>>> we're back now with some health news affecting more americans than ever before. there's been a surge in the last few years of baby boomers having knee replacement surgery, as their refusal to slow down takes a heavy toll on their aging bodies. our report tonight from our chief medical editor, dr. nancy snyderman.
>> reporter: decades of running, biking and competitive sports took their toll on sara kate loman's body.
>> the pain was grueling, grinding, constant. i could not get my knee in a position of comfort.
>> reporter: now at age 54, she's one of about four and a half million americans walking around with an artificial knee.
>> people at work said to me, you know, gosh, you know, we enjoy working with you but we've never seen you smile so much. they didn't know me out of pain.
>> reporter: loman is part of a growing trend, people opting for knee replacement surgery earlier in life.
>> there were times that this procedure was only done for people who were older than 65 years of age.
>> reporter: not anymore. the number of baby boomers getting total knee replacements tripled in a ten-year period. and even though there can be complications, nearly one in 20 americans over 50 is walking around with a new knee. what's behind the increased demand? well, experts point to a few factors. america's obesity epidemic, aging athletes not taking care of their aching joints, genetics, and the fact that we're living longer, more active lives.
>> previous generations, they were taught to live in pain and make some compromises. a lot of their attitude was about self-sacrifice. and today, people are less willing to do self-sacrifice. they're really very much in tune with their quality of life, want to maintain their quality of life.
>> reporter: and that can be hard to do with advanced knee problems.
>> i was basically sucking advil and really at the end it wasn't helping too much either.
>> reporter: less than one year out of surgery, 61-year-old ernie miller is back to working, traveling and spending time with his teenage son.
>> i have a new lease on that part of my life, that i can continue to grow this bond with him. maybe i can't beat him anymore at basketball, but at least i can play.
>> reporter: though he feels great, he admits the surgery made some things a little harder.
>> i can tell my wife i can't go shopping because my knee hurts. i still try to use that, but she doesn't buy it anymore.
>> reporter: dr. nancy snyderman, nbc news, mountain view, california.