Finite element analysis of three zygomatic implant techniques for the severely atrophic edentulous maxilla Hailin Wen, DDS, PhD,a Weihua Guo, DDS, PhD,b Rui Liang, ME,c Lin Xiang, DDS, PhD,d Gang Long, DDS, PhD,e Tingting Wang, MSD,f Meng Deng, MSD,g and Weidong Tian, DDS, PhDh Sichuan University, Chengdu, China Statement of problem. A variety of zygomatic implantation techniques currently exist; however, a consensus regarding the most suitable method has not yet been reached. Purpose. The purpose of this study was to evaluate and compare 3 zygomatic implantation techniques and to clarify the optimal number and position of zygomatic and dental implants for the reconstruction of the severely atrophied edentulous maxilla. Material and methods. A 3-dimensional finite element analysis craniofacial model was constructed from the computed tomography data of a selected patient with a severely atrophic edentulous maxilla. Modeled zygomatic implants were inserted into the craniofacial model with 3 surgical techniques (classic Brånemark, exteriorized, and extramaxillary), and with 3 model variations that involved the number and position of zygomatic and dental implants. The zygomatic implants were loaded with a vertical force of 150 N and a lateral force of 50 N. The stresses on and deformations of the bones and implants were then observed and compared. Results. No obvious differences in the amount and distribution of stress on the external craniofacial bones were detected in the models. The lowest stresses on the zygomatic implants were observed in the exteriorized technique group. The lowest deformations of the bone that surrounds zygomatic implants and dental implants were observed in the exteriorized technique and classic Brånemark technique groups. For the exteriorized technique group, the model with 1 dental implant in the site of the maxillary lateral incisor exhibited the lowest stress on the zygomatic implants and the least deformation of the bone surrounding the zygomatic and dental implants. This study was supported by National Basic Research Program (China, 2010CB944800), National High-Technology Research and Development Program (China, 2011AA030107), International Cooperation Program of China (China, 2013DFG32770 and 2011DFA51970), Nature Science Foundation of China (China, 81271095, 81271119 and 81200792), Doctoral Foundation of Ministry of Education of China (20110181120067 and 20110181110089), Key Technology R&D Program of Sichuan Province (2012SZ0013 and 12ZC0493) and Basic Research Program of Sichuan Province (2011JY0125 and 12JC0212). a
Postgraduate student, Department of Oral and Maxillofacial Surgery, West China College of Stomatology, Sichuan University; National Engineering Laboratory for Oral Regenerative Medicine, West China Hospital of Stomatology, Sichuan University; State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University. b Associate Professor, Department of Pedodontics, West China College of Stomatology, Sichuan University; National Engineering Laboratory for Oral Regenerative Medicine, West China Hospital of Stomatology, Sichuan University; State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University. c Postgraduate student, College of Architecture and Environment, Sichuan University; National Engineering Laboratory for Oral Regenerative Medicine, West China Hospital of Stomatology, Sichuan University; State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University. d Postgraduate student, Department of Implantology, West China College of Stomatology, Sichuan University; State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University. e Postgraduate student, Department of Implantology, West China College of Stomatology, Sichuan University; State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University. f Postgraduate student, Department of Endodontics, West China College of Stomatology, Sichuan University; State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University. g Postgraduate student, Department of Endodontics, West China College of Stomatology, Sichuan University; State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University. h Professor and Director, Department of Oral and Maxillofacial Surgery, West China College of Stomatology, Sichuan University; National Engineering Laboratory for Oral Regenerative Medicine, West China Hospital of Stomatology, Sichuan University; State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University.
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Conclusions. All 3 zygomatic implant techniques resulted in more or less homogeneous transference of force and thus could reconstruct the edentulous maxilla; however, the exteriorized technique with 1 dental implant in the lateral incisor appeared to be the most appropriate reconstruction method for the severely atrophied edentulous maxilla. (J Prosthet Dent 2013;-:---)
Clinical Implications These results can be clinically applied when choosing the appropriate zygomatic implant technique for the severely atrophied edentulous maxilla. The exteriorized technique could increase the long-term success rate of zygomatic implants from the biomechanical perspective.
The zygomatic implant has become a standard procedure in the treatment of maxillary defects,1-3 patients with edentulism and with severe maxillary atrophy4-10 and also has been advocated for patients with advanced periodontal disease.11 The zygomatic implant offers an efficient therapeutic process that avoids bone grafting and has a good success rate.12-15 The zygomatic implant was first used by Brånemark et al16 to rehabilitate maxillary defects. Since their introduction, zygomatic implantation techniques have been reported for rehabilitating severely absorbed edentulous maxilla by a number of researchers, including Stella and Warner,17 Migliorança et al,9 and Maló et al.18 Variations of the zygomatic implantation method are known as the Brånemark technique,16 the Stella or sinus slot technique,17 and the exteriorized or extrasinus technique.19 The technique introduced by Maló et al18 is referred to here as the extramaxillary technique. All of these zygomatic implantation techniques have been reported to perform well in short- and medium-term studies with implant survival rates of 82% to 100%.8,12,14,18,20-24 However, no consensus regarding the ideal technique for placing zygomatic implants has yet been reached, which is likely because of the lack of long-term clinical observations with extensive sampling and systemic comparisons among the different techniques. Each method offers certain advantages and disadvantages. For instance,
in both the Brånemark and Stella techniques,16,17 zygomatic implants are positioned through the sinus and fixed into both the zygomatic and maxillary bone. Clinical signs of maxillary sinus pathology must be considered by the clinician when using these 2 techniques.25-27 Although the Stella technique17 may improve upon implant orientation and reduce postoperative symptoms, BoyesVarley et al28 disagreed with the Stella technique17 because a lack of visibility may result in perforation of the posterior antral wall. The exteriorized technique increases the length of drill holes into the zygomatic bone, which may provide higher initial mechanical stability than other techniques.19 The extramaxillary technique reduces the size of the prosthesis that may reduce problems such as phonetic difficulties, dental hygiene concerns, and mechanical resistance from the prosthesis; however, there is reduced support and retention of the maxillary bone.18 The extrasinus technique may show improved biomechanical performance because it reduces the cantilever more than in the extramaxillary technique and supports the maxillary sinus more than the Brånemark technique.16 Therefore, a 3-dimensional finite element analysis (FEA) study was designed to provide biomechanical information to address the optimal technique for installing zygomatic implants and to clarify the ideal number and position of zygomatic and dental implants for the reconstruction of a severely atrophied edentulous maxilla.
