Endodontic tool for shaping a dental root canal, and method for making said endodontic tool

Abstract

The present invention relates to an endodontic tool for shaping a dental root canal, wherein said tool comprises a handle (13), a tang (11), and a stem (10) made of polymeric material and comprising a tip (12) and at least one helically developing cutting edge (100), and wherein said stem (10) develops along a longitudinal axis (A-A). The stem (10) has a substantially conical or tapering twisted shape, in particular said stem (10) having a solid cross-section with a diameter increasing from the tip (12) to the tang (11).

Description

[0001] ENDODONTIC TOOL FOR SHAPING A DENTAL ROOT CANAL, AND METHOD FOR MAKING SAID ENDODONTIC TOOL

[0002] DESCRIPTION

[0003] Field of the invention

[0004] The present invention relates to an endodontic tool for shaping a dental root canal; the present invention also relates to an endodontic kit comprising a plurality of endodontic tools for shaping a dental root canal; the present invention further relates to a method for making said endodontic tool.

[0005] Background art

[0006] The present invention finds application in the endodontics field, i.e. that branch of dentistry which deals with the tooth's internal tissues and dental pulp.

[0007] In this frame, it is known that, when the tissues around a dental root get damaged or diseased due to caries or trauma, it is necessary to resort to endodontic therapy.

[0008] Endodontic therapy, also referred to as canal treatment or root canal therapy, provides for removing pulp tissue and bacteria from the pulp canal by means of suitable tools that shape, in particular ream, the root canals, thus eliminating their vessel and nerve contents up to the tooth's apex.

[0009] After such shaping, the shaped endodontic spaces are usually subjected to a number of cleaning steps, said cleaning steps being aimed at significantly reducing the residual bacterial charge, followed by three-dimensional sealing of all canal structures (usually by means of thermoplastic rubber, e.g. gutta-percha, and a cement material, or by means of a sealing material for medical use) to seal the canal and deactivate any residual bacterial charge.

[0010] Once the canals have been sealed, the tooth can be reconstructed.

[0011] In this context, proper shaping of dental canals is the main objective of any endodontic treatment, because it determines the effectiveness of the subsequent three-dimensional cleaning and sealing steps.

[0012] The goals of a correct dental canal shaping procedure are:

[0013] - preserving the canal's initial anatomy, avoiding any transport phenomena that might significantly deviate the canal axis from its original configuration, thus weakening the tooth, which may then break;

[0014] - creating a frustoconical geometry easily accessible during the subsequent three-dimensional cleaning and sealing steps.

[0015] For both of such goals to be achieved, shaping tools must ensure good flexibility to adapt to the canal's initial geometry, while being sufficiently strong to withstand the stresses to which they are subjected in operation.

[0016] In particular, such tools are set in continuous rotation and / or reciprocating motion within the canal, the axis of which may have a variable number of bends (there may be one or more bends) with variable radii (bends characterized by different radii of curvature). Flexing under rotation results in fatigue stress, which is one of the main causes for tool failure in operation. Failure may also occur when the tool gets jammed due to obstacles in the canal's geometry, which prevent it from turning. When jamming occurs, the tools undergo torsional loads that may cause it to yield statically, when the tool is "stretched", or to break, when an actual fracture occurs.

[0017] It is known in the art that the shaping of a dental root canal is performed by means of suitable endodontic tools, called canal files or endodontic files, which comprise a handle (or grip), a tang (or shank), and a stem (or active part) ending with a tip (sometimes also referred to as "penetration guide").

[0018] Modern endodontic tools for shaping dental root canals are made of metallic material, in particular Ni-Ti alloy or stainless steel, and their stem has a substantially conical shape, i.e. a cross-section with a diameter increasing from the tip to the tang. In particular, the tip diameter may vary from a minimum diameter of 0.06 mm up to a maximum diameter of 1.20 mm, and conicity is generally variable along the tool, which ends with a diameter at the interface between the tang and the stem ranging from 0.70 mm to 1.30 mm; as concerns the length of the working zone, it may vary between approximately 15 mm and 20 mm.

[0019] Production processes that are typical of precision micromechanics are used for manufacturing metallic tools with such small dimensions. Therefore, the production costs and times necessary for manufacturing endodontic tools made of metallic material are currently significantly greater than would be possible with large-scale production.

