The human lumbar intervertebral disc : a multi-phase biological tissue approached by bio-chemo-mechanical considerations
Résumé
The intervertebral disc (ID) is an avascular and fibrocartilagenous joint thatseparates spinal vertebrae. It is composed of an outer-layer annulus fibrosus,a central proteoglycan-rich nucleus pulposus and 2 hyaline cartilage endplates.The proteoglycans (PGs) that concentration is set up by cellular activity, inflatethe nucleus by a high osmolarity and allow the disc to carry large compressiveloads. The ID loses and regains around 25% of its internal fluid over adiurnal cycle (low loads during rest and high load during day's activity). Duringdegeneration, it notably appears a decrease of cellular activity and a loss ofthe nucleus hydration associated with changes in disc mechanical stiffnessand cushioning properties. However the mechanisms for these changes whichimply biological and mechanical events remain still unclear.We propose in the present study to model the mechanical behavior of theID using a coupled chemico-mechanical model based on a fluid-transport approachand, taking into account cellular activity, physico-chemical environmentand interstitial fluid transport related to external loads. Basically, the modelsdeveloped in the literature assumed a constant permeability or dependenton solid matrix deformation using a classical Darcy's law. Our goal is toimprove these models considering that the permeability depends on hydrostaticand osmotic pressures related to nucleus cells activity via PGs and lactateconcentrations. These factors will be experimentally evaluated by means ofin vitro characterization of human lumbar ID extracted from cadavers. Thus,the chemo-mechanical couplings will be assessed and tested in numericalsimulations.Once established, the present model will help to analyze the changes inthe mechanical and physical properties of lumbar vertebral joints during agerelated,pathologic and traumatic degenerations.