BONE IMPLANT WITH COATED POROUS STRUCTURE

Description

[0001] The invention relates to a bone implant with a main body that has an open-cell porous lattice structure on its exterior, coated with a bone-growth-promoting layer comprising calcium phosphate. For implants, especially endoprostheses and bone grafts implanted in or on bone, a physiologically favorable and stable connection between the implant and bone is of particular importance. Furthermore, it is desirable that this connection be established as quickly as possible to achieve rapid remobilization of the patient. For this purpose, it is known to coat the implant with a bone-growth-promoting layer. This promotes the growth of bone cells and thus accelerates the ingrowth of the implant into or on the bone. It is essentially irrelevant whether the coating is applied to smooth or textured surfaces. In both cases, it fulfills its purpose.

[0002] Various materials are known for use as coatings. They all share the characteristic of being bioactive and, in particular, exhibiting bone growth-promoting properties. Depending on the material, they can be applied in different ways, for example, by plasma spraying, sputtering, or dipping. Calcium phosphate (CaP) is a material with favorable bone growth-promoting properties that has been known since the 1990s. It is particularly well-established in implantology for the clinical application of thin and soluble coatings.

[0003] Implants with coated, porous structures are known, for example, from US 2018 / 193152 A1, US 9456901 B2 and RU 2684617 C1.

[0004] More recently, implants manufactured using additive processes (e.g., 3D printing) have become known, allowing for the creation of a regular, macroporous structure. This is also known in the field as a trabecular structure. This structure inherently exhibits favorable osseointegration properties. However, this structure has only benefited to a limited extent from a coating with the well-known calcium phosphate. A special formulation has emerged in which calcium phosphate with a calcium / phosphate ratio of 1.1 is used as the coating, with a brushite content of at least 70% and a hydroxyapatite content of up to 30%.

[0005] The invention is based on the objective of creating a bone implant of the type mentioned above that exhibits improved ingrowth behavior.

[0006] The solution according to the invention lies in the features of independent claims 1 and 12. Advantageous further developments are the subject of dependent claims.

[0007] In a bone implant with a main body having an open-cell porous lattice structure in its exterior, formed from a plurality of regularly arranged unit cells, wherein the unit cells are designed as a constructed structure and each consists of an interior space and a plurality of interconnected struts surrounding the interior space, wherein the porous lattice structure is provided with a bone-growth-promoting coating comprising calcium phosphate, the invention provides that the calcium phosphate coating has a hydroxyapatite content of less than 1 wt% and forms an inner pore coating extending into the depth of the porous lattice structure.

[0008] First, some terms used will be explained: An "open-cell" porous lattice structure means that the pores are not isolated, but rather that the individual pores are interconnected. This results in an overall open cell structure, with the pores formed within the individual cells.

[0009] A "built structure" is understood to mean that the structure is additively manufactured. Various additive manufacturing processes are suitable for this purpose. 3D printing processes, such as electron beam melting or selective laser melting, are particularly well-suited.

[0010] The term "extending into the depths of the porous lattice structure" here means that the inner coating of the pores not only reaches the pores located at the surface, but also encompasses the pores located deeper within the material (i.e., further from the surface), especially those pores that lie several (pore) layers away from the surface. A "wurzite structure" is understood to be a structure formed according to the model of the wurzite crystal form (similar to how a diamond structure is understood to be a structure based on the model of the diamond crystal form).

[0011] The core of the invention is the idea of ​​modifying and improving the known calcium phosphate coating such that it is practically free of hydroxyapatite (less than 1 wt%). Typically, calcium phosphate coatings known from the prior art have a hydroxyapatite content of well over 20 wt% or more, as does the special formulation mentioned at the outset. By eliminating practically all hydroxyapatite from the calcium phosphate used for the coating, the invention achieves higher solubility. In a second step, the invention utilizes this advantage to coat the cavities created by the unit cells, even in their depth, thus making greater use of the high solubility.

[0012] The invention thus creates a special type of coating, departing from the previously prevailing concept of a relatively easy-to-apply and well-adhering coating on the implant surface. Due to the extremely low or even non-existent hydroxyapatite content, the invention foregoes the inherently good adhesion of the calcium phosphate material typically used for implant coatings. At first glance, this may seem counterintuitive; however, the invention recognizes that a decisive advantage can be realized from the apparent disadvantage of reduced adhesion. Specifically, it opens up the possibility of incorporating this inherently less adhesive calcium phosphate deeper into the open-cell structure, thereby making its bone-growth-promoting properties available even at depth within the open-cell structure.Surprisingly, this results in a decidedly positive effect, namely faster ingrowth of bone cells into the implant, without any loss of long-term effectiveness. This is unprecedented in the current state of the art.

