3-D Finite Element Stress Analysis And Experimental Research Of Micro-implant Anchorage In Orthodontics

The micro-implant anchorage (MIA), as absolute anchorage implanted in the maxilla or mandible, has already widely been applied to the oral orthodontic treatment. It is a special screw made in pure titanium with good biocompatibility. The screws are inserted into different sites of jawbone to resist the counterforce produced by orthodontic appliances. With small size, convenient practice and absolute anchorage effect, the micro-implant has been the development in the orthodontic treatment.The influences of the shape, diameter, length of MIA on the stress distribution on the interface between the implant and jawbone were studied by the finite element method (FEM). But most biomechanical studies were based on the assumption of full osseointegration without the interfacial pre-stress, an important character of interface between implant and jawbone by early loading. In this paper, apart from biomechanical experiment, the finite element stress analysis on MIA and the jawbone in the cases of osseointegration and immediate loading, as well as the related stress comparison and the parameter analysis were performed.In the experimental part, the maximal forces to pull out the MIA from the bone in the landscape orientation were obtained by the immediate loading experiment with the MIA implanted in the rabbit mandible. These forces were much larger than the orthodontic force commonly used in the clinic. The results indicate that the orthodontic force used in the clinic is extremely safe in the initial period of immediate loading.Finite element models with different thichnesses of cortical bone including MIA were developed in two cases of osseointegration and immediate loading. The results indicated that the thickness of cortical bone made no difference to the stress distributions on the contact surface in the case of osseointegration. And in the case of immediate loading, the thickness of cortical bone had influence on the stress distributions on the contact surface. By the increase of the thickness of cortical bone, the stress distributes of cortical bone became more evenly and the stress values became smaller.Then a mandible model with real geometry shape was developed. In the model, different friction coefficients were considered (from 0 to 0.5, in 0.1 increments). The result showed that the highest Von Mises stress on the interface of the MIA and the mandible was decreased with the increase of the friction coefficient. But the stresses changed slightly with the friction coefficient increased to a certain degree. Therefore the stress decreased with the increase of friction coefficient in the case of immediate loading, but the way of increasing friction coefficient to reduce the interfacial stress is inadvisable in practice. In the same model, different interferences were considered. When the interference was 0.01mm, the maximum stress of bone was equal to the ultimate intensity. So the maximum interference was 0.01mm in this model.Finally, the stresses on the contact surface were compared by changing the taper of MIA. The interfacial stress of the micro-implant without taper was observed to be the highest among those implants. And with the increase of taper, the interface stresses near the neck of MIA showed a trend of decrease, but stress above the junction of cortical and cancellous bone increased. So in the case of immediate loading, it's suggested that a micro-implant with appropriate taper should be chosen.