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Australian Journal of Basic and Applied Sciences, 8(7) May 2014, Pages: 52-57
AENSI Journals
Australian Journal of Basic and Applied Sciences
ISSN:1991-8178
Journal home page: www.ajbasweb.com
Corresponding Author: Muhammad Ikman Ishak, Faculty of Engineering and Technology, Multimedia University, Jalan
Ayer Keroh Lama, 75450 Melaka, Malaysia.
Phone: +(6)06 2523313; E-mail: ikman.ishak@mmu.edu.my @ mikman_ishak@yahoo.com.
Comparative Study of Different Conventional Dental Implant Numbers on Zygomatic
Implant Stability – A Finite Element Analysis
1Muhammad Ikman Ishak and 2Aisyah Ahmad Shafi
1Faculty of Engineering and Technology, Multimedia University, Jalan Ayer Keroh Lama, 75450 Melaka, Malaysia.
2Medical Implant Technology Group, Faculty of Biosciences and Medical Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor
Bahru, Johor, Malaysia.
A RT I C LE I NF O
A B ST R AC T
Article history:
Received 25 January 2014
Received in revised form
8 April 2014
Accepted 20 April 2014
Available online 10 May 2014
Keywords:
Atrophic maxilla, zygomatic implant,
implant numbers
Background: Zygomatic dental implants are extensively used for the treatment of
severely edentulous atrophic maxillae as an alternative to the previous protocol
treatment by bone grafting. In order to achieve a high stability in supporting the
prosthesis, these implants are commonly used in conjunction with conventional dental
implants placed in the anterior region. However, there is no consensus found on the
effects of different numbers of conventional dental implants towards bone stress and
zygomatic implant stability. Through this study, three-dimensional (3D) models of
craniofacial including soft tissue and prosthesis were constructed from computed
tomography (CT) image datasets. The implant models were developed using computer-
aided design (CAD) software and all models were analyzed via finite element analysis
(FEA) software. A 230 N of vertical occlusal load was applied on the top surface of
prosthesis in the first molar region and a masseter load of 300 N was applied at the
zygomatic arch. Objective: To investigate the effects of different number of anterior
conventional dental implants – 0, 1 and 2, for stress and displacement distribution
within bone and zygomatic implant body, respectively, by using 3D FEA. Results: The
result showed that the stability of zygomatic implant could be secured by the placement
of conventional dental implants in the premaxillary region. Conclusion: The use of one
conventional dental implant is preferable although it has significantly increased the
bone stress magnitude for about 1.5-fold. The conventional dental implants have also
reduced the tendency of zygomatic implant to highly displace from its original position.
© 2014 AENSI Publisher All rights reserved.
To Cite This Article: Muhammad Ikman Ishak and Aisyah Ahmad Shafi., Comparative Study of Different Conventional Dental Implant
Numbers on Zygomatic Implant Stability – A Finite Element Analysis. Aust. J. Basic & Appl. Sci., 8(7): 52-57, 2014
INTRODUCTION
The success rate of endosseous dental implants in the posterior region of maxilla is significantly lower as
compared to the other regions in the jaws due to low availability of bone volume as a result of high bone
resorption (Meyer et al., 2001, Sadowsky, 2007, Corrente et al., 2009). This situation could also be associated
by a poor bone quality and lower bone density of the maxilla than the mandible. An alternative method has thus
been suggested to treat severely atrophic posterior maxillae by an advance surgical procedure of bone
augmentation where the apparent problem of insufficient bone height may be reduced by this procedure (Meyer
et al., 2001, Cordaro et al., 2010, Al-Khaldi et al., 2011). Although this procedure can improve the
configuration for potential placement of implant to the affected maxillae, a lower implant success rate has been
reported as compared to the non-grafted maxillae owing to harvested bone morbidity (Palmer, 2005). On top of
that, the bone augmentation procedure also requires a long treatment time and longer healing time period
(Aparicio et al., 2008). Therefore, a new alternative for the treatment of atrophic maxillae was introduced by
Brånemark System® in 1988 utilizing zygomatic implant to minimize complications caused by the bone
augmentation procedure (Aparicio et al., 2008, Aparicio et al., 2010a, Aparicio et al., 2010b).
