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Effects of Bone Cement Volume and Distribution on Vertebral Stiffness After Vertebroplasty

Liebschner, Michael A.K. PhD,*; Rosenberg, William S. MD,† and; Keaveny, Tony M. PhD*‡

Biomechanics

Study Design. The biomechanical behavior of a single lumbar vertebral body after various surgical treatments with acrylic vertebroplasty was parametrically studied using finite-element analysis.

Objectives. To provide a theoretical framework for understanding and optimizing the biomechanics of vertebroplasty. Specifically, to investigate the effects of volume and distribution of bone cement on stiffness recovery of the vertebral body.

Summary of Background Data. Vertebroplasty is a treatment that stabilizes a fractured vertebra by addition of bone cement. However, there is currently no information available on the optimal volume and distribution of the filler material in terms of stiffness recovery of the damaged vertebral body.

Methods. An experimentally calibrated, anatomically accurate finite-element model of an elderly L1 vertebral body was developed. Damage was simulated in each element based on empirical measurements in response to a uniform compressive load. After virtual vertebroplasty (bone cement filling range of 1–7 cm3) on the damaged model, the resulting compressive stiffness of the vertebral body was computed for various spatial distributions of the filling material and different loading conditions.

Results. Vertebral stiffness recovery after vertebroplasty was strongly influenced by the volume fraction of the implanted cement. Only a small amount of bone cement (14% fill or 3.5 cm3) was necessary to restore stiffness of the damaged vertebral body to the predamaged value. Use of a 30% fill increased stiffness by more than 50% compared with the predamaged value. Whereas the unipedicular distributions exhibited a comparative stiffness to the bipedicular or posterolateral cases, it showed a medial–lateral bending motion (“toggle”) toward the untreated side when a uniform compressive pressure load was applied.

Conclusion. Only a small amount of bone cement (∼15% volume fraction) is needed to restore stiffness to predamage levels, and greater filling can result in substantial increase in stiffness well beyond the intact level. Such overfilling also renders the system more sensitive to the placement of the cement because asymmetric distributions with large fills can promote single-sided load transfer and thus toggle. These results suggest that large fill volumes may not be the most biomechanically optimal configuration, and an improvement might be achieved by use of lower cement volume with symmetric placement.

From the *Orthopaedic Biomechanics Laboratory, Department of Mechanical Engineering, and the Departments of

†Neurological Surgery, and

‡Bioengineering, University of California, Berkeley, California.

Supported by NSF BES-9625030, NIH AR41481, the University of California Academic Senate, and an unrestricted gift from Kyphon Inc., Santa Clara, California.

Acknowledgment date: September 12, 2000.

Acceptance date: December 14, 2000.

Address reprint requests to

Tony M. Keaveny, PhD

6175 Etcheverry Hall

University of California

Berkeley, CA 94720-1740

E-mail: tmk@me.berkeley.edu

Device status category: 1.

Conflict of interest category: 15.

© 2001 Lippincott Williams & Wilkins, Inc.