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Tensile radial stress in the spinal cord related to arachnoiditis or tethering: a numerical model

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Abstract

Spinal arachnoiditis comprises fibrous scarring of the subarachnoid space, following spinal trauma or inflammation, and is often associated with syringomyelia. We hypothesised that cord-to-dura attachments could cause transient tensile cord radial stress, as pressure waves propagate. This was tested in a fluid–structure interaction model, simulating three types of cord tethering, with ‘arachnoiditis’ confined to a short mid-section of the cord. The annular system was excited abdominally with a short transient, and the resulting Young and Lamb waves and reflections were analysed. Radial mid-section tethering was less significant than axial tethering, which gave rise to tensile radial stress locally when the cord was not fixed cranially. Simulated as inextensible string connections to the dura, arachnoiditis caused both localised tensile radial stress and localised low pressure in the cord as the transient passed. The extent of these effects was sensitive to the relative stiffness of the dura and cord. Tensile radial stress may create a syrinx in previously normal cord tissue, and transiently lowered pressure may draw in interstitial fluid, causing the syrinx to enlarge if fluid exit is inhibited. The suggested mechanism could also explain the juxtaposition of syrinxes to regions of arachnoiditis.

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Notes

  1. The concept of pressure here requires careful definition, since strictly there are only directed stresses in an elastic solid. Pressure is calculated as minus the average of the mutually orthogonal normal stresses. If the solids were microporous, with a fluid content component (as it is in reality in the case of cord tissue), the fluid would have this pressure.

  2. Further analysis of wave propagation in an infinitely long annular space bounded outside by a rigid conduit and inside by an elastic vessel containing another fluid is given by Cirovic et al. [5], particularly for Young waves. They also analyse a model consisting of two coaxial solid elastic annuli creating three coaxial fluid spaces within a rigid outermost tube.

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Correspondence to C. D. Bertram.

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Parts of this work were presented at the 5th World Congress of Biomechanics, Munich, Germany, 29 July–4 August 2006, and at the Conquer Chiari Research Symposium, Chicago, USA, 2 June 2007.

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Bertram, C.D., Bilston, L.E. & Stoodley, M.A. Tensile radial stress in the spinal cord related to arachnoiditis or tethering: a numerical model . Med Biol Eng Comput 46, 701–707 (2008). https://doi.org/10.1007/s11517-008-0332-0

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  • DOI: https://doi.org/10.1007/s11517-008-0332-0

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