Clinical significance of superior articular process as a reference point for free-hand pedicle screw insertion in thoracic spine

The trajectory of the pedicle screw perpendicular to the SAP (superior articular process) is consistent with the universal trajectory presented in the previous study of the entry point using computed tomography. The ideal entry point and trajectory of pedicle screw insertion have been a matter of considerable debate. We attempted to find the relationship between SAP and entry point and trajectory of the pedicle screw. Thoracic spine CT (computed tomography) scans of 9 volunteers were studied. A safe zone for the trajectory of the screw in the axial (Margin Ax) and sagittal (Margin Sag) was defined as the zone between lines perpendicular to the SAP along the medial and lateral cortex of the pedicle in the axial section, and the superior and inferior cortex in the sagittal section without violation of the pedicle walls. The midline of Margin Ax and Margin Sag was defined as the safe entry point of the trajectory in the axial and sagittal planes (Trajectory Ax and Trajectory Sag), respectively. Margin Ax and Margin Sag were 4.14±0.99 and 9.03±2.01mm, respectively. There was a statistically significant difference in Trajectory Ax between the upper and middle, and between the upper and lower (P=.0076 in both cases), but not between the middle and lower thoracic spine (P=.066). Trajectory Sag was within 1mm at the levels of T4, T8, T11 vertebrae and at 0mm at the other levels. Thus, the midpoint of Margin Sag was within 1mm from the SAP base. There was a constant angular relationship with the SAP and the pedicle axis; the line perpendicular to the SAP can act as a trajectory. Therefore, we suggest that the SAP might be the only accurate and safe reference for pedicle screw insertion in the thoracic spine perpendicular to the SAP using freehand technique.