A Change in Attractive Force of a Keeper Fabricated using a Reuse
Metal of Castable Magnetic Alloy
K. Yoshihara, T. Iwai, Y. Nakamura, H. Konno, T. Kogiso, T. Masuda, Y. Ohno, Y. Takada1, Y.
TanakaDepartment of Removable Proshodontics, School of Dentistry Aichi-Gakuin University
1
Division of Dental Biomaterials, Tohoku University Graduate School of Dentistry

Introduction
In clinical practice, a root cap with a ready-made stainless steel keeper is placed to a non-vital tooth,and a magnetic assembly is embedded in the denture base. The size of a magnetic attachment has
become smaller along with a reduction of the magnet size in recent years. However, there is often acase when a commercially available magnetic attachment cannot be placed due to the limited space,
inclined dental root, and the influence of adjacent teeth. A magnetic alloy root-cap was developed tobe replaced by a conventional root-cap with a keeper. This root-cap was fabricated using a castable
magnetic alloy, and, therefore, attachable to the magnetic assembly. The development of the root-cap
expanded the application of magnetic attachments in the limited space compared with a conventionalroot-cap with a keeper. Hearth formed by casting using a lost-wax method is essential to prevent
casting defects.
A mixture of hearth and virgin metal is used for casting in clinical practice. It is necessary toevaluate an influence of casting using hearth of dental magnetic alloy on the attachment force
between a keeper and a magnetic assembly.

Objective
The present study examined attractive force of a keeper and magnetic assembly fabricated using ahearth of dental magnetic alloy, and analyzed the constituent elements for verification.

Materials and Methods
1.
Materials
Castable magnetic alloy Attract P (TOKURIKI-HONTEN Co.) was used. Attract P is the only
commercially available dental cast magnetic alloy. Figure 1 and Table 1 show an ingot image andproperties of Attract P. The main component of the alloy is Ag-Co-Pd. It also includes Au, Zn, andIn.

Fig1．
「Attract P」（TOKURIKI HONTEN）
Table1．Composition and Properties of Attract P

2.

Experimental method

2.1
Sample fabrication method
Two metals were used for sample fabrication, Attract P ingot and hearth formed by casting thesample using Attract P ingot. The investment material and oxidized layers on the hearth surface wasremoved using a carborundum point, followed by sandblasting to reuse for casting.
Five different samples including a mixture of Attract P ingot and hearth at ratio of 1 : 3 (0) , 1 : 1(25) , and 3 : 1 (50) , ingot alone (75), and hearth alone (100) were prepared. The number of eachsample was 5 (Fig. 2). Figures 3 and 4 show completed samples and their size.

Fig2．Sample preparation methodFig3．Sample

Fig4．Sample Size
2.2
Attractive force measurement
Magnetic assembly GIGAUSS C600 was used, and attractive force between the samples andmagnetic assembly was measured using a compact table-top universal tester machine EZ test(SHIMAZU) 10 times for each sample at 5 mm / min crosshead speed. A custom-made jog devised
by Tanaka et al. (1)(2) was attached to EZ test (Fig. 5). Samples were fixed to the mold using instantadhesive so that magnetic assembly and keeper adsorption face are tightly adhered.

Fig5．Compact Table-top universal tester Ez-test & special JIG
2.3
Constituent element analysis method
Constituent element analysis was performed using an electron probe micro analyzer JXA-8340F(JEOL). Constituent elements were identified by point analysis, followed by the surface analysis of 2x 3 mm area using a probe with 9 μm diameter.
In addition to observation of the adsorption face of each sample, arbitrary face of the ingot surface
was observed as a control.
2.4
Statistical analysis
Attractive force measurement and constituent element analysis were performed for each sample.
Pearson’s product-moment correlation analysis was performed for each result to investigate factors
affecting the change of attractive force.

