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Chiropractic Research Review

Intradiscal Solid Phase Displacement

Solid and fluid phases comprise the intervertebral disc: collagen makes up the solid outside (annulus fibrosus) and proteoglycans and water constitute the fluid inside. It has been reported that fluid shifts within the disc when it is axially loaded.

A new study hypothesized that there should be an elaborate intradiscal matrix displacement, which generates a pressure gradient within the disc and causes a centripetal fluid shift. This study served two purposes: to clarify sequential solid phase displacement of the axially loaded intervertebral disc, and to determine the cause of centripetal fluid shift.

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 intradiscal solid phase displacement on the magnetic resonance images. Sequential displacement of the disc matrix was recorded during axial loading. Large centrifugal expansion at the inner layer of the annulus fibrosus was compared with less centrifugal expansion of the outer annulus fibrosus, and observed in accordance with gradual creep of the disc thickness.

The authors summarized their findings by stating: "Uneven displacement of the intradiscal solid phase expels the fluid phase from the inner annulus fibrosus, resulting in accumulation of fluid phase in the nucleus pulposus, which is surrounded by an annulus fibrosus with decreased water permeability caused by fluid loss." They also recommend a more detailed analysis to clarify topographic volumetric changes within the disc.

Kusaka Y, Nakajima S, Uemura O, et al. Intradiscal solid phase displacement as a determinant of the centripetal fluid shift in the loaded intervertebral disc. Spine 2001 (electronic version):26, ppE174-E181.

Chiropractic Research Review

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