Hard- and soft-tissue changes after adult Class II Division 1 orthodontic treatment: a retrospective CBCT-based observational study
Background Restoring facial balance is now central to adult orthodontic care, yet evidence describing hard-soft tissue coupling and short-term retention stability in non-growing adults with Angle Class II Division 1 malocclusion remains limited. Methods In this observational study, 236 consecutively treated adults (≥18 years) underwent clinically archived cone-beam computed tomography (CBCT) before treatment (T0) and within 2 weeks after debonding (T1). Ten hard- and five soft-tissue variables were measured on the T0 and T1 three-dimensional reconstructions and used for the treatment-change analysis. For the 12-month analysis (T2, n = 224) the T0 and T1 volumes were reformatted into orthogonally synthesized lateral cephalograms and re-measured, so that stability was assessed between two lateral cephalograms of the same geometrical type, the T2 record being the standardized digital lateral cephalogram taken at the routine retention review. Each T1–T2 change was judged against a variable-specific combined smallest detectable difference (SDD) derived from the method error of both cephalometric records, at group and at individual level. Paired tests with mean change, 95% CIs and Cohen's dz, correlations with multiplicity control, change-score correlations and multivariable linear regression evaluated change and hard–soft coupling. Results From T0 to T1, ANB, U1-NA and L1-NB decreased and the interincisal angle increased (all P < 0.001), while SNA and SNB were unchanged. The nasolabial and mentolabial angles and chin thickness increased, and both lips retracted toward the aesthetic plane (all P < 0.001). Between T1 and T2 no variable showed a mean change that reached its combined SDD, and 92.5% to 98.7% of the T0–T1 change was still present at 1 year; 2.2%−6.7% of individual patients nevertheless crossed the SDD in the relapse direction. In adjusted models, maxillary incisor retraction remained independently associated with nasolabial-angle increase (B = −0.96° per degree; 95% CI −1.20 to −0.72) and with upper-lip retraction (B = +0.24 mm per millimeter; 95% CI +0.18 to +0.30), and mandibular incisor retraction with lower-lip retraction (B = +0.16; 95% CI +0.10 to +0.22). Adjusted R2 ranged from 0.146 to 0.397 and the maximum variance inflation factor (VIF) was 1.76. Conclusions In non-growing adults, treatment was accompanied by predominantly dentoalveolar correction and by coupled soft-tissue change that was maintained at 1 year at group level, no mean change exceeding the error of the method. The coupling was consistent on average but explained a minority of the between-patient variance, and a small proportion of individuals showed measurable rebound, so the individual soft-tissue response remains difficult to predict. These observational data describe association and short-term stability, not causal benefit.