Patent Description:
Dental composites typically comprise a polymerisable resin matrix and one or more fillers. Dental composites are used in the treatment or restoration of teeth, e.g. to form a filling in a tooth. Typically, the dental composite is positioned and shaped, and is then polymerised (cured) to harden the dental composite.

Whether driven by public aesthetic demands, concerns over amalgam's toxicity or trends toward conservative management of dentitions, dental composites have become the material most often employed in the restoration of teeth.

Tooth-coloured dental composites are now widely considered as the material of choice for the restoration of teeth. However, the strength and toughness of these materials are considered by many practitioners to be less than ideal, especially for posterior teeth restorations. In addition, due to the inherent properties of the resins used (e.g. polymerization shrinkage and resultant interfacial debonding), their use can lead to post-operative sensitivity and recurrent decay.

Dental composites are typically composed of inorganic opaque particulate fillers dispersed in a resin matrix. The fillers provide the cured dental composite with wear resistant properties as well as aesthetic properties. Dental composites typically also comprise other components, such as colourants and initiators or catalysts to initiate or catalyse the polymerisation of the resin.

Dental composite materials are typically placed and manipulated directly in the mouth and are set (i.e. cured or hardened) with curing lights having a wavelength of approximately <NUM>-<NUM>. Their use in stress bearing areas of posterior teeth is becoming an acceptable option but brittleness and a tendency to edge fracture remain important limitations for such uses.

Although relatively uncommon clinically, the use of fibres in dental composites has been studied in some fixed and removable dental prostheses. The most commonly used and studied FRCs in dentistry are indirectly manufactured (i.e. portions of cured/hardened dental composite are formed out of the mouth and then either bonded or cemented onto tooth substrate). Indirect restorations usually require more aggressive tooth preparation than direct procedures since they need to allow for precise insertion pathways. Due to operator and/or patient based limitations (such as handling difficulties and/or salivary contamination), chairside or intra-oral reinforcement of dental resins with fibres is less common still. Such chairside procedures typically involve combining dental resin with sections of fibre cut from straight or woven ultra-high molecular weight polyethylene mats.

Higher stiffness, fracture toughness and flexural strengths have been claimed for some dental composites reinforced with randomly oriented short fibres. For example: <NUM> GPa, <NUM> MN/m<NUM>/<NUM> and <NUM> MPa respectively for a <NUM>% volume filled, <NUM>-<NUM> long E glass rod material [<NPL>]. This composite comprises milled E glass fibres which have a diameter of approximately <NUM>. The uncured composite is a very viscous material making it difficult to place in a cavity of a tooth. This composite has also been found to exhibit poor wear properties due to "plucking" of the surface exposed fibres and fillers.

Another directly packable dental composite material is "EverX Posterior" by GC, Japan, which comprises E-glass fibres approximately <NUM>-<NUM> long and approximately <NUM> in diameter. Although some of its properties are acceptable, such as fracture toughness, it also has some undesirable properties, such as it being a very sticky material and very difficult to place. In plane fibre alignment in EverX Posterior material also results in anisotropic mechanical properties. For example, testing has shown that EverX Posterior exhibits significant vertical shrinkage (resulting in high vertical stress).

<CIT> discloses a dental composite containing <NUM> to <NUM> % w/w of fibres, wherein the composite contains a population of 'short' fibres and a population of 'long' fibres. Having such a large proportion of fibres, the dental composite of this disclosure can be regarded as a soft packable composite.

<CIT> discloses dental compositions comprising <NUM> to <NUM>% by weight of fibrous, inorganic fillers with an average fibre length of <NUM> to <NUM> and a length to diameter ratio of <NUM>:<NUM> to <NUM>:<NUM>. This disclosure is generally directed to compositions containing fibres of very short lengths.

<NPL>) discloses a dental composite containing <NUM>% w/w of fibres, wherein the composite contains a population of 'short' fibres and a population of 'long' fibres. Having such a large proportion of fibres, the dental composite of this disclosure can be regarded as a soft packable composite.

<CIT> (A1) discloses a dental composite containing a 'short cut fiber filler, which has a length of <NUM> to <NUM>, i.e. relatively long fibres.

