WO2010009567A2

WO2010009567A2 - A preparation for being fastened on a natural tooth part or tooth and method of manufacturing such a preparation

Info

Publication number: WO2010009567A2
Application number: PCT/CH2009/000259
Authority: WO
Inventors: Jörg Mayer
Original assignee: Woodwelding AG
Current assignee: Woodwelding AG
Priority date: 2008-07-25
Filing date: 2009-07-21
Publication date: 2010-01-28
Anticipated expiration: 2011-01-25
Also published as: WO2010009567A3

Abstract

According to different aspects of the invention, a dental preparation is manufactured and provided, the dental preparation for being fixed on a natural tooth part or tooth, in particular for replacement of a load-bearing tooth part, said preparation being positionable in the natural tooth part or on the natural tooth part or tooth and comprising at least one region or one part of a material with thermoplastic properties, the at least one region forming at least part of the preparation surface, wherein the at least one part of a material with thermoplastic properties comprises a thermoplastic polymer forming a polymer matrix, and further comprising a plurality of filler elements (such as filler particles or fibers etc.) of a material that is solid at the melting temperature of the polymer matrix.

Description

A PREPARATION FOR BEING FASTENED ON A NATURAL TOOTH PART OR TOOTH AND METHOD OF MANUFACTURING SUCH A PREPARATION

Inventor: Jδrg MAYER, Lerchenweg 6, 5702 Niederlenz ,Switzerland, German citizen.

FIELD OF THE INVENTION

The invention lies in the field of dental medicine and relates to a preparation suitable for being fastened on a natural tooth part or tooth (for example by being anchored in it); in particular it is suitable as an artificial replacement of a load-bearing tooth part (e.g. endodontic restoration). The invention further relates to a method for manufacturing such a preparation.

BACKGROUND OF THE INVENTION

Tooth restorations which proceed from a tooth which is still at least partly present, i.e. from a natural tooth part and in which another natural tooth part is replaced by an artificial tooth part, are e.g. the filling of drilled-out teeth, the insertion of inlays, the placing of crowns, bridges or part-prostheses on natural tooth stumps, the fixing of root pins or similar means for fastening e.g. artificial teeth, bridges or tooth prostheses in natural tooth roots or parts thereof.

For filling drilled-out natural teeth, for placing crowns on natural tooth stumps and for fastening root pins in a natural tooth root according to the state of the art, parts of metals, polymers, ceramic materials or composite materials (filling body, crown, root pin) are fastened on or in the natural tooth part with the help of thermosetting composites and of polymeric cement, respectively. The cements are applied in a viscous condition and are then cured in situ for example with ultraviolet light. After complete curing, these cements can meet (in the same way as the part fastened with the help thereof) the demands of the high loading to which teeth are subjected.

The curing of the cements however often entails shrinkage. Such shrinkage often causes cracks between the natural and artificial tooth parts into which cracks moisture and bacteria penetrate. Also, due to the cross-linking, the polymers lose their ability to creep, and tensions may as a consequence not be released. Furthermore, the cement may swell up due to moisture such damaging the tooth irreversibly. The bacteria cause caries on the natural tooth part. Furthermore, the cured cements are usually very brittle and stress caused by shrinkage and/or swelling cannot be reduced or can be reduced by crack formation only. Due to the above mentioned phenomena tooth fillings fixed with the aid of e.g. polymeric cements are less toxic with respect to amalgam fillings but last less long.

In order to render the cement shrinkage as small as possible, the cements are applied already partly cross-linked. This solution of the shrinkage problem however has very great limitations since the more cross-linked the cement on applying it, the more viscous it is and thus the more difficult it becomes to securely place this cement such that it completely fills the cavities to be filled. In the publication US-5244933 it is suggested to use in the dental field polymeric cements which contain a high share of anorganic particles for improving their mechanical properties in the cured condition. These cements are highly viscous, and therefore it is suggested to bring them into an improved flowing condition in situ by applying high frequency vibrations. This effect is based on the thixotropic properties of such cements and the corresponding liquefaction does not entail heat development. However, the problem of shrinkage is not solved at all.

Shrinkage cannot be tolerated, e.g. in the case of tooth canal sealing. For this reason the above-mentioned cements are not used for this purpose but instead e.g. gutta-percha or other thermoplastic polymers with similar properties as gutta-percha are used. For being applied in the root canal, the polymer is warmed to be brought into a plastic condition and for re-solidification it is enough to let it cool. The shrinkage in such a case is significantly smaller than the shrinkage due to curing by cross-linking. Such methods for sealing root canals are for example described in the publications US-3919775 and US-4525147. According to these publications a gutta-percha plug is introduced into the root canal. It is then, in its entirety, brought into a plastic condition by heat and/or ultrasound and is pressed into the canal. This is possible without unreasonable thermal loading only if the material has a low softening temperature (gutta-percha: 70 to 100⁰C). The softening temperature thus limits the material choice. The function of the gutta-percha plug is the sealing of the root canal. The plug is not mechanically loaded at all. Indeed it would not be capable of being mechanically loaded due to the limited mechanical strength of the material, even if it were to contain carbon fibres as suggested in US 4525147.

