Fig. 2

a The red lines indicate the parallel lines of the upper and lower endplates. The green lines indicate the vertical lines of the red lines. Angle a is the angle between the two green lines which is indicated as the sagittal Cobb* angle in the study. b The red lines indicate the tangents of the connecting points (point e and point f) of the rod and the upper and lower pedicle screws. Angle b is the angle between the two red lines which is indicated as the bending rod angle in this study. c The preoperative unstable thoracolumbar fracture lateral radiography. d The postoperative unstable thoracolumbar fracture lateral radiography. The red lines indicate the tangents of the connecting points (point e and point f) of the rod and the upper and lower pedicle screws. Angle b is the angle between the two red lines which is indicated as the bending rod angle in this study.