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Decompression and paraspinous tension band: a novel treatment method for patients with lumbar spinal stenosis and degenerative spondylolisthesis
Accepted Manuscript Decompression and paraspinous tension band: a novel treatment method for patients with lumbar spinal stenosis and degenerative spondylolisthesis J N Alastair Gibson, MD, FRCS, Bart Depreitere, MD, PhD, Robert Pflugmacher, MD, Klaus J. Schnake, MD, Louis C. Fielding, MS, Todd F. Alamin, MD, Jan Goffin, MD, PhD PII:
S1529-9430(15)00004-2
DOI:
10.1016/j.spinee.2015.01.003
Reference:
SPINEE 56153
To appear in:
The Spine Journal
Received Date: 3 October 2014 Revised Date:
29 December 2014
Accepted Date: 2 January 2015
Please cite this article as: Gibson JNA, Depreitere B, Pflugmacher R, Schnake KJ, Fielding LC, Alamin TF, Goffin J, Decompression and paraspinous tension band: a novel treatment method for patients with lumbar spinal stenosis and degenerative spondylolisthesis, The Spine Journal (2015), doi: 10.1016/ j.spinee.2015.01.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Decompression and paraspinous tension band for degenerative spondylolisthesis
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J N Alastair Gibson, MD, FRCS
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Spinal Unit, Royal Infirmary of Edinburgh and the University of Edinburgh
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Edinburgh, Scotland
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Bart Depreitere, MD, PhD
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Decompression and paraspinous tension band: a novel treatment method for patients with lumbar spinal stenosis and degenerative spondylolisthesis
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Department of Neurosciences, Universitaire Ziekenhuizen KU Leuven
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Leuven, Belgium
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Robert Pflugmacher, MD
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Klinik und Poliklinik für Orthopädie und Unfallchirurgie, Universitätsklinikum Bonn
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Bonn, Germany
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Klaus J Schnake, MD
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Zentrum für Wirbelsäulentherapie, Schön Klinik Nürnberg Fürth
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Fürth, Germany
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Louis C Fielding, MS
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Simpirica Spine
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San Carlos, CA, USA
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Todd F Alamin, MD
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Department of Orthopaedic Surgery, Stanford University School of Medicine
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450 Broadway St | Pavillion A FL 1 MC6110 | Redwood City, CA 94063
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Jan Goffin, MD, PhD
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Department of Neurosciences, Universitaire Ziekenhuizen KU Leuven
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Leuven, Belgium
email: [email protected] | tel: +1 (650) 721-7616 | fax: +1 (650) 721-3409
Funding for this work was provided by Simpirica Spine, Inc.
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Decompression and paraspinous tension band for degenerative spondylolisthesis
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ABSTRACT
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BACKGROUND CONTEXT: Prior studies have demonstrated superiority of
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decompression and fusion over decompression alone for treatment of lumbar degenerative
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spondylolisthesis with spinal stenosis. More recent studies have investigated whether non-
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fusion stabilization could provide the durable clinical improvement after decompression and
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fusion.
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PURPOSE: To examine the clinical safety and effectiveness of decompression and
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implantation of a novel flexion restricting paraspinous tension band (PTB) for patients with
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degenerative spondylolisthesis.
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STUDY DESIGN/SETTING: Prospective Clinical Study
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PATIENT SAMPLE: Forty-one patients (7 men and 34 women) aged 45-83 years (68.2±9.0)
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were recruited with symptomatic spinal stenosis and Meyerding grade 1 or 2 degenerative
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spondylolisthesis at L3/4 (8) or L4/5 (33).
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OUTCOME MEASURES: Self-reported measures included visual-analog scales (VAS) for
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leg, back and hip pain as well as the Oswestry disability index (ODI). Physiologic measures
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included quantitative and qualitative radiographic analysis performed by an independent core
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laboratory.
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METHODS: Patients with lumbar degenerative spondylolisthesis and stenosis were
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prospectively enrolled at four European spine centers with independent monitoring of data.
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Clinical and radiographic outcome data collected pre-operatively were compared with data
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collected at three, six, 12 and 24 months after surgery. This study was sponsored by the PTB
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manufacturer (Simpirica Spine Inc., San Carlos, CA, USA), including institutional research
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support grants to the participating centers totaling approximately US $172,000.
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RESULTS: Statistically significant improvements and clinically-important effect sizes were
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seen for all pain and disability measurements. At 24 months follow-up, ODI scores were
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reduced by an average of 25.4 points (59%) and maximum leg pain on VAS by 48.1mm
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(65%). Back pain VAS scores improved from 54.1 by an average of 28.5 pts (53%). There
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was one postoperative wound infection (2.4%) and an overall reoperation rate of 12%.
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Eighty-two percent of the patients available for 24 month follow-up with a PTB in situ had a
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reduction in ODI of >15 points, and 74% had a reduction in maximum leg pain VAS
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>20mm. According to Odom's criteria the majority of these patients (82%) had an excellent
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or good outcome with all except one patient satisfied with surgery. As measured by the
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independent core laboratory, there was no significant increase in spondylolisthesis,
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segmental flexion-extension range of motion or translation and no loss of lordosis in the
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patients with the PTB at 2 years follow-up.
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CONCLUSIONS: Patients with degenerative spondylolisthesis and spinal stenosis treated
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with decompression and a PTB demonstrated no progressive instability at 2 years of follow-
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up. Excellent/good outcomes and significant improvements in patient-reported pain and
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disability scores were still observed at 2 years with no evidence of implant failure or
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migration. Further study of this treatment method is warranted to validate these findings.
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INTRODUCTION
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Decompression alone was the standard surgical treatment for lumbar degenerative
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spondylolisthesis with spinal stenosis [1,2] until several landmark articles suggested that
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decompression and fusion was a superior mode of treatment [3-5]. While there has been
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recent pressure to reduce rates of spinal fusion, payers and medical societies consistently
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recommend that fusion be determined medically necessary for lumbar degenerative
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spondylolisthesis with spinal stenosis [6-8], and it remains the dominant procedure for this
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indication in the US shown by a 95% fusion rate in the SPORT study [9]. However, fusion
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is clearly an imperfect treatment due to the short-term morbidity involved in adding a fusion
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to the decompression, as well as the longer term associated risk of adjacent-level
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degeneration and instability [10,11]. To circumvent these problems “dynamic stabilization”
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systems were introduced [12,13], but the majority are pedicle screw based and associated
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with many of the same problems as instrumented fusion, notably complex implant loading
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[14-18] and facet joint violation during screw placement [19-21]. For this reason simpler
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constructs have been suggested, such as that by Lee et al who reported a series of 65 patients
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with degenerative spondylolisthesis treated with decompression and posterior tension band
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stabilization using a figure of eight suture. They found that back pain relief and functional
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improvement were significantly correlated with achievement of total and segmental lumbar
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lordosis after a mean follow-up of 72.5 months [22] and equivalent outcomes could be
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achieved in comparison to posterior lumbar interbody fusion [23].
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Based on the need for segmental stabilization in this patient population, the relative draw
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backs of fusion and pedicle screw-based dynamic stabilization, and the promising outcomes
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of tension band stabilization reported by Lee et al, a novel paraspinous tension band (PTB;
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LimiFlex™ Spinal Stabilization System, Simpirica Spine Inc., San Carlos, CA, USA) has
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been developed to provide segmental sagittal plane stability through biasing the segment into
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lordosis. The facet joints are more engaged and afford more sagittal plane stability in
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segmental extension than in flexion due to the coronal orientation of the caudad portion of
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the joint [24], and thus their biomechanics allow for this indirect mechanism of providing
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stability. The PTB utilizes titanium coil tension springs to restore segmental flexion
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stiffness, and is attached to the spinous processes with ultra-high molecular weight
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polyethylene (UHMWPE) bands. A preclinical large-animal implantation study
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demonstrated that the PTB is well tolerated by the surrounding tissues, and retains its
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function after anatomical incorporation [25]. Cadaveric biomechanical testing has
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demonstrated restoration of the kinematics of a destabilized spinal segment to that of an
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intact segment, with increased flexion stiffness and reduced sagittal translation [26]. The
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device does not bear any axial loads, and therefore the forces transmitted by the PTB to the
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spinal elements are an order of magnitude less compared to pedicle screw-based systems.
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The PTB was designed to be easily implanted after a standard lumbar decompression with
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minimal additional exposure.
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We aimed to prospectively assess clinical and radiographic outcomes of patients treated with
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surgical decompression and stabilization with the LimiFlex PTB device over a two year follow-up period. We questioned whether these patients would become progressively more
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unstable from a radiographic standpoint over this length of follow-up, and whether their
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clinical improvement in back and leg pain would deteriorate. This study was intended to
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provide initial data to demonstrate the clinical feasibility of this treatment method.
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MATERIALS AND METHODS
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Patient Selection
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Between January 2010 and October 2011, 41 patients at four centers (Royal Infirmary of
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Edinburgh, Edinburgh, Scotland, UK; Universitaire Ziekenhuizen KU Leuven, Leuven,
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Belgium; Berufsgenossenschaftliche Unfallklinik, Frankfurt am Main, Germany;
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Universitätsklinikum Bonn, Bonn, Germany) were enrolled in clinical studies and followed
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for 24 months postoperatively. Informed consent was obtained from each of the patients in
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the series. All patients had degenerative spondylolisthesis (Meyerding grade 1 or 2) and
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spinal stenosis requiring decompression of at least one level (1-3 levels). Patients were
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excluded if they had spinous processes or posterior element anatomy inappropriate for
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implant fixation, severe osteoporosis (T-score ≤ -2.5 with fragility fracture) [27], isthmic
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spondylolisthesis of the segment to be instrumented, active systemic or local infection, or if
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they were pregnant or planning to become pregnant. These patients were felt to be otherwise
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candidates for lumbar fusion by their operating physician. The data presented here represent
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patients from this prospective, post-market case series that were approved by the Ethics
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Committees of each participating institution. This study was sponsored by the PTB
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manufacturer (Simpirica Spine Inc., San Carlos, CA, USA), including institutional research
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support grants to the participating centers totaling approximately US $172,000.