[0020] Moreover, the production process for manufacturing endodontic tools made of metallic material limits the possible geometries that can be used to improve cutting efficiency, especially in the region proximal to the tip. In particular, the currently adopted production technology strongly limits the possibility of creating large helix angles to improve cutting efficiency.

[0021] Another criticality which is important in canal treatment, particularly when performed by using endodontic tools made of metallic material, lies in potential failure of the tool during the shaping operation, which generally implies that part of the tool gets trapped inside the canal. In order to remove the trapped metal fragment, it is necessary to perform invasive and complex treatments. It is often impossible to remove the metal fragment without resorting to microsurgery (apicoectomy). Breakage of the tool inside the dental root canal occurs because of bending fatigue stresses generated by the curvature imposed by the canal under treatment and by tool rotation, and also because of static torsional stresses created by canal geometries stopping the rotation of the tool and generating torsional overloads.

[0022] Some proposals have been made over time in an attempt to solve the problems suffered by the tools known in the art.

[0023] For example, patent document EP 1377230 Bl illustrates an endodontic tool made of composite material, comprising a polymeric matrix reinforced with long carbon fibres oriented in a specific direction, e.g. twined around a longitudinal direction.

[0024] Patent document EP 2800537 Bl illustrates an instrument for cleaning the dental canal after shaping a root canal, which is made of polymeric material, and which comprises a plurality of fins and protrusions to provide such cleaning.

[0025] Utility model CN 206239536 U illustrates an endodontic tool made of polymeric material and having a hollow cross-section, for performing both canal reaming and canal cleaning. Object of the invention

[0026] In this frame, it is the main object of the present invention to provide an endodontic tool for shaping a dental root canal so conceived as to overcome the drawbacks of prior-art solutions.

[0027] In particular, it is one object of the present invention to provide an endodontic tool for shaping a dental root canal so conceived as to reduce, or even eliminate, the potential risk of tool failure inside the canal during the shaping operation.

[0028] It is another object of the present invention to provide an endodontic tool for shaping a dental root canal, wherein said tool is so realized as to improve cutting and debris removal efficiency due to geometries until now impossible to obtain. It is a further object of the present invention to provide an endodontic tool for shaping a dental root canal, wherein said tool is so realized as to ensure adequate flexibility to adapt to the canal's initial geometry, along with very good resistance to the stresses to which said tool is subjected during the shaping operation.

[0029] It is another object of the present invention to provide an endodontic tool for shaping a dental root canal, so conceived as to ensure a correct dental root canal shaping procedure, particularly in terms of proper preservation of the canal's initial anatomy and in terms of creation of a frustoconical geometry easily accessible during the subsequent cleaning and sealing steps, while at the same time drastically reducing the cost incurred for producing said endodontic tool.

[0030] It is yet another object of the present invention to provide an endodontic tool for shaping a dental root canal, wherein the manufacturing process requires less time and / or lower costs than prior-art processes.

[0031] Further objects, features and advantages of the present invention will become apparent in light of the following detailed description and of the annexed drawings, which are provided herein merely by way of non-limiting explanatory example.

[0032] Summary of the invention

[0033] The objects of the present invention are achieved through an endodontic tool for shaping a dental root canal, which comprises a handle and a stem. Said tool comprises also a tang interposed between, and integrally fixed to, the handle and the stem.

[0034] The stem, which extends along a longitudinal axis, comprises a tip and at least one helically developing cutting edge. The stem has a substantially conical or tapering twisted shape, and a solid cross-section with a diameter increasing from the tip to the tang. The stem of the endodontic tool of the invention is made of a polymeric material.

[0035] According to one embodiment, the polymeric material employed for making the stem of the endodontic tool of the invention has a ratio between fatigue limit and Young's modulus equal to or greater than 16.5 MPa / GPa, preferably greater than 20.0 MPa / GPa, and a yielding deformation equal to or greater than 4.4 % .

[0036] According to one aspect, the polymeric material is a thermoplastic or thermosetting material.

[0037] According to one aspect, the polymeric material comprises a reinforcement material.

