David A. Simon

Virtual fluoroscopy : Computer-assisted fluoroscopic navigation

Study Design In vitro accuracy assessment of a novel virtual fluoroscopy system. Objectives To investigate a new technology combining image-guided surgery with C-arm fluoroscopy. Summary of Background Data Fluoroscopy is a useful and familiar technology to all musculoskeletal surgeons. Its limitations include radiation exposure to the patient and operating team and the need to reposition the fluoroscope repeatedly to obtain surgical guidance in multiple planes. Methods Fluoroscopic images of the lumbar spine of an intact, unembalmed cadaver were obtained, calibrated, and saved to an image-guided surgery system (StealthStation; Medtronic Sofamor–Danek, Memphis, TN). A virtual fluoroscopy system (FluoroNav; Medtronic Surgical Navigation Technologies, Broomfield, CO) was used for the sequential insertion of a light-emitting diode–fitted probe into the pedicles of L1–S1 bilaterally. The trajectory of a “virtual tool” corresponding to the tracked tool was overlaid onto the saved fluoroscopic views in real time. Live fluoroscopic images of the inserted pedicle probe were then obtained. Distances between the tips of the virtual and fluoroscopically displayed probes were quantified using the image-guided computer’s measurement tool. Trajectory angle differences were measured using a standard goniometer and printed copies of the workstation computer display. The surgeon’s radiation exposure was measured using thermolucent dosimeter rings. Results Excellent correlation between the virtual fluoroscopic images and live fluoroscopy was observed. Mean probe tip error was 0.97 ± 0.40 mm. Mean trajectory angle difference between the virtual and fluoroscopically displayed probes was 2.7° ± 0.6°. The thermolucent dosimeter rings measured no detectable radiation exposure for the surgeon. Conclusions Virtual fluoroscopy offers several advantages over conventional fluoroscopy while providing acceptable targeting accuracy. It enables a single C-arm to provide real-time, multiplanar procedural guidance. It also dramatically reduces radiation exposure to the patient and surgical team by eliminating the need for repetitive fluoroscopic imaging for tool placement.

Accuracy requirements for image-guided spinal pedicle screw placement

Study Design Accuracy requirement analysis for image-guided pedicle screw placement. Objectives To derive theoretical accuracy requirements for image-guided spinal pedicle screw placement. Summary of Background Data Underlying causes of inaccuracy in image-guided surgical systems and methods for quantifying this inaccuracy have been studied. However, accuracy requirements for specific spinal surgical procedures have not been delineated. In particular, the accuracy requirements for image-guided spinal pedicle screw placement have not been previously reported. Methods A geometric model was developed relating spinal pedicle anatomy to accuracy requirements for image-guided surgery. This model was used to derive error tolerances for pedicle screw placement when using clinically relevant screw diameters in the cervical (3.5 mm), thoracic (5.0 mm), and thoracolumbar spine (6.5 mm). The error tolerances were represented as the permissible rotational and translational deviations from the ideal screw trajectory that would avoid pedicle wall perforation. The relevant dimensions of the pedicle model were extracted from existing morphometric data. Results As anticipated, accuracy requirements were greatest at spinal levels where the relevant screw diameter approximated the dimensions of the pedicle. These requirements were highest for T5, followed in descending order by T4, T7, T6, T3, T12, L1, T8, T11, C4, L2, C3, T10, C5, T2, T9, C6, L3, C2, T1, C7, L4, and L5. Maximum permissible translational/rotational error tolerances ranged from 0.0 mm/0.0° at T5 to 3.8 mm/12.7° at L5. Conclusions These results, obtained by mathematical analysis, demonstrate that extremely high accuracy is necessary to place pedicle screws at certain levels of the spine without perforating the pedicle wall. These accuracy requirements exceed the accuracy of current image-guided surgical systems, based on clinical utility errors reported in the literature. In actual use, however, these systems have been shown to improve the accuracy of pedicle screw placement. This dichotomy indicates that other factors, such as the surgeon’s visual and tactile feedback, may be operative.