Sports MedicineKneeSurgery 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 minutes | 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 cartilage cells, seeded onto a collagen scaffold, to repair full-thickness knee cartilage defects of 2–10 cm². It produces hyaline-like cartilage rather than the weaker fibrocartilage of microfracture, with 80–90% good-to-excellent outcomes at five years.

Why this approach — at MOS

MACI is what we reach for when a cartilage defect is too large for microfracture to hold up. The tissue microfracture produces is fibrocartilage (type I collagen, mechanically weaker than the cartilage it replaces), and for defects beyond roughly 2 cm² it reliably deteriorates. MACI regenerates hyaline-like tissue from the patient's own chondrocytes, and the randomized evidence shows that biological difference translating into better pain and function scores and fewer failures.

The two-stage structure is a feature, not a cost. The biopsy arthroscopy lets us characterize the lesion definitively before committing to a reconstruction, and the 3–5 week culture window is time we use to plan and stage concurrent work. Malalignment, ligamentous insufficiency, and meniscal deficiency are corrected at the time of implantation. An unstable or malaligned knee will destroy the graft, and we do not implant into one.

We are equally direct about what MACI asks of the patient: six to eight weeks of protected weight-bearing and a 12-to-18-month rehabilitation driven by cartilage biology rather than by how the knee feels. Return-to-sport clearance at MOS is criteria-based, not calendar-based, and rehabilitation runs through our in-house physical therapy team.

The SUMMIT randomized trial

The evidence behind the FDA approval is the SUMMIT trial (Saris et al., *American Journal of Sports Medicine*, 2014). This was a prospective, multicenter, randomized Phase 3 study of 144 patients (mean age 33.8, mean lesion size 4.8 cm²) comparing MACI with microfracture. At two years:

Responder rate: 87.5% with MACI vs. 68.1% with microfracture
Treatment failure: 12.5% vs. 31.9%
Significantly greater improvement in KOOS pain and function, with superiority also seen in activities of daily living, quality of life, and symptoms
The advantage was largest in lesions over 4 cm², in younger patients, and in trochlear lesions

Improvements seen at two years were maintained at the five-year extension.

Long-term durability

A systematic review of MACI outcomes at 10 years or more (Wang et al., AJSM, 2024) (168 patients, 188 defects) found durable improvement at 10 to 17 years, a 9.0% all-cause reoperation rate, and only 7.4% progression to knee replacement. In the longest ACI follow-up available, Peterson et al. (AJSM, 2010) reported that 92% of 224 patients at a mean of 12.8 years were satisfied and would have the procedure again.

A meta-analysis of six randomized trials (Dhillon et al., *Arthroscopy*, 2022) confirmed that third-generation ACI produces lower failure rates and greater improvement in patient-reported outcomes than microfracture.

What that means in practice

Good-to-excellent outcomes in roughly 80–90% of appropriately selected patients at five years
Best suited to defects larger than 2 cm² and to trochlear and patellofemoral lesions, where microfracture has historically underperformed
Conversion to knee replacement at 10 or more years is approximately 7–9%, which is what makes MACI a genuine joint-preserving option in a young, active knee

Who is a candidate?

Indications

A symptomatic, focal, full-thickness cartilage or osteochondral defect of the knee (ICRS Grade III or IV)
Defect size of roughly 2–10 cm². Smaller lesions are usually better served by microfracture or OATS; very large lesions may need an osteochondral allograft or a staged approach
Skeletally mature adults, typically under 55, for the best biological response
A contained lesion with stable, well-defined borders
Normal or correctable alignment — varus or valgus malalignment must be corrected, either at the same operation or in advance
A stable ligamentous environment. ACL or PCL insufficiency must be reconstructed
Adequate meniscal tissue — a meniscus-deficient knee needs concurrent or staged meniscal allograft transplantation
Failure of, or unsuitability for, marrow-stimulation techniques such as microfracture
Willingness and ability to complete a long, criteria-based rehabilitation

Contraindications

Advanced osteoarthritis (Kellgren-Lawrence Grade 3–4). MACI is not indicated for diffuse arthritis or for bipolar "kissing" lesions on opposing surfaces
Inflammatory arthritis (rheumatoid, psoriatic, gout) — per the FDA label
BMI over 35: the added mechanical load compromises graft maturation
Knee surgery within the past 6 months, excluding the MACI biopsy itself
Known hypersensitivity to gentamicin, aminoglycosides, or products of porcine or bovine origin
Uncorrected coagulation disorders
Inability to comply with the post-operative rehabilitation protocol
A prior failed ACI or MACI at the same site: a relative contraindication, considered case by case

How We Assess Candidacy

Evaluation at MOS includes standing long-leg X-rays to assess alignment, weight-bearing knee X-rays, and a dedicated cartilage MRI protocol (3T preferred, cartilage-sensitive sequences). Arthroscopy at the time of the Stage 1 biopsy allows the lesion to be characterized definitively before the reconstruction is planned.

