There are 2 menisci (plural of meniscus) in each knee. They are made of cartilage (like the soft tip of your nose) and act as shock absorbers in the knee. Each is a C-shaped washer between the femur (thigh bone) and the tibia (shin bone). The meniscus on the inner (medial) side of the knee is called the medial meniscus and the one on the outer (lateral) side of the knee is the lateral meniscus.
The meniscus can get torn. This can happen in one of 2 main ways.
One way is as part of an injury. This is one of the injuries what can happen when a person injures his/her knee. Usually this is a twisting knee injury with the knee in a bent or flexed position, but can happen in a variety of ways.
Another way is what happens when meniscal cartilage ages. Young meniscal cartilage is very flexible, like young rubber. As it gets older it becomes like old rubber that has been exposed to the air for years. It becomes hard, less flexible, more brittle and can develop cracks in it like the cracks that develop in old rubber. These cracks in older cartilage are called degenerative meniscal tears. These degenerative tears can occur with or without an injury.
They can vary greatly. Symptoms include just pain, or giving way of the knee, or catching and locking of the knee, water on the knee, etc. Often additional tests are necessary to determine the correct diagnosis and solve the problem and the most accurate and effective of these is arthroscopy by an experienced arthroscopist.
The most effective way to deal with a meniscal injury is through arthroscopy. Arthroscopy is not only the most accurate way of diagnosing the meniscal injury but also permits you to deal with the meniscal tear at the same time.
Normally, if a meniscal tear is present the arthroscopist will remove the torn portion that is causing the problem and smooth out the remaining part of the meniscus. If there are no other problems in that knee (arthritis, ligament injuries, etc) the results of doing this are excellent and the recuperation is just a matter of days.
An MRI (magnetic resonance image) is like an x-ray but uses magnetic waves instead of x-rays to produce the image. It allows you to see tissues as shadows of different densities. It allows you to see tissues that x-rays cannot show and is extremely useful especially in spines, shoulders, etc. I do not usually use MRI to deal with meniscal injuries.
The reason is simple. MRI is not totally accurate in diagnosing meniscal tears. I have seen cases where a tear was seen on MRI which on arthroscopy turned out not to be there and the opposite: no cartilage tear seen on MRI, but one found during arthroscopy.
The practical meaning of this is that if an MRI is done and shows a "cartilage tear" the result is that an arthroscopy needs to be done. If however an MRI is negative, but symptoms persist, an arthroscopy often still needs to be done because the MRI could be wrong. So the MRI does not change what needs to be done and at a cost of about $1,000 a test it is too expensive to do if it will not contribute usefully.
Meniscal injuries can occur either through an injury or as part of the aging process. The symptoms are very variable from person to person. Diagnosis requires an experienced orthopaedic surgeon and treatment usually consists of arthroscopy and can be very effective.