The disclosure US-5803736 also describes production of casts for root canals, in which production shrinkage cannot be tolerated either. It is suggested to use thermoplastic polymers which are applied in a heated and thus plastic condition. For preventing heat damage to the natural tooth part, the polymers to be used are limited to those with a softening temperature of 50 to 70⁰C. Polycaprolactone is suggested as being particularly advantageous as it has a softening temperature of 55 to 65° and a modulus of elasticity of approx. 400 MPa. It is clear that such a polymer cannot be applied for a load-bearing function in the dental field.

Many known root pins have a round cross section and a straight axis and are placed in a corresponding bore having a round cross section and a straight axis which bore is produced in the tooth root for placing such a pin. Since the tooth root neither has a round cross section nor a straight axis, the bore and pin may only have very limited dimensions and still, as the case may be, a significant part of the root must be removed by producing the bore. This sets narrow limits on the stability of the fixed root pin.

hi US Patent 6,955,540 a preparation and a method of fastening such a preparation on a natural tooth part or tooth, in particular for replacing a load-bearing tooth part, is disclosed. The preparation consists at least partly of a material with thermoplastic properties and it is designed in a manner such that it damps mechanical oscillations very little so that the material is liquefied by way of mechanical oscillation only at locations where stress concentrations arise, as is the case with ultrasound welding. The locations of the stress concentrations are produced by energy directors likewise known from ultrasound welding technology, which are to be provided or present on outer surfaces (also inner surfaces as the case may be) of the preparation, or on surfaces of the natural tooth part in contact with the preparation. The liquefied material comes into contact with a surface to which the material with thermoplastic properties is to be connected, namely a surface of a natural tooth part comprising dentin and/or enamel. These surfaces however have structures (macroscopic, microscopic and/or molecular) or are provided with such structures, with which the material liquefied by the mechanical oscillation comes into such intensive contact that after solidification is forms a positive- fit and/or material fit connection with these structures. WO 2008/080239 discloses a method for affixing a dental preparation to a surface of dentine, enamel or other hard tissue or hard tissue replacement material, which includes ultrasonically welding the preparation to pretreated surfaces of the dentine, enamel or other hard tissue or hard tissue replacement material. To this end, the pre-treated surfaces may comprise solid bodies of thermoplastic material, weldable to the thermoplastic material of the preparation.

SUMMARY AND DESCRIPTION OF THE INVENTION AND OF EMBODIMENTS THEREOF

The present invention deals with providing dental preparations with thermoplastic properties, and methods of their manufacturing, which dental preparations are for example suitable for being used as preparations of the kind and purpose disclosed in US Patent 6,955,540, the teaching of which is incorporated herein by reference in its entirety, and/or for being used as preparations of the kind and purpose disclosed in WO 2008/080239, also incorporated herein by reference in its entirety.

According to different aspects of the invention, a dental preparation is manufactured and provided, the dental preparation for being fixed on a natural tooth part or tooth, in particular for replacement of a load-bearing tooth part, said preparation being positionable in the natural tooth part or on the natural tooth part or tooth and comprising at least one region or one part of a material with thermoplastic properties, the at least one region forming at least part of the preparation surface, wherein the at least one part of a material with thermoplastic properties comprises a thermoplastic polymer forming a polymer matrix, and further comprising a plurality of filler elements (such as filler particles or fibers etc.) of a material that is solid at the melting temperature of the polymer matrix. Preferably, the preparation has oscillation properties with damping losses which are so small that local stress concentrations are required for liquefying the material by oscillation, and the preparation is designed in a manner such that the stress concentrations only occur in the region of the preparation surface.

Preferably, the modulus of elasticity of the one named part as a whole is 0.5GPa or more. The modulus of elasticity of the polymer matrix material should not exceed 8 GPa, preferably it should be lower than 6 Gpa, especially preferred between 0.9 GPa and 4GPa.

For restoration according to the method disclosed in US Patent 6,955,540, the preparation is positioned on the natural tooth part or tooth to be restored or in this tooth part and is then made to oscillate by suitable means, for example by the sonotrode of an ultrasound apparatus, and at the same time it is pressed against the natural tooth part or tooth. Suitable frequencies lie between 2 and 200 kHz (in particular 20 to 80 kHz), suitable oscillation amplitudes between 1 and 200 μm. Experiments have shown that good results are achieved with an oscillation power of 2 to 20 W per square millimetre of active surface. The thermoplastic material is locally (in particular on its surface in contact with the natural tooth part or close to this surface) liquefied by the oscillations transmitted to the preparation and is brought into intimate contact with the surface of the natural tooth part or tooth, by way of which the positive-fit and/or material-fit connection is produced. The optimal matching of amplitude and power to a specific type of preparation according to the invention is to be determined experimentally. Advantageously amplitude, power and preparation are to be matched to one another in a manner such that there is an optimal transmission of the sound power to the surface of the preparation. At least a part of the surface of the preparation according to the invention consists of the thermoplastic material, wherein on restoration, this surface region is in contact or is able to be brought into contact with a surface of the tooth part to be restored or with a tooth. Furthermore, the preparation may comprise shapings suitable as energy directors in the surface regions of the thermoplastic material, that is to say edges, tips, integrally formed parts or roughnesses which project at least 0.5μm beyond the rest of the surface. The function of the energy directors may however also be assumed by a suitable shaping of the tooth part to be restored or of another part of the preparation.

