Biomechanical Evaluation of Implant-Retained Overdenture

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Finite Element Analysis
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Overdenture combing with dental implant is the most common technique
nowadays, such as implant-retained overdenture, but the influences of biomechanics
in parameters of implant number, implant distribution, and cortical thickness are
unclear. Moreover, two-implant-retained overdenture, which was mainly inserted at
anterior region of dentition was widely used in edentulous patient. The reasons why
less implants were chosen and where the implants were inserted at the anterior area
should be further investigated. The purpose of this study is to investigate
biomechanical effects of parameters in implant number, implant distribution and
cortical thickness using complete anatomical structure of the implant-retained
overdenture model by three-dimensional finite element (FE) analysis.
FE model, which was consisted of maxilla, food, overdenture, mucosa,
attachments, implants and mandible, was constructed combining with three types of
implant distributions, implant numbers (insertion of two, three, and four implants),
and cortical bone thicknesses, total 19 models for investigating. The FE models of
this study were used to investigate biomechanical effect of implant-retained
overdenture comparing with three major parameters (such as implant number,
implant distribution and cortical thickness). The relationship between mechanical
index of FE models, such as stress, strain and stability, and fracture of
implant-retained overdenture were further investigated.
The results showed that the type of two-implant insertion at the anterior
dentation could provide a lower stress magnitude and better stability for all
components of implant-retained overdenture model, moreover, the type of multiple
implants retained overdenture was also demonstrated the tendency of more and
IV
more stress reduction of the overdenture components while the implant placements
more shifted to posterior region of the dentition. Periimplant bones of the FE
models were significantly evidenced the bone resorption effects due to 4000 micro
strain exceeding, but on the contrary the stress magnitudes of the overdenture,
attachments, and implants were too less to induce the failure. In addition, the effects
of cortical thickness to implants retained overdenture were less important than
implant distribution in this study.
Obviously the biomechanical benefit in the type of two implants retained
overdenture was better than the type of multiple implants retained overdenture. This
evidence is consistent with clinical outcomes which indicate the two implants
retained overdenture reconstruction can provide a better success rate.
Keywords: Implant-retained overdenture; Finite element analysis; Implant number;
Implant distribution; Cortical thickness

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1.1 Classification of Jawbone ................................................................................. 2
1.2 Treatment of Edentulism .................................................................................. 6
1.2.1 Conventional denture ......................................................................... 6
1.2.2 Implant Overdenture .......................................................................... 8
1.3.1 Implant-supported overdenture ........................................................ 11
1.3.2 Implant-retained overdenture ........................................................... 11
1.4 Literature Review............................................................................................ 15
1.4.3 Mechanical Adaptation in Bone ...................................................... 17
1.4.4 Experimental Study .......................................................................... 18
VIII
2.1 Research Procedures ....................................................................................... 22
2.2.2 Reconstruction of Overdenture ........................................................ 28
2.2.3 Integrated Model of Implant-Retained Overdenture ....................... 29
2.3 Element Type and Material Properties ........................................................... 31
2.4 Interface Connection, Loading and Boundary Condition ............................... 33
2.4.1 Definition of Interface Connection .................................................. 33
2.4.2 Loading Condition ........................................................................... 33
2.4.3 Boundary Condition ......................................................................... 35
2.5 Model Parameters of Implant Number, Distribution and Cortical Thickness 36
Chapter 3 Results .................................................................................................... 38
3.1.1 The Implant Distribution Based on Inserted 22 Position ................ 38
66 vs. 606 vs. 6226 vs. 6446 ..................................................................... 45
3.2 Stability ........................................................................................................... 52
3.3 Difference in Anterior and Posterior Implant Placement ............................... 59
3.4 Failure ............................................................................................................. 62
Chapter 4 Discussion .............................................................................................. 68
4.1.1 Implant Number and Distribution .................................................... 68
4.1.2 Effect of Implant Placement between Anterior and Posterior Region
................................................................................................................... 70
Table 1.1-1 Classification of bone density on jaw bone ............................................. 3
Table 1.4.1-1 Most common implant complications ................................................ 16
Table 2.3-1 Material properties ................................................................................ 32
Table 2.4.2-1 The X ,Y ,Z component of muscular force during jaw closing (Left
side) ........................................................................................................................... 34
Table 2.5-1 The scheme and name of different implant number and distribution ... 37
Table 3.5-1 convergent test ....................................................................................... 67
XI
Figure 1.1-1 Bone density distribution [6] .................................................................. 4
Figure 1.1-2 (A) Remodeling changes the shape in the mandible in relation to
edentulism (B) Classification of anterior mandible (base on the mental foramina) (C)
Classification of posterior mandible (base on the mental foramina) [8] .................... 5
Figure 1.2.1-1 Conventional denture .......................................................................... 7
Figure 1.3-1 The Difference between implant-retained overdenture(left) and
implant-supported overdenture(right) ....................................................................... 10
Figure 1.3.2-1 The structure of implant-retained overdenture ................................. 12
Figure 1.3.3-1 Attachment type (A) magnet attachment (B) bar and clip attachment
and (C) ball attachment ............................................................................................. 14
Figure 1.4.2-1 Distribution of contact area between the denture and mucosa under a
vertical incisor load. (The cold tone- close and tight, the warm tone - tilted and
separated from the mucosa) [14] .............................................................................. 17
Figure 1.4.3-1 The relationships between average peak strain and adaptive responses
[31] ............................................................................................................................ 18
Figure 1.4.4-1 Schematic diagram of the implant locations and different test
condition ................................................................................................................... 19
Figure 1.4.4-2 Mean bending moments on all implants for the different test
