MEDICAL IMPLANTS COMPRISING ANTI-INFECTIVE SURFACES

Abstract

A medical implant comprising: an implant body configured for use as a medical implant, a surface on the implant body, wherein the surface comprises a plurality of projections (40), each projection having a base proximal to the implant body, a peak distal to the implant body, and a side wall extending from the base to the peak, wherein the surface has a peak density in a range 50 to 500 peaks per 11 m.sup.2, and wherein the projections are tapered such that a width at the peak of each projection is less than a width at the base of each projection.

Claims

1. A medical implant comprising: an implant both configured for use as a medical implant, a surface on the implant body, wherein the surface comprises a plurality of projections, each projection having a base proximal to the implant body, a peak distal to the implant body, and a side wall extending from the base to the peak, wherein the surface has a peak density in a range 50 to 500 peaks per m2, and wherein the projections are tapered such that a width at the peak of each projection is less than a width at the base of each projection.

2. A medical implant according to claim 1, wherein the peak density is greater than 100, 150, or 200 peaks per m2.

3. A medical implant according to claim 1, wherein the peak density is less than 400, 300, or 250 peaks per m2.

4. A medical implant according to claim 1, wherein the surface has a kurtosis in a range 2.50 to 4.00.

5. (canceled)

6. (canceled)

7. A medical implant according to claim 1, wherein the surface has a skewness in a range 0.20 to +0.30.

8. (canceled)

9. (canceled)

10. A medical implant according to claim 1, wherein the surface has a surface roughness (Ra) in a range >5 nm to 18 nm.

11. A medical implant according to claim 10, wherein the surface roughness (Ra) of the surface is greater than 6 nm, 7 nm, or 7.5 nm.

12. A medical implant according to claim 10, wherein the surface roughness (Ra) of the surface is less than 14 nm, 12 nm, 10 nm, or 9 nm.

13. A medical implant according to claim 1, wherein the surface has an average feature size in a range 25 nm to 65 nm.

14. A medical implant according to claim 13, wherein the average feature size of the surface is greater than 30 nm.

15. A medical implant according to claim 13, wherein the average feature size of the surface is less than 55 nm, 45 nm, or 40 nm.

16. A medical implant according to claim 1, wherein the width at the base of each projection is at least 1.2, 1.4, 1.6, 1.8 or 2 times the width of each projection at th of a height of each projection.

17. A medical implant according to claim 1, wherein the peak of each projection is rounded.

18. A medical implant according to claim 17, wherein the rounded peak of each projection has a radius of curvature in a range 5 nm to 200 nm.

19. A medical implant at to claim 1, wherein the projections having a height from base to peak in a range 30 nm to 90 nm.

20. (canceled)

21. (canceled)

22. A medical implant according to claim 1, wherein the surface is formed by a coating on the implant body.

23. A medical implant according to claim 1, wherein the surface is formed of titanium or a titanium alloy.

24. (canceled)

25. A medical implant according to claim 1, wherein the implant body is a prosthetic joint or a component thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Embodiments of the present invention are described by way of example only with reference to the accompanying drawings in which:

[0024] FIG. 1(a) shows a schematic illustration of how a small, rigid bacteria cell and a large, deformable host (mammalian) cell interact with a prior art nano-columnar surface;

[0025] FIG. 1(b) shows a sample of a prior art nano-columnar coating;

[0026] FIG. 2(a) shows a schematic illustration of how a small, rigid bacteria cell and a large, deformable host (mammalian) cell interact with a tapered projection surface as described herein;

[0027] FIG. 2(b) shows a sample of a coating as described herein;

[0028] FIGS. 3 to 6 show four examples of surfaces as described herein (referred to as runs 18 to 21 respectively);

[0029] FIG. 7 shows the percentage surface coverage (hMSCs) following a race to the surface co-culture test with S. aureus for surfaces shown in FIGS. 3 to 6;

