Prediction on the chin advancement of the twin block functional appliance in growing Chinese patients using the cephalometric markers: a retrospective study.

In spite of the widespread use of functional appliances, broad variations were applied the treatment response. The aim of this study is to investigate the pre-treatment cephalometric predictors on the chin advancement of twin-block in growing Chinese patients with class II malocclusion. After screening, 90 patients treated by twin-block were included in the study. The treatment outcome was assessed by the alterations in the distance of skeletal pogonion (Pog) to the vertical reference plane perpendicular to the Frankfurt plane (ΔPog-VRP). Moreover, ΔPog-VRP was divided by the cranial growth indicated by the Nasion to Basion changes (ΔN-Ba) to minimize the growth discrepancy among individuals (adjΔPog-VRP). Patients with ΔPog-VRP/adjΔPog-VRP above the median value were categorized into good response group (GRG/adjGRG, N = 45), while the rest were poor response group (PRG/adjPRG, N = 45). Independent t-test was used to compare the pre-treatment cephalometric measurements between GRG/adjGRG and PRG/adjPRG. Stepwise multivariate regression models were used to determine the pre-treatment cephalometric predictors for the chin advancement. Generally, there were not any significant differences between GRG/adjGRG and PRG/adjPRG regarding age, gender and cervical stage before twin-block treatment. Patients from GRG had significantly reduced cephalometric measurements in the vertical dimensions, including ∠N-Go-Me, ∠Mandibular plane-Occlusal plane (∠MP-OP) and the sum of angles (p < 0.05) in comparison to PRG. When the individual growth was taken account, similar findings were observed. The patients from adjGRG had a significantly lower ∠Sella Nasion line-MP (∠SN-MP), ∠Ar-Go-Me and ∠N-Go-Me, as well as an increased Posterior facial height (PFH)/Anterior facial height (AFH) (p < 0.05) compared with their counterparts. ∠N-Go-Me variable was the independent predictor on Pog advancement with (β = -0.26, 95% CI: -0.06 to -0.01, p = 0.01) and without (β = -0.29, 95% CI: -0.06 to -0.01, p < 0.01) adjustments on individual growth. The results of this study showed that patients with a reduced N-Go-Me angle are more likely to experience a greater chin advancement following twin-block treatment.