ABS 104: Physical and Mechanical Characterization of Knee Synovial Fluid from Patients with Anterior Cruciate Ligament Tears and with Osteoarthritis

Hannah Johnson ¹ , Samuel Stealey ¹ , Nathan Ly ¹ , Robert H. Brophy ² , Muhammad Farooq Rai ³ , Silviya P. Zustiak ¹

¹ Department of Biomedical Engineering, Saint Louis University, Saint Louis, MO, United States of America
² Department of Orthopaedic Surgery, Washington University School of Medicine, Saint Louis, MO, United States of America
³ Department of Biological Sciences, Khalifa University, Abu Dhabi, United Arab Emirates

Van Wickle (2025) Volume 1, ABS 104

Introduction: Synovial fluid (SF) is a viscous, non-Newtonian fluid that resides within the synovial joint cavity and is essential for joint lubrication. The biological and mechanical characteristics of synovial fluid are important indicators of joint health, as these properties change significantly under pathological conditions such as anterior cruciate ligament (ACL) tears and osteoarthritis (OA). ACL tears cause acute changes in joint function, including altered physical and chemical properties due to inflammation, bleeding, and subsequent healing processes. In contrast, OA involves chronic changes in composition and functionality of SF associated with the gradual breakdown of cartilage. Prior research has compared the biology of SF from OA and ACL tears, but differences between the mechanical properties of SF under these conditions remain unclear. This study evaluated various properties of SF samples from 121 patients (83 ACL tear, 38 OA) collected via arthrocentesis. The color, surface adsorption, viscosity, and coefficient of friction (COF) of each sample was observed and compared to patient factors like age, sex, body mass index (BMI) and time from injury. For the mechanical testing, a custom tribo-rheology setup with a ball-on-three-ball geometry was used. SF from ACL tears exhibited significantly darker color, likely due to hemarthrosis, as well as increased hydrophobicity. Viscosity was lower and COF was higher in SF from ACL tear patients compared to that of OA patients, reflecting the differences in lubricious properties of SF between these two pathologies. As time from injury increased, SF from ACL tear patients showed increasing viscosity and decreasing COF, demonstrating how SF returns to homeostasis after an acute injury. COF was shown to increase with age in OA patients and with higher BMI in both groups. This study offers insights on how traumatic injuries and chronic conditions impact the mechanical properties of SF, guiding future research and treatments for joint pathologies.

Methods: Synovial fluid was collected from 121 patients (83 ACL tear, 38 OA) via arthrocentesis using superolateral suprapatellar approach. The samples were centrifuged to remove cells and particulates, and the supernatant was stored at -80°C until testing. The color and darkness of each sample was quantified into red, green, and blue (RGB) intensity values. Hydrophilicity was evaluated by pipetting samples directly onto a titanium plate and measuring the contact angle. The viscosity was measured using a Hagen-Poiseuille-type viscometer at a set shear rate. The dynamic coefficient of friction (DCOF) was measured using a custom tribo-rheology setup with a ball-on-three-ball geometry, with measurements taken at a set sliding speed and set normal force. Statistical differences in these parameters were analyzed against patient condition (ACL tear vs. OA), injury type (isolated ACL tear vs. combined ACL and meniscus tear), age, sex, body mass index (BMI), and time from injury, using GraphPad Prism software.

Results: Synovial fluid from ACL tear patients exhibited a significantly darker color, lower contact angle, decreased viscosity, and higher DCOF compared to that from OA patients. In older OA patients, DCOF was significantly increased, while no correlations were observed between age and ACL tear sample properties. For BMI, the ACL tear group exhibited decreased viscosity and increased DCOF with increasing BMI. Similarly, OA patient samples showed decreased viscosity with higher BMI. In the ACL tear group, viscosity increased and DCOF decreased over time from injury. No significant differences in these parameters were observed based on sex or injury type.

Discussion: These findings offer valuable insight into the effect of joint pathologies on synovial fluid properties. In ACL tear samples, hemarthrosis was suggested by the darker color and greater hydrophilicity, as blood cells would darken fluid and increase protein concentration. ACL tear samples returned to homeostasis as time from injury increased, while OA sample properties declined with age, highlighting differences between these acute and chronic conditions. A future direction will be quantifying protein concentration using a bicinchoninic acid assay. These insights into the physical and mechanical properties of synovial fluid can guide future research and treatments for joint diseases like OA.