Epidural application of spinal instrumentation particulate wear debris: a comprehensive evaluation of neurotoxicity using an in vivo animal model

Laboratory investigation

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Object

The introduction and utilization of motion-preserving implant systems for spinal reconstruction served as the impetus for this basic scientific investigation. The effect of unintended wear particulate debris resulting from micromotion at spinal implant interconnections and bearing surfaces remains a clinical concern. Using an in vivo rabbit model, the current study quantified the neural and systemic histopathological responses following epidural application of 11 different types of medical-grade particulate wear debris produced from spinal instrumentation.

Methods

A total of 120 New Zealand White rabbits were equally randomized into 12 groups based on implant treatment: 1) sham (control), 2) stainless steel, 3) titanium alloy, 4) cobalt chromium alloy, 5) ultra–high molecular weight polyethylene (UHMWPe), 6) ceramic, 7) polytetrafluoroethylene, 8) polycarbonate urethane, 9) silicone, 10) polyethylene terephthalate, 11) polyester, and 12) polyetheretherketone. The surgical procedure consisted of a midline posterior approach followed by resection of the L-6 spinous process and L5–6 ligamentum flavum, permitting interlaminar exposure of the dural sac. Four milligrams of the appropriate treatment material (Groups 2–12) was then implanted onto the dura in a dry, sterile format. All particles (average size range 0.1–50 μm in diameter) were verified to be endotoxin free prior to implantation. Five animals from each treatment group were sacrificed at 3 months and 5 were sacrificed at 6 months postoperatively. Postmortem analysis included epidural cultures and histopathological assessment of local and systemic tissue samples. Immunocytochemical analysis of the spinal cord and overlying epidural fibrosis quantified the extent of proinflammatory cytokines (tumor necrosis factor–α, tumor necrosis factor–β, interleukin [IL]–1α, IL-1β, and IL-6) and activated macrophages.

Results

Epidural cultures were negative for nearly all cases, and there was no evidence of particulate debris or significant histopathological changes in the systemic tissues. Gross histopathological examination demonstrated increased levels of epidural fibrosis in the experimental treatment groups compared with the control group. Histopathological evaluation of the epidural fibrous tissues showed evidence of a histiocytic reaction containing phagocytized inert particles and foci of local inflammatory reactions. At 3 months, immunohistochemical examination of the spinal cord and epidural tissues demonstrated upregulation of IL-6 in the groups in which metallic and UHMWPe debris were implanted (p < 0.05), while macrophage activity levels were greatest in the stainless-steel and UHMWPe groups (p < 0.05). By 6 months, the levels of activated cytokines and macrophages in nearly all experimental cases were downregulated and not significantly different from those of the operative controls (p > 0.05). The spinal cord had no evidence of lesions or neuropathology. However, multiple treatments in the metallic groups exhibited a mild, chronic macrophage response to particulate debris, which had diffused intrathecally.

Conclusions

Epidural application of spinal instrumentation particulate wear debris elicits a chronic histiocytic reaction localized primarily within the epidural fibrosis. Particles have the capacity to diffuse intrathecally, eliciting a transient upregulation in macrophage/cytokine activity response within the epidural fibrosis. Overall, based on the time periods evaluated, there was no evidence of an acute neural or systemic histopathological response to the materials included in the current project.

Abbreviations used in this paper:ABC = avidin-biotin complex; HAM = human alveolar macrophage; IL = interleukin; PCU = polycarbonate urethane; PEEK = polyetheretherketone; PET = polyethylene terephthalate; PTFE = polytetrafluoroethylene; TNF = tumor necrosis factor; UHMWPe = ultra–high molecular weight polyethylene; ZTA = zirconium toughened alumina.

Article Information

Address correspondence to: Bryan W. Cunningham, Ph.D., Orthopaedic Spinal Research Institute, University of Maryland St. Joseph Medical Center, 7601 Osler Dr., Jordan Suite 167, Towson, MD 21204. email: bcspine@gmail.com.