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Surgical Intervention
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Decompression
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All patients received a direct surgical decompression of up to three levels from L1-S1 to
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address their spinal stenosis, preserving at least 50% of the spinous processes. All
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decompressions were performed using an operating microscope, undercutting the facet joints
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and decompressing the foraminae as necessary. The structural integrity of the pars
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interarticularis was preserved and resection of the articular surface of the facet joints
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minimized. In some cases the midline supraspinous/ interspinous ligamentous complex was
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preserved and in other cases not according to the preference of the treating surgeon.
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Decompression characteristics are summarized in Table 2.
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Paraspinous Tension Band Implantation
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Following their decompression, all patients were implanted with a PTB at the level of the
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degenerative spondylolisthesis. The PTB comprises a pair of titanium coil springs secured
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to ultra-high molecular weight polyethylene (UHMWPE) straps (Fig. 1). The straps were
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passed around the cranial and caudal spinous processes, piercing the interspinous ligament of
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the segments adjacent to the treated segment. The patient was then repositioned, if necessary,
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by adjusting the amount of flexion on the spinal frame / table using intraoperative
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fluoroscopy so that local lumbar sagittal alignment approximated a standing lordotic
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position. This alignment was estimated by the treating surgeon; no fixed threshold values
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were used. The device could then be tensioned and secured using instruments that allowed a
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consistent nominal preload to the springs (approx. 20N). The decompression was reassessed
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after the application of the device.
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Outcome assessment
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Preoperatively, and at 3, 6, 12, and 24 months postoperatively patients rated their back, leg,
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and buttock/hip pain on a 0-100mm longitudinal VAS and their back pain disability (ODI)
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[28,29]. Patient outcomes were also assessed according to patient-reported satisfaction with
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their surgery and Odom’s criteria [30]. Clinical success was evaluated using the composite
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clinical success criteria utilized by Davis et al for this indication [31], whereby to be
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considered a clinical success a patient was required to meet all the following criteria: 15-
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point reduction in ODI, no reoperations, no major device-related complications, and no
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postoperative epidural injections.
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Intraoperative and postoperative complications were noted and compiled, and then attributed
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as either being related or unrelated to the device or the procedure by the study site
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investigator. All serious adverse events (SAEs) were adjudicated by an independent medical
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monitor who was a board certified orthopaedic spine surgeon.
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Diagnostic axial imaging studies (CT or MRI) were obtained preoperatively to confirm
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spinal canal stenosis and to evaluate degenerative lumbar pathology. Flexion, extension,
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standing lateral, and anterior/posterior (AP) radiographs were obtained preoperatively, as
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well as at 3, 6, 12, and 24 months postoperatively (Figs. 2,3). Quantitative and qualitative
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radiographic analyses were performed by a radiographic core laboratory using Quantitative
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Motion Analysis (QMA™, Medical Metrics, Houston, TX) [32]. Quantitative radiographic
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analysis included: •
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Static measurements of disc angle, anterior slip, disc height, and total lumbar (L1-S1) angle in flexion, neutral and extension.
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Dynamic measurements of rotation and translation in flexion-extension.
The qualitative radiographic analysis included assessments by a board-certified radiologist
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for device condition, device migration/dislocation, spinous process fracture, bone-implant
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interface remodeling, and exuberant bone formation.
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Statistical analyses
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Data were assessed for normality, and changes in ODI and VAS were assessed using 2-
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tailed, paired Student’s t-tests. A significance level of p≤0.05 was used. Confidence
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intervals for parametric statistics were calculated assuming a t-distribution. Binomial
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confidence intervals were calculated for success criteria using the Normal Approximation
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Method.
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RESULTS
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Patient demographics, operative results and follow-up
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Patient demographics are summarized in Table 1. The ratio of approximately 5:1 female to
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male and ages of the 41 patients are typical of those reported for degenerative
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spondylolisthesis [1,3]. These patients represent a relatively high-risk group, with a mean
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Charlson comorbidity index of 3.8 [33].
3 Operative results including decompression characteristics are summarized in Table 2. All
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forty-one patients received decompressions and were successfully implanted with the PTB
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with a mean implantation time of 28 minutes (range: 10-58 minutes), which included initial
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learning curve procedures. Thirty-four patients with the PTB in situ were available for 24-
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month follow-up evaluation. One died from a traumatic head injury and one patient was lost
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to follow-up. The device was explanted prior to final follow-up in five patients as described further below.
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11 Clinical outcomes
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Clinical outcomes are presented for the group of patients with all follow-up data, as well as
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for the sub-group of patients with the PTB in situ at 24 months. Statistically significant
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improvements and clinically important effect sizes were seen for all pain and disability
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measurements. Disability according to ODI was reduced by an average of 25.4 points (59%
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compared to baseline). Maximum leg pain VAS was reduced by an average of 48.1mm
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(65%). Back pain VAS averaged 54.1 preoperatively, and the postoperative value at two
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years was 22.6mm, representing an improvement of 28.5 mm (54%) (Table 3).
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Improvement in patient self-reported outcomes showed a significant improvement at the first
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(3 months) follow-up visit, which was sustained with a trend toward increased improvement
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throughout follow-up to 24 months (Figure 4). The clinical success rate according to the
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criteria established by Davis et al was 66.7% [31].
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Eighty-two percent of patients available for 24 month follow-up with the PTB in situ had a
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reduction in ODI of at least 15 points, and 74% had a reduction in maximum leg pain VAS
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of at least 20mm. Eighty-two percent (82%) of these patients were evaluated to have
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excellent or good outcomes according to Odom’s criteria, and 97% were satisfied with the
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surgery. (Table 4)
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Radiographic results
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Quantitative analysis
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No change in vertebral angulation or olisthesis was noted on device implantation or seen in
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radiographic measurements at 3 months follow-up. Segmental stability and lordosis were
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maintained to 24 months (Table 5). There were no significant increases in anterior slip,
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dynamic sagittal translation, or flexion-extension range of motion (ROM). Three-level
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ROM of the index, supradjacent and subjacent levels was relatively low preoperatively,
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indicating low patient effort in flexion-extension, with a trend towards increased overall
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motion at 24 months follow-up, in comparison to the index level ROM which showed no
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significant change during follow-up. Exemplary images are shown in Figures 2 and 3. Six
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patients with 24-month follow-up and the PTB in situ did not have all radiographic films
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necessary for quantitative analysis.
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Qualitative analysis
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Throughout the follow-up period, at all time-points, there were no x-ray findings of device
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breakage, disassembly, migration or dislocation; bone-implant interface remodeling; or
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exuberant bone formation. There were no findings of resorption at the strap-spinous process
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interface. There was one spinous process fracture with an unclear time of origin that will be
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discussed below, which was found on a postoperative CT scan.
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Complications
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Seven intra-operative adverse events were reported, including six dural tears and a
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superficial fracture of the tip of a spinous process. The dural tears all occurred during the
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decompressions prior to the implantation of the device, were all directly sutured at the time
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of decompression, and did not affect device implantation. There were no post-operative CSF
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leaks but there was one postoperative infection (2.4%) that was treated with irrigation and
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debridement, implant removal and fusion.
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During the follow-up period there were two other patients in the study who underwent
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conversion to fusion. One had persistent pain postoperatively and a sclerotic spinous process
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fracture was found on CT imaging 6 months after surgery. Instability was apparent on the
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flexion/extension films. It was not clear whether the fracture happened during the
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decompression or at a later date. An L4/5 fusion was performed. A second patient, who
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complained of persistent leg pain, was found on repeat imaging to have foraminal stenosis at
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the level below her L4/5 treated segment with no evidence of progressive instability or
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recurrent stenosis at L4/5. She was fused from L4 to S1.
3 Three days after surgery one patient presented with a symptomatic epidural hematoma
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leading to paraparesis. An emergency laminectomy was performed and the device removed.
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One patient developed a proximally migrated disc extrusion at the inferior adjacent segment
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nine months after surgery, and underwent discectomy. The device was removed to allow for
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easier access to the fragment. These complications represent an overall 12% reoperation
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rate.
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Explanted devices were sent to independent laboratories for examination (Exponent,
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Philadelphia, PA) and histopathology (Histion, LLC, Everett, WA). Examination of the
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removed implants found no device failures. Histopathology findings were that the implant
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was well integrated with fibrous tissue with no adverse tissue response.
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DISCUSSION
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All PTBs were easily implanted without implant-related complication and a median insertion
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time of 25 minutes. At two-year follow-up, this cohort of patients with degenerative
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spondylolisthesis showed significant clinical improvement in self-reported pain and
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disability scores from first follow-up at 3 months, with sustained improvement through the
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24 months follow-up period. Radiographically there was no evidence of progression in static
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or dynamic translation. Intraoperatively there was incidental durotomy in six of the 41
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patients (14.6%), probably a reflection of the elderly population studied (mean age 68yrs).
