Purpose

The synovial membrane lines the inner surface of the joint cavity and has an important role in controlling the synovial microenvironment. Recent studies show that synovitis promotes arthritic disease progression in the knee [1], however little is known on the role of the synovial membrane in aiding or hindering tissue regeneration following cartilage repair therapy. This study aims to evaluate the response of the synovial membrane to bone marrow stimulation (BMS) by microdrilling of a chronic cartilage defect [2].

Methods and Materials

Following ethics-approved protocols, 4x4 mm unilateral or bilateral cartilage defects were created in the femoral trochlea of skeletally mature NZW rabbits (N=9), brought to chronicity over four weeks, and then microdrilled with four 0.9 mm diameter holes at 2 mm depth. Supra-patellar synovial membranes were collected 1 week and 3 weeks after BMS. Starting at 1-week post-BMS, N=6 rabbits were submitted to 3 weekly intra-articular injections of saline or Synvisc®. Synovial membranes were analyzed for lining thickness, blood vessel density by stereology and macrophages infiltration by histomorphometry.

Results

Following a small medial arthrotomy and BMS, the supra-patellar synovial membrane exhibits typical signs of synovitis. The synovial lining was significantly thicker at 1-week and at 3-weeks post-operative compared to the synovium of intact knees (Fig. 1). RAM-11+ macrophage infiltration increased progressively through 3 weeks (Fig. 2A), but recruitment of wound-healing M2 macrophages to the synovial subintima was the highest at 1-week then diminished after 3-weeks of saline or Synvisc® IA-injection (Fig. 2B).

Conclusion

Following BMS, the supra-patellar synovial membrane displayed a strong inflammatory response similar to synovitis and was sustained up to 3-weeks post-operative. This response could be required or detrimental to cartilage repair processes and requires further study.

Acknowledgements: We thank Jun Sun. Funding sources: CIHR, NSERC.

References: [1] Sellam (2010) Nat Rev Rheumatol; [2] Dwivedi (2019) J Tissue Eng Regen Med.