WO2009063025A2

WO2009063025A2 - Implant comprising thermoplastic elastomer

Info

Publication number: WO2009063025A2
Application number: PCT/EP2008/065507
Authority: WO
Inventors: Darren Donald Obrigkeit; Atze Jan Nijenhuis
Original assignee: Dsm Ip Assets Bv
Priority date: 2007-11-13
Filing date: 2008-11-13
Publication date: 2009-05-22
Also published as: WO2009063025A3

Abstract

The invention relates to a small joint implant comprising a thermoplastic elastomer comprising a hard phase and soft phase. Preferably, the thermoplastic elastomer comprises a hard phase comprising a polymer chosen from the group consisting of polyester, polyamide, polystyrene, polyacrylate and polyolefin and a soft phase comprising a polymer chosen from the group consisting of polyether, polyester, polyacrylate, polyolefin and polysiloxane.

Description

IMPLANT COMPRISING THERMOPLASTIC ELASTOMER

The invention relates to a small joint implant. The invention further relates to the use of a thermoplastic elastomer (TPE) in a small joint implant and in procedures for small joint arthrodesis, arthroplasty and bone replacement.

Arthritis of the digital, mterpbalangeal, and intertarsa! joints is a painful condition arising from bone-on-bone contact duππg articulation of tho metacarpal or metatarsal bone against

1 ) the proximal phalangeal bone or proximal phalanx in the case of the metacarpophalangeal (MCP) or metatarsophalangeal (MTP) joint

2) tho proximal phalangeal bone against tho middle phalangeal bone or phalanx in the case of the proximal interphalangeal or PIP joint, or

3) the middle phalangeal bone against the distal phalangeal bone or distal phalanx in the case of the distal interphalangeal or DIP joint. Progression of the disoaso can also load to misalignment of tho bones at those joints.

Known treatments for arthritis include debridement of the articulating surfaces, excision or fusion of the affected joint or replacement of the joint with a prosthesis or arthroplasty.

Flexible, one-piece joint replacements have received wide medical acclaim and improved markedly the prognosis for restriction patients. In particular, one-piece designs, for example those listed in Table 1 , ease surgical implantation and produce a more stable implant with less joint dislocation.

Such one-piece joint replacements can for example be made of silicone rubbers, optionally with other materials. Oπo currently used arthroplasty implant, known as the Swanson prosthesis, has a central generally U-shaped hinge element and a pair of oppositely projecting intramedullary stems which are in use seated in holes drilled for tht? purpose in the ends of the bones which meet at the joint Tho device is made of a flexiblo silicone Apart from the danger of breakage or fracture of such a prosthesis, several other complications are associated with silicone rubbers. For example, high performance silicone rubber is used in space-filler type joints in artificial joint replacement. One of the problems that occurs with these artificial replacements is that they can fail because the silicone rubber used for their fabrication is a relatively weak material and shown to break apart and segment ("Preparation and bioactivity of novel multi-block thermoplastic elastomer/tricalcium phosphate composites", M. El Fray, Journal of Materials Science: Materials in Medicine, Volume 18, Number 3, March 2007, pp. 501-506(6)). Additionally, A ha& been observed that the silicone may be abraded with the resuit that silicone particles are produced Such parados may cause irritation to, or ovon destruction of the surrounding tissue. For example, silicone synovitis (long-term reaction of the osseous bed around silicone implants,

Publisher Springer,

Berlin / Heidelberg, 1434-3916 (Online) Volume 1 10. Number 3 / April. 1991 ) may result, as well as destructive bone changes, dislocation, adsorption of oxidized lipids, which causes swelling and slight dimensional change, and insufficient chemical stability of siloxane bonds in specific physiological environments. Moreover, immunological reactions to silicone rubber can also develop that can be local, regional due to silicone migration, or systemic. Migration of silicone rubber has been documented on numerous occasions in the literature. Systemic reactions, such as acute renal insufficiency and respiratory compromise, have been reported following the introduction of silicone rubber into the body (Biomedical application of commercial polymers and novel polyisobutylene-based TPE for soft tissue replacement, J. E. Puskas, Biomacromolecules, Vol. 5-4, July/Aug 2004).

