Injections for Knee Pain Are Common — But Do They Work?

Some people swear by them, but results can vary. Here’s what you need to know

Jeanne Dorin McDowell

AARP

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Published April 12, 2024

/ Updated September 06, 2024

Photo Collage: AARP (Source: Getty Images)

When Carole Porter’s* knees gave way to osteoarthritis in her late 50s, she started getting injections of hyaluronic acid (HA) gel every six months. For the next 10 years, Porter says, her knees were fine — until one day the injections stopped working. “I couldn’t walk 100 feet without terrible pain,” says the New York business executive, now 76. Permanent relief ultimately came by way of knee replacements.

Los Angeles architect Steven Ehrlich, 77, finds that every year or so, an injection of platelet-rich plasma (PRP) into his arthritic left knee provides some relief, enough for him to pursue an active lifestyle of golf, paddle tennis and heli-skiing. Although Ehrlich’s osteoarthritis is moderate, he’s relatively pain-free.

Kathy Cope had a very different experience. After suffering with osteoarthritis in her knees for 20 years, she tried HA injections in 2022. Taken over two months, the injections had no effect at all. Cope, 76, a retiree who splits her time between New York and Florida, soon went the knee-replacement route.

About 1 in 7 people diagnosed with knee osteoarthritis will receive some sort of injection to treat the painful, sometimes debilitating, symptoms of the disease. Yet studies in recent years have found that most of these injections actually do very little to improve osteoarthritis in the knees and are only slightly more effective than a placebo.

“They are safe, with minimal downsides; they may be helpful, and we don’t have many other great options,” says Scott Rodeo, M.D., codirector of the Orthopedic Soft Tissue Research Program at Weill Medical College of Cornell University in New York. But medical organizations have pulled back on their endorsements of knee injections.

In its 2021 guidelines, the American Association of Orthopaedic Surgeons (AAOS) “conditionally recommended” the use of one type of injection — corticosteroids — and advised against HA injections. Two other types — PRP and stem cell injections — are not fully approved by the Food and Drug Administration and are regarded as experimental by insurance companies.

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To help you wade (knee-deep) through the confusing world of injections, here’s a rundown of the most common kinds of shots and what the experts say about them.

Corticosteroids

What are they? After weight loss, exercise and anti-inflammatories, corticosteroids are usually the next option for treating knee pain. Corticosteroid shots are a combination of the steroid cortisone and a numbing agent that is injected into the knee for fast, temporary relief during a flare-up.

Do they work? Yes. They’re regarded as the fire extinguisher of knee injections — they usually start working within 48 hours to reduce pain and inflammation around the knee joint. Studies show corticosteroids can effectively provide short-term relief, though some people report success over a longer period.

“I’ve had patients who received injections in their knees every three months for several years,” says Cara Cipriano, M.D., an associate professor of adult reconstruction at the Perelman School of Medicine at the University of Pennsylvania. “Others feel that over time, the effects fade. In these situations, we move on to other treatments.”

What to know: Corticosteroids are typically administered up to four times a year. Although they’re relatively safe and effective, overuse may contribute to further deterioration of knee cartilage. People with diabetes or other metabolic conditions should be aware that they may increase blood sugar levels. Corticosteroid injections are covered by Medicare Part B and other insurance plans.

Hyaluronic acid

What is it? Hyaluronic acid is a natural lubricant that mimics the synovial fluid in a healthy knee joint. Generally given each week for three to five weeks, HA gel — or viscosupplementation — acts like a shock absorber that cushions the space around the knee joint and allows the bones to rub against each other with less pain.

Does it work? Anecdotally, yes. In his practice, Rodeo, head team physician for the New York Giants, says about two-thirds of his patients with mild to moderate knee osteoarthritis get a few months of relief using HA injections, though it takes a few weeks before they feel an improvement. But as research findings on the effectiveness of these injections come under scrutiny, medical experts voice a chorus of skepticism.

“I don’t think hyaluronic works at all to improve anything,” says Boston University professor of medicine David Felson, M.D., who chairs the Osteoarthritis Clinical Trial Network for the Arthritis Foundation. “We don’t have a lot to offer, so sometimes doctors and patients have been sold a bill of goods.”

“HA only remains in the joint a few days after being injected, at which point it’s no longer in the knee. We’re not sure if it works, or how; the research is not strong,” Cipriano says.

What to know: HA injections are covered by Medicare and other health plans, but authorization is usually required.

Platelet-rich plasma

What is it? PRP injections are developed by extracting blood from a person, then separating out the plasma, which is rich in platelets, from other parts through centrifugation. The concentration of platelets in the new mixture can be 5 to 10 times richer than in normal blood. Researchers say the shots promote healing and lessen inflammation, which should reduce pain and swelling.

Does it work? Research is ongoing, but several studies have found PRP to pack a powerful healing punch more effective than HA injections.

“There is good evidence for the use of PRP,” says Kenton Fibel, M.D., a nonoperative orthopedic sports medicine specialist at Cedars-Sinai Kerlan-Jobe Institute in Los Angeles and medical director for the Anaheim Ducks. “The injections last about a year, and pain and function improve.”

But “not all PRP is equal,” cautions Kenneth Mautner, M.D., a professor at Emory University’s Department of Orthopaedic Surgery and head team physician for the Atlanta Hawks. Platelets and growth factors in blood fluctuate based on time of day, recent meals, hormones, exercise and other factors. As a result, the composition of PRP can change day to day, even when it’s the same doctor using blood from the same patient.

What to know: The Arthritis Foundation recommends that PRP injections be done by a specialist, either an orthopedic surgeon or a sports medicine doctor. Ultrasound imaging is often used to help target the precise point in the joint space to insert the needle for maximum effectiveness. The procedure can be performed as a onetime injection or every week for three weeks. Because it’s not fully approved by the FDA, it’s generally not covered by insurance; a series of injections can cost around $2,000.

Stem cell therapy

What is it? In this process, stem cells are extracted from bone marrow or fat, then injected into an arthritic knee. Theoretically, the stem cells will promote tissue regeneration in the joint, though laboratory tests have shown that few stem cells survive or remain in the joint after being injected.

Does it work? The efficacy of stem cell injections to treat knee osteoarthritis has not been proven; benefits reported by patients may be the result of anti-inflammatory agents released from the bone marrow or fat.

“Injection therapies are not aimed at regenerating anything but rather trying to reduce inflammation and slow the progression of the disease,” says Andrew I. Spitzer, clinical chief of orthopedic surgery at Cedars-Sinai Medical Center in Los Angeles. “Injection therapies do not regenerate cartilage or change the course of osteoarthritis.”

What to know: The process for harvesting bone marrow for stem cells is carried out in a procedure room, not a doctor’s office, which makes it a less accessible treatment. Stem cell injections are expensive and not covered by insurance. An injection typically costs at least $3,000, which makes it an unaffordable option for many people.

The decision to try knee injections should be made in concert with a knowledgeable doctor and with the understanding that they may not work. In the next few years, researchers predict that more effective knee injections and effective regenerative injections will likely be refined or discovered.

“For now, injections all fit in the ‘might help, won’t hurt’ category,” Rodeo says. “We need better solutions.”

*Some names have been changed for privacy reasons.

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Jeanne Dorin McDowell writes about health, wellness culture and social issues for national print and digital publications. She was a correspondent for Time magazine for 20 years.