One of the issues negatively affecting the greater acceptance of dental composites by dental practitioners is the ease of use. For example, the dental materials currently available and recommended for use in stress bearing areas of the mouth are widely considered by practitioners to be difficult to place, highly technique sensitive and relatively difficult to finish.

It would be advantageous to provide alternative dental composites. It would be advantageous if at least preferred embodiments of the present invention were to provide a dental composite material having good strength (including flexural strength and/or elastic modulus) and durability when cured or hardened. It would also be advantageous if at least preferred embodiments of the present invention were to provide a dental composite material that is relatively easy for a practitioner to place and finish.

In a first aspect, the present invention provides a flowable dental composite according to claim <NUM> and comprising:.

In the invention, the diameter of the fibres is from about <NUM> to about <NUM>. In an embodiment, the mean fibre diameter is about <NUM>.

In an embodiment, the aspect ratio (AR) of the fibres is from about <NUM> to about <NUM>. In an embodiment, the aspect ratio (AR) of the fibres is from about <NUM> to about <NUM>.

In the invention, the fibres constitute from about <NUM>% to about <NUM>% w/w of the dental composite. In an embodiment, the fibres constitute about <NUM>% w/w of the dental composite.

In the invention, the fibres are composed of glass. In an embodiment, the glass is selected from aluminosilicate glass, barium glass, fluorine glass, quartz, fused silica, borosilicate glass, aluminofluorosilicate glass, high calcium glass, high magnesium glass and mixtures thereof. In an embodiment, the glass is S-glass. In an embodiment, the fibres are substantially randomly oriented in the dental composite.

In an embodiment, the fibres are fibres having a surface treated to enhance bonding with the resin. In an embodiment, the fibres are sulphuric acid-treated glass fibres, hydrochloric acid-treated glass fibres, sulphuric and hydrochloric acid-treated glass fibres or a combination of two or more of the above. In an embodiment, the fibres are silanated glass fibres.

In a second aspect, the present invention provides a dental restoration article (e.g. a filling) made by forming and curing the dental composite of the first aspect.

In a third aspect, the present invention provides a process for producing a dental composite, the process comprising a step of mixing a polymerisable resin and fibres having a diameter of from about <NUM> to about <NUM> and a length of from about <NUM> to about <NUM>, wherein more than <NUM>% w/w of the fibres in the dental composite have a length of from about <NUM> to about <NUM>.

In the invention, the fibres are composed of glass. In an embodiment, the glass is selected from aluminosilicate glass, barium glass, fluorine glass, quartz, fused silica, borosilicate glass, aluminofluorosilicate glass, high calcium glass, high magnesium glass and mixtures thereof. In an embodiment, the glass is S-glass.

In an embodiment, the fibres are substantially randomly oriented in the dental composite.

In an embodiment, the process further comprises a fibre preparation step wherein the fibres are treated, before mixing the resin and fibres, to provide a fibre surface having improved bonding with the resin. In the invention, the fibres are glass fibres and in an embodiment the fibre preparation step comprises contacting the fibres with a strong alkali, contacting the fibres with hydrofluoric acid, contacting the fibres with phosphoric acid, contacting the fibres with sulphuric acid, contacting the fibres with hydrochloric acid, contacting the fibres with a silanating agent, plasma etching the fibres or a combination of two or more of the above. In an embodiment, the glass fibres are contacted with sulphuric acid, hydrochloric acid or a mixture thereof, and then silanated.

In an embodiment, the process further comprises a step of sonicating the mixture of fibres and resin.

In a fourth aspect, the present invention provides a dental composite prepared by the process according to the third aspect.

In a fifth aspect, the present invention provides the use of a dental composite of the first and/or fourth aspect in the restoration of a tooth.

Embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings in which:.

In a first aspect, the present invention provides a dental composite comprising:.

The polymerisable resin may be any polymerisable resin suitable for dental applications. Typically, dental resins are synthetic resins comprising monomers (or prepolymers). There are many commercially available dental resins that are suitable for use in the dental composite of the present invention (e.g. those that are currently used, or have been used, in dentistry restorative materials or adhesives).

