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Orthopedic and Rehabilitation Engineering

Orthopedic and Rehabilitation Engineering - Poster Session D

Poster K8 - Physical and Mechanical Characterization of Knee Synovial Fluid from Patients with Anterior Crucial Ligament Tears and with Osteoarthritis

Friday, October 25, 2024
3:30 PM - 4:30 PM EST
Location: Exhibit Hall E, F & G

Introduction:  Synovial fluid is a viscous, non-Newtonian fluid that resides within the synovial joint cavity and is essential for joint lubrication and homeostasis. 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 often result in acute changes in joint function, with synovial fluid potentially exhibiting altered physical and chemical properties due to inflammation, bleeding, and subsequent healing processes. In contrast, OA, characterized by the gradual breakdown of cartilage, typically involves chronic changes in the composition and functionality of synovial fluid. Prior research has shown that the biology of synovial fluid differs between patients with ACL tears and OA. However, it remains unclear whether the mechanical properties of synovial fluid differ between these two conditions. To address this, we evaluated and compared the color, surface adsorption, viscosity, and coefficient of friction of human synovial fluid samples in patients with ACL tears and OA. Understanding these properties can enhance diagnostic precision, inform therapeutic interventions, and improve patient outcomes. Furthermore, these findings could contribute to the development of targeted treatments aimed at preserving or restoring the mechanical properties of synovial fluid in patients with ACL injuries or OA, ultimately enhancing quality of life for affected individuals.

Materials and

Methods:  Synovial fluid was collected from 121 patients (83 ACL tear, 38 OA) via arthrocentesis using a superolateral suprapatellar approach. Of those samples, 83 were from patients diagnosed with an ACL tear (average age of 28±13.46) and 38 samples were from patients diagnosed with OA (average age of 59±11.00). The samples were centrifuged to remove cells and particulates, and the supernatant was aliquoted and stored at -80°C until testing. The blue color intensity value of each sample was recorded to provide a quantification of the darkness of the samples, as it complements the yellow-red nature of synovial fluid. Hydrophilicity and surface adsorption properties were evaluated by pipetting samples directly onto a titanium (Ti-6Al-4V 9) plate and measuring the contact angle. The viscosity was measured using a Hagen-Poiseuille-type viscometer at a shear rate of 500 s-1. The dynamic coefficient of friction (DCOF) was measured using a custom tribo-rheology setup with a ball-on-three-ball geometry. This setup involved an upper steel ball mounted on a 20 mm cross-hatched parallel plate geometry, which was lowered onto three lower balls arranged in a triangle, forming a pyramid. DCOF measurements were taken at a sliding speed of 10 mm/s and a normal force of 5 N. Statistical differences in these parameters were analyzed based on condition (ACL tear vs. OA), injury type (isolated ACL tear vs. combined ACL and meniscus tear), age, sex, body mass index (BMI), time from injury, and presence or absence of bone bruise, using GraphPad Prism software.

Results, Conclusions, and Discussions: Synovial fluid from ACL tear patients exhibited a significantly darker color compared to that from OA patients, likely due to hemarthrosis, which is commonly associated with ACL tears (Figure 1A). Additionally, synovial fluid from ACL tear patients was found to be significantly more hydrophobic, as those samples exhibited lower contact angles (Figure 1B). The viscosity and DCOF measurements showed a coefficient of variance of less than 10% between individual runs. Synovial fluid from ACL tear patients was significantly less viscous (Figure 1C) but had a higher DCOF (Figure 1D) compared to fluid from OA patients. These findings offer valuable insight into how traumatic injuries like ACL tears can impact the lubrication properties of synovial fluid. As expected, an increase in viscosity and a decrease in DCOF were observed in the ACL tear group over time from injury. These trends may be attributed to the synovial fluid returning to homeostasis following the acute injury. While none of the parameters changed with age in ACL tear samples, DCOF was significantly increased in older patients with OA. Regarding BMI, the ACL tear group exhibited decreased viscosity and increased DCOF with increasing BMI. Similarly, samples from OA patients showed a decrease in viscosity with higher BMI. No significant differences were observed in these parameters based on sex, injury type, or the presence of a bone bruise. Overall, this study reveals notable differences in mechanical properties of synovial fluid between injured and diseased patients and highlights trends associated with various patient factors. These insights into the physical and mechanical properties of synovial fluid can guide future research and treatments for joint diseases like OA.

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