Stress analysis of a single prosthesis on a poorly positioned implant Análise de tensões em prótese unitária sobre implante mal posicionado Análisis de tensión en prótesis unitaria sobre implantación molly posicionada

The mechanical positioning behavior of a mal-positioned implant was evaluated in the finite element method. Models were created in SolidWorks Professional 2013 software with a single implant rehabilitation. The following were analyzed: Control Group, crown aligned to the implant long axis; Experimental Group, crown shifted 3 mm mesial proximally to the implant axis. Compressive stresses in cortical and medullary bone, and Von Mises stresses in implants and components were evaluated. With 100 N occlusal loading at 5 points. The peak von Mises stresses in the prosthetic screw of the experimental group (138.45 MPa) were 43.60% higher compared to the control group (96.41 MPa). The stresses in the prosthetic pillar were localized in the abutment region and the experimental group showed (875.63 MPa), 28% higher than the control group (683.88 MPa). Regarding the implant, the maximum stress peaks were located in the implant platform and the experimental group showed stress values of (1081.4) MPa and was 26.42% higher than the control 855.39 MPa. The cortical bone tissue of the experimental group showed shear stress values 10.81% higher than the control. The stress values were 151.36 MPa for the experimental group and (136.59 Mpa) for the control. The medullary bone showed shear stress (8.31 MPa) and was 12.29% higher than the control (7.40 MPa). A maximum peak was obtained in the cervical region of the medullary bone, adjacent to the cortical bone. The experimental group with a mal-positioned implant showed the highest maximum stresses for all simulated prosthetic components.


Introduction
In current dentistry, several rehabilitation possibilities such as fixed, removable prostheses or dental implants have proven to be effective in the face of loss of mastigatory function by edentulous patients. According to the Quirynen et al. (2015), Sargolzaie et al. (2017), Filius et al. (2018) when comparing an implant-retained prosthesis with removable prostheses, there is considerably increased patient satisfaction and quality of life with consequent psychological and social well-being, masticatory function, speech, nutrition, and stagnation in the resorption of the alveolar process.
Therefore, according to Torrecillas-martinez et al. (2014), Max et al. (2015) and Ozan & Kurtulmus-Yilmaz (2018) some conditions such as pneumatization of the maxillary sinuses or bone atrophies are impediments to a proper positioning of the implants, and in order to confer ideal interlocking and proper implant installation, many forms of regeneration, such as intraoral and extraoral grafts, have been proposed, bringing with them complications such as bleeding, infection, and paresthesia.
Thus, other options have been alternatives, such as short, inclined or cantilevered implants, which are defined as a beam in which any similar structure protrudes outward, supported only at one end.
However, according to Quirynen et al. (2015) and Drago (2016), cantilevers act as a lever arm causing greater structural bending and stress on implants, prosthetic components and perimplant bone, having the capacity for bone resorption as well as, prosthetic complications. Pjetursson et al. (2007), Max et al. (2015) and Premnath et al. (2020) concluded that cantilevers in implant prostheses although not related to loss of osseointegration have a negative influence on treatment prognosis, with an increase in technical complications, such as screw or component loosening, fracture of the covering material, bone resorption and perimplantitis.
According to Zurdo et al. (2009), Schmid et al. (2020, Papaspyridakos et al. (2020) and (Huang & Wang, 2019), although prosthetics on implants are clinically and scientifically consolidated, some challenges indicate the need for analysis and studies, since numerous technical and biological complications are common, such as screw loosening, porcelain cover fracture and bone loss. Aiming to understand these occurrences, this study evaluated, by means of three-dimensional finite element analysis (FEM), the biomechanical behavior of abutments, screws and bone in the face of a malpositioned implant in a single implant in the posterior region of the mandible, using as null hypothesis the non-increase of stresses in the evaluated elements.

