Effects of Different Abutment Connection Designs on the Stress Distribution Around Five Different Implants: A 3-Dimensional Finite Element Analysis

Titanium dental implants are widely used because of the advantages offered by their mechanical properties and because of their excellent anchorage in the jawbones.¹  The reliability and stability of the implant-abutment connection design and the surface properties of the fixture are crucial factors in maintaining the long-term functioning of the implant-bone interface.²  The basic knowledge concerning possible mechanical failures that may occur following prosthetic rehabilitation is an important part of treatment planning as it provides predictable success rates.

Animal experiments and clinical studies have shown that implant failures in the absence of plaque-related gingivitis might be associated with the disequilibrium of the forces acting on implants.^3–5  These adverse occlusal forces resulting from the functional components of mastication and nonfunctional occlusal contacts may result in mechanical malfunctions in the implant systems, particularly at the level of implant-abutment connection.⁶ 

There are various biomechanical techniques to evaluate the stress distribution of occlusal forces in bone around dental implants. The finite element method (FEM) is a mathematical model analysis that gives detailed qualitative solution of the interaction between prosthesis, implant, and surrounding bone.⁷  The FEM can present possible changes in shape, based on stress and strain values when the specific material properties, the forces applied, and the boundary conditions are predetermined. The aim of this study is therefore to observe the advantages and disadvantages of different types of implant-abutment connection systems under similar material properties and loading condition using FEM.

Five different implant systems having 5 different implant-abutment connections were chosen for biomechanical analysis. System-1 was 4.6 mm × 10.5mm size with external hexagonal implant-abutment connection, system-2 was 4.5 mm × 10 mm size having conical + internal hexagonal implant-abutment connection, system-3 was 4.1 mm × 10 mm size with the internal hexagonal implant-abutment connection, System-4 was 4.3 mm × 11 mm size with the tube in tube implant-abutment connection, and system-5 was 4.0 mm × 10 mm size with morse taper integrated screwed-in implant-abutment connection. The geometric modeling was done in CATIA V-5 (Dassault Systémes Inc., France) virtual design software as cylindrical implants without threads. These models were later transferred to MSC.Patran 2005 (MSC Software Corporation, USA) for postprocessing.

In this study, the resistance analysis of different implant systems in the crown above the implant, abutment, implant connection screw, and bone were carried out. The same material properties and loading conditions were used for all systems (Figure 1). All systems were considered to have been properly mounted. The pretension forces when screwing were not taken into account. The directions of the applied forces are presented in Figure 2. Material properties for bone and implant components were used according to previous studies (Table 1). The element and node numbers of the systems are given in Table 2. The 100 N vertical and 50 N horizontal and oblique forces were applied to the surface of the crown, which was supported by the implant systems. The results were analyzed by the distribution of the stresses in the abutment, implant, connection screw, and implant-bone interface area using the MSC.Nastran 2005 (MSC Software Corporation) computer software to obtain linear static solution.

FEM is generated by tetrahedron and triangle shell elements and spring elements. These elements are quadratic elements which have additional nodes on the midside of the elements and volume center with nodes on the element vertices. The curved faces are represented more accurately by using quadratic elements and this representation will increase the accuracy of solution results. The accuracy will increase by generating more element and node numbers in the model, but this will also increase the solution run time and the matrices which can build impossible-to-solve mathematical models with the current hardware. The finite element result does not change much after a certain number of elements and nodes regarding the convergence studies. In this study, the finite element model is generated with the element edge lengths which are varying between 0.1 mm to 0.5 mm. The components are connected to each other by using common nodes. The internal force transfer is providing by using common nodes between contacting parts.

All of the stress values are shown in Figures 3–7. When all of the surface of the implant and abutment was evaluated, it was found that the internal-hexagonal implant system generated the minimum stress while the external hexagonal connection had the highest. The maximum stress in the external hexagonal connection was found to be concentrated in the contact region of the implant and abutment. The maximum stress was in the neck of the external hexagonal system abutment, whereas it was minimal in the conical + internal hexagonal connection system abutment when the contact area of the implant and abutment is evaluated. The screw in the external hexagonal connection type showed the maximum stress levels while that in the internal hexagonal connection type showed the minimum. If we include the morse taper integrated screwed-in connection to this group, the lowest stress was seen in the morse taper integrated screwed-in type. The maximum stress in the implant was found in the external hexagonal connection system, while the minimum stress was found in the conical + internal hexagonal connection system. Stress in the bone was maximum in the external hexagonal connection system and minimal in the conical + internal hexagonal connection system. Stress concentration was in the contact area of bone and implant. Maximum stress values of all components of the system are shown in Figure 8.