Other currently used arthroplasty devices consist of two components with cooperating convex and concave surfaces which articulate against one another in the joint. Examples are listed in Table 2. The components are attached to their respective bones by rigid intramedullary stems which seat in holes formed in the ends of the bones. In some cases, a single component is fixed to one bone end to articulate against a shaped end of the other bone. In such arrangements it is possible for the stem(s) to work loose, compromising the effectiveness of the arthroplasty. Other articulating designs replace only one half of the joint; these also fail to provide the stability of a one-piece implant. In addition to silicone rubbers, other non-elastomeric engineering plastics such as PEEK can be used. Small joint designs made of such non-elastomeric engineering plastics require a pivot point in the implant to function. These do not, therefore, represent a significant improvement over existing pyrolytic carbon or other two-piece, rigid implant systems. A need therefore remains for an artificial implant for small joint arthroplasty, for example a metacarpophalangeal implant (MCP), a proximal (PIP) or distal (DIP) interphalangeal implant, a trapezium/metacarpal spacer implant, a metatarsalphalangeal implant (MTP), hammertoe or a great toe implant.

The aim of the invention is therefore to provide an implant for a small joint that does not show the aforementioned disadvantages, or at least shows them to a lesser extent.

This aim is achieved with a small joint implant comprising a thermoplastic elastomer (TPE) comprising a hard phase and soft phase.

Surprisingly it has been found that the small joint implant according to the invention has particular superior flex fatigue performance and crack growth resistance, high wear resistance, low creep, low compression set, high dimensional stability and high resistance to moisture, such that a compliant durable implant can be made. Such implant may comprise only one part without the need to articulate to preserve motion, such that wear due to articulation is avoided. Alternatively the implant may consist of two or more parts of which at least one part comprises the TPE according to the invention. As such, because of its superior adhesion properties the TPE can be combined with other elastomeric materials of different stiffness and flexibility and/or hard materials, such as metals and higher modulus polymers.

Other advantages of using the TPE according to the invention are that it has a crystalline (hard phase) component which is very resilient to mechanical forces, it can easily be processed to provide a variety of designs, by for example injection molding and that the shape of the implant according to the invention can easily be adapted during surgery to adapt it to the patient's anatomy.

The small joint implant according to the invention comprises a thermoplastic elastomer (TPE) comprising a hard phase and a soft phase.

The hard phase in the TPE comprises a rigid polymer phase with a melting temperature (Tm) or a glass transition temperature (Tg) higher than 35 ⁰C. The soft phase in the TPE comprises a flexible, amorphous polymer phase with a Tg lower than 35 ⁰C, preferably lower than 0 ⁰C. The TPE, used according to the invention, comprises, for example, blends of hard phase polymers with soft phase polymers and block copolymers. The hard and the soft phase can comprise one polymer type, but can also be composed of a mixture of two or more polymeric materials. Examples of TPE's according to the invention are styrenic TPE's, polyolefin-based TPE's including polyolefin blends, elastomeric alloys, thermoplastic polyurethanes, thermoplastic copolyesters and thermoplastic polyamides, TPE's based on halogen-containing polyolefins, polyether- ester elastomers, ionomeric TPE's and polyacrylate-based TPE's.

Preferably, the TPE, used according to the invention, comprises a hard phase comprising a polymer chosen from the group consisting of polyester, polyamide, polystyrene, polyacrylate and polyolefin and a soft phase comprising a - A -

polymer chosen from the group consisting of polyether, polyester, polyacrylate, polyolefin and polysiloxane.

More preferably, the TPE, used according to the invention, is a block- copolymer. When the TPE is a block-copolymer, the TPE used in the small joint implant comprises a thermoplastic elastomer comprising hard blocks and soft blocks, wherein the hard blocks comprise a polymer chosen from the group consisting of polyester, polyamide, polystyrene, polyacrylate and polyolefin and the soft blocks comprise a polymer chosen from the group consisting of polyether, polyester, polyacrylate, polyolefin and polysiloxane. Examples of TPE block-copolymers according to the invention are block-copolyesterester, block-copolyetherester, block-copolycarbonateester, block-copolysiloxaneester, block-copolyesteramide, block-copolymer containing polybutylene terephthalate (PBT) hard blocks and poly(oxytetramethylene) soft blocks, block-copolymer containing polystyrene hard blocks and ethylene butadiene soft blocks (SEBS).

