What's wrong with my knees?
September 21, 2016 9:26 AM   Subscribe

My knees showed up yellow on the airport scan today. I swear I'm not smuggling anything. What gives?

I went through airport security this morning and, for the first time, through one of those new body scanners. The lady waiting on the other side gestured me towards her and asked about my knees, and really, they showed up yellow on the scan, one slightly more so than the other. No other part of my body did.
They didn't find anything (of course) and let me through - I didn't even have to take my tights off. The lady said, in a charming rough Berlin tone, that if she were me, she'd be worried about my knees showing up weird.

In February, at the emergency room, when the doctors did some tests on me to determine brain function, they looked at my knees and commented that I must have hurt myself as a child. I have absolotuly no memory at all of any bigger injuries, and my parents confirmed that nothing more than the usual falling-from-bike injury ever occured. I'm guessing the doctor made that comment because of white pieces of skin on my knees. They're not scars, as far as I see them, because they're on the sides of my knees as well and not raised at all, just white. The biggest one is smaller than my smallest toe's nail. I have smaller white dots on my palms as well, and some slightly bigger ones on my arms, which I only discovered after getting a sunburn this year and those patches alone stayed white. I'm guessing they're some kind of vitiligo considering I have some thyroid weirdness.

What could this be, should I worry about it and if so, what kind of doctor should I see?
posted by LoonyLovegood to Health & Fitness (16 answers total) 1 user marked this as a favorite
My hips routinely trigger this kind of scanner and I can only assume it's the way my body is distributed - I don't have any plates, metal or significant injuries there and I don't wear belts/anything else with metal through the machines.

No one's ever mentioned it during pat-down as anything other than a weird quirk/false positive tendency of the machines, though, so the comments from the lady on the other side isn't necessarily something you need to act on if your knees feel otherwise okay.
posted by terretu at 9:39 AM on September 21, 2016 [1 favorite]

Random parts of me show up as yellow on the scanners, despite wearing simple clothing when I fly and emptying my pockets. I would not put too much credence in such a scan - they use relatively clumsy software to obfuscate the details of the body to indicate zones to possibly pat down. The scanners are designed to look outside the body rather than within it.
posted by Candleman at 9:40 AM on September 21, 2016

I've had random body parts show up on those scanners and there's nothing inorganic in there. My shin was yellow last time I went through Germany despite having totally bare legs. And actually, German airports are the worst, all the scanners etc are turned up to the max (particularly when there aren't many people around so they're bored). My bra underwire sets off the metal detector at the Hannover airport every time for example.

I wouldn't worry about it even a tiny bit.
posted by shelleycat at 9:40 AM on September 21, 2016 [7 favorites]

Those millimeter wave scanners throw up false positives constantly - I get the yellow dot about half the time I go through. They're thrown off by random folds of clothing, standing off-center in the machine, unusually bony wrists, etc. They don't see past the skin and they don't diagnose disease, so I wouldn't worry about this at all.
posted by theodolite at 9:40 AM on September 21, 2016 [3 favorites]

I actually do have metal implants in my body, and when I trigger the scanners (which happens maybe ΒΌ of the time), it's not where I have the metal.
posted by adamrice at 9:42 AM on September 21, 2016

Water (i.e. sweat) will often trigger the scanner. I get patted down every time I fly if it was warm outside while I was lugging the luggage. Honestly I wouldn't worry about it.
posted by restless_nomad at 9:42 AM on September 21, 2016 [1 favorite]

Runners sometimes get weird scan results at the airport as well:


It can be as simple as unusual bone density.

The color on the scanner isn't really meaningful because the output of the xray machines is greyscale. So there must be some sort of software interpretation going on intended to flag the attention of the TSA worker to anomalies.
posted by srboisvert at 9:43 AM on September 21, 2016

I wouldn't place any medical value in the results of an airport scanner, especially in a one-time thing.
I'd also not place any medical value in an odd comment from a security worker.

I've had my knee light up, my elbow, my ankle, etc.

It's not clear what type of device was used on you, but anything that "lights up" a body part is not presenting the results of an imaging scan to the security guards.

Rather, the machines are doing lots of sophisticated and opaque processing to decide if and when to light up a region. They attempt to light up a region based on a threat model, not due to any medical or physiological features.

Any such machine has rather high false positive rates. For example, "In Germany, the false positive rate [for millimeter wave scanning] was 54 percent".

Here are some additional articles relating to the high rates of false positives and generally random nonsense going on in airport security screening. There's just a lot of arbitrariness.

You were tagged by a device designed for security (and perhaps security theater).
You were not scanned medical imaging device.

If you have concern about the health of your knees, you could seek an orthopedic specialist, many of whom specialize in knees/hips/elbows. Such a doctor can also refer you to a dermatologist, if they feel a skin issue may be present.
posted by SaltySalticid at 9:48 AM on September 21, 2016 [2 favorites]

My upper left arm (never the right) apparently makes those machines think I'm smuggling something. It's my travel day dose of humiliation to have to explain that no, I'm just fat.
posted by cecic at 9:56 AM on September 21, 2016 [1 favorite]

Thanks, everyone! Germans are really paranoid - so paranoid they'd let an off comment from a security worker freak them out.
That said, I should probably tell my endocrinologist about the weird white patches and hope they are the part of thyroid symptoms that she does know about...
posted by LoonyLovegood at 9:59 AM on September 21, 2016 [3 favorites]

I just went through airport security this morning. My right ankle was yellow and all I had on was tight jeans and pressure knee-highs. My co-worker had her wrist flagged as yellow and she didn't have anything on, not even jewelry.
posted by computech_apolloniajames at 10:07 AM on September 21, 2016

Like restless_nomad, I've triggered a false positive because of sweat before.
posted by kevinbelt at 11:40 AM on September 21, 2016

I had to have my right calf, and only there, patted down this morning after my airport scan. Is "scanners are weird (shrug)" the new "cats are weird"?
posted by Sweetie Darling at 1:14 PM on September 21, 2016

I've gotten false positives all over my body. TSA told me "we know why, but we can't tell you".
posted by brujita at 1:34 PM on September 21, 2016 [1 favorite]

A month ago i tested positive for explosives.... also at an airport in Germany. The security lady had to repeat the test very slowly and carefully while two fully armed police officers cut off my escape routes :D She was cussing out the test machines all through this procedure :)

Fortunately it was at Berlin Tegel, where there is absolutely nothing else to do after security, so I didn't mind a bit of entertainment before boarding.
posted by Ender's Friend at 8:18 PM on September 21, 2016

I've had a large tattoo on my thigh trigger a pat-down at airport security!
posted by diffuse at 4:13 AM on September 25, 2016

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