Zygomatic implant was initially intended to rehabilitate the maxillectomy patients owing to tumour
resection, trauma or congenital defects. However, the function of this implant had been expanded for
rehabilitation of edentulous resorbed maxilla patients and has recorded a high survival rate ranges from 98.4%
to 100% based on numerous clinical follow-up studies (Ahlgren et al., 2006, Aparicio et al., 2006, Duarte et al.,
2007, Aparicio et al., 2008). The insertion path of zygomatic implant is usually from the alveolar ridge bone in
the second premolar or first molar region, going through maxillary sinus or its wall into the zygomatic bone.
53 Muhammad Ikman Ishak and Aisyah Ahmad Shafi, 2014
Australian Journal of Basic and Applied Sciences, 8(7) May 2014, Pages: 52-57
According to Nkenke et al., the success of implants placed in the zygoma could be achieved by crossing the
implant through four cortical layers (Nkenke et al., 2003). This is supported by Kato et al. who found the
presence of wider and thicker cancellous bone at the apical end of the fixture that could be used to promote
initial fixation (Kato et al., 2005).
From biomechanical point of view, zygomatic implants have a high tendency to bend under horizontal
loading due to increase in implant length when compared to conventional dental implants. The bending effect
may also be associated by insufficient bone quantity available in the maxillary alveolar crest to retain the
coronal part of zygomatic implant body. The most common treatment planning is by utilizing one zygomatic
implant placed bilaterally together with at least two conventional dental implants located in the anterior region
for additional retentions. For a severe bone resorption, two or more zygomatic implants that placed bilaterally
without any retention by conventional dental implants anteriorly are preferable for the treatment option.
To the best of authors’ knowledge, there is no specific study has been found, to date, to address the strength
of retention by conventional dental implants for the zygomatic implant stability. It is therefore a necessity for
the present study to highlight the role of anterior retention implants for the stability of prosthetic restoration.
MATERIALS AND METHODS
I. Three-dimensional Craniofacial Model Construction:
A series of CT image datasets of a real complete denture wearer with a high degree of maxillary bone
resorption was utilized to generate 3D model of bones, mucosa soft tissue and prosthesis using an image-
processing software of Mimics/Magics 10.01 (Materialise, Leuven, Belgium). The bone model was assumed to
be symmetrical for both sides, thus, only one side of the model would be analyzed. The selected region of
interest was on the left side covering the maxillary alveolar bone, palatal side, infrazygomatic crest, temporal
and frontal processes, zygomatic bone and the orbital floor surface. The cortical layer of maxillary alveolar bone
had a thickness ranging from 1.4 to 2.2 mm.
A partial prosthesis with flange was modeled based on the original patient's complete denture with 1.5 to
3.5 mm in thickness, 12.5 to 19.1 mm in width and 15.4 to 18.4 mm in height. The prosthesis model used in the
present study was a fixed restoration type which tightly connected to the implant abutment by screws. The gap
existed along the maxillary arch between the palatal surface of bone and the inside surface of complete
prosthesis was used to develop a soft tissue model with a thickness ranging from 0.4 to 5.58 mm.
II. Three-dimensional Implant Model Construction:
A 3D CAD software of SolidWorks 2009 (SolidWorks Corp., Concord, Massachusetts, USA) was utilized
to develop the implant models. The construction of implant model required a matched abutment to connect the
implant body to the prosthesis. Therefore, one 46.5 mm zygomatic implant body with a diameter of 4.5 mm and
a straight multi-unit abutment from Brånemark System® (Nobel Biocare AB, Gotebörg, Sweden) have been
modeled for the posterior anchorage as depicted in Figure 1. For the conventional dental implant, a 4.0 mm x
10.0 mm together with an angled multi-unit abutment 30° were chosen from the same manufacturer.
III. Virtual Surgery Simulation:
All reconstructed bone and implant models were individually exported as surface triangular elements in
stereolithography (STL) format into Mimics/Magics software to perform the implantation procedures virtually.
The conventional dental implants were placed adjacent to the lateral incisor or first premolar region whereas the
zygomatic implant was located in the first molar region. The implant configurations were in equally distributed
within the arch to achieve optimal support. As a result, three different cases were created – Case 1 (without
conventional dental implant support), Case 2 (one conventional dental implant support) and Case 3 (two
conventional dental implants support) as shown in Figure 2.
Fig. 1: Three-dimensional solid model of (a) conventional dental implant and (b) zygomatic implant.