Chewing forces of adult individuals with natural dentition and those with prosthetic rehabilitation are between 50 N and 2440 N, showing a decreasing pattern from molars to incisors.^9–16  In FEM studies, the application of such forces generally varies from 35 N to 178 N.^17–24  Based on these previous observations, we used the vectoral oblique force load, resulting from 100 N and 50 N of vertical and horizontal components, respectively. Combinations of the axial and horizontal forces that lead to maximum stress load in cortical bone should be taken into account in preclinical studies evaluating the stability of the implant-abutment connections since these are more realistic than using occlusal forces alone.^2,21,25 

In the external hexagonal connection systems, Jemt et al²⁶  observed that the connection screw receives all static and dynamic lateral force loads, which distribute throughout the surface. The authors concluded that such forces can cause loosening and/or fracture of the connection screw. Similarly, Levine et al²⁷  demonstrated that the external hexagonal connection system is more susceptible to screw loss than the solid conical abutment connection. Our findings are consistent with these previous reports as they suggest that the highest stresses are concentrated in the screw of the external hexagonal connection system, and the tension in the neck of the screw shows the maximum values.

Sutter et al²⁸  had shown that the conical angled design could reduce screw loosening by creating a friction lock. In addition, they found that the screw rotation is minimal in the morse taper integrated screwed-in thread abutment system when compared with the external hexagonal connection. Our results support these claims, as the morse taper integrated screwed-in implant system had the least stress values. Burguete et al²⁹  also observed that the conical angled implant abutment connection has higher mechanical properties than the external hexagonal connection design. According to their study, the screw is responsible for the stability of the implant-abutment connection in the external hexagonal connection system under functional forces. Similarly, in our study, the 3-dimensional models created using the conical design showed minimum stresses on the abutment screw. Furthermore, the stress yield was found to distribute evenly to its surface. In the external hexagon abutment system, however, higher stress values were observed to be concentrated highly on some points over the screw surface, indicating a higher possibility of loosening or even fracture at the corresponding regions.

Merz et al³⁰  considered the internal conical design and the use of 1-piece abutment as the main factors influencing the amount and distribution of the stress in implant systems. In the conical angled connection, the locking mechanisms were also reported to protect the abutment thread from excessive functional loads in this study, suggesting that a stable attachment in the conical design can be achieved by increasing the frictional forces.

Akça et al³¹  observed that the maximum stresses were found in the neck of the implant and the implant-abutment connection interface. In addition, the stress yield in vertical loading has been shown to accumulate on the conical surface of the abutment, whereas those produced during oblique loading concentrated at the neck of the implant and the abutment screw. They also found that the stresses on the screw surface and the implant neck decreased when the conical abutment was used. Maeda et al,³²  who compared the stress distribution of the internal and external hexagonal connection, concluded that the stress on the screw in the external hexagonal connection system accumulated in one spot with horizontal loads, although it was found to distribute in its surface. Besides, higher levels of stress in the neck region were observed in the external hexagonal connection system when compared with the internal hexagonal connection. In our models, we observed a similar stress distribution with the conical abutment. In addition to this, our findings had shown that the stresses became minimal in the conical abutment with the internal hexagonal connection system.

In conclusion, our findings indicated that the stress distribution at the level of the implant abutment connection is strongly associated with the design characteristics of the interface, which may vary according to the manufacturer. Since long-term successful clinical results depend on the decreasing magnitude of stress distribution in the bone surrounding the dental implants, the conical + internal hexagonal and screw-in implant abutment connection designs provide more biomechanically suitable prosthetic options than other systems, particularly in cases with increased vertical dimension in the posterior regions.

FEM

finite element method