The hard blocks in the thermoplastic elastomer consist of a rigid polymer, as described above, with a Tm or Tg higher than 35 ⁰C. In principle the different polymers as described above can be used as the hard blocks. Here and in the rest of the description a polycarbonate is understood to be a polyester. Also copolymers of esters, amides, styrenes, acrylates and olefins can be used as the hard polymer block as long as the Tm or Tg of the hard polymer block is higher than 35 ⁰C.

Preferably, the hard block of the TPE is a polyester block.

More preferably, in the TPE comprising a hard polyester block, the hard block consists of repeating units derived from at least one alkylene glycol and at least one aromatic dicarboxylic acid or an ester thereof. The alkylene group generally contains 2-6 carbon atoms, preferably 2-4 carbon atoms. Preferable for use as the alkylene glycol are ethylene glycol, propylene glycol and in particular butylene glycol. Terephthalic acid, 2,6-naphthalenedicarboxylic acid and 4,4'-diphenyldicarboxylic acid are very suitable for use as the aromatic dicarboxylic acid. Combinations of these dicarboxylic acids, and/or other dicarboxylic acids such as isophthalic acid may also be used. Their effect is to influence the crystallization behaviour, e.g. melting point, of the hard polyester blocks.

Most preferably, the hard block is polybutyleneterephthalate. The soft blocks in the thermoplastic elastomer consist of a flexible polymer, as described above, with a Tg lower than 35 ⁰C. In principle the polymers as described above can be used as the soft blocks. Here and in the rest of the description a polycarbonate is understood to be a polyester.

Also copolymers of ethers, esters, acrylates, olefins and siloxanes can be used as the soft polymer block as long as the Tg of the soft polymer block is lower than 35 ⁰C.

Preferably, the soft block comprises a polyester or a polyether; more preferably an aliphatic polyester or polyether. A particular advantage of TPE's comprising polyester, or polyether soft blocks is that aliphatic polyesters, and polyethers feature a high chemical stability. Especially, alkylene carbonates and aliphatic polyesthers are preferred as the soft block, which result in thermoplastic elastomers with particularly low moisture sensitivity and favourable adhesive properties. Preferably, the soft blocks in the TPE are derived from at least one alkylene carbonate and optionally, a polyester made up of repeating units derived from an aliphatic diol and an aliphatic dicarboxylic acid.

The alkylene carbonate can be represented by the formula

O

where

R = H, alkyl or aryl, x = 2 - 20. Preferably, R = H and x = 6 and the alkylene carbonate is therefore hexamethylene carbonate.

The aliphatic diol units are preferably derived from an alkylenediol containing 2 - 20 C atoms, preferably 3 - 15 C atoms, in the chain and an alkylenedicarboxylic acid containing 2 - 20 C atoms, preferably 4 - 15 C atoms. More preferably, the soft block comprises a polycarbonate.

It has been found that, with respect to their use in the small joint implant according to the invention, in particular the thermoplastic block-copolyesters (TPC-ET, as defined in ISO 18064: 2003) have many advantages over silicone rubber, because of their better mechanical properties, such as in particular superior flex fatigue performance as well as crack growth resistance, high wear resistance, low creep, low compression set, high dimensional stability and high resistance to moisture.

Most preferably, the TPE comprises a hard block comprising polybutyleneterephthalate and a soft block comprising polycarbonate. Optionally, this TPE is chain-extended with diisocyanate. Examples and the preparation of block-copolyether esters are for example described in the Handbook of Thermoplastics, ed. O.OIabishi, Chapter 17, Marcel Dekker Inc., New York 1997, ISBN 0-8247-9797-3, Thermoplastic Elastomers, 2nd Ed., Chapter 8, Carl Hanser Verlag (1996), ISBN 1-56990-205-4, and the Encyclopedia of Polymer Science and Engineering, Vol. 12, pp.75-1 17, and the references contained therein.

