The snap, crackle and pop of cracking knuckles really annoys some people. But is there a danger besides making a person unpopular among friends and coworkers?
Let's compare the knuckle-cracking process to the condition of arthritis. Joints are surrounded by a lubricating, nourishing soup called synovial fluid. This fluid contains fats, nutrients to maintain bone health and dissolved gases. Cracking the knuckles stretches the capsule that surrounds the fluid, and stretching the capsule increases its volume, which lessens the pressure inside. The gases expand under this lower pressure, forming bubbles that eventually pop. Fortunately for knuckle-cracking haters, it takes a while for the gases to reenter the solution, during which time the characteristic noises can't be made.
Arthritis -- specifically osteoarthritis -- is the degeneration of cartilage, which is the spongy cushion between bones in a joint. The exact cause isn't known, but injury, repetitive motion, genetics and increased stress on the joints from excessive weight are contributing factors. In theory, straining or breaking a knuckle while cracking it might put you at risk for arthritis, but no study has ever linked cracking knuckles directly with the disease.
Knuckles and other joints may also crack spontaneously due to natural changes in the ligament path over the bones. Cracking accompanied by pain, however, can signify other inflammatory conditions, including tendinitis and bursitis.