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

Abstract

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 θ_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 θ_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 (θ_ins) was checked and we determined whether it was between the θ_med and θ_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 (θ_AP) as sin⁻¹ l / L. We checked the θ_AP and θ_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 θ_ins values were greater than or equal to the θ_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 θ_ins was greater than or equal to the θ_med. Our suggested formula, θ_AP = sin⁻¹ l / L, seems to be useful for predicting the medial angle of the inserted CPS and for comparing it with θ_med during surgery based on an AP X-ray and preoperative CT scan.