BONE IMPLANT HAVING COATED POROUS STRUCTURE

Abstract

The invention relates to a bone implant, comprising a main body, which has, in its outer region, an open-cell porous lattice structure, which is formed from a plurality of regularly arranged elementary cells, the elementary cells being in the form of an assembled structure and each being composed of an interior and of a plurality of interconnected bars surrounding the interior. The porous lattice structure is provided with a bone-growth-promoting coating comprising calcium phosphate, the calcium phosphate coating having a hydroxylapatite proportion forming a pore inner coating extending into the depth of the porous lattice structure.

Claims

1. A bone implant, comprising a main body with an open-cell porous lattice structure in its outer region, said lattice structure comprising a plurality of regularly arranged unit cells, wherein the unit cells are an assembled structure and are constructed from an interior space and a plurality of interconnected bars surrounding the interior space, wherein the porous lattice structure is covered with a coating which promotes bone growth, comprising calcium phosphate, characterized in that the calcium phosphate coating has a hydroxyapatite content of less than or equal to 1 wt. %, and extends into the porous lattice structure.

2. The bone implant according to claim 1, wherein the calcium phosphate coating has a crystal phase which comprises brushite and monetite, and which is at least 90 wt. % wherein the brushite fraction is not less than is 65 wt. %.

3. The bone implant according to claim 1, wherein the calcium phosphate coating has a calcium/phosphate ratio in the range from 1.0 to 1.2.

4. The bone implant according to claim 1, wherein the interior spaces of the unit cells is between 10 and 25 μm.

5. The bone implant according to claim 1, wherein the calcium phosphate coating is unannealed.

6. The bone implant according to claim 1, wherein the calcium phosphate coating covers all sides of the assembled structure of the unit cells.

7. The bone implant according to claim 1, wherein the unit cells are arranged in layers to form an open-cell trabecular structure, and the unit cells are in a wurtzite structure.

8. The bone implant according to claim 1, wherein the open-cell porous lattice structure is a 3D printed structure, printed by means of electron beam melting (EBM) or selective laser melting (SLM).

9. The bone implant according to claim 1, wherein the main body is made of the same material as the open-cell porous lattice structure.

10. The bone implant according to claim 1, wherein the main body has a supporting region, said supporting region having a lower porosity than the porosity of the open-cell porous lattice structure.

11. The bone implant according to claim 1, wherein the inner spaces of the unit cells form macropores, the width of which is at least ten times the thickness of the coating (5), or the width of the pores is in the range between 0.4 and 2 mm and the coating has a thickness between 10 and 20 microns.

12. A method for producing a coated bone implant, having a main body which has an open-cell, porous lattice structure in its outer region, which lattice structure is formed from a plurality of regularly arranged unit cells, having the steps of: building up the regularly arranged unit cells as an assembled structure, each consisting of an interior space and a plurality of interconnected bars surrounding the interior space in such a way that the interior spaces are connected to each other, coating the porous lattice structure with a coating which promotes bone growth, comprising calcium phosphate, wherein the coating is produced with a hydroxyapatite content of less than or equal to 1 wt. %, and is applied into the porous lattice structure as an inner pore coating.

13. The method according to claim 12, wherein the coating has a crystal phase which comprises brushite and monetite, and which is at least 90 wt. %, and the fraction of brushite is not less than 65 wt. %.

14. The method according to claim 12, wherein the coating is applied to all sides of the porous lattice structure by an electrochemical method.

15. The method according to claim 14, wherein a current is used for the electrochemical method, which follows a current curve which, after an initial peak current, falls back to a lower working current.

16. The bone implant according to claim 10, wherein the main body has a solid supporting region and the open-call porous lattice structure are designed as a single unit.

Description

[0031] The invention is explained in more detail below with reference to the attached drawing, based on advantageous embodiments. In the figures:

[0032] FIG. 1 is an embodiment of an implant according to an embodiment of the invention;

[0033] FIG. 2 is a detailed view of a unit cell of the porous structure of the implant according to FIG. 1;

[0034] FIG. 3 is a schematic view of the unit cells and the elements forming them;

[0035] FIG. 4a, b are sectional views in two orthogonal directions of the porous structure;

[0036] FIG. 5 is an image of an augmentation according to a second embodiment of the invention;

[0037] FIG. 6a, b are a schematic side view and front view of the augmentation according to FIG. 5;

[0038] FIG. 7 is a comparative table showing bone ingrowth behavior; and

[0039] FIG. 8 is a diagram of the current curve during electrochemical coating according to the invention.

[0040] A first embodiment of an implant according to the invention is shown in FIG. 1. This implant is a cone 1 for the tibial component of a knee joint endoprosthesis (not shown).

[0041] The cone 1 forms a replacement for defective bone material at the proximal end of the tibia, so as to fill cavities which have arisen due to the absence of damaged bone material. In this way, a complete base is created upon which the tibial component of the knee joint endoprosthesis can be securely placed. For this purpose, the cone 1 is produced using the open-cell, porous lattice structure that is provided with a coating according to the invention to improve the growth of bone material into and/or onto the same. In this case, the open-cell, porous lattice structure 3 is applied to a main body 2.

[0042] Thanks in particular to the arrangement of this open-cell, porous lattice structure 3 on the outside of the cone 1, good ingrowth behavior of bone material from the surrounding tibia bone (not shown) can be achieved, resulting in the cone 1 being fixed quickly and securely in the tibia.

