Since the introduction of the Branemark

Transcription

1 RESEARCH Influence of the Implant Diameter With Different Sizes of Hexagon: Analysis by 3-Dimensional Finite Element Method Eduardo Piza Pellizzer, DDS, PhD, MSc 1 * Fellippo Ramos Verri, DDS, PhD, MSc 1 Sandra Lúcia Dantas de Moraes, DDS, MSc 1 Rosse Mary Falcón-Antenucci, DDS, MSc 1 Paulo Sérgio Perri de Carvalho, DDS, PhD, MSc 2 Pedro Yoshito Noritomi, PhD 3 The aim of this study was to evaluate the stress distribution in implants of regular platforms and of wide diameter with different sizes of hexagon by the 3-dimensional finite element method. We used simulated 3-dimensional models with the aid of Solidworks 2006 and Rhinoceros 4.0 software for the design of the implant and abutment and the InVesalius software for the design of the bone. Each model represented a block of bone from the mandibular molar region with an implant 10 mm in length and different diameters. Model A was an implant 3.75 mm/regular hexagon, model B was an implant 5.00 mm/regular hexagon, and model C was an implant 5.00 mm/ expanded hexagon. A load of 200 N was applied in the axial, lateral, and oblique directions. At implant, applying the load (axial, lateral, and oblique), the 3 models presented stress concentration at the threads in the cervical and middle regions, and the stress was higher for model A. At the abutment, models A and B showed a similar stress distribution, concentrated at the cervical and middle third; model C showed the highest stresses. On the cortical bone, the stress was concentrated at the cervical region for the 3 models and was higher for model A. In the trabecular bone, the stresses were less intense and concentrated around the implant body, and were more intense for model A. Among the models of wide diameter (models B and C), model B (implant 5.00 mm/regular hexagon) was more favorable with regard to distribution of stresses. Model A (implant 3.75 mm/regular hexagon) showed the largest areas and the most intense stress, and model B (implant 5.00 mm/regular hexagon) showed a more favorable stress distribution. The highest stresses were observed in the application of lateral load. Key Words: dental implants, biomechanics, finite element analysis INTRODUCTION Since the introduction of the Branemark system, the coronal aspect of the hexagon has gradually transformed to promote a better adaptation and an antirotational mechanism. 1 The design of the original external hexagonal was designed as a gear 1 Department of Dental Materials and Prosthodontics, Araçatuba School of Dentistry, São Paulo State University-UNESP, Brazil. 2 Department of Surgery and Integrated Clinic, Araçatuba School of Dentistry, São Paulo State University-UNESP, Brazil. 3 Renato Archer Information Technology Center, São Paulo, Brazil. * Corresponding author, ed.pl@uol.com.br DOI: /AAID-JOI-D mechanism and transfer of rotational torque to hold the implant during the surgical installation in the bone. 2,3 The hexagon had a height of 0.7 mm to allow for adjustment during installation of the implant. 2 Initially, the use of osseointegrated dental implants had been proposed only in fully edentulous patients. Gradually, that planning was expanded for partially edentulous patients and ultimately for the replacement of individual components. Prosthetic tooth implant solutions were the last to be developed by many implant systems. 4,5 From that moment on, a significant number of clinical complications began to emerge. 2,6 The increasing use of external hex implants Journal of Oral Implantology 425