The connections between the surface of the natural tooth part and the thermoplastic material which are created according to embodiments of the invention relating to the teaching of US Patent 6,955,540 are in particular positive-fit connections produced by pressing the liquefied material into pores and surface unevenness of the natural tooth part. There may however also be material-fit connections which are caused by adhesive exchange forces, hi all events, for creating the connections intimate wetting of the natural surfaces with the liquefied, thermoplastic material is required. For improving such wetting or for improving the connections to be created respectively the concerned natural surfaces may be suitably prepared. For example an enamel surface or a dentin surface may be roughened for achieving a positive fit (advantageous roughness: 0.5 to 500μm). For achieving improved positive-fit connections one may also create retention structures on the surfaces of the dentin or enamel, for example thread convolutions, furrows, recesses with depths of 0.1 to 2 mm and possibly undercut. Before positioning the preparation according to the invention, the natural surfaces may be pre-treated or etched with a suitable primer for improving wettability of the natural surfaces by the thermoplastic material and/or for achieving increased adhesive exchange forces. Applicable primer systems comprise in a per se known manner chemical reactive compounds reacting with the natural tooth material and/or molecular functionalities cooperating with corresponding functionalities on the side of the thermoplastic material. Such cooperating functionalities are for example matching of polarities of the two sides for increasing exchange forces, interpenetrating oligomers or reactive components which bind the two sides chemically when activated by the oscillations, by light, by heat or by chemical activation.

In all embodiments, the adhesion to dentine and enamel may be enhanced in addition to the measures taken in the two mentioned and herein incorporated references US 6,955,540 and WO 2008/080239. For example, the dentin and/or enamel may be pre-treated, for example to produce a suitable irregular, retentive surface. An example of such a pre-treatment step is acid etching. More in general, pre-treatment and aspects of dentin and enamel adhesion are described in Shalaby and SaIz (Eds.), "Polymers for

Dental and Orthopedic Applications", CRC Press, Boca Raton 2007, chapter 3, incorporated herein by reference.

In the following, aspects and examples of the invention are described referring to different subject matters, but all relating to preparations of the above kind comprising a preparation part of a thermoplastic material that includes a thermoplastic polymer matrix and fillers. The different aspects may be, and preferably but not necessarily are, combined with each other.

Matrix Materials and Filler Materials

The thermoplastic polymer may, in embodiments of all aspects of the invention, be for example polyolefins, polyacrylates, polymetacrylates (including PMMA), polyurethanes, polycarbonates, polyamides, polyesters, polyurethanes, polysulphones, polyaryl ketones, polyimides, polyphenyl sulphides (including polyphenylenesulfide PPS), liquid crystal polymers (LCPs), polyacetals, halogenated polymers, in particular halogenated polyoelefms, polyphenylene sulphides, polysulphones or polyethers. Corresponding copolymers or polymer mixtures are applicable also. An especially preferred material group is the polyamides. One example of a polyamide suitable as material for a dental preparation is polyamide 12 (PAl 2).

An other usable material, especially for heavily load bearing tooth parts to be replaced is Polyehteretherketone (PEEK). Pure (non-reinforced) PEEK is obtainable from different producers, an example being PEEK 450G from Victrex, Germany. A further usable material is PEEK by Invibio, a polymer that is accepted for medical applications.

A further preferred material is liquid crystalline polymers, that have been shown to be promising materials for high load applications in the medical field (see the PhD Thesis ETH No.13090 (ETH Zurich, 2000) by Pierre-Francois Kδver, the thesis being incorporated herein by reference in its entirety, chapter 12.6 and 12.7 and 12.8 and the references cited therein. Examplse of already tested LCPs are Vectra A950 and Vectra B950.

Yet further materials and blends are mentioned in the PhD Thesis ETH No.13090 by Pierre-Francois Kδver, for example in chapter 6, chapter 7.1., Fig. 59, and other text passages.

The materials or material systems which may be used in the preparation according to the invention must have sound-conducting properties and a sufficiently low damping so that the preparation is capable of oscillating. The loss factor should be sufficiently high for liquefaction in the regions of stress concentrations. It has been found that for the tooth restoration according to the invention thermoplastic materials with softening temperatures of up to approx. 350° (the melting temperature of PEEK) are applicable, wherein the mechanical oscillations are to be applied for approx. 0.1 to 10 sec. Heat quantity and exposure time remain so small that thermal damage of the dentin or the surrounding vital tissue is not to be expected.

The mentioned conditions with respect to modulus of elasticity and softening temperature are fulfilled by many materials with thermoplastic properties (thermoplasts or composite material with a thermoplastic component, in the following called thermoplastic materials) which materials can not only fulfil the mechanical requirements of a load-bearing or load-transmitting tooth part but are already used in other medical applications.

The regions of the preparation surface via which the oscillations are coupled into the preparation are advantageously to be designed such that no stress concentration arises there.

The filler elements may be filler particles of essentially heterogeneous or approximately round shapes, or they may be filler particles of whisker or fiber shape (short fibers or long fibers). According to a special embodiment, they may comprise non-particulate elements such as continuous fibers (preferably grouped together into at least one fiber strand) essentially ranging over an entire length of the preparation.

The filler materials may be glass, ceramic materials, metal oxides, carbon or polymers (the latter includes Aramid fibers).

Commercially available fillers (such as Aerosil silica fillers by Degussa or many other products, some of them mentioned in the following) or particularly designed and produced fillers may be used. Fibers that are known as fiber reinforcement materials for surgical implants are a class of suitable fiber fillers. Examples of such fillers may be found in chapters 2.2 and 2.3 (especially 2.3.2) of the PhD Thesis ETH No. 13177 by Marco Semadeni (Swiss Federal Institute of Technology Zurich, 1999), the thesis being incorporated herein by reference in its entirety. Suitable carbon fiber reinforcement fillers include AS4, IM7, T800, and TlOOO.

