Effect of Model Parameters on Finite Element Analysis of Micromotions in Implant Dentistry

Micromotion may be defined as a phenomenon that occurs at the interface of mating components belonging to one mechanical system leading to the displacement of one component relative to the second one. As a prerequisite, a non–force-fit contact, allowing for sliding movements, has to be present between the single components. In implant dentistry, micromovement of components may occur at two interfaces, the implant-abutment connection^1,2  and the bone-implant interface.³  Numerous articles dealing with the effects of micromotion at both levels mentioned can be found, although a sound mechanical description is often lacking.

Micromotion at the implant-abutment level has been described to result in microgap formation with subsequent bacterial colonization, which may ultimately cause inflammatory reactions of the tissues surrounding the implant.⁴  From a technical point of view, loosening of the abutment screw⁵  may occur, which may subsequently cause a fracture of the prosthetic component.^6,7  Micromotion at this interface predominantly has to be considered as an engineering issue, which should primarily be addressed by the implant manufacturers.

Micromotion at the bone-implant interface may occur when a nonosseointegrated implant is loaded, such as in an immediate-loading situation.⁸  In such a situation, occlusal forces lead to a displacement of the implant relative to the bony implant socket. It is generally accepted that such micromovement occurring during the healing phase may lead to fibrous encapsulation of the implant if a threshold displacement of 50 to 150 μm is surpassed.^8–14  The risk of jeopardizing osseointegration has originally been minimized by applying a late loading protocol in implant-supported reconstructions.¹⁵  Because of a paradigm shift in the loading protocols applied toward early and immediate loading,⁸  micromotion at the bone-implant interface has gained increased recognition as a potential risk factor.^15,16 

Current limitations of experimental approaches for determining micromotion at the implant-bone interface are related to limited applicability in the dental field as well as insufficient sensitivity and accuracy of the sensors. In addition, having dental implants available for biomechanical testing at different stages of osseointegration is difficult.^11,17,18  Similar to other mechanical problems in implant dentistry, finite element analysis (FEA) may be considered as a useful tool for gaining deeper understanding also in the field of micromotion.^9,10,19  The purpose of this FEA was to study the effect of macroscopic implant-related parameters (direction of loading, implant geometry) and modeling parameters (stage of osseointegration, friction between implant and bone) on FE simulations of implant micromotions.

Three-dimensional FE models of dental implants with and without threads were generated (Figure 1a), which were subsequently embedded in a bony socket consisting of cortical and trabecular bone and an intermediate layer surrounding the implant (Figure 1b). Combining both components, three-dimensional FE models (Figure 1c) were obtained, which allowed for simulating different stages of osseointegration by altering the elastic modulus of the intermediate bone layer,^10,20,21  for studying alterations in contact type between implant and bone¹⁰  as well as for loading the implants both in an axial and horizontal direction. Table 1 provides an overview of all parameters studied with the models described.

Isotropic linear model parameters were applied, ie, it was assumed that bone is a homogenous material showing direction-independent mechanical properties. The contact type between the different layers of bone was defined as “bond,” not allowing these components to be separated by any means. A friction-free contact was simulated between implant and bone, which only transferred compressive forces and allowed for sliding as well as for gap formation. Alternatively, the contact type between implant and bone could be changed to a friction contact and a force-fit contact, transferring loads from all aspects of the implant to bone. Augmented lagrange was applied for defining all contacts (ie, contacting components could not penetrate each other when loads were applied). Based on the results of previous investigations indicating that the size of the models was sufficient for evaluating micromotion, model dimensions were reduced to a minimum, and the borders of the models were fixed, not allowing any movements in these areas.^20,21  Depending on model type, 160 000 hexaeder elements and 600 000 to 650 000 nodes were used to set up the models using the elastic moduli given in Table 2. For simplification, for all materials, a Poisson's ratio of 0.3 was used. The geometry of the models was generated with a CAD program (SolidWorks 2011, SolidWorks Deutschland GmbH, Haar, Germany) and imported in a FE program (ANSYS Workbench 12, ANSYS Inc, Canonsburg, PA).

Results of all simulations were recorded as von Mises equivalent stress (N/mm² or MPa) in addition to contour plots of global total displacement (U) calculated according to the formula U = (U_x² + U_y² + U_z²)^1/2, where x, y, and z are the global coordinates of the FE models.

Furthermore, the displacement of reference landmarks on the implant and on the bone was recorded (Figure 2). As the displacement of the reference landmark on the implant represents displacement of both bone and implant, the displacement of the landmark on the bone was subtracted for evaluating micromotion between implant and bone for cylindrical and threaded implants after osseointegration, for threaded immediate implants, as well as with and without simulating friction between implant and bone.

