The Centers for Advanced Orthopaedics
Maryland Orthopedic Specialists
A Division of The Centers for Advanced Orthopaedics
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Surgery Center

MACI — Matrix-Induced Autologous Chondrocyte Implantation

Fellowship-trained cartilage restoration surgeons Christopher Raffo, MD and John Christoforetti, MD perform MACI for patients with symptomatic full-thickness cartilage defects who are candidates for biological joint preservation.

Duration: Stage 1: 30–45 min | Stage 2: 60–120 minutesAnesthesia: General or spinal
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What is maci — matrix-induced autologous chondrocyte implantation?

MACI (Matrix-Induced Autologous Chondrocyte Implantation) is a two-stage FDA-approved procedure that uses a patient's own harvested and cultured chondrocytes seeded onto a collagen scaffold to repair full-thickness knee cartilage defects of 2–10 cm². It produces hyaline-like cartilage superior to the fibrocartilage from microfracture, with 80–90% good-to-excellent outcomes at 5 years.

The procedure

What Is MACI?

Articular cartilage has virtually no capacity to self-repair. When full-thickness defects develop — whether from acute injury, repetitive stress, or osteochondritis dissecans — the resulting pain, swelling, and mechanical symptoms can be disabling, and untreated lesions may progress to early-onset osteoarthritis.

MACI (autologous cultured chondrocytes on porcine collagen membrane) is a third-generation autologous chondrocyte implantation (ACI) technique. It was approved by the U.S. Food and Drug Administration on December 13, 2016, manufactured by Vericel Corporation, and remains the only FDA-approved cellularized scaffold product for this indication in the United States.

How MACI differs from other cartilage procedures:

  • Microfracture — Fibrocartilage (type I collagen) — < 2 cm² — Mechanically inferior; results deteriorate over time in larger lesions
  • OATS / Mosaicplasty — Hyaline cartilage (plugs) — 1–4 cm² — Donor-site morbidity; limited fill geometry for irregular defects
  • Osteochondral Allograft — Hyaline cartilage (donor) — 2–10+ cm² — Allograft availability; potential immune considerations
  • MACI — Hyaline-like cartilage (type II collagen) — 2–10 cm² — Requires two-stage surgery; prolonged rehabilitation

The critical distinction of MACI — and of ACI technology generally — is the production of hyaline-like repair tissue rich in type II collagen, the hallmark of native articular cartilage. Microfracture, by contrast, stimulates marrow-derived mesenchymal cells that predominantly form type I collagen (fibrocartilage), which is biomechanically weaker and less durable under repeated joint loading. Multiple randomized controlled trials and systematic reviews confirm that this biological difference translates into measurably superior patient outcomes, particularly for defects larger than 2 cm².

The scaffold: The MACI implant consists of the patient's own chondrocytes seeded homogeneously onto a porcine-derived type I/III collagen membrane. This three-dimensional scaffold enables uniform cell distribution across the defect, conformable cutting to irregular defect shapes, and fixation with fibrin glue — eliminating the suturing required in earlier periosteal-patch ACI generations. Vericel is the sole licensed manufacturer and operates the U.S. cell-processing facility.

Frequently Asked Questions

1. How do I know if I'm a candidate for MACI?
MACI is best suited for active adults — typically under 55 — with a confirmed full-thickness cartilage defect of the knee (2–10 cm²), intact or repairable menisci, correct or correctable joint alignment, and stable ligaments. The most important first step is a dedicated cartilage MRI and an evaluation with one of our fellowship-trained surgeons, who will review your imaging, activity level, and prior treatment history. Many patients who come to us have already tried physical therapy or a prior procedure such as microfracture, and MACI offers a meaningful biological solution when those have not provided lasting relief.
2. How is MACI different from microfracture?
Microfracture works by creating small holes in the subchondral bone to release marrow stem cells, which then form a fibrocartilage "scar" patch over the defect. While microfracture is a good option for small defects, the fibrocartilage it produces is mechanically weaker than native cartilage and tends to deteriorate over time — particularly in defects larger than 2 cm² and in higher-demand patients. MACI uses your own cartilage cells (chondrocytes), cultured and placed back into the defect on a collagen scaffold, to regenerate hyaline-like cartilage — the same tissue type that lines healthy joints. Multiple randomized studies confirm that MACI produces superior pain relief, functional improvement, and lower failure rates compared to microfracture for larger defects.
3. Will I need two surgeries?
Yes — MACI is intentionally a two-stage procedure, and this is not a drawback but a feature of how the technology works. The first surgery is a short outpatient arthroscopy to harvest a small biopsy of healthy cartilage cells from a non-load-bearing area of your knee. Those cells are shipped to Vericel's laboratory, where they are grown on a collagen scaffold over 3–5 weeks. The second surgery — the actual implantation — then takes place. Between the two procedures, most patients have minimal activity restrictions and can prepare for the rehabilitation ahead. Many patients also have concurrent procedures performed at Stage 2, such as alignment correction or ligament reconstruction, reducing the overall number of separate surgeries needed.
4. How long until I can return to sport?
Full return to sport typically occurs at 12–18 months — and this timeline is non-negotiable because it reflects the biology of cartilage regeneration, not surgeon preference. The implanted cells need time to mature, integrate with surrounding cartilage, and develop the mechanical properties of native tissue. Returning too early risks graft failure. That said, you will be progressively active well before 12 months: most patients are walking normally by 3 months, swimming and cycling by 4–6 months, and beginning sport-specific training by 9–12 months. Our physical therapists use criterion-based milestones — not calendar dates — to guide advancement, meaning you progress when your knee is ready.
5. How long does MACI last — is this a permanent fix?
The evidence is encouraging. At 10-year follow-up, MACI patients demonstrate significant and durable improvements in pain and function, with approximately 93% maintaining their native knee without conversion to total knee replacement. At the longest follow-up studies available (10–20 years for first-generation ACI), 92% of patients reported they would undergo the procedure again. MACI does not "cure" the underlying biological susceptibility to cartilage damage, so protecting the repair with appropriate activity modification, maintaining a healthy weight, and completing rehabilitation fully are important for maximizing longevity. However, for appropriately selected patients, MACI offers the best available long-term biological restoration of cartilage function in a young, active knee.