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Surgeon assessing a patient for cartilage cell therapy

Cartilage cell therapy explained

Am I a candidate for MACI?

MACI suits focal knee defects up to about 4 cm². See who qualifies, and how STACi widens candidacy.

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Prof Paul LeeReviewed byProf Paul LeeLast reviewed 1 May 2026

Quick answer

MACI is designed for focal knee cartilage defects — clear holes in the cartilage with healthy tissue around the edges — up to about 4 cm², in a joint without significant arthritis. It is licensed for the knee only. STACi widens that: it treats larger and more complex defects, works across any joint, and is available in the UK.

Who MACi is designed for

MACi was developed to treat a specific kind of cartilage damage in a specific joint. The best candidates for it generally have:

  • A focal defect. A clear, contained hole or worn patch in the cartilage, with healthy cartilage around its edges to hold the repair. It is not designed for widespread wear.
  • A defect up to about 4 cm². MACi’s reliability is best for smaller, contained lesions; results tend to decline as the defect gets larger.
  • The knee. MACi is licensed for the knee only.
  • A joint without significant arthritis. Cartilage cell therapy repairs a defect within an otherwise reasonably healthy joint; it is not a treatment for advanced osteoarthritis.
  • Symptoms from the defect. Pain, swelling or catching that matches the damage seen on a scan.

If that describes you, the underlying principle of MACi is a good fit. The question then becomes how best to deliver it — and, in the UK, that is where availability matters, because MACi is not currently available here as a licensed product.

Where MACi reaches its limits

The features that define a good MACi candidate are also its limits. MACi is less suited — or not suited — when:

  • The defect is larger than about 4 cm² or is complex in shape or depth.
  • The damage is in a joint other than the knee — the hip, shoulder, ankle and others fall outside its licence.
  • You need treatment in the UK, where MACi is not currently available as a licensed product.

None of these means autologous cartilage cell therapy is off the table. It means the flat 2D-sheet delivery method MACi uses is the constraint — not the idea behind it.

How STACi widens who can be treated

STACi keeps everything that makes someone a good candidate for cartilage cell therapy — your own cells, regrowing cartilage in a defect within a viable joint — and removes several of MACi’s boundaries:

  • Larger and more complex defects. STACi’s three-dimensional scaffold supports growth in depth as well as area, so it was designed to treat damage beyond MACi’s ~4 cm² comfort zone.
  • Any joint. STACi is offered for the knee, hip, shoulder, ankle and beyond, not the knee alone.
  • Available in the UK. STACi is offered at London Cartilage Clinic, an ICRS Teaching Centre of Excellence, led by Professor Paul Lee.
  • Often a single operation. In most cases the lab step is brought into theatre, so there is no second surgery weeks later.

So a patient who is turned away from MACi for having too large a defect, or damage in a non-knee joint, may well be a strong candidate for STACi. For a fuller comparison of the two cell-therapy families, see ACi vs MACi.

The only way to know for sure

Suitability ultimately comes down to your imaging. The size, depth and location of the damage, the health of the surrounding cartilage and the joint as a whole all matter, and they can only be judged properly from your scans.

  • Free discovery call (15 minutes). A no-cost first conversation about your situation.
  • Consultation (£350). Professor Lee reviews your imaging and history and advises whether STACi, cartilage replacement, an injectable scaffold or a combination is right for you.
Cartilage cells cultured for cell therapy at London Cartilage Clinic
Your own cartilage cells, cultured and prepared — the principle behind MACi and STACi.
consulting-in-office-with-pen

Frequently Asked Questions

Am I a candidate for MACI?

You may be a good candidate if you have a focal knee cartilage defect — a contained hole with healthy cartilage around it — up to about 4 cm², in a joint without significant arthritis, causing symptoms. MACi is licensed for the knee only. Only your imaging can confirm suitability.

What size of defect can MACI treat?

MACi is best suited to focal defects up to about 4 cm². Its reliability tends to decline as the defect gets larger. STACi was designed to treat larger and more complex defects beyond that range.

Can MACI treat joints other than the knee?

MACi is licensed for the knee only. For cartilage damage in the hip, shoulder, ankle or other joints, the modern equivalent offered at London Cartilage Clinic is STACi, which works across any joint.

I have arthritis — can I have cartilage cell therapy?

Cartilage cell therapy repairs a focal defect within an otherwise reasonably healthy joint; it is not a treatment for advanced osteoarthritis. A consultation with imaging is the only reliable way to judge whether a repair is appropriate for you.

I was told my defect is too big for MACI — what now?

A defect larger than about 4 cm² is beyond MACi’s comfort zone, but not necessarily beyond treatment. STACi’s three-dimensional scaffold was designed for larger and more complex defects, so it is worth being assessed for STACi.

How do I find out if I qualify for STACi?

Suitability depends on your imaging — the size, depth and location of the damage and the health of the joint. Start with a free discovery call, then book a consultation for Professor Lee to review your scans and advise.

Still have more specific concerns?

Free Discovery Call

Wondering if you are a candidate?

MACi suits focal knee defects; STACi widens that to larger defects and any joint — and is available in the UK. Start with a free fifteen-minute discovery call, or book a consultation to have your imaging reviewed.

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