Evaluation of Orthodontic Mini-implant-anchored en masse Retraction of Maxillary Anterior Teeth with Reduced Bone Support: A Prospective Finite Element Analysis Study

Aim:The purpose of this study was to evaluate force systems to bring about the en masse retraction of maxillary anterior teeth having reduced bone levels using finite element analysis.Materials and methods: This is a prospective study.Three-dimensional finite element models of maxillary dentition having normal alveolar bone level and 2, 4, and 6 mm bone loss with first premolar extraction were constructed from a spiral CT scan of a skull.Archwire and brackets were modeled on the facial surfaces of teeth.Retraction force of 175 gm was applied from an orthodontic mini-implant placed bilaterally between the second premolar and first molar and 12 mm above plane of the archwire to anterior retraction hook (ARH) fixed at two heights of 6 and 10 mm above the archwire.Results: Maximum displacement and periodontal ligament (PDL) stress were calculated for different combinations of bone levels and ARH.As the bone loss increased, the tipping tendency, amount of intrusion, and maximum von Mises stress in PDL also increased, showing a direct correlation. Conclusion:To minimize tipping and PDL stress, the height of ARH should be increased in alveolar bone loss conditions to allow retraction force to pass through or even above the center of resistance of anterior teeth.Even then, pure bodily retraction may not be achieved, but tipping tendency can be reduced.Nevertheless, it may not be suitable to increase ARH beyond a limit owing to chances of irritation to the vestibular mucosa.Alternative methods should be contemplated to reduce the tipping behavior.Clinical significance: The alternative is to apply a lighter retraction force to reduce lingual tipping.A higher counter-moment in the archwire or bracket can also be incorporated.