GB2125803A - Medical casts - Google Patents

Abstract

Casts for surgical or orthopedic use and which are transparent so as to allow visual monitoring of the body- part covered by the cast are made from a blend of a cyclic ester polymer such as poly-epsilon caprolactone and a styrene-acrylonitrile copolymer containing not more than 30% by weight of acrylonitrile in the copolymer.

Description

SPECIFICATION Casts This invention relates to casts for surgical, orthopedic or like use on human or animal subjects. More particularly, the invention relates to casts made from thermoplastic polymeric materials. Herein, the term 'cast' is used without intending limitation to any particular method of formation of the shape of the cast.

Casts formed from certain thermoplastic polymeric materials, which materials have the property of being shapeable initially, and deformable for removal if required, at a temperature not unduly uncomfortable for the subject and providing a suitably firm support at ambient temperature are described in British Patent No 1366091. The cast materials exemplified in that patent do not give transparent casts. It would be of great benefit could transparent casts be produced since this would enable visual monitoring of the condition of the underlying body-part without removal of the cast. This is acknowledged indirectly in British Patent No 1366091 by stating that the cast material can be made transparent. However, no means of doing this is described in the patent or has been available to the public from any other source.

The present invention enables the provision of casts made from thermoplastic polymeric materials which have the property of transparency, such materials for use in the formation of casts for orthopedic, surgical or like treatments and, further, such materials 'per se' since, by virtue the of the particular combination of properties which render them suitable for cast formation, they are potentially suitable for other uses. The word 'transparency' herein is intended to have a practical rather than an absolute meaning. It is used in contrast to the word 'translucent' which is understood as indicating an ability to transmit light but not to see detail.

The transparent thermoplastic polymeric materials provided by the present invention consist essentially of one or more cyclic ester polymers characterised by the recurring unit: -O-(CR2)x-Az~(CR2)y~CO~ wherein R represents a substituent selected from hydrogen, alkyl, halo and alkoxy, A is the oxo group, x and y are integers from 1 to 4, z is zero or the integer 1, the total of x, y and z equalling 4 to 7 and the total number of substituents R which are other than hydrogen being 1 to 3, and a styrene-acrylonitrile copolymer characterised by a content of not more than 30%, by weight of the copolymer, of acrylonitrile.

Preferably R represents an alkyl or alkoxy group, particularly preferably the former, and/or on each occurence preferably contains not more than 12 carbon atoms, particularly preferably not more than 4 carbon atoms, and/or, in total, in all occurences in each recurring unit, preferably does not consist of more than 20 carbon atoms. Preferably the total number of substituents R is 1 or 2.

According to the present invention however, it is particularly preferred, though not essential, that the value of z in the above identified recurring unit is 0 and that x andy total 5 so that the unit is derived from an epsilon caprolactone the substitients R of which are as already defined, Preferably these substituents R are hydrogen atoms. Alternatively, of course, the recurring units may be derived from suitable valerolactones enantholactones or caprylolactones.

The preparation of the above identified cyclic ester polymers may be as described in British Patent No 1366091, that is, the ring opening polymerisation of the corresponding cyclic ester by means of a suitable catalyst the choice of which will depend on the presence or absence of a functional, e.g. active hydrogencontaining, initiator.

The catalyst may be suitably selected from, for example Lewis acids, organic acids, alkyl tin, alkyl titanium, or alkyl aluminium compounds or other suitable catalysts known in the art.

The ring opening may be accomplished at temperatures of from about 20"C to 200 C and the catalyst preferred depends on the temperature selected. When using higher temperatures within the last mentioned range the catalyst may suitably be an alkyl tin or an alkyl titanium compound, for example stannous octoate or dibutyl tin bisoctyl thioglycolate. Alternatively, when somewhat lower temperatures are used the catalyst may suitably be a Lewis acid for example boron trifluoride. The quantity of catalyst is preferably from 0.0005 to 1.0% by weight of the cyclic ester.

The functional initiator may be a hydroxyl containing compound, preferably a primary or secondary alcohol such as for example a glycol, or a lower aliphatic alcohol such, as methanol or octanol or an amino alcohol such as ethanolamine. At least so far as polyepsilon caprolactone is concerned this cyclic ester polymer is commercially available from Interox Chemicals Limited of Great Britain under the Trade Name CAPA.

The styrene - acrylonitrile copolymers used according to this invention critically contain a restricted proportion of acrylonitrile. While the precise upper limit of acrylonitrile may vary according to the degree of transparency regarded as necessary, and the particular cyclic ester polymers used, it is certainly found that, with polyepsilon caprolactone cyclic esters, the figure of about 30% by weight is the effective upper limit.