Other suitable fiber fillers are the carbon-based reinforceing additives discussed in chapter 13.8 of the PhD Thesis ETH No.13090 by Pierre-Francois Kδver. These materials include graphite whiskers, carbon fibers, and Fullerene-type carbon nanotubes.

Yet further suitable fillers are discussed in other chapters of the PhD Thesis ETH No.13090 by Pierre-Francois Kδver, such as chapter 6, or in section 2.2.2 of the above-mentioned book by Shalaby and SaIz, chapter 2 of this book being incorporated herein by reference in its entirety.

The technology of fiber reinforced plastics has produced a lot of further fiber materials suitable as fiber fillers in a thermoplastic matrix, all of which may be used together with the technology here presented, as long as they are acceptable, from a biocompatibilty point of view, in a dental preparation. The skilled person knows that for dental applications the requirements concerning biocompatibility are not as strict as for medical applications such as implantation.

Yet further suitable fillers are non-fibrous particulate fillers. Such fillers may include suitable oxides, such as Zirconia, Tantantalum Oxide, silica, Titanium Oxide (TiO₂) etc. An even further class of suitable fillers includes the minerals such as Bentonite or modified bentonite, for example of the kind referred to in chapter 13.9 of the PhD Thesis ETH No.13090 by Pierre-Francois Kόver.

Even further suitable fillers are the fillers mentioned in the following chapter "Inorganic particles suitable as the filling elements, and their manufacturing" hereinafter.

Of course, also modified variants, such as coated, or chemically activated or chemically passivated, or surface chemistry modified (functionalized) variants of the above materials are feasible, this includes silanisation, treatment with phosphate methacrylate, zirconates, or aluminozirconates. Further, bioactive restoratives may be released into the fillers (or also into the polymer matrix), this includes the release of fluoride, for example by way of providing fluorosilicates (such as Barium or Strontium fluorosilicate) or providing floride salts.

Other additives are possible, such as florescence dyes for making the preparation more easily visible for the dentist,etc.

Advantageously, the preparation comprises at least 40% (weight percent), preferably at least 50%, and especially preferred at least 55% or at least 60% or at least 65% filler elements. Preferred values for filler material maxima are 85%, especially preferred at most 80% or at most 75% filler material.

Examples of used and tested matrix filler-conglomerates: In all examples, the thermoplastic polymer material and the filler material was carefully mixed by hand before drying. After drying, the mixture was supplied to an injection molding setup (injection molding machine: Arburg 270S with a 18 mm screw auger. The following table shows the injection molding parameters for the investigated matrix materials.

The matrix material was in pure quality. The following fillers were used. "US-Fine" silicate fillers, Schottglass GM27884 K6 (3μm particles of a Ba-Al-Borosilicate), pure and mixed with Schottglass GM27884 UF0.4μm (0.4μm particles of a Ba-Al- Borosilicate) in a ratio of 4:1, Zeeospheres W-210 ex 3M₅ Silbond FW600 EST, spherical silica nanoparticles (size below 100 nm). Polycarbonate was only filled by the US-Fine filler, the other fillers were used both, to for a PA 12 matrix and a PVC matrix. A filling grade of 50% (weight percent) was chosen in all variants of this example.

Thin sections of the resulting samples were investigated by microscopy, especially for homogeneity. The homogeneity was found to be good for the materials with a PA 12 matrix and also for the Zeeospheres in PVC matrix material. For the other fillers in the PVC material as well as for the US-Fine filler in the PC material, the 'hand' mixing method was found to be insufficient, however, using the below taught further methods, results may be expected to be even better.

Further, the samples were investigated in vitro in terms of wear and abrasive alterations. Attritional wear was found to be low comparable to and even lower than wear of human enamel versus human enamel (attrition of specimens and antagonists added). For samples not subject to attritional wear but only to abrasion, small negative wear values indicative of water sorption were found. The best results in this respect were obtained with silica filled PVC samples. In summary, the tested materials were found to be well suited for long-time use in dental preparations.

Inorganic particles suitable as the filling elements, and their manufacturing

Inorganic nanoparticles are a promising group of inorganic fillers for dental preparations. For many applications, the dental preparation should preferably have a high transparency (in order to avoid visible dark spots or borders), and preferably they should have a high radioopacity in order to be well recognisable in x-ray images.

In PhD thesis ETH No. 17188 (Heiko Schulz), the thesis being incorporated herein by reference in its entirety, different particulate fillers for thermosetting (cross- linking) dental composites were investigated in view of radioopacity and filler transparency. It was found that a maximum transparency is reached for matching indices of refraction between the matrix and the fillers. The transparency is decreased as a function of the difference in the indices of refraction. The decrease was found the steeper the larger the particles. Also, a high crystallinity was found to have an adversary effect on the transparency. It is an insight of the inventor of the present application that similar considerations apply for fillers of thermoplastic polymers. Thus an according aspect of the invention, it is proposed to proceed as follows:

a) Choose a suitable polymer matrix and a filler material with indices of refraction that match as closely as possible (for example choose a polymer matrix material and choose a binary system such as (Ta₂OsVSiO₂ particles and adapt the relative concentration x for the binary system for the index of refraction to approximately correspond to the index of refraction of the matrix)

b) Produce the filler from the chosen filler material, the filler consisting of small particles, for example nanoparticles; the production method may for example include a thermal spraying method, such as flame spraying (according to a special embodiment, the production parameters are furthermore adapted so that the nanoparticles in themselves exhibit a low crystallinity).

c) Disperse the filler into the thermoplastic polymer matrix,

d) Carry out physical processes for maximising the dispersion of the filler in the polymer matrix and for eliminating filler coagulates,

e) Manufacture the dental preparation from the resulting material, optionally using further elements, such as a hard core etc.

