Intradiscal solid phase displacement as a determinant of the centripetal fluid shift in the loaded intervertebral disc

STUDY DESIGN: The movement of cross sections of the monofilament nylon threads inserted into the axially loaded intervertebral disc was traced with magnetic resonance imaging (MRI). This technique allowed the observation of the sequential solid phase displacement of the loaded intervertebral disc. OBJECTIVES: To clarify sequential solid phase displacement of the axially loaded intervertebral disc to elucidate the cause of centripetal fluid shift within a disc. SUMMARY OF BACKGROUND DATA: We already have reported that there is a centripetal fluid shift within the axially loaded intervertebral disc during the early phase of loading. We assumed that there should be an elaborate intradiscal matrix displacement that generates a pressure gradient within the disc to cause a centripetal fluid shift. METHODS: Thirteen freshly obtained bovine caudal intervertebral discs were prepared. Three to five monofilament nylon threads were inserted into each disc in the anterior-posterior direction to trace the intradiscal solid phase displacement on the midcoronal MR images. Sequential displacement of the disc matrix was recorded during a 294 N axial loading. RESULTS: Relatively large centrifugal expansion at the inner layer of the anulus fibrosus compared with less centrifugal expansion of the outer anulus fibrosus was observed in accord with gradual creep of the disc thickness. CONCLUSIONS: The uneven displacement of the intradiscal solid phase observed in the present study expels the fluid phase from the inner anulus fibrosus, thus resulting in accumulation of fluid phase in the nucleus pulposus. The present study suggests the presence of a mechanism that retains water within the normal intervertebral disc, in spite of an external load, because it forms a water-abundant nucleus pulposus, which is surrounded by an anulus fibrosus with decreased water permeability caused by fluid loss. A more detailed analysis is required to clarify topographic volumetric changes within the disc.