Straight-Forward Versus Anatomic Trajectory of Thoracic Pedicle Screws: A Biomechanical Analysis

PURPOSE: To investigate the optimal thoracic pedicle screw trajectory in terms of maximal insertional torque (MIT) and pullout strength (POS) for a straightforward (ST) trajectory (paralleling the vertebral endplate) versus an anatomic (AT) trajectory (directed along the true anatomic axis of the pedicle).

METHODS: Thirty fresh cadaveric thoracic vertebrae were harvested and evaluated with dual energy X-ray absorptiometry (DEXA) to assess bone mineral density (BMD). Matched, fixed-head pedicle screws were then randomly assigned (left versus right) and placed using the straight-forward or anatomic trajectory under direct and fluoroscopic visualization. MIT was recorded for each screw revolution with a digital torque wrench. Pullout strength testing to failure was conducted with a servohydraulic testing device.

RESULTS: Average BMD was 632 ± 0.25 mg/cm2 (471-794 mg/cm2). The MIT for the straight-forward technique was 2.58 ± .14 (SE) in-lbs while the anatomic trajectory averaged 1.86 ± 0.14 (SE) in-lbs. The insertional torque at the neurocentral junction for the ST technique averaged 1.89 ± .17 (SE) in-lbs. (73% of MIT), while the AT trajectory averaged 1.39 ± .11 (SE) in-lbs (75% of MIT). POS for the ST technique was 610.87 ± 49.73 (SE) in-lbs., while the POS of the AT averaged 480.52 ± 53.79 (SE) in-lbs. Thus, the straight-forward technique (paralleling the endplate) results in a 39% increase in MIT (p=0.0005), and a 36% increase in MIT at the neurocentral junction (p=0.007). Additionally, the average insertional torque at the neurocentral junction for the straight-forward trajectory was equivalent to the MIT for the anatomic trajectory. A 27% increase in POS (p=0.034) was seen with the straight-forward technique. BMD did not correlate with peak insertional torque for either technique (p=0.118), but did correlate with POS for both the AT (p=0.025) and ST techniques (p=0.027).

DISCUSSION AND CONCLUSION: The straight-forward technique results in a 39% increase in MIT (p=0.0005) and a 27% increase in POS (p=0.034) over the anatomic trajectory. The neurocentral junction provides approximately 75% of the MIT in the thoracic spine. BMD directly correlates with pullout strength, but not with insertional torque.