An intraoperative fluoroscopic method to accurately measure the post-implantation position of pedicle screws

Abstract

Purpose

Pedicle screw malplacement, leading to neurological symptoms, vascular injury, and premature implant loosening, is not uncommon and difficult to reliably detect intraoperatively with current techniques. We propose a new intraoperative post-placement pedicle screw position assessment system that can therefore allow surgeons to correct breaches during the procedure. Our objectives were to assess the accuracy and robustness of this proposed screw location system and to compare its performance to that of 2D planar radiography.

Methods

The proposed system uses two intraoperative X-ray shots acquired with a standard fluoroscopic C-arm and processed using 2D/3D registration methods to provide a 3D visualization of the vertebra and screw superimposed on one another. Point digitization and CT imaging of the residual screw tunnel were used to assess accuracy in five synthetic lumbar vertebral models (10 screws in total). Additionally, the accuracy was evaluated with and without correcting for image distortion and for various screw lengths, screw materials, breach directions, and vertebral levels.

Results

The proposed method is capable of localizing the implanted screws with less than 2 mm of translational error (RMSE: 0.7 and 0.8 mm for the screw head and tip, respectively) and less than \(2.3^{\circ }\) angular error (RMSE: \(1.3^{\circ }\) ), with minimal change to the errors if image distortion is not corrected. Breaches and their anatomical locations were all correctly visualized and identified for a variety of screw lengths, screw materials, breach locations, and vertebral levels, demonstrating the robustness of this approach. In contrast, one breach, one non-breach, and the anatomical location of three screws were misclassified with 2D X-ray.

Conclusion

We have demonstrated an accurate and low-radiation technique for localizing pedicle screws post-implantation that requires only two X-rays. This intraoperative feedback of screw location and direction may allow the surgeon to correct malplaced screws intraoperatively, thereby reducing postoperative complications and reoperation rates.