2 Influence of the Size of the Hexagon resulted in significant complications, mostly related to loosening or fracture of the abutment screw, mainly in unitary prostheses. 7 9 Thus, to minimize these difficulties, the external hexagon, transmucosal connections, and retaining screws underwent numerous modifications 2 to provide a mechanism for indexing and antirotational restorations. 10,11 A key factor for success or failure of dental implants is the way the tension is transferred to the surrounding bone. The overload of the implant may exceed the physiological limits and cause failures in rehabilitation or even the loss of osseointegration. 7,12 Studies on the biology of the bone suggest that the overload in the implant can lead to their failure. 13,14 When this load is greater than 2000 to 3000 microstrain, large deformations occur in the peri-implant bone, 15,16 exceeding the physiological tolerance of the bone and possibly causing microfractures in the bone-implant interface. 1,17 Therefore, it is essential to improve the chewing load distribution through the prostheses and from that to the implants and to the support bone. 17,18 Some authors 19,20 have suggested that the height of the hexagon could contribute to the stability of the union with the screw, and an optimal height could protect the screw from the effects of twist by the dispersion of forces to other components. Ohrnell et al 21 recommended that the external connection of the hexagon should have at least 1.2 mm in height, to provide lateral and rotational stability, particularly in single restorations. For Weinberg 20 and Ohrnell et al, 21 an external geometry is particularly vulnerable because of the limited gear with its external components and the presence of a short fulcrum point (small platform) when tilted forces are applied. On the world market, different companies offer options for different sizes of hex implants. Therefore, to obtain more data about the design of implants, this study proposes to evaluate the stress distribution in implants of regular platforms and of large diameter with different sizes of hexagon, using the 3-dimensional (3D) finite element method. MATERIALS AND METHODS TABLE 1 Models Model Specifications A Implant ( mm) with platform of 4.10 m and regular hexagon B Implant ( mm) with platform of 5.00 mm and regular hexagon C Implant ( mm) with platform of 5.00 mm and expanded hexagon For this study, 3D models (Table 1) were fabricated, represented by a section of jawbone type III, composed of trabecular bone in the center surrounded by 1 mm of cortical bone in the region corresponding to the mandibular molar with an implant of associated prosthetic component. The geometry of trabecular and cortical bone was obtained from the recomposition of a computerized tomography cross-molar region, made using the InVesalius software (CTI, São Paulo, Brazil), which allows the creation of virtual 3D models from cross-sectional tomography. Subsequently, the image was exported to the Rhinoceros 4.0 software (NURBS Modeling for Windows, Seattle, Wa) for geometry simplification and refinement of the design. The geometry of the implants was obtained from the Connection system (Master Screw, Conexão Sistemas de Prótese Ltda, São Paulo, Brazil) of 10 mm in length and its corresponding component prosthetic UCLA (abutment). All implants and components had their geometries simplified through the SolidWorks 2006 (SolidWorks Corp, Concord, Mass) and Rhinoceros 4.0 software (Table 2). The set of abutment-implant was inserted into the bony portion of the bone block in a centralized location. After the fabrication of the models, the geometries were exported to the FEMAP 10.0 finite element software (Siemens PLM Software Inc, Santa Ana, Calif) in STEP format. Then, finite element meshes with parabolic solid elements were generated. The corresponding mechanical properties of each material, Young s modulus and Poisson s ratio, were assigned to the mesh using the values found TABLE 2 Size of the hexagon of the implants Model Hexagon Width, mm A 3.10 B 3.10 C Vol. XXXIX / No. Four / 2013

3 Pellizzer et al Structure TABLE 3 Material properties Young s Modulus, GPa in the literature 22 (Table 3). All materials were considered isotropic, linearly elastic, and homogeneous. The contacts between the prosthetic component/screw, implant/cortical bone, implant/trabecular bone, cortical/trabecular bone, and implant/ screw were assumed to be bonded. Boundary conditions were established as prescribed in the 3 axes (x, y, and z) on the side surfaces of cortical and trabecular bone, with the rest of the set free from restrictions. The axial load applied in the model was 200 N, based on the literature, 23 and was applied to the surface of the abutment. The load was always divided into 4 points, in the form of force per area, in an area of approximately 0.17 mm 2. The analysis was then generated in the finite element software (FEMAP 10.0) and exported to the NeiNastran version 9.2 calculation software (Noran Engineering, Inc, Westminster, Calif) running on a workstation (Sun Microsystems Inc, São Paulo, Brazil). The results were then imported back into the FEMAP 10.0 software for viewing and postprocessing of the maps of von Mises stress and maximum principal stress maps. RESULTS Poisson s Ratio, l Von Mises stress maps Reference Trabecular bone Sertgoz et al 22 Cortical bone Sertgoz et al 22 Titanium Sertgoz et al 22 In the general map, we observed a low stress concentration for the axial load for the 3 models. In the application, loads and lateral oblique stress concentrated on the prosthetic component and the threads of the implant by dissipating a larger area and greater stress intensity on all models. In comparative analysis, model C had the highest stress (under the 3 loads), and this was located mainly in the narrowing of UCLA. In the von Mises stress maps of the implant, the application of the axial load showed that tensions were low for all 3 models. In applying the oblique and lateral loads, the 3 models presented stress concentration at the level of the platform and the threads in the cervical and middle third, at MPa. The stress on model A was concentrated on the first 3 threads of the implant, and in models B and C, strains were more evenly distributed. Comparing the models, it was observed that the strain was distributed by a larger area, respectively, in models C, A, and B and that the application side showed lower stress intensity than the opposite side. The models under side load showed higher concentrated areas of stress. The stress maps of the abutment in the application of axial load in the 3 models showed low concentrations of stress, being slightly higher at the middle third and upper abutment level, reaching MPa. In model C, this region presented the highest stress in the region of narrowing of UCLA, in the range of MPa. In the application of oblique and lateral loads, the models showed similar distribution patterns, with stress concentrations in the middle third and upper abutment. In model C, besides these regions, stress also was concentrated on the area of narrowing UCLA. Comparing the 3 models showed that the largest areas of stress concentration were under the load side and that the stress was higher for model C under axial, lateral, and oblique load. Maximum principal stress In a sagittal section cut of cortical and trabecular bone (Figures 1 3), stress concentration was observed in the application of axial load at the cervical and the first implant thread, where the 3 models presented similarity (Figure 1). In the application of oblique and lateral loading, the 3 models had areas of stress concentration in the cervical region, showing areas of tensile stress of 15 MPa. Model A had the largest areas of tension in both the surface and in the thickness of cortical bone. In comparative analysis, the largest areas were observed in the application of lateral load for the 3 models. The range of areas of compression were observed on the opposite side of the application of load and tensile areas on the opposite side (Figures 1 3). In an occlusal view (Figures 4 6), with the application of axial load, the models showed low tension, and this was concentrated in the area corresponding to the coronal region of the implant. In the application of oblique and lateral load, the models showed similar patterns of stress distribu- Journal of Oral Implantology 427

