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What do pimples have in common with frozen shoulders?

 •  • by Paul Ingraham
Weekly nuggets of pain science news and insight, usually 100-300 words, with the occasional longer post. The blog is the “director’s commentary” on the core content of a library of major articles and books about common painful problems and popular treatments. See the blog archives or updates for the whole site.

You can hardly crack a medical journal open these days without stumbling on another example of how wee beasties are guilty of causing unexpected trouble, a factor in yet another condition we didn’t realize was infectious. The canonical example is the role of H. pylori in stomach ulcers, and there have been many others since, like this fresh one: the infamous “plaques” of Alzheimer’s disease may be a form of immune reaction to an invader.

And now those examples may include frozen shoulder, of all things. Apparently sticking a probe into a shoulder can sometimes result in an alien invasion of the joint. “P. acnes was the most prevalent organism,” researchers reported in 2017. Yes, that’s the pimple bacteria, Propionibacterium acnes (although whether it’s actually responsible for acne is an interesting question).

Microscopic view of P. acnes bacterium.

Propionibacterium acnes, a bacterium that does not belong in shoulders.

How does a skin bacteria freeze a shoulder?

P. acnes is a “powerful nonspecific immune stimulant,” which probably does cause some cases of frozen shoulder, failures of prostheses, some arthritis, and perhaps even sciatica. Its role in post-operative infection is probably simply because it’s an endemic skin critter, nearly impossible to completely eliminate from the site of any puncture. Push just a few of them deep into a joint, and they find a new home and cause new kinds of trouble.


The infection is barely detectable at first, develops slowly, and never resembles a classic post-surgical infection. But the inflammation it causes can blossom into frozen shoulder, and then it sucks to be you.

Only surgery?

This seems to only apply to people who’ve had surgery, but I don’t think we can actually rule out the possibility of infection with other mechanisms. Like many conditions involving inexplicable inflammation of tissue, it’s possible that it only looks like the immune system is attacking our own tissues for no reason. How many “autoimmune” diseases are responses to infections we don’t understand yet? Pure speculation on my part — but plausible, I hope, and certainly very intriguing.

This is an abridged excerpt from an update to my guide to frozen shoulder, which is getting close to being launched as a full-blown book.

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