Above this limit the blends tend to be opaque. The presence of at least some acrylonitrile is also essential since polystyrene, itself, will not blend satisfactorily with the cyclic ester component of the blend and also gives an opaque material. Preferably the styrene-acrylonitrile copolymer contains at least 10% by weight, particularly preferably at least 15% by weight of acrylonitrile and, with a polyepsilon caprolactone cyclic ester polymer, particularly satisfactory transparent materials having been obtained between about 17% and 27% by weight and, particularly suitably, between 19% and 26% by weight of acrylonitrile in the copolymer.

The styrene-acrylonitrile copolymer may be produced by known techniques or purchased as a normal commercial product from Montedison under the Trade Name KOSTIL.

The proportions of cyclic ester polymer and the styrene-acrylonitrile copolymer are also important if suitable physical properties are to be attained. Preferably, the weight proportions of styrene - acrylonitrile copolymerto cyclic ester polymer of from 50: 50 to 75: 25. In the case where polyepsilon caprolactone is used as the cyclic ester polymers very suitable materials may be produced using weight proportions of styrene - acrylonitrile copolymer to cyclic ester polymers of from 55 : 45 to 70 : 30 particularly preferably from 60 : 40 to 70 : 30.Within the aforesaid ratios a lower proportion of styrene - acrylonitrile copolymer tends to lead to greater flexibility at ambient temperature, and a shortening of the transparent shelf life while a greater proportion tends to lead to increasing rigidity but also increasing brittleness and an increasing difficulty in working in the plastic temperature range.

The weight mean average molecular weight of the cyclic ester polymer is not found to be of significance between at the least the values of 30,000 and 150,000. At its broadest however, the said molecular weight may be between the values of 10,000 and 1 million subject to suitable application properties.

The molecular weight of the styrene acrylonitrile polymer is also not thought to be critical within wide limits and normal commercially available materials having weight mean average molecular weights of from about 50,000 to 5000,000 are thought to be suitable.

Small proportions of other materials, whether further copolymers included in or with the styrene acrylonitrile copolymer, or in or with the cyclic ester, or monomeric materials included in the polymer or copolymer chain or as additions to the blend may be allowable or even beneficial to the practice of this invention. For example an alkylene oxide, eg ethylene or propylene oxide monomer, maybe randomly dispersed in the cyclic ester polymer chain although preferably this is in less than 20%, particularly preferably less than 10% on a molar basis based on the number of cyclic ester units present.

According to British Patent No 1366091 a minor proportion of a further recurring unit may be included in the cyclic ester polymer. This further recurring unit is identified as -O-CHR-CHRwhere R may be hydrogen, alkyl, cycloalkyl, aryl, or chloroalkyl or, together with the ethylene moiety of the oxyethylene chain provided by a recurrence of these units a saturated cycloaliphatic hydrocarbon ring containing 4to 8 carbon atoms, but is preferably hydrogen. The presence of such a recurring unit is not excluded from the present invention. Preferably, however, the proportion of any co-polymeric material is less than 20% particularly preferably less than 10% and desirably less than 5% of the styrene-acrylonitrile copolymer or the cyclic ester polymer respectively.The relative proportions of the last mentioned copolymer or polymer are calculated without reference to any such further copolymeric material or other additives to the blend.

To improve the self-adherency of the materials according to this invention, which property is desirable when a cast is to be formed by winding sheet or strip material in self-overlapping arrangement, or to improve other physical properties, it is desirable to include in the said materials restricted quantities of substances known to have this effect in cyclic ester polymers such as gums or rosins, for example, a nitrile rubber gum which may contain, for example, from 25% to 50% by weight of acrylonitrile, or a styrene-butadiene rubber gum which may contain, for example, from 20% to 30% by weight of bonded styrene, or rosin esters, rosin itself, copolymers of styrene with vinyl toluene, styrene itself or methyl styrene.Preferably the proportion of such materials in the blend is not more than 15%, particularly not more than 10% for example, suitably, from 1% to 7% by weight.

It is also possible to tolerate in the blends according to the invention restricted quantities of other polymers although the proportion of such other polymers is preferably less than 10%, particularly preferably less than 5% by weight of the blend.

It is understood that any of the ingredients utilised in the blends according to the invention are included subject to not unduly limiting the transparency of the blends and their quantities may have to be restricted accordingly. The degree of transparency which is required will depend as a practical matter on the type of monitoring required and on the thickness of the case but, without limitation of the scope of the present invention thereto, an example of a suitable transparency would be one which allows the general condition of skin to be observed without undue difficulty through a total thickness of cast material of at least 2mm particularly preferably at least a 4mm. A degree of haziness can be tolerated provided that there is sufficient ability to see detail to give the practical effect required. Of course, for aesthetic reasons substantially full transparency is preferred and may be obtained according to at least the optimum embodiment of this invention.