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The overall re-operation rate (12%) was slightly higher than expected but may have simply
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reflected an initial experience with a new technique; complications were generally either
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unrelated or of indeterminate relationship to the PTB. The simplicity of the primary surgery
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avoiding damage to the facets at the instrumented level meant that any revision surgery was
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generally straightforward. Analysis of explanted devices found no device failures and that
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the implant was well integrated into the paraspinal tissue with no adverse tissue response.
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There were no radiographic findings of spinous process resorption at the strap interface,
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which is consistent with the large animal implantation study previously conducted on the
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PTB [25] and implying that there was no functional loss of device tension due to remodeling
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of the bone-implant interface.
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1 As this work represents a preliminary series from a small cohort without a control group, we
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have evaluated our results in context with published literature for general comparative
4
purposes. The 118 minute average surgical time for decompression and PTB implantation
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compares favorably to the 210.5 minutes and 202.7 minutes reported for the randomized and
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non-randomized cohorts of surgical patients with degenerative spondylolisthesis (treated
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with fusion in 95% of cases) in the SPORT study [9]. The in-patient stay reported here at a
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median of 5 days also compares favorably with the 7 to 9.5 day stays reported for
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posterolateral fusion in Europe [34,35]. In the US, decompression and PTB implantation
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would likely be performed as a short-stay procedure (<24hrs admission) similar to that
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typical of decompression alone [36], compared to the 3-5 day US hospital stay typical of
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instrumented fusion [37,38]. The mean 24.1 point improvement in ODI (58% reduction),
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44.7 mm reduction in mean VAS leg pain and 28.5 mm mean improvement in VAS back
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pain are similar to those obtained in other studies of decompression and fusion for
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degenerative spondylolisthesis [3,4,9,31]. Applying the composite clinical success criteria
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reported by Davis et al resulted in a 66.7% success rate for the patients in this series, which
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is comparable to their success rates of 62.8% for coflex® subjects and 62.5% for
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instrumented fusion controls for the subset of patients with degenerative spondylolisthesis in
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their randomized FDA study [31]. The maintained postoperative segmental stability was in
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contrast to the marked increased in olisthesis and translation in flexion-extension that has
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been seen in prior studies of decompression alone for this indication [3]. Mean disc height at
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baseline was 6.8mm; Blumenthal et al reported that disc height > 6.5 mm was associated
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with a 45% rate of reoperation at 3.6 years mean follow-up in patients receiving
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decompression without fusion for Grade I degenerative spondylolisthesis [39]. Review of
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the subset of patients with greatest preoperative instability (>2mm translation), all of whom
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were satisfied with their surgery, demonstrated similar sustained clinical improvement and
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stability at 24 months follow-up in the more unstable group (Fig. 5).
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This work represents the initial clinical feasibility of the PTB for postoperative stabilization
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after decompression for lumbar degenerative spondylolisthesis with spinal stenosis. While
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the patient sample size is small and follow-up duration limited to 24 months, we believe that
32
the results are encouraging but accept that Level I / II evidence is required. It is worth
33
noting that the PTB is not intended to correct or reduce the deformity associated with
34
degenerative spondylolisthesis, but rather to provide postoperative sagittal plane stability.
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This is in contrast to inter-spinous process devices that provide indirect or mechanical neural
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decompression through distraction of the posterior elements. As described previously, the
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device was designed to provide segmental stabilization, thereby optimizing the residual facet
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joint biomechanics through maximizing effective engaged articular surface area, minimizing
5
the risk of progressive translation and a poor clinical result. Perhaps of greater importance
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however is that surgical ‘insult’ from PTB insertion is minimal, making the procedure
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particularly attractive for the elderly and frail who represent a significant proportion of the
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population with this indication [1,3].
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There are several limitations of this non-randomized study without a control group. While
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the patients with degenerative spondylolisthesis represented a broad range of instability, the
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number of patients (41) was low with relatively short follow-up (2 years), and the small
13
sample size could create unintentional selection bias by the patient or institution. This work
14
was sponsored by the manufacturer of the PTB, and institutional research support was
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provided to the centers involved. The authors have also noted that preoperative ROM and
16
translation measured in flexion-extension were relatively low. There was however a small
17
ROM and translation of adjacent segments and that with the limitations in methodology
18
when obtaining standing radiographs probably indicate low levels of flexion effort rather
19
than a lack of instability. All subjects were deemed candidates for instrumented fusion by
20
their treating surgeons had they declined inclusion in the study or a PTB implantation.
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Seated flexion-extension radiography may allow for improved patient effort during image
22
acquisition without subjecting this elderly population to standing maneuvers. Whether
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flexion restriction stabilization will ultimately limit vertebral slippage and prevent
24
progressive stenosis or avoid adjacent segment degeneration in the long-term is unknown,
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but decompression and stabilization with PTB does seem to be a reasonable alternative for
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many patients who either do not want or are otherwise not good candidates for fusion and
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merits further investigation.
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CONCLUSIONS
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We report here encouraging results for patients with grade 1-2 lumbar degenerative
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spondylolisthesis and spinal stenosis treated with decompression and stabilization with a
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PTB flexion restriction device. Over the two year period of follow-up, the patient cohort
33
showed improvements in VAS, ODI and clinical success criteria consistent with those
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reported in the literature for patients with this pathology treated with decompression and
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fusion. Our series represents the preliminary clinical feasibility of this treatment for this
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patient population, and further investigation is merited.
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Fitzgerald JAW, Newman PH. Degenerative spondylolisthesis. J Bone Joint Surg Br 1976;58-B:184192. Herkowitz HN, Kurz LT. Degenerative lumbar spondylolisthesis with spinal stenosis. A prospective
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Fischgrund JS, Mackay M, Herkowitz HN, et al. Volvo Award winner in clinical studies.
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Weinstein JN, Lurie JD, Tosteson TD, et al. Surgical compared with nonoperative treatment for
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Blue Shield of California. Medical Policy. Spinal Fusion [Blue Shield of California Website]. March 5, 2012. Available at:
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Weinstein JN, Lurie JD, Tosteson TD, et al. Surgical versus nonsurgical treatment for lumbar degenerative spondylolisthesis. N Engl J Med 2007;356:2257-2270.
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10. Park P, Garton HJ, Gala VC, et al. Adjacent segment disease after lumbar or lumbosacral fusion: review of the literature. Spine 2004;29:1938-1944.
11. Lee CS, Hwang CJ, Lee S-W, et al. Risk factors for adjacent segment disease after lumbar fusion. Eur Spine J 2009;18:1637-1643.
12. Christie SD, Song JK, Fessler RG. Dynamic interspinous process technology. Spine 2005;30:S73S78. 13. Obernauer J, Kavakebi P, Quirbach S, Thomé C. Pedicle-Based Non-fusion Stabilization Devices: A Critical Review and Appraisal of Current Evidence. In: Schramm J, ed. Advances and Technical Standards in Neurosurgery 41. Cham, Switzerland: Springer; 2014:131-142. 14. Ianuzzi A, Kurtz SM, Kane W, et al. In vivo deformation, surface damage, and biostability of retrieved Dynesys systems. Spine 2010;35:E1310-E1316. doi: 10.1097/BRS.0b013e3181d6f84f
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15. United States Food and Drug Administration Orthopedic and Rehabilitation Devices Panel. FDA Executive Summary for Zimmer Spine’s Dynesys Spinal System. Background material provided to the public for Orthopaedic and Rehabilitation Devices Panel Meeting of 04 November 2009. 16. Impliant, Inc. Impliant Restarts Pivotal Clinical Trial for Patented TOPS™ Spine System. [PR Newswire, UBM plc. Website]. 2008. Available at http://www.prnewswire.com/newsreleases/impliant-restarts-pivotal-clinical-trial-for-patented-topstm-spine-system-57294052.html.
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Accessed 18 January 2014. 17. U.S. Food and Drug Administration. Class 2 Recall CD Horizon Agile dynamic stabilization device [FDA Website]. 2008. Available at:
http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfres/res.cfm?id=68075. Accessed January 30, 2014.
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18. Sjovold SG, Zhu Q, Bowden A, et al. Biomechanical evaluation of the Total Facet Arthroplasty System® (TFAS®): loading as compared to a rigid posterior instrumentation system. Eur Spine J 2012;21:1660-1673.
2007;32:E111-E120.
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19. Kosmopoulos V1, Schizas C. Pedicle screw placement accuracy: a meta-analysis. Spine
20. Babu R, Park JG, Mehta AI, et al. Comparison of superior-level facet joint violations during open and percutaneous pedicle screw placement. Neurosurgery 2012;71:962-970. doi: 10.1227/NEU.0b013e31826a88c8
21. Patel RD, Graziano GP, Vanderhave KL, et al. Facet violation with the placement of percutaneous pedicle screws. Spine 2011;36:E1749-E1752. doi: 10.1097/BRS.0b013e318221a800
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22. Lee SH, Lee JH, Hong SW, et al. Factors affecting clinical outcomes in treating patients with grade 1 degenerative spondylolisthesis using interspinous soft stabilization with a tension band system: a minimum 5-year follow-up. Spine 2012;37:563-572. 23. Lee SH, Lee JH, Hong SW, et al. Spinopelvic Alignment After Interspinous Soft Stabilization With a Tension Band System in Grade 1 Degenerative Lumbar Spondylolisthesis. Spine 2010;35:E691-701.
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24. Toyone T, Ozawa T, Kamikawa K, et al. Facet joint orientation difference between cephalad and caudad portions: a possible cause of degenerative spondylolisthesis. Spine 2009;34:2259-2262. doi: 10.1097/BRS.0b013e3181b20158
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25. Ehrhart EJ, Fielding L, Santoni AL, et al. In vivo response to a paraspinous tension band device with titanium coil. An ovine implantation study. Proceedings of the 40th Annual Meeting of the International Society for the Study of the Lumbar Spine, 13-17 May 2013, Scottsdale, AZ, USA. GP30.