The ratio of the soft and hard blocks in the TPE used in the small joint implant according to the invention may generally vary within a wide range but is in particular chosen in view of the desired modulus of the TPE. The desired modulus will depend on the structure and size (e.g. thickness) of the small joint implant and the functionality of the TPE in it. Generally, a higher soft block content results in higher flexibility and better toughness.

The TPE according to the invention may contain one or more additives such as stabilizers, anti-oxidants, colorants, fillers, binders, fibers, meshes, substances providing radiopacity, surface active agents, foaming agents, processing aids, plasticizers, biostatic/biocidal agents, and any other known agents which are described in Rubber World Magazine Blue Book, and in Gaether et al., Plastics Additives Handbook, (Hanser 1990). Suitable examples of fillers, e.g. radiopaque fillers and bone-mineral based fillers, and binders are described in U.S. Patent Number 6,808,585B2 in columns 8-10 and in U.S. Patent Number 7, 044,972B2 in column 4, I. 30-43, which are herein incorporated by reference.

Suitable commercially available TPE's include Arnitel^® TPE (DSM Engineering Plastics), in particular Arnitel^® E (polyether ester, PTMEG), Arnitel^® C (polycarbonate-ester, PHMC) and Arnitel^® P (polyether ester, polyols, polypropylene and polyethylene). Particularly suitable Arnitel^® grades include 55D, EL250, EM400, EM450, EM550, EM630, EL740, PL380, PL381 , PM381 , PL580, PM581 , 3103, 3104, and 3107.

TPE's, in particular thermoplastic block polyesters have been the subject of numerous FDA regulatory approvals. Specifically, Arnitel^® copolyesters have been listed under the Drug Master Files 13260, 13261 , 13263, 13264, 13259, and 13262. Additionally, these compositions have been cleared for use in permanent implants (510(k) K990952, K896946). According to the FDA MAUDE database, adverse events dating back to prior April, 2000 are mild and due to mechanical failure (see catalog number 8886441433, 447071 , 8886471011 V, and 8886470401 ). The absence of adverse effects due to material confirms the long-term biocompatibility of these compositions.

Arnitel^® E grades are in compliance with the code of Federal regulation, issued by the Food and Drug Administration (FDA) 21 CFR 177.2600 (rubber articles for repeated use) in the USA, the so-called FDA approval. Moreover, US Pharmacopoeia approvals were received for the following Arnitel^® grades: EM400, EM450, EM550, EM740, PL580 and 3104 (USP Class Vl), and PL380 and PM381 (USP Class IV).

Moreover multiblock poly(aliphatic/aromatic ester) (PED) copolymers as described in M. El Fray and V. Altstadt, Polymer, 44 (2003) pp. 4643-4650 can suitably be used as the TPE according to the invention. The small joint implant according to the invention can be produced in many different ways. Known techniques include (co-)injection molding, (co-)extrusion molding, blow molding or injection overmolding.

The temperature and other processing conditions at which the TPE can best be processed depends on the melting temperature, the viscosity and other rheological properties of the TPE and can easily be determined by the person skilled in the art once said properties are known. The above mentioned Arnitel^® grades have melting temperatures (measured according to ISO 11357-1/-3) between 180 and 221 ⁰C and are preferably processed at temperatures between 200 and 250 ⁰C.

The TPE's according to the invention, in particular Arnitel^® TPE's, can be sterilized by any known means.

The TPE's according to the invention can be cut with a fluid jet for customizing the implant shape to the patient's anatomy. Such fluid jets are described in patent US6960182 and are commercially provided by Hydrocision, Inc. (Billerica, MA). The ability to customize an implant with a fluid jet represents a significant advance over the current standard of practice, where grinding tools (e.g. Dremel) are used to abrade the surfaces of implants, which result in damaged implant surfaces, possible introduction of wear particles in the operating room, etc. This is particularly relevant for small joints. In fact the FDA database often cites wrong finger size as a reason for revision surgery (FDA MAUDE database). For small joint applications, implants are subjected to repeated cycles and rotations. For this type of applications hard-soft block systems are unique because they have a crystalline (hard block) component which is very resilient to mechanical forces. Moreover they are easily processible to provide a variety of designs and possess exceptional flex fatigue, which can be measured according to e.g. ISO 132 in which Arnitel^® TPE has been demonstrated to survive an excess of 15 million cycles.

