FIELD OF INVENTION 
     The present invention relates to a paste formulation, a tool, and a system for root canal treatment, and more particularly relates to the paste used for root canal filling, a paste injector, and a system used therewith. 
     BACKGROUND ART 
     In dental clinical treatments, root canal treatments are popularly operated. Among the root canal treatments, root canal filling is operated at the final stage of the root canal treatment, and then the root canal filling has large effects on prognosis of a damaged tooth. Since the length to an apical constriction varies from individual patients, a length of the root canal must be measured. A method for this measurement includes, conventionally to measure an actual length of the tooth by using a reamer, a file, or a scale as detection probes for an x-ray photograph. Recently, a method for measurement of the root canal length using an impedance is widely used, because there is a case that the x-ray exposure is not adequate for a pregnant woman and the impedance measurement is speedy and provides exact results. 
     The root canal length measurement by impedance conventionally uses a root canal meter. A working length is then determined to be the reamer length inserted in the root canal minus 1 mm which is indicated by a predetermined impedance previously measured depending on the conditions of the instruments for measurement when a top of the reamer contacts the periodontal membrane. Alternatively, the working length is determined as the length that gives current or resistance being different by a certain value from the value which indicates contact to the periodontal membrane. 
     A root canal filling material, or root canal filling agent such as a filler paste formulation conventionally used for the root canal filling includes a gutta percha point used together with various sealers, silver points and a filler in a paste formulation which may be formulated with a sustainable disinfectant or an accelerator for healing with osteoid scar. 
     Especially, a method for filling the paste formulation in the root canal portion includes filling the paste formulation through the reverse-rotated reamer operated by hand, filling by using a filling device with a helical shape such as Rentulo plugger, or filling the paste through a paste injector. However, the methods using the reamer and Rentulo plugger have disadvantages, because these methods require an extra procedure for measuring the root canal length and the special device. 
     FIG. 9 shows a conventional method for filling the paste formulation into the root canal by the injector. In the conventional root canal filling method using the injector, the injector  1  shown in FIG.  9 ( a ) filled with the paste formulation M is used. The injector  1  comprises a container portion  2  and an insert portion  3  which may be inserted into the container portion  2 . A tube for injection of the filler is disposed at a front end of the container portion  2  to form an injection portion  4 . In FIG.  9 ( a ), the tip of the paste injection tube is inserted into the root canal portion which has previously been spread widely. 
     In the conventional root canal filling method, as shown in FIG.  9 ( b ), the paste formulation M is injected into the root canal while inserting the filler injection tube into the root canal and pressing the insert portion  3  in the direction of the arrow A. When the filling is completed, the filler injection tube is drawn out from the root canal and then the root canal filling operation is completed. 
     When the root canal is filled by the injector, as described above, an advantage to complete the injection easily and quickly may be provided without the procedure to measure the root canal length previously. However, this method has some disadvantages, that is, filling of the paste to the apical constriction is unknown and the paste formulation may irritate root peripheral tissues when the paste formulation is injected beyond the apical constriction. When the filling is not enough, a dead space and/or residual pulps may be allowed to exist. Therefore, there are needs for the paste formulation, the paste injector, and the system for detecting the paste filling. 
     SUMMARY OF THE INVENTION 
     In the first aspect of the present invention, a paste formulation for dental use comprising electric conductive material may be provided. 
     In the first aspect of the present invention, the paste formulation is provided, wherein the electric conductive material is powder. 
     In the first aspect of the present invention, the paste formulation for dental use may be provided, wherein the electric conductive material is selected from the group consisting of metal powder, metal oxide powder, coated inorganic powder by metal and/or doped metal oxide, carbon powder, carbon whiskers, and whiskers integrated with metal. 
     In the first aspect of the present invention, the paste formulation for dental use may be provided, wherein the paste formulation includes at least one compound selected from the group consisted of calcium hydroxide, hydroxy apatite, tricalcium phosphate. 
     In the first aspect of the present invention, the paste formulation for dental use may be provided, wherein the paste formulation includes at least one compound selected from the group consisting of iodoform, barium sulfate, anhydrous zinc sulfate, aluminum sulfate, and zinc oxide. 
