X-shaped leg, or knee valgus, is a condition in which the lower limbs are naturally straightened or the knees touch each other when standing but the inner ankles of the two feet are separated. O-shaped leg is a condition in which the legs are similar to the shape of an O when the legs look forward when standing on both feet. X-shaped leg, i.e. knee valgus, is an abnormal phenomenon in which the lower limbs are naturally straightened or the knees touch each other when standing but the inner ankles of the two feet are separated, which is mostly caused by heredity, nutritional deficiencies, incorrect sitting and walking postures in childhood, and so on. O-shaped leg, i.e. knee inversion, refers to the abnormal phenomenon of the lower limbs naturally straightening or standing with the inner ankles of the two feet touching each other but the two knees can not be close together, which is mostly caused by the lack of nutrition and exercise, and standing too early. Whether it is X-shaped leg or O-shaped leg, it will cause damage to the knee joint and form arthritis, etc. It is necessary to go to the hospital for correction and treatment in time to avoid serious consequences.