Abstract: Each of two nuts includes two cylindrical recesses, the bottoms of which are pierced by openings of smaller diameter. Each of two jack bodies comprises, besides one of the nuts, two independent rods, the rectilinear ends of which are each fitted in one of the recesses and each include a blind hole serving as a guide for one of two guide pins connecting the nuts. By means of a screw having oppositely threaded ends screwed into tapped bores in the nuts, the jack bodies can be moved toward or away from one another, whereby forces situated strictly in the same plane can be exerted upon devices anchoring the appliance to the teeth. These anchoring devices are independent of one another and are each connected to only one tooth.