WO2024005673A1 - Method for the laser synthesis of endodontic instruments from nickel-titanium - Google Patents

Abstract

A method for manufacturing a hyperelastic self-adapting file from nickel-titanium powder by 3D printing includes analyzing and preparing an initial nickel-titanium powder material; preparing a thin-walled 3D model of a self-adapting file and converting same into an STL model with a single surface, the normal vectors of which are oriented toward the build axis; splitting the model into separate layers and preparing an execution file containing the vector coordinates for laser scanning in each layer; and synthesizing the self-adapting file layer-by-layer by selective laser melting (SLM) using a nickel-titanium substrate. A file comprises a cylindrical base and a lattice structure. Provided between the lattice structure and the cylindrical base are connectors. All of the elements of the file structure have angles of more than 35° between the overhanging surfaces and the plane of the substrate. The invention makes it possible to increase the resolution of SLM for synthesizing micro-objects, to provide for powder recycling, and also to produce a personalized file for a specific tooth canal type within a short period of time.

Description

METHOD FOR LASER SYNTHESIS OF ENDODONTIC INSTRUMENTS

FROM NONE KEL IDE TITANIUM

Technical field

The invention relates to the fields of additive manufacturing and dental instruments, in particular to a tool for cleaning and/or shaping and/or expanding a channel existing inside or passing through a solid object, and in particular to a method for producing self-adapting files by 3D printing from powder titanium nickelide with superelastic properties.

State of the art

In recent decades, the number of endodontic operations has increased significantly due to the predisposition of patients to endodontic treatment of teeth, in particular root canals, rather than prosthetics when severe pulpitis occurs. Classical endodontic treatment consists of preparation of the tooth canal, its mechanical cleaning and irrigation, followed by obturation to prevent the penetration of bacteria into the canal and infection of the tooth in the apical, hard-to-reach part (see [1] N.A. Yudina, Modern standards of endodontic treatment, Part 1, Problematic articles and reviews, 2012, pp. 5-9).

In the first stage, the endodontist prepares the tooth to access the root canal to a depth as close as possible to the apical region at the dentin-cementum boundary. The second stage involves thorough cleaning and disinfection of the root canal. The tool that is used in this step is called a file. Using this tool, an abrasive effect is exerted on the inside of the canal while moving the file up and down with high frequency.

Irregular shapes of the root canal significantly complicate treatment (see [2] N.A. Yudina, Modern standards of endodontic treatment, Part 2, Irrigation and obturation of root canals, 2012). Before the advent of the superelastic self-adjusting file (abbr. SAF), endodontists were forced to significantly increase the width of the canal to perform the operation with traditional instruments, which significantly reduced the likelihood successful dental treatment (see [3] K. Bansal, SAF: Paving A Way to Minimal Invasive Endodontics, 3, 2015, 144-149). However, with the help of self-adapting files, it has become possible to treat highly curvature or even C-shaped root canals using a minimally invasive approach, thereby significantly reducing dentin removal.

During the operation, the above-described instrument undergoes significant deformation. It is important that all deformations are reversible (elastic) to prevent tool breakage. The intermetallic phase of nickel titanium alloy has unique superelastic properties, high corrosion resistance, biocompatibility and low Young's modulus. These properties make endodontic instruments made from this material flexible, versatile and indispensable for the operations described above with irregularly shaped root canals.

Self-adapting files are currently produced using subtractive methods, namely by laser cutting of hollow tubes, which leads to the generation of large amounts of waste (up to 70% waste). Waste from such production significantly increases the cost of products, as a result of which endodontic treatment of severe forms of pulpitis becomes less accessible. Another disadvantage of the method is the limitation on the shape of the tool, namely the mesh structure cannot deviate from the shape of the tube and does not vary in thickness, which leads to an uneven distribution of the load during operation, as well as to the appearance of stress concentrators. Finally, in the case of laser cutting, an additional technological step is required to increase the surface roughness of the tools. This patent discusses the possibility of using additive technologies (AT) for the manufacture of these tools, namely the high-resolution Selective Laser Melting (SLM) method. During SLM, 95% of the material is used due to the possibility of recycling used powder up to 12-16 times. Also, this AT method has no restrictions on the shape of the product, and the surface roughness characteristic of products manufactured by the SLM method can be used for abrasive cleaning.

