Illustrated methods used to measure all parameters in the study. A) Cervical vertebra showing planes used for the Retro-construction image. B) Axial reconstruction image through the pedicle isthmus; OPW, IPW and PTA were measured. C) Reconstruction image: vertical plane perpendicular to the longitudinal pedicular axis (LPA); PA1 in (a); the OPH and IPH were measured D) Reconstruction image: vertical plane through the longitudinal pedicular axis (LPA); PA2 in (a); the PSA was measured. The abbreviations and description are in Table 1.