Abstract:
A method for making a cast of a tooth using a curable dental impression compound containing a ferromagnetic compound, preferably an iron powder additive as well as a method for guiding the compound into the sub-gingival areas of the tooth by the use of a magnetic field. The term “ferromagnetic” as used herein is used to describe a substance that is capable of being attracted by a magnetic field, in particular a substance that exhibits a high degree of magnetic permeability.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a Divisional application of U.S. application Ser. No. 12/523,868 filed 21 Jun. 2009, which is a National Stage application under 35 U.S.C. §371 of International application PCT/NO2008/00023, filed 24 Jan. 2008, which claims the benefit under 35 USC §119 to provisional application 60/887,617 filed 1 Feb. 2007. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to elastomeric compounds used for making impressions for the fabrication of dental prostheses, more specifically to a method for making dental impressions using such compounds. 
       BACKGROUND OF THE INVENTION 
       [0003]    In order to fabricate a dental prosthesis such as crowns and bridges, a mold of the teeth must first be prepared. Such a mold is prepared by making an impression of the teeth using a curable molding compound applied to the teeth by dental personal or dentist. In making such a mold, it is necessary that the impression also include the sub-gingival margins of the prepared tooth. This procedure is difficult, however, because of the small space between the tooth&#39;s surface and the gingival tissue. 
         [0004]    Previous methods for introduction of the molding compound into the sub-gingival area involved the widening of the sulcus, i.e. the sulcus and/or gingival tissue was retracted and the neck of the tooth exposed. One common method involves the insertion of a retraction cord into the sulcus. Other methods include the use of an expandable compound that causes a separation of the gingival tissue or an astringent or other substance causing tissue retraction, as well as surgical procedures for exposing the sub-gingival areas of the tooth. 
         [0005]    Most of the aforementioned methods involve substantial discomfort for the patient and potential complications. Furthermore, even when the gingival tissue is so widened, it has proven difficult to satisfactorily introduce the molding compound into the sub-gingival pockets in order to achieve a good impression of the subgingival margins of the tooth. A need therefore exists for a dental impression compound that can be used to make acceptable cast of the tooth and the sub-gingival areas regardless of the degree of gingival-widening or without gingival-widening, and that can be more effectively introduced into the sub-gingival areas. 
       SUMMARY OF THE INVENTION 
       [0006]    The current invention addresses the above-described disadvantages of the prior art by utilizing a curable dental impression compound comprising a ferromagnetic additive in a method for guiding the compound into the sub-gingival areas of the tooth by the use of a magnetic field. The term “ferromagnetic” as used herein is used to describe a substance that is capable of being attracted by a magnetic field, in particular a substance that exhibits a high degree of magnetic permeability. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The invention will be explained in more detail with reference to the attached figures, wherein: 
           [0008]      FIG. 1  is a cross sectional view of a tooth prepared for the taking of an impression 
           [0009]      FIG. 2  is a cross sectional view from  FIG. 1 , showing an amount of impression compound placed at the edge of the gingival tissue 
           [0010]      FIGS. 3 and 4  illustrates the use of a magnet device for guiding the impression compound downward into the gingival pocket. 
           [0011]      FIG. 5  illustrates an alternate method of using a magnet to guide impression compound into the gingival pocket 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    One aspect of the present invention utilizes an elastomeric compound for the taking of dental impression otherwise known in the art, to which is added a ferromagnetic compound such as iron powder or carbonyl iron powder in sufficient quantity such that the compound may be physically moved by a magnetic field. Another aspect of the invention provides for a method of using a magnet to guide the aforementioned compound into the sub-gingival pockets. 
         [0013]    Impression Compound 
         [0014]    According to a preferred embodiment, the impression compound utilized in the method comprises a light body/flowing addition polymerized silicone impression material commercially available. Carbonyl Iron powder, for example produced by BASF, Germany, is added to the impression compound in sufficient quantity as to provide the compound with the sufficient ferromagnetic property, but without negatively effecting the elasticity of the impression compound to a significant degree. The ferromagnetic property of the compound is preferably such that a magnetic field having a Tesla value within the range approved for human exposure will physically attract the compound, preferably a Tesla value of less than 2. 
         [0015]    According to one aspect of the invention the particle size of the ferromagnetic particles of the powder is between 0.3 μm-20 μm, more preferably between 0.5 μm-5 μm, and most preferably between 0.8 μm-2 μm. The powder preferably represents 10%-75% by weight of the compound, more preferably between 15-55% by weight and most preferably between 20%-30% by weight. 
