Research Papers

Biomechanical Evaluation of an Interfacet Joint Decompression and Stabilization System

[+] Author and Article Information
Jeremi M. Leasure

The Taylor Collaboration,
St. Mary's Medical Center,
San Francisco, CA 94117
San Francisco Orthopaedic Residency Program,
San Francisco, CA 94117
e-mail: jleasure@taylorcollaboration.org

Jenni Buckley

The Taylor Collaboration,
St. Mary's Medical Center,
San Francisco, CA 94117
University of Delaware Department of Mechanical Engineering,
Newark, DE 19716

1Corresponding author.

Manuscript received April 2, 2013; final manuscript received December 17, 2013; accepted manuscript posted December 30, 2013; published online May 22, 2014. Assoc. Editor: Brian D. Stemper.

J Biomech Eng 136(7), 071010 (May 22, 2014) (8 pages) Paper No: BIO-13-1170; doi: 10.1115/1.4026363 History: Received April 02, 2013; Revised December 17, 2013; Accepted December 30, 2013

A majority of the middle-aged population exhibit cervical spondylosis that may require decompression and fusion of the affected level. Minimally invasive cervical fusion is an attractive option for decreasing operative time, morbidity, and mortality rates. A novel interfacet joint spacer (DTRAX facet screw system, Providence Medical) promises minimally invasive deployment resulting in decompression of the neuroforamen and interfacet fusion. The present study investigates the effectiveness of the device in minimizing intervertebral motion to promote fusion, decompression of the nerve root during bending activity, and performance of the implant to adhere to anatomy during repeated bending loads. We observed flexion, extension, lateral bending, and axial rotation resonant overshoot mode (ROM) in cadaver models of c-spine treated with the interfacet joint spacer (FJ spacer) as stand-alone and supplementing anterior plating. The FJ spacer was deployed bilaterally at single levels. Specimens were placed at the limit of ROM in flexion, extension, axial bending, and lateral bending. 3D images of the foramen were taken and postprocessed to quantify changes in foraminal area. Stand-alone spacer specimens were subjected to 30,000 cycles at 2 Hz of nonsimultaneous flexion-extension and lateral bending under compressive load and X-ray imaged at regular cycle intervals for quantitative measurements of device loosening. The stand-alone FJ spacer increased specimen stiffness in all directions except extension. 86% of all deployments resulted in some level of foraminal distraction. The rate of effective distraction was maintained in flexed, extended, and axially rotated postures. Two specimens demonstrated no detectable implant loosening (<0.25 mm). Three showed unilateral subclinical loosening (0.4 mm maximum), and one had subclinical loosening bilaterally (0.5 mm maximum). Results of our study are comparable to previous investigations into the stiffness of other stand-alone minimally invasive technologies. The FJ spacer system effectively increased stiffness of the affected level comparable to predicate systems. Results of this study indicate the FJ spacer increases foraminal area in the cervical spine, and decompression is maintained during bending activities. Clinical studies will be necessary to determine whether the magnitude of decompression observed in this cadaveric study will effectively treat cervical radiculopathy; however, results of this study, taken in context of successful decompression treatments in the lumbar spine, are promising for the continued development of this product. Results of this biomechanical study are encouraging for the continued investigation of this device in animal and clinical trials, as they suggest the device is well fixated and mechanically competent.

Copyright © 2014 by ASME
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Fig. 1

A test specimen in the custom-designed jig used for positioning during radiographic examination of foraminal area specimens. This specimen is shown positioned in left axial rotation. The c-arm was positioned to encircle the specimen during its 3D data capture.

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Fig. 2

(left) A rendered image from the 3D c-arm system. The rendering is positioned for measurement of right C4/5 foraminal geometry. (right) Foraminal geometry was measured from 2D screen shots of the rendered images with the foraminal space in-plane with the image. The image shown is for foraminal area measurement.

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Fig. 6

Percent increase in foraminal area of distracted specimens only in neutral, flexed, and extended postures. All values represent mean percent change. Error bars represent one standard deviation. Positive values indicate distraction of the foramen. Negative values indicate contraction.

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Fig. 5

Mean multiaxial ROM for intact, destabilized, stand-alone device, and device + plate treatment groups. Error bars represent ±1 standard deviation across all samples. (*) p < 0.05 for difference versus Intact case using repeat measures ANOVA with post hoc adjustment for individual comparisons. Left and right axial rotation and lateral bending have been combined into single mean values as shown.

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Fig. 4

Measurement locations 1 through 3 on each device nut. Also shown are the reference points for back-out measurements. Ref 1 is the bony landmark that was used for one specimen, and Ref 2 corresponds to the screw tip that was used in five of six specimens.

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Fig. 3

(left) The test fixture used for fatigue testing. The c-arm encircled the test fixture. (right) Representative test specimen (09-094 C6–C7) loaded into the fatigue test frame. The red arrows indicate the directions of the applied torque and axial compressive force. The blue arrow shows how the specimen can be rotated to obtain a lateral X-ray image.

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Fig. 7

Previous studies of stand-alone cervical facet screw fixation versus DTRAX facet screw system stand-alone. Stiffness is normalized to intact values and presented as a multiplier, e.g., 2.5 = “stiffness with implant = 2.5x intact stiffness.” Left and right axial rotation and lateral bending have been combined into single mean values as shown.




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