GB1593288A

GB1593288A - Bone prosthetic material

Info

Publication number: GB1593288A
Application number: GB3920577A
Authority: GB
Original assignee: Institut National de la Sante et de la Recherche Medicale INSERM
Current assignee: Institut National de la Sante et de la Recherche Medicale INSERM
Priority date: 1976-09-20
Filing date: 1977-09-20
Publication date: 1981-07-15
Also published as: NL185392C; IT1086088B; NL7710315A; FR2364644B1; ES462630A1; CH624572A5; BE858815A; DE2742128A1; FR2364644A1; DE2742128C2; NL185392B

Abstract

A biocompatible polymer of high molecular mass and containing a minimum of oligomers receives an inorganic and/or organic filler in a proportion of 0.5 to 30 % with respect to the weight of the polymer, which is capable of stimulating resorption of the polymer. The resulting material is used for preparing bone prostheses in which the presence of the filler creates, in the polymer mass, points of attack giving microcavities in which the newly formed bone preferentially develops.

Description

(54) BONE PROSTHETIC MATERIAL (71) We, INSTITUT NATIONAL DE LA SANTE ET DE LA RECHERCHE MEDICALE, of 101, rue de Tolbiac 75013 Paris, France, a French Company, do hereby declare this invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a bone prosthetic material, as well as to the prosthetic members obtained by means of said material.

In the present application the term "prosthetic members" is understood to mean prosthetic members in the conventional sense and osteosynthetic members.

In bone surgery (e.g. orthopaedic, maxillofacial) the anchoring of rigid implants (e.g. articular prostheses, osteosyntheses) is at present effected either by means of a cement (e.g. methyl methacrylate polymerised in situ) or with metal screws as in osteosynthesis, or by force-fitting (e.g. centromedullary nail). All these solutions have various disadvantages and for many years research has been carried out with a view to the use of in vivo reabsorbable or porous materials for accelerating osseous rehabitation at the bone-implant interface.

The present invention solves the problem posed by the present research aimed at making osteosynthetic members (plates and screws) and the coating of prosthetic implants with substances which are biodegradable, which induce bone growth and, if necessary for the particular application envisaged, have adequate mechanical properties (modulus of elasticity close to that of bone and a breaking load which is adequate to prevent breaking in use).

The present invention relates to a material for bone prosthesis, comprising a biocompatible and reabsorbable polymer containing an adjuvant which is able to stimulate the reabsorption of the polymer for the benefit of a neoformed bone tissue.

The adjuvant, herein also referred to as the charge, may be constituted by i) salts based on phosphate anions ii) salts based on calcium or magnesium cations iii) borates, carbonates, fluorides or silicates of lithium sodium or potassium and their mixtures.

The material according to the invention therefore makes it possible to combine in the same prosthetic member the properties of reabsorbability and activation of osteogenesis of the contiguous tissue.

The initial mechanical strength of the prosthetic member is determined by the choice of an adequate polymer and the charge content, being optionally improved by the preparation of composite members in which the material according to the invention is associated with reinforcing members.

The polymers which can be used for producing the material according to the invention are preferably high molecular weight polymers (M.W. equal to or above 150,000 and preferably above 200,000). For the purposes of the invention, it is particularly recommended to use polymers containing a minimum of oligomers. Therefore, polymers are preferably used whose polydispersity index is low, for example below 2.

The polymers which can be used according to the invention can be homopolymers or copolymers.

Among the polymers which can be used for making the material according to the invention, polymers and copolymers of ahydroxy carboxylic acids and more particularly homopolymers and copolymers of glycollide and L-, D- and DL-lactides are particularly preferred, due to their excellent biocompatibility and particularly their biodegradability. Their chemical structure and properties give these polymers an im portant supporting function, whereby they are degraded into non-toxic products which can be excreted or metabolised and in the long term can be replaced by contiguous tissues belonging to the surrounding living organism. the Among the homopolymers which can be used, particular reference is made to homopolymers of glycollide, L-lactide and Dlactide.Among the copolymers which can be used, reference is made to the copolymerisation products of at least two of these monomers and in particular L-lactide/glycollide, D-lactide/glycollide, DL-lactide/glycollide and L-lactide/D-lactide polymers (including DL-lactide copolymers).