Simulating 200-N axial force acting on an osseointegrated cylindrical implant with no friction between implant and bone caused a symmetric loading situation of the bone surrounding the implant, with maximum loading occurring at the apical part of the implant (Figure 3). The addition of threads to a cylindrically shaped implant led to a decrease both in loading and displacement occurring at the apical part of the implant. Instead, greater distribution of loading and displacement was observed at the vertical walls of the implants (Figure 4). Reducing the elastic moduli of bone immediately surrounding the implant caused a substantial increase in displacement of the implant, but a more homogeneous load distribution at the thread tips was also noticed (Figure 5). Changing the contact type between implant and bone from friction free to force fit (rigid) led to a reduction of implant displacement of nearly 50% (Figure 6). In this situation, load transfer predominantly occurred in the cervical area of the implant, where cortical bone was modeled. The introduction of friction between bone and implant resulted in a more homogeneous distribution of loads as compared with the force-fit situation (Figure 7). Altering the direction of loading from pure axial loading to horizontal loading caused a change in loading patterns from a uniform loading surrounding the whole implant to localized loading at the cervical aspect of the implant (Figure 8). The simulation of both axial and horizontal loading still caused load concentrations at the neck of the implant in addition to minor loading of the whole implant circumference (Figure 9).

Calculating implant micromotion by subtracting displacement values for the bone and implant reference landmarks (Figure 10) confirmed the general trends observed by comparing the different FE models. Changing the contact type between implant and bone from friction free to friction led to a general decrease in implant micromotion by increasing the displacement of the bone landmark and decreasing the displacement of the implant landmark. The addition of threads to a cylindrically shaped implant also led to a decrease in implant micromotion. A considerable increase in displacement of the implant reference landmark and consequently in implant micromotion was seen when an immediate implant was modeled by reducing the elastic moduli of the bone immediately surrounding the implant.

In light of limited experimental capabilities, FEA has to be considered as an appropriate tool for evaluating micromovement phenomena in implant dentistry.^20,21  In this context, micromovement can be defined as relative displacement of a specific component within a mechanical system. As such, it appears that for an overall clinically relevant evaluation, the movement of both components (ie, bone and implant) have to be measured and taken into account. This is in contrast to Holst et al¹¹  and Goellner et al,²²  who solely used photogrammetric displacement measurements of implants for evaluating the effect of different types of provisional restorations on micromovements of implants.

In the study at hand, it could be shown that both implant- and model-related parameters have a substantial effect on the results of FE simulations on the micromotion of dental implants. Currently, numerous assumptions have to be made, particularly with respect to bone's material properties and contact type between implant and bone. Whereas homogeneous displacement and loading occurred in a friction-free situation, the introduction of a force fit (rigid contact) between implant and bone led to a reduction in implant displacement of about 50%, with force transfer predominantly occurring in the cervical area of the implant. These factors should be kept in mind when interpreting findings from FE studies in general.²⁰ 

For displaying force transfer, von Mises equivalent stress was used.^23,24  Although other stress states may also be used for describing mechanical systems, von Mises equivalent stress is most frequently applied in dental biomechanics and predicts yielding of materials under any loading condition, applying material parameters from uniaxial tensile tests. As a prerequisite, the materials considered have to show ductile behavior, which can be assumed for bone within certain limits.

Relative to the threshold values for micromovement reported in the literature,^8–14  the values described in the current FE study appear to be too low to constitute a potential risk for osseointegration, which may be seen as a potential limitation of this investigation. However, the basic findings of the study appear to correlate with clinical reports. In this context, increasing the elastic modulus of bone led to a reduction in micromovement, which is consistent with the findings reported by Trisi et al,¹⁶  who showed that in good-quality bone, the insertion torque of implants increases whereas micromovement decreases. A similar effect could be demonstrated by adding threads to a cylindrical implant. With the introduction of horizontal loading, stress concentrations in the cervical area occurred, which is consistent with the findings reported by Goellner et al²²  showing that the loading character had a significant influence on implant displacement in soft cancellous bone.

Within the limitations of this FEA, it can be concluded that both implant-related and bone-related factors determine the degree of micromotion of a dental implant during the healing phase. As excessive micromotion can cause problems in immediate-loading scenarios, a clinically applicable measurement device could provide guidance as to which loading protocol should be applied in a specific situation.

FEA

finite element analysis

This study has been generously supported by a grant from the American Academy of Implant Dentistry Research Foundation.