ITRM980476A1 - ODONTOSTOMATOLOGICAL USE OF NANOSTRUCTURED APATITIC BASED MATERIALS - Google Patents

Description

DESCRIPTION

accompanying a patent application for an invention entitled: "Odontostomatological use of nanostructured apatite-based materials"

The present invention relates to the odontostomatological use of nanostructured apatite-based materials. More specifically, the invention relates to the application in the fields of dentistry and dental hygiene of biocompatible crystalline products based on high specific surface apatite, with nanometric dimensions of the single constituent crystals, which are particularly effective in the treatment of hypersensitivity dentinal and remineralization of dental tissues.

As is known, dentinal hypersensitivity or sensitivity is a symptomatology characterized by pain and sensitivity to thermal, mechanical (tactile), chemical and osmotic stimuli caused by exposure of dentin to the oral environment. Numerous data on the epidemiology of dentinal sensitivity have been published over the years, revealing, among other things, that this clinical condition affects more than 15% of the population between the ages of 20 and 40, with a particular preference for women.

Dentin sensitivity can be primitive, i.e. not linked to any therapeutic intervention on the dental element, or secondary to previous periodontal interventions and / or traumas that have led to exposure of the dentin, or consequent to restorations (including prosthetic ones) with protruding margins and with incorrect profiles of the restorations themselves. Patients are frequent who after extensive periodontal surgery, especially resective surgery, report a hypersensitivity not previously present. In these cases, the syndrome is also referred to as root hypersensitivity or cervical sensitivity.

According to the theory formulated by Brànnstróm (hydrodynamic theory of pain induction, M. BrànnstrOm, A. Anstrom: The hydrodynamics of the dentini its possible relationship to dentinal pain. Int. Dent. J., 22: 219-227 (1972) ) and subsequently by Pashley (D.H. Pashley, Mechanisms of dentin sensitivity. Dent. Clin. North Am .; 34: 449-473 (1990)), the presence of exposed dentin and open dental tubules makes it possible to move the dentinal fluid to the 'interior of the tubules themselves. This displacement is induced by mechanical (for example the passage on the surface with a probe), osmotic and chemical (food, drink, etc.) and thermal causes (both hot and, above all, cold stimuli). However, it is not yet clear whether the displacement of the dentinal fluid causes a nerve stimulation (for example of intratubular nerve fibers) or whether it directly stimulates the extensions of the odontoblasts (which occupy about 2/3 of the entire length of the tubule).

In any case, the greater the surface of dentin exposed (with unobstructed tubules), the greater the possibility that the patient will feel sensitivity to any stimulus. In other words, the exposed dentin behaves like a semipermeable membrane, in which the dentinal tubules represent the pores of the membrane itself. Therefore, the diameter and number of exposed tubules largely affect the degree of hypersensitivity, as has been verified in vivo by Yoshiyama et al. (M. Yoshiyama, Y. Noiri, K. Oaki, A. Uchida, Y. Ishikawa, H. Ishida, Transmission electron microscopie characterization of hypersensitive human radicular dentin, J. Dent. Res .; 69: 1293-1297 (1990) ).

As a direct clinical consequence of this condition, any dental treatment that involves the removal of the enamel and the removal of dentin, and therefore the opening of the dentinal tubules, inevitably also leads to an increase in sensitivity. Therefore, the patient who has dentinal hypersensitivity results to have at the same time an area of exposed dentin (without enamel and / or gingival covering) with open tubular orifices and no type of superficial debris or smear layer (where this term means precisely that layer of debris that normally covers the dentin and occludes the tubules with caps, the so-called smear plugs) which obstruct (even only partially) the dentin tubules)].

Nutrition can play a leading role in determining the onset, or rather the exacerbation of dentinal sensitivity. In fact, many foods and drinks possess acidic properties as they contain citric, phosphoric and maleic acid, and can easily perform a chemical removal of the surface smear layer, opening the dentinal tubules. Other foods and drinks have also been reported to have effects on dentinal sensitivity, such as red wine, citrus fruits, unripe fruit in general, etc. Furthermore, as already noted, a particular condition can occur in patients undergoing resective periodontal surgery. , of which about 80% report dentinal sensitivity after periodontal therapy. Other periodontal procedures can also be related to the onset of dentinal sensitivity: for example, during the surgical procedures of scaling, root smoothing and surface cleansing, residual cement (often already absent) and infected dentin are removed, leading to removal of large root surfaces and exposure of a very high number of open dentin tubules!). In the event that this exposure allows the passage of intratubular fluid from inside the pulp chamber to the outside, with loss of piasmatic proteins and electrolytes and, vice versa, the passage of liquids and toxins from the outside to the inside, a transient pulpitis accompanied by painful symptoms noticeable a few hours and days after the operation. Very often the symptomatological picture tends to decrease spontaneously thanks to the spontaneous occlusion of the tubules, caused by the deposition of bacterial debris, by the precipitation of proteins and fibrinogen, as well as by the production of smear layer by the patient himself during normal hygiene procedures with the toothbrush (Pashley, 1990, cited above).

Since the presence of open dentin tubules!) Represents the decisive moment of dentinal hypersensitivity, the most accredited therapy basically aims to occlude said tubules. A different therapeutic approach, aimed at reducing receptor sensitivity, consists in increasing the intratubular concentration of potassium and strontium ions (through the application of toothpastes or other agents), in order to make the dentinal nerve fibers less excitable. However, the hypothesis of an activity of K <+> and Sr <2+> ions in influencing the conduction of the nerve stimulus has found little clinical evidence (D.G. Gillam, J.S. Bulman, R.J. Jackson, H.H. Newman, Efficacy of a potassium nitrate mouthwash in alleviating cervical dentine sensitivity (CDS), J. Clin. Periodontol. 23: 993-997 (1996)).

As regards home therapy, there are numerous toothpastes and specific gels for sensitive teeth on the market, the constituents of which are based, for example, on strontium chloride hexahydrate, potassium nitrate or citrate or stannous fluoride, and have also been proposed some mouthwashes and periodontal gels. The data provided by the literature show that many toothpastes have some desensitizing action, often independently of the type of active ingredient (D.G. Gillam, J.S. Bulman, R.J. Jackson, H.N. Newman, Comparison of 2 desensitizing dentifrices with a commercially available fluoride dentifrice in alleviating cervical dentine sensitivity, J. Periodontol., 67: 737-742, (1996)). Very often the other components of the toothpaste also play an important role, as they are themselves capable of occluding the dentinal tubules. Furthermore, the placebo effect plays a role that some define as fundamental.

