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Why can you suddenly tear your knee meniscus?

Physical Therapy in Grapevine for Knee

Q: I was with my elderly aunt when she tore her knee meniscus. She was just crossing the street with me and pop goes the weasel! The doctor says these things can happen just like that but there must be a reason. Do you know what is really going on?

A: It might help you understand what is happening if we review the anatomy of the meniscus (menisci for more than one). In each knee, the menisci sit between the femur (upper leg bone) and the tibia (lower leg bone). These structures are sometimes referred to as the cartilage of the knee, but the menisci differ from the articular cartilage that covers the surface of the joint.

The menisci of the knee are important for two reasons: (1) they work like a gasket to spread the force from the weight of the body over a larger area, and (2) they help the ligaments stabilize (hold) the knee.

Imagine the knee as a ball resting on a flat plate. The ball is the end of the thighbone as it enters the joint, and the plate is the top of the shinbone. The menisci actually wrap around the round end of the upper bone to fill the space between it and the flat shinbone. The menisci act like a gasket, helping to distribute the weight from the femur to the tibia.

The dynamic loading pattern differs from person to person due to alignment and surface structure (how curved the meniscus and joint surface are and how well they match up).
As we get older, the menisci can get pretty beat up. They start to dry out and stiffen up. This effect can alter the match up even more.

Degenerative changes within the joint such as bone spurs, thinning of the articular cartilage, frayed edges of the menisci change the way load and force are transmitted through the knee. If there are any alignment issues causing uneven wear on the joint, then these problems are amplified.

So it can look like an older adult spontaneously tears the meniscus without any injury at all, when in fact, these changes have been developing over a period of many years. For some people like your aunt, the simple act of walking was enough to create the injury. For others, an equally minor trauma (stepping off a curb, walking up stairs) might be the precipitating factor. Really, those events are just the final straw for a body part that was ready to go anyway.

Reference: Michael J. Salata, MD, et al. A Systematic Review of Clinical Outcomes in Patients Undergoing Meniscectomy. In The American Journal of Sports Medicine. September 2010. Vol. 38. No. 9. Pp. 1907-1916.