11. Influence of heating of a magnetic attachment on the attractive force

Miyata T., Niimi J., Ando A., Shouji k., Kumano H., Masuda T., Hasegawa N., Nakamura Y. and Tanaka Y.

The First Department of Prosthodontics, School of Dentistry, Aichi-Gakuin University

Introduction

A magnetic attachment has achieved excellent clinical results, and has been recognized as a clinically significantly tool with numerous advantages compared with a conventional mechanical retainer. A Nd magnet, recently available on the market, has been widely adopted in commercial magnetic attachments. The magnet has the temperature related characteristic of magnetic force loss at low relative temperatures. This is considered to be disadvantages of the Nd magnet. The clinical use conditions including the sterilization procedures and the laboratory processing should be thus considered for evaluation.

Objective

We compared the attractive force of each magnetic assembly before and after heating to investigate the influence of heating.

Materials and Methods

A magnetic assembly and a keeper used as experimental materials in the present study were GIGAUSS D 600 (GC) and PHYSIS MAGNET 35 (Hitachi Metals) (Fig. 1). The number of samples was 5 for each product.

Fig. 1: Magnetic attachments

Table.1 shows experimental items and conditions of the present study.

Experiment 1:

We performed a heating experiment to investigate the thermal characteristics of a magnetic assembly, and confirmed the influence of heating in a thermostat furnace on the attractive force.

Experiment 2:

We examined the influence of autoclave heating on a magnetic attachment on the assumption that a magnetic assembly and a keeper were sterilized.

Experiment 3:

We examined the influence of the heat generated during two resin curing process.

Experimental conditions for each experiment are as follows.

Experiment 1 and 2: open magnetic circuit of a magnetic assembly, and closed magnetic circuit comprising a keeper and a magnetic assembly.

Experiment 3: open magnetic circuit.

Table. 1: Experimental items and conditions

Attractive force measurement was conducted using EZ test, compact table-top universal tester with a 5kgf load cell and 5 mm/min crosshead speed.

Attractive force was measured 10 times for each magnetic assembly at each experimental condition, and the mean value was accepted as the attractive force. Attractive forces of the magnetic assembly before and after each experiment were compared.

The testing jig was devised by our department for use, assessments and measurement of the attractive forces (Fig. 2).

Fig. 2: Measuring device and custom made jig

In the Experiment 1, each sample was placed in the thermostat furnace (Laboclean QC-1) at 50 ℃ and 100 ℃, and subsequently raising the temperature by 20 ℃ per hour from 120 to 320 ℃ (Fig. 3).

Samples were cooled to room temperature in the atmosphere, followed by the attractive force measurement. Attractive forces between the samples at each temperature and those at room temperature were compared to investigate the influence of heating on attractive force.

Fig. 3: Thermostat furnace (Laboclean QC-1)

In the Experiment 2, samples were sterilized with dental autoclave (OSADA PUXHI CLAVE DA-5) at 135 ℃ at 0.23 MPa for 4 min, and cooled in the atmosphere for 24 hours, followed by attractive force measurement of each sample (Fig.4). Attractive forces before and after sterilization were compared to investigate the influence of sterilization on attractive force.

Fig. 4: Dental autoclave

(OSADA PUXHI CLAVE DA-5)

In the Experiment 3, we confirmed the influence of two resin curing methods (rapid heat curing method and wet heat curing method) on attractive force.

In the rapid heat curing method, resins were cured with microwave at 500 watt for 3 minutes. In the wet heat curing method, resins were cured at 70 ℃　for 60 minutes, and then 100 ℃ for 30 minutes. After being cured by each curing method, resins were cooled at room temperature for 24 hours, magnetic assemblies were removed from resins, and attractive force measurements were performed. Attractive forces before and after polymerization for each curing method were compared to investigate the influence of polymerization on attractive force (Table. 2).

Table. 2: Curing methods

Fig. 5 shows the process of sample fabrication used in the Experiment 3.

