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The basics


  • Full thickness defects of the articular cartilage in the knee, with bare bone exposed, can cause significant symptoms in the joint and, if left untreated, are likely to lead to further damage in the knee and,  possibly, eventually arthritis.
  • Partial thickness articular cartilage defects can be treated quite effectively with radiofrequency chondroplasty via knee arthroscopy.
  • Full thickness articular cartilage defects that are relatively small (<2cm2) can be treated quite effectively by microfracture, via knee arthroscopy.
  • Larger (>2cm2) full thickness articular cartilage defects tend to give less favourable results if treated by microfracture, and these defects tend to be more suitable for articular cartilage replacement techniques, such as articular cartilage transplantation (ACI or MACI) or Chondrotissue grafting.
  • Cartilage can be harvested from (taken out of) the knee joint and grown in a lab to produce large numbers of cartilage cells that can subsequently be reimplanted back into the knee to fill a cartilage defect. This is called ACI or MACI (further details below). However, this is a complex and expensive procedure that requires multiple actual operations.
  • Bioabsorbable artificial articular cartilage scaffolds (the Chondrotissue graft) are now available that allow new cartilage tissue to re-grow in the knee without the need for and cost of articular cartilage cells having to be cultured (grown) in a lab, and this has been shown to give similarly good results.


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The articular cartilage is the layer of tissue covering the ends of the bones in the knee. It is a few millimeters in thickness and it appears smooth, white, shiny and glistening. Articular cartilage is highly specialized tissue, and its function is to make the bone ends extremely smooth and very low friction, which allows millions of movement cycles a year without any significant wear.

When articular cartilage becomes damaged, unfortunately the tissue cannot repair itself to heal up properly, because the tissue does not actually have a blood supply. Therefore, any damage to the cartilage through trauma, from pressure overload, from general wear and tear or from osteoarthritis, tends to be permanent and cumulative.

Surgeons have tried for many years to find a suitable method for repairing articular cartilage defects in the knee. Unfortunately, none are quite ideal although over recent years there have been some significant advances that now provide surgeons with a variety of decent options.

Partial thickness articular cartilage damage is best treated by radiofrequency chondroplasty. Small patches of full thickness articular cartilage damage (<2cm2) are best treated by microfracture. However, the rest of this page focuses specifically on the treatment of larger (>2cm2) full thickness defects, for which microfracture does not work so well.



Autologous Chondrocyte Implantation (ACI)

This technique was developed initially by surgeons in Sweden. It is a difficult and complex procedure that involves a number of stages:-

Step 1: A knee arthroscopy is performed and samples (biopsies) of normal articular cartilage are taken from the knee. These are taken from the non-weight-bearing surfaces around the edge of the joint.

Step 2: The cells are put into a special culture medium and send to a company, where the cells are cultured in a lab. This multiplies the number of cells by many times, to give millions of the patient’s own living cartilage cells, ready for reimplantation. This step takes 6 weeks.

Step 3: A second operation is then performed, where the edges of the cartilage defect are tidied up and stabilized, and the base of the defect is scraped clean, down to the subchondral bone plate. A membrane (periosteum or collagen) is then fixed over the defect, stitching and/or gluing it to the surrounding edge of cartilage, and the patient’s own living cultured cartilage cells are injected under the membrane. The hole around the edge of the membrane where the cells were injected is then sealed with glue.

Step 4: After the implantation surgery, it is then vital that a specific slow careful rehab programme is followed, to allow the treated area to heal up properly. This involves 6 weeks on crutches and with a knee brace, followed by a few months of regular careful physio rehab. The tissue gradually matures with time, and most people agree that patients should not return to heavier activities (such as heavy weights and impact) until a full 9 months post-op.

The membrane

Initially, surgeons used to take a piece of periosteum (the fibrous layer of tissue covering the non-articular (outside of the joint) surface of a bone), harvested from the front of the tibia. However, surgeons then moved on to using membranes made of purified collagen, which is one of the main components of tendons, ligaments and other connective tissues. The collagen membranes (such as Chondro-Gide) were derived from pig collagen and removed the need to harvest periosteum from the patient’s own leg.