     In the first aspect of the present invention, the paste formulation for dental use may be provided, wherein the paste formulation includes at least one vehicle selected from the group consisting of silicone oil, guaiacol formaldehyde mixture, propylene glycol, dry ethanol, caster oil, liquid paraffin. 
     In the first aspect of the present invention, the paste formulation for dental use may be provided, wherein the paste formulation includes at least one compound selected from the group consisting of iodoform, barium sulfate, anhydrous zinc sulfate, aluminum sulfate, and at least one vehicle selected from the group consisting of silicone oil, guaiacol-formaldehyde mixture, propylene glycol, dry ethanol, caster oil, and fluid paraffin. 
     In the first aspect of the present invention, the paste formulation for dental use may be provided, wherein the paste formulation includes an x-ray contrast agent. 
     In the first aspect of the present invention, the paste formulation for dental use may be provided, wherein the electric conductive material is present between 5˜50 wt % based on a solid of the paste formulation. 
     In the first aspect of the present invention, the paste formulation for dental use may be provided, wherein the paste formulation includes at least one compound selected from the group consisting of calcium hydroxide, hydroxy apatite, tricalcium phosphate not less than 30 wt % based on a solid of the paste. 
     In the second aspect of the present invention, a paste injector may be provided. The paste injector comprises; 
     a container portion within which electric conductive paste is held, 
     an insert portion inserted into the container portion, 
     an injection portion for injecting the electric conductive paste formulation into a constricted portion, and 
     an electric conductive member for electrically connecting the electrically conductive paste formulation to an external electrode. 
     In the second aspect of the present invention, the paste injector may be provided, wherein the electric conductive member comprises a terminal exposed at an end face of the insert portion inserted into the paste container portion and contacting the electric conductive paste formulation, a terminal disposed at the insert portion and exposed outside the insert portion, and a lead line connecting between the terminals. 
     In the second aspect of the present invention, the paste injector may be provided, wherein the electric conductive member is inserted through the insert portion. 
     In the second aspect of the present invention, the paste injector may be provided, wherein the electric conductive member comprises an end extending through an inner path of the injection portion, an end drawn out of the paste injector, and a lead line connecting between the ends. 
     In the third aspect of the present invention, a system for detecting paste filling may be provided. The system comprises; 
     a paste injector comprising an electric conductive paste formulation and an electric conductive member for electrically connecting the electric conductive paste to an external electrode, 
     a current detector for detecting the current flowing through the electric conductive paste formulation injected into a constricted portion, and 
     a current display device for indicating the current flowing through the electronic conductive paste. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 shows the paste injector according to the present invention; 
     FIG. 2 shows details of the paste injector according to the present invention; 
     FIG. 3 shows a second embodiment of the paste injector according to the present invention; 
     FIG. 4 shows an arrangement of a conventional system for impedance measurement used for detecting the working length; 
     FIG. 5 shows a method for the root canal length measurement using the reamer and the impedance; 
     FIG. 6 shows an arrangement for filing electric conductive paste D into the root canal of the damaged tooth by the paste injector of a first embodiment according to the present invention; 
     FIG. 7 shows an arrangement for filing electronic conductive paste D into the root canal of the damaged tooth by the paste injector of a second embodiment according to the present invention in the same arrangement shown in FIG. 6; 
     FIG. 8 shows an electric circuit arrangement used for testing the paste according to the present invention; and 
     FIG. 9 shows a conventional root canal filling method by a conventional injector. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention provides an electric conductive paste formulation for dental treatment, i.e., an electric conductive paste formulation, a paste injector, and a system for detecting paste filling. Hereafter, the electric conductive paste formulation, the paste injector, and the system for detecting paste filling will be described using the drawings attached when necessary. 