Methodology
Two geometric models were generated using SolidWorks 2013 ® software (Solidworks Dassault Systems, Watham, Massachusetts, USA) for three-dimensional modeling, being a control group (in which the crown is positioned aligned with the longitudinal axis of the implant) and an experimental group (crown 3 mm off center, mesial to the longitudinal axis of the implant (Figure 1.a).
Based on anatomical references and measurements available in the literature (Table 1), a single rehabilitation in the posterior region of the mandible was simulated and to build the bone models, anatomical contours referring to a section of the posterior region of a mandible were used (Figure 1.b).
A 3.8 x 10m Infra System cone morse platform implant (Signovinces, Campo Largo, Parana, Brazil) was positioned in the region corresponding to the lower first molar, 1mm below the alveolar bone crest. A concentrically aligned conical abutment and, likewise, a prosthetic screw were positioned over it. To create the prosthetic crown, anatomical measurements were used to model a dental element corresponding to the lower first molar. To achieve a perfect fit between the crown and abutment base, the "entity loft" tool was used, which uses the abutment base as a reference for seating the crown. The parts were matched to each other using the subtraction matching feature, which allows the internal adjustment of the parts without generating interferences that could prejudice the numerical interpretation of the analysis. The experimental group was obtained by controlled mesial displacement of the prosthetic crown followed by adaptation to the abutment base (Figure 1.c).
The assembly was reconstructed in the assembly environment of SolidWorks software (Solidworks Dassault Systems, Watham, Massachusetts, USA) and exported for the analysis (Figure 1.d). The model was imported by AnsysWorkbench software (ANSYS Inc, Canonsburg, Pennsylvania, USA) into the static structural analysis environment. It was necessary to discretize the model which was performed by using mechanical properties such as modulus of elasticity and Poison's coefficient according to previously published works (Camargo et al., 2018), (Zheng et al., 2012), Dittmer et al. (2010. Previously to the analysis, it is necessary to divide the model into smaller elements by means of a mesh. These elements are interconnected to other elements by means of nodes. The present study used the mesh quality criterion, where consecutive refinements were performed in order to obtain a greater number of elements with a score equal to 1.0. The control group generated 962.480 nodes and 669.332 elements, whereas, the experimental group generated 955.030 nodes and 662.057 elements. The bone model was fixed on the lateral faces and base to simulate the union to the rest of the mandible. In addition, the purpose of the fixation is to prevent displacement of the model during load application. A load of 100 N was applied to the occlusal surface of the crown divided into 5 points. The loads were tilted 10 degrees relative to the long axis of the implant in order to simulate contact followed by sliding (Figure 2).
The data obtained were evaluated according to maximum shear stress criteria for cortical and medullary bone tissue, tensile stress for crown, and von Mises stress for implant, abutment, and prosthetic screw. The pattern of force distribution was compared qualitatively between the models, where the maximum peak (highest stress value) is represented by red color.

Material
Modulus of Elasticity   Source: Own authorship.

Results
After verification of the data through quantitative and qualitative analysis, the results were divided for individual evaluation of the stress generated in each region: bone tissues, implants, abutments, abutment screws, and prosthesis. The data obtained regarding maximum stress are shown in Table 2. Source: Own authorship.
The peaks of von Mises stress of the prosthetic screw, prosthetic abutment, and implant platform, and the corresponding qualitative test stress values presented in Figure 3.

