PLUG-SHAPED IMPLANT FOR THE REPLACEMENT AND REGENERATION OF BIOLOGICAL TISSUE AND METHOD FOR PREPARING THE IMPLANT

Abstract

A non-biodegradable plug-shaped implant (1) for the replacement and regeneration of biological tissue is described. The implant comprises a base section (2) configured for anchoring in bone tissue, and a top section (4) configured for growing cartilage tissue onto and into. The top section comprises a thermoplastic elastomeric material, which is porous. The thermoplastic elastomeric material comprises a linear block copolymer comprising urethane and urea groups, and may be substantially free of an added peptide compound having cartilage regenerative properties. The base section material further comprises one of a biocompatible metal, ceramic, mineral, such as phosphate mineral, and polymer, optionally a hydrogel polymer, and combinations thereof, wherein the thermoplastic elastomeric material further comprises carbonate groups.

Claims

1. A non-biodegradable implant for the replacement and regeneration of biological tissue in the shape of a plug, comprising a base section configured for anchoring in bone tissue, and a top section configured for replacing cartilage tissue of an intermediate and deep zone of a cartilage layer, and for growing cartilage tissue onto and into, thus regenerating a superficial zone of the cartilage layer, wherein the top section comprises a porous thermoplastic elastomeric material, wherein the thermoplastic elastomeric material comprises a linear block copolymer comprising urethane and urea groups, and wherein the base section material comprises one of a biocompatible metal, ceramic, mineral, and polymer, optionally a hydrogel polymer, and combinations thereof, wherein the thermoplastic elastomeric material further comprises carbonate groups.

2. The implant according to claim 1, wherein the thermoplastic elastomeric material is substantially free of an added peptide compound having cartilage regenerative properties.

3. The implant according to claim 1, wherein the thermoplastic elastomeric material comprises a poly-urethane-bisurea-alkylenecarbonate.

4. The implant according to claim 1, wherein the thermoplastic elastomeric material is aliphatic.

5. The implant according to claim 1, wherein the porous elastomeric material has an elastic modulus at room temperature of less than 8 MPa.

6. The implant according to claim 1, wherein the base section comprises a core of non-porous base section material and a circumferential shell of porous base section material, wherein the shell has a thickness that is less than 10% of a largest diameter of the base section.

7. The implant according to claim 1, wherein the base section extends between a top surface and a bottom surface, and comprises a layer of porous base section material, wherein the layer is adjacent to the top surface and has a thickness that is less than 10% of a largest height of the base section, and wherein the pores of the base section material in the layer comprise the biocompatible elastomeric material.

8. The implant according to claim 1, wherein the base section material comprises a metal, selected from titanium, zirconium, chromium, aluminum, stainless steel, hafnium, tantalum or molybdenum, and their alloys, or any combination thereof.

9. The implant according to claim 1, wherein the base section material comprises a ceramic or mineral, selected from oxides, nitrides, carbides and borides, or any combination thereof.

10. The implant according to claim 1, wherein the base section material comprises a (hydrogel) polymer, selected from collagen, poly(lactic-co-glycolic acid) (PLGA), polylactic acid (PLA), polycaprolactone (PCL), polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), polyacrylamide, polyurethane, polyethylene glycol (PEG), chitin, poly(hydroxyalkyl methacrylate), water-swellable N-vinyl lactams, starch graft copolymers, and derivatives and combinations thereof.

11. The implant according to claim 1, wherein the base section material comprises a non-hydrogel polymer.

12. The implant according to claim 11, comprising a substantially non-porous polyaryletherketone polymer with a porosity of less than 20%, relative to the total volume of the polyaryletherketone polymer.

13. The implant according to claim 11, wherein the base section comprises a non-porous polyaryletherketone polymer.

14. The implant according to claim 1, further comprising a contrast or radiopharmaceutical agent or body for medical imaging, preferably provided in the base section.

15. The implant according to claim 1, wherein the top surface of the base section comprises irregularities or undulations.

16. The implant according to claim 1, wherein the base section comprises a centrally located cavity that comprises the elastomeric material.

17. The implant according to claim 1, wherein the base section comprises an outer surface having irregularities or undulations.

18. The implant according to claim 1, wherein a height of the base section, and a height of the porous top section are selected such that a top surface of the implant comes to lie below a top surface of cartilage present on a osteochondral structure when implanted.

