Pelvic implant

Abstract

A pelvic implant for a hip joint prosthesis comprising: an acetabular cup (600) having an outer surface (605) for securing to the pelvis of a patient and an inner surface (603) for receiving a head of

Description

Field of the Invention The invention relates to a pelvic implant for a hip joint prosthesis. Background Hip joint prostheses are important medical devices that are used in joint replacement surgery to restore function of the hip joint and relieve pain. Around 100,000 hip prostheses are implanted each year in the UK [1], and this number is growing overtime, partly as a result of an aging population. Many hip implants currently in use may have an expected average survival of 15-20 years [2], This does not necessarily satisfy the increasing demand for younger and more active patients who need for these devices to last longer, ideally beyond their own lifetime. An example of a conventional total hip joint prosthesis 100 is illustrated in Figure 1. As shown in Figure la, the prosthesis 100 comprises a femoral implant 101 having a femoral head 102 connected to a femoral stem 103. The femoral head 102 may be metallic or, in some cases, ceramic. The femoral head 102 fits within an acetabular cup 108, which comprises a polymeric liner 104, typically polyethylene, which itself fits within an outer shell 105, typically metallic. The assembled prosthesis is shown in Figure lb. The acetabular cup 108 is implanted into the patient's pelvis 106 and the femoral stem 101 is implanted into the patient's femur 107, as shown in Figure 1c. When implanted, the femoral head 102 is contained within the liner 104, transferring load to the pelvis via the liner 104 and outer shell 105. In alternative examples, the acetabular cup 108 may be a single component. One of the main problems of current hip prostheses of the type illustrated in Figure 1 is the generation of wear particles during use. Such particles generally arise from wear of the polymeric acetabular cup liner 104 caused by sliding pressure on the liner from the femoral head 102. Polyethylene-based materials used for the liner 104 have good biocompatibility but relatively low wear resistance. Wear resistance may be improved by using Ultrahigh Molecular Weight Polyethylene (UHMWPE), cross-linked UHMWPE, surface coatings, heat treatment, blending additions, and improvement of the design such as the use of non-spherical bearing geometries, dual-mobility articulations to improve the rotation range to avoid severe wear due to impingement and edge-loading. Wear particles cannot, however, be entirely eliminated. This can cause problems particularly for long-term use of implants. Wear particles, which will tend to travel outside of the implant during use, can cause biological reactions in surrounding tissue that may result in as bone resorption [3], Trapped particles may also result in accelerated wear due to third body wear mechanisms that can then lead to early failure of the prosthesis [4], Reducing wear has therefore become an important strategy for improving the lifetime of hip joint implants. Over the past decade, the survival rate of hip implants has been significantly improved thanks to the new materials and designs. This has resulted in the 10-year revision rate being reduced from approximately 7% (2003-12) to around 5% (2008-19) in the UK [1], However, a plateau appears to have been reached. It would therefore be desirable to be able to extend the expected lifetime of hip implants beyond what is currently possible. Summary of the Invention According to a first aspect of the invention, there is provided a pelvic implant for a hip prosthesis, comprising: an acetabular cup having an outer surface for securing to the pelvis of a patient and an inner surface for receiving a head of a femoral implant, the acetabular cup comprising a fluid passage extending between the inner and outer surfaces along a path that is concentric with the inner surface of the acetabular cup, the fluid passage configured to collect wear particles from the hip prosthesis during use. The fluid passage may have first and second openings between the inner and outer surfaces at opposing sides of the acetabular cup. The acetabular cup may comprise an outer shell and an inner polymeric liner configured to fit within the outer shell, the inner polymeric liner having an outer surface configured to mate with an inner surface of the outer shell and the inner surface for receiving the head of the femoral implant. The fluid passage may be provided between the inner surface of the outer shell and the outer surface of the polymeric liner upon assembly of the polymeric liner and outer shell, the fluid passage extending around the outer surface of the polymeric liner. An advantage of the pelvic implant is that wear particles generated during normal use can be carried into the fluid passage and remain trapped therein, thereby preventing interaction of the wear particles with the surrounding tissue and implant. This can extend the lifetime of the hip prosthesis. The fluid passage may have first and second openings between the polymeric liner and the outer shell to permit fluid flow into and out of the fluid passage. The first and second openings may for example be located at diametrically opposing sides of the acetabular cup. In some arrangements the fluid passage extends into the polymeric liner. In other arrangements the fluid passage extends into the outer shell. The fluid passage may have a width of between around 2 mm and 5 mm. A total volume of the fluid passage may be between around 1 cm3 and 5 cm3. The fluid passage may comprise a textured wear particle collection region comprising a plurality of dimples. The dimples may extend into the polymeric liner, although in alternative arrangements the textured region may be provided in the outer shell. The dimples may have a re-entrant geometry, which can improve trapping of wear particles flowing through the fluid passage. The dimples may have a depth of between around 0.01 and 0.1 mm and a width of between around 0.1 and 1 mm [5], The fluid passage may comprise a series