Title: Computational Biomechanical Analysis of Engaging and Non-Engaging Abutments for Implant Screw-Retained Fixed Dental Prostheses Running title: Numerical Analysis of Engaging and Non-Engaging Abutments Authors: Roberto Savignano, MSc, PhD;1 Pooya Soltanzadeh, DDS, MS;2 Montry S. Suprono, DDS, MSD1 1 Center for Dental Research, Loma Linda University School of Dentistry, Loma Linda, CA 2 Division for General Dentistry, Loma Linda University School of Dentistry, Loma Linda, CA Corresponding author: Roberto Savignano Center for Dental Research, Loma Linda University School of Dentistry, 11092 Anderson St. Loma Linda, CA 92350 e-mail: rsavignano@llu.edu Disclosure: Preliminary results were presented as a poster at the 2020 IADR/AADR/CADR General Session 98th General Session with the title: 3D-FEA of Abutment Type Combinations for Implant Screw-retained Fixed-partial Dentures Conflict of Interest: The authors declare that there is no conflict of interest. Accepted date: November 12, 2020 This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/jopr.13282. This article is protected by copyright. All rights reserved. ABSTRACT Purpose: To evaluate the stress distribution, using 3-Dimensional Finite Element Analysis (FEA), on different implant components of a mandibular screw-retained fixed dental prosthesis (FDP) situation when using different combinations of engaging and non-engaging abutments. Material and Methods: A model of artificial bone was digitally designed. Dental implants were positioned in the lower right posterior area of teeth #’s 28 (premolar – pm) and 30 (molar – m). Restorative implant components were digitally designed and placed into the implant model. Four different implant abutment situations were simulated through FEA: 1) Both engaging abutments (pmE-mE), 2) both non engaging (pmNE-mNE), 3) premolar engaging and molar non-engaging (pmEmNE), and 4) premolar non-engaging and molar engaging (pmNE-mE). Thirty-five (35) Ncm preload to the abutment screws and 160 N static load at 45-degree angle to the occlusal plane were applied in each group. Results: The equivalent Von Mises stress was measured on each component. Stress distribution changed among the different configurations and ranged from 516.0 MPa to 1304.6 MPa in the implants, and from 554.6 MPa to 994.5 MPa with the abutments. Higher stress was found for the pmNE-mNE designs (1078.6-1106.9 MPa). Engaging and non-engaging abutments had different stress distributions on the screw (698.8-902.5 MPa). Peak stress areas were located on the upper part of the screws for the non-engaging configuration, and on the lower areas for the engaging abutments. The sum of the stress on both implants decreased in the following order: pmNE-mNE > pmNE-mE > pmE-mNE > pmE-mE. Conclusion: Under conditions of the present study, abutment design produced different stress patterns to the implant components. The lowest and most balanced stress distribution was found for the pmE-mE configuration followed by the pmE-mNE configuration. KEYWORDS: Finite Element Analysis; Dental Implants; Stress; Abutments This article is protected by copyright. All rights reserved. 2 Fixed implant-supported dental prostheses (FDPs) have become a routine treatment for restoring partially or fully edentulous patients. A stable implant-abutment interface (IAI) is vital to long term clinical success and depends on the stress and displacement of this system.1 Mechanical complications such as screw loosening and fracturing, and abutment fractures occurs when high stress is applied onto the prosthetic restoration and implant components.2,3,4-7 The patterns of stress in the implant-prosthesis-bone complex may be influenced by the macro-geometric shape of the implants (external versus internal indexed), abutment types (engaging versus non-engaging), implant component materials, position of the dental implants, masticatory forces, and fit of the prosthesis.8,9 Although screw-retained FDPs have several advantages over cement-retained, achieving passive fit can be more complex and difficult when restoring multiple, nonparallel, internally indexed implants.17 Retention failures have been reported and has been largely due to misfit of the prosthesis and inadequate torque placed on the prosthetic screws.10,2,3,11,12 Nevertheless, flexible options of using engaging and non-engaging abutments are available, and good survival and success rates have been reported in the literature.13 The design at the implant-abutment interface (IAI) can significantly reduce stress and strain on the abutment screw by increasing the contact area for engaging abutments. This results in better load distribution and decrease micro-movement that results in decrease chance of screw loosening.14 Although these features of internal indexed implants and engaging abutments enhances the stability of the prosthesis, the path of insertion or draw for multiunit prosthesis on nonparallel implants may be challenging. Non-engaging