The human body is capable of producing a whole array of peculiar noises. Think about your most recent knuckle cracking experience - you manipulate your finger, hear a pop, and enjoy the feeling of relief. That popping sound you heard was your joint passing gas! Knuckles produce that cracking sound when gas bubbles inside the joint burst.
Joints are covered by a layer of tissue called the joint capsule. The joint space (area within the joint capsule) contains synovial fluid, which lubricates the joint and provides nutrients for the adjacent bone surfaces. The synovial fluid contains dissolved gases. When you stretch or bend your fingers to crack your knuckle, the bones of the joint pull apart and stretch the joint capsule. When the joint capsule is stretched, the volume inside the joint space is increased, creating a vacuum. This causes dissolved gases in the joint fluid to become less soluble and form bubbles. When these bubbles burst, they produce the popping sound associated with knuckle cracking. It takes about 30 min for the gases to re-dissolve into the synovial fluid, which is why the same knuckle cannot be re-cracked immediately.
A common question that my patients ask is, “Does knuckle cracking cause arthritis?” The answer is no. In limited studies, there is no evidence to suggest habitual knuckle cracking as the cause of arthritis. However, experiencing pain during knuckle cracking may be due to a dislocating tendon, ligament injury, or arthritis.