Examples of polymerisable resins include resins comprising acrylates such as acrylate monomers (e.g. (mono-(meth)acrylate, di-(meth)acrylate, tri-(meth)acrylate) or dimethacrylate monomers (e.g. PEGMA, UDMA, HDDMA and TEGDMA. Other examples of polymerisable resins include ionomers such as glass ionomer cements. An ionomer is a polymer that comprises repeat units of both electrically neutral monomer units and a fraction of ionized monomer units (usually no more than <NUM> mole percent). Most ionomers are copolymers of neutral portions and ionized portions (usually carboxylic acid groups). Polymerisable resins are typically fluidic and easy to manipulate when in the unhardened state (uncured or wet state) and require some form of activation to initiate polymerisation to harden. Once polymerised and hardened, the resin should be insoluble in aqueous solutions (e.g. saliva) and resistant to wear (e.g. by movement of teeth).

The resins may be set or hardened (cured) by the methods known in the art. Common methods include, for example, exposure to light (e.g. at <NUM>-<NUM>), oxidation by exposure to air, oxidation by exposure to a chemical oxidant present in the resin (usually mixed into the resin immediately prior to use) or other forms of chemical reactions that initiate polymerisation of the resin.

There are many commercially available dental resins that may be used in the dental composite of the present invention. Examples of such commercially available resins include Beautifil II Universal Giomer Composite, Beautifil Flow Plus F03 Flowable Giomer and SDI Aura.

The dental composite of the present invention comprises fibres having a diameter of from about <NUM> to about <NUM> (e.g. from about <NUM> to about <NUM> or about <NUM>) and a length of from about <NUM> to about <NUM> (e.g. from about <NUM> to about <NUM>). The dental composite of the present invention comprises such fibres in an amount of from about <NUM>% to about <NUM>% w/w (e.g. from about <NUM>% to about <NUM>% w/w) based on the total weight of the dental composite. The fibres are mixed with, or dispersed in, the polymerisable resin.

In the invention, the inclusion of these amounts of fibres having these specific dimensions affords a dental composite that is flowable in the unhardened state (which allows the dental composite to be applied and finished more easily than EverX Posterior and the FRCs of Zakaria et al. ), and has good durability and acceptable wear resistance once hardened.

Dental composites comprising fibres in higher amounts may be more difficult to work, and composites comprising fibres in lower amounts may be less durable and have less wear resistance. For example, using fibres at less than <NUM>% w/w in the dental composite, a dental composite will be produced that in the hardened state is less durable and less wear resistant, whereas using more than <NUM>% w/w of fibres in the dental composite will result in a dental composite material that is more difficult to work with and may also suffer from reduced strength in the hardened state. In the invention, from about <NUM>% w/w to about <NUM>% w/w fibres is used in the dental composite of the present invention, more preferably about <NUM>% w/w of fibres is used in the dental composite.

In the invention, the fibre diameter and length have a significant effect on the properties of the dental composite. In some embodiments, the use of fibres from <NUM> to <NUM> provides for excellent mechanical properties of the cured/hardened dental composite and desirable handling properties of the uncured dental composite. Having a diameter of less than <NUM> may cause health concerns (e.g. particulate matter of less than <NUM> has previously been associated with health issues such as mesothelioma). Having a diameter greater than <NUM> negatively affects handling, manipulation and adaptation of the uncured dental composition. In the invention, the diameter of the fibres is from about <NUM> to about <NUM>. In an embodiment, the mean fibre diameter is about <NUM>.

Use of fibres longer than <NUM> or thicker than <NUM> also results in poorer surface qualities (e.g. voids, poor polishability, gloss and polish retention). The use of fibres having a length below about <NUM> generally provides poor mechanical properties of the cured/hardened dental composite, and the use of fibres having a length above about <NUM> generally gives dental composites having poor handling properties in the uncured state.

In some embodiments, the dental composite may also comprise some additional fibres having different dimensions to those specified above (i.e. fibres having a diameter less than <NUM> or greater than <NUM> and/or a length that is shorter than <NUM> or greater than <NUM>). In the invention, the fibres having a diameter of from about <NUM> to about <NUM> and a length of from about <NUM> to about <NUM> represent more than <NUM> % w/w of the total fibre content (for example, more than <NUM> % w/w, more than <NUM> % w/w, more than <NUM> % w/w or more than <NUM> % w/w of the total fibre content).

