Jacinta Yeo realised that something was wrong when she noticed that her front teeth had grown squarer and the edges sharper; she connected this to the fact that she seemed to spend all night trying to get comfortable.

"I would lie awake, aware of my face muscles and unable to get my jaw in a position where I ceased to be aware of it, she says. I got tired, tense and kept on getting backaches and headaches."

Then her jaw locked shut. As a Harley Street dentist, Yeo realised that her facial discomfort must be associated with tooth-grinding, which had got so bad that she had dislodged one of the cartilage discs in her jaw joint. Her muscles had locked her jaw to prevent further damage and she was unable to eat or speak.

She sought emergency help from a dental specialist, who gave her muscle relaxants and managed to insert a bite guard that let her teeth find an easy home against each other.

"Then I had somewhere to bite on properly. That reduced the pressure on the joint, so the swelling went down, and that allowed the disc to go back into position. Then the spasms stopped."

That was seven years ago, and Yeo has used a bite guard at night ever since. Her case is dramatic, but not unusual. Nigel Carter, a dentist, can recount cases where the teeth have been ground down to the gums.

But only five to 10 per cent of grinders have symptoms that they notice. The most common symptoms are sore jaw muscles, pain in the jaw joint, referred pain from the muscles to the teeth, restriction in opening the mouth, migraines, neck ache and backache.

"There is evidence of more and more people, particularly young people, grinding their teeth, and this is being put down to increased stress levels," says Dr Carter, the chief executive of the British Dental Health Foundation. "It's not all night-grinding. People also tend to clench in the day when they are concentrating. When people use computer keyboards, they tend to hold their teeth together, and then jiggle them over each other."

But although stress and concentration make grinding worse, it is not the underlying cause. Dentists believe that grinding is caused initially by a misalignment of the teeth. The brain, knowing that something is wrong, tries subconsciously to wear down the peaks that are preventing the jaws from meeting properly.

So determined is the brain to get rid of obstructions that massive forces are generated. A clench or grind can exert a pressure of 400lb per square inch — 20 to 30 times the force of a normal bite.

With such potentially destructive forces involved, it is important to get grinding and clenching treated. One answer is wearing a guard — if not in the day, then when especially stressed or sleeping. These individually tailored, horseshoe-shaped pieces of plastic present each tooth with a flat surface to work on. They con the brain into thinking that everything in the jaw is fine.

Orthodontic work is the more permanent alternative for those with a severe problem. But for most people, avoiding aggravating factors and using relaxation techniques can provide relief.

The British Dental Association recommends that anyone with symptoms of grinding or clenching should discuss them with their dentist. Most dentists can provide some help, but a referral to a dental specialist may be necessary.

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