WO2019224693A1

WO2019224693A1 - Method for teeth cleaning by means of a composition in the form of powder based on hydroxyapatite

Info

Publication number: WO2019224693A1
Application number: PCT/IB2019/054144
Authority: WO
Inventors: Chiara Colletti; Martina SIVIERI; Norberto Roveri; Marco Lelli; Sara DE FAVERI; Saverio MINUTOLI
Original assignee: Mectron S.P.A.
Priority date: 2018-05-21
Filing date: 2019-05-20
Publication date: 2019-11-28
Also published as: IT201800005561A1

Abstract

Method for teeth cleaning which comprises a step of applying on the dental surface a jet comprising micrometric particles of silica gel incorporating hydroxyapatite aggregates, wherein the hydroxyapatite aggregates have an average diameter (d50) from 50 nm to 500 nm, and the micrometric particles of silica gel have an average diameter (d50) from 1 pm to 150 pm. The incorporation of hydroxyapatite aggregates in the silica gel allows reducing undesired aggregating tendency of the hydroxyapatite and obtaining a biomimetic and bioactive composition.

Description

METHOD FOR TEETH CLEANING BY MEANS OF A COMPOSITION IN THE FORM OF POWDER BASED ON HYDROXYAPATITE

The present invention relates to a method for teeth cleaning by means of a composition in the form of powder based on hydroxyapatite. More particularly, the present invention relates to a method for teeth cleaning by means of an air-polishing technique wherein a composition in the form of powder is used which is based on hydroxyapatite incorporated in silica gel.

It is well known that oral hygiene is one of the cornerstones for the prevention and treatment of many oral diseases. The growing attention to oral hygiene is closely linked to the greater awareness that the condition of general health must also include the state of health of the oral cavity. In fact, there is a direct correlation between various systemic diseases and periodontal ones.

For this reason, scientific research is seeking to develop new tools and aids to support the work carried out by dental hygienists and dentists, in order to optimise the outcome of their work.

Among the factors that influence the health of the oral cavity, the most important are: presence of dental plaque, oral hygiene, odontostomatological treatments, personal physical conditions, and diet. The latter is in fact closely correlated with the intake of substances that strongly influence the proliferation rate of the oral cavity bacteria.

Dental plaque, or biofilm, is defined as a non- mineralised accumulation of microbial cells (bacteria) that colonise the tooth surface. By their nature, plaque deposits are only partially removed by brushing, especially in areas where cleansing is more difficult (for example: the gingival margin of the teeth, where it can cause caries and gingivitis; anatomical grooves and depressions in teeth; interdental area and surface of the upper third molar, difficult to reach) . Plaque is composed of an organic matrix derived from salivary glycoproteins and from extracellular microbial products. The bacteria that make up the plaque feed on the sugars consumed with the diet, which once converted to acid cause pH lowering and acidification of the oral cavity. The direct consequence of this acidification is the destruction of the interprismatic protein component of the hydroxyapatite of dental enamel. This destruction can lead to the onset of dentin hypersensitivity. The term dentin hypersensitivity is intended as a dental pathology that manifests itself as a painful symptomatology, generally localised to one or more dental elements presenting exposure of dentinal tissue, evoked by stimuli of various nature (thermal, tactile, chemical or osmotic) which act in the absence of other dental pathologies. This pathology is a consequence of the exposure of the dentinal tubules due to the removal of the enamel or the root cement that normally protect the dentin. The sense of pain occurs in response to a stimulus that induces the centrifugal displacement of the tubular fluid and is able to activate the nerve endings that produce the displacement of the fluid in the direction of the dental pulp. This phenomenon affects 10 to 30% of the population, mainly comprised between 20 and 50 years.

For preventive and/or therapeutic purposes the aforementioned pathogenic bacteria can be eliminated from the oral cavity thanks to professional mechanical cleaning techniques which consist in the removal of the plaque through the air-polishing technique and in the removal of tartar, or remineralised plaque, through tartar ablation.

