Who MACI is designed for

For most people researching MACI, the central question is straightforward: is this level of intervention actually warranted for my knee? The short answer depends on three things — lesion depth, lesion size, and life stage.

MACI is indicated for symptomatic, full-thickness cartilage defects of the knee in skeletally mature adults. In grading terms, that means ICRS grade III–IV lesions — damage that reaches all the way through the cartilage layer, sometimes to the underlying bone. Shallower surface-level wear does not typically meet the threshold for this approach.

Defect size matters equally. MACI is best suited to focal lesions in the 2–10 cm² range: large enough that microfracture or an osteochondral autograft (OATS) would struggle to provide reliable, durable cover, yet contained enough that cartilage restoration — rather than partial or total joint replacement — remains a realistic goal.

The biological rationale for intervening is that articular cartilage has very poor intrinsic regenerative capacity. Left untreated, focal full-thickness defects tend to enlarge over time, accelerating joint deterioration and, ultimately, the trajectory towards replacement surgery. MACI is therefore most clinically relevant for active adults — particularly those under 50 — for whom durable symptomatic relief and the preservation of the native joint are the priority.

What the original ACI procedure involved

Classic, first-generation ACI works in two distinct surgical visits. At Stage 1, the surgeon takes an arthroscopic biopsy of healthy cartilage from a low-load-bearing area of the knee — a relatively brief keyhole step — and sends the tissue to a laboratory. Over the following four to six weeks, chondrocytes are isolated and expanded in culture until sufficient cell numbers are ready.

Stage 2 is where the complexity accumulated. The re-implantation required a formal open arthrotomy — a full incision into the joint — rather than a keyhole approach. The surgeon then harvested a periosteal patch from the upper tibia, sutured it meticulously over the prepared defect to create a watertight seal, and injected the cultured cell suspension into the space beneath it. Each of those steps is technically demanding: the periosteal suture line must hold without leaking, the patch must conform precisely to defect geometry, and the cell suspension must distribute evenly in the enclosed space.

This complexity introduced specific failure modes. Periosteal hypertrophy — overgrowth of the patch itself — was reported in roughly 10–25% of classic ACI cases and sometimes required a further surgical débridement to resolve. Variability in suturing technique also meant that reproducibility across different surgeons was a recognised limitation of the first-generation approach.

The collagen membrane: what it replaces and why it matters

The decisive engineering shift in MACI happens not in the operating theatre but in the laboratory, weeks before any surgeon holds a scalpel.

After the Stage 1 biopsy — described in the previous section — the harvested chondrocytes are expanded in culture as in classic ACI. The difference is what comes next. Rather than being prepared as a liquid suspension for injection, the expanded cells are seeded directly onto a porcine type I/III collagen membrane and cultured there until they are integrated into the scaffold. By the time the implant is dispatched to the surgical team, it is a single, solid, cell-bearing construct: the cells are already in place, distributed evenly across the membrane surface in three dimensions.

This seemingly straightforward change eliminates three technically demanding elements that characterised the original procedure. There is no longer any need to harvest a periosteal patch from the upper tibia — the membrane substitutes for it entirely. There is no suturing of a patch across the defect to create a watertight seal. And there is no injection of a cell suspension into the space beneath that patch, with the associated risk of uneven distribution or leakage through an imperfect suture line. The surgeon at Stage 2 receives one engineered implant rather than managing a biological patch and a liquid simultaneously under the constraints of an open wound.

Removing periosteal harvest is particularly meaningful from a safety perspective. Periosteal hypertrophy — the overgrowth of the harvested patch reported in roughly 10–25% of classic ACI cases — was a recognised source of secondary procedures in the first-generation approach. Without a periosteal harvest, that complication pathway is closed.

Autologous sourcing of the chondrocytes is preserved in full: the cells remain the patient's own, so the immune rejection risk that accompanies allograft-based techniques does not arise.

What the implantation visit looks like now

MACI still involves two separate visits to theatre under anaesthesia. The first — an arthroscopic biopsy to harvest the cartilage sample — is followed, typically several weeks later, by the implantation visit once the laboratory has expanded and seeded the cells onto the membrane. When clinicians describe MACI as a 'single-stage membrane implant', they mean that Stage 2 itself has been radically simplified, not that biopsy and implantation happen under one anaesthetic.