[0038] According to a preferred embodiment of the above aspect, the reinforcement material comprises a plurality of non-continuous fibres, preferably non- continuous fibres shorter than 5 mm.

[0039] According to one embodiment, the stem of the endodontic tool of the invention comprises a surface coating consisting of particles of at least one material having high hardness and a low coefficient of friction.

[0040] According to one embodiment, the stem of the endodontic tool of the invention is so realized as to comprise a first diameter at the tip, in particular said first diameter ranging between approximately 0.06 mm and 1.20 mm.

[0041] According to one embodiment, the stem of the endodontic tool of the invention is so realized as to comprise a second diameter at an interface between the stem and the tang, in particular said second diameter ranging between approximately 0.70 mm and 1.30 mm.

[0042] According to one embodiment, the stem of the endodontic tool of the invention is so realized as to comprise a length from the interface between the stem and the tang to the tip, in particular said length ranging between approximately 15 mm and 20 mm. According to one aspect, the handle and the tang of the endodontic tool of the invention are made of a polymeric material, preferably the same polymeric material used for making the stem.

[0043] According to one aspect, the handle and the tang of the endodontic tool of the invention develop coaxially along the same longitudinal axis of the stem.

[0044] According to one embodiment, the stem of the endodontic tool of the invention extends along an axis with a three-dimensional development, preferably with a helical development.

[0045] The objects of the present invention are also achieved through an endodontic kit comprising a plurality of endodontic tools as previously described herein, wherein said plurality of endodontic tools comprise endodontic tools of different sizes, and wherein said different sizes relate to a first diameter at the tip, and / or a second diameter at the interface between the stem and the tang, and / or a length of the stem from the tip to the interface between the stem and the tang.

[0046] The objects of the present invention are also achieved through a method for making an endodontic tool for shaping a dental root canal as previously described herein, wherein said method comprises the step of making the stem of the endodontic tool by using a process of injection moulding of a polymeric material.

[0047] According to one embodiment, the method of the invention comprises the step of coating the stem with a surface coating consisting of particles of at least one material having high hardness and a low coefficient of friction.

[0048] According to one aspect of the above embodiment, the step of coating the stem with a surface coating consisting of particles of at least one material having high hardness and a low coefficient of friction is carried out by using a multimaterial injection moulding process.

[0049] According to one aspect which is alternative to the preceding one, the step of coating the stem with a surface coating consisting of particles of at least one material having high hardness and a low coefficient of friction is carried out by using a process of deposition of a thin layer of particles onto said stem.

[0050] According to a further aspect, which is alternative to both of the preceding aspects, the step of coating the stem with a surface coating consisting of particles of at least one material having high hardness and a low coefficient of friction is carried out by using a multimaterial three-dimensional printing process.

[0051] According to one embodiment, the method of the invention further comprises the step of strengthening the polymeric material with a reinforcement material.

[0052] According to one aspect of the above embodiment, the reinforcement material comprises a plurality of non-continuous fibres, preferably non-continuous fibres shorter than 5 mm.

[0053] Brief description of the drawings

[0054] The following detailed description of some embodiments of the invention will refer to the accompanying drawings, provided merely by way of non-limiting example, wherein:

[0055] - Figure 1 is a side view of an endodontic tool for shaping a dental root canal according to a first embodiment of the present invention;

[0056] - Figure 2 shows a portion (stem) of an endodontic tool for shaping a dental root canal according to a second embodiment of the present invention;

[0057] - Figure 3a shows a CAD model of a stem of an endodontic tool for shaping a dental root canal according to the first embodiment of the present invention;

[0058] - Figure 3b shows a computational mesh for the stem of Figure 3a, with boundary conditions for analysing the stresses arising when operating in a curved canal;

[0059] - Figure 3c shows a computational mesh for the stem of Figure 3a, with boundary conditions for analysing the stresses arising when the tool gets jammed in operation;

[0060] - Figure 4a shows the Von Mises equivalent tensile stresses generated by the simulation model in the configuration shown in Figure 3b;

[0061] - Figure 4b shows the Von Mises equivalent tensile stresses generated by the simulation model in the configuration shown in Figure 3c.

