Abstract:
A root canal instrument includes a twisted strip having a titanium-nickel alloy or a plastics material. The strip has a cross-section having three exterior surfaces or four exterior surfaces. A coating is disposed on at least one exterior surface, the coating includes abrasive particles. A method of making the root canal instrument includes making a basic plate having a thickness of less than one millimeter, coating the basic plate with a coating having the abrasive particles; dividing the basic plate into longitudinally extended strips, twisting the strip to form a root canal drill bit having a cutting edge with abrasive particles disposed on the cutting edge.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of and claims priority to U.S. Ser. No. 11/487,960, filed Jul. 17, 2006, which is pending, and which is hereby incorporated by reference in its entirety for all purposes. 
         [0002]    U.S. Ser. No. 11/487,960 claims priority to German Patent Application No. DE 10 2005 034 010.5 filed Jul. 18, 2005. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention The invention relates to a root canal instrument which has a core of a flexible elastic material having shape memory and which has a coating with abrasive particles on the core. 
         [0004]    2. Discussion of the Related Art 
         [0005]    A root canal instrument of such a kind is known from the publication U.S. Pat. No. 4,190,958. 
         [0006]    From that publication it is furthermore known that, in contrast to customary, very thick and inflexible dental drill bits, endodontic root canal instruments are very thin with a diameter of less than half a millimetre and are very flexible in order to be able to follow the curvature of the root canal in a tooth. There is accordingly proposed in that publication a drill bit which is made from a flexible elastic material having shape memory so that it returns to a straight position from a curved position, assumption of the curved position being necessary in order to be able to follow the curved root canal. In addition, it must have this shape memory while rotating in the curved position. 
         [0007]    The material proposed for the core in that publication is a standard material of a carbon-containing chromium steel, which is provided with a diamond coating. The abrasive particles of the diamond coating are fixed in an adhesion-producing agent which is electrolytically deposited or sintered or produced by standard methods. 
         [0008]    A disadvantage of a root canal instrument of such a kind is that, in the case of a small diameter of only about half a millimetre, a carbon-containing chromium steel wire coated with an electrolytically deposited or sintered adhesion-producing agent becomes so rigid that, despite its having a core of a flexible elastic material, it is not able to follow the curvature of root canals. It has therefore not been possible for such root canal instruments having an elastic core and a relatively rigid coating of diamond particles to become established in practice, because coating over a length of about from 10 to 12 mm on the core with an electrolytically deposited or sintered diamond-containing adhesion-producing agent practically takes away all the flexibility of a thin chromium steel wire. 
         [0009]    Since 1998, new instruments made of nickel-titanium alloys have been used in endodontistry. This material comprises about 55% by weight nickel and about 45% by weight titanium, it being possible for a small proportion of the nickel, about 2% by weight, to be replaced by cobalt or aluminium. In their stress-strain behaviour, the nickel-titanium alloys exhibit so-called superelasticity because, in addition to the Hooke&#39;s elasticity of the chromium carbon steels known from the publication U.S. Pat. No. 4,190,958, they have substantial shape memory which is not known in the case of chromium carbon steels. This shape memory results from the fact that this material, which was still entirely unknown in 1978, the year of filing of the publication U.S. Pat. No. 4,190,958, is capable of switching, in the event of deformation, from an austenitic structure to a partly martensitic structure and, when unloaded, of re-establishing the originally austenitic structure at room temperature and, with that, the original shape. 
         [0010]    Therefore, root canal instruments for endodontistry are shaped from twisted strips or rods of that new kind of alloy by grinding or machining However, this alloy too has disadvantages. The Vickers hardness HV of the alloy, at 303-362 HV, is, compared to carbon-containing chromium steel at 522-542 HV, almost one third less than carbon-containing chromium steel. It is therefore recommended in the prior art that, because of their greater cutting performance, steel instruments be used in regions where flexibility of the root canal instruments is not required. The limited rates of material removal due to the lower Vickers hardness have to be taken into account, however, in the case of root canal instruments made of nickel-titanium. In addition, root canal instruments made of nickel-titanium are usually used with torque-limited drive means in order to prevent the increased risk of breaking in the event of overloading. There is accordingly a need on the one hand to broaden the area of use of such endodontic instruments and on the other hand to eliminate the lack of sufficient hardness. 