In the invention, more than <NUM>% w/w (e.g. more than <NUM>% or more than <NUM>%) of the fibres in the dental composite have a length of from about <NUM> to about <NUM> (e.g. about <NUM> to about <NUM>).

It is believed that the aspect ratio (i.e. the ratio of the length of the fibres to the diameter of the fibres; "AR") also plays a role in providing a dental composite having desirable properties. Very low AR fibres (AR less than <NUM>), as found in milled glass fibres, provide less desirable physical properties to the hardened FRC than fibres having a higher AR. FRCs comprising fibres having a very high AR (AR greater than <NUM>), although strong in the hardened state, are difficult to manipulate and use in their unhardened state.

In an embodiment, the aspect ratio of the fibres is from about <NUM> to about <NUM>. In an embodiment, the aspect ratio of the fibres is from about <NUM> to about <NUM>.

In an embodiment, the fibres comprise two or more populations of fibres, wherein the mean aspect ratio (AR) of one population of fibres is different to the mean aspect ratio (AR) of at least one other population of fibres. By combining two or more populations of fibres, dental composites can be prepared having different properties in the uncured state and/or in the hardened state (e.g. flowability in the uncured state and strength and wear properties in the hardened state). In some embodiments, the difference in mean aspect ratio is greater than <NUM>% (e.g. <NUM>%-<NUM>%, <NUM>%-<NUM>%, <NUM>%-<NUM>%, <NUM>%-<NUM>%, <NUM>%-<NUM>%, <NUM>%-<NUM>%, <NUM>%-<NUM>%, <NUM>%-<NUM>% or <NUM>%-<NUM>%). In an embodiment, the mean aspect ratio (AR) of one population of fibres is from about <NUM> to about <NUM> (e.g. from about <NUM> to about <NUM>, from about <NUM> to about <NUM> or from about <NUM> to about <NUM>), and the mean aspect ratio (AR) of the at least one other population of fibres is from about <NUM> to about <NUM> (e.g. from about <NUM> to about <NUM>, from about <NUM> to about <NUM> or from about <NUM> to about <NUM>). In some embodiments, one population of fibres has the same diameter as another population of fibres, but differ in length (e.g. one population of fibres is <NUM> in diameter and about <NUM> in length (AR = <NUM>) and another population of fibres is <NUM> in diameter and about <NUM> in length (AR = <NUM>)). In some embodiments, the proportion of fibres in one population relative to the at least one other population of fibres is from about <NUM>:<NUM> to about <NUM>:<NUM> (e.g. about <NUM>:<NUM> to about <NUM>:<NUM>, about <NUM>:<NUM> to about <NUM>:<NUM> or about <NUM>:<NUM>). In some embodiments, about <NUM>% to about <NUM>% of the fibres have an AR of about <NUM> to about <NUM>, and about <NUM>% to about <NUM>% of fibres have an AR of about <NUM> to about <NUM>.

The fibres used in the dental composite of the present invention are glass fibres. There are various types of fibres that are commercially available and have differing diameters (and distributions of diameters). In an embodiment, the fibres are high strength fibres. High strength fibres are fibres having a tensile strength of above about <NUM> GPa (as measured by ASTM C1557 - <NUM>) and/or a flexural strength of greater than about <NUM> MPa (using ISO <NUM> three point flexural testing).

In some examples not part of the invention, the dental composite of the present invention comprises polymer fibres. Polymer fibres may help improve fracture toughness by utilizing controlled fibre pull out. Controlled fibre pull out is a process whereby excessive energy (generated by stresses on the tooth and/or restorative) is absorbed by the fibre being pulled out of the matrix, thereby avoiding catastrophic failure (i.e. fracture of the restorative).

In the invention, the fibres used in the present invention are glass fibres. Commercially available glass fibres generally have lengths greater than <NUM>. Glass fibres having a length of from about <NUM> to about <NUM>, and a desired length distribution within that range, can be prepared by a variety of techniques. The desired length of the glass fibres, and length distribution, may be obtained by, for example, cutting the long glass fibres individually. Alternatively, batch processes may be used to reduce the fibre length to the desired length. In the examples described below (Example <NUM> and <NUM>), both commercially cut S2 glass fibres and glass fibres cut using an in-house made glass fibre cutting tool were used. The in-house made glass fibre cutting tool was used to individually cut the fibres to the desired length (<NUM> to <NUM>) from lengths of glass fibre. The in-house made glass fibre cutting tool is a manually operated tool with high strength circular blades stacked together (with adjustable separation) to cut fibres to a length from <NUM> to <NUM>.