26. Fielding LC, Alamin TF, Voronov LI, et al. Parametric and cadaveric models of lumbar flexion instability and flexion restricting dynamic stabilization system. Eur Spine J 2013;22:2710-2718. doi: 10.1007/s00586-013-2934-y 27. WHO. Assessment of fracture risk and its application to screening for postmenopausal osteoporosis. Report of a WHO Study Group. World Health Organ Tech Rep Ser. 1994;843:1-129. 28. Price D, McGrath P, Rafii A, Buckingham B. The validation of visual analogue scales as ratio scale measurements for clinical and experimental pain. Pain 1983;17:45-56. 29. Fairbank JC, Pynsent PB. The Oswestry Disability Index. Spine 2000;25(22):2940-52.
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30. Odom GL, Finney W, Woodhall B. Cervical disk lesions. J Am Med Assoc 1958;166:23-28. 31. Davis R, Auerbach JD, Bae H, Errico TJ. Can low-grade spondylolisthesis be effectively treated by either coflex interlaminar stabilization or laminectomy and posterior spinal fusion? Two-year clinical and radiographic results from the randomized, prospective, multicenter US investigational device exemption trial: clinical article. J Neurosurg Spine 2013;19:174-184. doi: 10.3171/2013.4.SPINE12636
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32. Hipp JA, Wharton ND. Quantitative motion analysis (QMA) of motion-preserving and fusion technologies for the spine. In: Yue JJ, Bertagnoli R, McAfee PC, An HS, eds. Motion preservation surgery of the spine. Advanced techniques and controversies. Philadelphia, PA: Saunders Elsevier2008:85-96.
33. Charlson ME, Pompei P, Ales KL, MacKenzie CR. A new method of classifying prognostic
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comorbidity in longitudinal studies: Development and validation. Journal of Chronic Diseases 1987;40:373–83.
34. Fritzell P, Hägg O, Wessberg P, et al. 2001 Volvo Award Winner in Clinical Studies: Lumbar Fusion
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Versus Nonsurgical Treatment for Chronic Low Back Pain. A Multicenter Randomized Controlled Trial From the Swedish Lumbar Spine Study Group. Spine 2001;26:2521-2534. 35. Wilson-MacDonald J, Fairbank J, Frost H, et al. The MRC Spine Stabilization Trial. Surgical Methods, Outcomes, Costs and Complications of Surgical Stabilization. Spine 2008;33:2334-2340. 36. Best NM, Sasso RC. Outpatient lumbar spine decompression in 233 patients 65 years of age or older. Spine 2007;32:1135-1139.
37. Centers for Medicare and Medicaid Services. FY 2014 Final Rule Tables [CMS Website]. 2014.
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Available at: http://www.cms.gov/Medicare/Medicare-Fee-for-ServicePayment/AcuteInpatientPPS/FY-2014-IPPS-Final-Rule-Home-Page-Items/FY-2014-IPPS-Final-RuleCMS-1599-F-Tables.html?DLPage=1&DLSort=0&DLSortDir=ascending. Accessed April 30, 2014. 38. Lad SP, Babu R, Ugiliweneza B, et al. Surgery for Spinal Stenosis: Long-Term Reoperation Rates, Healthcare Cost, and Impact of Instrumentation. Spine 2014; 39:978-87. doi:
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10.1097/BRS.0000000000000314 39. Blumenthal C, Curran J, Benzel EC, et al. Radiographic predictors of delayed instability following decompression without fusion for degenerative grade I lumbar spondylolisthesis. J Neurosurg Spine
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Tables
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Table 1 Patient demographics
41 (7:34)
Age, mean±SD (range)
68.2±9.0 (45-83) years
Body Mass Index (BMI), mean±SD (range)
28.6±4.5 (21-40)
Patients with previous spinal procedures, N (%)
3 (7.3%)
Smoking history, N (%):
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Total patients enrolled (male:female)
Current smokers 10 (24%) Former smokers 14 (34%)
34 (82.9%) 5 24 10 6 21 4 20
Charlson comorbidity index, mean±SD
3.8±1.4
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Patients with relevant concurrent medical conditions, N (%) Renal/urinary Cardiovascular Endocrine Pulmonary Musculoskeletal Liver / Gastrointestinal Other 3 Table 2 Operative results
Total surgery time, mean±SD (median, range)
118±48.5 (107, 61-300) minutes
Implant insertion time, mean±SD (median, range)
28±13 (25, 10-58) minutes
Estimated blood loss, mean±SD (median, range)
271±255 (200, 50-1400) ml
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L3/L4 8 L4/L5 33
Levels decompressed (N)
1 level 31 2 levels 6 3 levels 4
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Level of degenerative spondylolisthesis and PTB implantation (N)
Decompression characteristics (PTB index level)
Bilateral 39 / 41 Unilateral 2 / 41 ISL/SSL resection 10 / 41
Hospital length of stay, mean±SD (median, range) 6.2±5.5 (5, 1-35) days 5 6
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Table 3 Summary of clinical outcomes (ODI and VAS)
All patients, all available follow-up data 24 months (N=37)
P (b)
Effect (c)
ODI (%) (a)
45.6±15.4 (40.7, 50.4)
20.3±19.2 (13.9, 26.7)
<0.001
-1.59
Maximum Leg Pain, VAS (0-100mm) (a)
74.5±21.1 (67.8, 81.1)
28.8±35.0 (17.1, 40.4)
<0.001
-1.13
Maximum Hip Pain, VAS (0-100mm) (a)
57.7±34.4 (46.9, 68.6)
21.8±28.5 (12.3, 31.3)
<0.001
-0.87
Back Pain, VAS (0-100mm) (a)
54.1±30.1 (44.6, 63.6)
22.5±26.0 (13.8, 31.1)
<0.001
-0.72
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Patients with PTB in situ at 24 month follow-up
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Baseline (N=41)
P (b)
Effect (c)
<0.001
-1.76
25.4±33.0 (13.8, 36.9)
<0.001
-1.25
20.5±27.8 (10.8, 30.2)
<0.001
-0.83
20.5±24.5 (12.0, 29.0)
0.001
-0.64
24 months (N=34)
ODI (%) (a)
43.4±15.2 (38.1, 48.7)
18.0 ± 17.6 (11.8, 24.1)
Maximum Leg Pain, VAS (0-100mm) (a)
73.5±22.5 (65.6, 81.3)
Maximum Hip Pain, VAS (0-100mm) (a)
55.5±34.8 (43.4, 67.7)
Back Pain, VAS (0-100mm) (a)
49.6±30.8 (38.8, 60.3)
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Baseline (N=34)
(a) mean±SD (t-distribution 95% confidence interval lower limit, upper limit) (b) 2-tailed paired student’s t-test of baseline vs. 24 months (c) Standardized effect size (group difference in means divided by SD of difference) 2
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Table 4 Clinical success rates at 24 months follow-up
Quantitative endpoint
Success rate (95% CI (a))
ODI reduction ≥ 15 points
28/34, 82% (70%-95%) (b) 29/37, 78% (65%-92%) (c)
VAS leg pain reduction ≥ 20mm
25/34, 74% (59%-88%) (b) 26/37, 70% (56%-85%) (c)
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N (%) (b) 17 (52%) 10 (30%) 6 (18%) 0 (0%)
N (%) (c) 17 (47%) 10 (28%) 7 (19%) 2 (6%)
Satisfied 32 (97%) Not satisfied 1 (3%)
33 (92%) 3 (8%)
Subjective endpoints
Patient outcome per Odom’s criteria at 24 months follow-up
Self-reported patient satisfaction at 24 months follow-up Composite endpoint
Excellent Good Fair Poor
Success rate (95% CI (a))
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Composite Clinical Success per Davis et al [Error! Reference source not found.] (a) (b) (c) (d)
26/39, 66.7% (52%-81%) (d)
Binomial confidence interval, normal approximation method Patients with PTB in situ at 24 months follow-up All patients, all available follow-up data Subjects with evaluable composite clinical success
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1 Table 5 Quantitative radiographic results
Baseline 3 months 24 months P(a) (N=38) (N=34) (N=28) Index level ROM 2.9° ± 2.9° 2.6° ± 2.5° 2.8° ± 2.7° 0.66 (1.9°, 3.8°) (1.8°, 3.4°) (1.8°, 3.9°) Index level ROM, as % 28% ± 15% 24% ± 29% 25% ± 14% 0.39 of three-level ROM (b) (23%, 33%) (13%, 34%) (20%, 30%) Three-level ROM (b) 10.5° ± 8.9° 10.4°± 8.3° 12.1° ± 8.0° 0.20 (7.7°, 13.4°) (7.7°, 13.4°) (9.1°, 15°) Anterior slip (mm) 4.7 ± 2.9 4.8 ± 3.3 4.7 ± 3.9 0.36 (3.8, 5.6) (3.7, 5.9) (3.2, 6.1) Translation (mm) (c) 0.9 ± 0.9 0.6 ± 0.6 0.8 ± 0.7 0.84 (0.6, 1.2) (0.4, 0.8) (0.5, 1.0) Standing disc angle 7.6° ± 4.5° 7.2° ± 5.1° 8.5° ± 5.4° 0.96 (lordosis > 0°) (6.2°, 9.1°) (5.4°, 8.9°) (6.5°, 10.5°) Standing disc height 6.8 ± 2.1 6.5 ± 2.8 6.7 ± 2.4 0.50 (mm) (6.1, 7.5) (5.6, 7.5) (5.8, 7.6) All results presented as mean ± SD (t-distribution 95% CI) (a) 2-tailed paired student’s t-test, 24 months vs. baseline (b) Sum of flexion-extension ROM at index, supradjacent and subjacent levels (c) Measured in flexion-extension images as described by Hipp et al [31]
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Figure Legend
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Fig. 1 Paraspinous tension band (LimiFlex Spinal Stabilization System, Simpirica Spine, Inc., San Carlos, CA, USA)
4 Fig. 2 Patient with L4-L5 Grade 1 degenerative spondylolisthesis and spinal stenosis with translational instability and 12.5° of degenerative lumbar scoliosis. Preoperative 4.9mm slip in standing radiographs is
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reduced in the supine MRI and fluid is present in the facet joints. At 24 months postoperatively there is 44% reduction of translation and no progression of the spondylolisthesis. The landmarks used by the radiographic core laboratory are shown on the endplates of the index and adjacent segments.
