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Characteristics of Pedicle Screw Loading: Effect of Surgical Technique on Intravertebral and Intrapedicular Bending Moments

McKinley, Todd O. MD*; McLain, Robert F. MD; Yerby, Scott A. PhD; Sharkey, Neil A. PhD§; Sarigul-Klijin, Nesrin PhD; Smith, Tait S. MS*

Biomechanics

Study Design. A static nondestructive bending analysis of pedicle screws inserted into vertebral analogues was conducted. Pedicle screw load was studied as a function of variables in insertion technique.

Objectives. To determine how the sagittal bending moment in pedicle screws is affected by changes in pedicle screw length, insertional depth, and sagittal placement.

Background Data. An unexpectedly high rate of clinical failure has been observed in pedicle screws used in short-segment instrumentation for unstable burst fractures. The majority of screws fail in sagittal bending within the pedicle. Little is known of the insertion technical factors that affect in situ loads incurred by pedicle screws.

Methods. Synthetic vertebral analogues were fabricated. Pedicle screws internally instrumented with strain gauges were used as load transducers to determine screw bending moments within the pedicle and body of the analogue. Analogues were loaded in compression to simulate loading of an unstable burst fracture.

Results. Screw bending moments within the pedicle increased 33% and 52% when screws were left 3 mm and 5 mm short of full insertion. Intrapedicular moments increased 20% to 29% in screws inserted superiorly or inferiorly within the pedicle. Thirty-five-millimeter screws developed intrapedicular moments 16% higher than 40-mm and 45-mm screws.

Conclusions. In situ pedicle screw loads increased significantly as a direct result of variations in surgical technique. Screws left short of full insertion, placed off center in the sagittal plane of the pedicle, or less than 40 mm long developed increased intrapedicular bending moments.

From the *Department of Orthopaedic Surgery, University of California, Davis, Sacramento, California, the †Department of Orthopaedic Surgery, Cleveland Clinic Foundation, Cleveland, Ohio, the ‡VA Palo Alto Health Care System, Palo Alto, California, the § Center for Locomotion Studies, Penn State University, University Park, Pennsylvania, and the ∥Department of Mechanical and Aeronautical Engineering, University of California, Davis, Davis, California.

Supported by a grant received from AO North America.

Acknowledgment date: October 17, 1997.

Acceptance date: April 13, 1998.

Device status category: 5.

Address reprint requests to: Todd McKinley, MD; R. Adams Cowley Shock Trauma Center; University of Maryland; 22 S. Greene St.; Baltimore, MD 21201.

© 1999 Lippincott Williams & Wilkins, Inc.