In an embodiment, the glass fibres may be selected from aluminosilicate glass, barium glass, fluorine glass, quartz, fused silica, borosilicate glass, aluminofluorosilicate glass, high calcium glass, high magnesium glass and mixtures thereof. In an embodiment, the glass is an S-glass (structural glass). S-glass is typically an alumino silicate glass having negligible CaO content and a high MgO content. S-glass is so named for "stiff glass", due to its high tensile strength or modulus. Examples of other types of glass, glass fibres or fibreglass that may be used include E-glass (an aluminoborosilicate glass with less than <NUM>% w/w alkali oxides), A-glass (an alkali lime glass with little or no boron oxide), E-CR glass (an electrical/chemical resistant alumino-lime silicate glass having less than about <NUM>% w/w alkali oxides), C-glass (an alkali-lime glass having high boron oxide content), D-glass (a borosilicate glass named for its low dielectric constant) and R-glass (alumino silicate glass having negligible MgO and CaO content used for high mechanical requirements as reinforcement).

Glass fibres are commercially available in many different diameters. Obtaining glass fibres having the desired aspect ratio typically involves selecting glass fibre having the desired diameter and cutting the length accordingly.

In some embodiments, the fibres are fibres having a surface treated to enhance bonding with the resin (e.g. surface modification). In some embodiments, the treatment to enhance bonding with the resin provides an increase in interfacial shear strength (IFSS) of the fibre in the resin in the hardened/cured state. In some embodiments, increased IFSS improves the strength and/or wear resistance of the cured dental composite. In some embodiments, the fibres are processed glass fibres, wherein the glass fibres have been processed by one or more of: contacting the glass fibres with a strong alkali (such as NaOH and KOH); contacting the glass fibres with hydrofluoric acid; contacting the glass fibres with phosphoric acid; contacting the glass fibres with sulphuric acid; contacting the glass fibres with hydrochloric acid or contacting the glass fibres with a silanating agent; especially contacting the glass fibres with sulphuric acid or hydrochloric acid. In some embodiments, the glass fibres are contacted with alkali, hydrofluoric acid, phosphoric acid, sulphuric acid, hydrochloric acid or a combination of two or more of the above. In some embodiments, the glass fibres are strong alkali-treated glass fibres, hydrofluoric acid-treated glass fibres, phosphoric acid-treated glass fibres, sulphuric acid-treated glass fibres, hydrochloric acid-treated glass fibres or a combination of two or more of the above. In some embodiments, the glass fibres are sulphuric acid-treated glass fibres, hydrochloric acid-treated glass fibres, sulphuric and hydrochloric acid-treated glass fibres or a combination of two or more of the above. In some embodiments, the glass fibres are silanated glass fibres. As referred to herein, "silanated" refers to the treatment (of the fibre) with a silanating agent (e.g. silane compound, especially a bi-functional silane compound such as <NUM>-(trimethoxysilyl) propyl methacrylate (TMSPMA)). In some embodiments, the glass fibres are processed (e.g. treated with the alkali or acid referred to above, e.g. hydrochloric acid and/or sulphuric acid) and then silanated.

In some embodiments, the fibres are substantially randomly oriented in the dental composite. In other words, the fibres are not lined up in a continuous fashion and not aligned to any appreciable extent. Having fibres in a randomly oriented alignment leads to dental composites that do not shrink in one direction further than in other directions to an appreciable extent (isotropic strength and/or shrinkage). In other embodiments, a proportion (e.g. more than about <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>% or <NUM>%) of fibres are aligned to within about <NUM>° degrees of each other (e.g. within about <NUM>°, <NUM>°, <NUM>°, <NUM>° or <NUM>° of each other) to provide anisotropic strength and/or shrinkage. Such anisotropy (e.g. alignment/parallelism) may be advantageous in certain applications, such as in the restoration and reinforcement of the middle section of the tooth (pulp chamber).

