11. The Effect of Abutment Tooth Connection with Extracoronal Attachment using
the Three Dimensional Finite Element Method- Part 2. The Construction of Finite Element Model from CT Data Ando A., Nakamura Y., Kanbara R., Kumano H., Miyata T., Masuda T., Ohno Y. and Tanaka Y.
Removable Prosthodontics, School of Dentistry , Aichi-Gakuin University

Introduction
An extracoronal magnetic attachment retainer design for partial denture prosthetic
applications has been developed. Restorative limitations of space and physiologic
function requirements may complicate esthetic placement of attachments in a partial
denture design. Our previous reports have presented the advantages of an extracoronalmagnetic attachment design for extracoronal applications with vital teeth. The
traditional retainer design requirements satisfying retention and resistance form andwhile providing an applications that is esthetic has shown good patient feedback1-3).
In a distal extension partially edentulous treatment, a patient may refuse to wear
bilateral denture for several reasons, including esthetic concerns and satisfactory hardand soft tissue adaptation and fit.
In applications of distal extension and cantilever, an extracoronal attachment has
potential to cause overload. The optimal and proper use of any remaining teeth
adjacent to the edentulous area for use as supporting abutments is of great concern.
Any decision in the selection of abutment number or interconnected splinting of
supporting abutment teeth is based on the best knowledge based upon comprehensive
dental evaluation. However, there are clinical situations where minimal supporting
information exists to appropriately render these decisions.
Finite Element Method (FEM) has been well applied in stress evaluations forprosthodontic dental studies. We have previously reported the use of two-dimensionalmodels of prosthodontic treatment situations. This study utilized the application of
three dimensional modeling to evaluate the applications and stress distribution patterns
for extracoronal magnetic attachment retainers for partial dentures.
Objective
The aim of the present study was to fabricate three-dimensional finite element modelfrom actual patient data. We investigated the effect of interconnected abutment
number in case of unilateral use of extracoronal magnetic partial denture retainers usingthe FEM model.
Materials and Methods
1. Materials
A subject was selected frompatients who were diagnosed
and treated in the departmentof implant dentistry at ouruniversity hospital.
Theselected patient was given
informed consent before thestudy.
The CT data at
diagnosis and the study model
Fig . 1 Materials
were used in the present study(Fig. 1).
The patient was a 42-year old woman who visited our implant department due tothe left mastication disorder. At first visit, lower left first and second molars weremissing, and no problem was found in the bone and periodontal tissue.
2. Methods
1) Geometry data formation
Table.1 shows the flow chart of the geometry data formation.

Table . 1 Flow Chart untill The Geometry DATA

(1) CT data process

The DICOM data obtained from CT was processed using
three-dimensional image processing and editing software (Mimics,Materialise), and geometry was extracted from CT concentration value.
Segmentation of the teeth, cortical and cancellous bones was performed.
Constructed 3D geometry data was output as a STL format (Fig. 2).

Fig . 2 Managements of CT DATA

(2) Fabrication of a sample for CT scanA sample for CT scan was fabricated to reproduce complex geometry suchas extracoronal attachment and denture. The following is a fabrication
process:○1 Fabrication of a copy model
A copy model of the study model made atdiagnosis was fabricated, and used as a work
model (Fig. 3).
Simulate fabrication of a denture andcrowns were performed on this work model
representing an oral cavity.

Fig . 3 A copy of Study Model○2 Simulate preparation of abutments and fabrication of resin crowns
Simulate preparation of abutments was performed on the work model (Fig.
4-a), followed by the build-up of resin crowns (Fig. 4-b). Scanning resin(Yamahachi Dental MFG, Co.) was used to fabricate a sample. This radiopaquequick cure resin is suitable for reading the geometry since radiopaque contrast
agent is homogenously dispersed.

a . preparationb . resin crownFig . 4 Simulate Preparation○3 Placement of an attachment
The EC keeper tray plastic pattern wasduplicated with scanning resin, and wasplaced on the distal part of the second
premolar following the conventional method.
Milling was performed on the lingual surfaceof the second premolar, and the male part for
an interlock was created between the lingualsurface of the second premolar and thecrown of the first premolar (Fig. 5).

Fig . 5 Complete of Part of Male

○4 Build-up of a metal frame
A removable metal frame that fits into theplaced attachment was fabricated (Fig. 6).
Fig . 6 Part of Metal Flame○5 CT scan and three-dimensional construction
A sample was scanned with Micro-focus X-ray CT (Shimazu Corp.) in Tiffformat.
The data was then processed using three-dimensional image
processing and editing software (Mimics, Materialise), and geometry wasextracted (Fig. 7-a).

○6 Build-up of a denture base

A denture base part was built-up on the metal frame part, and its morphology
was modified (Fig. 7-b).

a . Part of Removable Dentureb . 3D Constructions (MIMICS)

Fig . 7 Complete of Sample and 3D Constructions

2) Finite element Modeling
All geometry data wasimported into Patran (MSCSoftware Corp.) The cervicalmargin of the crown obtained
from CT data and the cervicalmargin of the fabricated crown
were adjusted to coincide in
goodbalanceinthethree-dimensional space basedon the occlusal plane andpanoramic radiography (Fig. 8).

