Is MACI the Right Cartilage Repair for a Larger Knee Defect?

Overview

MACI is a two-stage cartilage repair that uses a patient's own cartilage cells, grown on a collagen membrane, to resurface a full-thickness knee defect. Dr. Raffo uses it for larger, contained chondral lesions in patients who want a durable, biologic repair. It requires an initial biopsy, several weeks of cell culture, then a second surgery to implant the graft.

Who this is for

MACI is FDA-indicated for "the repair of symptomatic, single or multiple full-thickness cartilage defects of the knee with or without bone involvement in adults" (FDA — MACI product page). The indication is not restricted to one part of the knee — a large US real-world series notes that "MACI has approval for any lesion within the knee joint," which is why patellar and trochlear defects are commonly treated on-label alongside femoral condyle lesions (Milliron/Flanigan, Cartilage 2025). Good candidates typically have a defect at or above roughly 3 cm² — the size threshold enrolled in the pivotal trial — since that is where MACI has demonstrated a clear advantage over microfracture (Saris et al. 2014). MACI is not the right choice when there is substantial bone loss requiring restoration of the osteochondral unit, or in a knee with underlying osteoarthritis, neither of which the MACI label is built to address.

How I approach it

I reach for MACI when a defect is large enough that microfracture is unlikely to hold up, and the bone underneath is fundamentally intact. The pivotal SUMMIT trial showed why this matters: at 2 years, treatment failure occurred in 12.5% of MACI patients versus 31.9% of microfracture patients, a difference that held up statistically (Saris et al. 2014). What I find more convincing than the 2-year data is that the advantage was still present at 5 years — KOOS pain and function scores remained significantly better for MACI (P = .022) (Brittberg et al. 2018).

I'm also candid with patients that MACI is a two-stage commitment. There's a biopsy surgery, a waiting period of roughly 4 to 6 weeks while the cells are cultured onto the membrane, and then the implantation itself. That timeline matters for patients trying to plan around a season or a work schedule, and I walk through it explicitly before we book the first stage.

The operation

The first stage is a brief arthroscopic procedure to harvest a small cartilage biopsy from a non-weight-bearing area of the knee. The biopsy is sent to the manufacturer, where chondrocytes are isolated and cultured onto a porcine collagen membrane over roughly 4 to 6 weeks. The second stage is the implantation itself: the defect is prepared through a small arthrotomy, sized precisely, and the cultured membrane is trimmed to match and secured in place, typically with fibrin glue. If the knee also needs a concomitant procedure — a tibial tubercle osteotomy for patellar maltracking, for example — it is generally performed at the same time as implantation.

Risks and honest tradeoffs

MACI is generally well tolerated, but it is not risk-free and it is not universally successful. In a 10-year Australian cohort of 204 patients, overall graft failure occurred in 10.8%, with 7 patients progressing to total knee replacement (Ebert et al. 2024). Patellofemoral lesions carry a distinct set of tradeoffs: in a 10-15 year patellofemoral series, 35.4% of patients required a concomitant tibial tubercle osteotomy to address maltracking, and 3 of 82 patients eventually progressed to TKA (Ebert et al. 2024, patellofemoral series). A separate U.S. patellofemoral series found survivorship fell to 85.7% by 4 years (Palhares et al. 2025). Return-to-sport rates are real but sobering compared to how MACI is sometimes marketed: in one cohort, only 49.5% of patients returned to their desired sport level, and in another, only 66% were satisfied with their return to sport at 2 years (Weishorn et al. 2024; Ebert et al. 2020).

Recovery and rehabilitation

Recovery from MACI has two distinct phases relative to the two surgeries. After the biopsy, recovery is brief since it is a minor arthroscopic procedure. After implantation, the timeline is longer and protects the developing graft.

• 0–6 weeks post-implantation: Protected, limited weight-bearing with a brace, and range-of-motion exercises are introduced early.
• 6–12 weeks: Progressive weight-bearing and strengthening as the graft matures.
• 3–6 months: Return to low-impact activity and conditioning for most patients.
• 9–13 months: Assessment for return to higher-demand sport; outcomes continue to improve through the first 2 years before typically plateauing, with little further change from 2 to 10 years (Ebert et al. 2024).

1. 0–6 weeks post-implantation

   Protected, limited weight-bearing with a brace, and range-of-motion exercises are introduced early.

2. 6–12 weeks

   Progressive weight-bearing and strengthening as the graft matures.

3. 3–6 months

   Return to low-impact activity and conditioning for most patients.

4. 9–13 months

   Assessment for return to higher-demand sport.

   outcomes continue to improve through the first 2 years before typically plateauing, with little further change from 2 to 10 years (Ebert et al. 2024).

Alternatives I considered

For a similarly sized lesion with meaningful bone involvement, I use osteochondral allograft instead, since it restores the bone and cartilage together as a single unit rather than resurfacing cartilage alone. For a patient with mild-to-moderate arthritis already present — a population MACI's label doesn't address — I evaluate whether Agili-C's FDA indication, which explicitly covers Kellgren-Lawrence grade 0-3, is a better fit.

Biologics used