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Use of Ultrasound in Spinal Arthrodesis: A Rabbit Model

Glazer, Paul A., MD*; Heilmann, Moira R., MS; Lotz, Jeffrey C., PhD; Bradford, David S., MD

Basic Science

Study Design. The biomechanical and histologic characteristics of posterolateral spinal fusion in a rabbit model with and without the application of low-intensity ultrasound were analyzed.

Objectives. To evaluate the use of ultrasound to improve the spinal fusion rate and biomechanical characteristics of the fusion mass in a rabbit model.

Summary of Background Data. This is the first study in which the benefits of ultrasound in spinal fusion have been assessed. Posterolateral intertransverse process fusion in the rabbit has a pseudarthrosis rate similar to that recorded in humans (5-40%).

Methods. Fourteen New Zealand White rabbits were randomly assigned to each of two groups to undergo spinal fusion using autologous bone with ultrasound or autologous bone without ultrasound. A specially designed plastic constraint was used to focus the ultrasound over the rabbits' lumbar spine 20 minutes per day. Animals were killed at 6 weeks for biomechanical and histologic testing.

Results. The rate of pseudarthrosis, evaluated radiographically and manually in a blinded fashion, decreased at a statistically significant rate (from 35% to 7%) with ultrasound. Biomechanical analysis of the fusion mass showed that ultrasound resulted in statistically significant increases in stiffness (33%; P = 0.03), area under the load displacement curve (25%; P = 0.05), and load to failure of the fusion mass (24%; P = 0.04). Qualitative histologic assessment showed increased bone formation in those fusions exposed to ultrasound.

Conclusions. Lumbar spinal fusion is a complex biologic process. The results of the current study demonstrate the reproducibility of a rabbit fusion model and the ability of ultrasound to induce a statistically significant increase in fusion rate, stiffness, area under the load displacement curve, and load to failure of the fusion mass. These results provide a basis for continued evaluation of biologic improvement of spinal arthrodesis with the use of ultrasound.

From the *Department of Orthopaedic Surgery, Beth Israel-Deaconess Medical Center, Harvard University, Boston, Massachusetts; and the †Department of Orthopaedic Surgery, University of California, San Francisco.

Supported, in part, by Exogen, Inc., Piscataway, NJ, and the National Orthopaedic Surgery Fellows Foundation Grant, 1995, Department of Orthopaedic Surgery, University of California, San Francisco.

Acknowledgment date: April 9, 1997.

First revision date: August 6, 1997.

Acceptance date: November 3, 1997.

Device Status category: 4.

Address reprint requests to: Jeffrey C. Lotz, PhD; Department of Orthopaedic Surgery; University of California, San Francisco; Box 0728; San Francisco, CA 94143-0728; E-mail:

© Lippincott-Raven Publishers.