TPE's offer the unique advantage of improved wear and fatigue resistance over traditional materials (e.g. silicone). This allows the creation of an implant with both 1.) the superior stability of a one-piece implant and 2.) the superior durability of a two-piece performance. Therefore, the use of TPE is especially advantageous in one-piece prostheses requiring good flex fatigue resistance such as MCP, PIP, DIP, MTP, Great Toe, etc (Tables 1 and 2). For these implants, improved flex fatigue resistance especially reduces the incidence of implant fracture at the hinge (for example when used in implant parts as disclosed in US387559, Fig. 2, 12; GB1192960, Fig. 1 , 12; US4871367, Fig. 2, 14; Fig 3. 34; US5824095, Fig. 2, 11 ; US6869449B2, Figs. 1-4, 10; US4634445 3; US4758080, Figs 1-2, 2; US6319284B1 , Fig 2-7, 20). However, it also provides significantly harder grades in wear-sensitive applications such as trapezium implants, hammertoe implants and CMC implants. Examples of these implants are described in US20G5/G251265A1 ; US5326364; US4164793; US4198712, US4955915 and US4969908. Small joint implants comprising TPE's according to the invention can be produced in radiopaque versions for easy visualization of implant under X-ray. This can be accomplished by one skilled in the art of polymeric fillers and biocompatible materials. For example, barium sulfate, zirconium dioxide, hydroxyapatite, tricalcium phosphate, and other substances which impart radiopacity are described in US6808585 and US7044972 and incorporated here by reference.

Moreover it is possible to produce a fully MRI/CT - compatible implant by making it entirely of the TPE according to the invention. This is particularly relevant for small joints, where soft tissues are critical for providing proper joint function. Therefore, the ability to image soft tissues with MRI and/or CAT scans is advantageous. This would provide an advantage for implants which are currently also only offered as metal implants (for example US4969908, US 4966860).

As already mentioned above, a particular advantage of the use of a TPE according to the invention, in particular a block-copolyester, is its very good adhesion to different materials, for example to a different TPE, e.g. a TPE with a different stiffness or modulus, or a metal. This makes the material particularly suitable for application with for example a metal, for example Ti₆AI₄V, in overmolding. This property is expressed as a high peel strength. Preferably the peel strength is higher than 6 N/cm, measured according to ISO/IEC standard 7810. This is particularly advantageous for implants which employ grommets as described, for example, in US6319284B1 Fig. 4, 84, 86.

In Biomaterials, 1992 13(9), pp 585-593 it was demonstrated that the hydrolytic stability of the TPE accordfing to the invention clearly outperforms that of polyurethanes.

The use of TPE's according to the invention provides the ability to meet requirements without articulating surfaces, which minimizes the occurrence of wear, particles and/or reactions.

Examples of known small joint implant designs that can be made partially or completely from the TPE according to the invention, or that can be partially or completely overmolded with the TPE according to the invention are a metacarpophalangeal implant, a proximal or distal interphalangeal implant, a trapezium/metacarpal spacer implant and a metatarsalphalangeal implant. A more detailed overview and specific examples of said known small joint implants, of which some are commercially available, are listed in Table 1 and 2.

The known one-piece small joint implants according to Table 1 can be entirely made of TPE. The two-component small joint implants according to Table 2 can be entirely or partly made of the TPE according to the invention, for example by using different grades of TPE with different properties, or by combining TPE with other materials, for example metals, PEEK or pyrolitic carbon.

JaMβ..l..E.xdrnpj.es.M..QfIβ-pigcβjojnlτ.epJacemenls,.

Table 2. Examples of two-component joint replacements.

