There are three main reasons for the swelling and pain in the arm. 1. The arm may be stimulated by trauma, such as collision with external forces, resulting in bruising, swelling and inflammatory edema in the bones, muscles, tendons, ligaments, fascia, skin and other tissues, which can cause significant swelling and painful stimulation of the peripheral nerves. Especially when the arm activity is carried out, it will lead to more obvious pain. 2. It may be due to the thrombosis of the deep vein in the arm, once the thrombosis is formed, it will lead to a serious decrease in venous return, and a large amount of venous blood will accumulate in the distal end. This can lead to significant swelling of the arm and painful stimulation of the surrounding nerve structures. 3. It is possible that the inflammatory lesions in the proximal arm, such as the shoulder joint, can stimulate the peripheral circulation and lead to decreased circulation. After the decrease in venous return, the same swelling in the arm can be caused, but this swelling is much less severe than that of deep vein thrombosis. In addition, the inflammatory irritation in the periapical area also involves soreness in the muscle tissue of the arm, so this swelling as well as painful symptoms also occur.