[0062] Detailed description of some embodiments of the invention

[0063] An endodontic tool 1 for shaping a dental root canal according to a first embodiment of the present invention is shown in Fig. 1.

[0064] Reference numeral 10 designates a stem of an endodontic tool 1 (which can also be defined as canal file or endodontic file) for shaping a dental root canal.

[0065] Said stem 10 comprises a tip 12. The stem 10 develops along a longitudinal axis A-A (schematically shown as a dashed-dotted line in Figure 1) and has a substantially conical or tapering twisted shape, i.e. it has a cross-section (with reference to said longitudinal axis A-A) with a diameter that increases from the tip 12 to the interface 15 between the stem 10 and the tang 11.

[0066] Said cross-section is solid, i.e. it has no empty volumes or holes inside.

[0067] The stem 10 further comprises at least one helically developing cutting edge 100.

[0068] In this context, the stem 10 is so realized as to comprise:

[0069] - a first diameter DI at the tip 12, said first diameter DI preferably ranging between approximately 0.06 mm and 1.20 mm;

[0070] - a second diameter D2 at the beginning of the interface 15 between the stem 10 and the tang 11, said second diameter D2 preferably ranging between approximately 0.70 mm and 1.30 mm;

[0071] - a length L from the interface 15 to the tip 12, said length L preferably ranging between approximately 15 mm and 20 mm.

[0072] The endodontic tool 1 for shaping a dental root canal according to the present invention may also comprise a handle 13 associated with the tang 11. The tang 11 is interposed between, and integrally fixed to, the handle 13 and the stem 10. The handle 13 is so configured as to facilitate the use of the tool 1 during manual shaping operations; as an alternative, the handle 13 may be so realized as to comprise coupling means configured to attach the tool 1 to a motorized device that sets the tool in continuous or reciprocating rotation.

[0073] The tang 11 may be so realized as to comprise circumferential markings substantially perpendicular to the longitudinal axis A-A, said markings being spaced apart at regular intervals to allow a user / endodontist to know the depth to which the tool (in particular, the stem 10) has penetrated the dental root canal. The stem 10 of the endodontic tool 1 for shaping a dental root canal of Figure 1 is made from thermoplastic or thermosetting polymeric material. Furthermore, the stem 10 has an outer surface with at least one helically developing cutting edge 100 ensuring high cutting efficiency. The stem 10 has a surface treatment to increase surface hardness and reduce friction against the walls of a dental root canal to be shaped. According to the embodiment illustrated in Figure 1, the handle 13 and the tang 11 are also made of a polymeric material, preferably the same polymeric material used for making the stem 10; in addition, the handle 13 and / or the tang 11 develop coaxially along said longitudinal axis A-A; basically, the handle 13 and / or the tang 11 are substantially coaxial to the stem 10.

[0074] The polymeric material used for making the stem, the handle and / or the tang may be, as aforesaid, either of the thermoplastic type or of the thermosetting type. The polymeric material may optionally comprise a reinforcement material, e.g. fibres or particles (flakes). Whether it consists of fibres or particles, the reinforcement material is generally disposed randomly, i.e. it is not oriented in specific directions.

[0075] For example, the reinforcement material may comprise a plurality of non- continuous fibres, preferably non-continuous fibres shorter than 5 mm, or anyway shorter than the length L of the stem 10.

[0076] For the purpose of validating the suitability of polymeric materials for the fabrication of endodontic tools for shaping a dental root canal, a preliminary finite-element analysis was carried out which made it possible to assess, with good approximation, the stresses to which such tools are subjected in critical conditions, i.e. when the tool gets jammed while shaping a canal with extended curvature. The geometry of the simulated tool was inspired by commercially available canal shaping tools. Stresses and constraints were applied in accordance with the literature.

[0077] Figure 3a shows the CAD model of the tool, with basic dimensions in millimetres and the discretization mesh for the finite-element analysis.

[0078] The finite-element model was simulated in the Hypermesh environment with static and linear analyses. A convergence study was also performed to identify the dimension of the tetrahedral elements of the finite-element model, which turned out to be 0.1 mm. In the analyses, an elastic linear material with an elastic modulus of 1 GPa and a Poisson's coefficient of 0.3 was considered. Assuming the device to operate in the elastic range, so that the effect superposition principle holds, two analyses were carried out.

