Abstract: BACKGROUND: Human umbilical cord-derived mesenchymal stem cells have been identified as a potential therapeutic agent for osteoarthritis; however, the precise dose-efficacy relationship remains to be elucidated.
OBJECTIVE: To observe the efficacy of varying doses of human umbilical cord mesenchymal stem cells in treating osteoarthritis in rats, offering insights for optimal dose selection in human umbilical cord-derived mesenchymal stem cells therapy for osteoarthritis. 
METHODS: The osteoarthritis rats induced by monosodium iodoacetate were divided into the following groups: negative control group, hyaluronic acid control group, low-dose group (2.5×105 cells/joint), medium-dose group (8.0×105 cells/joint), and high-dose group (2.5×106 cells/joint). Normal rats without modeling were included as the conditional control group. Each group consisted of 16 rats. In the stem cell group, rats received an intra-articular injection of 25 μL of different doses of human umbilical cord-derived mesenchymal stem cells into the right hind limb knee joint cavity, followed immediately by an injection of 25 μL of 0.2% hyaluronic acid solution. In the hyaluronic acid control group, rats received an intra-articular injection of 25 μL of 0.2% hyaluronic acid solution and 25 μL of saline solution into the right hind limb knee joint cavity. In the condition control group and negative control group, rats received an intra-articular injection of 50 μL of sodium chloride solution into the right hind limb knee joint cavity. At 2, 4, 6, 8, 10, and 12 weeks after administration, assessments were conducted for paw pressure pain threshold, plantar pain threshold, difference in knee pressure pain threshold between left and right limbs, and spontaneous locomotor activity. At 6 and 12 weeks after administration, the right hind limb knee joints were collected for macroscopic examination of articular cartilage, hematoxylin-eosin staining, and safranin O-fast green staining, followed by grading and scoring. 
RESULTS AND CONCLUSION: (1) At all tested time points, the negative control group exhibited lower paw withdrawal thresholds, plantar pain thresholds, and spontaneous locomotor activity ability compared with the conditional control group, while demonstrating higher differences in knee pressure pain thresholds between left and right limbs, macroscopic articular cartilage scores, and tissue damage scores, which confirmed successful establishment of the osteoarthritis rat model. (2) Compared with the negative control group, medium- and high-dose groups showed varying degrees of improvement in all these parameters. These results suggest that a single intra-articular injection of human umbilical cord mesenchymal stem cells ranging from 8.0×105 to 2.5×106 cells significantly increased the pain threshold and enhanced spontaneous locomotor activity in osteoarthritis rats, while also repairing pathological cartilage damage. These results indicate that human umbilical cord mesenchymal stem cells can improve joint function and alleviate cartilage damage in osteoarthritis rats. 

Key words: human umbilical cord mesenchymal stem cells, osteoarthritis, pain, function, cartilage damage, dose, efficacy, rat