“…This changes the conditions of equilibrium. The forces on the lingual surfaces of the teeth, from the tongue, predominate, which leads to proclination of the incisors as reported by amongst others, Bergersen (1972), Bjerregaard et al (1980), Osborn et al (1991), Soo and Moore (1991), and Nevant et al (1991). A significant and clinically important effect of the lip bumper is also the transverse expansion of the dental arch (Cetlin and Ten Hoeve, 1983;Osborn et al, 1991;Nevant et al, 1991).…”