DOI:10.14311/APP.2021.30.0081 Acta Polytechnica CTU Proceedings 30:81–86, 2021 © Czech Technical University in Prague, 2021 available online at http://ojs.cvut.cz/ojs/index.php/app NUMERICAL COMPARISON OF TRANSGINGIVAL AND SUBGINGIVAL DENTAL IMPLANTS IN REGARD TO THEIR STRESS DISTRIBUTIONS Luboš Řehouneka, ∗, Aleš Jíraa, Gabriela Javorskáa, Daniel Bodlákb a Czech Technical University in Prague, Faculty of Civil Engineering, Department of Mechanics, Thákurova 7, 166 29 Prague 6, Czech Republic b ProSpon spol. s.r.o., Jiřího Voskovce 3206, 272 01 Kladno, Czech Republic ∗ corresponding author: lubos.rehounek@fsv.cvut.cz Abstract. Most modern dental implants differentiate in regard to the fixation of the abutment into two main categories - the external or internal hexagon or octagon. We performed mechanical tests according to the ČSN EN ISO 14801 standard on a dental implant variant using the external hex. We found that failure of all implant specimens occured below the screw head. To improve the current geometry, we performed numerical analysis of an alternative variant (internal hex) and compared it with analysis of the current geometry (external hex). It was found that the stress distribution of the variant with internal hex is preferable to the old variant. Although extreme values of shear stress in the corresponding plane of loading are higher, they do not concentrate below the screw head, where the screw itself is thinner and more prone to breaking. Therefore, it seems that the new variant of the dental implant is stronger, which is still to be proven by mechanical tests. Keywords: External hexagon, FEM, implant-abutment interface, internal hexagon, stress. 1. Introduction The implant-abutment connection is one of the most important interfaces of a dental implant. It provides a means to apply torque required to screw the implant into bone and also serves as a connection between the intraosseous part and the abutment. There are many requirements and functions of this interface that may be less or more propagated in regard to the form of connection (internal or external). However, one of the most important functions is the prevention of rotation between individual parts [1]. This particular property is greatly influenced by the character of the interface. From a mechanical standpoint, there are several geometrical solutions among dental implants in re- gard to their placement and the fixation of the abut- ment. They include the Internal Morse Taper, Exter- nal Morse Taper, internal lobing, tube-to-tube sys- tems and external and internal polygons. These last 2 main categories comprise the vast majority of clini- cally used implants as well as the majority of market share. The most common variants of the polygonal fixation are the hexagon or the octagon. Our study includes 2 different variants of the polygonal variant – the external hexagon (transgingival variant) and the internal hexagon (subgingival variant). Their names suggest their placement in the bone, as shown in Fig. 1. Historically, the first manufactured titanium dental implants by Brånemark had the external hexagonal connection. It is a variant that is very simple a ro- bust, however, it has some deficiencies, such as insuf- ficient anti-rotational resistance or insufficient resis- Figure 1. Comparison of placement of the 2 anal- ysed dental implant variants inside the bone. Left – the internal hexagon variant (subgingival placement) and right – the external hexagon variant (transgingi- val placemenent). The lenght L indicates the portion of the intraosseous part that exceeds the bone after implantation. tance to intraoral forces with greater offset from the connection (e.g. top of the crown). To improve lat- eral and rotational stability, many other variants were developed (internal hex, spline connection, Morse Ta- per, tapered hexagon and other variants) [2]. 1.1. Motivation for development The literature suggests that internal connections have mechanical advantages. It was found that the bend- ing moments of zirconia implants are superior when paired with internal connection, compared to exter- 81 http://dx.doi.org/10.14311/APP.2021.30.0081 http://ojs.cvut.cz/ojs/index.php/app L. Řehounek, A. Jíra, G. Javorská, D. Bodlák Acta Polytechnica CTU Proceedings Figure 2. Documentation of the old external (EH) connection variant of the implant.The intraosseous part is the subject of a patent [7]. nally connected abutments [3, 4]. Research suggests that external connections lead to greater stresses in- side the components of the implant, mainly at the connection interface [5]. Also, they pose as an al- ternative to external connections, which have proven to break in the abutment screw [6]. The abutment screw can be identified as a very weak component of the system as it is thin while being exposed to shear forces during mastication. The motivation to develop other variants of con- nection also stems from the fact that external connec- tions have proven to decrease levels of crestal bone at the peri-implant area after 1 year of loading and also exhibit greater bone remodelling and bone loss when compared to internal connections within the same set of circumstances [8]. Furthermore, internal abutment connections have better lateral stability as the lever arm of forces is shorter, thus reducing the value of maximum force that can be applied in the lateral di- rection. According to literature, this results in better stress distributions, less micromovement and higher survival probability [9]. Other factors, such as sur- face treatment, anodization, implant length, abut- ment material etc. also influence bone loss. Another aspect of potential improvement over the external connection types is improving the transfer of loads in the surrounding bone and minimizing the gaps at the implant-abutment interface which may Figure 3. Documentation of the new internal (IH) connection variant of the implant.The intraosseous part is the subject of a patent [7]. lead to bacterial colonization [10] and subsequent biofilm accumulation [11]. 