The shared starting point: why both techniques use two stages

ACI and MACI share the same biological logic: because adult cartilage contains too few chondrocytes to repair a meaningful defect on its own, the surgeon must first harvest a small sample of healthy cells and allow them to multiply in a laboratory before there are enough to fill the damaged area. That cellular arithmetic drives the two-stage structure common to every generation of the technique.

In Stage 1, an arthroscopic biopsy removes a small plug of cartilage from a low-load region of the knee — typically the intercondylar notch or the periphery of the condyle. The harvested chondrocytes are then sent for laboratory expansion over several weeks. Stage 2 is the implantation operation, in which those cultured cells are introduced into the prepared defect and secured in place.

MACI is the third generation of ACI, not a separate procedure. The underlying cell biology — autologous chondrocytes, laboratory-expanded, reimplanted into the patient's own joint — is unchanged. Both techniques are indicated for focal, full-thickness cartilage defects (ICRS grade III–IV), typically in the 2–10 cm² range where smaller, single-stage options such as microfracture or OATS are unlikely to provide durable repair.

What changed between the generations was the delivery method — and that difference, it turned out, mattered considerably.

The technical problems first-generation ACI introduced

The delivery mechanism that first-generation ACI relied on created a series of practical problems that had little to do with the quality of the cells themselves.

Once the expanded chondrocytes were ready, they were returned to the patient as a liquid suspension. The surgeon harvested a strip of periosteum — the fibrous tissue lining the shin bone — sutured it over the cartilage defect to form a sealed pocket, and then injected the cell suspension underneath it. Achieving a watertight seal across an irregular defect surface, under operating conditions, left little margin for error. Cell leakage was a recognised risk, and because a liquid does not distribute itself evenly beneath an uneven surface, uniform coverage across the defect was difficult to guarantee.

The suturing step itself added operative time, introduced trauma to the tissue surrounding the defect, and required a second harvest site at the tibia — creating an additional wound with its own recovery burden.

The most significant complication, however, was graft hypertrophy: an overgrowth of scar-like tissue beneath the periosteal cover that could cause mechanical symptoms and frequently required further surgery to address. This was common enough with the periosteal technique to become a recognised reason for revision.

Second-generation ACI substituted an off-the-shelf synthetic collagen membrane for the periosteal harvest, sparing the tibia donor site and reducing — though not eliminating — hypertrophy. The fundamental limitation remained: surgeons were still injecting a liquid suspension and relying on sutures to contain it.

What MACI's collagen membrane actually changed

Rather than arriving in theatre as a liquid, the chondrocytes in MACI come pre-seeded onto their scaffold. During manufacture, the patient's expanded cells are distributed uniformly across a resorbable porcine collagen membrane — a bilayered matrix of Type I and III collagen, supplied commercially as Chondro-Gide. By the time the membrane reaches the operating table, the cells are already embedded within it, not suspended in fluid waiting to be contained.

Intraoperatively, the surgeon measures the prepared defect using a sterile aluminium template, cuts the membrane to match its exact contour, and presses it into place. Fibrin glue holds it there. No sutures are placed at the defect rim, no periosteal harvest is needed, and the risk of cell leakage during fixation is substantially reduced — because the cells are already fixed within the scaffold before the surgeon touches them.

Each of those changes addresses a specific failure point identified in the earlier generations. Uniform seeding during manufacture replaces the unpredictable distribution of an injected suspension. Fibrin glue fixation replaces the technically demanding, time-consuming suturing step, reducing operative time and the trauma that suture tension imposed on surrounding cartilage and bone. Removing the periosteal patch eliminates the tibial donor-site wound that first-generation ACI required.

The effect on graft hypertrophy is the most clinically measurable improvement: the collagen membrane reduces postoperative tissue overgrowth to approximately 1%, compared with the considerably higher rates that made periosteal-cover hypertrophy a recognised indication for revision surgery.

The two-stage commitment described above remains, as does the need for a structured rehabilitation programme — the scaffold changed what happens in the operating theatre, not the biological timeline of repair.

Which defects MACI is suited to and why sizing matters

Deciding whether MACI is appropriate depends less on the technique itself than on the size, shape, and location of the lesion — and on whether the mechanical environment is suitable for a graft to survive.

The clearest indicator is defect area. MACI is generally indicated for focal, full-thickness chondral defects in the range of 2 cm² to 10 cm². Below roughly 2–4 cm², single-stage procedures such as OATS or, historically, microfracture carry a lower procedural burden; for those smaller lesions, the two-stage commitment MACI requires — with its laboratory expansion period and second operation — is harder to justify on the available evidence. The SUMMIT trial, which demonstrated MACI's superiority over microfracture at two and five years, enrolled patients with defects of at least 3 cm², which is precisely where the case for a cell-based approach becomes strongest.

