It seems that it could just be a fatty lump, or lipoma, if the joint is freely mobile and squishy. The general rule for a lipoma is to monitor it and, if it hurts and grows rapidly so as to interfere with his mobility, it should be removed. Equally it could also be a fluid-filled bruise, forming from bumping the area and causing swelling and fluid build-up as protection. The fluids tend to be reabsorbed by the body and the lump goes away with time; alternatively it may burst and drain a red fluid. The best thing to do is to rest the joint, so fewer walks and no running or jumping. Bursitis could be the cause of the overheating of the joint, caused by the production of synovial fluid, but this commonly affects much larger animals. If the dog is old then it may be an onset of arthritis, in which case a vet can properly treat the dog; other symptoms are a gradual limping, shortness of breath and general irritation. As a boxer, perhaps excess weight is also a cause so a diet may be the best course of action. To relieve the pain on the joint, if the warmth is still there, add an ice pack or, conversely if the heat has gone down but the swelling remains, soak a towel in warm water and apply two or three times a day, being careful that the pack is not too warm. There is a chance that the kneecap has luxated and its ridges have worn down, which would explain the excess of heat. Surgery would be an option to deepen the groove that the kneecap fits into, known as a trochlear modification, or have the kneecap tied down on the outside, all of which will have a recovery period of up to two months.
