A torn meniscus is one of the most common knee injuries. Any activity that causes you to forcefully twist or rotate your knee, especially when putting your full weight on it, can lead to a torn meniscus. Each of your knees has two C-shaped pieces of cartilage that act like a cushion between your shinbone and your thighbone. A torn meniscus causes pain, swelling and stiffness. You also might feel a block to knee motion and have trouble extending your knee fully.