An endodontic instrument for cleaning/forming the root canal of a tooth is known from the prior art (see [4] EP3071139B1, IPC A61 C5/42, published 09/01/2021), containing an elongated rod consisting of a porous material. The rod has a proximal end portion, a distal end, and a conical working portion, the working portion extending from the proximal end portion to the distal end, the material being made from titanium nickelide. A method for manufacturing an endodontic instrument includes obtaining a porous material with a porosity from 15% to 90% and forming the product by grinding, additive manufacturing, 3D printing, etching and/or combinations thereof. The disadvantages of this analogue are the non-optimized shape of the product for AT, and also the specific additive manufacturing technology and the method to achieve the necessary resolution for the production of the tool are not considered.

Also known from the prior art is a self-adapting spiral-shaped tool (see [5] US2009130638A1, IPC A61C 3/02, published 05/21/2009) in the manufacture of which titanium nickelide and its superelastic properties are used. This analogue only mentions the possibility of using AT for manufacturing, but does not specify the AT method, and also does not consider a specific method of how to provide the necessary resolution for the production of a tool.

Also known from the prior art is a method for manufacturing thin-walled parts using selective laser melting technology (see [6] CN109622963A, IPC B22F3/105, published 04/16/2019) containing the following steps: initially the three-dimensional model consists of two parts, in which one part is a three-dimensional solid object created by traditional modeling method, and the other is a thin-walled part; then the executive file for the first part of the 3D model is created in the traditional way, and for the second part the model is equated to the supporting structures, due to which single laser passes are generated along the outer boundary of the model, after which the executive files are combined for further synthesis using selective laser melting technology and the final product layer by layer is formed. In this analogue, the manufacturing method is not aimed at a specific application, in particular self-adapting files, but a specific method for increasing the resolution of the SLP is considered. The disadvantages of this analogue are: multi-stage, and as a result, the complexity of preparing an as-built file for printing, while there is a need for manual processing of parts of the model where high resolution is required, as well as the inability to control the distance of the laser pass from the outer surface of the 3D model.

Since at the moment the state of the art does not present specific work on the production of files from titanium nickelide using the DES method, but there are only references to the possibility of using AT, the state of the art contains analogues for the production of other micro-objects from titanium nickelide using the DES method, namely coronary stents. Coronary stents are also objects with an optimized mesh structure to distribute the load evenly within the cavity, but the application area differs significantly. The prior art knows a method for manufacturing an intravascular stent from titanium nickelide based on automatic powder supply technology combined with laser processing, which is a prototype of selective laser melting technology (see [7] CN105033252A, IPC B22F3/105, publ. 11/11/2015). The method includes the following steps: an executive file is prepared containing laser motion trajectories in accordance with the three-dimensional model of the stent; Nickel and titanium powders are added to the dosing system in a certain proportion, after which a layer of a mixture of powders is formed in the construction area. Next, the powder layer is scanned with a laser beam in accordance with the prepared executive file, and this operation is repeated layer by layer until the final formation of the intravascular stent. At the next stage, electrochemical polishing is carried out to the required surface roughness. The disadvantages of this analogue are: the use of additional multi-stage processing to obtain the desired roughness of the stent surface; the use of a mechanical mixture of powders in the required proportion, rather than an atomized alloy, which cannot provide the same chemical composition and, as a consequence, the same temperature of the martensitic phase transition throughout the volume of the product.

Also known from the prior art is a method for selective laser melting of a nickel-titanium alloy with a high nickel content (see [8] CN113134627A, IPC B22F10/28, published July 20, 2021), including the formation of a layer of powder with a nickel content of 53-57%, laser processing of the layer in accordance with the parameters of the printing process. Repeat the above steps until the nickel-titanium alloy part is formed. In the laser melting process, the laser power is 80-150 W, the laser scanning speed is 150-450 mm/s, and the layer thickness is 30-120 μm. The disadvantage of this analogue is the lack of guarantee of complete conversion of the mechanical mixture of nickel and titanium into the intermetallic phase - titanium nickelide. Residual pure nickel is toxic to living organisms (tissues). An increase in the nickel content in the chemical composition, on the one hand, lowers the temperature of the martensitic phase transformation, which ensures the achievement of superelastic properties, but on the other hand, increases the likelihood of the appearance of metastable phases that worsen the mechanical characteristics.

Also known from the prior art is a method for manufacturing an intravascular stent using 3D printing technology (see [9] CN104224412A, IPC A61 F2/90, published December 24, 2014), which includes several stages. First, a 3D model of the intravascular stent is created; set technological parameters and prepare an executive file for the ZO printer. Next, prepare a powder material consisting of nickel- titanium powder and a binder based on stearic acid, to form an intravascular stent blank. Next, layer-by-layer 3D printing is performed to obtain a workpiece made of titanium nickelide and a binder. At the next stage, the binder is sequentially removed, vacuum sintering and cooling of the workpiece to obtain an intravascular stent. The disadvantage of this analogue is the low resolution technology of sintering the powder with a binder and further homogenizing annealing of the product. In addition, uneven distribution of stearic acid can lead to uncontrolled porosity in products.

