MACI

MACI (matrix-induced autologous chondrocyte implantation) is the third generation of the ACI lineage and the first FDA-approved tissue-engineered cartilage product using a scaffold (Vericel MACI, December 2016). The aim: repair tissue closer to hyaline cartilage than the fibrocartilage marrow stimulation produces.

How it works

Two operations. An arthroscopic biopsy harvests your chondrocytes, which are culture-expanded and seeded onto a porcine collagen I/III membrane. Weeks later, at a second surgery, the cell-loaded membrane is cut to shape and fixed into the prepared defect with fibrin glue.

The randomized evidence

The SUMMIT trial (144 patients, defects ≥3 cm²) is the anchor: MACI was superior to microfracture on KOOS pain and function at 2 years, and the advantage was maintained at 5 years in the 128-patient extension. An independent smaller RCT (60 patients) found the same direction: both arms improved at 24 months, but MACI did significantly better on Lysholm, Tegner, and ICRS scores.

Honest caveats on the anchor: SUMMIT was open-label and sponsor-funded (Genzyme/Vericel), and its only comparator was microfracture — a procedure documented elsewhere to fade after 2–5 years. No RCT compares MACI to OATS or osteochondral allograft. Against membrane ACI (ACI-C), MACI showed no clinical superiority at 1 year; its advantage there is surgical practicality.

Durability

A systematic review of 578 patients at minimum 5 years found sustained improvement with 9.7% failure overall — 12.4% for tibiofemoral defects, 4.7% patellofemoral. The longest view is an uncontrolled prospective cohort of 87 patients (99 grafts) at mean 13.1 years: outcomes still significantly improved, 9.1% graft failure, 88.5% satisfaction, with a nonsignificant decline in MRI tissue infill from year 2.

The sobering counterweight from the wider ACI literature: first-generation ACI proved no better than microfracture at 14–15 years in a randomized trial. MACI's own randomized evidence stops at 5 years; whether it escapes that long-horizon null result is unknown.

Who it suits, and the logistics

SUMMIT enrolled defects of 3 cm² and larger; label and registry use extend from roughly 2 to 10+ cm². Larger and multiple defects are MACI's home turf — small defects have cheaper single-stage options. Two surgeries and a cell-culture stage make this the most logistics- and cost-heavy option among the cartilage repairs.

Rehab is faster than the folklore: a randomized trial of 6- versus 8-week return to full weight bearing found no outcome differences out to 5+ years. Expect protected weight bearing for 6–8 weeks and return to impact sport typically at 9–12+ months.

Questions for your surgeon

• Is my defect in the studied range — SUMMIT itself required 3 cm² or larger?
• How many MACI procedures do you do a year?
• What is your graft failure rate, and what is the revision plan if the graft delaminates?
• For a patellofemoral defect: what is your experience there, since randomized patellofemoral evidence is absent?
• Which rehab timeline do you use — the 6-week accelerated protocol has randomized support?

Safety

Standard surgical risks, twice over. Graft-specific risks: delamination or failure (roughly 8–10% by 5–13 years), graft hypertrophy (much rarer than with the old periosteal ACI), and arthrofibrosis. There is no donor-site morbidity and no allograft disease-transmission risk. Whether higher cell density improves on standard MACI is the question behind the ICC / CEMTRO variant — see that entry.