19. The implant according to claim 1, wherein a height of the base section, and a height of the porous top section are selected such that a bottom surface of the top section comes to lie about level with a bottom surface of cartilage present on a osteochondral structure when implanted.

20. The implant according to claim 1, comprising a top section with a slightly curved top surface, having a radius of curvature in a sagittal plane and/or in a medial-lateral plane ranging from 15 mm to 150 mm.

21. The implant according to claim 1, wherein the base section material comprises a reinforcing material selected from the group consisting of fibrous or particulate polymers and/or metals.

22. A method for the preparation of an implant, comprising: a) providing in a mold at room temperature a base section that comprises base section material comprising one of a biocompatible metal, ceramic, mineral, and polymer, optionally a hydrogel polymer, and combinations thereof; and granules of a thermoplastic elastomeric material on top of the base section, the thermoplastic material comprising a linear block copolymer comprising urethane and urea groups; b) closing the mold and heating the above assembly to a temperature of between 100° C. and 250° C. under a pressure of between 1 and 2 GPa, such that the thermoplastic elastomeric material melts and fuses with the base section; and c) cooling the assembly to room temperature to consolidate the thermoplastic elastomeric material and opening the mold; d) providing a top section of the thermoplastic elastomeric material with pores either before or after opening the mold.

23. The method according to claim 22, wherein the thermoplastic elastomeric material is substantially free of an added peptide compound having cartilage regenerative properties

24. The method according to claim 22, wherein after step b) the mold is opened and additional granules of the thermoplastic elastomeric material are added to the mold, and step b) is repeated.

25. Osteochondral structure comprising an implant in accordance with claim 1, wherein a top surface of the implant lies below a top surface of the cartilage layer on the osteochondral structure.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0069] The invention will now be further elucidated by the following figures and examples, without however being limited thereto. In the figures:

[0070] FIGS. 1A to 1D show a schematic side view of four embodiments of an exemplary implant according to the present invention;

[0071] FIG. 2A shows a schematic perspective view of a base section according to an embodiment of the invention;

[0072] FIG. 2B shows a schematic cross-section of the embodiment of FIG. 2A;

[0073] FIGS. 2C and 2D show a schematic detailed view of parts B and C of the embodiment of FIG. 2B;

[0074] FIG. 3 shows a schematic representation of a possible synthetic route to the thermoplastic polycarbonate material according to an embodiment of the invention;

[0075] FIG. 4 shows a .sup.1H-NMR spectrum of the thermoplastic polycarbonate material according to an embodiment of the invention;

[0076] FIGS. 5A to 5C show DSC thermograms of the thermoplastic polycarbonate material according to an embodiment of the invention at different heating rates;

[0077] FIGS. 6A to 6C show a schematic representation of a defect in an osteochondral structure (6A), the osteochondral structure comprising an implant according to an embodiment of the invention (6B) and the same osteochondral structure after on-/ingrowth of cartilage (6C);

[0078] FIGS. 7A to 7D show a schematic side view of four embodiments of an implant according to yet another embodiment of the present invention; and finally

[0079] FIGS. 8A to 8C show a schematic representation of a defect in an osteochondral structure (8A), the osteochondral structure comprising an implant according to another embodiment of the invention (8B) and the same osteochondral structure after on-/ingrowth of cartilage (8C).

[0080] Referring to FIG. 1A, a side view of an embodiment of an exemplary implant according to the present invention is shown. The implant 1 in the shape of a plug comprises a base section 2, configured for anchoring in bone tissue, and a porous top section 4 configured for replacing cartilage tissue and growing cartilage tissue onto and into. The top section 4 comprises a thermoplastic elastomeric material in porous form. The thermoplastic elastomeric material in this embodiment comprises a poly-urethane-bisurea-hexylenecarbonate, the preparation and properties whereof will be elucidated further below. The base section 2 comprises a non-porous polyaryletherketone polymer, which, in the embodiment shown is a non-porous PEKK polymer. The implant 1 is cylindrical and has a diameter 10 of 6 mm. The height 20 of the base section 2, and the height 40 of the top section 4 add up to a total height of 6 mm.