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MATERIAL AND METHODS The research protocol was approved by the institutional review board of Sichuan University. A 3-dimensional finite element solid model of a human skull was constructed from the clinical computed tomography data from a 69-year old Chinese patient with a severely absorbed edentulous maxilla. The skull was scanned in the transverse plane (Computed Tomography Scanner; Siemens) with a 1-mm-slice thickness and 1-mm scan increment in 240 slice images. Three-dimensional external shapes of the craniofacial structures were generated by image-processing software (Mimics 10.01; Materialise NV). Output data were imported to software designed for surface reconstruction and the design of zygomatic and dental implants (Pro/ENGINEER WildFire 4.01; PTC Corp). The data obtained were then processed by software (ANSYS Workbench 12.01; ANSYS Inc) for conversion into a 3-dimensional FEA solid model. Because of the similarity between the Brånemark and Stella techniques,16,17 the classic Brånemark technique was chosen to represent both the Brånemark and Stella techniques in the study. The 3 zygomatic implantation techniques considered in this study served to define 3 model groups: the Brånemark (B group), exteriorized (Ed group), and extramaxillary (Ey group) groups. In total, the FEA study worked with 9 reconstructed models, which were divided into 3 groups according to the surgical
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1 Views of zygomatic implants and dental implant models for 9 model variations. B, Brånemark technique group. Ey, extramaxillary technique group; Ed, exteriorized technique group; green, craniofacial bones; pink, maxillary sinus, nasal cavity, and other cavities; yellow, gold alloy bar; gray, zygomatic implants and dental implants; blue, cancellous bone; dark green, cortical bone.
technique modeled. In the Brånemark technique group (B group),16 all zygomatic implants were inserted from the site of the maxillary first molar. Model LP consisted of 1 zygomatic implant and 2 dental implants in the maxillary lateral incisor and the first premolar on each side. Both model P and model L in the Brånemark technique16 group had 1 zygomatic implant on each side, with 1 dental implant in the maxillary first premolar in model P or the lateral incisor in model L. In the extramaxillary technique group model LP included 1 zygomatic implant in the maxillary first molar and 1 dental implant in the lateral incisor on each side. Model P had 2 zygomatic implants on both sides, inserted from the site of the first molar and the second premolar. Model L had 1 dental implant in the canine on the base of model P. In the exteriorized technique group, model LP, model P, and model L were the same as those described for the Brånemark technique.16
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A 45-mm zygomatic implant (Nobel Biocare AB) with an abutment 5 mm in height and 2 regular dental implants (3.75 13.0 mm; Nobel Biocare AB) with abutments 5 mm in height were modeled. The appropriate length of the zygomatic implant was selected according to the distance between the crest of the alveolar process of the maxilla near the palate and the jugal point (JU) of the zygomatic bone.29 Prosthetic superstructures were modeled as a gold alloy symmetrically curved bar with a section 10 8 mm30 and were constructed with Pro/ ENGINEER WildFire 4.01 software (PTC Corp). The different numbers of zygomatic and dental implants in models LP, P, and L with the 3 different surgical techniques are shown in Figure 1. In this study, the mechanical properties of implants and bones were all treated as isotropic, homogeneous, and linear elastic. The elastic modulus and Poisson ratio values of titanium, gold alloy, and cortical and cancellous bone
(Table I) were obtained from the literature.31-41 The bone-implant contact area was assumed to be complete osseous integration and so the contact area was simulated by using a surface-to-surface option fully bonded for the cemented
Table I. Mechanical properties of materials used in FEA models Elastic Modulus (GPa)
Poisson Ratio
Titanium
110.0*
0.33
Cortical bone
13.7y
0.30
Materials
Cancellous bone Gold alloy
1.37y 80z
FEA, finite element analysis. *From Refs. 39,41. y From Refs. 31-38. z From Refs. 40,41.
0.30 0.33
4
Volume versions.30,42-45 Nine solid models were transferred to the ANSYS Workbench 12.01 (ANSYS Inc) to generate the FEA models with 10 node tetrahedral elements. An element size of 1 mm was used as the meshing requirement for all FEA models.46 Mesh generation resulted in a total of approximately 274 000 elements and 484 000 nodes. To simulate occlusal force, a vertical load of 150 N was applied to the occlusal surface along the zygomatic implant axis, and a lateral load of 50 N was applied to the palatal surface of the superstructure.30,47-49 To simulate the action and pass downward and backward, a distributed occlusal force of 300 N was applied to the insertion area of
the masseter muscle on the zygomatic arch and zygomatic process of the maxilla.30,50 For the top cutting plane, the restraint was chosen to simulate the presence of the remainder of the skull. Both loading and boundary conditions of the FEA models are shown in Figure 2.
RESULTS Comparisons were made between the von Mises stress and deformation magnitude and distribution in craniofacial models rehabilitated by zygomatic and dental implants with each of the 3 techniques. Stress and deformation were visualized with pseudocolors on
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the FEA model (Figs. 3 to 8). Unstressed regions were represented by dark-blue areas. Red areas represent regions of maximum stress. Each item was compared by using the scale shown on the left side of these images. Stresses caused by occlusal forces were found to be primarily supported by the zygoma, with transfer through the infrazygomatic crest. The maximum stress in craniofacial bone was 20 to 25 MPa both under vertical loading and lateral loading (Fig. 9). No obvious differences in stress distribution and magnitude in the craniofacial bone of any of the 9 models were noted. The distribution of stress in the craniofacial bone was somewhat homogeneous.
2 Front and lateral views of craniofacial and implant models under loading conditions (red).
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3 Front view of implant models under vertical loading.
4 Front view of implant models under lateral loading.
The craniofacial bones and components that provided the majority of mechanical support for the zygomatic and dental implants were the maxillary bone, the zygomatic bone, and the zygomatic arch. The maximum stress values in each of these 3 components are shown in Figure 9. In all models, except model LP in the Ey group, the maximum stress in the maxillary bone was approximately 10 MPa under vertical loading and 7 MPa under lateral loading, and appeared where the
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zygomatic implant perforated the maxillary bone or in the surrounding alveolar bone (Fig. 9). The maximum value in the zygomatic bone was approximately 20 MPa and concentrated in the JU bone landmark (Fig. 9). In all models, stress was transferred along the zygomatic arch with a value that ranged from 7 to 10 MPa under both vertical and lateral loading (Fig. 9). The amount and distribution of stresses on zygomatic and dental implant components are shown in Figures 3, 4,
and 10. The zygomatic implant of model L in the Ed group had an average value of maximum stress on 2 sides of approximately 45 MPa under vertical loading and approximately 18 MPa under lateral loading (Fig. 10). All other models had a similar maximum stress of approximately 100 MPa under vertical loading and approximately 24 to 35 MPa under lateral loading (Fig. 10). The primary regions of maximum stress were the inner side of the joint of the implant abutmen, and some stress was
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5 Stress distribution and magnitude on surrounding bones under vertical loading.
6 Stress distribution and magnitude on surrounding bones under lateral loading. maximized on the middle and end portions of the zygomatic implants (Figs. 3, 4). The posterior zygomatic implant of model P in the extramaxillary
technique group (model P Ey) supported more stress and transferred larger forces than the anterior zygomatic implant (Figs. 3, 4).
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The standard dental implant of model L in the Brånemark technique group (model LB) and model L in the exteriorized technique group (model LEd)
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7 Deformation of surrounding bones under vertical loading.
