1http://dx.doi.org/10.20396/bjos.v19i0.8659191 Volume 19 2020 e209191 Original Article 1 Department of Biosciences, Piracicaba Dental School, University of Campinas, São Paulo, Brazil. 2 Department of Prosthodontics and Periodontology, Piracicaba Dental School, University of Campinas, São Paulo, Brazil. 3 Department of Structural and Functional Biology, Institute of Biology, University of Campinas, São Paulo, Brazil Corresponding author: Paulo Henrique Ferreira Caria Department of Structural and Functional Biology, Institute of Biology, University of Campinas, Monteiro Lobato St, 255 – Zip code: 13083-862 Campinas, São Paulo, Brazil. Phone/fax;+55193521-6184 E-mail address: phcaria@unicamp.br Received: April 16, 2020 Accepted: August 31, 2020 How many implants are needed for mandibular full-arch rehabilitation? Karina Giovanetti1 , Ricardo Armini Caldas2 , Paulo Henrique Ferreira Caria3,* Aim: To analyze the stress distribution at the peri-implant bone tissue of mandible in full-arch implant-supported rehabilitation using a different number of implants as support. Methods: Three-dimensional finite element models of full-arch prosthesis with 3, 4 and 5 implants and those respective mandibular bone, screws and structure were built. ANSYS Workbench software was used to analyze the maximum and minimum principal stresses (quantitative analysis) and modified von Mises stress (qualitative analysis) in peri-implant bone tissue after vertical and oblique forces (100N) applied to the structure at the cantilever site (region of the first molars). Results: The peak of tensile stress values were at the bone tissue around to the distal implant in all models. The model with 3 implants presented the maximum principal stress, in the surrounding bone tissue, higher (~14%) than the other models. The difference of maximum principal stress for model with 4 and 5 implants was not relevant (~1%). The first medial implant of the model with 5 implants presented the lower (17%) stress values in bone than model with 3 implants. It was also not different from model with 4 implants. Conclusion: Three regular implants might present a slight higher chance of failure than rehabilitations with four or five implants. The use of four implants showed to be an adequate alternative to the use of classical five implants. Keywords: Dental implants. Finite element analysis. Mouth rehabilitation. Prosthodontics. http://orcid.org/0000-0001-6279-7121 https://orcid.org/0000-0002-5362-4744 https://orcid.org/0000-0001-8829-6704 2 Giovanetti et al. Introduction The implant-supported prostheses are a successful form of treatment, presenting a high survival rate and favorable biomechanical conditions1. Among the possibilities of treatment for edentulous patients, the full-arch fixed prosthesis presents better stabil- ity and masticatory efficiency when compared to complete denture or overdentures2. The Brånemark Novum concept (oral implant protocol) indicates the use of three wide implants to support a full-arch fixed prosthesis in the edentulous mandible3. From this, technical variations were developed by changing the size, position, and num- bers of implants4. The success of new protocols depends on several factors, such as implant inclination, bone quality, bone quantity, and distribution of masticatory loads to the prosthetic system (prosthesis, framework, and prosthetic components)5. As an alternative to improve the biomechanical behavior, it has been suggested to increase the implant diameter and tilting the distal implants to reduce the cantilever length6. Also, the stress distribution in the periimplant bone is directly related to occlusion, masticatory force, number, and position of the implants7. Short and medium-term clinical reports have demonstrated the successful use of four implants, even inclined or parallel8. Thus, simplified protocols become cheaper and provide less morbidity to patients. Full-arch rehabilitation in the edentulous mandible supported by 3, 4 or 5 regular implants has been described by several studies showing high success rates9,10. However, long-term studies are necessary to evaluate the biological complica- tions, survival rates, implant failures, and technical complications of these rehabilitations9. To understand the bone behavior in rehabilitations with dental implants, several studies with different methods have been performed. The use of finite element method (FEM) allows investigating the biomechanical behavior on specific three-dimensional models, making it possible to predict and quantify the stresses induced throughout the biolog- ical system11. Therefore, this study aimed to evaluate the stresses transmitted to the peri-implant bone tissue in 3D finite elements models with three, four, or five dental implants built for the oral rehabilitation of the mandible full-arch fixed prosthesis. Materials and methods Three-dimensional models of full-arch prosthesis were constructed, varying from 3 to 5 implants (Figure 1). C B A D C B A E D C B A Model 1 Model 2 Model 3 Figure 1. Three-dimensional computer models of three full-arch prosthesis evaluated, varying from 3 to 5 implants. 