Model to assess duration of distraction compared with degree of incisal crowding in symphyseal distraction osteogenesis

Our aim was to evaluate symphyseal distraction in relation to duration of expansion, and to propose an optimised procedure. Eighteen patients (mean (SD) age 19 (7) years) with transverse mandibular deficiency were treated by symphyseal distraction osteogenesis. The mean (SD) anterior dental crowding measured was 6.8 (4.2) mm. We retrospectively compared the width of expansion between the canines (ICE) and expansion between the first molars (IFME) after symphyseal distraction according to the distraction time. The median (range) duration of expansion was 13.6 (7–21) days. The median (range) ICE distraction width was 5.5 (3.4–8) mm and IFME width 3.3 (1–7.9) mm (p < 0.001). There was a significant correlation between expansion width and distraction time (p < 0.001). The transverse anterior and posterior expansion widths differed significantly until 14 days after expansion. The expansion ratio (ICE:IFME) decreased as expansion time increased. The results suggest that the duration of activated expansion can be predicted from the degree of incisal crowding using the formula: distraction time (days) = 0.84 + 3.4 × [IC (mm)] − 0.2 × [IC (mm)]2.