Knock-knee. Coloured X-ray of the legs of a 2- year-old child suffering from knock-knee, also known as genu valgum. This condition causes the inward curving of the legs so that the knees touch, and the feet are displaced outwards. It is common between the ages of three to five years; in children it usually requires no treatment unless it persists after the age of ten. Common causes are rickets, osteoarthritis, or bone fracture in the knee area. Heel wedges in shoes may help correct the line of the leg but most people require orthopaedic surgery (osteotomy), in which each tibia bone is cut and realigned to straighten the legs.