of grooves forming one or more Tesla valves extending along the fluid passage. The series of grooves may extend into the polymeric liner, or in other arrangements into the outer shell. The grooves may have a depth of between 0.1 and 0.5 mm. The polymeric liner may be composed of a polycarbonate polyurethane (PCU), polyether ether ketone (PEEK), UHMWPE or cross-linked UHMWPE material, which are approved by the U.S. Food and Drug Administration (FDA). The outer shell may be composed of a stainless steel, titanium alloy or fibre-reinforced PEEK material, approved by the FDA. According to a second aspect there is provided a hip prosthesis comprising: a femoral implant comprising a femoral stem and a femoral head; a pelvic implant according to the first aspect. Detailed Description The invention is described in further detail below by way of example and with reference to the accompanying drawings, in which: figure la is a drawing of an example hip prosthesis assembly; figure lb is a drawing of the example hip prosthesis assembly with the femoral head within the polymeric liner, which is in turn within the acetabular cup; figure 2a is a drawing of an example polymeric liner of a pelvic implant; figure 2b is a drawing of an example outer shell of an acetabular cup of a pelvic implant; figure 2c is a drawing of an example acetabular cup comprising the liner of figure 2a and the outer shell of figure 2b; figure 3 is a drawing of an example polymeric liner comprising tesla valves and dimples within a fluid passage extending around an outer surface of the polymeric liner; figure 4 is a schematic cross-sectional drawing of a dimple for the polymeric liner of figure 3; figure 5 is a drawing of an example alternative polymeric liner comprising a fluid passage extending around an outer surface of the polymeric liner; and figure 6 is a drawing of a further example alternative acetabular cup comprising a fluid passage extending through the cup. Figures 2a, 2b and 2c illustrate an example acetabular cup 200 comprising an outer shell 201 and a polymeric liner 202. The polymeric liner 202 is configured to fit within the outer shell 201 and has an inner surface 203 for receiving a femoral head of a femoral implant such as the femoral implant 101 illustrated in figure 1. A fluid passage 206 is provided between the inner surface 204 of the outer shell 201 and an outer surface 205 of the polymeric liner 202. The fluid passage 206 may extend into the outer surface 205 of the polymeric liner 202, into the inner surface 204 of the acetabular cup 201 or a combination of the two. Figure 2a shows the fluid passage 206 extending into the outer surface 205 of the polymeric liner 202. The fluid passage 206 extends around the outer surface 205 of the polymeric liner 202, in this example in the form of a circumferential groove around the polymeric liner 202. The fluid passage 206 thereby extends along a path that is concentric with the inner surface 204 of the outer shell 201. Figure 2b illustrates an alternative in which a groove 207 is provided extending into the inner surface 204 of the outer shell 201. Openings 208a-f are provided between the liner 202 and the outer shell 201 to permit fluid flow into and out of the fluid passage 206. The openings 208a-f are illustrated in figures 2a and 2c as extending into the polymeric liner 202. In alternative arrangements the openings may extend into the inner surface 204 of the outer shell 201. The openings 208a-f shown in figures 2a and 2c are uniformly distributed around an outer edge of the polymeric liner 202. In an example arrangement, at least two openings are provided, which may be located at diametrically opposing sides of the polymeric liner 202. This allows for flow of fluid, together with any wear particles, through the fluid passage 206. More than two openings 208a-f may be provided, as in figure 2c in which six openings 208a-f that are regularly spaced around the outer circumference of the liner 202. The function of the fluid passage 206 between the liner 202 and outer shell 201 is to move wear particles away from the moving surfaces of the femoral head 102 (Figure la) and the polymeric liner inner surface 203 and the surrounding tissue so that biological reactions such as bone reabsorption due to inflammation can be reduced or avoided. With a metal or ceramic femoral head articulating within a polyethylene based acetabular cup liner, polyethylene wear particles of sizes around 0.001-0.1 mm (1-100 pm) can result. Such particles are particularly biologically active and can be found in high levels in third body wear [3], It may be possible to prolong the lifetime of the implant by reducing wear and host tissue responses through transporting these wear particles away from the possibility of interaction with surrounding tissue and sliding surfaces of the implant. During use of the implant, bodily fluid around the implant flows due to sliding of the femoral head relative to the acetabular cup. This results in wear particles being carried by the fluid into the passage 206 driven by pressure differences in the joint capsule during joint motion. The wear particles can then become trapped in the fluid passage 206 by interaction with the passage surface, either on the polymeric material or the (typically) metallic surface of the outer shell 201. A typical diameter of the inner surface 203 of the polymeric liner 202 is around 36 mm. The liner 202 may have a thickness of around 4 mm. The outer shell 201 may have a diameter of around 44.8 mm of the inner surface 204 and a thickness of around 5 mm. The fluid passage may have a width of between around 2 mm and 5 mm. A total volume of the fluid passage may be between around 1 and 5 cm3. The total volume of the passage needs to be sufficient to be able to store the expected wear particles generated over the lifetime of the implant. Under normal operating conditions, a wear rate of a polyethylene liner may be around 10 mm3 per year [6], The polymeric liner 202 may be made from various materials such as PCU, PEEK, UHMWPE or cross-linked UHMWPE. The outer shell 201 may be made from various