The collagenase-induced osteoarthritis (CIOA) model: Where mechanical damage meets inflammation.

Objective: To characterize inflammatory and mechanical changes in the collagenase-induced OA (CIOA) model in rats.

Design: Skeletally mature, 6-month-old Wistar rats received unilateral intraarticular injections of saline, 500 U or 1000 U of collagenase on days 0 and 2 of the study. Joint tissues were harvested on either day 4 or 70 to evaluate the acute and long-term changes. Blood biomarkers, gait asymmetry and mechanical hyperalgesia were assessed repeatedly up until day 70.

Results: The intraarticular injection of collagenase triggered an increase in cartilage degeneration and bone resorption over time, particularly for 1000 U. Similarly, mild synovitis was observed on day 70 with an increased number of synovial lining cells, increased fibrosis, and infiltration of peripheral macrophages. Mechanistically, these findings were linked to a dose-related mechanical weakening of the anterior cruciate ligament (ACL), which caused persistent joint destabilization throughout the study. Furthermore, the collagenase injection triggered acute inflammation and swelling of the synovium on day 4 and an acute systemic inflammatory response with increased cytokine and peripheral blood immune cell levels. While mild synovitis persisted until day 70, the systemic inflammatory response returned to control levels after 8 days. Similarly, the observed acute changes in gait and mechanical hyperalgesia also returned to baseline after 21 days.

Conclusion: By evaluating inflammatory and mechanical factors at different doses and timepoints, our characterization enables a more targeted study design and increases the clinical relevance of future studies involving the CIOA model.