People normally open their mouths about four centimetres to eat but some burgers measure eight centimetres in height. ((CBC))

Fast food chains should shrink their burgers since consumers may over-extend and injure their mouths trying to eat them, a media report said Thursday.

Many diners had to see dentists for jaw-related problems after eating large beef burgers served by two fast food chains, Hsu Ming-lun, associate professor of the School of Dentistry of National Yang-Ming University, told China Daily.

The reported problems included sore jaws and difficulties opening the mouth, both symptoms of temporomandibular joint dysfunction, Hsu said. The temporomandibular joint connects the jaw to the side of the head.

People normally open their mouths about four centimetres to eat but the burgers measured eight centimeters in height, dentist Chen Yun-chih said in the paper.

Opening the mouth that wide might hurt temporomandibular joint, Chen added.

The dentists called on fast food chains not to sell such large foods to attract customers.

Specialist Dentist Group, a dental clinic in Singapore, also advises its patients that big foods require a large mouth opening.

"It is important to understand that Asian[s] in general have smaller mouths," the group's website says. "When you want to have that super-sized burger next time, may be you want [to] choose one that is normal size. They taste equally good anyway."

Jaw pain from temporomandibular joint dysfunction may go away with little or no treatment.

Treatment may range from simply eating soft foods, applying ice packs, or taking pain medicine, according to the U.S. National Institute of Dental and Craniofacial Research. In rare cases, surgery may be needed, the institute said.