abutments is another option that allows up to a certain degree of correction with achieving a good fit of the prosthesis to multiple, nonparallel internally indexed implants. The caveat with non-engaging abutments is that greater stress has been reported around the abutment screws and the IAI when eccentric forces are applied. The choice for using engaging or non-engaging largely depends on the implant positions and angulations.8,15,16 Therefore, different combinations of engaging and non-engaging abutments have been evaluated and proposed as an alternative to regain some advantages of utilizing the internal connection with an engaging abutment, while achieving the required passive fit with a non-engaging abutment.8 Biomechanical aspects of implant-supported FDPs and stress distribution in the components have been investigated in a number of in vitro, animal and clinical studies to predict the clinical behavior of implant-supported restorations.17-20 Photoelastic analysis and finite element analysis (FEA) have been utilized in dentistry to investigate the biomechanical behavior of teeth, biomaterials, orthodontic appliances and dental implants.21,22 The wide application of FEA is due to its cost effectiveness, predictability, and continuously improved accuracy. The response of an implant-prosthesis-bone complex to mechanical loading can be analyzed through FEA. Development of FEA models that are able to predict in vitro and clinical outcomes are still lacking.23,24 These models are needed to compare a large number of designs and configurations that would otherwise not be feasible with in vitro testing. To date, no study has This article is protected by copyright. All rights reserved. 3 evaluated different implant abutment configurations for screw-retained implant FDPs using 3D FEA. Therefore, the aim of the present study was to use numerical analysis to compare the mechanical response of different engaging and non-engaging abutment configurations for FDPs in the region of the posterior mandible. The primary focus was to evaluate the stress values and distributions of the IAI. Materials and methods An artificial bone block (45mm in length x 20mm in width x 20 mm in height) and a cortical shell that had a homogeneous thickness of 1.5 mm was designed using Ansys 19 software (Ansys Inc., PA).25 Digital models of 2 bone level internally indexed endosteal dental implants (∅ 4.1mm x 10mm) were created and designed according to an implant manufacturer (SLActive®, RC – Regular CrossFit®, Straumann, MA). The Finite Element Model (FEM) of the teeth was used to position the implants at the level of the bone block in teeth #’s 28 (premolar – pm) and 30 (molar – m) locations. The implant components (screw and abutments) were designed using Geomagic Wrap (3D systems, SC) and Ansys 19 (Ansys Inc., PA). The implant screw-retained FDP was created from a digital typodont model through CAD modelling using Geomagic Wrap and exported as an IGES (Initial Graphics Exchange Specification) model (Fig 1). Four implant screw-retained FDP (teeth #’s 28-x-30) groups with different abutment combinations (Straumann, MA) were created (Fig 2): dual engaging (pmE-mE), dual non-engaging (pmNE-mNE), premolar engaging and molar non-engaging (pmE-mNE), and premolar non-engaging and molar engaging (pmNE-mE) abutments. The implant components, implants and bone block were aligned to the screw-retained FDPs using rigid roto-translatory movements. Using Boolean operations, the bone block was remodeled to create the space for the implants. Constant frictional contacts (0.3)26 were applied between all implant FDP components except for the abutment-FDP contact surfaces, which were bonded. To simulate complete osseointegration, the implant-bone block contacts were also set as bonded.27 The non-linear contact surfaces were solved using the “Pure penalty” settings with a maximum allowed penetration of 0.005 mm. The choice of simulating complete osseointegration was to control the variable and to focus on the different abutment designs on the IAI. All bodies were meshed with solid elements and were assigned linear elastic mechanical properties (Table 1).28 According to manufacturer’s guidelines, thirty-five (35) Ncm preload were applied to the screws, and the mesial and distal extremities of the bone block were fixed in all directions.29,30 Using Ansys 19 (Ansys Inc., PA), the simulation was divided into two steps: preload application and occlusal load application. The 35 Ncm preload was simulated applying a 583 N preload onto the screws using the “Bolt Pretension” function. To simulate a more realistic condition, a 160 N load at a 45-degree angle to the occlusal plane31 was applied in each group and distributed on the occlusal This article is protected by copyright. All rights reserved. 