Retentive force and Magnetic flux leakage of small sized keeper

 

C. Nagoya¹, Y. Umekawa^1,2, T. Ishigami^1,2, O. Kujirai³, H. Toyoma^1,2, M. Takamura¹, M. Tsuyumu¹ and S. Endo¹

 

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

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

³GC Corporation

Introduction

Dental magnetic attachments of various types and sizes that have satisfactory retentive force and stability are now commercially available^1,2. From among the various sizes of magnetic attachments available, the magnetic attachment is usually selected according to the size and shape of cross-section of the retained root^3,4. Additionally, there are some difficulties to apply circle formed attachment to a shaped root with an elliptical cross-section. Following to this clinical problem, GC produced a new magnetic attachment GIGAUSS C800 (C800). This minor axis of keeper is 0.2 mm smaller than the corresponding magnetic attachment.

Objective

This study sought to evaluate the retentive force and magnetic flux leakage of C800, as compared with D800.

Materials and Methods

The magnetic attachments used in this study and the equipments are presented in Table 1. Two cylindrical magnetic assemblies, GIGAUSS C800 (C800; GC, Tokyo, Japan) and GIGAUSS D800 (C800) were assessed in this study. The sizes of the magnetic attachment are shown in Figure 1.

 

 

Retentive force was measured in a universal testing machine (EZ-TEST; SHIMADZU Co, Kyoto, Japan) using a retentive force testing jig (Nichidaigata; Tokyogiken), and C800 and D800 magnet-keeper combinations were tested at a crosshead speed of 5 mm/min. The vertical retentive force testing jig consisted of a linear ball slide (Liner Ball Slide; THK America, Inc, Schaumburg, Ill) and a universal joint to regulate traction in the perpendicular direction (Fig. 2). The jig was installed in a universal testing machine. Rectangular parallelepiped acrylic resin blocks (15 × 15 × 20 mm) were set in the bottom and traction side of the jig. The keeper and magnetic assembly were attached with cyanoacrylate adhesive (Aron Alpha; TOAGOSEI CO, Tokyo, Japan) in the center of the acrylic resin block. Without allowing for any space between the keeper and magnet assembly, the magnetic assembly was held in place by its force of attraction. The magnetic retentive force of each attachment was measured by attaching the magnetic assembly to the keeper and then dislodging it. Each magnet-keeper combination in a group was tested 5 times, and the mean values were compared.

 

 

Magnetic flux leakage was measured using a Gaussmeter (F.W Bell 5180; Sypris Test and Measurement, Orlando, FL) and dedicated measuring probe (Ultra Thin Transverse Probe STB1X-0201; Sypris Test and Measurement, Orlando, FL); Tektronix Services Solutions (Table I). Ten measurements (2 groups × 5 measurements), were made at 3 points: A, at the outside interface of the keeper and magnetic assembly; B, beside the keeper; and C, at the bottom of the keeper. The probe has an active area that is located 0.3 mm from the tip surface, and the measurement was performed when the probe was in contact with the specimen (Fig. 3).

 

 

Data were analyzed with a 1-way analysis of variance (ANOVA), and differences between the groups were analyzed with Tukey’s Honestly Significant Difference (HSD) post hoc test (α = .05).

Results

The mean retentive force was 700.0 gf for D800 and 719.1 gf for C800. C800 was significantly higher than D800 (p <.05) (Fig. 4).

 

 

The results for magnetic flux leakage are shown in Figure 5. D800 of magnetic leakage was significantly higher at B than at A and C (p<.05). C800 of magnetic flux leakage showed significantly higher at A than at B and C (p <.05). At the measurement point of A on D800, the mean magnetic flux leakage was 29.8 mT and at the point of A on C800 was 27.7 mT, there was no significant different between D800 and C800 (p <.05). At the measurement point of B, the mean magnetic flux leakage was 32.0 mT for D800 and 14.6 mT for C800, (p<.05). C800 showed significantly lower magnetic flux leakage at B (p <.05).

 

 

Discussions

Any deleterious effects on marginal tissues have not been reported, however, there is a need to clarify how the long-term use of magnetic attachments in the oral area might affect patients. The magnetic flux strength decreased in proportion to the square of the distance. As the magnetic attachment is not attached directly to oral tissue because the keeper is attached to a structure composed of metal dental materials, there is sufficient distance from the magnetic attachment to the marginal gingival tissue.

The results indicated that C800 has stronger attractive force, and C800 is lower magnetic flux leakage than D800 at every measurement points, nevertheless C800 has smaller keeper than D800. This considerable reason for these results in this study is differences in shapes of C800 and D800 magnetic assemblies. D800 and C800 have undercut groove for retention. However, circle shaped D800 needs higher retention than elliptic shaped C800. Therefore, the undercut of D800 is bigger than C800. It seems like this groove effect of the magnetic circuit.

Furthermore, the tendency of the magnetic flux at C800 is large at upper surface of the magnetic assembly, in contract to C800, D800 of the magnetic flux is large at attached side of the keeper and magnetic assembly, where is the marginal gingival tissue area. In this study, both C800 and D800 of magnetic field leakage at 0.03 mm from the magnetic assembly were not above 40 mT, and, therefore, its recommended usage should not adversely affect the human body. Anyway, C800 is lower magnetic flux leakage than D800 at every measurement points. Magnetic assembly has undercut area for retention. Undercut area of D800 is bigger than C800.It is suggested that undercut area of C800 had better magnetic circuit than D800, therefore, C800 has lower magnetic flux and stronger attractive force.

Taken together, the retentive force and magnetic flux leakage of C800, this now attachment GUGAUSS C800 is useful to apply for a lot of clinical situations.

Conclusions

Within the limitations of this study, the following conclusions were drawn:

1. Retentive force of GIGAUSS C800 was higher than D800.

2. Magnetic flux strength of GIGAUSS C800 was lower than D800.

References

1. Sasaki H, Kinouchi Y, Tsutsui H, Yoshida Y, Karv M, Ushita T.Sectional prostheses connected by samarium-cobalt magnets. J Prosthet Dent 1984;52:556-8.

2. Akaltan F, Can G. Retentive characteristics of different dental magnetic systems. J Prosthet Dent 1995;74:422-7.

3. Maeda Y, Nakao K, Yagi K, Matsuda S. Composite resin root coping with a keeper for magnetic attachment for replacing the missing coronal portion of a removable partial denture abutment. J Prosthet Dent 2006;96:139-42.

4. Boeckler AF, Morton D, Ehring C, Setz JM. Mechanical properties of magnetic attachments for removable prostheses on teeth and implants. J Prosthodont 2008;17:608-15.