Accuracy of the freehand (fennell) technique using a uniform entry point and sagittal trajectory for insertion of thoracic pedicle screws: A computed tomography-based virtual simulation study

Abstract

Background: Thoracic spine has complex pedicle anatomy with a narrow canal diameter which makes pedicle screw insertion challenging. Fennell et al. have described a simple freehand technique of thoracic pedicle screw placement. We have tested the accuracy of Fennell technique using computed tomography-based (CT-based) simulation model with pedicle screw simulator (PSS).

Methods: Normal CT thoracic spine obtained from CT thorax data of five patients were used in the 3D slicer environment using PSS for simulation. Entry points and axial trajectory as described by Fennell et al. and a sagittal trajectory parallel to the superior endplate were used for simulating the freehand technique using EA (entry angle) mode in the PSS. An ideal trajectory through the midsection of the pedicle from the same entry point and a sagittal trajectory parallel to the superior endplate were simulated using the ET (Entry Target) mode. Angle predicted by the software for an ideal axial trajectory was compared with the Fennell technique and this angle difference was noted at all the levels. Presence of pedicle breach was noted while simulating the Fennell technique.

Results: A total of 240 thoracic pedicle screw insertions were simulated, 120 screws by each technique. A sagittal trajectory parallel to the superior endplate caused no pedicle breach in the cranial-caudal direction at any level. No medial or lateral breach was noted while using an axial trajectory of 30° at T1-T2 and 20° from T3-T10. A 20° axial trajectory at T11 and T12 resulted in a breach of the medial cortex and the ideal mean axial angles at T11 and T12 were 2.8° and 6.5°, respectively.

Conclusions: Fennell technique was effectively simulated using PSS. A uniform entry point and sagittal trajectory parallel to the superior endplate serves as a useful guide for freehand insertion of thoracic pedicle screws. At T11 and 12, ideal axial trajectories are less than 10°.

Keywords: Freehand technique; pedicle screw simulator; thoracic pedicle screws.