Prior to step c), a surface functionalization may take place, for example by chemically modifying the surface properties of the filler particles. For example, the surface may be provided with functional groups that are capable of incurring hydrogen bonds with the polymer matrix material and thus produce an affinity of the filler to be well dispersed in the polymer matrix. A teaching of how the surfaces of Ta₂OsZSiO₂ particles can be modified, by wet chemical steps or by functionalization in the gas phase, for can be found in the named PhD Thesis ETH 17188 by Heiko Schulz. While this teaching provides ways to surface functionalize the particles to provide good chemical bonds during crosslinking of a thermosetting polymer, the skilled person, in view of the teaching of the present application, will find ways to modify the teaching to provide an other surface function, such as the named hydrogen bonding.

A special class of suitable inorganic fillers are, therefore, oxide nanoparticles, such as Ta-oxide-Si-Oxide particles. A method of their manufacturing can be found in the PhD Thesis ETH 17188 by Heiko Schulz, chapter 2, and pages 54 and 83. In accordance with this further aspect of the invention, Tantalum Oxide or Zirconium Oxide and/or Titanium Oxide and/or Silicon Oxide based nano-particulate filler materials are proposed. Nanoparticles in the context of this text are paricles of sizes between 1 nm and 1000 nm, preferably between 1 ran and 100 ran. Especially, flame spray pyrolysis made nanoparticles with 0-100% wt.% Ta₂Os further containing SiO₂ are proposed. In the PhD Thesis ETH 17188 by Heiko Schulz, the particles were found to remain amorphous up to a Ta₂Os content in the filler material of about 35% (up to this content, a good Ta dispersion in the Si-O-Matrix was observed).

The determination of a suitable material and its manufacture may according to an example therefore comprise at least some of the steps of:

Determine an index of refraction for visible light of the thermoplastic polymer matrix, Tailor the Ta₂Os content of Ta₂OsZSiO₂ particles of a desired size distribution to match the refractive index of the polymer matrix,

- Produce a powder of Ta₂OsZSiO₂ nanoparticles of the tailored properties, if necessary attending to a modification of the surface chemical properties to cause the particles not to be repelled by particles of the matrix material, especially in an appropriate solvent,

If necessary dry the powder,

- Disperse the powder and a powder of the thermoplastic polymer into an appropriate solvent, such as an aliphatic compound, for example alcohol, for example isopropanol or a higher order aliphatic alcohol,

Carry out a mechanical blending (that may be a 'mechanical alloying' step) step to blend the thermoplastic polymer with the powder, for example in an attritor, for example using mechanically hard particles (such as stainless steel spheres) for enhancing the blending effect,

- Dry the resulting mixture

Optionally, sintering andZor calcining step if drying does not produce sufficient results, the sintered product may be coarsely grinded before being fed to an auger (extruder); If not grinded, optionally granulate the mixture, for example by dribbling it on a rotating disc

Extrude the resulting mixture through an appropriate jet nozzle and carry out an injection molding process to manufacture the preparation.

In this, the mechanical blending step is a distinctive feature of the use of an approach according to the invention. The blending step may include ultrasonication, but preferably includes the charging of the mixture with higher forces, such as in an attritor or similar. The next section of this text deals with the preparation of a material conglomerate; the teaching of the next section may be combined with the above, but is also useful if applied to other particulate filler materials, for example non-transparent, filler materials.

Depending on the chosen polymer, translucency of the matrix material itself is also an issue. One approach to obtain a good translucency is to use amorphous polymers, such as PA in amorphous qualities.

Methods of preparing a theπnoplastic-polymer-filler conglomerate for manufacturing a dental preparation

The teaching of the following embodiments that deal with the preparation of a material conglomerate to be injected into the mold for an injection molding process may also be used for other methods for fabricating dental preparations than injection molding, such as for extrusion-based methods. As thermoplastic polymer matrices are, even above their melting point, more viscous than available thermosetting materials in an uncured state, the adding of filler elements in a high concentration of for example more than 50 weight % to the thermoplastic polymer matrix, as is advantageous for the here disclosed dental preparations, is not trivial.

According to a first embodiment, a conglomerate subsequently used for manufacturing a dental preparation is manufactured by providing a strand of continuous fibers impregnated by a thermoplastic polymer matrix (not unlike a 'prepreg' material, but with a thermoplastic material), and then the strand is subsequently chopped into small pellets. Before the chopping into pellets, the fiber strand may optionally be consolidated at an elevated temperature, for example in a hot press. The plurality of resulting chopped pellets may optionally be kept at a temperature above the melting temperature and stirred in an according vessel, before being cooled or directly fed to a injection molding setup. As an alternative, the chopped pellet conglomerate may directly be feed to a conveying unit that injects the conglomerate into the mold.

