Polymerizable cement compositions

A polymerisable cement composition, particularly for dental and biomedical uses, comprises a mixture of polymerisable monomer materials including between 5 and 95% by weight tetrahydrofurfuryl methacrylate (THFMA), and at least 5% by weight secondary monomer, preferably a dimethacrylate; and active filler material, preferably powdered fluoroaluminosillicate glass, capable of undergoing an acid-base reaction in the presence of water with acid or acid derivative groups in the composition. The composition is conveniently in the form of a resin-modified glass-ionomer cement or a compomer composition. The use of THFMA as a monomer material has a number of advantages. THFMA has low shrinkage on polymerisation, good biological acceptability and advantageous water uptake properties in comparison to other monomer systems. The invention also covers a method of preparing a polymerisable cement, and a method of dental treatment.

FIELD OF THE INVENTION
 This invention concerns polymerisable cement compositions, particularly
 polymerisable cement compositions suitable for dental and biomedical
 applications.
 BACKGROUND OF THE INVENTION
 Cement compositions are widely used in dental and biomedical applications.
 Typical dental uses include restoration of teeth by filling cavities
 following destruction by decay, cementing crowns, inlays and orthodontic
 devices in place, providing a base and/or lining in a tooth cavity etc. A
 wide range of cement compositions have been developed and are commercially
 available. These fall into a number of different types.
 Glass-ionomer cements are acid-base reaction cements that typically set by
 the interaction of an aqueous solution of a polymeric acid with an
 acid-degradable glass,eg as disclosed in GB 1316129. The principal setting
 reaction is the slow neutralisation of the acidic polymer solution to form
 a polysalt matrix. The acid is typically a polycarboxylic acid (often
 polyacrylic acid) and the glass is typically a fluoroaluminosilicate. The
 setting reaction begins as soon as the components are mixed, and the set
 material has residual glass particles embedded in interconnected polysalt
 and silica matrices.
 The advantages of glass-ionomer cements include their adhesion to tooth
 tissue, thus allowing the use of conservative techniques, and sustained
 fluoride release, thus imparting to the tooth an increased resistance to
 acid demineralisation.
 However the disadvantages associates with glass-ionomer cements are that
 the immature cement is sensitive to moisture contamination and hence
 requires protection to ensure optimum final mechanical properties are
 achieved. The nature of the setting reaction means that the strength of
 the glass-ionomer develops with time, consequently the immediate strength
 of the glass-ionomer is not as high as that of other materials. Finally
 the glass-ionomer is not as tough as some other dental cements.
 Composite resin cements set by the free-radical polymerisation of a resin
 (monomer) component. The cements usually include a non-active filler
 (usually a glass and/or silica) which is not involved in the setting
 mechanism of the materials, although the filler is generally bound into
 the matrix via a difunctional silane coupling agent. The monomers are
 generally large molecule aromatic or urethane dimethacrylates, selected
 with the aim of minimising polymerisation shrinkage of the material.
 However these monomers have quite high viscosities and consequently smaller
 dimethacrylate monomers are used as diluents to lower the viscosity and
 thus increase the capacity for filler incorporation.
 Composite resins are supplied as either one or two paste systems, depending
 upon the method used for initiation of the polymerisation reaction. The
 reaction may be initiated by an external energy source (one paste) usually
 high intensity blue light (470nm) eg using an .alpha. diketone with an
 amine reducing agent as the initiator system. Alternatively the
 polymerisation may be initiated by mixing the components (two paste) with
 eg a peroxide and tertiary amine as the initiator system.
 The advantages of the composite resin are good aesthetics, good mechanical
 strength nd good wear resistance. However because of the nature of their
 setting reaction they have the disadvantages of polymerisation shrinkage,
 polymerisation exotherm, water sorption and monomer leaching. Shrinkage
 during curing is a particular problem because it allows microleakage
 around a restoration which can cause further decay of the tooth. It also
 means that stresses can be set up in the filling or the tooth.
 Resin-modified glass-ionomer cements (RMGICs) were introduced with the
 intention of overcoming the problems associated with the conventional
 glass-ionomer, eg uncontrolled chemical set and tendency towards brittle
 fracture, whilst still retaining its advantages, eg fluoride release and
 adhesion. To achieve this the technologies of the acid-base and resin
 cements were combined. See, eg, EP 0323120, U.S. Pat. No. 4872936 and U.S.
