XO-leg is not caused by bent bones, but by knee valgus and knee inversion; X-leg is caused by knee valgus, so that when the body stands naturally or when the legs are straightened naturally, the two inner knees can come together, but the two inner ankles of the feet cannot come together and are separated, making the lower limbs look like the letter X. O-leg is the opposite, because O-leg is caused by knee inversion. The causes of O-leg are similar, and include acquired malnutrition, such as rickets, poor sitting or walking posture at an early age, and premature walking. This disease has hereditary characteristics.