Osteosynthesis system with bone plate and bone anchor made of magnesium alloys

Abstract

System for osteosynthesis comprising a bone plate (2) having at least one receiving opening (8) and a bone anchor (3) comprising an anchor head (6), wherein the anchor head (6) is configured for fixation in the receiving opening (8) by forming a positive-locking and / or friction-locking connection with the bone plate (2), wherein the bone plate (2) is made of a magnesium alloy at least in a section comprising the receiving opening (8) and the bone anchor (3) is made of a magnesium alloy at least in a section comprising the anchor head (6), wherein the magnesium alloy of the bone anchor (3) and the magnesium alloy of the bone plate (2) have different hardnesses, characterized in that • the magnesium alloy with greater hardness has a material structure achieved at least in one step by cold forming and • the bone plate (2) has a coating (12) on a base body made of the respective magnesium alloy at least in a section encompassing the receiving opening (8) and / or the bone anchor (3) has a coating (12) on a base body made of the respective magnesium alloy at least in a section encompassing the anchor head (6).

Description

[0001] The invention relates to a system for osteosynthesis with a bone plate having at least one receiving opening and with a bone anchor comprising an anchor head with a connecting structure, wherein the anchor head is arranged for fixation in the receiving opening by forming a positive and / or frictional connection with the bone plate by means of the connecting structure of the anchor head.

[0002] Such a system is known from WO 00 / 066 012 A1. Fixation can be achieved through the interaction of threads on the anchor head and the bone plate (as internal threads in the receiving opening). These threads are not complementary, so that when the threaded connection is formed, the material of one of the connecting partners is deformed. This results in a high loosening torque for the threaded connection, thus preventing unintentional loosening.

[0003] To achieve controlled deformation of the material of one of the connecting partners in such an osteosynthesis system, the bone plate and the bone anchor are typically made of different materials, differing particularly in hardness. A thread formed from the harder material can thus engage precisely with the softer material of the other connecting partner (see US 2021 / 0378723A1 and EP 3081180A1). It is known to use different titanium alloys for the connecting partners, for example, Grade 4 and Grade 5.

[0004] The publication “New findings on self-dissolving implants” by the Helmholtz Center Hereon describes the use of magnesium alloys for implants, whereby the resorption behavior of the implants can be influenced by the gadolinium content in the magnesium alloy.

[0005] The invention is based on the objective of improving such a system for osteosynthesis.

[0006] This problem is solved in a system according to claim 1. Advantageous embodiments thereof are the subject of further claims and / or will become apparent from the following description of the invention.

[0007] According to the invention, in an osteosynthesis system comprising at least one bone plate having at least one receiving opening and at least one bone anchor having an anchor head, wherein the anchor head is configured for fixation in the receiving opening by forming a positive-locking and / or friction-locking connection with the bone plate, the bone plate, at least in a section encompassing the receiving opening, and the bone anchor, at least in a section encompassing the anchor head, are each made of a magnesium alloy, wherein the magnesium alloy of the bone anchor and the magnesium alloy of the bone plate have different hardnesses. Preferably, the magnesium alloy of the bone anchor has a greater hardness than the magnesium alloy of the bone plate.

[0008] According to the invention, the magnesium alloy with the greater hardness is further provided that it has a microstructure achieved by cold forming. Cold forming is an advantageous method for increasing the hardness of a magnesium alloy. To achieve the hardness difference between the magnesium alloys according to the invention, it is then preferable that the magnesium alloy with the lesser hardness does not have a microstructure achieved by cold forming. However, different hardnesses can generally be achieved in magnesium alloys through different cold forming processes, so that the magnesium alloy with the lesser hardness can also have a microstructure achieved by cold forming.

[0009] According to the invention, the bone plate, at least in a section comprising the receiving opening, and / or the bone anchor, at least in a section comprising the anchor head, has a coating on a base body made of the (respective) magnesium alloy. Such a coating can be used, in particular, to influence and, in particular, to slow down the corrosion and thus, if applicable, also the resorption behavior of the magnesium alloy of the associated base body. By coating the respective base body, at least in a section of the bone plate comprising the receiving opening and / or in a section of the bone anchor comprising the anchor head, the areas of the connection between the bone plate and the anchor head, through which relatively high forces and moments are typically transmitted, can be kept stable for as long as possible.

[0010] A "bone plate" is understood to be a component designed and intended for attachment to a bone. The component can be designed to be flat (flat or curved), such that its length and width are each greater, in particular at least two, five, or ten times greater, than its height.

[0011] A "bone anchor" is understood to be a component that is designed and intended for fixation within a bone.

