Patent Publication Number: US-2007105062-A1

Title: Expansion screw for correcting misalignment of teeth

Description:
The present invention relates to an expansion screw having the features defined in the preamble of claim  1 . An expansion screw of that kind has been known from DE 824 832. The known expansion screw comprises two bodies the spacing of which can be varied by means of a spindle. The spindle comprises an actuator element and, projecting from the latter, two threaded portions which extend in opposite directions one relative to the other and which are provided with a left-hand thread and a right-hand thread, respectively. Each of the threaded portions is rotatably seated in one of the bodies and engages an inner thread in the body. The inner threads are formed in two nuts, each of which is received in a recess in each of the two bodies, where it is permitted to slide, while being fixed against twisting. The nuts are subjected to the effect of a spiral spring that acts between the nut and the end of the respective expansion screw body. In order to prevent twisting of the two bodies of the expansion screw one relative to the other as the spindle rotates, two guide pins are provided in parallel to the spindle, which pins are fixed in matching bores in the one expansion screw body and are slidably guided in lengthwise direction in correspondingly aligned bores in the other expansion screw body.  
      EP 0 817 596 B1 discloses another expansion screw having the features defined in the preamble of claim  1 , where the springs are made from a form-memory alloy that is pseudoelastic at the temperatures prevailing in a person&#39;s mouth.  
      There are further known expansion screws where the spindle is screwed directly into an inner thread of an expansion screw body so that a direct, non-cushioned connection exists between the spindle and the expansion screw bodies.  
      It is important for the progress of the treatment that can be achieved using an expansion screw in correcting misalignment of teeth that the spindle will remain in the position adjusted by the treating orthodontist and will not alter its adjustment by itself while located in a person&#39;s mouth. Consequently, a sufficient blocking effect must be obtained between the spindle and the nut thread. For reasons of corrosion-resistance, the expansion screw bodies formerly were made from German silver, while the guide pins and, in some cases, the nuts and the spindles were made from stainless steel. It was possible in this case to achieve sufficient blocking by selecting a sufficiently small pitch and tight tolerances between the spindle and the nut thread engaged by it. For reasons of biocompatibility, however, German silver in expansion screws meanwhile has been replaced by stainless steel. This had the effect to considerably reduce the blocking action between the spindle and its nut thread.  
      In the past, various attempts have been made in connection with orthodontic expansion screws, made from stainless steel throughout, to improve the blocking effect of the spindle in the nut thread. A known solution consists in squeezing the nut thread a little together. Although this actually improves the blocking effect, it is a disadvantage that the extent of the blocking effect can be reproduced only with difficulty so that considerable differences with respect to the blocking effect are encountered within a series of expansion screws.  
      It has further been known to inject a curable plastic material into the gap between the spindle and its nut thread. In this case, it is necessary that the play between the spindle and its nut thread must not be excessively small. But even then, the plastic material will penetrate into the nut thread over a fraction of its length only. This has the result that in spite of the injection of plastic material a certain play of the spindle will be left, which is undesirable in use of the expansion screw. However, injecting a plastic material into the gap between the spindle and its nut thread increases the production cost of the expansion screw quite considerably.  
      Another known possibility consists in providing a plastic sleeve between the spindle and the nut thread. It is a disadvantage of that solution that a larger gap is required between the threaded portions of the spindle and the nut thread and that the plastic sleeve must be manufactured and assembled separately. This considerably increases the production cost of the expansion screw and leads to gaps that cannot be cleaned and that may form a source of bacterial contamination.  
      Now, it is the object of the present invention to show in connection with expansion screws where the expansion screw body, the spindle, the nut thread and, if applicable, any springs present are made from steel, how the blocking effect between the spindle and the nut thread can be improved to a reproducible level at low cost.  
      This object is achieved by an expansion screw having the features defined in claim  1 . Advantageous further developments of the invention are the subject-matter of the sub-claims.  
      According to the invention, the spindle of the expansion screw is coated at least on its threaded portion or, if two threaded portions are present, on at least one of the two threaded portions, with a second metal or a second alloy that differs in composition and/or in hardness from the at least one first metal or from the at least one first alloy. That solution offers considerable advantages: 
          The coating actually increases the blocking effect to a sufficient degree.     The thickness of the coating can be made easily reproducible if the conditions under which the coating is applied are kept constant.     The coating can be applied on a basic material of the spindle, especially on stainless steel, either directly or over a thin intermediate adhesive layer, and will adhere to that material with sufficient strength. Gold, which is preferred as a coating material, adheres to stainless steel directly and strongly.     Experience has shown that gold, being the preferred material, is corrosion-resistant and biocompatible under the conditions prevailing in a person&#39;s mouth.     The desired blocking effect is achieved already by a very thin coating. Good results have been achieved with a gold plating having a mean thickness as small as 0.3 μm to 0.5 μm. The lower limit for the thickness of the coating, especially a gold plating, preferably should be 0.1 μm. That thickness already provides a closed layer that improves the blocking effect. Preferably, the coating, especially the gold plating, has a mean thickness of maximally 1 μm. Greater layer thickness no longer have the effect to considerably improve the blocking effect, but may be suited to bridge greater tolerances between the threaded portion of the spindle and the nut thread. More conveniently, however, the tolerances between the threaded portions of the spindle and the nut thread should be kept small.     The coating can be applied at extraordinarily low cost, especially by a galvanic separation process. Still, it is also possible, though connected with somewhat higher cost, to apply the coating using a chemical separation process, without any use of external current, a PVD process or by sputtering.        

