What happens when you crack your knuckles?

Does the sound people hear when someone cracks their knuckles come from a bubble popping that existed in the joint or from a bubble being created? That was the question researchers answered during an annual meeting of the Radiological Society of North America.

To study this, researchers examined 40 men and women 18 to 63 years old, some avid knuckle-crackers and others who reported never intentionally cracking their own knuckles. A sonographer recorded video images as participants tried to crack their knuckles at the base of each finger. Static images of the knuckle before and after participants attempted to crack the joint were also captured.

“We’re confident that the cracking sound and bright flash on the ultrasound are related to the dynamic changes in pressure associated with a gas bubble in the joint,” said Dr. Robert D. Boutin, professor of radiology at University of California, Davis Health System, who compared the bright flash to a firework exploding.

Researchers said further exploration is needed to determine whether the cracking sound or the flash of light come first.

Dr. Harun Durudogan, an orthopedic surgeon at Advocate Christ Medical Center in Oak Lawn, Ill., describes what is going on when people crack their knuckles.

“Around each joint, a capsule forms to contain fluid,” says Dr. Durudogan.“This synovial fluid lubricates the joint and cushions the cartilage on either side. Multiple gasses, such as oxygen, nitrogen and carbon dioxide, are found within this fluid. When a joint is extended, the gases form a bubble. Rapid collapse of the gas bubble is believed to be the audible popping sound.”

While many people crack their knuckles for a sense of relief or as a habitual act, Dr. Durudogan says others refrain from doing the habit out of a dislike for the sound or because they’re worried about the long-term effects of the action.

The study didn’t analyze long-term effects, but no immediate pain, swelling or disability was evident in the participants who cracked their knuckles.

Dr. Durudogan says joints can make other sounds as well, which involve crepitus — a grating sound caused by friction between bone and cartilage, tendon snapping and clicking.

“While the popping of your knuckles appears to be a normal physiological phenomena that does not require specific treatment, sounds from your joints that involve pain should be evaluated and possibly treated,” he says.

Dr. Durudogan offers the following tips for maintaining healthy joints:

• Contact your physician when joint sounds are associated with pain, swelling, locking or decreased movement.
• Stay at a healthy weight in order to keep from overloading specific body parts. This will lessen the pressure on your joints.
• Support your joints by doing low-impact exercises with stretching and high rep, low weight strengthening exercises.
• Focus on prevention, as it is often more important than waiting for a problem to develop that requires treatment.