To: The Editor, The Angle Orthodontist

Re: Efficiency of upper arch expansion with the Invisalign system. Ning Zhou; Jing Guo. Angle Orthod. 2020; 90: 23–30.

Thank you to the authors of this study evaluating the efficiency of upper arch expansion with aligners. The paper makes several useful recommendations for controlling torque as a preventive measure to avoid excessive tipping during expansion. However, in the protocol for the patients in the study, it was unclear what mechanics were specified to be included in the expansion prescriptions. It would be helpful if you could elaborate on the following details regarding the mechanics prescribed:

1. How was rotation of the molars considered in evaluating the amount of expansion needed for each patient. Were molars rotated as part of the expansion procedure and, if so, how was that accounted for in the measurements?

2. Regarding the transverse amount of expansion prescribed for each patient, were the teeth prescribed to translate transversely with the crown and root moving the same amount? Or, was tipping or reverse torque built into the prescription? Was the protocol the same for all patients in the study?

Thank you for taking the time to clarify and elaborate on these points from your study.