[0081] FIG. 1B schematically represents a side view of another embodiment of an implant according to the present invention. The embodied implant 1 in the shape of a plug again comprises a base section 2, configured for anchoring in bone tissue, and a top section 4 configured for replacing cartilage tissue and growing cartilage tissue onto and into. The top section 4 comprises the same porous poly-urethane-bisurea-hexylenecarbonate material. The base section 2 comprises a substantially non-porous PEKK polymer with a porosity of less than 20%, relative to the total volume of the PEKK polymer. The base section 2 of this embodiment in particular comprises a core 21 of non-porous PEKK polymer and a circumferential shell 22 of porous PEKK polymer. The shell 22 has a thickness 23 of about 8% of the diameter 10 of the base section 2 (and implant 1). The base section 2 further extends between a top surface 24 and a bottom surface 25, and comprises a layer 26 of porous PEKK polymer, which layer 26 is adjacent to the top surface 24 and has a thickness 27 of about 8% of the height 20 of the base section 2. The pores of the PEKK polymer in the layer 26 comprise the biocompatible poly-urethane-bisurea-hexylenecarbonate which originates from the top section 4 and has infiltrated the pores of the PEKK polymer in the layer 26 during manufacturing. A method for manufacturing the implant will be elucidated further below. As with the embodiment of FIG. 1A, the implant 1 is cylindrical and has a diameter 10 of 6 mm. The height 20 of the base section 2, and the height 40 of the top section 4 add up to a total height of 6 mm.

[0082] FIG. 1C schematically represents a side view of yet another embodiment of an implant according to the present invention. The embodied implant 1 in the shape of a plug again comprises a base section 2, configured for anchoring in bone tissue, and a top section 4 configured for replacing and growing cartilage tissue onto and into. The top section 4 comprises a poly-urethane-bisurea-hexylenecarbonate material, which is porous in the top section 4. The base section 2 comprises a substantially non-porous PEKK polymer with a porosity of less than 20%, relative to the total volume of the PEKK polymer. The base section 2 of this embodiment in particular extends between a top surface 24 and a bottom surface 25, and comprises a layer 26 of porous PEKK polymer, which layer 26 is adjacent to the top surface 24 and has a thickness 27 of about 8% of the height 20 of the base section 2. The pores of the PEKK polymer in the layer 26 comprise the biocompatible poly-urethane-bisurea-hexylenecarbonate which originates from the top section 4 and has infiltrated the pores of the PEKK polymer in the layer 26 during manufacturing. The dimensions and shape are the same as in the embodiments of FIGS. 1A and 1B.

[0083] FIG. 1D schematically represents a side view of yet another embodiment of an implant according to the present invention. The embodied implant 1 in the shape of a plug corresponds to the one shown in FIG. 1C. In addition, the porosity of the elastomeric material in the top section 4 A p increases in a transverse direction 30 of the plug-shaped implant 1 from a low value of about 35 vol. % at a center line 3 of the plug-shaped implant towards a higher value of about 55 vol. % at an outer side of the implant 1. Further, the porosity of the elastomeric material in the top section 4 increases in a longitudinal direction 31 of the plug-shaped implant 1 from a low value of about 35 vol. % at a bottom surface of the top section 4 (corresponding with the top surface 24 of the base section 2) towards a higher value of about 55 vol. % at a top surface 41 of the top section 4. Further, the base section 2 comprises a layer 26 of porous PEKK polymer, which layer 26 is adjacent to the top surface 24 and has a thickness 27 of about 5% of the height 20 of the base section 2. The pores of the PEKK polymer in the layer 26 comprise the biocompatible poly-urethane-bisurea-hexylenecarbonate which originates from the top section 4 and has infiltrated the pores of the PEKK polymer in the layer 26 during manufacturing. The base section 2 further comprises a core 21 of non-porous PEKK polymer and a circumferential shell 22 of porous PEKK polymer. The shell 22 has a thickness 23 of about 5% of the diameter 10 of the base section 2 (and implant 1). Finally, the base section 2 also comprises a layer 28 of porous PEKK polymer, which layer 28 is adjacent to the bottom surface 25 and has a thickness 29 of about 5% of the height 20 of the base section 2. The dimensions and shape are the same as in the embodiments of FIGS. 1A to 1C.

[0084] Please note that in FIGS. 1B, 1C, and 1D the circumferential shells (22, 32) are shown in cross-section to show their respective thicknesses (23, 33). In a side view, they would extend over the complete diameter 10 of the implant 1.