The air-polishing technique was introduced in the late 1970s and aims to remove plaque from the tooth surface and remove extrinsic stains on tooth enamel caused by tea, coffee, red wine, tobacco and other pigmenting substances. It is an effective technique that is minimally invasive and can be repeated over time, since it has a delicate action on the hard and soft tissues of the oral cavity.

The air-polishing technique consists in performing a teeth cleaning treatment thanks to an abrasive jet of powders mixed with compressed air. The abrasive jet is surrounded by an annular water jet which has the purpose of confining the abrasive jet and washing the dental surfaces. Usually this technique uses a device capable of emitting the abrasive jet surrounded by a jet of water. The type of powder is selected based on the type of cleaning treatment to be performed.

With the same device, same usage time of the device and same angle of incidence of the jet on the dental surface, the parameters that most influence the efficiency and the erosive rate of the treatment are: density, hardness, dimensions and shape of the particles that make up the powder .

Commonly used powders are sodium bicarbonate, glycine, calcium phosphosilicate, aluminium hydroxide, calcium carbonate and erythriol. Sodium bicarbonate and glycine are nowadays the most commonly used powders. Sodium bicarbonate, in particular, has been used in the dental field since 1980. When used for air-polishing, its particles generally have an average diameter (d50) of 150 pm and the crystals have a chiselled rectangular and/or square-type shape. It effectively removes the supragingival bacterial plaque and stains from the enamel surface. However, it must be used with caution on dentin and root cement as it can cause demineralisation, strong abrasions and loss of healthy tissue.

Glycine is an amino acid. When used for air polishing, its particles generally have an average diameter (d50) of 25 pm and it is currently used in the clinical practice of non-surgical periodontal therapy. This powder is indicated for patients with systemic diseases, subject to a low-salt diet, as in the case of patients with hypertension and renal insufficiency, where the use of sodium bicarbonate as an air-polishing powder would be contraindicated. Furthermore, glycine has a lower abrasive power, compared to sodium bicarbonate, on root cement, enamel, dentin and exposed implants .

The powders known to the expert in the field, commonly used in air-polishing, in addition to the characteristic of inducing the mechanical removal of plaque and stains, do not comprise other beneficial characteristics for the treated tooth, such as for example biocompatibility, biomimicry and bioactivity. The term "biocompatibility" means a characteristic of the powder that guarantees its non-toxicity for the treated patient. The term "biomimetics " means, instead, that the powder has chemical, physical, morphological and mechanical properties similar to those of the tooth structure. The term "bioactivity" means, finally, that the powder is able to chemically interact with the tooth surface, leading to a self-repairing and remodelling effect on the tooth surface itself.

The Applicant has therefore posed the problem of developing a powder composition adapted to be used with the air-polishing technique which does not present the aforementioned drawbacks, which has a delicate action on the hard and soft tissues of the oral cavity, so that the treatment can be frequently repeated. The Applicant has also posed the problem of developing an air-polishing powder that can reduce dental hypersensitivity, reduce problems related to demineralisation and is biocompatible, biomimetic and bioactive.

The Applicant has found that these problems can be solved by using a powder composition comprising micrometric particles of hydroxyapatite aggregates as defined below.

Hydroxyapatite is a mineral having the formula Caio (PO4) _{6 (}OH) 2 and belongs to the apatite group. The OH- group can be replaced by fluoride ion, chloride ion or carbonate ion. The crystals have a prismatic shape with a hexagonal base. Hydroxyapatite is the main component of dental enamel and dentin.

For example, the article by Juan de DiosTeruel et al, Archives of Oral Biology, Vol.60, 2015, 768-775, in fact describes that the enamel of human teeth has a highly mineralised chemical composition, having values very close to those of hydroxyapatite.

