#3 Influence of the Measuring Methods on the Attractive Force of Magnetic Attachments

Y. Nakamura, K. Shoji, R. Kanbara, H. Kumano, A. Ando, T. Iwai, T. Kogiso, Y. Ohno, Y. Tanaka
	Removable Prosthodontics, School of Dentistry, Aichi-Gakuin University

Introduction

As handing of a removable denture, most of dentists usually instruct a patient that a denture should be removed overnight. However, on patientｓ with bruxism, dentures are recommended to wear for protecting teeth occasionally.　Meanwhile, in spite of proposing to use dentures while asleep, bruxism may be caused inThere has been an increasing use of magnets in the clinical dental setting. Magnetic attachments are excellent retaining devices with many functional applications and aesthetic benefits. These attachments have also had high satisfaction ratings for the treatment results when used in conjunction with dental implant treatments. A magnetic attachment consists of a magnetic assembly and a keeper. The optimum attractive force relationships between these two components are of prime importance. Therefore, a careful evaluation of the relationship of attractive force and magnetic assembly mechanism is required. Our department has been conducting studies on magnetic attachments such as attractive force measurements to elucidate magnetic attachment properties. Several different methods have been reported on magnetic attachment measurement techniques. A specialized jig orientation measuring device was designed, reported and tested upon at Department of Removable Prosthodontics, School of Dentistry, Aichi Gakuin University. The validity of the attractive force measuring method using this jig was confirmed.

Objectives

A simply designed device for the measurement of the attractive force between a magnet and a keeper was proposed by the Australian magnet research institute at the 2009 ISO conference. The comparison to existing measurement techniques was desired. In the present study, we compared the results of the attractive force measurement method proposed by Australian magnetic institute with our measurement method to verify the validity of the newly proposed method.

Materials and methods

1.Materials

GIGAUSS C (GC) was used as a magnetic assembly testing subject(Fig. 1). Attractive force was measured using a compact　tabletop　EZ-test tensile tester (Shimazu) (Fig. 2).

Fig 1: GIGAUSU C600 (GC)

Fig 2: EZ test (SIMAZU)

2.Methods

1) Experimental items
(1) Attractive force measurement using a special jig 　　

Attractive force of a magnetic assembly was measured using a special jig shown in Fig. 3. In obtaining measurements of attractive force measurement using a conventional jig, there have been vertical and horizontal restrictions on the attractive surface between a magnetic assembly and a keeper when vertically separating a magnet potentially interfering with vertical measurement accuracy. Repeated accurate repositioning of the magnetic assembly and a keeper to an original position is also very difficult. A special jig device was designed, consisting of a pair of upper and lower holding members. Bilateral bars guide the movement of a jig, preventing unwanted axial and horizontal deviations during the elevating movements on a magnetic assembly. Two horizontal bars in the upper member of the jig can be fit inserted into the guide holes of the lower member jig part. The holes in the bottom are longitudinal bearing structure to avoid the friction resistance during the measurement. A magnetic assembly and a keeper can be returned to the original position by using a mold and a guide bar shown in Fig. 4. The accurate attractive force measurement was achieved by using this special jig design.

Fig 3: special jigu

Fig 4: mold and a guide bar

(2) Attractive force measurement proposed by the Australian magnetic institute.

The following is the attractive force measurement design proposed by the Australian magnetic institute in 2009 ISO conference. A simple jig is used instead of a complex previously reported design jig regulating the sideslip on the attractive surface between a magnetic assembly and a keeper. Reliable measurement can be achieved by using cotton string for flexible traction of a magnetic assembly. 　　Based on the Australian design proposal, another simple jig to correct sideslip was designed (Fig. 5). It is difficult to control the movement between a magnetic assembly and a keeper due to the small size
(GIGAUSS C 600 - major axis 4.1 mm and minor axis 3.3 mm). Therefore, a magnetic assembly and a keeper were attached to a holding mold. A simple guide was fabricated using acrylic cylinder around the mold. Adjustment was made between a guide and a mold to avoid friction resistance. Cotton string was used for flexible traction of a magnetic assembly.

Fig 5: simple jig based on the Australian design proposal

(3) Attractive force measurement without vertical and horizontal restrictions

Attractive force measurement without vertical and horizontal restriction was applied to compare the attractive force measurement methods between the Australian magnetic institute and the specialized jig measuring method (Fig. 6). Horizontal and vertical restrictions were eliminated by removing the holding mold from the magnetic measurement assembly, and the cotton string for traction, respectively.

Fig 6: attractive force measurement without vertical and horizontal restrictions

2) Measurement conditions 　　
Attractive force measurement was repeated 10 times for each of the 5 samples using the EZ test.
The cross head speed was 5 mm/minutes.