[0085] Referring to FIG. 7A, a side view of another embodiment of the implant according to the present invention is shown. The implant 1 in the shape of a plug comprises the same materials and sections as shown in FIG. 1A. The dimensions of the implant of FIG. 7A are the same as those of the implant of FIG. 1A with one exception. Instead of having a flat top surface 41 of the top section 4 (and the implant 1), as in FIG. 1A, the top surface 41a of the top section 4 is spherical with a radius of curvature R of about 28 mm (not drawn to scale).

[0086] Referring to FIG. 7B, a side view of another embodiment of the implant according to the present invention is shown. The implant 1 in the shape of a plug comprises the same materials and sections as shown in FIG. 1B. The dimensions of the implant of FIG. 7B are the same as those of the implant of FIG. 1B with one exception. Instead of having a flat top surface 41 of the top section 4, as in FIG. 1B, the top surface 41a of the top section 4 is spherical with a radius of curvature R of about 28 mm (not drawn to scale).

[0087] Referring to FIG. 7C, a side view of another embodiment of the implant according to the present invention is shown. The implant 1 in the shape of a plug comprises the same materials and sections as shown in FIG. 1C. The dimensions of the implant of FIG. 7C are the same as those of the implant of FIG. 1C with one exception. Instead of having a flat top surface 41 of the top section 4, as in FIG. 1C, the top surface 41a of the top section 4 is spherical with a radius of curvature R of about 28 mm (not drawn to scale).

[0088] Referring to FIG. 7D, a side view of another embodiment of the implant according to the present invention is shown. The implant 1 in the shape of a plug comprises the same materials and sections as shown in FIG. 1D. The dimensions of the implant of FIG. 7D are the same as those of the implant of FIG. 1D with one exception. Instead of having a flat top surface 41 of the top section 4, as in FIG. 1D, the top surface 41a of the top section 4 is spherical with a radius of curvature R of about 28 mm (not drawn to scale).

[0089] Again note that in FIGS. 7B, 7C, and 7D the circumferential shells (22, 32) are shown in cross-section to show their respective thicknesses (23, 33). In a side view, they would extend over the complete diameter 10 of the implant 1 (not drawn to scale).

[0090] Referring to FIGS. 2A to 2D, an embodiment of a base section 2 of the invented implant 1 is schematically shown. The base section 2 shown is essentially cylindrical-shaped with a diameter 10, and a height 20. The top surface 24 of the base section has a circumferential flat rim part 240 that gradually extends into a centrally located cavity 241. The cavity 241 is provided with locking parts 242 that have a larger diameter than the diameter of the cavity 241. A shown in detail in FIG. 2C, the locking parts 242 of the cavity 241 are disk-shaped whereby the outer rim of the disk makes an angle 246 with the longitudinal direction 247 of the base section 2 of between 1° and 20°, more preferably between 5° and 15°. The cavity 241 (and parts 242) during manufacturing of the implant fills with part of the biocompatible elastomeric material to provide an adequate locking of the top section 4 to the base section 2. As discussed above, the base section 2 comprises a PEKK polymer which may be non-porous or substantially non-porous, the latter embodiment including the examples disclosed above. The base section 2 is further seen to comprise an outer surface having irregularities or undulations. In the present embodiment, these comprise circumferential ridges 243 which, in cross-section, are saw-tooth-shaped, as shown in detail in FIG. 2D. The angle 244 under which the saw-tooth flanks extend with respect to the transverse direction 245 of the base section 2, is preferably between 70° and 85°, more preferably between 75° and 80°.

[0091] Preparation of the Elastomeric Material of the Top Section

Example 1: Polycarbonate—aliphatic: Poly(hexylene carbonate urethane)-bis-urea biomaterial MVH313, See Table 1 Below

[0092] This one-pot two-step produced Biomaterial MVH313 was prepared by functionalization of 1.0 molar equivalent of poly(hexylene carbonate) diol (MW=2000) with 2.0 molar equivalents of 1,6-diisocyanatohexane (step 1), and subsequent chain extension using 1.0 molar equivalent of 1,6-diaminohexane (step 2).

