Intervertebral disc (IVD) degeneration can be associated with cartilage endplate (CEP) calcification (Roberts et al. 1996) and a corresponding reduction in permeability, but any causal mechanism is yet to be established. The permeability of the CEP may play an important role in IVD degeneration by controlling the convective and diffusive transport of metabolites into the nucleus pulposus (NP) (Urban et al. 1982).A poroelastic model of the IVD, which includes osmotic pressure and strain-dependent permeability, has been shown to accurately describe the behaviour of non-degenerate IVDs (Riches et al. 2002). This model was modified to incorporate the CEP by utilising a boundary condition that equated the fluid flux through the top of the disc with the fluid flux through a rigid porous layer representing the CEP. Other model parameters remained unchanged. The effect of a loss of CEP permeability on the mechanics of the NP was predicted by using three different CEP permeabilities: (1×10-15 m4/Ns, 5×10-16 m4/Ns and 1×10-16 m4/Ns). Two fresh IVDs with Thompson grade 3 degeneration were subjected to 5 cycles of 1MPa loading consisting of 20 minutes compression followed by 40 minutes expansion (Dhillon et al. 2001). Experimental results were compared to the model.Figure 1 shows the model height and internal IVD permeability distribution for a CEP permeability of 1×10-16 m4/Ns. The change in height with time is effectively linear for both compression and expansion. Conversely, the deformation behaviour for degenerated IVDs was not linear (Figure 2).Whilst IVD degeneration reduces the osmotic pressure and the water content of the IVD, IVD degeneration does not affect tissue permeability (Iatridis et al. 1998). Therefore, the time-dependent deformation behaviour for degenerate and non-degenerate IVDs should be similar, unless the permeability of the CEP is reduced to less than that of the NP, as shown in this study. Since our preliminary data do not exhibit a linear height change, we can conclude, at least for these two IVDs, that even after significant IVD degeneration, the permeability of the CEP is greater than that of the NP, and as a consequence there may be limited or no hinderance to NP nutrition. The large height variation seen experimentally suggests a reduced aggregate modulus and a reduced osmotic pressure compared to non-degenerate IVDs
University of Glasgow (2004) J Physiol 557P, C72
Communications: Intervertebral disc nutrition via the endplate may not be affected by degeneration
P. Riches (a),N.Dhillon (c) and D. McNally (b)
(a) Department of Applied Physiology, University of Strathclyde, Glasgow, UK, (b) University of Nottingham, Nottingham, UK and (c) University of California at San Francisco, San Francisco, CA, USA
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Where applicable, experiments conform with Society ethical requirements.