Autologous chondrocyte implantation (ACI) is a medical procedure used to repair damaged cartilage in joints, particularly the knee. It involves harvesting healthy cartilage cells, called chondrocytes, from the patient's own joint. These cells are then cultured in a laboratory to increase their number before being implanted back into the damaged area. During the procedure, the surgeon creates a small incision to access the joint and prepares the damaged cartilage site. The cultured chondrocytes are then injected into this area, where they can grow and regenerate new cartilage, helping to restore joint function and reduce pain.