The Effects of Geometrical Parameters of the Pedicle Screw on Its Pullout Strength: In-Vitro Animal Tests

Pedicle screw instrumentation is one of the most commonly used forms of stabilization. Achieving solid screw fixation within the bone presents a significant challenge to spinal surgeons. Although pure pullout is not the mode of failure seen in clinical situations, pullout testing is thought to be a good predictor of pedicle screw fixation strength for spinal fusion.
To investigate the effects of varying lengths and thread depths of pedicle screws, as well as its insertion angle relative to the sagittal line on pullout strength, and stiffness of the pedicle screws and adjacent bone.
Six fresh-frozen bovine lumbar vertebrae (L5) were examined. Pedicle screws with the lengths of 35, 40, and 45 mm, and the pedicle screws with thread depths of 0.9, 1.1, and 1.15 mm were inserted in pedicles by an orthopedic surgeon. Axial pullout tests were performed by a pullout apparatus and force-displacement curves were plotted.
The 45 mm length screw showed the maximum pullout strength (1746 N) and stiffness (564.7 N/mm) in the case of constant thread depth of 0.9 mm. The 1.15 mm thread depth screw showed the maximum pullout strength (1719 N) and stiffness (646.4 N/mm) in the case of constant length of 40 mm, and the screw insertion angle of 25° resulted in maximum pullout strength (1251 N) and stiffness (249.6 N/mm) in the case of constant thread depth of 0.9 mm and constant length of 35 mm.
Increasing the length and the thread depth of the screws leads to an increase in the pullout strength, as well as the bone-screw construct stiffness. Pedicle screw pullout strength and the bone-screw construct stiffness were also affected by the insertion angle of the screw. There are other factors such as the insertion technique employed by the surgeon, and bone mineral density, which affect the pullout strength.