Hydroxyapatite is therefore an excellent candidate for the preparation of air-polishing powder compositions which, in addition to the purely mechanical plaque removal action, can have the aforementioned characteristics of biocompatibility, biomimicry and bioactivity .

However, hydroxyapatite has high hygroscopic properties and therefore, when made in the form of small particles, these tend to aggregate with each other, due to absorption of water molecules present in the surrounding environment, with consequent formation of aggregates that hinder a regular operation of the air- polishing device. In fact, in order to correctly emit the abrasive jet, this device exploits a turbulent regime generated inside a chamber present in the device itself, which has the purpose of maintaining the particles in disaggregated form. Once mixed with compressed air, the particles are emitted at high speed by a nozzle of the device. However, the Applicant has ascertained that, using a powder composition based on hydroxyapatite as such, the turbulent regime is not able to maintain the particles in disaggregated form, so that difficulties are observed in the emission of a high speed abrasive jet that is uniform and constant over time.

The Applicant has therefore posed the problem of developing a method for cleaning teeth that uses a powder composition based on hydroxyapatite, suitable to be used with air-polishing devices without affecting the correct operation of the device and therefore the effectiveness of the technique.

The Applicant has now found that this problem, and others which will be better explained below, can be solved by a teeth cleaning method which uses a jet of a powder composition comprising micrometric particles of silica gel incorporating hydroxyapatite aggregates, having average diameters as better defined below.

In a first aspect, the present invention therefore relates to a method for teeth cleaning which comprises a step of applying on the dental surface a jet comprising micrometric particles of silica gel incorporating hydroxyapatite aggregates, wherein the hydroxyapatite aggregates have an average diameter (d50) from 50 nm to 500 nm, preferably from 100 nm to 250 nm, and the micrometric particles of silica gel have an average diameter (d50) from 1 pm to 150 pm, preferably from 5 pm to 70 pm .

Preferably the hydroxyapatite aggregates are aggregates composed of hydroxyapatite crystals wherein each crystal has a nanometric size, that is an average size not exceeding 100 nm.

In a second aspect, the present invention relates to a composition in the form of micrometric particles of silica gel incorporating hydroxyapatite aggregates, wherein the hydroxyapatite aggregates have an average diameter (d50) from 50 nm to 500 nm, preferably from 100 nm to 250 nm, and the micrometric particles of silica gel have an average diameter (d50) from 1 pm to 150 pm, preferably from 5 pm to 70 pm.

In a third aspect, the present invention relates to a composition which comprises micrometric particles of silica gel incorporating hydroxyapatite aggregates, for use in dental prophylaxis through air-polishing, wherein the hydroxyapatite aggregates have an average diameter (d50) from 50 nm to 500 nm, preferably from 100 nm to 250 nm, and the micrometric particles of silica gel have an average diameter (d50) from 1 pm to 150 pm, preferably from 5 pm to 70 pm. In the context of the present description and the attached claims, "average particle diameter" refers to, unless otherwise indicated, diameter d50 (median value), i.e. the value of the diameter below which there is 50% by volume of the particle population (see "A Guidebook to Particle Size Analysis" published by Horiba Instruments Inc. - 2016, available at https : / /www . horiba . com/fileadmin/uploads/ Scientific/eMa g/PSA/Guidebook/pdf/PSA_Guidebook . pdf ) . It can be determined through a laser diffraction technique, according to ISO 13320:2009.

Preferably, the particles comprise from 5% to 60% by weight, more preferably from 10% to 50% by weight, of hydroxyapatite, and from 40% to 95% by weight, more preferably from 50% to 90% by weight, of silica gel, the percentages being expressed with respect to the weight of the particles.