8 Deformation of surrounding bones under lateral loading. shared similar minimal stress values of approximately 10 MPa under vertical loading and 5 MPa under lateral loading (Fig. 10). However, the dental implant in model LP of the extramaxillary technique
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group (model LPEy) showed the maximum stress values of approximately 22 MPa under vertical loading and 18 MPa under lateral loading (Fig. 10). However, stress values on the standard dental
implant were much smaller than those on the zygomatic implants (Figs. 3, 10). The maximum stress on the bones that surround the zygomatic implants of model L in Ed group was approximately
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Volume Craniofacial Bone Model Under Vertical Load LP
25.0
P
30.0
L
20.0 15.0 10.0 5.0 0.0
B
Ey
LP
25.0
10.0 5.0 B
15.0 10.0 5.0
P
4.0 2.0
10.0 5.0
LP
15.0 10.0 5.0 B
2.0 Ey
Ey
12.0
L
Stress (MPa)
Stress (MPa)
P
6.0 4.0
B
L
Ed
Zygomatic Arch Model Under Lateral Load
8.0
0.0
P
20.0
Zygomatic Arch Model Under Vertical Load LP
Ed
0.0
Ed
12.0 10.0
Ey
25.0
L
15.0
Ey
L
Zygomatic Bone Model Under Lateral Load
20.0
B
P
6.0
B
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Stress (MPa)
LP
LP
8.0
Zygomatic Bone Model Under Vertical Load
0.0
Ed
0.0
Ed
25.0
Ey
10.0
L
Stress (MPa)
Stress (MPa)
P
20.0
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L
Maxillary Bone Model Under Lateral Load
LP
B
P
15.0
Maxillary Bone Model Under Vertical Load
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LP
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P
L
8.0 6.0 4.0 2.0 0.0 B
Ey
Ed
9 Mean value of maximum stress in craniofacial bone models and maximum stress in maxillary bone, zygomatic bone, and zygomatic arch models on both sides under vertical and lateral loading. 7 MPa under vertical loading and 5.5 MPa under lateral loading (Fig. 11). Meanwhile, model LP in the Ey group had a maximum stress of approximately 16 MPa (others were approximately 10 MPa under vertical loading); all of the models except model L in the Ed group were similar to a maximum stress of 7 to 8 MPa (Fig. 11). The maximum stress on the bones that surround the dental
implants was almost 2 to 4 MPa under vertical loading and 1 to 2 MPa under lateral loading except for the models of the Ey group (Fig. 11). The maximum stress on the bones that surround the implants was at the site where zygomatic implants perforated the maxillary bone, maxillary sinus, or the site near the bone landmark JU point, where the end of the zygomatic implants was fixed
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both under vertical loading and under lateral loading (Figs. 5, 6). The maximum deformation of surrounding bones in the B and Ed groups was approximately 10 mm, and the maximum deformation of the Ey group was approximately 15 mm under vertical loading (Fig. 12). Under lateral loading, the maximum deformation was 12 mm in the B group, 14 mm in the Ed group,
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P
80.0 60.0 40.0 20.0 0.0
40.0
L
100.0
Stress (MPa)
Stress (MPa)
120.0
Left Zygomatic Implant Model Under Lateral Load
B
Ey
10.0
P
40.0
L
100.0 80.0 60.0 40.0 20.0 0.0
B
Ey
P
10.0
B
Ey
P
Ed
L
20.0 10.0 Ey
Ed
Standard Dental Implant Model Under Lateral Load 20.0
15.0 5.0 0.0
LP
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L
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Stress (MPa)
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25.0
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Standard Dental Implant Model Under Vertical Load 30.0
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Right Zygomatic Implant Model Under Lateral Load Stress (MPa)
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Right Zygomatic Implant Model Under Vertical Load 120.0
P
30.0
0.0
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10 Mean maximum stress on zygomatic implant and dental implant models under vertical and lateral loading. and 18 mm in the Ey group (Fig. 12). The maximum deformation of bones that surround dental implants was approximately 2 to 5 mm, both under vertical loading and under lateral loading (Fig. 12). The deformation of surrounding bones in the B and Ed groups was similar. The deformation of bones that surround dental implants was much less than that surrounding zygomatic implants. The maximum deformation region was near the bone landmark JU point where the end of zygomatic implants was fixed (Figs. 7, 8).
DISCUSSION No obvious differences in the magnitude and distribution of stress in the total and external craniofacial bone models were found for any of the 3
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techniques. However, the lowest stress and deformation on the zygomatic implants and in the bones that surround the zygomatic implants were found in the exteriorized technique group. Stress in the total and external craniofacial bone models indicated that the 3 techniques all resulted in more or less homogeneous transference of force. Moreover, transference of force in the external craniofacial bone was consistent with those of Ujigawa et al.30 The zygomatic implant was found to bear most of the occlusal force. The stress on the standard dental implant was so small that it could essentially be ignored because titanium alloys are known to tolerate stresses up to 900 N/ mm2 without irreversible deformation51; therefore, a force of 150 N would not likely result in dental implant failure.
However, because concentrated stress could lead to the absorption of surrounding bone,7 the stress and deformation of the bone that surrounds zygomatic and dental implants was taken into consideration. The stress of the bone that surrounds the zygomatic implants was much larger than that of the bone that surrounds dental implants. Because the static strength of bone is approximately 150 MPa in tension and approximately 250 MPa in compression,52 rehabilitating the edentulous maxilla with zygomatic implants and dental implants should not risk overstressing the surrounding bone. Deformation is determined by the force of loading and the material’s mechanical properties. Because there is much cancellous bone near the landmark JU and stress in this region was maximal, maximum deformation of the
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Volume Craniofacial Bone Around Left Zygomatic Implant Under Vertical Load LP
P
9.0 6.0 3.0 0.0
25.0
L
12.0
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Ey
Craniofacial Bone Around Left Zygomatic Implant Under Lateral Load P
6.0 4.0 2.0 0.0
B
Ey
P
4.5 3.0 1.5 0.0
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Ed
10.0 5.0 B
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P
L
8.0 6.0 4.0 2.0 B
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Craniofacial Bone Around Dental Implant Under Lateral Load 7.5
L
Stress (MPa)
Stress (MPa)
LP
6.0
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Craniofacial Bone Around Dental Implant Under Vertical Load 7.5
P
15.0
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8.0
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Craniofacial Bone Around Right Zygomatic Implant Under Lateral Load Stress (MPa)
Stress (MPa)
LP
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20.0
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Craniofacial Bone Around Right Zygomatic Implant Under Vertical Load Stress (MPa)
Stress (MPa)
15.0
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LP
P
L
6.0 4.5 3.0 1.5 0.0 B
Ey
Ed
11 Maximum stress on bones that surround zygomatic implants and dental implants under vertical loading and lateral loading.
surrounding bone was concentrated on this region. Stress on external and total bone was influenced by occlusal force and the force of the masseter. Stress on the surrounding bone was only influenced by the occlusal force. Occlusal force was much less than the masseter, and stress distributions on external and total bone were somewhat homogeneous. However, some differences were found between the stresses of the surrounding bones. Because stress distribution in external and total craniofacial bone was rather homogeneous and stress on the dental implants was low, the biomechanics of the zygomatic implant were determined to be the main factor that influenced the success rate of the surgery. Minimal stresses on the zygomatic implant, under
both vertical and lateral loading, were found in model L in the exteriorized technique. The surrounding bones of zygomatic implants with both the Brånemark technique16 and the exteriorized group presented less deformation than found in the extramaxillary group, which meant that the surrounding bone of zygomatic implants in the extramaxillary group was more susceptible to bone absorption. When taking into consideration the surgical procedure and potential complications,25-28 the exteriorized or extrasinus technique may be the optimal method. When considering the bone contact length of zygomatic implants, the exteriorized technique increases the contact length more than the Brånemark technique does.19 Moreover, it was obvious
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that the contact length of the extramaxillary technique was much shorter than that of the exteriorized and Brånemark techniques.16 Zygomatic implants inserted by using the extramaxillary technique represent a long cantilever, which was also a risk factor of failure. The results of this FEA study were consistent with these outcomes. However, when there is insufficient alveolar bone and maxillary bone for the exteriorized and Brånemark techniques,16 the extramaxillary technique is a proper choice. As in every FEA study, some limitations apply. One limitation was that the zygomatic implants and dental implants were simplified to facilitate modeling and calculations. Another was that, in the present study, 100% osseointegration between implants and
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18.0
LP
P
L
15.0 12.0 9.0 6.0 3.0 0.0
B
Ey
Craniofacial Bone Around Right Zygomatic Implant Under Vertical Load Deformation (µm)
Deformation (µm)
Craniofacial Bone Around Left Zygomatic Implant Under Vertical Load
Ed
20.0
LP
P
L
16.0 12.0 8.0 4.0 0.0
B
Ey
Ed
P
L
12.0 8.0 4.0 0.0
B
Ey
Ed
L
12.0 8.0 4.0 0.0
B
Ey
Ed
20.0
LP
P
L
16.0 12.0 8.0 4.0 0.0 Ey
Ed
Craniofacial Bone Around Dental Implant Under Lateral Load Deformation (µm)
Deformation (µm)
LP
16.0
P
16.0
B
Craniofacial Bone Around Dental Implant Under Vertical Load 20.0
LP
Craniofacial Bone Around Right Zygomatic Implant Under Lateral Load Deformation (µm)
Deformation (µm)
Craniofacial Bone Around Left Zygomatic Implant Under Lateral Load
20.0
15.0
LP
P
L
12.0 9.0 6.0 3.0 0.0 B
Ey
Ed
12 Maximum deformation of bones that surround zygomatic implants and dental implants under vertical loading and lateral loading.