         [0011]    The problem of the invention is to provide a root canal instrument which has a core of a flexible elastic material having shape memory and which has a coating with abrasive particles on the core but which overcomes the disadvantages in the prior art in respect of becoming rigid and the problem of the lower cutting and drilling performance of root canal instruments based on nickel-titanium alloys. 
       SUMMARY OF THE INVENTION 
       [0012]    In accordance with the invention, there is provided a root canal instrument which has a core of a flexible elastic material having shape memory and which has a coating with abrasive particles on the core. For the purpose, either the core is made from a nickel-titanium alloy or it comprises a plastics material, preferably a carbon-fibre-reinforced plastics material. In addition, the flexibility of the coating with abrasive particles is matched to the flexibility of the core. 
         [0013]    This flexibility of the coating can be achieved by adhesion-producing agents, in which the abrasive particles are anchored, the adhesion-producing agents themselves having high flexibility and consequently being able to follow the changes in shape of the core of flexible elastic material. For the purpose, rubber-elastic or elastomeric plastics materials, for example based on silicone, are suitable, the abrasive particles on the one hand being held therein and on the other hand projecting sufficiently far out from the adhesion-producing mass that they can perform a cutting function. 
         [0014]    As abrasive particles there are used preferably diamond particles and/or ceramic particles such as corundum particles and/or boron nitride particles and/or boron carbide particles and/or silicon particles and/or silicon nitride particles and/or silicon carbide particles. Whereas hard particles such as diamond particles are preferably used for root canal instruments for cutting and grinding, softer particles such as cerucides, iron oxides and/or chalk particles are used as polishing agents. 
         [0015]    In the case of an electrolytically deposited rigid adhesion-producing mass, for example of bronze, or a sintered rigid adhesion-producing mass, for example of sintered aluminium masses, the core is provided to have coated and uncoated regions in alternating manner, preferably in periodically alternating manner, so that adhesion-producing masses that are structured in regions, for example in the manner of a link chain or spiral, with abrasive particles are applied so that the regions that are free of coating retain the flexibility of the root canal instrument. 
         [0016]    For the purpose, the root canal instrument can preferably be structured so that it has a core of the nickel-titanium alloy or of an electrically conductive plastics material, preferably of a carbon-fibre-reinforced electrically conductive plastics mass, which core has a structured metal coating as anchoring adhesion-producing mass with abrasive particles. As a result of the structuring of the metal coating, which preferably consists of bronze, the above-mentioned flexibility is retained, because the structured metal coating is restricted solely to partial regions of the surface of the flexible elastic core of the root canal instrument. This metal coating as anchoring adhesion-producing mass for the abrasive particles can both be electrodeposited on a core of the nickel-titanium alloy, which has good electrical conductivity, or can also be produced on a core of a plastics mass to which electrically conductive particles, such as silver particles, have been added. 
         [0017]    If such cores of an electrically conductive material are not available, it is possible to use, on a core of plastics material such as carbon-fibre-reinforced plastics material, preferably an adhesion-producing mass made from plastics material instead of the electrodeposited metal coating. This adhesion-producing mass of plastics material can simultaneously hold together the fibres of the core, such as carbon fibres, and anchor the abrasive particles in the plastics mass, part of the abrasive particles projecting out from the outer surface of the root canal instrument. 
         [0018]    This is achieved by means of the fact that the fibre-containing core is compressed in an extrusion method with supply of an extrudable mixture of plastics material and abrasive particles in injection-moulding to form a composite component. Subsequently, the tips of the abrasive particles can be exposed, for example by removal of material by laser or dissolution, in such a manner that the abrasive particles remain anchored in the plastics material. The flexibility of the embedding plastics material for the core is, in the process, advantageously matched to the flexibility of the core without the need for coating-structuring measures, which are needed in the case of the above-mentioned metallic adhesion-producing masses. 
         [0019]    In addition, injection-moulding or extrusion of a mixture of plastics material and abrasive particles ( 9 ) can be carried out. Subsequently, the tips of the abrasive particles ( 9 ) can be freed of the plastics material. Alternatively, by means of co-extrusion or two-stage injection-moulding, the plastics core can be sheathed in a mixture of plastics material and abrasive particles ( 9 ) and subsequently the tips of the abrasive particles ( 9 ) can be freed of the plastics material. 
