BACKGROUND: Studies have shown that metformin has anti-inflammatory, anti-tumor, anti-aging and vasoprotective effects, and can inhibit the progression of osteoarthritis, but its specific mechanism of action remains unclear. OBJECTIVE: To investigate the mechanism of metformin on cartilage protection in a rat model of osteoarthritis. METHODS: Forty male Sprague-Dawley rats were randomly divided into four groups (n=10 per group): blank, control, sham-operated, and metformin groups. The blank group did not undergo any surgery. In the sham-operated group, the joint cavity was exposed. In the model group and the metformin group, the modified Hulth method was used to establish the osteoarthritis model. At 1 day after modeling, the rats in the metformin group were given 200 mg/kg/d metformin by gavage, and the model, blank, and sham-operated groups were given normal saline by gavage. Administration in each group was given for 4 weeks consecutively. Hematoxylin-eosin staining, toluidine blue staining, and safranin O-fast green staining were used to observe the morphological structure of rat knee joints. Immunohistochemical staining and western blot were used to detect the protein expression of SOX9, type II collagen, a disintegrin and metalloproteinase with thrombospondin motifs 5 (ADAMTS5), Beclin1, P62, phosphatidylinositol 3-kinase (PI3K), p-PI3K, protein kinase B (AKT), p-AKT, mammalian target of rapamycin (mTOR), and p-mTOR in rat cartilage tissue. RESULTS AND CONCLUSION: The results of hematoxylin-eosin, toluidine blue and safranin O-fast green staining showed smooth cartilage surface of the knee joints and normal histomorphology in the blank group and the sham-operated group, while in the model group, there was irregular cartilage surface of the knee joint and cartilage damage, with a decrease in the number of chondrocytes and the content of proteoglycans in the cartilage matrix. In the metformin group, there was a significant improvement in the damage to the structure of the cartilage in the knee joints of the rats, and the cartilage surface tended to be smooth, with an increase in the number of chondrocytes and the content of proteoglycans in the cartilage matrix. Immunohistochemistry staining and western blot results showed that compared with the control and sham-operated groups, the expression of SOX9, type II collagen, and Beclin1 proteins in the cartilage tissue of rats in the model group was significantly decreased (P < 0.05). Conversely, the expression of ADAMTS5, P62, as well as p-PI3K, p-AKT, and p-mTOR proteins was significantly increased (P < 0.05). Furthermore, compared with the model group, the expression of SOX9, type II collagen, and Beclin1 proteins in the cartilage tissue of rats in the metformin group was significantly increased (P < 0.05), while the expression of ADAMTS5, P62, as well as p-PI3K, p-AKT, and p-mTOR proteins was significantly decreased (P < 0.05). To conclude, Metformin can improve the autophagy activity of chondrocytes and reduce the degradation of cartilage matrix in osteoarthritis rats by inhibiting the activation of PI3K/AKT/mTOR signaling pathway, thus exerting a protective effect on articular cartilage. [ABSTRACT FROM AUTHOR]