Updated on June 20, 2024.
Knee replacement surgeries are on the rise in the United States. For people younger than 65, the main reason behind the trend appears to be obesity, a condition which is also increasing in prevalence.
Learn more about knee replacement surgery, who might be at higher risk for needing it, and what you can do to lower your chances of having the procedure.
What is knee replacement surgery?
Knee replacement is a surgery that replaces all or part of your knee joint. It’s a type of arthroplasty, or joint replacement surgery. Other types of arthroplasty include hip or shoulder replacements.
During the surgery, a surgeon will remove the damaged parts of the knee joint, including cartilage and bone. They will then replace those damaged parts with artificial joint parts or an entire artificial joint, usually made of plastic or metal. To replace torn or damaged cartilage, a surgeon will insert a device called a spacer made of plastic. (Cartilage is the spongy tissue that provides cushion in joints between bones.) A surgeon may also reshape the kneecap so that everything fits well together.
Knee replacement surgeries are fairly fast procedures, taking only about one hour. They’re generally effective and long-lasting. Almost all people who have had the surgery enjoy improved knee function for more than a decade.
Why would someone get knee replacement surgery?
People usually have this surgery when they experience stiffness, swelling, pain, and mobility issues in the knee joint. Surgery is usually prescribed when nonsurgical approaches—such as medication, physical therapy, weight loss, and more minor procedures—haven’t been able to help.
Osteoarthritis (OA) is the most common condition that contributes to the need for surgery. It’s a form of arthritis caused by wear and tear on joints. Other conditions that may lead to surgery include rheumatoid arthritis (RA), bone fractures, or other traumas to the knee, such as those from an accident or injury. The goal of the surgery is to reduce disability and to allow people to be active again without pain.
Risk factors for knee arthritis and knee replacement surgery
There are several risk factors for knee arthritis, according to the Centers for Disease Control and Prevention (CDC). These coincide with risk factors for needing knee replacement surgery. Some are risk factors you cannot control, such as:
- Age: The risk of OA goes up with age.
- Sex: Women and people assigned female at birth are more likely to have OA, particularly beyond age 50.
- Race: Knee OA appears in higher numbers among Black people.
- Genetics: Those with a family history of OA are at higher risk of having it.
- Socioeconomic status: Knee OA appears in higher numbers in people with fewer economic resources.
- Preexisting OA: Having OA in other parts of the body, particularly in the hand, increases the risk of developing it in the knees.
- Medical history: Past joint injuries can raise the risk of OA.
There are, however, several risk factors you may be able to control. You can help reduce your risk of knee pain by:
- Avoiding repetitive stress on the knee or overusing the joint
- Getting infections checked out by a healthcare provider (HCP) right away. (Certain infections can cause joints to swell and become inflamed. These include Lyme disease, staphylococcus infection, and gonorrhea.)
- Making modifications to your work habits if your job involves repeatedly squatting and bending your knees
Having obesity also multiplies your overall risk of knee OA by four to five times. This makes it important to work with your HCP to try to reach and maintain a healthy weight as much as possible.
Obesity plays a large role in knee arthritis
The number one modifiable risk factor for knee arthritis is overweight or obesity. Carrying excess weight makes people more likely to develop knee OA. It can also worsen OA for people who’ve already developed it.
When a person is carrying excess weight, more stress is placed on their joints. This is especially true for their hips and knees, which do a lot of the work of holding up the weight of the body.
Walking on a flat surface puts the force of 1.5 times your body weight on your knees. Walking up or down stairs can cause pressure up to 3 times your body weight. When you squat down to pick something up from the ground or put on a shoe, it causes your knees to take on the pressure of up to 5 times your body weight. All this pressure adds to wear and tear on the knees over time.
Weight gain can also cause inflammation in the body, potentially causing additional stress on the joints.
Why are knee replacement surgeries on the rise?
The number of total knee replacement surgeries for people 65 years and older insured by Medicare in the U.S. increased by about 156 percent on average between the years 2000 and 2019. That’s according to a 2023 study published in JBJS Open Access using information from the Medicare database.
That same study predicts that by the year 2060, the population of people aged 65 and older who will need knee replacement will have increased by 469 percent. This figure doesn’t include people who are not on Medicare or who are under age 65, so the actual total number of surgeries, both past and forecast, is likely much larger.
Why are these surgeries increasing so dramatically?
As with knee arthritis, obesity is a major reason. Research has repeatedly drawn a clear link between obesity and needing knee surgery. For example, in a 2022 study published in ANZ Journal of Surgery, researchers found that more than half of 56,000 knee surgery recipients had obesity and nearly 32 percent were overweight but not obese.
In a 2022 systematic review published in Diabetology & Metabolic Syndrome, researchers studied the risks of developing weight-related complications, depending on body weight. They found evidence that the risk of total knee surgery went up in people with obesity and those who were overweight, alongside other known risks like cardiovascular disease or diabetes.
Rising obesity rates
Obesity is on the rise, which suggests more knee replacement surgeries. According to the CDC, between 1999 and 2020, the prevalence of obesity in the U.S. went up by more than 10 percent, from 30.5 percent to 41.9 percent. The prevalence of severe obesity nearly doubled, from 4.7 percent to 9.2 percent.
Other demographics are changing, too. For example, one study reported that younger patients (those under age 65) who have knee replacement surgery tend to have nearly twice the rate of obesity as those over age 65. Other research has pointed to a rise in younger patients receiving knee replacement surgery.
The demographic outlook seems to point to an ever-growing need for knee replacement across the population. A 2021 study published in Knee Surgery, Sports Traumatology, Arthroscopy predicted that the number of such expected surgeries between 2020 and 2050 will increase by nearly 90 percent. Increasing numbers of patients will be men and more younger patients aged between 50 and 65.
Manage weight to ease pain and keep your knees healthy
Even being 10 pounds overweight can add much more force and pressure to your knees. One way to help avoid knee replacement surgery is by losing weight if you are overweight. According to the Johns Hopkins Arthritis Center, studies are increasingly showing that losing weight can make tangible differences in the risk of developing knee OA, as well as in lessening pain.
To aim for and maintain a healthy weight while keeping your joints fit and flexible, try these tips:
- Exercise. Getting regular exercise can help you maintain a healthy weight. Staying strong and flexible can also help alleviate knee pain.
- Tweak your habits. Making little lifestyle changes here and there can help you drop pounds.
- Get on the mat. Yoga can help you gently stretch out achy joints. Other gentle exercises—such as walking, swimming, or practicing tai chi—can also help you lose weight and reduce pain.
Losing weight and keeping it off is hard. If you’re having trouble maintaining a healthy weight, speak with an HCP. They may recommend other methods, including medications for weight loss, depending on your health profile.
