GB2315688A - Orthopaedic splinting material - Google Patents

Abstract

An orthopaedic splinting material comprises a flexible substrate carrying a resin system comprising at least a water hydrolysable organo-oxy silane functionalised prepolymer where the organo-oxy silane functional group is a cyclic organo-oxy silane functional group.

Description

ORTHOPAEDIC MATERIALS This invention relates to orthopaedic casting and splinting materials. In particular this invention relates to casting and splinting materials based on synthetic resins which, upon contact with water will cross-link and harden to form a weight bearing support for a limb or body portion and to such resins which on contact with water will cross-link and harden.

Resin based splinting and casting materials find wide use in the immobilisation of limbs, for example the fixation of fractured bones, immobilisation of injured joints and for the support of ligaments and muscles where it is necessary to encase the limb in a partially or completely surrounding rigid form or cast.

There are several major considerations for a casting or splinting material. A suitable material should be easily handleable, with a reasonable setting time to allow sufficient time in the case of casting bandages to mould the bandage about the limb and it should be flexible and free of offensive chemicals which may affect the patient or practitioner applying the material. In addition it is desirable that during the forming of the cast the material does not generate an uncomfortable exothermic reaction and that it sets within a relatively short time under mild conditions.

Splinting and casting materials comprising water activated synthetic polymers in which a cross-linkable prepolymer resin system is coated onto a suitable substrate are well known. The most favoured and commercially developed systems are those based on prepolymers which contain isocyanate functional groups and which in the presence of cold water will cross-link to form urea bridges. The resin systems usually also contain a catalyst to speed up the cross-linking reaction and so that a weight bearing cast may be formed in as shorter time as possible consistent with requirements for moulding and shaping the splint.

The isocyanate cross linking reaction is usually an exothermic reaction. In meeting the working requirements for such resin systems the chemistry has to be carefully controlled so as to allow the prepolymers to cure in a period of time which is acceptably short and yet not cure under conditions in which so much heat is evolved that the applied cast is uncomfortable to the wearer.

A disadvantage of such systems is the relatively high exotherm generated on curing and that there is a perceived health hazard with the use of orthopaedic bandages comprising isocyanate functionalised prepolymers. It is thus desirable to make suitable splinting materials without utilising isocyanate functionalised prepolymers.

Examples of this approach include US Patent No. 4411262 (von Bonin et al) wherein there are disclosed splinting and casting materials comprising substrates impregnated or coated with a reactive one component system, wherein inter alia the system includes organic compounds with molecular weights greater than 10,000, comprising reactive groups which may be alkoxysilane groups. It is taught that the organic compounds themselves were produced by an isocyanate functionalised reaction and thus any perceived health risk associated with isocyanate functionalised resins is not entirely removed without ensuring that the resins are fully reacted.

US Patent No. 5,423,735 also describes materials of this type, where the water reactive prepolymer is an alkoxysilane fu nctionalised polyu rethanelpolyurea prepolymer, produced by reacting isocyanate functionalised precursors. Such prepolymers potentially have the same problems which may be associated with other prior prepolymers produced from isocyanate functionalised precursors.

The use of alkoxysilane groups as the water activated reactive hydrolysable groups, more specifically the use of methoxysilane as the hydrolysable group has been proposed in the art.

The methoxysilane group has the advantage of being highly reactive with water without the need for catalysis, whereas higher molecular weight alkoxy substituents attached to the silane such as ethoxysilane or propoxysilane react more slowly and are not suitable for uncatalysed splinting and casting materials.

One possible disadvantage with the use of methoxysilane functionalised materials however, is the evolution of methanol as a by-product during hydrolysis of the methoxysilane groups as illustrated below:

Methanol is a toxin in man, with potential to cause serious irreversible effects to the liver, central nervous systems and visual centres. It is a volatile compound, and thus patients and more possibly, plaster technicians could be exposed via inhalation as well as skin contact following its release from a setting casting material.

The rate of metabolism and excretion of methanol is very low in man and thus repeated exposure to methanol may be regarded as having a cumulative effect.

There would probably be only a single exposure for the patient but a casting technician would be repeatedly exposed as each new casting material was applied and therefore is deemed unacceptable.

Thus an object of the invention is to provide a casting system which does not suffer from the aforementioned disadvantages.

A method for preventing the production of volatile alcohols during the curing process is to use higher molecular weight alkoxy groups on the silane, however as herein before described this results in the rate of hydrolysis becoming unacceptably slow.

