Ultrasonographic analysis of the effects of fixed lingual retainer placed after orthodontic treatment on periodontal phenotype

Gingival phenotype is one of the key determinants of the biological resilience of periodontal tissues and their adaptive capacity following orthodontic treatment. In individuals with a thin phenotype, gingival recession and alveolar bone resorption are more frequently observed, underscoring the clinical importance of evaluating the long-term effects of fixed retention appliances. This study included 53 individuals who applied to the Department of Periodontology, Faculty of Dentistry, Selçuk University, had completed orthodontic treatment, and were provided with fixed lingual retainers. A total of 636 anterior teeth were evaluated. Sagittal inclination changes were assessed on lateral cephalometric radiographs. According to the baseline maxillary incisor–Sella-Nasion angle (Mx1–SN) and mandibular incisor–Gonion-Gnathion angle (IMPA), subjects showing an increase were classified as the proclination group, whereas those showing a decrease were classified as the retroclination group. The study population was divided into four subgroups: maxillary retroclination (U-Ret, n=150), maxillary proclination (U-Pro, n=168), mandibular retroclination (L-Ret, n=138), and mandibular proclination (L-Pro, n=180). Measurements were obtained at baseline (T0), 3 months (T1), and 6 months (T2). The gingival phenotype was determined using a color-coded phenotype probe set and the probe transparency method. Hard- and soft-tissue measurements were performed using a high-frequency (18 MHz) intraoral ultrasonography device (Versana Active, GE Healthcare) with high spatial resolution. Significant changes were observed in both hard and soft tissue parameters in the time-dependent (T0–T1–T2) and intergroup (U-Ret, U-Pro, L-Ret, L-Pro) comparisons. In the maxillary groups, probing depth and keratinized gingival height values were significantly higher than in the mandibular groups (p < 0.001). The buccal bone thickness (BKK), which was measured 1 mm apical to the marginal bone level, was highest in the U-Ret group and lowest in the L-Pro group. The marginal bone level (MBL), defined as the distance between the cemento-enamel junction and the marginal bone crest, was lowest in the U-Pro group at the sixth month. For all soft tissue parameters, GT1 to GT3 represented gingival thickness measured 1, 2, and 3 mm apical to the marginal bone respectively, and GT4 represented gingival thickness measured 1 mm apical to the marginal bone level overlying the bone (BKK). The highest values were recorded in the U-Ret group, while the L-Pro and L-Ret groups showed the lowest mean values (p < 0.001). In the U-Pro and L-Pro groups, all parameters except MBL showed a time-dependent decrease (p < 0.05), whereas BKK, GT1, and GT2 increased in the L-Ret group (p < 0.05).In the subgroup analysis, the U-Pro group showed the greatest soft tissue thickness in the central incisors and the lowest in the canines. In the U-Ret group, the highest GT4 values were found in the central incisors, whereas the highest mucosal level, which represents the distance between the marginal gingiva and the marginal bone, was observed in the canines. According to the ROC analysis, the threshold values discriminating the thin phenotype were determined as 0.495 mm for BKK, 0.715 mm for GT1, and 3.895 mm for mucosal level (SDY). Based on these findings, ultrasonography can be considered an effective and reliable method for monitoring periodontal tissues in the post-orthodontic period. The long-term biological effects of fixed retention appliances should be evaluated from a phenotype-based perspective. In individuals with a thin gingival phenotype, the use of fixed retainers should be carefully planned and continuously monitored through regular clinical follow-ups.