3 Giovanetti et al. The groups: Model 1: total of 3 implants, two positioned 5 mm mesial to each mental foramen (15 degrees angulated to distal) and the third implant vertically at midline; Model 2: total of 4 implants, two positioned 5 mm mesial to each mental foramen (15 degrees angulated to distal) and two implants vertically, 9 mm from midline to each side; Model 3: total of 5 implants, two positioned 5 mm mesial to each mental foramen (15 degrees angulated to distal), two implants vertically 9 mm from midline and one implant vertically at midline. In all models, the framework was 4 mm distant from the alveolar process, with a cross section of 4.3 mm width x 3.6 mm height, and 8 mm in each side cantilever. The bone geometry was con- structed based on an edentulous mandible of an approximately 60-year-old man from a Cone Bean computerized tomography (images with 0.25 mm range) Mimics 17.0 (Materialise, Leuven, Belgium). The desired bone density was selected to cre- ate the image mask and then regularized the remaining alveolar ridge of the mandi- ble. Furthermore, the model was simplified by removing mandibular ramus, with no influence on the results. Also, models of external hexagonal cylindrical implants with dimensions of Ø4.1 mm x 11.5 mm in length Nobel Biocare (Yorba Linda, CA, USA), screws, prosthesis frame- work and complete assemblies were made in the software SolidWorks® (Dassaul Sys- teme, Waltham, MA, USA). The 3D models are available at the Supplementary data (file format: .OBJ). The 3D models were imported to software ANSYS Workbench® 14, (Canonsburg, PA, USA). All the materials were set to homogeneous, isotropic and linear elastic. The material properties are shown in Table 1, as previous stud- ies12. In addition, the contact between implants and framework was set to frictional (µ = 0.3)13 and all other contacts between different materials were set to be bonded. Then, the meshes were set as 10-node tetrahedrons and refined to a point where it does not considerably affect the obtained results. The mesh was checked for element quality and refined in the regions of interest, resulting in about 400,000 elements and 600,000 nodes per model. The posterior sur- face of mandible was set to fixed (zero degrees of freedom). The mechanical loading was performed with vertical or oblique (45° to vestibular) forces of 100 N applied at the framework’s cantilever in different analysis, simulating bite forces14. The data of the maximum and minimum principal stresses (quantitative analysis) and modified von Mises stress (qualitative analysis) were realized by the ANSYS Wor- bench® 14 software in the 3D finite element models15. Results At surrounding bone tissue of implant A, the model 1 presented the maximum princi- pal stress ~14% higher than model 2 and 3 (Table 1). The maximum principal stress difference for model 3 to 2 was not relevant (~1%). At implant B, the model 3 present the lower stress values in bone, being 17% lower than model 1, and also not relevant different from model 2 (Table 1). 4 Giovanetti et al. Table 1. Comparison of vertical loading (maximum and minimum principal stress) to implant A and B, on the three full-arch prosthesis evaluated. 20 10 0 MPa -10 -20 -30 -40 Model 1 Model 2 Model 3 Max principal Min principal Implant A Max principal Min principal Implant B The compressive stress values (minimum principal stresses) at vertical loading showed inverse relationship to number of implants. However, these differences of stress values were not relevant (≤1%) (Table 1). For oblique loading, model 1 presented minimum principal stress 10% higher than model 2 and 3 at implant A (Table 2). Table 2. Comparison of oblique loading (maximum and minimum principal stress) to implant A and B, on the three full-arch prosthesis evaluated. 20 10 0 MPa -10 -20 -30 -40 Model 1 Model 2 Model 3 Max principal Min principal Implant A Max principal Min principal Implant B No relevant differences were observed for maximum principal stress at implant A. The stress field presented by modified von Mises indicates a similar behavior around implant A for all models (Figure 2). 5 Giovanetti et al. Vertical Oblique Y Z X 15.6 13.9 12.1 10.4 8.7 6.9 5.2 3.5 1.7 0.0 MPaModel 1 Model 2 Model 3 Figure 2. The stress field by vertical and oblique loading, represented by von Mises analysis, around implant A, for the three models of full-arch prosthesis evaluated. The highest values were at distal side of implant A for vertical loading and directed to buccal side for oblique loading. As the higher stress values concentrated at implant A and B, the implants C, D and E were not included in the comparison. For a better understanding of stress location at bone tissue (tensile and compressive), the figure 3 indicates the movements of the cantilever after the mechanical loading. The vectors indicate the movement direction and magnitude. Vertical Oblique Y Y Z Z XX Deformation Max Min Figure 3. The deformation of the cantilever during the vertical and oblique loading. The vectors indicate the movement direction and the respective magnitude. 6 Giovanetti et al. Discussion This study evaluated the peri-implant bone stresses with a different number of implants supporting a full-arch oral rehabilitation. The results showed different stresses val- ues for the studied cases. The decreasing in the number of implants used for full- arch rehabilitation has been subject to laboratory and clinical studies9,16,17, aiming to minimize treatment costs and patient morbidity8. Literature data have shown high success rates for total rehabilitations with 3, 4 and 5 dental implants8-10,17. In an obser- vational study with 33 patients with an 18-month follow-up of a complete fixed man- dibular prosthesis supported by three implants, an adequate option was developed with peri-implant bone loss that has been described for prostheses of the same type supported by larger numbers of implants9. In a systematic review, it was observed survival rate for more than 24 months of 99.8% in full arch 4 mandibular implants10. However, there is a lack of sufficient long-term data with follow-ups of at least 5 years to evaluate the biological complications, survival rates, implant failures, and technical complications of these rehabilitations with three implants9,18. Also, some studies con- cerns about the influence of reducing implants’ number in the survival rate of pros- thetic components9,19. The present study did not evaluated the influence of reduced number of implants at prosthetic components and its long-term consequence. Nev- ertheless, it is reported that the use of a lower number of implants could overload the prosthetic components, resulting in higher rates of screw loosening and leading to a larger number of follow-up appointments17. According to the literature17, the tensile and compressive stresses generated during function were not sufficient to immediately damage the bone, presenting stress val- ues lower than ultimate compressive (167 MPa)20 and tensile (100 MPa)21 strengths. However, these stresses can be harmful at the long-term analysis9,19. In a full-arch rehabilitation, the greatest loads are received by the distal implants, regardless of the total numbers of implants22. In the present study, the stresses generated in the peri-implant bone (implant A) are in agreement with Silva-Neto et al.17; they reported that reducing the number from 5 to 3 implants relevantly increased the stress in the peri-implant bone. In our study, during axial forces the rehabilitation with 3 implants showed higher ten- sile stresses (~14% higher) at peri-implant bone tissue than other analyzed models. The compressive stresses followed the same pattern, but with no relevant difference at stress values among models (≤1%). Also, for oblique loading, the compressive stress was 10% higher at the model with three implants17. The stresses at implant B presented influence of the implant number, with the reduction of number of implants increasing the bone stress. The rehabilitation with 5 implants showed a tensile stress 17% lower than the 3-implant rehabilitation. Our data did not present relevant differences between the use of four or five implants. Even with differences among models, all results were lower than critical for bone frac- ture, and lower than the stress in implant A. As the risk of implant failure is directly related to peri-implant stresses, the main concern relies at implant A. As one of the alternatives to number of implants, literature suggests that increasing in stresses provided by three regular implants could be avoided placing wider implants3,17. 7 Giovanetti et al. The results of this study suggest that it is not necessary to use of the traditional 5 implants to support a mandibular full-arch rehabilitation9,17. Other studies have demonstrated satisfactory success rates with the use of 4 implants for full-arch fixed mandibular prosthesis10,23. A longitudinal study of the survival of all-on-4 implants in the mandible with up to 10 years of follow-up showed prostheses’ survival rate was 99.2 percent and implant-related success rates 93.8 percent23. Although studies have demonstrated satisfactory survival rates of full-arch prosthesis fixed by 3 implants9, the use of 4 implants is in general safer, since in case of loss of some implant other than the distal one, there will still be a favorable biomechanical condition, not requiring a new surgical procedure. Moreover, if the implant is lost after function, it is possible to use an existing prosthesis8,10. The use of osseointegrated implants in Dentistry provides stability and comfort to den- ture wearers, especially in the mandibular arch8. Implants are used as pillars and when exposed to excessive functional loading can transmit harmful stresses directly to the periimplant bone, which may cause failure in osseointegration8. Around two thousand implant options are available for any clinical situation. Studies about implants use have to be done to orientate the clinic procedures form dentists. In vitro tests can be divided into surface analysis and mechanical assessment. Differ- ent methodologies can present results that dentists may find difficult to understand their clinical application. The in vitro testing has limitations, however, current evidence presents new analysis as scanning electron microscopy useful to inform about the implant surface topography. Atomic force microscopy, single-cell tests, 3D imaging, and gene expression tests also could be used to the assessment of cellular and phys- io-biochemical properties of the implants24. As well as 3D finite element analysis has been used in the evaluation of mechanical properties of dental implants, as on the distribution of stresses in bone regions and prosthetic components25. The FEM allows to evaluate similarly to what happens in vivo by specific three-dimensional models, enabling prediction and quantification of stresses induced by the entire system11. The FEM is a computational analysis with restrictions because it cannot reproduce some variables from the oral environment. The results obtained with that analysis can be useful to understand the behavior of prosthesis supported by implants, which could not be done in vivo. In a study similar to ours26, the authors concluded that the greater amount of implants supporting a complete arch prosthesis, promotes less stress concen- tration during simulated loading, and that decreasing the number of implants in a rehabilitation is harmful. Further experimental studies and clinical trials should be performed to verify the effects of such arrangements on the longevity of this type of rehabilitation. The present results suggest that full-arch rehabilitations in the edentulous mandi- ble with 3 regular implants present a slightly higher chance to failure than rehabili- tations with 4 or 5 implants because of the higher stress concentration presented in some analysis. The use of 4 implants presented promising results suggesting to be adequate substitute to the classical technique that uses 5 implants. 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