Knee Cartilage
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Cartilage repair options for lasting joint health
A practical overview of cartilage repair, injection therapy, rehabilitation planning, and the clinical evidence that guides joint preservation care.
Cartilage cell therapy compared
ACI vs MACI: How the Two Cell-Based Cartilage Repairs Compare
Both grow your own cartilage cells over two operations. The difference is how the cells are held in place — and where you can have them.
Reviewed byProf Paul Lee MBBch, FRCS (Tr & Orth), PhDLast reviewed 1 May 2026Quick answer
ACI and MACI are both two-stage, cell-based cartilage repairs used mainly in the knee. In ACI the grown cells are sealed under a periosteal flap taken from your shin; in MACI they sit on a flat 2D collagen sheet, removing the shin harvest. MACI is not currently available in the UK. STACi modernises both into a single-stage, any-joint procedure.
ACI and MACI are close cousins — both grow your own cartilage cells over two operations, mainly for the knee. The main difference is how the cells are held in place: ACI uses a flap of tissue from your shin, MACI a flat collagen sheet. MACI is not currently available in the UK. STACi is the modern single-stage evolution of both.
ACI vs MACI, side by side
| What to compare | ACI | MACI | STACi |
|---|---|---|---|
| How the cells are held in place | Sealed under a periosteal flap from the shin | Seeded on a flat 2D collagen sheet | 3D sponge-like scaffold, closer to natural cartilage |
| Number of operations | Two | Two | One in most cases |
| Extra tissue taken from elsewhere | Yes — periosteum from the shin | No shin harvest | No extra harvest |
| Joints it is used for | Mainly the knee | Licensed for the knee only | Knee, hip, shoulder, ankle and beyond |
| Range of damage | Focal defects up to ~4 cm² | Focal knee defects | The same, plus larger and more complex defects |
| Wait between operations | 4–6 weeks of lab cell growth | 4–6 weeks of lab cell growth | None, when single-stage |
| UK availability | A few specialist / tertiary centres | Not currently available in the UK | London Cartilage Clinic only |
ACi — periosteal flap
Cells sealed under a thin flap of shin tissue.
MACi — 2D collagen sheet
Cells seeded across a flat collagen membrane.
STACi — 3D scaffold
Cells grow through the full depth of a sponge-like scaffold.
What ACI and MACI have in common
Start with how similar they are. Both ACI and MACI are autologous chondrocyte implantation — they use your own cartilage cells (chondrocytes, pronounced KON-droh-sites), grown in a specialist laboratory over four to six weeks, to repair a focal area of cartilage damage. Both are two-stage: a keyhole biopsy first, then a second operation to implant the grown cells. Both are used mainly in the knee, for clear focal defects with healthy cartilage around the edges. MACI came later and was, in effect, an engineering upgrade to the same idea.
The real difference: how the cells are held in
The distinction between them is in the delivery. In classical ACI, the cells are placed into the defect and sealed in by a thin flap of periosteum — the protective membrane that covers bone — usually harvested from the top of your shin. That flap can occasionally overgrow and need further treatment, and taking it means a second small surgical site. MACI removed that step: the cells are grown on a flat, two-dimensional collagen membrane, which is then shaped to the defect and secured in place, with no shin harvest. It was a genuine refinement — but both remain two-dimensional in concept, seating cells across a surface rather than through the depth of real cartilage.
Why MACI isn’t currently a UK option
There is a practical catch with MACI in the UK: it is not currently available here as a licensed product. Its European marketing authorisation was suspended in 2014 following the closure of its EU manufacturing site, and while it was later approved by the US FDA in 2016 for knee cartilage repair, that does not make it available on this side of the Atlantic. So for a UK patient weighing ACI against MACI, MACI is often not a live choice at all — which is part of why the conversation has moved on to what is available and current.
When ACI or MACI made sense
For a focal knee defect in an otherwise healthy joint, both ACI and MACI were reasonable, evidence-backed choices in their time — MACI with the advantage of no shin harvest. Where either is genuinely on the table and suits the joint, they remain legitimate cartilage repairs.
When STACi is the better route
STACi is the modern single-stage evolution of both. It seats the cells inside a true three-dimensional scaffold rather than under a flap or on a flat sheet, treats larger and more complex defects, works across any joint rather than the knee alone, needs no shin harvest, and in most cases is done in a single operation — removing the four-to-six-week wait between two surgeries. For a UK patient, it is also actually available. That is why we route both comparisons to STACi.
Frequently Asked Questions
What is the difference between ACI and MACI?
Both grow your own cartilage cells over two operations, mainly for the knee. The difference is delivery: ACI seals the cells under a periosteal flap taken from your shin, while MACI seeds them on a flat 2D collagen sheet, avoiding the shin harvest. Both are two-dimensional in concept.
Is MACI better than ACI?
MACI refined ACI by removing the shin-tissue harvest and using a collagen membrane, which some find more straightforward. Clinically the two are closely comparable. The bigger practical point in the UK is that MACI is not currently available here, so it is often not a live choice.
Is MACI available in the UK?
Not currently as a licensed product. MACI’s European marketing authorisation was suspended in 2014 after its EU manufacturing site closed. It was approved by the US FDA in 2016 for knee cartilage repair, but that does not make it available in the UK. See MACI UK availability.
Are both ACI and MACI two-stage procedures?
Yes. Both involve a first keyhole operation to biopsy cartilage cells, a four-to-six-week wait while the cells are grown in a laboratory, and a second operation to implant them. STACi is single-stage in most cases, removing that wait and the second operation.
What is the modern alternative to ACI and MACI?
STACi — the single-stage, scaffold-based evolution of both. It seats cells in a three-dimensional scaffold rather than under a flap or on a flat sheet, treats larger and more complex defects, works in any joint, and is done in one operation in most cases. It is available in the UK at London Cartilage Clinic.
Which should I choose — ACI, MACI or STACi?
For a UK patient, MACI is generally not available, and both ACI and MACI are two-stage. STACi offers the same cell-based principle in a single operation for any joint. The right choice depends on your imaging — book a consultation to have it reviewed.
Still have more specific concerns?
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