We have found that the use of cyclic alkoxysilanes instead of trialkoxysilanes, on hydrolysis give higher molecular weight alcohols which are less volatile and further, the rate of hydrolysis is enhanced due to the presence of strain energy within the cyclic structure.

Therefore in accordance with the present invention there is provided an orthopaedic splinting material comprising a flexible substrate and a water curable resin system where the resin system comprises at least a water curable cyclic organo-oxy silane functionalised prepolymer, wherein said cyclic organo-oxy silane functional group is selected from the group comprising:

i) where PP = prepolymer ii) where Z = -OR3, -O-C(O)R3, -R3, -H, or halide and iii) where R', R2, R3 may be the same or different, substituted or unsubstituted and are groups based on the following Ci - C12 alkyl, cycloalkyl, heteroalkyl all of which may be optionally interrupted by -O-, -S-, -NH- andlor-C(O)-.

Organo-oxy silanes are suitably defined as a group including alkoxysilanes [-Si-O-R], acyloxysilanes [-Si-O-C(O)-R], cyclic alkoxy and cyclic acyloxy silanes.

References hereinafter to organo-oxy silane functionalised prepolymers include functional groups comprising at least one cyclic organo-oxy silane group.

The term "splinting material" used herein is intended to embrace splints, bandages, casts, supports and braces where such materials do not necessarily surround the whole limb or body portion.

During the curing reaction of the organo-oxy functionalised prepolymer (PP) the organo-oxy groups are hydrolysed to form hydroxysilane groups which condense together to give siloxane [-Si-O-Si- 1 links.

The nature of the hydrolysable group that is released depends on the nature of the links between the hydrolysable group and the silane, for example: i) if all are Si-O-R links, the hydrolysable group may be released as a high molecular weight non-volatile alcohol as illustrated below:

ii) if there is one Si-R link, where the alkyl is linked directly to the silicon, and at least one Si-O-R link, the hydrolysis reaction results in the cleavage of the Si-O bonds only, leaving the alcohol attached to the silicon, as illustrated below:

The hydrolysis reaction of the alkoxy or acyloxy silane group may be catalysed. Suitable catalysts include organotin salts, chloroacetic acid, methane sutphonic acid (MSA), phosphoric acids, 1, 5-diazabicyclo[4.3.0]non-5-ene (DBN), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), p-toluene sulphonic acid and bismorpholino dialkyl ethers such as dimorpholinodiethylether.

Further suitable catalysts comprise water soluble halides, for example organic and inorganic halides from the group comprising lithium fluoride, sodium fluoride, potassium fluoride and tetrabutylammonium fluoride.

The catalyst may be present as a single chemical species or a mixture and, when used, is aptly present in an amount of up to 20% wiw, more aptly up to 10% w/w and most preferably up to 5% w/w of the resin. Aptly if used, it should be at least 0.01% wlw of the prepolymer.

The prepolymers used in the invention may be precured by the addition of water to cross-link a small percentage of the available organo-oxy silane groups. The advantage gained would be an increase in molecular weight, leading to an increase in viscosity.

Examples of organo-oxy silane functionalised prepolymers include but are not limited to: organo-oxy silane functionalised polyurethanes, one component organo-oxy silane functionalised prepolymers made by nucleophilic addition reactions and organooxy silane functionalised ionic based prepolymers.

Nucleophilic addition one component organo-oxy silane functionalised prepolymers may be prepared by for example the reaction of nucleophiles with activated carbon-carbon based moieties or Michael-type reactions, where at least one of the reagents is terminated with a hydrolysable organo-oxy silane group.

Patent Application PCTIGB96/00231 is incorporated herein by reference.

Michael - type reactions may be defined as the reaction of a nucleophile with an a, -unsaturated carbonyl moiety. Michael - type reactions may be exemplified by the reaction of acrylate or acrylamide reagents with reagents terminated with -SH, -N H2, -NHNH2, or > NH moieties.

Preferred Michael-type reactions include the reactions of aminoalkylsilanes with acrylates or acrylamides.

Suitable acrylates and acrylamides may be any well known in the art, or such materials may be custom made to impart preferred physical properties to the prepolymer. For example acrylamides may be prepared by the reaction of alkylazlactones with amine terminated precursors.

Apt nucleophilic reactions also comprise the reaction of nucleophiles with carbon-carbon based moieties, which are activated by ring strain or electron withdrawing effects. These include the reaction of reagents terminated with three membered heterocycles with reagents terminated with -SH, -N H2, -NH-NH2, > NH moieties where at least one of the reagents is further terminated with hydrolysable organo-oxy silane group.

