For the first 48 to 72 hours after a knee injury, use a cold pack to ease swelling and numb the pain. A plastic bag of ice or frozen peas works well. Use it for 15 to 20 minutes three or four times a day. Wrap your ice pack in a towel to be kind to your skin. After that, you can heat things up with a warm bath, heating pad, or warm towel for 15 to 20 minutes, three or four times a day.