[0093] In particular, the aliphatic poly-urethane-urea-hexylene carbonate biomaterial of the top section 4 was manufactured as follows (with reference to FIG. 3). Poly(hexylene carbonate) diol (MW=2000; 23.9 g, 11.9 mmol) was weighed in a 500 mL 3-necked flask and dried by heating to 75° C. overnight under vacuum, after which it was allowed to cool to room temperature. Under an argon atmosphere, 1,6-diisocyanatohexane (4.1 g, 23.9 mmol), DMAc (20 mL) and a drop of Sn(II)bis(2-ethylhexanoate) were added, after which the mixture was heated and stirred for 3 hours upon which the viscosity increased. The mixture was allowed to cool to room temperature, was diluted with DMAc (100 mL) and a solution of 1,6-diaminohexane (1.4 g, 11.9 mmol) in DMAc (50 mL) was added at once under thorough mixing. A gel was immediately formed upon addition and mixing. The mixture was further diluted with DMAc (150 mL) and was heated in an oil bath of 130° C. to acquire a homogeneous viscous slurry. After cooling to room temperature, the mixture was precipitated in a water/brine mixture (2.75 L water+0.25 L saturated brine) to yield a soft white material. This material was cut into smaller pieces and was stirred in a 1:5 mixture of methanol and water (3 L) for 64 hours. After decanting the supernatant, the resulting solid was stirred in a 2:1 mixture of methanol and water (0.75 L) for 6 hours. Decanting of supernatant, stirring in a 2:1 mixture of methanol and water (0.75 L) for 16 hours, decanting of the supernatant, and drying of the solid at 70° C. in vacuo yielded a flexible, tough elastomeric polymer.

[0094] .sup.1H NMR spectroscopy was performed on the resulting polymer, using a Varian 200, a Varian 400 MHz, or a 400 MHz Bruker spectrometer at 298K. DSC was performed using a Q2000 machine (TA Instruments). Heating scan rates of 10° C./min and 40° C./min were used for the assessment of the melting temperature (Tm) and the glass transition temperature (Tg), respectively. The Tm was determined by the peak melting temperature and the Tg was determined from the inflection point.

[0095] All reagents, chemicals, materials, and solvents were obtained from commercial sources and were used without further purification. The used poly(hexylene carbonate) diol had an average molecular weight of approximately 2 kg/mol. FIGS. 4 and 5 show the .sup.1H NMR spectrum and DSC thermograms of the obtained polymer, respectively. The .sup.1H NMR spectrum results may be summarized as follows: .sup.1H NMR (400 MHz, HFIP-d2): δ=4.23 (m, n*4H, n˜14.3), 4.10 (m, 4H), 3.17 (m, 12H), 1.87-1.32 (multiple signals for aliphatic CH2 methylenes) ppm. The average molecular weight of the repeating hard/soft block sections is about 2.5 kDa. The DSC results may be summarized as follows: DSC (10° C./min, FIG. 5A): Tm (top)=20.9° C. (soft block melt); DSC (40° C./min, FIG. 5B): Tg=−38.0° C. No second melting point for the hard block was observed up to 200° C. However, in a final heating run up to 250° C. at 10° C./min (FIG. 5C), a small and broad melting transition was observed at ca. 227° C. In the DSC-diagrams, the endothermic melting peaks are plotted downwards, whereas the exothermic crystallizations are plotted upwards.

[0096] The non-porous aliphatic poly-urethane-urea-hexylene carbonate biomaterial had an elastic modulus according to ASTM D638 of 3.6±0.03 MPa.

Example 2: Polyether—aromatic: Poly(tetrahydrofuran urethane)-bis-urea biomaterial MVH309B, See Table 1 Below

[0097] In a similar one-pot two-step experimental procedure as described in detail for Biomaterial MVH313, Biomaterial MVH309B was also produced. Particularly, Biomaterial MVH309B was prepared by functionalization of 1.0 molar equivalent of poly-tetrahydrofuran diol (MW=2000) with 1.33 molar equivalents of bis(4-isocyanatophenyl)methane (MDI) (step 1), and subsequent chain extension using 0.33 molar equivalent of 1,6-diaminohexane (step 2). Biomaterial MVH309B was isolated as a white, flexible, tough elastomeric polymer.

