How does congenital cervicothoracic scoliosis bring about early trunk tilt and coronal imbalance during curve progression: a radiographic analysis to dissect the mechanism of proximal takeoff phenomenon.

PURPOSE: To dissect the mechanism of how congenital cervicothoracic scoliosis (CTS) drive the occurrence of early trunk tilt, namely proximal takeoff phenomenon (PTO) during curve progression. METHODS: CTS patients were stratified into case and control groups according to the presence of PTO. The radiographic deformity parameters of head-neck-shoulder complex were measured and compared between the two groups. The main risk factors for PTO were identified through multiple linear regression analysis. RESULTS: 16 CTS patients with PTO were recruited, and the non-PTO group consisted of 19 CTS patients without PTO. The average Cobb angle was 64.9 ± 19.8° in PTO group and 57.7 ± 21.9° in control group (p > 0.05). Significant difference could be observed for head shift, neck tilt, trunk inclination, apex-C7 deformity angular ratio (DAR), apex translation ratio, C6 tilt, clavicle angle (CA), radiographic shoulder height (RSH), head-neck translation and coronal balance distance (CBD) (All p < 0.05) but not head tilt (p > 0.05). Multiple linear regression analysis revealed that head shift, but not neck tilt correlated significantly with the severity of trunk inclination (ß = 0.106, p = 0.003), while apex-C7 DAR and apex translation ratio were the two factors contributing significantly to the severity of head shift (ß = 0.620, p = 0.020; ß = - 0.371, p = 0.004). CONCLUSIONS: Development and progression of head shift rather than neck tilt is a significant causative factor initiating the occurrence of trunk tilt and proximal takeoff in CTS. A higher apex-C7 DAR representing a short angular upper hemi curve and a lower apex translation ratio representing poor proximal coronal compensation are key risk factors predisposing to head shift.