Letters From Our Readers

To: Editor, The Angle Orthodontist

Re: Orthodontic treatment stability predictors: A retrospective longitudinal study. The Angle Orthodontist . 2016. Online Early

We would like to thank Dr Yi for the interest in our research.

Regarding the first question about the association between extractions and instability of the midline, we would like to highlight some points:

When analyzing our database, we have observed that there were no statistically significant differences (chi square P = 0.591) in the midline deviation between the extraction and non-extraction cases at the beginning of the treatment.

Likewise, the percentage of patients whose midline showed an improvement after treatment was 64.6% in extraction cases, and 59.1% in non-extraction cases, showing again no significant differences (chi square P = 0.659).

The multivariate logistic regression model showed that extractions were predictors for midline instability (OR 4.76; 95% CI 1.05–21.6). However, we cannot clear up the reasons why extraction cases are less stable with regard to the midline, since the sample size was not big enough to divide the patients into subgroups with an adequate statistical power. Further investigations are needed in order to explain the specific causes for midline instability in extraction cases.

Therefore, according to our results, we consider that extractions could be a risk factor for long-term stability. Although the reasons for extracting teeth are diverse, it would be logical to assume that if extractions are performed to correct midline deviations, that deviation should improve substantially, however, special attention must be paid regarding the risk of relapse.

In respect to the second question about the pre-treatment Angle Class, the sample distribution was as follows: 35% Class I, 55% Class II and 10% Class III. The Angle Class at the beginning of treatment did not show association with instability (chi square P = 0.346), thus we conclude that the Angle Class variable does not play an important role in stability.

We hope we have answered your questions adequately.

Please do not hesitate to contact us if further concerns arise.