Cold therapy is usually used to reduce pain and swelling. It works by closing down blood vessels. This helps reduce swelling by decreasing the amount of fluid volume near the joint.
Cold also slows down the tissue metabolism. This means the tissues need fewer nutrients. It also stops the release of enzymes. This reduces damage to the tissues. There's less swelling and less pain.
Using heat on a joint that's already painful and swollen can cause the symptoms to get worse. Heat opens up the blood vessels and brings more blood to the area. More fluid puts pressure on all the soft tissues in the area. There is a place for heat when a joint or
injury is more chronic. A new (acute) condition responds better to cold.