Fig. 4

Measurement of range of safe angle (RSA). RSA indicated the difference of the Max TA and the Min TA, namely, the intersection angle between EI and EO. EI was the line crossing the entry point (E) and the inner cortex in the pedicle isthmus. EO was the line crossing the point E and the outer cortex in the pedicle isthmus. The point E was the crossing point of the pedicle axis and the surface cortex of the pedicle. The Max TA was the angle between the line EI and the midline of the vertebral body. The Min TA was the angle between the line EO and the midline of the vertebral body (The diameter of pedicle screw was neglected)