At Stage 2, the surgeon trims the pre-prepared membrane to fit the defect precisely, then places and fixes it with fibrin glue. That fixation step replaces the periosteal suturing described in the previous section, removing much of the operative complexity from this visit. Depending on where the defect sits in the knee and how large it is, delivery may be by mini arthrotomy or — in suitable cases — by full arthroscopy, placing MACI considerably closer to a minimally invasive keyhole procedure than the open operation required for first-generation ACI.

Arthroscopic delivery is not universally available: defect location and size determine whether that route is appropriate, and the surgeon's assessment at the planning stage guides that choice.

The simplified implantation does not shorten the biological timeline of the repair itself — cartilage-protective rehabilitation remains a matter of months, consistent with any cell-based restoration procedure.

Clinical evidence: what the SUMMIT trial showed

The pivotal trial underpinning MACI's clinical positioning is the SUMMIT randomised controlled trial (Saris et al., American Journal of Sports Medicine, 2014), which compared MACI against microfracture in adults with symptomatic full-thickness knee cartilage defects. At two-year follow-up, MACI demonstrated statistically greater pain relief and functional improvement than microfracture — and those gains were maintained through five years, providing evidence of durability that the microfracture arm did not match.

That durability distinction carries weight when set alongside what is known about microfracture's longer-term biology. Microfracture stimulates fibrocartilage infill rather than hyaline-like cartilage restoration, and documented concerns about fibrocartilage breakdown and subchondral bone plate changes at two to three years help explain why the functional advantage of a membrane-based cell therapy tends to widen over time. The SUMMIT five-year data were part of the evidence base that supported FDA approval of MACI (BL 125603) for the repair of symptomatic, full-thickness cartilage defects of the knee in adults.

One limitation in the published trial landscape is worth stating plainly: the available randomised data position MACI against microfracture rather than against first-generation periosteal-patch ACI. A granular head-to-head RCT comparing the two ACI generations has not been established in the published literature, so conclusions about relative outcomes between MACI and classic ACI rest on procedural and mechanistic reasoning rather than direct comparative trial evidence. The microfracture comparator is itself a clinically meaningful benchmark — microfracture remains the most widely performed marrow-stimulation procedure — but it means the evidence base does not yet fully resolve every question a patient or clinician might reasonably ask about the generational step from ACI to MACI.

Where MACI sits in the broader cartilage repair pathway

MACI occupies a specific rung on the cartilage repair ladder: the cell-based, two-stage tier for focal full-thickness defects broadly in the 2–10 cm² range, where single-stage options — such as AMIC (matrix-augmented microfracture) or OATS (osteochondral autograft transfer) — are technically unsuitable or the defect exceeds what those approaches can reliably address.

At either end of that range the calculus shifts. For very large or osteoarticular defects, fresh osteochondral allograft (OCA) is generally the more appropriate pathway. For patients not yet at surgical threshold, scaffold-based injection options may form part of the joint-preservation assessment before a cell-based procedure is considered.

The next evolutionary step is already taking shape. Single Treatment ACI (STACI) aims to combine biopsy and implantation under one anaesthetic, removing the multi-week interval between stages. Taylor and Lee outlined the rationale in their 2019 paper 'Single Treatment Autologous Chondrocyte Implantation: The Next Generation of ACI', but STACI remains investigational with a limited evidence base — a direction of travel rather than a currently available standard.

For a patient working through these options, MACI's practical significance is this: it delivers cell-based restoration with five-year durability evidence at a procedural complexity considerably lower than first-generation ACI — a meaningful step forward for the right candidate, at a point in the repair ladder where single-stage techniques are insufficient but large-scale allograft reconstruction is not yet warranted. Defect size, depth, bone involvement, prior procedures, and overall joint condition all bear on whether that description fits a given knee.

At London Cartilage Clinic, Professor Paul Y. F. Lee and the clinical team assess these decisions within a joint-preservation framework; to discuss whether MACI is the appropriate pathway for your knee, a consultation can be arranged via londoncartilage.com.

1. [1] Autologous cultured chondrocytes on porcine collagen membrane (MACI). https://en.wikipedia.org/wiki/Autologous_cultured_chondrocytes_on_porcine_collagen_membrane https://en.wikipedia.org/wiki/Autologous_cultured_chondrocytes_on_porcine_collagen_membrane
2. [2] Knee cartilage replacement therapy. https://en.wikipedia.org/wiki/Knee_cartilage_replacement_therapy https://en.wikipedia.org/wiki/Knee_cartilage_replacement_therapy
3. [3] Autologous chondrocyte implantation. https://en.wikipedia.org/wiki/Autologous_chondrocyte_implantation https://en.wikipedia.org/wiki/Autologous_chondrocyte_implantation