Patient-specific three-dimensional finite-element analysis of miniscrew implants during orthodontic treatment

The objective of this study is to demonstrate the potential of 3D modeling and finite-element analysis (FEA) as clinical, patient-specific, pre-operative planning tools for orthodontic treatment using a temporary skeletal anchorage device (TSAD). Anatomically accurate 3D models reconstructed from cone-beam computed tomography (CBCT) scans were used to simulate clinically relevant loads on both an inserted miniscrew implant and a single canine with PDL extracted from the same patient-specific mandible. Forces of 100-200 cN were used to retract the mandibular canine using the inserted miniscrew implant as temporary anchorage. Detailed stress distributions in the implant and peri-implant bone were investigated. In addition, the effect of orthodontic bracket hook length and angle of force application on the resulting stress response in the periodontal ligament (PDL) were determined. The equivalent von Mises stress on the miniscrew under a 200-cN tangential load reached 42 MPa at the first thread recession, whereas von Mises stress in the bone only reached 11 MPa below the neck. The majority of bone and miniscrew stresses occurred in the first 2 threads at the most slender cross-sections and in the top 2.5 mm of cortical bone. The tension side of the miniscrew, and the compressive side of the peri-implant bone experienced the highest level of stress. High tightening loads of 200 N˙mm torsion and 460 cN axial compression resulted in much greater bone and implant von Mises stresses than tangential loading, exceeding 17 GPa and 1.3 GPa in the implant and cortical bone, respectively. In the canine-PDL simulation, increasing hook length effectively reduced the PDL stress from 80 kPa with no hook to 22 kPa using a 7-mm-long hook. Angulating the force apically downward from 0° to 30° had a less significant effect on the PDL stress profile and initial canine deflection. Results suggest that stress on miniscrew implants under load is sensitive to changes in diameter and loading of force on miniscrews. Overtightening a miniscrew after insertion may be a more likely cause of fracture failure and bone trauma than application of orthodontics. In addition, reduction of stress within the PDL as a result of increasing hook length may account for steadier tooth translation by force application closer to the canine's center of resistance. Finally, the relatively minor effect of force angulation on PDL response suggests that the vertical placement of miniscrews in keratinized or non-keratinized tissue may not significantly affect tooth movement.