Relations between synovial fluid and serum concentrations of osteocalcin and other markers of joint tissue turnover in the knee joint compared with peripheral blood.

OBJECTIVE: To determine if osteocalcin (OC) is locally produced in the joint and to study the relation between markers of bone, cartilage, and synovial tissue turnover.
METHODS: The concentrations of OC, keratan sulphate epitope (5D4), and hyaluronate (HA) were measured in paired serum and synovial fluid in 10 healthy volunteers and 15 patients with osteoarthritis (OA) and 16 with rheumatoid arthritis (RA). OC was measured with a commercial immunoradiometric assay and concentrations of 5D4 and HA were measured using enzyme linked immunosorbent inhibition assays.
RESULTS: Synovial fluid OC was found to be significantly lower than serum (p < 0.001) in all patients and controls. Synovial fluid OC concentrations were directly correlated with serum concentrations (r = 0.63, p < 0.001) and with age (r = 0.48, p < 0.01). There were also some relations between OC, HA, and 5D4 in patients with OA and RA. The OC concentrations were directly correlated with HA (r = 0.68, p < 0.01) in OA serum and there was a similar correlation in RA synovial fluid (r = 0.69, p < 0.01). A weak negative correlation was found between OC and 5D4 in OA serum (r = -0.55, p = 0.035) while a weak positive correlation was found in RA serum (r = 0.53, p = 0.034).
CONCLUSIONS: These results show that more OC is present in the circulation than in knee joint fluids suggesting that synovial fluid OC may be derived from the blood.