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The Fracture Risk of Adjacent Vertebrae Is Increased by the Changed Loading Direction After a Wedge Fracture

Aquarius, René, MSc*; Homminga, Jasper, PhD; Verdonschot, Nico, PhD*,†; Tanck, Esther, PhD*

doi: 10.1097/BRS.0b013e3181f0f726
Biomechanics
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Study Design. In vitro biomechanical study.

Objective. To measure the effect that off-axis vertebral loading has on the stiffness and failure load of vertebrae.

Summary of Background Data. Adjacent level vertebral fractures not only are common in patients who received a vertebroplasty treatment but also occur in patients with conservatively treated wedge fractures. The wedge-like deformity, which is present in both groups, changes the spinal alignment. The load of vertebrae adjacent to the fractured vertebra will change from perpendicular to the endplate to a more shearing, off-axis, load. This change may induce a higher fracture risk for vertebrae adjacent to wedge-like deformed vertebrae.

Methods. Twenty vertebrae, harvested from one osteopenic cadaver spine and three osteoporotic cadaver spines, were loaded until failure. The vertebrae were loaded either perpendicular to the upper endplate, representing vertebrae in a spine without wedge fractures (0° group, n = 10), or at an angle of 20°, representing vertebrae adjacent to a wedge fracture (20° groups, n = 10). Vertebral failure load and stiffness were the most important outcome measures.

Results. The failure load was significantly higher (P = 0.028) when tested at 0° (2854 N, SD = 622 N), compared with vertebrae tested at 20° (2162 N, SD = 670 N). Vertebrae were also significantly stiffer (P < 0.001) when tested at 0° (4017 N/mm, SD = 970 N/mm) than those tested at 20° (2478 N/mm, SD = 453 N/mm).

Conclusion. The failure load of osteopenic/osteoporotic vertebrae was 24% lower under off-axis loads (20°) than under axial loads (0°). This study may lead to a better understanding of the etiology of adjacent vertebral fractures after wedge-like deformities and demonstrates the importance of height reconstruction of wedge fractures in order to normalize the loading conditions on adjacent vertebrae.

Human vertebrae from four diff erent cadavers were tested under either an axial load (0°) or an off-axis load (20°). Failure load of osteopenic/osteoporotic vertebrae was 24% lower during off-axis loading than during axial loading. These data help to better understand the etiology of adjacent vertebral fractures after wedge-like deformities

*Orthopaedic Research Laboratory, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands.

University of Twente, Enschede, The Netherlands.

Address correspondence and reprint requests to Esther Tanck, PhD, Orthopaedic Research Laboratory, Department of Orthopaedics, Radboud University Nijmegen Medical Centre, P.O. Box 9101, 6500 HB, Nijmegen, The Netherlands; E-mail: e.tanck@orthop.umcn.nl

Acknowledgment date: February 17, 2010. First revision date: May 6, 2010. Second revision date: June 9, 2010. Third revision date: July 1, 2010. Acceptance date: July 6, 2010.

The manuscript submitted does not contain information about medical device(s)/drug(s). Foundation 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.

© 2011 Lippincott Williams & Wilkins, Inc.