 Pat. No. 5154762. One attempt to achieve this advocated simply replacing
 some of the water in a conventional glass-ionomer cement with a
 hydrophilic monomer. Another approach also replaced some of the water in
 the formulation, but in addition modified the polymeric acid so that some
 of the acid groups were replaced with unsaturated species, so that the
 polymeric acid could also take part in the polymerisation reaction.
 Resin-modified glass-ionomers have two setting reactions: the acid-base
 reaction of the glass-ionomer, and the polymerisation of the composite
 resin. The monomer systems used in resin-modified glass-ionomers are not
 generally the same as those in composite resins. This is because the
 monomer must be compatible with the aqueous acid-base reaction of the
 glass-monomer components.
 Resin-modified glass-ionomers have the advantage of improved aesthetics
 compared with conventional glass-ionomers but they also have the potential
 for fluoride release and the adhesion of the conventional material
 although it should be noted that some materials are supplied with a
 bonding agent similar to that used with composite materials. The fracture
 toughness of the resin-modified material is higher than that of
 conventional glass-ionomers and in some cases the resin-modified material
 is higher than that of conventional glass-ionomers and in some cases the
 resin-containing materials have higher strengths. However, because of the
 polymerisation reaction involved, resin-modified glass-ionomers have the
 disadvantages of polymerisation shrinkage and exotherm, water sorption and
 loss of free monomer. These disadvantages are far more of a problem than
 they are for composite resins because of the small toxic monomers
 currently used in resin-modified glass-ionomers.
 Acid-modified composite resins (compomers) set by photopolymerisation of
 their monomer system. However, the systems include monomers with acid
 character not found in conventional composite resins. The filler in these
 materials is typically made up, at least in part, of acid-degradable glass
 as used in glass-ionomers. Consequently, in the presence of water, the
 monomer should be capable of undergoing a glass-ionomer type reaction with
 the glass. Unlike conventional and resin-modified glass-ionomer cements,
 compomers are supplied as one paste systems. To achieve this the water,
 essential for the acid-base reaction, is excluded from the formulation.
 Once in situ the cement will take up water.
 This water could then initiate the acid-base reaction potentially
 permitting a glass-ionomer cement style sustained fluoride release from
 the material. The aesthetics of the compomers are good but their fluoride
 release rate is lower than that of a glass-ionomer. The bonding system
 supplied for use with the material assumes that the material will behave
 like a composite.
 The present inventors have carried out experiments to assess alternative
 monomer materials for use in polymerisable cement compositions, and have
 discovered that good results are obtained by use of a mixture of monomers
 including an amount of tetrahydrofurfuryl methacrylate (THFMA).
 THFMA is known for use as a monomer material in polymers for a number of
 purposes, including a composite resin cements for use as provisional or
 temporary crown-and-bridge resin (WO81/02022 and U.S. Pat. No. 4264489),
 for use in the construction of dentures, dental bridges and crowns and as
 bone cements (GB 2107341), and for use in compositions for promoting
 tissue repair (WO93/09819).
 U.S. Pat. No. 5154762 and AU 46717/89 both concern resin-modified
 glass-ionomer cements employing polymerisable unsaturated organic
 compounds, particularly various acrylates and methacrylates. These
 documents refer to a large number of possible polymerisable compounds,
 including THFMA, but do not include illustrative examples of use of this
 material and there is no evidence that THFMA has hitherto been used in
 resin-modified glass-ionomer cements.
 In experiments with THFMA, the present inventors have been unable
 effectively to polymerise 100% THFMA (as broadly disclosed in U.S. Pat. No
 5154762 and AU 46717/89) under clinically relevant conditions, but have
 found that on inclusion of at least 5% by weight of a suitable secondary
 monomer with the THFMA, polymerisation does occur, and that such monomer
 mixtures are useful in polymerisable cement compositions for dental and
 biomedical applications.
 SUMMARY OF THE INVENTION
 In one aspect, the present invention therefore provides a polymerisable
 cement composition comprising a mixture of polymerisable monomer materials
 including between 5 and 95% by weight tetrahydrofurfuryl methacrylate
 (THFMA), and at least 5% by weight secondary monomer; and active filler
 material capable of undergoing an acid-base reaction in the presence of
 water with acid or acid derivative groups in the composition.