the surrounding bone was assumed. In clinical situations, the percentage of osseointegration could be reduced by such things as local inflammation, drugs, and osteoporosis. Further investigations are needed to assess the osseointegration of zygomatic implants in different situations.
CONCLUSIONS Within the limitations of this study, the following conclusions were drawn: 1. The classic Brånemark, the exteriorized, and the extramaxillary zygomatic implantation techniques resulted in more or less homogeneous transference of force and thus could be used to reconstruct the edentulous maxilla.
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2. In terms of minimizing stress on the zygomatic implant structures, and when considering the stress and deformation of the bones that surround the zygomatic implants, the optimal reconstruction method was model L (a zygomatic implant on each side, with a dental implant in the lateral incisor position) of the exteriorized technique group.
REFERENCES 1. Uckan S, Oguz Y, Uyar Y, Ozyesil A. Reconstruction of a total maxillectomy defect with a zygomatic implant-retained obturator. J Craniofac Surg 2005;16:485-9. 2. Hu YJ, Hardianto A, Li SY, Zhang ZY, Zhang CP. Reconstruction of a palatomaxillary defect with vascularized iliac bone combined with a superficial inferior epigastric artery flap and zygomatic implants as anchorage. Int J Oral Maxillofac Surg 2007;36:854-7.
3. Kreissl ME, Heydecke G, Metzger MC, Schoen R. Zygoma implant-supported prosthetic rehabilitation after partial maxillectomy using surgical navigation: a clinical report. J Prosthet Dent 2007;97: 121-8. 4. Malevez C, Daelemans P, Adriaenssens P, Durdu F. Use of zygomatic implants to deal with resorbed posterior maxillae. Periodontol 2000 2003;33:82-9. 5. Bedrossian E, Stumpel LJ III. Immediate stabilization at stage II of zygomatic implants: rationale and technique. J Prosthet Dent 2001;86:10-4. 6. Boyes-Varley JG, Howes DG, Lownie JF. The zygomaticus implant protocol in the treatment of the severely resorbed maxilla. SADJ 2003;58:106-9, 113e4. 7. Farzad P, Andersson L, Gunnarsson S, Johansson B. Rehabilitation of severely resorbed maxillae with zygomatic implants: an evaluation of implant stability, tissue conditions, and patients’ opinion before and after treatment. Int J Oral Maxillofac Implants 2006;21: 399-404.
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Volume 8. Davo R, Malevez C, Rojas J. Immediate function in the atrophic maxilla using zygoma implants: a preliminary study. J Prosthet Dent 2007;97:44-51. 9. Migliorança RM, Coppedê A, Dias Rezende RC, de Mayo T. Restoration of the edentulous maxilla using extrasinus zygomatic implants combined with anterior conventional implants: a retrospective study. Int J Oral Maxillofac Implants 2011;26:665-72. 10. Cordero EB, Benfatti CA, Bianchini MA, Bez LV, Stanley K, de Souza Magini R. The use of zygomatic implants for the rehabilitation of atrophic maxillas with 2 different techniques: Stella and Extrasinus. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2011;112:49-53. 11. Binon P. Immediately loaded fixed maxillary implant treatment for a patient with advanced periodontal disease: a clinical report. J Prosthet Dent 2010;104:353-8. 12. Malevez C, Abarca M, Durdu F, Daelemans P. Clinical outcome of 103 consecutive zygomatic implants: a 6-48 months follow-up study. Clin Oral Implants Res 2004;15:18-22. 13. Balshi SF, Wolfinger GJ, Balshi TJ. A retrospective analysis of 110 zygomatic implants in a single-stage immediate loading protocol. Int J Oral Maxillofac Implants 2009;24:335-41. 14. Stiévenart M, Malevez C. Rehabilitation of totally atrophied maxilla by means of four zygomatic implants and fixed prosthesis: a 6-40-month follow-up. Int J Oral Maxillofac Surg 2010;39:358-63. 15. Sherry JS, Balshi TJ, Sims LO, Balshi SF. Treatment of a severely atrophic maxilla using an immediately loaded, implantsupported fixed prosthesis without the use of bone grafts: a clinical report. J Prosthet Dent 2010;103:133-8. 16. Parel SM, Brånemark PI, Ohrnell LO, Svensson B. Remote implant anchorage for the rehabilitation of maxillary defects. J Prosthet Dent 2001;86:377-81. 17. Stella JP, Warner MR. Sinus slot technique for simplification and improved orientation of zygomaticus dental implants: a technical note. Int J Oral Maxillofac Implants 2000;15: 889-93. 18. Maló P, Nobre Mde A, Lopes I. A new approach to rehabilitate the severely atrophic maxilla using extramaxillary anchored implants in immediate function: a pilot study. J Prosthet Dent 2008;100:354-66. 19. Corvello PC, Montagner A, Batista FC, Smidt R, Shinkai RS. Length of the drilling holes of zygomatic implants inserted with the standard technique or a revised method: a comparative study in dry skulls. J Craniomaxillofac Surg 2011;39:119-23. 20. Aparicio C, Ouazzani W, Garcia R, Arevalo X, Muela R, Fortes V. A prospective clinical study on titanium implants in the zygomatic arch for prosthetic rehabilitation of the atrophic edentulous maxilla with a follow-up of 6 months to 5 years. Clin Implant Dent Relat Res 2006;8: 114-22.
