0
Technical Briefs

Validation of an Improved Method to Calculate the Orientation and Magnitude of Pedicle Screw Bending Moments

[+] Author and Article Information
Andrew L. Freeman1

 Excelen Center for Bone and Joint Research, 700 10th Ave. S., Minneapolis, MN 55415afreeman@excelen.org

Mina S. Fahim

 University of Minnesota, 312 Church St. SE, Minneapolis, MN 55455

Joan E. Bechtold

 University of Minnesota, 312 Church St. SE, Minneapolis, MN 55455; Excelen Center for Bone and Joint Research, 700 10th Ave. S., Minneapolis, MN 55415

1

Corresponding author.

J Biomech Eng 134(10), 104502 (Oct 01, 2012) (7 pages) doi:10.1115/1.4007629 History: Received March 29, 2012; Revised August 30, 2012; Posted September 25, 2012; Published October 01, 2012; Online October 01, 2012

Previous methods of pedicle screw strain measurement have utilized complex, time consuming methods of strain gauge application, experience high failure rates, do not effectively measure resultant bending moments, and cannot predict moment orientation. The purpose of this biomechanical study was to validate an improved method of quantifying pedicle screw bending moment orientation and magnitude. Pedicle screws were instrumented to measure biplanar screw bending moments by positioning four strain gauges on flat, machined surfaces below the screw head. Screws were calibrated to measure bending moments by hanging certified weights a known distance from the strain gauges. Loads were applied in 30 deg increments at 12 different angles while recording data from two independent strain channels. The data were then analyzed to calculate the predicted orientation and magnitude of the resultant bending moment. Finally, flexibility tests were performed on a cadaveric motion segment implanted with the instrumented screws to demonstrate the implementation of this technique. The difference between the applied and calculated orientation of the bending moments averaged (±standard error of the mean (SEM)) 0.3 ± 0.1 deg across the four screws for all rotations and loading conditions. The calculated resultant bending moments deviated from the actual magnitudes by an average of 0.00 ± 0.00 Nm for all loading conditions. During cadaveric testing, the bending moment orientations were medial/lateral in flexion–extension, variable in lateral bending, and diagonal in axial torsion. The technique developed in this study provides an accurate method of calculating the orientation and magnitude of screw bending moments and can be utilized with any pedicle screw fixation system.

FIGURES IN THIS ARTICLE
<>
Copyright © 2012 by American Society of Mechanical Engineers
Your Session has timed out. Please sign back in to continue.

References

Figures

Grahic Jump Location
Figure 4

Pedicle screws were carefully aligned parallel to the spinous processes using a pin placed through a small hole in the screw. This procedure ensured that the strain gauges were correctly oriented with respect to the anatomic planes.

Grahic Jump Location
Figure 5

(a) White marks were painted on the screw to indicate the direction of a positive bending moment for each strain channel. (b) The signs of each strain channel were compared to determine the orientation of the resultant bending moment vector. (c) Resultant bending moment vectors were calculated using the Pythagorean theorem and orientations were calculated using basic trigonometry.

Grahic Jump Location
Figure 7

(Left) Bending moment direction vectors in lateral bending for each of the four pedicle screws during cadaveric testing. Vector length is equal to the bending moment magnitude. (Right) Orientation of the corresponding resultant forces experienced by each pedicle screw.

Grahic Jump Location
Figure 10

Change in orientation of the bending moment on the right L1 pedicle screw during lateral bending flexibility testing of the spinal motion segment in the bilateral pedicle screw condition. Vector length is equal to the bending moment magnitude.

Grahic Jump Location
Figure 11

Change in orientation of the bending moment on the right L1 pedicle screw during axial torsion flexibility testing of the spinal motion segment in the bilateral pedicle screw condition. Vector length is equal to the bending moment magnitude.

Grahic Jump Location
Figure 12

ROM and average (±SD) bending moment magnitude on the four strain gauged pedicle screws during cadaveric testing

Grahic Jump Location
Figure 1

(a) Pedicle screws were instrumented with four strain gauges to measure biplanar bending moments. Gauges on parallel faces were wired together such that each screw had two independent strain channels. An alignment hole was drilled perpendicular to one of the milled surfaces to allow for screw positioning. (b) Forces applied to the head of the pedicle screw by the fusion rods induced bending moments in the screw. The output of the strain gauges was used to calculate the correct bending moment magnitude and direction.

Grahic Jump Location
Figure 2

A custom fixture was used for pedicle screw calibration and angular orientation

Grahic Jump Location
Figure 3

(a) Posterior and (b) lateral views of the cadaveric spinal motion segment implanted with the strain gauge instrumented pedicle screws

Grahic Jump Location
Figure 6

(Left) Bending moment direction vectors in flexion–extension for each of the four pedicle screws during cadaveric testing. Vector length is equal to the bending moment magnitude. (Right) Orientation of the corresponding resultant forces experienced by each pedicle screw.

Grahic Jump Location
Figure 8

(Left) Bending moment direction vectors in axial torsion for each of the four pedicle screws during cadaveric testing. Vector length is equal to the bending moment magnitude. (Right) Orientation of the corresponding resultant forces experienced by each pedicle screw.

Grahic Jump Location
Figure 9

Change in orientation of the bending moment on the right L1 pedicle screw during flexion–extension flexibility testing of the spinal motion segment in the bilateral pedicle screws condition. Vector length is equal to the bending moment magnitude.

Tables

Errata

Discussions

Some tools below are only available to our subscribers or users with an online account.

Related Content

Customize your page view by dragging and repositioning the boxes below.

Related Journal Articles
Related eBook Content
Topic Collections

Sorry! You do not have access to this content. For assistance or to subscribe, please contact us:

  • TELEPHONE: 1-800-843-2763 (Toll-free in the USA)
  • EMAIL: asmedigitalcollection@asme.org
Sign In