 Under suitable conditions, the monomer materials polymerise by free radical
 polymerisation. A number of different initiation systems may be employed
 for initiating polymerisation, eg as are well known in the prior rt,
 including cold chemical cure systems eg using benzoyl peroxide (BP) as
 initiator and NN dimethyl p toluidine (DMPT) as activator and
 photochemical cure systems eg using camphorquinone (CQ) as initiator and
 DMPT as activator with exposure to light of suitable wavelength.
 Further, the active filler is capable of undergoing acid-base reaction with
 the acid or acid derivative groups in the presence of water, constituting
 a second setting mechanism.
 The secondary monomer can be any species that is capable of polymerising
 with or in the presence of THFMA and has biological properties suitable
 for the intended use of the composition. Suitable materials include
 acrylates, diacrylates, methacrylates, dimethacrylates,
 spiroorthocarbonates and ormecers, with currently preferred materials
 being dimethacrylates including biphenol-A-glycidyl dimethacrylate
 (BisGMA), urethane dimethacrylate (UDMA) and tri ethylene glycol
 dimethacrylate (TEGDMA). Mixtures of secondary monomer materials may be
 used.
 The active filler material may be any suitable organic or inorganic filler,
 eg as are known in the prior art for use in dental cements.
 Suitable inorganic active filler materials include metal oxides, metal
 salts, glasses and ceramics that contain metal compounds, zeolites, and
 oxidisable metals, as well as products obtained by sintering such
 materials. Preferred metal oxides include barium oxide, calcium oxide,
 magnesium oxide and zinc oxide. Preferred metal salts include salts of
 multivalent cations, for example aluminum acetate, aluminum chloride,
 calcium chloride, magnesium chloride, zinc chloride, aluminum nitrate,
 barium nitrate, calcium nitrate, magnesium nitrate, strontium nitrate and
 calcium fluoroborate. Preferred glasses include borate glasses, phosphate
 glasses and fluoroaluminosilicate glasses. Fluoroaluminosilicate glasses
 are particularly preferred as they provide a source of fluoride ions that
 leach from the filler on reaction with the acid groups, with consequent
 dental benefits. Mixtures of filler materials may be used.
 The active filler material should be in finely divided, particulate or
 powdered form, for ease of inclusion in the composition, ease of use and
 ease of reaction. The filler material preferably has an average particle
 diameter of less than 45 microns.
 The active filler material may optionally be surface treated in known
 manner, including treatment with a polymerisable silane to promote bonding
 to the resulting polymer, and washing with a dilute acid solution which
 increases cement hardness (and reduces setting speed).
 The active filler material should be present in an amount suitable to
 provide a composition having good mixing and handling properties before
 polymerisation and good performance after polymerisation. The filler
 material conveniently constitutes between 5% and 85% of the total weight
 of the composition before polymerisation.
 The acid or acid derivative groups in the composition may be present in a
 number of different forms.
 For example, the composition may include an acidic polymer, preferably in
 the form of a homopolymer or copolymer of vinyl phosphonic acid or an
 alkenoic acid such as acrylic acid, itaconic acid and maleic acid. One
 currently preferred source of acid groups is polyacrylic acid. Suitable
 polymers are readily available commercially. The polymer should have an
 appropriate molecular weight to provide good storage, handling and mixing
 properties with the molecular weight conveniently being in excess of 5000.
 A mixture of acidic polymers may be used.
 Alternatively, acid groups may be present on one or more of the monomer
 materials, with such monomers constituting bifunctional molecules
 containing both acidic and unsaturated species. Suitable bifunctional
 molecules include those disclosed in U.S. Pat. No. 4872936, eg as
 described in column 3, lines 5 to 20 and column 3, line 28 to column 4,
 line 10, and U.S. Pat. No. 5218070, eg as described generally in column 1,
 line 67 to column 2, line 8, such as butan-1,2,3,4-tetracarboxylic acid,
 bis (2-hydroxyethyl methacrylate) ester, as described in Example 2.
 A mixture of sources of acid groups may be used.
