METHOD AND SYSTEM FOR THE PRODUCTION OF MOLDED BODY BASED ON LINKED UHMWPE AND MOLDED BODY BASED ON LINKED UHMWPE

Description

[0001] The present invention relates to a process for producing one or more molded bodies based on cross-linked UHMWPE.

[0002] The invention further relates to a molded body based on crosslinked UHMWPE, which is produced according to this process.

[0003] Furthermore, the invention relates to a system for producing one or more molded bodies based on cross-linked UHMWPE with a receiving device for one or more molded bodies made of compacted UHMWPE.

[0004] Joint endoprostheses that incorporate molded bodies made of ultra-high molecular weight polyethylene (UHMWPE) as sliding surface elements have been known for some time. The UHMWPE sliding surface element typically interacts with a corresponding element made of a metallic material, resulting in a low coefficient of friction. In a hip joint prosthesis, the UHMWPE sliding surface element is typically designed as an inlay of the femoral head implant, whereas in a knee joint prosthesis, it is typically attached to the tibial component and is also referred to as a meniscal element. A UHMWPE sliding surface can also be used in a kneecap prosthesis (patellar prosthesis).

[0005] The sliding surface element must meet certain requirements regarding its mechanical properties, wear resistance, and aging resistance, whereby the material properties of UHMWPE required for this are partly contradictory. To increase wear resistance, UHMWPE can be cross-linked with ionizing radiation, as described, for example, in EP 0 995 450 A1. At the same time, however, cross-linking reduces the material's ductility, thereby increasing the risk of structural fatigue, delamination, and potentially failure of the sliding surface element.

[0006] Since free radicals are created during radiation crosslinking of UHMWPE and partially remain in the material, this tends to lead to poorer aging resistance due to oxidative processes. This problem can be at least partially counteracted by the addition of antioxidants such as α-tocopherol (vitamin E). The antioxidant is either added to the UHMWPE prior to the production of a molded article or introduced into the molded article by diffusion after crosslinking (see, for example, EP 3 111 895 A1). Given that increasingly younger patients are being treated with joint endoprostheses, and that life expectancy is simultaneously increasing and patients remain active even into old age, the aging resistance of UHMWPE in joint endoprostheses is becoming increasingly important.

[0007] The properties of molded articles made of cross-linked UHMWPE are significantly influenced by the type of ionizing radiation and the radiation dose used for irradiation. For example, the use of gamma radiation has the advantage over beta radiation that the radiation dose used is more easily controllable, resulting in higher reproducibility of the radiation cross-linking result. Due to the significantly lower radiation intensity of gamma radiation, irradiation times are typically in the range of several hours, compared to several seconds or minutes with beta radiation. As an alternative to gamma radiation, cross-linking can also be achieved with X-rays, as described, for example, in EP 3 510 084 B1.

[0008] To achieve the highest possible wear and aging resistance of molded articles made of cross-linked UHMWPE, it is generally desirable to achieve a high degree of cross-linking with the lowest possible radiation dose. To achieve this, it is advantageous if the irradiation is carried out at an elevated temperature of the molded article, particularly at a temperature in the range of the melting temperature of the UHMWPE or above, as this leads to greater mobility of the polymer chains and a more effective cross-linking reaction. Since direct heating of the molded articles in the irradiation device is difficult to achieve, the prior art predominantly involves first preheating the molded articles to be irradiated to a specific temperature and then irradiating them.However, it is unavoidable that the molded bodies cool down again during irradiation, and the extent of this cooling cannot be precisely controlled (for example, during transfer from an oven to the irradiation device). This has a negative impact on the reproducibility of radiation crosslinking. DE 10 2013 113 781 A1 proposes placing the molded bodies in a thermally insulating packaging unit during irradiation to limit cooling during irradiation. However, this only partially solves the problem of lack of reproducibility due to the temperature change of the molded bodies.

[0009] US 2012 / 016051 A1 and US 2015 / 151866 A1 disclose a process for producing one or more molded bodies based on cross-linked UHMWPE.