Among the best known and studied products for home use are toothpastes based on strontium chloride hexahydrate, which, as already noted, is used more for its occlusive capacity towards the dentinal tubules than for those depressing the stimulation of the relative fibers. nervous. It is believed that the Sr <2+> ion reacts with the apatitic matrix of dentin forming with it an insoluble strontiumapatite, which is deposited in the form of a layer of microcrystals capable of reducing the functional diameter of the dentinal tubules. Stannous fluoride would also act by forming insoluble complexes with dentin (fluoroapatite or calcium fluoride) capable of obstructing the tubular lumen. As for potassium citrate, it has been shown that the citrate ion chelates the calcium of dental hydroxyapatite by precipitating a soluble calcium citrate (Misra D.N., Interaction of cifrate Acid with Hydroxyapatite: Surface exchange of ions and precipitate of calcium cifrate, J. Dent. Res., 75 (6), 1418-1425, June 1996).

In this context, toothpastes and other desensitizing preparations have also been proposed which are based not on products that react in the oral environment to form insoluble precipitates, but on the direct use of apatite, that is, the basic material constituting dentin, in the microcrystalline state. For example, US Patent No. 4,634,589 (Wurttembergische Parfumerie-Fabrik) describes the use of crystalline apatite, in particles of size <8 pm, as an abrasive and polishing agent, active for the treatment of dentinal hypersensitivity and also endowed with a remineralizing action towards the dental tissues. The proposed toothpaste formulation contains at least 15% by weight of this component. Similarly, European patent application No.

0346957 (Unilever) describes the use of hydroxyapatite as an abrasive desensitizing agent for dental products. In this case the hydroxyapatite particle size is 1-15 µm, and the presented formulation also includes a potassium and / or strontium source (as an additional desensitizing agent).

Home treatments, however, have the drawback of requiring long periods (often over 4 weeks) before reaching any clinical result, and this result is often only partial, as it requires considerable collaboration from the patient. The treatment of choice is therefore the outpatient one, to be performed directly on the tooth or teeth involved, also in this case with the main objective of occluding the open dentinal tubules. Depending on the degree of severity of the symptoms, various techniques and materials have been proposed, schematically attributable to four main groups: 1) mechanical treatment of the sensitive surface to obtain a new smear layer; 2) application of agents capable of occluding the dentinal tubules, either directly or by forming insoluble precipitates; 3) impregnation and obstruction of the dentinal tubules with adhesives; 4) application of restorative materials with possible application of intraoral devices with slow fluoride release. In the most serious cases and reluctant to any type of therapy, restorative and endodontic treatment are often included as a definitive resolution of the picture.

As regards the first of the treatments indicated above, given that in the presence of an adequate smear layer the dentin is not permeable, often the simple application of an abrasive paste (such as Nupro paste) and the instrumentation of the dentinal surface with silicone cups or rubber allows the formation of a thin and uniform layer of smear layer and the closure of the tubules. It is a simple but not long-lasting treatment, as the smear layer is soluble (especially in the addo environment) and can therefore be easily removed by the patient, often simply with incorrect brushing or with the intake of acid liquids (juices fruit, soft drinks, etc.). Therefore, this treatment is reserved only in mild cases, such as after periodontal therapy but with modest dentinal surfaces exposed and without any anamnesic history of dentinal sensitivity.

The second type of outpatient treatment is carried out, for example, through the application of oxalates (potassium oxalate, iron oxalate, aluminum oxalate and oxalic acid) which can interact with the dentinal decapatile constituents forming insoluble microcrystals of calcium oxalate both on the surface than within the dentinal tubules. The solutions can be easily applied to the dentinal surface with small brushes and left in place for 2-3 minutes, enough for the aforementioned crystals to form. It should be noted, however, that some problems may exist in relation to bone toxicity. Also silver nitrate, used for a long time, bases its mechanism of action on the formation of precipitates (of Ag chloride) which occlude the dentinal tubules.

This framework also includes the direct application of microcrystalline apatite already mentioned in relation to products for home therapy and, more specifically, the use of an ultramicronized apatite, described for example in the Italian patent No. 1,271,874, in which the dimensions of about 70% of the microcrystals are below 1 pm (0.2 -1 pm}. Such a product is considered more efficient than the micronized ones previously described, based on the simple consideration that the dentinal tubules have average diameters of about 1.3-3.5 µm, and therefore crystals of just submicron size can, theoretically, penetrate inside them by operating a better mechanical occlusion, while those described in the two initially cited patent documents can only create an occlusive layer on the dentine wall, without penetrating into the tubules.

At the level of outpatient treatments, they are however more valid (and techniques of the third category mentioned above, that is, those that operate the impregnation of dentin with adhesive resins (resin impregnation technique). This technique, already proposed by Nordenvall and Brànnstróm (K.J. Nordenvall, M. Brànnstróm, In vivo resin impregantion of dentinal tubules, J. Prosthet. Dent; 44: 630-637 (1980)), became extremely valid only after the introduction of dentinal adhesive systems consisting of highly hydrophilic resins and capable of infiltrate the dentin by penetrating the interior of the dentinal tubules for a few tens of microns. It has been shown that the dentine surface treated with etching acid (for example with phosphoric acid or malic acid) is demineralized for 4-10 pm and rich only in partially collapsed collagen fibers . This treatment facilitates the penetration of resin (usually a primer) inside the dentinal tubules and inside the collagen coll assato. The resin, penetrating into the tubules, forms long saps (called resin tags) which occlude the tubular lumen and reduce dentine permeability. The lateral canaliculi are also infiltrated and occluded by the resin (as recently confirmed by some studies with a scanning electron microscope), contributing to the formation of a dense network of resin capable of stably blocking any movement of dentinal fluid. In this way, a hybrid layer is created consisting of resin and collagen. As regards the mechanism of action, it is not clear whether other factors contribute to reducing sensitivity in addition to the closure of the tubules with a reduction in permeability (probably the main mechanism). It has been hypothesized that some of the components of the dentine primers also have a pharmacological action such as to reduce symptoms.

