ABSTRACT
Porous Media ........................................................................................... 130 6.3 A Specialized Triphasic Formulation for Charged Hydrated Soft Tissue ................. 131
6.3.1 Governing Equations ............................................................................................ 132 6.3.2 Boundary Conditions ............................................................................................ 134 6.3.3 The Weak Formulation for the Finite Element Implementation ..................... 135
6.4 Multiphasic FEMs of Human IVD ................................................................................... 136 6.4.1 Human IVD under Axial Compression during Stress Relaxation ................. 136
6.4.1.1 IVD Geometry, Loading Conguration, and FE Implementation ... 137 6.4.1.2 Proles of Physical Signals .................................................................... 139 6.4.1.3 Effects of Porosity and FCD ................................................................... 139 6.4.1.4 Solute Transport ...................................................................................... 139 6.4.1.5 Summary .................................................................................................. 141
6.4.2 Human IVD under Physiological Diurnal Cyclic Loading ............................. 143 6.4.2.1 Nutrient Consumption Rate and Metabolite Production Rate ......... 144 6.4.2.2 Concentration Proles of Nutrient and Metabolite ............................ 146 6.4.2.3 Impact of Cyclical Loading .................................................................... 147 6.4.2.4 Effects of CEP Calcication ................................................................... 148 6.4.2.5 Effects of NP Cell Injection in Cell-Based Therapy ........................... 149 6.4.2.6 Summary .................................................................................................. 151
6.5 Conclusion and Future Perspective ................................................................................ 151 Acknowledgments ...................................................................................................................... 153 References ..................................................................................................................................... 153
ABSTRACT A three-dimensional inhomogeneous nite element model for charged hydrated soft tissues containing charged/uncharged solutes was developed based on the triphasic theory. It was applied to analyze the mechanical, chemical, and electrical signals within the human intervertebral disc (IVD) during axial unconned compression and physiological loading conditions. The human IVD was modeled as an inhomogeneous mixture consisting of a charged elastic solid, water, ions (Na+ and Cl−), and nutrient solute (oxygen, glucose, and lactate) phases. The effects of tissue properties and boundary conditions on the physical signals and the transport of uid and solute were investigated. In addition, the effects of the endplate calcication and cell injection were simulated by a reduction of the tissue porosity and increasing the cell density. The numerical simulation showed that, during disc compression, the uid pressurization and the effective (von Mises) solid stress were more pronounced in the annulus brosus (AF) region near the interface between the AF and nucleus pulposus (NP). The electrical signals were very sensitive to xed charge density. Changes in material properties of the NP (water content, xed charge density, and modulus) affected uid pressure, electrical potential, effective stress, and solute transport in the disc. The physiological diurnal cyclic loading did not signicantly change the nutrient environment in the human IVD during the day and night. Calcication of the cartilage endplate signicantly reduced the nutrient levels in the human IVD. In cell-based therapies for IVD regeneration, excessive amounts of injected cells may cause further deterioration of the nutrient environment in the degenerated disc. This study is important for understanding the disc biomechanics and pathology of IVD degeneration and providing new insights into cell-based therapies for low back pain.
