Sagittal Trajectory Variation to Salvage Failed Thoracic Pedicle Screws: A Biomechanical Analysis

Purpose: To investigate the ability to safely place a thoracic pedicle screw with adequate maximal insertional torque (MIT) using the anatomic (AT) trajectory (directed along the true anatomic axis of the pedicle) after purposeful failure/medial violation of the pedicle using the straight-forward (ST) trajectory (paralleling the vertebral endplate).

Methods: Twenty-three fresh cadaveric thoracic vertebrae were harvested and evaluated with dual energy X-ray absorptiometry (DEXA) to assess bone mineral density (BMD). Fixed-head 5.0 mm stainless steel pedicle screws were then placed using the ST trajectory under direct and fluoroscopic visualization after undertapping by 1 mm, followed by creation of a medial pedicle wall violation. Screws were then placed using the AT trajectory after undertapping by 1 mm in an attempt to salvage pedicle fixation at that level. MIT was recorded for each screw revolution with a digital torque wrench.

Results: BMD for the vertebrae averaged 732 g/cm2 (620-884 g/cm2). The MIT for the straight-forward technique without pedicle violation was 2.61 + 0.19 (SE) in-lbs., while that of the salvage procedure after medial wall violation (AT) averaged 1.62 + 0.12 (SE) in-lbs. Thus, the anatomic technique (AT) achieved 62% (p=0.027) of the fixation strength (in terms of MIT) during salvage after failure/medial violation of the pedicle in the thoracic spine. MIT for both the ST* and AT trajectories correlated with both global BMD of the vertebrae (p=0.008*, p=0.004) and regional BMD of the vertebral body (p=0.044*, p=0.023).

Discussion and Conclusion: The anatomic technique (AT) achieved 62% (p=0.027) of the MIT during salvage of a failed/violated pedicle if properly placed. BMD correlated with both the initial and salvage techniques. The anatomic trajectory may be used in a salvage situation to safely place a thoracic pedicle screw with adequate insertional torque following medial pedicle wall violation.