The possible causes of external rotation of the tibia are congenital factors, tibia fracture and osteoarthritis. 1. Congenital factors: External rotation may be caused by congenital underdevelopment of the tibia. 2. Tibia fracture: It may be caused by traumatic tibia fracture. If the tibia is not properly repositioned during the healing process, it will easily lead to deformity of the bones, which will lead to external rotation of the tibia. 3. Osteoarthritis: External rotation of the tibia may be caused by osteoarthritis of the knee joint. Part of the knee joint may be swollen due to osteophytes or joint effusion, and the cartilage and subchondral bone may be worn down, resulting in deformity of the joint and the formation of an “X-shaped leg”. There may also be other reasons for external rotation of the tibia. It is recommended to go to the hospital in time for a complete examination to clarify the cause of the problem, and then give targeted treatment or therapy under the guidance of the doctor.