Clinical crown height changes in mandibular anterior teeth retained with two types of fixed retainers over two years: findings from a randomized clinical trial

This study aimed to assess longitudinal changes in lingual clinical crown height (CCH) in patients treated with two types of fixed mandibular retainers bonded to all six anterior teeth. This is a secondary analysis of a randomized, parallel-group clinical trial conducted in a single center, including 133 orthodontic patients (median age 24.6 years; interquartile range: 17.2-32.4 years). Participants were allocated in a 1:1 ratio to receive either a three-strand round twisted (RT) wire or an eight-strand rectangular braided (RB) wire retainer. The primary outcome in this analysis was the change in CCH, defined as the vertical distance from the incisal edge (or canine cusp tip) to the most apical point of the gingival margin on the lingual surface. Measurements were obtained from dental casts at baseline (T0), at retainer placement (T1), and at 3, 6, 12, 18, and 24 months thereafter. Measurement reproducibility was high, with intra- and inter-examiner agreement exceeding 0.89 (ICC). Across the observation period, CCH values showed minimal variation, with differences between consecutive time points remaining below 0.3 mm. Regression analysis indicated that retainer design was not a significant predictor of CCH changes. No adverse effects were observed during follow-up. Within the limits of this study, fixed retention bonded to all mandibular anterior teeth does not appear to influence lingual clinical crown height over a 24-month period.Trial registration: The trial was registered in ClinicalTrials.gov (NCT07476950).