The present invention will now be illustrated by the following examples that by no means limit the scope of the invention. EXAMPLES

Materials

• Arnitel^® 55D (hard block: polybutylene terepthalate (PBT), soft-block: polycarbonate, modulus 140 MPa) from DSM N.V.

• Arnitel^® EL250 (hard block polybutylene terepthalate (PBT), soft-block: polytetramethyleneoxide (PTMO), modulus 25 MPa) from DSM N.V.

• Arnitel^® EM400 (hard block: polybutylene terepthalate (PBT), soft-block: polytetramethyleneoxide (PTMO), modulus 50 MPa) from DSM N.V. • Arnitel^® EM460 (hard block polybutylene terepthalate (PBT), soft-block: polytetramethyleneoxide (PTMO), modulus 100 MPa) from DSM N.V.

• Arnitel^® EM550 (hard block polybutylene terepthalate (PBT), soft-block: polytetramethyleneoxide (PTMO), modulus 200 MPa) from DSM N.V.

• Arnitel^® EM630 and 630-H (hard block polybutylene terepthalate (PBT), soft-block: polytetramethyleneoxide (PTMO), modulus 310 MPa) from DSM

N.V. (H means heat stabilized)

• Arnitel^® EM740 (hard block polybutylene terepthalate (PBT), soft-block: polytetramethyleneoxide (PTMO), modulus 1 100 MPa) from DSM N.V. • Arnitel^® PL380 (hard block polybutylene terepthalate (PBT), soft block:

Polyethyleneoxide-polypropyleneoxide-polyethyleneoxide (PEO-PPO-PEO), modulus 60 MPa from DSM N.V.

• Elastollan^® 119OA TPU; (a polyether-urethane polymer, modulus 31 MPa) from BASF A. G. • Elastollan^® 1195A TPU; (a polyether-urethane polymer, modulus 41 MPa) from BASF A. G.

• Silastic HP100; a silicone rubber of Dow Corning.

• Silastic Q7-4565; a silicone rubber of Dow Corning.

Example I Crack-growth resistance

The crack-growth resistance was measured according to ASTM D 1052 on a Ross tester, measuring the resistance of a material against crack growth under cyclic loading. A through thickness crack (2.5 mm width) was made in a sample with dimensions 25.4 x 6.4 x 152.4 mm³. The sample was flexed between 0° and 90° and the growth of the crack was monitored as a function of the number of cycles.

According to this test, Silastic Q7-4565 shows 500 % increase in crack length after 250,000 cycles (D. T. Hutchinson et al., J. Biomed. Mat. Res., vol 37, no. 1 , pp 94-99 (1998)). Silastic HP100 shows 500 % increase in crack length after 390,000 cycles, and complete failure of the sample after 950,000 cycles (K.M. Savory et al., J. Biomed. Mat. Res., vol 28, No. 10, pp 1209-1219 (1994)).

For a TPE block copolymer, with a polyester hard block and a polyether soft block (modulus 35 MPa) 500 % crack length increase is not reached after 1 ,000,000 cycles. For a TPE block copolymer, with a polyester hard block and a polyether soft block (modulus 200 MPa) 500 % crack length increase is reached after 800,000 cycles.

The above results show that both thermoplastic polyether ester elastomer block copolymers, which are examples of the TPE according to the invention, perform significantly better on crack-growth resistance than the silicone rubbers tested.

Example Il Biocompatibility

Samples of Arnitel^® EL250, EM400, and EM740 were tested under

GLP conditions according to ISO 10993 parts 3, 5, 6, 7, 10, and 11 : ISO10993-3 Tests for genotoxicity, carcinogenicity, and reproductive toxicity.

ISO 10993-5 Tests for in vitro cytotoxicity.

ISO 10993-6 Test for local effects after implantation.

ISO 10993-7 Ethylene oxide residuals.

ISO 10993-10 Test for irritation and delayed-type hypersensitivity. ISO 10993-11 Test for systemic toxicity

Each of these material grades passed all of the above biocompatibility tests, demonstrating the safety of Arnitel^® TPE as an implant material.