The dental composite may also comprise one or more other components, such as fillers (e.g. silica particles, quartz particles, glass particles, zirconia, ion-releasing particles, polymer particles and powdered composite particles), colourants (e.g. titanium dioxide and iron oxides), bonding agents (e.g. agents that bond or improve bonding of fibres/fillers/colourants etc. to the cured resin, such as bi-functional silane compounds), photoinitiators to initiate polymerisation of the resin (e.g. camphorquinone/amine, TPO and BAPO), catalysts to catalyse the polymerisation of the resin (e.g. photocatalysts such as titanium dioxide) and preservatives (e.g. stabilisers or inhibitors, such as BHT).

In the invention, the dental composite of the present invention is flowable prior to polymerisation of the resin (i.e. the uncured dental composite is flowable). This gives advantageous properties such as being easy to manipulate, position and shape by a practitioner. In addition, having a flowable dental composite may lead to better and/or easier finishes, as the surface energy, optionally with the aid of gravity, allows the dental composite to bridge across gaps and fill cavities in teeth as well as advantageously allowing the formation of naturally shaped contours. In addition, such flowable dental composites may need less grinding and/or polishing in order to get a desired finished look once placed and cured in a cavity in a tooth. Previous flowable dental composites have not been recommended for use in stress bearing areas of the mouth due to strength and durability concerns. Advantageously, the dental composites of the present invention may be produced that, when hardened, have properties similar to dentine. For example, dentine typically has flexural strength of about <NUM> MPa and flexural modulus of about <NUM> GPa.

The present invention encompasses the dental composites in their flowable unhardened (uncured) state, as well as the dental composites in their hardened (cured) state.

In a third aspect, the present invention provides a process for producing a dental composite, the process comprising a step of mixing a polymerisable resin and fibres having a diameter of from about <NUM> to about <NUM>, and a length of from about <NUM> to about <NUM>.

The mixing may be achieved by any suitable means, such as, for example, introducing fibres to the resin or adding the resin to fibres and subsequent mixing with, for example, a spatula. The method of mixing can affect the properties of the dental composite. For example, mixing by a method that introduces air bubbles may lead to dental fibres that include those air bubbles and therefore afford cured dental composites that have a lower strength. Alternatively, some methods of mixing may orient the fibres in a particular direction, which may lead to dental composites that have anisotropic strength properties. In some embodiments, FRCs comprising randomly oriented fibres are obtained by mixing techniques involving sonication (e.g. a sonicator or an ultrasonic mixer). Simple flow of the uncured dental composite tends to align the fibres. Since fibres will usually self-align in the direction of flow, methods of mixing that end with extrusion may lead to a degree of anisotropy. In some circumstances, randomly-oriented fibres in an uncured dental composite may become aligned during the material placement in a tooth. For example, fibre orientation may be affected by the delivery of the flowable material through a nozzle with a specified diameter (e.g. about <NUM> to about <NUM>, with smaller diameter nozzles affording greater fibre alignment).

It is believed that shorter fibres are more resistant to alignment during placement and tend to remain more randomly oriented compared to longer fibres. It is also believed that longer fibres tend to align themselves along the cavity walls and that this is more pronounced in smaller cavities.

In an embodiment, the resin is mixed with an amount of the fibres to provide a dental composite comprising the fibres in an amount of from about <NUM>% to about <NUM>% w/w of the dental composite (e.g. from about <NUM>% to about <NUM>% w/w), based on the total weight of the dental composite. In an embodiment, the process further comprises a fibre preparation step wherein the fibres are treated before mixing the fibres and resin to provide a fibre surface having improved bonding with the resin. Fibres may not always adhere strongly to the resin and including a fibre preparation step acts to prepare the surface of the fibre in order to make it form stronger bonds with the resin, thereby enhancing or improving the bonding of the fibres in the resin. This can lead to fibres being more securely bonded to the resin in the hardened/cured state and improve the strength and/or wear resistance of the cured dental composite. In an embodiment, the fibres are glass fibres and the fibre preparation step is selected from contacting the glass fibres with a strong alkali, contacting the fibres with hydrofluoric acid, contacting the fibres with phosphoric acid, contacting the fibres with sulphuric acid, contacting the fibres with hydrochloric acid, contacting the fibres with a silanating agent, plasma etching the fibres or a combination of two or more of the above. The fibre preparation step may involve affecting the physical properties of the surface (e.g. roughening the surface) or may involve modifying the surface chemistry of the fibres (e.g. transforming or reacting the chemical functional groups present on the surface of the fibres) or both. Chemical transformation/modification of the surface groups can also lead to improved chemical bonding within the resin. In the invention, the fibres are glass fibres and optionally the glass fibres are contacted with sulphuric acid, hydrochloric acid or a mixture thereof, and optionally, then silanated.