　Parrafin wax with 35 x 60 mm in width and 5 mm in thickness was invested in a flask, and wax elimination was performed following the conventional method. A magnetic assembly was adhered with alpha-cyanoacrylate adhesive (Aron Alpha) into the space created by wax elimination. Acron MC Live Pink no. 8 was packed into a flask in the rapid heat curing method, and Acron Live Pink no. 8 was packed in the wet heat curing method. A magnetic assembly was removed from the resin place after polymerization, and used as a sample.

Fig. 5: Process of sample fabrication

Results

1. Experiment 1.

In the open magnetic circuit, attractive force started to decrease from 120 ℃ both in GIGAUSS D 600 and PHYSIO MAGNET 35. At 220 ℃, attractive force decreased to less than half of that of the room temperature. On further raising the temperature, attractive force was lost at 260 ℃ in GIGAUSS D 600, and at 300 ℃ in PHYSIO MAGNET 35 (Fig. 6).

In the closed magnetic circuit, attractive force started to decrease from 220 ℃ both in GIGAUSS D 600 and PHYSIS MAGNET 35. Unlike the case in the open magnetic circuit, both products showed the attractive force more than 400 gf until 260 ℃. On further raising the temperature, attractive force was lost at 300 ℃ in GIGAUSS D 600, and at 320 ℃ in PHYSIO MAGNET 35 (Fig. 7).

Fig. 6: Open magnetic circuit

Fig. 7: Closed magnetic circuit

2. Experiment 2.

In the open magnetic circuit, attractive force decreased both in GIGAUSS D 600 and PHYSIO MAGNET 35 after sterilization (Fig. 8).

On the other hand, no difference was found in the attractive force before and after sterilization in the closed magnetic circuit (Fig. 9).

Fig. 8: Open magnetic circuit

Fig. 9: Closed magnetic circuit

3. Experiment 3.

In the rapid heat curing method, attractive force decreased both in GIGAUSS D 600 and PHYSIO MAGNET 35 after polymerization (Fig. 10).

The same results were obtained in the wet heat curing method (Fig. 11).

Fig. 10: Rapid heat curing

Fig. 11: Wet heat curing

Discussions

The results of the present study suggested that attractive force decreases with application of heat to a magnetic attachment. This finding was particularly significant when a magnetic assembly was heated in the open magnetic circuit. The decrease in the magnetic force by heating a magnet is considered to be due to the directional change of an atomic magnet. Permeance coefficient determined by the shape of the magnet is also an important factor causing irreversible thermal change. The results showed the high potential of the decrease in attractive force by heating of a magnetic assembly, suggesting the risk of sterilization process and laboratory work under such condition.

Conclusions

In the present study, we heated a magnetic assembly with several possible heating methods in clinical practice to confirm the influence of heating on attractive force. Attractive forces before and after heating were compared, and the following results were obtained.

1. The heating experiment performed in a thermostat furnace showed that attractive force of a magnetic assembly decreased above a certain temperature. The decreasing patterns were different in each product, and in different circuit.

2. When a magnetic assembly was sterilized in a autoclave, no influence was observed before and after sterilization in the closed magnetic circuit. In contrast, attractive force of each product decreased after sterilization in the open magnetic circuit, and a significant difference was observed before and after sterilization.

3. As for the influence of each curing method, attractive force of each product decreased both in the rapid heat curing method and the wet heat curing method, and a significant difference was observed before and after polymerization.

References

1. Jackson,T.R.: The application of rare earth magnetic retention toosseointegrated implants. Int. J. Oral ＆ Maxill. Imp., 1: 81-92, 1986.

2. Tanaka, Y.: Dental Magnetic Attachment, Q＆A, Ishiyaku Publishers, Inc.(Tokyo),1995.

3. Nakamura, K. Hiroshi, M. Fukazawa, N. and et al: Influence of Heat Treatments on Attractive Force of Magnetic Attachments. J J Mag Dent. 6: 63-70, 1997.