Autologous chondrocyte transplantation, or ACI ( autologous chondrocyte implantation) technique, which uses autologous chondrocytes expanded and cultured in vitro, arthrotomy 2-stage cell implantation, and periosteal or collagen membrane covering to close the implanted chondrocytes, which produces a mixture of hyaline and fibrocartilage cartilage. It is used to treat traumatic lesions of the weight-bearing joint surface (mainly the femoral condyle). 1.Method: Arthroscopically take the cartilage from the intercondylar fossa of the femur or the proximal medial part of the talus, isolate and culture the chondrocytes, expand them and prepare them for use; treat the cartilage defect (all the unstable degenerated cartilage in the defect area must be removed so that a vertical edge formed by normal cartilage tissue is formed around it; remove the calcified cartilage layer and do not destroy the normal subchondral bone plate), cover the surface with periosteum or collagen membrane, suture, inject chondrocytes after expansion. 2. Indications: FDA approved in 1997 for repairing local traumatic injuries on the weight-bearing surface of the femoral condyle. It is suitable for patients aged 15-55 years with large (2-10cm2) focal, symptomatic cartilage defects, good alignment and stability of the lower extremity, and high functional requirements. (Note: subchondral bone defect should not be deeper than 8mm, otherwise bone grafting of the defect area should be performed first and cartilage grafting should be feasible only after complete healing) 3. Contraindications: Rheumatic or other systemic arthritis, poor knee force lines, extensive osteoarthritic changes and multifocal lesions, etc. In addition, lesions on both sides of the joint surface and cartilage degeneration of more than 3 degrees on the corresponding joint surface are also contraindications to surgery. 4. Post-operative rehabilitation: The post-operative rehabilitation program of ACI is slow and should gradually increase the amount of activity. The basic principles are to maintain mobility, protect the graft, exercise the muscles and gradually increase weight bearing. It is roughly divided into three phases, both the proliferative phase, the transitional phase and the shaping and reconstruction phase. In the first phase (0-6 weeks), 6-8 hours of continuous passive activity per day for 6 weeks, and the patient is on crutches with toe-point light weight bearing; in the second phase (7-12 weeks), there is a gradual transition to full weight bearing and closed-chain strength training is started; in the third phase (after 12 weeks), daily activities are gradually resumed. Avoid strenuous exercise, including running, for 12-18 months after surgery. 5. Efficacy: Basad et al. selected large cartilage defects of 4 cm2 or more as study subjects and compared ACI and microfracture techniques through a randomized controlled clinical trial, which found that the clinical results of ACI were significantly better than those of microfracture. a recently published randomized controlled clinical trial study by Saris et al. found that even for small cartilage defects (mean 2.6 cm2 ), ACI The histological and functional improvement results were significantly better than those of microfractures. 6. Advantages: no size limitation of the damaged area; the repair tissue is a mixture of fibrocartilage and hyaline cartilage (glass-like cartilage); chondrocytes are autologous tissue and there is no immune rejection problem. Disadvantages: limited source of autologous chondrocytes, inconvenient extraction (arthroscopic or surgical extraction is required), limited ability of conventional culture to pass on and proliferate, in vitro culture is prone to dedifferentiation, and its activity decreases significantly with age; two surgeries are required for extraction and transplantation, which is traumatic; rehabilitation is very complicated, and the time required to resume sports is long (12-18 months) and expensive. In addition to the methods listed above, for patients with poor joint force lines, other treatments should be considered after osteotomy and orthopedic correction of inversion deformity; in addition, surgical methods for treating cartilage injuries include tissue engineering cartilage transplantation, joint surface replacement, etc., which will not be discussed here. In conclusion, the choice of surgery should be based on the size of the cartilage defect, and whether 2 cm2, 3 cm2, or 4 cm2 is used as the cut-off point for choosing different surgical methods depends on the subjective decision of the surgeon and the specific situation of the patient. For example, for a male patient of 190 cm in height, a cartilage defect of 3 cm2 represents a small portion of the damage to the weight-bearing part of the condyle, and therefore can be considered a small defect. However, a defect of the same size in a female patient of 160 cm may represent a full range of injury at the condylar weight-bearing site and thus should be counted as a large defect.