Also known from the prior art is a method for manufacturing a stent from a nickel-titanium alloy using additive manufacturing (see [10] CN112427654A, IPC A61 L31/02; B22F10/28, publ. 03/02/2021). The method includes the following stages: at the first stage, the size and shape of the stent is designed in accordance with three-dimensional data about a specific vessel, at the second stage, using software, the designed stent is divided into layers, and at the third stage, 3D printing of nickel-titanium alloy powder is performed in according to the stent model; Ni content in nickel-titanium alloy powder is 50-52 at. %Ni. Parameters of the 3D printing process: power 50-4000 W, scanning speed 200-3000 mm/s, scanning distance 50-200 microns. The disadvantages of this analogue are the low resolution of this approach, and there is also no information on the limitations and optimal geometric shape of the stent for manufacturing using DES technology.

Also known from the prior art is a method of 3D printing with a ternary alloy based on nickel, titanium and zirconium (see [11] CN111842888A, IPC B22F3/105, published 10.30.2020), characterized by the fact that the method uses selective laser melting technology for printing parts with shape memory effect. Adding zirconium to the system makes it possible to increase the temperature of the martensitic phase transition, and varying the technological parameters makes it possible to locally control this temperature. The starting material from the NiTiZr alloy is a powder with a particle size of 15-53 microns. In this case, parts are printed layer by layer in specified zones. The disadvantage of this analogue is the inability to obtain superelastic properties at room temperature due to the addition of zirconium to the alloy, which limits the number of potential applications of the method.

From the above analogues we can conclude that at the moment there are no patents for the production of self-adaptive files using specific additive methods. However, there are patents for the manufacture of titanium nickelide stents using DES technology, which contain several stages. Initially, the size and shape of the product are designed in accordance with the end use of the medical device, then using computer software software breaks the file model into layers. Laser scanning trajectories are formed using the corresponding cross-section profiles. The data is loaded into the corresponding interface of the SLP installation, laser processing parameters are set: laser power, laser scanning speed, layer thickness, overlap parameter (if applicable). Nickel titanium alloy powder is prepared and placed into the dosing hopper. In the synthesis stage, the powder is directed from the dosing hopper to the build area using a powder feeder, and the material is evenly distributed on the substrate using a squeegee, after which the laser scans the powder layer, the cycle is repeated layer by layer until the final shape of the product is obtained.

However, despite all the advantages of the additive approach, SLM is a complex physicochemical metallurgical process. Achieving successful fusion of materials such as nitinol requires in-depth understanding and fine-tuning of the process due to the numerous possible defects. Additionally, printing endodontic instruments poses several technological challenges. The first problem relates to the insufficient resolution of the classical DES method for printing at such a scale. The diameter of the product is only 2 mm, and the size of the jumper in the mesh reaches 100 microns. The second difficulty relates to the need to optimize the technological parameters of the SLM in order, on the one hand, to achieve a minimum melt pool, and on the other, to maintain the mechanical properties of the product.

The essence of the invention

The technical challenge facing the invention is a special high-resolution selective laser melting technology for the production of self-adapting files from titanium nickelide, including original solutions to increase the resolution of the SLM installation and the production of files with a minimum characteristic size factor of 100 microns.

The technical result of the claimed invention is to increase the resolution of the SLM for the synthesis of micro-products (self-adaptive files) from titanium nickelide, reduce the cost of the final product due to the possibility of recycling the powder, reduce production time and the possibility of obtaining a personalized file for a specific type of dental canal.

The technical problem is solved, and the technical result is achieved through a method for manufacturing superelastic self-adapting files, including: analysis and preparation of the initial nickel-titanium powder material; preparation of a thin-walled 3D model of a self-adapting file using CAD and conversion to an STL model; dividing the model into separate layers using specialized Software and preparation of an executive file containing the coordinates of scanning vectors for the laser in each layer; loading the resulting executive file into the SLP installation, loading the powder into the installation’s dispenser hopper; layer-by-layer synthesis of a self-adapting file using the SLM method until the finished product is obtained, while when preparing the as-built file for the installation of selective laser melting, an STL model with a single surface is used, the normal vectors of which are facing the construction axis; when synthesizing a self-adapting file in a SLM installation, a titanium nickelide substrate is used; each layer consists of single laser passes to create the final self-adapting file layer by layer.