21. Duarte LR, Filho HN, Francischone CE, Peredo LG, Brånemark PI. The establishment of a protocol for the total rehabilitation of atrophic maxillae employing four zygomatic fixtures in an immediate loading system: a 30-month clinical and radiographic follow-up. Clin Implant Dent Relat Res 2007;9:186-96. 22. Pi Urgell J, Revilla Gutiérrez V, Gay Escoda CG. Rehabilitation of atrophic maxilla: a review of 101 zygomatic implants. Med Oral Patol Oral Cir Bucal 2008;13: 363-70. 23. Aparicio C, Ouazzani W, Hatano N. The use of zygomatic implants for prosthetic rehabilitation of the severely resorbed maxilla. Periodontol 2000 2008;47: 162-71. 24. Aparicio C, Ouazzani W, Aparicio A, Fortes V, Muela R, Pascual A, et al. Extrasinus zygomatic implants: three year experience from a new surgical approach for patients with pronounced buccal concavities in the edentulous maxilla. Clin Implant Dent Relat Res 2010;12:55-61. 25. Becktor JP, Isaksson S, Abrahamsson P, Sennerby L. Evaluation of 31 zygomatic implants and 74 regular dental implants used in 16 patients for prosthetic reconstruction of the atrophic maxilla with cross-arch fixed bridges. Clin Implant Dent Relat Res 2005;7: 159-65. 26. Davó R, Malevez C, López-Orellana C, Pastor-Beviá F, Rojas J. Sinus reactions to immediately loaded zygoma implants: a clinical and radiological study. Eur J Oral Implantol 2008;1:53-60. 27. Sato FR, Sawazaki R, Berretta D, Moreira RW, Vargas PA, de Almeida OP. Aspergillosis of the maxillary sinus associated with a zygomatic implant. J Am Dent Assoc 2010;141:1231-5. 28. Boyes-Varley JG, Howes DG, Lownie JF, Blackbeard GA. Surgical modifications to the Brånemark zygomaticus protocol in the treatment of the severely resorbed maxilla: a clinical report. Int J Oral Maxillofac Implants 2003;18:232-7. 29. Uchida Y, Goto M, Katsuki T, Akiyoshi T. Measurement of the maxilla and zygoma as an aid in installing zygomatic implants. J Oral Maxillofac Surg 2001;59:1193-8. 30. Ujigawa K, Kato Y, Kizu Y, Tonogi M, Yamane GY. Three-dimensional finite elemental analysis of zygomatic implants in craniofacial structures. Int J Oral Maxillofac Surg 2007;36:620-5. 31. Borchers L, Reichart P. Three-dimensional stress distribution around a dental implant at different stages of interface development. J Dent Res 1983;62:155-9. 32. van Zyl PP, Grundling NL, Jooste CH, Terblanche E. Three-dimensional finite element model of a human mandible incorporating six osseointegrated implants for stress analysis of mandibular cantilever prostheses. Int J Oral Maxillofac Implants 1995;10:51-7. 33. Meijer GJ, Starmans FJ, de Putter C, van Blitterswijk CA. The influence of a flexible coating on the bone stress around dental implants. J Oral Rehabil 1995;22:105-11.
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34. Papavasiliou G, Kamposiora P, Bayne SC, Felton DA. Three-dimensional finite element analysis of stress-distribution around single tooth implants as a function of bony support, prosthesis type, and loading during function. J Prosthet Dent 1996;76:633-40. 35. Sertgöz A. Finite element analysis study of the effect of superstructure material on stress distribution in an implant-supported fixed prosthesis. Int J Prosthodont 1997;10: 19-27. 36. Menicucci G, Lorenzetti M, Pera P, Preti G. Mandibular implant-retained overdenture: finite element analysis of two anchorage systems. Int J Oral Maxillofac Implants 1998;13:369-76. 37. Teixeira ER, Sato Y, Akagawa Y, Shindoi N. A comparative evaluation of mandibular finite element models with different lengths and elements for implant biomechanics. J Oral Rehabil 1998;25:299-303. 38. Saab XE, Griggs JA, Powers JM, Engelmeier RL. Effect of abutment angulation on the strain on the bone around an implant in the anterior maxilla: a finite element study. J Prosthet Dent 2007;97: 85-92. 39. Quaresma SE, Cury PR, Sendyk WR, Sendyk C. A finite element analysis of two different dental implants: stress distribution in the prosthesis, abutment, implant, and supporting bone. J Oral Implantol 2008;34: 1-6. 40. Lewinstein I, Banks-Sills L, Eliasi R. Finite element analysis of a new system (IL) for supporting an implant-retained cantilever prosthesis. Int J Oral Maxillofac Implants 1995;10:355-66. 41. Geng JP, Tan KB, Liu GR. Application of finite element analysis in implant dentistry: a review of the literature. J Prosthet Dent 2001;85:585-98. 42. Miyamoto S, Ujigawa K, Kizu Y, Tonogi M, Yamane GY. Biomechanical threedimensional finite-element analysis of maxillary prostheses with implants. Design of number and position of implants for maxillary prostheses after hemimaxillectomy. Int J Oral Maxillofac Surg 2010;39:1120-6. 43. Baggi L, Cappelloni I, Di Girolamo M, Maceri F, Vairo G. The influence of implant diameter and length on stress distribution of osseointegrated implants related to crestal bone geometry: a three-dimensional finite element analysis. J Prosthet Dent 2008;100: 422-31. 44. Okumura N, Stegaroiu R, Kitamura E, Kurokawa K, Nomura S. Influence of maxillary cortical bone thickness, implant design and implant diameter on stress around implants: a three-dimensional finite element analysis. J Prosthodont Res 2010;54: 133-42. 45. Rothstock S, Uhlenbrock A, Bishop N, Laird L, Nassutt R, Morlock M, et al. Influence of interface condition and implant design on bone remodeling and failure risk for the resurfaced femoral head. J Biomech 2011;44:1646-53.
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46. Wu T, Liao W, Dai N, Tang C. Design of a custom angled abutment for dental implants using computer-aided design and nonlinear finite element analysis. J Biomech 2010;43: 1941-6. 47. Korkmaz FM, Korkmaz Y, Yalug S, Korkmaz T. Impact of dental and zygomatic implants on stress distribution in maxillary defects: a 3-D FEA study. J Oral Implantol 2012;38:557-67. 48. Gross MD, Arbel G, Hershkovitz I. Threedimensional finite element analysis of the facial skeleton on simulated occlusal loading. J Oral Rehabil 2001;28:684-94.
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13 49. Cattaneo PM, Dalstra M, Melsen B. The transfer of occlusal forces through the maxillary molars: a finite element study. Am J Orthod Dentofacial Orthop 2003;123:367-73. 50. Tuxen A, Bakke M, Pinholt EM. Comparative data from young men and women on masseter muscle fibres, function and facial morphology. Arch Oral Biol 1999;44:509-18. 51. Zarone F, Apicella A, Nicolais L, Aversa R, Sorrentino R. Mandibular flexure and stress build-up in mandibular full-arch fixed prostheses supported by osseointegrated implants. Clin Oral Implants Res 2003;14: 103-14.
52. Currey JD. Bone strength: what are we trying to measure? Calcif Tissue Int 2001;68: 205-10. Corresponding author: Dr Weidong Tian No. 14, 3rd Section, Renmin South Chengdu CHINA E-mail: [email protected] Copyright ª 2013 by the Editorial Council for The Journal of Prosthetic Dentistry.
