Journal September 26, 2026 6 min read

What does the research say about synovial fluid?

Synovial fluid is the dialysate of plasma and hyaluronic acid that fills diarthrodial joints. Research on it spans osteoarthritis, septic arthritis, periprosthetic joint infection and rheumatoid arthritis, with most mech

peptides-pro Editorial

Synovial fluid (synovia, joint fluid) is the viscous, non-Newtonian dialysate of plasma that fills the cavities of diarthrodial joints, tendon sheaths and bursae. It is a plasma ultrafiltrate to which hyaluronic acid secreted by synovial fibroblasts is added, giving the fluid its characteristic viscosity and lubricating properties. The evidence block for this page does not supply a sequence, formula or molecular weight for synovial fluid as a whole; those identifiers apply to its individual protein and glycosaminoglycan constituents, not to the fluid itself. The work summarised here centres on the NLRP3–TXNIP–iNOS inflammatory axis and on the cytokine and acute-phase protein networks (TNF-α, IL-1β, alpha-defensin) that can be measured in synovial fluid.

Synovial fluid is not a single analyte but a matrix. Research uses it as a sampling compartment for inflammatory mediators, as a diagnostic specimen for infection, and as a target for injected preparations intended to modify the intra-articular environment. The studies below are grouped by the mechanism, tissue or model each one examines.

Research Studies

REJENERA and the NLRP3–TXNIP–iNOS axis in a rat osteoarthritis model

A comparative study evaluated REJENERA, a multi-component bioflavonoid-based formula containing olive leaf bioflavonoids (ZeyEX, quercetin and luteolin), S-allylcysteine, palmitoylethanolamide, L-proline, hyaluronic acid and boron, against ZeyEX, NPROC (collagen-rich eggshell membrane combined with olive leaf extract) and ibuprofen in a rat model of knee osteoarthritis. Osteoarthritis was established by intra-articular injection of monosodium iodoacetate (MIA; 3 mg) into the right knee joint. The study reported that REJENERA alleviated osteoarthritis and provided chondroprotection by regulating the NLRP3–TXNIP–iNOS axis and inflammation [1]. This is an animal model; the findings have not been established in human joints.

Hyaluronic acid injection and TNF-α and IL-1β in knee osteoarthritis

A registered clinical study is determining the effect of intra-articular hyaluronic acid injection therapy on pre- and post-treatment TNF-alpha and IL-1beta levels and on methylation changes in blood and synovial fluid in patients with knee osteoarthritis, compared with a placebo group. The same study evaluates the effects of HA injection on pain, functional status and quality of life, and relates these to changes in TNF-alpha and IL-1beta [3]. This is a human study; results are not yet reported in the evidence block.

Autologous bone marrow aspirate and combined LAM + LP-PRP in knee osteoarthritis

The ABLE OA trial is a Health Canada authorised phase II/III, multi-centre, prospective, double-blinded, randomised, placebo-controlled adaptive trial evaluating two minimally manipulated autologous cellular preparations: bone marrow aspirate (BMA) injection, and combined lipoaspirate micronized (LAM) plus leukocyte-poor platelet-rich plasma (LP-PRP) injections, for the treatment of knee osteoarthritis. BMA, LAM from lipoaspirate and LP-PRP from whole blood are prepared using the Cervos Marrow Cellution Bone Marrow Aspiration System, Cervos LIPO-PRO Adipose Transfer System and Cervos KEYPRP Platelet Separator System [4]. This is a human trial; the evidence block does not report outcomes.

Collagen peptide supplementation and connective tissue remodelling after total knee arthroplasty

A registered study is examining the effects of collagen peptide supplementation on connective tissue remodelling, functional outcomes and wound healing after total knee arthroplasty (TKA). Recruitment is performed by checking the calendar for scheduled TKA procedures more than 8 weeks ahead in the UAMS orthopedic clinic, with clinic staff screening basic inclusion and exclusion criteria in the electronic medical record and obtaining electronic consent through REDCap [2]. This is a human study; the evidence block does not report results.

Serum and synovial markers in periprosthetic joint infection

Total joint replacement is an increasingly effective orthopaedic procedure, but failure due to aseptic or septic loosening remains an important problem, often driven by patient predisposing factors that determine the need for revision surgery. In light of open questions about the diagnostic accuracy of serum and synovial fluid markers in periprosthetic joint infection (PJI), a diagnostic clinical study on patients hospitalised for revision is evaluating the diagnostic accuracy and cost-effectiveness of combining serum and/or synovial markers in the diagnosis of PJI [6]. This is a human diagnostic study.

Alpha-defensin and synovial proteins in paediatric septic arthritis

Differentiating septic arthritis from other causes of joint inflammation in paediatric patients is challenging and important, because septic arthritis requires surgical debridement as part of treatment. The current gold standard for diagnosis in children is a positive synovial fluid culture, but joint cultures may take several days to return, and surgeons frequently must decide whether to debride a joint before culture results are available. There is no single laboratory test or clinical feature that reliably distinguishes the two, which motivates the evaluation of alpha-defensin and synovial proteins to improve detection [8]. This is a human diagnostic study.

Acthar and synovial inflammation in rheumatoid arthritis

In a study of synovial inflammation in rheumatoid arthritis, patients are assigned to receive either Depo Medrol or Acthar treatment. A synovial biopsy, blood draws, synovial fluid aspiration and physician assessments are performed before and after initiating treatment [7]. This is a human study; the evidence block does not report outcomes.

Borrelia B-cell diagnostics and synovial-adjacent immune sampling

The BRILLIANT study is a single-centre, prospective observational study in children with Lyme disease, assessing the immune response against Borrelia burgdorferi. Its main objectives include development of a Bb-specific ASC ELISpot as a new test method for diagnosis of early Lyme disease, addressing the low sensitivity of current standard two-tier testing in recently infected patients and the false positives that can arise from long-term persistence of antibodies against Bb [5]. This is a human diagnostic study; it is included here because synovial fluid is one compartment in which Borrelia-related joint inflammation is assessed.

Allogenic PRP in the equine LPS model of joint inflammation

A randomised controlled trial investigated the short-term effects of intra-articular allogenic platelet-rich plasma in the lipopolysaccharide (LPS) model of joint inflammation in equine radiocarpal joints. A preliminary study using a 0.0625 ng LPS dose was performed in the middle carpal joint and compared within animal to a saline control to confirm that synovitis was induced [9]. This is an animal model; findings are not directly transferable to human joints.

Smoking history and inflammatory biomarkers in synovial fluid

A study examined the association between smoking history and inflammatory biomarkers in the synovial fluid of patients undergoing arthroscopic knee surgery for meniscal injury. Smoking has been linked to alterations in cellular inflammatory pathways and adverse outcomes in orthopaedic procedures, and it was unclear how patient smoking affects the intra-articular microenvironment in the setting of symptomatic meniscal tears [10]. This is a human observational study.

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