         [0020]    In a preferred embodiment of the invention, at least the proximal end of the core is uncoated. This has the advantage that the root canal element follows the curvature of the root canal, and the uncoated proximal end is directed by the surrounding dental cementum of the root canal and does not bore its way out of the root canal through the surrounding tooth cementum. The uncoated proximal end accordingly guides the root canal instrument automatically along the softer tissue of the root canal without damaging the surrounding harder dental cementum. It is only by means of the abrasive coating that follows on from the proximal end of the root canal instrument that the dental cementum is processed, subjected to removal of material or polished, depending on the size and nature of the particles used. 
         [0021]    In a further preferred embodiment of the invention, the core has, on its outer surface, a coating of an adhesive, in which the abrasive particles are anchored and out from which the abrasive particles project. A coating of an adhesive of such a kind has the advantage that abrasive particles can be held on the outer surface of the core irrespective of the material of the core. This means that a layer of an adhesive of such a kind with abrasive particles can be applied both on top of a core of a nickel-titanium alloy and on top of a core of plastics material, especially of glass-fibre-reinforced or carbon-fibre-reinforced plastics material. 
         [0022]    In a further preferred embodiment of the invention, the core comprises carbon fibres embedded in an adhesion-producing mass of polypropylene, polyethylene or epoxy resin, the adhesion-producing mass of the carbon fibres forming a sheath, which anchors the abrasive particles and out from which the abrasive particles project. This root canal instrument structure has the advantage that it can be produced by a single injection-moulding procedure, because the adhesion-producing mass for the abrasive particles also simultaneously provides the adhesive bond for the carbon fibres. 
         [0023]    There will now be presented hereinbelow differently structured coatings for cases when the adhesion-producing agent has a tendency to hinder the flexibility of the root canal instrument. 
         [0024]    In such cases, the root canal instrument can preferably have at least one further uncoated region of elliptically shaped or round regions on the outer surface of the core. The effect of those elliptically shaped or round regions, which are kept free of coating, is that the coating does not substantially limit the flexibility. In addition, the cutting performance is maintained over a relatively long period, because removed tooth material blocks up the relatively large spaces of the root canal instrument relatively slowly. 
         [0025]    Conditions for matched flexibility between the coating and core are even more advantageous when the structured metal coating comprising abrasive particles comprises circular or elliptical structures which are surrounded by regions without metal coating. As a result of this coating structure, a continuous area of coating-free core surface material is achieved, so that minimal impairment of flexibility is to be expected from this structure. 
         [0026]    Preference is given to the coating being arranged in a helical shape on the core and helically shaped parts of the core not being coated. This helically shaped structuring has the advantage of a continuously alternating phase of coated and uncoated core surface regions in the longitudinal direction. Furthermore, such a helically shaped structuring of the coating can be produced without great manufacturing outlay. As a result of the helically shaped structure, removed tooth material is advantageously conveyed in the apical-to-distal direction. 
         [0027]    In a further preferred embodiment of the invention, provision is made for the core to be coated in strips so that coated and uncoated strips alternate on the core surface. Finally, provision is made for the coating to surround the core in a ring-shaped or elliptically shaped arrangement, so that coated regions and uncoated regions alternate in a ring-shaped or elliptically shaped arrangement on the core in the longitudinal direction. This structure too has an advantage because an elliptically shaped ring has the additional advantage that, on rotation, there is no possibility of ring-shaped tracks grinding into the root canal. 
         [0028]    Provision is furthermore made for the coating to comprise lozenges surrounded by two oppositely extending helically structured regions of the core without coating. A lozenge structure of such a kind can be produced very simply by means of two oppositely extending helical structures, which are introduced into a coating by means of removal of material. That removal can consist of removing, by lasers or other selective removal or dissolution methods, the adhesion-producing mass of the coating. 