Fig . 8 FEM Modeling3) Analysis
Mechanical property value was input into the constructed finite element model.
The boundary condition was applied, followed by the calculation of an analysis
solver (Marc2005r3, MSC Software).
(1)

Analysis modelFig. 9 shows an overall view of the FEM model. Four models with #4-5,3-4-5, 2-3-4-5, and 1-2-3-4-5 interconnected abutments were constructed.
Element and nodal point numbers are shown in Table.2.

b . Attachment

a . Perspective

c . R.P.D
Fig . 9 FEM Model

Table . 2 Number of Elements and Nodes

(2)Analysis condition
Contactconditions
wereintroduced between a denture andthe mucosa, and an abutment andattachment. Friction type was
Coulomb friction, and the friction
coefficient μ was set at 0.01.
Analysis type was linear elastic
stressanalysis,
andthree-dimensional tetrahedron andpentahedron
element
wereemployed. DELL PRECISION
470 (DELL) was used for theanalysis.

Table . 3 Components and Mechanical Properties○1 Components and mechanical propertiesTable. 3 shows components and mechanical properties of the model. Samemechanical property value was applied for a crown, attachment, and metal frame.
For the periodontal membrane and mucosa, a preliminary experiment was
performed so that the vertical displacement of teeth against the applied load isclose to the known value.

○2 Constraining and loading conditions
Lower bilateral coronoid processes was a complete constraint. A total of 10 Nof vertical loads to the occlusal plane was applied on the occlusal surface ofdenture teeth (Fig. 10).

Fig . 10 Fix and Load Conditions

Results
1 . Distributions
Stress distribution pattern demonstrated con Mises equivalent stress distribution.
1) Abutment
Abutments from the tooth a . 45interconnected#1 to 4 were extracted fromtheanalysis
model.
Figures.11a-d show thestress distribution of eachmodel. All models showedstress concentration in the b . 345interconnectedcrown margin of the mostposterior
abutment.
However, stress distribution
was observed extensively in
the #4-5 interconnectedmodel when focusing on thec . 2345interconnectedroot of the most posterior
tooth #5.
The modelsinclude the tooth #3 as an
abutment demonstrated littlechange in the stress
distribution although slightd . 12345interconnectedstressrelaxation
wasobserved.
2) Cortical bone
Figures 12 a-d show
stress distributions of thecortical bone. The data ofFig . 11 Stress Distributions of Abutment Tooththe cortical bone wasextracted from the model.
In the 4-5 interconnected model, stress concentration was observed throughout thecompact bone around abutment teeth. As the number of abumtment teeth increased,
stress distribution was shifted towards the mesial area.

a . 45interconnectedb . 345interconnectedc . 2345interconnectedd . 12345interconnected

Fig . 12 Stress Distributions of Cortical Bone2 . Displacement
1) Abutment
Figure. 13 shows the distal displacement of abutments. Measuring points were cusp
tips of the most posterior abutments. The results showed a decrease in thedisplacement as the number of interconnected abutment increased, especially in themodels including the tooth #3 as an abutment. The 3-4-5 interconnected model
demonstrated a decrease by 50% compared with the 4-5 model.

Fig . 13 Displacement of abutment

2) Denture
Figure. 14 shows the vertical displacement of a denture. The measuring point wasposterior margin of a denture. The vertical displacement decreased as the number ofinterconnected abutment increased, especially in the models including the tooth #3 as anabutment. The 3-4-5 model interconnected model demonstrated a decrease by 35%
compared with the 4-5 model.

Fig . 14 Displacement of Denture

Discussion

1.

Modeling
We have been investigating the influence of the number of interconnected abutment
on the stress distribution in the tissue around the denture in order to provide theoreticalevidence for the decision-making of the number of interconnected abutment. In
previous studies, we performed the analysis using a simple designed model due to thedifficulty in constructing a complex designed model. However, the analysis results
could not be used as clinical evidence due to the simplicity of the model althoughanalysis results were consistent. The model construction method that we used in thepresent study was non-invasive, and allowed to construct finite element models withaccurate size and structure by fabricating simulated restorations. Each patient has adifferent oral cavity construction. We believe that Finite Element Method has a great
potential for providing theoretical evidence for the optimal denture design in the clinicalsettings by using this method to easily fabricate the patient’s model.

2.

Analysis results
The distal displacement of the most posterior abutment and vertical displacement of adenture significantly decreased by interconnecting three abutments between the second
premolar and canine. No significant decrease was observed by increasing the numberof interconnected abutment more than three. These results are presumably due to anincrease of the number of interconnected abutment, and long canine root.
Conclusion
In the present study, more accurate analysis model was constructed by using actual
patient’s data and a study model.
In the unilateral application of extracoronal attachment, the stress on abutments wasmitigated and the vertical displacement of a denture decreased by increasing the numberof interconnected abutment.
A significant stress relaxation was observed by including the canine as an abutment.
No significant change was observed by increasing the number of interconnected
abutment more than three.
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