[0010] It is the object of the present invention to propose a process for producing one or more molded bodies based on crosslinked UHMWPE, which is as efficient and reproducible as possible with regard to the degree of crosslinking of the molded bodies produced.

[0011] This object is achieved according to the invention in that the method comprises the following steps: Compacting UHMWPE in powder form into one or more shaped bodies; introducing the shaped body(s) into a receiving space of a receiving device, wherein the receiving device comprises a heating means for heating the shaped body(s) located in the receiving space; irradiating the shaped body(s) in the receiving device with gamma or X-ray radiation to crosslink the UHMWPE; and removing the shaped body(s) from the receiving space, wherein the receiving device is designed to control a predetermined temperature profile of the shaped body(s) before, during and after irradiation.

[0012] The key innovation of the process according to the invention is that the molded body(s) are arranged within a holding device during irradiation, which enables active heating and control of the temperature profile of the molded body(s). In this way, the temperature at which the irradiation takes place can be specified and adjusted very precisely, so that the crosslinking of the UHMWPE occurs under defined and reproducible conditions.

[0013] The receiving device, in the sense of the present invention, is a separate unit independent of the irradiation device. Thus, the invention eliminates the need for a technically complex and expensive modification of the irradiation device used for the method. Instead, the receiving device used can be used with conventional irradiation devices and is introduced into their irradiation chamber during the method. It is only necessary to ensure that the receiving device is appropriately dimensioned and that its walls are made of a material permeable to gamma and X-ray radiation.

[0014] It is preferred if the molded body(s) are heated to a target temperature after being introduced into the receiving space and prior to irradiation by the heating element. The target temperature can be selected and specified depending on various factors, whereby it is particularly advantageous to consider the intended radiation dose and / or irradiation time when selecting the target temperature. Due to the ability to precisely coordinate these essential process parameters, the process according to the invention allows for particularly efficient crosslinking of the molded body(s).

[0015] The target temperature is ideally in the range of the melting temperature of UHMWPE (approximately 137 °C) or higher. Due to the higher mobility of the polymer chains in this temperature range, radiation crosslinking is generally more effective than at temperatures below the melting range.

[0016] The target temperature in the process according to the invention is preferably from 135 to 150 °C, more preferably from 137 to 145 °C.

[0017] In a preferred embodiment of the method, the molded body(s) are kept substantially constant at the target temperature during irradiation. Alternatively, the temperature of the molded body(s) can also be changed during irradiation.

[0018] After irradiation and before removal from the receiving chamber, the molded body(s) are preferably allowed to cool. Optionally, the cooling time can be extended by activating the heating medium to a certain extent during this phase.

[0019] Thus, according to the method according to the invention, not only is a constant target temperature specified and controlled during irradiation, but it is also possible, in principle, to specify and control the entire temperature profile of the molded body(s) from their introduction into the receiving space (typically at ambient temperature) to their removal from the receiving space. For this purpose, the receiving device advantageously comprises a control means and one or more temperature sensors.

[0020] As a heating means, the receiving device can in principle comprise any type of device that enables heating of the molded body(ies) located in the receiving space. In particular, the heating means can comprise a hot air blower, an infrared radiator, or an electric heating element.

[0021] The receiving device preferably comprises an ambient-independent power supply, in particular a rechargeable battery. The rechargeable battery must be sufficiently dimensioned to heat the molded body(ies) to the target temperature and maintain the corresponding temperature at least during the irradiation period. Alternatively, the receiving device can also be connected to an external power supply.

[0022] The holding device preferably has an outer wall made of a thermally insulating material. This reduces the energy required for heating and maintaining the temperature. The material of the outer wall of the holding device must also be transparent to gamma and X-ray radiation, as already mentioned above.

[0023] The irradiation of the molded body(s) is typically carried out with a radiation dose of 25 to 45 kGy, preferably 27 to 33 kGy, more preferably approximately 30 kGy. It has been shown that with a radiation dose of this magnitude, very favorable properties of the cross-linked molded body can be obtained, particularly with regard to wear resistance and ductility.

[0024] The irradiation of the shaped body(s) with gamma or X-ray radiation is preferably carried out over a period of 2 to 6 hours, preferably 3 to 4 hours. With a preferred radiation dose of 30 kGy, this corresponds to a dose rate in the range of 5 to 15 kGy / h.