         [0029]    The orders of magnitude of the root canal instruments will now be dealt with hereinbelow, a crucial variable being the length l of such a core, because it has to extend from the crown of the tooth to the end of the root canal. With regard thereto, the length l of the root canal instrument is preferably from 10 to 40 mm. The diameter d of the core of the root canal instrument can become narrower towards the proximal end, but resulting in a diameter over the entire length of the core which preferably is from 0.1 to 3 mm. For the thickness h of the adhesion-producing mass, in which the abrasive particles are anchored, an order of magnitude of from 0.1 to 50 μm is provided. The cutting performance of a root canal instrument is highly dependent on the particle size k of the abrasive particles, the particle size k being in the range from 1 to 500 μm. The larger and/or harder the particle, the greater is the material removal rate and the roughness of the worked surface of the root canal. The smaller and/or softer the particle, the smoother and more uniform is the surface of the root canal. In the process, the adhesion of bacteria can be advantageously reduced by a high degree of polishing. 
         [0030]    Such root canal instruments are preferably used for treating the roots of teeth. To that end, the tooth enamel is normally already partially destroyed in the upper region of the teeth so that the dentine of the tooth is exposed and it is possible to carry out treatment on the tooth through the dentine and into the root canals. 
         [0031]    A first method for the production of a root canal instrument comprises the following method steps. First, a sub-millimetre thick core of the above-mentioned order of magnitude is produced from a nickel-titanium alloy or an electrically conductive plastics material, preferably a carbon-fibre-reinforced plastics material. There are then covered over those regions of the outer surface of the core which are to be protected from electrodeposition of an adhesion-producing mass. For that purpose preference is given to the selective application of electrically insulating lacquers. A coating a few micrometres thick of an adhesion-producing mass with abrasive particles is then deposited on those regions of the outer surface of the core which are not covered by the insulating layer. Afterwards, the insulating layer can be removed. 
         [0032]    Such a method has the advantage that a root canal instrument can be produced in three reliable method steps, the core being produced in the first method step and the structuring being prepared in a second method step and the coating already being performed in a third method step. 
         [0033]    An alternative method for the production of a root canal instrument comprises the following method steps. First, a sub-millimetre-sized core is again produced, but this time from fibre-reinforced electrically non-conductive plastics material. An adhesion-producing mass of a flexible layer of an adhesive or of a flexible plastics mass is then applied to partial regions of the surface of the core. Subsequently, abrasive particles are anchored in the adhesion-producing mass to the extent that they project out from the adhesion-producing mass. This method has the advantage that, depending on the properties of the layer of the adhesive or the plastics mass, which hold the abrasive particles, structuring can be provided or not. From a manufacturing point of view it is advantageous if the layer of the adhesive or plastics mass is fully matched in terms of its flexibility to the flexibility of the core so that structuring of the adhesion-imparting coating for the abrasive particles is not necessary. 
         [0034]    In a preferred means of implementing the invention, co-extrusion or co-injection-moulding of plastics material and abrasive particles on the plastics core is carried out. The tips of the abrasive particles are then freed of the plastics material so that a high cutting capability is produced. Co-extrusion and co-injection-moulding of adhesion-producing mass and abrasive particles simplifies production and yields relatively economical root canal instruments. 
         [0035]    Furthermore, preference is given to the production of a sub-millimetre thick core of a carbon-fibre-reinforced plastics material being carried out by means of pre-prepared compression moulds or prepregs of carbon fibres pre-coated with plastics material. Such prepregs of carbon fibres can be processed, without great manufacturing outlay, into sub-millimetre thick cores, to which an appropriate coating with abrasive particles can then be applied. 
         [0036]    A further example of implementing the method provides for an adhesion-producing mass of a flexible layer of an adhesive being produced by immersion of the fibre-reinforced plastics core in a solution of an adhesive. Abrasive particles can then be applied by rolling the adhesive-coated core in a particle powder, which particles are anchored in the adhesive as a result of full hardening of the latter. This method too is suitable for mass production. 