The reported results of ACI are good, with a success rate of about 80% at 5-year follow-up. However, ACI does not grow quite normal articular cartilage (which is also called ‘hyaline’ cartilage), but instead it grows what is referred to as ‘hyaline-like’ cartilage, which is not quite as good (does not have quite the same mechanical or biological properties) as normal cartilage tissue. The procedure does, however, reduce pain, increase function and keep patients’ knees going for longer, either delaying or even avoiding the need for further subsequent surgery in the future, such as a knee replacement.



Matrix-induced Autologous Chondrocyte Implantation (MACI)

The next step in the evolution of articular cartilage surgery was that instead of injecting the cultured cartilage cells as a paste underneath a membrane in the knee, people started culturing the cartilage cells and actually implanting the cells into a porous felt-like membrane, which acted as a scaffold. This allowed for a simpler and more robust surgical technique, and published results have shown MACI to give equivalent outcomes to ACI, but with a slightly easier implantation procedure.



Autologous Matrix-Induced Chondrogenesis (AMIC)

After the introduction of MACI, some surgeons then tried treating larger (>2cm2) articular cartilage defects by extensive microfracture combined with covering over the microfractured area with an absorbable porcine collagen membrane. This technique allowed the rich healthy blood from the subchondral bone (full of stem cells) to seep into the porous scaffold, which allowed the cells to then mature into cartilage cells. The membrane is slowly absorbed, to leave behind healthy ‘hyaline-like’ cartilage, in a very similar fashion to ACI and to microfracture but without the need for culturing cartilage cells (in ACI) and with better quality tissue formation than is seen with microfracture.


Chondrotissue grafting

The latest development in the field of articular cartilage transplantation / regeneration is the use of the Chondrotissue graft. This is a synthetic composite scaffold that looks similar to a sheet of felt. It is made of a combination of polyglycolic acid and hyaluronan in a highly porous membrane/sheet that is cut to size to fit a cartilage defect and which is fixed in place over a microfractured area of subchondral bone in exactly the same way as is done with AMIC. Studies have shown excellent regeneration of new articular cartilage tissue and excellent clinical results for reduction of pain, reduction of swelling and for increased joint function.




CLICK HERE  for more information about Chondrotissue articular cartilage grafting.



What would I have done?

There are pros and cons with each of the potential different surgical options for articular cartilage replacement:-



  • Ideal for small defects (<2cm2)
  • Can be performed fully arthroscopically at the same time as any arthroscopy


  • Poor results for bigger defects (>2cm2)
  • Creates fibrocartilage, not normal hyaline articular cartilage




  • Better for larger cartilage defects
  • Creates more normal looking new articular cartilage


  • Very expensive
  • Complicated procedure and complicated processes/logistics
  • Requires 2 separate operations (1 to harvest the cartilage cells, 1 to implant the cultured cells)
  • Has been associated with problems of cartilage overgrowth, which can necessitate further surgery in up to 40% of patients.

Chondrotissue grafting


  • Ideal for larger (or even very large) cartilage defects
  • No cell harvesting/culture required, therefore can be done as a single-stage procedure (rather than 2 separate operations)
  • Considerably less complicated procedure and logistics than ACI/MACI
  • Considerably cheaper than ACI/MACI
  • Histological studies demonstrate regeneration of excellent new hyaline-like articular cartilage.


  • Still can be somewhat expensive, though considerably less so than ACI or MACI


McDermott’s firm preference for the treatment of articular cartilage defects that are greater than 2cm2 or for smaller defects that have failed attempts at previous microfracture is to use the Chondrotissue graft, which in his hands has given the best results.



Written by 
Mr Ian McDermott
Consultant Knee Surgeon London
Last updated 24-6-15