Approaches of optimizing and increasing (compared to commercially available short fiber reinforced polymers for injection molding) the fiber content of conglomerates, using a thin-screw extrusion, are provided in the PhD Thesis ETH No. 13177 by Marco Semadeni (Swiss Federal Institute of Technology Zurich, 1999), the thesis being incorporated herein by reference in its entirety. Especially, chapter 2.3 of the named thesis describes in detail an extrusion setup for producing pellets with a high fiber content of up to 68 weight percent fiber material (IM7/Hercules carbon fibers), and the properties of such material. An other description of approaches of optimizing material properties of fiber reinforced thermoplastics can be found in chapter 2 of the PhD Thesis ETH No. 13695 by Marc Andre Riner (ETH Zurich 2000), the thesis also incorporated herein by reference in its entirety. According to a second embodiment, that may optionally be combined with the first embodiment (as a second mixing step), the following method of preparing a thermoplastic-polymer-filler conglomerate is suggested:

a) Providing the polymer matrix material in a powdered or granulate or liquid (the latter includes paste-like) form,

b) Providing the filler particles (for example being any filler particles of a kind mentioned in the different chapters previously in this text),

c) Mixing the polymer matrix material and the filler particles thus producing a polymer matrix-filler particle mixture, and

d) Mechanically treating the polymer matrix-filler particle mixture by causing portions oft the mixture to be compressed between bodies the hardness of which exceeds at least the hardness of the polymer matrix material.

For example, in step c) the mixture is suspended in a solvent, which solvent is removed (after step d), for example by drying at room temperature or at an elevated temperature. Such a solvent may be chosen so that the Zeta potential of the polymer matrix material and of the filler particles impedes agglomeration at least of the filler particles. For example, the Zeta potential may also be such that the polymer matrix and the filler particles do not repulse each other. For example, the Zeta potential of the filler material may have an absolute value of at least 25 mV, and the Zeta potential of the polymer matrix may have the opposite sign. The bodies between which portions of the mixture are compressed in step d) include, for example, hard spheres, such as steel spheres; there may also be mechanisms where one of the bodies between which the material is compressed is a vessel wall. In step d), methods known from metallurgy for mechanical alloying may be used, for example milling (there exist various milling machines, including planetary milling machines, pan milling machines, etc., attritor processing, vibrating apparatus with at least one hard body (such as a sphere) in the vibrating vessel, grinding, etc.

Following step d), the resulting mixture may be granulated and supplied to an extruder, which may optionally be used for feeding the resulting conglomerate to an injection molding apparatus.

An example of a set-up for carrying out a method of preparing a thermoplastic- polymer-filler conglomerate for manufacturing a dental preparation is illustrated in Figure 1. The reference numerals denote: 1 : polymer matrix material (granulate), 3: polymer matrix material grinder, 5: vessel for pre-mixing the constituents, 7: solvent, 9: filler powder, 11 : attritor, 13: rotating disc for granulation, 15: resulting conglomerate granulate, 17: extruder. Not shown in the figure is a drying stage which may be between the attritor and the granulation stage.

The following further approaches are suitable also fibrous and non-fibrous fillers:

"Reversed integration": Adding the polymer matrix to the filler which is immersed in a solvent, such as Ethanol, instead of pouring the filler powder into the polymer material powder immersed in a solvent. After the adding of the polymer matrix, the resulting conglomerate is stirred and/or treated by ultrasound processing and/or treated in an attritor (see below). After the mixing and prior to the injection molding or extrusion, the mixture may be dried to remove the possible solvent.

- Ultrasound processing: The effect of mechanical stirring may be enhanced by subjecting the mixture to ultrasound while stirring (or during the attritor processing).

- Vacuum compounding,

- Spray drying (helps to avoid the powder nests),

- Kneading.

An analysis of some processes for the homogeneous integration of a filler material into a polymer can also be found in Chapters 7.2, 7.10, and 8.2 of the named PhD

Thesis ETH No.13090 by Pierre-Francois Kδver. Alternative processes are described in chapter 12.2 of the same thesis. The thesis deals with the integration of organobentonite into PEEK, however, the teaching can be transferred onto the integration of other fillers and/or onto the integration into other thermoplastic matrices.

Of course, also conglomerates containing both, fibrous and non-fibrous fillers (such as nanoparticles or microparticles) are within the scope of the invention. In this case, for example, the non-fibrous fillers may be blended with the thermoplastic polymer matrix at an early stage, and one or more of the above-mentioned steps and approaches may be used to mix them. The fibrous fillers in contrast may be added at a late stage, for example by being supplied (for example pre-impregnated with the polymer matrix) only to the extruder screw that is for example used to produce a granulate or to directly feed the conglomerate to the mould. In the named PhD Thesis ETH No.13090 by Pierre-Francois Kδver, an optimized segmented screw suitable for homogenizing powder and for processing conglomerates of either a polymer matrix and at least one fiber filler, a polymer matrix and at least one non-fibrous filler, or a polymer matrix containing both, fibrous and non-fibrous fillers is described in chapter 7.2.2.

Completely different, alternative approaches include the polymerization of the polymer matrix from monomers or oligomers in the presence of dispersed filler elements (this polymerization is preferably carried out in factory, though in-situ polimerization in the mold is not excluded in principle). Basic materials for carrying out polymerization of a thermoplastic material that has been mixed with the filler material is already available on the market, for example PAl 1 and PA 12 by Ems.

