Stresses around a miniscrew. 3-D analysis with the finite element method (FEM)

Miniscrews used for absolute anchorage may induce stresses in the surrounding tissues that are dependent on their proximity to the miniscrew.

To determine the stresses in the buccal walls of the sockets of lower molars adjacent to a miniscrew under load when the position and angulation of the miniscrew are changed.

Five 3-D FEM models containing the first and second lower molars, their periodontal ligaments and the surrounding spongy and cortical bone, were modelled in SolidWorks 2006 (SolidWorks, Concord, MA, USA) and transferred to the ANSYS Workbench (ANSYS Inc., Southpointe, Canonsburg, PA, U.S.A.). A tensile force of 2 N, decomposed in 3-D space, was applied to a miniscrew inserted between the lower first and second molars. The von Mises (equivalent) stresses along the buccal walls of the sockets of the first and second molars were derived following changes in miniscrew position and angulation. No direct force was applied to the molars.

When the miniscrew was inserted at right angles to the bone and midway between the molars the stress in the crestal area was 0.093 MPa. This stress increased proportionally in the first molar socket as the miniscrew was moved towards the first molar and declined when the miniscrew was tipped towards the second molar. Stresses also decreased in the crestal area of the second molar as the miniscrew was moved towards the first molar, but increased when it was tipped towards the second molar. A 30-55 per cent increase in crestal stress in the first molar socket was detected.

Stress occurred in the tissues surrounding a miniscrew subjected to a force vector. Changes in the position or angulation of a miniscrew can affect the stress in the socket walls of adjacent teeth.