The Applicant believes that incorporating hydroxyapatite aggregates in silica gel makes it possible to reduce the undesirable aggregating tendency of hydroxyapatite, thus making it particularly suitable for application with air-polishing. Moreover, the fact of using hydroxyapatite aggregates as defined above advantageously allows obtaining a biomimetic and bioactive composition having the ability to chemically interact with the dental surface, leading to a self repairing and remodelling effect of the dental surface treated with air-polishing, thus reducing hypersensitivity. One of the advantages of the method and of the composition according to the invention is therefore the ability to reduce dentin hypersensitivity through artificial occlusion of exposed tubules. It can also help to artificially restore demineralised areas caused by fractures or abrasions of the dental surface.

In a preferred embodiment, the hydroxyapatite is a hydroxyapatite of the general formula:

Ca_(10-a)M_a (PO4) _(6-b) (X)b(OH) 2-cYc (I) where :

M is a substituent selected from: Na, Mn, K, Zn,

Mg, Sr;

X is a substituent selected from: SiCy, CO3;

Y is a substituent selected from: Cl, F;

a is zero or is comprised from 0.030 to 1.0% wt/wt, extremes included;

b is zero or is comprised from 1.0 to 5.0 % wt/wt, extremes included;

c is zero or is comprised from 0.01 to 1.0% wt/wt, extremes included.

The Applicant believes that the presence of Zn⁺², Sr⁺², Mg⁺², K⁺, Mn²⁺, Na⁺ ions in partial substitution of the Ca⁺² ions, and of the SiCy ⁴ e CCy ² ions, in partial substitution of the PCy ³ ions, is advantageous as it makes the hydroxyapatite crystals even more similar to dental enamel, thus favouring biomimicry and bioactivity .

Preferably, the micrometric particles of silica gel incorporating the hydroxyapatite aggregates are coated with a C6-C24 fatty acid salt, preferably a stearic acid salt (Cie), even more preferably with magnesium stearate, sodium stearate or calcium stearate.

This advantageously allows further reducing hygroscopicity of the particles and thus further improving flowability index. The quantity of said fatty acid salt is comprised from 1% to 20% by weight, more preferably from 5% to 15% by weight, with respect to the weight of the micrometric particles of silica gel incorporating hydroxyapatite aggregates.

Preferably the particles of silica gel incorporating hydroxyapatite aggregates, as such i.e. in the absence of any coating agent, have a surface area (BET) comprised from 50 m²/g to 600 m²/g, preferably from 60 m²/g to 550 m²/g.

Preferably the particles of silica gel incorporating hydroxyapatite aggregates, superficially treated with a fatty acid salt as indicated above, have a surface area (BET) comprised from 2 m²/g to 500 m²/g, more preferably from 5 m²/g to 450 m²/g. Reduction of the surface area is an indicator of the flowability of the particles, which is further improved by the presence of the coating with a fatty acid salt.

The surface area (BET) can be determined according to IS09277: 2010.

Preferably, the particles of silica gel incorporating hydroxyapatite aggregates have an apparent density (bulk density) comprised from 0.2 g/ml to 2.0 g/ml, more preferably from 0.3 g/ml to 1.5 g/ml.

The apparent density can be determined according to method 1 of "EUROPEAN PHARMACOPOEIA 7.0 - 01/2009:0308 corrected 6.8" (see "METHOD 1: MEASUREMENT IN A GRADUATED CYLINDER, 2.9.34 Bulk density and tapped density of powders") .

Preferably, the micrometric particles of silica gel incorporating the hydroxyapatite aggregates have a pH value comprised from 3 to 7.

The pH value can be determined according to known techniques, for example according to the following method. An aliquot of particles is mixed with distilled water in a ratio, respectively, of 1:2.5. The suspension obtained is stirred for 15 minutes and then left to rest for 30 minutes, in order to allow sedimentation of the particles. The pH value of the supernatant aqueous solution is then measured using a pH meter.

In carrying out the method according to the present invention, the jet comprises, in addition to the micrometric particles of silica gel incorporating the hydroxyapatite aggregates, compressed air that acts as a means for transporting the particles and an annular jet of water that confines the particles and allows the washing of the dental surface treated. The compressed air is preferably emitted at a pressure from 0.9 bar to 10 bar, more preferably from 1.5 bar to 8 bar.