     The electric conductive paste formulation according to the present invention may be formulated by mixing a suitable electric conductive material with conventional compositions for forming of calcium hydroxide paste, zinc eugenol paste, iodoform paste, paraform-formaline paste. These conventional paste compositions may include Triozinc pasta which is a mixture of paraform aldehyde, anhydrous zinc sulfate, aluminum sulfate, potassium sulfate, zinc oxide mixed with cresol, phenol, and creosote; Kri 1 which is a mixture of iodoform powder, parachlorophenol campher, menthol, lanolin, glycerin; Carbitar which is a mixture of iodoform powder, calcium hydroxide powder, sulfathiazole powder, guanofuracin with a vehicle such as T-caine and guanofuracin; Bitapex which is a mixture of calcium hydroxide powder and iodoform powder with silicone oil; FR which is a mixture of calcium hydroxide powder, zinc oxide powder, barium sulfate powder, and a vehicle such as guaiacol-formaldehyde mixture, propylene glycol, dry ethanol, caster oil, liquid paraffin; Sealapex which comprises a polymer resin and calcium hydroxide. 
     Other paste formulation such as Endofil which uses a polymeric material as a base and the paste formulations which use hydroxy apatite or tricalcium phosphate may be useful, because such paste formulations exhibit affinity to live tissues and accelerate hard tissues formation. These paste formulations may provide advantages that the formulation may be absorbed by the tissues if the paste formulation runs out from the apical constriction. The above described calcium hydroxide, hydroxy apatite, tricalcium phosphate may be included in the electric conductive paste in an amount not less than 30 wt %, more preferably not less than 50 wt % based on the weight of solid contents in the electric conductive paste formulation. 
     The electric conductive material suitably used in the present invention may include electric conductive powder so as to be mixed sufficiently with the above described paste formulations. Such electric conductive powder may have a particle size between 200 Å˜100 μm, more preferably, between 0.01˜80 μm, most preferably between 10˜50 μm in order to achieve a good filing performance. Shapes of the electric conductive powder may be a spherical shape, a needle shape, and a scale shape. 
     The powder may include metal powder such as, for example, gold powder, silver powder, copper powder, nickel powder, metal oxide powder such as, for example, ZnO, SnO 2 , I 2 O 3 , ITO, various doped metal oxides such as antimony doped SnO 2 , BaSO 4  coated with metal, SnO 2 , or other electric conductive materials, TiO 2  powder coated by Ag, TiO 2  powder coated with SnO 2 , electric conductive carbon black, powder having high aspect ratio such as, for example, potassium titanate whisker coated thereon by Ag or Pd, whisker coated thereon by SnO 2 , electric conductive carbon whiskers. Particularly, needle type powder may provide an improved volume resistivity of the electric conductive paste formulation with low contents. 
     The above described electric conductive paste may have volume resistivity from 10 −2  Ωcm to 10 6  Ωcm, and it is preferred that the resistance of the electric conductive paste formulation when injected in the root canal portion may exhibit resistance in an order of the resistance through the periodontal membrane. As described below, if the volume resistivity is too high, the detection of the paste injection may not provide sufficiently exact results. 
     The electric conductive powder maybe present in the electric conductive paste formulation so as to provide the required volume resistivity to the electric conductive paste formulation, and the powder may present from 5 wt %˜50 wt % based on the solid of the paste formulation, more preferably present from 10 wt %˜30 wt % based on the solid of the paste formulation. 
     The vehicles may present in the electric conductive paste formulation from 10 wt %˜70 wt %, more preferably from 30 wt %˜60 wt %. 
     Hereinbelow, an exemplary embodiment of the paste formulation including calcium hydroxide as its major component and electric conductive metal powder will be explained. 
     First, calcium hydroxide is prepared. This calcium hydroxide may be used under the specification of Japanese Pharmacopoeia having the particle size from 10˜50 μm. 
     In this electric conductive paste, iodoform may preferably be present, because an anti-bacterial activity may be extremely enhanced, and an x-ray imaging performance may be also improved. 
     In addition, a vehicle described above such as silicone oil etc. may be added to calcium hydroxide. Addition of the silicone oil may provide an anti-corrosion property to the paste formulation as well as providing an adequate flow performance. 
     Other additives to the calcium hydroxide may be selected from various types. For example, x-ray contrast agents including bismuth bicarbonate, bismuth sulfate, zirconium silicate may improve the x-ray imaging performance of the paste. 
