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Updated: May 6, 2026
A Spine Robotic-Assisted Navigation System for Pedicle Screw Placement
Published on: May 11, 2020
Free-hand insertion of S2AI screw: what can we learn from O-arm navigation?
Changsheng Fan1, Jie Li1, Chen Ling2
1Division of Spine Surgery, Department of Orthopedic Surgery, Nanjing Drum Tower Hospital, The Affiliated Hospital of Medical School, Nanjing University, Zhongshan Road 321, Nanjing, 210008, China.
Objective:
To characterize the actual entry point and trajectory of the S2AI screw following O-arm navigation from the postoperative CT scans.
Methods:
Postoperative CT scans from 66 patients of adult degenerative scoliosis who received S2AI fixation under O-arm navigation were investigated. 3D image processing was applied to analyze and compare two entry points of the S2AI screw: point A was the reference point which was located at 1 mm lateral and 1 mm distal to the S1 dorsal sacral foramen; point B was the actual entry point, and the relative positions of point A and point B were compared with the S1 entry point. The following trajectory of the screw between two points was compared: the screw length (SL), the transverse angle (TA), the sagittal angle (SA) and the skin distance (SD). For point B, compared the difference of the entry point and the trajectory of screws between the two groups: the low PT group (Low PT) was defined as having a baseline PT/PI <25th percentile, and the high PT group (High PT) was defined as having a PT/PI > 25th percentile.
Results:
On the left side point A was located at the lateral (5.26 ± 0.84) mm and the caudal (22.10 ± 1.26) mm of the S1 entry point and the right side was (5.23 ± 0.88) mm lateral and (22.13 ± 1.66) mm caudal of the S1 entry point. For point B, the location was at the lateral (8.08 ± 1.39) mm and the caudal (24.47 ± 2.20) mm of the S1 entry point, and the location of the right side is symmetrical to the left side. The actual entry point was 3 mm more lateral and 2 mm more caudal than the reference one. The TA of the actual point is smaller than the reference one and the SL of the actual point is longer than the reference one, whilst the rest of the trajectory parameters are comparable. The 25th PT/PI percentile of all patients was 0.5, 17 patients were divided into the Low PT group and 49 patients were divided into the High PT group. There was no significant difference between the two groups for the entry point and the trajectory of S2AI screws.
Conclusions:
This study used O-arm navigation to determine the optimal entry point for S2AI screws, which is located at 3 mm caudal and 4 mm lateral to the S1 dorsal foramen. The optimal transverse angle is 46°, and the sagittal angle is 39°. These parameters with low dispersion, offer a reliable reference for the free hand to insert the S2AI screws.
