What Is Matrix-induced Autologous Chondrocyte Implantation (MACI)?

By Sabrina M. Strickland, MDAbigail Pyne

Matrix-induced autologous chondrocyte implantation or MACI is a procedure in which a patient’s own cells are used to regrow new cartilage for the knee joint. Because of this, it is a two-part procedure.

  • At first, a cartilage biopsy is performed arthroscopically. This allows the surgeon to look inside the knee with a camera, remove a bit of the patient’s cartilage, and then send it to a lab to grow.
  • When the cartilage is ready, the surgeon takes the cartilage implant and cuts it to fit the defect in the patient’s knee perfectly. The cartilage cells then grow and strengthen during recovery.

Who is a candidate for MACI?

  • Patients with symptomatic cartilage lesions in their knee
  • Patients with patellar instability that has caused painful cartilage degeneration in their patellofemoral joint

Symptoms of these conditions include sharp or dull pain in the front of the knee that is amplified by sitting for long periods of time, climbing and descending stairs, or any activities in day to day life or working out that involve a squatting motion. Patellar instability symptoms include the kneecap coming off its track and subluxing or dislocating. At HSS, candidates for MACI will be evaluated by their surgeon using both a physical examination as well as magnetic resonance imaging (MRI) to provide the clearest picture of the cartilage injury present in the knee.

What happens during MACI surgery?

During the first operation, the surgeon views the knee through two small portals (one medial, one lateral). Using a very small camera, the surgeon can look inside the knee to examine the condition of the cartilage. If necessary, the surgeon may debride—remove damaged tissue—from the knee, and then will take a biopsy of the patient’s cartilage. This procedure takes around 30 minutes. The biopsy is then sent to the lab to grow on a collagen matrix for approximately one month. This allows the cartilage to be relatively stable when it is inserted back into the knee during stage two of the procedure.

During the second operation, the surgeon cuts the MACI implant grown in the lab to the exact size of the defect in the patient’s knee. The implant is then glued onto the defect. This procedure takes approximately one hour and involves a 2-inch incision. The second operation is where a combined procedure would take place (such as tibial tubercle osteotomy or MPFL reconstruction) if needed.

What is recovery from MACI surgery like?

Since MACI is a two-step procedure, there will be two recoveries following each of the stages.

  • Stage 1: The patient returns home from the hospital the same day they receive the procedure. The patient can immediately bear weight as tolerated, and most patients are off crutches within a few days. Patients can progress with activity as tolerated. Most patients can return to work within a day or two.
  • Stage 2: The patient returns home from the hospital the same day they receive the procedure. Patients will wake up from the surgery in a straight-locked brace and will be expected to wear the brace for 6 weeks. During those 6 weeks, patients may be allowed to bear weight as tolerated (if they do not have an osteotomy) and unlock the knee brace during activities like sitting. Patients will also be instructed to use a continuous passive motion machine (CPM) to minimize scar tissue, encourage even cartilage growth, and keep the joint moving without actively using the patients’ muscles. Physical therapy starts at 4-6 weeks after the second operation. Additional procedures, such as MPFL reconstruction and tibial tubercle osteotomy, alter the recovery.

What results can I expect from MACI surgery?

Patients who are good candidates for MACI should expect significant improvement in knee pain and function. Goals should be discussed with your surgeon prior to selecting a procedure, but patients are often able to return to sports and activities that their knee pain was preventing them from participating in. As with any surgery, there are risks that should be discussed with your surgeon before undergoing MACI surgery.


Sabrina M. Strickland, MD
Associate Attending Orthopedic Surgeon, Hospital for Special Surgery
Associate Professor of Orthopaedic Surgery, Weill Cornell Medical College

Abigail Pyne
Research Coordinator, Hospital for Special Surgery

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