Human articular cartilage is orthotropic where microstructure, micromechanics, and chemistry vary with depth and split-line orientation

Abstract

Objective: Quantitative, micrometer length scale assessment of human articular cartilage is essential to enable progress toward new functional tissue engineering approaches, including utilization of emerging 3D bioprinting technologies, and for improved computational modeling of the osteochondral unit. Thus the objective of this study was to characterize the structural organization, material properties, and chemical composition of human skeletally mature articular cartilage with respect to depth and defined morphological features: normal to the articulating surface, parallel to the split-line, and transverse to the split-line. Method: Three samples from the lateral femoral condyles of 4 healthy adult donors (55e61 years old) were evaluated via histology, second harmonic generation, microindentation, and Raman spectroscopy. All metrics were evaluated as a function of depth and direction relative to the split-line. Results: All donors presented with intact and healthy tissue. Collagen fiber orientation varied significantly between testing directions and with increasing depth from the articular surface. Both compressive and tensile modulus increased significantly with depth and differed across the middle and deep zones and depended on orthogonal direction relative to the split-line. Similarly, matrix components varied with both depth and direction, where chondroitin sulfate steadily increased with depth while collagen prevalence was highest in the surface layer. Conclusions: Microscale measurements of human articular cartilage demonstrate that properties are both depth-dependent and orthotropic and depend on the underlying tissue structure and composition. These findings improve upon existing knowledge establishing more accurate measurements, with greater degree of depth and spatial specificity, as inputs for tissue engineering and computational modeling.

Acellular Cartilage-Bone Allografts Restore Structure, Mechanics, and Surface Properties In Vivo, but Limit Recellularization and Integrative Repair

“…The implant and defect repair manifested key differences in composition. We compared the amplitude of common peaks identified from Raman spectra 15–18 for collagen, chondroitin sulfate, phosphate, and non-collagenous proteins on the surface of three regions in each joint: native cartilage, integration zone, and tissue (i.e., implant or defect) repair ( Figure 3 ). We found consistency across all native tissue, despite whether the joint had an implant or an untreated defect.…”

“…In each region, spectral maps consisting of sixteen measurements covering a 40 μm x 40 μm area were collected. At each point in the spectral map, cosmic rays were removed from the collected spectra, a linear baseline was subtracted and intensity normalized, and spectra were smoothed 18 . The sixteen spectral acquisitions that composed each spectral map were averaged, key peaks of interest were identified, and peak amplitude was calculated using custom R code following a previously established Raman spectra analysis approach 53 .…”

Strategy insight: Mechanical properties of biomaterials’ influence on hydrogel-mesenchymal stromal cell combination for osteoarthritis therapy

“…Interestingly, the mechanical properties of cartilage depend on its anatomical position, and cartilage is regional heterogeneity. ECM tissues’ biochemical components and structure can also significantly affect intraarticular biomechanics ( Fischenich et al, 2020 ). Due to the orthotropic material properties of articular cartilage, the measurement reported by different groups varied in methods and results.…”

Cellular therapy and tissue engineering for cartilage repair

“…To generate successful implanted constructs, cartilage produced by cellular therapy or tissue engineering must have the characteristics of native articular cartilage. That is, regenerated cartilage must contain appropriate mechanical, compositional, and structural anisotropies [13], to be able to withstand compressive forces and enable tribological movement of the joint, as well as integrate with bone and surrounding cartilage. Crosstalk at the osteochondral junction [14] is also important for construct success.…”