[0030] FIG. 8 shows the density of peaks for the four surfaces shown in FIGS. 3 to 6;

[0031] FIG. 9 shows the average feature size (nm) for the four surfaces shown in FIGS. 3 to 6;

[0032] FIG. 10 shows the surface roughness Ra (nm) for the four surfaces shown in FIGS. 3 to 6;

[0033] FIGS. 11(a) and 11(b) illustrated the surface parameters of skewness and kurtosis respectively;

[0034] FIG. 12 shows kurtosis data for the four surfaces shown in FIGS. 3 to 6;

[0035] FIG. 13 shows skewness data for the four surfaces shown in FIGS. 3 to 6;

[0036] FIG. 14 illustrates a sputtering coating technique;

[0037] FIG. 15 illustrates surface processing steps including etching and coating; and

[0038] FIG. 16 shows a stem of a prosthetic hip joint to which surface processing as described herein has been applied.

DETAILED DESCRIPTION

[0039] FIG. 1(a) is a schematic illustration of how a small, rigid bacteria cell 10 and a large, deformable host (mammalian) cell 20 interact with a prior art nano-columnar surface 30. The nano-columnar surface comprising a plurality of column-like projections 30. Typical prior art nano-columnar coatings have a column height between 100 nm and 300 nm, a column diameter in a range 30 to 100 nm, and a surface roughness R.sub.a of less than 5 nm. The columnar structures are generally formed by a glancing angle deposition technique which results in columnar projections with an inclination angle up to 30 relative to vertical. The inclination angle is not shown in the schematic of FIG. 1 but can be seen in the image of an actual prior art nano-columnar coating shown in FIG. 1(b).

[0040] As can be seen in FIG. 1(a), a bacterial cell is small with a rigid cellular wall and has a low contact surface area with such a nano-columnar coating. As such, adhesion of bacterial cells is low. In contrast, host cells such as human Mesenchymal stem cells (hMSCs) are large and deformable and extend partially down the side walls of the columnar projections thus having a larger contact surface area and a higher associated adhesion. That said, the adhesion of host cells to such a nano-columnar surface is still reduced when compared to a planar surface. Furthermore, as described in the background and summary of invention sections, the thin columnar projections are relatively fragile and can be damaged and fragmented during storage or in use. This may result in nanoparticles being released into a patient's system and may cause adverse effects on organs, tissue, and cells. In addition, physical damage to the topography of the surface during storage of the implant or in use can adversely affect the functional performance of the surface in terms of reducing bacterial adhesion.

[0041] FIG. 2(a) shows a schematic illustration of how a small, rigid bacteria cell 10 and a large, deformable host (mammalian) cell 20 interact with a tapered projection surface 40 as described herein. The surface has a peak density in a range 50 to 500 peaks per m.sup.2 and the projections are tapered such that a width at the peak of each projection is less than a width at the base of each projection. An actual sample comprising such a surface structure is shown in FIG. 2(b).

[0042] As with the prior art nano-columnar structure, a bacterial cell is small and rigid and has a low contact surface area with such a surface resulting in low bacterial cell adhesion. Host cells such as human Mesenchymal stem cells (hMSCs) are large and deformable and extend partially down the side walls of the columnar projections thus having a larger contact surface area and a higher associated adhesion. However, in contrast to the nano-columnar structure of FIG. 1, the tapered projections increase the contact surface area to a level not far removed from a planar surface such that adhesion of the host cell is not significantly reduced. As such, the ratio of host cell adhesion to bacterial cell adhesion is improved when compared with the nano-columnar structure. Further still, the smaller tapered projections are more mechanically robust and less prone to damage and fragmentation. This improves the reliability of functional performance in use and reduces the risk of fragmented nanoparticles causing adverse local or systemic tissue reactions.