         [0037]    The methods explained hereinbefore are based on a round core, which preferably becomes narrower towards the proximal end. The following method example is based on a basic plate, which is first coated and is then cut into suitable strips, which can then in turn be finished by means of twisting to form coated root canal drill bits. For this purpose, the method for the production of a root canal instrument comprises the following method steps. First, a sub-millimetre thick basic plate of a titanium-nickel alloy or of a fibre-reinforced plastics material is produced. The basic plate is then provided with a coating comprising abrasive particles. Finally, the basic plate is divided up into longitudinally extending strips of four-sided or three-sided cross-section in such a manner that narrow sides of the four-sided cross-sections or one side of the three-sided cross-sections have/has the coating. Then, twisting of the strips is carried out to form a root canal drill bit having cutting edges comprising abrasive particles. This method has the advantage that it yields root canal drill bits according to the invention which are a match for chromium steel drill bits in respect of their cutting performance and yet are sufficiently flexible for introduction into a root canal. 
         [0038]    The four-sided cross-sections preferably comprise a rectangle or a parallelogram. The parallelogram is formed when the dividing line is introduced into the coated basic plate not vertically with respect to the surface but at a slant or on an inclination with respect to the surface. After division, the strips having a cross-section in the form of a rectangle and/or a parallelogram and/or a triangle can be twisted, the parallelogram having the advantage that it yields clearly salient cutting edges when twisted. 
         [0039]    For the purpose of coating the basic plate, the plastics mass comprising abrasive particles can be applied on both sides so that the cutting performance of the root canal drill bit is further improved. For coating, electrodeposition can be performed on the sub-millimetre plate in an appropriate electrolyte bath. This has the advantage that coating is simultaneously possible for a large number of root canal drill bits. The mean particle size k of the abrasive particles in that case is in the range from 1 to 500 μm. Division of the prepared basic plate into longitudinally extending strips is preferably carried out by means of high-speed saws having air bearings and diamond saw blades having a thickness of up to 100 μm and a cutting depth t where t is up to 1 mm. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]    The invention will now be explained with reference to the accompanying Figures. 
           [0041]      FIG. 1  is a schematic diagram of a root canal instrument according to a first embodiment of the invention in use. 
           [0042]      FIG. 2  is an enlarged schematic diagram of the root canal instrument according to  FIG. 1 . 
           [0043]      FIG. 3  is a schematic diagram of a root canal instrument according to a second embodiment of the invention. 
           [0044]      FIG. 4  is a schematic diagram of a root canal instrument according to a third embodiment of the invention. 
           [0045]      FIG. 5  is a schematic diagram of a root canal instrument according to a fourth embodiment of the invention. 
           [0046]      FIG. 6  is a schematic diagram of a root canal instrument according to a fifth embodiment of the invention. 
           [0047]      FIG. 7  is a schematic diagram of a root canal instrument according to a sixth embodiment of the invention. 
           [0048]      FIG. 8  is a schematic diagram of a root canal instrument according to a seventh embodiment of the invention. 
           [0049]      FIG. 9  shows a diagrammatic cross-section through the root canal instrument according to  FIG. 8  along the line of section A-A in  FIG. 8 . 
           [0050]      FIG. 10  shows a diagrammatic cross-section through a variant of the root canal instrument according to  FIG. 8  along the line of section A-A in  FIG. 8 . 
           [0051]      FIG. 11  shows a diagrammatic cross-section through a further variant of the root canal instrument according to  FIG. 8  along the line of section A-A in  FIG. 8 . 
           [0052]      FIG. 12  is a schematic diagram of a root canal instrument according to an eighth embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0053]      FIG. 1  is a schematic diagram of a root canal instrument  1  according to a first embodiment of the invention in use. The root canal instrument  1  has a push-in coupling  27  to a drive device (not shown), which brings about torque-protected rotation of the push-in coupling  27  in the direction of rotation B. The root canal instrument  1  furthermore has a thin core  7 , which becomes narrower towards the proximal end  10  of the root canal instrument  1 . The distal end  28  of this core  7  is embedded in the material of the push-in coupling  27  by means of a shape-based and friction-based connection. In an upper, distal region  29 , the outer surface  12  of the core  7  is uncoated, and in a lower, proximal region  31  the core  7  has a coating  8  with abrasive particles. 