      An especially advantageous solution is achieved when the at least one first metal and/or the at least one first alloy, from which the spindle and the elements that contain the nut threads for the spindle are made, and the second metal and/or the second alloy with which at least one threaded portion of the spindle is coated, are clearly different in hardness. Related to the harder material, the difference in hardness should be at least 30%. Preferably, the difference in hardness is 50% or more.  
      A material especially well suited for coating the spindle is gold as the second metal and/or a gold alloy as the second metal alloy, especially one with a high gold content of at least 70% by weight, especially a gold alloy free from nickel and copper. Gold is advantageous because of its inert behavior, and has been found to perform well in a person&#39;s mouth. Other metals and alloys are, however, likewise imaginable provided they are stable under the conditions prevailing in a person&#39;s mouth: Stable to saliva, food and beverages, and stable to the generation of electrochemical potentials relative to other materials that may be installed in the person&#39;s mouth.  
      Preferably, the spindle is coated with pure gold. On the one hand, this can be realized at low cost, while on the other hand it also leads to the best possible results, as regards the improvement of the blocking effect. Although the use of a gold alloy with high gold content, especially a nickel-free and copper-free gold alloy having a gold content of at least 75% by weight, is like possible, pure gold is preferred.  
      No coating is required on the actuator element of the spindle. Still, the spindle preferably will be coated over its full outside as coating the whole spindle will be cheaper, even in the case of gold, than leaving certain parts of the spindle uncoated. 
    
    
      The attached drawing shows an oblique view of an expansion screw according to the invention, illustrating the screw in  
       FIG. 1  in closed condition and in  
       FIG. 2  in expanded condition. 
    
    
      The expansion screw illustrated in the drawing comprises a first body  1  with a threaded bore  2  and two cylindrical bores  3  and  4 , extending in parallel to the threaded bore  2 . A second body  5  of the expansion screw comprises two cylindrical bores  6  and  7 , aligned with the bores  3  and  4 . Cylindrical guide pins  8  and  9  are fitted in non-sliding relationship, for example are pressed into, the cylindrical bores  6  and  7 , while being slidably fitted in the bores  3  and  4  of the first body  1 . The second body  5  comprises a recess  17  in which a threadless shaft  10  of a spindle  11  is retained in such a way that the spindle  11  is prevented from moving in its lengthwise direction in the second body  5 . The spindle  11  comprises a threaded portion  12  which engages the threaded bore  2  of the first body  1  and which thereby performs the function of a nut thread for the spindle  11 . The shaft  10  separates the actuator element  13  from the threaded portion  12  of the spindle  11  and has a diameter smaller than the diameter of the actuator element  13  and the threaded portion  12 . The actuator element  13  has a cylindrical head in this case and comprises a hexagonal hole  14  in which a spanner can be fitted for turning the spindle  11 .  
      The second body  5  of the expansion screw comprises two extensions  15  and  16  that serve as retention arms.  
      All parts of the expansion screw are made from a stainless steel, for example a number  14305  stainless steel. The spindle  11  is coated with gold all over, the gold having been applied, especially, by galvanic separation in a mean layer thickness of 0.3 μm to 0.4 μm. Due to the electrolytic metal separation process, the thickness of the gold layer is subject to variations as more metal will be deposited on peaks and edges due to the higher disconnection field strengths present in those areas, compared with plain or screened surface areas. With regard to those variations, the thickness of the coating therefore has been stated as “mean” values in the claims.  
     LIST OF REFERENCE NUMERALS  
     
         
           1 . first body  
           2 . threaded bore  
           3 . cylindrical bore  
           4 . cylindrical bore  
           5 . second body  
           6 . cylindrical bore  
           7 . cylindrical bore  
           8 . cylindrical guide pins  
           9 . cylindrical guide pins  
           10 . threadless shaft  
           11 . spindle  
           12 . threaded portion  
           13 . actuator element  
           14 . hexagonal hole  
           15 . extensions  
           16 . extensions  
           17 . recess