     A dispersion method for the powder may be selected from any well-known conventional dispersion methods. Especially, when the electric conductive powder having the particle size between 10 and 50 μm is used as described above, it is possible to apply various mixing and/or dispersion methods, because aggregation of the powder may not be so severe. In the preparation of the electric conductive paste formulation according to the present invention, the paste formulation may be prepared, for example, by providing a paste formulation which is premixed with calcium hydroxide, iodoform, and silicone oil, adding the electric conductive powder, mixing the resulting paste formulation again to formulate the electric conductive paste formulation, then filling the resulting electric conductive paste formulation into the injector in a form of a cylinder for use when necessary. However, it is possible to mix a non-conductive powder such as calcium hydroxide and the electric conductive powder together with the vehicle such as silicone oil at the same time so that as the paste formulation according to the present invention may be obtained. 
     In the above described embodiment, the conductivity/resistivity described above may be obtained by adding electric conductive powder such as gold, silver, copper, and nickel having the particle size described above. Since the electric conductivity is provided with the paste formulation, and the paste formulation may be injected into the root canal by an injector implemented with an electric conductive member, the adequate injection of the paste formulation to the root canal may be proved by detecting the resistance. Further, since the electric conductive powder is added to the paste, and the thermal and electric conductivities of the paste are improved, good thermal and electric properties may be obtained. When carbon powder is used, it is expected to absorb formaldehyde included in form-cresol which is used to treat the root canal and is also included as the major component of form-cresol. 
     Referring to figures, an embodiment of the paste injector, which is able to inject the electric conductive paste formulation into the root canal and simultaneously to measure the electric resistance, will be explained in detail. 
     FIG. 1 shows an exploded view of a first embodiment of the paste injector  1  according to the present invention. The paste injector  1  comprises the container portion  2  for holding the fill electric conducting paste formulation, the insert portion  3  being inserted into the container portion  2 , the injection portion  4  formed as a tapered tube toward the tip and disposed at the front end of the container portion  2 , and the seal member  5  disposed at the end of the insert portion  3  which is inserted to the container portion  2 . 
     The container portion  2  and the insert portion  3  may be made of glass, or synthetic resin such as polypropyrene, polycarbonate, polyvinyl chloride. However, when handling performance such as damage is considered, the portions  2 ,  3  may preferably be made of a synthetic resin such as polypropyrene, polycarbonate, polyvinyl chloride. The container portion  2  has a hollow inside portion within which the electric conductive paste formulation is held. The container portion  2  comprises a projection  2   a  at the end opposite to the injection portion  4  with which insertion of the insert portion  3  is made easy when fingers are rested thereon. Materials for forming the injection portion  4  may be selected from any suitable materials so that the shape of the tapered tube of the injection portion  4  is formed to be thin and flexible. 
     The insert portion  3  is used to extrude the electric conductive paste formulation held in the container portion  2 . The seal member  5  disposed at the front end of the insert portion  3  improves the seal between the inner face of the container portion  2  and insert portion  3 . Installation of the seal member  5  onto the insert portion  3  may be achieved through various methods including engaging the seal member  5  in the shape of pipe onto a recess formed to the insert portion  3 , or molding the seal member  5  made of a synthetic resin integrally around the insert portion  3 . 
     At the end of the insert portion  3  opposite to the end inserted into the container portion  2  there is included a projection  3   a  extending beyond the diameter of the insert portion  3  so as to make the insert easy by exerting force by fingers when the electric conductive paste formulation is injected into the root canal. The projection  3   a  may be formed separately from the container portion  3  and then may be adhered by some types of adhesives as well as being integrally formed with the container portion  3 . 
     When the electronic conductive paste formulation is injected into the root canal, the above described paste injector  1  is held by one hand, and holding the projections  2   a ,  3   a  between the fingers to exert the force to extrude the electric conductive paste formulation into the root canal. 
     As shown in FIG. 1, the first embodiment of the insert portion  3  of the paste injector  1  according to the present invention is implemented with the conductive member. The conductive member comprises the end  6 , the terminal  7 , the end  8 , the terminal  9 , and the lead line  10  extending between the end  6  and the end  8 . These components form the conductive member by the end  6  being connected to the terminal  7  formed on the side of the seal member  5  which contacts the electric conductive paste formulation. Another end  8  is connected to the terminal  9  extending outside the insert portion  3  from the position adjacent projection  3   a  formed on the insert portion  3 . The lead line  10  extends through the insert portion  3 , thereby the electric conductive paste formulation is connected electrically outside the insert portion  3 . The lead line  10  and the terminals  7 ,  9  may be formed by insert molding when the insert portion  3  is molded, or by inserting them after molding thereof. 
