Background: Loading of the compromised periodontium with orthodontic forces produces different results than those achieved in patients with healthy periodontal support. Determining the force value at a level preventing further deterioration of the patient's periodontal status, thus delivering the most precisely individualized "dose" of loading, seems to be crucial for the successful intrusion of teeth with reduced periodontal support.
Objectives: The aim of the study was to determine the range of force values efficiently intruding maxillary incisors without further compromising the initially-impaired periodontal status. Finite element analysis (FEA), allowing estimation of the stress and strain distribution, was the method of choice.
Material and methods: The CT scans of a periodontally-compromised patient were segmented using InVesalius software. A model - based on NURBS surfaces - was adjusted to the CT scans in order to obtain both smooth and natural curvatures of each model segment. All relevant tissues were modeled as separate volumes. The geometric model was discretized in order to create a numerical model for applying Ansys software (v. 15.07) and using APDL. The central incisors were loaded with external intrusive forces, ranging from 0.1 to 0.4 N.
Results: The simulation, performed iteratively, showed that even the lowest force value - 0.1 N - causes stress changes in the alveolus and on the root surfaces, with a tendency of stress increasing towards the bottom of the alveolus and root apex. It is also notable that during the application of forces of equal magnitude, the stress/strain distribution was significantly higher around tooth 21, which displayed the highest range of PDL reduction. Application of the same force level created a higher stress-strain response around tooth 21, and the characteristics were less homogenous.
Conclusions: A force value of 0.1 N applied in vivo might produce the most effective tooth intrusion and bone modeling which favors bone defect regeneration.
Keywords: FEM; PDL; bone defect; intrusion; orthodontic forces.