"Introduction: Synovial fluid is a viscous liquid found inside the joints of bones. This fluid helps in minimizing the bone to bone contact and facilitates movements of joints via lubrication. The key component of synovial fluid which gives its viscous property is hyaluronic acid (HA). Apart from HA it also consists of various proteins like albumins, gamma globulins etc. Synovial fluid has a unique viscoelastic property by which it protects the cartilage tissue from tearing down from joint movements. Alteration of this viscoelastic property leads to different arthritic conditions. There are different types of arthritic conditions starting from osteoarthritis, rheumatoid arthritis, gouty arthritis etc. It has been found that in every arthritic condition synovial fluid lactate level were different. Specially in case of rheumatoid arthritis, synovial lactate levels were found to be elevated. The presence of elevated lactic acid is might be due to from anaerobic respirations of some infectious bacteria. The presence of elevated lactic acid leads to decrease in pH conditions. This low pH environment might decrease the molecular weight of hyaluronic acid which eventually decreases the viscosity of hyaluronic acid. In order to know how lactic acid affects the synovial fluid viscoelasticity, microrheology as well as bulk rheology experiments were performed for hyaluronic acid and prepared model synovial fluid which consists of albumin and gamma globulins in addition to HA.
Materials and Methods: HA concentration was used 3.4 mg/ml prepared in 10mM phosphate buffer at pH 7.4. For model synovial fluid preparation, 3.4 mg/ml of HA, 10 mg/ml of bovine serum albumin (BSA) and 0.5mg/ml γ- Globulin were mixed. For the study of effects of Lactic acid, L-Lactic acid with different concentrations ranging from 1mg/ml to 5mg/ml were used. Microrheology as well as Bulk rheology experiments were done using Diffusing Wave Spectroscopy (DWS) and Anton Par MCR 301 Interfacial rheometer respectively.
Results and Discussions: In the microrheology experiment, there is a slight decrease in decay time of normalized intensity correlation function (ICF) of HA with increasing lactic acid concentrations. From Bulk rheology experiment, the viscosity of HA was also found to be reduced with Lactic acid concentrations. This might be due to decrease in chain length of HA. On the contrary, DWS of model SF, there is drastic change in ICF decay, where Lactic acid treatment significantly decreases the time for ICF decay. Further Bulk rheology experiment shows the notable decrease in viscosity of Lactic acid treated samples.
Conclusion: From microrheology as well as bulk rheology experiments it was established that Lactic acid treatment creates an environment where protein binds with HA and coprecipitates. This might be the reason behind the decreased synovial fluid viscosity in rheumatoid arthritis conditions.
Acknowledgements: This work was funded by ICMR, Grant no. 5/3/8/20/2019-ITR and MeitY - Grant no. 5(1)/2021-NANO, Government of India."