Postgraduate student, Department of Oral and Maxillofacial Surgery, West China College of Stomatology, Sichuan University; National Engineering Laboratory for Oral Regenerative Medicine, West China Hospital of Stomatology, Sichuan University; State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University. b Associate Professor, Department of Pedodontics, West China College of Stomatology, Sichuan University; National Engineering Laboratory for Oral Regenerative Medicine, West China Hospital of Stomatology, Sichuan University; State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University. c Postgraduate student, College of Architecture and Environment, Sichuan University; National Engineering Laboratory for Oral Regenerative Medicine, West China Hospital of Stomatology, Sichuan University; State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University. d Postgraduate student, Department of Implantology, West China College of Stomatology, Sichuan University; State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University. e Postgraduate student, Department of Implantology, West China College of Stomatology, Sichuan University; State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University. f Postgraduate student, Department of Endodontics, West China College of Stomatology, Sichuan University; State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University. g Postgraduate student, Department of Endodontics, West China College of Stomatology, Sichuan University; State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University. h Professor and Director, Department of Oral and Maxillofacial Surgery, West China College of Stomatology, Sichuan University; National Engineering Laboratory for Oral Regenerative Medicine, West China Hospital of Stomatology, Sichuan University; State Key Laboratory of Oral Diseases, West China Hospital of Stomatology, Sichuan University.
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Conclusions. All 3 zygomatic implant techniques resulted in more or less homogeneous transference of force and thus could reconstruct the edentulous maxilla; however, the exteriorized technique with 1 dental implant in the lateral incisor appeared to be the most appropriate reconstruction method for the severely atrophied edentulous maxilla. (J Prosthet Dent 2013;-:---)
Clinical Implications These results can be clinically applied when choosing the appropriate zygomatic implant technique for the severely atrophied edentulous maxilla. The exteriorized technique could increase the long-term success rate of zygomatic implants from the biomechanical perspective.
The zygomatic implant has become a standard procedure in the treatment of maxillary defects,1-3 patients with edentulism and with severe maxillary atrophy4-10 and also has been advocated for patients with advanced periodontal disease.11 The zygomatic implant offers an efficient therapeutic process that avoids bone grafting and has a good success rate.12-15 The zygomatic implant was first used by Brånemark et al16 to rehabilitate maxillary defects. Since their introduction, zygomatic implantation techniques have been reported for rehabilitating severely absorbed edentulous maxilla by a number of researchers, including Stella and Warner,17 Migliorança et al,9 and Maló et al.18 Variations of the zygomatic implantation method are known as the Brånemark technique,16 the Stella or sinus slot technique,17 and the exteriorized or extrasinus technique.19 The technique introduced by Maló et al18 is referred to here as the extramaxillary technique. All of these zygomatic implantation techniques have been reported to perform well in short- and medium-term studies with implant survival rates of 82% to 100%.8,12,14,18,20-24 However, no consensus regarding the ideal technique for placing zygomatic implants has yet been reached, which is likely because of the lack of long-term clinical observations with extensive sampling and systemic comparisons among the different techniques. Each method offers certain advantages and disadvantages. For instance,
in both the Brånemark and Stella techniques,16,17 zygomatic implants are positioned through the sinus and fixed into both the zygomatic and maxillary bone. Clinical signs of maxillary sinus pathology must be considered by the clinician when using these 2 techniques.25-27 Although the Stella technique17 may improve upon implant orientation and reduce postoperative symptoms, BoyesVarley et al28 disagreed with the Stella technique17 because a lack of visibility may result in perforation of the posterior antral wall. The exteriorized technique increases the length of drill holes into the zygomatic bone, which may provide higher initial mechanical stability than other techniques.19 The extramaxillary technique reduces the size of the prosthesis that may reduce problems such as phonetic difficulties, dental hygiene concerns, and mechanical resistance from the prosthesis; however, there is reduced support and retention of the maxillary bone.18 The extrasinus technique may show improved biomechanical performance because it reduces the cantilever more than in the extramaxillary technique and supports the maxillary sinus more than the Brånemark technique.16 Therefore, a 3-dimensional finite element analysis (FEA) study was designed to provide biomechanical information to address the optimal technique for installing zygomatic implants and to clarify the ideal number and position of zygomatic and dental implants for the reconstruction of a severely atrophied edentulous maxilla.
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MATERIAL AND METHODS The research protocol was approved by the institutional review board of Sichuan University. A 3-dimensional finite element solid model of a human skull was constructed from the clinical computed tomography data from a 69-year old Chinese patient with a severely absorbed edentulous maxilla. The skull was scanned in the transverse plane (Computed Tomography Scanner; Siemens) with a 1-mm-slice thickness and 1-mm scan increment in 240 slice images. Three-dimensional external shapes of the craniofacial structures were generated by image-processing software (Mimics 10.01; Materialise NV). Output data were imported to software designed for surface reconstruction and the design of zygomatic and dental implants (Pro/ENGINEER WildFire 4.01; PTC Corp). The data obtained were then processed by software (ANSYS Workbench 12.01; ANSYS Inc) for conversion into a 3-dimensional FEA solid model. Because of the similarity between the Brånemark and Stella techniques,16,17 the classic Brånemark technique was chosen to represent both the Brånemark and Stella techniques in the study. The 3 zygomatic implantation techniques considered in this study served to define 3 model groups: the Brånemark (B group), exteriorized (Ed group), and extramaxillary (Ey group) groups. In total, the FEA study worked with 9 reconstructed models, which were divided into 3 groups according to the surgical
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1 Views of zygomatic implants and dental implant models for 9 model variations. B, Brånemark technique group. Ey, extramaxillary technique group; Ed, exteriorized technique group; green, craniofacial bones; pink, maxillary sinus, nasal cavity, and other cavities; yellow, gold alloy bar; gray, zygomatic implants and dental implants; blue, cancellous bone; dark green, cortical bone.
technique modeled. In the Brånemark technique group (B group),16 all zygomatic implants were inserted from the site of the maxillary first molar. Model LP consisted of 1 zygomatic implant and 2 dental implants in the maxillary lateral incisor and the first premolar on each side. Both model P and model L in the Brånemark technique16 group had 1 zygomatic implant on each side, with 1 dental implant in the maxillary first premolar in model P or the lateral incisor in model L. In the extramaxillary technique group model LP included 1 zygomatic implant in the maxillary first molar and 1 dental implant in the lateral incisor on each side. Model P had 2 zygomatic implants on both sides, inserted from the site of the first molar and the second premolar. Model L had 1 dental implant in the canine on the base of model P. In the exteriorized technique group, model LP, model P, and model L were the same as those described for the Brånemark technique.16
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A 45-mm zygomatic implant (Nobel Biocare AB) with an abutment 5 mm in height and 2 regular dental implants (3.75 13.0 mm; Nobel Biocare AB) with abutments 5 mm in height were modeled. The appropriate length of the zygomatic implant was selected according to the distance between the crest of the alveolar process of the maxilla near the palate and the jugal point (JU) of the zygomatic bone.29 Prosthetic superstructures were modeled as a gold alloy symmetrically curved bar with a section 10 8 mm30 and were constructed with Pro/ ENGINEER WildFire 4.01 software (PTC Corp). The different numbers of zygomatic and dental implants in models LP, P, and L with the 3 different surgical techniques are shown in Figure 1. In this study, the mechanical properties of implants and bones were all treated as isotropic, homogeneous, and linear elastic. The elastic modulus and Poisson ratio values of titanium, gold alloy, and cortical and cancellous bone
(Table I) were obtained from the literature.31-41 The bone-implant contact area was assumed to be complete osseous integration and so the contact area was simulated by using a surface-to-surface option fully bonded for the cemented
Table I. Mechanical properties of materials used in FEA models Elastic Modulus (GPa)
Poisson Ratio
Titanium
110.0*
0.33
Cortical bone
13.7y
0.30
Materials
Cancellous bone Gold alloy
1.37y 80z
FEA, finite element analysis. *From Refs. 39,41. y From Refs. 31-38. z From Refs. 40,41.
