Abstract
Background and Purpose
Dental implants are considered to be one of several treatment options that can be used to replace missing teeth. The objective of the study is to examine and compare the biomechanics of zygomatic and pterygoid implants planned on the atrophic maxilla with three different bone types.
Materials and Methods
An in vitro finite element study was conducted on a three-dimensional model of zygomatic and pterygoid implants. In a total of 24 implants, two bilateral zygomatic and pterygoid implants with two anterior dental implants were inserted in models. 150 N vertical occlusal and 300 N load on masseter and medial pterygoid were simulated on the modeled prosthesis. The data were processed with ANSYS software. The stress on and deformations of the bones and implants were observed and compared.
Results
When comparing the D4, D3, and D2 bones in subgroup I with zygomatic implants, the D2 bone was subjected to less stress compared to D3 and D4. The smallest displacement (0.125784 mm) was seen in D4 followed by the largest displacement (0.74073 mm) in D2. Similarly, when comparing the D2, D3, and D4 bone in subgroup II with pterygoid implants, the D2 bone in the atrophic maxilla received the least amount of stress from the pterygoid implants compared to D3 and D4. Furthermore, the smallest displacement (0.030934 mm) was seen in D2, and the largest (0.046319 mm) in D4.
Conclusion
Results suggest firstly, that the overall stress was better distributed in D2 bone and secondly, the pterygoid implant showed higher stress concentration than the zygomatic implant.
Keywords: Atrophic maxilla, Displacement, Finite element analysis, Pterygoid implant, Stress, Von Mises, Zygomatic implant
Introduction
The use of dental implants to replace missing teeth has been in practice for more than half a century. They have shown a high success rate after replacing the missing teeth [1]. The introduction of the osseointegration concept by Prof. P. I. Branemark in the early 1960s paved the way for the immense growth and success of implants in dentistry. Osseointegration is a term originally coined by Brånemark [2, 3]. Rehabilitation of severely atrophic maxilla is challenging due to increased sinus pneumatization, poor quality, and decreased bone quantity [1–3]. The possibility of increasing the maxillary dimension with autologous grafting from a bone and the possibility of graft failure often necessitate the use of general anesthesia. Limiting factors include the marked resorption of bone harvested from the iliac crest during healing and the limited volume of bone afforded by calvarial grafts.
The discomfort experienced by patients in the donor region during the postoperative period constitutes another important factor [4]. Zygomatic and pterygoid implants are an alternative procedure that avoids invasive surgical protocols such as bone augmentation [5]. Figure 1a shows the anterior view of the skull with a zygomatic implant and prosthesis. Figure 1b shows an anterior mesh view of a skull model with a pterygoid implant and prosthesis. The theoretical successes of angled implants are dependent on several factors, such as cortical anchorage, implant length, and implant distribution within the arch. There will be an increase in the bone-implant contact interface with a longer duration of use of implants. Table 1 shows the material properties of bone and various materials used for implants. Immediate loading can be secured when the implant body achieves one or more cortical anchorage points [6].
Fig. 1.
a The anterior view of the skull with a zygomatic implant and prosthesis. b An anterior mesh view of a skull model with a pterygoid implant and prosthesis. The theoretical successes of angled implants are dependent on several factors, such as cortical anchorage, implant length, and implant distribution within the arch
Table 1.
Material properties
| Material | Elastic modulus (GPa) | Poison’s ratio |
|---|---|---|
| Cortical bone | 13.7 | 0.3 |
| Cancellous bone | 13.7 | 0.3 |
| Pure titanium bar | 117 | 0.3 |
| Ti6AI4V implant | 110 | 0.33 |
| Resin | 2.7 | 0.35 |
It is crucial to estimate the peak level of stress for the success of rehabilitation of a prosthesis. This is because excessive stress at the implant-bone interface is the potential cause of peri-implant-bone loss and failure of osseointegration. Finite element analysis divides complex problems into smaller and simpler parts or elements. It provides numerical results useful for implant biomechanics as a result of implant loading [4]. Studies comparing the biomechanics of zygomatic and pterygoid implants in posterior atrophic maxilla are limited. Hence, this study aims to biomechanically evaluate zygomatic and pterygoid implants in atrophic maxilla using finite element analysis.
Materials and Methods
Study Design and Sample
An in vitro, comparative study was conducted on three-dimensional models of zygomatic and pterygoid implants. This was done to evaluate the biomechanics of zygomatic versus pterygoid implants in the atrophic maxilla with type 2, type 3, and type 4 bone types.
For this purpose, a human skull was generated based on CT data measurement of a real case scenario, and the maxillary prosthesis was modeled with CBCT scanner and Solid Edge Software Version 19. Six three-dimensional finite element computer models of skulls with three different bone types were also constructed to mimic various case scenarios. In a total of 24 implants, two bilateral zygomatic implants with two anterior dental implants were inserted in one set of models. Two bilateral pterygoid implants with two anterior dental implants were inserted in another set of models.
The inclusion criteria were: 3d Finite Element Computer Models of Partial Atrophic Maxilla and those with Bone Type 1 were excluded from the study. The Human Cadaveric Model with Atrophic Maxilla was also not taken for this study.
The exclusion criteria were as follows: 3D finite element computer models of partial atrophic maxilla, 3D finite element computer models of atrophic maxilla with bone type 1, and human cadaveric model with atrophic maxilla.
Data Collection Method
The computed tomography data of a polyurethane model scanned using Mimics software version 14 were used to create a skull model. Solid Edge software version 19 was used to create 3D models of zygomatic implants and pterygoid implants based on the surface morphology and dimensions of the Norris medical implant system. Table 2 shows the cortical thickness along with the number of elements and nodes for zygomatic and pterygoid models. Bilateral zygomatic implant at 450° angulations with 45 mm length and 4.2 mm diameter, as well as bilateral PteryFit- at 500 angulation with length 18 mm and diameter 4.2 mm number, coupled with two anterior tuff implants with a length of 10 mm and a diameter of 4.2 mm that were saved as STEP files and afterward exported into a human skull model file. Figure 2a shows a schematic view of the zygomatic implant. Figure 2b shows a schematic view of the pterygoid implant.
Table 2.
Number of elements and nodes
| Cortical thickness (mm) | Elements | Nodes | |
|---|---|---|---|
| Zygomatic model | 0.5 | 928,726 | 1,158,273 |
| 0.75 | 957,829 | 1,178,277 | |
| 1.5 | 977,273 | 1,202,838 | |
| Pterygoid model | 0.5 | 802,545 | 1,059,366 |
| 0.75 | 832,738 | 1,089,237 | |
| 1.5 | 852,838 | 1,102,882 |
Fig. 2.
a A schematic view of the zygomatic implant. b A schematic view of the pterygoid implant
According to Lecholm and Zarb's classification, the skulls have 1.5-mm-, 0.75-mm-, and 0.5-mm-thick cortical layers, which indicate three unique bone types (D2, D3, and D4). The rectangular bar attachment was modeled and put 1.5–2 mm above the crestal bone as shown in Fig. 3. The bar attachment was given the material qualities of pure titanium. It was supposed that the bar attachment and the implant were assumed to be merged connections. A maxillary model was used to create a dentate prosthesis, which was then scanned with a CBCT scanner, and the data were uploaded to Solid Edge software version 19 for hybrid prosthesis modeling.
Fig. 3.
The rectangular bar attachment was modeled and put 1.5–2 mm above the crestal bone
The tooth dimensions were obtained using the tooth anatomy of ‘Wheelers’ as shown in Fig. 4. The material qualities of acrylic were assigned to the model region once the prosthesis was modeled. A bar attachment connects the entire arch prosthesis to all implants. Hyper mesh V11 was used to mesh the models, which were separated into four-point tetrahedral-shaped micro-components. In the X-, Y-, and Z-axes, the skull was thought to be immobile. The models were then sent to ANSYS for analysis.
Fig. 4.
The tooth dimensions were obtained using the tooth anatomy of 'Wheelers’
After analysis, 150 N was applied vertically to the contact locations of the first premolar, second premolar, and first molar, which is known as the physiologic occlusal loading level. Figure 5 shows the physiologic occluding load 150 N applied vertically on contact points of posterior teeth in the skull model containing both zygomatic and pterygoid implants. A simulated masseter load of 300 N was applied to the insertion area of the masseter muscle and the zygomatic arch and zygomatic process of the maxilla on the left and right sides of the bone, with force components of − 62.1 N on the X-axis, − 125.7 N on the Y-axis, and 265.2 N on the Z-axis, for subgroup I.
Fig. 5.
Physiologic occluding load 150 N is applied vertically on contact points of posterior teeth in the skull model containing both zygomatic and pterygoid implants
Similarly, for subgroup II, the force was applied to the medial pterygoid muscle insertion area and between the pterygoid plates on the left and right sides of the bone. Figure 6a shows a 300 N masticatory load applied for the skull model with a zygomatic implant. Figure 6b shows a 300 N masticatory load applied for the skull model with a pterygoid implant. Both numerical and visible color scatter scales were used to assess the maximal von Mises stress surrounding and in the implant. For each model, displacement values were obtained and evaluated.
Fig. 6.
a A 300-N masticatory load applied for the skull model with a zygomatic implant. b A 300-N masticatory load applied for the skull model with a pterygoid implant
Results
For analysis, the final models with prostheses were transferred to the ANSYS software. A 300 N masticatory load and a 150 N occlusal load were applied. The maximum von Mises stress around and in the implants and total displacement were assessed by both numerical and visualized color scales in which warm colors represent higher values and cold colors with lower values.
Discussion
Dental implants are considered to be one of several treatment options that can be used to replace missing teeth. Several implant-supported treatment options have been used successfully to replace a single tooth and multiple teeth, as well as a completely edentulous jaw [1]. The process of placement of an implant in the atrophied posterior maxillary region may be challenging because of the restricted vertical height of alveolar bone or due to sinus pneumatization, the lower density of bone, and difficulty in accessing surgically [2]. A case with extreme maxillary atrophy was reported. Six osseointegrated dental implants, two in the canine region, two zygomatic implants placed using the sinus slot approach, and two in the pterygomaxillary region, were placed. A high-definition computed tomography scan was used to make a stereolithographic model. The surgical approach was simulated, and a precise surgical template was used to transfer the surgical approach for positioning implants to real bone. Four months after surgery, the implants were loaded, with no increase in bone volume from bone grafts. Results demonstrate that zygomatic implants are a valid alternative to bone grafting in treating severe atrophic and edentulous maxillae [7]. There are various treatment modalities available for placement of the implant-supported prosthesis in the posterior atrophic maxilla such as augmentation of the floor of the sinus, placement of short-length implant systems, and using zygomatic or pterygoid implants.
The current study evaluated the biomechanics of zygomatic implants and pterygoid implants planned on the atrophic maxilla with three different bone types. For this analysis, the final models with prostheses were transferred to the ANSYS program. A 300 N masticatory load and a 150 N occlusal load were applied. The zygomatic implant and pterygoid implants were 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 deformation. Therefore, a force of 150 N would not likely result in dental implant failure. The stress of the bone that surrounds the zygomatic implants and pterygoid 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, rehabilitating the edentulous maxilla with these implants should not risk overstressing the surrounding bone. Table 3 demonstrates the stress and deformities caused by zygomatic and pterygoid implants in various bone types. It is known that the success of angled implants depends on factors such as the size of the implant, the amount of cortical anchorage, and the distribution of the implant within the crest [6]. Also, in the literature, it is stated that if the amount of cortical anchorage is increased, the success of the implant is also greater.
Table 3.
The stress and deformation caused by zygomatic and pterigoid implant in various bone types
| Model | Cortical thickness (mm) | Maximum deformation (mm) | Location of deformation | Maximum stress (Mpa) | Location of stress |
|---|---|---|---|---|---|
| Zygomatic | 0.5-D4 | 0.164399 | Zygomatic arch | 106.58 | Zygomatic implant neck and crestal bone interface |
| 0.75-D3 | 0.125784 | Zygomatic arch | 99.0686 | Zygomatic implant neck and crestal bone interface | |
| 1.5-D2 | 0.74073 | Zygomatic arch | 86.1836 | Zygomatic implant neck and crestal bone Interface | |
| Pterygoid | 0.5-D4 | 0.046319 | Pterygoid plates bilaterally | 125.775 | Pterygoid implant neck and crestal bone interface |
| 0.75-D3 | 0.035242 | Retromolar area and Pterygoid plates bilaterally | 127.663 | Pterygoid implant neck and crestal bone interface | |
| 1.5-D2 | 0.030934 | Retromolar area and Pterygoid plates bilaterally | 131.388 | Pterygoid implant neck and crestal bone Interface |
The higher the cortical layer thickness less stress it takes which correlates with the current study. D2 bone is denser concerning D3 and D4 bone. For subgroup I with zygomatic implants, minimum cortical stress was seen in the D2 bone with 34.8356 Mpa which slightly increased in the D3 bone and maximum was seen in the D4 bone with 77.2404 Mpa as shown in Figs. 7a–c. Cortical layer anchorage directly affects the stability of the prosthesis and implant, it has been reported that trabecular bone anchorage has no effect on implant stability, and it may not be directly proportional to cortical stress [8]. An FEA study was conducted to examine the biomechanics of zygomatic implants placed on an atrophic maxilla with five different buccal defects. Two bilateral zygomatic implants with two anterior implants were inserted in all models. 150-N vertical occlusal and 300-N masseteric loads were stimulated on the modeled prosthesis, and maximum von Mises stress was found in a skull modeled with a type 4 defect and D3 bone type. Minimal stress values were found in type 2 buccal bone defect and D2 bone type. The study concluded, saying that cortical bone anchorage and bone type of zygomatic implant positively affect their biomechanics [9]. Minimum cancellous stress was seen in the D2 bone followed by the D3 bone, and the highest cancellous stress was seen in the D4 bone as shown in Figs. 8a–c. Similarly in subgroup II with pterygoid implants, it was determined that the D4 bone has the least total stress, and the D2 bone has the least displacement as shown in Figs. 9a, b, 10 and 11.
Fig. 7.
For subgroup I with zygomatic implants, minimum cortical stress was seen in the D2 bone with 34.8356 Mpa (7a) which slightly increased in the D3 bone (7b) and maximum was seen in the D4 bone with 77.2404 Mpa (7c)
Fig. 8.
Minimum cancellous stress was seen in the D2 bone (8a) followed by the D3 bone (8b), and the highest cancellous stress was seen in the D4 bone (8c)
Fig. 9.
In subgroup II with pterygoid implants, it was determined that the D4 bone has the least total stress and the D2 bone has the least displacement
Fig. 10.
In subgroup II with pterygoid implants, it was determined that the D4 bone has the least total stress and the D2 bone has the least displacement
Fig. 11.
In subgroup II with pterygoid implants, it was determined that the D4 bone has the least total stress and the D2 bone has the least displacement
Table 4 demonstrates various stresses across D4, D3, and D2 bones in the zygomatic model. The results suggested that cortical stress is more in D4 compared to D3 and D2 bones. Moreover, displacement was greater in D4 compared to D3 and D2 bones. Therefore, D2 bone with zygomatic implant prosthesis showed minimum overall stress and minimum displacement when compared to D3 and D4 bone. Table 5 demonstrates various stresses across D4, D3, and D2 bones in the pterygoid model. The results suggested that cortical stress is more in D4 compared to D3 and D2 bones. Moreover, displacement was greater in D4 compared to D3 and D2 bones. Therefore, D2 bone with pterygoid implant prosthesis showed minimum overall stress and minimum displacement when compared to D3 and D4 bone. Figures 12 and 13 show the displacement values with zygomatic and pterygoid implants.
