The causes of O-shaped legs include rickets, congenital epiphyseal growth disorder, trauma and fracture, arthritis and so on. 1. Rickets: rickets can be caused by vitamin D deficiency, resulting in a decrease in calcium formation and bone softening, and the bones of the lower limbs can not withstand the pressure of upright weight-bearing, gradually resulting in inward angular deformity between the femur and the tibia, which in turn manifests O-shaped legs. It is mostly seen in children and adolescents. 2. Congenital epiphyseal dysplasia: Congenital epiphyseal dysplasia is caused by bone lesions, mostly bilateral, which affects the normal development of knee joints and leads to joint deformities, and O-shaped legs can be formed when bilateral inversion deformity develops. 3. Traumatic fracture: Traumatic fracture leads to the destruction of the medial bone quality, which is untreated and heals abnormally, resulting in an inversion deformity. 4. Arthritis: late arthritis can cause entropion deformity due to the wear and tear of the medial articular cartilage, and when entropion deformity occurs on both sides, O-shaped leg is formed. O-shaped legs are mainly seen in the elderly. O-shaped legs may be normal before the age of 6, and will recover with age. If it cannot be recovered, it is pathologic and should be treated in the hospital as soon as possible.