A Study of MACI in Patients Aged 10 to 17 Years With Symptomatic Chondral or Osteochondral Defects of the Knee

Inclusion Criteria: * Age 10 to 17 years at the time of planned randomization * Symptomatic cartilage or osteochondral defects * One or more International Cartilage Repair Society (ICRS) Grade III or IV chondral or unsalvageable osteochondral defects located on the femoral condyles and/or trochlea amenable to treatment with the surgical procedure determined at randomization (MACI or microfracture). 1. Concurrent ICRS Grade I and II defects are acceptable on the tibia if they remain untreated (or are treated with debridement only) at the time of study treatment. 2. Participants with a potentially re-attachable osteochondral fragment, identified by magnetic resonance imaging (MRI) and that has not previously been repaired, may be consented to the study and undergo baseline assessments per protocol and prior to confirmatory arthroscopy (Visit 2). At arthroscopy, an attempt will be made to reattach the osteochondral fragment. If this is successful, the participant will be excluded from the study and deemed an intra-operative screen failure. If the attempted reattachment fails and no other exclusion criteria apply, the participant may continue in the PEAK study: a MACI biopsy will be taken, the participant will be randomized and treated per protocol. * At least 1 defect size ≥1.5 cm2 on the patella, femoral condyles and/or the trochlea; defects include OCD lesions with a bone lesion depth of ≤6 mm and does not require a bone graft. * Salvageable OCD lesions must have failed prior conservative and a reparative treatment, and the participant still experiences pain associated with the osteochondral lesion. Participants with defects as a result of trauma or unknown etiology are eligible regardless of prior treatments. * Stable target knee (i.e., anterior and posterior cruciate ligaments should be free of laxity as well as stable and intact). Ligament repair or reconstruction procedures are allowed prior to screening arthroscopy. * Intact meniscus or partial meniscus (at least 50% of functional meniscus remaining) in the target knee. Meniscal repair or resection might be performed either staged or concurrent with the cartilage repair procedure provided that the surgeon was able to confirm that at least 50% of functional meniscus would remain after the corrective meniscal treatment. * Written informed consent Exclusion Criteria: * Any surgery on the target knee joint within 6 months prior to Screening (not including diagnostic arthroscopy) * Previous investigational drug or device use within 3 months prior to Screening * Avascular necrosis of the target knee (other than potential OCD lesion to be treated) * Participants with uninvolved open growth plates where the surgery necessitates crossing the growth plate, or where the depth of the lesion crosses the growth plate * ICRS Grade III or IV chondral or unsalvageable osteochondral defects located on the tibia or any lesion that is bipolar to the index lesion * Malalignment requiring concurrent osteotomy to correct tibial-femoral or patella-femoral alignment (prior guided growth treatment is not exclusionary) * Symptomatic musculoskeletal conditions in the lower limbs that could impede measurement of efficacy for the target knee joint * Concomitant inflammatory disease or other conditions that affects the joints (eg, rheumatoid arthritis, metabolic bone disease, psoriasis, symptomatic chondrocalcinosis) * Known history of septic arthritis in the index knee joint * Current malignancy or treatment for malignancy within the past 5 years * Known history of hypersensitivity to gentamicin, other aminoglycosides, or products of porcine or bovine origin * Females who are pregnant or lactating * Participants who, in the opinion of the Investigator, have significant medical or psychosocial problems that warrant exclusion. Examples of significant problems included but are not limited to: * Any condition that has potential for negatively impacting intra or postoperative course * Conditions that limit compliance with rehabilitation program * Any condition that has potential for significantly limiting participant's ability to assess postoperative knee function * Any condition, psychiatric or otherwise, that would preclude informed consent/assent, consistent follow-up, or compliance with any aspect of the study * Participant is currently abusing drugs or alcohol or, in the opinion of the Investigator, at high risk for poor compliance