a keeper for magnetic attachment for replacing the missing coronal portion of a removable partial denture abutment. J Prosthet Dent 2006;96:139-42.

5. van Waas MA, Kalk W, van Zetten BL, van Os JH. Treatment results with immediate overdentures: an evaluation of 4.5 years. J Prosthet Dent 1996;76:153-7.

6. Obatake RM, Collard SM, Martin J, Ladd GD. The effects of sodium fluoride and stannous fluoride on the surface roughness of intraoral magnet systems. J Prosthet Dent 1991;66:553-8.

7. Boeckler AF, Morton D, Ehring C, Setz JM. Mechanical properties of magnetic attach-ments for removable prostheses on teeth and implants. J Prosthodont 2008;17:608-15.

8. Riley MA, Walmsley AD, Harris IR. Magnets in prosthetic dentistry. J Prosthet Dent 2001;86:137-42.

9. Gonda T, Ikebe K, Dong J, Nokubi T. Effect of reinforcement on overdenture strain. J Dent Res 2007;86:667-71.

10.Kokubo Y, Fukushima S. Magnetic attach-ment for esthetic management of an over-denture. J Prosthet Dent 2002;88:354-5.

11.Ohashi N, Koizumi H, Ishikawa Y, Furuchi M, Matsumura H, Tanoue N. Relation between attractive force and keeper surface characteristics of iron-neodymium-boron magnetic attachment systems. Dent Mater J 2007;26:393-400.



14.National Radiological Protection Board (NRPB). Review of the scientific evidence for limiting exposure to electromagnetic fields (0-300 GHz). Doc NRPB 2004;15:1-215.

15.Miyakoshi J. Effects of static magnetic fields at the cellular level. Prog Biophys Mol Biol 2005;87:213-23.

16.Noble D, McKinlay A, Repacholi M. Effects of static magnetic fields relevant to human health. Prog Biophys Mol Biol 2005;87:171-372.

17.World Health Organization. Environmental health criteria monograph no. 232. Static fields. Geneva: World Health Organization; 2006. p. 112-23. Available at: http://www.who.int/peh-emf/publications/reports/ehc-static/en/index.html

Corresponding author:Dr Mikage HasegawaDepartment of Partial Denture ProsthodonticsNihon University School of Dentistry1-8-13 Kanda Surugadai, Chiyoda-kuTokyo 101-8310JAPANFax: +81-3-3219-8350E-mail: [email protected]

Copyright © 2011 by the Editorial Council for The Journal of Prosthetic Dentistry.

Documents

Retentive force and magnetic flux leakage of magnetic attachment in various keeper and magnetic assembly combinations