OBJECTIVE To determine whether differences existed in the viscoelastic properties of synovial fluid samples from the metacarpophalangeal, intercarpal, and distal interphalangeal joints of orthopedically normal athletic horses. ANIMALS 45 warmblood horses and 30 Thoroughbreds (age range, 4 to 16 years). PROCEDURES Synovial fluid samples were aseptically obtained via arthrocentesis from 1 metacarpophalangeal, intercarpal, and distal interphalangeal joint of each horse, and nucleated cell counts were performed. A commercial ELISA was used to measure sample hyaluronic acid concentrations, and full rheological characterization of samples was performed to measure the elastic or storage modulus G' and viscous or loss modulus G“ at 37.5°C (representing the body temperature of horses). Findings were compared among joints and between breed groups by means of ANOVA. RESULTS Significant differences in synovial fluid G' and G“ values were identified between Thoroughbreds and warmblood horses for the metacarpophalangeal joint, between the metacarpophalangeal and intercarpal joints of Thoroughbreds, and between the metacarpophalangeal and distal interphalangeal joints and intercarpal and distal interphalangeal joints of warmblood horses. No significant differences were identified between breed groups or among joints in synovial fluid hyaluronic concentrations or nucleated cell counts. CONCLUSIONS AND CLINICAL RELEVANCE Viscoelastic properties of the forelimb joints of orthopedically normal Thoroughbreds and warmblood horses differed within and between these 2 groups, mainly as a function of the evaluated joint. To the authors' knowledge, this was the first study of its kind, and additional research is warranted to better understand the viscoelastic properties of synovial fluid in horses to optimize their locomotive function.

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Osteoarthritis (OA) is the most common major disabling disease in humans and horses. Hyaluronic acid (HA), naturally abundantly present in synovial fluid (SF), is thought to have crucial impact on the functional rheological and biochemical features of SF in healthy and osteoarthritic joints. Here we present comparative measurements of HA concentration in SF from 35 normal and osteoarthritic equine joints, between two different approaches. On the one hand, an established biochemical HA-specific Enzyme–Linked Immunosorbent Assay (ELISA) assay was employed, which determined that SF in healthy and osteoarthritic equine joints is characterized by HA concentration of ca 0.3–2 mg/mL and 0.1–0.7 mg/mL respectively. On the other hand the same SF samples were also examined with a new exploratory approach of finding out HA concentration, which is based on SF rheology. This was done following “calibration” using appropriate model HA solutions. Comparative analysis of the results obtained by both the biochemical and the rheological approaches, revealed that in most cases the rheological approach greatly overestimates HA concentration in SF, by ca 3 to 8 times and 6 to 11 times, in healthy and diseased SF respectively. Overall these findings support the notion that, contrary to the established view, HA may not be the major contributor of equine SF rheology. This should be taken into account for the development of new more effective preventive strategies, as well as more effective early-stage interventions in osteoarthritis.

The objective of the present study was to investigate the viscoelastic properties in synovial fluid between normal horses and horses with naturally occurring OA and to detect factors affecting synovial fluid viscosity. In total, 105 horses were included in this study. Synovial fluid samples were obtained from 60 mature horses with mild to moderate osteoarthritis in the 2nd interphalangeal, the metacarpophalangeal or the intercarpal joint. Forty-five horses were used as controls. Full rheological sample characterization was performed in order to measure the elastic G’ and viscous G’’ moduli. For determining hyaluronic acid concentrations a commercially available ELISA kit was used. The results of the linear mixed effect (LME) model revealed statistically significant (p < 0.001) effect of HA concentration, on the mean values of logG’ and logG” measurements. The ANOVA findings of the final model revealed statistically significant effect of joint type (p < 0.001) on the mean values of viscoelastic measurements. Interpreting the coefficients of the covariates osteoarthritis (p < 0.001) and age (p = 0.013), a negative correlation was detected on the response logG’ and logG” measurements. Geldings seemed to present lower viscous properties compared to mares. To the authors’ knowledge this is the first multivariate study to quantitatively evaluate the several factors that affect the viscoelastic properties of equine synovial fluid. Horses with osteoarthritis seemed to present lower viscoelastic properties compared to the healthy subjects that are joint type dependant. Finally, considering the multifactorial nature of osteoarthritis, one should expect an emerging need of personalized disease-modifying treatments.