 The acid groups should be present in sufficient amount for reaction with
 the filler material. For example, in embodiments using polyacrylic acid as
 the source of acid groups and ion-leachable glass as the filler material,
 satisfactory results have been obtained with glass to acid weight ratio in
 the range 10:1 to 1:1 preferably 8:1 to 2:1. A glass acid ratio of 4:1 is
 currently favoured for dental restorative cements.
 The composition may optionally include one or more further heterocyclic
 monomer materials, in addition to THFMA. Suitable heterocyclic monomers
 include 2.3-epoxypropyl methacrylate, tetrahydropyranyl methacrylate,
 tetrahydropyran-2-ylmethyl methacrylate, isobornyl methacrylate (IBMA) and
 tetrahydrofurfurylacrylate. By selection of a suitable mixture of
 heterocylic monomers, the composition can be tailored to have desired
 properties suited to particular intended uses thereof.
 The THFMA (and other optional heterocyclic monomer material, if present) is
 preferably present in an amount of at least 30%, more preferably at least
 40% by weight of the total weight of the monomer mixture. For mixtures of
 THFMA/BisGMA, the THFMA content is preferably in the range 65% to 85% by
 weight; good results have been obtained with a monomer mixture comprising
 about 70% by weight THFMA and about 30% by weight BisGMA. For mixtures of
 THFMA/UDMA, the THFMA content is preferably in the range 40% to 80% by
 weight, and the currently preferred monomer mixtures are 60% by weight
 THFMA and 40% by weight UDMA and 50% by weight THFMA and 50% by weight
 UDMA. In three part systems with IBMA replacing some of the THFMA, the
 preferred percentage of THFMA may be lower than in the two component
 monomer systems.
 The composition may also optionally include non-active filler material,
 that is filler material that does not undergo an acid-base reaction with
 the acid groups in the composition under aqueous conditions. Suitable
 non-active filler materials are known in the prior art, and include quartz
 powder, microfine silicic acid, aluminum oxide, barium glasses etc. The
 non-active filler material should be in finely divided form, which may or
 may not be of comparable particle size to the active filler material. A
 mixture of non-active filler materials may be used. The number, type and
 amount of non-active filler materials can be selected in known manner to
 provide desired properties, eg enhanced mechanical or chemical resistance,
 radio-opacity etc.
 The composition may include water (distilled, deionised or tap water)
 either included in the compositions as sold or added on use. The amount of
 water is chosen to provide required handling and mixing properties, and to
 permit ion transport for the acid-base reaction of the active filler. When
 included, water is conveniently present in an amount of at least about 1%
 of the total weight of the composition preferably 3% to 45%, more
 preferably 3 to 30%.
 In some cases, the monomer materials can polymerise without use of a
 polymerisation initiator, eg by exposure to a high energy pulsed xenon
 source. However, it is preferred to use in known manner one or more
 polymerisation initiators that act as a source of free radicals when
 activated. Such initiators can be used alone or in combination with one or
 more accelerators, activators and/or sensitisers.
 The initiator may be a photoinitiator that promotes polymerisation on
 exposure to light of a suitable wavelength (eg visible light, ultra violet
 light, laser light etc) and intensity. The initiator should be
 sufficiently stable to provide an acceptable shelf life and to permit
 storage and use under normal dental/biomedical conditions.
 Preferred visible light-induced initiators include camphorquinone (which
 typically is combined with a suitable hydrogen donor such as an amine),
 diaryliodonium simple or metal complex salts, chromophore-substituted
 halomethyl-s-triazines and halomethyl oxadiazoles. Particularly preferred
 visible light-induced photoinitiators include an .varies.- diketone, eg.
 camphorquinone, with a hydrogen donor (such as sodium benzene sulphonate,
 amines and amine alcohols).
 Preferred ultraviolet light-induced polymerization initiators include
 ketones such as benzyl and benzoin, and acyloins and acylion ethers.
 Preferred commercially available ultraviolet light-induced polymerization
 initiators include 2,2-dimethoxy-2-phenylacetophenone (available under the
 Trade Mark IRGACURE 651) and benzoin methyl ether
 (2methoxy-2-phenylacetopheneone), both from Ciba-Geigy Corp.
