TAIPEI, Thursday 8 July 2010 (AFP) - Taiwanese dentists have urged fast food chains to drop over-sized hamburgers from their menus following a growing number of jaw injury cases, a report said Thursday.

Patients have developed sore jaws or had difficulties opening their mouths after consuming giant burgers, the China Post quoted professor Hsu Ming-lung of the School of Dentistry at National Yang-Ming University as saying.

Problems can arise when fast-food fans tuck into hamburgers larger than eight centimetres (three inches) high, Hsu said according to the paper.

A human mouth is designed to gape over objects measuring up to four centimetres, and overextending can hurt the joint between the jawbone and the temporal bone in front of the ears, Hsu said.