conditions. ................................................................................................................. 19
Figure 2.2.1-1 Segmentation of mandibular contours by Avizo 7.0 ........................ 25
Figure 2.2.1-2 The muscular attached region of mandible ....................................... 25
Figure 2.2.1-3 Surface model convert to solid model .............................................. 26
XII
Figure 2.2.1-4 The three types of cortical bone thicknesses to reflect three of
edentulous mandible, the average thickness (A) 1.04mm (decreased thickness) (B)
2.04 mm (patient from CT construction) (C) 3.04 mm (increased thickness) .......... 27
Figure 2.2.2-1 Nextengine scanner and turntable ..................................................... 28
Figure 2.2.2-2 To coat an unreflective layer on the denture surface. ....................... 29
Figure 2.2.3-1 A complete model of implant-retained overdenture for investigating in
the finite element analysis. ........................................................................................ 30
Figure 2.3-1 SOLID187 element is a higher order 3-D, 10-node element. .............. 31
Figure 2.4.2-1 The orientation of muscular force during closing jaw (A)
Front View (B) Back View ....................................................................................... 34
Figure2.4.3-1 The boundary condition ..................................................................... 35
Figure 2.5-1 The serial number of tooth position ..................................................... 36
Figure 3.1.1-1 Maximum von Mises stress on components with different number and
distribution based on 22 position .............................................................................. 39
Figure 3.1.2-1 Maximun von Mises stress in (A) denture, (B) attachment, (C)
abutment on 4224model ............................................................................................ 41
Figure 3.1.2-2 Maximum von Mises stress on denture with different number and
Figure 3.1.2-3 Maximum von Mises stress on attachment with different number and
Figure 3.1.2-4 Maximum von Mises stress on abutment with different number and
Figure 3.1.2-5 Maximum microstrain on cortical bone with different number and
XIII
Figure 3.1.2-6 Minimum principal stress on cortical bone with different number and
Figure 3.1.3-1 Maximum von Mises stress on denture with different number and
distribution ................................................................................................................ 46
Figure 3.1.3-2 Maximum von Mises stress on attachment with different number and
distribution ................................................................................................................ 46
Figure 3.1.3-3 Maximum von Mises stress on abutment with different number and
distribution ................................................................................................................ 47
Figure 3.1.3-4 Maximum von Mises strain on cortical bone with different number and
distribution ................................................................................................................ 47
Figure 3.1.3-5 Minimum principal stress on cortical bone with different number and
distribution ................................................................................................................ 48
Figure 3.1.3-6 Maximum von Mises tress and minimum principal stress on
components with different implant number of anterior mandible ............................ 48
Figure 3.1.3-7 The distribution of minimum principal stress at the each model (Blue
arrow refers to the position of minimum value) ....................................................... 49
Figure 3.1.3-8 The distribution of maximum von Mises stress on attachment (red
arrow indicated the position of maxmum value) (A) 6226 (B) 6446 ....................... 50
Figure 3.1.3-9 The distribution of maximum von Mises stress at the dentur (A)
6226 (B) 6446 ........................................................................................................... 51
Figure 3.2.1-1 Deformation in type of 2-implants model at the 1mm cortical thickness
................................................................................................................................... 53
Figure 3.2.1-2 Deformation in type of 3-implants model at the 1 mm cortical
thickness .................................................................................................................... 53
XIV
Figure 3.2.1-3 Deformation in type of 4-implants model at the 1mm cortical thickness
................................................................................................................................... 54
Figure 3.2.1-4 Deformation of 1mm cortical thickness comparing with different
implant number ......................................................................................................... 54
Figure 3.2.1-5 The maximum deformation in the denture at the 1 mm cortical
thickness (A) 22 (B) 606 (C) 6446 ........................................................................... 55
Figure 3.2.2-1 Deformation in type of 3-implants model at the 3mm cortical thickness.
................................................................................................................................... 56
Figure 3.2.2-2 Deformation in type of 4-implants model at the 3mm cortical thickness.
................................................................................................................................... 57
Figure 3.2.2-3 Deformation of 3mm cortical thickness ............................................ 57
Figure 3.2.2-4 Comparison of the best stability in two types of 3- and 4-implants
retained overdenture with 1 and 3mm cortical thickness ......................................... 58
Figure 3.3-1 Comparison with model 22and 66 in the maximum von Mises stress. 60
Figure 3.3-2 Comparison with model 202, 404 and 606in the maximum von Mises
stress. ......................................................................................................................... 60
Figure 3.3-3 Comparison with model 4224, 6226 and 6446in the maximum von
Mises stress. .............................................................................................................. 61
Figure 3.4.1-1 Maximum von Mises strain of two implants at the 1mm cortical
thickness .................................................................................................................... 63
Figure 3.4.1-2 Maximum von Mises strain of three implants at the 1mm cortical
thickness .................................................................................................................... 63
Figure 3.4.1-3 Maximum von Mises strain of four implants at the 1mm cortical
thickness .................................................................................................................... 64
XV
Figure 3.4.2-1 Maximum von Mises stress on the denture, attachment and abutment
in each implanted types. ............................................................................................ 65
Figure 3.5-1 The curve of convergent test in the denture ......................................... 66
Figure 4.1.1-1 Comparison with one- and two-implants model in the maximum von
Mise stress ................................................................................................................. 69
Figure 4.1.1-2 The deformation in comparing with one- and two-implants ............ 69
Figure 4.1.2-1 Influence of bolus position in the type of 6446 insertion ................. 70
Figure 4.1.3-1 Schematic diagram of two-implants-retained overdenture in the
reaction force. ........................................................................................................... 72
reaction force. ........................................................................................................... 72
Figure 4.1.3-3 The enclosing area of implant-retained overdenture with different
implant number and distribution (Blue region). ....................................................... 73
Figure 4.1.5-1 Cantilever arm accompanied with different implant distributions ... 75

Tooth loss is a multifactorial and complex interaction of multiple comorbidities.
