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Effect of Various Tapping Diameters on Insertion of Thoracic Pedicle Screws: A Biomechanical Analysis

Kuklo, Timothy R. MD; Lehman, Ronald A. Jr, MD

doi: 10.1097/01.BRS.0000084665.31967.02
Biomechanics

Study Design. A biomechanical cadaver study to assess the effect of various tapping diameters on thoracic pedicle screw insertional torque.

Summary of Background Data. Thoracic pedicle screws are now commonly used for deformity and nondeformity cases. The optimal insertion techniques, however, have not been determined.

Purpose. To investigate the effect of various tapping techniques before insertion of thoracic pedicle screws in terms of maximal insertional torque (MIT) or screw pullout.

Materials and Methods. Thirty-four fresh cadaveric thoracic vertebrae were harvested and evaluated with dual-energy radiograph absorptiometry (DEXA) to assess bone mineral density (BMD). Twenty-three matched, fixed-head, 5.0-mm pedicle screws (group 1) were placed using the straight-forward (ST) trajectory (paralleling the endplate) at various thoracic levels after random side selection using either line-to-line tapping (5.0-mm tap) or 1-mm undertapping (4.0-mm tap) under direct and fluoroscopic visualization. After this, 11 matched 5.0-mm pedicle screws (group 2) were placed comparing undertapping by 0.5 mm (4.5-mm tap) with 1 mm undertapping (4.0-mm tap). MIT was recorded for each screw revolution with a digital torque wrench.

Results. BMD averaged 0.732 g/cm2 (0.620–0.884 g/cm2) for group 1, and 614 g/cm2 (0.533–0.697 g/cm2) for group 2. In group 1, the average MIT was 0.153 ± 0.009 (SE) Nm for line-to-line tapping and 0.295 ± 0.021 (SE) Nm for 1-mm undertapping, a 93% increase in MIT (P < 0.0005). In group 2, the average MIT was 0.138 ± 0.009 (SE) Nm for 0.5 mm undertapping and 0.202 ± 0.018 (SE) Nm for undertapping by 1 mm, a 47% increase in MIT (P = 0.03). BMD correlated with undertapping by 1 mm in group 1 (P < 0.0005), but not with undertapping by 0.5 mm (P = 0.087), although there appeared to be a trend in osteoporotic specimens. There were no noted differences in MIT between thoracic regions/levels, despite small differences in thoracic pedicle widths (P = 0.193).

Discussion and Conclusion. Undertapping the thoracic pedicle by 1-mm increases MIT by 47% (P = 0.03) when compared with undertapping by 0.5 mm, and by 93% (P < 0.0005) when compared with tapping line-to-line.

From the Department of Orthopaedic Surgery and Rehabilitation, Walter Reed Army Medical Center, Washington, DC; the Department of Surgery, Uniformed Services University of the Health Sciences, Bethesda, MD; and the Union Memorial Hospital, Baltimore, MD.

Investigation performed at Walter Reed Army Medical Center, Washington, DC and Union Memorial Hospital, Baltimore, MD.

Acknowledgment date: May 20, 2002.

First revision date: October 7, 2002.

Acceptance date: March 28, 2003.

The opinions or assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the United States Army or the Department of Defense. Some authors are employees of the United States government. This work was prepared as part of their official duties and as such, there is no copyright to be transferred. The Department of Clinical Investigation at Walter Reed Army Medical Center has supported this protocol/manuscript.

Address correspondence to Timothy R. Kuklo, MD, 15619 Thistlebridge Lane, Rockville, MD 20853; e-mail: kuklot@aol.com

© 2003 Lippincott Williams & Wilkins, Inc.