Results

1. Attractive force measurement method using a special jig

	Fig 7 shows the results of the attractive force measurement using a special jig. Minor variation was noted in measurements between each sample. The maximum attractive force was 570 gf, and the minimum attractive force was 543 gf. It was confirmed that the standard deviation for measurement precision was subtle in 10 measurements of each sample.
		Fig 7: attractive force (special jigu)


2. Attractive force measurement proposed by Australian magnetic institute
	Fig 8 shows the results of attractive force measurement proposed by the Australian magnetic institute using a simple guide. A significant variation in measurements and standard deviation were observed between each sample. The maximum attractive force was 424 gf, and the minimum attractive force was 337 gf.
		Fig 8: attractive force (simple guide)


3. Attractive force measurement without vertical and horizontal restrictions
	Fig 9 shows the results of attractive force measurement without vertical and horizontal restrictions. Restriction-free measurements were achieved by eliminating the magnetic assembly holding mold and using a cotton string traction. 　　The variation in measurements between each sample was larger than other two measuring methods. The maximum attractive force was 437 gf, and the minimum attractive force was 351 gf. Standard deviation between each sample was larger than the method using a special jig.
		Fig 9: attractive force (measurement without vertical and horizontal restrictions)


4. The influence of the measuring methods on the attractive force of magnetic attachments
	The influence of the measuring methods on the attractive force was investigated by comparing attractive force measurements of 5 samples obtained by 3 measuring methods. Multiple comparison test was performed using one-way analysis of variance and Scheffe’s test to determine differences between the measuring method using a special jig and other methods.( Fig 10) 　　A significant difference was observed in the mean attractive force measurements between three measuring methods. The difference in the measurements was subtle between the measuring method using a simple guide and the measuring method without restriction.
		Fig 10: differences between the measuring method using a special jig and other methods

Discussions

1. The design of the simple guide in the measuring method proposed by the Australian magnetic institute
　　We fabricated a simple guide based upon a design presented by the Australian magnetic institute. However, exact duplication was unknown since there was no specific information regarding guide design. One application method considered direct application of a keeper and a magnetic assembly attractive surfaces. However, frictional resistance during the traction would likely adversely affect accurate measurement. Correct guide placement was repeatedly difficult as the combination of a keeper and a magnetic assembly were only 2 mm in thickness. 　　Recommendations and considerations for the optimal measurements of magnetic attachments include indirect guides for placement positioning when using the special jig testing apparatus reported. In the present study, measurements were performed by using a mold in a magnetic assembly and a keeper, and a guide was placed indirectly to the mold. This design seeks to minimize the friction resistance between a mold and a guide. Inter-guide lubricans were considered but were not used. The effect of added lubricants could not be accurately assessed.

2. Measurements
　　A significant difference was observed in the obtained attractive force methods using a simple guide and another method without vertical and horizontal restrictions compared with the special jig method. The method using a special jig showed the highest attractive force measurement values for the identical magnetic device.

1) Measuring method of the attractive force using a special jig
　　Greatest measurement accuracy was achieved by using the special jig design testing method. Unlike other measuring methods, there was vertical and horizontal axis control of the magnetic attachments during separation. Friction resistance during traction was not encountered due to the ball bearing structure design and minimal measurement error. Disadvantages of the special jig design remain including high cost and high maintenance requirements. Considered of these issues lead to a simple measuring method. The jig design is of importance for accurate measurement of magnetic assembly attractive forces. The present study demonstrated the role of a special jig in the attractive force measurement, and confirmed the measurement accuracy.

2) Attractive force measurement proposed by Australian magnetic institute (Simple guide)
　　The results of the attractive force measurement using a simple guide showed lower magnetic force measurements. This finding is thought to be due to the lack of vertical control during the traction of a magnet assembly. The traction using a cotton string produces not only vertical but also horizontal stress between a magnetic assembly and a keeper in the guide, causing a non-axial rotational force. This force creates rotational movement when a magnetic assembly and a keeper detatch, resulting in a decrease in measureable attractive force.

3) Attractive force measurement without vertical and horizontal control
　　A decrease in the attractive force was observed due to the rotational movement caused by traction with a cotton string. The results of the attractive force measurement without vertical and horizontal control showed similar measurement values as the degree of attractive force in the simple guide method (Fig 10). The results suggested that the rotational force caused by cotton string traction affects the results of the attractive force measurement more than the horizontal restriction design.

Conclusions

The influence of the measuring methods on the attractive force of magnetic attachments was investigated. Our prior report described the attractive force measurement of a magnetic assembly using a special jig, but a more simple method was proposed by Australian magnetic institute. In the present study, the attractive force of magnetic attachments using a simple guide was measured according to the specific designs proposed by the Australian magnetic institute, and the results were compared. The following conclusions were drawn:
1. The measurement error was the smallest in the measuring method using a special jig that controls vertical and horizontal direction during traction, and the highest attractive force was measured.
2. Although attractive force measurement using a simple guide fabricated according to the proposal appears to provide horizontal displacement. The effectiveness of this design component was not shown.

References

1. Tanaka,Y.: Dental Magnetic Attachment,Q&A,Ishiyaku Publishers,Inc.(Tokyo),1995.
2. Gillings,B.R.D.: Magnetic retention for complete and partial overdentures, Part.J.Prosthet.Dent.,45(5):484-491,1981.
3. Jackson,T.R.: The application of rare earth magnetic retention toosseointegrated implant.Inc.J.Oral & Maxill.Imp.,1:81-92,1986.

Discussion Board