4 Influence of the Size of the Hexagon FIGURES 1 3. FIGURE 1. Maximum principal stress of cortical and trabecular bone, axial load. FIGURE 2. Maximum principal stress of cortical and trabecular bone, oblique load. FIGURE 3. Maximum principal stress of cortical and trabecular bone, lateral load. tion, and the tensile stresses were located on the side of the load application and the areas of compression on the opposite side. A comparative analysis showed that model A presented the largest areas of tension for all load applications and that the 3 models showed the highest levels of compressive and tensile stress with side loading. On the maps of maximum principal stress of the trabecular bone (Figures 7 9) for all load applications, tensions were located around the threads of the implant, and we observed a larger area with the highest voltages for model A, model B, and model C, respectively, and values were in the range of MPa. For all 3 models, the highest stress intensity was located at the platform level of the implant. The largest areas of tensile stress were observed in the 3 models in the application of lateral load. In general, taking into account the 3 applied forces, a lower compressive stress and tension were observed in a smaller area in the models of large diameter in both the cortical bone and trabecular bone. DISCUSSION Analyzing the stress maps of the abutment in all applications of load, there was a greater area of high stress intensity for model C, unlike other models. The stress was concentrated in the area of 428 Vol. XXXIX / No. Four / 2013

5 Pellizzer et al FIGURES 4 6. FIGURE 4. Maximum principal stress of cortical bone (occlusal view), axial load. FIGURE 5. Maximum principal stress of cortical bone (occlusal view), oblique load. FIGURE 6. Maximum principal stress of cortical bone (occlusal view), lateral load. narrowing of UCLA, and this is probably due to the width of the hexagon being greater than the other models, which reduced the settlement area on the platform, decreasing the thickness of the UCLA and thus producing a higher stress concentration. This was explained by Bidez and Misch, 24 who reported that the magnitude of stress is dependent on 2 variables: the magnitude of force and the area over which the force is dissipated. Therefore, a smaller area would lead to greater stress concentration. The stress in model A was located in the first thread of the implant in the oblique load on the side with the largest areas of stress intensity, which is reflected in cortical areas showing higher tensile (red areas) and shear, as observed in other studies and theoretical analysis. 17,19,24 As for models B and C, both had a better layout with smaller areas of compression and tension. According to Bidez and Misch, 24 compression force tends to maintain the integrity of the bone-implant interface, while the tensile forces tend to separate the interface, being the most destructive. According to Martin et al, 25 on the cortical bone, the maximum stress is higher in compression (170 MPa) than tensile stress (100 MPa). Furthermore, the resistance of trabecular bone is the same in compression and tensile stress, being approximately 2 5 MPa. Thus, considering the citations of these authors, the values obtained in this study (0 1.5 Journal of Oral Implantology 429

6 Influence of the Size of the Hexagon FIGURES 7 9. FIGURE 7. Maximum principal stress of trabecular bone, axial load. FIGURE 8. Maximum principal stress of trabecular bone, oblique load. FIGURE 9. Maximum principal stress of trabecular bone, lateral load. MPa) are compatible with the values of these studies and appear to be within physiological limits. Comparing models B and C, model C generated a greater area of tension because the settlement area of the abutment was lower, allowing a greater displacement of the last, transferring tension to the surrounding bone (cortical and trabecular). On trabecular bone, the stresses were low compared with that of cortical bone. It is known that cortical bone has a higher elastic modulus and therefore a lower deformation, concentrating higher stress. On the other hand, the trabecular bone, because it has a smaller Young s modulus, concentrated less stress around the implant body, 24 which was verified in the results of this study. However, the stress areas were higher in model A because of a smaller area (diameter) for the stress distribution. Between models B and C, model C showed larger stress areas probably due to the difference in the diameter of the hexagon. In the models analyzed, it was observed that the axial load presented a lower stress concentration; with the application of oblique and lateral loads, the models showed higher stress concentrations, which is in agreement with studies that reported that external hexagon implants are vulnerable to oblique and lateral loads. 19,20 The results of the present study are significant since it was demonstrated that the higher the hexagon width in relation to the implant platform, the higher the stress transferred to the bone tissue. Clinically, the insertion of implants with large 430 Vol. XXXIX / No. Four / 2013

