What causes the "pop" when you pop your back?

Matt, you ignorant slut:

I've talked to various experts about what makes the popping sound when you crack your back, and I've gotten a different definitive answer from each of them. So much for expertise among experts. The only guy who gave me a credible answer (an excellent physical therapist) said no one really knows for sure. That's not the answer I was looking for, but it sure beats the nonsense the top MDs in San Diego were feeding me. So, knower of all knowers, what does make the popping sound when you crack your back? Specific details are required. If you try to pass off the old "it's gas escaping" theory (which several big-time MDs claimed was impossible), I want to know the exact composition of that gas and how it gets back in there after I crack my back.

-- Rob (a.k.a. Paco), PB

So, by "excellent" physical therapist you mean the guy who's married to your wife's old college roommate and who, by the way, happens to be a physical therapist? And the only people who actually get to hold conversations with "big-time MDs" are other big-time MDs and perhaps the occasional Porsche dealer. If you were a big-time MD, you wouldn't be asking this question. I rest my case. (And it's pronounced Por-sha, please, pleeeeeez.)

Too bad you don't buy the gas-bag theory. If there is a generally agreed-upon explanation, that's it. I'm sure there are some who say it's bunk, but I couldn't come up with any scientific arguments against it. If your "big-time MDs" would care to submit a paper, we'll consider it. Some of the research into the phenomenon has appeared in journals for chiropractors, which big-time MDs use for coasters, I think.

The first definitive studies date from 1945 and 1971 and were published in the Journal of Anatomy and Annals of Rheumatic Diseases. In one of them, sadistic bioengineers hung weights from people's fingers and measured how much was required to create the crack sound, while they watched X-ray pics of the whole sordid experiment. What they and others have found is that the weight (or pulling on or stretching a joint) separates the bones and elongates the surrounding joint capsule. The capsule is air tight, and internal pressures are slightly sub-atmospheric. Inside the capsule is a joint oil called synovial fluid in which gasses, mostly carbon dioxide, are dissolved. The mechanical force exerted on the joint raises the pressure on the fluid, the bones finally separate rapidly, at which point the gasses are forced out, clumping together in a bubble that can be seen on X ray. The energy released when the gas is forced out causes the popping sound. The process is called cavitation. One reason it's difficult to re-pop your knuckles or your back right away is because it takes perhaps a half hour for the capsule pressure to return to normal and the gasses to redissolve. In the meantime, the science guys say, your joints are remarkably wobbly.

Other types of joint pops, especially in your knees, can be caused by frayed or torn cartilage rubbing across bone, or bones slipping across one another. Thank God, not everyone can be a knuckle-cracker. If your joints are too stiff or already too flexible, it won't work. And joint-cracking doesn't cause arthritis, despite what your mother told you. Okay? Think I got that one in a knock-out. Now I'll get the elves out of your socks.