An intra-oral device comprising upper and lower jaw applicators, the upper jaw applicator comprising upper left and upper right components, and the lower jaw applicator comprising lower left and lower right components； a transverse adjustment mechanism for simultaneously adjusting a distance between the upper left and upper right components, and the lower left and lower right components wherein the transverse adjustment mechanism comprises: a threaded rod having one end portion engageable with the upper left and lower left components, and another end portion engageable with the upper right and the lower right components.
