#treatment - Knee

Articular cartilage repair & replacement

  • Articular cartilage has no blood supply, and hence it does not repair itself or regrow / regenerate on its own.
  • Unstable areas of articular cartilage damage in a knee can be smoothed off and stabilised relatively easily via keyhole surgery (knee arthroscopy), by arthroscopic radiofrequence collation chondroplasty (with a small ‘welding’ probe).
  • Small focal patches of cartilage loss can be treated by microfracture / nano-drilling.
  • Larger focal patches of articular cartilage loss can be treated by articular cartilage grafting.
  • Autologous Chondrocyte Implantation (ACI) is good surgical technique for articular cartilage grafting, which involves taking cartilage cells from a knee, growing them in a lab to multiply them, and then implanting the cultured cells back into the knee. However, this required 2 x separate operations, and the cell culture is very expensive.
  • Autologous Membrane-Induced Chondrogenesis (AMIC) is an alternative procedure for articular cartilage grafting, which relies on nano-drilling combined with the use of a bio absorbable membrane that new cartilage cells grow into. It is a single-stage procedure (1 op instead of 2), it is much cheaper and large studies have shown that it gives results that are directly compatible to (no different from) ACI grafting, and AMIC has now been approved by NICE.
  • Articular cartilage grafting is not appropriate for knees where there is widespread articular cartilage loss (i.e. fully-blown arthritis), and it is not an alternative to knee replacement surgery.

Articular cartilage damage in the knee

Articular cartilage damage / loss in a knee can cause pain, swelling, stiffness and loss of function. Articular cartilage has no blood supply, and hence damaged cartilage does not repair or grow back; instead, cartilage damage is something that only ever normally tends to get worse with time, not better. With time, progressive cartilage loss can end up leading to bare bone exposed in a knee: i.e. arthritis.

There are a number of different surgical techniques for treating articular cartilage damage in a knee. Importantly, however, it should be noted that articular cartilage grafting is specifically for the treatment of focal full-thickness cartilage defects, and it is not suitable in knees where there is fully-blown arthritis, and it is not an alternative to knee replacement surgery in arthritic knees.

Arthroscopic radiofrequency coblation chondroplasty

If there is symptomatic articular cartilage damage in a knee, with rough or unstable flaps or damaged edges, and if the damage is not too severe / not too far gone (i.e. if the knee is not yet fully arthritic), then it can sometimes be appropriate to perform arthroscopic radiofrequency coblation chondroplasty.

If you picture the damaged cartilage in a knee as being like flaky paint on a rusty gate, where every time you rub something against the surface you are breaking off more flakes of paint… then radiofrequency chondroplasty is like using a blow-torch on that flaky paint, to melt the surface and to smooth off and stabilise what’s left.

A more ‘crude’ way of smoothing things off in a knee is to use an arthroscopic shaver – this is a bit like using an electric sander on your flaky paint: yes, it can get things smoother… but it does so by taking away the paint. The depth of damage to the articular cartilage from using a radiofrequency probe is significantly less than what is seen with an arthroscopic shaver, and the radiofrequency probe leaves things much smoother and more stable.

It has been found that if someone has an unstable articular cartilage defect in their knee, and if you simply smooth it off and stabilise it (by arthroscopic radiofrequency chondroplasty), then up to 40% of those patients then end up not actually needing to take things further with, for example, articular cartilage grafting.

CLICK HERE for further information about radiofrequency chondroplasty.

Microfracture / Nano-drilling

If there is a small (<2cm2 in surface area) full-thickness articular cartilage defect in a knee, then microfracture / nano-drilling can sometimes be an appropriate treatment.

Articular cartilage sits on a hard surface called the subchondral bone plate, under which is normally healthy bone. The articular cartilage has no blood supply (and hence it has minimal healing capacity). The bone underneath, however, has a very good blood supply, and the bone marrow is full of cells, including stem cells.

Microfracture / nano-drilling involves making tiny holes in the subchondral bone plate to allow the blood and bone marrow from the bone to ‘leak’ through to the surface. This then forms a ‘super-clot’ in the defect. The ‘super-clot’ then solidifies and this then matures into what’s referred to a ‘fibrocartilage’, which is half way between normal cartilage and scar tissue. Whilst this is not normal perfect cartilage, it’s better than exposed bare bone.

Microfracture is the old-fashioned way of achieving ‘bone marrow stimulation’, and it involved hammering a sharp metal awl into the surface of the bone with a small hammer, to create roughly 2mm-diameter holes in the subchondral bone plate. This tends to cause some damage to the subchondral bone, and it can lead to the formation of ‘subchondral osteophytes’, which are raised islands of new bone that form (instead of fibrocartilage).

Small full-thickness cartilage defect being microfractured with an awl.

Microfractured area heals over with a layer of fibrocartilage.

Nano-drilling is the new and better version of microfracture. With nano-drilling, we use a flexible drill bit to drill tiny holes into the surface of the bone that are just 0.9mm in diameter. This creates less damage to the subchondral bone, and it has been shown to lead to significantly less formation of raised subchondral osteotophytes, with, hence, better outcomes.

Focal full-thickness cartilage defect in a knee, with the edges stabilised by arthroscopic radiofrequency.

Base of defect being nano-drilled with a Stryker Phoenix Nanodrill to create multiple tiny holes in the subchondral bone plate.

Microfracture / nano-drilling has a success rate in the region of approximately 80% at 5-year follow-up, which is fairly good. However, the outcomes do then tend to deteriorate and drop with time, after that (i.e. the longer-term results tend to be somewhat poorer).

