knock knee (genu valgum) Condition in which there is space between the ankles when the legs are held together and extended. Young children often have knock knees after the third year of life. These usually disappear of their own accord. They may however persist in, for example, cases of flat feet. Because of their wider pelvis, women have knock knees three times more frequently than men. Knock knees may also occur in bone disorders such as rickets and osteomalacia, or in paralysis of the leg muscles. They can also arise by overstraining the ligaments of the joints through obesity or lifting heavy weights.