Injury or degeneration of bone or cartilage can cause a piece of bone or cartilage to break off and float in the joint space and affect the joint movement. Or the ligament that extends from the outside of your pelvic bone to the outside of your tibia becomes so tight that it rubs against the outer portion of your femur or thighbone. This is common with distance runners.
The knee cap can also become dislocated, slipping out of place to the outside of the knee and not slip back into place. Sometimes athletes or those with arthritis develop a chronic pain between the knee cap and the thighbone that may be the result of the misalignment of the kneecap. Problems with your hip or foot may change the way you walk and end up putting more stress on the knee joint.