If the problem left continued, it may progress to complete edentulism. According to
American college of prosthodontists, edentulism is defined as the absence and
complete loss of all natural dentition [1]. However, edentulism is a common oral
disease in elderly in Taiwan. According to the statistics of National Health
Insurance Bureau from 2003 to 2005 [2], the remaining teeth number was 14.35 for
over 65-years-old people in Taiwan, while the prevalence of edentulous individuals
was 13.3%, and females (29.2%) were higher than males (23.1%). Furthermore, the
prevalence of edentulous for over 75-years-old people was reached 17.4%. In June
2012, the newest statistics of DGBAS indicated that the population whose aged
over 65 years had exceeded 2.5 million in Taiwan and the proportion of the
edentulism up to 26.1% [3]. According to the Department of Statistics in Ministry
of Interior shown in April 2013 that over 65-years-old population had reached
2,628,881 people in Taiwan , it was 11.27% of the total population [4]. According
to the prediction of government, the older population will be 14.4% of the total
population in the 110 years of the Republic of China; as a result of the increase in
edentulism, the demand for treatment will increase.
Edentulism do not only affect facial appearance but also the occlusal stability
and pronunciation. However, the weakened chewing ability will change the choices
of daily diet and nutrient absorption, and cause an evasive frame mind in patients.
Therefore, oral problems in the elderly need treat urgently. Currently implant
2
overdenture is the first choice treatment for edentulous patients. However, there are
many factors to affect the success rate of dental implants, such as bone anatomy,
quality and quantity, implant number, implant placement, implant length and
diameter, occlusal habits, etc. In order to understand the influences of factors, this
study will investigate the biomechanical effects in parametric analysis of
implant-retained overdenture in the edentulous subject.
1.1 Classification of Jawbone
The alveolar bone is the thickened ridge of bone that contains the tooth sockets
on bones that bear teeth, therefore the amount and bone mineral density of alveolar
bone is one of the important factors for primary implant stability.
According to the Wolff’s law [5].- “Every change in the form and function of
bone or of its function alone is followed by certain definite changes in the internal
architecture, and equally definition alternation in its external conformation, in
accordance with mathematical laws.” Therefore, extending the two kinds of
phenomena:
(1) Bone modeling is the process by which osteoclasts break down bone and
release the minerals, resulting in the change of the shape or size of bone. It can also
be the result of disuse and the lack of stimulus for bone maintenance.
(2) Bone remodeling is a process of resorption and formation at the same site that
replaces the previously existing bone. .
Ba
Therefo
classifi
classifi
Fine trabecular
bone respo
Zarb prop
aw morph
, D2, D3,
bone dens
ior la, sity n
1.2 Treatment of Edentulism
As the world population aging, the edentulous population has continued to
grow. It’s an important issue that how to effectively treat the edentulous symptoms.
There are two kinds of treatment type for edentulous patient: (1) Conventional
denture (2) Implant overdenture.
1.2.1 Conventional denture
In the past 100 years, conventional complete denture (Figure 1.2.1-1) was the
only treatment to heal the edentulous patients [10, 11]. The traditional treatment
modality of edentulism has been the fabrication of removable, tissue-supported
complete dentures. The treatment does not need to perform an operation, so the
elderly patients are more acceptable to this treatment type. In functional, it provides
the requirement of simple chewing, but it has lack of tissue between dentures and
mandible to stable and support the denture. Although the denture has used denture
adhesive at the interface, but it still can’t replace the retention of natural teeth.
Therefore, the denture will have the risk to move or fall out when speaking or
chewing.
However, the periodontal ligament plays an intermediate cushion role to
buffer the occlusal loads in natural teeth. Once the teeth have been lost, the bone
will no longer be stimulated by the tooth roots and begin to resorb. Therefore, when
the teeth were extracted or wore traditional dentures through long-term, the
phenomenon of alveolar absorption was unavoidable. Residual alveolar bone will
continue to produce resorption and be damaged, and the retained force and
7
supported force of denture will become worse. Patients who wear conventional
denture can only restore about 10% to 20% of chewing forces.
In 1972, Tallgren et al. in a 25 years long- term follow-up showed that the
absorption of alveolar volume was about 0.4 mm/year for conventional denture
wearer especially on mandible denture case [12, 13]. And absorption rate on
alveolar bone ridge was about four times greater in mandible than in maxilla, this
phenomenon have a great impact for denture retention and stability and also
increase the difficulty of the treatment in the future.
In addition, the alveolar bone has continuous reduction of the residual
alveolar ridges and the soft tissue structures have constantly changed by bearing
excessive load in the long-term [12]. In order to keep the facial features and
compensate for the bone loss, the denture base must be continually increased in size
to fit the shape of alveolar bone. This treatment type just can be short-term
improvement for patients but it’s unable to obtain complete treatment.
Figure 1.2.1-1 Conventional denture
1.2.2 Implant Overdenture
In recent years, the treatment of a fully edentulous mandible by implant
overdenture has become a common technique [14, 15]. Edentulous patients who
used the conventional prostheses can be benefit from implant overdentures [14,
16].
Currently, the endosseous implant of root form is most commonly used in
dental implant. Endosseous implant can be traced to the 1960s in Sweden, Dr.
Brånemark, an anatomy and experimental biologists who demonstrated the
phenomenon “osseointegration”, whereby a biocompatible metal which was
titanium could be structurally integrated into living bone without inflammation [17,
18]. In the early 1980s, Zarb introduced the concept of osseointegration to North
America in Canada conference. In 1985, the American Dental Association also
approved that dental implant used in clinics in the Americas [19]. The earliest
studies about implant overdenture was traced back to 1985 in Swedish, Stalblad et
al. proposed that implants could apply to edentulous patients [19]. In 1987, Van
Steenberghe et al. reported on the possibility of using overdentures supported by
two Brånemark implants to treat 43 mandibular denture patients. After the 52
months clinical follow-up, the result showed that the success rate was up to 98%
[20, 21]. “In 2002, the McGill consensus statement suggested that an overdenture
retained by two implants should be the first choice of treatment for the edentulous
mandible” [22].
The patient obtains several advantages with implant overdenture, such as (1)
minimum anterior bone loss, (2) improved esthetics, stability, occlusion, chewing
efficiency, retention, support, and speech, (3) decrease in soft tissue abrasions [23].
