An Assessment of the Medial Angle of Inserted Subaxial Cervical Pedicle Screw during Surgery: Practical Use of Preoperative CT Scanning and Intraoperative X-rays

The most important factor for cervical pedicle screw placement (CPS) is creating a sufficient medial angle. We aimed to know the medial angle of the inserted subaxial CPS during surgery using intraoperative AP X-rays. From March 2012 to September 2014, we performed posterior cervical fusions using CPS on 75 patients, including a total of 389 CPS insertions. Using preoperative CT scanning, we determined the theta(lat) (i.e., an angle between a vertical line and a line to connect the planned entry point and the axial middle point of the pedicle) and theta(med) (i.e., an angle between a vertical line and a line to connect a new medial entry point and the axial middle point of the pedicle; this angle was regarded as minimally acceptable and a safe medial angle). The actual inserted medial angle (theta(ins)) was checked and we determined whether it was between the theta(med) and theta(lat) in the accurately placed CPS, and not in the laterally violated CPS. We measured the horizontal distance of the CPS body (l; using an intraoperative AP X-ray). If the actual screw length (L) was known, we could calculate the medial angle (theta(AP)) as sin(-1) l/L. We checked the theta(AP) and theta(ins) for all of the same levels. Intra-and inter-observer agreement was analyzed. Among 368 accurately inserted CPSs, we found that 360 of the theta(ins) values were greater than or equal to the theta(med) on the same level (P <0.001). The intra-observer agreements were 0.781 and 0.847. The inter-observer agreements were 0.917 and 0.949. It was important that theta(ins) was greater than or equal to the theta(med). Our suggested formula, theta(AP) = sin(-1) l/L, seems to be useful for predicting the medial angle of the inserted CPS and for comparing it with theta(med) during surgery based on an AP X-ray and preoperative CT scan.