The adjuvant or charge added to the polymer for producing the material according to the invention is given above. Among the preferred charges, reference is made to calcium and magnesium phosphates and in particular tricalcium phosphate Ca3 (PO4)2.

The charge may contain or be constituted by any material, within the appended claims, which has a local stimulating action on bone growth.

In general manner, the charge is present in a quantity which is sufficient to enable it to fulfil the functions indicated hereinbefore. The charge content is generally 0.5 to 30%, based on the polymer weight. However, in numerous applications and particularly when it is desired to produce prosthetic members having a relatively high mechanical strength, for example, osteosynthetic members for long bones, materials are preferably used in which the charge is present in quantities of 0.5. to 5%, based on the polymer weight.

Preferably, the charge is added in the form of a powder whose particles have a dimension which can vary between 1 and 20 micrometres.

The presence of a charge in the prosthetic material of the present application has original action: - the presence of the charge creates in the polymers material microheterogeneities which facilitate the action at certain points of the material, thus modifying the reabsorbability of the latter. Thus, a physical action takes place. The particles of the charge constitute preferred action points on the surface with the formation of microcavities in which the neoformed bone is developed in a preferred manner.

The formation of new bone which is aided is detrimental to the formation of a fibrous capsule which constitutes one of the disadvantages observed with conventional prosthetic members. Finally, in the material of the present application, the charge is distributed throughout the mass and not only on the surface, thus constituting a reserve of basic material for the neoformed bone as the reabsorption progresses.

This in particular leads to the following advantages.

The preferred action at certain points of the material leads to a rough surface, providing a larger interface, which explains the intimate connection of the material and the bone shown by histological section. This also explains how a stable anchoring of the prosthesis is obtained.

The reabsorption of the material is not accompanied by an embrittlement of the bone because due to the habitation of the action points by neoformed osseous tissue, the solidity and mechanical properties are not decreased.

The polymers or copolymers used may be compositions prepared by ring-opening polymerisations in accordance with per se known methods. However, particular attention is preferably paid to the purity of the monomers and to the polymerisation which should be performed in such a way as to obtain high molecular weight materials (for example M.W.) 200,000; intrinsic viscosity CHCl3 > 1.6, polydispersity index I < 2) free or largely free from unpolymerised monomers or oligomers. It is for example possible to purify the crude polymer by washing in a solvent for the monomers and/or oligomers.

The incorporation of the charge (also called adjuvant) can be performed by per se known methods, i.e.: addition of the charge in the form of a powder to polymer solutions, followed by precipitation, melt mixing, mixing of the pulverulent compositions (polymer and charge) followed by hot mixing, introduction of the charge into the polymerisation medium at the start of or during the operations, or any other method which leads to the formation of a homogeneous mixture of the adjuvant and the polymer mass.

As described in greater detail hereinafter, the material according to the invention can be used for making machined or moulded members, or in the form of granules. Thus, the invention relates to the material both in the form of solid blocks and in the form of granules or powder (moulding powder).

In the moulding powders, the particle size of the polymer is preferably 1 to 100 micrometres and more preferably 10 to 50 micrometres.

An aspect of the invention is the application of the material to the production of bone prosthesis wholly or partly based on said material.

Certain special embodiments of this application, together with the advantages provided by the material according to the invention in certain special applications are described hereinafter: a) Production of solid members to be implanted. These members are for examples screws, plates, centromedullary nails for osteosynthesis and for fixing fractured bones in an anatomical position. Subsequent reabsorption prevents the embrittlement of the synthesised bones and also avoids a second operation for removing the object. If the initial mechanical strength of the osteosynthesised assembly is inadequate to withstand the stresses of normal mobilization, it is possible to complete the treatment by temporary immobilization by means of plaster or splints.

Such osteosynthetic members have in particular the following advantages. Their reabsorption avoids a second surgical operation generally performed one year after the fracture to remove the osteosynthetic members. Furthermore, their progressive degradation leads to a gradual transfer of stresses from the osteosynthetic member to the bone, thus obviating embrittlement of the bone protected by the osteosynthetic plate, which is generally observed in conventional osteosynthetic treatments. In fact, the bone becomes "spongy" when the dynamic loads to which it is normally subjected are supported by the much more rigid osteosynthetic plate. Finally, embrittlement of the bone is prevented during the period of filling the hole after removing the metal screws between the bones.

