Nearly 1 in 20 US adults over 50 have fake knees (AP)

In this Feb. 4, 2012 photo, Donna Brent, 63, works out her leg at her home in Deerfield, Ill. Brent says decades of racket ball, tennis, softball and other sports took a toll on her knees, but she got used to living with the pain, even when she became bowlegged and developed a limp. When pain started getting in the way of some of her sports, she gave in to her doctor's advice and had a knee replacement operation last June on her right knee. Nearly 1 in 20 Americans older than 50 have bionic knees, or more than 4 million people, according to the first national estimate showing how common these replacement joints have become in an aging population. (AP Photo/Nam Y. Huh)AP – Nearly 1 in 20 Americans older than 50 have artificial knees, or more than 4 million people, according to the first national estimate showing how common these replacement joints have become in an aging population.