Discussion
Planning for implant installation is essential, however, often due to anatomical limitations such as pronounced resorption of the maxillae and consequent approximation of important structures such as the maxillary sinus, nasal fossa, mentual foramen and even the inferior alveolar nerve, This procedure aims to simplify planning, thus avoiding extensive surgeries, minimizing costs and morbidity (Alom et al., 2021;da Silva et al., 2018;Manea et al., 2019;Muangsisied et al., 2021).
The null hypothesis of absence of increased stresses in the evaluated elements was refuted in view of the results achieved in the present study, that in an extension of 3 mm, stresses increased due to the incorrect mesio-distal positioning of the implant.
There was an increase in the peak von Mises stress of 40.6% in the prosthetic screw and 28% in the abutment (Figures 3.a and   3.b). These results converge with studies showing the screw and prosthetic abutment regions as the most affected with complications such as loosening and fracture (Ramos et al., 2014;Pjetursson et al., 2007;Drago, 2016;Rodrigues et al., 2019).
Although Torrecillas-martinez et al. (2014) and Camargo et al. (2018) argued that despite minute technical intercurrences, the presence of extension did not affect the success rate of the prostheses.
The von Mises stress of the prosthetic abutment was localized in the transmucosal region, close to the implant platform.
Uniform distribution in the rest of the abutment body (Figure 3.c). The experimental group showed 875.63 MPa and was 28% greater than the control group (683.88 MPa), a result similar to that of Alom et al. (2021), who, when analyzing a buccal cantilever in a single prosthesis, found greater stress in the abutment of the group tested.
In the present study, an increase in shear stress in the cortical bone of 10.81% was obtained in the experimental group, in the cervical region of the implant near the first threads, and in the medullary bone of 12.29%, which may suggest an increased risk of osseointegration loss in poorly positioned implants (Figure 3.d, 3.e 3.f). Schmid et al. (2020) and Zurdo et al. (2009) reported that bone loss tends to increase in cantilever-associated implants, however Hasan et al. (2014) stated that although there is maximization of stresses in cancellous bone under functional loading, greater importance should be given to the bone thickness in which the implant will be installed.
In this study, the peak stress of the prosthetic abutment was located in the transmucosal region, close to the implant platform. The experimental group presented with incorrect implant positioning, 28% higher than the control group. In a similar study, Tang et al. (2012) applied a 170N force with 45° angulation and obtained increased stresses, mainly in the gingival profile region of the abutment and in the first threads of the implant, although they concluded that it is unlikely that this magnification results in failures that make the rehabilitation unfeasible. Regarding the abutment, Suedam et al. (2016) suggest minimizing the abutment length in order to reduce stresses, decreasing the lever arm, in addition to using stiffer alloys for making the prosthesis with the purpose of favoring a better distribution of stresses through the implant and components. Desai et al. (2015), when performing an FEM study to evaluate the best thread configuration in several implants found in the market, stated that the key factor for the collapse or success of an implant is the way in which stresses are transferred to the perimplant bone, and trapezoidal threads obtained the best biomechanical performance. However, Mosavar et al. (2015), using a similar methodology, concluded that the various thread configurations did not affect stress distribution patterns in the bone, and that this was not the predominant factor. In the implant used in this study, the threads are trapezoidal, with the cortical bone tissue of the experimental group exhibiting 10.81% higher shear stress when compared to the control group. The maximum stress was located in the first threads of the implant in both the medullary and cortical bone (Figure 3.d; 3.e and 3.f). Aglietta et al. (2012) understand that there is no fear in the use of cantilevers, in which they corroborate Premnath et al.
(2020), Camargo et al. (2018) and (Greenstein & Cavallaro JR, 2010) stating that the cantilever is another alternative in planning for rehabilitation in the face of conditions in which the absence of bone and anatomical structures prevent the installation of an implant for each crown. Halg et al. (2008) concluded that there is no clinical correlation between the use of cantilever extensions and bone loss or failure of the implants supporting them, and that this increase in stress is tolerated by the system and dissipated Research, Society andDevelopment, v. 10, n. 11. e579101119815, 2021 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v10i11.19815 by the numerous materials it is composed of: titanium, abutments, screws, infrastructure (CrCo) or cover materials (ceramic or resin). Although the results found do not correlate perimplant bone loss with cantilever, the present study, with increased stresses in all elements analyzed, suggest risk in rehabilitations with cantilever mainly regarding prosthetic complications such as screw loosening, abutment loosening and material fracture probably accompanied by long-term clinical compromise. (da Silva et al., 2018) (Storelli et al., 2018) This study was exclusively laboratory, thus clinical studies are needed to elucidate the gap of how the influence of the cantilever due to improper positioning of the single implant, acts on the long-term rehabilitation.

Conclusion
In the present study it can be stated that in the experimental model with the implant in the wrong position, the influence of the cantilever showed the highest stresses for all components involved in the prosthetic rehabilitation, simulated in comparison to the control group. Complementary studies addressing the clinical effect of this increase in tensions may be of great importance in rehabilitation.