Influence of keeper tilt angle on retentive force of magnetic attachment

Yoshitada Umekawa¹, Mayu Kokubu¹, Eiichi Nagai^1,2, Kenji Ohtani^1,2, Toshiki Fujimoto¹, Yuusuke Katakura¹, Masaaki Takamura¹, Setsuko Sakaguchi³, Tomohiko Ishigami^1,2

¹Department of Partial Denture Prosthodontics, Nihon University School of Dentistry

²Division of Clinical Research, Dental Research Center, Nihon University School of Dentistry

³Dental Technician Training School, Nihon University School of Dentistry

Introduction

The magnetic attachment shows maximum retentive force when the direction of detachment is perpendicular to the joint surface of the keeper and the assembly. Further, the keeper is placed parallel to the occlusal plane of the overdenture, and the prosthesis is supported against the occlusal force. However, an anterior tooth is tilted in a direction perpendicular to the occlusal plane and has a possible harmful influence on the occlusal force when the keeper is placed parallel to the occlusal plane. (Fig. 1)

Orthogonalizing the keeper surface in the direction of the anterior root of the overdenture is recommended; however, there are few studies on the retentive force of the inclined magnetic attachment.1)

Case I is a maxillary complete overdenture with five copings. The keeper surface is parallel to the occlusal plane. However, the tooth axis of 23 is inclined buccally; therefore, the keeper surface is not orthogonal to the tooth axis. (Fig. 2)

Case II is a maxillary partial overdenture for a hemimandibulectomy patient. The teeth axes of 31 and 32 are inclined lingually; therefore, the keeper surfaces are tilted in order to reduce the occlusal stress. (Fig. 3) These cases are frequently observed in clinical practice.

Objective

The purpose of this study was to evaluate the retentive force of the magnetic attachment at various keeper angles.

Materials and Methods

1. Magnetic attachment

The magnetic attachment that was tested consisted of a magnetic assembly and KB keeper of GIGAUSS C600 (GC).

2. Tensile test

A rectangular parallelepiped (10×10×30 mm) column was prepared by using acrylic resin. The acrylic resin rectangular column was cut at the center at angles ranging from 0° to 75°. (Fig. 4) The magnetic attachment was placed at the center of the cross-sectionally cut surface of the acrylic resin rectangular column. (Fig. 5)

The retentive force of the magnetic attachment was investigated using the testing jig and measured by a digital force gauge (NIDEC-SHIMPO Co.). (Fig. 6) This testing jig contains four ball bearings to prevent sidewise movements of the specimen.

3. Statistical analysis

Statistical analysis was performed using the SPSS ver. 12 software. Statistical comparison was performed using ANOVA, and the significance of differences between each group was evaluated by Tukey's test.

Results

1. Retentive force

Fig. 7 shows the retentive force at various keeper angles. The retentive force of the magnetic attachment decreased from 5.6 N at 0° to 1.6 N at 75°.

There was no significant difference in the retentive force between keeper angles of 30° and 45°.

Discussions

No significant difference is indicated between the magnetic attachment at keeper angles of 30° and 45°. It may be thought that the retentive force observed in this investigation was the result of shear and tensile forces. Therefore, further investigation using more angles is required in order to clarify the equivalence between the angles of 30° and 45°.

The retentive force is reduced at an increased keeper angle, and the occlusal force is directed to the root direction.

Assessment of the abutment teeth is required to reduce the keeper angle to the occlusal plane.

Conclusions

1. The retentive force of GIGAUSS C600 reduced with increase in the setting angle of magnetic attachment.

2. No significant difference was indicated in the retentive force at keeper angles of 30° and 45°.

3. In this study, the experimental testing jig was useful in reducing the slippage of the magnetic attachment.

Acknowledgment

This study was supported by the NEDO Grant (05IS1, 2005) and the Sato Fund, Nihon University School of Dentistry.

References

1) Nakamura Y, Stress analysis of an overlay denture and a magnetic attachment using finite element method, J Jpn Prosthodont Soc 42(2):234-245, 1998. (in Japanese)

Discussion Board