[0025] In a preferred embodiment of the process according to the invention, the UHMWPE in powder form is mixed with an antioxidant before compaction. As already mentioned above, an antioxidant can improve the aging resistance of radiation-crosslinked UHMWPE by reacting with the free radicals remaining after irradiation.

[0026] Various antioxidants approved for medical use can be used as antioxidants added to UHMWPE. The antioxidant is preferably selected from tocopherols, tocotrienols, ascorbic acid, polyphenolic antioxidants such as flavonoids, butylhydroxytoluene (BTH), and butylhydroxyanisole (BTA).

[0027] In a preferred embodiment of the invention, the antioxidant is α-tocopherol. This is also referred to as vitamin E, although the term vitamin E broadly encompasses all tocopherols, tocotrienols, and other fat-soluble antioxidants.

[0028] The proportion of antioxidant added to the UHMWPE is advantageously from 0.05 to 0.15 wt.%, based on the UHMWPE, preferably from 0.09 to 0.11 wt.%.

[0029] The UHMWPE used in the invention typically has a molecular weight in the range of 5 10 6 to 10 7 g / mol (determined from the intrinsic viscosity) and a density in the range of 0.92 to 0.95 g / cm 3 . A suitable UHMWPE in powder form is available, for example, from Ticona GmbH under the name GUR 1020, or as a mixture with 0.1 wt.% α-tocopherol as an antioxidant under the name GUR 1020-E.

[0030] The compaction of the UHMWPE into a shaped body is typically carried out by compression molding or RAM extrusion, preferably to a density of the shaped body of 0.92 g / cm 3 or more, ie essentially to the theoretical density of the UHMWPE.

[0031] One or more of the actual end products can be manufactured from a compacted UHMWPE molded body by material-removing machining, particularly by milling. Within the scope of the present invention, these end products are, in particular, a sliding surface element for a joint endoprosthesis or other medical implants made of cross-linked UHMWPE.

[0032] The material-removing machining of the molded body can be carried out within the process according to the invention, namely after compacting the UHMWPE and before inserting the molded body(s) into the receiving device. In this case, the molded bodies irradiated with gamma or X-ray radiation already correspond in their spatial shape to the sliding surface elements or other medical implants to be produced.

[0033] Alternatively, the material-removing processing of the molded body(s) can also be carried out after irradiation and removal from the receiving space, i.e., following the process according to the invention. In this case, the molded bodies produced according to the invention are blanks made of cross-linked UHMWPE, e.g., in the form of plates or cylinders.

[0034] The present invention further relates to a molded article based on cross-linked UHMWPE, which is produced by the process according to the invention.

[0035] The particular advantages and preferred embodiments of the shaped body according to the invention have already been explained in connection with the process according to the invention.

[0036] The molded body according to the invention is preferably a sliding surface element for a joint endoprosthesis, in particular for a hip joint endoprosthesis or a knee joint endoprosthesis. According to a further embodiment, the molded body according to the invention is a blank for producing such a sliding surface element by material-removing machining.

[0037] The present invention further provides a system for producing one or more molded bodies based on crosslinked UHMWPE, comprising a receiving device. The receiving device comprises a receiving chamber for receiving one or more molded bodies made of compacted UHMWPE and a heating means for heating the molded body(s) located in the receiving chamber. It also comprises an irradiation device for irradiating the molded body(s) (16) in the receiving device (10; 30; 40) with gamma or X-ray radiation in order to crosslink the UHMWPE. The receiving device is designed to control a predetermined temperature profile of the molded body(s) before, during, and after irradiation.

[0038] Significant advantages and preferred embodiments of the system according to the invention have also already been described in connection with the method according to the invention. The features of the receiving device explained in this context can be implemented individually or in any combination within the scope of the invention, unless the respective context dictates otherwise.

[0039] The present disclosure further relates to the use of the system according to the invention in a process for producing one or more shaped bodies based on crosslinked UHMWPE, in particular in the process according to the invention.

