Attractive Force Analysis of Implant Magnetic Keeper using Three-Dimensional Finite Element Method

− The effect of surface screw hole pattern design for magnetic attachments −

 

H. Kumano, T. Masuda, Y. Nakamura, R. Kanbara, T. Iwai, Y. Ohno, K. Yoshihara, Y. Tanaka

Department of Removable Prosthodontics, School of Dentistry, Aichi-gakuin University

Introduction

The clinical applications of magnetic attachments with implant overdentures has become more common with progressive advances in implant materials and methods of use. The use of the magnetic attachments non-mechanical retentive design and an ease of concealment helps to reduce lateral forces and results in the controlled load transfer to a supporting abutment. The application and use of magnetic attachments with dental implants permits prosthetic flexibility in implant location and orientation, helping the overall esthetic outcome.

An implant magnetic attachment is secured to a implant fixture using a retaining screw. All screw designs require a superior surface access hole on the the magnetic keeper for placement and removal. These keeper screw access holes used may be of different dimensions depending upon the proprietary instrument size requirements for each design. Few studies are available regarding the influence of a screw hole on attractive force. The finite element method is an effective method for the solution of problems with non-linear material behaviors. The dynamics of a magnet interior is visualized by using this method, and permits simulation under changing conditions.

Objective

The purpose of the present study was to analyze the effects of differential screw access hole dimensions to an implant keeper upon attractive force and magnetic field using the three-dimensional (3D) finite element method.

Materials and Methods

1.   Analysis model

Figure 1 shows a implant keeper prototype used as a reference of the analysis model.

An abutment screw is made of magnetic stainless steel, and serves as an implant keeper. An abutment is fixed to the fixture with this screw. A screw hole is located in the center of a keeper. The magnetic assembly is designed to adhere to a magnetic attachment (GIGAUSS D 600; GC). Interior configuration of a magnetic assembly was measured prior to modeling procedures. Actual and proprietary measurements of an attachment were compared to estimate the external shape of the attachment. Internal configuration data was also required for modeling, but was not available. The sample attachment was embedded, sectioned using a diamond cutter, and then internally measured using VF-7510 to determine internal shape and measurement configurations. Figure 2 shows measurements of a magnetic assembly.

The magnet embedded in the assembly was a round-shape configuration. Secondly, three keepers with different screw hole locations including a keeper without a screw hole, a keeper with a screw hole in the center, and a keeper with a screw hole in the edge were designed. Figure 3 shows a diagram and size of an implant keeper designed in the present study.

An analysis model was constructed using Femap (UGS), and μ-MF (μ-TEC) was used for the analysis. Identical element count and nodal point were used for all models. The element type designation was three-dimensional pentahedral and hexahedral elements. Figure 4 shows a constructed finite element model.

A quarter model was created for the purpose of evaluating axial symmetry. Analysis range was peripheral 3 mm of a keeper and magnetic assembly. An element breakdown was conducted. A keeper without a screw hole,a keeper with center screw hole, and a keeper with a edge access screw hole are referred to as Models 1, 2, and 3, respectively.

1.   Analysis condition

The magnetic property value was determined based on the thermal property of the test magnet (GIGAUSS D 600) obtained from a prior study and compared with proprietary information. (Miyata et al.) Although fabrication of the original yoke and keeper material is of SUSXM 27 alloy, the proprietary information is not released nor available. Similar steel property values were thus selected for functional similarity of  magnetic properties. (SUS447J1 steel material) As the SUS447J1 steel material values were assigned, B-H curve was then approximated and selected, for designation of magnetic properties (Table 1). Analysis results were evaluated in terms of magnetic flux density distribution and attractive force.

Results

1.      Magnetic flux density distribution

Figure 5 shows the magnetic flux density distribution. No significant difference in the magnetic flux density distribution inside a magnetic assembly was observed. However, on the adhesive surface between a magnetic assembly and a keeper, a high magnetic flux density was observed around the Model 2 screw hole. As for a magnetic flux density inside a keeper, a magnetic flux distribution extended to the inferior part of a keeper in Models 1 and 3.

2.      Attractive force

Figure 6 shows an attractive force of each model. An attractive force was the highest in Model 1 (520 gf), followed by Model 3 (490 gf), and Model 2 (440 gf). Attractive force decreased by 16% in the Model 2 with a screw hole in the middle, and 6% in the Model 3 with a screw hole in the edge compared to the Model 1 without a screw hole.

Discussions

Although magnetic forces and magnetic fields can be measured using specialized measuring devices, it is difficult to design a magnet for maximum magnetic force based solely these values, and to also verify optimal properties with minimal field leakage. The finite element method is an effective way to examine some of these various issues.

Since the space has a magnetic distribution, an integration path of the space around the analysis model and also the interface between the magnetic assembly and keeper needs to be subdivided for evaluation. A preliminary analysis was performed to calculate figures with minimal influence. A subdivided analysis is considered very accurate for these purposes.

The magnetic properties of the magnetic stainless steel and magnet are important for the analysis. However, the detailed magnetic properties are unknown. Therefore, the SUS447J1 steel material values that have similar magnetic properties as SUSXM27 were assigned. The B-H curve was approximated from these values and used in the analysis. Future challenges lie in accurate value measurement and a search for materials with closer magnetic properties as SUSXM27.

The magnetic assembly used in the present analysis was cup yoke type. Magnetic flux density concentrates on the center of a keeper adsorption surface in this type of assembly. The air apace layer in the non-magnetic area screw hole blocks the magnetic flux, and creates high magnetic flux distribution in the surrounding area, resulting in the oversaturated magnetic flux density distribution of the keeper in the Model 2 compared to the Model 1. In the Model 3 which has a screw hole in the edge, magnetic flux was not blocked due to the side location of a screw hole. Therefore, the magnetic flux distribution of the Model 3 was the same as Model 1.   

Attractive force is calculated by square magnetic flux density and facing area. The Model 2 with a screw hole in the center showed the biggest decrease in attractive force. This is considered to be due to a decrease in facing area and oversaturated magnetic flux density around a screw hole. In the Model 3 with a screw hole in the edge, a decrease in the attractive force was suppressed despite a decrease in the facing area. This is considered to be due to the small influence of the magnetic flux density inside the keeper. It has been reported that a decrease in attractive force can be prevented by making a screw hole smaller in the Model 2. Further analysis is required to obtain optimized implant keeper configuration.

Conclusions

An influence of the screw hole configuration on an implant abutment attachment magnetic keeper surface was analyzed using three-dimensional finite element method, and the following results were achieved.

1.       Oversaturated magnetic flux density was observed inside a keeper around a screw hole in the model with a screw hole in the middle.

2.       Attractive force decreased by 16% in the model with a screw hole in the center, and 6% in the model with a screw hole in the edge compared to the model without a hole.

 

References

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

2. Nakamura, Y. Tanaka, Y. Ishida, T. and et al : Dynamic Analysis of a Magnetic Attachment using Finite Element Method –Comparison of the two dimensional analysis with the three dimensional one-. J J Mag Dent.8:57-62,1999

3. Miyata, T. Niimi, J. Ando, A. and et al : Influence of heating of a magnetic attachment on the attractive force. J J Mag Dent.17:44-50,2008