Please include this information when citing this paper: published online June 28, 2013; DOI: 10.3171/2013.5.SPINE13166.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    A: Low-angle laser light scattering histogram demonstrating the volume-based size distribution for PEEK material. Particles exhibited a mean diameter of 14.6 μm (range 0.5–88 μm) with 24% of particles less than 5 μm and 3% less than 1 μm. B: Low-angle laser light scattering histogram demonstrating the number-based size distribution for PEEK material. Particles had a mean diameter of 0.8 μm (range 0.5–18.5 μm); 99% of particles were less than 5 μm and 84% were less than 1 μm.

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    Representative scanning electron microscopic image (A) and energy-dispersive x-ray energy analysis profile (B) obtained for the PEEK material. A: The scan showed the PEEK particle morphology to be rough, irregular, granular, or flaky, with no fibers at any size range. Magnification 5-μm intervals. B: The profile demonstrated a signature consistent with that produced by PEEK material.

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    A: Intraoperative posterior views of the L5–6 level following resection of the spinous process and interspinous ligaments and subsequent transection of the ligamentum flavum. The spinal cord and membranous covering were directly visible prior to application of wear particulate, which was implanted in dry, sterile format. B and C: Intraoperative images showing PCU (B) and PET (C).

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    Histopathological gross posterior views of the spinal cord at necropsy demonstrating an operative control (sham) specimen (A) and titanium alloy treatment specimen (B). Note the extent of epidural fibrosis and dissemination of metallic material along the dural region and bordering the exiting nerve root in the experimental treatment specimen.

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    Comparative levels of activated IL-6 cytokines in the epidural fibrosis and spinal cord at the 3-and 6-month intervals following implantation of the materials. Statistical significance between the 3-month treatments is as follows: ~ versus all except titanium alloy and UHMWPe; ** versus all except stainless steel and UHMWPe; # versus all except stainless steel, titanium alloy, and UHMWPe; * versus all except stainless steel and titanium alloy (p < 0.05). Bar height indicates mean value and error bars minus 1 SD (1-way ANOVA, F = 19.8, p = 0.000).

  • View in gallery

    Comparative levels of activated macrophages in the epidural fibrosis and spinal cord at the 3-and 6-month time intervals following implantation of the materials. Significance between the 3-month treatments is as follows: ^ versus all except titanium alloy, cobalt chrome, UHMWPe, PCU, and PET. Significance between the 6-month treatments is as follows: # versus all except cobalt chrome; ~ versus all except stainless steel. Bar height indicates mean value and error bars minus 1 SD (1-way ANOVA, F = 3.76, p = 0.000).

  • View in gallery

    Histological axial sections from 6-month control specimens prepared with H & E (A) and HAM-56 macrophage (B) stains. Histopathological evaluation indicated normal distribution of myelin and intracellular neurofibrillary networks and characterized all control specimens as without significant pathological changes at both the 3-and 6-month time intervals. Original magnification 10×.

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    A and B: Histological sections demonstrating the epidural layer and spinal cord from a 6-month postoperative cobalt chromium (CoCr) specimen. In many cases, a particulate layer formed along the dura mater and consisted of unphagocytosable particles, which were encapsulated by an organized fibrous connective tissue layer. Note the particulate debris (B) and intrathecal dissemination of particles localized subdurally (A). This diffusion process was coupled with histiocytic macrophage response consistent with a localized, chronic inflammatory reaction (macrophages indicated by red regions). Original magnification 40× (A) and 200× (B). C and D: Histological sections demonstrating membrane-bound or intracellular IL-6 cytokines, which produced a yellow to brown chromogen label localized in the epidural fibrosis layer as shown in these 3-month postoperative cobalt chrome treatments. Note the proximity of particulate debris to IL-6–expressing macrophages. ABC–horseradish peroxidase technique for interleukin-6, original magnification 100× (C) and 200× (D).

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    Histological sections demonstrating macrophage reactivity to polymeric debris localized within regions of epidural fibrosis. Agglomeration of the UHMWPe polymeric treatments is evident in A and B, with particle sizes ranging from 50 to 300 μm. PCU (C) and PET (D) macrophage reactivity and particulate aggregation are also demonstrated. Each aggregate contained a dense macrophage reaction along the periphery as shown in these light microscopic views. HAM-56 macrophage stain, original magnification 100× (A) and 200× (B–D).

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