Research Papers

Mechanical Characterization of a Viscoelastic Disc for Lumbar Total Disc Replacement

[+] Author and Article Information
Edward C. Benzel1

Department of Neurosurgery, Neurological Institute, Cleveland Clinic, 9500 Euclid Avenue, S40, Cleveland, OH 44195benzele@ccf.org

Isador H. Lieberman

Scoliosis and Spine Tumor Center, Texas Back Institute, Texas Health Presbyterian Hospital Plano, 6020 West Parker Road, Plano, TX 75093ilieberman@texasback.com

E. Raymond Ross

 Salford Royal NHS Foundation Trust University Teaching Hospital, Stott Lane, Salford M6 8HD, UKersross@hotmail.co.uk

Raymond J. Linovitz

 CORE Orthopaedic Medical Group, 332 Santa Fe Drive, Suite 110, Encinitas, CA 92024doclino@sbcglobal.net

James Kuras

 AxioMed Spine Corporation, Garfield Heights, OH 44125jkuras@axiomed.com

Kari Zimmers

 AxioMed Spine Corporation, Garfield Heights, OH 44125kzimmers@axiomed.com


Corresponding author.

J. Med. Devices 5(1), 011005 (Mar 08, 2011) (7 pages) doi:10.1115/1.4003536 History: Received August 17, 2010; Revised January 13, 2011; Published March 08, 2011; Online March 08, 2011

A viscoelastic artificial disc may more closely replicate normal stiffness characteristics of the healthy human disc compared with first-generation total disc replacement (TDR) devices, which do not utilize viscoelastic materials and are based on a ball and socket design that does not allow loading compliance. Mechanical testing was performed to characterize the durability and range of motion (ROM) of an investigational viscoelastic TDR (VTDR) device for the lumbar spine, the Freedom® Lumbar Disc. ROM data were compared with data reported for the human lumbar disc in the clinical literature. Flexibility and stiffness of the VTDR in compression, rotation, and flexion/extension were within the parameters associated with the normal human lumbar disc. The device constrained motion to physiologic ranges and replicated normal stress/strain dynamics. No mechanical or functional failures occurred within the loads and ROM experienced by the human disc. Fatigue testing of the worst case VTDR device size demonstrated a fatigue life of 50 years of simulated walking and 240 years of simulated significant bends in both flexion/extension and lateral bending coupled with axial rotation, with no functional failures. These results indicate that the VTDR evaluated in this mechanical study is durable and has the ability to replicate the stiffness and mechanics of the natural, healthy human lumbar disc.

Copyright © 2011 by American Society of Mechanical Engineers
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Figure 1

The FLD VTDR device consisting of titanium alloy end caps and retaining plates with a viscoelastic polymer core

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Figure 2

(a) Test configuration for the axial compression and torsion ROM testing using the INSTRON 8874 bi-axial table top servohydraulic dynamic testing system (INSTRON, Canton, MA) in an environmental chamber holding PBS. (b) Test configuration for dynamic shear compression tests using the INSTRON 8874 axial table top servohydraulic dynamic testing system (INSTRON, Canton, MA) in PBS.

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Figure 3

Results of static testing in axial compression. Data points are plotted and appear as a line. Device stiffness increases with an increasing load. Vertical dashed line at 0.4 mm marks estimated boundary of neutral (high flexibility) (left of line) versus elastic (high stiffness) (right of line) zone.

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Figure 4

Axial compression fatigue curve for the VTDR. Data points and 95% confidence interval are shown. Functional failures occurred in axial compression at nonphysiologic loads of 7000–17,500 N.

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Figure 5

Dynamic stiffness of the VTDR in axial compression over 10×106 and 50×106 cycles. Dynamic stiffness of the device remained constant throughout testing, even at the nonphysiologic loads of 6000 N and 7000 N.

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Figure 6

45 deg compressive shear testing fatigue curve for the VTDR. Data points and 95% confidence interval are shown. The device demonstrated an endurance limit load of 1200 N in 45 deg compressive shear. 45 deg compressive shear loads of 1200–2000 N correspond to loads in anterior shear of 1697–2828 N.




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