9
According to the literatures, anterior residual ridge where place the implants has
minimum bone resorption. An average of 4-mm vertical bone loss occurs during
the first year after the extraction of mandibular teeth. The bone under an
overdenture may resorb as little as 0.6mm vertically over 5 years; After healing of
residual ridge, annual rate of reduction in height is about 0.1-0.2 mm in mandible
[23, 24]. In 2003, Kordatzis reported that the estimated average reduction of
residual ridge in height was 1.63 mm for conventional denture and 0.69 mm for
implant overdenture, ie, almost 1 mm less in the overdenture [25]. Based on the
several advantages, implant overdenture has become a major treatment in recent
years. And implant overdenture treatment can divide into the implant-retained
overdenture and implant-supported overdenture.
Implant-Supported Overdenture
Base on the implant numbers and supported types, the treatment can be
divided into two categories of implant-retained and implant-supported overdenture.
According to the terminology of implant prostheses [26]implant-retained
overdenture was referred to that the number of implants were less than four and the
biting force will be shared by the implant and mucosa. The implant-supported
overdenture was referred to that the number of implants was 4 to 6 and the biting
force will be entirely born by the implants (Figure 1.3-1).
Figure 1.3-1 The Difference between implant-retained overdenture(left) and
implant-supported overdenture(right)
In 2005, Misch proposed standard indications for the patients [23]
1. Tissue serious defect in posterior areas,
2. Lack of retention and stability
3. Difficult in conversation,
4. Soft tissue inflammation
Implant - supported overdenture suffer the occlusal force entirely by implant
rather than the tissue, hence it can reduce bone absorption, increase the bone
volume of posterior, and constrain the lateral movement of denture. But this
treatment has increased the stability by increasing implant number, so the cost is
relatively high. If cost don’t mainly consider, the treatment will be a good choice
for patients.
The implant retained overdenture implant used with less implants placement,
the cost is cheaper for patients and excellent therapeutic effect was evidenced after
treatment. The implant-retained overdenture for mandible was generally consisted
of dental implant, the abutment containing one half of the attachment system, and
the overdenture prosthesis, which houses the other half of the attachment system, as
shown Figure 1.3.2-1.
part
the
vide
orm
13
and function (Figure 1.3.3-1). Three designs have different ideas and theories, but
most authors consider that it’s no difference between the satisfaction of patients
[27]. The magnet was combined with overdenture by using magnetic poles attract
abnormal way, so the retention of axial was stronger than lateral. When the denture
bear lateral force, the attachment would slide and cause the denture dislocation.
Therefore, it should be avoided producing lateral force in this system. When
compared with bar attachment and ball attachment, bar attachment needed more
requirements to repair and derived hygiene problems. Since ball attachment is
considered a simplified and cost-effective treatment as compared to bar and clip
type implant overdentures and it has fewer complications and more retention force.
Therefore, ball attachment type combining with the overdenture was selected in
this study to investigate the biomechanical effects of the implant-retained
overdenture.
14
Figure 1.3.3-1 Attachment type (A) magnet attachment (B) bar and clip attachment
and (C) ball attachment
1.4 Literature Review
1.4.1 Clinical Review
Mericske-Stern, Jemt, Naert, Behneke et al. reported survival rates of implants
supporting an overdenture ranging from 94.5% to 98.8% up to 5 years .In 2002,
Meijer et al. reported a prospective study that 30 edentulous patients were treated
with two endosseous implants in the interforaminal region of the mandible. The
5-year survival rate of implants in this study was 98.3% to 100% [28].
Although scholars used different implant system and attachment, but its studies
had shown high success rates for implant. Therefore, implant-retained mandibular
overdenture treatment was a high success rate and predictable treatment for
edentulous patients.
Although the implant-retained overdenture has a high success rate in clinical,
there are still many complications. In 2003, Charles et al. Reported many implant
complications such as overdenture attachment fracture, overdenture fracture, etc [29].
“It was not possible to calculate an overall complications incidence for prostheses
because there were not multiple clinical studies that simultaneously evaluated most
of complications. “
1.4.2 Finite Element Analysis
In 1996, Atilla Sertgoz and Sungur Guvener constructed a 3D finite element
model of simplified fixed complete dentures, and loaded a 25N horizontal force and a
75 N vertical force on the distal end of dental bridge to analyze the stress distribution.
The partial results shown that stresses were concentrated at the most distal
piri-implant bone and increasing cantilever length resulted in increased stress values
at the bone/implant interface [30].
In 2012, Jinyin Liu et al. constructed four 3D finite element model of
mandibular overdentures which used 1-4implants.Three loading type were applied:
100N vertical load on left first molar, 100N inclined load on left first molar and a
100N inclined load on the lower incisors. The models were constrained at the nodes
on the mesial and distal bone in all degrees of freedom. The result was shown that
when functioning with the anterior teeth, three- and four-implant model were
steadier than two-implant model. Because the implant placed in anterior zone could
avoid the intrusion of the anterior portion of the denture towards the tissues [14].
17
Figure 1.4.2-1 Distribution of contact area between the denture and mucosa under a
vertical incisor load. (The cold tone- close and tight, the warm tone - tilted and
separated from the mucosa) [14]
1.4.3 Mechanical Adaptation in Bone
Frost reported a model of four zones for compact bone as it related to
mechanical adaptation to strain before spontaneous fracture (Figure 1.4.3-1) [23,
31]. The microstrain of bone for trivial loading was reported to be 0 to 50
microstrain. The range of strain value would lose mineral density and occur to
disuse atrophy. The lower limit of the physiological strain might be around 200
microstrain. Modeling was stimulated at strains above 2,500 microstrain, whereas
remodeling was stimulated when strains fall below about 200 microstrain. The mid
overload zone (1,500-3,000 microstrain), high deformations occurred in
peri-implant bone. Pathologic overload zone were reached 4,000 microstrain,
which m
1.5 Motivation and Objective
Treatment types for edentulous patients can be considered two types of
conventional denture and implant-involved overdenture to reconstruct occlusal
function. Overdenture combing with dental implant is the most common technique
nowadays, such as implant-retained overdenture, but the influences of
biomechanics in parameters of implant number, implant placement distribution,
and cortical thickness are unclear. Moreover, two-implant-retained overdenture,
placement was mainly located at anterior region of dentition, was widely used in
edentulous patient. The reason why less implants were chosen and where the
implants were inserted at the anterior area should be further investigated.
