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Relaxation of Forces Needed to Distract Cervical Vertebrae After Discectomy: A Biomechanical Study

Aryan, Henry E. MD*; Newman, C. Benjamin MD; Lu, Daniel C. MD, PhD*; Hu, Serena S. MD; Tay, Bobby K. MD; Bradford, David S. MD; Puttlitz, Christian M. PhD§; Ames, Christopher P. MD*

doi: 10.1097/BSD.0b013e318168d9c0
Original Articles

Study Design In vitro and in vivo biomechanical stress measurements are made of the intervertebral disc segment distraction force during anterior cervical discectomy.

Objective The purpose of this study is to determine the short-term force relaxation of the native intervertebral disc segment and to determine the short-term force relaxation of the segment after removal of the intervertebral disc, as is commonly performed in anterior cervical discectomy with fusion and arthroplasty.

Summary of Background Data No published data examine the issue of intraoperative distraction force of the cervical intervertebral disc segment. This is a novel research in this area.

Methods In vitro and in vivo studies under institutional review board approval were performed to determine the mechanical behavior of the normal and diseased cervical functional spinal unit. Seven in vitro and 11 in vivo spines were studied. Strain measurements between distracting Caspar-type pins were made before, at various points during, and after discectomy to assess how removal of the disc and other spinal components affects the force-displacement behavior of the spinal unit.

Results The in vitro data show progressive reduction in force needed for distraction after discectomy and uncovertebral joint resection. Greatest reduction is noted after discectomy. The in vivo data indicate that, on average, the cervical functional spinal unit requires 20 N less force to achieve the same degree of distraction after removal of the intervertebral disc.

Conclusions A sharp reduction in the strain across the intervertebral space occurs after distraction. The removal of the cervical intervertebral disc significantly reduces the viscoelastic response of the cervical motion segment. The long-term force used to stabilize intervertebral grafts or implants is less than what is achieved at the time of distraction. The exact magnitude of the resultant force on graft or device at a given distraction force is unknown and would depend also upon fit.

Departments of *Neurological Surgery

Orthopedic Surgery, University of California, San Francisco

Division of Neurosurgery, University of California, San Diego, CA

§Department of Biomedical Engineering, Colorado State University, Fort Collins, CO

Disclosures: The authors have not received financial assistance or own stock (or stock options) in a commercial company or institution related directly or indirectly to the subject of this manuscript.

The FDA has not cleared the medical device for the use described in this manuscript.

Reprints: Charles Benjamin Newman, MD, Division of Neurosurgery, University of California, San Diego, 200 West Arbor Drive, Mailcode 8893, San Diego, CA 92103 (e-mail:

Received for publication August 16, 2007; accepted January 5, 2008

© 2009 Lippincott Williams & Wilkins, Inc.