Jennifer Stacey was 13 when she first noticed the clicking sound on the left side of her jaw whenever she ate. Twenty-four years later, it’s still clicking. “My jaw gets tired and sore if I eat something that requires a lot of chewing,” she says. “It feels like an overworked muscle, a dull ache.”

Stacey, an airport security badging coordinator who lives in Vancouver, Wash., suffers from temporomandibular joint and muscle disorder, known as TMD (also often called TMJ), which impairs the jaw joint and the muscles that control it, causing pain as well as embarrassment. It can affect one or both sides of the jaw.

It’s rarely serious, but it can be debilitating if it becomes chronic, as it is with Stacey. “When I open wide — for example, at the dentist — it can be quite painful,” she says.

Her father had it, too. The clicking “was so loud when he would eat that I would sometimes have to leave the room because it irritated me so much,” she says. After she developed it, “I’d actually make excuses to not eat in front of people.”

The temporomandibular joint connects the lower jaw (or mandible) to the temporal bone at the side of the head. The jaw joint’s position and movement are controlled by muscles that surround it. Opening the mouth causes the rounded ends of the lower jaw (known as condyles) to glide along the joint socket of the temporal bone. They slide back when we close our mouths.

Why the click? A soft cartilage disk cushions the two bone surfaces. In people with TMD, the disk often is out of its normal position. When the mouth is opened, the disk snaps back into its normal location, causing a click; when the mouth is closed, it becomes displaced again, causing another click.

Estimates suggest that more than 10 million Americans have experienced TMD. For most people, this usually means a single episode or a few isolated bouts.

“It’s one of the most common pain disorders, after low back pain and headache,” says John Kusiak, acting deputy director of the National Institute of Dental and Craniofacial Research. “Fortunately, most first-onset cases of TMD will resolve with either no treatment or minimal care.”

About 10 percent of people with TMD go on to develop long-term symptoms that affect the quality of their daily lives, Kusiak says. Experts usually define chronic TMD as consistent pain in the jaw area that lasts beyond three months, he says.

“The jaw is very important for a number of things, including how we eat, for smiling, for talking, for singing and for kissing,” Kusiak says. “People may have difficulty talking, and smiling, difficulty interacting with others. As a result, they may develop emotional and psychological problems that can lead to the inability to work or communicate.”

Scientists don’t know what causes it, although trauma to the jaw or temporomandibular joint is a clear risk factor. Most of the time, TMD develops for no obvious reason.

Because the condition is more common in women, scientists are exploring its possible connection to female hormones. They also are studying possible genetic links, a relationship that would not surprise Stacey. “With my father having it, I’m assuming that’s why I developed it, too,” she says.

Research suggests that TMD risk factors also might include teeth grinding, which can aggravate the joint, smoking and sleep dysfunction — insomnia or sleep apnea, “anything that disturbs the normal cycle of sleep,” Kusiak says — but there is no evidence that “a bite that is off, or constant chewing on one side” causes TMD.

While there is no cure, recommendations to treat symptoms include eating soft foods, applying ice packs to ease pain or swelling, and avoiding extreme jaw movements such as wide yawning, loud singing and gum chewing.

Sometimes, learning relaxation and stress-reduction techniques can help. Short-term use of over-the-counter or prescription pain medications also may provide some relief. There is no evidence that splints or night guards work as a first-line treatment, Kusiak says.

Experts advise against having surgery or other treatments that result in permanent changes in the bite or the jaw, among them crown and bridge work to balance the bite, orthodontics to change the bite, grinding down teeth to bring the bite into balance (occlusal adjustment) and repositioning splints, which permanently change the bite. Total or partial joint replacement can be especially risky, Kusiak says.

“In some cases, it can make the joint worse, and pain and discomfort worse, in that it can induce inflammation that can lead to a lot of serious problems,” he says. Also, “in a minority of cases, it can lead to more-serious complications and multiple surgeries.”

Stacey has learned to live with the disorder. “I’d never been told it was treatable,” she says. “Every time I go to a new dentist, it’s usually the first thing they say to me when I open my mouth — ‘Oh, you have TMJ.’ I’ve learned how to hide it or minimize it while in social settings where food is involved. I chew very slowly and take small bites.”