Evaluation of stress pattern in periodontium and change in inclination during en masse retraction: Finite element analysis

Objective: Understanding the effects of retraction accompanied by intrusion on the periodontium of the anterior teeth along with the change in displacement during the en masse retraction of the maxillary anteriors using orthodontic mini-implants (OMIs) and retraction hooks at different levels. Design: Finite element analysis. Materials and method: The finite element model of maxillary dentition was made using a computed tomographic scan. The implants of 1.5 x 8.0-mm dimensions were inserted at an angle of 90 degrees at distance of 7 mm (low OMI) and10 mm (high OMI) with anterior retraction hooks (ARH) at 3, 6, and 9 mm. Six models were constructed for the study. After the boundary conditions were defined, 200 g of retractive force was applied from ARH at low and high OMI traction. The stresses on periodontal ligament (PDL), bone, bone surrounding implant, and cementum, along with change in inclination and displacements of anterior and posterior teeth along the y- and z-axis were determined. Results: An overall tipping and extrusion was seen in all the cases. Low OMI placement with retraction hooks at 3 mm and 6 mm showed more tipping movements, whereas high OMI with 9-mm ARH showed more bodily movement. Both high and low OMI with 9-mm ARH showed more root movement. The stresses in PDL, bone, and cementum were less at 3-mm ARH in both low and high OMI groups. The stresses were more in cortical bone than cancellous bone and also more on lateral incisors than central incisors. Conclusion: If controlled lingual tipping of anterior teeth is required with minimum torque loss, high OMI with low ARH is recommended, whereas if lingual root movements with retraction is required, high OMI and high ARH is recommended. (C) 2019 Published by Elsevier Inc. on behalf of World Federation of Orthodontists.