 Cold cure (or chemical) initiator systems, not dependent on exposure to
 heat or light are also known and can be used, eg benzoyl peroxide
 initiator with NN dimethyl p toluidine activator.
 The initiator system ingredients should be present in an appropriate amount
 to provide the desired rate and extend of polymerisation and are typically
 present in an amount between 0.01% and 15% of the total weight of liquid,
 preferably between 0.5% and 5% of this weight. In the filled systems, good
 results have been obtained with initiator system ingredients each present
 in the amount 5% by weight of liquid.
 The ratio of solid eg powdered materials to liquid materials may be varied
 and is typically in the range 10:1 to 1:15, preferably 8:1 to 1:1. Good
 results have been obtained with a solid:liquid ratio of 3:1 for a dental
 restorative cement.
 The composition may include water and/or be mixed with water on use so that
 both the free radical polymerisation reaction and the acid base reaction
 take place on use of the composition, in known manner. In this case the
 composition may be in the form of a resin modified glass ionomer
 composition (RMGIC). Alternatively, the composition may be in the form of
 a non-aqueous composition that initially sets on use by the free radical
 polymerisation reaction and that undergoes a slow acid-base reaction in
 situ over an extended period of time (in some instances many months) on
 take-up of water from the surroundings, possibly accompanied by release of
 fluoride or other useful ions. In this case the composition may be in the
 form of a compomer composition.
 The composition may, for example, be in the form of a two part formulation
 or a single part, non-aqueous formulation, although other variations are
 possible.
 Various additives may be optionally included in the composition in known
 manner, such as anti-oxidants, stablizers including UV inhibitors and
 polymerisation inhibitors, pigments, therapeutic agents such as
 antibiotics, corticosteroids and other medicinal agents eg metal ions etc.
 Compositions in accordance with the invention find application in a range
 of dental and biomedical uses including use as bone cement. The
 compositions find particular application in dentistry, including use as a
 filling material for restoring teeth following destruction by decay and
 for cementing inlays and crowns into place in a tooth, for providing a
 base and/or lining in a tooth cavity, for providing temporary fixing of
 orthodontic devices to teeth and for sealing root canals after endodontic
 treatment.
 The composition is used in conventional manner. In a typical case
 consisting of a two part RMGIC formulation for use as a dental cement, the
 two parts are mixed, possibly with addition of an appropriate amount of
 water, to produce a workable mixture that rapidly becomes putty-like or
 rubbery in consistency and can be readily used eg by being placed in a
 tooth cavity. If appropriate, the composition is exposed in situ to a
 suitable light source to initiate polymerisation. The composition then
 sets hard in situ in a clinically acceptable time, eg 10 minutes for a
 dental restorative cement. Maximum hardness may develop over time.
 The compositions adhere to teeth via chemical interaction of the acid
 groups, as with conventional glass-ionomer cement compositions, and as
 noted above release fluoride ions, if present in the active filler
 material without degradation of the cement. The properties of the
 compositions when set, including strength, hardness, etc. are well suited
 to dental uses.
 The use of THFMA as a monomer material has a number of advantages. THFMA
 has low shrinkage on polymerisation, good biological acceptability and
 advantageous water uptake properties when compared to other monomer
 systems.
 In a further aspect the invention provides a method of preparing a
 polymerisable cement, particularly a dental or biomedical cement,
 comprising mixing the ingredients of the composition of the invention, and
 causing the mixture to set.
 The invention also includes within its scope a method of dental treatment,
 comprising applying to a tooth a composition in accordance with the
 invention, and causing the composition to set.

A series of initial experiments were carried out to establish that mixtures
 of THFMA and secondary monomers, such as BisGMA, polymerise, to
 investigate different initiator systems and to investigate model RMGIC
 compositions.
 Section 1 - The potentials for auto- and photo-polymerisation of THFMA and
 monomer mixtures containing THFMA.
 EXAMPLE 1 (THFMA and THFMA/BisGMA)
 Experiments were carried out with monomers comprising 100% THFMA and
 mixtures of THFMA with BisGMA in the following weight ratios: 70%/30%,
 80%, 20%, 90%, 10% and 95/5%.
