Deflection and stress distribution around mini-screw implants: A finite element investigation into the effect of cortical bone thickness, force magnitude and direction

Abstract

The purpose of this investigation was to quantitatively evaluate mini-screw deflection and stress distribution associated with two different cortical bone thicknesses (CBTs) under loading with two applied forces at four different angles. Four finite element models (ANSYS 11) of bone with mini-screws (Dentos) inserted at 90° angulation to the cortical surface were made as follows: model A: CBT 1·5 mm, load 200 g; model B: CBT 2 mm, load 200 g; model C: CBT 1·5 mm, load 300 g; model D: CBT 2 mm, load 300 g. In each model, the loading force was applied at angulations of 70, 90, 110 and 130° to the long axis of the mini-screw. The elastic modulus of the cortical bone, cancellous bone and mini-screw were kept at 15, 1·5 and 114 GPa, respectively. The maximum equivalent stress (MES) distribution and maximum deflection (MD) at the mini-screw to bone interface was evaluated in the four models at each angulation. In each model, MES and MD of the mini-screw decreased when the angulation of loading increased from 70 (MES = 2·81 MPa) to 130° (MES = 1·92 MPa). Mean MES with model A = 2·21 MPa, model B = 1·83 MPa, model C = 3·06 MPa and model D = 2·67 MPa. The MES and MD both decrease as the angulation of mini-screw loading increases, with increasing load differences occur in MD but the MES remains similar, whilst differences were observed in both MES and MD as CBT increased.