Retentive force of combined sandwich-type magnetic assemblies
M. Takahashi1, G. Togawa1, M. Kanyi2, K. Ishikawa1, 3 Y. Takada1
1

Division of Dental Biomaterials, Tohoku University Graduate School of Dentistry, JAPANMoi Teaching and Referral Hospital, KENYA
3Tohoku University Hospital, JAPAN2Introduction
Dental magnetic attachments are often used as retentive elements in partial and complete dentures. They
can also be utilized in implant over dentures such as 2-IOD and 4-IOD. From 2021, the Japanese insurance
system committed to cater for cost of dental magnetic supported appliances1). Its usage is therefore expected
to rise in future. Magnetic attachments manufactured in Japan are predominantly a combination of magnetic
assembly and keeper. A magnetic assembly exhibits closed magnetic circuit which is characterized by highretentive force despite its small size. Leakage of magnetic field associated with the closed circuit is minimal.
On the other hand, products made in other countries utilize both closed and open magnetic circuit typemagnets. To overcome poor corrosion resistance of rare–earth magnets, magnets used in dentistry are covered
with stainless steel or titanium2). A magnet is simple and easy to manufacture compared to magnetic assembly.
However, its open magnetic circuit feature is associated with high leakage of magnetic field, and low retentive
force relative to its size. In order to compensate for the weak retentive force of magnet-keeper combination,
it may be beneficial to combine two magnets. In practice, there are three possible combinations of dental
magnetic attachments: magnetic assembly to keeper, magnet to keeper, and magnet to magnet.
The magnet to magnet combination exhibits a retentive force greater than that of magnet-keeper
combination. It is therefore postulated that two magnetic assemblies combined would also yield higher
retentive force than magnetic assembly to keeper. However, there is no research on retentive force whencombining two magnetic assemblies.
The two types of magnetic assemblies for application as dental magnetic attachments which exist are cupyoke and sandwich type3). It is difficult to combine two cup-yoke type of magnetic assemblies because thecontact mating faces are of similar polarity causing repulsion (Fig. 1). Sandwich types are easy to combine
due to the attraction force exhibited by the unlike poles which make up mating faces (Fig. 2). This study
explored the possibility and characteristics of a new combination that involves two sandwich-types ofmagnetic assemblies.

NS

S
N
Fig. 1 Combination of cup-yoke type of magnetic assemblies

SN

NS

Fig. 2 Combination of sandwich type of magnetic assemblies

Objective
The aim of this study was to explore possibility of combining two sandwich-type magnetic assemblies andinvestigate the retentive force characteristics. A combination of magnetic assembly and keeper, which forms
the ordinary magnetic attachment, was used for comparison.

Materials and Methods
1. Dental magnetic attachment
Oval sandwich-type dental magnetic attachments (MAGFITEX600W, 3.8 mm major axis and 2.8 mm
minor axis length, Aichi steel Corp.) were used in this study. The magnetic assembly was either combinedwith a keeper or another magnetic assembly. The mating faces were placed in contact with different polaritiesacross each other (Fig. 2). Magnetic keeper was paired up with magnetic assembly to represent the ordinary
magnetic attachment.
2. Procedure of retentive force measurement
The retentive force measuring device used in this study matches the description in ISO 13017:20203). The
device was connected to a digital force gauge (ZPS, Imada) and retentive force measured at crosshead speedof 2 mm/min. recording was done at a sampling rate of 1 kHz. Applying the known time and speed values,distance was calculated then a retentive force curve generated. The experiment begins when the magnetic
assembly has its center aligned to that of keeper or second magnetic assembly. The measurements were done
at intervals of 100 µm horizontal displacement, along the major and minor axis direction, of keeper or second
magnetic assembly. The measurement is terminated when the magnetic assembly completely separates fromits keeper or second magnetic assembly.

Results and Discussion
1. Retentive force of aligned magnetic attachments
The retentive force of a magnetic assembly paired with keeper or second magnetic assembly was 5.26 Nand 6.45 N, respectively. Therefore use of a second magnetic assembly instead of keeper raised the retentive
force by 1.23 times. This is attributed to the shorter distance between the magnetic poles of two magnetic
assemblies (Fig. 3b) compared to magnetic assembly and its keeper (Fig. 3a). Additionally, the volume of themagnet is doubled, and the energy of the magnet increased causing an enhanced effect. However, due tosaturation of the cup yoke’s magnetic flux density, the retentive force is not doubled. Magnets whose design
involves closed magnetic circuit are characterized by small size and relatively large retentive force. The force
can be further enhanced through combination of two magnetic assemblies.

