Establishment and validation of a novel rat model of early-stage patellofemoral osteoarthritis induced by patellar instability.

OBJECTIVE: This study aimed to establish and validate a rat model of patellofemoral osteoarthritis (PFOA) induced by patellar instability and to characterize its early degenerative features and compartment specificity. METHODS: Twenty male Sprague-Dawley rats were assigned to four groups (n = 5 per group): a control group and PFOA model groups evaluated at 1, 2, and 3 weeks after surgery. PFOA was induced in the left knee by transecting the medial one-third of the patellar ligament and medial patellar retinaculum, after which the patella was gently displaced laterally. With the patella maintained in a mildly lateralized position, the lateral peripatellar soft tissues were sutured at intervals and secured to the lateral joint capsule. Cartilage degeneration was assessed by hematoxylin and eosin staining and Mankin scoring. Cartilage matrix changes were evaluated using Safranin O-Fast Green and toluidine blue staining. Synovial lavage fluid and serum levels of procollagen II C-terminal propeptide (PIICP), matrix metalloproteinase-13 (MMP-13), C-terminal telopeptide of type II collagen (CTX-II), and cartilage oligomeric matrix protein (COMP) were measured by ELISA. RESULTS: At 1 week after surgery, patellofemoral cartilage showed surface irregularity, disrupted zonal architecture, and abnormal chondrocyte arrangement, with progressive worsening over time. In contrast, tibiofemoral changes were milder and appeared later. Safranin O-Fast Green and toluidine blue staining showed earlier and more pronounced loss of proteoglycans and glycosaminoglycans in patellofemoral cartilage than in tibiofemoral cartilage, accompanied by an earlier reduction in positively stained area. Compared with controls, PIICP levels in synovial lavage fluid and serum decreased at all postoperative time points, whereas MMP-13, CTX-II, and COMP levels increased (P < 0.05). These changes were more pronounced at 2 and 3 weeks than at 1 week, with no significant difference between 2 and 3 weeks, suggesting that the model reached a relatively stable early degenerative phase by approximately 2 weeks after surgery. CONCLUSION: Combined partial transection of the patellar ligament and medial patellar retinaculum with lateralized patellar tracking successfully established an early-stage rat PFOA model with preferential patellofemoral involvement. This model rapidly induced cartilage metabolic imbalance and histological degeneration, showing compartment specificity and potential value for studying early PFOA pathogenesis and interventions.