The blends according to the present invention may be produced by mixing the cyclic ester polymer with the styrene-acrylonitrile copolymer in the form of solids, together with any other ingredients, and heating the mixture to above the melting point of the polymers under the action of mixing or blending equipment known in the art. Very suitably this may be accomplished in a Banbury (Trade Mark) or a similar mixer.

The resulting mixture may be formed into sheets or strips of a suitable thickness, for example from 1 to 3mm in thickness, in which form it may be stored. For use the sheets or strips, or the buik polymeric blend if such have not been formed, may be softened by heating to at least about 60"C, preferably at least about 70"C, for example, by immersion in warm water, at which temperature it becomes shapeable and tends to retain this characteristic even when cooled to a somewhat lower temperature, for example 50"C, at which temperature a human subject will not experience undue discomfort.The sheet, strip or bulk material may be caused to conform to the shape of the subject and in supportive relationship thereto and allowed to cool further to below its solidification temperature to give a firm support depending, of course, on the use of a suitable cast thickness, whether by using a relatively thick sheet or strip or a relatively large number of layers or otherwise. The case may be removed by immersing it in warm water and unwinding it when it has softened sufficiently or may be removed by cutting.

The present invention will now be illustrated by the following examples of which Examples 1 to 4 and 9 to 15 are according to the invention and Examples 5 to 8 and 16 are comparative tests of blends not according to the invention.

Examples 1 to 5 and 13 illustrate varying proportions of styrene and acrylonitrile in the copolymer and of the cyclic ester polymer.

Examples 6 to 8 and 16 illustrate the inoperability of other clear polymers used in place of the styrene-acrylonitrile copolymer.

Examples 9 to 12, 14 and 15 illustrated the addition of minor quantities of modifying materials.

Example 1 65 parts by weight styrene-acrylonitriie copolymer containing 21.5% by weight acrylonitrile was blended with 35 parts by weight polyepsilon-caprolactone (as the cyclic ester polymer) of approximate molecular weight 50,000 using a small internal mixer heated at 1 50"C. The product was ciear and transparent. It was pressed into sheets of 1 .5mm and 3mm thickness at 1 500C and cooled to retain this shape. The sheets were cut into strips varying between 1.5cm and 4cm wide. The strips were immersed in water at 900C for 20 seconds, were removed and wiped dry and were wound around the finger and/or arm of several subjects as soon as the surface has cooled sufficiently.The strips hardened on cooling to give support similar to that of a conventional cast while being tough, lightweight, transparent and water resistant.

Example 2 A blend was prepared as in Example 1 except that 60 parts by weight of styrene-acrylonitrile copolymer was used together with 40 parts by weight of polyepsilon-caprolactone. The sheets strips and casts formed were more flexible and less supportive than in Example 1 and, although initially transparent and so according to the invention, gradually become opaque over a period of from 1 to 3 weeks.

Example 3 A blend was prepared as in Example 1 except that 70 parts by weight of stryene-acrlonitrile copolymer was used with 30 parts by weight of polyepsilon-caprolactone. The sheets and strips formed were transparent although more rigid than in Example 1. However they were more difficult to mould into casts which were relatively brittle when compared with those produced by Example 1.

Example 4 A blend was prepared as in Example 1 except that the styrene-acrylonitrile copolymer contained 25.9% by weight acrylonitrile. The sheets, strips and casts formed were transparent although less so than in Example 1.

Example 5 A blend was prepared as in Example 1 except that the styrene-acrylonitrile copolymer contained 31.5% by weight acrylonitrile. The sheets, strips and casts formed were completely opaque.

Example 6 A blend was prepared as in Example 1 except that the styrene-acrylonitrile copolymer was replaced by transparent rigid polyvinyl chloride bottle compound. The casts formed would not adequately retain the required shape after moulding from strip.

Example 7 A blend was prepared as in Example 1 except that the styrene-acrylonitrile copolymer was replaced by transparent polystyrene. The sheets and strips formed were opaque and had poor physical properties.

Example 8 A blend was prepared as in Example 1 except that the styrene-acrylonitrile copolymer was replaced by transparent polymethylmethacrylate. The sheets and strips formed were opaque and had poor physical properties.

