Whether the thoracic cage deformity in adolescent idiopathic scoliosis (AIS) can be sufficiently treated with vertebral derotation alone, has been quite controversial. Our aim is to control the hypothesis that the rib cage deformity (RCD) may be adequately corrected when only vertebral derotation is applied. We studied retrospectively patients treated for AIS with posterior spinal fusion without costoplasty. The RCD was assessed on lateral radiographs by rib index (RI). The correction of RI after surgery was calculated. Of the 103 patients that were finally included in our study, 29 patients (22 females and 7 males; mean age, 14.5 ± 2.1 years) represented Group A (Harrington rod instrumentation - no derotation), while 74 patients (61 females and 13 males; mean age, 14.1 ± 2.4 years) were operated with either a full pedicle screw system or a hybrid construct with hooks and pedicle screws (Group B-derotation). RI was significantly corrected after surgery in both groups. RI was significantly greater in Group A after surgery. Whatsoever, the correction of RI, thereby the RCD correction, did not significantly differ among groups. In conclusion, it cannot be suggested by the present study that vertebral derotation alone can offer an absolute correction of the deformity of the thoracic cage in patients with Lenke Type 1 AIS, and it seems also that the development of RCD may not exclusively result from the spinal deformity, thus questions can be further raised regarding scoliogeny per se.