 The following polymerisation systems were used:
 1. Chemical cure (cold cure)
 Benzoyl peroxide (BP) - initiator
 N.N-dimethyl-p-toluidine (DMPT) - activator
 2. Photo-chemical (light cure)
 Camphorquinone (CQ) - initiator
 N.N-dimethyl-p-toluidine (DMPT) - activator
 Various combination and amounts of initiators (CQ or BP) and activator
 (DMPT) were added to the monomer mixtures. For cold cure, amounts of BP
 ranging from 0.5% to 5% and amounts of DMPT ranging from 0.5% to 5% were
 used. For light cure the amount of CQ ranged from 0.5% to 5% and the
 amount of DMPT was between 0.5 and 10%. In all cases the amounts of
 initiator and activators are % by weight relative to the total weight of
 monomer.
 The monomers were then poured into a disc-shaped (10mm x 1mm rubber mould
 and covered with a glass microscope slide, thus reducing oxygen inhibition
 of the polymerisation reaction. The chemically-cured monomers were allowed
 to set in the mould for 5 minutes. The photo-initiated monomers were cured
 for 60s using a Luxor (Luxor is a Trade Mark) visible light-curing unit
 (from ICI operating at 460-470nm. The set polymers were removed from the
 mould. Polymerisation of the polymers was arbitrarily determined after 5
 minutes by reference to the hardness of the surface of the specimens
 tested with a spatula: if the surface of the specimen was hard, this
 indicated that polymerisation had taken place.
 With monomer comprising 100% THFMA no polymerisation was obtained with
 light cure and for cold cure polymerisation was only obtained either using
 unacceptably high amounts of initiator and activator or in an unacceptably
 long time scale to be clinically acceptable or useful. However,
 potentially clinically useful cure regimes were established for all of the
 THFMA/BisGMA monomer mixtures and results are given in FIGS. 1 and 2.
 The following broad conclusions were reached concerning the amounts of
 initiator and activator.
 For photo-initiation for the monomer mixtures THFMA 70%/BisGMA 30% and
 THFMA 80%/BisGMA 20%, using CQ 0.5% and DMPT 0.5% was sufficient for the
 polymerisation of the monomer. Mixtures having a lower percentage of
 BisGMA required higher concentrations of the initiator and activator.
 For chemical cure, best polymerisation occurred in the THFMA 70% BisGMA 30%
 mixture where the lowest concentration of the initiator and activator
 could be used (BP 0.5% and DMPT 0.5%). Using a lower percentage of BisGMA
 in the mixture required higher concentrations of the initiator and
 activator.
 In view of the failure of 100% THFMA usefully to polymerise, no further
 work was carried out on 100% THFMA and further experiments were carried
 out on mixtures of THFMA with secondary monomers.
 EXAMPLE 2 (THFMA/UDMA)
 Following the procedure generally as described in Example 1, further
 experiments were carried out on mixtures of THFMA with UDMA in the
 following weight ratios: 95%/5%, 90%/10%, 80%/20%, 70%/30%, 60%/40%,
 50%/50%, 40%/60%, 30%/70%, 15%/85% and 5%/95%. Both chemical cure
 (BP/DMPT) and light cure CQ/DMPT systems were used in various amounts, as
 described in Example 1, although for mixtures containing 50% or more UDMA,
 the BP or CQ was generally first dissolved in THFMA (a good solvent) and
 low levels of initiator (0.15 wt%) and activator (2 wt%) were used. The
 hardness of the resulting specimens was determined using a Wallace
 microhardness tester. The depth of the impression on a specimen surface is
 measured and shown on an indicator dial which is graduated into 100
 hardness units, each representing a depth of 0.0001 inch. The method of
 testing is as follows.
 A minor load of 1g was applied initially and this was followed by the
 application of a major load of 300g for 10 seconds. The depth of the
 resulting indentation was measured with three values per specimen being
 recorded. Measurements were made 3, 10 and 60 minutes after mixing, the
 specimens being stored in the interim in darkness at 23.degree.C.
 Typical results for chemical cure systems are shown in FIG. 3.
 For light cure systems, typical results of average Wallace Hardness number
 (WHN) at times up to 60 minutes after the start of light curing are as
 follows:

Set by (mins)
 [CC] Set Mean
 % DMPT % DMPT after 2 mins? Initial
 SYSTEM T B ratio CC LC BP* CQ* [LC] WHN
 11 70/30 CC 1.0/1.0 -- 3 137
 12 70/30 LC -- 1.0/1.0 SET 188
 * = % total monomer
 Replacing 20% of the THFMA content of the THFMA/UDMA mixtures did not
 significantly affect the setting time or the initial WHN of the systems.
