Retentive force of magnetic attachments applied to the proximal surface–Part 2–

 

Y. Miyama, Y. Mizuno, D. Okano^*1, F. Tsuchida^*1, Y. Takayama^*1, N. Takishin^*1, M. Abe^*1, C. Ohkubo^*1 and T. Hosoi^*2

 

Dental Technician Training Institute, Tsurumi University School of Dental Medicine

*1 Department of Removable Prosthodontics, Tsurumi University School of Dental Medicine

*2 Tsurumi University

Introduction

A clinical technique in which magnetic attachments are applied to the proximal surface of abutment teeth of removable partial dentures is employed in Tsurumi University Dental Hospital^1,2). The advantages of this technique are that it can be used for vital teeth without preparation and provides good esthetics. It is more effective if multiple magnetic attachments are applied or when combined with other retainers. (Fig. 1)

 

Objective

The efficiency of magnetic attachments applied to the proximal surface of abutment teeth has been evaluated³⁾. The purpose of this study was to investigate the effects of the fixation angle of the keepers and the location of loading points on retentive force.

 

Materials and Methods

A free-end saddle model, which was made of acrylic resin, was employed with the mandibular first premolar as the abutment tooth. Mesio-distal width of the abutment tooth was 7.1 mm, according to the average size of natural teeth. The framework was made of cobalt-chromium alloy. A space of approximately 0.5 mm was given between the abutment tooth and the bracing arm of the framework. Therefore, the framework was in contact with the model only at the rests and rest seats, and lateral force was not generated during the tensile test. (Figs. 2 and 3)

PHYSIO MAGNET 35 (Nissin Co., Ltd., Kyoto, Japan) was used in this study (Table 1). Keepers were fixed to both mesial and distal surfaces of the abutment tooth. The fixation angles of both keepers were 2°, 4° or 6° to the direction of removal. Small, flat planes were prepared on both proximal surfaces of the abutment tooth by a milling machine and keepers were fixed using PATTERN RESIN (GC Co., Ltd., Tokyo, Japan). (Fig. 4)

The retentive force was measured under two conditions. In condition-1, magnetic attachments were applied to both the mesial and distal surfaces of the abutment tooth. In condition-2, a magnetic attachment was applied to only the distal surface of the abutment tooth. On the mesial surface, a keeper was fixed to the framework to simulate the relationship between the guiding plane and proximal plate.

Retentive force was measured by tensile testing conducted with a digital force gauge (FGC-1, NIDEC-SHIMPO Co., Kyoto, Japan) at a crosshead speed of 5 mm/min. The maximum load (N) was recorded. Tensile testing was conducted at five points, namely, the central and the distal points of the abutment tooth, the second premolar, and the first and second molar regions. Testing points and the locations are shown in Table 2 and Fig. 5. A distance of 5.0 mm was between points B and C, 10.5 mm between points B and D and 20.0 mm between points B and E.

 

Measurements were repeated five times under each condition. Statistical analysis was performed using Student’s t-test and Scheffés test (SPSS ver. 12, SPSS Japan Inc., Tokyo, Japan) at a significance level of α＝0.05.

 

Results

Results of condition-1 are shown in Fig. 6.

When the fixation angle of the keeper was 2°, the retentive force was 1.97 N at point A, and 1.65 N at point B. Tensile testing could not be conducted at points C, D or E. Following the tensile testing, the framework tended to tilt and a mechanical interlocking action occurred. When the fixation angle of the keeper was 4°, the same action occurred at point E.

As the fixation angle increased or the loading points moved distally, the retentive force tended to decrease. There was a significant difference between 2° and 4° at point A, and between 4°and 6° at point D. There was a significant difference between point A and E when the fixation angles were 6°.

Results of condition-2 are shown in Fig. 7.

When the fixation angle of the keeper was 2°, the retentive force was 0.89 N at point A, and 0.85 N at point B. Mechanical interlocking action occurred at points C, D or E and the tensile testing could not be conducted. The same action occurred at point E, when the fixation angles were 4°.

As in condition-1, the retentive force tended to decrease as the fixation angle increased or the testing points moved distally. There was a significant difference between points A and D when the fixation angle was 4° and between points C and E when the fixation angle was 6°.

Comparing condition-1 with condition-2, effects of the fixation angles or the location of testing points on retentive force showed a similar tendency. The retentive force in condition-1 was approximately two times that of condition-2. When magnetic assemblies were applied on both proximal surfaces of the abutment tooth, the retentive force seemed adequate.

 

Conclusions

When magnetic assemblies were applied to both proximal surfaces of the abutment tooth and the fixation angle was 2° or 4°, the retentive force was adequate. Mechanical interlocking was observed when the fixation angles of the keepers were small, which would be effective for the retention of removable partial dentures.

References

1) Tanaka R., Tsuchida F., Abe M. et al. Magnetic　Attachment on the Proximal Surface of an Abutment Tooth.  J J Mag Dent 13(2): 33-37, 2004. (in Japanese)

2) Mutou R., Abe M., Tsuchida F. et al. Magnetic Attachment on the Proximal Surface of an Abutment Tooth: Second Report.  J J Mag Dent 16(2): 62-66, 2007. (in Japanese)

3)Miyama Y., Tsuchida H., Takishin N. et al. Studies on the retentive force of the magnetic attachment applied on the proximal surface –part 1–. J J Mag Dent 17(1): 30-35,2008. (in Japanese)