         [0054]    A root canal instrument  1  of such a kind is guided through an opening  33  in the tooth enamel  5  of the crown  6  of a tooth  3  and through the dentine  4  to a tooth root  34 , and, by virtue of its high flexibility, it follows the curvature of the root canal  2  of the tooth. For that purpose, the proximal end  10  of the root canal instrument  1  remains free of a coating  8  with appropriate abrasive particles, in order to ensure that the proximal end  10  of the root canal instrument  1  guides the lower, proximal region  31  of the core  7 , which region is occupied by abrasive particles, through that material of the tooth root canal  2  which is softer than the tooth cementum  32 , along the curvature of the canal, without prematurely penetrating through the surrounding dentine  4  and tooth cementum  32  as a result of the rotatory and grinding movement of the root canal instrument  1  and not following the curvature of the root canal  2 . 
         [0055]      FIG. 2  is an enlarged schematic diagram of the root canal instrument  1  according to  FIG. 1 . In the case of this root canal instrument  1 , the upper, distal region  29  of the core  7  is free of a coating  8  with abrasive particles  9  over a length l 1  of the overall length l of the core  7  of the root canal instrument  1 . The diameter d 1  at the distal end  28 , which is embedded in the push-in coupling  27 , is from 0.1 to 3 mm. The coating-free region  11  in the upper, distal region  29  has a length l 1  of about 5 mm, whereas the region occupied by abrasive particles has a length l 2  of preferably from 0.5 to 25 mm. The core  7  narrows towards the uncoated region  15  of the proximal end  10  to a diameter d 2 , the diameter d 2  being from 0.1 to 1.2 mm. 
         [0056]    In this first embodiment of the invention, the core  7  is made from a carbon-fibre-reinforced plastics material, the plastics material on the outer surface  12  of the core  7  in the lower, proximal region  31  anchoring the abrasive particles  9  in such a way that they project out from the outer surface  12  of the plastics material. The plastics material, which holds the carbon fibres of the core together, accordingly serves at the same time for anchoring abrasive particles  9  on the outer surface  12  in the lower, proximal region  31  of the root canal instrument  1 . By that means it is ensured that the flexibility of the coating  8  with abrasive particles  9  is optimally matched to the flexibility of the core  7 . 
         [0057]    The curvature, visible in  FIG. 2 , of the lower, proximal region  31  of the core  7  coated with abrasive particles follows the curvature of a root canal; it does not show the root canal instrument  1  in its position of rest. In its position of rest, by virtue of the superelasticity of the carbon fibres, the root canal instrument  1  returns to its original longitudinally extended rectilinear shape indicated here by the broken line  35 . 
         [0058]      FIGS. 3 to 8  show different embodiments of the invention, especially in respect of the structuring of the coating  8  in the lower, proximal region  31  of the root canal instrument  1 . Components having the same functions as in  FIGS. 1 and 2  are marked with the same reference symbols in  FIGS. 3 to 8  and are not separately explained. 
         [0059]      FIG. 3  is a schematic diagram of a root canal instrument  30  according to a second embodiment of the invention. This root canal instrument  30  has a core  7  which consists of an electrically conductive material. 
         [0060]    This electrically conductive material can be a nickel-titanium alloy, which comprises about 45% nickel by weight and about 55% titanium by weight and which is distinguished by its superelasticity, which is characterised in that, in addition the Hooke&#39;s elasticity, as chromium carbon steels are known to have, an additional elasticity due to the shape memory of this alloy also comes into play, wherein temporarily as a result of mechanical loading caused by deformations a hexagonal structure called martensite forms in the cubic host structure called austenite, the martensitic structure re-forming again into the host structure on unloading. 
         [0061]    As an adhesion-producing agent for the abrasive particles  9 , a metal coating can be electrodeposited on such metallic materials for the core  7 , the abrasive particles  9  being deposited on the outer surface  12  of the core  7  at the same time as the electrodeposition. In order to achieve the ring-shaped structure of deposited coating which is shown in  FIG. 3 , the uncoated regions  15 , which are likewise ring-shaped in this embodiment, can be protected with an insulating layer in the electrodeposition bath. The insulating layer can subsequently be removed after deposition of the ring-shaped structured coating  8 . 
         [0062]    Instead of a metallic core  7 , a core  7  of plastics material can also be prepared for electrodeposition, either by coating the outer surface  12  with conductive particles by, for example, sputtering or by including a filler of metallic particles such as silver with the plastics material and thereby making an electrically conductive core  7 . Instead of the circular rings of the structured coating  9  in  FIG. 3 , elliptically shaped rings can also be deposited on the surface  12  of the core. 