Example 3: Polyether—aliphatic: Poly(tetrahydrofuran urethane)-bis-urea biomaterial MVH312, See Table 1 Below

[0098] In a similar one-pot two-step experimental procedure as described in detail for Biomaterial MVH313, Biomaterial MVH312 was also produced. Particularly, Biomaterial MVH312 was prepared by functionalization of 1.0 molar equivalent of poly-tetrahydrofuran diol (MW=2000) with 2.0 molar equivalents of 1,6-diisocyanatohexane (step 1), and subsequent chain extension using 1.0 molar equivalent of 1,6-diaminohexane (step 2). Biomaterial MVH312 was isolated as a flexible, tough elastomeric polymer.

Example 4: Polycarbonate—aromatic: Poly(hexylene carbonate urethane)-bis-urea biomaterial MVH311, See Table 1 Below

[0099] In a similar one-pot two-step experimental procedure as described in detail for Biomaterial MVH313, Biomaterial MVH311 was also produced. Particularly, Biomaterial MVH311 was prepared by functionalization of 1.0 molar equivalent of poly(hexylene carbonate) diol (MW=2000) with 1.33 molar equivalents of bis(4-isocyanatophenyl)methane (MDI) (step 1), and subsequent chain extension using 0.33 molar equivalent of 1,6-diaminohexane (step 2). Biomaterial MVH311 was isolated as a flexible, tough elastomeric polymer.

Mechanical Properties of the Elastomeric Material of the Top Section without Pores

[0100] Stress Relaxation Testing was performed on the two aromatic and two aliphatic polymers of Examples 1-4, as well as on three equine cartilage specimens obtained from the Utrecht Medical Centre. A description of the specimens (e.g. polymer classes) and their dimensions are listed in Table 1. Using an Instron Electropulse E10000, each specimen was compressed at a strain rate of 0.005 s-1 up to a strain of 0.05 mm/mm which remained constant for 1800 s. All tests were done in triplicate. During the tests, load, displacement and time were recorded and afterwards, stress relaxation curves were obtained from the data. Stress relaxation is shown by determining the stress relaxation modulus G(t) at the onset of stress relaxation (G(0)) and 1800 s after the onset of stress relaxation (G(1800)) using the following equation: G(t)=σ(t)/ε.sub.0, where σ(t) is the compressive stress and ε.sub.0 is the set (constant) strain.

TABLE-US-00001 TABLE 1 Overview of the stress relaxation tests. All tests were done in triplicate. Test Code Description Dimensions 1 EC Equine cartilage ∅8.5 × 1.55 × 0.28 mm (average of three specimens) 2 MVH309B Polyether based 10.8 × 10.5 × 3.0 mm (l × w × h) aromatic polymer 3 MVH311 Polycarbonate based 12.0 × 11.1 × 2.9 mm (l × w × h) aromatic polymer 4 MVH312 Polyether based 12.4 × 11.3 × 3.0 mm (l × w × h) aliphatic polymer 5 MVH313 Polycarbonate based 13.5 × 13.5 × 3.0 min (l × w × h) aliphatic polymer

[0101] The results are shown in Table 2 below.

TABLE-US-00002 TABLE 2 Stress relaxation moduli of the materials at and after 1800 s after the onset 9 of stress relaxation. Stress relaxation modulus [MPa] Test Code G.sub.(0) G.sub.(1800) 1 EC 1.32 ± 0.58 0.03 ± 0.02 2 MVH309B 0.85 ± 0.04 0.65 ± 0.04 3 MVH311 12.29 ± 0.30  10.84 ± 0.39  4 MVH312 10.36 ± 0.61  7.42 ± 0.28 5 MVH313 3.60 ± 0.03 3.14 ± 0.05

