Why Do Joints Ache When It Rains?

Last year, 31 years of treating my body like a rental car finally caught up to me.  Unexpectedly though, it was one of my hip joints and not my liver that crapped out.  From what I understand, years of bike riding and a malformed femur bone had gradually mashed the cartilage in my hip joint into a very painful paste.  After trying a bunch of stuff that didn't work for six months, I eventually had to have arthroscopic surgery to do something about the bits of dangling flesh that were getting pinched whenever my hip joint moved.  One painful operation, three months of physical therapy, and an assload of painkillers later I'm mostly fine again, with one exception: the joint aches like hell when it's raining or about to rain.

If I undestood the nice doctor correctly, the labrum (latin for "horrible pain flesh") had to be sewn or trimmed back so the femur would stop squishing the edge of it.  Don't get old, kids.  

Everyone's heard old people complain about this, but old people love to complain about all kinds of things so I never took it that seriously.  Thinking about it a little bit though, it's very weird-- common sense would dictate that it's either the barometric pressure drop or spike in relative humidity you get with a rainstorm that causes the joint pain, but I'm at a loss to explain why either of those would do it.  There also seems to be a time-delay effect at work-- right now I'm sitting two stories underground, in a clean room with its temperature, pressure, and humidity regulated to the second decimal place, and somehow my hip still seems to know it's raining out.  

It doesn't make sense that humidity would have any effect on a joint-- your body is mostly water and your joint is buried deep in your body and behind your somewhat-impermeable-to-water skin, so it's hard to imagine a slight increase in the partial pressure of water vapor in the air being something a joint could "feel." Barometric pressure is a little bit more believable-- a change in the pressure differential between the inside and outside of your body could cause slight decompression or compression of the joint.  Not much, but since joint tissue is apparently made entirely of angry nerves, it might be enough to feel.

The first article in my Google search results for this question was on WebMD.  Given that, I expected it to blame weather-related joint pain on HIV, since that seems to be their explanation for most other things that can go wrong with your body, but the article was actually pretty decent.  Apparently, the jury is still out on exactly what causes weather-based joint pain; it's one of those things that has loads of anecdotal evidence but not enough incentive to actually bother studying in detail (kind of like baby colic, the subject of a future post).

The article pretty much agrees with me that it's got to be a pressure thing.  Barometric pressure is a pretty good leading indicator of weather; you can often detect a front coming through by a drop in pressure long before any physical indicators of it, like rain clouds, become obvious.  As mentioned above, a drop in ambient pressure is going to change the pressure differential between the inside of your body and the environment, which means your body will expand outward like a balloon very slightly (the extreme case of this would be explosive decompression in space, or that experiment where you stick a knotted balloon in a vacuum chamber).  In this case, the expansion is very, very minor.  Like I said though, damaged joints are basically tiny globs of connective tissue and hate; even a slight change that causes a tiny pressure to be put on the tissue is enough to cause noticeable discomfort.

One thing I've learned as the owner and operator of a bona fide Bad Hip is that once something makes it hurt, it's going to keep hurting for awhile.  Anything that causes joint pain is also going to cause inflammation of the tissue, which makes it come into more contact with the surrounding bone, which makes it hurt worse, which makes it get inflamed more, etc.  That's probably the explanation for why, even down here in my little climate-controlled hole in the ground, my leg still knows it's raining-- it had 12 hours at "rainstorm" pressure to get good and inflamed, and it's damned well not going to stop doing that just because the original cause is no longer present.  Thanks, leg!

WebMD has listed a number of helpful "solutions" to this problem, including "Apply a heating pad to the joint" and "Try to improve your mood".  While I'm sure sitting at work with a heating pad jammed down the front of my pants would help the pain, the resulting sexual harassment lawsuit would probably drop my mood down a couple of notches and I'd be back at square one, so that's not that useful.  I've found anti-inflammatories like aspirin help a little, but mostly the cure seems to be just waiting it out (luckily it doesn't seem to rain more than two or three times a year in Minnesota now).  In conclusion, being old sucks and if any of you have a bionic leg you're looking to sell, drop me a line.