[0013] Preferably, the calcium phosphate coating is configured to have a crystalline phase comprising brushite and monetite. This combined crystalline phase constitutes at least 90% by weight, preferably at least 95% by weight.

[0014] The extremely small proportion of hydroxyapatite according to the invention allows the combined crystalline phase of brushite / monetite to have a very high proportion, possibly even 99% or more. It is preferably provided that the brushite content is no less than 65% by weight. There is no lower limit for the monetite content. Since brushite is more readily broken down by the organism receiving the bone implant, the minimum brushite content, in particular, ensures optimal solubility and thus bone ingrowth.

[0015] Advantageously, the average thickness of the calcium phosphate coating is dimensioned such that the interiors of the unit cells remain interconnected, preferably between 10 and 25 µm, and more preferably 15 µm ± 5 µm. This preserves the open-cell structure despite the coating, which facilitates the penetration of osteoblasts into the cells. This improves the osteointegration behavior.

[0016] Furthermore, the calcium phosphate coating preferably has a calcium / phosphate ratio in the range of 1.0 to 1.2, preferably 1.05 to 1.15. The higher phosphate content (compared to the prior art ratio of approximately 1.6) ensures greater solubility, which in turn, especially together with the coating extending deep into the structure, promotes bone ingrowth. The calcium phosphate coating also preferably has a brushite phase comprising at least 90% by weight, preferably at least 95% by weight (and optionally up to 100% by weight). This ratio allows for a high brushite content in the calcium phosphate with a minimum or no hydroxyapatite content, resulting in the advantageous effects described above. In particular, this ensures that the calcium phosphate coating is applied to all sides of the constructed unit cell structure, especially its ribs.In particular, an omnidirectional (all-around) lining of the pores with the calcium phosphate coating can be formed, even in pores that have complex or undercut structures.

[0017] Furthermore, it is expedient that the calcium phosphate coating is untempered. This simplifies manufacturing and has the advantage of avoiding an undesirable reduction in the brushed particle content, which would be caused by tempering.

[0018] Advantageously, the unit cells are arranged in layers, with several layers stacked on top of each other, preferably as an open-cell trabecular structure. This allows for the creation of a deeper open-cell structure, which in turn facilitates deeper ingrowth through osteointegration. The calcium phosphate coating is also expediently incorporated into a deeperA layer, preferably in all layers, is applied. A deeper layer is defined here as a layer that is not directly on the surface, but deeper within the material. This further improves the bond between the implant and the surrounding bone, thus promoting both short- and long-term stability.

[0019] Advantageously, the open-cell porous lattice structure is produced using 3D printing, preferably by electron beam melting (EBM) or selective laser melting (SLM). This allows for the efficient, rapid, and controlled production of components from metallic materials, including those with complex and numerous undercuts and cavities. The structure of the unit cells can be precisely defined, enabling a defined arrangement of the cells and their constituent elements, particularly their struts. These methods are especially suitable for manufacturing the implant from biocompatible material, particularly metallic material selected from a group comprising pure titanium, titanium alloys, cobalt-chromium, tantalum, stainless steel, and zirconium, preferably titanium grade 2 or 4.

[0020] Advantageously, the main body is made of the same material as the open-cell porous lattice structure. This allows the same cost-effective, biocompatible material to be used for the main body as well. Furthermore, this enables a seamless and, if necessary, stepless transition between the open-cell porous structure and the main body itself. It also allows for more efficient manufacturing. This is especially true if the main body also has a load-bearing area. This area can also advantageously exhibit a certain degree of porosity, which, however, typically differs from that of the open-cell lattice structure and is preferably lower. It is particularly advantageous if the load-bearing area is made of solid material.This not only results in higher mechanical strength, but also allows a barrier effect to be achieved in the sense of a partition wall, for example to separate inner and outer areas or to prevent the passage of materials such as bone cement and / or body fluids.

[0021] It is particularly advantageous if the bearing surface can be designed as a single unit with the open-cell porous structure. This allows for particularly efficient manufacturing and a seamless transition. The latter, in particular, offers the advantage of minimal irritation to the surrounding tissue and thus further promotes ingrowth.

