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Influence of Screw Augmentation in Posterior Dynamic and Rigid Stabilization Systems in Osteoporotic Lumbar Vertebrae: A Biomechanical Cadaveric Study

Sven, Hoppe MD*; Yannick, Loosli; Daniel, Baumgartner; Paul, Heini MD§; Lorin, Benneker MD*

doi: 10.1097/BRS.0000000000000198
Biomechanics

Study Design. Biomechanical cadaveric study.

Objective. To determine whether augmentation positively influence screw stability or not.

Summary of Background Data. Implantation of pedicle screws is a common procedure in spine surgery to provide an anchorage of posterior internal fixation into vertebrae. Screw performance is highly correlated to bone quality. Therefore, polymeric cement is often injected through specifically designed perforated pedicle screws into osteoporotic bone to potentially enhance screw stability.

Methods. Caudocephalic dynamic loading was applied as quasi-physiological alternative to classical pull-out tests on 16 screws implanted in osteoporotic lumbar vertebrae and 20 screws in nonosteoporotic specimen. Load was applied using 2 different configurations simulating standard and dynamic posterior stabilization devices. Screw performance was quantified by measurement of screwhead displacement during the loading cycles. To reduce the impact of bone quality and morphology, screw performance was compared for each vertebra and averaged afterward.

Results. All screws (with or without cement) implanted in osteoporotic vertebrae showed lower performances than the ones implanted into nonosteoporotic specimen. Augmentation was negligible for screws implanted into nonosteoporotic specimen, whereas in osteoporotic vertebrae pedicle screw stability was significantly increased. For dynamic posterior stabilization system an increase of screwhead displacement was observed in comparison with standard fixation devices in both setups.

Conclusion. Augmentation enhances screw performance in patients with poor bone stock, whereas no difference is observed for patients without osteoporosis. Furthermore, dynamic stabilization systems have the possibility to fail when implanted in osteoporotic bone.

Level of Evidence: N/A

Augmentation increases pedicle screw performance in osteoporotic vertebrae. Relevance and quantification of the benefit supplied by this method has been investigated using 2 different caudocephalic compression test setups (stable and dynamic) with a stepwise increase of load. Both setups have revealed significant ameliorations of the screw-bearing capacity (up to 58.7%).

*Department of Orthopaedic Surgery, Inselspital, University of Bern, Bern, Switzerland

RMS Foundation, Bettlach, Switzerland

Laboratory of Biomechanics, ETHZ, Zurich, Switzerland; and

§Clinic Sonnenhof, Bern, Switzerland.

Address correspondence and reprint requests to Sven Hoppe, MD, Department of Orthopaedic Surgery, Inselspital, University of Bern, 3010 Bern, Switzerland; E-mail: sven.hoppe@insel.ch

Acknowledgment date: September 3, 2013. First revision date: October 18, 2013. Second revision date: December 16, 2013. Acceptance date: December 18, 2013.

The device(s)/drug(s) is/are FDA-approved or approved by corresponding national agency for this indication.

No funds were received in support of this work.

No relevant financial activities outside the submitted work.

© 2014 by Lippincott Williams & Wilkins