Specially angled insoles may not do much to relieve pain for people with knee osteoarthritis, suggests a new review of past studies.
Researchers found that across 12 trials, people who used so-called lateral wedge insoles rated their pain about two points lower on a 20-point scale than those who used flat insoles or none at all.
But when the study team looked only at higher quality trials, including ones that accounted for the placebo effect of simply having insoles, any significant benefit went away.
"It doesn't mean there can't be effective treatments that we put in shoes," Dr. David Felson, from the Boston University School of Medicine, said. "There may be research which can identify for us those people who are likely to respond to this treatment, or that develops a newfangled approach to put a wedge in a shoe that might work a little better," said Felson, who worked on the new study.
Heel wedges fit underneath the sole of the shoe and are thicker on the outside than the inside, tilting the foot inward. They are designed to relieve pain caused by a certain kind of knee arthritis by transferring loads to different parts of the joint.
The wedged insoles can be bought over the counter for about $10 or can be specially made.
Because they are easy to use, heel wedges are "an attractive treatment," Matthew Parkes from the University of Manchester Institute of Inflammation and Repair in the UK, who was also part of the study team, said.
"There's a lot of promise to suggest that this should work," he added.
For their analysis, the researchers reviewed 12 studies that looked at the effects of lateral heel wedge insoles or shoes.
Those included 885 people with knee osteoarthritis, 502 of whom were randomly assigned to use the wedges - which typically had an angle of five to six degrees. The other participants either used a flat insole or no insole.
Participants wore their assigned insole or shoe for anywhere from two weeks to two years, depending on the study.
The researchers found any benefits seen with the insoles were in studies that compared wedge insoles to a normal shoe alone, according to findings published in the Journal of the American Medical Association.
"If you give one group nothing and one group something, then you see an effect," Parkes said.
However, the two-point improvement across all studies shrunk to a 0.1-point benefit in the seven trials that compared angled insoles to flat ones. Those studies are best able to account for a possible placebo effect of insoles, researchers said.
Dr. Gillian Hawker, who has studied treatment of osteoarthritis at the University of Toronto, said the results were "not surprising at all."
"As is true with really all non-drug therapies for osteoarthritis… the better the studies get, the larger the trials, unfortunately, the less often they show a significant treatment benefit," she said.
But Hawker, who was not involved in the new research, said that may be because many people with arthritis pain report feeling better with sham treatment.
"One of the explanations for the results is, it's the same as placebo. And placebo works."
Although the researchers said their findings "do not support the use of lateral wedges" for knee arthritis, Hawker believes the insoles are still worth a try.
"The risk with lateral wedges is pretty darn low," she said. And, "In the occasional patient, they will make a big difference."
Felson said one of the more effective conservative treatments for knee osteoarthritis is wearing a knee brace - although those can be bulky and uncomfortable. The other is exercise.
"Exercise has been shown in many studies to be effective for arthritis," he said.