[0022] Preferably, the unit cells are designed in a wurcite structure. This differs from the known diamond structure in that the diamond structure exhibits the same stiffness in all three spatial dimensions, while the wurcite structure exhibits varying stiffness in the spatial directions. This allows for better adaptation of the stiffness behavior to anatomical conditions using the wurcite structure, thereby increasing the biocompatibility of the implant.

[0023] Advantageously, the unit cells are macroporous. In this context, this means, in particular, that their interior spaces form macropores with a diameter between 0.4 and 2 mm, preferably 0.7 to 1.5 mm. The depth of the porous structure is expediently chosen such that at least two layers of unit cells lie on top of each other. Such a macroporous, open-cell structure, with its large interconnected spaces, offers particularly favorable conditions for the cross-linked ingrowth of bone.

[0024] Preferably, in contrast to the relatively large size of the pores formed by the unit cells, the coating is comparatively thin. Advantageously, the coating has a thickness of only between 10 and 20 µm. With such a thin coating, a kind of inner lining of the macropores formed by the unit cells can be achieved, such that the porosity and, in particular, the open-cell structure (i.e., the connection between the individual spaces) are perfectly preserved. This is particularly advantageous with regard to bone ingrowth behavior, both for osteoinduction and osteoconduction. Advantageously, the ratio between the width of the space formed by the unit cells, on the one hand, and the thickness of the coating, on the other, is chosen such that the width of the space is at least ten times, and preferably between 30 and 200 times, the thickness of the coating.

[0025] The invention further extends to a method for producing a correspondingly coated implant with a main body having an open-cell porous lattice structure in its exterior, formed from a plurality of regularly arranged unit cells, comprising the steps of: constructing the regularly arranged unit cells as a built structure consisting of an interior space and a plurality of interconnected struts surrounding the interior space such that the interior spaces are connected to each other; coating the porous lattice structure with a bone growth-promoting coating comprising calcium phosphate, wherein, according to the invention, the coating is produced with a hydroxyapatite content of less than 1 wt% and is applied as an inner pore coating to the depth of the porous lattice structure.Advantageously, the coating is applied such that it has a crystalline phase comprising brushite and monetite, comprising at least 90% by weight, preferably at least 95% by weight, wherein the brushite content is not less than 65% by weight. For further explanation, reference is made to the preceding description.

[0026] Preferably, the coating is applied to the porous lattice structure on all sides as a precipitation, preferably by means of an electrochemical process. This allows for an omnidirectional internal pore coating, i.e., lining of the pores with the bone-growth-promoting coating. Advantageously, the electrochemical process uses a current that follows a current curve, which falls back to a lower operating current after an initial peak current. The invention has recognized that this reduced current leads to an improved precipitation reaction, particularly of the combined crystalline phase with brushite / monetite of calcium phosphate, on the structural elements of the unit cells, especially at depth within the structure. Furthermore, this method reliably achieves a uniform, thin coating.

[0027] Preferably, subsequent tempering after electrochemical processing is omitted. This prevents undesirable crystal transformation, ensuring that the brushite phase of the calcium phosphate retains the desired high proportion.

[0028] For further advantageous designs and a more detailed description, reference is made to the above explanation of the implant, which also applies accordingly to the procedure.

[0029] In summary, it can be stated that the coating according to the invention can achieve improved bone ingrowth, as tests have shown.

[0030] The invention is explained in more detail below with reference to the accompanying drawing and advantageous embodiments. The drawing shows: Fig. 1 an embodiment of an implant according to an embodiment of the invention; Fig. 2 a detailed view of a unit cell of the porous structure of the implant according to Figure 1 Fig. 3 a schematic view of the unit cells and the elements forming them; Fig. 4a, b sectional views in two orthogonal directions of the porous structure; Fig. 5 an illustration of an augmentation according to a second embodiment of the invention; Fig. 6a, b schematic side view and frontal view of the augmentation according to Figure 5 ; Fig. 7 a comparison table showing the bone ingrowth behavior; and Fig. 8 a diagram of the current flow during electrochemical coating according to the invention.

[0031] A first embodiment of an implant according to the invention is shown in Figure 1 This is a cone 1 for the tibial component of a knee joint endoprosthesis (not shown).

