Hot hands: a surprising test of powered mittens for arthritis

The British Medical Journal gave us a weird Christmas gift! This is a study I didn't think I’d ever see in one of the big, general medical journals: not just a study of heat for hand arthritis, but the unusually practical and folksy option of electrically heated mittens. Power mittens.

“Mittens with inbuilt battery driven heat elements are now commercially available and offer an easy means of delivering heat therapy to patients with hand osteoarthritis in an accessible manner outside of clinics.”

The goal of Bartholdy et al was to give us some evidence on this topic that’s actually good for once, and I think they more or less did it. It’s biggish, rigorous, well-planned. The paper is also open-access and quite readable. This is valuable new science about the most basic and popular of all self-care options for pain: the comfort of warmth.

Obviously hand osteoarthritis is just about as common as painful problems get. It makes life harder for hundreds of millions of people. Although heat is a staple treatment, the evidence is of course super lame — as usual for this field, as my first post of 2025 explained. It’s rare but nice to see a study like this in a top-tier journal, because they often immediately become the best evidence available for many years to come, out-ranking a bunch of junkier studies.

And sometimes they don’t.

This one does have a huge flaw: Bartholdy et al compared hot mittens them to cold ones. Which begs a burning question: who did they think they were fooling? How exactly is a unheated mitten an effective sham for a hot one? Wouldn’t people be rather disappointed to get the cool ones? Or perhaps too enthusiastic about the niceness of the hot ones? Isn’t that a glaring confounder?

Yes, and they knew it:

“It was not possible to mask the participants throughout the trial, which is a major limitation. This is not surprising as heated mittens would become warmer shortly after switching on the heating element, and the control mittens would not, despite the LED lights being active. Unsuccessful masking and use of patient reported outcomes could have led to an overestimation of effects, which is an inherent weakness and challenge when assessing effects of physical treatments in clinical trials.”

But science is messy, flawed data is better than none, and everything else was done quite well. And so let’s crack on…

The heated mittens were far from perfect: "more concentrated heat around the joints would have been preferable. … Developing mittens specifically for people with hand osteoarthritis, with a better, possibly customised fit and easier battery recharging system, might help to enhance the effect of heated mittens. Image by Bartholdy et al. & BMJ, CC BY-NC 4.0.

They measured pain and function in a couple hundred older Danish patients with bad hands. Everyone got a pair of mittens, with or without power, and were asked to wear them for at least 15 minutes/day for six weeks, “preferably when symptoms were worst (typically in the morning).” Another study by Kominami et al showed that it takes about that long to fully heat up. Actual wear times were somewhat longer.

But of course everyone knew what they got! The researchers made a diligent effort to conceal allocation up until the start of the trial, but the jig was up as soon as anyone put them on.

The results

Hot mittens didn’t help. Boooo.

There was no important difference between the groups in pain or function, and what little difference there is probably attributable to the frustrebo effect, more negative reporting from the “frustrated” cold-mitten group … and a more standard enthusiasm-powered placebo for the hot-mitten group.

“Many participants expressed a positive experience from use of heat. It is therefore reasonable to assume that participants allocated to the intervention group were more enthusiastic about the mittens, which also could have contributed to an overestimation of the true effect.”

Note that “a positive experience” is not the same thing as analgesia and boost in function. We see this a lot in the world of pain treatment: things that feel nice and therefore reduce “suffering,” but not pain. It’s easy to mistake sedation, euphoria, and/or a pleasant sensory experience for meaningful pain relief. Suffering and pain overlap and interact, but they aren’t the same thing.

P.S. Just one adverse effect reported: itching. 😜