Comparison of FlouroNav and standard fluoroscopy for placement of thoracic pedicle screws

Abstract

Andrew T. Dailey, MD, Darrel S. Brodke, MD, Matthew Rondina, BS, Kent N. Bachus, PhD, Salt Lake City, UT, USA; Thomas H. Jansen, MS, Memphis, TN, USAIntroduction: Despite its wide use in the lumbar spine, pedicle screw fixation has not been universally accepted in the thoracic spine. Morphometric data have shown the thoracic pedicles to be very narrow in transverse diameter, particularly in the T4–T7 region. However, studies have appeared in the literature illustrating the use of pedicle screws for trauma and deformity correction with pedicle perforation rates of 15% to 25%. Recently, the combination of image-guided surgery and fluoroscopy or virtual fluoroscopy has been introduced to improve the ease and accuracy of pedicle screw placement and reduce radiation exposure to the surgeon. This study is an in vitro comparison of a freehand fluoroscopically guided technique with that using virtual fluoroscopy (FlouroNav) in the placement of thoracic pedicle screws.Methods: Eight cadaveric spines with ribs intact were instrumented from T1 to T12. Preoperative computed tomography (CT) scans were performed to assess pedicle size and orientation. Pedicles larger than 5.5 mm were instrumented with 5.5 mm diameter screws, and those ranging from 4.5 to 5.5 mm had 4.5-mm screws placed. Any pedicles less than 4.5 mm were not instrumented. The freehand technique was performed using fluoroscopic images (OEC 9600) in the lateral and anteroposterior (AP) planes to guide the pedicle finder. An entry point at the intersection of the superior border of the transverse process and the lateral two thirds of the facet was used to start the pedicle finder. In the spines instrumented with FlouroNav, the dynamic reference arc was applied to the spinous process of the vertebrae, AP and lateral fluoroscopy images were acquired (OEC 9600 with calibration device attached), and then three adjacent vertebrae were instrumented using the image-guided pedicle awl and finder. Postoperatively, CT scans were performed using 3-mm cuts with 1 mm overlap through the entire vertebral body to assess accuracy of screw placement. Results were compared using chi-squared tests with significance set at p <.05.Results: A total of 158 pedicles were instrumented in both groups (n = 84, freehand; n = 74, FlouroNav). Screw placement was graded on CT as follows: Grade 0, no pedicle perforation; Grade 1, threads out only (less than 2 mm); Grade 2, core screw diameter outside the pedicle (2 to 4 mm) and Grade 3, screw entirely outside the pedicle. Accuracy was confirmed with visual inspection of the dissected spine. In the freehand spines, 16% of screws had Grade 2 perforations, 24% had Grade 1 perforations and 60% were Grade 0. With FlouroNav guidance, there were no Grade 2 perforation, 16% Grade 1 perforations and 84% were Grade 0 (p <.05). Combining Grade 0 and 1 perforations, accuracy was 84% with freehand versus 100% with FlouroNav (p <.05). Only 5% of perforations using either technique were medial perforations, and all were Grade 1 perforations. The amount of fluoroscopy time was reduced by more than 60% using the FlouroNav technique (FlouroNav, 1.2 seconds/screw; freehand, 3.1 seconds/screw).Discussion: Virtual fluoroscopy significantly improved accuracy of screw placement in the thoracic spine and reduced the amount of radiation exposure secondary to C-arm fluoroscopy. Although the biomechanical strength of thoracic pedicle screws that do not have an ideal trajectory remains to be determined, the avoidance of neurological and vascular complications is clinically relevant. Further studies will reveal the utility of these systems in the clinical setting.