[0043] The porous structure 3 is formed by a plurality of regularly arranged unit cells 4. A detailed view of an unit cell 4 and its integration into the surrounding unit cells is shown in FIG. 2. The unit cell 4 has an interior space 40 which is connected to the interior space 40′ of adjacent unit cells 4′. The unit cells are arranged regularly along a layering plane 49. Advantageously, several layer levels are arranged one above the other.

[0044] The regular arrangement of the unit cells can be seen particularly well from the side views in FIG. 4a, b. These figures show isometric views along the two orthogonal axes (see axes x, y in FIG. 3) which define the layering plane 49. It can be seen that different cross-sectional views, in particular with regard to the form of the interior 40, are produced in the two directions. This is a special property of the crystal structure used, namely the wurtzite structure. It ensures that the open-cell, porous lattice structure formed in this way has different compression stiffnesses in different spatial directions, which is favorable in terms of adaptation to the anatomical conditions of the bone. It can also be seen there that adjacent inner spaces 40 are connected to each other, such that the macropores formed by the unit cells 4 with their inner spaces 40 are connected to each other in an open-celled manner (they form so-called “interconnected pores”).

[0045] The actual structure of the unit cells 4 is shown schematically in FIG. 3. In the embodiment shown, the unit cells 4 are formed from basic elements 45, which are each designed as tetrapods. It should be understood that basic elements other than tetrapods can also be provided. Each of these tetrapods has four legs 41, 42, 43, 44 designed as bars which are each connected to each other at one end, and thus form a node there. The tetrapods can be formed regularly or irregularly, with equal leg lengths or different leg lengths. Shown is a regular embodiment, where the legs are of equal length and each leg forms the same angle with each of the other legs. In the arrangement of the tetrapods in a flat layering, three legs 41, 42, 43 are arranged standing up on a plane, while the fourth leg 44 is oriented perpendicular to the plane. This fourth leg thus represents a connection to the tetrapods of a layering level arranged above it (see FIG. 3).

[0046] By choosing the number of layering levels, the depth of the open-cell porous structure can be controlled. For example, three or four or five superimposed layers can be provided (see FIG. 4a, b), but typically at least two superimposed layers are provided. A titanium alloy or pure titanium is preferably used as the material for the open-cell porous structure.

[0047] A second embodiment is shown in FIGS. 5 and 6. FIG. 5 is a photographic image. It shows a cylindrical augmentation 1′, such as can also be used for filling bone defects, or optionally also for the purpose of fusing neighboring bone elements, in particular vertebral bodies. It has a substantially sleeve-shaped main body 2′, which is generally cylindrical in shape. The main body 2′ is provided with the open-cell, porous lattice structure 3′ on its shell surface. As can be seen particularly well in the schematic view in FIG. 6a, b, it is also formed from unit cells 4 with their interior spaces 40 connected to each other, the unit cells 4 in turn consisting of tetrapods as basic elements 45.

[0048] As can be seen particularly well from the photographic image in FIG. 5, the open-cell, porous lattice structure 3′ formed by the unit cells 4 is provided with a coating 5 that appears somewhat rough in the image. The coating 5 is applied over the surface of the open-cell porous lattice structure 3′ and the two end regions of the main body 2′, and further also in the depth of the structure 3′ in the interior spaces 40 of the unit cells 4.

[0049] Exemplary dimensions for the length and width of the cylindrical sleeve-like main body 2′ are 12 mm in length and 6 mm in diameter as width. The inner spaces 40 of the unit cells 4 forming the open-cell porous structure 3′ have a width of approximately 700 μm, and the depth of the open-cell porous structure 3′ extends over approximately 2000 μm. Viewed in unit cells 4, this results in a depth of almost three layers of unit cells 4.

[0050] The coating 5 has a combined crystal phase of brushite and monetite with a fraction of 95 wt. %, the fraction of brushite being at least 65 wt. %. Furthermore, the coating 5 completely sheathes the unit cells 4 with their cavities 40, not only in the uppermost layer but also in the layers below.

[0051] According to the invention, this results in significantly improved ingrowth of bone material during the process of osteointegration and osteoconduction. Results for a comparison experiment with a comparison implant that has an open-cell porous structure of the same shape, but without a coating 5 according to the invention, are shown in FIG. 7. In the figure, a quantitative histomorphometric analysis is shown; the bone/implant contact ratio expressed as a percentage is plotted along the Y-axis for two different regions (ROI1 and ROI2). The two pairs of columns on the left represent the comparison implant (“C1”), and the two pairs of columns on the right represent the tested implant (“T”) according to the invention. The left column in each pair of columns shows the short-term ingrowth (measured at 4 weeks), and the right column in each pair of columns shows the long-term ingrowth (measured at 26 weeks). One can clearly see that with the implant (“T”) according to the invention, excellent ingrowth of bone material is already achieved after 4 weeks, with the comparative example only achieving a similar value after a good six times as long, namely after 26 weeks. This impressively demonstrates the bone growth-promoting property of the coating according to the invention.

[0052] An electrochemical process is expediently used for the coating. The profile of the current during the electrochemical coating is shown in FIG. 8.

[0053] It can be seen that a high peak current is initially set, which is then reduced to a lower working current. With this current profile, a particularly good precipitation reaction of the calcium phosphate, which is particularly suitable for the thin and uniform coating, can be achieved, with the combined brushite/monetite phase being formed with its high proportion of 95%.