54 Muhammad Ikman Ishak and Aisyah Ahmad Shafi, 2014
Australian Journal of Basic and Applied Sciences, 8(7) May 2014, Pages: 52-57
Fig. 2: Configuration of different conventional dental implant numbers used in (a) Case 1, (b) Case 2 and (c)
Case 3 shown in occlusal view.
All models had been converted from surface triangular into solid tetrahedral elements in the FEA software
of MSC/MARC 2007 (MSC Software, Santa Ana, California, USA) with four nodes element type and three
degrees of freedom. A single mesh pattern with 0.5 mm triangular element size has been assigned to all models,
which is corresponding with the size used in Cattaneo et al. study (Cattaneo et al., 2003). For convergence
purposes, the chosen element size was almost three times smaller than the one suggested by Lin et al. (Lin et al.,
1999). The total number of tetrahedral elements for Case 1, Case 2 and Case 3 were about 383,000, 392,000 and
404,000, respectively.
IV. Contact Modeling:
All contacting surfaces of implant and prosthesis were simulated via friction coefficient, µ, of 0.3 to
represent the immediate loading function (Huang et al., 2008). As the threaded part of all implant designs were
ignored through the preparation of models, it was accordingly simulated via contact properties with a friction
coefficient of 0.5 to represent its strong attachment to the bone. The contact surfaces between cortical-
cancellous and cortical-mucosa soft tissue were assumed to be as perfectly bonded by merging the nodes
between the two contacted models, therefore, no frictional contacts were assigned.
V. Material Properties Assignment:
All the finite element models were assumed to be isotropic, homogenous, static and linearly elastic
throughout the analysis. The material properties (Young’s modulus and Poisson’s ratio) of all models are
defined as follow: cortical bone, 13,400 MPa/0.30 (Ujigawa et al., 2007); cancellous bone, 1,000 MPa/0.30
(Meyer et al., 2001); mucosa soft tissue, 2.8 MPa/0.40 (Cheng et al., 2010); prosthesis, 100,000 MPa/0.30
(Ujigawa et al., 2007) and implants, 110,000 MPa/0.33 (Geng et al., 2001).
VI. Loading Conditions:
A static vertical occlusal load of 230 N (Cheng et al., 2010) was applied on the top surface of prosthesis in
the first molar region to represent the chewing action. Moreover, a masseter load of 300 N (Cattaneo et al.,
2003, Miyamoto et al., 2010) with the force components of 62.12 N along the x-axis, -265.20 N along the z-axis
and 125.69 N along the y-axis was applied to the muscle attachment area on the zygomatic bone. For the
boundary conditions, the posterior (x-z plane), midsagittal (y-z plane) and top cutting planes (x-y plane) were
constrained in the x, y and z directions to prevent any movements (Figure 3).
Fig. 3: Applied loadings and boundary conditions on the finite element models as viewed from (a) frontal and
(b) sagittal plane.
55 Muhammad Ikman Ishak and Aisyah Ahmad Shafi, 2014
Australian Journal of Basic and Applied Sciences, 8(7) May 2014, Pages: 52-57
Results:
I. Von Mises Stress Result within the Bones:
Our results showed that the placement of conventional dental implants in the anterior region of maxilla has
significantly increased the magnitude of stress generated within the bones as depicted in Figure 4. This could be
shown by a larger stress distribution area was developed where the stress was highly concentrated at the alveolar
crest bone around zygomatic implant head and also within the zygomatic bone. The maximum stress (173.26
MPa – 270.33 MPa) in all cases was found within the zygomatic bone. The use of one (Case 2) and two (Case 3)
conventional dental implants increased the maximum cortical bone stress about 36% and 31%, respectively as
compared to model without conventional dental implant (Case 1). There was less discrepancy of stress value
between Case 2 and Case 3 which merely 5%.
Fig. 4: von Mises stress distribution within the bones in different conventional dental implant numbers for (a)
Case 1 (without conventional dental implant), (b) Case 2 (one conventional dental implant) and (c) Case
3 (two conventional dental implants).