Yet a further variant is the precipitation of fillers out of a polymer melt in an appropriate solvent.

Preparing a fiber-reinforced thermoplastic dental preparation with predetermined fiber orientation by injection molding

According to a first aspect of the invention, a method of manufacturing a dental preparation suitable of being fastened on a natural tooth part or tooth, and a dental preparation manufactured by such method are provided, the method including injection molding the dental preparation from a thermoplastic polymer and at least one filler comprising a plurality of filler elements. Injection of the thermoplastic polymer into the mold is carried out at a temperature at which the thermoplastic polymer is liquid (the here used definition of the term "liquid" includes all states in which the viscosity of the thermoplastic polymer makes possible deformation to a degree that allows injecting the polymer into the mold), and at which the filler elements are solid. The deformable thermoplastic polymer material and the filler elements may be pre-mixed before they are injected into the mold. As an alternative, at least a portion of the filler elements may be added to the mold before the thermoplastic polymer is injected.

Accordingly, the method according to this aspect of the invention comprises the steps of:

- providing a mold:

providing a thermoplastic polymer having a polymer melting temperature or melting temperature range;

- providing a plurality of filler elements being solid at the polymer melting temperature or melting temperature range;

- putting the polymer in a deformable state and the filler elements into the mold, so that the mold is at least partially filled by a conglomerate of the filler elements embedded in a matrix of the deformable polymer;

causing the mold with the composite to cool down; and

- removing the mold from the cooled conglomerate, the cooled composite thereby forming the dental preparation.

The method according to this aspect of the invention allows to pre-fabricate dental preparations of a filled polymer matrix, which preparations approximately fit the shape of a cavity to be filled or a tooth part to be replaced. Because of their thermoplastic properties, the method of US Patent 6,955,540 or of WO 2008/080239 may be used to in situ tailor the dental preparation to the shape of the cavity to be filled and/or to the shape of the remaining tooth part, and to anchor the dental preparation at the remaining tooth part.

In one embodiment, the orientation of the fiber fillers is optimized in the injection molding process. Especially, it is possible to orient the fiber parallel to a mould surface along a flow direction.

Preparation with thus oriented fibers are especially advantageous and useful if they are of a shape and purpose that includes a certain anisotropy. A special class of such preparations are the root pins, in which it has been found to be desirable under certain circumstances that the fibers are oriented parallel to the proximodistal axis.

To this end, one or a combination of the following measures may be taken:

the opening in the mold cavity into which the pre-mixed fiber-matrix- conglomerate (the "melt") is injected may be arranged at a proximal or distal end thereof;

The temperature of the mold is kept below the melting temperature of the thermoplastic matrix during injection molding; The temperature of the melt is close to its melting temperature and not substantially above it, for example the temperature of the melt is not higher than

Tmehing+30 ;

The injection velocity/the flow volume is/are kept low;

The holding pressure is high.

In addition or as an alternative, the mold may comprise, in addition to a first opening in which the melt is injected, a second opening at an opposite side of the mold. The injection molding may then take place in a flow-through manner, thus melt is injected from one side, and is pushed through the mold and exits on the other side. As a first alternative, this may be done in a unidirectional push-out injection molding process, where some of the molten material that has flown via the first opening into the mold and through it exits through the second opening and into a overflow volume. This may, according to different variants, be done into a sufficiently large overflow volume that does not restrict the volume of overflow material, or in a manner in which an out-flow-gate is controlled and an outflow rate is adjustable. As a second alternative, the flow-through injection molding may be done by push-pull injection molding, where after melt injection, the melt is subject to a plurality of push-pull strokes from the two different mold openings (the portion of the melt that has not yet solidified is moved to and fro in the mold).

By such processes, the fiber orientation has found to be enhanced. A systematical study of injection molding techniques and of the influence of the injection molding process design on fiber orientation can be found in chapters 2.2 and 2.6 of the mentioned PhD Thesis ETH No. 13177 by Marco Semadeni. The thesis deals with manufacturing hip joint endoprothesis stems. It is a major insight of the inventors of the present invention that such teachings that are relevant for hip joints (where thermoplastic materials have been discussed previously) and for the accordingly comparably large molded parts, may also be applicable in view of the completely different requirements to be met for the small dental preparations for which the requirements are fundamentally different. Further information, also related to hip endoprotheses, are found in EPl 151732.

Figures 2, 3 and 4 show variants of flow-through injection molding of dental preparations, particularly of root pins. In the figures, the mold 21 has two openings 23, 25 serving as inlet and outlet openings. The variant of Figure 2 comprises a single injection unit 17 (extruder), and an overflow volume 27 in which material exiting through the second opening may flow. The overflow volume 27 distinguishes the set-up from classical injection molding setups. The variant of Figure 3 is of the push-pull type with an injector 17 on both sides. The variant of Figure 4 is of a same principle as the one of Figure 2, but with means 39 for adjusting the outflow, for example valve means, and/or a piston in the overflow volume.

