Characterizing the Baseline Regional Biphasic Mechanical Properties of Cervical Intervertebral Discs.

Purpose: While the regional viscoelastic biomechanical properties of lumbar intervertebral disc tissues are well documented, equivalent tissue-level characterizations for human cervical discs remain unexplored. This study aimed to quantify biphasic mechanical properties of the nucleus pulposus (NP), annulus fibrosus (AF), and cartilaginous endplate (CEP) in cervical discs.

Methods: A previously established confined compression testing technique was used to measure swelling pressure, equilibrium aggregate modulus, and hydraulic permeability in cervical NP, AF, and CEP tissues. Specimen-specific porosity was also assessed and correlated with these properties. A finite element model was used to simulate unconfined compression.

Results: Swelling pressure (154.50 ± 89.47 kPa) and aggregate modulus (0.677 ± 0.671 MPa) were significantly higher in the CEP compared to the NP (p = 0.0308 and p = 0.0227, respectively) or AF (p = 0.0338 for aggregate modulus), with no significant differences observed between NP and AF. Permeability did not differ significantly among regions. Porosity showed negative correlations with both swelling pressure (r = - 0.55, p = 0.0006) and aggregate modulus (r = - 0.53, p = 0.001). Finite element analysis revealed a relatively uniform von Mises stress distribution between NP and AF, with higher magnitudes concentrated in the CEP.

Conclusion: Cervical NP and AF exhibit relatively homogeneous biomechanical properties, whereas the CEP is found to have greater stiffness and swelling pressure. These findings indicate unique tissue-level adaptations in cervical discs to support greater mobility. These data could also inform future studies investigating region-specific degeneration and aging effects on cartilaginous tissue function in cervical discs and enhance the representation of viscoelasticity in computational modeling of the IVD.