Method for treating a polymer workpiece for use in a joint implant

Abstract

The present invention provides a method for treating a polymer workpiece (1) for use in a joint implant. It comprises the steps of placing the polymer workpiece in an explosion chamber, introducing a combustible gas mixture into the explosion chamber and igniting the combustible gas mixture. Igniting the gas mixture in the explosion chamber produces a temperature that lies above the melting point of a polymer of the polymer workpiece.

Claims

1. Method for treating a polymer workpiece (1) comprising the steps: a. placing the polymer workpiece (1) in an explosion chamber, b. closing the explosion chamber, c. introducing a flammable gas mixture into the explosion chamber, d. igniting the flammable gas mixture, wherein a temperature that lies above the melting point of a polymer of the polymer workpiece (1) and in a range of 1500° C. to 2800° C. is produced in the explosion chamber by igniting the gas mixture for smoothening the surface of the polymer workpiece, wherein igniting the flammable gas mixture in the explosion chamber also treats the polymer workpiece by deburring, and wherein the polymer workpiece is for use in a joint implant.

2. Method according to claim 1, where the polymer workpiece (1) comprises a thermoplastic.

3. Method according to claim 1, where the gas mixture to be exploded is introduced at a pressure of 1.5 to 2.1 bar.

4. Method according to claim 1, where the polymer workpiece (1) is pre-treated by means of deburring by hand, deburring by milling, deburring by turning, and combinations thereof.

5. Method according to claim 2, where the polymer workpiece (1) is pre-treated by means of deburring by hand, deburring by milling, deburring by turning, and combinations thereof.

6. Method according to claim 1, where the temperature produced by exploding the gas mixture is in a range of 1500° C. to 2800° C. and is maintained over a period of 1 ms to 10 ms.

7. Method according to claim 3, where the temperature produced by exploding the gas mixture is in a range of 1500° C. to 2800° C. and is maintained over a period of 1 ms to 10 ms.

8. Method according to claim 1, where the gas mixture comprises oxygen and methane.

9. Method according to claim 3, where the gas mixture comprises oxygen and methane.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The following figures illustrate the following detailed explanation of preferred embodiments of the present invention, wherein

(2) FIG. 1 shows a polymer workpiece for use in a joint implant,

(3) FIG. 2 shows a section of the polymer workpiece from FIG. 1 following machining treatment,

(4) FIG. 3 shows a section comparable to the section from FIG. 2 following manual deburring by hand,

(5) FIG. 4 shows a section comparable to the section from FIG. 2 following treatment of the workpiece with the method according to the invention,

(6) FIG. 5 shows an image taken with a scanning electron microscope, with FIG. 5a) showing the surface of the workpiece in an edge area following manual deburring by hand and FIG. 5b) showing the surface of the workpiece in a comparable edge area after using the method according to the invention,

(7) FIG. 6 shows an image taken with a scanning electron microscope in a higher resolution compared to FIG. 5, with FIG. 6a) illustrating the surface of the workpiece in an edge area following manual deburring by hand and FIG. 6b) illustrating the surface of the workpiece in a comparable edge area after using the method according to the invention, and

(8) FIG. 7 shows workpieces in a holder prior to carrying out explosion deburring.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(9) FIG. 1 shows a polymer workpiece 1 provided for use in a joint implant. The polymer workpiece 1 shown is a hip-joint insert of UHMWPE, which can be inserted into a metal hip socket which is in turn provided for implantation into the acetabulum of a patient. To put it differently, the joint insert illustrated in FIG. 1 is anchored in the bone tissue using a not-shown hip socket of a polymer material.

(10) The complexity of the external geometry that a joint insert can have also becomes clear from FIG. 1. The hip joint insert shown in FIG. 1 is just one example of the many varied application possibilities of the method for joint implantation. It can be used for the most varied joints, such as for example hip sockets or inserts, tibia plateaus of knee joints, knee-joint inserts, shoulder-prosthesis components, ankle-joint components, elbow-joint components, megaprosthesis components and can be used for instruments made from sterilizable thermoplastics.

(11) Owing to the precise and reliable removal of irregularities that is substantially carried out without material removal from the target geometry, the method can also be used particularly advantageously for smaller joint components, such as, for example, finger joints like the thumb-saddle joint. Particularly with these comparatively small joints, and thus smaller polymer workpieces 1, deburring by hand leads to a relatively strong change in the workpiece geometry.

(12) Irregularities as part of this invention are understood as projections created owing to the manufacture of the polymer workpiece 1. These include ridges G and chips S generally generated by cutting processes, but also polymer particles 22 pressed into or against the workpiece surface (compare FIG. 5a). It is also possible to reduce irregularities on the surface introduced by cutting processes.

(13) Workpieces 1 to be used in a joint replacement are preferably cast by means of injection molding. Consequently, a thermoplastic that also has the advantages already listed above in connection with the method is preferred as a polymer material. In a normal case, machining treatment methods will also be used, i.e. whether a geometry is to be drilled or milled from solid material or if projections remaining after casting, such as for example mold seams, are to be removed.

(14) A polymer workpiece 1 pre-fabricated in this way and illustrated as an example in FIG. 1 is thereafter fixed on a holder 12 in an explosion chamber or an explosion chamber insert 10 for applying the method (see FIG. 7). With the explosion chamber insert 10 illustrated in FIG. 7, three polymer workpieces 1 can be treated simultaneously. The explosion chamber is formed by inserting the explosion chamber insert 10 into a corresponding opening. The free volume of the explosion chamber to be filled with the gas mixture that is determined by this is approximately 10 times to 30 times, preferably 15 times to 25 times, the volume of the polymer workpieces 1.