Suitable reactions include the reaction of organo-oxy terminated aminoalkylsilanes with reagents terminated with three membered heterocycles such as epoxides or aziridines.

Hydrolysable organo-oxy silane functionalised ionic based prepolymers are a single component system where the group bearing the hydrolysable organo-oxy silane group is ionically bound to the resin, rather than covalently bound to the resin.

The ionic based resins may suitably be made by for example the reaction of alkyl carboxylic acids with alkyl amines, where one or both of the reagents is terminated by a water hydrolysable organooxy silane group, to produce a salt complex, where a salt complex is defined as a prepolymer containing ionic linkages.

The advantage conferred by ionic based prepolymers is that the wetting out characteristics of the prepolymer are improved.

Wetting out is defined asthe ability of the prepolymer to take up water which in turn affects the rate at which the functionalised prepolymers react with water.

Hydrolysable organo-oxy silane functionalised prepolymers, where the prepolymer carries pendant hydrolysable organo-oxy groups and is additionally terminated with reactive amines may be further modified with an acid such as acid chlorides or acid anhydrides, for example, acetyl chloride.

Acetyl chloride reacts with the terminating reactive amine groups liberating hydrogen chloride as a by-product, thereby producing an acidic environment which may enhance the rate of hydrolysis as herein before described.

Preferred formulations of the resin system may include a variety of additives conventional in the art. These additives may comprise fillers, pigments, fragrances, surfactants, lubricants or mixtures thereof.

Suitable fillers include but are not limited to talc, calcium carbonate, fumed silica sold under the trade name Cab-o-Sil , alumina and fibrous reinforcing fillers such as wollastonites (calcium metasilicate), to impart desirable viscosity and handling characteristics.

The fillers may be present as single chemical species or as mixtures and, when used, are aptly present in an amount of up to 50% w/w, preferably up to 20% w/w and aptly at least 1.0% wIw of the resin.

The resin system used for splinting materials according to the invention may be carried on any substrate suitable for a casting, splinting, support or bracing material.

The resin system may be coated, laminated, sprayed or impregnated onto a suitable substrate using conventional methods known in the art. Aptly the casting materials of the invention are prepared by nip-coating the resin on to the substrate.

A preferred substrate is a flexible fabric carrier which may be a woven, knitted or non woven fabric which can carry enough of the prepolymer to ensure that the resultant cast has adequate strength.

The substrate should be sufficiently porous to allow water to come into contact with the carried prepolymer when the formed bandage is immersed in water. The substrate may be in the form of tapes, bandages, sheets or other conventional forms, apt for preparing orthopaedic casting bandages, splinting materials or braces.

Suitable materials for forming the substrate include polyester, nylon, polypropylene, polyamides, polyolefins and glass fibre or mixtures thereof. Examples of such substrates are disclosed in Patent Nos. US 4,427,002, US 4,627,424 and EP 326, 285.

Aptly the substrate may be a mesh having openings through it to enable the water to penetrate into the rolled bandage to contact all of the prepolymer. The openings will also permit circulation of air and aid evaporation of moisture from the skin beneath the cured cast.

Preferably the mesh is of a loose weave or knit so as to allow at least partial impregnation as well as coating by the prepolymer.

The amount of prepolymer carried by the substrate before curing may vary depending on the intrinsic properties of the prepolymer making up the resin system and should be sufficient to ensure that the resultant cast has adequate strength.

Suitable amounts range from 30 to 80% w/w which are calculated using the equation: weight of (substrate + prepolymer) - weight of (substrate) x 100% weight of (substrate + prepolymer) Preferably 35 to 70% wlw and most preferably 40 to 65% w/w of prepolymer are used.

A hardened cast may be formed by wetting and shaping the wet material around a body member or part thereof and allowing the material to cure.

Aptly wetting is achieved by immersing the material in water, and removing any excess water, for example, by squeezing the material several times before application to the body member.

When removed from the water the material can be readily wrapped about a limb or body member to be treated. Preferably the body member is surrounded by a conventional underlying stockinette or padding, before application of the material.

An alternative method for forming a cast or splint comprises applying the material to the body member followed by spraying the material with water.

