Which Type of Cartilage (Meniscus or Articular)?
Confusion in describing meniscal tears in the knee commonly arises because there are two kinds of cartilage in the knee. Footballers tear their “cartilages” commonly. This is properly termed a meniscal tear since these cartilages are like little shock absorbers between the thigh and shin bone. They each form a horseshoe, one on the inner side and one on the outer side of the knee between the femur and the tibia which makes the flat top surface of the tibia into a bit of a socket to take the curved bottom end of the femur. The meniscus looks a bit like an incomplete tap washer or gasket. The cartilages work by redistributing the forces by making the bottom end of the femur fit better, ie: be more congruent with the shape of the top end of tibia.
The second kind of cartilage is the gristle ending of the bone. This a layer which coats the “ivory” bone surface. This is the bearing surface and made out of quite a different material from the meniscus, perfectly designed for near frictionless movement. When all the articular cartilage is gone the bearing is worn out and this is what occurs in osteoarthritis.
The meniscus (cartilage) is very most commonly torn, either in twisting injuries in young people or commonly in older individuals in their thirties or forties without any trauma. This is because the cartilage get a little bit brittle and degenerate as we get older and small splits will often occur and not declare themselves until the piece of gristle moves out of place and gets caught leading to the pain. This needs operation to either repair it or remove the torn piece.
To Take out the Meniscus?
Soon after the war it was thought that the cartilages were something you could do without, and footballers complaining of pain in the knee would often have an open operation to remove the cartilage in case it was causing trouble. Soon after this it was realised that it was not a good idea and that removing cartilages increased the pressure across the surface of the joint, leading to premature degenerative changes in some patients. The emphasis then moved in the sixties and seventies particularly with the invention of arthroscopy (key hole surgery) to removing as little as the cartilage as possible. That is just removing the bit of cartilage which was torn and not removing the normal rim. More recently still the emphasis has turned towards not even doing this, but instead making every effort to repair back the torn bit of cartilage. This can now be done through the key hole with a very high success rate in appropriate cases. Our success rate overall around 85% and this is obviously an ideal situation in that it returns the knee to near normal rather than taking away one of the major shock absorbers. Repair is only possible in some cartilage tears and particularly the fresh tears associated with a damaged ligament which can be dealt with at the same time.
We occasionally do have a problem with young professional athletes who have a repairable meniscus in that they will tell us that they want to have a bit of cartilage removed so that they can get back on the field in two to three weeks rather than having a repair procedure which will keep them out for two to three months. This is a bit of a difficult one since it may be allowing a short term gain at the expense of long term pain and premature osteoarthritis.
To Replace the Meniscus?
Grafts taken from dead donors (“allografts” have been used since the mid 1980s with reasonably good results, but have quite a number of disadvantages quite apart from many patient’s distaste for the idea! Sterilisation and preparation are problematic and the quality of tissue scaffold is often poor. It is difficult to find the right size especially as they do tend to shrink postoperatively.
A better idea may be a tissue-engineered solution either from cow collagen or more recently a biosynthetic polymer. Both of these were CE marked for use in the UK during 2008 and show great promise with or without the use of host stem cells to kick-start the biological incorporation. Both these devices are available “off-the-shelf” for use whenever required, but it is early days in their use and we are trying to study the results very carefully. It is by no means a solution for all patients as yet, not least because the recovery is fairly prolonged. They are very expensive (the Collagen Meniscus Implant “Menaflex” and European “Actifit” polymer implants alone cost between £1,500 & £2500 each, approximately the same as the cost of the implant used in a full knee replacement), & we are working with NHS purchasers and private insurers to make the case for their funding in appropriate cases.