Kneecap replacement is called patellar resurfacing. Most of today's new patellae are made of plastic (polyethylene). Metal backed implants have been used but the metal debris caused problems in the joint.
Plastic backed kneecaps can still wear out or wear unevenly over time. They can be very difficult to remove and replace years later. The biggest plus to patellar resurfacing is the decrease in knee pain after surgery. Chronic knee pain is more common when the kneecap isn't replaced.
Problems can occur if the kneecap isn't replaced and the blood supply is cut off during surgery. The bone can actually die, a condition called osteonecrosis. This doesn't happen very often. It's more likely to occur when large amounts of soft tissue must be cut or released.
If the muscles around the knee are out of balance, the patella may not move up and down as it should. Tracking problems can occur in both kinds of patellae. More and more studies support the use of resurfaced patellae. Long-term studies (more than five years) may offer extra information in years to come.