[0043] FIGS. 3 to 6 show four examples of surfaces as described herein (referred to as runs 18 to 21 respectively). All the surfaces share the common feature of having a peak density in a range 50 to 500 peaks per m.sup.2 and the projections are tapered such that a width at the peak of each projection is less than a width at the base of each projection. The surfaces vary in terms of their more detailed structure as further described later in this specification.

[0044] A test was developed to analyze the adhesion of bacteria on the prepared surfaces using a modified Atomic Force Microscopy (AFM) probe. The technique requires adhesion of a single bacterium on to the AFM probe, the probe is then brought into contact with the surface coating allowing the bacteria to form an attachment to the surface and then the probe is removed. The force required to remove the bacteria was measured and recorded. A study was conducted investigating the adhesion of S. epidermidis and P. aeruginosa to titanium alloy (Ti6Al4V) surfaces following either a polished surface finish or magnetron sputter coating process (Runs 18 and 21). Results are shown in Table 1 below.

TABLE-US-00001 S. epidermidis P. aeruginosa Proportion Proportion strong % Strong strong % Strong adhesion adhesion adhesion adhesion Sample (>1.5 nN) (>1.5 nN) Sample (>1.5 nN) (>1.5 nN) No 0/239 0 No 0/239 0 Bacteria Bacteria Polished 23/257 8.9 Polished 26/191 13.6 control control Run 24 20/227 8.8 Run 24 10/187 5.3 (18) (18) Run 25 36/187 19.3 Run 25 29/241 12.0 (21) (21)

[0045] The results indicate that Run 18 encountered the least number of strong adhesions from AFM probing of either strain of bacteria.

[0046] A race to the surface test was conducted, used to determine the rate at which an introduced bacterial strain colonizes and envelopes a surface in competition with human mesenchymal stem cells (hMSCs). This is measured by quantifying the surface coverage of hMSCs following 24 hours of dual incubation of bacteria and hMSCs. In this test Ti6Al4V surfaces were exposed to S. aureus bacteria suspensions (510.sup.2 bacteria/ml) for 60 minutes under 100% humidity. Samples were removed from the suspensions, removing any un-attached bacteria. hMSCs were seeded on the bacterial-coated samples (310.sup.4 cells/ml) and maintained at 37 in a humidified 5% CO.sub.2 atmosphere for 24hrs. The hMSC surface coverage at the 24 hour time point is presented in FIG. 7 along with a control sample (sm.contr.).

[0047] From analysis of the results it appears that Run 18 is most successful at resisting the proliferation of the S. aureus bacteria strain at the 24hr period. It is important to keep in mind that in these tests, if given sufficient time, the bacterial strain is inevitably going to out-compete the hSMCs for coverage of the surfaces. This is due to the virility of the bacterial strain and the lack of immune response available to resist surface biofilm formation.

[0048] FIG. 8 shows the density of peaks (per m.sup.2) for the four surfaces shown in FIGS. 3 to 6. All the surfaces have a peak density in a range 50 to 500 peaks per m.sup.2 with the most successful sample, sample 18, having a density of peaks of approximately 225 peaks per m.sup.2 representing a preferred value for this parameter, at least within the context of this study.

[0049] The data of FIG. 8 was generated using analysis software which derives summits from peaks in the surface height data. The analysis software defines a peak as any point, above all eight nearest neighbours. Summits are constrained to be separated by at least 1% of the minimum X or Y dimension comprising the 3D measurement area. Additionally, summits are only found above a threshold that is 5% of the depth at the lowest point above the mean plane.

[0050] FIG. 9 shows the average feature size (nm) for the four surfaces shown in FIGS. 3 to 6. All surfaces have an average feature size in a range 25 nm to 65 nm with the most successful sample, sample 18, having an average feature size of approximately 35 nm representing a preferred value for this parameter, at least within the context of this study. In this regard, the average feature (or grain) size is calculated using commercially available software (Gwyddion software) following an image processing threshold operation which segments the image in order to identify representative grains, exclude very small features, and then calculate the average feature size by averaging the equivalent square size of the identified grains.