II. Displacement and Deformation of Zygomatic Implant:
When the results were interpreted in terms of displacement of zygomatic implant, it was clearly observed
that Case 3 has shown considerably lower implant displacement value than the one in Case 2 and Case 1 (Figure
5). The highest value of zygomatic implant displacement was 0.0128 mm (Case 1), followed by 0.0119 mm
(Case 2) and 0.0057 mm (Case 3). These results were in accordance with the deformation of implant body
where the most significance bending effect was noted in Case 1, Case 2 and the least in Case 3. The coronal part
of implant body showed a greater deformation than the apical part towards buccal and mesial direction.
Fig. 5: Comparison of displacement magnitude of zygomatic implant in (a) Case 1, (b) Case 2 and (c) Case 3.
The implants are also shown in deformation scale with the magnification factor of 200. The pink color
outlines showing the undeformed shape or the original position of implant body.
Discussion:
Prosthetic design plays a vital role in determining the success of zygomatic implants either in a short-term
or long-term performance evaluation. Among parameters majorly contribute in the prosthetic restoration
associated with zygomatic implants are stability, precision and barrier. The prosthetic design stability is defined
as the potential of bridge framework to sustain the implants position by having minimum implant movement
under physiological function whilst the precision is referred to the strength of connecting screw joints between
prosthesis and abutment. The design criteria of prosthetic restoration have also to decrease the bending moments
as the bending moments may cause the deformation of implant body that leading to the failure of implants or
screw loosening.
Based on the results, the restoration of zygomatic implant together with conventional dental implants in the
anterior maxilla has considerably increased the maximum bone stress magnitude approximately 1.5-fold. The
increase in bone stress might be due to the presence of more opening holes for the placement of implants which
allowing internal stress to concentrate at the sharp edges relevant to the coronal part of implant body. In
comparison, the placement of one and two conventional dental implants exhibited less significant difference of
bone stress value (5%) in which therefore, it is suggested that only one conventional dental implant (Case 2) is
preferable for the additional retention anteriorly. The rationale behind this is also in relation to reduce the
56 Muhammad Ikman Ishak and Aisyah Ahmad Shafi, 2014
Australian Journal of Basic and Applied Sciences, 8(7) May 2014, Pages: 52-57
treatment cost as well as to avoid bone damage due to high stress concentration. According to Ujigawa et al., the
combination of zygomatic and conventional dental implants could distribute the functional loading, however,
the stress concentration at the implant-abutment connection could not be avoided under both vertical and lateral
loadings (Ujigawa et al., 2007). The present results also parallel with literatures where at least two implants are
required in the anterior maxilla for an optimal prosthetic components stabilization, considering full dental arch
restoration (Ujigawa et al., 2007, Maló et al., 2008, Stiévenart and Malevez, 2010).
In terms of implant displacement, it is noteworthy that the increase in conventional dental implant numbers
has significantly decreased the tendency of zygomatic implant to move from its original position. The additional
implants in the anterior region of maxilla may secure the zygomatic implant position by preventing the
rotational load effects that could initiate rotational displacements. On top of that, the implants can also reduce
the bending moment effects on zygomatic implants as more stresses are dissipated and spread out in a larger
region. According to Stievenart et al., the success rate of zygomatic implants is highly dependent on the cortical
bone anchorage (Stiévenart and Malevez, 2010). The zygomatic implant body in all three cases have a low
potential to failure as the value of micromotion between 50 and 150 µm could negatively influence
osseointegration and bone remodeling at the bone-implant interface (Javed and Romanos, 2010). Moreover, the
apical part of implant body showed less deformation when compared to the coronal part that possibly due to a
high strength of anchorage in the zygomatic bone (Nkenke et al., 2003).
In all models tested, the highest stress value was recorded within the zygomatic implant body. This could be
probably due to a high titanium alloy modulus of elasticity of 110,000 MPa as compared to the bones. The
maximum stress values generated within the zygomatic implant bodies have no tendency to the implant failure
as titanium alloy is known can tolerate stresses up to 900 MPa (Koca et al., 2005).
Conclusion
In conclusion, the prosthetic restoration of zygomatic implants associated with the treatment of severely
atrophic maxillae could be more stable if the implants are rigidly connected with the conventional dental
implants in the anterior region. The increase in conventional dental implant numbers is proportional to the bone
stress result whilst the reverse is observed for the displacement of zygomatic implant.
ACKNOWLEDGEMENT
An appreciation is given to Medical Implant Technology Group (MediTeg), Universiti Teknologi Malaysia
and Faculty of Engineering & Technology, Multimedia University.
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