In the United States, according to a study by Blue Cross Blue Shield, a typical knee replacement surgery can range between $12,000 to $70,000 depending on what part of the country you live in.
And then there's the growing popularity of the surgery: The American Academy of Orthopedic Surgeons projects knee replacements in the US alone will grow by up to 189% in the next decade, for a projected 1.28 million procedures by 2030.
The US population of baby boomers is aging, as are their knees, but those numbers may be partially driven by the rise in knee replacements among those under the age of 65. A 2012 study found total knee replacement more than tripled for people aged 45 to 64 between 1999 and 2008; for those over 65, it only doubled. The cost for all those operations, the study found, was more than $9 billion.
Not everyone believes that such an objective approach will succeed in the health care environment.
"I would say this paper looks at the issue from the perspective of the experts and not necessarily from a patient perspective," said Dr. Bart Ferket, an assistant professor of population health science and policy at the Icahn School of Medicine at Mount Sinai Hospital in New York City.
"It's an attempt to objectify things that are subjective," said Mount Sinai orthopedic surgeon Dr. Edward Adler, who, like Ferket, was not involved in the study.
Pain, for example, is subjective and could interfere with the algorithm's ability to assess knee stability and a patient's reported levels of pain.
"Some people will allow you to move their knee even though their knee hurts a lot," Adler said. "They can have a lot of pain, you wouldn't know it. They function well.
"There are other people who have a little bit of pain and everybody around them has to know about it," he added. "So it's fairly subjective as to how much you tolerate before you get your knee replaced."
Ghomrawi agrees there could be excellent subjective reasons why a person might decide to get an early knee transplant instead of deciding to wait.
One scenario, he says, for a transplant at a younger age, for example, could stem from financial considerations. A candidate for the surgery may elect to go through with it, thinking, "I'm the only financial support for my family; I'm maintaining my functional level so that I can continue to be the breadwinner for my family."
Or perhaps an older person has a very painful knee, "but they're bearing with it because they're taking care of their spouse," Ghomrawi said.
Still, studies show many people aren't happy with the outcome of their knee replacement; a 2010 study found almost 20% said they were dissatisfied.
Objective or subjective, Adler said there needs to be a realistic assessment by each person of what a new knee can really accomplish. If a person has the surgery before the onset of severe or significant pain, the patient may not see enough improvement.
"Knee replacements are not really made for tennis and running," he said, "They are made for walking long distances and performing activities of daily living.
"What God gave you is not necessarily the same as what I can give you," Adler said."if your goal is to be normal, that's a difficult thing to obtain when your knee is coming out of a box."