Example 9 A blend was prepared as in Example 1 except that 5 parts by weight of the styrene-acrylonitrile copolymer was replaced by nitrile rubber gum containing 33.3% acrylontrile by weight. The sheets and strips formed were particularly easy to form into casts. However the casts were on the limit of useful transparency.

Example 10 A blend was prepared as in Example 1 except that 5 parts by weight of the styrene-acrylonitrile copolymer was replaced by styrene-butadiene rubber gum containing 24% by weight of bonded styrene and having a viscosity ML4 (100 C) of 34. The sheets and strips formed were particularly easy to form into casts. The casts were hazy in appearance although reasonably transparent and therefore according to the invention.

Example 11 A blend was prepared as in Example 1 except that 5% by weight of the styrene-acrylonitrile copolymer was replaced by polyester thermoplastic rubber of 40 Shore D hardness and made from dimethyl terephthalate polytetramethylene ethylene glycol and 1, 4-butane diol. The resulting sheets and strips could be formed into casts. The cases were transparent although hazy in appearance.

Example 12 A blend was prepared as in Example 1 except that the cyclic ester polymer used had a molecular weight of 30,000. The resultant sheets, strips and casts were made in a similar manner two Example 1 and had similar properties to that example.

Example 13 A blend was prepared as in Example 1 except that the cyclic ester polymer had a molecular weight of 150,000. The resultant sheets, strips and casts were made in the same manner as Example 1 and had similar properties to Example 1.

Example 14 A blend was prepared as in Example 1 except that 5% of the styrene acrylonitrile copolymer was replaced by a rosin of acid value 156, saponification value 171 and ball and ring softening point 60"C. The resultant sheets and strips were transparent although slightly harder than in Example 1 and were easily formed into casts. After 6 to 8 weeks storage they lost some transparency and became brittle.

Example 15 A blend was prepared as in Example 1 except that 5% of the styrene-acrylonitrile was replaced by a hydrogenated ester of rosin with pentaerythritol. The resultant sheets and strips were transparent and were easily formed into casts. After 6 - 8 weeks storage they became brittle and lost some transparency.

Example 16 An attempt was made to prepare a blend from polyepsilon caprolactone and polycarbonate. It was not possible to use the same means of blending as in Example 1 due to the high melting point of polycarbonate.

In this case the blends were prepared on a single screw extruder using a barrel temperature of 200 - 270"C and a die temperature of 200 C. It was found that it was not possible to prepare a transparent blend containing more than 20% polyepsilon caprolactone. The blends thus prepared could not be heat formed at a temperature low enough to allow contact with the skin of the subject.

Claims (12)

1. A transparent thermoplastic polymeric composition consisting essentially of one or more cyclic ester polymers characterised by the recurring unit: -O-(CR2)x-Az~(CR2)y~CO~ wherein R represents a substituent selected from hydrogen, alkyl, halo and alkoxy, A is the oxo group, x and y are integers from 1 to 4, z is zero or the integer 1, the total ofx, y and z equalling 4 to 7 and the total number of substituents R which are other than hydrogen being 1 to 3, and a styrene-acrylonitrile copolymer characterised by a content of not more than 30%, by weight of the copolymer, of acrylonitrile.

2. A composition as claimed in claim 1 wherein the cyclic ester polymer is poly-epsilon caprolactone.

3. A composition as claimed in claim 1 or 2 wherein the composition contains the styrene-acrylonitrile copolymer in a weight proportion, relative to the cyclic ester polymer, of from 50:50 to 75:25.

4. A composition as claimed in claim 3 wherein the weight proportion of the styrene-acrylonitrile copolymer relative to the cyclic ester polymer is from 60:40 to 70:30.

5. A composition as claimed in any one of claims 1 to 4 wherein the content of acrylonitrile in the styrene acrylonitriie copolymer is at least 10% by weight.

6. A composition as claimed in claim 5 wherein the content of acrylonitrile in the styrene-acrylonitrile copolymer is from 17% to 27% by weight.

7. A composition as claimed in any preceding claim containing other polymeric material in less than 20% by weight.

8. A composition as claimed in any preceding claim containing a gum or rosin in less than 15% by weight.

9. A composition as claimed in claim 1 and substantialiy as described herein with reference to any one of Examples 1 to 4 or 9 to 15 herein.

10. A surgical or orthopedic cast material comprising a composition as claimed in any one of claims 1 to 9.

11. A cast composed of one or more layers of a material as claimed in claim 10 in sheet or strip form.

12. A method of forming a cast comprising applying to a subject a composition as claimed in claim 10 having a temperature above its solidification temperature, causing the composition to conform to the shape of the subject in supportive relationship thereto and allowing the composition to cool to below its solidification temperature.