0.30 0.33
4
Volume versions.30,42-45 Nine solid models were transferred to the ANSYS Workbench 12.01 (ANSYS Inc) to generate the FEA models with 10 node tetrahedral elements. An element size of 1 mm was used as the meshing requirement for all FEA models.46 Mesh generation resulted in a total of approximately 274 000 elements and 484 000 nodes. To simulate occlusal force, a vertical load of 150 N was applied to the occlusal surface along the zygomatic implant axis, and a lateral load of 50 N was applied to the palatal surface of the superstructure.30,47-49 To simulate the action and pass downward and backward, a distributed occlusal force of 300 N was applied to the insertion area of
the masseter muscle on the zygomatic arch and zygomatic process of the maxilla.30,50 For the top cutting plane, the restraint was chosen to simulate the presence of the remainder of the skull. Both loading and boundary conditions of the FEA models are shown in Figure 2.
RESULTS Comparisons were made between the von Mises stress and deformation magnitude and distribution in craniofacial models rehabilitated by zygomatic and dental implants with each of the 3 techniques. Stress and deformation were visualized with pseudocolors on
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the FEA model (Figs. 3 to 8). Unstressed regions were represented by dark-blue areas. Red areas represent regions of maximum stress. Each item was compared by using the scale shown on the left side of these images. Stresses caused by occlusal forces were found to be primarily supported by the zygoma, with transfer through the infrazygomatic crest. The maximum stress in craniofacial bone was 20 to 25 MPa both under vertical loading and lateral loading (Fig. 9). No obvious differences in stress distribution and magnitude in the craniofacial bone of any of the 9 models were noted. The distribution of stress in the craniofacial bone was somewhat homogeneous.
2 Front and lateral views of craniofacial and implant models under loading conditions (red).
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3 Front view of implant models under vertical loading.
4 Front view of implant models under lateral loading.
The craniofacial bones and components that provided the majority of mechanical support for the zygomatic and dental implants were the maxillary bone, the zygomatic bone, and the zygomatic arch. The maximum stress values in each of these 3 components are shown in Figure 9. In all models, except model LP in the Ey group, the maximum stress in the maxillary bone was approximately 10 MPa under vertical loading and 7 MPa under lateral loading, and appeared where the
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zygomatic implant perforated the maxillary bone or in the surrounding alveolar bone (Fig. 9). The maximum value in the zygomatic bone was approximately 20 MPa and concentrated in the JU bone landmark (Fig. 9). In all models, stress was transferred along the zygomatic arch with a value that ranged from 7 to 10 MPa under both vertical and lateral loading (Fig. 9). The amount and distribution of stresses on zygomatic and dental implant components are shown in Figures 3, 4,
and 10. The zygomatic implant of model L in the Ed group had an average value of maximum stress on 2 sides of approximately 45 MPa under vertical loading and approximately 18 MPa under lateral loading (Fig. 10). All other models had a similar maximum stress of approximately 100 MPa under vertical loading and approximately 24 to 35 MPa under lateral loading (Fig. 10). The primary regions of maximum stress were the inner side of the joint of the implant abutmen, and some stress was
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5 Stress distribution and magnitude on surrounding bones under vertical loading.
6 Stress distribution and magnitude on surrounding bones under lateral loading. maximized on the middle and end portions of the zygomatic implants (Figs. 3, 4). The posterior zygomatic implant of model P in the extramaxillary
technique group (model P Ey) supported more stress and transferred larger forces than the anterior zygomatic implant (Figs. 3, 4).
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The standard dental implant of model L in the Brånemark technique group (model LB) and model L in the exteriorized technique group (model LEd)
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7 Deformation of surrounding bones under vertical loading.
8 Deformation of surrounding bones under lateral loading. shared similar minimal stress values of approximately 10 MPa under vertical loading and 5 MPa under lateral loading (Fig. 10). However, the dental implant in model LP of the extramaxillary technique
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group (model LPEy) showed the maximum stress values of approximately 22 MPa under vertical loading and 18 MPa under lateral loading (Fig. 10). However, stress values on the standard dental
implant were much smaller than those on the zygomatic implants (Figs. 3, 10). The maximum stress on the bones that surround the zygomatic implants of model L in Ed group was approximately
8
Volume Craniofacial Bone Model Under Vertical Load LP
25.0
P
30.0
L
20.0 15.0 10.0 5.0 0.0
B
Ey
LP
25.0
10.0 5.0 B
15.0 10.0 5.0
P
4.0 2.0
10.0 5.0
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15.0 10.0 5.0 B
2.0 Ey
Ey
12.0
L
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Stress (MPa)
P
6.0 4.0
B
L
Ed
Zygomatic Arch Model Under Lateral Load
8.0
0.0
P
20.0
Zygomatic Arch Model Under Vertical Load LP
Ed
0.0
Ed
12.0 10.0
Ey
25.0
L
15.0
Ey
L
Zygomatic Bone Model Under Lateral Load
20.0
B
P
6.0
B
Stress (MPa)
Stress (MPa)
LP
LP
8.0
Zygomatic Bone Model Under Vertical Load
0.0
Ed
0.0
Ed
25.0
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10.0
L
Stress (MPa)
Stress (MPa)
P
20.0
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Maxillary Bone Model Under Lateral Load
LP
B
P
15.0
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Stress (MPa)
30.0
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LP
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L
8.0 6.0 4.0 2.0 0.0 B
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Ed
9 Mean value of maximum stress in craniofacial bone models and maximum stress in maxillary bone, zygomatic bone, and zygomatic arch models on both sides under vertical and lateral loading. 7 MPa under vertical loading and 5.5 MPa under lateral loading (Fig. 11). Meanwhile, model LP in the Ey group had a maximum stress of approximately 16 MPa (others were approximately 10 MPa under vertical loading); all of the models except model L in the Ed group were similar to a maximum stress of 7 to 8 MPa (Fig. 11). The maximum stress on the bones that surround the dental
implants was almost 2 to 4 MPa under vertical loading and 1 to 2 MPa under lateral loading except for the models of the Ey group (Fig. 11). The maximum stress on the bones that surround the implants was at the site where zygomatic implants perforated the maxillary bone, maxillary sinus, or the site near the bone landmark JU point, where the end of the zygomatic implants was fixed
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both under vertical loading and under lateral loading (Figs. 5, 6). The maximum deformation of surrounding bones in the B and Ed groups was approximately 10 mm, and the maximum deformation of the Ey group was approximately 15 mm under vertical loading (Fig. 12). Under lateral loading, the maximum deformation was 12 mm in the B group, 14 mm in the Ed group,
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9 Left Zygomatic Implant Model Under Vertical Load LP
P
80.0 60.0 40.0 20.0 0.0
40.0
L
100.0
Stress (MPa)
Stress (MPa)
120.0
Left Zygomatic Implant Model Under Lateral Load
B
Ey
10.0
P
40.0
L
100.0 80.0 60.0 40.0 20.0 0.0
B
Ey
P
10.0
B
Ey
P
Ed
L
20.0 10.0 Ey
Ed
Standard Dental Implant Model Under Lateral Load 20.0
15.0 5.0 0.0
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B
L
20.0
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0.0
Ed
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Stress (MPa)
LP
25.0
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Standard Dental Implant Model Under Vertical Load 30.0
B
Right Zygomatic Implant Model Under Lateral Load Stress (MPa)
Stress (MPa)
LP
L
20.0
Right Zygomatic Implant Model Under Vertical Load 120.0
P
30.0
0.0
Ed
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LP
P
L
15.0 10.0 5.0 0.0 B
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10 Mean maximum stress on zygomatic implant and dental implant models under vertical and lateral loading. and 18 mm in the Ey group (Fig. 12). The maximum deformation of bones that surround dental implants was approximately 2 to 5 mm, both under vertical loading and under lateral loading (Fig. 12). The deformation of surrounding bones in the B and Ed groups was similar. The deformation of bones that surround dental implants was much less than that surrounding zygomatic implants. The maximum deformation region was near the bone landmark JU point where the end of zygomatic implants was fixed (Figs. 7, 8).