In a modification of these flow-through techniques, the material properties of the material injected may be chosen to vary as a function of time (and thus as a function of filling of the mold). Especially, the filler content may vary from an initially low grade (of for example between 0% and 50% (filler contents are weight percents in this text)) to a later high grade (of for example between 50% and 80%). In this way, the resulting preparation will have a shell of a thermoplastic polymer rich conglomerate and a core region in which the filler material predominates. The thermoplastic polymer shape may be beneficial in view of the fastening process by means of mechanical vibrations liquefying portions of the preparation. In addition or as an alternative, the mold may be only partially filled by the conglomerate leaving a hollow pre-fabricated element which may be filled with further material (such as a thermosetting polymer core), or which may be put over a solid core, etc.

In addition or as yet another alternative to the above, the filler elements may include continuous fiber strands. Such continuous fiber strands may be provided in the form of pre-impregnated fiber strands placed in the mold cavity before the injection process and held therein by appropriate holding means (such as holding means of the cavity). Such fiber strands may increase the mechanical stability considerably, and may be suitable for example for dental preparations that should withstand pulling forces.

In addition, or as an even further alternative, the filler elements may comprise at least one of an inlay of Titanum or Zirconia or PEEK/IM7 and/or elements (fabricated) according to the teachings of WO 99/61081 or WO 96/19336.

Continuous fibers in the context of the present are fibers with fiber lengths corresponding to a length of the preparation and ranging essentially through a full cross section of the preparation.

In all embodiments of the invention, the preparation may be subject to residual stress annealing after the injection molding process.

Prefabricated standard preparations The invention features, according to a further aspect, a method of carrying out a dental restoration, the method comprising the steps of providing a plurality of preparations, for example made by any one or any combination of the above- described methods, the preparations being of defined, different shapes, of determining the at least approximate size and shape of a cavity to be filled or tooth part to be replaced, the preparations comprising thermoplastic material, of choosing an approximately fitting preparation of the plurality of preparations, and of fastening the chosen preparation to a remaining tooth part using mechanical vibrations to at least partially liquefy the thermoplastic material while at least a portion of the thermoplastic material is in contact with dentine and/or enamel and/or an already- produced filling body.

Claims

WHAT IS CLAIMED ES:

A dental preparation, at least a part of which is made of a thermoplastic material, the thermoplastic material comprising a thermoplastic polymer matrix material having a melting temperature of between 80⁰C and 400⁰C, and between 40 weight % and 85 weight %, preferably between 50 weight % and 80 weight % of a filler material comprising a plurality of filler elements dispersed in the polymer matrix, the filler elements being solid at the melting temperature of the polymer matrix.

2. The dental preparation according to claim 1, wherein the modulus of elasticity of the polymer matrix material is between 0.5 GPa and 10 GPa.

3. A dental preparation, for example according to claim 1 or 2, comprising a thermoplastic polymer matrix and nanoparticles as filler elments.

4. The dental preparation according to claim 3, wherein the nanoparticles comprise Tantalum oxide, Zirconia, Silicon oxide, and/or Titanium oxide.

5. A method of manufacturing a dental preparation, for example according to any one of the previous claims, comprising the steps of:

Choosing a thermoplastic polymer matrix material and a filler material, the index of refraction of which matches at least approximately the index of refraction of the polymer matrix; Producing the filler from the chosen filler material, the filler consisting of a plurality of filler material particles;

Dispersing the filler into the thermoplastic polymer matrix,

Carrying out physical processes for maximising the dispersion of the filler in the polymer matrix and for eliminating filler coagulates; and

Manufacturing the dental preparation from the resulting material, optionally using further elements.

6. A method of manufacturing a dental preparation, for example according to any one of claims 1-4, for example according to claim 5, comprising the steps of:

- Providing a thermoplastic polymer matrix material in a powdered or granulate or liquid form;

Providing a plurality of filler particles, being solid at a polymer melting temperature or melting temperature range of the polymer matrix material;

Mixing the polymer matrix material and the filler particles thus producing a polymer matrix-filler particle mixture; and

Mechanically treating the polymer matrix-filler particle mixture by causing portions oft the mixture to be compressed between bodies the hardness of which exceeds at least the hardness of the polymer matrix material.

7. A method, for example according to claim 5, of manufacturing a dental preparation suitable of being fastened on a natural tooth part or tooth, for example according to any one of claims 1 -4, the method comprising the steps of: providing a mold:

providing a plurality of filler elements being solid at the polymer melting temperature or melting temperature range;

putting the polymer in a deformable state and the filler elements, for example prepared by a method according to claim 6, into the mold, so that the mold is at least partially filled by a conglomerate of the filler elements embedded in a matrix of the deformable polymer;

- causing the mold with the composite to cool down; and

removing the mold from the cooled conglomerate, the cooled composite thereby forming the dental preparation.

8. The method according to claim 7, wherein the polymer and at least a portion of the fillers are mixed with each other prior to be put into the mold.

9. The method according to claim 7 or 8, wherein at least a portion of the fillers are fibers.

10. The method according to any one of the claims 7-9, wherein the mold comprises at lest two mold openings, wherein the conglomerate is injected through at least one of the openings, and wherein a portion of the conglomerate is caused to exit through an other one of the openings.

11. The method according to claim 10, wherein conglomerate materia] portions adjacent a mold wall are caused to solidify while material portions closer to a center of the mold are still in flow.

12. The method according to claim 11, wherein the conglomerate is injected into the mold with a filler concentration that varies as a function of the time.

13. Use of an injection molding process for producing a dental preparation with a thermoplastic polymer matrix.

14. Use of a process of intermixing a thermoplastic polymer matrix with a plurality of filler elements for producing a material for a dental preparation.