Skeletal, dental, periodontal, and upper airway changes after Hyrax rapid maxillary expansion with different premolar anchorage: a retrospective CBCT study

Abstract Background To compare skeletal, dental, periodontal, and upper airway changes after tooth-borne Hyrax rapid maxillary expansion (RME) with first-premolar/first-molar anchorage (group 46) or second-premolar/first-molar anchorage (group 56), and to explore associations among dentoskeletal changes. Methods This single-center retrospective study included 52 patients aged 12–15 years treated at Lanzhou University Stomatological Hospital between 2021 and 2025 (26 per group). Cone-beam computed tomography (CBCT) scans obtained before expansion (T0) and after 3–6 months of retention (T1) were used to assess skeletal, dental, periodontal, and upper airway measures. Paired tests assessed within-group changes, and independent-samples tests compared change scores between groups. Correlation and exploratory multivariable linear regression analyses assessed associations among transverse skeletal and dentoalveolar changes. Results Both groups showed increases in maxillary basal width and midpalatal suture opening (all P < 0.05). Group 56 showed greater basal expansion (2.01 vs 1.48 mm, P = 0.007) and anterior, middle, and posterior suture opening (0.89 vs 0.45, 1.02 vs 0.47, and 0.78 vs 0.54 mm, respectively; all P < 0.05). Most maxillary dental expansion measures did not differ significantly between groups, except for greater second-premolar interapical expansion in group 56 (P = 0.01). Second-premolar buccal bone thinning was greater in group 56, whereas palatal bone thickening at some first-premolar sites was greater in group 46 (P < 0.05). Oropharyngeal and nasopharyngeal volumes increased in group 46 (0.99 and 0.68 cm³; P = 0.02 and 0.004, respectively), but airway changes did not differ significantly between groups. Correlation and exploratory multivariable regression analyses identified positive associations among basal, alveolar, and sutural changes. Conclusions Both anchorage configurations produced marked transverse maxillary expansion. In this sample, group 56 showed greater maxillary basal expansion and midpalatal suture opening, whereas overall dental expansion was similar to that in group 46. Periodontal remodeling occurred predominantly around the respective anchor teeth. Changes in airway volumes and minimum cross-sectional areas did not differ significantly between groups. Correlation and exploratory multivariable regression analyses identified positive associations among maxillary basal expansion, alveolar expansion, and sutural opening.