Reliability of torque expression by the Invisalign ® appliance: A retrospective study

Introduction: Incisal torque in orthodontics has always been important due to its relationship with smile aesthetics and the stability of a final treatment outcome. The aim of this study was to quantify the accuracy of torque expression predicted by ClinCheck ® planning associated with Invisalign ® treatment, compared to clinical outcomes. Methodology: Forty adult patients consecutively treated using the Invisalign appliance were selected. Torque and the interincisal angle (IIA) were measured at T 0 (pretreatment), T 1 (predicted post-treatment), and R (end of initial aligner sequence) stages on STL files using metrology software (Geomagic Control X). Results: The difference between the predicted and achieved torque (torque differential) was statistically significant ( P < 0.001) in the labial crown torque group with a mean of 6.43° (SD = 7.09) for the upper central incisor, 5.06° (SD = 7.32) for the upper lateral incisor and 2.75° (SD = 5.7) for the lower incisor sub-groups. In contrast, the corresponding lingual/palatal crown torque sub-groups did not show a statistically significant torque differential ( P >0.05). Similar observations were made for the IIA differential (difference between predicted and achieved IIA) with a mean IIA differential of 9° (SD = 10.73; P < 0.001) and -3.4° (SD = 7.9; P > 0.05) in the labial and lingual/palatal crown torque IIA groups, respectively. Conclusions: Torque was under-expressed when the teeth were moving labially, and fully- or over-expressed when moved lingually, indicating Invisalign’s inefficiency in torqueing incisors in the labial direction.