The simplest way to distinguish rotund legs, also known as O-leg, is to have the baby in supine position with bilateral ankles together, and when they are together, if the bilateral knee joint interval is greater than 3cm, it can be initially determined as O-leg. If the inversion of the knee is greater than 25 degrees and the baby is older than 2 years old, the baby will have O-leg, which is mostly caused by rickets and may also be caused by low muscle strength of both lower limbs. If it is caused by rickets, vitamin D and calcium supplements can be given; if it is caused by low muscle strength and muscle tone, muscle strength exercises of both lower limbs should be carried out, and massage should be given, and orthopedic braces should be worn if necessary.