Fig. 3 Patient with L3-L4 degenerative spondylolisthesis (7.2mm) and spinal stenosis as well as bulging intervertebral disc. At 24 months postoperatively there is a 39% reduction of dynamic translation and no progression of the spondylolisthesis.
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Fig. 4 Summary of clinical outcomes (mean and 95% confidence interval)
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(Mean and SD)
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Fig. 5 Summary of clinical and radiographic results for subjects with >2mm translation at preoperative baseline
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Acknowledgements: Institutional research support was provided by Simpirica Spine, Inc. (San Carlos, CA, USA). None of the institutional investigators hold a financial interest in the company or device. The authors would like to acknowledge Christine Beadle, Dominike Bruyninckx, Sarah Winkelkotte, Rahel Bornemann, MD, and Tom R
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Jansen, MD for their contributions to this work.
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S1529-9430(15)00004-2
DOI:
10.1016/j.spinee.2015.01.003
Reference:
SPINEE 56153
To appear in:
The Spine Journal
Received Date: 3 October 2014 Revised Date:
29 December 2014
Accepted Date: 2 January 2015
Please cite this article as: Gibson JNA, Depreitere B, Pflugmacher R, Schnake KJ, Fielding LC, Alamin TF, Goffin J, Decompression and paraspinous tension band: a novel treatment method for patients with lumbar spinal stenosis and degenerative spondylolisthesis, The Spine Journal (2015), doi: 10.1016/ j.spinee.2015.01.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Decompression and paraspinous tension band for degenerative spondylolisthesis
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J N Alastair Gibson, MD, FRCS
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Spinal Unit, Royal Infirmary of Edinburgh and the University of Edinburgh
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Edinburgh, Scotland
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Bart Depreitere, MD, PhD
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Decompression and paraspinous tension band: a novel treatment method for patients with lumbar spinal stenosis and degenerative spondylolisthesis
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Department of Neurosciences, Universitaire Ziekenhuizen KU Leuven
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Leuven, Belgium
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Robert Pflugmacher, MD
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Klinik und Poliklinik für Orthopädie und Unfallchirurgie, Universitätsklinikum Bonn
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Bonn, Germany
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Klaus J Schnake, MD
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Zentrum für Wirbelsäulentherapie, Schön Klinik Nürnberg Fürth
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Fürth, Germany
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Louis C Fielding, MS
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Simpirica Spine
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San Carlos, CA, USA
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Todd F Alamin, MD
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Department of Orthopaedic Surgery, Stanford University School of Medicine
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450 Broadway St | Pavillion A FL 1 MC6110 | Redwood City, CA 94063
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Jan Goffin, MD, PhD
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Department of Neurosciences, Universitaire Ziekenhuizen KU Leuven
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Leuven, Belgium
email: [email protected] | tel: +1 (650) 721-7616 | fax: +1 (650) 721-3409
Funding for this work was provided by Simpirica Spine, Inc.
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Decompression and paraspinous tension band for degenerative spondylolisthesis
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ABSTRACT
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BACKGROUND CONTEXT: Prior studies have demonstrated superiority of
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decompression and fusion over decompression alone for treatment of lumbar degenerative
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spondylolisthesis with spinal stenosis. More recent studies have investigated whether non-
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fusion stabilization could provide the durable clinical improvement after decompression and
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fusion.
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PURPOSE: To examine the clinical safety and effectiveness of decompression and
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implantation of a novel flexion restricting paraspinous tension band (PTB) for patients with
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degenerative spondylolisthesis.
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STUDY DESIGN/SETTING: Prospective Clinical Study
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PATIENT SAMPLE: Forty-one patients (7 men and 34 women) aged 45-83 years (68.2±9.0)
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were recruited with symptomatic spinal stenosis and Meyerding grade 1 or 2 degenerative
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spondylolisthesis at L3/4 (8) or L4/5 (33).
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OUTCOME MEASURES: Self-reported measures included visual-analog scales (VAS) for
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leg, back and hip pain as well as the Oswestry disability index (ODI). Physiologic measures
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included quantitative and qualitative radiographic analysis performed by an independent core
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laboratory.
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METHODS: Patients with lumbar degenerative spondylolisthesis and stenosis were
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prospectively enrolled at four European spine centers with independent monitoring of data.
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Clinical and radiographic outcome data collected pre-operatively were compared with data
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collected at three, six, 12 and 24 months after surgery. This study was sponsored by the PTB
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manufacturer (Simpirica Spine Inc., San Carlos, CA, USA), including institutional research
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support grants to the participating centers totaling approximately US $172,000.
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RESULTS: Statistically significant improvements and clinically-important effect sizes were
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seen for all pain and disability measurements. At 24 months follow-up, ODI scores were
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reduced by an average of 25.4 points (59%) and maximum leg pain on VAS by 48.1mm
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(65%). Back pain VAS scores improved from 54.1 by an average of 28.5 pts (53%). There
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was one postoperative wound infection (2.4%) and an overall reoperation rate of 12%.
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Eighty-two percent of the patients available for 24 month follow-up with a PTB in situ had a
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reduction in ODI of >15 points, and 74% had a reduction in maximum leg pain VAS
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>20mm. According to Odom's criteria the majority of these patients (82%) had an excellent
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or good outcome with all except one patient satisfied with surgery. As measured by the
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independent core laboratory, there was no significant increase in spondylolisthesis,
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segmental flexion-extension range of motion or translation and no loss of lordosis in the
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patients with the PTB at 2 years follow-up.
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CONCLUSIONS: Patients with degenerative spondylolisthesis and spinal stenosis treated
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with decompression and a PTB demonstrated no progressive instability at 2 years of follow-
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up. Excellent/good outcomes and significant improvements in patient-reported pain and
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disability scores were still observed at 2 years with no evidence of implant failure or
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migration. Further study of this treatment method is warranted to validate these findings.
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INTRODUCTION
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Decompression alone was the standard surgical treatment for lumbar degenerative
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spondylolisthesis with spinal stenosis [1,2] until several landmark articles suggested that
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decompression and fusion was a superior mode of treatment [3-5]. While there has been
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recent pressure to reduce rates of spinal fusion, payers and medical societies consistently
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recommend that fusion be determined medically necessary for lumbar degenerative
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spondylolisthesis with spinal stenosis [6-8], and it remains the dominant procedure for this
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indication in the US shown by a 95% fusion rate in the SPORT study [9]. However, fusion
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is clearly an imperfect treatment due to the short-term morbidity involved in adding a fusion
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to the decompression, as well as the longer term associated risk of adjacent-level
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degeneration and instability [10,11]. To circumvent these problems “dynamic stabilization”
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systems were introduced [12,13], but the majority are pedicle screw based and associated
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with many of the same problems as instrumented fusion, notably complex implant loading
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[14-18] and facet joint violation during screw placement [19-21]. For this reason simpler
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constructs have been suggested, such as that by Lee et al who reported a series of 65 patients
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with degenerative spondylolisthesis treated with decompression and posterior tension band
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stabilization using a figure of eight suture. They found that back pain relief and functional
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improvement were significantly correlated with achievement of total and segmental lumbar
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lordosis after a mean follow-up of 72.5 months [22] and equivalent outcomes could be
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achieved in comparison to posterior lumbar interbody fusion [23].
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Based on the need for segmental stabilization in this patient population, the relative draw
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backs of fusion and pedicle screw-based dynamic stabilization, and the promising outcomes
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of tension band stabilization reported by Lee et al, a novel paraspinous tension band (PTB;
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LimiFlex™ Spinal Stabilization System, Simpirica Spine Inc., San Carlos, CA, USA) has
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been developed to provide segmental sagittal plane stability through biasing the segment into
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lordosis. The facet joints are more engaged and afford more sagittal plane stability in
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segmental extension than in flexion due to the coronal orientation of the caudad portion of
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the joint [24], and thus their biomechanics allow for this indirect mechanism of providing
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stability. The PTB utilizes titanium coil tension springs to restore segmental flexion
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stiffness, and is attached to the spinous processes with ultra-high molecular weight
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polyethylene (UHMWPE) bands. A preclinical large-animal implantation study
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demonstrated that the PTB is well tolerated by the surrounding tissues, and retains its
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function after anatomical incorporation [25]. Cadaveric biomechanical testing has
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demonstrated restoration of the kinematics of a destabilized spinal segment to that of an
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intact segment, with increased flexion stiffness and reduced sagittal translation [26]. The
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device does not bear any axial loads, and therefore the forces transmitted by the PTB to the
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spinal elements are an order of magnitude less compared to pedicle screw-based systems.
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The PTB was designed to be easily implanted after a standard lumbar decompression with
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minimal additional exposure.