[0032] The cone 1 forms a replacement for defective bone material at the proximal end of the tibia, thus filling cavities created by the lack of damaged bone material. In this way, a complete base is created on which the tibial component of the knee endoprosthesis can be securely positioned. For this purpose, the cone 1 is manufactured using an open-cell porous lattice structure, which is provided with a coating according to the invention to improve the ingrowth of bone material. The open-cell porous lattice structure 3 is applied to a main body 2.

[0033] In particular, thanks to the arrangement of this open-cell porous lattice structure 3 on the outside of the cone 1, good ingrowth of bone material from the surrounding tibia (not shown) can be achieved, resulting in fast and secure fixation of the cone 1 in the tibia.

[0034] The porous structure 3 is formed by a multitude of regularly arranged unit cells 4. A detailed view of a unit cell 4 and its integration into surrounding unit cells is shown in Figure 2 The unit cell 4 has an interior space 40, which is connected to the interior space 40' of neighboring unit cells 4'. The unit cells are arranged regularly along a layering plane 49. Advantageously, several layering planes are arranged one above the other.

[0035] The regular arrangement of the unit cells is particularly evident from the side views in Figure 4a, b Clearly visible. They 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 result in the two directions, particularly with regard to the structure of the interior 40. This is a special property of the crystal structure used, namely the wurzite structure. It ensures that the open-cell porous lattice structure thus formed exhibits different compression stiffnesses in different directions of space, which is advantageous with regard to its adaptation to the anatomical conditions of the bone. It can also be seen that adjacent interior spaces 40 are interconnected, so that the macropores formed by the unit cells 4 with their interior spaces 40 are interconnected in an open-cell manner (they form so-called "inter-connected pores").

[0036] The actual structure of the unit cells 4 is shown schematically in Figure 3As illustrated, the unit cells 4 are formed from basic elements 45, each configured as a tetrapod. It is understood that basic elements other than tetrapods can also be provided. Each of these tetrapods has four legs 41, 42, 43, 44 configured as webs, each connected at one end to form a node. The tetrapods can be regular or irregular, with legs of equal or different lengths. A regular embodiment is shown, in which the legs are of equal length and each leg forms the same angle with each of the other legs. When the tetrapods are arranged in a planar layer, three legs 41, 42, 43 are arranged upright 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 layered plane arranged above it (see . Figure 3 ).

[0037] The depth of the open-cell porous structure can be controlled by selecting the number of layering levels. For example, three, four, or five superimposed layers can be provided (see Figure 4a, b ), typically at least two layers are provided.

[0038] A titanium alloy or pure titanium is preferably used as the material for the open-cell porous structure.

[0039] A second embodiment is described in the Figures 5 and 6 shown. At Figure 5This is a photographic illustration. It shows a cylindrical augmentation 1', which can also be used to fill bone defects or, if necessary, for the purpose of fusing adjacent bone elements, especially vertebral bodies. It has a main body 2' essentially designed as a sleeve, which is generally cylindrical in shape. The surface of the main body 2' is provided with the open-cell porous lattice structure 3'. This structure is shown, in particular, in the schematic view in Figure 6a, b It can be clearly seen that it is also formed from unit cells 4 with their interconnected interiors 40, the unit cells 4 in turn being made up of tetrapods as basic elements 45.

[0040] As can be seen in particular from the photographic illustration in Figure 5As can be clearly seen, 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 figure. The coating 5 is applied over the entire surface of the open-cell porous lattice structure 3', as well as to the two end regions of the main body 2' and further into the depth of the structure 3' in the interior spaces 40 of the unit cells 4.

[0041] Exemplary dimensions for the length and width of the cylindrical main body 2' are 12 mm in length and 6 mm in diameter, respectively. The interior 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 to approximately 2000 µm. In terms of unit cells 4, this results in a depth of almost three layers of unit cells 4.

[0042] The coating 5 has a combined crystalline phase of brushite and monetite with a proportion of 95 wt%, where the proportion of brushite is at least 65 wt%. Furthermore, the coating 5 completely lines the unit cells 4 with their cavities 40, not only in the uppermost layer but also in the underlying layers.