The water used to wet the material may contain a soluble catalyst for the hydrolysis reaction The curing reaction of the prepolymer should be sufficiently slow to allow the material to be positioned and shaped before the material becomes unworkable. Suitable working times are aptly 1 to 6 minutes more aptly 2 minutes to 4 minutes. The curing reaction of the prepolymer should, however, be sufficiently fast to permit the formed cast or splint to become supportive and load-bearing as soon as possible after completion of working. Aptly the material will set and become supportive between 5 and 30 minutes, more aptly within 15 minutes and particularly in the case of a cast, will aptly become load-bearing within 60 minutes, more aptly after 10 minutes.

The prepolymers employed in the present invention possess the advantage that no methanol is generated during the curing reaction, and the further advantage in that the curing reaction is only slightly exothermic thus causing no harm or discomfort to the patient.

The cast may be readily removed by conventional means such as by cutting with a conventional vibrating sawtooth disc.

The orthopaedic splinting material should be protected during storage from water and moisture vapour to prevent a premature setting taking place. The material can be conventionally packaged in heat sealed pouches such as metal foil polyethylene laminate pouches.

The invention will now be described by way of example only and it should be understood that normal precautions for excluding moisture during chemical reactions were employed.

Example 1.

Preparation of a cyclic alkoxysilane amine terminated precursor: 3-Aminopropylmethyldiethoxysilane (191 g, 1.0mop) and 1,4-butanediol (909, 1.0mop) were combined in the presence of titanium isopropoxide (5% by weight of silane) and then distilled at atmospheric pressure under nitrogen. The alcohol (ethanol) was collected at 700C leaving a viscous oil. The oil was dried at 500C in vacuo to remove any residual ethanol, leaving the precursor.

Preparation of a cyclic alkoxysilane amine terminated prepolymer.

The precursor (0.8mol) as prepared above was subsequently reacted with methylene bisacrylamide (0.4mol), by stirring under nitrogen for 5 days at 60 OC, to give a cyclic alkoxysilane terminated prepolymer.

The prepolymer was coated onto a dry glass fibre bandage by passing the bandage through the prepolymer followed by passing the coated bandage through a nip roller, adjusted to a suitable pressure for obtaining a coating weight of 5060% w/w coating. The prepolymer was set of by dipping the coated bandage in cold water, and squeezing several times before application to a mandrel representing a limb.

Claims (11)

1. An orthopaedic splinting material comprising a flexible substrate and a water curable resin system where the resin system comprises at least a water curable cyclic organo-oxy silane functionalised prepolymer, wherein said cyclic organo-oxy silane functional group is selected from the group comprising:

i) where PP = prepolymer ii) where Z = -OR3, -O-C(O)R3, -R3, -H, or halide and iii) where R', R2, R3 may be the same or different, substituted or unsubstituted and are groups based on the following C, - C12 alkyl, cycloalkyl, heteroalkyl all of which may be optionally interrupted by -O-, -S-, -NH- andlor -C(O)-.

2. A material according to claim 1 wherein R', R2, R3 may be the same or different, substituted or unsubstituted and are groups based on the following Ci - C6 alkyl, cycloalkyl, heteroalkyl all of which may be optionally interrupted by -0--S-, -NH- and/or -C(O)-.

3. A material according to claim 1 wherein the organo-oxy silane functionalised prepolymers may include include organo-oxy silane functionalised polyurethanes, one component organo-oxy silane functionalised prepolymers made by nucleophilic addition reactions and organo-oxy silane functionalised ionic based prepolymers.

4. A material according to claim 1 including a catalyst to catalyse the polymerisation reaction of the silane functionalised prepolymer with water.

5. A material according to claim 4 wherein said catalyst is from the group comprising: methane sulphonic acid, 1 ,5-diazabicyclo(4.3.0] non -5-ene, 1,8-diazabicyclo[5.4.0]undec-7-ene, ethyl titanate, potassium fluoride, p-toluene sulphonic acid, dibutyltindilaurate, or mixtures thereof.

6. A material according to claim 1 including additives comprising pigments, fragrances, surfactants, lubricants, fillers or mixtures thereof.

7. A material according to claim 1 wherein the resin system is coated onto the flexible substrate.

8. A method for applying an orthopaedic splinting material according to claim 1 comprising wetting the material, shaping the material around a bodymember and allowing the resin system to cure.

9. An orthopaedic splinting material comprising a flexible substrate carrying a water curable silane functionalised resin as defined in claim 1.

10. An article comprising a water curable silane functionalised resin as defined in claim 1.

11. The use of a water curable silane functionalised resin as defined in claim 1 as an orthopaedic casting bandage or splint.