Occlusal overloading is one of the causes of peri-implant bone resorption, and many studies on stress distribution in the peri-implant bone by three-dimensional finite element analysis (3D FEA) have been performed. However, the FEA models previously reported were simplified and far from representing what occurs in clinical situations. In this study, 3D FEA was conducted with simulation of the complex structure of dental implants, and the influences of neck design and connections with an abutment on peri-implant bone stress and abutment micromovement were investigated.
Three types of two-piece implant CAD models were designed: external joint with a conical tapered neck (EJ), internal joint with a straight neck (IJ), and conical joint with a reverse conical neck (CJ). 3D FEA was performed with the setting of a “contact” condition at the component interface, and stress distribution in the peri-implant bone and abutment micromovement were analyzed.
The shear stress was concentrated on the mesiodistal side of the cortical bone for EJ. EJ had the largest amount of abutment micromovement. While the von Mises and shear stresses around the implant neck were concentrated on the labial bone for IJ, they were distributed on the mesiodistal side of the cortical bone for CJ. CJ had the least amount of abutment micromovement.
Implants with a conical joint with an abutment and reverse conical neck design may effectively control occlusal overloading on the labial bone and abutment micromovement.
In accordance with the increase in their success rate, dental implants have become a popular treatment option for missing teeth. One of the criteria for successful dental implant treatment is radiographic vertical peri-implant bone loss of <2.0 mm . Remodeling of the peri-implant bone occurs once the implant is exposed to the oral environment by a second surgical procedure or by the immediate placement of an abutment after implant surgery. Factors that negatively affect the remodeling process and result in marginal bone resorption include traumatic surgical technique , excessive loading conditions , microbial contamination of the microgap between an implant and an abutment , micromovement of an implant and abutment , and repeated screwing and unscrewing . Exclusion of one or more of these factors is important for successful implant treatment.
Osteoclastic bone resorption occurs in areas where microcracks are produced by occlusal dynamic stress and osteocyte apoptosis . Bone is resistant to compressive stress but is susceptible to tensile and shear stresses, and the threshold values of tensile and shear stresses that cause resorption of cortical bone are reportedly about 30% and 65% less than the threshold value of compressive stress, respectively . Therefore, reducing the shear stress caused by occlusal loading effectively controls peri-implant bone resorption . Considering the benefits of eliminating an excessive occlusal loading condition , much attention has been paid to biomechanical evaluation of implants. Finite element analysis (FEA) is useful for numerical stress analysis of multiple structured materials and has been widely used for the analysis of dental implants. However, because dental implants have a complex structure, most of the previous FEA studies utilized simplified simulation such as two-dimensional models or three-dimensional (3D) models of one-piece implants coupled with an abutment . While advantageous in terms of calculation costs, limited information on stress distribution can be provided by two-dimensional models. The 3D models are more informative and clinically relevant. However, one-piece models do not reflect the real structure of implants, and stress distribution analysis of each component of an implant is not possible.
In this study, 3D two-piece implant CAD models were created, and FEA was performed by a CAD software add-in. In addition, different implant body and abutment connection types were designed, including the reverse conical neck by which bone platform switching is achieved. The purpose of this study was to investigate the influences of the neck design of an implant body and types of implant–abutment joints on peri-implant bone stress by 3D FEA of two-piece structured implants. Under simulation of the complex structure of implants, the influences of these designs on stress in the abutment screw and abutment micromovement were also evaluated.
Materials and methods
3D CAD model
Three 3D models were created by CAD functions in computer-aided engineering software (SolidWorks Simulation 2011; SolidWorks Corporation, MA, USA) ( Fig. 1 A–C ). The diameter and length of the implant body, the shape of the neck, and the types of implant–abutment joints of the three models are shown in Table 1 . The pitch and shape of the threads were the same in all groups. Each abutment had the same shape and size with the exception of the type of connection with the implant body. The implant and abutment were connected with an abutment screw.
|Diameter and length of implant body (mm)||Shape of neck||Joint with an abutment||Code|
|4.5 × 13||Conical tapered||External||EJ|
|4.3 × 13||Straight||Internal||IJ|
|4.3 × 13||Reverse conical||Conical||CJ|
A CAD model of an anterior maxillary bone with a 1.5-mm-wide cortical bone was prepared, and each 3D implant model was placed ( Fig. 1 D). The mechanical properties of bone and titanium used for the finite element (FE) models are shown in Table 2 . For simulations of osseointegrated implants, a “fixed bond” condition was set at the interface of the bone and the implant. A “contact” condition, which accepts possible microscopic sliding, was set at the interfaces among the components of the implants. The bottom part of the maxillary bone and both sectional surfaces of the bone segment were fixed, and a static load of 100 N was applied to the basal ridge surface of the abutment 45° obliquely to the long axis of the implant ( Fig. 1 E). Elements for FEA were tetrahedrons. To determine the mesh size that offer an accurate result in a reasonable amount of computation time, the number of elements was increased until the maximum principal stress converged. The results of convergence analysis are shown in Table 3 and Fig. 2 . FEA was performed by SolidWorks Simulation SolidWorks 2011 add-in (SolidWorks Corporation).
|Component||Young’s modulus (MPa)||Poisson ratio|
Peri-implant bone stress
By FEA, distributions of the principal, shear, and von Mises stresses in the peri-implant bone were assessed. The value of the maximum principal stress was recorded.
Strain distribution in abutment screw
Distribution of the equivalent strain in the abutment screw was assessed by FEA. In the middle section of mesiodistal plane, the equivalent strain distribution was determined.
Micromovement of abutment
In the mesiodistal plane, distribution of the principal stress on the abutment in the x -axis direction and shear stress at the interface of the implant and the abutment in the y -axis direction were assessed. In this study, micromovement of the abutment was calculated as the ratio of the deviation component of volume by following equation:
Micromovement = V after − ( V before ∩ V after ) V before ∪ V after × 100 ( % )