Based on the concept of biomechanics, we know that numerous
numbers of the implants and dispersed placement of the implants could provide a
better stability and stress distribution to increase in success rate. There is a
conflict biomechanical concept and clinical application in prosthesis of
implant-retained overdenture. Therefore, the purpose of this study is to investigate
the biomechanical effects of the parameters in implant number, implant distribution
and cortical thickness to complete anatomical structure of the implant-retained
overdenture model using three-dimensional finite element analysis.
Specific aims:
1. To evaluate the fracture sites of the implant-retained overdenture in the
different parameters of the finite element models..
2. To understand the better stability in combinations of the parameters for
implant-retained overdenture .
21
3. To investigate the bone resorption around implant holes under biting force
applied.
4. To explain whether the anterior placement of the implant is good for clinical
operation.
22
2.1 Research Procedures
In this study, the process could be summarized into reconstruction of
three-dimensional finite element model and biomechanical analysis of
implant-retained overdenture model. The process in this study was shown in
Figure 2.1-1.
Reconstruction of 3D Model
The models of maxillary tooth and mandible were reconstructed from CT
images of edentulous patient by using Avizo 7.0 software which could identify the
boundary contour of mandible (including cortical bone and cancellous bone) and
stratify 2D images to 3D model. Then the data were exported as surface model of
STL format, and then imported to Geomagic to reconstruct the cortical bone
thickness.
The surface model was obtained from the shape of the overdenture of the
patients by using Nextengine 3D scanner. The model was imported to Geomagic
software to convert into solid model.
Based on the size and shape of implants of the ITI manufactured company,
solid model of attachment and implant were constructed by CAD procedure and all
components were merged by Solidworks.
Reconstruction and Analysis of Finite Element Model
These models were imported into Ansys-workbench software to mesh and
convert to the FE model, which was included with maxilla, food, overdenture,
mucosa, attachment, implants and mandible.
23
The model of this study was used to investigate biomechanical effect of
implant-retained overdenture with different parameters (implant number, implant
distribution and cortical thickness). The indexes focus on the von-Mises stress in
the distribution on overdenture, attachment and implants, as well as bone of
von-Mises strain.
Model
2.2.1 Reconstruction of Maxillary and Mandible Bone
To obtain the accurate geometry of mandible, CT images were selected from
a 68-ages female who was edentulism. The images were utilized to build the
mandible and maxilla geometry (maxillary tooth, cortical bone and cancellous
bone of mandible). Detail reconstructed processes are showed as following:
1. To import the CT images into medical image reconstruction
software-Avizo7.0, and established both the upper and the lower threshold of
pixel intensity. The contours of mandible and maxilla could be identified by
segmentation mask (Figure 2.2-1).
2. All contours were reconstructed for 3D surface model.
3. The surface model were exported as “.stl” file and imported to Geomagic
software. The surface model was smoothed to provide the better mesh
quality.
4. To create the insertion area of elevator masticatory muscles on the surface of
the mandible (Figure 2.2-2) [33].
5. The solid model was exported as “IGES” file and further imported into
Solidworks software to covert and operate (Figure 2.2-3).
Figure 2.2
orks
27
Figure 2.2.1-4 The three types of cortical bone thicknesses to reflect three of
edentulous mandible, the average thickness (A) 1.04mm (decreased thickness) (B)
2.04 mm (patient from CT construction) (C) 3.04 mm (increased thickness)
(A)
(B)
(C)
ner
rate
for
g
olid
her
29
Figure 2.2.2-2 To coat an unreflective layer on the denture surface.
2.2.3 Integrated Model of Implant-Retained Overdenture
Based on the size and shape of the ITI implant, a solid model was constructed
by CAD software of Solidworks 2010. The soft tissue of mucosa between denture
and mandible was also reconstructed to reflect precise anatomy of the mandible
for the 3D finite element analysis. In this 3D finite element analysis, the food
bolus, an ellipsoid built 15 mm in major axis and 12 mm in minor axis, was
simulated as mouth chewing located at the first molar between maxillary and
mandibular bone. The implants and prosthetic components were merged with the
mandible, mucosa, and denture in the Solidworks software.. Finally, the complete
implant-retained overdenture models with different parameters were imported to
the ANSYS software of the finite element analysis to simulate the influences of the
parameters of the implant-retained overdenture (Figure 2.2.3-1).
Figure
Cancellous
Tooth 84100 0.33 [36]
Bolus 21.57 0.45 [36]
33
The interface connections, loading conditions and boundary condition were
defined for pre-processing of finite element analysis. Its settings were described as
below:
2.4.1 Definition of Interface Connection
According to anatomical condition, the coefficient of friction in 0.334 was
assumed between the overdenture and mucosa [14]; the coefficient of friction of the
interface between overdenture and bolus was assumed to be 0.2 [36]. Moreover, the
interfaces between attachment (cap) and abutments (ball) were set to no separation
with sliding between two parts of the models. Total bonding between bone (cortical
bone and cancellous bone) and implants was assumed to simulate a complete
implant osseointegration.
2.4.2 Loading Condition
This study was simulated the motion of closing jaw when eating food, and the
muscles of closing jaw was contained masseter muscle, temporalis muscle, medial
pterygoid. The muscle forces were identified the coordinates of the muscles’
insertion and origin site according to human anatomy. The unit vectors of muscles
could be calculates by the coordinates of insertion and origin site. The magnitude
of X, Y, Z components of muscular force were selected and applied by previous
literatures (Table 2.4.2-1) [37-40]. Then the components were applied on the
surfaces of mandible where the contact area of muscles marked out by Geomagic
(Figure 2.4.2-1).
