What causes the popping sound?
Kiffin's blog quotes a pretty good answer on this question.
The original website is unfortunately gone.
The mechanism by which clicking noises can be produced by extreme pulling, twisting, flexion, or extension of joints is well established. When a joint is deformed in this way, the pressure in the joint space decreased, and a CO2 filled cavity forms in the synovial fluid. The pressure in the cavity is lower than that in the surrounding fluid, so the fluid quickly rushes into the cavity. This sudden implosion of the cavity is thought to be what causes the distasteful cracking sound. Interestingly, tiny bubbles of CO2 remain in the synovial fluid, taking about 15 minutes to be reabsorbed. This explains why a knuckle cannot be recracked immediately.
I found another interesting article on this question which also takes tendons and liagments into account.
The following was posted on October 26, 2001 by Raymond Brodeur in the Ergonomics Research Laboratory at Michigan State University
To understand what happens when you "crack" your knuckles, or any other joint, first you need a little background about the nature of the joints of the body. The type of joints that you can most easily "pop" or "crack" are the diarthrodial joints. These are your most typical joints. They consist of two bones that contact each other at their cartilage surfaces; the cartilage surfaces are surrounded by a joint capsule. Inside the joint capsule is a lubricant, known as synovial fluid, which also serves as a source of nutrients for the cells that maintain the joint cartilage. In addition, the synovial fluid contains dissolved gases, including oxygen, nitrogen and carbon dioxide.
The easiest joints to pop are the ones in your fingers (the interphalangeal and the metacarpophalangeal joints). As the joint capsule stretches, its expansion is limited by a number of factors. When small forces are applied to the joint, one factor that limits the motion is the volume of the joint. That volume is set by the amount of synovial fluid contained in the joint. The synovial fluid cannot expand unless the pressure inside the capsule drops to a point at which the dissolved gases can escape the solution; when the gases come out of solution, they increase the volume and hence the mobility of the joint.
The cracking or popping sound is thought to be caused by the gases rapidly coming out of solution, allowing the capsule to stretch a little further. The stretching of the joint is soon thereafter limited by the length of the capsule. If you take an x-ray of the joint after cracking, you can see a gas bubble inside the joint. This gas increases the joint volume by 15 to 20 percent; it consists mostly (about 80 percent) of carbon dioxide. The joint cannot be cracked again until the gases have dissolved back into the synovial fluid, which explains why you cannot crack the same knuckle repeatedly.
But how can releasing such a small quantity of gas cause so much noise? There is no good answer for this question. Researchers have estimated the energy levels of the sound by using accelerometers to measure the vibrations caused during joint popping. The amounts of energy involved are very small, on the order of 0.1 milli-joule per cubic millimeter. Studies have also shown that there are two sound peaks during knuckle cracking, but the causes of these peaks are unknown. It is likely that the first sound is related to the gas dissolving out of solution, whereas the second sound is caused by the capsule reaching its length limit.
A common, related question is, Does popping a joint cause any damage? There are actually few scientific data available on this topic. One study found no correlation between knuckle cracking and osteoarthritis in the finger joints. Another study, however, showed that repetitive knuckle cracking may affect the soft tissue surrounding the joint. Also, the habit tends to cause an increase in hand swelling and a decrease in the grip strength of the hand.
Another source of popping and cracking sounds is the tendons and ligaments near the joint. Tendons must cross at least one joint in order to cause motion. But when a joint moves, the tendon's position with respect to the joint is forced to change. It is not uncommon for a tendon to shift to a slightly different position, followed by a sudden snap as the tendon returns to its original location with respect to the joint. These noises are often heard in the knee and ankle joints when standing up from a seated position or when walking up or down the stairs.
Your joints can make a variety of sounds: popping, cracking, grinding, and snapping. The joints that "crack" are the knuckles, knees, ankles, back, and neck. There are different reasons why these joints "sound off".