In the current state of knowledge, the resin impregnation treatment of the exposed dentinal surface is to be considered specific for patients in whom few elements have medium and severe hypersensitivity.

Finally, in the presence of widespread and severe dentinal sensitivity, it is necessary to distinguish between lesions with periodontal tissue deficiency, in which surgical treatment is the treatment of choice, and lesions with severe and extensive loss of enamel, where conservative reconstructive treatment is indicated. The materials that can be used are both the dentine adhesive systems with the technique described above, obviously followed by the application of the composite, and the application of photopolymerizable glass ionomer cements. This type of intervention, as already noted, is reserved only for cases with substance deficiency or when hypersensitivity is extremely severe. The next therapeutic step is, of course, the endodontic treatment of the tooth itself.

In addition to the above-mentioned consolidated treatments, other especially patent literature proposes the use (in addition to the micronized and ultramicronized apatitic materials already mentioned), of calcium phosphates in amorphous form, both with the aim of remineralizing (and, optionally, fluorinated) the dental tissues, both with that of reducing dentinal sensitivity. US Patent No. 5,268,167 (American Dentai Association Health Foundation) describes for this purpose the use of amorphous calcium phosphates (ACP or amorphous Ca phosphate, ACPF or amorphous Ca fluorophosphate, ACCP or amorphous Ca carbonate-phosphate ). According to the text, these salts in amorphous form, or solutions that can originate them by precipitation, are applied to the surface of the dental tissues and are deposited on it and inside the tissues themselves, where they are converted into crystalline apatite carrying out an action remineralizing and reducing dentinal sensitivity. Along the same lines, the international patent application PCT No. WO 94/04460 (of the same owner) also presents the phosphate ACCPF (fluorophosphate-carbonate of amorphous Ca) as a new compound of particular interest for the same purpose.

In light of the state of the art summarized above, the present invention therefore proposes the aim of providing a biocompatible product with a substantially apatitic basis that is able to satisfactorily respond to various therapeutic needs in the odontostomatology field and, first of all, is capable of to effectively and lastingly counteract the phenomenon of dentinal sensitivity, occluding and cementing the dentinal tubules in depth, and carrying out a desensitizing action that also resists the attack of chemically aggressive agents present in the oral environment, such as drinks and addi food.

According to the present invention, it has now been found that the apatitic materials characterized by a nanocrystalline structure (in contrast to the known amorphous and microcrystalline structures, including the ultramicronized ones) lend themselves to the purposes indicated above in a considerably better way, allowing to achieve remarkable therapeutic results , up to the complete elimination of dentinal sensitivity and, moreover, with an appreciable remineralization of the dental tissues. These results are possible thanks to the unique peculiarities of crystalline nanostructures, which make them advanced materials of great potential in many fields of application.

In general, nanocrystalline materials are artificially synthesized materials, characterized by a constitutive phase or by granular structures modulated on a length scale normally less than 100 nm. Depending on the number of dimensions in which they have a nanometric structure, they are classified with zero dimensionality (clusters of atoms - for example dispersed in a non-nanocrystalline matrix -, filaments or tubules), with one dimensionality (multilayers, i.e. nanometric layers in the only thickness direction), two dimensionality (granular, ultrafine overlays or buried layers), or three dimensionality (nanophasic materials, in which all the constituent phases are of nanometric proportions over three dimensions) (R.W. Siegei, in Materials Science and Technology, Voi . 15: Processing of Metals and Alloys, R.W. Cahn, 583 (1991)). The particular properties of nanocrystalline materials compared to traditional ones derive from the combination of their three fundamental and distinctive characteristics: atomic domains (clusters, grains, layers or phases) spatially limited to less than 100 nm, significant atomic fractions associated with interface environments (i.e. grain boundary, interfaces and heterophases and free surfaces) and interactions between their constitutive domains.

Currently, clusters of atoms in the nanometric dimensionality regime, containing from hundreds to tens of thousands of atoms, in such a number that they can be assembled into materials that can be obtained advantage by incorporating a multiplicity of dimension-related effects into a single material. The latter range from quantum-dimensional electronic effects caused by the spatial confinement of valence electrons to the suppression of lattice defect mechanisms, such as dislocation generation and migration to limited granular dimensions. The basis of the particular performance of nanostructured materials is in the fact that a physical property of matter is altered when the entity or mechanism responsible for that property (or their combination) is confined within a space (defined by the size of the set of atoms) less than a certain critical length associated with that entity or mechanism. Thus, for example, a metal that is traditionally ductile by virtue of the usual ease in creating and moving dislocations across its crystal lattice will become significantly harder by reducing the grain size to a critical point where the dislocation sources are no longer able. to function at low levels of applied stress.

In addition to the size of their ultrafine domain (grains or layers), nanocrystalline materials are dominated by the large number of interfaces. Since the interfaces present in them are much more numerous than in traditional materials, an appropriate control, in the synthesis, of the nature of the interfaces created between the constitutive phases leads to a control over the nature of the interactions through them. To get an idea of the importance of the interfacial environment in a nanocrystalline material, it is sufficient to consider, for example, that in a material with an average granular diameter of 5-10 nm, the percentages of atoms in the grain boundaries vary between iM5 and 50%.

In light of the general characteristics of the nanocrystalline materials mentioned above, for the proposed use according to the invention, the apatite-based nanostructured materials prove to possess not only the correct granular dimensions to be able to easily penetrate deep inside the dentinal tubules (which, as mentioned, have diameters of the order of 1.3-3.5 pm), but also of particular chemical-physical properties linked to their nanocrystalline nature, including the remarkable chemical surface reactivity and hygroscopicity, which ensure that the material not only mechanically penetrates inside the tubules, but cements the internal walls, both swelling and reacting promptly with the natural tissues, recrystallizing as a result of the metastable character of the nanocrystalline system which in the specific case may present a <~> high defects.

