Numerical analyses of biomechanical behavior of various orthodontic anchorage implants

The quality and quantity of the alveolar process are considered important influential factors affecting the anchorage effectiveness of orthodontic mini-implants. The objective of this study was to establish the effect of various material parameters in regard to various implant types, sizes, and load directions using the finite element method (FEM).

FE models of the following 16 implants by six different manufacturers were made in idealized jaw bone segments with the program system MSC.Marc/Mentat: Aarhus Mini-Implant (American Orthodontics), AbsoAnchor (Dentos), Dual-Top (Jeil Medical), LOMAS (Mondeal), OrthoImplant (IMTEC), tomas (Dentaurum). The intra-osseous parts of the mini-implants had lengths ranging between 6.7 mm and 10.0 mm, and diameters between 1.2 mm and 2.0 mm. Cortical thicknesses of 1 mm and 2 mm were simulated. The Young's modulus of cancellous bone was varied between 100 MPa and 1 GPa, the load direction of 0 degrees to 45 degrees in a buccal direction for a load of 5 N. In each case we determined the deflection of the implant head as well as the distribution of stresses and strain in the cortical and cancellous bone.

Deflections of the implants varied between 2 microm (Aarhus Mini-Implant 11.6 mm x 2.0 mm, 2 mm cortex) and 20 microm (AbsoAnchor 12.5 mm x 1.2 mm, 1 mm cortex), the deflection was between 4 microm and 10 microm for most of the implants. The deflections of the implant increased as Young's modulus of the cancellous bone dropped with a cortical thickness of 1 mm. We did not observe such a correlation with a cortical thickness of 2 mm. We measured the highest loads in the bone in all models when the cortical thickness measured 1 mm and with a Young's modulus of cancellous bone of 100 MPa. When the load direction was tilted in a buccal direction, the stresses and amount of strain were reduced by as much as 35%.

We have demonstrated that the cortical thickness is a decisive parameter for the stability of these mini-implants. When the cortical bone is thinner, the mobility becomes increasingly dependent on the Young's modulus of the cancellous bone. Moreover, the greatest stress and amount of strain occur in the bone when the cortical bone is less thick and Young's modulus of cancellous bone lower.