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