     FIG. 2 shows an enlarged cross section of the terminal connecting portion of the terminal  9  of the first embodiment of the paste injector  1  according to the present invention. FIG.  2 ( a ) shows an enlarged cross section of the insert portion  3  comprising the terminal  9 . As shown in FIG.  2 ( a ), the terminal  9  extends outside the insert portion  3  from a position adjacent to the projection  3   a  and electrode  106  is connected to the terminal  9  through the clip. FIG.  2 ( b ) shows that the terminal  9  is disposed in the projection  3   a , and protrudes outside the insert portion  3  from the lateral side of the projection  3   a . FIG.  2 ( c ) shows that the terminal  9  has the opening  11  facing outside the insert portion  3  into which a needle electrode may be inserted or screwed. FIG.  2 ( d ) shows that the terminal  9  is formed as a band which extends circumferentially and radially about the insert portion  3  at a position adjacent to the projection  3   a  of the insert portion  3 . The arrangement of the terminal  9  illustrated in FIG. 2 may be selected depending on easiness of handling and operation for the root canal filling, and the constructions shown in FIG. 2 may be used in any combinations each other. 
     FIG. 3 shows a second embodiment of the paste injector  1  according to the present invention. FIG.  3 ( a ) shows a partial cut away cross section of the paste injector  1 , and the paste injector  1  comprises the container portion  2  and the insert portion  3  which is inserted into the container portion  2 . The injection portion  4  is formed as a tapered tube and is disposed at the front end of the container portion  2 . In FIG.  3 ( a ), the container portion  2  is partly cut away to show the electric conductive paste formulation D. In the injection portion  4  of the paste injector  1  of the second embodiment according to the present invention, the lead line  10  extends outside the paste injector  1  at about the curved portion thereof so that the distance between a tip of the injection portion  4  and the end of the lead line  10  may be substantially decreased with respect to the first embodiment so as to reduce adverse effects of the resistance of the electric conductive paste formulation D on the resistance measurement. 
     In FIG.  3 ( b ), an enlarged cross section of the curved portion of the injection portion  4  attached to the paste injector  1  is shown. The second embodiment of the paste injector  1  shown in FIG.  3 ( b ) has the lead line  10  extending through inside path  4   a  adjacent to the tip from the lead line inlet position formed at the injection portion  4  as close as possible such that the end  6  may be located adjacent to the tip. from which the electric conductive paste D is injected. When such construction is adopted, the distance between the end  6  and another electrode  104  (not shown) through the electric conductive paste formulation D may be reduced, thereby reducing the effect on the resistance measurements due to the resistance of the electric conductive paste formulation D. 
     When the lead line  10  is drawn out of the paste injector  1  of the second embodiment according to the present invention, the terminals  9  shown in FIG. 2 may be used. Alternatively, without providing the terminal  9 , the lead line  10  is stripped at the both ends to obtain sufficient conductivity such that one end is disposed in the inner path  4   a . Another end may be drawn outside the wall of he injection portion  4   c  while maintaining seal performance with a heat-seal or the adhesive  4   b . It is of course possible to mold the lead line  10  integrally when the injection portion  4  is molded. The lead line  10  drawn out of the paste injector  1  may be extended to the projection  2   a  formed on the container portion  2  along to the outer lateral side so as not to contact the stripped end to an oral cavity, and thereafter be connected to an electrode of the apparatus for measuring resistance. The lead line used may have any thickness. However, to obtain good handling performance while not degrading the injection through the injection portion  4 , the lead line  10  may preferably be sufficiently thin. The lead line  10  may have a cover, or may not have a cover. 
