Orthodontic spreader
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an orthodontic spreader, which comprises an actuating screw and pressure-transmitting nuts screwed on both end portions of the screw and is mainly intended for correcting the positions of molars in the upper jaw. 
     2. Description of Prior Art 
     Such orthodontic spreaders which comprise an actuating screw and pressure-transmitting nuts screwed on both end portions of the screw and are mainly intended for correcting the positions of molars in the upper jaw are known, in which said actuating screw has end portions provided with oppositely handed screw threads and is operable to move said pressure-transmitting nuts along parallel guide rods. For instance, one known jaw-expanding spreader comprises two housing members, which are adjustable by means of an actuating screw and are guided by a single pin or by two pins. Each guide pin is an interference fit in one housing member and a sliding fit in the other housing member. In that arrangement the force-fitted guide pins increase the shortest length to which the spreader can be contracted. In order to reduce that shortest length, orthodontic spreaders have been proposed, wherein a spreader can be contracted to a shorter length because a guide extension extends from the screw-threaded portion of the first housing member toward the second housing member so that the length required for the fixation of the guide pins is reduced. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an orthodontic spreader in which force is applied at points which are as close as possible to the center of resistance and which has a high pressure-applying and spreading capacity and is as convenient as possible for the patient. 
     In an orthodontic spreader which comprises an actuating screw and pressure-transmitting nuts screwed on both end portions of the screw and is mainly intended for correcting the positions of molars in the upper jar, that object is accomplished in that the pressure-transmitting nuts are adapted to be extended from the actuating screw in mutually opposite directions, guide rods are axially slidably mounted in the pressure-transmitting nuts, and stops are provided for limiting the longitudinal movement of the guide rods relative to the pressure-transmitting nuts. This arrangement ensures that even short pressure-transmitting nuts will be guided over large lengths. 
     Specifically, the stops may consist of stop collars, which are provided at the ends of the guide rods and may be integrally formed therewith. These stop collars limit the outward movement of the pressure-transmitting nuts by engaging the same on an axially outwardly facing surface. 
     The stops carried by the guide rods and limiting the outward movement of pressure nuts. The guide rods may also co-operate with the actuating portion of the actuating screw. In that case the actuating portion of the actuating screw may be laterally enlarged and extend into annular grooves formed on the guide rods, which may have a flat on one side so that the guide rods can be assembled with the screw and can subsequently be rotated so as to be fixed in position by the enlarged actuating portion extending into the annular grooves. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Two preferred embodiments of orthodontic spreaders according to the invention will now be described with reference to the drawings, in which 
     FIGS. 1 and 2 are top plan views showing a first embodiment of such orthodontic spreader in contracted and extended positions, respectively, 
     FIG. 3 is a top plan view showing that spreader in use at the teeth of the upper jaw, 
     FIGS. 4 and 5 are, respectively, a top plan view and side elevation showing a second embodiment of the spreader and 
     FIGS. 6 and 7 are, respectively, a top plan view and a side elevation showing the spreader of FIGS. 4 and 5 in use at the teeth of the upper jaw. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As is apparent from FIGS. 1 to 3 of the drawing, the first embodiment of the orthodontic spreader for correcting the positions of teeth of the upper jaw comprises two pressure-transmitting nuts 1, 2 screwed on an actuating screw 3 having oppositely handed end portions on opposite sides of a laterally enlarged actuating portion 4. Two parallel guide rods 5 are disposed on opposite sides of the screw 3 and are provided with stops, which limit movement of the nuts relative to the rods 5. In this embodiment these stops consist of stop collars 6, which are formed on each guide rod at opposite ends thereof and limit the outward movement of the pressure-transmitting nuts 1, 2 by engaging the latter at axially outwardly facing surfaces. In a preferred embodiment, the pressure-transmitting nuts 1, 2, are formed at their outer ends with recesses 7, which are capable of receiving the stop collars 6 to permit a further extension of the pressure-transmitting nuts. The peripheral surfaces of the recesses 7 may be provided with a plastic liner 10 as the plastic pressure-applying members 8, 9 (FIG. 3) are molded around the pressure-transmitting nuts 1 and 2, respectively. These liners 10 will present a frictional resistance to the movement of the end collars 6 of the guide rods 5 into the recesses 7. This will eliminate the gradual development of a lateral backlash, which would otherwise require the use of a new spreader. upper jaw. As shown, the spreader is mountable on a palate plate 9&#39; having the pressure applying members 8 and 9. As can be seen, the guide rods 5 are shiftable during the installation of the pressure applying members into the plate 9&#39;. In other words, the guide rods 5 are moved to extreme position in one direction until the stop collars 6 abut the pressure nut 1 inside its recess (not shown), as evident in FIG. 3. 
     As is apparent from FIGS. 4 to 7, the second embodiment of the orthodontic spreader for correcting the positions of teeth in the upper jaw also comprises two pressure-transmitting nuts 11, 12 screwed on an actuating screw 13 having oppositely handed end portions and a laterally enlarged actuating portion 14 between said end portions. Two parallel guide rods 15 are disposed on opposite sides of the screw 13. The actuating portion 14 extends into annular grooves 16 formed in the guide rods 15 so that the parts 14, 15 are held against axial movement relative to each other. To permit an assembling of the spreader, each guide rod 15 is provided with a flat 17 on one side. When the nuts 11, 12, the screw 13 and the guide rods 15 have been assembled, the guide rods 15 are rotated to interengage with the actuating portion 14 at the grooves 16, as is shown in FIG. 5. 
     The flats 17, as seen in FIG. 4, are facing outwardly. If the two guide rods 15 are rotated inwardly toward each other by 90°, the flats 17 will be facing each other as interposed by actuating portion 14. In this position, the actuating means 14 is free of contact with the guide rods 15. When the flats 17 occupy the position shown in FIG. 5, the actuating means 14 is in engagement with the two guide rods 15. The pressure nuts 11, 12 possess, at their exterior side, recesses 7&#39;, similar to the recesses 7 shown in FIG. 1. The guide rods 15 possess stops (not shown) similar to the stop collars 6 shown in FIG. 1. Consequently, the pressure nuts 11, 12 can move outwardly to each other by a limited distance. 
     An intermediate portion 18 of a pressure-applying bracket is mounted, e.g., by laser welding, on each pressure transmitting nut 11 or 12 and extends transversely to the direction A (FIG. 6) in which pressure is applied. Each of said brackets has two arms, which are bent from said intermediate portion at opposite ends thereof at right angles thereto and serve to apply pressure to the teeth Z. As is shown in FIG. 6 these arms are bent so that the bracket comprises an additional transverse member 19 for engaging the teeth. In the embodiment shown in FIG. 7 the framelike brackets 18, 19 are upwardly curved in the region B so that they generally conform to the palate G and the pressure-transmitting nuts 11 and 12 also conform somewhat to the shape of the palate G. The brackets 18, 19, are secured to the teeth Z by means of cuffs 20.