4 points (Fig 1-C).32 A convergence test was performed on the pmNE-mNE model to ensure that the mesh size was appropriate for the specific analysis. The Von-Mises stress on the molar implant was used as a reference and a variation of <5% of its value was considered acceptable. The mesh size of 0.075mm edge length was found to be appropriate and the total number of elements and nodes for the four simulated designs ranged from 501,617 and 858,088 for the pmNE-mNE model to 665,830 and 1,138,859 for the pmE-mE model. Von-Mises stresses were measured for all FDP components with a particular focus on the implant screws and the IAI. The maximum principal stresses were measured for the cortical and cancellous bone, as it is a more effective parameter to predict failure for brittle materials. A schematic workflow is depicted in Figure 3. RESULTS The peak Von-Mises stress values were calculated for each of the FDP components (Fig 3) and Table 2 presents the peak Von-Mises stress values calculated for each of the implant FDP components and the peak maximum principal stress for cancellous and cortical bone. Von-Mises stress values ranged from 173.5 MPa with the pmNE-mNE configuration, to 231.2 MPA with the pmE-mNE configuration. The areas with the highest stress were located at the connection with the abutment of tooth # 28 in the pmE-mE configuration, and in the area of the prosthetic connector between pontic tooth # 29 and abutment tooth # 30 for the other configurations (Fig 4). The equivalent stress on the abutments ranged from 554.62 MPa for abutment # 28 in the pmNE-mE configuration to 994.5 MPa for abutment # 30 in the same configuration. Abutment # 30 had the highest stress values for all scenarios and for both abutments, higher stress was found for surfaces that contacted the screw. The implants were subjected to stress values that ranged from 516.0 MPa to 1304.6 MPa. Differences in stress patterns were found between engaging and non-engaging abutments (Fig 4). For non-engaging abutments, higher stress was found around the implant collars. For the prosthetic screws, different stress and distributions (range of 698.8 MPa to 902.5 MPa) were found between engaging and non-engaging configurations (Fig 4). The cancellous bone showed the lowest principal stress value (2.5 MPa) with the pmNE-mNE configuration and the highest (4.1 MPa) with the pmNE-mE configuration. For the cortical bone, the lowest principal stress value (22.2 MPa) was found with the pmE-mE configuration, while the highest (51.5 MPa) was found with the pmNE-mNE configuration. Discussion The results of this study showed that abutment design (engaging and non-engaging) and location can affect the stress distribution with implants and restorative components. Out of all tested abutment combinations, the dual non-engaging (pmNE-mNE) abutment configuration had higher This article is protected by copyright. All rights reserved. 5 stress in the area of the IAI and could be attributed to the decreased extension of the abutment into the internal connection of the implant. This increase in localized stress could possibly cause material wear and loss, which could lead to the breakdown of the implant-prosthetic assembly.33 This study uses 3D FEA to evaluate the stress distribution in different implant components. The dual engaging (pmE-mE) abutment configuration had the lowest overall stress distributions while dual non-engaging (pmNE-mNE) abutment configuration had the highest stress distributions when an oblique static load was applied to the model. When comparing both hemi-engaging (pmNE-mE and pmE-mNE) configurations, a more balanced stress distribution among the components was found when the engaging abutment was in the more anterior position. Previous reports have used in vitro testing to analyze the mechanical behavior and the fatigue response of different implant restorations. Dogus et al.8 presented an in vitro study analyzing the effect of internal engaging abutment position in a cantilevered fixed-splinted 3 unit prosthesis. Their analysis showed that the engaging abutment should be placed further away from the heaviest expected load for the best fatigue response. Although no direct comparisons can be made, this study also found that for hemi-engaging abutments, the pmE-mNE had the best stress distribution when compared to the dual non-engaging and pmNE-mE abutment configurations. Different stress distributions for the screws were found with the various abutment combinations (Fig 4). For non-engaging abutments, higher stress was found near the neck of the screw, close to the abutment. For engaging abutments, two regions of high stress were found: on the lower part of the screws, near the screw threads and near the neck of the screw. It is expected that this different stress distribution can affect the failure mode, because for the non-engaging abutment screws the stress was more distributed all over the screw, instead of being concentrated around the screw neck. There appears to be a relationship between abutment design and stress around the bone (Table 2). The cortical bone showed a stress of 22.2 MPa with both