The procedure

What Is MACI?

Articular cartilage has virtually no capacity to repair itself. Full-thickness defects (from acute injury, repetitive loading, or osteochondritis dissecans) cause pain, swelling, and mechanical symptoms, and untreated lesions can progress to early osteoarthritis.

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

How MACI Compares to Other Cartilage Procedures

Microfracture produces fibrocartilage (type I collagen) and suits defects under 2 cm². It is mechanically inferior and deteriorates over time in larger lesions.
OATS / mosaicplasty transplants plugs of the patient's own hyaline cartilage for defects of 1–4 cm², at the cost of a donor site and limited ability to fill irregular defects.
Osteochondral allograft uses donor hyaline cartilage and bone for defects of 2 cm² and up, and depends on graft availability.
MACI produces hyaline-like cartilage (type II collagen) for defects of 2–10 cm². Its costs are two operations and a long rehabilitation.

The defining feature of MACI, and of ACI technology generally, is the hyaline-like repair tissue rich in type II collagen, the collagen of native articular cartilage. Microfracture, by contrast, recruits marrow-derived cells that lay down mostly type I collagen, which is weaker under repeated joint loading. Randomized trials and systematic reviews confirm that this biological difference produces measurably better outcomes, particularly for defects larger than 2 cm².

The Scaffold

The MACI implant is the patient's own chondrocytes seeded evenly onto a porcine type I/III collagen membrane. The three-dimensional scaffold distributes cells uniformly across the defect, can be cut to match an irregular defect shape, and is fixed with fibrin glue. This eliminates the sutures required by earlier periosteal-patch ACI, which carried a real risk of graft hypertrophy and delamination.

The Two-Stage Procedure

MACI is performed in two operations separated by roughly 3–5 weeks of cell culture.

Stage 1 — Cartilage Biopsy (Arthroscopic)

Performed at an ambulatory surgery center under general or spinal anesthesia, taking approximately 30–45 minutes.

A diagnostic arthroscopy is performed first, confirming the diagnosis and characterizing the defect (size, borders, depth, bone involvement) and assessing the menisci, ligaments, and opposing surfaces to plan Stage 2.

200–300 mg of healthy cartilage is then harvested from a low-load region of the knee (typically the margins of the trochlear notch or the intercondylar notch wall), where removing cartilage has minimal functional consequence.

The biopsy is shipped overnight to Vericel's facility, where the chondrocytes are isolated and expanded in culture over roughly 3–5 weeks and seeded onto the collagen membrane. Each patient-specific implant undergoes quality release testing before it is returned to the surgical center.

Most patients have minimal restrictions after Stage 1 and resume normal activity within one to two weeks. That interval is used for pre-operative rehabilitation, education, and planning of any concurrent procedures.

Stage 2 — MACI Implantation

Performed under general or spinal anesthesia, taking roughly one to two hours depending on defect complexity and any concurrent procedures.

Stage 2 requires a mini-open or open arthrotomy rather than arthroscopy. The scaffold has to be handled directly. The approach depends on where the lesion is: a medial or lateral parapatellar mini-arthrotomy for femoral condyle lesions, a medial parapatellar arthrotomy for trochlear and patellofemoral lesions, and a compartmental exposure for the uncommon tibial lesion.

Step 1: Defect preparation. Unstable and calcified cartilage is debrided back to stable, perpendicular borders. Removing the calcified layer is essential for integration, while the subchondral bone plate is preserved. If bone involvement is deeper than roughly 6–8 mm, bone grafting is performed concurrently or in a prior staged procedure.

Step 2: Template and sizing. A foil template is shaped to the exact geometry of the debrided defect and used to cut the scaffold to a precise fit.

Step 3: Scaffold placement. The scaffold is placed cell-side down into the defect and its orientation confirmed.

Step 4: Fibrin glue fixation. The scaffold is secured with fibrin glue around and beneath its perimeter. No sutures are used.

Step 5: Seal check. Saline is injected to confirm the scaffold neither migrates nor lifts under pressure.

Concurrent Procedures

A real strength of MACI is that corrective procedures can be done in the same setting: tibial or femoral osteotomy for malalignment, ACL reconstruction, meniscal repair or allograft transplantation. Correcting the biomechanics at the time of implantation reduces the total number of operations and gives the graft the mechanical environment it needs to mature.

Recovery timeline

Weeks 1–6 (Protection)

Continuous passive motion, non-weight-bearing or toe-touch weight-bearing on crutches, and range-of-motion exercises. CPM nourishes the graft, which has no blood supply of its own, through diffusion of synovial fluid.

