Protective Effect of Normal Synovium on Damaged Cartilage

OBJECTIVES: Traumatic joint injury results in an acute inflammatory response. If uncontrolled, this can progress to chronic inflammation, which can expedite the onset of post-traumatic osteoarthritis (PTOA). Normal synovium plays an important protective role for cartilage, while inflamed synovium can lead to degenerative changes. Previous studies have investigated the cartilage-synovium in limited conditions, but the effect of normal synovium on damaged cartilage remains unknown. The purpose of this study is to investigate the effect of normal synovium on two types of damaged cartilage: IL-1β or impaction-induced. METHODS: Fresh human tali and distal femoral condyles were obtained from 7 human donors (mean age; 39 years-old, 4 male and 3 female) with no history of joint disease. Gross morphology was assessed using Collins grading 0-4 and only normal joints of grade 0-1 were used. Two damage models were introduced: 1) treatment with IL-1β (10ng/ml) at 48 hours before co-culture; or 2) controlled mechanical impaction (600 N within 2 ms). Cartilage explants (8mm) and grossly normal synovium (8 mm) from both joints were harvested and randomized to one of six treatment groups (n = 5 in each group): non-treated cartilage without or with synovium (Group 1 or 2), IL-1β-treated cartilage without or with synovium (Group 3 or 4), impacted cartilage without or with synovium (Group 5 or 6). Samples were collected at 0, 2, and 14 days and assessed for the percentage of live cells and histology with Safranin O staining. Cell survival was measured using calcein AM and ethidium bromide homodimer-1. Image analysis was performed on the superficial and middle/deep zone. Live cells were counted using Image J. Histological grading was based on modified Mankin score. The level of significance for all analysis was set at P < .05. RESULTS: The percentage of live cells in Group 1 was mostly unchanged, 79.9 - 87.9% in the superficial layer and >90% in the middle/deep layer with no significant differences between Group 1 and 2 (Figure 1). All damaged cartilage displayed elevated cell death, especially in the superficial layer at all time points (Group 3; 56.4 ± 20.0%, 44.5 ± 11.5%, and 57.6 ± 16.3%, Group 5; 14.8 ± 5.3, 12.8 ± 13.2%, and 20.1 ± 4.1%, respectively). In the presence of synovium, chondrocyte survival significantly increased at 2 and 14 days (Group 4; 75.3 ± 4.7% and 77.8 ± 7.8%, P < 0.01 and 0.02, Group 6; 31.8 ± 13.7% and 33.4 ± 14.3%, P = 0.03 and 0.04, respectively). By contrast, the percentage of cells in the middle/deep layer remained unchanged for both damaged cartilage with or without synovium. For histological analysis, non-treated cartilage (Group 1 and 2) had a normal structure through 14 days. Mankin score progressively increased in damaged cartilage (Group 3; 1.0 ± 1.41, 2.2 ± 1.8, and 3.25 ± 1.64, Group 5; 3.0 ± 1.2, 3.6 ± 1.1, and 4.6 ± 0.5 at 0, 2, and 14 days). In the presence of synovium, Mankin score decreased at 2 and 14 days (Group 4; 0.8 ± 1.3, and 0.8 ± 0.8, P = 0.087 and < 0.01, Group 6; 1.0 ± 0.0 and 1.6 ± 0.5, P =0.02 and < 0.01, respectively). CONCLUSION: These results demonstrate that healthy synovium is protective of damaged cartilage as evidenced by increased cell viability, especially in the superficial layer. These findings suggested that the inhibition of inflammation by normal synovium may lead to both the recovery of damaged cartilage and prevention of PTOA. In the future, the underlying mechanism responsible for the interaction between the normal synovium and damaged cartilage needs to be addressed.