 Although the THFMA/BisGMA (70/30) specimens generally produced the best
 results, systems 5 and 9 (THFMA, UDMA .+-. IBMA-light-cured) produced
 specimens with comparable hardness, and the chemically-cured systems 1, 2
 and 6 to 8 (THFMA, UDMA .+-. IBMA) produced acceptable specimens.
 EXAMPLE 4 (THFMA/IBMA/BisGMA)
 Experiments similar to Example 3 were carried out to investigate mixtures
 of IBMA/THFMA/BisGMA to ensure that the polymerisation patterns of the
 monomer mixes were not unduly affected by the addition of IBMA, rather
 than looking at IBMA as a monomer to enhance the hardness or setting
 patterns of them. The following monomer mixtures were tested:

T/B 70/30
 I/T/B 10/60/30
 I/T/B 20/50/30
 T/B 80/20
 I/T/B 10/70/20
 I/T/B 20/60/20
 Method
 (i) Chemical Cure
 The following evaluations were carried out on the mixed material:
 a) The time taken to reach the `jelling` stage (when the liquid mixture
 begins to look lumpy rather than smooth)
 b) The appearance of the mixture at 5 minutes after the start of mixing
 Levels of initiator were 1% of the monomer system for both BP and DMPT.
 (ii) Light Cure
 The specimens were light-cured for 60s. At 2 mins after the start of
 light-curing, each specimen was examined to assess whether polymerisation
 had been achieved.
 Basic composition of systems:

JELLING STARTED
 MONOMER (s)
 T/B 70/30 90
 I/T/B 10/60/30 120
 I/T/B 20/50/30 90
 T/B 80/20 155
 I/T/B 10/70/20 125
 I/T/B 20/60/20 120
 b) Appearance of system at 5 mins after start of mixing
 At 5 mins after the start of mixing, all the systems had set to a clear,
 hard and almost colourless material.
 in small amounts, IBMA thus does not significantly affect the setting times
 using chemical cure when added to THFMA/BisGMA mixtures.
 (ii) Light Cure
 At 2 mins after the start of light-curing, all systems had set to a clear,
 hard material - slightly yellow around the edge of the specimens.
 IBMA thus does not affect ability of the specimens to polymerise after 60s
 of light-curing.
 Section 2 - The effects of the incorporation of active and non-active
 fillers on the polymerisation of monomer systems.
 EXAMPLE 5 (THFMA/BisGMA)
 Mixtures of THFMA and BisGMA monomers were used as in Example 1, that is
 with ratios 70%/30%, 80%/20%, 90%/10% and 95%5%. The various monomer
 mixtures were mixed with either powdered ion-leachable
 fluoroaluminosilicate glass (active filler) or non-active filler material.
 Various combination of initiators (CQ or BP) and activator (DMPT) were
 added to the mixtures. Disc specimens were prepared as described
 previously.
 The glass used was obtained from the commercial glass ionomer cement
 Opusfil W (Opusfil W is a Trade Mark) from Davis Schottlander & Davis
 Limited using the normal set formulation. Opusfil W is supplied as a
 powder which contains both glass and a dried form of polymeric acid. To
 obtain the glass alone for use in these experiments, the acid must be
 removed from the powder. This was achieved by "washing" the powder with an
 excess of methanol. The acid dissolves into the methanol, and the glass is
 then filtered out of the solution. This process is repeated until all the
 acid has been removed. The glass is then washed with an excess of
 distilled water to remove any trace of acid. Finally the glass is allowed
 to dry and then sieved (sieve about 150.mu.m) to break down any
 agglomerates.
 The non-active filler was an SiO.sub.2, BaO, B.sub.2 O.sub.3, Al.sub.2
 O.sub.3 glass of median particle size 0.78.mu.m (from Schott, Landshut,
 Germany).
 The chemically-cured monomers were allowed to set in the mould for 5
 minutes. The photo-initiated monomers were cured for 60s using a
 light-curing unit. The set specimens were removed from the mould. The
 polymerisation of the specimens was arbitrarily determined by the surface
 hardness as in Example 1.