[0079] The first analysis evaluated the effect of canal curvature by constraining the base nodes of the tool and applying a displacement of 5 mm to the tip (the displacement is indicated by letter v in Figure 3b). The bending stresses thus obtained represent the alternate component of the fatigue stress to which the tool is subjected while shaping the canal.

[0080] The second analysis evaluated the effect of a jam occurring while shaping the canal. The nodes lying on the helical lateral faces of the tools were constrained, and a twisting moment Mt of 2.5 Nmm (Newton millimetre) was applied to the base nodes, as shown in the literature (Figure 3c). The torsional stresses thus obtained represent the extreme case of the entire tool jamming while executing the shaping operation. Figure 4a shows, for the analysis carried out, the tension field (Von Mises equivalent stresses) with reference to the stresses shown in Figure 3b, while Figure 4b shows, for the analysis carried out, the tension field (V on Mises equivalent stresses) with reference to the stresses shown in Figure 3c. As can be inferred from the data shown in Figures 4a and 4b, the maximum stress (V on Mises equivalent stress) due to bending amounts to 20 MPa (for the IGPa material taken into account), and represents the alternate component of the fatigue stress (Figure 4a), while the Von Mises equivalent stress induced by jamming amounts to 70 MPa (Figure 4b). Based on the simulation results, it is possible to estimate, in an approximate but conservative manner, the following characteristic quantities: the ratio between fatigue limit and Young's modulus necessary for shaping the canal, wherein the fatigue limit is estimated considering the maximum stress in the bending model; the yielding deformation necessary for shaping the canal, obtained as the ratio between the yield stress and the Young's modulus of the material, wherein the yield stress is estimated starting from the equivalent maximum stress reached during the load cycle and considering a safety coefficient of 1.5, in accordance with the literature.

[0081] Such minimum characteristic quantities can be summarized as follows: Ratio between fatigue limit and Young's modulus, OD / E [MPa / GPa]: 20.0 Yielding deformation, EY [%]: 4.4

[0082] The above-summarized minimum characteristic quantities ensure, in the conditions simulated by the model, the structural integrity of the tool while shaping the dental root canal.

[0083] At any rate, the above-mentioned values of the minimum characteristic quantities, obtained by simulation, are conservative estimates, and less stringent values may be sufficient in real conditions.

[0084] It must also be pointed out that, according to the Ashby charts and the material tables contained in the Granta EduPack software, the metallic materials currently used for endodontic tools according to the prior art have, on average, yielding deformation values of less than 8% and ratios between fatigue limit in MPa and elasticity modulus in GPa below 4. Therefore, the imposed curvature being equal, any material having yielding deformation values higher than 8% and ratios between fatigue limit in MPa and elastic modulus in GPa greater than 4 has characteristics of static and fatigue strength that are equal to or greater than those of the best metallic materials currently in use for manufacturing endodontic tools. Based on such considerations, and starting from a study of the Ashby charts (not shown herein, since they are known to those skilled in the art) and using the databases available in the Ansys Granta Edupack program, it is possible to identify families of polymers, possibly (but not necessarily) reinforced, which can reach characteristic quantities OD / E and EY greater than or equal to those resulting from finite-element modelling, and which can provide better performance than the metallic materials currently in use.

[0085] Based on such criteria, the polymeric material can be selected among, for example, at least one of the following materials: thermoplastic polyurethane (TPU), possibly (but not necessarily) reinforced with a percentage of randomly disposed glass fibres ranging between 30% and 40% by weight, thermosetting polyimide (PI), hard rubber (ebonite).

[0086] In general, the surface hardness of the tool should be greater than or equal to that of dentin, which ranges between 60 HV and 70 HV. A good surface hardness of the tool is an important factor.

