Patellar fracture and patellectomy (removal of the kneecap) are fairly rare events. Since there aren't very many cases reported, information about results is limited.
Doctors from the Mayo Clinic recently reported on eight cases of patellectomy after total knee replacement. Their patients were in a leg cast for six to eight weeks. After the cast was taken off, the patients started putting weight on the leg and began range of motion exercises.
Half the patients reported pain relief from this operation. Three people still had mild pain. One person had severe pain. Although the patellectomy gave them pain relief, motion and function were very poor even months to years later.