Knee Cartilage
Demo Author One
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 explained
ACi: Autologous Chondrocyte Implantation
ACi — autologous chondrocyte implantation — is a surgery that repairs damaged cartilage using your own cartilage cells. First reported in 1994, it was the very first procedure of its kind, and it proved something important: that a joint's own cells can be grown and used to regrow real cartilage. This page explains, in plain English, what ACi is, who it helps, whether you can get it on the NHS, and how the modern single-stage version — STACi — improves on it. STACi is the procedure London Cartilage Clinic offers, for any joint.
Reviewed byProf Paul Lee MBBch, FRCS (Tr & Orth), PhDLast reviewed 1 May 2026Quick answer
ACi (autologous chondrocyte implantation) repairs cartilage using your own cells. A small sample is taken, grown in a lab for four to six weeks, then implanted into the damaged area at a second operation. It works but needs two surgeries. London Cartilage Clinic offers the modern single-stage evolution — STACi — which does the same job in one operation, for any joint.
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What ACi is
and where it came from
ACi treats a damaged area of cartilage using your own cartilage cells. Cartilage is the smooth, slippery coating that lets a joint glide; once it is damaged it does not usually heal by itself. The cells that build and maintain it are called chondrocytes (pronounced KON-droh-sites).
The procedure was developed in the late 1980s and first reported in a landmark 1994 study — the first time anyone had grown a patient’s own cartilage cells and used them to repair a joint. It was designed for the knee, where cartilage damage is most common, but the same principle has since been applied to other joints. In ACi, a small sample of cells is taken from the affected joint by keyhole surgery, sent to a specialist laboratory to multiply over four to six weeks, then placed back into the damaged area at a second operation. There they are sealed in by a thin flap of tissue called periosteum — the protective layer that covers bone — usually taken from the top of your shin.
Key facts about ACi
Two operations
A keyhole biopsy first to take cartilage cells, then a second operation four to six weeks later to put the grown cells back into the damaged area.
Focal defects up to ~4 cm²
Designed for clear holes in the cartilage with healthy tissue around the edges. Most often performed in the knee.
Sealed under a tissue flap
The cells are held in by a small flap of periosteum taken from the top of the shin — which can occasionally overgrow and need further treatment.
ACi proved cell therapy could regenerate cartilage. The thirty years since have been about delivering those cells better — first MACi replaced the shin flap with a collagen sheet, and now STACi uses a true three-dimensional scaffold, extending the same principle reliably to any joint.
Can you get ACi on the NHS?
Sometimes — but the bar is high. The National Institute for Health and Care Excellence (NICE) recommends ACi on the NHS only when all of the following apply: the cartilage damage is larger than 2 cm²; you have not already had surgery to repair that cartilage; you have minimal osteoarthritis in the joint; and the procedure is carried out at a specialist tertiary referral centre. If any one of those is not met — for example, if you have already had a microfracture, or the joint shows more than minimal wear — NHS-funded ACi is usually not available, and waiting lists at the few centres that offer it can be long.
- Defect larger than 2 cm²
- No previous cartilage repair surgery
- Minimal osteoarthritis
- Treated at a tertiary referral centre
All four met
Eligible for NHS ACi
Any one not met
Consider private STACi
This is a common reason patients look privately. It is also why the modern single-stage STACi matters: it is designed to treat exactly the larger and more complex damage that the older ACi — and its strict NHS criteria — often turn away.
Criteria per NICE Technology Appraisal TA477. Read more on ACi on the NHS.
How STACi improves on ACi
point by point
STACi keeps the principle ACi proved: use your own cartilage cells to regrow damaged cartilage. What it changes is how those cells are delivered. Instead of being sealed under a flap of shin tissue, the cells sit inside a three-dimensional scaffold — a sponge-like structure that supports growth in depth as well as across the surface, much closer to the way natural cartilage is built. There is no need to harvest periosteum from your shin.
That single design change lets STACi do everything ACi does, plus what ACi cannot. It suits the same patients ACi was designed for — and also those who would have been turned away because their defect was too large or too complex. And because the scaffold approach is reliable across joints, STACi is offered for the knee, hip, shoulder, ankle and beyond, not the knee alone. In many cases it can be done in a single operation, removing ACi’s weeks-long wait for lab-grown cells.