engaging abutments but increased to 51.5 MPa with both non-engaging abutments. A plausible explanation is that concentrated areas of stress were found in the area of the implant necks, which may be transferred through the implant surface and to the alveolar bone. This warrants further investigation. The IAI consists of the implant platform, the prosthetic screws, and the restorative abutments. The higher peak stresses found with engaging abutments could be due to the more restricted configuration (i.e. Extending into the internal aspect of the implant). However, studies have described more favorable dissipation of forces with engaging abutments.9,29 It was interesting to observe that the most distal prosthetic screw (implant #30) had higher stress values than the more anterior screw for all configurations except for the pmE-mNE configuration. The most distal abutment also had higher stress values for all configurations. These observations could be partially explained by the occlusion that was designed for this study in that higher occlusal forces, depicted as stress values, are generated in the posterior area. This article is protected by copyright. All rights reserved. 6 The present study showed how the abutment design combination and location can strongly affect the stress in the implants and restorative components. When the stress distribution on the FDP components becomes higher than the yield strength of the material, the components incur a plastic deformation which can consequently lead to screw loosening. Moreover, high stress concentrations can result in deformation and wear between the components.34 Stress in the area of the implant platforms for non-engaging abutments exceeded the yield strength (880 Mpa) of titanium alloy.35 This was also observed for the implant abutment #30 (994.5 MPa) for the pmNE-mE configuration. Considering all tested abutment combinations, the dual non-engaging (pmNE-mNE ) configuration had the most stress on the implants while the dual engaging (pmE-mE) configuration was the most balanced for all components. Restoring a screw-retained FDP with dual engaging abutments requires meticulous planning, adequate bone to allow precise parallel placement of the dental implants, and achieving passive fit with the prosthesis; all of which are difficult to achieve for a number of reasons.14,36 Based on the results of this study, the dual engaging configuration was the best in stress distribution and is recommended if all criteria can be achieved. However, to allow a certain degree of divergence between dental implants and achieve passive fit, an alternative is to use either dual non-engaging or hemi-engaging combinations of abutments. To control the variables that may affect the results of our study, a more simplified model was designed. A realistic model should account for non-complete osseointegration and a real bone geometry simulating friction instead of bonded contacts. In a real clinical situation, the actual bone model should be reconstructed possibly by using microcomputed tomography to digitize the actual complex trabecular and cortical bone structure.32 Additionally, the actual bone should be characterized by anisotropic mechanical behavior, which is often simplified as homogenous in FEA studies.37 In the present study, a comparative biomechanical analysis was performed, and according to previous studies, the comparison is not affected by the simplified modelling.27 A study by Wang et al.38 demonstrated through electronic strain measurements that FEA is an accurate tool to measure the mechanical response of FDP. For this reason, the present study focused on the comparison of all possible engaging and non-engaging abutment configurations for screwretained FDPs in the mandibular posterior region, which is a common area for performing biomechanical testing due to reported high success and survival of dental implants, occlusal loads, and is a common area for missing teeth.39 An advantage of FEA allows for preliminary analysis that could provide hints for future in vitro experiments. For clinical and in vitro testing, it is important to apply the appropriate preload to the implant screws to prevent loosening or fracturing and for proper study design. For 3D FEA analysis, it is essential to include a preload condition to the screws in order to replicate the aforementioned. Modeling the preload will help achieve a more realistic biomechanical model, creating an initial stress between the FDP components.29 The preload is responsible for the stability of the prosthetic system. The optimum preload should not exceed the yield strength of the screws and was reported to be ideally 75% of the yield strength. When the external forces overcome the preload, the This article is protected by copyright. All rights reserved. 