7 Pellizzer et al diameter and smaller hexagon may be more favorable since they allow fitting of the prosthetic component to a larger area of the implant platform. It also allows for the platform-switching technique when abutments with smaller diameter are used (the hexagon dimension is similar for regular- and large-diameter implants). CONCLUSION Based on the methodology, the following conclusions were made: Among the models of wide diameter (models B and C), model B was more favorable in the distribution of stresses. Model A (implant 3.75 mm/regular hexagon) showed the largest areas and the most intense stress, and model B (implant 5.0 mm/regular hexagon) showed a more favorable stress distribution. The highest tensions were observed in the application of lateral load. REFERENCES 1. Binon PP. The effect of implant/abutment hexagonal misfit on screw joint stability. Int J Prosthodont. 1996;9: Binon P. Implants and components: entering the new millennium. Int J Oral Maxillofac Implants. 2000;15: Lazzara RJ. Restorative advantages of the coronally hexed implant. Compendium. 1991;12: 924, Jemt T. Failures and complications in 391 consecutively inserted fixed prostheses supported by Branemark implants in edentulous jaws: a study of treatment from the time of prosthesis placement to the first annual checkup. Int J Oral Maxillofac Implants. 1991;6: Lekholm U, van Steenberghe D, Herrmann I, et al. Osseointegrated implants in the treatment of partially edentulous jaws: a prospective 5-year multicenter study. Int J Oral Maxillofac Implants. 1994;9: Adell R, Lekholm U, Rockler B, et al. A 15-year study of osseointegrated implants in the treatment of the edentulous jaw. Int J Oral Surg. 1981;10: Smith DE, Zarb GA. Criteria for success of osseointegrated endosseous implants. J Prosthet Dent. 1989;62: Ekfeldt A, Carlsson GE, Börjesson G. Clinical evaluation of single-tooth restorations supported by osseointegrated implants: a retrospective study. Int J Oral Maxillofac Implants. 1994;9: Eckert SE, Wollan PC. Retrospective review of 1170 endosseus implants placed in partially edentulous jaws. J Prosthet Dent. 1998;79: English CE. Externally hexed implants, abutments, and transfer devices: a comprehensive overview. Implant Dent. 1992;1: Beaty K. The role of screws in implant systems. Int J Oral Maxillofac Implants 1994;9: Eskitascioglu G, Usumez A, Sevimay M, et al. The influence of occlusal loading location on stresses transferred to implantsupported prostheses and supporting bone: a three-dimensional finite element study. J Prosthet Dent. 2004;91: Kim Y, Oh TJ, Misch CE, et al. Occlusal considerations in implant therapy: clinical guidelines with biomechanical rationale. Clin Oral Implants Res. 2005;16: Carlsson GE. Critical review of some dogmas in prosthodontics. J Prosthodont Res. 2009;53: Stanford CM, Schneider GB. Functional behaviour of bone around dental implants. Gerodontology. 2004;21: Frost HM. Mass and the mechanostat : a proposal. Anat Rec. 1987;219: Rangert B, Jemt T, Jörneus L. Forces and moments on Brånemark implants. Int J Oral Maxillofac Implants. 1989;4: Weinberg LA. Reduction of implant loading with therapeutic biomechanics. Implant Dent. 1998;7: Weinberg LA, Kruger B. A comparison of implant/prosthesis loading with four clinical variables. Int J Prosthodont. 1995;8: Weinberg LA. The biomechanics of force distribution in implant-supported prostheses. Int J Oral Maxillofac Implants. 1993; 8: Ohrnell L, Hersh J, Ericsson L, et al. Single tooth rehabilitation using osseointegration: a modified surgical and prosthodontic approach. Quintessence Int. 1988;19: Sertgöz A. Finite element analysis study of the effect of superstructure material on stress distribution in an implantsupported fixed prosthesis. Int J Prosthodont. 1997;10: Morneburg TR, Pröschel PA. Predicted incidence of occlusal errors in centric closing around arbitrary axes. Int J Prosthodont. 2002;15: Bidez MW, Misch CE. Clinical biomechanics in implant dentistry. In: Misch CE, ed. Contemporary Implant Dentistry. 2nd ed. St Louis, Mo: Mosby; 1999: Martin RB, Burr DB, Sharkey NA. Skeletal Tissue Mechanics. New York, NY: Springer; Journal of Oral Implantology 431