(CNN) -- Living with knee pain? A new study has found that 90% of Americans with osteoarthritis suffer too long before having a knee replacement that could improve their quality of life.
"When people wait too long, they lose more and more function and can't exercise or be active, thus leaving them open to weight gain, depression and other health problems," said lead investigator Hassan Ghomrawi, associate professor of surgery at Northwestern University's Feinberg School of Medicine.
In addition, the surgery may not be as successful, Ghomrawi said.
"There are multiple studies that have shown that patients who do surgery when their function is very deteriorated may improve quite a bit, but their improvement is still not to the average," Ghomrawi said. "They lag behind in optimal benefit."
On the flip side, the study also found that 25% of people who do choose knee surgery are getting it too early, running significant risks, including potential complications, while incurring the cost of major surgery potentially without getting much extra benefit in mobility.
"There are a million knee surgery procedures occurring in the United States each year," Ghomrawi said, "and 25% of those are premature. That's a lot of patients."
Because artificial knees wear out after 20 years or so, early adopters are also setting themselves up for yet another knee replacement later in life, Ghomrawi said, which is typically a much more difficult surgery with a poorer outcome than the original.
An objective algorithm
The study, published Monday in the Journal of Bone and Joint Surgery, followed over 8,000 people with symptoms of knee osteoarthritis for up to eight years.
While other studies have looked at people who underwent the knife, this study is believed to be the first to examine the timeliness of knee replacement among people who might benefit from the procedure, Ghomrawi said.
The study applied an objective measure to determine the "ideal timing" of knee replacement. It used an algorithm first developed in Europe in 2003, then updated in 2014 by Virginia Commonwealth researchers who analyzed data from a smaller study of 200 people and found a third had surgery too early.
"There are 16 unique combinations that can be assigned based on age, knee stability, and whether the patient has slight, moderate, intense or severe pain," Ghomrawi said.
Knee stability is defined as the not only the ability to bend, but also how "wiggly" the knee is due to loose tendons, and also takes into account clicking and grinding sounds.
In addition, the measurement looks at the severity of the osteoarthritis on X-rays -- "if it's bone on bone" -- as well as how many parts of the knee are affected: the femur (thigh bone), tibia (shin bone), and patella (kneecap).
After factoring all of these elements, Ghomrawi and his team assigned patients in the study to three categories: timely -- they had the surgery within two years of the replacement becoming potentially appropriate; delayed -- no surgery or surgery that waited until after those two years; and premature.
The cost of premature surgery
This isn't the first study to try and apply an objective criteria to what has been a traditionally subjective conversation between a patient and doctor. The UK's National Health Service commissioned a study last year to see if they could apply objective measures to the decision.
The effort is partly driven by cost -- In the UK the cost can range from 11,000 pounds ($14,300) to 15,000 pounds ($19,467) and, according to a 2015 study, if there are complications or the surgery must be redone, it can rise to 75,000 pounds ($97,313).
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.
Can an objective algorithm work?
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."
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