     Now, a system for detecting the paste filling will be explained hereinbelow with referring to FIG.  4 -FIG.  7 . FIG. 4 shows a conventional arrangement of a system for measuring the root canal length by the resistance or the impedance. When the root canal length is conventionally measured by the resistance, the operation thereof comprises steps of; first spreading the root canal portion of the damaged tooth T, next cleaning the spread root canal  21  by hydrogen peroxide while retaining hydrogen peroxide within the root canal  21 , and then inserting the reamer  20  into the root canal  21 . 
     The resistance measuring device  100  comprises the current detector  101 , the power supply  102 , the control device  103  including relay circuit, the electrode  104  connected to the damaged tooth through the lip, the current display device  105  including a current indication device and/or an alarm device, and the electrode  106  connected to the end of the reamer  20 . The resistance measuring device  100  actuates the current display device  105  through the control device  103  based on the current signal detected by the current detector  101 . The resistance measuring device  100  acknowledges to the operator that the resistances or the current values become predetermined values, or the top of the reamer reaches to the apical constriction P, or the top of the reamer reaches to the position having a predetermined distance, i.e., about 1 mm from the apical constriction P. 
     Usually, when the reamer  20 , reaches the periodontal membrane, the electric circuit formed between the reamer  20  and the electrode  104 , which is connected to a tube contacting a lip and being extended from a dental pump, has a resistance about 6.5 kΩ including an internal resistance of the system for resistance measurement. FIG. 5 shows a schematic illustration of the resistances. In FIG. 5, the resistance of the reamer  20  is shown by the reference numeral R 1 . As shown in FIG. 5, the reamer  20 , the periodontal membrane, and the electrode  104  usually connected to the dental tube form an electric circuit including a resistance R 1 . Usually, the inside of the root canal in which the reamer  20  is inserted is cleaned and is almost dried under the measurement. Therefore, the current may not substantially flow in the circuit. However, the current begins to flow due to effects of exudate, and/or conductivity of hydrogen peroxide used in the cleaning process and remained therein as the top of the reamer  20  moves toward the periodontal membrane. When the reamer  20  reaches the periodontal membrane, the entire circuit resistance becomes the above explained resistance of about 6.5 kΩ. 
     The present invention uses the electric conductive paste formulation D as a probe electrode rather than using the reamer  20  as the probe electrode for the measurement of the root canal length. 
     FIG. 6 shows an arrangement when the electric conductive paste formulation is filled in the root canal  21  of the damaged tooth T using the paste injector  1  according to the first embodiment of the present invention. As shown in FIG. 6, the terminal  9  is connected to the electrode  106  of the resistance measurement device  100 , and the electrode  104  of the resistance measurement device  100  is connected to the dental tube. In the condition illustrated in FIG. 6, no current, or very low current flows through the current detecting device  101 , and then the resistance is held high. 
     FIG. 7 shows the filling process of the electric conductive paste formulation D into root canal  21  using the same arrangement shown in FIG. 6 while the paste injector  1  according to the second embodiment of the present invention is used. As shown in FIG. 7, as the filling of the electric conductive paste formulation D proceeds and the electric conductive paste formulation D becomes closer to the apical constriction P, a week current starts to flow through the electric conductive paste formulation D. Finally the electric conductive paste D contacts the periodontal membrane, and then the resistance R 1  of the reamer  20  is replaced to the resistance R 2 . The resistance R 2  depends on the distance between the position where the lead line  10  contacts the electric conductive paste formulation D within the injection portion  4  and the periodontal membrane. Therefore, the detection of the filing of the electric conductive paste formulation D to the apical constriction P is determined by monitoring the resistance to be (6.5 kΩ−R 1 +R 2 ). Alternatively, the relation between the distance from the periodontal membrane to the electric conductive paste formulation D including conductive metals, or conductive metal oxides and the resistance is previously measured by x-ray photographs, and then the distance from the periodontal membrane may be estimated when the resistance reaches a certain resistance higher than (6.5 kΩ−R 1 +R 2 ). As described above, since the present invention uses the paste formulation filled in the root canal as the current probe, once the resistance reaches the above certain value, it is determined that the filling of the root canal by the paste formulation is completed. The construction described above allows the reduction of the complexity of root canal filing and/or injection operation that previously requires duplex steps; measuring the root canal length by the impedance, and next spreading the root canal portion and then inserting the gutta percha point to complete the filling of the root canal. In addition, the detection of whether or not the paste formulation is sufficiently filled within the root canal becomes easy, whereas such detection is regarded to be difficult in the conventional root canal filling/injection operation. Therefore, the present invention can provide a system for detecting paste filling which makes the measurement more speedy and easier, and that insures the filling of the paste formulation. 