[0043] According to the invention, this results in a significantly improved ingrowth of bone material within the framework of osteointegration and osteoconduction. Results from a comparative test with a reference implant that has a similarly designed open-cell porous structure, but without the coating 5 according to the invention, are presented in Figure 7The figure shows a quantitative histomorphometric analysis, where the percentage bone / implant contact ratio is displayed along the Y-axis for two different regions (ROI1 and ROI2). The two left-hand columns represent the comparison implant ("C1"), and the two right-hand columns represent the tested implant according to the invention ("T"). The left column in each pair shows the short-term ingrowth behavior (measured after 4 weeks), and the right column in each pair shows the long-term ingrowth behavior (measured after 26 weeks). It is clearly evident that excellent bone ingrowth is achieved with the implant according to the invention ("T") after only 4 weeks, whereas the comparison implant only reaches a similar value after more than six times the time, namely after 26 weeks. This impressively demonstrates the bone growth-promoting property of the coating according to the invention.

[0044] An electrochemical process is expediently used for the coating. The current profile during electrochemical coating is described in Figure 8 depicted.

[0045] It can be seen that a high peak current is set at the beginning, which is then reduced to a lower operating current. This current regime allows for a particularly good precipitation reaction of calcium phosphate, which is especially suitable for thin and uniform coatings, resulting in the combined brushite / monetite phase with its high proportion of approximately 95%.

Claims

1. A bone implant, having a main body (2) with an open-cell porous lattice structure (3) in its outer region, said lattice structure being formed from a plurality of regularly arranged unit cells (4), wherein the unit cells (4) are embodied as an assembled structure and are constructed from an interior space (40) and a plurality of interconnected bars (41, 42, 43, 44) surrounding the interior space (40), wherein the porous lattice structure (3) is provided with a coating (5) which promotes bone growth, comprising calcium phosphate, characterized in that the calcium phosphate coating (5) has a hydroxyapatite content of less than 1 wt%, and forms an inner pore coating extending deep into the porous lattice structure (3).

2. The bone implant according to claim 1, wherein the calcium phosphate coating (5) has a crystal phase which comprises brushite and monetite, and which is at least 90 wt%, preferably at least 95 wt%, wherein the brushite fraction is not less than is 65 wt%.

3. The bone implant according to claim 1 or 2, wherein the calcium phosphate coating (5) has a calcium / phosphate ratio in the range from 1.0 to 1.2, preferably 1.05 to 1.15.

4. The bone implant according to any one of the preceding claims, wherein the average thickness of the calcium phosphate coating (5) is dimensioned such that the interior spaces of the unit cells (4) remain interconnected, preferably being between 10 and 25 µm, more preferably 15 µm ± 5 µm.

5. The bone implant according to any one of the preceding claims, wherein the calcium phosphate coating (5) is unannealed.

6. The bone implant according to any one of the preceding claims, wherein the calcium phosphate coating (5) is applied to all sides of the assembled structure of the unit cells (4), in particular its bars (41, 42, 43, 44), preferably with an omnidirectional inner pore coating being formed.

7. The bone implant according to any one of the preceding claims, wherein the unit cells (4) are arranged in layers, with several layers arranged on top of one another, preferably as an open-cell trabecular structure, and / or the unit cells (4) are embodied in a wurtzite structure.

8. The bone implant according to any one of the preceding claims, wherein the open-cell porous lattice structure (3) is embodied as a 3D printed structure, preferably by means of electron beam melting or selective laser melting.

9. The bone implant according to any one of the preceding claims, wherein the coating (5) is formed using a current which follows a current curve which, after an initial peak current, falls back to a lower working current.

10. The bone implant according to any one of the preceding claims, wherein the main body (2) has a supporting region, said supporting region having a lower porosity than the porosity of the open-cell porous lattice structure (3), the supporting preferably being embodied in a compact manner, and / or the supporting region and the open pore lattice structure being embodied in a unitary manner.

11. The bone implant according to any one of the preceding claims, wherein the interior spaces (40) of the unit cells (4) form macropores, the width of which is at least ten times, preferably between 30 times and 200 times, the thickness of the coating (5), or the width of the pores is in the range between 0.4 and 2 mm, preferably 0.7 to 1.5 mm, and the coating (5) has a thickness between 10 and 20 µm.

12. A method for producing a coated bone implant according to any of the preceding claims, wherein the bone implant comprises a main body which has an open-cell, porous lattice structure in its outer region, said lattice structure being formed from a plurality of regularly arranged unit cells, the method including the steps of: building the regularly arranged unit cells into 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, characterized in that the coating is produced with a hydroxyapatite content of less than 1 wt%, and is applied deep 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%, preferably at least 95 wt%, wherein the brushite fraction is not less than 65 wt%.

14. The method according to claim 12 or 13, wherein the coating is applied to all sides of the porous lattice structure as a precipitate, preferably by means of 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.