Interspecies Comparison of Multilayer Mechanical Properties of Synovium Using Atomic Force Microscopy

The synovium is a loose connective tissue that separates the intra-articular (IA) joint compartments of all diarthrodial joints from the systemic circulation. It can be divided into two layers: the intima, a thin and cell-dense layer atop a more heterogeneous subintima, composed of collagen and various cell types. The subintima contains penetrating capillaries and lymphatic vessels that rapidly clear injected drugs from the joint space which may vary not only with drug size and charge but also with the microstructure and composition of the intima and subintima of the synovium. Prior work has measured the mechanical properties and solute diffusivities in the synovium of porcine, bovine, and human joints. Here, we measured the Young's moduli of synovium from smaller joints of the rat knee, as well as pig and human, using atomic force microscopy (AFM). The format for AFM enabled testing of intima and subintimal regions of synovium in all three species. The Young's moduli of the subintimal regions were similar across all three species (1-1.5 kPa). Furthermore, there was little evidence of differences in Young's moduli between synovium from the intima and subintima in each species. A general similarity of data from AFM testing with moduli measured with bulk testing of pig and human synovium suggests that AFM can be useful to measure the mechanical properties of smaller joint synovium and spatial variations in stiffness with depth. Enzymatic digestion of synovium tissue from the pig was also performed with findings of lower moduli values following treatment with chondroitinase ABC but not collagenase. Although the molecular composition of the synovium is not yet fully characterized and may vary across species, these findings suggest that noncollagenous species contribute to AFM-measured properties in synovium. These are some of the first data to measure mechanical properties in small joint synovium and will be useful in models studying IA drug clearances in joints with pathology and following treatment.