By C. CLAIBORNE RAY
Published: October 28, 2013
The New York Times

Full Story:  http://www.nytimes.com/2013/10/29/science/what-causes-a-neck-to-crack.html?ref=science&_r=2&

Q. Why do necks crack?

A. “Because the joints boil when they are stretched,” said Dr. Christopher J. Visco, director of sports medicine at NewYork-Presbyterian/Columbia University. “It’s true,” he added; the mechanism, involving the release of gas bubbles, “works for any joint in the body surrounded by a capsule, such as knuckles.”

The neck has a series of paired joints, called facet joints, that run up and down each side. Each is filled with a bit of fluid and is surrounded by a capsule.

“When someone bends his or her neck, the capsule surrounding each joint is stretched out,” Dr. Visco said. “This decreases the pressure on the fluid in the joint.”

The lower the pressure on a fluid, the closer it gets to changing from liquid to gas.

“This is called boiling,” Dr. Visco said. “Another term is cavitation.”

“If someone decreases the pressure on any fluid enough, eventually that fluid will release its gas,” he said. “And in the case of the neck pop, you hear a crack as the gas pops out of the fluid.”

Generally, joint cracking is not harmful, Dr. Visco said, but when extreme stretching becomes a habit (to relieve tension, for instance), it can cause irritation of soft tissue or cartilage. And because important nerves and blood vessels run through the neck, amateur neck-cracking is ill advised. A gentle stretching routine is preferable.