Escaping gases: Scientists explain that synovial fluid present in your joints acts as a lubricant. The fluid contains the gases oxygen, nitrogen, and carbon dioxide. When you pop or crack a joint, you stretch the joint capsule. Gas is rapidly released, which forms bubbles. In order to crack the same knuckle again, you have to wait until the gases return to the synovial fluid.
Movement of joints, tendons and ligaments: When a joint moves, the tendons position changes and moves slightly out of place. You may hear a snapping sound as the tendon returns to its original position. In addition, your ligaments may tighten as you move your joints. This commonly occurs in your knee or ankle, and can make a cracking sound.
Rough surfaces: Arthritic joints make sounds caused by the loss of smooth cartilage and the roughness of the joint surface.
Hah, we already proved this one wrong. Many of us can crack many more joints than those listed! :lol:
Bud rang in to ask, "What happens when you crack your joints, and is it bad for you?"
Some scientists wanted to learn more about knuckle cracking, so they actually stuck a sensitive microphone onto a finger. They found that there wasn't just one single sound when you cracked a finger joint - there were actually two separate sounds. The joint space is the space between the bones. There is a liquid in this space, and there are ligaments on each side, holding the bones together. As you pull on the joint, you first drop the pressure in the joint space - and the ligaments get sucked in. Once this pressure gets low enough, a bubble pops into existence - making a popping sound, which is the first of the two sounds.
Now this bubble has a certain size - on average, about 15% of the now-bigger joint space. Because the joint space suddenly has a bubble in it, the liquid, just as suddenly, pushes on the ligaments - snapping them back to their original position. This snapping back of the ligaments is the second sound.
The energy set loose inside the joint is only about 7% of what you need to damage the cartilage. But if you crack your knuckles often enough, you can end up with swollen ligaments.
Another study looked at 300 people who had been cracking knuckle joints for 35 years. They had slightly swollen joints (which is no big deal). But the real surprise was that their hands were weaker - their grip strength was one quarter as strong as it should been!
So cracking your knuckles won't bother you in the short term, but 35 years from now, you might not be able to open a jar of Vegemite!
The following references are given to investigate this further:
o Jearl Walker, The Flying Circus of Physics, by Jearl Walker, ISBN 0-471-02984-x, 1975, p. 226. Jorge Castellanos & David Axelrod, 'Effect of habitual knuckle cracking on hand function', Annals of Rheumatic Diseases, Vol. 49, 1990, pp. 308-309.
o Sam W. Wiesel, M.D. etal, 'Occipitoatlantal Hypermobility', Spine, Vol. 4, May/June 1979, pp. 187-189.
o John F. Rothrock, M.D., 'Vertebral artery occlusion and stroke from cervical self-manipulation', Neurology, October 1991, p. 1696. Raymond Brodeur, D.C., Ph.D., 'The Audible Release Associated with Joint Manipulation', Journal of Manipulative and Physiological Therapeutics, Vol. 18, No. 3, March/April, 1995, pp. 155-164.
Another article by the Orthopaedic Surgery and Sports Medicine at the University of Washington on multiple causes for joint popping sounds:
Joints can make different noises–some are serious and some are not.
**Some people learn how to "pop their knuckles." By pushing or pulling a joint in a certain way, an air bubble can suddenly appear in the joint with a "pop." Once the bubble is there the joint cannot be popped again until the air has been reabsorbed.
Some joints crack as the ligaments and tendons that pass over them slide past bumps on the bones. Individuals who "crack their neck" make noise in this way.**
Other joints lock up intermittently–often with a loud pop--because something gets caught in between the joint surfaces. A torn cartilage in the knee or a loose piece of bone or cartilage in the joint can do this. Once a joint is stuck in this way, it may need to be wiggled around to unlock it. This may also cause a pop.
Finally, joints that are arthritic may crack and grind. These noises usually occur each time the joint is moved. This noise is due to the roughness of the joint surface due to loss of the smooth cartilage.
I took the liberty to mark the two types in bold which are the common ones in this community.
Wow good links JointCracker, thanks