Apatite-based materials characterized by a nanocrystalline structure have so far been somehow described only in the international patent application PCT No, WO 97/17285 (Etex Corp,), relating to the low temperature synthesis of low crystallinity apatite to be used in transplants of bone tissues. In fact, the definition of “low crystallinity material” given in this text generically includes both amorphous and nanocrystalline materials (with crystalline domains overflowing of nm or A). This document, however, exclusively concerns the production of resorbable synthetic bone materials, which, for their intended use, are formed in molds to give solid elements, and for which the only requirement of interest is the capacity of the material to mimic natural bone tissues, so that it can be integrated and reabsorbed in said tissues. The applications of this material in the orthopedic and odontostomatology fields differ in that in the first case the material must satisfy requirements of stability, osteointegrative and in some cases osteoinductive (direct relationship between material and cells), while in the second case the material it must interact with mineral tissues other than bone for chemical and structural characteristics and with different organic components, so the material's capabilities in terms of surface reactivity and metastability must prevail. Furthermore, the orthopedic field that involves interventions on the bone represents a system that interacts with the bloodstream and is not subject to environmental variations due to contact with the outside (pH variations, compositional variations of various types), exactly as opposed to what occurs in the oral area and therefore on the surface of the tooth.

Therefore, the specific object of the present invention is the use of nanostructured materials with an apatitic base having a general formula:

where: M is a cation other than Ca <2+>, B is an anion other than the PO4 ion <3> ·, A is chosen from O <2->, C03 <2> ", F <'> and Cl -, x is a number between 0 and 9, y is a number between 0 and 5, 2 is a number between 0 and 2, said numbers may also be non-integers, with the condition that the sum of the charges of the cations Ca and M is equal to the sum of the charges of the anions P04 <3> ", B and A,

having an average crystallite size between 0.5 and 200 nm, for the production of preparations for odontostomatological applications, useful for the repair and protection of dental tissues, and specifically for the therapy of dentinal sensitivity and for the remineralization of enamel and dentin. Specifically, said nanostructured materials can exhibit reticlular deformations and defects.

As is known, apatites represent the main inorganic process of calcification of normal tissues (enamel, dentin, cement, bone) and are found associated with other minerals, phosphates and not, in pathological calcifications. The main among these, hydroxyapatite or hydroxylapatite (HA), whose stoichiometric formula is Ca-, o (P04) and (OH) 2 (or Ca5 (P02) 30H) and which, in the synthetic (biocompatible) form, is the commercially most exploited apatite for various indications in dentistry, orthopedics and maxillofacial surgery, is never found pure in biological tissues. This is due to the possible isomorphic substitutions of Ca <2+>, PO4 <3> - and OH- ions. The calcium ion can be totally or partially replaced by numerous cations generally (but not exclusively) with oxidation number 2; I phosphate ion (site B) can be replaced by carbonate, hydrogen phosphate, pyrophosphate, sulfate, aluminate and silicate ions, and the hydroxyl ion (site A) can be replaced by halide, carbonate and oxide ions.

Compared to the value of 1.67 relative to the stoichiometric compound, biological apatites have a molar Ca / P ratio between 1.53 and 1.74 (in particular, between 1.53 and 1.64 for dental enamel, between 1, 62 and 1.68 for dentin and between 1.72 and 1.80 for bone). Overall, the causes of oscillation of the Ca / P ratio can be indicated in: 1) reticular holidays; 2) ions isomorphically substituted or adsorbed on the surface of the lattice (for example, the replacement of the anion PO4 <3> · with hydrogen phosphate, divalent, leads to a reduction of the calcium content); 3) coexistence or presence of possible precursors consisting of phosphates with reverse Ca / P ratio. Among these, in particular, β-tricalciophosphate (β-TCP, Ca3 (P04) 2, an orthophosphate also known as tribasic calcium phosphate), amorphous calcium phosphate (ACP), octacalcium phosphate (OCP, Ca8H2 (P04 ) and 5H20), dicalcium phosphate dihydrate (DCPD, CaHP04-2H20, also known as calcium hydrogen phosphate, dibasic calcium phosphate or dicalcium orthophosphate). As can be seen from the relative formulas, the values of the Ca / P molar ratio for some of the phosphates considered are the following:

More correctly, biological apatites can be described as non-stoichiometric carbonate-apatites, generally containing different ions as impurities. For these reasons they show variations in morphology, crystallinity and chemical and chemical-physical properties.

Among the possible phosphates and apatitic materials present in nature and obtainable synthetically that have an interest in the production of materials useful in the dental and biomedical fields, it has already been noted that hydroxyapatite [CasiPC ^ feOH] is the best known material. It has a structure in which the phosphate ions and calcium ions are arranged in a first approximation according to a hexagonal prism; in the elongation direction (crystallographic axis c) the prism is crossed by a channel with a diameter of about 3-3.5 A, within which the OH- groups or any other substituent ions (eg fluorine and chlorine) are positioned. HA can crystallize in two forms: a monoclinic (space group P21 / b) and a hexagonal (space group Pe ^ m). In the monoclinic form, along the c axis there is a binary axis of symmetry, while in the hexagonal form the axis of symmetry becomes hexagonal.

Some authors have hypothesized for the biological samples a simultaneous presence of amorphous calcium phosphate (ACP) and low crystalline HA. These two phosphates would be differentiated by a different intensity of the diffracted X-rays, even with the same widening of the peaks. The same authors hypothesize that the bone tissue contains these two phases, of which the first to be deposited would be the ACP; over time, this last phase would undergo a transformation into microcrystalline HA according to a partial solubilization process with subsequent renucleation. The transformation process of ACP seems to be regulated by environmental factors (ATP, Pyrophosphates, Mg <2+>, etc.) which stabilize one of the two phases with their presence.

Also rottacalciofosfa was

detected in biological samples. Its crystalline structure is very similar to that of hydroxyapatite. In fact, the OCP can transform into HA through a simple chemical mechanism such as the in situ hydrolysis of the crystals already formed. This type of transformation, considered irreversible, allows the incorporation of foreign ions in the crystal lattice. This process could also be regulated by a "layer by layer" transformation, in the sense that as OCP is deposited in layers of the thickness of an elementary cell, these hydrolyze and transform into two layers consisting of two elementary cells of HA. OCP can be considered as a mediator between the aqueous and apatitic phases. The type of product obtained (OCP or HA) depends on the relative rates of the precipitation process of OCP and transformation into HA.