Shape and containment matter separately from size. Because the collagen membrane is trimmed to a custom template, it can conform to irregularly bordered or uncontained defects that plug-based autograft techniques such as OATS cannot easily address — those require a cylindrical, well-contained recipient site.

Anatomical reach is broad: the medial femoral condyle, lateral femoral condyle, trochlea, and patella are all treatable, and more than one compartment can be addressed in the same implantation procedure.

Alignment is a further consideration that sits alongside defect sizing. Where significant varus or valgus malalignment is placing asymmetric load on the repair site, an osteotomy to correct the mechanical axis may be needed alongside MACI — without that correction, the graft bears the same overload that damaged the original cartilage.

What the clinical evidence shows at 2, 5, and 10 years

The SUMMIT trial's findings remain the clearest head-to-head benchmark: patients with femoral condyle defects of 3 cm² or larger who received MACI returned significantly better KOOS pain and function scores than those treated with microfracture at both two and five years. Those margins, sustained across a five-year window, provide the strongest controlled evidence that MACI produces durable functional gains rather than early-phase improvement that later fades.

Published reviews confirm that both ACI and MACI deliver improved pain and activity levels over preoperative baseline in the short to medium term, extending the evidence base across procedure generations and patient groups.

The most reassuring durability signal comes from a prospective patellofemoral cohort of 82 patients followed for more than ten years. Across both patellar and trochlear graft sites (41 patients each), KOOS, SF-36, and VAS pain scores all remained significantly improved at 2, 5, and beyond 10 years — with no significant difference between the two graft locations. That continuity across a decade suggests the regenerated tissue holds up under real-world loading conditions.

That said, the picture isn't uniformly settled. A 2025 matched-pair analysis (48 patients across three groups) comparing MACI, AMIC, and arthroscopic minced cartilage found no significant differences in KOOS-Pain, KOOS-Symptoms, VAS, or quality-of-life scores at two years; all three approaches showed statistically significant improvement from baseline. At short follow-up, newer and technically simpler alternatives appear to match MACI's results — which is clinically useful context, even if it doesn't alter the indication for larger defects where MACI's longer-term evidence base is strongest.

One gap deserves open acknowledgement: direct head-to-head RCT evidence comparing first-generation ACI with MACI is limited. Most claims about generational improvement rest on observational series and mechanistic reasoning rather than controlled trials, and that distinction matters when weighing the strength of any comparative conclusion.

Where MACI sits now and what the next generation may offer

By 2025, MACI is described in the peer-reviewed literature as the reference standard for larger focal cartilage defects — a standing built on the procedural refinements covered in earlier sections and now supported by outcome data extending beyond a decade. That position is earned rather than simply asserted.

Yet the field continues to move. A fourth-generation, single-stage variant — sometimes termed STACI or next-generation ACI — has begun to attract early research interest, with its proposed advantage being the elimination of the laboratory expansion phase entirely, which would remove the two-stage burden that MACI retains. Evidence is limited at this stage, and STACI remains an area of active development rather than an established clinical pathway.

MACi is therefore best understood as an important intermediate chapter rather than the final word in cell-based cartilage repair. Whether it holds its current position long-term or is eventually succeeded by technically simpler single-stage approaches depends on data — including long-term cost-effectiveness comparisons with newer alternatives — that are still being gathered.

For any individual patient, the practical question is not which technique is most advanced but which is best matched to their defect size, location, mechanical alignment, and prior treatment history. Those factors cannot be resolved by general guidance alone; they require evaluation by a consultant with specific expertise in cartilage preservation and joint restoration.

1. [1] Cartilage Repair with Autologous Chondrocytes (ACI Generations 1-4). (2024). https://doi.org/10.1016/j.csm.2024.08.003 https://doi.org/10.1016/j.csm.2024.08.003
2. [2] Favorable Short-Term Outcomes of MACI for Osteochondral Lesions of the Talus: A Systematic Review. (2025). https://doi.org/10.1016/j.arthro.2025.07.045 https://doi.org/10.1016/j.arthro.2025.07.045
3. [3] Prospective Clinical and Radiological Follow-Up Beyond 10 Years in Patients Undergoing MACI in the Patellofemoral Joint. (2024). https://doi.org/10.1177/2325967124s00196 https://doi.org/10.1177/2325967124s00196
4. [4] Comparison of Three Different Techniques — MACI vs AMIC and Arthroscopic Minced Cartilage: 2-Year Follow-Up. (2025). https://doi.org/10.3390/jcm14072194 https://doi.org/10.3390/jcm14072194