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Biomechanics of Prophylactic Vertebral Reinforcement

Sun, Kay BSc; Liebschner, Michael A. K. PhD

Spine:
doi: 10.1097/01.BRS.0000128760.94173.90
Biomechanics
Abstract

Study Design. The effects of bone cement placement, volume, and bone density on the degree of biomechanical reinforcement on cadaveric vertebral bodies were studied using experimentally calibrated detailed finite element models.

Objectives. To investigate the efficacy of prophylactic vertebroplasty on intact vertebral bodies with respect to biomechanical recovery and fracture risk reduction.

Summary of Background Data. Vertebroplasty is a potentially effective fracture prevention treatment, but the risk of complications due to cement leakage must be minimized. Therefore, the least amount of bone cement required to improve vertebral strengths to low fracture risk levels need to be determined.

Methods. Six different polymethyl methacrylate volumes—1, 2.5, 3.5, 5, 7.5 and 9 cm3—were virtually implanted into previously validated vertebral body finite element models, following bipedicular and posterolateral vertebroplasty approaches. Stiffness and fracture load of the treated and untreated vertebral body models under uniaxial compression were determined.

Results. Greater augmentation effects were observed for vertebral bodies with average quantitative computed tomography densities below 0.1 g/cm3 injected with polymethyl methacrylate volumes higher than 20% compared to lower injection volumes and higher bone densities, as well as for the bipedicular approach versus posterolateral. Vertebral bodies at high risk of fracture required at least 20% fill of polymethyl methacrylate to improve the mechanical integrity of vertebral bodies to low fracture risk levels, whereas 5% to 15% polymethyl methacrylate volumes were needed for the medium-risk vertebral bodies.

Conclusion. Prophylactic vertebroplasty can be effective in reducing fracture risk. However, for the polymethyl methacrylate volume (20%) required for the successful reinforcement of high-risk vertebral bodies, the risk of complications will be as high as that for current vertebroplasty procedure for fracture repair. Therefore, alternative materials have to be investigated for prophylactic vertebroplasty. Furthermore, bipedicular vertebroplasty is the recommended approach due to its higher strengthening effect and easier surgical access than the posterolateral case.

In Brief

Finite element models of whole vertebral bodies were used to investigate the biomechanical effects of bone cement (polymethyl methacrylate) volume and placement, as well as the initial bone density on prophylactic vertebroplasty. Greater strengthening and stiffening effects were generally observed for the bipedicular vertebroplasty approach compared to posterolateral and for vertebral bodies with low bone densities. For successful reinforcement of high fracture risk vertebral bodies, about 20% volume fill of polymethyl methacrylate is needed, which does not reduce current complications of vertebroplasty. As such, material properties of polymethyl methacrylate may not be optimal for vertebral reinforcement, and alternative materials need to be investigated.

Author Information

From the Department of Bioengineering, Rice University, Houston, Texas.

Financial support for this work was provided by the Whitaker Foundation.

Acknowledgment date: May 30, 2003. First revision date: August 14, 2003. Acceptance date: September 4, 2003.

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

Institutional 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.

Address correspondence and reprint requests to Michael A.K. Liebschner, PhD, Department of Bioengineering, MS-142, Rice University, 6100 Main Street, Houston, TX 77005; E-mail: Liebschner@rice.edu

© 2004 Lippincott Williams & Wilkins, Inc.