Regarding the possible substitution of cations in the apatite formula, although this problem has been studied in the literature, the only information of some interest is that solid solutions are structurally more ordered when the substituent cation is large. The substitution capacity of calcium with crystallochemically similar ions cannot be predicted a priori, and even when this occurs, the isomorphic substitution field can be partial. However, it is ascertained that the method of preparation has a substantial influence on the extent of the replacement.

In aqueous environment and under conditions similar to physiological ones, Mg <2+> ions inhibit the precipitation of hydroxyapatite and promote the formation of β-tricalciophosphate. In fact, when the magnesium concentration exceeds 10%, there is the simultaneous formation of HA and β-TCP, which is the only product to precipitate when the concentration exceeds 25%. The finding that magnesium replaces calcium in β-TCP is evidenced by the displacement of the X diffraction maxima of the dust spectra in the same samples.

However, in biological apatites the magnesium content is very low (around 1%), and is limited to a superficial deposition. This is evidenced by the fact that during the initial phase of dissolution of biological apatites (bone and dental enamel) a prevalent release of magnesium ions can be observed. In magnesium-substituted apatites, the lattice parameters show a slight contraction, as do the bands of the I.R. they move to lower frequencies coherently with the lower atomic mass of magnesium and with the different interaction energy Mg-O.

Strontium is present in biological apatites only at the level of impurities and can replace calcium causing an expansion of the a and c axes. The presence of this element in apatite for odontostomatological use is considered important in relation to a possible cariostatic effect (in addition to the hypothesized effect of reducing dentinal sensitivity), and gives apatite less solubility and greater resistance to heat treatments.

As regards barium, in hydroxyapatite this cation does not replace calcium in an isomorphous manner throughout the concentration range. In fact, the variation of the lattice parameters involves an expansion of the sides of the cell.

As already noted, the basic structure of hydroxyapatite can also be replaced with anionic groups. In particular, in carbonate-apatites the carbonate ion can replace the hydroxide ion (site A) or the phosphate ion (site B) or both. Although the substitution of the carbonate ion at site B is preferential in biological samples, it is however possible to obtain carbonate-apatites of type A synthetically by means of high temperature reactions. On the other hand, by precipitation from solutions mainly type B or mixed type A + B carbonate-apatites are obtained.

The presence of the carbonate ion in the different sites, difficult to detect by X diffractometry, can be highlighted by I.R. since the position of the carbonate ion absorption bands is in direct dependence on the occupied site. The presence of the carbonate ion in the hydroxyapatite modifies the reticular dimensions: if this occupies site A there is an increase in the parameter a as a consequence of the greater dimensions of the C03 <2> 'ion compared to the OH- ion; if instead it occupies the site B, the same parameter a undergoes a contraction due to the shorter distances 0-0 in the ion CO3 <2> 'with respect to the ion PO ^. Furthermore, it has been observed that the inclusion of the carbonate ion in apatite leads to a decrease and a different morphology of the crystals, which are no longer needle-like but with edges of comparable size. Solubility also increases, while thermal stability decreases.

In the fluoro- and choroapatites, the F- and Cl- ions, respectively, replace the hydroxyl; this replacement, theoretically, can be total. Fluoroapatite is characterized by an increase in the size of the crystal, a decrease in the parameter a of the unit cell, a lower solubility and increased thermal stability. The current use of the fluorine ion in the therapy of bone pathologies and dental caries derives from the lower solubility, and therefore from the greater reticular stability of the fluorapatites.

Chloroapatite is characterized by an expansion of the a side and a contraction of the c side of the unit cell. The different reticular behavior of chloroapatite with respect to fluoroapatite derives from the considerable difference in the ionic radius of the two halogens: in fluoroapatite the fluorine ion is located on the senary axis located on the plane consisting of the three calcium ions, while in the cioroapatites the chlorine ion detaches itself slightly from the plane of metal ions. This phenomenon involves the aforementioned parametric variations of the chloroapatite, while the crystallinity does not seem to be significantly affected, even if the thermal stability decreases. It is possible to observe the presence of solid solutions in the whole concentration range between fiuoroapatites and apatites and also between cioroapatites and apatites; solid solutions can also be found between fiuoroapatites and cioroapatites.

The non-stoichiometry of biological apatites can also be caused by the presence of the hydrogen phosphate ion HP04 <2> -; this ion is contained in a particular way in the dental enamel, in quantities between 5% and 15% and has the effect of increasing the solubility of hydroxyapatite. The presence of hydrogen phosphate in biological and synthetic apatites is not easily detectable, as the carbonate ion, almost always present, masks the I.R. and causes a similar variation of the cell parameters. The original presence of HP04 <2> 'can be highlighted by the formation of pyrophosphate, by heating the apatitic samples between 400X and 500 ° C.

According to some specific embodiments of the invention, therefore, the cation M is selected from the following: H <+>, Na <+>, Mg <2> *, K <+>, Sr <2+>, Ba <2+> and Fé <2-> * ·, and preferably the value x is between 0 and 2. The anion B is instead chosen, preferably, from C03 <2> ', HPO4 <2>', HC03 <*>, P2O7 <4> ", while the value y can be, for example, between 0 and 2. As regards the anion A, according to a specific choice, it is absent, the value of z being equal to 0 ( hydroxyapatites), while according to another specific choice it is F ~, with z = 2 (fluoroapatites).

The nanostructured apatite object of the invention can be produced by any of the numerous methods known and already used for the production of nanocrystalline materials, such as synthesis methods from atomic or molecular precursors (e.g. chemical or physical vapor phase deposition, gas condensation, chemical precipitation, aerosol reactions), the methods of processing precursors in bulk (e.g. by mechanical friction, by crystallization from the amorphous state by phase separation), and those borrowed from nature (biological imitation systems) .

The traditional deposition of layers of materials by electrolytic processes or by vapor condensation has been exploited in recent times to lay down materials of nanometric dimensions with remarkable control and precision. Among the new or improved methods are, in particular, increasingly sophisticated single- or multi-bath systems for electrolytic deposition and new chemical or physical vapor deposition methods such as molecular beam epitaxy (MBE) and chemical metal-organic vapor deposition (MOCVD). Such methods are not only capable of exercising precise chemical and thickness control of the deposited layers on a nanometer scale, but can also, in certain cases, control the nature of the interfaces within the layers themselves (L.E. McCandlish, D.E. Polk, R.W. Siegei, and B. H. Kear, Multicomponent Ultrafine Microstructures: Mater. Res. Soc. Symp. Proc. 132 (1989); G. Gumbs, S. Luryi, B. Weiss, and G.W. Wicks. Growth, Processing and Characterization of Semiconductor Heterostructures : Mater. Res. Soc. Symp. Proc., 326 (1994)).

