The influence of the insertion technique on the pullout force of pedicle screws: an experimental study.
METHODS
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.


BACKGROUND
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.


CONCLUSIONS
Undertapping of a pilot-hole either using a tap or another screw can increase the pullout strength of self-tapping pedicle screws.
Unit of Biomechanics, Department of Mechanics, School of Applied Mathematical and Physical Sciences, National Technical University of Athens, 5 Heroes of Polytechnion Avenue, Athens, Greece.
Panagiotis E
PE
Chatzistergos
George
G
Sapkas
Stavros K
SK
Kourkoulis
D001863:Bone Screws; D004867:Equipment Design; D019544:Equipment Failure Analysis; D016268:Internal Fixators; D008422:Materials Testing; D013314:Stress, Mechanical; D013718:Tensile Strength