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We aimed to prospectively assess clinical and radiographic outcomes of patients treated with
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surgical decompression and stabilization with the LimiFlex PTB device over a two year follow-up period. We questioned whether these patients would become progressively more
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unstable from a radiographic standpoint over this length of follow-up, and whether their
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clinical improvement in back and leg pain would deteriorate. This study was intended to
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provide initial data to demonstrate the clinical feasibility of this treatment method.
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MATERIALS AND METHODS
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Patient Selection
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Between January 2010 and October 2011, 41 patients at four centers (Royal Infirmary of
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Edinburgh, Edinburgh, Scotland, UK; Universitaire Ziekenhuizen KU Leuven, Leuven,
19
Belgium; Berufsgenossenschaftliche Unfallklinik, Frankfurt am Main, Germany;
20
Universitätsklinikum Bonn, Bonn, Germany) were enrolled in clinical studies and followed
21
for 24 months postoperatively. Informed consent was obtained from each of the patients in
22
the series. All patients had degenerative spondylolisthesis (Meyerding grade 1 or 2) and
23
spinal stenosis requiring decompression of at least one level (1-3 levels). Patients were
24
excluded if they had spinous processes or posterior element anatomy inappropriate for
25
implant fixation, severe osteoporosis (T-score ≤ -2.5 with fragility fracture) [27], isthmic
26
spondylolisthesis of the segment to be instrumented, active systemic or local infection, or if
27
they were pregnant or planning to become pregnant. These patients were felt to be otherwise
28
candidates for lumbar fusion by their operating physician. The data presented here represent
29
patients from this prospective, post-market case series that were approved by the Ethics
30
Committees of each participating institution. This study was sponsored by the PTB
31
manufacturer (Simpirica Spine Inc., San Carlos, CA, USA), including institutional research
32
support grants to the participating centers totaling approximately US $172,000.
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Surgical Intervention
2
Decompression
3
All patients received a direct surgical decompression of up to three levels from L1-S1 to
4
address their spinal stenosis, preserving at least 50% of the spinous processes. All
5
decompressions were performed using an operating microscope, undercutting the facet joints
6
and decompressing the foraminae as necessary. The structural integrity of the pars
7
interarticularis was preserved and resection of the articular surface of the facet joints
8
minimized. In some cases the midline supraspinous/ interspinous ligamentous complex was
9
preserved and in other cases not according to the preference of the treating surgeon.
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Decompression characteristics are summarized in Table 2.
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Paraspinous Tension Band Implantation
12
Following their decompression, all patients were implanted with a PTB at the level of the
13
degenerative spondylolisthesis. The PTB comprises a pair of titanium coil springs secured
14
to ultra-high molecular weight polyethylene (UHMWPE) straps (Fig. 1). The straps were
15
passed around the cranial and caudal spinous processes, piercing the interspinous ligament of
16
the segments adjacent to the treated segment. The patient was then repositioned, if necessary,
17
by adjusting the amount of flexion on the spinal frame / table using intraoperative
18
fluoroscopy so that local lumbar sagittal alignment approximated a standing lordotic
19
position. This alignment was estimated by the treating surgeon; no fixed threshold values
20
were used. The device could then be tensioned and secured using instruments that allowed a
21
consistent nominal preload to the springs (approx. 20N). The decompression was reassessed
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after the application of the device.
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Outcome assessment
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Preoperatively, and at 3, 6, 12, and 24 months postoperatively patients rated their back, leg,
26
and buttock/hip pain on a 0-100mm longitudinal VAS and their back pain disability (ODI)
27
[28,29]. Patient outcomes were also assessed according to patient-reported satisfaction with
28
their surgery and Odom’s criteria [30]. Clinical success was evaluated using the composite
29
clinical success criteria utilized by Davis et al for this indication [31], whereby to be
30
considered a clinical success a patient was required to meet all the following criteria: 15-
31
point reduction in ODI, no reoperations, no major device-related complications, and no
32
postoperative epidural injections.
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Intraoperative and postoperative complications were noted and compiled, and then attributed
3
as either being related or unrelated to the device or the procedure by the study site
4
investigator. All serious adverse events (SAEs) were adjudicated by an independent medical
5
monitor who was a board certified orthopaedic spine surgeon.
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8
Diagnostic axial imaging studies (CT or MRI) were obtained preoperatively to confirm
9
spinal canal stenosis and to evaluate degenerative lumbar pathology. Flexion, extension,
10
standing lateral, and anterior/posterior (AP) radiographs were obtained preoperatively, as
11
well as at 3, 6, 12, and 24 months postoperatively (Figs. 2,3). Quantitative and qualitative
12
radiographic analyses were performed by a radiographic core laboratory using Quantitative
13
Motion Analysis (QMA™, Medical Metrics, Houston, TX) [32]. Quantitative radiographic
14
analysis included: •
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Static measurements of disc angle, anterior slip, disc height, and total lumbar (L1-S1) angle in flexion, neutral and extension.
•
Dynamic measurements of rotation and translation in flexion-extension.
The qualitative radiographic analysis included assessments by a board-certified radiologist
19
for device condition, device migration/dislocation, spinous process fracture, bone-implant
20
interface remodeling, and exuberant bone formation.
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Statistical analyses
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Data were assessed for normality, and changes in ODI and VAS were assessed using 2-
24
tailed, paired Student’s t-tests. A significance level of p≤0.05 was used. Confidence
25
intervals for parametric statistics were calculated assuming a t-distribution. Binomial
26
confidence intervals were calculated for success criteria using the Normal Approximation
27
Method.
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RESULTS
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Patient demographics, operative results and follow-up
31
Patient demographics are summarized in Table 1. The ratio of approximately 5:1 female to
32
male and ages of the 41 patients are typical of those reported for degenerative
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Decompression and paraspinous tension band for degenerative spondylolisthesis
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spondylolisthesis [1,3]. These patients represent a relatively high-risk group, with a mean
2
Charlson comorbidity index of 3.8 [33].
3 Operative results including decompression characteristics are summarized in Table 2. All
5
forty-one patients received decompressions and were successfully implanted with the PTB
6
with a mean implantation time of 28 minutes (range: 10-58 minutes), which included initial
7
learning curve procedures. Thirty-four patients with the PTB in situ were available for 24-
8
month follow-up evaluation. One died from a traumatic head injury and one patient was lost
9
to follow-up. The device was explanted prior to final follow-up in five patients as described further below.
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11 Clinical outcomes
13
Clinical outcomes are presented for the group of patients with all follow-up data, as well as
14
for the sub-group of patients with the PTB in situ at 24 months. Statistically significant
15
improvements and clinically important effect sizes were seen for all pain and disability
16
measurements. Disability according to ODI was reduced by an average of 25.4 points (59%
17
compared to baseline). Maximum leg pain VAS was reduced by an average of 48.1mm
18
(65%). Back pain VAS averaged 54.1 preoperatively, and the postoperative value at two
19
years was 22.6mm, representing an improvement of 28.5 mm (54%) (Table 3).
20
Improvement in patient self-reported outcomes showed a significant improvement at the first
21
(3 months) follow-up visit, which was sustained with a trend toward increased improvement
22
throughout follow-up to 24 months (Figure 4). The clinical success rate according to the
23
criteria established by Davis et al was 66.7% [31].
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Eighty-two percent of patients available for 24 month follow-up with the PTB in situ had a
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reduction in ODI of at least 15 points, and 74% had a reduction in maximum leg pain VAS
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of at least 20mm. Eighty-two percent (82%) of these patients were evaluated to have
28
excellent or good outcomes according to Odom’s criteria, and 97% were satisfied with the
29
surgery. (Table 4)
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Radiographic results
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Quantitative analysis
2
No change in vertebral angulation or olisthesis was noted on device implantation or seen in
3
radiographic measurements at 3 months follow-up. Segmental stability and lordosis were
4
maintained to 24 months (Table 5). There were no significant increases in anterior slip,
5
dynamic sagittal translation, or flexion-extension range of motion (ROM). Three-level
6
ROM of the index, supradjacent and subjacent levels was relatively low preoperatively,
7
indicating low patient effort in flexion-extension, with a trend towards increased overall
8
motion at 24 months follow-up, in comparison to the index level ROM which showed no
9
significant change during follow-up. Exemplary images are shown in Figures 2 and 3. Six
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patients with 24-month follow-up and the PTB in situ did not have all radiographic films
11
necessary for quantitative analysis.
12
Qualitative analysis
13
Throughout the follow-up period, at all time-points, there were no x-ray findings of device
14
breakage, disassembly, migration or dislocation; bone-implant interface remodeling; or
15
exuberant bone formation. There were no findings of resorption at the strap-spinous process
16
interface. There was one spinous process fracture with an unclear time of origin that will be
17
discussed below, which was found on a postoperative CT scan.
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Complications
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Seven intra-operative adverse events were reported, including six dural tears and a
21
superficial fracture of the tip of a spinous process. The dural tears all occurred during the
22
decompressions prior to the implantation of the device, were all directly sutured at the time
23
of decompression, and did not affect device implantation. There were no post-operative CSF
24
leaks but there was one postoperative infection (2.4%) that was treated with irrigation and
25
debridement, implant removal and fusion.
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During the follow-up period there were two other patients in the study who underwent
28
conversion to fusion. One had persistent pain postoperatively and a sclerotic spinous process
29
fracture was found on CT imaging 6 months after surgery. Instability was apparent on the
30
flexion/extension films. It was not clear whether the fracture happened during the
31
decompression or at a later date. An L4/5 fusion was performed. A second patient, who
32
complained of persistent leg pain, was found on repeat imaging to have foraminal stenosis at
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Decompression and paraspinous tension band for degenerative spondylolisthesis
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the level below her L4/5 treated segment with no evidence of progressive instability or
2
recurrent stenosis at L4/5. She was fused from L4 to S1.