     The power supply  102  used may be selected from a direct current type or an alternating current type, and the value of R 2  and the internal resistance etc. may be selected as requested. The measurement etc. may be achieved through resistance measurement or impedance measurement depending on the kinds of the power supplies used. It may possible to alarm the operator to pay attention when the predetermined working length is obtained. 
     Hereafter, the present invention will be explained in detail by examples, however, these examples are provided only for explanation of the present invention and do not intend to limit the scope of the present invention. 
     EXAMPLES 
     Example 1 
     To a solid component including 50 wt % of calcium hydroxyde, iodoform 20 wt %, 30 wt % of silicone oil was added and mixed to prepare a paste formulation. 
     To this paste formulation, further 10 wt % of a silver powder based on the weight of the above solid component was added and mixed to form a paste formulation according to the present invention. The current of 5 mA was flowed in the resultant electric conductive paste formulation as measured by an ampere meter. Next, a lead line having 0.1 mm diameter was inserted into an injection portion of a commercially available paste injector so as to prepare the paste injector according to the present invention as shown in FIG.  3 . The electric conductive paste as prepared above was filled in the paste injector, and the paste discharge through the tip portion was confirmed. 
     A circuit illustrated in FIG. 8 was constructed using a pulpless tooth and injecting the electronic conductive paste formulation into the root canal  21  using the paste injector in order to operate the root canal filling. The front end of the electrode  104  of the resistance measurement device  100  was located inside the root canal  21  slightly above the apical constriction P. As the injections of the electric conductive paste formulation was started and proceeded, the current detector  101  detected the current through the circuit shown in FIG.  8 . 
     Thereafter, the injection portion of the paste injector was pulled out carefully from the pulpless tooth, and an x-ray photograph was taken to observe the filling condition of the electric conductive paste formulation. It was confirmed that the electric conductive paste formulation reached to the apical constriction P. That is, the detection of the circuit formation by the electric conductive paste formulation reached to the apical constriction P indicated the completion of the root canal filling. 
     Example 2 
     40 wt % of hydroxy apatite, 10 wt % of barium sulfate, 50 wt % of silicone oil were mixed together to form the paste formulation. Au powder of 25 wt % was mixed to the above paste formulation. The current of 3 mA flowed in the resultant paste as measured by the ampere meter. The resultant electric conductive paste formulation was used in the same test as described in the Example 1. It was confirmed that the electric conductive paste was filled in the apical constriction P of the damaged tooth. 
     INDUSTRIAL AVAILABILITY 
     As described above, since the present invention uses the paste formulation filled in the root canal as the current probe, the completion of the paste formulation into the root canal is immediately detected when the resistance becomes the above described value. Therefore, conventional duplex procedures comprising measuring the root canal length by the impedance measurement, next spreading the root canal, and then inserting the gutta percha point so as to fill the root canal is not required. In addition, the detection whether or not the paste is entirely filled in the root canal becomes easy, which was difficult in the conventional root canal filling operation using paste. The present invention may provide the system for detecting of paste filling which is carried out more speedy and easier while insuring the sufficient filling of the paste. 
     The present invention has been explained by non-limiting exemplary examples, however, it is appreciated by a person skilled in the art that many other variations, modifications, and omissions may be possible within the scope of the invention as described in the appended claims. True scope of the present invention will only be limited by the claims.
A paste formulation for dental use, a paste injector, and a system for detecting paste filling are disclosed. The paste formulation includes electric conductive material. The present invention also discloses the system for detecting paste filling comprising a paste injector containing electric conductive paste and an electric conductive member for electrically connecting the electric conductive paste to an external electrode, a current detector for detecting the current flowing through the electric conductive paste held in the container portion, and a current display device for indicating the current flowing through the electric conductive paste.