 Incorporation of either the glass or the filler required greater
 concentration of the initiator and activator to achieve polymerisation of
 most of the monomer mixtures, but the 70/30 THFMA/BisGMA mixture was only
 slightly affected by the presence of glass or filler.
 Section 3 - Investigation of the setting reaction of experimental RMGICs.
 EXAMPLE 6 (THFMA/BisGMA, HEMA/BisGMA, HEMA)
 A model glass-ionomer system was established, using polyacrylic acid
 (having a molecular weight in the range 40,000 to 55,000), distilled water
 and powdered ion-leachable fluoroaluminosilicate glass. The glass was as
 described in Example 5.
 A system comprising glass:acid of 4:1 and powder:liquid of 3:1 was selected
 as convenient for further testing.
 Further experiments were carried out to test the effect in such a system of
 replacement of the distilled water with increasing percentages of monomer
 mixture (THFMA:BisGMA 70:30) (10% to 100% in steps of 10%) on the
 acid-base reaction. The effect of the presence of monomer was monitored by
 the changes in working and setting times of the cement. As expected, the
 presence of the monomer increased the working and setting times of the
 cements, and results are shown in FIG. 4. From these results it was
 concluded that an optimum monomer concentration that could be used in
 cement composition would not be in excess of 50 wt% of the liquid.
 Section 4 - The effect of the concentration of initiators and activators on
 the hardness of model RMGIC specimens.
 EXAMPLE 7 (THFMA/BisGMA)
 Further experiments were carried out using model RMGIC systems, generally
 as described in Example 6, using the same glass and polyacrylic acid.
 The glass was mixed with polyacrylic acid in the weight ratio of 4 to 1
 (G:A=4:1). The powder mixtures were then mixed with a liquid mixture of
 distilled water 50% and monomer (THFMA 70%/BisGMA 30%) 50% in the weight
 ratio of 3:1 (P:L=3:1). (This represents the maximum concentration of
 monomer that produced a workable cement). The mixtures were cured either
 by light cure or cold cure using the initiators and activators in various
 combinations and amounts as described in Example 1. The hardness of the
 specimens were determined using the Wallace hardness tester, as described
 in Example 2.
 Results are shown in FIG. 5 for light cure and FIG. 6 for chemical cure.
 Up to 24h, all specimens polymerised using the initiator/activator system
 were significantly harder at the corresponding time than the cement set by
 acid-base reaction alone, i.e. 0.0 wt% initiator/activator concentration.
 The hardness of all the specimens also increased with time.
 For light cure (photoinitiation), there were no significant differences for
 the hardness of the specimens when the concentration of the initiator (CQ)
 and the activator (DMPT) were 2.5/2.5, 2.5/5.0, 5.0/5.0, 7.5/5.0 and
 10.0/5.0%. The hardnesses were less when the concentrations increased over
 this level.
 For cold cure (chemical cure), there were no significant differences for
 the hardness of the specimens when the concentrations of the initiator
 (BP) and the activator (DMPT) were 2.5/2.5, 2.5/5.0, and 5.0/5.0%.
 Section 5 - The effects of non-active filler content on hardness of model
 RMGIC specimens.
 EXAMPLE 8 (THFMA BisGMA)
 Further experiments were carried out using model RMGIC systems, generally
 as described in Example 6, using the same glass and polyacrylic acid.
 The glass was mixed with polyacrylic acid in the weight ratio of 4 to 1
 (G/A=4:1). Non-active filler material, as in Example 5, was then added at
 amounts of 0%, 5%, 10%, 15% and 20% as a % by weight of the total weight
 of the powder. The mixtures were then mixed with a mixture of distilled
 water 80% and monomer (THFMA 70% and BisGMA 30%) 20% in the weight ratio
 of 3:1 (P/L=3.1). Both light cure and dark cure systems were used. The
 hardness of the specimens were determined using the Wallace hardness
 tester, as described in Example 2. Results are given in FIG. 7, which
 shows the hardness of the specimens was not adversely affected by the
 presence or amount of filler.
 Section 6 - Dental cement formulations
 EXAMPLE 9
 Prototype RMGIC formulations are as follows: