Evaluation of treatment options for atrophic mandible under trauma forces: a 3D finite element analysis treatment options for atrophic mandible under trauma forces

Abstract

Objective

This study aimed to assess the stress distributions at different treatment options in edentulous and severely atrophic mandibula under trauma forces.

Material and method

In this study, four different treatment methods were applied to a severely atrophic edentulous mandibular model. In Model 1, inferior alveolar nerve lateralization was performed, followed by the placement of six implants. Model 2 utilized the all-on-four approach with implants placed. Model 3 used subperiosteal implants made of PEEK, while Model 4 used titanium subperiosteal implants. Each subperiosteal implant model included 14 osteosynthesis screws positioned in key areas of the mandible. 2000 Newton trauma force was applied anterior side of the mandibula. The maximum principal stress (Pmax), minimum principal stress (Pmin), and Von Mises (VMs) stress values are measured as MPa.

Results

In this study, Model 4 showed the highest Pmax in the symphysis region, while the other models had similar values. The mandibular condyle exhibited the highest Pmax in Model 1 and the lowest in Model 2. The Pmin values were comparable across all models in the symphysis region. Regarding VMs stress values, balanced stress was observed in the metal frameworks of all models. Model 3 had the most balanced and lowest VMs stress in abutments and abutment screws, while Model 4 had the highest.

Conclusion

This FEA study found that both the all-on-4 and PEEK subperiosteal implants are more resistant under trauma forces compared to titanium subperiosteal implants and the 6-implant IANL approach. The PEEK subperiosteal implant offers a less invasive treatment option for fractured mandibles.

Introduction

As age increases, muscle mass, cognitive and visual skills decline, and comorbidities or systemic diseases start to be observed in the elderly population. The physical effects and occurrence of trauma could be seen in older individuals than younger ones. It was reported that the number of fractures increased after the age of 55, especially in women [1]. The mandibula is the mobile bone that has a bilateral joint in the maxillofacial region. Due to its location in the skull, the mandibula could be affected by various trauma forces. When trauma forces are taken from the anterior side of the mandibula, the fractures can be observed mostly in symphysis and condyle neck regions. The fractured areas could be varied according to the direction, intensity, and location of the impact point of trauma forces [2]. In cases of rehabilitated atrophic mandibula, numbers, angle, location, materials of implants, and many other application approaches could be affected by stress accumulation in the mandibula [3]. Due to the choice of ideal design implant applications could be crucial at severely atrophic jaws.

The loss of teeth results in a decrease in alveolar bone volume; however, the physiological resorption pattern of jaws does not always allow intraosseous implant applications. The rehabilitation of atrophic mandibula in the elderly population could be challenging. Augmentation procedures could be demanding and have low success rates in this population [4]. To avoid augmentation, many different techniques are identified in the literature as inferior alveolar nerve lateralization, all-on-four concept, subperiosteal implants, mono-implant, short implants, etc [5,6,7]. There are various studies evaluating survival rates, success rates, and postoperative complications, such as fracture risks of intraosseous implant applications with different approaches in mandibula. However, there is insufficient data about the subperiosteal implant approach in the mandibula, especially in response to the trauma forces [4, 7].

In this study, four different treatment designs were chosen to provide more data about subperiosteal implants in the mandibula and compare them with other conventional techniques under trauma forces. These are Polyarylethyletheretherketone (PEEK) and titanium subperiosteal implants, all-on four concepts, and 6 implant applications with inferior alveolar nerve lateralization. The stress distributions were assessed during trauma in the atrophic and edentulous mandible. This study hypothesized that different treatment designs in atrophic mandibula wouldn’t change the stress distributions on mandibula.

Material and methods

This study was conducted in collaboration between Hacettepe University Faculty of Dentistry and Tinus Technologies. The arrangement of the three-dimensional mesh structure and its transformation into a mathematically appropriate solid mesh structure, the creation of three-dimensional finite element analysis models, and the finite element stress analysis were performed on HP workstations with INTEL Xeon E-2286 processors operating at 2.40 GHz and 64 GB ECC memory.

The bone model was obtained from tomography data using the 3DSlicer software. The. stl model from tomography data was also obtained using 3DSlicer. Reverse engineering and three-dimensional CAD activities were carried out using ANSYS SpaceClaim software, while the activities for preparing solid models for the analysis environment and creating an optimized mesh structure were performed using ANSYS Workbench software. The LS-DYNA solver was used to solve the finite element models created.

Modeling of cortical bone and trabecular bone

To create the mandibular bone model used in the study, a tomography scan was performed on a fully edentulous adult individual. The tomography data was reconstructed with a slice thickness of 0.1 mm. The tomography data obtained from the reconstruction was transferred to the 3DSlicer software in DICOM (.dcm) format. The CT data in DICOM format was segmented in 3DSlicer software according to appropriate Hounsfield values and converted into three-dimensional models. The models were exported in.stl format.

A 2 mm thick mucosa model was created by applying a 2 mm outward offset to the mandibular bone. A 2 mm thick cortical bone model was created by applying a 2 mm inward offset to the mandibular bone. The trabecular bone was obtained by referencing the inner surface of the three-dimensional cortical bone with the adjusted thickness. All prepared models were placed in the correct coordinates in 3D space using ANSYS SpaceClaim software, and the modeling process was completed (Fig. 1A). In the analyses, the linear material properties of the materials, defined by their elastic modulus and Poisson's ratio, were used. The material properties of the analyzed model were numerically defined (Table 1).

Fig. 1

A The atrophic mandibula model. B The force of 2000 N was applied perpendicularly to the mandibular prosthetic restoration from the labial to the lingual side at the symphysis region. The designs of (C) Model 1 (D) Model 2 (E) Model 3 (F) Model 4

Table 1 Mechanical properties

Modeling of implant, abutment, subperiosteal structure, screw, framework and prosthesis, and creation of working models

The implants, abutments, subperiosteal structure, screws to be used in the subperiosteal system, occlusal screws, basal screws, and framework to be used in the study were modeled in ANSYS SpaceClaim software.

A six-tooth superstructure model was created according to Wheeler's atlas data to match the models. All models were modeled in ANSYS SpaceClaim software. To ensure force transmission between the models, alignment of the mesh structures was performed in ANSYS Workbench software.

Models and converge test results

In this research, a mesh convergence test was carried out to verify the accuracy and reliability of the finite element model employed in the biomechanical analysis. The primary goal was to identify an optimal mesh density that strikes a balance between computational efficiency and solution precision, with an acceptable error margin below 3%.

The biomechanical finite element model was developed based on the"all-on-six, all-on-four and subperiosteal implant"concept in the mandible, incorporating anatomical bone structure, implants, abutments, prosthetics, screws, and related components. Finite element meshes with varying levels of refinement, ranging from coarse to fine, were generated for the study. Each mesh was analyzed under consistent loading and boundary conditions to ensure fair and reliable comparisons.

Von Mises stresses in the implants and principal stresses in the surrounding bone were selected as evaluation criteria, as these parameters are critical in assessing the biomechanical performance of the system.

The results from progressively refined meshes were compared to evaluate the variations in these parameters. The relative error between consecutive mesh results was determined using the following formula:

$$\mathrm{Relative}\;\mathrm{Error}\;\left(\%\right)=\left[\left[\mathrm{Value}\left(\mathrm{Refined}\;\mathrm{Mesh}\right)-\mathrm{Value}\left(\mathrm{Previous}\;\mathrm{Mesh}\right)\right]/\mathrm{Value}\left(\mathrm{Refined}\;\mathrm{Mesh}\right)\right]\times100$$

This iterative process was repeated until the relative error between two consecutive meshes fell within the defined threshold of 2–3%. The numerical results are summarized in the Tables 2 and 3.

Table 2 Convergence test results for implants

Table 3 Convergence test results for bone surrounding the implant

The validated mesh density determined through this process will be utilized in subsequent analyses to ensure reliable outcomes.

Triangular 2D and tetrahedral 3D mesh types were utilized during the model preparation process. These mesh configurations are particularly well-suited for organic structures like bone, as they are better equipped to represent intricate geometries and curved surfaces.

For all models, the mesh quality was evaluated based on criteria such as skewness exceeding 80° and a minimum edge length of 0.001. Any meshes that did not meet these standards were subsequently revised to ensure compliance.

Four different treatment methods were designed and applied to a severely atrophic edentulous mandibular model.

• ♦ In model 1, inferior alveolar nerve lateralization was performed; after that, 6 implants were applied. 4,1 × 12 mm (Straumann®, Institute Straumann AG, Basel, Switzerland) implants were applied to the lateral incisor and first premolar region. 4,1 × 10 mm implants were applied to the first molar region (1,263,808 nodes, 5,184,021 elements), (Fig. 1C).

• ♦ In model 2, all-on four approach was performed. 4,1 × 12 mm implants were applied to the lateral incisor region. 4,1 × 14 mm implants were applied to the premolar region with a 30-degree angle. (1,211,572 nodes, 4,988,566 elements), (Fig. 1D)

• ♦ In model 3, three pieces of subperiosteal implant which was made by Polyetheretherketone (PEEK) were applied (626,331 nodes, 2,503,575 elements), (Fig. 1E).

• ♦ In model 4, three pieces of subperiosteal implant, which was made of titanium, were applied (626,331 nodes, 2,503,575 elements), (Fig. 1F).

In subperiosteal implant models, a total of 14 osteosynthesis screws were applied at lingual and buccal areas. At least four osteosynthesis screws were applied for each subperiosteal implant fragment [7, 8]. The subperiosteal implant’s screw was applied in the mandibular symphysis area, the genial tubercle, the external oblique ridge, and the areas closed to the mandibular basis at the buccal and lingual side [9, 10]. Slot areas were created for the abutments by performing osteotomy on the alveolar bone crest of the mandibula [7]. In the prostheses, the metal infrastructure is modeled from cream cobalt material and the upper part is modeled from Polymethyl methacrylate (PMMA). The abutments of all implants used were 4.1 mm in diameter, based on the platform-matching concept. All subperiosteal implants had a thickness of 1.8 mm, made of 5-grade titanium. Each subperiosteal implant was stabilised using titanium screws measuring 7 mm in length and 2.3 mm in width. For all analyses, a BONDED contact definition has been applied between interacting components. This approach assumes that the components move in full correlation during motion.

Loading scenarios and boundary conditions

The models were fixed at the condyle regions on both sides of the cortical bone, preventing movement in all axes at the corresponding nodes. Forces were distributed across the surrounding nodes to avoid singularities of stress in the loading areas. Since the mandible is a structure that can move in all three axes, some muscle groups were modeled as spring elements in the study to simulate this movement more accurately from a biomechanical perspective. These muscles were identified as the masseter muscle, lateral pterygoid muscle, medial pterygoid muscle, anterior temporal muscle, and posterior temporal muscle, with spring stiffness values taken from the literature as 16.35 N/mm, 12 N/mm, 15 N/mm, 14 N/mm, and 13 N/mm, respectively. Under the specified force and boundary conditions, a total of eight linear static analyses were performed.

For each model, a loading scenario simulating trauma forces was created, and a force of 2000 N was applied perpendicularly to the mandibular prosthetic restoration from the labial to the lingual side at the symphysis region [3] (Fig. 1B). The stress values were measured in the alveolar crest region of intraosseous implants while slot regions of in subperiosteal implant models the stress distributions were assessed at contact points of subperiosteal implants and cortical bone because trauma forces mostly reabsorbed by cortical bone volume. Furthermore, in mathematical models, for accurate analysis and reliable results, it is essential to define the surface interactions between the components of the model within the analysis software [3]. Also, the stress values symphysis and condylar neck region of mandibula, subperiosteal implants and intraosseous implants’ abutments, and their abutments screws and the metal framework were assessed in megapascal (MPa) units (N/mm²). The analysis visually represents areas with high stress levels in red and low stress levels in blue, providing insights into stress distribution patterns.

Results

The maximum principal stress (Pmax) and minimum principal stress (Pmin) on the mandibula

While the highest Pmax stress accumulation in the symphysis region was observed in Model 4, other models’ values were similar. In mandibular condyle, the highest and lowest Pmax stress values were detected in Model 1 and Model 2, respectively. Total Pmax values on the alveolar crest, while the highest measurements were observed in Model 4, the other Models’ values are comparable (Table 4), (Fig. 2A-D).

Table 4 Principle minimum (Pmin) and maximum (Pmax) stress values on the bone

Fig. 2

The maximum principal stress (Pmax) values were recorded for each model as follows: (A) Model 1, (B) Model 2, (C) Model 3, and (D) Model 4

The Pmin stress accumulation in the symphysis region was comparable for all models. In the condyle neck region, the highest and lowest Pmax stress values were detected in Model 1 and Model 2, respectively. While total Pmin values on the alveolar crest the highest measurements were observed in Model 2, the lowest values were seen in Model 4 (Table 4), (Fig. 3A-D).

Fig. 3

The minimum principal stress (Pmin) values were determined for the following models: (A) Model 1, (B) Model 2, (C) Model 3, and (D) Model 4

The Von Mises (VMs) stress values of implants, abutments, abutment’s screws and metal frameworks

While the VMs stress values were higher at the lateral incisor and first premolar region in Model 2 than Model 1 at the coronal area of conventional implants, the total VMs stress values in Model 2 was comparable with Model 1 at the coronal area of conventional implants (Table 5), (Fig. 4A, B). Although the most balanced VMs stress values at metal frameworks were observed in Model 1, the total VMs stress values at metal frameworks were similar in all models. The most balanced and lowest VMs stress values of abutments were observed in Model 3, while the highest VMs stress values of abutments were detected in Model 4 (Table 5). The lowest VMs stress value was observed on abutments screws in Model 3. In Model 1, 2 and 4 the stress values were comparable. Also, their value was about 9 times that of the Model 3 (Table 5), (Fig. 5A-D).

Table 5 Von Mises stress values of implants, metal framework, abutments and abutment screws

Fig. 4

The von Mises (VMs) stress values in the coronal region of the implants were evaluated for (A) Model 1 and (B) Model 2. Sectional views along the axis of the posterior and anterior implants are shown for (C) Model 1 and (D) Model 2

Fig. 5

The von Mises (VMs) stress values observed in the abutments were evaluated for the following models: (A) Model 1, (B) Model 2, (C) Model 3, and (D) Model 4

Discussion

In some cases, the atrophic mandibular overdenture prosthesis was mostly applied in past years; however, with developing techniques, fixed denture prosthesis has been demanding in recent years from the elderly population. Although the fracture of rehabilitated atrophic mandibula with dental implants was reported at 0.2% [11], the increased application of dental implants has advanced the incidence of surgeons encountering mandibular fractures [12, 13]. Even though there are various surgical fixation techniques, advancing age, decreased vascularization and blood flow, limited bone quality and quantity, and absence of teeth complicate immobilization of the jaws [14]. Due to this, it is critical to prevent fractures before they occur in elderly patient populations.

