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Enhancing Pedicle Screw Fixation in the Aging Spine With a Novel Bioactive Bone Cement: An In Vitro Biomechanical Study

Zhu, Qingan, PhD*; Kingwell, Stephen, MD*; Li, Zhaoyang, PhD; Pan, Haobo, PhD; Lu, William W., PhD; Oxland, Thomas R., PhD*,‡

doi: 10.1097/BRS.0b013e31825a676e
Biomechanics
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Study Design. A paired biomechanical study of pedicle screws augmented with bone cement in a human cadaveric and osteoporotic lumbar spine model.

Objectives. To evaluate immediate strength and stiffness of pedicle screw fixation augmented with a novel bioactive bone cement in an osteoporotic spine model and compare it with polymethylmethacrylate (PMMA) cement.

Summary of Background Data. A novel bioactive bone cement, containing nanoscale particles of strontium and hydroxyapatite (Sr-HA), can promote new bone formation and osteointegration and provides a promising reinforcement to the osteoporotic spine. Its immediate mechanical performance in augmenting pedicle screw fixation has not been evaluated.

Methods. Two pedicle screws augmented with Sr-HA and PMMA cement were applied to each of 10 isolated cadaveric L3 vertebrae. Each screw was subjected to a toggling test and screw kinematics were calculated. The pedicle screw was subjected to a pullout test until failure. Finally, the screw coverage with cement was measured on computed tomographic images.

Results. Screw translations in the toggling test were consistently larger in the Sr-HA group than in the PMMA group (1.4 ± 1.2 mm vs. 1.0 ± 1.1 mm at 1000 cycles). The rotation center was located closer to the screw tip in the Sr-HA group (19% of screw length) than in the PMMA group (37%). The only kinematic difference between Sr-HA and PMMA cements was the screw rotation at 1000 cycles (1.5° ± 0.9° vs. 1.3° ± 0.6°; P = 0.0026). All motion parameters increased significantly with more loading cycles. The pullout force was higher in the PMMA group than the Sr-HA group (1.40 ± 0.63 kN vs. 0.93 ± 0.70 kN), and this difference was marginally significant (P = 0.051). Sr-HA cement covered more of the screw length than PMMA cement (79 ± 19% vs. 43 ± 19%) (P = 0.036).

Conclusion. This paired-design study identified some subtle but mostly nonsignificant differences in immediate biomechanical fixation of pedicle screws augmented with the Sr-HA cement compared with the PMMA cement.

The biomechanical performance of pedicle screws augmented with a novel bioactive bone cement containing nanoscale particles of strontium and hydroxyapatite was evaluated and compared with screws augmented with polymethylmethacrylate cement. Under toggling and pullout tests, this study identified some subtle but mostly nonsignificant differences in immediate kinematics and pullout strength.

*Departments of Orthopaedics, University of British Columbia and Vancouver Coastal Health Research Institute, Vancouver, British Columbia, Canada

Department of Orthopaedics, University of Hong Kong, Hong Kong, China

Mechanical Engineering, University of British Columbia and Vancouver Coastal Health Research Institute, Vancouver, British Columbia, Canada.

Address for correspondence and reprint requests to Thomas R. Oxland, PhD, Department of Orthopaedics, University of British Columbia, 5460-818 West 10th Ave., Vancouver, BC V5Z 1M9; E-mail: toxland@exchange.ubc.ca

Acknowledgment date: September 30, 2011. Revision date: March 9, 2012. Acceptance date: March 16, 2012.

The legal regulatory status of the device(s)/drug(s) that is/are the subject of this manuscript is not applicable in my country.

No funds were received in support of this work.

No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

© 2012 Lippincott Williams & Wilkins, Inc.