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A Comparison of Pullout Strength for Pedicle Screws of Different Designs: A Study Using Tapped and Untapped Pilot Holes

Pfeiffer, Ferris M., MS; Abernathie, Dennis L., MD*

doi: 10.1097/01.brs.0000244658.35865.59

Study Design. The pullout strengths of various pedicle screw designs are compared using tapped and untapped pilot holes.

Objective. The objective of this study is to compare the pullout strength of various pedicle screw designs. The designs are compared using tapped and untapped pilot holes. By using several different screw designs, it is possible to gain an understanding of whether there is a correlation between tapping a pilot hole and the ultimate pullout strength.

Summary of Background Data. Most bone screws originally developed were intended to be installed in a pretapped pilot hole. This same technology has been carried over to the development of more modern bone screws for use in spinal fixation applications. Many pedicle screws in use today are still intended to be installed in a tapped hole. Preparing the vertebrae and tapping of a pilot hole involve additional trauma to the patient as well as increased operating time.

Methods. Pedicle screws from various manufacturers are installed in tapped and untapped pilot holes and then loaded to failure. A uniform synthetic material was used to provide a consistent test of each screw design by eliminating variability seen in bone.

Results. Tapping pilot holes did not increase the pullout strength of the screws tested in this study. It was observed during testing that tapping some of the holes degraded the material. This degradation led to pullout strengths that were lower than in the untapped case, and generally larger standard deviations.

Conclusions. The pullout strength was not increased by tapping for the screws in this study. Screws placed in untapped holes generally had higher pullout strengths and lower standard deviations. The results of this study suggest that tapping does not increase pullout strength in bone with densities near 20 lb/ft3, which correlates with low density cancellous or osteoporotic bone.

Many pedicle screw systems in use today are intended to be installed into a pretapped pilot hole. Tapping of pilot holes requires additional surgical time and increases patient trauma. In this study, it is shown that pretapping pilot holes does not increase the pullout strength of the screw designs examined in this study in synthetic cancellous bone.

From the *Columbia Orthopaedic Group, Columbia, MO; and †Brenoak Labs LLC, Columbia, MO.

Acknowledgment date: March 24, 2006. First revision date: May 1, 2006. Second revision date: June 1, 2006. Acceptance date: June 2, 2006.

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

No funds were received in support of this work. Although one or more of the author(s) has/have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this manuscript, benefits will be directed solely to a research fund, foundation, educational institution, or other nonprofit organization which the author(s) has/have been associated.

Address correspondence and reprint requests to Ferris M. Pfeiffer, MS, 10 Hickory Grove, Boonville, MO 65233; E-mail:

© 2006 Lippincott Williams & Wilkins, Inc.