2. Materials and Methods The means of comparison in this study are FEM anal- yses of 2 selected dental implant variants in regard to their stress distribution. The selected variants are the external hexagon (EH) and the internal hexagon (IH), as shown in Fig. 2 and Fig. 3. The simula- tions were performed on both variants on the basis of conditions and results of mechanical tests of the EH variant so as to verify the viability of the newer IH variant in regard to stress distribution. 2.1. Mechanical Tests We performed mechanical tests on the old EH vari- ant according to the ISO 14801 Dentistry–Implants– Dynamic fatigue tests for endosseous dental implants standard [12] on a total of 6 specimens of dental implants (Fig. 4). The tests were performed on a double-column pneumatic press LiTeM Vertical Dou- ble Column VDC A-6. The machine complies with ISO 7500-1 [13] and enables for constant crosshead displacement. A sample P-D diagram of the tested EH speci- mens is shown in Fig. 6. All specimens were embed- ded in the Dentacryl fixating medium (methylacry- late resin for technical use). The resin has good me- chanical properties and thermal insulation properties. Its bending strength is 50 MPa, tensile strength is 20 82 vol. 30/2021 Comparison of Transgingival and Subgingival Implants Figure 4. A disassembled specimen of the EH vari- ant used for the mechanical tests. Although the body of the implant is different, the locking system includ- ing the abutment and screw are the same. Specimen Number Force [N] 1 1278.7 2 1387.6 3 1301.2 4 1486.0 5 1275.8 6 1233.2 Mean Value 1327.1 Table 1. Maximum static load force for all EH spec- imens. MPa and compressive strength is 80 MPa. Its elastic modulus is 3.2 GPa. With these properties, it com- plies with the mentioned standards for static testing. As the ISO 14801 standard [12] requires a 30° slanted force, a fixation apparatus is needed to slant the specimens as the LiTeM machine can only apply force in the vertical direction. We needed to make this preparation ourselves as the manufacturer does not provide any further preparations for the machine. It was modelled in NetFabb and subsequently 3D printed on the Sinterit Lisa Pro 3D printer by us- ing the SLS (Selective Laser Sintering) technology. The used material was the PA12 polyamide, whose strength is comparable with aluminum. The fixating preparation is shown in Fig. 7. After the fixation, the implant is connected with the abutment by the abutment screw with a M2.2 thread using a 250 Nm momentum by a special ratchet supplied by the manufacturer of application instrumentation. The mechanical tests were con- trolled by a constant speed of displacement of 4 mm/min until specimen failure in the LiTeM ma- chine. The mean maximum force of the implant specimens at failure was 1327.1 N (Table 1) and the failure oc- cured below the head of the abutment screw for all EH specimens, as shown in Fig. 5. This test con- firmed the hypotheses that the abutment screw would be the weakest part of the assembly and also corrobo- Figure 5. Two EH specimens after mechanical tests. The images show failure of the specimens right be- low the head of the abutment screw. Note that all specimens failed exactly in this manner. Figure 6. A sample P-D diagram of specimen n. 2 from the mechanical testing. The maximum force for this specimen was 1387.6 N. rated the research of [6]. From Fig. 5, shear failure of all specimens is apparent. The failure of all implants occured by shear failure of the screw connecting the intraosseous part and the abutment. This connection proved to be a limiting factor for the tested EH speci- mens. A positive finding was the fact that the bodies of the implants themselves remained intact. 2.2. FEM Analyses As all of the EH specimens achieved failure below the head of the abutment screw, we set out to model these results using ANSYS Workbench 2020 R1. The over- all approach was to model the older EH variant, ver- ify the model by comparing it with mechanical tests and subsequently analyse a newer version of the im- plant with an internal hexagonal connection to con- firm whether distributions of stresses are better. The geometrical models were imported into AN- SYS in the form of a parasolid (.x_t extension) and are shown in Fig. 9. According to the previously conducted mechanical experiments, we chose a force with the magnitude of 900 N as it is approximately the highest force that is still in the linear region of the P-D diagram (Fig. 6). The force is situated on top of the abutment rim and distributed equally among a few nodes of the mesh (Fig. 9). To accurately represent the conditions of the experiment and the standard [12], we fixed the lower part of the implant that was embedded in the Dentacryl resin and left 83 L. Řehounek, A. Jíra, G. Javorská, D. Bodlák Acta Polytechnica CTU Proceedings Figure 7. The mechanical testing assembly with the LiTeM machine and a 3D-printed preparation for an- choring the specimens at a 30° angle. a 2 mm gap between the top of the fixed area and the theoretical level of bone (Fig. 9). As the goal of the simulation was to compare stress distributions during the linear behavior, we did not consider non- linear behavior in the model and chose the same force of 900 N