Causes of meniscus tears

by Nathan Wei, MD, FACP, FACR

Nathan Wei is a nationally known board-certified rheumatologist and author of the Second Opinion Arthritis Treatment Kit. It's available exclusively at this website... not available in stores.

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There are two menisci in the knee. They sit between the femur (upper leg bone) and the tibia (lower leg bone).

The ends of the femur and tibia are covered with hyaline cartilage. These help with cushioning and gliding. In addition, the two menisci, made of fibrocartilage, also assist with cushioning, gliding, and rotation of the knee. Essentially, the menisci add to the stability of the joint.

The medial meniscus sits on the medial tibial plateau; the lateral meniscus rests on the lateral tibial plateau.

The menisci are nourished by small blood vessels, but the central portion of the menisci have an area that has no direct blood supply (avascular). The avascular areas tend not to heal.

The two most common causes of a meniscus tear are traumatic injury and degenerative processes. Meniscus tears can occur in all age groups. Traumatic tears are most common in active younger people. Traumatic meniscal tears are usually radial or vertical (picture a piece of bread that can be sliced one way or the other)and are more likely to produce a fragment that can catch in the knee and therefore require surgical treatment.

Traumatic meniscus tears occur when the knee joint is bent and the knee is then twisted. It is not uncommon for the meniscus tear to occur in combination with injuries to the anterior cruciate ligament (ACL) and the medial collateral ligament (MCL). These three problems occurring together are known as the "unhappy triad." This triad is seen in sports like football when the player is hit on the outside of the knee, after his foot is planted.

Individuals who experience a meniscus tear usually experience pain and swelling as their primary symptoms. Another common complaint is joint locking, or the inability to completely straighten the joint. This is due to a piece of the torn cartilage preventing the normal flexion and extension of the knee.

Degenerative tears are most common in people from age 40 upward. Degenerative meniscal tears are thought to occur as part of the osteoarthritis process. Collagen fibers within the meniscus break down and are subjected to shear forces that are magnified by the presence of osteoarthritis. Degenerative tears are usually horizontal. They are usually not unstable like traumatic meniscus tears. Therefore, they are less likely to produce mechanical symptoms of catching or locking.

For some reason, degenerative meniscal tears are also more common in smokers.

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