In an embodiment, the process further comprises a step of sonicating the mixture comprising the fibres and resin. Sonicating the mixture helps to mix the fibres and resin more uniformly. Sonicating the mixture of fibres and resin may also lead to a more random distribution of fibre orientation within the resin. Sonicating the mixture of fibres and resin also leads to a mixing that does not necessarily involve the entraining of air into the mixture. For example, sonication may mix the fibres and the resin mixture without introducing air into the mixture and may also serve to expel any entrained air bubbles from the mixture during the mixing process. Bubble removal (and improved surface wetting of fibres) may also be performed by mixing under reduced pressure.

The present invention is further described below by reference to the following non-limiting Examples.

A "control" dental composite was used in Example <NUM> and Example <NUM>. The "control" dental composite consisted of:.

This "control" dental composite was used as a control sample and was also used in the preparation of the fibre reinforced dental composite samples <NUM> to <NUM> described below in Examples <NUM> and <NUM>.

A SEM image of the unprocessed S-glass fibres (representative of the S-glass fibres used in Examples <NUM> to <NUM>) is presented in <FIG>.

<FIG> is a graph of the distribution of fibre lengths of the glass fibres used in Samples <NUM> to <NUM>, <NUM> and <NUM> (described below in Examples <NUM> to <NUM>).

A fibre reinforced dental composite (sample <NUM>) was prepared using the control dental composite and processed glass fibres (<NUM>% w/w based on the total weight of the dental composite). The processed glass fibres (etched with sulphuric acid and silanated by treatment with <NUM>-(trimethoxysilyl) propyl methacrylate (TMSPMA) were <NUM> long and <NUM> micron in diameter. An SEM image of the processed glass fibres is presented in <FIG>.

The control dental composite was mixed with the glass fibres manually (hand mixing with a spatula). The dental composite was then photocured using a curing lamp (wavelength <NUM>) for <NUM> seconds. Curing shrinkage strain of the fibre reinforced dental composite (sample <NUM>) and the "control" is shown in <FIG>.

Samples <NUM> to <NUM> were prepared in an analogous manner to sample <NUM> described above in Example <NUM>. Samples <NUM> to <NUM> varied in the amount and dimension of glass fibres used, as set out below.

<FIG> is a graph of the distribution of fibre lengths of the glass fibres used in Samples <NUM> to <NUM>. An SEM image of the glass fibres used in Samples <NUM> to <NUM> is presented in <FIG>.

Control dental composite with <NUM>% w/w <NUM> long by <NUM> micron diameter S2 glass.

The flexural modulus of the "control" (i.e. no fibre) and samples <NUM> to <NUM> was then measured using the ISO <NUM> three point flexural test method and the results are summarised in <FIG>.

A second control dental composite ("Control <NUM>") was used in Example <NUM> and Example <NUM>. The "Control <NUM>" dental composite consisted of:.

The "Control <NUM>" dental composite was used as a control sample and was also used in the preparation of the fibre reinforced dental composites described in Example <NUM>, examining the effect of glass fibre surface modification on interfacial properties, and also used in the preparation of samples <NUM> to <NUM> described below in Example <NUM>, where a fixed percentage of the filler (strontium glass) in the control composite was replaced with fibres.

