Knee Cartilage
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Cartilage cell therapy explained
Am I a Candidate for ACI? Who ACI Is — and Isn’t — For
Who ACI suits, who it does not, and how single-stage STACi widens candidacy to larger defects and any joint.
Reviewed byProf Paul Lee MBBch, FRCS (Tr & Orth), PhDLast reviewed 1 May 2026Quick answer
ACI suits adults with a clear, focal area of cartilage damage — most often in the knee — surrounded by healthy tissue, with minimal osteoarthritis. It is less suited to widespread wear, very large defects or non-knee joints. STACi widens candidacy, treating larger and more complex defects in any joint, so many people ACI turns away still have options.
Who ACI was designed for
ACI was designed for a fairly specific patient: an active adult with a clear, focal “pothole” of cartilage damage in an otherwise healthy joint. The ideal candidate has a defect with good-quality cartilage around its edges to contain the repair, minimal or no osteoarthritis, a stable joint, and — because ACI grows living cells — enough biological health for those cells to thrive. It was developed and is most often used in the knee, where focal cartilage damage is most common. Age is less a hard cut-off than a guide: younger, biologically healthy joints tend to respond best.
When ACI is not the right fit
- Widespread osteoarthritis. ACI repairs focal damage; it is not a treatment for a joint that is broadly worn.
- Very large or complex defects. Classical ACI was designed for defects up to around 4 cm². Larger or awkwardly shaped areas stretch what it can reliably do.
- Non-knee joints. ACI is overwhelmingly a knee procedure; other joints have historically had fewer options.
- Previous cartilage repair on the same defect. Prior surgery to that area, such as microfracture, can affect suitability — and rules out the NHS route.
- An unstable or poorly aligned joint. Underlying instability or malalignment usually needs addressing first, or alongside, for any cartilage repair to last.
Why candidacy is decided on your imaging, not a checklist
No web page can tell you whether you are a candidate. Suitability for cartilage repair depends on the exact size, depth and location of the damage, the health of the surrounding cartilage, the alignment and stability of the joint, and your own goals and activity level — all of which come from your imaging and history, not a list of yes/no questions. Two people with “a hole in the knee cartilage” can need very different plans. That is what a consultation is for: to look at your scans and match the procedure to your joint.
How STACi widens who can be treated
Many people who are told they are not ideal ACI candidates are not out of options — they have simply outgrown what the older procedure was designed to do. STACi keeps the same principle, growing your own cartilage cells to repair the joint, but delivers them inside a three-dimensional scaffold. That lets it treat the same patients ACI was built for and also those turned away because their defect was too large or too complex, and it is offered for the knee, hip, shoulder, ankle and beyond rather than the knee alone. In most cases it is done in a single operation.
“Not a candidate for ACI” often means your defect is larger, more complex, or in a joint ACI was never designed for. Those are precisely the situations STACi was built to treat — so it is worth being assessed before assuming nothing can be done.
Frequently Asked Questions
Am I a candidate for ACI?
You may be if you have a clear, focal area of cartilage damage — usually in the knee — with healthy cartilage around it and minimal osteoarthritis. Widespread wear, very large defects or non-knee joints make classical ACI less suitable. Only a review of your imaging can confirm candidacy.
What makes someone unsuitable for ACI?
Widespread osteoarthritis, a very large or complex defect, an unstable or poorly aligned joint, or previous cartilage repair on the same area can all make classical ACI unsuitable. Many of these patients are still candidates for STACi, which treats larger and more complex damage.
Is there an age limit for ACI?
There is no single fixed age cut-off. Because ACI grows living cells, biologically healthier joints — often, though not always, in younger patients — tend to respond best. Suitability is judged on the health of the joint rather than age alone.
Can ACI treat joints other than the knee?
ACI was developed for the knee and is used there most. The cell-therapy principle has been applied elsewhere, but options for non-knee joints have historically been limited. At London Cartilage Clinic, STACi is offered for any joint, so non-knee enquiries are typically assessed for STACi.
I was told my defect is too big for ACI — what now?
A defect too large for classical ACI is often a candidate for STACi, which uses a three-dimensional scaffold to treat larger and more complex damage. Book a consultation to have your imaging reviewed and the right procedure matched to your joint.
How do I find out for certain if I’m a candidate?
Have your imaging reviewed. A free fifteen-minute discovery call is a good first step, and a full consultation with Professor Lee assesses your scans and history to confirm which procedure suits your joint.
Still have more specific concerns?
Free Discovery CallWant to know if you’re a candidate?
A free fifteen-minute discovery call is the easiest place to start. To have your imaging reviewed and candidacy confirmed, book a consultation with Professor Lee.
London Cartilage Clinic
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