Properly termed genu valgum, this is a deformity of the lower limbs causing the legs to diverge from one another when the limbs are straightened. As a result, the knees knock against each other during walking. The amount of knock-knee is measured by the distance between the medial malleoli of the ankles, with the inner surfaces of the knee touching and the knee-caps facing forwards. The condition is so common in children between the ages of 2–6 years that it may almost be regarded as a normal phase in childhood. When marked, or persisting into later childhood, it can be corrected by surgery (osteotomy).