Influence Of Different Shoulder Of Platform Switching Configuration On Stress Distribution In The Implant-bone Interface

The resulting crestal bone levels around dental implants following restoration has been used as a reference for evaluating long-term implant success, and remains the principal determinant of the esthetic result. However, crestal bone resorption close to the first thread of osseointegrated implants is frequently observed during initial loading. In recent years, the concept of"platform switching"has been introduced which refers to the use of smaller-diameter abutment on a larger-diameter implant platform. Some clinical and radiographic observations have indicated that platform switching can limit crestal resorption and preserve peri-implant bone level. Currently, some foreign scholars have performed the biomechanical analysis on platform switching, but no study has evaluated the influence of different shoulder of platform switching configuration on stress distribution in the implant-bone interface. The optimal design of shoulder width and angle is not clear.According to the experimental design, the three-dimensional finite element models were created based on the physical properties of the maxilla anterior part, implant and prosthesis. Experimental group was divided into two parts. Group A: 3.5×11mm implants with shoulder widths of 0.25mm, 0.5mm, 0.75mm and shoulder angles of 0°, 15°, 30°, and 60°; Group B: 4.5×11mm implants with shoulder widths of 0.75mm, 1.0mm, 1.25mm and shoulder angles of 0°, 15°, 30°, and 60°. Control group were 3.5×11mm and 4.5×11mm implants without shoulder. The alveolar bone, prosthesis attachment, implant and abutment material properties were considered as homogeneous, isotropic and linearly-elastic in the analyses. Implant-to-bone contact was assumed to be 100%, indicating complete osseous integration. After the model had been meshed, two types of 100N loads (axial direction and 30°inclined to the long axis of the implant) were applied on the abutments supporting single tooth restorations. Values of von Mises stress at the implant-bone interface were computed using the finite element analysis for all variations. Evaluated stress distribution characteristics in the implant-bone interface to determine the optimal shoulder design, and provide instruction for clinical application.The results showed that:In general, maximum stress areas were located around the implant neck, and the highest von Mises stresses were found on the buccal face of the cortical bone for all models with both axial and oblique loads. A decrease in the von Mises stresses from the cervical region to the apex was also seen in the cortical alveolar bone crest. In the trabecular bone, the von Mises stresses were much lower than in the cortical bone, and similar in both experimental and control groups. Higher stress were found in the apical region of the trabecular bone under axial load, and in the cervical region of the trabecular bone under oblique load. The maximum von Mises stresses in both cortical and trabecular bone were found to be higher under oblique load than under axial load for all models.Implants with small shoulder angles provided for more favorable stress distributions compared to the control groups, and the stress concentration were more and more obvious with the increasing of shoulder angle. The maximum von Mises stresses in implant-bone interface were found to be lower for the 0°and 15°models compared to the control groups, and decreased in inverse proportion to an increase in shoulder width. The values of maximum von Mises stresses were higher for part of the 30°and 60°models compared to the control groups, and increased with an increasing shoulder width for 60°models. The stresses were found to be lowest for the models with smallest shoulder angle and largest shoulder width in both group A and B. Otherwise, the stress around the implant with a diameter of 4.5mm was smaller than 3.5mm.According to the results above, we can get conclusion as follow:1.The maximum stress areas were located in the cervical cortical bone, which may explain the crestal bone loss during initial loading.2.The maximum stress was higher under oblique load than under axial load for the same model. It is neccessery to design a good placement for implant and superstructure to minimize oblique load.3.The increase in the implant diameter decreased the maximum von Mises stress in implant-bone interface.4.The shoulder width and angle of platform switching configuration could have significant effects on the stress distribution in implant-bone interface. Implants with smaller shoulder angles and larger shoulder width provided for more favorable stress distributions.From a biomechanical perspective, the optimum choice was an implant with maximum possible diameter and platform switching configuration with 0 degree angle and large width shoulder.