         [0063]      FIG. 4  is a schematic diagram of a root canal instrument  40  according to a third embodiment of the invention. In this case the flexibility of the coating  8  is matched to the flexibility of the core  7  by means of a helically shaped coating  8  structure. A helically shaped coated structure accordingly alternates with helically shaped uncoated regions  15  in the lower, proximal region  31  of the root canal instrument  40 . 
         [0064]      FIG. 5  is a schematic diagram of a root canal instrument  50  according to a fourth embodiment of the invention. In this embodiment, the coating  8  has been so structured that lozenges  16  are occupied by abrasive particles, which are surrounded by two oppositely extending helically shaped uncoated regions  15 . This pattern of lozenges  16  can be produced by means of suitable preparation as has already been discussed for  FIG. 4 . 
         [0065]      FIG. 6  is a schematic diagram of a root canal instrument  60  according to a fifth embodiment of the invention. In this embodiment of the invention, elliptically shaped islands  13 , which are surrounded by regions without coating  15 , are occupied by abrasive particles. In this case the uncoated regions  15  form a continuous region, which improves the flexibility of this embodiment of the invention. 
         [0066]      FIG. 7  is a schematic diagram of a root canal instrument  70  according to a sixth embodiment of the invention. In this sixth embodiment of the invention, circular round regions  14  within the coating  8  have been kept free of particle coating. The flexibility of the coating  8  can likewise be matched to the flexibility of the core  7  by means of these uncoated circular regions  14 . 
         [0067]    In principle, mixed forms of the coating structures as shown in  FIGS. 2 to 7  are also possible. 
         [0068]      FIG. 8  is a schematic diagram of a root canal instrument  80  according to a seventh embodiment of the invention. In this case, production of the core  7  does not start from a prefabricated conical rod which narrows towards the proximal end  10  as the core but rather starts from a basic plate produced using a core material such as nickel-titanium alloy and/or a plastics material. The basic plate is coated on one side or on two sides with an adhesion-producing mass  17  comprising abrasive particles. This basic plate can subsequently be divided into strips of rectangular or parallelogram-shaped or triangular cross-section. As a result of twisting the strips, the root canal instrument  80  shown in  FIG. 8 , having a root canal drill bit  21 , can be produced. In principle it is also possible first to produce the strips from an uncoated nickel-titanium plate and then to carry out coating and finally to twist the strips or bars. 
         [0069]    For the purpose there is used, in the case of a carbon-fibre-reinforced plastics basic plate, a thermoplastic material, which is heated up for the purpose of twisting after the strips have been produced. When twisting a metallic strip of a nickel-titanium alloy, this is likewise heated up in order to retain the austenitic structure. As a result of coating of the basic plate with an adhesion-producing mass including a filler of particles  17 , in a thickness h of from 0.5 to 50 μm, this root canal drill bit  21  gives rise to a root canal instrument  80 , which has abrasive particles on its cutting edges  22  and  23 . 
         [0070]      FIG. 9  shows a diagrammatic cross-section through the root canal instrument  80  along the line of section A-A in  FIG. 8 . The four-sided cross-section  18  in the shape of a rectangle  24  having the narrow sides  19  and  20  arises as a result of dividing the basic plate up into individual strips, with sawing being carried out in a perpendicular direction to the coatings  8 . In order that both narrow sides  19  and  20  can be occupied by abrasive particles  9 , the basic plate is coated on two sides with an adhesion-producing mass  17  comprising abrasive particles  9 . The adhesion-producing mass  17  can be a bronze layer  26 , in which the abrasive particles  9  are embedded. As a result of the abrasive particles  9 , the cutting edges  22  and  23  become effective on rotation of the root canal drill bit  21  in the direction of arrow C, while in the opposite direction D the cutting edges  36  and  37  located opposite bring about removal of material. 
         [0071]      FIG. 10  shows a diagrammatic cross-section through a variant of the root canal instrument  80  according to  FIG. 8  along the line of section A-A in  FIG. 8 . Components having the same functions as in  FIG. 9  are marked with the same reference symbols and are not separately explained. 