Importantly, nano-drilling is only appropriate for small discrete focal cartilage defects, and it is not a suitable treatment for knees with larger cartilage defects, with meniscal deficiency or with actual arthritis.

CLICK HERE for further information about microfracture / nano-drilling.


Autologous Chondrocyte Implantation (ACI) was first performed all the way back in 1987! … with the first pilot study being published in 1994. So, it is not new technology!

ACI is a 2-stage operation:

  • The first operation involves an arthroscopy, to evaluate a cartilage defect, to stabilise its edges and to harvest some cartilage tissue from the edges of the joint.
  • The harvested cartilage cells are then sent to a lab where they are cultured, to produce at least 20-fold more cells, which takes approximately 6 weeks.
  • A 2nd-stage procedure is then performed to implant the new cultured cells back into the knee, with the cells being placed either under a patch or else embedded in a scaffold.

Operation #1. Healthy cartilage tissue is harvested from around the edges of the knee and are sent to a lab.

Operation #2. 6 weeks later, the cultured (multiplied) cartilage cells are reinserted back into the knee either underneath (ACI) or embedded within (MACI) a membrane, to fill the cartilage defect.

With 1st generation ACI, the a patch of periosteum (the soft fibrous surface layer on a bone, outside of a joint) was taken from the patient, and the cultured cartilage cells were injected under this patch, with the patch being sewn in place. This sometimes caused issues with over-growth of new tissue.

2nd generation ACI involved using collagen patches (obtained from animal collagen) instead of periosteum.

With 3rd generation ACI, the cultured cartilage cells are provided embedded into a collagen membrace / scaffold.

With all of these procedures, the reported success rate is approximately 80% at 5-year follow-up, which is good, but…

  • ACI involves the need for 2 operations (rather than just 1),
  • the logistics of the cell culture process are very onerous indeed and
  • the procedure is very expensive, with the culturing of the cells alone costing in the region of about £10,000 to £17,000, before you then have to add in the cost of 2 x separate operations on top of that!

For this reason, some surgeons in specialised centres offer AMIC (Autologous Membrane-Induced Chondrogenesis) articular cartilage grafting to their patients instead, as this is a single-stage procedure that is much cheaper but that actually gives exactly the same clinical outcomes.

CLICK HERE for further information about ACI.

AMIC single-stage cartilage grafting

Autologous Membrane-Induced Chondrogenesis (AMIC) was developed as an alternative procedure to ACI/MACI.

An AMIC procedure involves:

  • first, the edges of a cartilage defect are stabilised with a radiofrequency probe,
  • next the base of the defect is nano-drilled, and then
  • the defect is covered with a bioabsorbable scaffold.

The scaffold absorbs the bone marrow and stem cells released from the subchondral bone by the nano-drilling, and these cells then grow new cartilage-like tissue within the scaffold, and the scaffold is then absorbed and disappears with time.

AMIC single-stage articular cartilage grafting. The new gold-standard for articular cartilage grafting in the knee.

The advantages of AMIC over ACI/MACI are that:

  • AMIC is a single-stage surgical procedure (whereas ACI/MACI requires 2 separate operations) and
  • AMIC is very significantly cheaper, with the scaffolds costing in the region of about £1000 to £2000 or so (as opposed to the cell culture for ACI/MACI costing up to £17,000), and with there being the cost of just 1 single op instead of 2, and
  • AMIC is equally effective as ACI/MACI, in terms of clinical outcomes, but it is more cost-effective.

There are three main types of scaffold that are commonly used for AMIC grafting, and these are:

  • Chondrotissue (hyaluronic acid impregnated woven polyglycholic acid)
  • Hyalofast (a single 3D fibrous layer of a benzyl ester of hyaluronic acid)
  • Chondrogide (a bilayer collagen I/III membrane made from highly refined porcine collagen).

Importantly, research has shown that the outcomes from AMIC grafting are identical to ACI/MACI, but with far less hassle for the patient and far less cost:

In April 2024, NICE (the National Institute for Health and Care Excellence) issued Draft Recommendations approving the use of AMIC grafting for the treatment of articular cartilage defects in the knee:

CLICK HERE for further information about AMIC cartilage grafting.

Other important relevant considerations

  • Articular cartilage grafting is contra-indicated in a compartment of the knee (medial or lateral) if the meniscal cartilage ‘shock absorber’ is badly damaged or actually missing in that side of the joint. If a meniscus is missing, then it might also need to be replaced as well, at the same time, by meniscal allograft transplantation.
  • Articular cartilage grafting is also contraindicated if there is malalignment of the knee – in which case realignment osteotomy might be required first (or patellar realignment, if the articular cartilage damage is in the patellofemoral compartment at the front of the knee, and if there is lateral patellar maltracking).
  • If there is ligamentous instability in a knee (e.g. with an ACL rupture), then the knee will need to be stabilised with a ligament reconstruction either prior to or, potentially, at the same time as any articular cartilage grafting.

Can you run or go back to impact sport after articular cartilage grafting?

A better and more-appropriate question is not ‘can’ you do impact / running / sport after articular cartilage grafting… but, rather, should you?? … And the common sense answer to that is NO!

Articular cartilage grafting is very much a salvage procedure, not truly restorative, and if you are going to go through all the pain, hassle and risks of reconstructive surgery, then the only sensible approach afterwards is to then protect the knee, sticking to just light, non-impact, non-pivoting cardio exercise only.

You wouldn’t go back to smoking after a lung transplant!

You wouldn’t go back to drinking after a liver transplant!

So, similarly, it’s just not sensible to go back to running / jumping / impact type sport after articular cartilage grafting in a knee.