Rheological and interface adhesive properties of osteoarthritic synovial fluids

Lubrication by synovial fluid is vital in maintaining the physiological operation of diarthrodial joints, which can be compromised due to changes in synovial fluid as osteoarthritis progresses, resulting in a lubrication deficiency. We studied the rheological and interface adhesive properties of osteoarthritic synovial fluid samples (49 in total), aiming to establish the correlation with medical conditions experienced by patients who underwent knee or hip Total Joint Replacement due to osteoarthritis. The steady-state rheology and the interfacial adhesion energy of those samples were examined, of which the results were compared across various patient characteristics. Seniority (60–79 years old) and obesity (Body Mass Index above 30) were identified as the dominant risk factors for the degeneration of synovial fluid and, thus, the potential development of osteoarthritis. At low shear rates where the interactions are more prominent than high shear rates, synovial fluid viscosity decreased by 58% with age and 38% with Body Mass Index, while the shear-thinning index increased by 40% and 7%, respectively. Additionally, synovial fluid interfacial adhesion properties deteriorate, as shown by the adhesion energy results. The adhesion energy, which is positively correlated to the coefficient of friction, increased by 172% with age and by 234% with Body Mass Index. The changes in both rheological and interface adhesion properties of synovial fluid samples support the relationship between the physico-chemical properties and the clinical observation. The AFM based surface adhesion method could be considered as a new approach to evaluate the tribological characteristics of natural synovial fluids as it requires small volume of samples.