(a)

(b)

Fig. 3 Schematic diagram of magnetic flux flow in the magnetic attachments (a) combination of magnetic
assembly and keeper, (b) a pair of magnetic assemblies
2. Decline of retentive force upon complete separation of mating face
Retentive force curves for the two set up combinations are shown in Fig. 4. The displacement position ata time when the mating faces separate is denoted as 0 mm. For both scenarios, the retentive force values
decreased with increasing distance which corresponds to displacement of the magnetic assembly away fromkeeper or second assembly. However, the rate of decline was moderate when two magnetic assemblies werepaired up compared to the one with keeper. Although magnetic flux volume emitted from the assembly yokes
is same, magnetic field lines are concentrated more towards the second magnetic assembly yoke than thosethat flow solely within the first assembly as shown in Fig 5. A combination of two magnetic assemblies offers
a wider range of influence on the attractive force and is stable against air gap.

10.0
two magnetic
assemblies

Force (N)8.0magnetic assemblyand keeper

6.0
4.0
2.0

0.0
-10

1
2
3
Distance (mm)4Fig. 4 Retentive force curve(a)(b)Fig. 5 Images of magnetic flux flow after separation of mating faces. (a) magnetic assembly and keeper
combination, (b) pair of magnetic assemblies

Retentive force (N)3. Changes in retentive force against lateral displacement
Fig. 6 shows the change in retentive force when the second magnetic assembly was shifted in the major
axis direction. The retentive force decreased gradually with increasing displacement. The loss in retentive
force was linear at a displacement of 1.2 to 2.5 mm. This pattern was similar to that of the combination of
the magnetic assembly and the keeper4). The retentive force is therefore dependent on the contact surface
area between the two magnetic assemblies upon displacement along the major axis direction.

7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
0.0
1.02.0
3.0
4.0
Horizontal displacement (mm)Fig. 6 Retentive force against displacement along major axis direction

Retentive force (N)Fig. 7 shows the changes in retentive force when displacement occurs along the minor axis direction. The
retentive force decreased sharply upon displacement to reach 0 N at 1.5mm. Further displacement generated
negative values which imply a repulsion of magnetic assemblies. This phenomenon was not observed in themagnetic assembly and keeper set up4).
As illustrated in Fig. 3, congruently matched magnetic assemblies have unlike poles in contact thereby
producing an attractive force. Displacement along the minor axis direction, as shown in Fig. 8, creates varying
scenarios in which some parts of magnetic assemblies are in attraction whereas others are in repulsion. The
resultant force acting in a direction perpendicular to the mating face is measured and represents the retentiveforce. At the initial stages of displacement, the attractive force exceeds the repulsive force. At about 1.5 mm
displacement, the two forces are equal resulting into 0 N retentive force. Further displacement yields a higher
repulsive force compared to attractive force.

7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
-1.0
0.0
1.0
2.0
3.0
Horizontal displacement (mm)Fig.7 Retentive force against displacement along minor axis direction

SSSN

N

S

attractive repulsive
force > force

NNN

attractive repulsive
force = force

NSS

attractive repulsive
force < force

Fig.8 Images of forces between two magnetic assemblies in response to displacement along the minoraxis direction
Besides the attractive and repulsive forces explained above; there also exists (lateral) forces that act in adirection parallel to the mating faces. Fig 9 demonstrates how attraction and repulsion forces act in a manner
similar to closed and open coil springs respectively. A slight displacement (a→b) is accompanied by restoring
force which acts like springs taking the set up back to the original position (b→a). Therefore, combining two
magnetic assemblies yields a "restoring force" which counters the effect of minor lateral displacement.

NS

S

NN
(a)

NS

S(b)

Fig. 9 Schematic drawing to illustrate spring effect of repulsion and attraction forces (a) well aligned, (b)slight displacement
4. Clinical application of combined magnetic assemblies
Dental magnetic attachment set up that involves two magnetic assemblies provides salient features whichhave meaningful applications in dentistry especially complete dentures. The spring like action by restoring
force acts as a guide in proper alignment of magnetic attachments. It eases the procedure of incorporating
magnetic assembly into denture. Higher retentive forces associated with magnetic assemblies compared tokeeper of the same size offers improved denture stability. Erroneous inclusion of air gap has minimal effectas the consequent decrease in retentive force is gradual. The restoring force further promotes denture stability
as minor denture displacement during function is automatically corrected. Major displacement of themagnetic attachment against abutment tooth in a horizontal direction triggers the repulsion forces which aid
in removal of denture from the oral cavity. The concept of combining two magnetic assemblies isadvantageous compared to use of assembly and keeper in terms of retentive force and stability.

Conclusion
The retentive force of a pair of magnetic assemblies was higher than that of magnetic assembly combinedwith keeper. The decline in retentive force was more gradual for the set up involving two magnetic assemblies
compared to assembly and keeper. A pair of combined magnetic assemblies generates a restoring andrepulsive force upon small and large displacements along the minor axis, respectively. It was establishedadvantageous to combine two magnetic assemblies.

Acknowledgments
The authors greatly acknowledge Aichi steel Corp. for providing the MAGFIT EX600W.

References
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