In a further aspect, the present invention relates to a process for producing a composition in the form of micrometric particles as defined above, comprising the steps of :

- synthesising hydroxyapatite in the form of an aqueous suspension;

- adding a hydrolysable silicate to said aqueous suspension;

- maintaining said aqueous suspension thus added at a temperature from 30°C to 100°C until obtaining a silica gel incorporating hydroxyapatite;

- drying and grinding the silica gel thus obtained.

In an alternative embodiment of the process, the present invention relates to a process for producing a composition in the form of micrometric particles as defined above, comprising the steps of:

- providing a hydroxyapatite in powdered form; - suspending said hydroxyapatite in an aqueous solution of a hydrolysable silicate;

- maintaining said aqueous suspension thus obtained at a temperature from 30°C to 100°C until obtaining a silica gel incorporating hydroxyapatite;

- drying and grinding the silica gel thus obtained.

Preferably, the hydrolysable silicate is in the form of an aqueous solution, wherein the solvent comprises a mixture of water and alcohol, more preferably ethanol .

Preferably, the hydroxyapatite is a hydroxyapatite of general formula (I) . The synthesis of hydroxyapatite can be carried out according to methods known in the art, in particular wet, hydrothermal, electrochemical, sol-gel synthesis.

As a source of calcium ions it is possible to use a compound selected from: calcium hydroxide, calcium oxide, calcium carbonate, calcium nitrate, calcium chloride, calcium acetate, calcium sulphate, calcium silicate, or mixtures thereof.

As a source of phosphate ions it is possible to use a compound selected from: phosphoric acid, NH₄H₂PO₄, (NH₄)2HP0₄, H2KPO4, HK2PO4, H₂NaP0₄, HNa₂P0₄, H2KPO4, HK2PO4, or mixtures thereof.

As a source of carbonate ions it is possible to use a compound selected from: carbon dioxide, calcium carbonate, zinc carbonate, magnesium carbonate, strontium carbonate, or mixtures thereof.

As a source of zinc ions it is possible to use a compound selected from: zinc acetate, zinc oxide, zinc hydroxide, zinc carbonate, zinc phosphate, zinc chloride, zinc PCA, zinc gluconate, zinc nitrate, or mixtures thereof.

As a source of magnesium ions it is possible to use a compound selected from: magnesium chloride, magnesium carbonate, magnesium acetate, magnesium oxide, magnesium hydroxide, magnesium sulphate, or mixtures thereof.

As a source of strontium ions it is possible to use a compound selected from: strontium oxide, strontium hydroxide, strontium chloride, strontium acetate, strontium carbonate, or mixtures thereof.

As a source of silicate ions it is possible to use a compound selected from: sodium silicate, potassium silicate, calcium silicate, or mixtures thereof.

As a source of sodium ions it is possible to use a compound selected from: sodium chloride, sodium acetate, sodium sulphate, or mixtures thereof.

As a source of potassium ions it is possible to use a compound selected from: KOH, KF, KNa, KC1, K₂SO₄, KHSO₄, or mixtures thereof.

As a source of fluorine ions it is possible to use a compound selected from: NaF, KF, HF, ammonium fluoride or mixtures thereof.

Further details on the preparation of a hydroxyapatite in aqueous suspension are reported for example in WO 2007/045954, WO 2007/137606, WO 2012/159675, WO 2013/068020 and WO 2015/075644.

Preferably, the hydrolysable silicate is a tetraalkyl orthosilicate of formula (RO)4Si, where R is a C₁-C₄ alkyl, linear or branched, for example: tetramethyl orthosilicate (TMOS), tetraethyl orthosilicate (TEOS).

The hydrolysable silicate is usually used in the form of a solution in a water-soluble organic solvent, in particular an alcohol such as ethyl alcohol or methyl alcohol .