Particularly advantageous are the favorable foreign body response and fibrous tissue encapsulation reactions observed in ISO 10993-6.

Example III Sterilization testing

Samples of Arnitel^® types 55D, EL250, EM550, EM630, EM630-H,

EM400, EM460, EL630, and EL740 were tested for the effects of gamma sterilization up to 100 KGray (roughly 4 times a typical sterilization dose). These samples were subsequently mechanically tested to determine the effects on E-modulus, Stress at Break, and Strain at Break. In all instances little or no changes in the material properties were observed.

Example IV Flex fatigue testing

Arnitel^® EM400 and Elastollan^® 1190A TPU were tested according to the ISO 132 deMattia test. The results showed comparable crack growth numbers for Arnitel^® EM400 and Elastollan^® 1 190A.

Example V Comparison of tensile and creep properties The tensile modulus and the creep properties were determined at room temperature according to ISO 527. The sample used was type 5A.

The determined tensile modulus of three materials was similar.

Stress level: 2.5 MPa

Material Initial strain Strain after 1 hour

Arnitel^® PL380 5 .8 % 7.4 %

Arnitel^® EM400 6 .9 % 9.0 %

Elastollan^® 5 .5 % 10.5 % 1195A Stress level: 5.0 MPa

Example Vl Dynamic creep properties

Experiment

Cylindrical samples having a 13 mm diameter and 6 mm height were mounted between the plates of a MTS 810-11 servo-hydraulic tensile tester. The samples were loaded force controlled by a harmonically time varying compressive force. The cycle frequency of the force signal was 0.25 Hz. The maximum compressive stress during a cycle was 4 MPa whereas the minimum compressive stress was 0.4 MPa. The experiments were carried out in an oven at 37°C. The stress levels that were applied were derived from ASTM 2423-05, and were chosen to be higher by a factor 4.

Results

The sample compression at the maximum and minimum stress during a cycle was monitored as a function of cycle number. The results are summarized below.

Arnitel^® EM400

Claims

1. Small joint implant comprising a thermoplastic elastomer comprising a hard phase and a soft phase.

2. The small joint implant according to claim 1 , wherein the hard phase comprises a polymer chosen from the group consisting of polyester, polyamide, polystyrene, polyacrylate and polyolefin and the soft phase comprises a polymer chosen from the group consisting of polyether, polyester, polyacrylate, polyolefin and polysiloxane.

3. The small joint implant according to Claim 2, wherein the hard phase and the soft phase are present in a block copolymer, wherein the hard blocks are chosen from the group consisting of polyester, polyamide, polystyrene, polyacrylate and polyolefin and the soft blocks are chosen from the group consisting of polyether, polyester, polyacrylate, polyolefin and polysiloxane.

4. The small joint implant according to Claim 3, wherein the hard block is a polyester hard block.

5. The small joint implant according to Claim 4, wherein the polyester hard block consists of repeating units derived from at least one alkylene glycol and at least one aromatic dicarboxylic acid or an ester thereof.

6. The small joint implant according to Claim 5, wherein the polyester hard block is polybutyleneterephthalate.

7. The small joint implant according to any one of Claims 1-6, wherein the soft block is an aliphatic polyester or polyether.

8. The small joint implant according to Claim 7, wherein the soft block comprises polycarbonate.

9. The small joint implant according to Claim 8, wherein the hard block is polybutyleneterephthalate and the soft block comprises polycarbonate.

10. Use of a thermoplastic elastomer, comprising a hard phase and soft phase, in a small joint implant.

11. Use of a thermoplastic elastomer, comprising a hard phase and soft phase, in procedures for arthrodesis, arthroplasty and bone replacement.

12. The small joint implant according to any one of Claims 1-9 or the use according to claim 10 or 11 , wherein the small joint implant is applied for small joint arthroplasty, for example a metacarpophalangeal implant, a proximal interphalangeal implant, a trapezium/metacarpal spacer implant or a metatarsalphalangeal implant.

13. Use of the small joint implant according to according to any one of Claims 1-9 in procedures for arthrodesis, arthroplasty and bone replacement.