How Age Impacts Lower Arch Changes After Orthodontic Treatment: A One-Year Study

Background: Post-orthodontic retention is crucial for stabilizing teeth, as shifts can occur after treatment, especially in younger patients with ongoing growth. Understanding age-related differences in lower arch stability is important for improving retention strategies and reducing relapse rates. This study aimed to compare changes in the lower arch within 12 months after orthodontic treatment across different age group patients using a standardized fixed retainer (FR). Methods and Results: A longitudinal study analyzed 30 dental cast models from patients treated for mild crowding without extractions, all of whom had an FR bonded to the lower arch. Cast models were collected one day after debonding and again one year later. Measurements included the little irregularity index (LII), arch perimeter (AP), premolar width (PW), and intercanine distance (IcD). The sample comprised 15 growing patients of 14–16 years (Group 1) and 15 adults of 19–25 years (Group 2). In the selected cast models, the male-to-female ratio was 0.58:1. Female cast models were significantly more represented than male models, with a difference of 26.6% (P=0.041). Twelve months after treatment and placement of an FR, the mean increase in LII was 0.18±0.24. Group 1 showed a greater increase in LII (0.26±0.26; Z=2.074; P=0.038) and significantly higher AP (Z=3.837; P=0.0001) and PW (Z=4.106; P=0.0001) than Group 2. No significant difference was found for IcD between Groups 1 and 2 (Z=4.106; P=0.967). A significant decrease in LII, AP, and PW was associated with increasing age (R=-0.421 [p=0.021], R=-0.618 [P=0.0003] and R=-0.657 [P=0.00008], respectively). No significant correlation was found between age and IcD. Conclusion: Growing patients experienced notable changes in the lower arch during the first post-treatment year, emphasizing the need for tailored retention strategies. While IcD remained stable during the 12-month follow-up, the findings raise questions about the long-term effectiveness of non-extraction methods in preserving mandibular arch widths, indicating that changes may predominantly affect the posterior arch. The study results allow for improved retention strategies and reduced relapse rates, improving long-term outcomes.