The hydrolysis of the hydrolysable silicate (that is the step of keeping the aqueous suspension added with the silicate at a temperature from 30°C to 100°C) is preferably carried out at a pH value from 1 to 10. More preferably, this step is carried out at a pH value of 5 to 7, a range within which the hydroxyapatite is particularly stable.

According to an embodiment of the process according to the invention, the aqueous suspension of the hydrolysable silicate further comprises an acid or basic catalyst, preferably an acid catalyst. The acid catalyst can be selected for example from mineral acids or organic acids, such as: hydrochloric acid, nitric acid, sulphuric acid, phosphoric acid, acetic acid, or mixtures thereof. The basic catalyst can be selected from: NH₃, NaOH, KOH, amines, or mixtures thereof.

The hydrolysis and then the gelation of the silica is advantageously accelerated by the presence of the catalyst, and generally requires a time comprised from 1 to 15 hours, preferably from 2 to 10 hours.

It has been observed that the use of an acid catalyst leads to the formation of a powder composition having greater compactness, avoiding the formation of a powdery composition, characterised by a very low density which makes it uneffective for use in the air-polishing devices .

Preferably the step of drying the silica gel is carried out, for example in a thermostatically- controlled oven, at a temperature comprised from 40 °C to 85°C, more preferably from 50°C to 80°C, for a period of time variable within wide margins, for example between 24 hours and 100 hours.

The step of grinding the dried silica gel can be carried out according to known techniques, for example by means of a ball or hammer mill or by micronisation processes .

After grinding, the gel is usually sieved according to known techniques, for example by means of a vibrating sieve shaker, so as to obtain particles of the desired granulometry.

To the powder composition thus obtained it is possible to add powder additives such as: amino acids, sugars, aluminium oxides, sodium phosphosilicates , silica, carbonates and bicarbonates, in order to improve flow properties. It is also possible to add any powdered fragrances to improve taste and/or flavour of the powder composition when it is used. Finally it is also possible to mix with the powder composition of the invention other powders commonly used for air-polishing, such as for example: sodium bicarbonate, glycine, calcium phosphosilicate, aluminium hydroxide, calcium carbonate and erythriol.

The following embodiment examples are provided merely to illustrate the present invention and should not be construed in a sense that would limit the scope of protection defined by the claims.

EXAMPLE 1: Synthesis of hydroxyapatite.

The following products were introduced into a laboratory flask, vigorously stirred for 3 hours:

water: 1500 g;

calcium hydroxide: 250 g;

strontium hydroxide octahydrate: 0.2 g; zinc carbonate: 0.4 g;

magnesium carbonate: 0.3 g.

In parallel, a solution of phosphoric acid 75% w/w (280 g) and water (1000 g) was prepared. The phosphoric acid and water solution was slowly added through a dropping funnel into the above flask, with a dripping speed of 1 ml/s, under vigorous stirring, until a pH of about 6 was obtained. The mixture thus obtained was maintained under vigorous stirring for further 6 hours.

After 6 hours, 20 g of silicon oxide were added to the mixture and the flask was kept under vigorous stirring for a further hour. At the end of the hour, the stirring was stopped and the final product thus obtained (25% w/w, percentage expressed with respect to the total weight of the suspension) was an aqueous suspension comprising hydroxyapatite substituted with zinc, magnesium, silicate, strontium and carbonate.

The synthesised hydroxyapatite can be represented by the following formula:

Ca₍9,64₎Zno,oi4Sro, oisMgo, 30 (PO4 ) _{(5,69) (} SiCL ) 0,010 (CO3 ) 0, 30 (OH) 2 The final product obtained is an aqueous suspension comprising hydroxyapatite aggregates having an average diameter (d50) of 200 nm.

EXAMPLE 2: Synthesis of silica gel incorporating hydroxyapatite .