Table 4.
Various stresses across D4, D3, and D2 bones in zygomatic model
| Zygomatic model | D4-0.5 mm | D3-0.75 mm | D2-1.5 mm |
|---|---|---|---|
| Cortical stress | 77.2404Mpa | 62.1383Mpa | 34.8352Mpa |
| Cancellous stress | 3.13602Mpa | 2.36836Mpa | 2.41233Mpa |
| Overall von Mises stress | 106.58Mpa | 99.0686Mpa | 86.1836Mpa |
| Displacement | 0.164399 mm | 0.125784 mm | 0.74073 mm |
Table 5.
Various stresses across D4, D3, and D2 bones in pterygoid model
| Pterygoid model | D4-0.5 mm | D3-0.75 mm | D2-1.5 mm |
|---|---|---|---|
| Cortical stress | 41.672Mpa | 36.5244Mpa | 22.7928Mpa |
| Cancellous stress | 3.88186Mpa | 3.72588Mpa | 3.36815Mpa |
| Overall von Mises stress | 125.775Mpa | 127.663Mpa | 131.388Mpa |
| Displacement | 0.046319 mm | 0.035242 mm | 0.030934 mm |
Fig. 12.
Displacement values with zygomatic implants
Fig. 13.
Displacement values with pterygoid implants
The highest von Mises stress was found in bone type D4 when compared to bone type D3 and D2 both in zygomatic and pterygoid implants. The minimum overall von Mises stress was seen in the D2 bone. Additionally, maximum strain distributions were detected toward the implant neck for D2, D3, and D4 bone under occlusal and masticatory loading. Figures 14 and 15 depict von Mises stress on D2, D3, and D4 bone with zygomatic and pterygoid implants, respectively. Therefore, the course and position of implant anchorage within bones, as well as the final prosthesis configuration, can have a significant impact on the overall stability of prosthetic components.
Fig. 14.
Von Mises stress on D2, D3, and D4 bone with zygomatic implants
Fig. 15.
Von Mises stress on D2, D3, and D4 bone with pterygoid implants
Conclusion
Finite element analysis is a computer-based numerical solution method that seeks solutions to various problems with an acceptable approach. It is risky and difficult to test the tension in living tissues as a result of the applied force. Therefore it has been suggested that stress analysis studies should be conducted on models. The overall stress was better distributed in the D2 bone. This shows that The success of the implant depends on the bone quality and the increase in density of bone affects the biomechanical properties in the implant-bone interface positively. The pterygoid implant showed higher stress concentration than the zygomatic implant.
Limitations of the Study
Load application is static whereas in a real-life situation during mastication, variations in applied load are present due to variations in muscle force, the shape of the bone, and complex jaw and temporomandibular joint movements.
In this study, variations in the thickness of cortical and cancellous bones were not considered while measuring stress.
All implants were assumed to be completely osseointegrated; hence, real-time stress distribution cannot be correctly assessed.
Although the anatomical structures and chewing forces were simulated optimally, because the study was performed in in vitro conditions, an exact reflection of oral conditions is limited. Despite the limitations, this study is helpful and can provide foresight to physicians before zygomatic and pterygoid applications in a biomechanical sense. Therefore, long-term clinical studies or 3D finite element analysis are necessary for the determination of the influence of observed stress levels on the functionality of the tissue and prosthesis.
Funding
Nil
Declarations
Conflict of interest
All authors declare that they have no conflict of interest.
Footnotes
Publisher's Note
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