[0051] FIG. 10 shows the surface roughness Ra (nm) for the four surfaces shown in FIGS. 3 to 6. All surfaces have a surface roughness (Ra) in a range >5 nm to 18 nm with the most successful sample, sample 18, having a surface roughness of approximately 8 nm representing a preferred value for this parameter, at least within the context of this study.

[0052] FIGS. 11(a) and 11(b) illustrated the surface parameters of skewness and kurtosis respectively. As previously described in the summary section, skewness is a measure of asymmetry in a histogram of projection height distribution whereas kurtosis is a measure of whether the surface is peaked or flat relative to the mean.

[0053] FIG. 12 shows kurtosis data for the four surfaces shown in FIGS. 3 to 6. All surfaces have a kurtosis in a range 2.50 to 4.00 with the most successful sample having a value of approximately 3.00 which represents a preferred value for this parameter, at least within the context of this study.

[0054] FIG. 13 shows skewness data for the four surfaces shown in FIGS. 3 to 6. All surfaces have a skewness within a range 0.20 to +0.30 with the most successful sample, sample 18, having a value of approximately 0.075 which represents a preferred value for this parameter, at least within the context of this study.

[0055] In addition to the above, it should also be noted that sample 18 in particular exhibits rounded peaks. As previously indicated, the combination of a relatively high peak density in combination with rounded peaks, and the fact that staphylococcus bacterial bodies are generally spherical and relatively rigid, actually results in low bacterial adhesion. Furthermore, the surface is more amenable to adhesion of larger, deformable host cells which can result in an overall improvement in performance.

[0056] As described in the summary section, surfaces as described herein can be formed by a coating on an implant body. However, it is also envisaged that such surfaces can be formed directly into the implant body by, for example, etching. The surfaces can be formed of titanium or a titanium alloy such as a titanium aluminium vanadium alloy. Such materials are consistent with those used presently for implant bodies such as prosthetic joints and components thereof.

[0057] One such method utilizing a sputtering coating technique. FIG. 14 illustrates a sputtering coating technique. An ion source directs argon ions into a target which can be formed of titanium or a titanium alloy and which functions as a sputtering source. Atoms are ejected from the target and coated on a suitably placed substrate, which in the present case is an implant body such as prosthetic joint or a component thereof. The method can utilize etching (e.g. chemical and/or ion etching), passivation, and coating steps. FIG. 15 illustrates the surface processing steps including etching and coating. The table below lists a number of different processes used to fabricate samples 18 to 21 which have been previously discussed.

TABLE-US-00002 Ion etch Chemical during process etching passivation deposition Run number 1 x x 18, 24 2 x x x 19, 22 3 x x 20, 23 4 x 21, 25

[0058] The specific conditions for each step can be tailored to achieve the desired final surface finish. The specific operational parameters values will vary according to the equipment used. However, a person skilled in the art will be able to tune the operating parameters to achieve a desired final surface finish relatively easily given the teachings as provided herein and their common general knowledge of etching and deposition equipment. The critical feature is knowing what surface structure is desired for a particular application.

[0059] FIG. 16 shows a stem of a prosthetic hip joint to which the surface processing has been applied. The part has been segmented to allow for better characterisation of the surface structure. The stem comprises both non-porous regions and also regions formed of a porous coating to promote bone adhesion and ingrowth. Coatings as described herein can be applied to both the porous and non-porous regions of the medical implant. Furthermore, it is possible to reliably coat relatively complex three dimensional implant components compared to prior art methods of fabricating nano-columnar surfaces using a glancing angle deposition technique.

[0060] The present specification thus enables the provision of advanced titanium implants with controlled nanotopographies for dual regulation of bacterial and mammalian cell adhesion. While the invention has been described in relation to certain embodiments it will be appreciated that various alternative embodiments can be provided without departing from the scope of the invention which is defined by the appending claims.