3 Three days after surgery one patient presented with a symptomatic epidural hematoma
5
leading to paraparesis. An emergency laminectomy was performed and the device removed.
6
One patient developed a proximally migrated disc extrusion at the inferior adjacent segment
7
nine months after surgery, and underwent discectomy. The device was removed to allow for
8
easier access to the fragment. These complications represent an overall 12% reoperation
9
rate.
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Explanted devices were sent to independent laboratories for examination (Exponent,
12
Philadelphia, PA) and histopathology (Histion, LLC, Everett, WA). Examination of the
13
removed implants found no device failures. Histopathology findings were that the implant
14
was well integrated with fibrous tissue with no adverse tissue response.
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DISCUSSION
17
All PTBs were easily implanted without implant-related complication and a median insertion
18
time of 25 minutes. At two-year follow-up, this cohort of patients with degenerative
19
spondylolisthesis showed significant clinical improvement in self-reported pain and
20
disability scores from first follow-up at 3 months, with sustained improvement through the
21
24 months follow-up period. Radiographically there was no evidence of progression in static
22
or dynamic translation. Intraoperatively there was incidental durotomy in six of the 41
23
patients (14.6%), probably a reflection of the elderly population studied (mean age 68yrs).
24
The overall re-operation rate (12%) was slightly higher than expected but may have simply
25
reflected an initial experience with a new technique; complications were generally either
26
unrelated or of indeterminate relationship to the PTB. The simplicity of the primary surgery
27
avoiding damage to the facets at the instrumented level meant that any revision surgery was
28
generally straightforward. Analysis of explanted devices found no device failures and that
29
the implant was well integrated into the paraspinal tissue with no adverse tissue response.
30
There were no radiographic findings of spinous process resorption at the strap interface,
31
which is consistent with the large animal implantation study previously conducted on the
32
PTB [25] and implying that there was no functional loss of device tension due to remodeling
33
of the bone-implant interface.
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1 As this work represents a preliminary series from a small cohort without a control group, we
3
have evaluated our results in context with published literature for general comparative
4
purposes. The 118 minute average surgical time for decompression and PTB implantation
5
compares favorably to the 210.5 minutes and 202.7 minutes reported for the randomized and
6
non-randomized cohorts of surgical patients with degenerative spondylolisthesis (treated
7
with fusion in 95% of cases) in the SPORT study [9]. The in-patient stay reported here at a
8
median of 5 days also compares favorably with the 7 to 9.5 day stays reported for
9
posterolateral fusion in Europe [34,35]. In the US, decompression and PTB implantation
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would likely be performed as a short-stay procedure (<24hrs admission) similar to that
11
typical of decompression alone [36], compared to the 3-5 day US hospital stay typical of
12
instrumented fusion [37,38]. The mean 24.1 point improvement in ODI (58% reduction),
13
44.7 mm reduction in mean VAS leg pain and 28.5 mm mean improvement in VAS back
14
pain are similar to those obtained in other studies of decompression and fusion for
15
degenerative spondylolisthesis [3,4,9,31]. Applying the composite clinical success criteria
16
reported by Davis et al resulted in a 66.7% success rate for the patients in this series, which
17
is comparable to their success rates of 62.8% for coflex® subjects and 62.5% for
18
instrumented fusion controls for the subset of patients with degenerative spondylolisthesis in
19
their randomized FDA study [31]. The maintained postoperative segmental stability was in
20
contrast to the marked increased in olisthesis and translation in flexion-extension that has
21
been seen in prior studies of decompression alone for this indication [3]. Mean disc height at
22
baseline was 6.8mm; Blumenthal et al reported that disc height > 6.5 mm was associated
23
with a 45% rate of reoperation at 3.6 years mean follow-up in patients receiving
24
decompression without fusion for Grade I degenerative spondylolisthesis [39]. Review of
25
the subset of patients with greatest preoperative instability (>2mm translation), all of whom
26
were satisfied with their surgery, demonstrated similar sustained clinical improvement and
27
stability at 24 months follow-up in the more unstable group (Fig. 5).
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29
This work represents the initial clinical feasibility of the PTB for postoperative stabilization
30
after decompression for lumbar degenerative spondylolisthesis with spinal stenosis. While
31
the patient sample size is small and follow-up duration limited to 24 months, we believe that
32
the results are encouraging but accept that Level I / II evidence is required. It is worth
33
noting that the PTB is not intended to correct or reduce the deformity associated with
34
degenerative spondylolisthesis, but rather to provide postoperative sagittal plane stability.
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Decompression and paraspinous tension band for degenerative spondylolisthesis
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This is in contrast to inter-spinous process devices that provide indirect or mechanical neural
2
decompression through distraction of the posterior elements. As described previously, the
3
device was designed to provide segmental stabilization, thereby optimizing the residual facet
4
joint biomechanics through maximizing effective engaged articular surface area, minimizing
5
the risk of progressive translation and a poor clinical result. Perhaps of greater importance
6
however is that surgical ‘insult’ from PTB insertion is minimal, making the procedure
7
particularly attractive for the elderly and frail who represent a significant proportion of the
8
population with this indication [1,3].
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There are several limitations of this non-randomized study without a control group. While
11
the patients with degenerative spondylolisthesis represented a broad range of instability, the
12
number of patients (41) was low with relatively short follow-up (2 years), and the small
13
sample size could create unintentional selection bias by the patient or institution. This work
14
was sponsored by the manufacturer of the PTB, and institutional research support was
15
provided to the centers involved. The authors have also noted that preoperative ROM and
16
translation measured in flexion-extension were relatively low. There was however a small
17
ROM and translation of adjacent segments and that with the limitations in methodology
18
when obtaining standing radiographs probably indicate low levels of flexion effort rather
19
than a lack of instability. All subjects were deemed candidates for instrumented fusion by
20
their treating surgeons had they declined inclusion in the study or a PTB implantation.
21
Seated flexion-extension radiography may allow for improved patient effort during image
22
acquisition without subjecting this elderly population to standing maneuvers. Whether
23
flexion restriction stabilization will ultimately limit vertebral slippage and prevent
24
progressive stenosis or avoid adjacent segment degeneration in the long-term is unknown,
25
but decompression and stabilization with PTB does seem to be a reasonable alternative for
26
many patients who either do not want or are otherwise not good candidates for fusion and
27
merits further investigation.
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CONCLUSIONS
30
We report here encouraging results for patients with grade 1-2 lumbar degenerative
31
spondylolisthesis and spinal stenosis treated with decompression and stabilization with a
32
PTB flexion restriction device. Over the two year period of follow-up, the patient cohort
33
showed improvements in VAS, ODI and clinical success criteria consistent with those
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Decompression and paraspinous tension band for degenerative spondylolisthesis
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reported in the literature for patients with this pathology treated with decompression and
2
fusion. Our series represents the preliminary clinical feasibility of this treatment for this
3
patient population, and further investigation is merited.
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Macnab I. Spondylolisthesis with an intact neural arch; the so-called pseudo-spondylolisthesis. J Bone Joint Surg Br 1950;32-B:325-333.
2.
Fitzgerald JAW, Newman PH. Degenerative spondylolisthesis. J Bone Joint Surg Br 1976;58-B:184192. Herkowitz HN, Kurz LT. Degenerative lumbar spondylolisthesis with spinal stenosis. A prospective
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study comparing decompression with decompression and intertransverse process arthrodesis. J Bone Joint Surg Am. 1991;73:802-808. 4.
Fischgrund JS, Mackay M, Herkowitz HN, et al. Volvo Award winner in clinical studies.
Degenerative lumbar spondylolisthesis with spinal stenosis: a prospective, randomized study
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comparing decompressive laminectomy and arthrodesis with and without spinal instrumentation. Spine 1997;22:2807-2812. 5.
Weinstein JN, Lurie JD, Tosteson TD, et al. Surgical compared with nonoperative treatment for
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Blue Shield of California. Medical Policy. Spinal Fusion [Blue Shield of California Website]. March 5, 2012. Available at:
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Aetna. Clinical Policy Bulletin: Spinal Surgery: Laminectomy and Fusion [Aetna website]. December
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10. Park P, Garton HJ, Gala VC, et al. Adjacent segment disease after lumbar or lumbosacral fusion: review of the literature. Spine 2004;29:1938-1944.
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12. Christie SD, Song JK, Fessler RG. Dynamic interspinous process technology. Spine 2005;30:S73S78. 13. Obernauer J, Kavakebi P, Quirbach S, Thomé C. Pedicle-Based Non-fusion Stabilization Devices: A Critical Review and Appraisal of Current Evidence. In: Schramm J, ed. Advances and Technical Standards in Neurosurgery 41. Cham, Switzerland: Springer; 2014:131-142. 14. Ianuzzi A, Kurtz SM, Kane W, et al. In vivo deformation, surface damage, and biostability of retrieved Dynesys systems. Spine 2010;35:E1310-E1316. doi: 10.1097/BRS.0b013e3181d6f84f
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18. Sjovold SG, Zhu Q, Bowden A, et al. Biomechanical evaluation of the Total Facet Arthroplasty System® (TFAS®): loading as compared to a rigid posterior instrumentation system. Eur Spine J 2012;21:1660-1673.