materials such as stainless steel, a titanium alloy or a carbon fibre reinforced PEEK material. Wear particles from the polymeric liner 202 may be negatively charged, while a metallic outer shell 201 may have a positively charged inner surface, causing the wear particles to be attracted to the inner surface of the outer shell and remain trapped within the fluid passage 206. A roughened or dimpled texture along a surface of the fluid passage can further assist in trapping wear particles that enter the fluid passage 206. An example of how this may be achieved is illustrated in the example polymeric liner 302 illustrated in figure 3. The fluid passage 306 comprises a textured wear particle collection region 311, which comprises a plurality of dimples 312. The dimples may have a depth of between around 0.01 and 0.1 mm and a width of between around 0.1 and 1 mm. As wear particles are transported along the fluid passage 306 during normal use of the implant, the particles can become trapped in the dimples 312 and thereby prevented from interacting with the surrounding tissue and implant surfaces. To assist flow of fluid and wear particles through the fluid passage 306, a series of grooves 313 may be provided that form Tesla valves 317a, 317b extending along the passage 306. The operating principle of a Tesla valve is described in US patent 1,329,599. In brief, the geometry of the Tesla valve results in a preferential fluid flow direction, which in figure 3 is indicated by arrows 314, 315. The preferential fluid flow direction causes the fluid, with wear particles, to be directed towards the textured region 311 so that the wear particles can be trapped. The grooves 313 may be a depth of between around 0.1 and 0.5 mm. Fluid flows into and out of the passage 306 via openings 308a, 308b. The openings 308a, 308b, Tesla valves and textured region 311 may be arranged such that a preferential fluid flow direction is set up from one opening to another opening. As illustrated in figure 3, the textured region 311 and Tesla valves can be formed in the polymeric liner 302 and may be formed by either machining or moulding. In alternative arrangements the textured region and / or Tesla valves may be formed in the outer shell. In further alternative arrangements the textured region and / or Tesla valves may be provided in both the liner 302 and the outer shell. Figure 4 illustrates in sectional view an example form of a dimple 312 in the polymeric liner 302. The dimple 312 has a re-entrant geometry, i.e. having a greater width below the surface of the polymeric liner 302. This may result in wear particles 316 becoming entrapped in the dimple 312, preventing the particles 316 from re-entering the fluid flow through the passage 306. Figure 5 illustrates an alternative example of a polymeric liner 502 with a fluid passage 506 extending between diametrically opposed openings, one of which 508 is visible in Figure 5. Rather than being oriented parallel to a face of the acetabular cup 200 as in the example illustrated in Figures 2 and 3, the fluid passage 506 extends transverse to the face of the acetabular cup, in this example extending orthogonal to the face of the acetabular cup. The various features of fluid passage as described above in relation to Figures 2 and 3 may also apply to the fluid passage 506. Figure 6 is a drawing of an alternative pelvic implant comprising a unitary acetabular cup 600. Instead of having an outer shell and inner liner, the acetabular cup 600 is composed of a single component. A fluid passage 606 extends between inner and outer surfaces 603, 605 of the acetabular cup 600 along a path that is concentric with the inner surface 603.The fluid passage 606 extends between openings 608a, 608b on a face 609 of the acetabular cup, the openings 608a, 608b being provided on diametrically opposed sides of the acetabular cup 600. As with the example in Figure 5, the fluid passage 606 is oriented transverse to the face 609 of the acetabular cup 600, in this case oriented orthogonal to the face 609. The various features of the fluid passage described above in relation to the examples in Figures 2, 3 and 5 may also apply to the fluid passage 606. In this case, detailed surface geometries may be more difficult to machine or mould in an acetabular cup of unitary construction and instead an engineered surface roughness of the fluid passage, which may be possible through 3-D printing techniques such as direct metal deposition, may achieve the required surface that entraps wear particles during use of the implant as fluid flows through the passage 606. As with the other examples described above, the fluid passage 606 may have a diameter or width or between around 2 to 5 mm, for example around 3 mm. Other embodiments are intentionally within the scope of the invention as defined by the appended claims. References | I | “National Joint Registry the 18th Annual Report,” 2021. (Page 62-63, Figure 3.H4) [2] J. T. Evans, J. P. Evans, R. W. Walker, A. W. Blom, M. R. Whitehouse, and A. Sayers, “How long does a hip replacement last? A systematic review and meta-analysis of case series and national registry reports with more than 15 years of follow-up,” Lancet (London, England), 2019, vol. 393, no. 10172, pp. 647^654. [3] A. Liu, L. Richards, C. L. Bladen, E. Ingham, J. Fisher, and J. L. Tipper, “The biological response to nanometre-sized polymer particles,” Acta Biomater., 2015, vol. 23, pp. 38-51. [4] H. J. Lundberg, D. R. Pedersen, T. E. Baer, M. Muste, J. J. Callaghan, and T. D. 5 Brown, “Effects of implant design parameters on fluid convection, potentiating third-body debris ingress into the bearing surface during THA impingement / subluxation,” J. Biomech., 2007, vol. 40, no. 8, pp. 1676-1685. [5] L. Gao, P. Yang, I. Dymond, J. Fisher, and Z. Jin, “Effect of surface texturing on the elastohydrodynamic lubrication analysis of metal-on-metal hip implants,” Tribol. 10 Int., 2010, vol. 43, no. 10, pp. 1851-1860. [6] L. M. Jennings et al., “(iv) Enhancing the safety and reliability of joint replacement implants,” Orthop. Trauma, 2012, vol. 26, no. 4, pp. 246-252.