The technical result is also achieved due to the fact that the selective laser melting installation uses an ytterbium fiber laser with a spot diameter of 30-55 microns.

The technical result is also achieved due to the fact that the nickel-titanium powder material is obtained by atomization of the alloy, and the powder has a median particle diameter of 15-25 microns.

The technical result is also achieved due to the optimized shape of the self-adapting file obtained according to the claimed method, including a cylindrical base and a mesh structure, and between the mesh structure and the cylindrical base there are additional jumpers and all structural elements of the self-adaptive file are made with angles between the overhanging surfaces and the substrate plane of more 35°, which makes the structural elements of the self-adapting file self-supporting.

Brief description of drawings

In FIG. 1 — Optimized form of a self-adapting file for SLP technology, a) isometric view; b) side view; c) frontal view; d) development of a thin-walled file, pos. (1) - additional jumpers between the mesh structure and the cylindrical base.

In FIG. 2 - Strategy for shading the powder layer, a) section A-A on the file scan; b) section of the rear model by plane A-A, pos. (1) — generated trajectories of laser movement in single passes, pos. (2) - a single surface of the STL model.

In FIG. 3 — Synthesized self-adaptive files using the SLP method. a) appearance of files on the substrate, pos. (1) - synthesized files, pos. (2) - titanium nickelide sheet, pos. (3) — substrate for installing SLP; b) panoramic SEM image of the entire file; c) the top of the file; d) file tip; f) jumper in the file structure. Carrying out the invention

The method for manufacturing superelastic self-adapting files consists of several stages: analysis and preparation of the initial nickel-titanium powder material; preparation of a thin-walled 3D model of a self-adapting file using a computer-aided design (CAD) system and conversion to an STL model; dividing the model into separate layers using specialized software and preparing an executive file containing the coordinates of the scanning vectors for the laser in each layer; loading the resulting executive file into the SLM installation, loading the powder into the dispenser hopper of the installation and layer-by-layer synthesis of the self-adapting file using the SLM method until the finished product is obtained. The method is implemented as follows.

The first stage of the invention includes the analysis and preparation of the starting powder material. The recommended method for producing nickel-titanium powder is gas atomization of titanium nickelide alloy. It is worth noting that it is impossible to use a mechanical mixture of Ni and Ti powders due to the formation of various metastable phases during the synthesis of SLM. The powder material must have the following chemical composition: 55.5-55.8 wt. % Ni, 44.5-44.2 wt. % Ti, with an oxygen content of no more than 0.05 wt. %. The chemical composition in the powder must be homogeneous in particle volume; sphericity coefficient 0.85 and higher; granulometric composition with a median particle diameter of 15-25 microns and a Gaussian particle size distribution; flowability of 100 g of powder material is no more than 35 s through a certified Hall funnel with a hole diameter of 4 mm.

The second stage includes the design of a thin-walled 3D model of a self-adapting file using CAD and conversion to an STL model. The recommended design of the model is shown in Fig. 1a)-c), a scan of a thin-walled file is also presented (see Fig. 1d)), where additional jumpers in the structure are indicated (see item (1) Fig. 1d)), which gives additional rigidity to the structure and avoids the use of supporting structures. It is worth noting that all structural elements of the self-adapting file are made with angles between the overhanging surfaces and the substrate plane of more than 35°, which makes the structural elements of the self-adapting file self-supporting. An important circumstance is the dependence of the critical angle on the scale of the part. It was experimentally revealed that the smallest angle for this particular micro-object is 35°; as the part is enlarged, this angle will tend to 45°.

At the third stage, the model is divided into separate layers using specialized software and an executive file is prepared containing the coordinates of the scanning vectors for the laser in each layer. Final quantity layers in a product directly depends on the optimal layer thickness, which is determined based on the granulometric composition of the original powder material. With a median particle diameter of 25 µm, a layer thickness of 30 µm was used, respectively, the model of a self-adapting file with a height of 18 mm has 600 layers. A distinctive feature of the method is the unit vector scanning technique. To implement this technique, surfaces with normals facing the inside of the product are removed from the STL model (see item (2) Fig. 2b)). To maintain the exact dimensions of the product, it is necessary to assign zero values to the parameters “offset from the model boundary” and “offset compensating for the width of the melt pool”. Next, a special algorithm is used that generates unit vectors (laser motion trajectories) in each layer (see Fig. 2a), pos. (1) Fig. 2b)), and not zones with contours and internal shading, as in the prior art. This operating algorithm is not typical for SLP installations, and commercial solutions do not support such capabilities at the moment. When generating zones with closed contours using a standard algorithm, excessive filling of the layer occurs (and then excessive fusion of the powder by the laser), which reduces the resolution of the SLM.

