Application-Specific Accuracy Requirements for Image Guided Spinal Surgery

Introduction:

Previous studies have examined the clinical accuracy of particular IG systems, underlying causes of inaccuracy in these systems, and methods for quantifying this inaccuracy. However, application–specific accuracy requirements have not been adequately addressed. The purpose of this study was to derive theoretical translational and rotational accuracy requirements for image–guided (IG) spinal screw placement using existing morphometric data.

Methods:

We developed a geometric model relating spinal anatomy to accuracy requirements for IG surgery. Using this mathematical model, which treated the pedicle as a cylinder, we determined the accuracy required to avoid pedicle wall perforation when placing screws with clinically relevant diameters.

Results:

An inverse relationship was found between allowable translational and rotational errors for safe screw placement. As anticipated, accuracy requirements were greatest at levels where the requisite 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 allowable error ranged from 0.0 mm translation/0.0( rotation at T5 to 3.8 mm translation/22.7( rotation at L5.

Conclusions:

This study demonstrates that extremely high accuracies are required to safely place pedicle screws at certain levels of the spine. In some cases, these accuracies are beyond the capabilities of existing IG systems. We hypothesize that other factors, such as the mechanical constraints imposed by the pedicle wall and the surgeon's tactile feedback, are critical for achieving the improved clinical accuracy of image guidance systems demonstrated in the literature.