How ACI and MACI actually differ

The names suggest two distinct treatments, but ACI (autologous chondrocyte implantation) and MACI (matrix-induced ACI) share the same underlying biology — and the same number of operations. What separates them is a single engineering decision made at the second stage.

Both procedures begin with an arthroscopic biopsy: a small sample of healthy cartilage (around 200–300 mg) is removed from a low-load area of the knee. The living cartilage cells — chondrocytes — are sent to a specialist laboratory, where they are cultured and multiplied over roughly three to six weeks before being reimplanted at a second procedure.

In first-generation ACI, those expanded cells are delivered as a liquid suspension. A surgeon harvests a small patch of periosteum (the fibrous tissue covering the upper tibia), sutures it over the defect to create a sealed pocket, and injects the cell liquid beneath it. This creates a separate wound at the donor site, demands precise suturing, and risks the cells pooling unevenly rather than spreading across the defect.

MACI removes each of those drawbacks. In the laboratory, the cells are seeded directly and uniformly onto a porcine collagen Type I/III membrane — a material known commercially as Chondro-Gide — which is then shaped to the exact dimensions of the defect at surgery and fixed in place with fibrin glue. No periosteal harvest, no sutures, and no liquid pooling. Because fibrin glue requires no sutured seal, the incision can be smaller, and in suitable cases the implantation can be performed entirely arthroscopically — the approach sometimes called MACI Arthro.

Why MACI has largely replaced first-generation ACI

Surgeons have, in practice, already made the choice. First-generation ACI has been largely phased out in favour of MACI, and the clinical data help explain why.

The most direct comparison comes from a 2024 systematic analysis by Manjunath and colleagues (PubMed 38739659), which found that patients treated with MACI achieved greater reductions in pain scores than those who received first-generation ACI. Activity levels tell an equally clear story: ACI patients showed a decline in postoperative activity, whereas MACI patients maintained higher levels — a gap felt most acutely by younger, active patients aiming to return to sport.

Complication and reoperation figures sharpen that picture further. Rates for first-generation ACI sit at approximately 29%, compared with around 10% for MACI. Earlier ACI was also associated with higher rates of graft hypertrophy and scar-tissue formation within the repair site — both contributors to the symptom recurrence that sometimes prompted reoperation. Across the broader ACI/MACI family, Nuffield Health cites an overall success rate of approximately 85%.

The current weight of evidence therefore favours MACI on complication and reoperation rates. That said, first-generation ACI carries a longer published track record — some series extend beyond nine years — while very long-term data for MACI are still accumulating given the relative recency of its widespread adoption. What the mid-term evidence does consistently show is fewer technical complications and better maintenance of function, which is why MACI has become the default form of cell-based cartilage repair for most patients assessed today.

Who is a good candidate for MACI or ACI

Eligibility broadly comes down to three questions: where is the damage, how far has it spread, and is the rest of the knee stable enough to support a repair?

The typical candidate is aged 16–55 with a focal, full-thickness defect larger than 2 cm² — meaning the cartilage loss is confined to one clearly defined zone rather than distributed across the joint surface. That distinction matters clinically: a focal defect on an otherwise healthy knee responds very differently to MACI than the diffuse, low-grade damage associated with osteoarthritis, which is generally outside the scope of cell-based repair. MACI can address defects across several knee locations — the medial and lateral femoral condyle, the patella, and the trochlea — including uncontained defects that earlier techniques struggled to manage.

Published prognostic data (Krishnan et al., 2006) point to three factors associated with better outcomes: younger age, a higher pre-operative function score, and a symptom history of less than two years. None of these is an absolute gate, but they help explain why earlier assessment tends to produce better results.

Certain conditions need to be resolved before or alongside the procedure rather than ruling it out permanently. Mechanical malalignment — such as bow-legged or knock-kneed mechanics that would overload the repair — and ligament instability are typically corrected first. High BMI is associated with lower success rates. For MACI specifically, a known allergy to porcine products or gentamicin is a contraindication, as the collagen membrane and culture medium contain both.

