**4.1 Autologous chondrocyte implantation (ACI)**

The technique initially described by Brittberg in 1994 nowadays is the most used for cartilage regeneration [60]. ACI is a two-stage procedure; arthroscopy is initially performed to evaluate the lesion, and three to four CA chondral biopsies are taken from non-weight-bearing surfaces of the joint (intercondylar notch, peripheral edges of the femoral condyles). The sample is then sent to the laboratory, where chondrocytes are isolated with an enzymatic process. The chondrocytes are then cultured for 3–4 weeks until the volume increases by 30 times for implantation (12 million chondrocytes approx.). Usually, after 6 weeks of the initial surgery, the second procedure is performed [59–66].

#### *4.1.1 First generation*

Access with medial or lateral patellar arthrotomy is performed in association with defect debridement. A periosteal flap is removed from the proximal region of the tibia or medial femoral condyle. The flap is then attached to the defect (with its cambium layer facing the bone) on all sides, except at the top. The cultured chondrocytes are then injected under the flap, and, finally, the flap is then attached superiorly as well. The fibrin glue can be used to seal the edges of the flap [60].
