In-vitro comparison of the pullout strength of 3 different thoracic screw fixation techniques

Dvorak et al in 1993 and Husted et al in 2003 reported 2 different screw fixation techniques at the thoracic spine as alternatives to transpedicular screws. So far, there is no investigation comparing the pullout stability of all 3 possible screw fixation techniques.

To evaluate the stability of possible alternatives for transpedicular screw fixation.

A biomechanical human cadaver investigation of the transpedicular and 2 different extrapedicular techniques was performed in the form of a comparative pullout test.

Eighteen human vertebral bodies from Th7 to Th9 were harvested from 6 donors, dissected from surrounding tissue, and matched to 3 different fixation groups. As alternatives for transpedicular screw fixation, an extrapedicular supratransverse screw insertion from posterolateral and a tricortical screw fixation technique, penetrating the transverse process and reentering the vertebral body at the pedicle base were evaluated biomechanically. A unilateral screw fixation was performed in one of the described techniques. Axial pullout strength was measured using a Zwick Z50 servoelectric testing machine.

The average pullout strength of the pedicle screws was 400 N, whereas the supratransverse and the pertransverse screw fixation resisted 370 N pullout force on average. There was neither a statistic significant difference between the pullout forces of the 3 groups nor a significant correlation of pullout strength and bone mineral density measured by quantitative computed tomography.

In-vitro pullout resistance of thoracic screw fixation does not differ significantly in intrapedicular and extrapedicular insertion techniques.