Orthodontic loading influence on root resorption susceptibility: Finite element analysis

Abstract

Purpose: (1) To compare the strains and the stresses values within the periodontal ligament (PDL) following the application of three types of loading regimes: pure intrusion (PI), buccal tipping (BT), and intrusion combined with buccal tipping (IT). (2) To verify the correlation between these results and the available clinical findings regarding root resorption (RR) occurrence. Methods and materials: A tridimensional finite element (FE) model was constructed with axisymmetric geometry to simulate the tooth root. The model includes root, cementum, PDL, cortical and trabecular bone. All materials were considered linear elastic, except the PDL that was modeled as a nonlinear hypo-elastic material. Based on the literature that evaluates the influence of force magnitude on the RR occurrence, the loading magnitude was defined: PI simulated by applying 25-g or 225-g of axial loading; BT by applying the same force magnitudes 90◦ to the long axis; IT combining PI and BT. The displacement of the root within the PDL was calculated as percentage of the original thickness of the PDL to verify the amount of PDL deformation. Strains and stresses at the nodes at the PDL–cementum interface at cervical, middle and apical thirds were also assessed. Results: For the PI movement, the root displaced vertically within the PDL uniformly (9% of the PDL total thickness for 25-g and 36% for 225-g). For the PI and BT, the maximum root displacement occurred at the cervical buccal third (8% and 13% for 25-g, and 33% and 57% for 225-g; respectively). Higher forces generated higher values of strains and stresses within the PDL. For the PI movement, the apical and middle thirds showed higher values of strains than the cervical third. For the BT, the apical and cervical thirds showed higher values of strains than the middle third. For the IT, the cervical and middle thirds at the buccal side showed the highest values of strain. The maximum stresses were seen at the cervical third for the PI, and at the cervical and buccal lingual sides for the BT and IT (tensile sides). Conclusion: As expected, higher orthodontic forces generated higher values of strains and stresseswithin the PDL. From clinical findings, it is possible to verify that the RR occurred mostly in areas under compression. Therefore, our results support that RR might be related with changes of the blood supply associated with the constriction of the PDL at the compression sides.