In Example <NUM>, fibre reinforced dental composites were prepared using the control dental composite "Control <NUM>" and glass fibres processed by differing methods. Glass fibres (<NUM> long by <NUM> micron diameter S2 glass, AR = <NUM> were processed using sulphuric acid or hydrochloric acid for <NUM> or <NUM> hours before being silanated by treatment with <NUM>-(trimethoxysilyl) propyl methacrylate. SEM images of the unprocessed and processed glass fibres under these different etching conditions are presented in <FIG>. These glass fibres were then incorporated into composites using a process analogous to the process described below in Example <NUM>, to provide fibre reinforced dental composites.

To evaluate the effect of glass fibre surface modification on interfacial properties, the changes in interfacial shear strength (IFSS) of fibre and resin were investigated using microdroplet pull-out tests. Microdroplet specimens were prepared by depositing a droplet of resin onto the single fibre using a micro-manipulator. After light-curing, an embedded length of the droplet was measured and ranged from <NUM> to <NUM> µm. The droplet was positioned inside a micro vise (gap maintained at <NUM> µm) and pull-out force was measured. IFSS was calculated using the equation: <MAT> where τi refers to IFSS, F is the maximum load prior to the interface debonding, df is the fibre diameter and le is the fibre embedded length. <FIG> shows the interfacial shear strength of unprocessed and processed fibres (HCl, <NUM>; HCl, <NUM>; H<NUM>SO<NUM>, <NUM>; H<NUM>SO<NUM>, <NUM>), where an increase is observed for the processed fibres compared to the unprocessed fibres.

To prepare Samples <NUM> to <NUM>, "Control <NUM>" dental composite was mixed with processed glass fibres using an automated mixer under vacuum. The dental composite was then photocured using a curing lamp (wavelength <NUM>) for <NUM> seconds. <FIG> is an SEM image showing the distribution of processed fibres within the composite. <FIG> shows that the fibres are uniformly dispersed and randomly oriented in the cured composite.

Samples <NUM> to <NUM> were prepared using the method described in the above paragraph, with differing amounts and differing dimensions of glass fibres, as set out below.

"Control <NUM>" dental composite with <NUM> %w/w of strontium glass filler (based on the total weight of the composite) replaced with <NUM>% w/w (based on the total weight of the composite) <NUM> long by <NUM> micron diameter S2 glass (AR = <NUM>) resulting in a fibre reinforced dental composite having <NUM> %w/w strontium glass filler and <NUM> %w/w glass fibres (based on the total weight of the composite). The fibres used in Sample <NUM> had been etched using hydrochloric acid for <NUM> hours and then silanated.

"Control <NUM>" dental composite with <NUM> %w/w of strontium glass filler (based on the total weight of the composite) replaced with <NUM>% w/w (based on the total weight of the composite) <NUM> long by <NUM> micron diameter S2 glass (AR = <NUM>) resulting in a fibre reinforced dental composite having <NUM> %w/w strontium glass filler and <NUM> %w/w glass fibres (based on the total weight of the composite). The fibres used in Sample <NUM> had been etched using hydrochloric acid for <NUM> hours and then silanated.

"Control <NUM>" dental composite with <NUM> %w/w of strontium glass filler (based on the total weight of the composite) replaced with <NUM>% w/w (based on the total weight of the composite) <NUM> long by <NUM> micron diameter S2 glass (AR = <NUM>) resulting in a fibre reinforced dental composite having <NUM> %w/w strontium glass filler and <NUM> %w/w glass fibres (based on the total weight of the composite). The fibres used n Sample <NUM> had been etched using hydrochloric acid for <NUM> hours and then silanated.

The flexural strength and modulus of "Control <NUM>" (i.e. no fibre) and samples <NUM> to <NUM> were then measured using the ISO <NUM> three point flexural test method and the results are summarised in <FIG>.

All the dental composites of the present invention (Samples <NUM> to <NUM> and <NUM> to <NUM>) exhibited a higher flexural modulus than the control, and all of Samples <NUM> to <NUM> exhibited higher flexural strength than the control, indicating the greater suitability of the dental composites of the present invention for use in high stress areas.

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

Claim 1:
A flowable dental composite comprising:
- a polymerisable resin; and
- from about <NUM>% to about <NUM>% w/w of glass fibres having a diameter of from about <NUM> to about <NUM> and a length of from about <NUM> to about <NUM>, wherein more than <NUM>% w/w of the fibres in the dental composite have a length of from about <NUM> to about <NUM>.