There are three main reasons why a cast can lead to swelling, as follows: 1, because the patient in a cast is usually a fracture or other soft tissue structure injury, after the injury will lead to local bruising and oozing and the formation of inflammatory edema, and this inflammatory edema will be progressive aggravation, about 5-7 days to reach the peak, so the swelling will certainly increase during this period. Therefore, the limb in the cast should be elevated above the level of the heart to effectively reduce local edema and bruising and swelling by gravity. 3. In this process, it will lead to further slowing down of venous return, which will also aggravate the swelling.