The short answer most patients need first

If your surgeon has mentioned MACI and you've since come across the older term ACI, the key thing to know is that they are not two competing treatments — MACI is simply the most recent, refined version of ACI. Think of it as the third generation of the same idea, rather than a separate technology altogether.

The core difference comes down to how the cells are delivered. In conventional ACI, the cultured chondrocytes are injected as a liquid under a patch that is sutured over the defect. MACI skips the suturing entirely: the cells are pre-seeded onto a thin collagen membrane in the laboratory, and that ready-made construct is trimmed to fit the defect and fixed in place with fibrin glue. Same biology, different delivery.

In practice, most UK centres have moved to MACI as their standard approach, and conventional ACI is now rarely offered. Patients are therefore unlikely to face a direct choice between the two, but both terms still appear in referral letters, clinic notes, and online research — which is why understanding the relationship matters.

One shared feature worth knowing early: both procedures use the patient's own cartilage cells, so immune rejection is not a concern with either approach.

What both procedures have in common

Both procedures follow an identical two-stage structure, which is worth understanding before the differences become relevant.

Stage 1 begins with a short arthroscopic procedure. The surgeon takes a small biopsy — typically 200–300 mg of cartilage — from a low-load region of the knee where the joint surface is intact. That tissue is sent to a specialist cell-processing laboratory, where the chondrocytes are isolated and cultured. The expansion period runs to roughly 3–5 weeks, long enough to multiply the harvested cells to the therapeutic numbers needed for implantation. Patients should factor this interval into their planning: both procedures require a firm commitment to a two-appointment surgical pathway before any implantation can take place.

Stage 2 is the implantation itself. The prepared defect is cleaned back to stable cartilage margins, and the cultured cells are introduced into that space — the method at this point is where ACI and MACI diverge, as the next section explains.

Both techniques are designed for focal, full-thickness cartilage defects, typically in the range of 2–10 cm², in a knee that is otherwise mechanically stable. Neither is appropriate for diffuse or end-stage joint disease.

Where ACI and MACI part ways: the implant itself

The evolution from first-generation ACI to MACI is a story of successive problem-solving, with each generation designed to fix a specific drawback of the one before.

First-generation ACI used cells in their simplest form: a liquid suspension injected directly into the prepared defect. To stop the cells washing out, surgeons sutured a thin strip of periosteum — tissue harvested from the surface of the tibia — over the top as a physical seal. The approach worked, but the periosteal patch frequently caused overgrowth of scar tissue at its edges, a complication known as periosteal hypertrophy. In a number of patients this required a further procedure to trim away the excess.

Second-generation ACI (sometimes called CACI) addressed that specific problem by swapping the periosteal patch for a biodegradable collagen membrane. Hypertrophy effectively disappeared — but the membrane still had to be sutured in place, which introduced its own difficulties: surgical complexity, the risk of microtrauma to the repair zone, and the possibility of cells leaking through or around the suture line.

MACI — the third generation — removes suturing from the process entirely. Instead of injecting cells as a liquid, the laboratory pre-seeds them uniformly onto a resorbable porcine Type I/III collagen membrane before surgery. The surgeon trims this ready-made scaffold to match the exact shape of the defect and fixes it using fibrin glue. Because no sutures are needed, the fixation technique also makes an arthroscopic implantation variant — MACI Arthro — clinically feasible, an option that sutured ACI could not practically support.

Invasiveness, surgery, and recovery

On surgery day itself, the differences become very practical. Conventional ACI — both the first- and second-generation variants — required an open arthrotomy: a longer incision to access the joint fully, with the greater soft-tissue disruption and longer anaesthetic time that entails. MACI Arthro, where the defect site and technique allow it, uses arthroscopic portals instead, meaning a substantially smaller surgical footprint and, in eligible cases, a day-case rather than inpatient stay.

That reduction in invasiveness has a measurable downstream effect on rehabilitation. Published evidence supports an accelerated postoperative protocol with MACI compared with conventional open ACI, with patients typically returning to weight-bearing and beginning structured physiotherapy earlier following the arthroscopic route. A 2021 review in the orthopaedic surgery literature described a simplified surgical technique and accelerated rehabilitation as two of MACI's principal advantages over its predecessors.

Defect size remains the most significant modifier of individual recovery: a small isolated lesion and a near-10 cm² defect in the same compartment carry very different rehabilitation demands regardless of which implantation method was used. Any concurrent procedure — an alignment-correcting osteotomy to offload the repaired area, for example — will also extend the overall timeline. A pre-operative assessment that accounts for the whole joint picture, not just the cartilage lesion itself, is what produces a realistic and patient-specific recovery plan.

Clinical evidence: what the data show

The SUMMIT trial provides the most cited controlled benchmark. In that study, patients with focal knee cartilage defects of 3 cm² or greater who received MACI achieved superior scores on the KOOS — a validated questionnaire measuring knee pain, stiffness, and physical function — compared with those who underwent microfracture, with the advantage present at both two-year and five-year follow-up. That finding matters because defects in that size range are precisely where microfracture has historically performed least well at longer timepoints.

Beyond that specific trial, MACI is broadly recognised in the orthopaedic literature as the current clinical gold standard for focal, full-thickness cartilage repair in the knee. Numerous long-term outcome studies support its durability, and published comparative data indicate that patients treated with MACI demonstrated greater reductions in pain scores than those treated with earlier-generation ACI.

The honest qualification is that direct, long-term randomised controlled trials comparing ACI head-to-head with MACI are sparse. Much of the case for MACI's superiority over its predecessors rests on generational comparisons and mixed-cohort data rather than single blinded trials with uniform follow-up. Similarly, current evidence does not formally establish MACI's efficacy in joints other than the knee, or in patients over the age of 55. Those are real gaps — but they sit alongside a reasonably robust body of evidence for the setting in which MACI is most commonly used.

Who is a suitable candidate, and what are the limits

Practical candidacy questions tend to cluster around defect size, location, patient age, and timing.

The clinical and regulatory scope for cell-based repair — including MACI's FDA approval — covers full-thickness defects of the femoral condyle and patella, broadly in the 2–10 cm² range. Smaller lesions may suit less resource-intensive approaches; extensive multi-compartment damage typically sits outside the scope of cartilage repair techniques altogether.

Age and joint type carry specific evidence gaps worth raising directly with a specialist. MACI's efficacy has not been formally established in patients over 55 or in joints outside the knee in clinical trials. That is a limitation of current trial populations rather than an automatic contraindication — an experienced surgeon will weigh individual bone quality and activity level — but patients in those groups should ask specifically what evidence applies to their presentation.

One technical consideration that rarely appears in patient-facing discussions is cell density. Standard MACI delivers chondrocytes at a lower density per cm² than first-generation ACI's injected suspension, which can produce softer regenerated tissue. High-Density ACI (HD-ACI), seeding five million chondrocytes per cm², is in development to address this and has shown hyaline-like rather than fibrocartilage formation in experimental models. It is worth mentioning so patients can pose informed questions, not because it represents a routine clinical choice.

For patients where the two-stage timeline is a practical barrier, single-stage alternatives such as the Agili-C scaffold avoid the separate cell-culture period entirely and carry their own emerging evidence profile. Conventional first-generation ACI is no longer the standard offer at most centres; patients assessed today will almost certainly be directed towards MACI.

If a defect is full-thickness, roughly 2–10 cm², and sits in a mechanically sound knee, the published evidence supports MACI well. Outside that profile — older patient, non-knee joint, unusual defect geometry — the key question for a consultant is: what data specifically supports this approach for someone with my age, joint, and defect characteristics?