Ligaments and muscles give the knee stability and hold it together. Surprisingly, the knee joint is structurally quite weak. For this reason, the knee is vulnerable to a variety of injuries. Fifty million Americans suffer from knee pain or injuries some time in their lives. The most common knee injuries are sprains, cartilage tears, arthritis, and overuse.
Knee sprains can occur by wrenching or twisting or by a violent blow.
One or more of the ligaments of the knee joint may be torn. Ligaments are strong but not very flexible. They may tear slightly, or completely pull away from the bone. A tear in the cartilage can occur from a sudden move or twist when there is weight on the knee. Long-term wear and tear can also break down the cartilage.