[0087] When complex and narrow canals characterized by extended curvature are involved, in fact, the stresses generated by flexion and torsion may reach values incompatible with the yield strength of a polymeric material or a polymeric- matrix material. The stresses generated by torsion are particularly critical because they are induced by friction between the canal walls and the rotating tool. If wall pressure is high (as is the case, for example, in narrow canals), the torsional load can reach values resulting in yielding of the polymeric or polymeric-matrix tool. The torsional load due to frictional contact between the canal walls and the tool generates stresses in the tool which, since they are not dependent on the Young's modulus, cannot be reduced through the use of highly pliable materials such as polymeric ones. On the contrary, the flexion induced by the canal's curvature is less critical for polymeric materials, since it generates stresses that, since they are dependent on the Young's modulus of the material, are not critical for highly pliable materials. In conclusion, the hardness of dentin and the stresses induced by the torsional load generated by frictional contact between the canal walls and the tool are two aspects that must be taken into account when using polymeric materials for endodontic tools.

[0088] In order to overcome the above-described possible problems, the polymeric material used for making the stem of the endodontic tool or the entire endodontic tool is preferably subjected to specific treatments aimed at increasing the surface hardness of the stem and / or of the entire tool.

[0089] In a preferred embodiment, the stem may comprise a surface coating consisting of particles of at least one material having high hardness and a low coefficient of friction; such a solution improves the performance of the polymeric material used for making the endodontic tool 1 according to the present invention, since it increases its hardness and yield strength values.

[0090] In general, the stem and / or the entire endodontic tool 1 can be manufactured by using a process of injection moulding of the selected polymer, whether reinforced or not.

[0091] When the stem comprises a surface coating like the one described above, the surface-coated stem can be produced by using a multimaterial moulding process, e.g. injection co-moulding or co-extrusion.

[0092] In such a case, the stem may, for example, have an inner structure made of nonreinforced TPU and an outer layer made of TPU reinforced with 30-40% by weight of glass fibres. This solution ensures compatibility between the inner structure and the outer layer, optimizing both stiffness and surface strength.

[0093] As an alternative, the surface-coated stem may be manufactured by using a multimaterial three-dimensional printing process.

[0094] In this case, the stem may be made by using FDM technology, e.g. by combining non-reinforced PLA (for the inner structure) with PLA reinforced with fibres (e.g. carbon / glass fibres) for the outer layer, or standard nylon (for the inner structure) and carbon nylon (nylon reinforced with carbon fibres) for the outer layer.

[0095] Still with reference to the case of multimaterial three-dimensional printing, it is also possible to use PolyJet (photopolymers) technology, which allows combining, in one component, materials having different mechanical properties, e.g. a rigid material like VeroClear for the inner structure and an elastomer material like Agilus30 for the outer layer, both produced by Stratasys. Such a combination also provides good control over the final properties.

[0096] As a further alternative, the stem provided with said surface coating may be manufactured by deposition of said surface coating, in particular by depositing a thin layer of particles having high hardness and a low coefficient of friction onto said stem. For example, such deposition may be Plasma assisted Chemical Vapor Deposition (PaCVD), essentially involving the deposition of a surface coating less than 3pm thick, consisting of particles of a material, e.g. ta-C (Tetrahedral Amorphous Carbon) having high hardness and a low coefficient of friction (thickness 0.5 pm, hardness 5,000-7,000 HV, coefficient of friction 0.1, deposition temperature 100°C).

[0097] As is known in the art, the shaping of dental root canals is usually performed by means of a plurality of differently sized endodontic tools, so that said shaping can be effected by using in succession tools gradually increasing in size. In this context, it is therefore apparent that the present invention also concerns a kit of endodontic tools made in accordance with the teachings of the present invention, said kit being such as to comprise a plurality of endodontic tools of different sizes, in particular as regards the first diameter DI at the tip 12, and / or the second diameter D2 at the interface 15 between the tang 11 and the stem 10, and / or the length L from the interface 15 to the tip 12.

[0098] The advantages of the present invention are apparent from the above description. Indeed, the peculiar provisions of the present invention result in an endodontic tool for shaping a dental root canal so conceived as to drastically reduce production costs thanks to: the use of raw materials that, the produced volume being equal, are up to twenty times less expensive than Nickel-Titanium (Ni-Ti) alloys and four times less expensive than the steel types currently employed for manufacturing the tools known in the art; the utilization of large-scale production processes, such as injection moulding adapted to small-size products. The small size permits, in fact, the achievement of high accuracy values and processing tolerances as low as 0.01 mm, which values are wholly compatible with the tolerances currently accepted for endodontic tools.