34
Table 2.4.2-1 The X ,Y ,Z component of muscular force during jaw closing
(Left side)
Figure 2.4.2-1 The orientation of muscular force during closing jaw
(A) Front View (B) Back View
Muscle Position X-Force(N) Y-Force(N) Z-Force(N)
Masster Superficial 64.65 -87.1 250.306
Temporalis Anterior -66.473 66.358 293.33
Posterior -99.873 171 100.33
Cortical Thickness
This study is to investigate the biomechanical effects of the implant-retained
overdenture using the 3D finite element models which were constructed combining
with three types of implant distributions (for example position of 22, 202, and
4224), three types of implant numbers (such as two, three, and four implants), and
three types of cortical bone thicknesses (such as 1mm, 2mm, and 3mm thicknesses).
Implant-retained overdenture was commonly technique, especially using two
implants placed at anterior zone in clinical operation. Integrated the above
parameters the finite element models were total number of 19 models. For total 19
models were denominated according to implant number and implant position that
the number in front and behind of dash represented implant number and implant
position respectively (Figure 2.5-1). The schematic diagram of the implant number
and implant distribution were shown on the Table 2.5-1, and the implant position
was named A, B, C and D by the sequence from working side (food block) to
non-working side.
1 2
3 4
The type of two-implant-retained overdenture was mainly located at anterior
region of dentition was widely used in edentulous patient in clinic. But some
literatures reported that increased in the implant number and wider implant
distribution could enhance the stability of structure. This section will compare the
effect between implant number and distribution.
3.1.1 The Implant Distribution Based on Inserted 22 Position
22 vs. 202 vs. 4224 vs. 6226
Base on the position of lateral incisor, there has investigated the different in
implant number and position at anterior region. However, the three kinds of cortical
thickness have the same trend of distribution, so cortical thickness of 1mm
represents to describe in detail. In four models, the maximum von Mises stress were
concentrated on the overdenture, attachment and abutment of peri-implant of
working side. Whether microstrain or minimum principal stress, the maximum
value were also concentrated on peri-implant bone at loading side (Figure 3.1.1-1).
The maximum von Mises stress was 681.69 MPa on attachment, 349.98 MPa on
the neck of abutment in 4224 model. The minimum principal stress is -302.13 MPa
on the peri-implant bone of loading side in 4224 model.
Comparing the implant number of anterior zone and posterior zone (22 vs. 202
vs. 4224), the result showed that the increase of the implant numbercould increase
the value of stress in components.
W
3 3 2 2 1 1
1 1 2 2 3 3 4 4 5 5 6 6 7 7
vo n M
P a)
When compa
ult showed
plants but
350 300 250 200 150 100 50 0
50 100 150 200 250 300 350 400 450 500 550 600 650 700 750
C u sp A
C D A B
24 vs. 6226
value on bo
404 vs. 4224 vs.6446
Base on the position of first premolar, there has investigated the different in
implant number and position on anterior and posterior regions of mandible. All
value of indexs were the highest on loading side. In total view, 4 implants retained
overdenture which inserted in 4224 model have the higher von Mises stress value
on denture (45.817 MPa), attachment (349.98MPa), and abutment (681.69 MPa).
Maximum microstrain is 15797 on the cortical bone of working side and minimum
pricipal stress is -302.13 MPa (Figure 3.1.2-1).
When compared the implant number of anterior area (404 vs. 4224), the result
showed that the value of 4-implants was higher than 3-implants in attachment,
abutment and bone on the loading site.
The result showed that the value of stress or microstrain would increase when
increased implant number, especially inserted in anterior mandible, but the value
would decrease when the implant move to posterior area(Figure 3.1.2-2~6).
41
Figure 3.1.2-1 Maximun von Mises stress in (A) denture, (B) attachment, (C)
abutment on 4224model
vo n M is e s tr e ss (M
P a)
M i
t (M
P a)
P a)
Maximum v
P a)
aximum vo
p ri n ci p al st re ss (M
P a)
M in im
u m p ri n ci p al s tr e ss (M
P a)
Minimum p
and d
66 vs. 606 vs. 6226 vs. 6446
Base on the position of first molar, there has investigated the different in
implant number and position at anterior region. However, the three types of cortical
thicknesses have the same trend of distribution, hence, cortical thickness of 1mm
represents to describe in detail. In four models, the maximum von Mises stress was
concentrated on the overdenture, attachment and abutment of peri-implant of
working side. Whether microstrain or minimum principal stress, the maximum
value are also concentrated on peri-implant bone at loading side (Figure 3.1.3-1~5).
The maximum von Mises stress was 683.93MPa in neck of abutment, and the
maximum microstrain is 13585 at the 6226 model. The minimum principal stress is
-264.58 MPa on the loading side at the 6226 model.
When compared the implant number of anterior area (66 vs. 606 vs. 6226), the
result showed that the increased the implant number in anterior region, the value of
stress would increase in components and microstrain on bone, but decreased in the
non-loading side (Figure3.1.3-6~7).
When compared two types of implant inserted on anterior mandible (6226 vs.
6446), the result showed that more distal placement could reduce the value on bone
and implants, but increase the value on denture and attachment of loading side
(Figure3.1.3-8~9). And the von Mises stresses on the loading side of denture were
22.756 and 25.527 MPa at the distal and mesial region respectively.
