There may be some cause for concern. Temperature increases in the joint can cause cell death and fibrous tissue to form. These changes can lead to pain and implant loosening.
A recent study at the University of Washington (Seattle) compared the temperature inside three types of knees. They measured the knees of normal adults, adults with arthritis, and patients with total knee replacements. The normal knees increased temperature by about one-degree after 20 to 40 minutes of activity (biking, walking).
Some of the implants had 2 to 3-degree changes. Knee replacements made of cobalt-chromium seem to generate the greatest amount of heat. Find out what kind of implant you have and what's normal for that replacement. Make sure you go to your follow-up appointments with the surgeon. X-rays can be used to detect any early problems with the implant. Warmth can also be a sign of infection, which should be evaluated by your doctor.