         [0072]    The difference in the case of this variant lies in the four-sided cross-section  18  of the core of the root canal drill bit  21 . This cross-section comprises a parallelogram  25 , which arises as a result of the basic plate, having been coated on two sides with an adhesion-producing mass  17 , being divided up into strips at an angle of inclination with respect to the coatings. In this embodiment of the invention, a cutting action of the root canal drill bit  21  is obtained solely in the direction of rotation C, in which the cutting edges  22  and  23  with their abrasive particles  9  promote the removal of material. 
         [0073]    In their material removal action, such root canal drill bits  21  provided with abrasive particles  9  on their cutting edges  22  and  23  surpass those root canal drill bits made of a nickel-titanium alloy which are known from the prior art and which, because of their reduced Vickers hardness, have disadvantages in their material removal rate compared to chromium carbon steels. Accordingly, this root canal instrument combines, in ideal manner, the high flexibility of nickel-titanium alloys and/or of plastics materials with the high cutting and polishing capability of abrasive particles to form a new, highly effective root canal instrument. 
         [0074]      FIG. 11  shows a diagrammatic cross-section through a variant of the root canal instrument  80  according to  FIG. 8  along the line of section A-A in  FIG. 8 . Components having the same functions as in  FIG. 8 ,  9  or  10  are marked with the same reference symbols and are not separately explained. 
         [0075]    The difference in the case of this third variant lies in the triangular cross-section  18  of the core of the root canal drill bit  21 . This cross-section comprises a triangle  38 , which arises as a result of the basic plate, having been coated on two sides with an adhesion-producing mass  17 , being divided up into strips at two angles of inclination with respect to the coatings. In this embodiment of the invention, a cutting action of the root canal drill bit  21  is obtained both in the direction of rotation C and also in the direction of rotation D, in which the cutting edges  22  and  23  with their abrasive particles  9  promote the removal of material. 
         [0076]      FIG. 12  is a schematic diagram of a root canal instrument  90  according to an eighth embodiment of the invention. The root canal instrument  90  has a core ( 7 ) of a flexible elastic material having shape memory. At its proximal end  10  there is arranged a grinding or polishing body  39 , which comprises abrasive particles  9 . This grinding or polishing body  39  is a part of the core ( 7 ) or is screwed, bonded, soldered or welded to the core  7  or is electrodeposited on the proximal end  10  of the core  7  or applied to the proximal end  10  by means of injection-moulding technology. 
       LIST OF REFERENCE SYMBOLS 
       [0000]    
       
           1  root canal instrument 
           2  root canal 
           3  tooth 
           4  dentine 
           5  tooth enamel 
           6  crown of tooth 
           7  core 
           8  coating 
           9  abrasive particles 
           10  proximal end 
           11  further uncoated region 
           12  outer surface of core 
           13  elliptically shaped island 
           14  round region 
           15  region without coating 
           16  lozenge 
           17  adhesion-producing mass 
           18  four-sided cross-section 
           19  narrow side 
           20  narrow side 
           21  root canal drill bit 
           22  cutting edge 
           23  cutting edge 
           24  rectangle 
           25  parallelogram 
           26  bronze layer 
           27  push-in coupling 
           28  distal end 
           29  upper, distal region 
           30  root canal instrument (second embodiment) 
           31  lower, proximal region 
           32  tooth cementum 
           33  opening 
           34  tooth root 
           35  broken line 
           36  cutting edge 
           37  cutting edge 
           38  triangle 
           39  grinding or polishing body 
           40  root canal instrument (third embodiment) 
           50  root canal instrument (fourth embodiment) 
           60  root canal instrument (fifth embodiment) 
           70  root canal instrument (sixth embodiment) 
           80  root canal instrument (seventh embodiment) 
           90  root canal instrument (eighth embodiment) 
         A-A line of section 
         B direction of rotation 
         C direction of rotation 
         D direction of rotation 
         d diameter of core 
         d 1  diameter of core at the distal end 
         d 2  diameter of core at the proximal end 
         h thickness of the adhesion-producing mass 
         l length of the core 
         l 1  length of the core in the distal region 
         l 2  length of the core in the proximal region