A measurement technique to evaluate the macroscopic permeability of the vertebral end-plate

The remodelling response of vertebral segments in idiopathic scoliosis or disc degeneration, shows a modification of the disc hydration. The investigation of mass transport between the disc and the vertebral body is relevant to understanding the normal and pathological behaviour of the spine. The measurement method we adopted, to derive the macroscopic permeability of the vertebral end-plate, used the relaxation pressure due to a transient-flow rate into the biological structure. Our approach mimicked the in vivo conditions of mass transfer between the disc and the vertebral bodies. It minimized the flow-induced matrix compaction and it allowed the boundary conditions of the specimen to be controlled. We proposed a specific theoretical method and an associated device according to poroelastic theory. A preliminary evaluation with a controlled porous medium, preceded a pilot study in an animal model (lumbar L(4)-L(5) segment of a 4 months old female pig). The macroscopic permeability of multilayered cylindrical specimens including the trabecular bone, subchondral bone and cartilage end-plate was measured, using a 'cartilage-towards-bone' fluid flow direction. A histologic evaluation completed the biomechanical approach. Results showed that the central zone was more permeable than the periphery and this concurred with qualitative studies from the literature.