Influence of the Axial Surface of Keeper Coping on theCircumferential Tissue Using Three-Dimensional Finite Element
N. Shibuya1, D. Yamanaka1, T. Uchida1, T. Ohyama1,2, S. Nakabayashi1,2, S. Tadokoro1,H. Toyoma1,2, K. Ohtani1,2, T. Fujimoto1, M. Ishijima1 and T. Ishigami1,2
1
Department of Partial Denture Prosthodontics, Nihon University School of Dentistry2
Division of Clinical Research, Dental Research Center, Nihon University School of Dentistry

Introduction
When the keeper coping is applied to the abutment tooth for an overdenture, the shape of the axial
surface of the keeper coping may affect the stress distribution to the abutment tooth and thecircumferential tissue. The aim of this study was to evaluate the influence of the axial surface
shape of the keeper coping on the circumferential tissue, and to analyze the biomechanical behaviorof the cortical bone under the denture base during three different loading conditions using
three-dimentional finite element method.

Materials and Methods
A complete overdenture model with keeper coping delivered on mandibular right canine wasevaluated. The outline of the abutment tooth and mandible were modeled on the basis ofthree-dimentions data from a multi-detector CT (Asteion Super4 Edition, Toshiba, Japan). The
periodontal ligament, cortical bone, cancellous bone, and alveolar mucosa shapes were modeled withreference to anatomical measurements. The analysis models constructed were tooth, cortical bone,
cancellous bone, denture base, keeper coping, periodontal ligament and alveolar mucosa. For thisstudy, Rhinoceros (Version 1.0, Robert McNeil & Associates, U.S.A) and ANSYS (Version 12.1,Ansys Inc., U.S.A.) were used. Table 1 shows the Young modulus and Poisson’s ratio.

Table 1: Material Properties
Fig.1 shows the design of keeper coping and analysis models.
The height of the keeper coping was 2.5mm from the lingual alveolar crest.
The abutment tooth was set to incline 15 degrees for the occlusal plane, and the top surface of thekeeper coping was set perpendicularly to the tooth axis.
Model A was set the inclination angle on axial surface is 0 degree.
Model B was set the inclination angle on axial surface is 30 degrees.
Model C was set the form of axial surface was curved.

Fig.1: The Design of Keeper Coping and Analysis Model
The three different loading simulations (clenching in bilateral occlusion, left lateral occlusion andright lateral occlusion) was set up with the vector of masficatory muscles.
Table2 shows the loading conditions.

Table. 2: The Loading Conditions
The paths of each mandibular condyle ware constrained. Guidance by the articular eminence wassimulated with a planar constraint. It permitted rotations and translations in the specified plane.
The constraining plane was angled 30 degree forward and downward relative to the occlusal plane,
and it was canted 5 degrees medially. The inclination angle of buccal cusp of artificial teeth was set
30 degrees and the lingual cusp of the artificial tooth was 20 degrees.
The balanced occlusion was simulated by ten occlusal stops on complete overdenture (Fig.2).

Fig.2: Constraint Conditions

Results

Figure 3-5 shows the stress distribution of the abutment tooth clenching in bilateral occlusion, leftlateral occlusion and right lateral occlusion. There were no differences in the stress distribution of
the abutment tooth among three kinds of analysis models in three different loading conditions.
The stress concentration was detected on the labial and lingual side of the abutment tooth and thecenter of the mandible.
Figure 6-8 shows the stress distribution graph of the top surface of cortical bone clenching in threedifferent loading conditions. The thirteen analysis points were set on the top surface of cortical bone(analysis points:a-m). There were no significant differences with all models in bilateral occlusion
and right lateral occlusion. It was showed the difference of stress distribution around the abutmenttooth in left lateral occlusion. The stress concentration around the abutment tooth (mesial side anddistal side) was decreased in model C.

Fig.3:Bilateral occlusion

Fig.4:Left lateral occlusion

Fig.5:Right lateral occlusion
The Stress Distribution(Minimum Principal Stress)

Fig.6:Bilateral occlusion

Fig.7:Left lateral occlusion

Fig.8:Right lateral occlusion
The Stress Distribution Graphof The Top Surface of Cortical Bone

Discussions
In model A during left lateral occlusion, the lowest stress concentration was found around theabutment tooth. It could be thought that the axial surface of keeper coping curved was available toreduce the force loading on the abutment tooth.

Conclusions
Within the limitations of this study- it was thought the curved form of the axial surface reduced thebiomechanical risks of the circumferential tissue around the abutment tooth in overdenture.

References
1. Korioth TW, Hannam AG, Deformation of the Human Mandible During Simulated Tooth
Clenching, J Dent Res 73:56-66, 1994.
2. G.E.J. Langenbach, A.G. Hannam, The Role of Passive Muscle Tensions in A Three-dimensional
Dynamic Model of the Human Jaw, Arch Oral Biol 44:557-573, 1999