Q: Sometimes my knee locks up and I cant bend or straighten it. Eventually, I can move it, but its sore. Whats wrong with my knee?A: There are several possibilities, but the most likely one is a torn meniscus. The meniscus is a piece of cartilage in the knee joint. There are two types of cartilage in the knee. Articular cartilage covers the ends of the upper leg bone (femur) and the bone at the front of the lower leg (tibia).