[0012] The hardness of magnesium materials can be determined in particular as Vickers hardness (HV) according to DIN EN ISO 6507-1:2018 to -4:2018.

[0013] Magnesium and magnesium alloys, particularly rare-earth magnesium alloys, magnesium-calcium-zinc alloys, and magnesium-aluminum alloys with or without the addition of yttrium, exhibit favorable biocompatibility, making them advantageous for use in osteosynthesis systems. Simultaneously, magnesium alloys possess sufficient mechanical properties (especially regarding strength) to adequately withstand the forces typically exerted on the components of such a system during use. A further, particularly relevant advantage of magnesium alloys is their resorbability, meaning that at least the magnesium component dissolves gradually over an extended period through interaction with endogenous substances. This allows for the removal of an implanted osteosynthesis system after the bone fracture has healed.Accordingly, it may preferably be provided that the magnesium alloy of the bone plate and / or the magnesium alloy of the bone anchor is / are resorbable.

[0014] If a system according to the invention comprises several bone plates and / or bone anchors, it may preferably be provided that all of these components are partially or completely made of magnesium alloys.

[0015] According to a preferred embodiment of a system according to the invention, it can be provided that the hardness of the magnesium alloy with the lower hardness is between 40 and 80 HV or between 30 and 50 HV or between 35 and 45 HV and / or the hardness of the magnesium alloy with the higher hardness is at least 85 HV.

[0016] A particularly pronounced increase in the hardness of a magnesium alloy can be achieved by hammering, so that it is preferably intended that the magnesium alloy with the greater hardness has a material structure achieved by hammering. The magnesium alloy with the lesser hardness, on the other hand, preferably does not have a material structure achieved by hammering.

[0017] Hammering, also known as rotary die forging or rotary swaging, is characterized in that two or more tools (A) arranged circumferentially of a workpiece (B) exert forming strokes directed radially towards the workpiece center, while the workpiece (B) rotates around the workpiece center relative to the tools (A), as schematically shown in the Fig. Figure 1 shows a relative movement between the workpiece (B) and the tools (A) along a longitudinal axis (C) of the workpiece (B). This enables continuous or discontinuous forming machining of workpieces (B) whose dimensions along the longitudinal axis (C) are larger than the corresponding dimensions of the tools (A). The relative movement between the workpiece (B) and the tools (A) along the longitudinal axis (C) of the workpiece (B) can preferably be achieved by moving (only) the workpiece (B) longitudinally. Alternatively or additionally, a corresponding movement of the tools (A) can also be provided. The strokes performed by the tools (A) can be relatively short (e.g., between 0.25 mm and 3 mm, particularly between 0.3 and 1.5 mm) and relatively high-frequency (e.g., 1000 per minute and more).It may also be preferably provided that the tools (A) surround the workpiece (B) almost completely, i.e. with only minimal distances between the tools (A).

[0018] According to a preferred embodiment of a system according to the invention, it can be provided that the magnesium alloy of the bone plate on the one hand and the magnesium alloy of the bone anchor on the other hand have the same chemical composition, i.e., the chemical elements of which the magnesium alloys are mainly composed (i.e., each with a proportion of at least 0.1 wt.%; since elements whose respective proportion is less than 0.1 wt.% are considered impurities according to the invention) are, on the one hand, the same and are also present in equal proportions ("equal" according to the invention means with respect to the alloy proportions with a maximum relative deviation of 10% based on the absolutely larger of the compared proportions.Different hardnesses of the magnesium alloys for the bone plate on the one hand and the bone anchor on the other hand can be achieved through different processing and / or treatment of the magnesium alloys.

[0019] The use of magnesium alloys with the same chemical composition for both the bone plate and the bone anchor, preferably for the entire system, enables simple and cost-effective manufacturing. A particular advantage is that, if required, proof of chemical, toxicological, and biological safety or the required biocompatibility according to DIN EN ISO 10993:2016 only needs to be provided for one magnesium alloy (defined in certain areas by its chemical composition), and there is no need to address the interaction of different materials with regard to certification as a medical device.

[0020] According to a preferred embodiment, the magnesium alloy of the bone plate and / or the magnesium alloy of the bone anchor can be an Mg-Y-RE-Zr alloy, and particularly preferably an Mg-Y-Nd-Zr alloy, also known as WE43 alloy. In particular, the rare earth elements (RE) Dy, Y, Nd, and Gd exhibit low toxicity when used as alloying elements in magnesium alloys and possess advantageous mechanical and corrosion properties when used clinically in corresponding alloys as implant material.

[0021] The magnesium alloy of the bone plate and / or the magnesium alloy of the bone anchor can advantageously also be an Mg-Y-Nd alloy with or without the addition of Zr. In a preferred embodiment, such a magnesium alloy has a yttrium content between 3 wt.% and 5 wt.% and an Nd content between 2 wt.% and 4 wt.%.

