S2 alar-iliac (S2AI) screws likely represent the best option for spinopelvic fixation, though their placement can be technically difficult. There is a lack of good anatomic and fluoroscopic landmarks to guide their placement, and determining if the screw is entirely within bone using fluoroscopy can be difficult. Navigation and robotic drill guide placement are newer techniques that hold promise to improve the accuracy of screw placement in spine surgery. In order to determine if a robot improves the accuracy of S2AI screw placement, investigators at Columbia University compared S2AI screw accuracy between 59 screws placed using a free hand (i.e. no image guidance) and 46 screws placed using a robot-positioned drill guide. They found an 8.5% breach rate using the free hand technique and a 4.3% breach rate using the robot, though this difference was not statistically significant. Moderate-severe breaches (>3 mm) occurred in 5.1% of free hand screws and 2.2% of robot-directed screws. They also compared the trajectory of the screws and found that the robot-directed screws aimed somewhat more caudally. No screws caused neurovascular or visceral injury, and there were no inferior breaches into the sciatic notch. Based on these data, the authors concluded that the free hand and robot-directed technique had comparable results.
The authors have done a nice job creating a comparative study looking at these two S2AI techniques. While the authors work at a busy spinal deformity center, they included only 51 patients who underwent surgery over two years. This indicates that it is very difficult to put together a large series of these cases as even busy deformity centers are not putting in large numbers of S2AI screws. The relatively low numbers involved and relatively low breach rate resulted in an underpowered study. The free hand technique had twice the breach rate as compared to the robot-directed technique, yet this difference was not close to statistically significant. Based on a lack of statistical significance, the authors concluded that accuracy was similar for both techniques, yet the data indicate that the robot might cut the breach rate in half. The only way to prove that would be to do a large, likely multicenter trial, which is always challenging and expensive. The data suggest that one of the robot-directed screws was markedly off target, likely by a centimeter or more. It would be interesting to know the mechanism of failure for that screw given that the others seemed to be placed quite accurately. Stereotactic navigation is another option for placing S2AI screws, and the authors did not comment on that. In my experience, it can be difficult for the cameras to capture the stereotactic array on the pedicle finder due to the caudal angulation required to place the screw. With all of the new technology related to placing screws more accurately, it seems as though the placement of S2AI screws can be made easier and more accurate by using the technology. It remains to be seen what technique (free hand, fluoroscopy guided, navigation, 3D printed drill guides, robots, etc.) will lead to the most accurate placement while maintaining efficient workflow. It seems unlikely that most spine surgeons will ever be able to match Dr. Lenke's skill with free hand screw placement, and even he had a 5% moderate-severe breach rate. Navigation and robotics seem to be here to stay, and it will be up to the spine surgery community to figure out how to best use these tools.
Please read Dr. Shillingford's article on this topic in the November 1 issue. Does this change how you view the role of robotics in S2AI screw placement? Let us know by leaving a comment on The Spine Blog.
Adam Pearson, MD, MS
Associate Web Editor