When the top front teeth sit too far forward over the bottom front teeth it is known as an increased overjet. It may be caused by thumb sucking, having a naturally forward tongue posture, sucking of the lower lip, or simply having a shorter lower jaw than upper jaw. Having the upper teeth protruding forward in such a way can make them very susceptible to accidents and injury. It can also cause problems with lip closure, gum problems, and also chewing.