These members can also be used for making good osseous substance losses (following trauma or resection of osteomas, etc). The final fitting of the preoperative members can for example be obtained by grinding them or hot shaping them if the materials are thermoplastic, as is the case with polylactic or polyglycolic acids and their copolymers.

b) Coating solid members with an inert material. This is for example effected on the femoral cauda of a hip prosthesis which permits the temporary fixing by force-fitting in the medullary canal and aids the final fixing by stimulating the osseous growth in contact with the implant. These prosthesis coatings replace the cement interface, which is the main source of unsealing and consequently the main cause of failures of these prostheses.

c) Impregnation of inert porous materials members or coatings made from porous material (ceramic, metal, etc). The reabsorption of the impregnation material stimulates osseous growth in the open pores of the permanent material if the average dimensions of the interconnected pores are of the order of 0.1 to lmm.

d) Bringing into the form of granulesof a few tenths of a mm to a few mm usable for filling osseous substance losses. Initially osseous growths take place in the intergranular spaces, and then continue on the grains being reabsorbed. The improvement of the mechanical strength of the assembly is accelerated by the stimulation of osseous growths.

An aspect of the invention is also the application of the material according to the invention consisting of replacing osseous substances losses and filling them with granules as described hereinbefore.

Thus, the invention in particular relates to the application of the material to the preparation of osseous prosthetic members, whereby solid members are produced or the solid members are coated with an inert material, and alternatively the inert porous matenal members by means of the osseous implant material as defined hereinbefore.

The transformation into finished objects (osteosynthetic plates, screws, etc) can be performed by any known method for the transformation of thermoplastic polymeric materials, such as injection moulding, compression, machining blocks, etc, depending on the characteristics of the materials desired which can also be composite materials.

The invention also relates to bone prosthetic members wholly or partly based on the material according to the invention and in particular solid members, coated members and impregnated members described hereinbefore, as well as composite members in which the mechanical properties of the material according to the invention are improved with biocompatible reinforcing members, for example Dacron (Trade Mark) wires (a polyethylene terephthalate).

It should be noted that the osteosynthetic plates of the invention permit the use, as a complementary treatment, of the technique of electrical stimulation of osteogeny, without deformation of the electrical signal.

The following examples illustrate the invention without limiting the same.

Example 1 (Preparative example) 100g of high purity L-lactide (recrystallised four times in methylethylketone) are placed in a tube for polymerisation. The dry polymerisation catalyst is added (250mg of zinc powder). The mixture is perfectly degassed by hot/cold and nitrogen/vacuum cycles, then the tube is sealed under vacuum. The assembly is placed in a heating enclosure and left at 140"C for 90 hours. The recovered polymer is washed for several hours with dioxan. 90g of a polymer of high viscosity is obtained having all the characteristics of a poly-L-lactide with a high molecular weight (pF = 174"C, crystalline at RX, n CHCl3 = 1.74, Young's modulus E = 360 kg/mm-).

Example 2 20 g of copolymer containing 75% of L-lactic units and 25% of D-lactic units in the form of a very fine powder (particle size 10 to 50 micrometres) and ig of pulverulent dry calcium phosphate are cold mixed in such a way as to obtain a homogeneous powder based on the two constituents.

This powder is placed in a compression mould and a 90 x 50 x 4mm sheet is moulded at a temperature of 100 to 1200C under a pressure of 200 bars, working so as to prevent the formation of air bubbles.

Example 3 20g of glycollide are treated in the same way as described in example 1 but prior to degassing of the monomer/catalyst mixture, the desired calcium phosphate quantity, i.e.

0.6g is added.

Polymerisation is carried out under the same conditions as in example 1, but it is necessary to increase the reaction time to 8 to 10 days.

A macroscopically perfectly homogeneous material is obtained which can then be transformed into prosthetic members by any suitable method.

