Taiwanese dentists are urging 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 bite into hamburgers larger than three inches high, Hsu said.

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