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Optimization of Pedicle Screw Depth in the Lumbar Spine: Biomechanical Characterization of Screw Stability and Pullout Strength

Buckenmeyer, Laura
http://rave.ohiolink.edu/etdc/view?acc_num=toledo1365099627

Abstract Details

2013, Master of Science in Engineering, University of Toledo, Bioengineering.
Objective: While much is known about the clinical outcomes of spinal fusion, questions remain in our understanding of the biomechanical strength of lumbar pedicle screw fixation as it relates to screw sizing and placement. Biomechanical analyses examining ideal pedicle screw depth with current pedicle screw technology are limited. In the osteoporotic spine, optimized pedicle screw insertion depth may improve construct strength, decreasing the risk of loosening or pullout. The purpose of this study was to test the pullout strength of transpedicular pedicle screw insertion depth subsequent to cyclic loading, which simulates post-operative loosening in the osteoporotic lumbar spine. Methods: Ten osteoporotic lumbar spines were dissected free of soft tissue, imaged by computed tomography and potted in polyester resin to ensure rigid fixation. Pedicles were assigned to three experimental groups. Accordingly, screws were inserted to mid-body, pericortical and bicortical depths and underwent cyclic loading and pullout using a custom MTS Bionix servohydraulic testing system. Insertion depth was confirmed by fluoroscopy. Screw insertion depth was randomized between specimens, as well as vertebral level and side. A pure moment was applied to each screw head to simulate flexion-extension forces prior to pull out. Ten preliminary and ten post-fatigue cycles were run to determine a specimen-specific displacement range and reduction in angular stiffness of the screw-bone interface due to fatigue. A specimen specific range of displacement was determined, and each specimen was cycled through for 5000 cycles at 1 Hz using a fixed displacement rate of 0.25 mm/sec along an 80 mm lever arm until the desired load was reached to apply a ±2 Nm moment. Motion was tracked using 4 infrared light-emitting diodes (irLED) markers attached to each pedicle to determine fulcrum location. To stimulate pull out forces, axial distraction was applied to each screw head at a rate of 5 mm/min for 40 mm. Load-displacement curves were analyzed to determine failure loads and energy absorption. Results: Pre-fatigue and post-fatigue angular stiffnesses were, respectively, 86.4±63.4 Nm/rad and 65.9±58.2 Nm/rad for mid-body screws; 115.0±64.6 Nm/rad and 96.8±62.2 Nm/rad for pericortical screws; and 159.9±84.9 Nm/rad and 141.0±81.0 Nm/rad for bicortical screws. There was a statistically significant difference between the pre-fatigue and post-fatigue cycles for mid-body (p<0.001), pericortical (p<0.001) and bicortical (p<0.001) groups. Angular stiffness reduction for mid-body, pericortical and bicortical groups was 25.6±17.9%, 20.8±14.4%, 14.0±13.0% (p=0.012), respectively. The reduction in angular stiffness was statistically significant between mid-body and bicortical screws (p=0.009). Average pedicle length was measured to be 21.5±2.7 mm. The screw fulcrum point was 25.7±7.9 mm, 24.8±6.1 mm and 14.8±5.1 mm anterior of the insertion point for mid-body, pericortical and bicortical groups (p <0.001), respectively. Fulcrum position for mid-body and pericortical screw insertion depths was found to be significantly different from the fulcrum position of bicortical screws (p<0.001). The maximum pullout load for mid-body, pericortical and bicortical groups was 583±306 N, 713±321 N, and 797±285 N, respectively. The difference in fixation strength between mid-body and bicortical screws was found to be statistically significant (p=0.005). Energy absorption was not significant between groups (p=0.063). Conclusion: Increased fixation and decreased loosening were observed with increasing depth of pedicle screw placement in the osteoporotic lumbar spine. We demonstrated that additional purchase of anterior cortex significantly improved fixation strength when compared to mid-body screws that extend halfway through the vertebral body. An understanding of these results may demonstrate a critical advantage in identification of optimal pedicle screw depth for achieving greater screw fixation in the management of osteoporotic patients undergoing spinal surgery.
Vijay Goel (Advisor)
Constantine Demetropoulos (Committee Member)
Scott Molitor (Committee Member)
71 p.
Biomechanics; Biomedical Engineering; Biomedical Research
Pedicle Screws; Screw Pullout; Cyclic Loading; Fatigue; Osteoporosis; Lumbar Spine; Screw Fixation

Recommended Citations

Citations

  • Buckenmeyer, L. (2013). Optimization of Pedicle Screw Depth in the Lumbar Spine: Biomechanical Characterization of Screw Stability and Pullout Strength [Master's thesis, University of Toledo]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1365099627

    APA Style (7th edition)

  • Buckenmeyer, Laura. Optimization of Pedicle Screw Depth in the Lumbar Spine: Biomechanical Characterization of Screw Stability and Pullout Strength. 2013. University of Toledo, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=toledo1365099627.

    MLA Style (8th edition)

  • Buckenmeyer, Laura. "Optimization of Pedicle Screw Depth in the Lumbar Spine: Biomechanical Characterization of Screw Stability and Pullout Strength." Master's thesis, University of Toledo, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1365099627

    Chicago Manual of Style (17th edition)

toledo1365099627
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© 2013, some rights reserved.Creative Commons License Optimization of Pedicle Screw Depth in the Lumbar Spine: Biomechanical Characterization of Screw Stability and Pullout Strength by Laura Buckenmeyer is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by University of Toledo and OhioLINK.