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Technical Briefs

Novel Synthetic Total Disk Model for Mechanical Testing of Nucleus Replacement Devices

[+] Author and Article Information
David Spenciner

Department of Orthopaedics, Brown Medical School/Rhode Island Hospital, Providence, RI 02903dspenciner@lifespan.org

Jared W. Walkenhorst, Dennis Y. Lee

 SpineWave, Inc., Shelton, CT 06484

David Paller

Department of Orthopaedics, Brown Medical School/Rhode Island Hospital, Providence, RI 02903

J. Med. Devices 2(1), 014502 (Mar 18, 2008) (6 pages) doi:10.1115/1.2884194 History: Received August 22, 2007; Revised January 24, 2008; Published March 18, 2008

We are presenting a novel, non-biologic model of the healthy human annulus. This lumbar Total Disc Model (TDM) ideally would be both biofidelic and sufficiently robust to withstand long-term fatigue testing without breaking down or tearing apart. Mechanical validation testing was performed to confirm that the compressive and torsional properties were similar to literature values of denucleated human lumbar discs in two-body constructs. Long-term fatigue tests were performed to establish the durability of the model. We have reported data for both our empty TDM and the TDM filled with a representative nucleus replacement device (NRD). The silicone model is geometrically equivalent to the healthy human lumbar disc, including the discoid cavity present following a total nuclectomy, the annulus fibrosis with micro-annulotomy, and the cartilaginous endplates. The pressure transmitted through the center of the disc went from negligible when the TDM was empty to over 40% when the TDM was filled. The compression stiffness was 992±15 N/mm for the empty TDM and 1583±136 N/mm for the filled TDM. The torsional stiffness was 0.505±0.024 Nm/° for the empty and 0.550±0.056 Nm/° for the filled TDM. Lastly, the only mechanical damage suffered by either empty or filled TDMs during dynamic testing came from debonding from the endplates at higher torque levels. No damage was seen during dynamic compression testing. After determining the appropriate geometry for the TDM, validation testing was performed to ensure that the load sharing, compressive, and torsional properties were similar to the native human disc. The silicone model was durable enough to avoid tearing or mechanical failure at physiologic loads. This study demonstrated that a silicone total disc model was developed with appropriate properties necessary for determining the mechanical degradation properties of a NRD and will not mechanical fail at physiologic loads.

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Copyright © 2008 by American Society of Mechanical Engineers
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References

Figures

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Figure 7

Digital scan of the filled TDM showing the pattern of transmitted pressure

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Figure 8

Comparison of the empty TDM to literature results for the denucleated disk (19-20)

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Figure 9

Comparison of the filled TDM to the literature results for the intact disk (21-24)

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Figure 6

Digital scan of the empty TDM showing the pattern of transmitted pressure

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Figure 5

Horizontal section of TDM

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Figure 4

Typical midsagittal sections of a filled TDM (a) and the native human disk (b)

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Figure 3

Schematic of the silicone annulus after the wax has been removed

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Figure 2

Schematic of the mold with a wax insert for making the TDM

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Figure 1

Schematic of the wax insert used to create the nuclear cavity in a TDM

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