A specialist assessment, including imaging and often an arthroscopic review, is what confirms whether MACI or ACI is the right pathway for a given patient.

What the two-stage process and recovery involve

Once the biopsy is complete and the cells are in culture (covered in the section above), the practical planning begins. The gap between Stage 1 and Stage 2 typically spans three to six weeks — time the laboratory needs to multiply the harvested chondrocytes to sufficient numbers. Patients should factor this waiting period into work, travel, and caring commitments before the biopsy takes place, not after.

The second procedure is meaningfully shorter and less invasive than first-generation ACI's equivalent stage. Because the chondrocytes arrive pre-seeded on the collagen membrane and are fixed with fibrin glue rather than sutured into place, the implantation can often be performed through a smaller incision — and in some cases arthroscopically.

Recovery milestones

Recovery from MACI follows a structured, personalised programme that typically spans nine to twelve months or more, with timelines varying according to defect size, location, and individual patient factors:

• Weeks 1–2: crutches, hinged knee brace, and a continuous passive motion (CPM) machine to encourage early joint movement without loading the repair.
• Weeks 8–12: gradual progression to full weight-bearing without crutches.
• Months 3–6: supervised strength rehabilitation — squats, lunges, and resistance work.
• Months 6–9: jogging and treadmill running, subject to clinical review.
• Months 9–12+: return to sport, typically guided by functional testing rather than calendar dates alone.

Physiotherapy throughout this period is a clinical requirement, not optional aftercare. Evidence on MACI rehabilitation emphasises that the quality and consistency of the rehab programme directly influences how well the repair matures — the surgical procedure creates the conditions for recovery, but the patient's engagement with physiotherapy determines the outcome.

When ACI and MACI are not the right fit

Defects smaller than 2 cm² generally do not warrant a two-stage cell-based approach. Techniques such as OATS (osteochondral autograft transfer) or AMIC (autologous matrix-induced chondrogenesis) offer single-stage alternatives for these smaller lesions — either transplanting an osteochondral plug from the patient's own knee or augmenting microfracture with a collagen scaffold to support healing.

Microfracture alone has historically been the first-line choice for small defects, but its use is declining. Evidence indicates it produces mechanically inferior fibrocartilage rather than true hyaline cartilage; this tissue can break down within two to three years and may damage the subchondral bone plate in ways that limit the effectiveness of any subsequent repair procedure.

At the other end of the scale, larger or structurally complex defects — particularly those involving bone loss as well as cartilage — may call for STACi (a three-dimensional scaffold-based next-generation ACI) or fresh osteochondral allograft (OCA). STACi applies the same autologous cell principle across a wider range of defect sizes and joint types, though its evidence base remains limited relative to MACI.

Diffuse osteoarthritis across the joint surface sits outside the scope of cartilage repair altogether; in those cases the clinical pathway moves toward joint-preservation or replacement options.

Significant mechanical malalignment also needs to be addressed before any cartilage repair is likely to hold. Where load is chronically misdirected — varus deformity loading the medial compartment, for example — a corrective osteotomy such as HTO (high tibial osteotomy) or DFO (distal femoral osteotomy) may be planned alongside or ahead of the cartilage procedure.

Finding a knee cartilage repair specialist

Surgeons who perform MACI and ACI typically work within dedicated cartilage-restoration services and carry a higher caseload in these procedures than a general knee surgeon. When booking an initial assessment, it is reasonable to ask how many MACI or ACI procedures the surgeon performs annually, and whether pre-operative MRI and a biomechanical assessment — evaluating alignment and ligament stability — form part of their standard work-up.

Search MSK lists knee cartilage repair specialists across the UK; filter by region and specialty to identify a surgeon offering these procedures near you.

A specialist assessment will confirm whether MACI, an alternative technique such as OATS or STACi, or a different pathway entirely is most appropriate — based on the defect's size and location, the condition of the surrounding joint, and the mechanical factors that need to be in place for any repair to hold.