[0040] The following embodiments serve to explain the invention in more detail without limiting it in any way.

[0041] They show in detail: Figure 1: Schematic representation of a first exemplary embodiment of a system according to the invention with a receiving device; Figure 2: Schematic representation of a second exemplary embodiment of a system according to the invention with a receiving device; Figure 3: Schematic representation of a third exemplary embodiment of a system according to the invention with a receiving device; and Figure 4: Diagram illustrating a predetermined temperature profile according to an exemplary embodiment of the method according to the invention.

[0042] The Figure 1schematically shows a first embodiment of a system according to the invention with a receiving device 10. The receiving device 10 has an outer wall 12 made of a material that is permeable to gamma and X-ray radiation and is preferably thermally insulating. Within the receiving device 10 is a receiving space 14 into which one or more molded bodies 16 are introduced through a closable opening in the outer wall 12 (not shown in the figure).

[0043] Within the receiving device 10, two hot air blowers are also arranged as heating means 18, which are supplied with electrical energy by an accumulator 20. In this embodiment, the accumulator 20 is arranged outside the wall 12 of the receiving device 10; however, an arrangement within the wall 12 is also possible.

[0044] By means of the hot air blower 18, the molded bodies 16 arranged in the receiving space 14 can be heated to a predetermined temperature and maintained at this temperature. When carrying out the method according to the invention with the receiving device 10, it can be provided, for example, that the molded bodies 16 are heated to the target temperature before the start of the irradiation, maintained at this target temperature during the irradiation period, and allowed to cool within the receiving device after the end of the irradiation.

[0045] To control or regulate such a temperature profile, the receiving device 10 further comprises three temperature sensors 22 in the immediate vicinity of the molded bodies 16 and a control means not shown in the figure.

[0046] The Figure 2schematically shows a second embodiment of a system according to the invention with a receiving device 30. This largely corresponds to the receiving device 10 of the first embodiment, with the difference that in this case two infrared radiators are provided as heating means 18. The infrared radiators 18 are arranged in spatial proximity to the molded bodies 16 so that their direct heating is possible.

[0047] The Figure 3 shows a schematic representation of a third exemplary embodiment of a system according to the invention with a receiving device 40. The receiving device 40 also essentially corresponds to the receiving devices of the first two exemplary embodiments, but with the difference that in this case, an electric heating element is provided as the heating means 18. The heating element 18 can, for example, be designed in the form of a heating blanket arranged in the immediate vicinity of the molded bodies 16.

[0048] The receiving devices 10, 30, and 40 according to the exemplary embodiments described above can be used in particular for carrying out the method according to the invention for producing one or more molded bodies based on crosslinked UHMWPE. UHMWPE in powder form, preferably with the addition of an antioxidant, is compacted into one or more molded bodies 16. These are introduced into the receiving space 16 of the receiving device 10, 30, or 40 and heated by the heating means 18, preferably to a predetermined target temperature.

[0049] The receiving device 10, 30, or 40 with the molded bodies 16 is placed in an irradiation device, and the molded bodies 16 are irradiated with gamma or X-ray radiation. During the irradiation, the target temperature of the molded bodies 16 is preferably maintained by the heating means 18 with the aid of the temperature sensors 22 and the control means. After the end of the irradiation, the molded bodies 16 are allowed to cool in the receiving space 14, wherein the cooling rate can be influenced by the heating means 18.

[0050] An exemplary temperature profile when carrying out the method according to the invention is shown in the diagram of Figure 4In this example, the molded bodies are first heated to a target temperature of approximately 140 °C (preheating), which is slightly above the melting temperature of UHMWPE of approximately 137 °C. This target temperature of the molded bodies is maintained during the irradiation period of 3 to 4 hours. After irradiation, the molded bodies are cooled to ambient temperature.