for both simulations. Simple tetrahedral linear elements and linear ma- terial behavior with using the Ti-6Al-4V material model were used to model the numerical problem. The mesh was designed so as to represent at least 5 elements across the width of the stabilizing ribs of the implant substructure and was further refined by size in the region of interest – the abutment screw. As the assembly consists of 3 solids – the abutment, the screw and the intraosseous part – the model con- tains basic contact conditions with specific distance break criteria. The contact conditions are necessary as some parts have conical shapes, e.g. the fit is as- sured by screwing the screw into the abutment coni- cal cavity, the screw passing through the intraosseous part of the implant etc. The contact conditions are shown in Fig. 8. Due to the complicated geometry of the real screw, we decided to only model its threaded body as a cylin- der as the goal of the simulation was not to observe stress distributions in individual threads or details, but rather stress distributions across the whole screw body, i.e. compare the stress concentrations or lack thereof. Figure 8. Contact regions of the new IH variant inside the ANSYS environment. Figure 9. The two geometrical models inside the ANSYS environment. Left– the old EH variant. Right – the new, IH variant. Blue shows the area of fixation (2 mm below the theoretical level of bone), red shows the location of the 30° slanted force. Boundary conditions comply with previous mechani- cal tests and with ISO 14801 [12]. 84 vol. 30/2021 Comparison of Transgingival and Subgingival Implants Figure 10. The final results of both analyses (EH and IH variant) of the FEM analyses displayed on the abutment screws. A – section through the abutment screw of the EH variant, typical stress concentration marked red. B – a detail of the area highlighted in "A". C – the new IH geometrical solution with significantly reduced shear stress concentrations. 3. Results The initial hypothesis was that by introducing a mod- ified geometry of the implant-abutment connection, we can reduce the shear stress concentrations and therefore either eliminate stress concentrations or in- crease the maximum force at which the screw will eventually break (for the EH variant, the mean force was 1327.1 N, see Table 1). The results from the simulation of the old EH vari- ant are shown in Fig. 10-A and B. The image shows that the model behaves in the same way as the spec- imens in mechanical tests. Although we did not en- compass the region of failure as our analysis is only linear, it is apparent that the concentration of shear stresses occur exactly at the same place – the thin cross-section of the abutment screw below its head. With this information, we constructed the second model in the exact same way and compared the anal- yses. Moreover, another region with significant stress concentration emerged at the bottom of the abutment screw. However, this occurrence was not accounted for as it did not have any direct impact during the mechanical tests. The results from the simulation of the new IH vari- ant are shown in Fig. 10-C. The shear stress concen- trations were diminished and extremes moved from the thin cross-section of the screw into its head. Al- though the overall values of shear stresses are higher across the whole body of the abutment screw, the concentration points were eliminated by incorporat- ing the internal connection. 4. Conclusions The initial hypothesis of this experiment was that by replacing the external hexagonal (EH) connec- tion with internal (IH) connection, we can eliminate stress concentrations that emerge with the use of EH because of its geometry. The mechanical tests have shown that all of the dental implant specimens reached failure during the tests by shear failure of the screw below the its head. Von-Mises criteria and principal stresses were also considered, but their re- sults were not included in this study as their evalua- tion has proven to be non-essential for confirming the hypothesis. The numerical analyses have shown that the EH variant induces concentrations of shear stress in the screw below its head. Under the same conditions, we 85 L. Řehounek, A. Jíra, G. Javorská, D. Bodlák Acta Polytechnica CTU Proceedings performed analysis of the IH variant and found a more viable distribution of shear stress with no apparent concentrations in the body of the abutment screw. Therefore, this analysis confirms our initial hypoth- esis that the internal connection might be more favor- able and corroborated the results provided by litera- ture [6]. The trend to innovate in the area of implant- abutment connections can be seen in the market share of dental implants, where more and more companies offer alternatives to the tried-and-true external con- nection, which proves to have its shortcomings, such as low anti-rotational resistance, bone loss at the peri-implant area, lower lateral stability, microgaps at the abutment-implant interface leading to bacte- ria colonisation and biofilm formation, etc. To further prove if this concept is true and finish the research, we need to manufacture specimens of this new geometrical solution with internal connec- tion and subject them to mechanical tests to deter- mine whether the character of failure changes or the maximum force required to break the abutment screw increases. Acknowledgements The financial support provided by the by the Fac- ulty of Civil Engineering, CTU, Prague, project n. SGS20/155/OHK1/3T/11 is gratefully acknowledged. References [1] L. Gaviria, J. P. Salcido, T. 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