Mandibular distraction osteogenesis improves mandibular hypoplasia in hemifacial microsomia, but postoperative relapse may arise from different anatomical processes over time. This study characterized stage-specific changes in mandibular ramus height and its condylar and subcondylar components.

This single-center retrospective study included patients with unilateral hemifacial microsomia who underwent mandibular distraction osteogenesis between 2007 and 2024 and had computed tomography examinations at preoperative baseline (T0), distractor removal (T1), and at least one postoperative follow-up (T2–T6). Affected-side mandibular ramus height, condylar height, subcondylar ramus height, and the affected-to-unaffected mandibular ramus height ratio were evaluated. Post-removal follow-up examinations were categorized as T2 (<0.5 years), T3 (0.5–<1.5 years), T4 (1.5–<2.5 years), T5 (2.5–<3.5 years), and T6 (3.5–<4.5 years). Paired t tests with Holm adjustment and piecewise linear mixed-effects models with a 0.5-year breakpoint were used.

Forty-three patients were included. From T0 to T1, condylar height decreased by 1.60 ± 2.97 mm, whereas subcondylar ramus height increased by 12.85 ± 4.22 mm (both P < 0.001). During the T2 interval after removal, mandibular ramus height and subcondylar ramus height decreased at −7.83 and −8.76 mm/year, respectively (both P < 0.001), while condylar height showed a nonsignificant upward trend (+1.03 mm/year; P = 0.197). After 0.5 years, mandibular ramus height (+0.88 mm/year; P = 0.006) and CH (+0.58 mm/year; P = 0.004) increased, whereas subcondylar ramus height showed no significant further change. The affected-to-unaffected mandibular ramus height ratio remained significantly lower than T1 throughout follow-up.

Ramus height changes after mandibular distraction osteogenesis in hemifacial microsomia are stage-specific. Condylar height decreases during treatment, consistent with transient condylar compression or remodeling, whereas early postoperative mandibular ramus height loss is mainly associated with shortening of the subcondylar ramus. The affected-to-unaffected mandibular ramus height ratio declines predominantly during the early postoperative period and remains reduced thereafter, suggesting that relapse of relative mandibular asymmetry occurs mainly early rather than progressing continuously over time.