MACi is a surgery that uses your own cartilage cells to repair damaged cartilage in a joint. It needs two operations several weeks apart, with the cells grown in a specialist lab in between. Most often used in the knee, the same approach can be applied to any joint with a focal cartilage defect. STACi is the modern version of the same idea: it does everything MACi 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
MACi 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). MACi 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.
A small sample of those cells is taken from the affected joint, sent to a specialist laboratory to multiply over four to six weeks, then attached to a thin collagen sheet (a flexible membrane made from natural body protein). At a second operation that sheet is fitted into the damaged area, a bit like patching a worn-out area on a tyre.
The defining features of how MACi delivers cartilage cells, how many operations it needs, and what kind of cartilage damage it was designed for.
A small keyhole biopsy first, then a second operation four to six weeks later to fit the cell-loaded sheet 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 cultured cells are seeded onto a thin collagen sheet, then secured into the defect with a tissue glue (fibrin) or fine sutures.
MACi is a proven technology for the right kind of cartilage damage. Its main limitations are how big a defect it can reliably treat, the fact that it relies on a flat 2D sheet rather than a 3D structure, and the requirement for two operations several weeks apart.
STACi keeps everything that works about MACi: it uses your own cartilage cells, grown and placed back into the damaged area to repair it. What it changes is how those cells are delivered. Instead of being attached to a flat collagen sheet, 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.
That single design change means STACi can do everything MACi does, plus the things MACi cannot. It is suitable for the same patients MACi was designed for, and also for those who would have been turned away from MACi 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 MACi has historically been most used.
The same principle, delivered in a way that does more for more patients.
| What to compare | MACi | STACi |
|---|---|---|
| How the cells are held in place | Flat 2D collagen sheet | 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 MACi 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 |
| 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
MACi (Matrix-Assisted Autologous Chondrocyte Implantation) is a two-stage cartilage regeneration procedure. Cartilage cells are biopsied arthroscopically from the affected joint, expanded in a specialist laboratory over four to six weeks, seeded onto a Type I/III collagen membrane, and implanted into the prepared cartilage defect at a second operation.
MACi was developed and is most often used in the knee, where focal cartilage damage is most common. The same matrix-assisted cell-therapy principle has been applied in other joints including the ankle, hip and shoulder. At London Cartilage Clinic the modern equivalent (STACi) is offered for any joint where cartilage can be damaged, so patients enquiring about MACi for a non-knee joint are typically assessed for STACi.
STACi delivers the cells within a three-dimensional scaffold rather than on a two-dimensional collagen membrane. The 3D scaffold supports cartilage cell growth in depth as well as area, allows treatment of larger and more complex defects, can be performed as a single-stage procedure in selected cases, and is reliably applicable across joints. MACi remains a two-stage procedure with a laboratory wait between operations and is most commonly used in the knee.
Native hyaline cartilage is a three-dimensional tissue with depth and zonal organisation. A 2D membrane carrier delivers cells across the surface of the defect but does not provide structural support through the depth of the regenerating tissue. A 3D scaffold gives the cells a framework that more closely mimics native cartilage architecture, supporting more durable regeneration.
Yes. MACi outcomes deteriorate as defect size increases, particularly above four square centimetres. STACi was designed to treat larger and more complex defects than the membrane technique allows, 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 MACi always requires.
London Cartilage Clinic has moved to STACi as the standard cell-based cartilage regeneration procedure. Patients enquiring about MACi 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 MACi-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.
