Meniscus Tear internal derangement of the knee
A meniscus is a wedge of rubbery cartilage that acts as a shock absorber in the knee joint.  There are two c-shaped menisci in each knee--one on each side of the joint.  The meniscus can be torn if it is injured.
A meniscus tear may occur as the result of a pivoting or twisting injury.  Meniscus tears can also be degenerative, meaning that they result from wear and tear. 
The meniscus is made of cartilage which does not heal particularly well because of its poor blood supply.  The torn part of the meniscus may displace (move out of place).  This can cause pain and mechanical symptoms such as catching, locking, and giving out.
Meniscus tears are diagnosed based on a history, physical exam, and MRI.  Patients usually have a history of a twisting or pivoting injury.  In younger patients, this usually occurs while playing sports or doing something else active.  In older patients it may occur with a seemingly minor twist of the knee or fall.
During the physical exam, the joint line just over the torn meniscus may be very tender.  There may be catching or clicking.  Stress is put on the meniscus with McMurray’s maneuver or the Apley grind test.  If these tests cause pain, there may be a torn meniscus. 
Meniscus tears are not visible on x-rays, but they are often obtained to look for evidence of fractures or arthritis.  If a meniscus tear is suspected, an MRI may be obtained.  Meniscus tears are usually identified on the MRI.
People with small meniscus tears and older people with arthritis may do well with non-surgical treatment.  Treatment options include anti-inflammatory medications such as ibuprofen, steroid injections, ice, elevation, and activity modification.
People with larger meniscus tears, especially young, active people, will do better with surgery.  Surgery is usually done arthroscopically (through a knee scope).  If the tear is located near the outer edge of the meniscus where it attaches to the joint capsule, it may be repaired with sutures.  If the tear is located near the inner edge of the meniscus, a repair would not be beneficial because the tissue does not heal well.  If the tear can be repaired, it should be to preserve as much of the healthy meniscus as possible.  If not, the torn portion is debrided.  After a meniscus repair, the patient must remain non-weightbearing with crutches for a few weeks to allow the meniscus to heal.  If the torn portion is simply removed, the patient may bear full weight immediately following surgery.
Meniscus Tears
MRI normal meniscus
meniscus tear
Meniscus Surgery