DISCUSSION No obvious differences in the magnitude and distribution of stress in the total and external craniofacial bone models were found for any of the 3
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techniques. However, the lowest stress and deformation on the zygomatic implants and in the bones that surround the zygomatic implants were found in the exteriorized technique group. Stress in the total and external craniofacial bone models indicated that the 3 techniques all resulted in more or less homogeneous transference of force. Moreover, transference of force in the external craniofacial bone was consistent with those of Ujigawa et al.30 The zygomatic implant was found to bear most of the occlusal force. The stress on the standard dental implant was so small that it could essentially be ignored because titanium alloys are known to tolerate stresses up to 900 N/ mm2 without irreversible deformation51; therefore, a force of 150 N would not likely result in dental implant failure.
However, because concentrated stress could lead to the absorption of surrounding bone,7 the stress and deformation of the bone that surrounds zygomatic and dental implants was taken into consideration. The stress of the bone that surrounds the zygomatic implants was much larger than that of the bone that surrounds dental implants. Because the static strength of bone is approximately 150 MPa in tension and approximately 250 MPa in compression,52 rehabilitating the edentulous maxilla with zygomatic implants and dental implants should not risk overstressing the surrounding bone. Deformation is determined by the force of loading and the material’s mechanical properties. Because there is much cancellous bone near the landmark JU and stress in this region was maximal, maximum deformation of the
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Volume Craniofacial Bone Around Left Zygomatic Implant Under Vertical Load LP
P
9.0 6.0 3.0 0.0
25.0
L
12.0
B
Ey
Craniofacial Bone Around Left Zygomatic Implant Under Lateral Load P
6.0 4.0 2.0 0.0
B
Ey
P
4.5 3.0 1.5 0.0
B
Ey
Ed
10.0 5.0 B
Ey
Ed
LP
P
L
8.0 6.0 4.0 2.0 B
Ey
Ed
Craniofacial Bone Around Dental Implant Under Lateral Load 7.5
L
Stress (MPa)
Stress (MPa)
LP
6.0
L
0.0
Ed
Craniofacial Bone Around Dental Implant Under Vertical Load 7.5
P
15.0
10.0
L
8.0
LP
Craniofacial Bone Around Right Zygomatic Implant Under Lateral Load Stress (MPa)
Stress (MPa)
LP
-
20.0
0.0
Ed
10.0
Issue
Craniofacial Bone Around Right Zygomatic Implant Under Vertical Load Stress (MPa)
Stress (MPa)
15.0
-
LP
P
L
6.0 4.5 3.0 1.5 0.0 B
Ey
Ed
11 Maximum stress on bones that surround zygomatic implants and dental implants under vertical loading and lateral loading.
surrounding bone was concentrated on this region. Stress on external and total bone was influenced by occlusal force and the force of the masseter. Stress on the surrounding bone was only influenced by the occlusal force. Occlusal force was much less than the masseter, and stress distributions on external and total bone were somewhat homogeneous. However, some differences were found between the stresses of the surrounding bones. Because stress distribution in external and total craniofacial bone was rather homogeneous and stress on the dental implants was low, the biomechanics of the zygomatic implant were determined to be the main factor that influenced the success rate of the surgery. Minimal stresses on the zygomatic implant, under
both vertical and lateral loading, were found in model L in the exteriorized technique. The surrounding bones of zygomatic implants with both the Brånemark technique16 and the exteriorized group presented less deformation than found in the extramaxillary group, which meant that the surrounding bone of zygomatic implants in the extramaxillary group was more susceptible to bone absorption. When taking into consideration the surgical procedure and potential complications,25-28 the exteriorized or extrasinus technique may be the optimal method. When considering the bone contact length of zygomatic implants, the exteriorized technique increases the contact length more than the Brånemark technique does.19 Moreover, it was obvious
The Journal of Prosthetic Dentistry
that the contact length of the extramaxillary technique was much shorter than that of the exteriorized and Brånemark techniques.16 Zygomatic implants inserted by using the extramaxillary technique represent a long cantilever, which was also a risk factor of failure. The results of this FEA study were consistent with these outcomes. However, when there is insufficient alveolar bone and maxillary bone for the exteriorized and Brånemark techniques,16 the extramaxillary technique is a proper choice. As in every FEA study, some limitations apply. One limitation was that the zygomatic implants and dental implants were simplified to facilitate modeling and calculations. Another was that, in the present study, 100% osseointegration between implants and
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2013
11
18.0
LP
P
L
15.0 12.0 9.0 6.0 3.0 0.0
B
Ey
Craniofacial Bone Around Right Zygomatic Implant Under Vertical Load Deformation (µm)
Deformation (µm)
Craniofacial Bone Around Left Zygomatic Implant Under Vertical Load
Ed
20.0
LP
P
L
16.0 12.0 8.0 4.0 0.0
B
Ey
Ed
P
L
12.0 8.0 4.0 0.0
B
Ey
Ed
L
12.0 8.0 4.0 0.0
B
Ey
Ed
20.0
LP
P
L
16.0 12.0 8.0 4.0 0.0 Ey
Ed
Craniofacial Bone Around Dental Implant Under Lateral Load Deformation (µm)
Deformation (µm)
LP
16.0
P
16.0
B
Craniofacial Bone Around Dental Implant Under Vertical Load 20.0
LP
Craniofacial Bone Around Right Zygomatic Implant Under Lateral Load Deformation (µm)
Deformation (µm)
Craniofacial Bone Around Left Zygomatic Implant Under Lateral Load
20.0
15.0
LP
P
L
12.0 9.0 6.0 3.0 0.0 B
Ey
Ed
12 Maximum deformation of bones that surround zygomatic implants and dental implants under vertical loading and lateral loading.
the surrounding bone was assumed. In clinical situations, the percentage of osseointegration could be reduced by such things as local inflammation, drugs, and osteoporosis. Further investigations are needed to assess the osseointegration of zygomatic implants in different situations.
CONCLUSIONS Within the limitations of this study, the following conclusions were drawn: 1. The classic Brånemark, the exteriorized, and the extramaxillary zygomatic implantation techniques resulted in more or less homogeneous transference of force and thus could be used to reconstruct the edentulous maxilla.
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2. In terms of minimizing stress on the zygomatic implant structures, and when considering the stress and deformation of the bones that surround the zygomatic implants, the optimal reconstruction method was model L (a zygomatic implant on each side, with a dental implant in the lateral incisor position) of the exteriorized technique group.
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52. Currey JD. Bone strength: what are we trying to measure? Calcif Tissue Int 2001;68: 205-10. Corresponding author: Dr Weidong Tian No. 14, 3rd Section, Renmin South Chengdu CHINA E-mail: [email protected] Copyright ª 2013 by the Editorial Council for The Journal of Prosthetic Dentistry.
