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 degrees at T5 to 3.8 mm/12.7 degrees 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.