ACi vs STACi: side by side
| What to compare | ACi | STACi |
|---|---|---|
| How the cells are held in place | A flap of tissue (periosteum) taken from the shin | 3D sponge-like scaffold, closer to natural cartilage |
| Number of operations | Two operations | One operation in most cases |
| Range of damage it treats | Best for defects up to ~4 cm² | The same, plus larger and more complex defects |
| Joints it is offered for | Most often the knee | Knee, hip, shoulder, ankle and other joints |
| Extra tissue taken from elsewhere | Yes — periosteum from the shin | No extra harvest |
| Wait between operations | 4–6 weeks of lab cell growth | None, when done as a single operation |
| UK availability | A few specialist centres | London Cartilage Clinic only |
The same idea ACi pioneered, delivered in a way that does more for more patients. Read the full STACi page for the procedure detail, suitability, recovery and pricing — or compare MACi and STACi.
You may have more options than you think
Most patients have more treatment options than they have been told
At London Cartilage Clinic we follow a structured clinical framework across four areas of treatment. Before recommending a single procedure, we assess which combination of approaches gives you the best outcome.
Preserve
Protect what you have. Slow degeneration and manage symptoms.
Repair
Fix specific damage. Torn tissue, unstable joints, structural problems.
Regenerate
Rebuild lost tissue. Biological treatments that stimulate new growth.
Replace
When other options are exhausted. Joint replacement as a last resort.
This treatment can be applied across multiple joints. Select yours to see the full range of options we offer, organised by clinical approach.
Explore All Treatment Options
Frequently Asked Questions
What is ACi?
ACi (autologous chondrocyte implantation) is the original cell-based cartilage regeneration procedure, first reported in 1994. Cartilage cells are biopsied by keyhole surgery, grown in a specialist laboratory over four to six weeks, and reimplanted into the cartilage defect under a periosteal patch (a flap taken from the shin) at a second operation.
Is ACi available on the NHS?
Sometimes, but only if strict criteria are met: the defect is larger than 2 cm², you have not had previous cartilage repair surgery, there is minimal osteoarthritis, and it is done at a specialist tertiary referral centre (NICE TA477). Many patients do not meet all four and look at private options such as STACi. See ACi on the NHS.
How much does ACi cost privately in the UK?
London Cartilage Clinic offers STACi — the modern single-stage evolution of ACi — from £28,000, all-inclusive. This covers theatre, the scaffold, biological augmentation, the consultant anaesthetist and twelve months of follow-up. See the STACi cost guide and ACI cost in the UK.
Which joints can ACi be used in?
ACi was developed for the knee and is most often used there. The same cell-therapy principle has been applied to the ankle, hip, shoulder and elbow. At LCC the modern equivalent, STACi, is offered for any joint — so patients enquiring about ACi for a non-knee joint are typically assessed for STACi.
How is STACi different from ACi?
STACi delivers the cells inside a three-dimensional scaffold rather than under a periosteal patch. That supports better-quality regeneration, treats larger and more complex defects, removes the shin-tissue harvest, can be done in a single operation, and works reliably across joints. Read the full STACi page.
What is recovery like after ACi or STACi?
The early pathway is similar: protected weight-bearing with crutches for six to eight weeks, early range-of-motion with physiotherapy, low-impact activity from four to six months and higher-impact sport from nine to twelve months. The main practical difference is that STACi can be single-stage, removing the wait between two operations. See ACI recovery time.
Does London Cartilage Clinic offer classical ACi?
LCC has moved to STACi as its standard cell-based cartilage regeneration procedure. Patients enquiring about ACi are assessed for STACi and the wider range of regenerative options at consultation.
I’ve been recommended ACi — what should I do?
Book a consultation. The team will review your imaging and history and advise whether STACi, cartilage replacement (OATS, OCA), an injectable scaffold such as ChondroFiller, or a combination is best for your defect.
Still have more specific concerns?
Free Discovery CallNot sure if ACi or STACi is right for you?
A free fifteen-minute discovery call is the easiest place to start. If you would rather be assessed, book a consultation and have your imaging reviewed by Professor Lee.
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