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21. Patel RD, Graziano GP, Vanderhave KL, et al. Facet violation with the placement of percutaneous pedicle screws. Spine 2011;36:E1749-E1752. doi: 10.1097/BRS.0b013e318221a800
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26. Fielding LC, Alamin TF, Voronov LI, et al. Parametric and cadaveric models of lumbar flexion instability and flexion restricting dynamic stabilization system. Eur Spine J 2013;22:2710-2718. doi: 10.1007/s00586-013-2934-y 27. WHO. Assessment of fracture risk and its application to screening for postmenopausal osteoporosis. Report of a WHO Study Group. World Health Organ Tech Rep Ser. 1994;843:1-129. 28. Price D, McGrath P, Rafii A, Buckingham B. The validation of visual analogue scales as ratio scale measurements for clinical and experimental pain. Pain 1983;17:45-56. 29. Fairbank JC, Pynsent PB. The Oswestry Disability Index. Spine 2000;25(22):2940-52.
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33. Charlson ME, Pompei P, Ales KL, MacKenzie CR. A new method of classifying prognostic
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Versus Nonsurgical Treatment for Chronic Low Back Pain. A Multicenter Randomized Controlled Trial From the Swedish Lumbar Spine Study Group. Spine 2001;26:2521-2534. 35. Wilson-MacDonald J, Fairbank J, Frost H, et al. The MRC Spine Stabilization Trial. Surgical Methods, Outcomes, Costs and Complications of Surgical Stabilization. Spine 2008;33:2334-2340. 36. Best NM, Sasso RC. Outpatient lumbar spine decompression in 233 patients 65 years of age or older. Spine 2007;32:1135-1139.
37. Centers for Medicare and Medicaid Services. FY 2014 Final Rule Tables [CMS Website]. 2014.
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10.1097/BRS.0000000000000314 39. Blumenthal C, Curran J, Benzel EC, et al. Radiographic predictors of delayed instability following decompression without fusion for degenerative grade I lumbar spondylolisthesis. J Neurosurg Spine
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2013;18:340–6.
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Tables
2
Table 1 Patient demographics
41 (7:34)
Age, mean±SD (range)
68.2±9.0 (45-83) years
Body Mass Index (BMI), mean±SD (range)
28.6±4.5 (21-40)
Patients with previous spinal procedures, N (%)
3 (7.3%)
Smoking history, N (%):
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Total patients enrolled (male:female)
Current smokers 10 (24%) Former smokers 14 (34%)
34 (82.9%) 5 24 10 6 21 4 20
Charlson comorbidity index, mean±SD
3.8±1.4
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Patients with relevant concurrent medical conditions, N (%) Renal/urinary Cardiovascular Endocrine Pulmonary Musculoskeletal Liver / Gastrointestinal Other 3 Table 2 Operative results
Total surgery time, mean±SD (median, range)
118±48.5 (107, 61-300) minutes
Implant insertion time, mean±SD (median, range)
28±13 (25, 10-58) minutes
Estimated blood loss, mean±SD (median, range)
271±255 (200, 50-1400) ml
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L3/L4 8 L4/L5 33
Levels decompressed (N)
1 level 31 2 levels 6 3 levels 4
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Level of degenerative spondylolisthesis and PTB implantation (N)
Decompression characteristics (PTB index level)
Bilateral 39 / 41 Unilateral 2 / 41 ISL/SSL resection 10 / 41
Hospital length of stay, mean±SD (median, range) 6.2±5.5 (5, 1-35) days 5 6
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Table 3 Summary of clinical outcomes (ODI and VAS)
All patients, all available follow-up data 24 months (N=37)
P (b)
Effect (c)
ODI (%) (a)
45.6±15.4 (40.7, 50.4)
20.3±19.2 (13.9, 26.7)
<0.001
-1.59
Maximum Leg Pain, VAS (0-100mm) (a)
74.5±21.1 (67.8, 81.1)
28.8±35.0 (17.1, 40.4)
<0.001
-1.13
Maximum Hip Pain, VAS (0-100mm) (a)
57.7±34.4 (46.9, 68.6)
21.8±28.5 (12.3, 31.3)
<0.001
-0.87
Back Pain, VAS (0-100mm) (a)
54.1±30.1 (44.6, 63.6)
22.5±26.0 (13.8, 31.1)
<0.001
-0.72
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Patients with PTB in situ at 24 month follow-up
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P (b)
Effect (c)
<0.001
-1.76
25.4±33.0 (13.8, 36.9)
<0.001
-1.25
20.5±27.8 (10.8, 30.2)
<0.001
-0.83
20.5±24.5 (12.0, 29.0)
0.001
-0.64
24 months (N=34)
ODI (%) (a)
43.4±15.2 (38.1, 48.7)
18.0 ± 17.6 (11.8, 24.1)
Maximum Leg Pain, VAS (0-100mm) (a)
73.5±22.5 (65.6, 81.3)
Maximum Hip Pain, VAS (0-100mm) (a)
55.5±34.8 (43.4, 67.7)
Back Pain, VAS (0-100mm) (a)
49.6±30.8 (38.8, 60.3)
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Baseline (N=34)
(a) mean±SD (t-distribution 95% confidence interval lower limit, upper limit) (b) 2-tailed paired student’s t-test of baseline vs. 24 months (c) Standardized effect size (group difference in means divided by SD of difference) 2
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Table 4 Clinical success rates at 24 months follow-up
Quantitative endpoint
Success rate (95% CI (a))
ODI reduction ≥ 15 points
28/34, 82% (70%-95%) (b) 29/37, 78% (65%-92%) (c)
VAS leg pain reduction ≥ 20mm
25/34, 74% (59%-88%) (b) 26/37, 70% (56%-85%) (c)
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N (%) (b) 17 (52%) 10 (30%) 6 (18%) 0 (0%)
N (%) (c) 17 (47%) 10 (28%) 7 (19%) 2 (6%)
Satisfied 32 (97%) Not satisfied 1 (3%)
33 (92%) 3 (8%)
Subjective endpoints
Patient outcome per Odom’s criteria at 24 months follow-up
Self-reported patient satisfaction at 24 months follow-up Composite endpoint
Excellent Good Fair Poor
Success rate (95% CI (a))
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Composite Clinical Success per Davis et al [Error! Reference source not found.] (a) (b) (c) (d)
26/39, 66.7% (52%-81%) (d)
Binomial confidence interval, normal approximation method Patients with PTB in situ at 24 months follow-up All patients, all available follow-up data Subjects with evaluable composite clinical success
2
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1 Table 5 Quantitative radiographic results
Baseline 3 months 24 months P(a) (N=38) (N=34) (N=28) Index level ROM 2.9° ± 2.9° 2.6° ± 2.5° 2.8° ± 2.7° 0.66 (1.9°, 3.8°) (1.8°, 3.4°) (1.8°, 3.9°) Index level ROM, as % 28% ± 15% 24% ± 29% 25% ± 14% 0.39 of three-level ROM (b) (23%, 33%) (13%, 34%) (20%, 30%) Three-level ROM (b) 10.5° ± 8.9° 10.4°± 8.3° 12.1° ± 8.0° 0.20 (7.7°, 13.4°) (7.7°, 13.4°) (9.1°, 15°) Anterior slip (mm) 4.7 ± 2.9 4.8 ± 3.3 4.7 ± 3.9 0.36 (3.8, 5.6) (3.7, 5.9) (3.2, 6.1) Translation (mm) (c) 0.9 ± 0.9 0.6 ± 0.6 0.8 ± 0.7 0.84 (0.6, 1.2) (0.4, 0.8) (0.5, 1.0) Standing disc angle 7.6° ± 4.5° 7.2° ± 5.1° 8.5° ± 5.4° 0.96 (lordosis > 0°) (6.2°, 9.1°) (5.4°, 8.9°) (6.5°, 10.5°) Standing disc height 6.8 ± 2.1 6.5 ± 2.8 6.7 ± 2.4 0.50 (mm) (6.1, 7.5) (5.6, 7.5) (5.8, 7.6) All results presented as mean ± SD (t-distribution 95% CI) (a) 2-tailed paired student’s t-test, 24 months vs. baseline (b) Sum of flexion-extension ROM at index, supradjacent and subjacent levels (c) Measured in flexion-extension images as described by Hipp et al [31]
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Figure Legend
2 3
Fig. 1 Paraspinous tension band (LimiFlex Spinal Stabilization System, Simpirica Spine, Inc., San Carlos, CA, USA)
4 Fig. 2 Patient with L4-L5 Grade 1 degenerative spondylolisthesis and spinal stenosis with translational instability and 12.5° of degenerative lumbar scoliosis. Preoperative 4.9mm slip in standing radiographs is
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reduced in the supine MRI and fluid is present in the facet joints. At 24 months postoperatively there is 44% reduction of translation and no progression of the spondylolisthesis. The landmarks used by the radiographic core laboratory are shown on the endplates of the index and adjacent segments.
Fig. 3 Patient with L3-L4 degenerative spondylolisthesis (7.2mm) and spinal stenosis as well as bulging intervertebral disc. At 24 months postoperatively there is a 39% reduction of dynamic translation and no progression of the spondylolisthesis.
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Fig. 4 Summary of clinical outcomes (mean and 95% confidence interval)
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(Mean and SD)
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Fig. 5 Summary of clinical and radiographic results for subjects with >2mm translation at preoperative baseline
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Acknowledgements: Institutional research support was provided by Simpirica Spine, Inc. (San Carlos, CA, USA). None of the institutional investigators hold a financial interest in the company or device. The authors would like to acknowledge Christine Beadle, Dominike Bruyninckx, Sarah Winkelkotte, Rahel Bornemann, MD, and Tom R
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Jansen, MD for their contributions to this work.
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