O-shaped leg is medically known as knee inversion, which refers to the inward rotation of the tibia bone in the knee joint area. The main causes of O-shaped leg are: 1. Developmental factors: due to the lack of calcium, phosphorus and other nutrients, resulting in abnormal bone development, bone deformation or poor development of articular cartilage, which may be due to insufficient intake caused by nutritional deficiencies or intestinal disorders resulting in impaired absorption of calcium and phosphorus elements. 2. Imbalance factors: due to poor posture, including sitting posture, standing posture, etc., incorrect exertion, such as standing with the center of gravity on one leg, etc., causing imbalance in the mechanics of the muscles that govern the lower limbs, which leads to displacement of the joints. 3. Trauma and other diseases: Any disease that destabilizes the knee joint, such as injury to the lateral collateral ligament caused by trauma, may lead to O-shaped leg. There are other possible causes of O-shaped legs. Once you have O-shaped legs, you should go to the hospital as soon as possible to seek professional treatment.