Vertebral-apex angle
The angle between the line perpendicular to the vertebral body and the line from the vertebral body to the apex was defined as the “vertebral-apex angle” (V-A angle) in end-diastolic phase of CT scan. The V-A angle was individually measured by two surgeons, and the average value was obtained. This angle reflected the rotation of the apex to the ipsilateral thorax (Fig. 1 ). Changes in the V-A angle before and after the operation were evaluated. Furthermore, the preoperative (α) and postoperative (β) V-A angles, and the ratio of the post- to preoperative angle (β/α) were compared between the PVO and non-PVO groups.