Intervertebral discs (IVDs) in the lumbar spine are among the largest avascular tissues in the human body, crucial for weight bearing, shock absorption, and the coordination of complex three-dimensional motions (Adams and Roughley, 2006, Alonso and Hart, 2014). Each disc is interfaced with adjacent vertebrae via a thin layer of hyaline cartilage endplate (CEP) tissue covering its superior and inferior ends. CEP is considered vital for supplying nutrients to the disc and maintaining its mechanical integrity as it ages or degenerates (Urban et al., 1982, Roberts et al., 1989, Urban and Roberts, 1995, Adams and Roughley, 2006). Changes to CEP permeability during calcification for instance, has been implicated in the disruption of internal nutrient solute gradients critical for disc cell viability (Roberts et al., 1993, Roberts et al., 1996, Soukane et al., 2007, Jackson et al., 2011, Wu et al., 2013, Huang et al., 2014). Other changes in the CEP, such as lesion formation, have been linked to heightened inflammatory responses in the disc and collagen matrix catabolism, leading to altered IVD kinematics and further injury (Lotz and Ulrich, 2006, Risbud and Shapiro, 2014, Dowdell et al., 2017).

The CEP has been previously characterized as having a greater compressive modulus and lower hydraulic permeability compared to other IVD regions in bovine tissue (Wu et al., 2015). In human, CEP aggregate modulus is nearly four times greater than that for NP tissue and up to thirteen times greater than that for the AF, while hydraulic permeability has been found to be up to an order of magnitude lower in the CEP than in the AF (Cortes et al., 2014, DeLucca et al., 2016). These findings have led to the general understanding that CEP tissue minimizes stress concentration at the IVD-bone interface, acting as a transition layer between the softer disc tissue and more rigid vertebral bone. Moreover, the lower CEP permeability has been hypothesized to support interstitial fluid pressurization within the disc, creating a barrier to fluid convection which may help the disc to better sustain compressive loads (Wu et al., 2015). This may be especially true of the bordering nucleus pulposus (NP) region at the center of the IVD, which is gel-like in structure and is densely embedded with electrically charged proteoglycan molecules that promote disc swelling (Buckwalter, 1995, Perie et al., 2006). In regions where the CEP makes contact with the AF, which is less hydrated and more organized in its matrix (Buckwalter, 1995), it may have additional roles in resisting tension and shear to prevent the AF from experiencing collagen fiber tears or herniation injury (Iatridis et al., 2005, Fields et al., 2018, Zhou et al., 2021).

Other characterizations of mechanical and solute transport properties in the CEP have further highlighted its uniqueness and material complexity in the IVD. Viscoelastic tensile properties have been measured in the CEP, with average equilibrium tensile moduli shown to be comparable to that of adult femoral articular cartilage (Fields et al., 2014). CEP hydraulic permeability has been found to decrease in value in degenerated IVD, while aggregate modulus stays at a similar magnitude (DeLucca et al., 2016). From our previous studies on nutrient solute (glucose), metabolite (lactate), and ion (Na+ and Cl−) diffusivities in the CEP, regionally dependent (i.e., central, lateral, anterior, and posterior) solute transport rates have been observed (Wu et al., 2016, 2017). Furthermore, qualitative differences in cartilage endplate histomorphology suggest a regionally distinct interface structure as well (Wu et al., 2016). However, region-dependent structure function relationships between CEP matrix structure, biochemical composition, and biphasic mechanical properties remain unclear.

Given the regional dependence in human CEP diffusion phenomena and its histological pattern (Wu et al., 2016, 2017), we hypothesized that CEP biphasic mechanical properties are variable between central, lateral, anterior, and posterior regions owing in part to unique tissue composition and structure. Therefore, the primary objectives of this study were to 1) spatially quantify biphasic properties from healthy human CEPs using a previously established confined compression technique; 2) correlate regionally obtained CEP biochemical compositions with biomechanical data from the same specimens; and 3) assess the CEP-bone interface morphology and disc collagen fiber insertion structure in each endplate region through label-free multiphoton confocal microscopy. This work aims to enhance current knowledge of healthy baseline CEP biomechanics and lay a groundwork for further understanding CEP heterogeneity and its biomechanical functions with disc degeneration progression.