Cartilage lesions - areas of cartilage damage - are regularly identified during arthroscopy. They may occur on their own or in association with damage to other structures, and it is may be difficult to decide if the concurrent pain and disability is due to the cartilage damage per se or to the damage to the other structures.
When cartilage is damaged it heals poorly because of low metabolic activity and lack of blood supply. Areas of damage may progress to general osteoarthritis (OA). It is for this reason that attempts are made to repair isolated full thickness areas of cartilage damage before there is progression to OA.
If a cartilage injury is suspected MRI should be considered to determine the extent of the damage.
The authors give a brief historical overview of the various options, and it makes good and succinct reading if you want to get the original article from your library. The rest of the paper focus on the MACI technique.
In both ACI and MACI cartilage tissue is harvested from the patient in a first-stage procedure and sent to the laboratory to expand the material by tissue culture of the cartilage cells (chondrocytes). The harvest is of a full-thickness bit of cartilage from a non-weightbearing region such as the area around the intercondylar notch or the lateral (outer) border of the trochlea. This first-stage surgical procedure also allows a full evaluation of the inside of the knee.
In the laboratory enzymes are used to digest away the cartilage matrix and release the chondrocytes, which are then cultured to obtain 15-20 million cells over about 4 weeks. They are grown at this stage in a mono-layer and are undifferentiated.
A few days before implantation the cartilage cells are 'seeded' onto a more structured bio-degradable scaffold, which then allows them to partially differentiate into their definitive cartilage form and start to produce the cartilage matrix components that will rebuild and give strength to the repaired cartilage.
There are a number of companies that offer this service, and they differ in the nature of the scaffold material. Only the MACI technique is available in the USA, and it has the largest clinical experience and the majority of published reports, including two randomised clinical trials.
Surgical technique of MACI
The original defect is debrided to the layer just under the cartilage and the edges are prepared to render them stable and vertical.
The implant is then trimmed to exactly match the defect in size and shape and then it is glued in place with a fibrin glue. The cell-seeded layer faces the bottom of the defect. Pressure is applied for several minutes to ensure fixation.
If defects are large, bio-degradable bone anchors or sutures may be used to hold the implant in place.
The procedure is quicker than ACI, and it may be easier to deal with any other problems in the knee (eg torn ligaments) at the same time than it is with ACI.
There is a short initial period of immobilisation followed by CPM which stimulates the production of matrix. Then there are 8-12 weeks of limited weight bearing, while rehab focuses on progressive ROM and then progressive advancement of activity.
Full return to sport is not usually permitted for at least 18 months.
Results and State of the Evidence
The authors tabulate the results of 8 studies, including comparisons of MACI and microfracture, and MACI and ACI. Although many studies demonstrate improvement, the authors point out that "the vast majority of clinical evidence regarding MACI is based on small case series using a variety of techniques on heterogeneous patient populations, the results of which are evaluated with a plethora of incomparable outcome measures. These factors impair the ability to compare results between studies, which are often contradictory".
MACI complications include tissue hypertrophy (the most common), infection, the need for subsequent surgery and procedure failure - but the incidence of complications is low. If hypertrophy occurs, the excess growth can be trimmed via arthroscopy.
The authors found the comparative studies of the various cartilage repair procedures fail to date to demonstrate any one procedure to be better than any other. In the four studies available at the time of this publication where MACI was compared to other procedures it appears that MACI offers better results than marrow stimulation techniques, but there is no data available comparing the long term outcome of MACI versus conservative management (that is, no surgery). The authors found no data in the literature demonstrating the ability of MACI to prevent or delay the onset of osteoarthritis.
Their final conclusion was that basically the jury was still out on this one.