The knee joint is deceptively simple. At first glance, it appears its only job is to bend and straighten. How hard can that be? To truly understand this joint, however, it's important to understand that the end of the thigh bone (femur) is irregularly shaped. This shape dictates that if the lower leg is fixed (for instance, when the foot is in contact with the ground), then the femur must rotate inward as the knee joint bends (flexes) and outward as it straightens (extends). If the lower leg is not fixed on the ground (as when kicking a soccer ball), the lower leg usually rotates instead. This little bit of rotation is where the knee gets into trouble because the large muscles on the front and back of the thigh bend and straighten the knee well, but they are not designed to adequately control knee joint rotation. Instead, the joint surface, meniscus, and ligaments guide rotation.
More Answers from Rick Olderman