Results
Attractive force decreased with an increase of hearth content. Constituent element analysis revealed
that Co content in samples decreased and segregation of samples is more likely to happen with anincrease of hearth content.
Regarding Co content in samples, Co w% on the observation surface in sample was employed as an
approximate value of Co content in samples.
Since the segregation phase is rich in Ag, the area with more than 45% Ag content in the Ag
mapping image was regarded as the segregation phase. Segregation phase area ratio on theadsorption face in samples was calculated based on Ag content.
Statistical analysis result revealed a moderate correlation between attractive force and Co content,
attractive force and area ratio of the segregation phase on adsorption face (segregation phase ratio),
hearth and Co contents, and attractive force and Co at 1% significance level. There was a minor
correlation between hearth content and segregation phase ratio (Table 2, Figures 6, 7, 8, 9 and 10).
Table2．Attractive force, Co contents，segregation phase ratio，hearth content

Fig6．Attractive force - Co contents

Fig7．Attractive force - hearth content

Fig8．hearth content - Co contents

Fig9．segregation phase ratio - hearth content

Fig10．hearth content - Attractive force
Constituent element analysis of an ingot confirmed the area with multiple segregations (Fig. 11).

Fig11．SEM of an Ingot
Point analysis of the segregation phase showed that the main component of the segregation phase
was Ag-Pd, whereas that of base alloy was Co-Pd (Fig. 12).

Fig12．Result of point analysis

Discussions
1.
Hearth and Co contents
A moderate correlation between hearth and Co content is considered to be due to the lost Co
contained in the removed oxidized layer. Oxidized layers are formed not only on the surface of thehearth but also on the contact surface with the investment material, and were observed on the surfaceof the cast. (Fig. 13) It is recommended to perform the process from melting to casting in argon
atmosphere to prevent oxidization of the metal.

Fig13．Oxidized layers
2.
Hearth content and segregation phase ratio
Segregation was observed on all the sample surfaces. The correlation between the size and amountof segregation and hearth content was weak. It is considered that composition of Attract P easily
forms segregation regardless of the use of hearth.
Considering segregation phase was commonly observed in the ingot, the influence of biased
segregation caused by coagulation condition of a cast is higher than hearth content.
3.
Attractive force and Co content
Co is a strong magnetic body, and is considered to be involved in magnetic property. Okamoto etal(3) reported that Au-Co alloy shows magnetic property in 8 ? 82 wt%, and Pd-Co alloy showsmagnetic property in more than 5 wt%.
Considering there was a moderate correlation between Co content and attractive force, Co content
is considered to affect attractive force.
The Co amount of the adsorption face in samples was analyzed and evaluated in the present study.
The overall Co content may change if larger segregation exists in the deeper area. Melting samples
and measuring accurate Co content may confirm stronger correlation.
4.
Attractive force and segregation phase ratio
The segregation phase was observed only on the adsorption face, and the size of segregation phasevaried from 10 μ to 500 μm in diameter. Although the influence of segregation phase with a small
diameter on attractive force may be small since the master alloy is situated just below thesegregation phase, that with a large diameter may cause decrease in attractive force in a similar way
as air gap.
A moderate correlation was observed between attractive phase and segregation phase ratio in thepresent study. Although the size of segregation phase was not considered, the influence on attractiveforce may be different between multiple segregation phases with a small diameter and a few
segregation phases with a large diameter despite of the same area ratio.

The size of segregation phase affecting attractive force needs to be investigated. It is also necessary
to confirm the relationship between segregation phase with a large diameter and attractive force.

Conclusions
Moderate correlation was observed between hearth and Co contents, attractive force and Co content,and hearth content and attractive force. The results suggested that Co content decreased afterrecasting using a hearth, resulting in a decrease in attractive force.
A hearth in the present study was formed after casting once from the virgin metal. However, ahearth is repeatedly used in clinical practice, and it is concerned that composition of the alloy
significantly changes.
The composition of the base alloy after casting using a repeatedly-used hearth needs to beconfirmed.

References
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Force of Megnetic Attachments．JJ Mag Dent 19 (2):10-15，2010．
2. Y.Terao，Y.Nakamura，T.Isihda，et al：Mesuring Methods of the Attractive Force of MagneticAttachment．JJ Mag Dent 16(2):14-19, 2007．
3. K. Okamoto, K. Eda, M. Miyazaki et al: Basic research regarding casting magnetic alloy. Dental
Materials Journal, 17 (5) 309-314, 1998