ACi is a surgery that uses your own cartilage cells to repair damaged cartilage in a joint. First performed in 1994, it was the very first procedure of its kind, most commonly used in the knee but applicable to any joint where cartilage can be damaged. STACi is the modern version of the same idea: it does everything ACi does and more, with the option of a single operation in many cases, and it is offered for any joint that needs it. STACi is the only such procedure available in the UK and is offered exclusively at London Cartilage Clinic. This page compares the two so you can see why STACi is now the preferred choice.
Reviewed byProf Paul Lee MBBch, FRCS (Tr & Orth), PhDLast reviewed 1 May 2026
ACi treats a damaged area of cartilage in a joint using your own cartilage cells. Cartilage is the smooth, slippery surface that lets a joint glide. Once it is damaged it does not usually heal on its own. The cells that build and maintain it are called chondrocytes (pronounced KON-droh-sites). The procedure was developed for the knee, where cartilage damage is most common, but the same approach can be used in any joint with a focal cartilage defect.
First performed in 1994, ACi was the very first procedure of its kind. A small sample of cells is taken from the affected joint, sent to a specialist laboratory to multiply over four to six weeks, then placed back into the damaged area at a second operation. They are sealed in by a small flap of tissue (called periosteum, the thin protective layer that covers bones), usually harvested from the top of your shin.
The defining features of how ACi delivers cartilage cells, how many operations it needs, and what kind of cartilage damage it was designed for.
A small 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.
Designed for clear holes in the cartilage with healthy tissue around the edges. Most often performed in the knee, applicable across joints.
The cells are sealed in by a small flap of periosteum (a thin protective layer) 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 of refinement since have improved how the cells are delivered: first MACi replaced the periosteal flap with a collagen sheet, and now STACi uses a true three-dimensional scaffold, extending the same principle reliably to any joint.
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, they sit inside a three-dimensional scaffold (a sponge-like structure) that supports cell 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 means STACi can do everything ACi does, plus the things ACi cannot. It is suitable for the same patients ACi was designed for, and also for those who would have been turned away from ACi 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 other joints, not only the knee where ACi has historically been most used.
The same principle, delivered in a way that does more for more patients.
| 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 structure |
| Number of operations | Two operations | One operation in most cases |
| Range of damage it can treat | Best for defects up to around 4 cm² | Suitable for the same defects ACi treats, plus larger and more complex ones |
| Joints it is offered for | Most often used in the knee | Knee, hip, shoulder, ankle and other joints |
| Extra tissue taken from elsewhere | Yes, periosteum harvested from the shin | No extra tissue harvest |
| Wait between operations | 4 to 6 weeks of laboratory cell growth | No wait when performed as a single operation |
| Where it is available in the UK | A small number of specialist centres | London Cartilage Clinic only |
Read the full STACi page for the procedure detail, who it is suitable for, recovery pathway and pricing, or book a consultation to have your imaging reviewed.
You may have more options than you think
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.
Protect what you have. Slow degeneration and manage symptoms.
Fix specific damage. Torn tissue, unstable joints, structural problems.
Rebuild lost tissue. Biological treatments that stimulate new growth.
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
ACi (Autologous Chondrocyte Implantation) is the original cell-based cartilage regeneration procedure, first reported in 1994. Cartilage cells are biopsied arthroscopically from the affected joint, expanded in a specialist laboratory over four to six weeks, and reimplanted into the cartilage defect under a periosteal patch harvested from the upper tibia at a second operation.
ACi was developed for the knee, where focal cartilage damage is most common, and the knee remains its most frequent application. The same cell-therapy principle has been used in other joints including the ankle, hip, shoulder and elbow. At London Cartilage Clinic the modern equivalent (STACi) is offered for any joint where cartilage can be damaged, so patients enquiring about ACi for a non-knee joint are typically assessed for STACi.
STACi delivers the cells within a three-dimensional scaffold rather than under a periosteal patch. The scaffold provides structural support during regeneration, allows treatment of larger and more complex defects, removes the need to harvest periosteum from the tibia, can be performed as a single-stage procedure in selected cases, and is reliably applicable across joints. ACi remains a two-stage procedure with a laboratory wait between operations and is most commonly used in the knee.
A periosteal patch is essentially a biological seal over a cell suspension. It does not provide structural support to the cells through the depth of the defect, can hypertrophy and require further surgery, and adds donor-site morbidity at the harvest area. A 3D scaffold gives the cells a framework that mimics native cartilage architecture and removes the periosteal harvest entirely.
Yes. ACi outcomes deteriorate as defect size increases, particularly above four square centimetres, and the periosteal-patch technique limits how reliably cells can be retained across larger areas. STACi was designed to treat larger and more complex defects, including defects that would otherwise be referred for cartilage replacement (OATS, OCA) or earlier joint replacement.
The early rehabilitation pathway is similar: protected weight-bearing for six to eight weeks, early range of motion under physiotherapy, return to low-impact activity from four to six months and higher-impact sport from nine to twelve months. The main practical difference is the option of a single-stage STACi procedure in selected cases, removing the laboratory wait that ACi always requires.
London Cartilage Clinic has moved to STACi as the standard cell-based cartilage regeneration procedure. Patients enquiring about ACi are assessed for STACi and the wider range of regenerative options as part of the consultation pathway.
Book a consultation. The team will review your imaging and clinical history and advise whether STACi, classical ACi-style cell therapy, cartilage replacement (OATS, OCA), an injectable scaffold such as ChondroFiller, or a combination is the best option for your defect.
STACi is priced from £28,000 at London Cartilage Clinic. The price list page sets out what is included; the final cost is confirmed after assessment.
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Clinical updates, cartilage treatment guidance, and recovery-focused articles from our specialist team.
Most new knee, shoulder and back pain is managed first with structured rehabilitation, not surgery or immediate scanning. Recurrent giving way, marked weakness, or red flags such as unrelenting night pain, fever, weight loss, trauma or neurological symptoms prompt earlier specialist review.
ChondroFiller is a cell-free type I collagen scaffold placed into focal cartilage defects to support repair, and small knee studies report improved pain and function with no major implant-related complications at 12 months. Its main attraction is a lighter surgical footprint than microfracture, OATS or osteochondral allograft, but long-term evidence remains limited.
Worn knee cartilage does not reliably grow back to normal hyaline cartilage, while ACL and meniscus injuries can raise the risk of post-traumatic osteoarthritis. Repeated swelling, giving-way, catching or true locking point to ongoing joint stress and a need for specialist assessment.