Several new opportunities also exist in the field of manufacturing nanophasic materials assembled from atomic clusters synthesized by physical and chemical methods. For example, chemical precipitation represents one of the traditional methods of synthesis of ultrafine powders or colloidal suspensions that has been successfully applied in the synthesis of nanometer-sized clusters with narrow dimensional distributions, for example, by applying the technique of sol-gel synthesis or the inverse micelle method. Currently, many high-temperature gas reaction methods are also available for the synthesis of nanometric clusters, or of larger powders with nanometric structures (R.W. Siegei, 1991, cited above).

As regards the latter group of techniques, the synthesis of nanocrystalline materials through the in situ consolidation under vacuum of ultrafine particles condensed in the form of a nanometric gas begins when a precursor material, be it an element or a compound, is evaporated in a gas maintained at low pressure, generally well below 1 atmosphere. The evaporated atoms lose energy through collisions with the atoms or molecules of the gas and undergo a homogeneous condensation capable of forming clusters of atoms in the highly supersaturated area close to the source of the precursor. In order to keep cluster sizes small by minimizing accretion with additional atoms or molecules and cluster-to-cluster coalescence, previously nucleated clusters from the highly supersaturated region must be rapidly removed. Since the clusters are already suspended in the condensation gases, this can be quickly achieved by preparing the conditions for moving this gas, for example by natural convection or forced flow. The clusters suspended in the gas are then conveyed to a collection surface (which occurs by means of thermophoresis) and then positioned in a piston-anvil device for consolidation. It is also possible to carry out, for example, a direct deposition of clusters as thin films or filaments.

Only three fundamental quantities, which function in relation to each other, control the formation of clusters of atoms in the condensation process in the form of gas described above: (McCandiish et al., 1989, cited above) the rate of supply of atoms towards the supersaturation area in which condensation occurs, the speed of removal of energy from hot atoms through the condensation medium, i.e. the gas, and (Siegei, 1991, cited above) the speed of removal of the clusters previously nucleus from the 'supersaturation area.

With the mechanical friction method (reticular destabilization) nanostructures are produced not through the assembly of clusters, but through the decomposition of large-grain structures caused by severe mechanical deformation. The nanometer-sized grains form a core within sliding bands of highly deformed precursor materials, transforming a large-grain structure into a nanophasic one. Strong deformation is normally caused by high-energy crushing, but can also occur as a result of surface wear phenomena (E. Hellstern, H.J. Fecht, Z. Fu and W.L. Johnson., J. Appi. Phys., 65 (1989 ); C.C. Koch, Nanostructured Mater., 2, 109 (1993)), or with other methods of introducing high deformation densities (R. Valiev, in Mechanical Properties and Deformation Behavior of Materials Having Ultra-Fine Microstructures, M. Nastasi , D.M. Parkin, and H. Gleiter. 303 (1993)). During the heavy mechanical work on precursors it is also possible to make different materials react so that they form new phases and compounds.

This rather direct and relatively simple method to implement allows ready access to the dimensions of fine grains useful for the purposes of the invention and allows to produce commercial quantities of materials, and has therefore been used for the synthesis of the nanostructured apatitic materials of the invention which were subjected to the comparative experimentation described below. The nanostructured apatitic material according to the invention was produced starting from apatite with a microcrystalline structure by reticular destabilization in a controlled, high-energy environment, subjecting the starting material to a treatment with a high transfer of mechanical energy, which takes place inside a cylindrical reaction chamber by means of high energy impacts by rigid spheres contained within the chamber itself. The kinetic energy of the spheres is generated through the rotation of the chamber with respect to its main axis and revolution with respect to an axis parallel to the main axis. The transfer of kinetic energy into mechanical energy of lattice destabilization occurs through collisions between the spheres and the starting material. The chamber can work both in air, both in inert gas and under vacuum, up to a pressure value of 1C <6> torr or with liquids (alcohols, ethers, oils and other organic molecules).

The system reduces the size of the crystallites through the subsequent introduction of lattice defects (measurable by X diffraction techniques). In particular, the nanoapatitic material obtained with the described synthesis method and used in the reported applications has an average crystallite size around 15 nm, with an average strain between 10 <'5> and 10 <' 2> .

The apatite-based nanocrystalline products of the present invention represent a material that simulates the composition of dentin and is perfectly compatible, both biologically and structurally, with dental tissues, being able to integrate efficiently and stably with dentin, and to make it completely impervious. Thanks to the dimensional characteristics, the surface activity, the defect content of the nanoapatites obtained preferably by the method described above, both the mechanical filling of the tubules and the fixing reaction to the tubule wall (remineralization) and the dentine sensitivity are easily obtained. consequent to the hydraulic conductance through the tubular structure of the dentin is practically eliminated. As already noted, this permeability is a determining factor in the theory of painful dentine stimulation, because the presence of exposed and open tubules determines, in the presence of adequate external stimuli, the movement of the dentinal fluid perceived at the level of the nerve structures as a pain stimulus.

According to a preferred embodiment of the invention, the nanocrystalline apatitic materials are protected from acid attack by subjecting them to a treatment with protective aqueous solutions based on tartaric acid and / or its salts, for example with aqueous solutions based on sodium potassium tartrate (or Seignette salt, or Rochelle salt, NaOOC (CHOH) 2COOK-4H20) and hydrated calcium acetate Ca (CH3C00) 2 * H20), or with a solution based on tartaric acid (HOOC (CHOH) 2 COOH Preferably, a 0.1-1.0 M sodium potassium tartrate solution and a 0.1-1.0 M hydrated calcium acetate solution are used in succession, in which the material of the invention is immersed, preferably at 37 ° C, and for themes ranging from 5 to 30 minutes Specifically, the nanoapatite is placed in the first solution by holding it there for 5 minutes, then it is taken from the first solution and immediately placed in the second solution for 20-30 minutes.

