Biomechanics

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

An Experimental Study

Chatzistergos, Panagiotis E. MS^*; Sapkas, George PhD^†; Kourkoulis, Stavros K. PhD^*

From the *Unit of Biomechanics, Department of Mechanics, School of Applied Mathematical and Physical Sciences, National Technical University of Athens, Athens, Greece; and †First Department of Orthopaedics, Athens University Medical School, Athens, Greece.

Acknowledgment date: May 6, 2009. Revision date: May 30, 2009. Acceptance date: July 19, 2009.

The device(s)/drug(s) is/are FDA-approved or approved by corresponding national agency for this indication.

This paper is part of the 03ED292 research project implemented within the frame work of the “Reinforcement Programme of Human Research Manpower” (PENED) and co-financed by National and Community Funds (25% from the Greek Ministry of Development—General Secretariat of Research and Technology and 75% from E.U. European Social Fund). No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

Address correspondence and reprint request to Stavros K. Kourkoulis, Unit of Biomechanics, Department of Mechanics, School of Applied Mathematical and Physical Sciences, National Technical University of Athens, Theocaris Building, 5, Heroes of Polytechnion Avenue, 157 73 Zografou Campus, Athens, Greece; E-mail: [email protected]

Spine 35(9):p E332-E337, April 20, 2010. | DOI: 10.1097/BRS.0b013e3181ba0b0c

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.

Increase Your Article's Exposure

Interested in increasing awareness of your research? Visit www.wkauthorservices.com to learn more about creating infographics, videos and other features that can help increase your article's exposure.

Secondary Logo

Journal Logo

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