Autologous chondrocyte implantation (ACI) is a surgical procedure involving the injection of a person's own cultured joint cartilage cells back into a prepared defect in the cartilage.
In one operation (eg arthroscopy) a bit of healthy cartilage is removed. The matrix is broken down in the laboratory and the chondrocytes (cartilage cells) are multiplied.
The defect is cleaned up and then the chondrocytes are then injected behind a membrane that is sewn onto the defect
This is known as a two-stage procedure, because the surgeon needs one procedure to harvest some healthy cartilage from a less important area and send the sample to a laboratory for culture (where they are multiplied and make many cells), and then there is a second procedure to implant the expanded cells into the damaged area to grow into healthy cartilage.