         [0016]    Method 
         [0017]    As shown in  FIG. 1  a tooth  10  is first prepared for the dental prosthesis (not shown). The relevant anatomical features of the tooth for the purposes of the following description include alveolar bone  11 , the attached gingival tissue  12 , the free gingival tissue  13 , the gingival pockets  14  and the sub-gingival margins  15 . 
         [0018]    After the tooth is prepared, a small amount of impression compound  16  according to the first aspect of the invention is applied around tooth  10  at the edge of the free gingival tissue  13  as shown in  FIG. 2 . As shown in  FIGS. 3 and 4 , a magnet  17 , preferably attached to a handle  18 , is thereafter moved vertically against the gingival tissue from above the gum line to below the border between tooth and alveolar bone  11 , thus drawing the compound down into the gingival pockets  14 . After the pockets have been filled, the remainder of the compound necessary to complete the mold is applied to the tooth, and the compound polymerized according to techniques known in the art. In a preferred embodiment the magnet that is used is a cylindrical Neodymium Iron boron magnet, approximately 1 cm long and 0.5 cm in diameter, attached to a handle  18 . One or several stronger or weaker magnet may also be employed, so long as the magnet exhibits a magnetic field of a strength approved for the use on humans. 
         [0019]    In an alternate embodiment shown in  FIG. 5 , a syringe  19  is used to inject the impression compound  16  under the gum line while the magnet  17  is held in place below the level of the sub-gingival preparing margins  15  of the tooth. The magnetic attraction between the magnet and the ferromagnetic additive will thus cause the compound to flow down into the gingival pocket  14  as compound exits the syringe. 
         [0020]    The compound material will begin to overflow the gingival edge when the pockets are filled. In this embodiment, it would be possible to either use a hand-held magnet as described above, or to use another type of fixture comprising a magnet attached to the teeth or around the jaw. 
         [0021]    The above described method can also be used in conjunction with prior art methods for widening the sulcus, and can be used in a two-step process where a non magnetic compound is utilized for the remainder of the impression after the magnetic compound has been used to fill the gingival pockets. 
       EXAMPLES 
       [0022]    The invention will be further illustrated by the following examples. 
         [0023]    President® impression compound was mixed with Carbonyl Iron powder from BASF having an average particle size of 1 μm. The Powder represented 23% of the finished mixture. 
         [0024]    The elasticity of the 23% mixture was tested in accordance with the guidelines of the ISO 4823:2000(E) standard. Specimens of the 23% mixture were compressed 4 millimeters in 1 second and gradually released during 5 seconds. Their lengths were measured after 70 seconds of elastic recovery. The constant (K) was derived, with a K-value over 96.5 being regarded as acceptable. In the samples tested, the K-value of the 23% mixture was shown to be 99.599 compared to 99.78 for pure compound. 
         [0025]    The ability of the 23% iron compound to reproduce detail was also tested. A standard test block with three lines carved into it as described in ISO 4823:2000(E) was used. By pouring the compound over the block an impression of the carved lines was formed. If the material was to be regarded as acceptable, all three lines should be visible with the use of a microscope. Actual observation confirmed a positive result. 
         [0026]    In order to test the improved ability of the compound to penetrate into the sub-gingival areas, a clinically-relevant model of gingival pockets was constructed by stretching an elastic membrane over the outside of a glass beaker. Copper wires having 1 mm diameter were placed vertically between the membrane and the outside surface of the beaker, and arranged in intervals around the circumference of the vessel. The copper wires thus displaced the membrane from the surface of the glass, creating pockets mimicking gingival pockets ranging from 1 mm to zero in width. Using a force meter the elasticity of the artificial gingival “tissue” was measured to be a clinically-relevant 10 g per millimeter in the horizontal plane. By placing the model on digital scales and then penetrating the pockets with a periodontal probe the vertical force needed for penetration was measured as a clinically-relevant force of over 40 g. 
         [0027]    Six samples of the impression compound/iron mixture was applied to the gingival edge of the model. Then during 15 seconds a Neodymium Iron boron magnet, approximately 1 cm long and 0.5 cm in diameter, attached to a handle was moved vertically from the edge downwards ca. 2 cm while contacting the compound, guiding the compound downwards into the pockets. The motion continued downwards and away from the model in a circular fashion. The same procedure was performed on six control samples of President® impression compound without the added iron powder. The amount of penetration of the compound into the artificial gingival pockets was recorded for both the test samples and the control samples by measuring the distance from the gingival edge to the deepest penetration into the gingival pocket. The examples of the compound containing iron showed an average of five times greater degree of penetration than the pure compound.