Stress analysis of the tissue around the abutment tooth by thedifference in the keeper angles of the most posterior molar inmagnetic attachment dentures
R. kanbara, H. Kumano, Y. Nakamura, K. Shoji, S. Tanaka and J. Takebe
Department of Removable Prosthodontics, School of Dentistry, Aichi Gakuin University
Abstract
Application of the magnetic attachment to the most posterior molar is effective as the mostposterior support region of the denture. However, a good clinical prognosis is rarely provided for theabutment tooth is located under the denture base with poor self-cleaning property. In this study, weinvestigated the mechanical influence of angular difference of the keeper attractive surface applied tothe most posterior molar to the abutment teeth and surrounding tissues using three dimensional finite
element method.
The analysis model was constructed using a mandibular cast model manufactured by Nissin Co.,
Ltd. In the prosthetic design, the overlay prosthesis was made by applying a magnetic attachment to
the mandibular left second molar. Analysis items were modeled by setting the keeper attractive
surface parallel to the occlusal plane and a model in which the keeper attractive surface was setperpendicular to the tooth axis of the abutment.
From the analysis results, relaxation of the stress distribution in the mandibular left second molar
was observed in the model that the keeper attractive surface was set perpendicular to the tooth axisof the abutment. In addition, decrease of the periodontal ligament burden area and increase of
mucous burden area could be confirmed.

Introduction
Application of the magnetic attachment to the most posterior molar is effective as the mostposterior support region of the denture and furthermore the retention force of the denture can beobtained by the attractive force of the magnetic attachment. The change in attractive force with thechange in angle of the keeper attractive surface angle of the magnetic attachment exerts themaximum attractive force in the direction perpendicular to the keeper attractive surface. 1 For thisreason, it is recommended that the keeper attractive surface of the root cap is basically set parallelto the occlusal plane so that the maximum attractive force can be exerted. However, by setting it
parallel to the occlusal plane in order to acquire the maximum attractive force, the occlusal force isreceived in a direction different from the tooth axis and clinical symptoms may be confirmed in thetissue around the abutment tooth.

Objective
The purpose of this study was to investigate the mechanical influence of the difference in angleof the keeper attractive surface applied to the most posterior molar to the surrounding tissue of theabutment for the mandibular removable partial denture by using the three dimensional finite
element method.

Material and Methods
1. Analysis model
The mandibular model used in this study is shown in Fig.1. For model construction, a mandibular
plaster model and a skull model (P10 - SB.1) manufactured by Nissin Co., Ltd. were used (Fig. 2).
Initially, this mandibular plaster model was scanned using a model and impression scanner (7series, Dental Wings, Montreal, Canada) tomake model shape data in STL format. Next, askull model was CT photographed and theobtained CT data was prepared in STL format
with mandibular bone data and tooth root
shape data using three-dimensional
construction soft (Mimics version l1.0，Fig.1 : The mandibularFig.2 : a mandibular plastermodel used in this study
model and a skull model
Materialise，Leuven，Belgium). We
imported these data into computer aidedengineering（CAE）pre／post processing software (Patran2013 windows 64bit, MSC software, Los Angels, USA) andconstructed a model. The thickness of the residual ridge
mucosa and periodontal ligament were set with reference tothe literature values and the morphology of theFig.3 : The design of the removable partial denture
mandible was simple form. 2The design of the removable partial denture is shown inFig.3. The magnetic attachment was designed for themandibular left second molar and the RPI clasp was appliedon the both sides first premolar as the direct abutment
device.
The constructed analysis model is shown in Fig.4. The
Fig.4 : the constructed analysis model
magnetic attachment applied in this research was GIGAUSSB
A
D1000 and accurately reproduced its high diameter andwidth for model construction.
2. Analysis Items
Analysis items are shown in Fig.5.
The analysis items were an occlusal plane parallel modelthat the keeper attractive surface was set parallel to theFig.5 : the analysis items
occlusal plane and a tooth axis vertical model that theA : occlusal plane parallel model
keeper attractive surface was set perpendicular to the tooth
B : tooth axis vertical model
axis of the mandibular left
Table1 : the mechanical property values Table2 : the material constantsecond molar.
conversion program
The mechanical property valuesof the analysis model are shown in
Table 1. About the periodontal
ligament and the residual ridge
mucosa, these nonlinear
viscoelastic properties wereintroduced by material constantconversion program（Table 2）.
3. Analysis Conditions
The load conditions are shown in Fig.6. The loading site was a
total of 6 places on the mandibular bilateral denture occlusal surface
and the loading direction was perpendicular to the occlusal
plane. Based on the literature value, the load amount was set
to 300 N in total.3 The inferior border of the mandible wasdefined as a constraint condition in the x, y, and z directions.
In the contact condition, the contacting relationship with theFig.6 : the load conditions
tooth and the mucosa in contact with the denture was added by
Coulomb friction and the coefficient of defined Coulomb friction was set at μ = 0.090 for this study.
4Analysis Results
The stress in this analysis was evaluated by Von Mises
stress.
1. Mandibular alveolar boneFig. 7 shows the stress distribution of the alveolar cavityof the mandibular left second molar. Compared to theocclusal plane parallel model, stress relaxation wasconfirmed in the tooth axis vertical model.
2. Mandibular left second molar
A stress distribution of the mandibular left second
molar is shown in Fig.8. Both models showed stressspreading from the mesial root to the furcation. Especiallyin the mesial root, relaxation of stress was confirmed in thetooth axis vertical model compared with the occlusal plane
parallel model.
3. Right supporting denture base mucosa
Fig. 9 shows the stress distribution of the rightsupporting denture base mucosa. Expansion of the stress
distribution was confirmed in the tooth axis verticalmodel compared to the occlusal plane parallel model．An
increase in mucosal burden zone was confirmed.

AB

Fig.7 : the stress distribution of the alveolarcavity of the mandibular left second molar
A : occlusal plane parallel model
B : tooth axis vertical model
ABFig.8 : the stress distribution of themandibular left second molar
A : occlusal plane parallel model
B : tooth axis vertical model
A
B

Discussion and Conclusion
In the finite element analysis studied this time, comparedto the design the keeper attractive surface is set parallel tothe occlusal plane, stress relaxation was confirmed in the Fig.9 : the stress distribution of the rightdesign the keeper attractive surface is set to be supporting denture base mucosa
A : occlusal plane parallel model
perpendicular to the abutment tooth axis. It was also
B : tooth axis vertical model
confirmed that in the burden form of the dental prosthesis,
the region of the periodontal ligament burden decreases and the mucosal burden zone increases.
The difference in the angle of the keeper attractive surface examined this time is 15 °. According
to report of Nakabayashi, the reduction rate of the attractive force is about 17%.1
Based on this result, when applying a magnetic attachment to the most posterior molar, it isextremely important to clarify the condition of the abutment to determine whether to give priority to
the attractive force of magnetic attachment or to give priority to stress relief to the abutment tooth.

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