Joint immobilization reduces synovial fluid hyaluronan concentration and is accompanied by changes in the synovial intimal cell populations

Objectives. Synovial fluid (SF) of normal joints contains high hyaluronan ( HA) concentrations. However, the mechanism by which these are controlled an d how they are influenced by articulation and loading are not established. In this study, we have examined whether immobilization influences SF HA con centration and whether this is associated with alterations in the synovial lining. Methods. Hock joints of five adult sheep were immobilized by external fixat ion. Twelve weeks later, SF and synovium samples were collected. The HA con centration in SF was assayed using an ELISA-based method. Non-specific este rase (NSE) and uridine diphosphoglucose dehydrogenase (UDPGD) activities we re assessed in cryostat sections of snap-chilled synovial samples using cyt ochemical techniques, and UDPGD activity per cell was measured in synovial lining cells by scanning and integrating microdensitometry. Results. We found that the SF HA concentration was decreased from 1.65 +/- 0.25 mg/ml in control joints to 0.68 +/- 0.16 mg/ml in immobilized joints. Synovial intimal cell UDPGD activity decreased from 18.0 +/- 2.7 U/cell in control joints to 12.2 +/- 1.5 after immobilization. There was also a decre ase in UDPGD-positive intimal cell numbers. Intimal surfaces in controls co ntained numerous NSE-positive cells, which were rarely observed in intima f rom immobilized joints. Conclusions. These results suggest that immobilization decreases SF HA leve ls and that this is associated with reduced intimal cell UDPGD activity (es sential for HA formation). Immobilization also decreased the prevalence of (NSE-positive) intimal macrophages. These findings suggest that mechanosens itive homeostatic mechanisms exist within the synovial intima.