Knee Replacement and Revision Surgeries on the Rise
Two new studies examine the changing trends in age at knee replacement surgery and causes for revision surgeries.
Two new studies presented recently at the annual meeting of the American Academy of Orthopaedic Surgeons (AAOS) are adding to the body of knowledge on total knee replacements.
One study examined who’s getting total knee replacements and revision surgeries – that is, a second surgery to make corrections after a first joint replacement surgery fails – and how fast those numbers are growing. The other study zeroed in to find out more about the causes of revision surgeries.
“We observed [in our practice] that joint replacement patients were becoming younger,” says lead author of the first study, Jacob Drew, MD, explaining why he and his colleagues at the University of Massachusetts Medical School (UMMS), in Worcester, undertook the study.
The study provided the proof. The researchers found the number of total knee replacements rose sharply over the past decade, with the greatest increase among people younger than 65. Using the Nationwide Inpatient Sample database, they reviewed hospital discharge data for more than 2 million total knee replacement patients to determine how many knee replacements and revisions were performed between 2000 and 2009. (A separate study, also presented at the AAOS conference found 4.7 million Americans are currently living with artificial knees.)
The researchers found the overall number of total knee replacements performed per year rose from more than 282,000 in 2000 to more than 621,000 in 2009 – an increase of 120 percent. The overall increase is steep, but even more so in younger age groups. While they found an 89 percent increase for those ages 65 to 84, they found a startling 188 percent increase for 45- to 64-year-olds.
Dr. Drew says the increase in total knee replacements isn’t due solely to population growth, although that is one factor. The driving force is a rise in the rate – that is, the percentage – of people in the under-65 age group opting for the procedure.
Although knee replacement surgeries are considered to have a good track record, “new knees” don’t last forever; earlier studies have found that 85 percent of knees last 20 years and the AAOS estimates 10 percent of patients at some point need a revision for one reason or another. The younger the patients are when they have the first surgery and the longer they live afterward, the more likely they will be to need revision surgery.
“We had some theoretical concerns about how young is too young,” says Dr. Drew.
The fast rate of growth of knee surgery among younger patients seems to support the concerns of Dr. Drew and his colleagues: The researchers found the number of revision surgeries increased 133 percent overall during the study period, with a higher rate of revisions among younger than older patients.
Some revision surgeries are relatively minor – for instance, when one component of the implant is exchanged for another. But many entail removing and replacing the entire implant device, the ends of which are affixed to the thigh and shin bones. Revisions are long and complex, require special surgical skills and are rarely as successful as the first operation in terms of restoring normal function and range of motion.
And the reason for increased rates among younger patients? It’s not what many people may think, says senior study author David Ayers, MD, chair of the department of orthopaedics and physical rehabilitation at UMMS.
“People assume that most knee replacement patients are young, active baby boomers who have damaged their knees through sports and other activities. But when we look at the representative set of patients age 45 to 64, they aren’t anything like that; they are actually fairly unhealthy, are more obese and have more comorbidities than the over-65 group,” he explains.
That creates a clinical dilemma. “If a patient has debilitating pain and loss of function at age 47, do we ask him to wait as long as possible before undergoing total knee replacement so the need for revision is less likely? Or do we perform the procedure, relieve the pain and get the person back in the workforce, knowing that he may need a revision 20 years down the road?” asks Dr. Drew. "That question is very difficult to answer because we don’t know what’s going to happen for a particular patient. That unpredictability hampers us in weighing the short- and long-term risks and benefits.”
William Robb, MD, director of the Illinois Bone and Joint Institute, an orthopaedic practice in the Chicago area, agrees with the study findings but thinks some key information is missing.
“This is [hospital discharge] data and doesn’t provide an in-depth understanding of factors such as comorbidities or implant type” – factors that may influence revision rates, he says. “It appears that revision rates are growing in a younger age group, but the value is to focus on understanding why these patients are at increased risk.”
Understanding why total knee replacements fail is exactly what researchers at the University of California, San Francisco (UCSF) set out to do in a separate effort. Using the same database used in the Massachusetts study, they evaluated the reasons for nearly 302,000 revision procedures performed between 2005 and 2010.
Like the Massachusetts group, the San Francisco researchers found a big jump in total number of revision surgeries – from more than 48,000 in 2006 to more than 67,000 in 2010. Revisions were more common in women and in patients ages 65 to 74, and the growth rate of revisions was higher among younger patients.
The most frequent causes of revisions were joint infections (25 percent) and implant loosening (18 percent). Far less common were fractures in the bones around the implant (1.6 percent) and osteolysis (2.9 percent), which occurs when debris generated by the implant damages or destroys surrounding tissue. Most of the revisions (37 percent) required replacement of the full implant device.
Lead study author Kevin Bozic, MD, a professor of orthopaedic surgery and vice-chair of the department of orthopaedic surgery at UCSF, says better implants and better surgical techniques have reduced rates of implant loosening (which happens when the bond between implant and bone fails), but infection rates haven’t improved in decades.
“Infections are the result of both surgical and patient factors,” he explains. “We are working hard to eliminate surgical site infections. We must be meticulous about sterile techniques and minimize the duration of surgery. We are also trying to understand and optimize patient health prior to surgery. We know that smoking, diabetes, obesity and [poor] nutrition increase the risk of infection. In fact, the risk of all complications increases exponentially in patients with a body mass index [BMI] of 40, and even those with a BMI of 30 to 40 are at increased risk.”
Treating implant infections can be challenging, Dr. Bozic says. If the infection is superficial, patients may improve with oral or intravenous antibiotics or a washout of the knee, but more often the implant must be removed and replaced with an antibiotic-impregnated cement spacer. Later, in yet another surgery, the spacer is removed and another implant inserted.
The study showed infections and fractures in the bones around the implant were associated with the longest hospital stays, and fractures were the most expensive complication to treat.
“What we are learning is that a lot of knees have to be replaced, and patients need to be aware of that. Even though there are claims that a prosthetic knee will last forever, it won’t. It will wear out. But we now also know that the implant [itself] isn’t the whole story. Surgeons and patients play a role, too,” Dr. Bozic says.