At the final fourth stage, the resulting executive file is loaded into the SLM installation, the powder is loaded into the dispenser hopper of the installation and the layer-by-layer synthesis of the self-adapting file by the SLM method is completed until the finished product is obtained. The installation uses a ytterbium fiber laser operating in continuous mode, with a nominal power of 200 W, with a Gaussian power density distribution (TEM00), with a wavelength of 1070 nm, and the diameter of the laser spot at the focal length is 30-55 μm. To ensure the fusion of titanium nickelide powder, a substrate is used (see item (3) Fig. 3) on which a sheet of titanium nickelide is fixed (see item (2) Fig. 3)), with a difference in chemical composition from the powder material no more than 0.2 wt. %Ni. The installation chamber is provided with an inert atmosphere of argon with an oxygen content of no more than 100 ppm. To ensure the necessary compaction of the powder layer, a silicone squeegee is used.

The resolution of SLM technology, in addition to the fraction of the initial powder material, the laser spot diameter, and the synthesis strategy, also depends on the synthesis modes (laser power, laser speed, beam diameter and layer thickness). To ensure high resolution of SLM technology, it is necessary, on the one hand, to achieve the smallest size of the melt pool, and on the other hand, to maintain the mechanical properties of the product by optimizing the technological parameters of SLM. Possible technological parameters of the SLM process for nickel-titanium powder are: laser power in the range of 50-200 W; scanning speed in interval 100-1500 mm/s; and the thickness of the powder layer is in the range of 20-30 microns. The optimal parameters for the synthesis of a self-adaptive file are: laser power of 70 W, scanning speed of 800 mm/s with a powder layer thickness of 30 microns. These modes were optimized by measuring the width and depth of the melt pool in single laser passes, as well as mechanical tensile tests on samples cut from thin walls that were synthesized using the considered single laser pass strategy. The combination of synthesis using the unit vector method and the use of modes that provide the smallest size of the melt pool while achieving the required mechanical properties guarantee the maximum resolution of the technology, which made it possible to print self-adapting files (see item (1) of Fig. 3), Fig. 3b)-e)).

The technical result is achieved through the use of an original strategy for shading the powder layer with a laser; using a laser with a smaller spot diameter at a focal length of 30-55 microns; the use of fine nickel-titanium powder, namely with a median particle diameter of 15-25 microns; using an optimized form of a self-adapting file for the SLM method, in which all structural elements are made with angles between the overhanging surfaces and the substrate plane of more than 35°, which makes the structural elements of the self-adapting file self-supporting; application of optimized technological parameters: laser power of 70 W, scanning speed of 800 mm/s with a powder layer thickness of 30 microns.

Claims

CLAIM 1. A method for manufacturing superelastic self-adapting files from titanium nickelide, including: analysis and preparation of the initial nickel-titanium powder material; preparation of a thin-walled 3D model of a self-adapting file using CAD and conversion to an STL model; dividing the model into separate layers using specialized software and preparing an executive file containing the coordinates of the scanning vectors for the laser in each layer; loading the resulting executive file into a selective laser melting (SLM) installation, loading nickel-titanium powder material into the installation’s dispenser hopper; layer-by-layer synthesis of a self-adapting file using the SLM method until obtaining a finished product, characterized in that when preparing an as-built file for a selective laser melting installation, an STL model with a single surface is used, the normal vectors of which are facing the construction axis; when synthesizing a self-adapting file in a SLM installation, a titanium nickelide substrate is used; each layer consists of single laser passes to create the final self-adapting file layer by layer. 2. The method according to claim 1, characterized in that the selective laser melting installation uses an ytterbium fiber laser with a spot diameter of 30-55 microns. 3. The method according to claim 1, characterized in that the nickel-titanium powder material is obtained by atomization of the alloy, and the powder has a median particle diameter of 15-25 microns. 4. A self-adapting file obtained using the method according to claim 1, including a cylindrical base and a mesh structure, characterized in that there are additional jumpers between the mesh structure and the cylindrical base and all structural elements of the self-adaptive file are made with angles between the overhanging surfaces and the plane of the substrate more than 35°, which makes the structural elements of the self-adapting file self-supporting.