The following products were introduced into a laboratory beaker, stirred for 1 hour:

tetraethyl orthosilicate (TEOS): 650 g;

ethyl alcohol: 500 g.

At the same time a solution of hydrochloric acid (0.5 g) and water (150 g) was prepared. This solution was mixed with the products contained in the beaker for 90 minutes at room temperature.

After 90 minutes, the reagents of the beaker added with the hydrochloric acid and water solution were brought to a temperature of 70°C (thanks to a thermostated plate) and left under vigorous stirring for 10 minutes .

At the end of the 10 minutes, 180 g of the aqueous hydroxyapatite suspension of Example 1 were added with 3 g of NaOH and then added to the contents of the beaker. The content of the beaker added with the hydroxyapatite suspension of Example 1 was left under vigorous stirring, at a temperature of 70°C, until the completion of the gelation .

Once the silica gel incorporating hydroxyapatite aggregates was obtained, it was transferred into a Pyrex glass or steel container and placed inside a thermostatically-controlled oven for 24 hours at a temperature of 70°C.

EXAMPLE 3: Production of the powder composition with a hydroxyapatite base incorporated in silica gel.

After the 72 hours of drying, the silica gel of Example 2 was ground with a hammer mill. The powder thus obtained was collected and sieved in a vibrating sieve shaker. The sieving step had a total duration of 2 hours, at the end of which a fraction of the particles was selected characterised by an average granulometry comprised between 10 and 25 pm.

EXAMPLE 4: Coating of the particles of the powder composition with magnesium stearate.

920 g of the particles of the powder composition of Example 3 were mixed with 80 g of magnesium stearate inside a stirrer for 16 hours, in order to coat the particles with magnesium stearate.

EXAMPLE 5: Use of the powder composition in an air polishing device.

20 g of the powder composition of Example 4 were inserted into an air-polishing device in order to verify its functionality. The powder composition showed suitable chemical-physical characteristics for use inside the device. Once activated, the device in fact demonstrated being able to correctly emit the jet of compressed air added with the powder composition of Example 4, at a pressure comprised from 1.5 to 3 5 bar. EXAMPLE 6 : Test of the powder composition in the removal of a layer of synthetic plaque from a tooth.

Using the VITA Farbskalen scale as a reference scale and SpectroShade as an instrument for colorimetric measurement, the degree of pigmentation of the surface of a bovine tooth was determined. The degree of pigmentation of the analysed tooth was classified as "Al" .

Subsequently the bovine tooth was covered with a thin layer of a dye and was dried at room temperature for 1 minute. Once dried, the dye layer showed characteristics of resistance to removal similar to those of dental plaque.

At this point the tooth covered with the layer of synthetic plaque was subjected to air-polishing technique through the use of a device and the powder composition of Example 5.

Once the air-polishing treatment was completed, the degree of pigmentation of the tooth surface was again determined (VITA Farbskalen scale and SpectroShade instrument), the degree was classified as "Al". The test therefore demonstrated that the powder composition of Example 5, when used in an air-polishing device, effectively removes plaque from the dental surface without damaging the tooth, in fact bringing the degree of pigmentation of the dental surface back to the initial value of "Al".

Claims

1. Method for teeth cleaning which comprises a step of applying on the dental surface a jet comprising micrometric particles of silica gel incorporating hydroxyapatite aggregates, wherein the hydroxyapatite aggregates have an average diameter (d50) from 50 nm to 500 nm, and the micrometric particles of silica gel have an average diameter (d50) from 1 pm to 150 pm.

2. Method according to claim 1, wherein the particles comprise from 5% to 60% by weight, more preferably from 10% to 50% by weight, of hydroxyapatite, and from 40% to 95% by weight, more preferably from 50% to 90% by weight, of silica gel, the percentages being expressed with respect to the weight of the particles.

