Stress Analysis of Extracoronal Magnetic Attachment using Finite Element Method

Kumano H., Nakamura Y., Ishida T., Masuda T., Miyata T., Ando A., Shoji K., Tanaka Y.

The First Department of Prosthodontics, School of Dentistry, Aichi-Gakuin University

Introduction

A conventional magnetic attachment is generally designed for use with non-vital teeth. A recently developed extracoronal attachment utilizes missing tooth space, and therefore can be applied with vital teeth as well. The introduction of an extracoronal magnetic attachment markedly expands the range clinical applications and uses of magnetic attachments. However, developments in the technical and laboratory applications of these designs have continued to been shown.

Fig. 1 shows the basic shape in case of using EC keeper tray. A markedly rigid mechanism is acquired by using EC keeper tray in the conventional method, and forming a pair of slits on the distal part, and an interlock on the medial side of the abutment tooth, as shown in the left figure. When the shape of the alveolar ridge and the condition of the missing teeth area adjacent to the abutment tooth have mechanical advantage, a slightly lower rigidity is achieved. Laboratory design provides the correct insertion and removal of the denture placement path. These are achieved by placing the guide plane on the distal surface and the rest seat on the mesial part of the abutment tooth, as shown in the right figure.

OBJECTIVE

The condition of the missing teeth areas adjacent to the abutment determines the shape and size of the extracoronal attachment required. There have been various discussions about the shape and size of the attachment required in terms of retentive attachment forces and periodontal disease. In this study, we performed the analysis of the extracoronal magnetic attachment using FEM by modification and adjustment shape and size of different parts of a newly-developed “GIGAUS C600 EC keeper tray”.

ANALYSIS CONDITION

1. Analysis model and Analysis method

Fig. 2 show an original and recently developed analysis model of “GIGAUSS C600 EC keeper tray”.

Analysis model components consisted of 1.EC keeper tray, 2.keeper, and 3. luting cement. Linear elastic stress analysis was used for the analysis. (Fig. 3)

Table 1 shows the analysis condition and mechanical property value. As for values, we selected values that we have been using in our department. The loading conditions were as follows: 1. loading site-the center of the keeper, 2. loading direction-vertical and 3. loading amount-9 points, 100 N in total. As for the constraint condition, lateral surface of the keeper tray was complete constraint in the direction of axes X, Y and Z (Fig. 4).

2. Analysis items

Analysis 1

We performed the analysis when changing the base thickness of a keeper tray with a slit. The base thickness was decremented sequentially from 1.2 mm to 0.9 mm, 0.6 mm and 0.3 mm, and stress distributions were compared and investigated (Fig. 5, 6).

Analysis 2

The modeling of GIGAUSSC600 EC keeper tray without slit was performed. This tray is slightly lower in rigidity, but requires easy laboratory works. The same analysis as analysis 1 was performed, and stress distributions were compared and investigated (Fig. 7).

RESULT

Analysis 1

A comparison of stress distribution figures (Fig. 8) demonstrated high stress concentrations in the neck and slit base areas (indicated by arrows) in each analysis model. The range of stress concentrations extends as the base thickness become thinner. Next, we compared cross section stress distribution figures (Fig. 9), and found high stress concentration in the neck and base part areas. As the base thickness becomes thinner, stress continuity between the neck part and the base is gradually established, and stress distributions are demonstrated as an indicator of nonlinear or rapture analyses.

Analysis 2

A comparison of stress distribution figures (Fig. 10) showed high stress concentrations in the neck part and the base under the neck part. As the base thickness became thinner, expanding in stress distribution area and the stress continuity between the neck part and the base were confirmed. Next, we compared stress distribution figures (Fig. 11) at cross section, and found high stress concentration in the neck part and the base. As the base thickness became thinner, stress continuity between the neck part and the base was observed. A significant stress concentration was observed at the keeper connecting site in the thickness of 0.6 mm and 0.3 mm.

DISCUSSION

Stress continuity and high stress concentration were observed at the neck part and the base in analysis models with the thickness under 0.6 mm in each analysis. These results suggest that fracture starts from the neck part towards the base where the stress continuity is found. In clinical practice, we also have experienced a fracture starting from this region. This fact corroborates the analysis results.

CONCLUSION

It requires a considerable effort to design and manufacture a device of satisfactory strength, which may also transmit minimal stresses to an abutment tooth, resistant to hygiene area problems. The evaluative and pre-production design concerns require considerable preparation and time. A time saving and cost efficient way to develop a new device is to explore reasonable variations and uses with a FEM analysis during the design phase prior to material manufacture or fabrication.

REFERENCES

1. Jackson,T.R.: The applicaiton of rare earth magnetic retention toosseointegrated implants. Int. J. Oral & Maxill. Imp., 1: 81-92, 1986.

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

3. Nakamura, Y.: Stress analysis of overlay denture and a magnetic attachment using finite element method. J Jpn Prosthodont Soc, 42: 234-245, 1998.

Discussion Board