Biomechanical Characteristics Of Micro-implant For Anchorage In Orthodontics – A Three-dimensional FEM Study

INTRODUCTION

In orthodontic treatment, anchorage control is essential for success¹. If there is an imbalance of force over counter force, unwanted tooth movement occurs. A recent development, stationary anchorage (micro-implants) eliminates one of the uncertainties of orthodontic tooth movement by offering absolute control over potentially undesirable counter movements². A key factor for the success or failure of a dental implant is the manner in which stresses are transferred to the surrounding bone¹. Finite element analysis (FEA) allows us to predict stress distribution in the contact area of the implants with cortical bone and around the apex of the implants in trabecular bone³.

One of the main uses of implants is to retract anteriors in maximum anchorage cases. Implants are usually placed at 30º to 60º angulations to the long axis of the teeth in maxillary arch between maxillary 1^st molar and maxillary 2^nd premolar for en-mass retraction. This will improve retention while reducing the risk of striking a root⁴.

The objective of the study was to establish a 3-D finite element model for micro-implant and to analyze the influence of different angulations to the long axis of the teeth (30º to 90º) on the biomechanical characteristics of orthodontic anchorage implant-bone interface.

MATERIALS AND METHODS

A 3-dimensional, orthogonal, Cartesian model was generated by using the NISA-II of 64,000 nodes software system. The model was composed of an orthodontic implant with standard dimensions; the implant simulated the position in between upper first permanent and second premolar of average size, with periodontal ligament (PDL) and cortical and cancellous bone. The model was divided into 14,953 nodes and 34,109 elements: all of them were tetra 4-R type, a solid element with 4 faces and 4 nodes. Each node had 6 degrees of freedom—3 rotations and 3 translations. Homogeneous, isotropic, and linearly elastic behavior was assumed for all the materials. Boundary conditions were located at the floor of the nasal cavity and maxillary sinus and at the lateral surface of the maxillary bone.

The implants were simulated in four different angulations of 30⁰, 45⁰, 60⁰ and 90⁰ to the long axis of the maxillary first molar. A simulated orthodontic force, which was 200gms was loaded mesio-distal to the mathematical model. The stresses and deflections were determined at different mesio-distal and occluso-gingival levels. In addition, specific stress values were evaluated in the different nodes of the root surface, PDL, cortical bone, and implant from the apical zone to the cervical margin. The stress and displacement on the implant – bone interface was analyzed.

RESULTS

The behavior of the evaluated structures under single force was linearly or directly proportional to the applied load. Von Mises stresses in the evaluated loads showed the highest stress on the implant and the cortical bone at the cervical third. The lowest stress appeared at the apical third of the implant.

Von Mises stress values at implant-bone interface are show in Table 1, Graph 1 and Fig 1. Displacement values at implant-bone interface are show in Table 2, Graph 2 and Fig 2. The largest stress and deformation was seen in cortical bone and upper region of trabecular bone. Stress and deformation increased as the angulations of the implant to the long axis of the tooth increased.

DISCUSSION

One of the remodeling theories argues that local mechanical signals stimulate or induce regulating cells to trigger remodeling events^5. This implies the importance of studying stress levels from orthodontic forces in the root surface, PDL, and cortical bone and those that occur in the anchor unit like bones, or, as in this case, in an Osseo-integrated implant.⁶

The aim of the present study was to investigate the deformation of the bone surface around an implant in response to force application in different angulations of implant placement. Abnormally high stress concentration in the supporting tissue can result in pressure necrosis and subsequently in implant failure.

FEM models were used to evaluate the load transfer from the mini-screw to the surrounding bone. The primary component of the load transfer takes place at a single revolution of the mini-screw thread with in the cortex. Under the assumed loading condition, the mini-screw is displaced in a tipping mode, causing tensile stress in the direction of the force⁷. In general stress levels are higher in the cortical bone than in the trabecular bone. The thickness of the cortical bone determines the overall load transfer from the mini-screw to bone and stiffness of the trabecular bone plays only a minor role.⁸ The cortical surfaces of the maxilla are thinner and less compact than those of the mandible.³ The finite element method was adapted largely to clinical conditions by selecting parameters such as implant and bone shape, stress, and its angulations. By applying the finite element method, the influence of different angulations to the long axis of the maxillary first molar was observed.⁹

During tooth movement, nonlinear elastic, plastic, and viscoelastic phenomena can occur. In this article, only linear elastic behaviors of the tooth- periodontal structures are considered.¹⁰ Therefore, in the future, additional modelling may be needed along with nonlinear elastic analysis. However, the model does provide quantitative results of the complex 3-dimensional stresses caused by mesiodistal forces during orthodontic treatment. The model allows areas of high stress to be accurately located, and these are considered to be relevant in understanding the long-term behavior of orthodontic tooth movement

CONCLUSION

          Micro-implant can be safely loaded with 200gms of mesio-distal orthodontic force.

          As the angulations of the implant to the long axis of maxillary first molar increased stress and deformation was recorded.

          Maximum stress and displacement was recorded when implant was placed perpendicular to the long axis of maxillary first molar.