3. Method according to claim 1 or 2, wherein the hydroxyapatite is a hydroxyapatite of general formula:

Ca_(10-a)M_a (PO4) _(6-b) (X)b(OH) 2-cYc (I) where :

M is a substituent selected from: Na, Mn, K, Zn,

Mg, Sr;

X is a substituent selected from: SiCg, CCg;

Y is a substituent selected from: Cl, F;

a is zero or is comprised from 0.030 to 1.0% wt/wt, extremes included;

b is zero or is comprised from 1.0 to 5.0 % wt/wt, extremes included;

c is zero or is comprised from 0.01 to 1.0% wt/wt, extremes included.

4. Method according to any one of the preceding claims, wherein the micrometric particles of silica gel incorporating hydroxyapatite aggregates, as such i.e. in the absence of any coating agent, have a surface area (BET) comprised from 50 m²/g to 600 m²/g, preferably from 60 m²/g to 550 m²/g.

5. Method according to any one of the preceding claims, wherein the micrometric particles of silica gel incorporating hydroxyapatite aggregates are coated with a C6-C24 fatty acid salt, preferably a stearic acid (Cis) salt .

6. Method according to claim 5, wherein the amount of said fatty acid salt is comprised from 1% to 20% by weight, preferably from 5% to 15% by weight, with respect to the weight of the micrometric particles of silica gel incorporating hydroxyapatite aggregates.

7. Method according to claim 5 or 6, wherein the micrometric particles of silica gel incorporating hydroxyapatite aggregates, superficially treated with a C6-C24 fatty acid salt, have a surface area (BET) comprised from 2 m²/g to 500 m²/g, preferably from 5 m²/g to 450 m²/g.

8. Method according to any one of the preceding claims, wherein the micrometric particles of silica gel incorporating hydroxyapatite aggregates have an apparent density (bulk density) comprised from 0.2 g/ml to 2.0 g/ml, preferably from 0.3 g/ml to 1.5 g/ml.

9. Method according to any one of the preceding claims, wherein the micrometric particles of silica gel incorporating hydroxyapatite aggregates have a pH value comprised from 3 to 7.

10. Method according to any one of the preceding claims, wherein the jet comprises compressed air, preferably at a pressure from 0.9 to 10 bar, more preferably from 1.5 bar to 8 bar.

11. Composition in the form of micrometric particles of silica gel incorporating hydroxyapatite aggregates, wherein the hydroxyapatite aggregates have an average diameter (d50) from 50 nm to 500 nm, preferably from 100 nm to 250 nm, and the micrometric particles of silica gel have an average diameter (d50) from 1 pm to 150 pm, preferably from 5 pm to 70 pm.

12. Composition according to claim 11, wherein the particles are defined according to any of the claims from 2 to 9.

13. Composition according to claim 11 or 12, for use in dental prophylaxis by air-polishing.

14. Process for producing a composition in the form of micrometric particles according to claim 11 or 12, comprising the steps of:

- synthesising hydroxyapatite in the form of an aqueous suspension;

- adding a hydrolysable silicate to said aqueous suspension;

- drying and grinding the silica gel thus obtained.

15. Process for producing a composition in the form of micrometric particles according to claim 11 or 12, comprising the steps of:

- providing a hydroxyapatite in powdered form;

- suspending said hydroxyapatite in an aqueous solution of a hydrolysable silicate;

- drying and grinding the silica gel thus obtained.

16. Process according to claim 14 or 15, wherein the hydrolysable silicate is a tetraalkyl orthosilicate of formula (R0)₄Si, where R is a C1-C4 alkyl, linear or branched .

17. Process according to any one of claims from 14 to 16, wherein, in the step of maintaining said suspension at a temperature from 30°C to 100°C, the aqueous suspension has a pH value from 1 to 10, preferably from 5 to 7.

18. Process according to any one of claims from 14 to 17, wherein the aqueous suspension of hydrolysable silicate further comprises an acid or basic catalyst.