For short and long legs, lower limb length is usually assessed by the straight line distance connecting the anterior superior iliac spine to the medial ankle. If the straight line distance of the lower limbs is not equal bilaterally, it can mostly be judged that the lower limbs are unequal in length bilaterally. The most common cause of bilateral lower limb unequal length is congenital hip dysplasia. The hip dysplasia side may have joint dislocation or subluxation, and the femoral head will be displaced upward, which often causes shortening of the lower limbs on the affected side. In addition, it can also be seen in knee dysplasia, such as knee inversion or eversion deformity, as well as secondary diseases of the knee, such as degenerative osteoarthritis of the knee, bone tumor, bone tuberculosis, etc., which can appear inside and outside of the knee, causing unequal length of the lower limbs.