Weeks 6–12 (Early Loading)

Progressive weight-bearing, weaning off crutches, and low-load closed-chain strengthening. Non-weight-bearing runs 6–8 weeks for femoral condyle defects; patellofemoral lesions follow a modified protocol.

Months 3–6 (Active Rehabilitation)

Full weight-bearing walking, cycling, swimming, and progressive neuromuscular training. No impact activity in this phase.

Months 6–12 (Sport-Specific Training)

Low-impact sport-specific drills, with straight-line jogging typically introduced around 4–6 months and progressed as strength and symptoms allow.

Months 12–18 (Return to Sport)

Full return to sport, contingent on criteria-based clearance (including quadriceps symmetry of at least 90%, proprioception, and functional movement quality) rather than on elapsed time.

MACI rehabilitation is driven by biology, not by pain. Chondrocytes on the scaffold follow a predictable sequence: proliferation and matrix synthesis through the first six weeks, remodeling and integration over months 2 to 12, and final maturation of hyaline-like tissue from 12 to 24 months. Loading the graft before it is ready disrupts that sequence, which is why the protected phase is not negotiable.

Physical therapy is central to the outcome. MOS provides in-house physical therapy with therapists experienced in cartilage restoration protocols, and clearance to advance is criteria-based. Patients progress when the knee meets the milestone, regardless of the date on the calendar.

The timeline is longer than most orthopedic procedures because newly implanted cells need 12 to 18 months or more to mature into mechanically competent tissue. Patients who commit to the rehabilitation consistently do better in the long run.

Frequently Asked Questions

How do I know if I'm a candidate for MACI?

MACI suits active adults (typically under 55) with a confirmed full-thickness knee cartilage defect of 2–10 cm², intact or repairable menisci, correct or correctable alignment, and stable ligaments. The 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. Many patients come to us having already tried physical therapy or a prior microfracture; MACI is often the biological option when those have not held up.

How is MACI different from microfracture?

Microfracture makes small holes in the bone to release marrow cells, which form a fibrocartilage patch over the defect. It works well for small lesions, but fibrocartilage is mechanically weaker than native cartilage and tends to break down over time, particularly in defects larger than 2 cm² and in higher-demand patients. MACI uses your own cartilage cells, cultured and implanted on a collagen scaffold, to regenerate hyaline-like cartilage, the tissue that lines healthy joints. Randomized studies show better pain relief, better function, and fewer failures than microfracture for larger defects.

Will I need two surgeries?

Yes, and the two stages are how the technology works. The first is a short outpatient arthroscopy to take a small biopsy of healthy cartilage from a non-load-bearing part of your knee. Those cells are grown on a collagen scaffold at Vericel's laboratory over 3–5 weeks, and the second operation implants them. Between the two you have minimal restrictions. Concurrent procedures such as alignment correction or ligament reconstruction are usually done at Stage 2, which keeps the total number of operations down.

How long until I can return to sport?

Full return to sport is typically 12–18 months, and that timeline reflects the biology of cartilage maturation rather than surgeon preference. Returning early risks the graft. You will be active well before then: most patients walk normally by three months, swim and cycle by four to six months, and begin sport-specific training around nine to twelve months. Our therapists advance you on criteria-based milestones rather than dates, so you progress when the knee is ready.

How long does MACI last? Is this a permanent fix?

The long-term data are encouraging. At ten years or more, MACI patients show durable improvement in pain and function, with roughly 93% still on their own knee rather than a replacement. In the longest ACI follow-up available, 92% of patients said they would have the procedure again. MACI does not remove the underlying susceptibility to cartilage injury, so activity modification, a healthy weight, and completing rehabilitation matter for longevity. For well-selected patients it remains the best available biological restoration of cartilage in a young, active knee.

Meet the surgeons

Christopher S. Raffo, MD

Orthopedic Surgery · Sports Medicine · Knee & Shoulder Arthroscopy · Joint Replacement

Meet Dr. Raffo →

John J. Christoforetti, MD

Orthopedic Surgery · Sports Medicine · Hip, Knee & Shoulder Arthroscopy · Shoulder Replacement

Meet Dr. Christoforetti →

Related conditions

Chondral (Articular Cartilage) Defects Osteochondritis Dissecans (OCD) of the Knee Patellar Instability / Dislocation

Related procedures

CARTIHEAL Agili-C — Aragonite Biphasic Scaffold Microfracture (Cartilage Repair)OATS: Osteochondral Autograft Transfer Osteochondral Allograft Transplantation (OCA)

Medically reviewed by Christopher S. Raffo, MD

Last reviewed July 13, 2026