The knee seems like a simple joint, but it plays a large role in your mobility and stability. Your knee bears your weight and must coordinate the tibia (thigh bone) with the femur (shin bone) and the patella (kneecap) so you can walk straight and fluidly.
The bony parts of the knee work contain the ligaments that stabilize the joint. The muscles above (quadriceps) and below (calf) move the knee, with the ligaments holding everything together. If you add into the mix, the cartilage, also called meniscus, you have another part that can be injured. This lies across the bone and helps the weight be evenly distributed. Because there is so much going on, there is more potential for injury to any of the ligaments, bone, or muscle.