7 deformation and loosening of the screw begin, ultimately resulting in prosthetic complications and failure.30 Long-term success depends on the stress distribution over the entire implant-restorative assembly. It is difficult to evaluate these stress distributions clinically. In vitro biomechanical tests have provided evidence of material fatigue, wear, and deformation. However, both clinical and in vitro tests are costly, time consuming, and have limitations in controlling certain variables. The use of 3D FEA is most suitable to evaluate the complex geometries of dental implants and restorative components, and the alveolar bone. It can provide additional information that can support and enhance our understanding of the mechanisms of implant complications. This study is based on numerical settings under ideal testing conditions. Additional studies are needed for comparisons, as well as validation with in vitro mechanical testing. Future studies should evaluate both static and cyclic forces in occlusal and oblique directions. Another limitation is that this study evaluated only one implant design and components. Expanding this to include other designs and components would be beneficial and could provide further insights into the mechanisms of implant complications and failures. Conclusion Different combinations of engaging and non-engaging abutment designs resulted in different stress patterns with the implants and restorative components. The dual engaging abutment design had the best stress distribution, followed by the hemi-engaging (pmE-mNE) abutment design where the engaging abutment was placed in the more anterior implant position. Non-engaging abutments resulted in higher stress areas at the implant platform and the prosthetic screws, which exceeded the yield strength of titanium alloy. References 1. Jung RE, Pjetursson BE, Glauser R, et al.: A systematic review of the 5-year survival and complication rates of implant-supported single crowns. Clin Oral Implants Res 2008;19:119130 2. Goodacre CJ, Bernal G, Rungcharassaeng K, et al.: Clinical complications with implants and implant prostheses. J Prosthet Dent 2003;90:121-132 3. Pjetursson BE, Thoma D, Jung R, et al.: A systematic review of the survival and complication rates of implant-supported fixed dental prostheses (FDPs) after a mean observation period of at least 5 years. Clin Oral Implan Res 2012;23:22-38 4. 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Jorneus L, Jemt T, Carlsson L: Loads and designs of screw joints for single crowns supported by osseointegrated implants. Int J Oral Maxillofac Implants 1992;7:353-359 35. Boyer R, Welsch G, Collings EW, et al. Materials properties handbook titanium alloys. In: Materials Park, OH: ASM International; 1994: http://app.knovel.com/web/toc.v/cid:kpMPHTA002/. 36. Abduo J, Bennani V, Waddell N, et al.: Assessing the fit of implant fixed prostheses: a critical review. Int J Oral Maxillofac Implants 2010;25:506-515 37. Geng JP, Tan KBC, Liu GR: Application of finite element analysis in implant dentistry: A review of the literature. J Prosthet Dent 2001;85:585-598 38. Wang GQ, Zhang S, Bian CR, et al.: Verification of finite element analysis of fixed partial denture with in vitro electronic strain measurement. J Prosthodont Res 2016;60:29-35 39. Rossi AC, Freire AR, Prado FB, et al.: Photoelastic and finite element analyses of occlusal loads in mandibular body. Anat Res Int 2014;2014:174028 FIGURE LEGENDS Figure 1. Mesial view of the digital finite element model (A); deconstructed view of all components (B); occlusal contacts used to distribute the load (C). This article is protected by copyright. All rights reserved. 11 Figure 2. Representation of the four different engaging and non-engaging abutment configurations. Figure 3. Schematic workflow. This article is protected by copyright. All rights reserved. 12 Figure 3. Graphical representation of the maximum Von-Mises stress for each implant FDP component and configuration. Figure 4. Color map of implant fixed dental prosthesis (FDP) components for each simulation. This article is protected by copyright. All rights reserved. 13 Table 1. Mechanical properties assigned to each component. Component Young’s modulus (Gpa) Poisson’s ratio Abutment/screw/implant (Ti6Al4V) 110 0.33 FDP (Zirconia Y-TZP) 209.3 0.32 Cancellous Bone 13.7 0.3 Cortical Bone 1.37 0.3 This article is protected by copyright. All rights reserved. 14 Table 2. Von-Mises stress values for each implant FDP component and the maximum principal stress for the bone. Abutment Configurations Stress Premolar EngagingMolar Engaging Premolar NonEngaging-Molar Non-Engaging Premolar NonEngaging-Molar Engaging Premolar Engaging-Molar Non-Engaging Screw 28 698.8 860.4 772.2 790.9 Screw 30 804.5 902.5 874.4 745.3 FDP 220.3 173.5 231.2 226.9 708.1 612.3 554.62 560.7 779.3 637.1 994.5 573.1 Implant # 28 581.9 1078.6 1123.4 580 Implant # 30 516 1106.9 615 1304.6 3.4 2.5 4.1 3.7 22.2 51.5 44.1 32.3 Components Von-Mises Abutment # equivalent 28 (MPa) Abutment # 30 Maximum Cancellous principal (MPa) Cortical This article is protected by copyright. All rights reserved. 15