The material can also be treated individually with one of the following agents: calcium gluconate mono-, bi-, tri-, tetra-, polycarboxylate in aqueous solution at a concentration of 0.01-5% by weight, or alcoholic or hydroalcoholic to a concentration varying between 0.01 and 15% by weight, or with an acetic solution of chitosan at a concentration varying between 5 and 20% by weight, or with a solution of bovine, porcine or porcine tendon collagen turkey, isotonic and not, at a concentration ranging from 0.5 to 5% by weight. These treatments determine a protection of the nanostructured material against acid environments. However, the material can also be used without treatments.

The invention further relates to compositions for odontostomatological use including the nanostructured apatite-based materials described above, together with any additional ingredients and excipients of the type used in preparations for hygiene and oral therapy. The compositions, which are preferably in paste, gei or suspension form, preferably contain 0.5 to 50% by weight of nanocrystalline apatite material, whether treated with protective agents or untreated.

Some specific embodiments of the invention are described by way of example below, together with the results of the experiments carried out on the proposed nanostructured products and in comparison with other non-nanocrystalline apatitic materials.

EXAMPLE 1

A microcrystalline apatitic material, carbonated and out of stoichiometry as regards the hydroxyl, was subjected to a reticular destabilization treatment in a controlled environment, at high energy, using the reaction chamber described above. The resulting product (nanoapatite) was characterized by an average crystallite size of 15-20 nm and an average microstrain content between 10 <-4> and 10 <-5>.

A part of the nanocrystalline apatite thus obtained (which will also be referred to below as nanoapatite 1 or nano 1) was also subjected to treatment with tartrate in order to increase its resistance to acid attack in the conditions of use in the oral cavity. For this purpose, the nanoapatite was placed in contact with a 0.1 M aqueous solution of sodium potassium tartrate (Seignette salt, Carlo Erba, Milan) at 37 ° C for 5 minutes and then with a 0.1 M solution of hydrated calcium acetate (Carlo Erba, Milan) at 37 ° C for 20 minutes.

EXAMPLE 2

An apatitic material of the same nature and composition as that of Example 1 was subjected to a similar reticular destabilization treatment, with a different value of the energy content transferred. The resulting nanoapatite was characterized by an average crystallite size of 10-14 nm and an average microstrain content of between 5-10 <-4> and 10 <-4>.

Also in this case, a part of the obtained nanocrystalline apatite (nanoapatite 2 or nano 2) was subjected to treatment with sodium potassium tartrate and hydrated calcium acetate, according to the same procedure as in Example 1.

• Dentin permeability tests

The efficacy of the nanocrystalline products according to the invention in the treatment of dentinal sensitivity was evaluated in terms of reduction of the hydraulic conductance inside the dentinal tubules, according to a well-coded protocol (Pashley, 1990, already cited) in accordance with the international literature. To perform this test, healthy human molars were used, extracted for orthodontic reasons from young subjects, to avoid the problem of sclerotic dentin. Each tooth was suitably separated from the root and sectioned to obtain a crown segment deprived of the occlusal enamel and, after removing the pulp tissue, it was fixed with an adhesive on a Plexiglas support with the flat surface of occlusal dentin facing the 'aito. The thickness of the support was crossed by a tubular steel segment that emerged inside the pulp chamber for the hydraulic connection, below the plane of the Plexiglas support, with the hydraulic conductance detection system. The latter was made with a simple hydrodynamic device consisting of a series of capillaries filled with deionized water and connected through the tubular steel segment to the pulp chamber. The passage of water from the pulp chamber to the occlusal surface through the dentin was highlighted by the displacement of an air bubble in a graduated microcapillary placed in the hydrodynamic system.

The tests reported below were carried out, with the device described, by applying on the exposed dentinal surface a carboxymethyl cellulose-based gei to which the active ingredients under test were added from time to time. Each quantity was obtained by solubilizing 6 g of carboxymethyl cellulose at 50 ° C in an aqueous solution containing 0.2% by weight of parabens, and continuing to mix until obtaining a transparent gel of the right consistency. The various test products were mixed at 3 ml aliquots of the gel thus obtained, according to the following scheme:

1. white, consisting of the gel vehicle only;

2. commercial microcrystalline hydroxyapatite (Merck, Darmstadt, Germany, Art. 2196);

3. commercial microcrystalline hydroxyapatite treated with sodium potassium tartrate and calcium acetate hydrate as described in example 1; 4. nanoapatite 1, produced according to example 1;

5. nanoapatite 1 treated with sodium potassium tartrate and hydrated calcium acetate according to the same example;

6. nanoapatite 2, produced according to example 2;

7. nanoapatite 2 treated with sodium potassium tartrate and hydrated calcium acetate as described in example 1.

According to the codified procedure referred to above, the following steps were carried out for each test:

* Formation of a smear layer by rubbing on abrasive paper under manual pressure, followed by washing with deionized water; * Application of 0.5 M EDTA (ethylenediamino-tretraacetic acid, Sigma, St. Louis, USA) at pH 7.4 for 5 minutes. Permeability was measured after letting it stand for 2 min. and washed the sample. The value obtained was taken as a reference for the subsequent tests, associating it with a permeability of 100%.

* Treatment with the white gel or gel containing the apatite under test (treated or untreated), brushing the tooth for 3 min. This time was chosen in relation to the correct duration of brushing in normal tooth brushing operations. The tooth was brushed and not brushed to avoid the formation of a smear layer, which would make it difficult to assess the effectiveness of the product.

* Etching with 37% by weight orthophosphoric acid (Merck, Darmstadt) for 1 min. and rinse. The permeability was evaluated after waiting one minute.

The permeability tests were performed seven times for each set time (30sec., 60sec., 120sec.) After treatment with each single gel and repeating each of the steps listed above.

The results obtained are presented in the following Table 1 for all the steps of the experiment, while to highlight the behavior of each gel towards acid attack, the data relating to the measurement at 60 minutes after application of the gel and after treatment with orthophosphoric addus are also reported in the histogram of the attached Figure.

(table follows)

From the examination of the table above draw the following conclusions, which are also clearly evident from the attached histogram: with the white (carboxymethylcellulose gel) there is no positive response and the tooth is therefore very permeable, as well as particularly susceptible to acid attack , so much so that after treatment with H3P04 permeability values higher than 100% are obtained (see Figure).

