The strain-dependent osmotic pressure and stiffness of the bovine nucleus pulposus apportioned into ionic and non-ionic contributors

Elucidation of the load-bearing mechanism of the nucleus pulposus (NP) facilitates understanding of the mechanical and metabolic functioning of the intervertebral disc and provides key data for mathematical models. Negatively charged proteoglycans in the NP generate an ionic osmotic pressure, πi, which contributes to the tissueʹs resistance to load and, moreover, is the main mechanism by which the unloaded disc rehydrates. Functionally important, πi has seldom been investigated in situ and, crucially, its variation with strain has not been reported. In a confined compression apparatus, we aimed to apportion the strain-dependent load-bearing mechanism of the NP at equilibrium to the tissue matrix and ionic osmotic pressure; and to determine whether any proteoglycan loss occurs during confined compression testing. Forty-eight confined compression experiments were conducted in isotonic (0.15 M NaCl) and hypertonic (3.0 and 6.1 M NaCl) external solutions in single and multiple step-strain protocols. The 6.1 M NaCl external solution was needed to eliminate as much of the ionic effects as possible. The ionic osmotic pressure was well described by πi=19.1λ−1.58 (R2=0.992), and was approximately 70% of the applied load at equilibrium, independent of λ. The effective aggregate modulus, , also increased with strain: . Concentrations of sulphated glycosaminoglycans were obtained for the samples tested in isotonic NaCl with no proteoglycan loss detected from the confined compression tests. These results highlight the non-linearity of the stress–strain response of NP tissue and the necessity to include a non-linear function for osmotic pressure in mathematical models of this tissue.