When compared two types of 606 and 6446 models, the resultsshowed that
there were no difference between two models at the bone and implant of loading
side even though the model 6446 have more implants than the model 606 (1.592%
in von M
is e s st re ss (M
P a)
is e s st re ss (M
P a)
is e s st re ss (M
P a)
aximum vo
Maximum vo
M in im
u m p ri n ci p al s tr e ss (M
P a)
er
49
Figure 3.1.3-7 The distribution of minimum principal stress at the each model
(Blue arrow refers to the position of minimum value)
50
Figure 3.1.3-8 The distribution of maximum von Mises stress on attachment (red
arrow indicated the position of maxmum value) (A) 6226 (B) 6446
(A)
(B)
51
Figure 3.1.3-9 The distribution of maximum von Mises stress at the dentur
(A) 6226 (B) 6446
3.2 Stability
Cortical bone thickness is one of the most important factor to influence the
primary stability of implant , so this section is focus to investigate the maximum
deformation of denture and bone to compare the effect of stability both the worst
and the best bone thickness.
3.2.1 Thin Cortical Thickness (1 mm)
The result in type of 2-implants model showed that the implants inserted in
anterior mandible could significantly reduce the deformation on the denture and
bone, as shown Figure 3.2.1-1.
The result in type of 3-implants model showed that wider implant distribution
(606) could more reduce deformation almost 50% than closer implant distribution
(202), as shown Figure 3.2.1-2.
In type of 4-implants model had the same trend as type of 3-implants model,
in other words, the deformation of wider implant distribution (6446) was lower
than the narrower implant distribution (4224) as shown Figure 3.2.1-3.
According to the foregoing comparisons, we obtained the effect of implant
distribution in different implant number groups for the stability. Furthermore, the
smallest deformation values in each group was obtained and compared again to
obtain the best stability from implant number (Figure 3.2.1-4~5). And the result
showed that the stability of 2-implants retained overdenture was better than 3-,
4-implants retained overdenture, but there was no difference between 3-and
4-implants retained overdenture.
0
1
2
3
4
5
6
7
A
Deformat
Deformati
A
D
B
C
0
0.5
1
1.5
2
2.5
3
3.5
4
Deformat
Deformatio
2A2B
6A0B6C
6A4B4C6D
4A2
6A2
6A4
nt
2B2C4D
2B2C6D
4B4C6D
55
Figure 3.2.1-5 The maximum deformation in the denture at the 1 mm cortical
thickness (A) 22 (B) 606 (C) 6446
Unit: mm
rtical Thic
0
0.5
1
1.5
2
2.5
3
3.5
4
2 Deformat
gure 3.2.2-3
A B
at io n (m
Model 22 vs. 66 in 1mm
The same cortical thickness was used to investigate the effects of implant
insertion in the anterior and posterior area. The results indicated that the stress of
the denture, attachment and abutment in the model 66 was higher than model 22
(Figure 3.3-1).
Model 202 vs. 404 vs. 606 in 1mm
The result showed that model 202 was the lowest von Mises stress on the
denture, attachment and abutment of 14.173MPa, 77.422MPa and 165.15MPa
respectively. And furthermore, model 404 was detected the highest stress on
denture and attachment (Figure 3.3-2).
Model 4224 vs. 6446 vs. 6226 in 1mm
Comparing with three types of inserted distribution of four implants, the
results displayed that the four-implants retained overdenture with closer
distribution in the posterior region was the better ability of dispersed stress (Figure
3.3-3).
Figure
Figure
is e s st re ss (M
P a)
P a)
mparison w
mparison w
Cusp A
P a)
mparison w
Implant-retained overdenture was used with repeated biting force fora
long-term duration, the overdenture complex would be damaged and caused bone
resorption, which was an unavoidable phenomenon .This section was focused on
failure investigation of bone and overdenture complex.
3.4.1 Bone Resorption
This section assessed whether the bone would be produced resorption by
excessive force for a long term. Bone resorption caused the implant loosening
and the structure unstable. In the finite element method, equivalent strain on the
bones is used to determine whether the bone would cause bone resorption to infer
the probability of occurrence. In this section, cortical thickness of 1 mm was
selected to evaluate as the worse situation of bone resorpion. Figure 3.4.1-1 ~
3.4.1-3 showed that the equivalent strain changed with different implant number
and distribution. Horizontal axis was implant placement and the vertical axis was
the value of equivalent strain and the maximum equivalent strain appeared in the
bone around the implant side. It also could be found by Figure 3.4.1-1, overdenture
was inserted 2 implants, implant insertion more anteriorly could significantly
reduce the equivalent strain. To compared with Figure 3.4.1-2 and Figure 3.4.1-3,
insertion of multiple implants found that implant inserted more transferred in the
posterior region could indeed reduce the equivalent strain.
Figur
Figure
is e s st ra in
500
1000
1500
2000
2500
3000
is e s st ra in
Maximum
is e s st ra in
Maximum v
ent study,
he loading
maximum s
ectional ar
is e s st re ss (M
P a)
gure 3.4.2-
3.5 The convergent test
In finite element analysis, the amount of elements numbers will affect the
results of the model by numbers of meshing elements. More and more elements
number seemed to increase the accuracy of the FE analysis.. Therefore, the
convergent test can provide a more appropriate number of elements, to avoid
consuming in calculated time, in addition, this test could increase the reliability of
the FE model.
All models reached convergent criteria, the error percentage of von Mises
stress less than 3% (Figure 3.5-1). According to convergent test of the FE models,
we known that the element number 529,078 in the FE model has been converged,
whichrepresented the element number by mesing procedure in this FE model have
enough reliability (Table 3.5-1).
Figure 3.5-1 The curve of convergent test in the denture
15
15.5
16
16.5
17
17.5
18
18.5
19
vo n M
p a)
Nodes number
67
Stress Percentage of convergence
4.1.1 Implant Number and Distribution
In general, we believed that the greater implant number and wider implant
distribution would be dispersed the stress, but this effect was only found in
multiple-implants retained overdenture in the present study. Two implants
supported the overdenture could move backward or forward when the food
mastication was applied. Patients’ occlusal habits can influence of above effect,
hence, different patient will reflet varied stress distribution in the overdenure
model. Actually, the type of the 2-implants-retained overdenture, especially
inserted more anterior region, could provide a better steady of the overdenture
structure, the result of two-implants-retained overdenture of the FE analysis was
evidenced. The implant distribution based on 66 insertion in the multiple implant
retained overdenture shown that the implants insertion in the first molar such as
three-implants and four-implants retained overdenture, had the similar closed area
to disperse the stress, therefore, three-implants retained overdenture was enough
to sustain under biting force.
