Pathological features of established osteoarthritis with hydrathrosis are similar to rheumatoid arthritis

The prevalence of rheumatoid arthritis (RA) and knee osteoarthritis (OA) is increasing with our aging society. Some reports suggest that OA with effusion synovitis develops into RA and early OA patients with effusion are pathologically similar to those with RA. The purpose of this study was to examine the relationship between histological features of established knee OA with or without effusion and RA.

Seventy-nine patients in which synovial specimens were obtained during total knee arthroplasty were included. Patients were divided into an RA group, OA with effusion (OA+) group, and OA without effusion (OA-) group. The Rooney synovitis score and serum matrix metalloproteinase (MMP)-3 levels were compared among groups. We also examined the correlation between the Rooney synovitis score and its sub-scores with MMP-3 levels.

The total Rooney score was significantly higher in the RA group than in the OA+ and OA- groups (25.4 vs 17.1, p < 0.01; 25.4 vs 13.5, p < 0.001, respectively). This score also was significantly higher in the OA+ group than in the OA- group (p < 0.05). The proliferating blood vessels score, perivascular infiltrates of lymphocytes score, focal aggregates of lymphocytes score, and diffuse infiltrates of lymphocytes score were significantly higher in the RA group than in the OA- group (7.05 vs 3.29, 4.95 vs 3.43, 3.29 vs 1.46, and 2.26 vs 1.18, respectively; p < 0.05), but not compared with the OA+ group. The total Rooney score demonstrated a significantly positive correlation with serum MMP-3 levels in the RA group (r = 0.61; 95% CI: 0.28 to 0.81; p < 0.01) and in the OA+ group (r = 0.57; 95% CI: 0.24 to 0.78; p < 0.01).

Previous reports showed the histological similarity between RA and early OA with effusion. We confirmed this histological similarity, in particular the distribution of lymphocytes, between RA and established OA with effusion. It is possible that cases diagnosed as OA with effusion might progress to overt RA.

• Histological similarity was observed between RA and established OA with effusion.