The Influence of the Insertion Technique on the Pullout Force of Pedicle Screws

Study Design: The pullout strength of a typical pedicle screw was evaluated experimentally for different screw insertion techniques.

Objective: To conclude whether the self-tapping insertion technique is indeed the optimum one for self-tapping screws, with respect to the pullout strength.

Summary of Background Data. It is reported in the literature that the size of the pilot-hole significantly influences the pullout strength of a self-tapping screw. In addition it is accepted that an optimum value of the diameter of the pilot-hole exists. For non self-tapping screw insertion it is reported that undertapping of the pilot-hole can increase its pullout strength. Finally it is known that in some cases orthopedic surgeons open the threaded holes, using another screw instead of a tap.

Methods: A typical commercial self-tapping pedicle screw was inserted into blocks of Solid Rigid Polyurethane Foam (simulating osteoporotic cancellous bone), following different insertion techniques. The pullout force was measured according to the ASTM-F543–02 standard. The screw was inserted into previously prepared holes of different sizes, either threaded or cylindrical, to conclude whether an optimum size of the pilot-hole exists and whether tapping can increase the pullout strength. The case where the tapping is performed using another screw was also studied.

Results: For screw insertion with tapping, decreasing the outer radius of the threaded hole from 1.00 to 0.87 of
the screw’s outer radius increased the pullout force 9%. For insertion without tapping, decreasing the pilot-hole’s diameter from 0.87 to 0.47 of the screw’s outer diameter increased its pullout force 75%. Finally, tapping using another screw instead of a tap, gave results similar to those of conventional tapping.

Conclusion: Undertapping of a pilot-hole either using a tap or another screw can increase the pullout strength
of self-tapping pedicle screws.