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Pedicle Screw Design and Cement Augmentation in Osteoporotic Vertebrae: Effects of Fenestrations and Cement Viscosity on Fixation and Extraction

Choma, Theodore J., MD*; Pfeiffer, Ferris M., PhD; Swope, Ryan W., DO*; Hirner, Jesse P.

doi: 10.1097/BRS.0b013e3182740e56
Biomechanics
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Study Design. Experimental, human cadaveric study.

Objective. To assess the fixation effects of injecting cement augmentation before screw insertion or after insertion of fenestrated screws; the effect of modulating cement viscosity; and the effects of these techniques on screw removal.

Summary of Background Data. It seems clear that cement augmentation can enhance pedicle screw fixation in osteoporotic bone. What remains to be demonstrated is the aspects of optimal technique such that fixation is enhanced with the greatest safety profile.

Methods. Part I: Human osteoporotic vertebrae were instrumented with solid (nonaugmented) screws, solid screws with polymethylmethacrylate (PMMA), partially cannulated fenestrated (Pfen) screws, or fully cannulated fenestrated (Ffen) screws through which PMMA was injected. Screw fixation was tested in pullout. Part II: Ffen screws were augmented with standard low-viscosity PMMA versus high-viscosity PMMA. Part III: Sample cohorts were extracted from vertebrae to assess required torque and characterize difficulty of extraction.

Results. Part I: Pfen screws demonstrated the greatest fixation with mean failure force of 690 ± 182 N. All methods of cement augmentation demonstrated significant increases in screw fixation. Part II: Ffen screws did not demonstrate a significant difference in pullout strength when high-viscosity PMMA was used as compared with low-viscosity PMMA. Part III: Mean extraction torque values for solid augmented screws, Ffen screws, and Pfen screws were 1.167, 1.764, and 1.794 Nm, respectively, but these differences did not reach significance. None of the osteoporotic vertebrae sustained catastrophic failure during augmented screw extraction.

Conclusion. Polymethylmethacrylate cement augmentation clearly enhances pedicle screw fixation in osteoporotic vertebrae when tested in pure pullout. The technique used for cement injection and choice of specialty screws can have a significant impact on the magnitude of this effect. Fenestrated screws have the capacity to confine cement placement in the vertebral body and may provide enhanced safety from cement extrusion into the spinal canal. It is feasible to inject high-viscosity PMMA through this fenestration geometry, and higher-viscosity cement may enhance the fixation effect.

Experimental, osteoporotic cadaveric study comparing cement augmentation in solid and fenestrated pedicle screws. Polymethylmethacrylate cement augmentation clearly enhances pedicle screw fixation in osteoporotic vertebrae when tested in pure pullout. Fenestrated screws have the capacity to confine cement in the vertebral body. It is feasible to inject high-viscosity polymethylmethacrylate through this fenestration geometry, which may enhance the fixation effect.

*Department of Orthopaedic Surgery

Comparative Orthopaedic Laboratory; and

School of Medicine, University of Missouri, Columbia, MO.

Address correspondence and reprint requests to Theodore J. Choma, MD, Department of Orthopaedic Surgery, and Missouri Spine Center, University of Missouri, 1100 Virginia Ave, DC 0.053, Columbia, MO 65212; Email: chomat@missouri.edu

Acknowledgment date: May 29, 2012. Revision date: July 30, 2012. Acceptance date: September 5, 2012.

The device(s)/drug(s) that is/are the subject of this manuscript is/are not FDA-approved for this indication and is/are not commercially available in the United States.

Depuy Spine grant funds (University of Missouri grant 00023145) were received to support this work.

Relevant financial activities outside the submitted work: consultancy, travel/accommodations/meeting expenses.

© 2012 Lippincott Williams & Wilkins, Inc.