[0102] Preparation of Biomaterial-Capped Pekk Bone Anchors

[0103] The implant 1 was manufactured by attaching the top section 4 to a PEKK base section 2 which serves as bone anchor. In a method according to an embodiment of the invention, PEKK bone anchors were capped with the poly-urethane-urea-hexylene carbonate biomaterial by pressing small granules of the aliphatic polycarbonate polymer on top of and into the PEKK anchors. For this purpose, a custom press setup was used. Various temperatures (100° C. to about 150° C.), compressive forces (2 kN to about 4 kN) and methods have been tested. The best results were obtained using a two-step procedure, employing a temperature of 150° C. and using a compressive force of 40 kN (4 tons, or 4000 kg; corresponding to a pressure of 1.4 GPa). Lower temperatures than 150° C. seemed to give less homogenously pressed poly-urethane-urea-hexylene carbonate biomaterial layers (sections 3 and 4), while higher temperatures are less desired as the urea groups in the poly-urethane-urea-hexylene carbonate biomaterial may then degrade to some extent. In the first step, ca. 50 mg of the polymer 12 was pressed onto and into the PEKK bone anchor for 15 minutes, while in the second step, ca. 2 mg of polymer 12 was added to the setup and the sample was pressed for another 15 minutes under the same conditions (150° C. and 40 kN). The samples were subsequently removed from the compression setup and were then allowed to cool. After the second pressing step, the surface of the poly-urethane-urea-hexylene carbonate biomaterial layer (sections 3 and 4) on top of the base section 2 seemed to be substantially flat. The biomaterial was almost transparent and colorless. The edges of the biomaterial showed some fringes or frays, and these were removed using a scalpel.

[0104] A central hole (241, 242) of the base section 2 was about 4.5 mm deep and about 2 mm in diameter. The hole was substantially filled with the poly-urethane-urea-hexylene carbonate biomaterial, and the attachment of the biomaterial to the PEKK base section 2 seemed quite strong and robust. Removing the biomaterial from the PEKK base section by force, or loosening the connection at the PEKK-biomaterial interfaces, proved practically impossible. All used equipment and accessories that were intended to come into contact with the PEKK base section 2 and/or with the elastomeric biomaterial were rinsed with ethanol or isopropanol and were thereafter dried. After pressing, and cutting the frays, the PEKK-biomaterial plug implant was rinsed with isopropanol and dried. The plugs may also be produced in a sterilized environment, if needed.

[0105] As assessed by measuring, the PEKK base section was 6 mm in diameter and 6 mm tall (a height of 6 mm). The central cavity in the base section was about 2 mm in diameter and about 4.5 mm deep. The elastomeric biomaterial (the aliphatic polycarbonate) positioned onto the PEKK base section was about 6 mm in diameter and about 1 mm high. Accordingly, the total PEKK-biomaterial plug implant was about 7 mm tall.

[0106] The top section 4 was provided with pores by drilling holes in it with an average diameter of 300 micron, to a final porosity of 50 vol. %. The porous aliphatic poly-urethane-urea-hexylene carbonate biomaterial of the top section 4 had an elastic modulus according to ASTM D638 of 0.9±0.2 MPa.

[0107] The implant 1 may be implanted into an osteochondral defect 8 as shown in FIGS. 6A to 6C. In a typical method, a cartilage defect extending into the subchondral bone (FIG. 6 A) is drilled out and a plug-shaped implant 1 is implanted into the drilled hole under some pressure (‘press fit’), as shown in FIG. 6B. Bone then grows onto, and in some embodiments into, the PEKK base section 2, anchoring the implant 1. Surrounding native cartilage 5 grows onto a top side 41 of the top section 4 and new cartilage 5a is generated on top of the implant 1, as shown in FIG. 6C. As is also shown in FIG. 6C, the height 20 of the base section 2, and the height 40 of the porous top section 4 are selected such that a top surface 41 of the implant 1 comes to lie below a top surface 50 of cartilage 5 present on an osteochondral structure (5, 6) when implanted, preferably over a distance 51 of between 0.1-1 mm. In the present case, this distance was about 0.5 mm. The osteochondral structure (5, 6) comprises subchondral bone 6 and a cartilage layer 5 on top of it. A synovial cavity 7 is generally also present.

[0108] As also shown in FIGS. 6B and 6C, the height 20 of the base section 2, and the height 40 of the porous top section 4 are selected such that a bottom surface 24 of the top 4 (or top surface 24 of the base section 2) comes to lie about level with a bottom surface 51 of the cartilage layer 5 of the osteochondral structure (5, 6) when implanted.

[0109] Finally, the implant according to the embodiment shown in FIGS. 7A to 7D may also be implanted into an osteochondral defect 8 as shown in FIGS. 8A to 8C. Due to a spherical top surface 41a of the top layer 4, this embodiment may regenerate a new cartilage layer 5a on the top surface 41a of the top section 4 of the implant 1 of about equal thickness across the top surface 41a. The result may be a radius of a top surface 50 of the regenerated cartilage 5a that is about the same as the radius of the surrounding native cartilage layer 5 next to the implant, thereby showing a continuity in radius.