This 3-D finite element analysis aimed to evaluate four different treatment options for atrophic mandibula in terms of biomechanical responses at the bone and the implant structures under trauma forces. In finite element analysis (FEA), evaluating the distribution of forces on jaws and dental implants is challenging because of the bones'heterogeneous structure and the difficulty in accurately simulating the effects of muscles and soft tissues on the bones [15]. However, with advancements in FEA software, the effects of dental implants and the human jaw's soft tissues and muscles have been accurately simulated in this study. Recent studies have compared FEA analyses of mandibular fractures with actual clinical cases and found FEA to be a precise, non-invasive, and reproducible method for examining the biomechanical behavior of human mandibles under mechanical loads. Consequently, from an ethical standpoint, FEA reduces the need for animal and cadaveric studies [16, 17].

An atrophic mandibular augmentation procedure could be challenging for gaining fixed prosthetic rehabilitation. Various complications could occur between 25.9% and 1.7% incidence in these procedures [4]. To prevent extensive augmentation surgeries, inferior alveolar nerve lateralization (IANL) is one of the alternative treatment options in the literature. Even though there is a higher incidence of long-lasting nerve damage in IANL, higher implant success rates were reported than in the augmented bone region [18]. Furthermore, the all-on 4 concept developed by Malo is one of the most popular options for edentulous atrophic mandibular rehabilitation due to preventing augmentation procedures [19]. Because of these, IANL and the all-on four concepts were preferred for treatment options when applying intraosseous implants. According to our study results, although the total Pmin stress values on the alveolar bone were higher in all-on 4 (Model 2) than the IANL approach (Model 1), the Pmax stress values on the condyle neck region were lower in Model 2 than in Model 1. During anterior trauma forces, the most affected side was the condyle neck region in the mandibula [2, 20]. Reducing loading in the condylar region under trauma forces could be crucial.

Krennmair et al. reported that under trauma forces four implant applications to the atrophic mandibula, the stress values on the condyle region were lower than those of two implant applications. The authors indicated that since the force load from a frontal trauma to an implant-treated mandible is absorbed by the bone regions adjacent to the implants, increasing the number of implants from two to four will lead to a significant reduction in stress on the condylar neck, thereby lowering the risk of fractures in that area [21]. In contrast, an increasing number of implants raised stress accumulation on the condylar neck region in the current study (Fig. 2A, B). This result could be associated angle and the implant length at the premolar side in Model 2. Because of the angle, the bone could resist the trauma force not only in the vertical dimension but also in the horizontal dimension. Helping this biomechanical behavior, the trauma force could be transferred to the surrendering bone of tilted implants. Another cause of these results could be associated with the difference between spongious and cortical bone biomechanical behavior. Ayali et al. [3] stated that while spongious bone distributes the load, the cortical bone absorbs. In Model 1, there is more spongious bone contact than in Model 2. Therefore, the total von Mises stress values which were around the implants neck region were higher in Model 2 than Model 1 and the trauma force could be transferred to the condylar neck region in Model 1 than in Model 2. Also, Rubo et al. reported that von Mises stresses in the peri-implant bone increased by 30–37% as the cantilever length increased [22]. In this study, in Model 2, the prosthesis has a cantilever length, so the trauma force may have been interrupted by this cantilever length. In future studies, the biomechanical advantages and disadvantages of different angled inclined implant designs in both lateral incisors and premolar region and different cantilever lengths can be evaluated under traumatic forces.