[0099] Another advantage of the provisions of the present invention lies in the fact that they make it possible to create stem geometries with a solid cross-section and a helical cutting edge, e.g. by precision machining of moulds for injection moulding, which cannot otherwise be obtained from the solutions currently known in the art.

[0100] For thermoplastic polyurethanes (TPU), injection moulding can be effected at injection pressures ranging between 100 and 150 MPa.

[0101] For polyimides (PI), injection moulding can be effected at injection pressures ranging between 150 and 200 MPa.

[0102] For ebonites, injection moulding can be effected at injection pressures ranging between 150 and 250 MPa.

[0103] Injection may occur either in a heated mould or in a mould at room temperature. This latter solution is useful to minimize the thermal stresses undergone by the tools.

[0104] Thanks to the new geometries that can be produced, it is possible to significantly increase the cutting and debris removal efficiency of the endodontic tool according to the present invention beyond the current limits imposed by the technology required for manufacturing metallic endodontic tools.

[0105] The new geometries may also have non-rectilinear axes. This means that the axes must not necessarily be two-dimensional (one-dimensional axis in Figure 1) or sinusoidal (two-dimensional axis) as currently adopted for metallic tools, but may have a three-dimensional development, e.g. three-dimensional helices (stem 20 shown in Figure 2), which improve debris removal without requiring cleaning during the shaping process, and which ensure better adaptability to the canal's anatomy because of the variability of the total envelope during the process.

[0106] For example, tools with a three-dimensional axis like the one shown in Figure 2 can exhibit better resistance to fatigue stresses. In terms of minimum characteristic quantities, the above-described simulation model shows, for tools having a three-dimensional axis, a reduction by about 20% in the value of the ratio between fatigue limit and Young's modulus, OD / E [MPa / GPa], which settles to 16.5 as opposed to 20.0 for a tool having a one-dimensional axis: therefore, the three-dimensional axis geometry widens the range of possible polymeric materials that can be used.

[0107] A further advantage of the endodontic tool for shaping a dental root canal according to the present invention lies in the fact that it makes it possible to reduce, or even completely eliminate, the criticalities connected to failures of prior-art tools inside the dental root canal thanks to:

[0108] - the wide selection of (reinforced) polymeric materials having mechanical properties (in particular, in terms of yield, ductility, static and fatigue strength) which are even better than those of the metallic tools currently in use;

[0109] - the possibility of modifying the mechanical properties of the surface (in particular, in terms of hardness and friction) by, for example, injection moulding, which permits the creation of a surface polymeric layer with mechanical characteristics which are different from those of the core, or by multimaterial 3D printing, which permits co-moulding tools made of different materials in specific regions of interest, or, lastly, by deposition of surface coatings by using cold techniques such as PaCVD. In particular, by modifying the mechanical properties of the surface of the endodontic tool according to the present invention it is possible to: i) significantly reduce the friction against the canal walls, thereby decreasing the torsional load applied to the tool and improving the latter's reliability; ii) improve fatigue strength by adding a beneficial surface precompression layer; iii) improve the effectiveness of the helical cutting edges due to increased hardness;

[0110] - the possibility of softening a polymeric fragment trapped within a dental root canal by heating it to temperatures that are not traumatic for a patient, through the use of procedures similar to those currently employed for filling dental canals, followed by canal cleaning by irrigation;

[0111] - the identification of (reinforced) surface-treated polymeric materials capable of exceeding the performance of the metallic materials currently in use.

[0112] The endodontic tool described herein by way of example may be subject to many possible variations without departing from the novelty spirit of the inventive idea; it is also clear that in the practical implementation of the invention the illustrated details may have different shapes or be replaced with other technically equivalent elements.

[0113] It can therefore be easily understood that the present invention is not limited to the above-described endodontic tool, but may be subject to many modifications, improvements or replacements of equivalent parts and elements without departing from the inventive idea, as clearly specified in the following claims.

