Synovial fluid is an ultrafiltrate of blood and, as well as HA, also contains a significant amount of the serum proteins globulin (≈ 0.7 wt.%) and albumin (≈ 1.1 wt.%). In healthy SF the HA concentration is around 0.3 wt.% and has a molecular weight of a few million Daltons.

Synovial fluid is predominantly a semi-dilute solution of the glycosaminoglycan hyaluronic acid (HA), a high molecular weight linear polysaccharide. Extensional rheological measurements have also shown that SF is significantly strain-hardening under elongational deformations, which is likely to provide shock-dampening when joints experience sudden compressive loading, as occurs in the knees during running and jumping for example. In particular, the rheology of SF, which is highly shear-thinning and viscoelastic –, contributes to the low friction between joint surfaces undergoing flexion. Synovial fluid (SF) plays a vital role in the protection and maintenance of healthy function in animal joints. These interactions influence FIB-derived macromolecular parameters such as the relaxation time and the molecular weight distribution and therefore must be minimized for the best validity of this method as an analytical technique, in which non-interaction between molecules is assumed. However a systematic shift in the FIB response as the PSF and t-PSF samples are progressively diluted indicates that HA-HA interactions remain significant at the concentrations tested. This helps support the validity of the analysis of synovial fluid HA content, either by microfluidic or by other techniques where the synovial fluid is first diluted, and suggests that the HA and protein content in synovial fluid must be higher than a certain minimum threshold concentration before HA-protein or protein-protein interactions become significant.

The results show that, at the synovial fluid concentrations tested, the protein content plays a negligible role in either the fluid's bulk shear or extensional flow behaviour. Equivalent experiments are also carried out using trypsin-treated PSF (t-PSF) in which the protein content is digested away using an enzyme. The PSF sample is diluted to 10× 20× and 30× its original mass in a phosphate-buffered saline and its FIB response measured as a function of the strain rate at the stagnation point of the cross-slots. In this study, a microfluidic cross-slot device is used to examine the extensional flow response of diluted porcine synovial fluid (PSF) samples using flow-induced birefringence (FIB) measurements.

Synovial fluid viscosity test