Is It "In" or "Out"? The Optimal Fluoroscopic Views for Intraoperative Determination of Proper Lateral Mass Screw Placement.

Abstract

Cadaveric. Determine optimal fluoroscopic views for detecting cervical lateral mass screw (LMS) violations. Single plane intraoperative x-rays are commonly used but frequently inadequate due to its complex trajectory. Fluoroscopy can be taken in multiple planes, but the ideal fluoroscopic view to assess malposition is not known: depending on the view, any given screw may look "in" or "out." C3-6 LMS were inserted in three cadavers. To evaluate neuroforaminal violation, LMS were inserted into the foramen with the tip penetrating the anterior cortex by 0, 2, and 4 mm. To assess facet joint violation, LMS were inserted toward the subjacent facet joint with the tip penetrating the anterior cortex by 0 and 2 mm. Fluoroscopic views were taken 0°, 10°, 20°, 30°, and 40° to the lateral plane. Views were independently evaluated by three blinded spine surgeons. Twenty-degree oblique view correctly identified a 2 mm penetration into the neuroforamen in 79%, and a 4 mm penetration in 86%, for a sensitivity of 83% and specificity of 90%. Thirty-degree view had lower sensitivity (76%) but slightly higher specificity (93%). Twenty-degree and 30° views were significantly more sensitive than the other views. Zero-degree view correctly identified a 2 mm penetration into the facet joint in 93%, for a sensitivity of 93% and specificity of 92%. Ten-degree view had lower sensitivity (72%) but higher specificity (100%). The 0° view was significantly more sensitive than the other views. Twenty-degree and 30° oblique views significantly provided the most sensitive assessment of LMS potentially violating the neuroforamen, whereas the 0° neutral lateral view significantly provided the most sensitive assessment of facet violations. The specificities were also high (in the 90% range) for these views. We recommend the use of these views intraoperatively when assessing proper placement of LMS fluoroscopically. N/A.