This study was also constructed a set of one-implant-retained overdenture
with different bone thickness, and further compared with two-implant-retained
overdenture to obtain the biomechanical effects of the implant number. The result
showed that two-implants retained overdenture still had better stability and lower
stress value, obviously, 2-implants model can provide a line share to resist the
biting force. In contrast with two-implants model, one-implant supported the
overden
is e s tr e ss (M
P a)
at io n (m
at io n (m
4.1.3 Stability
The effect of distribution and the number of the implants had the same trend
in the stability in two kinds of cortical thickness. The two-implants retained
overdenture, the food bolus was constrained at the first molar, in the model 66
was applied closing mouth movement to cause the major deformation on the
overdenture, at the same time, anterior part of the overdenture generated tilt. In
the model 22, the occlusal force oppressed the mucosa on the end of overdenture,
moreover, two implants located at the anterior region to support dentures and
produce inhibition reaction force to avoid the denture posterior reclining, hence,
this type of insertion could produce the smaller deformation than that of model 66
(Figure 4.1.3-1). Furthermore, in order to verify whether the effect bolus position
caused different results, this study simulated the bolus position on incisors model as
a control,. Effect of occlusal position was found indeed influential, but stability of
the model 66 was still poor than the model 22 (Figure 4.1.3-2).
The implant retained overdenture, which used more than 3 implants was
sufficient to constitute a plane, according to σ = F / A which constituted the larger
area could more evenly distributed stress and increased the structural stability. The
influenced factors of number and larger distribution area were evidenced with
the better stability (Figure 4.1.3-3).
Figur
Figur
at io n (m
Figure 4.1.3-3 The enclosing area of implant-retained overdenture with different
implant number and distribution (Blue region).
74
4.1.4 Failure
Previous article reported that the equivalent strain of bone was more than 4000
microstrain might cause micro-fracture, such as bone resorption, at the
bone-implant interface. moreover, cortical bone fractures occurred at
10,000-20,000 microstrain. In the present study, all peri-implant bones at the
loading side were discovered over 4000 microstrain, to reach pathological overload
zone. This study was applied the maximum biting force during chewing and
performed a load step of static simulation, hence, the results of the FE models
could appear a exceed strain magnitude to induce the bone resorption.
The concept was the same as section 4.1.3, thus the larger area could more
evenly distributed stress and increased the structural stability for mutiple
implants-retained overdenture. But particular, two- implant-retained overdenture
should be suggested to inserte away from biting side to avoid direct compression
on the implant.
4.2 Overview
The thicknesses of the cortical bone were always considered as an important
factor to influence the biomechanical effect of implant-retained overdenture from
previous studies, but this factor was not significantly influence in this
three-dimensional FEA. In other words, the implant distribution was more
important than the cortical thickness in this study.
In the two-implants retained overdenture, the implant placement should be
inserted more forward anterior region to provide better stress reduced and stability
incresaed. In multiple-implants retained overdenture, the implant placement should
be dispersed in the posterior region, where closed to the biting force to obtain a
better biomechanical effect. In the present study, we found that different food bolus
constraint reflected different stress distribution, this parameter should be
examined in the further study.
From the present results, the two-implants retained overdenture was inserted
at the anterior region to maintain a better postoperative result for short term
observation. And the reason why the implant inserted in the more anterior
mandible may be due to (1) bone quality of anterior mandible is better than
posterior mandible, (2) the sapce of opening mouth during surgery is limited to
insert implant at the posterior area, and (3) the mental foramens are located under
first premolar in the anterior region of the mandible. Therefore, in order to avoid
injuring the nerve, implant is often inserted in front of the first premolar in clinic.
77
4.3 Limitation
1. To simplify the material properties were assumed to be homogeneous and
isotropic which was not consisted with human body.
2. The loading condition of muscular forces in direction and attachment area was
simplified.
3. Real implant should be two-piece structures, but this study was eliminate the
interface between abutment and implant as one-piece structure.
4. The interface between the cap of attachment and the ball ofabutment was
simplified without clasp.
5. The bolus was placed at the first molar to simulate grinding the foodstuff.
78
1. The effects of cortical thickness for investigating the implant-retained
overdenture models were not more important than implant distribution in this
preliminary study, strictly speaking, 1mm step size to increase and decrease
cortical thickness as an average thickness applying to subject’s cortex seemed
insufficient to reflect the cortical thickness effect of the mandible in this study.
2. The implant-retained overdenture in two implants type was shown the better
stability comparing with other finite element models, and furthermore the
model 22 was evidenced more stable than the model of 66. The reason could be
explained that the model 22, implants were mainly inserted alveolar bone
between mental foramens, have conjectured a seesaw effect of lateral side for
providing a better stability.
3. The bone resorption around implants was significantly evidenced by micro
strain index in all of the finite element models. On the contrary, the stress results
of the overdenture, attachments, and implants did not to reflect a fracture trend
in the finite element analysis.
4. For investigating an oppression of mucosa, increasing implant number and
more contacted region between overdenture and mucosa in the
79
implants-retained overdenture could be prevented ulcer and should be more
dispersed stress distribution and better stability.
5. The constraint of food bolus was very sensibility to influence the results of the
finite element analysis, hence different types of food bolus constrain should be
further examined for understanding the biomechanical effects of the
implant-retained overdenture models.
6. The preliminary results of the complete anatomical implant-retained
overdenture of the 3D finite element analysis was evidenced the same
tendency with a clinical treatment option, thus why two implant inserted and
anterior region placement was applied more popular.
80
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Biomechanical Evaluation of Implant-Retained Overdenture