In past years, subperiosteal implant applications were performed in severely atrophic jaws; however, because of 2 two-stage surgery processes, unfitting design of implants, and postoperative complications, the applications were decreased [10]. In the last decade, enhancing technology, software, and tomography images additively manufactured subperiosteal implants were developed. Although there are various subperiosteal implant designs in the maxilla, there are insufficient designs in the mandibula in literature [23]. In cases of intraosseous implant applications couldn’t be applied in the mandibula. The subperiosteal implants could be a reliable option for treatment. In this study, a 3-piece subperiosteal implant design was performed, which was made of titanium and PEEK materials. According to our study results, although the stress values on the condylar neck and symphysis region were comparable in Model 3 and 4, the total stress values on the alveolar crest area were higher in Model 4 than in Model 3. The VMs stress values in abutments and abutments screws were also higher in Model 4 than in Model 3. These results could be associated with different elastic modules of materials because PEEK materials has a lower elastic modulus (3.8 GPa), which makes it more similar to bone. That is, PEEK distributes the incoming load more homogeneously and provides a load transfer closer to the bone. PEEK can absorb more load than titanium because it is more flexible. This is particularly useful when osseointegration with bone is important. By absorbing part of the load, it does not overload the bone. Titanium, on the other hand, offers a high elastic modulus (110 GPa) and high strength. These properties cause titanium to transmit the incoming load directly to the bone [24, 25]. Furthermore, FEA studies, which are investigating intraosseous implant application with PEEK and titanium materials, identified that PEEK implants transmitted trauma stresses less than titanium implants to the bone and act as bone [3, 26]. Additionally, PEEK implant materials exhibit excellent cell adhesion, proliferation, biocompatibility, and osteogenic properties. PEEK also positively impacts the biofilm structure, reducing the likelihood of peri-implant inflammation [24]. According to all of these advantages, PEEK subperiosteal implants could be a better option than titanium subperiosteal implants.

In this study, slot areas were created for abutment parts of subperiosteal implants in the alveolar mandibular crest. Because in literature it was stated that the abutments should always be positioned within specially created slots in the alveolar crest, ensuring they rest on the basal bone, which is less prone to resorption over time [7]. This approach could provide more resistance to the trauma forces. In the literature, there is no ideal screw number for subperiosteal implants. In this presented study, 4 or 5 osteosynthesis screws were applied for every piece of subperiosteal implants because at least four osteosynthesis screws are recommended to ensure rigid fixation in favorable mandibular fractures [7, 8]. The subperiosteal implant’s screw locations were chosen in the areas of greatest resistance of the jaw, following the physiological distribution lines of the masticatory and muscular forces specified by Champy [9, 27]. These areas are mandibular symphysis, where the genioglossus and geniohyoid muscles attach to form the genial tubercle, where the buccinator muscle attaches to form the external oblique ridge, and nearest areas of mandibular basis at the buccal and lingual side which is identified as an optimal site for screw application due to its resistance to resorption over time [9, 10]. In literature studies also recommended internal oblique ridge and mandibular angle for supporting subperiosteal implants rigid fixation [28].

In this study, subperiosteal and intraosseous implants were compared. Our findings indicate that the all-on-four (Model 2) and PEEK (Model 3) subperiosteal implant groups exhibited greater advantages when compared to the other two groups. When we compare these groups, although the stress values were similar in many regions, Model 2 showed slightly less stress accumulation than Model 3 in the condylar neck. However, in trauma forces, fracture formation is sometimes inevitable, and the fracture can be occurring different locations in the mandibula, not just the condylar neck [20]. In cases of mandibular fracture, subperiosteal implant designs could be more beneficial than intraosseous implants because screw and wing locations of subperiosteal implants were chosen in resistant points of jaws and physiological distribution lines of the masticatory and muscular forces [7, 9]. With this approach, more optimal healing of the fracture site can be achieved by using non-invasive techniques instead of invasive techniques during the treatment of mandibular fractures. Furthermore, for enhancing PEEK subperiosteal approach in further studies the subperiosteal implants made by carbon fiber reinforced PEEK material and all-on-4 approach could be compared. According to our study results, the hypothesis that different treatment designs in atrophic mandibula wouldn’t change the stress distributions on mandibula was rejected.

Conclusion

In this FEA, all-on-four and PEEK subperiosteal implant applications were observed to be more beneficial than titanium subperiosteal and six implants with an IANL approach under trauma forces. Also, in cases of fractured mandibula, the PEEK subperiosteal implant approach can provide a less invasive treatment model for patients. Nevertheless, in-vivo, in-vitro, and clinical studies on the subject are still needed.