Raymond Brodeur in the Ergonomics Research Laboratory at Michigan State University responds:
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.