Introduction: Optical coherence tomography (OCT) is a high-resolution imaging modality which can be used to acquire detailed elastograms of biological tissue. In this investigation, we demonstrate the use of OCT to generate µm-scale strain maps of articular cartilage (AC) under compressive and shear deformations. AC is a dense connective tissue which provides a low-friction surface in synovial joints. The specific alignment of collagen fibrils and proteoglycans (which contribute primarily to shear and compressive stiffness, respectively) give rise to depth-dependent mechanical properties.
Methods: Six 6mm diameter samples of articular cartilage were harvested from a calf femur. A custom-designed biaxial loading apparatus applied compressive and shear displacements. Three-dimensional images of the tissue were obtained using a spectral-domain OCT system as the sample was loaded at constant rate of displacement. Both speckle-tracking and phase-shift methods were used to generate strain maps from these images.
Results: Under both shear and compressive loading, clear differences in local strain distribution were observed between the superficial, transitional, and radial zones of the cartilage. In shear, the superficial/transitional zones are stiffest while in compression these regions are more compliant than the radial zone. These distributions correspond with existing literature and the known orientation of collagen in the AC.
Conclusion: It is feasible to rapidly acquire strain maps in AC using OCT. This technique may be extended to high-throughput screening to nondestructively determine the functionality and failure modes of engineered AC as compared to native tissue.