Osteoarthritis (OA) is a degenerative joint disease that affects humans and animals worldwide. Its early diagnosis remains challenging due to subtle clinical signs and late radiographic changes. This study aimed to explore candidate biomarkers associated with spontaneous OA and to investigate their correlation with ultrasonographic scores to support early diagnosis. Clinical, radiographic, and ultrasonographic evaluations were performed on 52 equine metacarpophalangeal joints, with and without OA, allowing joint scoring and classification into osteoarthritis (OAG) and control groups. Synovial fluid samples were analyzed for cartilage degradation (C2C), untargeted ¹H NMR-based metabolomics, and lipid peroxidation (TBARS). Statistical analyses included Student’s t-test, Mann–Whitney U test, univariate and multivariate metabolomic analyses, and Spearman’s correlation (p < 0.05). Ultrasonography revealed higher scores in the synovial fold, membrane, and fluid, indicating synovitis as the predominant finding in the acute phase. C2C and TBARS concentrations were significantly higher in the OAG. Seven metabolites differed between groups, with citrate and TBARS showing the strongest correlations with ultrasonographic scores. These findings suggest increased metabolic activity and lipid peroxidation in early OA and highlight citrate and TBARS as potential auxiliary biomarkers for early diagnosis associated with synovitis.

Synovitis is an inflammatory disorder in horses that contributes to cartilage degradation and osteoarthritis. This study evaluated the effects of intra-articular administration of high-molecular-weight (HMW), linear (non-cross-linked) hyaluronic acid (HA) in an equine model of lipopolysaccharide (LPS)-induced acute synovitis. In this randomized blinded study, acute synovitis was induced in radiocarpal joints of 16 adult horses using LPS (0.25 ng). After 12 h, joints were treated intra-articularly with either 2 mL of phosphate-buffered saline (PBS; control group) or 2 mL of HMW-HA (20 mg/mL; 2280 kDa; treatment group). Horses underwent orthopedic, ultrasonographic, and synovial fluid evaluations. Horses treated with HMW-HA exhibited lower synovial membrane thickening and reduced lameness scores compared with controls. Synovial fluid analysis demonstrated increased concentrations of HMW-HA during the acute inflammatory phase in treated joints. Reduced chondroitin sulfate release was also detected following HMW-HA administration. Lower lipid peroxidation levels (TBARS) were observed in the treated group at 24 h and 14 days. Intra-articular administration of high-molecular-weight linear hyaluronic acid was associated with increased synovial availability of HMW-HA during the acute inflammatory phase, attenuation of pain-related clinical signs and synovial membrane thickening, and modulation of the redox environment. These findings support a modulatory role of linear HMW-HA in the intra-articular environment.