The minimal important difference in orthodontic treatment duration: a survey across adult patients

The minimal important difference (MID) is defined as the smallest difference that the patient perceives as important. Furthermore, the smallest worthwhile effect (SWE) is the important change measured with the benefit-harm trade-off method. The aim of this study was to evaluate the MID in orthodontic treatment duration to inform the decision regarding seeking procedures to accelerate orthodontic tooth movement and reduce treatment duration.

We constructed a survey eliciting views of the MID from adult participants from four countries undergoing orthodontic treatment. Ten questions addressed reduction in the treatment duration for both durations 12 and 24 months, and four questions were related to the reduction in treatment duration that the patients would require to undergo surgical or non-surgical adjunctive procedures. We applied a univariable random effects logistic regression model to examine the association between the participants' characteristics and the MID. Then, we fitted a multivariable logistic random effects regression including significant predictors.

Four hundred and fifty adults, with a median age of 21 (interquartile range: 19-24), undergoing orthodontic treatment participated in the survey. Of the respondents, 60% considered 15 days as a trivial reduction from 12 months duration of therapy and 70% considered 15 days a trivial reduction from 24 months. Of the respondents, 48% considered the period of 2 months a moderate reduction from 12 months, and 60% considered 2 months a moderate reduction from 24 months. From these results, we inferred that patients considered reductions of approximately 1 month as the MID in the treatment duration for both 12 and 24 months. However, SWE was considerably more than the MID for most of the participants to decide undergoing surgical adjunctive procedures to reduce the time of therapy. The participants required smaller SWE to undergo non-surgical procedures compared to surgical procedures.

The MID in the treatment duration is one month for both treatment durations 12 and 24 months. Patients require a greater SWE than the MID to undergo adjunctive procedures to shorten the duration, particularly for surgical procedures.