Example 4 20g of the polymer obtained in example 1 are ground until a fine powder is obtained (diameter of particles 10 to 50 micrometres). This powder is mixed with 0.2g of tricalcium phosphate in the form of a powder (diameter of the paricles 1 to 20 micrometres). A powder is obtained which can be used as a moulding powder. Under the action of heat, this powder can also be transformed into a syrupy mass which by means of extrusion and cutting up gives granules with dimensions of about 0.5 to lmm.

Example 5 On the femoral cauda of a dislocated hip prosthesis of anodised titanium/aluminium/ vanadium alloy is an applied 0.5-1.00 mm layer of a mixture of the polymer of Example I, with 1% of calcium phosphate, in the molten state. The prosthetic member thus coated is then fixed forcibly in the medullary cavity. In a similar manner the thread of an osteosynthetic screw is fixed with the polymer mixture plus adjuvant.

Example 6 The convex part of a cupule (cotyle prothesis) formed of sintered porous ceramic hydroxyapatite:, is impregnated under vacuum with a polymer identical to that obtained in Example 3 but to which is added as a adjuvant not only calcium phosphate but also sodium fluoride. The impregnation mixture is applied in the molten state.

In a similar manner are impregnated (articulated prosthetic joints (knees or shoulders) to be inserted in the respective medullary cavities for anchoring of the prosthesis.

Example 7 In sheep there have been implanted without osteotomy osteosynthetic screws and plates produced with the polymer of Example I mixed with 10% of calcium phosphate.

At the end of the experiment, observation by optical microscope of histological sections has revealed the absence of discernible fibrous capsule in the interfaces. There is thus direct contact between the neoformed bone tissue and the polymer. It should be noted that the osteosynthetic plates and screws were subjected to physiological constraints of the walking action of the animals, which were at liberty.

WHAT WE CLAIM IS: 1. Bone prosthetic material, which comprises a biocompatible, reabsorbable polymer containing a biocompatible adjuvant able to stimulate the reabsorption of the polymer to the benefit of neoformed osseous tissue and comprising a salt selected from: i) salts based on phosphate anions ii) salts based on calcium or magnesium cations iii) borates, carbonates, fluorides or sili cates of lithium, sodium or potassium.

2. A material according to claim 1, wherein the polymer has a molecular weight equal to or above 150,000.

3. A material according to either of the claims 1 and 2, wherein the molecular weight is above 200,000.

4. A material according to any one of the preceding claims, wherein the polymer used is a polymer whose polydispersity index is low, for example below 2.

5. A material according to any one of the preceding claims, wherein the polymer is a homopolymer or copolymer of an hydroxy carboxylic acid.

6. A material according to claim 5, wherein the polymer is chosen from homopolymers of glycollide, L-lactide and Dlactide and copolymers resulting from the copolymerisation of at least two of these monomers, including DL-lactide copolymers.

7. A material according to any one of the preceding claims, wherein the adjuvant contains or is constituted by a material being a salt as set out in claim 7, having a local stimulating action on osseous growth.

8. A material according to any one of the preceding claims, wherein the charge contains a phosphate of calcium or magne

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

RX, n CHCl3 = 1.74, Young's modulus E = 360 kg/mm-).

Example 2

20 g of copolymer containing 75% of L-lactic units and 25% of D-lactic units in the form of a very fine powder (particle size 10 to 50 micrometres) and ig of pulverulent dry calcium phosphate are cold mixed in such a way as to obtain a homogeneous powder based on the two constituents.

This powder is placed in a compression mould and a 90 x 50 x 4mm sheet is moulded at a temperature of 100 to 1200C under a pressure of 200 bars, working so as to prevent the formation of air bubbles.

Example 3 20g of glycollide are treated in the same way as described in example 1 but prior to degassing of the monomer/catalyst mixture, the desired calcium phosphate quantity, i.e.

0.6g is added.

Polymerisation is carried out under the same conditions as in example 1, but it is necessary to increase the reaction time to 8 to 10 days.

A macroscopically perfectly homogeneous material is obtained which can then be transformed into prosthetic members by any suitable method.

Example 4 20g of the polymer obtained in example 1 are ground until a fine powder is obtained (diameter of particles 10 to 50 micrometres). This powder is mixed with 0.2g of tricalcium phosphate in the form of a powder (diameter of the paricles 1 to 20 micrometres). A powder is obtained which can be used as a moulding powder. Under the action of heat, this powder can also be transformed into a syrupy mass which by means of extrusion and cutting up gives granules with dimensions of about 0.5 to lmm.