(15) Following placement in the explosion chamber, this is closed and the flammable, preferably pre-mixed gas mixture such as for example that mentioned above, is introduced.

(16) The gas mixture is thereafter ignited so that an explosion-like combustion similar to that of an internal combustion engine takes place. The combustion process is controlled here in particular by adding a corresponding quantity of the gas mixture in such a way that temperatures of 1500° C. to 2800° C., and preferably of 2000° C. to 2500° C., are reached in the explosion chamber.

(17) The period over which the temperatures of said temperature range are reached is selected in such a way that existing irregularities are removed by combustion or evaporation. The period required for this normally lies within the millisecond range and in particular within a range of 1 ms to 10 ms, preferably 1 ms to 5 ms, and more preferably 1 to 2 ms. A very short period such as, for example, the latter will suffice for substantially removing existing irregularities. Said longer periods will also lead to an increased smoothing effect on the surface of the workpiece 1.

(18) The overpressure generated by the combustion is vented from the explosion chamber in a controlled way. The entire explosion deburring process only takes approximately 1 minute. In other words, approximately 1 minute will elapse from one ignition to the next ignition, with several workpieces 1 being able to be treated simultaneously with each ignition, as illustrated in FIG. 7.

(19) The advantages achieved with the method will be illustrated below with reference to FIGS. 2 to 6 by way of an example of treating a polymer workpiece 1 as shown in FIG. 1.

(20) FIG. 2 shows the pole section P from FIG. 1 in an enlarged presentation following machining treatment, with which the complex geometry illustrated in FIG. 1 is produced. In FIG. 2, the resulting ridges G and chips S can be seen which were generated by the machining treatment and which have not detached themselves from the workpiece 1.

(21) One treatment method normally used for such workpieces 1 is manual deburring, which is mostly carried out by hand. The result of such a deburring by hand is illustrated in FIG. 3, which shows a section that equals the section shown in FIG. 2 at the same enlargement.

(22) Deburring is also a machining process. In other words, a deburring by hand allows for the possibility that chips and ridges will not only remain, but will be newly created. One of these reasons has led to chip S still being present in FIG. 3.

(23) The change in workpiece geometry following manual deburring is also particularly evident in FIG. 3. The triangular or cake-wedge shaped tips on the pole side P of the polymer workpiece 1 of FIG. 1 are here repositioned and blunted owing to manual post-processing. This localized size reduction in the geometry increases the probability that a relative movement is possible between the polymer workpiece 1 shown and the associated anchoring component, i.e. in this case a hip socket. Such movements within the micrometer range can also detach polymer particles. This once again favors the occurrence of osteolysis in the area of bone tissue that lies in the vicinity of the artificial joint replacement, i.e. the anchoring component. This is true in particular for the interface area between the polymer workpiece 1 and the anchoring component connected therewith that faces the tissue.

(24) Unlike FIG. 3, the method according to the invention was used as a treatment method for the section of a polymer workpiece 1 shown in FIG. 4. It can be clearly seen from this section, which in turn corresponds to the section of the untreated polymer workpiece 1 shown in FIG. 2, that the workpiece geometry remaining after applying the method according to the invention comes much closer to the ideal geometry, namely that envisaged by the construction. Consequently, a higher fit accuracy can be realized with sections for a connection between the polymer workpiece 1 and the anchoring component, and thus the micro-movements mentioned above can be prevented.

(25) FIGS. 5 and 6 show an enlarged section of an edge area of the polymer workpiece 1 from FIG. 1. FIGS. 6a and 6b show a section like that of FIGS. 5a and 5b, although in a view that has been enlarged yet again. FIGS. 5a and 6a illustrate the result achieved with deburring by hand, whilst FIGS. 5b and 6b show the treatment result achieved with the method according to the invention.

(26) Manual deburring was carried out on the workpiece 1 shown in FIGS. 5a and 6a, where only the macroscopic irregularities recognizable in FIG. 2 could, however, be removed due to the deburring process and the properties of the polymer material. It does become clear from the scanning electron microscopic images in FIGS. 5a and 6a, however, that the remaining ridges or chips cannot be removed completely during deburring. The deburring, which is a cutting process in itself, also creates a new, if somewhat smaller ridge. This is illustrated in the upper area of FIG. 5a, and is once again illustrated more easily recognizable in an enlarged form in the upper area of FIG. 6a.

(27) It is also clear in FIG. 5a that on the surface polymer particles 22 are pressed into the workpiece surface due to the deburring process. These pose the risk that they may loosen when subjected to mechanical loads following implantation and could cause the disadvantages mentioned above.

(28) The sections of a polymer workpiece 1 shown in FIGS. 5b and 6b, which show a workpiece surface produced with the method, are, however, of a much more even appearance. No irregularities comparable to those in FIGS. 5a and 6a can be found on the workpiece surface in FIGS. 5b and 6b. Instead the surface shown has an almost smooth appearance created with the method by the brief high temperature increase in the explosion chamber. This also prevents a subsequent breakout or detachment of polymer particles 22 from the polymer workpiece 1 of the implanted artificial joint.

(29) A breakout of polymer particles 22 is in particular prevented especially in the edge area of a joint surface. It is exactly in this area in which the polymer workpiece 1 is subjected to great loads due to spatial contact with its joint partner in its implanted condition that the risk of a breakout of polymer particles is particularly high, but can be lowered substantially by using the present method.

(30) Overall not only are cost advantages realized due to the omission of complex deburring by hand and a clear acceleration of processing, but qualitative advantages are also achieved by the increased accuracy and comprehensive treatment of the workpiece surface.

REFERENCE NUMBERS

(31) 1 Polymer workpiece 10 Explosion chamber insert 12 Holder for workpiece 22 Polymer particle P Pole side G Ridge S Chip