[0051] The process according to the invention thus enables a consistent and reproducible temperature profile of the molded bodies, allowing the crosslinking of the UHMWPE resulting from irradiation to be carried out efficiently and reproducibly. In particular, the process according to the invention prevents intermediate cooling of the molded bodies after preheating and before irradiation, as typically occurs in prior art processes. List of reference symbols

[0052] 10Holding device 12Wall 14Holding chamber 16Moulded body made of UHMWPE 18Heating medium 20Accumulator 22Temperature sensors 30Holding device 40Holding device

Claims

1. A method for the manufacture of one or more moulded bodies (16) based on cross-linked UHMWPE, comprising the steps: • compressing UHMWPE in powder form into one or more moulded bodies (16); • introducing the moulded body or bodies (16) into a receiving space (14) of a receiving device (10; 30; 40), wherein the receiving device (10; 30; 40) comprises a heating means (18) for heating the moulded body or bodies (16) located in the receiving space; • irradiating the moulded body or bodies (16) in the receiving device (10; 30; 40) with gamma - or X-rays, in order to cross-link the UHMWPE; and • removing the moulded body or bodies (16) from the receiving space (14), wherein the receiving device (10; 30; 40) is set up to control a pre-determined temperature behaviour of the moulded body or bodies (16) before, during and after the irradiation.

2. A method according to claim 1, wherein the moulded body or bodies (16) are heated to a target temperature after the introduction into the receiving space (14) and before the irradiation by the heating means (18).

3. A method according to claim 2, wherein the target temperature lies in the region of or over the melting temperature of the UHMWPE, wherein the target temperature is preferably from 135 to 150°C, further preferably from 137 to 145°C.

4. A method according to claim 2 or 3, wherein the moulded body or bodies (16) are held substantially constantly at the target temperature during the irradiation; or wherein the temperature of the moulded body or bodies (16) is changed during the irradiation.

5. A method according to any one of the preceding claims, wherein the receiving device (10; 30; 40) comprises a control means and one or more temperature sensors (22) for controlling the temperature behaviour of the moulded body or bodies (16) before, during and after the irradiation.

6. A method according to any one of the preceding claims, wherein the heating means (18) comprises a hot air blower, an infrared radiator or an electrical heating element.

7. A method according to any one of the preceding claims, wherein the irradiation of the moulded body or bodies (16) is executed with a radiation dose of 25 to 45kGy, preferably of 27 to 33kGy, further preferably of approximately 30kGy.

8. A method according to any one of the preceding claims, wherein the radiation is executed with gamma - or X-ray radiation over a time period of 2 to 6 hours, preferably of 3 to 4 hours.

9. A method according to any one preceding claims, wherein the UHMWPE in powder form is cross-linked before the compression with an antioxidant, which is preferably selected from tocopherols, tocotrienols, ascorbic acid, polyphenolic antioxidants such as for example flavonoids, butylated hydroxytoluene and butylated hydroxyanisole, and wherein the antioxidant is further preferably α-tocopherol.

10. A method according to claim 9, wherein the UHMWPE is cross-linked with 0.05 to 0.15wt% of the antioxidant, preferably with 0.09 to 0.11wt%.

11. A moulded body (16) based on cross-linked UHMWPE, which is manufactured according to the method according to any one of the preceding claims.

12. A moulded body (16) according to claim 11, wherein the moulded body (16) is a sliding surface element for a joint endoprosthesis, in particular for a hip joint endoprosthesis or a knee joint endoprosthesis.

13. A system for the manufacture of one or more moulded bodies (16) based on cross-linked UHMWPE with a receiving device (10; 30; 40), comprising a receiving space (14) for receiving one or more moulded bodies (16) of cross-linked UHMWPE and a heating means (18) for heating the moulded body or bodies (16) located in the receiving space (14), and with an irradiation device for irradiating the moulded body or bodies (16) in the receiving device (10; 30; 40) with gamma - or X-ray radiation, in order to cross-link the UHMWPE, wherein the receiving device (10; 30; 40) is set up to control a predetermined temperature behaviour of the moulded body or bodies (16) before, during and after the irradiation.

14. A system according to claim 13, wherein the receiving device (10; 30; 40) comprises a control means and one or more temperature sensors (22) for controlling the temperature behaviour of the moulded body or bodies (16) before, during and after the irradiation.

15. A system according to claim 13 or 14, wherein the heating means (18) comprises a heating blower, an infrared radiator or an electrical heating element.