Claims

1. A pelvic implant for a hip prosthesis, comprising:an acetabular cup having an outer surface for securing to the pelvis of a patient and an inner surface for receiving a head of a femoral implant, the acetabular cup comprising a fluid passage extending between the inner and outer surfaces along a path that is concentric with the inner surface of the acetabular cup, the fluid passage configured to collect wear particles from the hip prosthesis during use.

2. The pelvic implant assembly of claim 1, wherein the fluid passage has first and second openings between the inner and outer surfaces at opposing sides of the acetabular cup.

3. The pelvic implant of claim 1, wherein the acetabular cup comprises an outer shell and an inner polymeric liner configured to fit within the outer shell, the inner polymeric liner having an outer surface configured to mate with an inner surface of the outer shell and the inner surface for receiving the head of the femoral implant.

4. The pelvic implant of claim 3, wherein the fluid passage is provided between the inner surface of the outer shell and the outer surface of the polymeric liner upon assembly of the polymeric liner and outer shell, the fluid passage extending around the outer surface of the polymeric liner.

5. The pelvic implant of claim 4, wherein the fluid passage has first and second openings between the polymeric liner and the outer shell to permit fluid flow into and out of the fluid passage.

6. The pelvic implant of claim 5, wherein the first and second openings are located at diametrically opposing sides of the acetabular cup.

7. The pelvic implant of any one of claims 4 to 6, wherein the fluid passage extends into the polymeric liner.

8. The pelvic implant of any one of claims 4 to 7, wherein the fluid passage extends into the outer shell.

9. The pelvic implant of any preceding claim, wherein the fluid passage comprises a textured wear particle collection region comprising a plurality of dimples.

10. The pelvic implant of claim 9, wherein the dimples extend into the polymeric liner.

11. The pelvic implant of claim 9 or claim 10, wherein the dimples have a re-entrant geometry.

12. The pelvic implant of any one of claims 9 to 11, wherein the dimples have a depth of between around 0.01 and 0.1 mm and a width of between around 0.1 and 1 mm.

13. The pelvic implant of any one of claims 4 to 12, wherein the fluid passage comprises a series of grooves forming one or more Tesla valves extending along the fluid passage.

14. The pelvic implant of claim 13, wherein the series of grooves extend into the polymeric liner.

15. The pelvic implant of claim 13 or claim 14, wherein the grooves have a depth of between 0.1 and 0.5 mm.

16. The pelvic implant of any one of claims 3 to 15, wherein the polymeric liner is composed of a PCU, PEEK, UHMWPE or cross-linked UHMWPE material.

17. The pelvic implant of any one of claims 3 to 16, wherein the outer shell is composed of a stainless steel, titanium alloy or fibre-reinforced PEEK material.

18. The pelvic implant of any preceding claim, wherein the fluid passage has a width of between around 2 mm and 5 mm.

19. The pelvic implant of any preceding claim, wherein a total volume of the fluid passage is between around 1 cm3 and 5 cm3.

20. A hip prosthesis comprising:a femoral implant comprising a femoral stem and a femoral head; a pelvic implant according to any preceding claim.

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