[0110] Preparation of Biomaterial-Capped Metallic Bone Anchors

[0111] Another embodiment of the implant 1 was manufactured by attaching the top section 4 to a titanium base section 2 which serves as bone anchor. The titanium used was alloy Ti6A14V, which is readily commercially available. The titanium base section was provided with pores having an average pore size of about 300 microns. In a method according to an embodiment of the invention, titanium bone anchors were capped with a poly-urethane-urea-hexylene carbonate biomaterial by pressing small granules of the aliphatic polycarbonate polymer on top of and into the pores of the titanium anchors. For this purpose, the same custom press setup as used in the previous example was used. Optimum results were again obtained using a two-step procedure, employing a temperature of 150° C. and using a compressive force of 40 kN (4 tons, or 4000 kg; corresponding to a pressure of 1.4 GPa). In the first step, ca. 50 mg of the elastomeric polymer was pressed onto and into the titanium bone anchor for 15 minutes, while in the second step, ca. 2 mg of the elastomeric polymer was added to the setup and the sample was pressed for another 15 minutes under the same conditions (150° C. and 40 kN). The samples were subsequently removed from the compression setup and were then allowed to cool. After the second pressing step, the surface of the poly-urethane-urea-hexylene carbonate biomaterial layer (sections 3 and 4) on top of the base section 2 seemed to be substantially flat. The biomaterial was almost transparent and colorless. Some edges of the biomaterial showed fringes or frays, which were removed using a scalpel.

[0112] As with the PEKK base anchor, the titanium base anchor was also provided with a central hole (241, 242) with the same dimensions. The hole was substantially filled with the poly-urethane-urea-hexylene carbonate biomaterial, and the attachment of the biomaterial to the titanium base section 2 was satisfactory.

[0113] The titanium base section 2 had the same dimensions as the PEKK base section. Since the same mold was used, the elastomeric biomaterial (the aliphatic polycarbonate) positioned onto the titanium base section was about 6 mm in diameter and about 1 mm high. Accordingly, the total titanium-biomaterial plug implant was about 7 mm tall.

[0114] The top section 4 was provided with pores by drilling holes in it with an average diameter of 300 micron, to a final porosity of 50 vol. %. The porous aliphatic poly-urethane-urea-hexylene carbonate biomaterial of the top section 4 had an elastic modulus according to ASTM D638 of 0.9±0.2 MPa.

[0115] The implant 1 may be implanted into an osteochondral defect 8 as shown in FIGS. 6A to 6C, as was already described above. In a typical method, a cartilage defect extending into the subchondral bone (FIG. 6 A) is drilled out and a plug-shaped implant 1 is implanted into the drilled hole, as shown in FIG. 6B. Due to the relatively high stiffness of the titanium base section 2, a press fit was not appropriate. Instead, the dimensions of the drilled out subchondral bone was slightly larger than the dimensions of the titanium base section 2. Bone is seen to grow onto the titanium base section 2, anchoring the implant 1.

[0116] Surrounding native cartilage 5 grows onto a top side 41 of the top section 4 and new cartilage 5a is generated on top of the implant 1, as shown in FIG. 6C. As is also shown in FIG. 6C, the height 20 of the base section 2, and the height 40 of the porous top section 4 are selected such that a top surface 41 of the implant 1 comes to lie below a top surface 50 of cartilage 5 present on an osteochondral structure (5, 6) when implanted, preferably over a distance 51 of between 0.1-1 mm. In the present case, this distance was about 0.5 mm. The osteochondral structure (5, 6) comprises subchondral bone 6 and a cartilage layer 5 on top of it. A synovial cavity 7 is generally also present.

[0117] As also shown in FIGS. 6B and 6C, the height 20 of the base section 2, and the height 40 of the porous top section 4 are selected such that a bottom surface 24 of the top section 4 (or top surface 24 of the base section 2) comes to lie about level with a bottom surface 51 of the cartilage layer 5 of the osteochondral structure (5, 6) when implanted.

[0118] It will be apparent that many variations and applications are possible for a skilled person in the field within the scope of the appended claims of the invention.