With the use of commercial microcrystalline hydroxyapatite, partial occlusion of the dentinal tubules may occur with a lower permeability than in the smear layer formation phase, but after the use of H3PO4 no appreciable resistance is noted. A certain resistance is instead evident if the microcrystalline hydroxyapatite is treated with solutions of Seignette salt and calcium acetate. The behavior before and after the acid attack, in the absence and in the presence of protective treatment, is evident in the attached Cistogram.

With the use of nanoapatites 1 and 2 the dentinal permeability is considerably reduced so that, from 100% permeability after treatment with EDTA, it passes to about 40% with nanoapatites, to further improve in the time of 120 sec. the acid attack in this case produces no appreciable negative effect, and this is certainly the most interesting data.

The efficacy of the treatment with nanocrystalline apatite according to the invention is even more evident with the use of nanoapatites treated with Seignette salt and calcium acetate, which not only produce a considerable reduction in dentinal permeability even in the absence of acid attack but, above all, they offer an even more significant reduction after acid attack for H3PO4, with permeabilities that do not rise more than about 35% of the value after treatment with EDTA.

Although the statistical analyzes did not reveal any significant difference between nanoapatites 1 and 2, it can be emphasized that nanoapatite 2 gave a lower standard deviation of the measurements, and therefore more reproducible results.

From the above experimental results it can be appreciated how the nanostructured apatitic materials under study are validly suitable as biocompatible materials active in prolonged reduction of the hydraulic conductance inside the dentinal tubules and able to integrate stably and deeply into the dentinal tissues. Therefore, these materials can be used with advantage, for example, in toothpastes, mouthwashes, gels, resins and cements, or as such, for the protection of dentin, for the therapy of dentinal hypersensitivity, for the remineralization of dental tissues and support, for closing the dentinal tubules or reducing their functional diameter, for the protection of the dental pulp and prosthetic abutments and for use as bases and substrates for amalgam restorations, cements for endodontic fillings, for dental prostheses, cements orthodontics and sealants for enamel and dentin.

The present invention has been described with reference to some of its specific embodiments, but it is to be understood that variations or modifications may be made to it by those skilled in the art without thereby departing from the relative scope of protection.

Claims (22)

CLAIMS 1. Use of apatitic based nanostructured materials of general formula: where: M is a cation other than Ca <2+>, B is an anion other than the PO4 <3> ion ", A is chosen from O <2 '>, C03 <2'>, F <-> and CI <">, x is a number between 0 and 9, y is a number between 0 and 5, z is a number between 0 and 2, said numbers may also be non-integers, with the condition that the sum of the charges of the cations Ca and M is equal to the sum of the charges of the anions P04 <3->, B and A, having an average crystallite size between 0.5 and 200 nm, for the production of preparations for odontostomatological applications. Use according to claim 1, wherein said nanostructured materials exhibit deformations and / or lattice defects. Use according to claims 1 02, wherein M is selected from H <+>, Na <+>, Mg <2+>, K <+>, Sr <2+>, Ba <2+> and Fe2 < +>. Use according to any one of claims 1-3, wherein x is between 0 and 2. Use according to any one of claims 1-4, wherein B is selected from C03 <2> ", HPO4 <2>", HC03 "and P2O7 <4> ·. Use according to any one of claims 1-5, wherein x is between 0 and 2. Use according to claim 1, wherein x, y and z are 0 (hydroxyapatite). Use according to claim 1, wherein x and y are 0, A is F ~ and z is equal to 2 (ftuoroapatite). 9. Use according to any one of the preceding claims in which the average dimensions of said cristaltites are comprised between 1 and 50 nm. 10. Use according to each of the preceding claims, in which said preparations for odontostomatological applications are powders, pastes, gels, mouthwashes, resins, cements and suspensions for the protection of the hard tissues of the tooth, for the therapy of dentinal hypersensitivity and for remineralization of dental and supporting tissues, as well as for use in protecting the dental pulp and prosthetic abutments and as bases and substrates for restorations. 11. Use according to any one of the preceding claims, in which said nanostructured apatitic materials are treated, in said production, with protective aqueous solutions based on tartaric acid and / or its salts. 12. Use according to claim 11 wherein a first aqueous solution of 0.1-1.0 M sodium potassium tartrate and a second aqueous solution of 0.1-1.0 M calcium acetate hydrate are used in sequence. 13. Use according to claim 12 wherein said material is first immersed in the first solution for 5 minutes at 37 ° C, then it is withdrawn from said solution and immediately placed in said second solution at 37 ° C, holding it there for 20-30 minutes. 14. Use according to any one of claims 1-10, wherein said nanostructured apatitic materials are treated, in said production, with protective aqueous solutions of calcium gluconate mono-, bi-, tri-, tetra-, polycarboxylate at a concentration 0.01-5% by weight, or alcoholic or hydroalcoholic at a concentration varying between 0.01 and 15% by weight. Use according to any one of claims 1-10, wherein said nanostructured apatitic materials are treated, in said production, with a protective acetic solution, of chitosan at a concentration varying between 5 and 20% by weight. Use according to any one of claims 1-10, wherein said nanostructured apatitic materials are treated, in said production, with a protective aqueous solution of isotonic or non isotonic bovine tendon collagen collagen varies from 0.5 to 5% by weight. 17. Composition for odontostomatological use comprising apatite-based nanostructured materials defined as in claim 1. 18. Composition according to claim 17, wherein said apatite-based nanostructured material is hydroxyapatite. 19. Composition according to claims 17 or 18, wherein the average dimensions of the crystallites of said nanostructured material are comprised between 1 and 50 nm. 20. Composition according to any one of claims 17-19 in the form of a paste, gel or suspension containing from 0.5 to 50% by weight of said apatite-based nanostructured material. 21. Composition according to any one of claims 17-20 wherein said nanostructured apatite-based material has been treated with protective solutions based on tartaric acid and / or its salts in aqueous solution, or with mono-, bi-, tri -, tetra-, polycarboxylate in aqueous, alcoholic or hydroalcoholic solution, or of kiosan in acetic solution, or of bovine, porcine or turkey tendon collagen in aqueous solution. 22. Odontostomatological use of nanostrutured apatite-based materials and compositions containing them according to claims 1-21, substantially as described and illustrated above.