Injection infection? Another surprising way to freeze a shoulder

Back in February, I wrote about infection as a possible cause of frozen shoulder: a good example of an unexpected mechanism for a common painful problem, a slow-burn infection of the joint with Propionibacterium acnes, introduced by surgery. P. acnes is endemic on our skin and in our pores, tough to get rid of entirely, so even a properly disinfected incision site isn’t always P. acnes free.

The causality isn’t proven, but it does rest on some evidence (previously discussed).

If surgery can do it, then there’s probably another way for P. acnes to catch a ride into the joint capsule…

Shoulder injury related to injections

The number one candidate for a non-surgical mechanism of infection is injection. While injections cause a lot fewer complications than surgery, anything that breaks the skin is still “invasive” and involves some infection risk. The most common injections are vaccinations, which is why this phenomenon is often referred to as “shoulder injury related to vaccine administration” (SIRVA). But this has nothing to do with vaccination — this is about needles, not what’s in them, and the last thing the world needs now is more demonization of vaccinations. So let’s not call it SIRVA, because that’s a bullshit term that seems to go out of its way to emphasize the wrong thing.

Most injections are in the shoulder, because most injections are intramuscular, and the shoulder is a convenient spot with a bunch of muscle. But sometimes shoulder injections miss: the needle goes into the joint capsule instead of the muscle tissue. Oops. Within a day or two, it becomes clear that the shoulder is much more sore than it should be. The symptoms may drag on and on: “Patients will often visit their physicians months later because they are not able to carry out daily tasks that were possible before the vaccination” (Bancsi). “The prolonged pain and stiffness…is distinct—in other words, much worse—than typical soreness from shots” (Zhang). The conventional wisdom is that this is an ordinary minor injury, which is often mistaken for conditions like bursitis, rotator cuff tears, and adhesive capsulitis. Bancsi et al.:

During physical examination and on ultrasound scan, SIRVA will not appear to be any different from routine shoulder injuries. The only difference is that the shoulder symptoms will have started within days of a vaccination.

But is that the “only difference”? It’s not really normal for a minor physical trauma to drag on for weeks and months. It seems like something more might be going on here.

Unfortunately, it’s plausible that injections can trigger genuine adhesive capsulitis by the mechanism described above: the needle drags P. acnes into the joint capsule, just like a surgery can, and then excessive and prolonged inflammation ensues, more than minor physical trauma would ever cause, creating a genuine case of frozen shoulder.

Citation needed, but where there’s smoke…

Injection injuries are certainly a real thing, and frozen shoulder is a real thing, but do they overlap? Is there any evidence that injection injury specifically can cause frozen shoulder, via the mechanism of P. acnes infection? Not direct evidence, no. This is the closest we’ve got: a 2015 paper reporting three cases of injection injury leading to a confirmed diagnosis of frozen shoulder (Saleh).

But this is also about as where-there’s-smoke-there’s-fire as you can get. Infection as a complication of both surgery and injection are plausible, and there’s good evidence of the first. And it’s also plausible that such infection could lead to frozen shoulder, and there’s also both direct and indirect evidence for that (again, see previous post). From those premises, it’s just a short, reasonable hop to the hypothesis that injection injuries can cause frozen shoulder: far from proven, definitely possible.

Injection safety

The next time I get an injection — probably a vaccination, which I obviously will do, because the benefits dramatically outweigh the risks — I’m going to drown my shoulder in rubbing alcohol immediately beforehand. I want exactly zero surviving P. Acnes on my skin when that needle goes in!

“I say we take off & nuke the entire site from orbit. It’s the only way to be sure.”

Ellen Ripley, Aliens

Or just don’t get shots in the shoulder! Surgeon Dr. Michael Skyhar: “This is all avoidable by simply asking that the [injection] be placed in a different location, such as the upper/outer buttocks.”

I’ve updated my frozen shoulder tutorial with this strange and troubling idea.