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

OBJECTIVE: 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. MATERIAL AND METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.