Clinical evaluation of center of resistance of maxillary anterior teeth during en masse retraction with mini implant anchorage

Objectives: To clinically evaluate the center of resistance of maxillary anterior teeth and check the validity and applicability of power arm of proposed length for bodily translation of maxillary anterior teeth during en masse retraction using mini-implants as anchorage.
Materials and Methods:: Thirty patients with Angles Class I or Class II malocclusion, who were undergoing standard preadjusted edgewise technique (MBT prescription, 3M Unitek), including extraction of maxillary first premolars were selected and retraction forces were applied through power arm of length 5 mm soldered to the arch wire between lateral incisor and canine with mini implants placed between maxillary first molar and second premolar as anchorage, assuming that the force will act through the Center of resistance (CRe). Pre retraction and post retraction lateral cephalograms of each patient were taken, traced, superimposed and treatment outcomes, tooth movements and efficacy of mini implants as anchorage were evaluated. Paired and unpaired-t-tests were used for the data analysis.
Results: Translation was not seen in all the cases. Among 30 subjects, only 7 showed bodily movement, 11 showed controlled tipping and 12 showed uncontrolled tipping with retraction. Some amount of intrusion and distal tipping of maxillary first molars was observed with use of mini implants as anchorage units.
Conclusions: Location of CRe of consolidated unit of maxillary anterior teeth varies amongindividuals and depends on number of factors such as alveolar bone support, root morphology and tooth inclinations. Until strong conclusive clinical evidence is put forth, generalization of center of resistance location and assigning standard retraction hook dimensions to produce bodily movement of consolidated unit of maxillary anterior teeth is not advised.