Example 5 On the femoral cauda of a dislocated hip prosthesis of anodised titanium/aluminium/ vanadium alloy is an applied 0.5-1.00 mm layer of a mixture of the polymer of Example I, with 1% of calcium phosphate, in the molten state. The prosthetic member thus coated is then fixed forcibly in the medullary cavity. In a similar manner the thread of an osteosynthetic screw is fixed with the polymer mixture plus adjuvant.

Example 6 The convex part of a cupule (cotyle prothesis) formed of sintered porous ceramic hydroxyapatite:, is impregnated under vacuum with a polymer identical to that obtained in Example 3 but to which is added as a adjuvant not only calcium phosphate but also sodium fluoride. The impregnation mixture is applied in the molten state.

In a similar manner are impregnated (articulated prosthetic joints (knees or shoulders) to be inserted in the respective medullary cavities for anchoring of the prosthesis.

Example 7 In sheep there have been implanted without osteotomy osteosynthetic screws and plates produced with the polymer of Example I mixed with 10% of calcium phosphate.

At the end of the experiment, observation by optical microscope of histological sections has revealed the absence of discernible fibrous capsule in the interfaces. There is thus direct contact between the neoformed bone tissue and the polymer. It should be noted that the osteosynthetic plates and screws were subjected to physiological constraints of the walking action of the animals, which were at liberty.

WHAT WE CLAIM IS: 1. Bone prosthetic material, which comprises a biocompatible, reabsorbable polymer containing a biocompatible adjuvant able to stimulate the reabsorption of the polymer to the benefit of neoformed osseous tissue and comprising a salt selected from: i) salts based on phosphate anions ii) salts based on calcium or magnesium cations iii) borates, carbonates, fluorides or sili cates of lithium, sodium or potassium.

2. A material according to claim 1, wherein the polymer has a molecular weight equal to or above 150,000.

3. A material according to either of the claims 1 and 2, wherein the molecular weight is above 200,000.

4. A material according to any one of the preceding claims, wherein the polymer used is a polymer whose polydispersity index is low, for example below 2.

5. A material according to any one of the preceding claims, wherein the polymer is a homopolymer or copolymer of an hydroxy carboxylic acid.

6. A material according to claim 5, wherein the polymer is chosen from homopolymers of glycollide, L-lactide and Dlactide and copolymers resulting from the copolymerisation of at least two of these monomers, including DL-lactide copolymers.

7. A material according to any one of the preceding claims, wherein the adjuvant contains or is constituted by a material being a salt as set out in claim 7, having a local stimulating action on osseous growth.

8. A material according to any one of the preceding claims, wherein the charge contains a phosphate of calcium or magne

sium or a mixture of these salts.

9. A material according to any one of the preceding claims, wherein the charge is constituted by tricalcium phosphate.

10. A material according to any one of the preceding claims, wherein the charge is present in a quantity of 0.5 to 30%, based on the polymer weight.

11. A material according to any one of the preceding claims, wherein the charge is present in a quantity of 0.5 to 5%, based on the polymer weight.

12. A material according to any one of the preceding claims, wherein the charge is added to preformed polymer.

13. A material according to claim 12, wherein the charge is added as a powder whose particles have dimensions between 1 and 20 micrometres.

14. A material according to any one of the preceding claims, in the form of solid blocks, granules or powder.

15. An application of the material as defined in any one of claims 1 to 13 wherein the solid osseous prostheses are produced by moulding or machining blocks of the material.

16. An application of the material as defined in any one of claims 1 to 14 wherein osseus substance losses are made good by filling them by means of the material, said material being in the form of granules.

17. Bone prosthetic members constituted wholly or partly by the materials deined in any one of the claims 1 to 14.

18. Bone prosthetic members according to claim 17 in the form of solid members, composite members, coated inert material members or impregnated material members.

19. A bone prosthetic material, substantially as described hereinbefore with particular reference to any one of the Examples.

20. An application of the bone prosthetic material substantially as described hereinbefore with particular reference to any one of the Examples.