Three-dimensional MR images of the medial tibial cartilage revealed a progression of cartilage wear along the inner edge of the medial meniscus, accompanied by medial meniscal extrusion. This association highlights the importance of meniscal integrity, as meniscus injuries can lead to extrusion and often necessitate meniscectomy, a procedure that increases the risk of osteoarthritis in middle-aged and older patients. Therefore, developing therapeutic methods to preserve meniscal function and expand repair indications is crucial. Our research, using a microminipig model, demonstrated that transplanting synovial mesenchymal stem cells (MSCs) onto repaired menisci promoted healing. Ten minutes after placement, a portion of the cells had adhered to the injured tissue. Encouraged by these findings, we conducted a clinical study and a physician-sponsored clinical trial to investigate the transplantation of MSCs following meniscal repair surgery in middle-aged and elderly patients. This approach resulted in improved clinical scores. We are currently conducting a multicenter trial of patients with flap tears. We are objectively evaluating the clinical outcomes, as well as the healing status of the meniscus, using a novel arthroscopic scoring system that we have developed for assessing the healing of meniscal flap tears.