Claims

CLAIMS1. Endodontic tool (1) for shaping a dental root canal, said tool comprising a handle (13); a stem (10; 20); and a tang (11) interposed between, and integrally fixed to, the handle (13) and the stem (10; 20); wherein the stem (10; 20) comprises a tip (12) and at least one helically developing cutting edge (100), wherein said stem (10; 20) extends along a longitudinal axis (A-A), and wherein said stem (10; 20) has a substantially conical or tapering twisted shape, said stem (10; 20) also having a solid cross-section with a diameter increasing from the tip (12) to the tang (11), and being made of a polymeric material.

2. Endodontic tool (1) according to claim 1, characterized in that the polymeric material has a ratio between fatigue limit and Young's modulus equal to or greater than 16.5 MPa / GPa and a yielding deformation equal to or greater than 4.4 % .

3. Endodontic tool (1) according to any one of claims 1 or 2, wherein the polymeric material is a thermoplastic or thermosetting material.

4. Endodontic tool (1) according to any one of the preceding claims, wherein the polymeric material comprises a reinforcement material.

5. Endodontic tool (1) according to claim 4, wherein the reinforcement material comprises a plurality of non-continuous fibres, preferably non- continuous fibres shorter than 5 mm.

6. Endodontic tool (1) according to any one of the preceding claims, characterized in that the stem (10; 20) comprises a surface coating consisting of particles of at least one material having high hardness and a low coefficient of friction.

7. Endodontic tool (1) according to one or more of the preceding claims, characterized in that said stem (10; 20) is so realized as to comprise a first diameter (DI) at the tip (12), in particular said first diameter (DI) ranging between approximately 0.06 mm and 1.20 mm.

8. Endodontic tool (1) according to one or more of the preceding claims, characterized in that said stem (10; 20) is so realized as to comprise a second diameter (D2) at an interface (15) between the stem (10; 20) and the tang (11), in particular said second diameter (D2) ranging between approximately 0.70 mm and 1.30 mm.

9. Endodontic tool (1) according to one or more of the preceding claims, characterized in that said stem (10; 20) is so realized as to comprise a length (L) from the interface (15) between the stem (10; 20) and the tang (11) to the tip (12), in particular said length (L) ranging between approximately 15 mm and 20 mm.

10. Endodontic tool (1) according to one or more of the preceding claims, characterized in that said handle (13) and said tang (11) are made of a polymeric material, preferably the same polymeric material used for making the stem (10; 20).

11. Endodontic tool (1) according to one or more of the preceding claims, characterized in that said handle (13) and said tang (11) develop coaxially along said longitudinal axis (A- A).

12. Endodontic tool (1) according to one or more of the preceding claims, characterized in that the stem (20) extends along a non-rectilinear axis, preferably with a three-dimensional development, even more preferably with a helical development.

13. Endodontic kit comprising a plurality of endodontic tools (1) according to one or more of claims 1 to 12, wherein said plurality of endodontic tools (1) comprise endodontic tools of different sizes, and wherein said different sizes relate to a first diameter (DI) at the tip (12), and / or a second diameter (D2) at the interface (15) between the stem (10; 20) and the tang (11), and / or a length (L) from the interface (15) to the tip (12).

14. Method for making an endodontic tool (1) for shaping a dental root canal according to any one of claims 1 to 12, said method comprising the step of making the stem (10; 20) by using a process of injection moulding of a polymeric material.

15. Method according to claim 14, further comprising the step of coating the stem (10; 20) with a surface coating consisting of particles of at least one materialhaving high hardness and a low coefficient of friction.

16. Method according to claim 15, wherein the step of coating the stem (10; 20) with a surface coating consisting of particles of at least one material having high hardness and a low coefficient of friction is carried out by using a multimaterial injection moulding process.

17. Method according to claim 15, wherein the step of coating the stem (10; 20) with a surface coating consisting of particles of at least one material having high hardness and a low coefficient of friction is carried out by using a process of deposition of a thin layer of particles onto said stem (10; 20).

18. Method according to claim 15, wherein the step of coating the stem (10; 20) with a surface coating consisting of particles of at least one material having high hardness and a low coefficient of friction is carried out by using a multimaterial three-dimensional printing process.

19. Method according to one or more of claims 14 to 18, further comprising the step of strengthening the polymeric material with a reinforcement material.

20. Method according to claim 19, wherein the reinforcement material comprises a plurality of non-continuous fibres, preferably non-continuous fibres less than 5mm long.* * * * * * *

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