[0022] The magnesium alloy of the bone plate and / or the magnesium alloy of the bone anchor may advantageously also comprise calcium and zinc, preferably as an Mg-Ca-Zn alloy or as an Mg-Zn-Ca alloy, each with or without the addition of Zr. The Ca and Zn contents may preferably each be less than 1 wt.%, less than 2 wt.%, or less than 5 wt.%.

[0023] According to a preferred embodiment of a system according to the invention, the receiving opening of the bone plate may, in an initial state (i.e., before initial connection with the bone anchor), lack a connection structure that is complementary to a connection structure of the anchor head. Alternatively, the anchor head may, in an initial state, lack a connection structure that is complementary to a connection structure of the receiving opening. Connecting the bone plate and the bone anchor thus results in plastic deformation in each case.

[0024] Preferably, the connection structure of the anchor head, if provided, may include or be a thread, in particular an external thread. If the receiving opening of the bone plate in such a system according to the invention does not have a connection structure complementary to the connection structure or the thread of the anchor head, it may preferably be provided that it is threadless in the initial state or provided with a thread incomplementary to the thread of the anchor head due to different thread parameters, in particular an internal thread.

[0025] It can also be provided that the connection structure of the receiving opening, if provided, comprises or is a thread, in particular an internal thread. If the anchor head in such a system according to the invention does not have a connection structure complementary to the connection structure or the thread of the receiving opening, it can preferably be provided that it is threadless in the initial state or provided with a thread, in particular an external thread, that is incomplementary to the thread of the receiving opening due to different thread parameters.

[0026] The bone anchor of a system according to the invention can preferably be designed as a bone screw and, for this purpose, have an anchor shaft with a shaft thread designed to engage or cut into bone material when the anchor shaft is (optionally only partially) positioned within an opening in the bone. This allows a connection between the bone anchor or bone screw and the bone to be achieved that is easy to establish and simultaneously secure. It is further preferably provided that the thread of the anchor head and the shaft thread have at least partially different thread parameters (in particular, different thread pitches and / or thread divisions and / or flank angles and / or core or outer diameters), thereby reducing the stability of the fixation in the bone material and / or the torque required for screwing into the bone material.can be controlled.

[0027] Preferably, the coating may comprise or be an oxide layer, in particular a magnesium oxide layer. Such an oxide layer may be produced, or have been produced, particularly preferably by plasma electrolytic oxidation (PEO). This is a combined process from the fields of plasma technology and electrochemistry, by which the surfaces of components made of so-called valve metals can be provided with a surface layer of an oxide ceramic (see WO 2015 / 090 267 A1). Suitable valve metals include, in particular, native barrier layer formers such as magnesium. The production of the surface layer can be carried out, in particular, in aqueous electrolytes. The component(s) to be oxidized can be immersed in the electrolyte as an electrode, together with one or more other components that act as counter electrodes.When using direct current, the component(s) to be coated are anodically polarized. With alternating current or bipolar pulsed currents, the component(s) to be coated act as "electrodes," and the surface layer only forms in those current segments where the component(s) are the anode. Therefore, with symmetrical pulsed or alternating currents (i.e., equal current flow in both directions), both the component(s) used as "electrodes" and the component(s) used as "counter electrodes" can be alternately anodically connected and thus coated with a surface layer. During PEO (Plastic Electrode Formation), the component(s) to be coated initially form a purely chemically induced passive layer. The growth of this passive layer can be achieved by applying a potential between the anodically polarized component and the cathode.In this process, the passive layer of the component to be coated is locally broken down, triggering plasma-chemical solid-state reactions, i.e., spark discharges. PEO is therefore also referred to as "anodic oxidation by spark discharge" (ANOF). The spark discharges are not generated over a large area but only locally at those points where the thickness of the oxide layer, and thus the local electrical resistance, is lowest. Since the plasma reactions always occur at those points on the passive layer with the smallest local thickness, and thus cause layer thickness growth, the surface is coated with a very uniform surface layer. To permanently break down the increasing dielectric properties of the growing oxide layer with a breakdown voltage, the applied electrical potential is increased until the desired surface layer thickness is reached.

[0028] According to a preferred embodiment of a system according to the invention, the receiving opening and / or the anchor head may have a conical or conical or frustoconical basic shape, thereby enabling advantageous transmission of forces and moments between the bone plate and the bone anchor. This applies in particular to a non-parallel and especially also non-coaxial orientation of the bone anchor within the receiving opening of the bone plate, i.e., when a central longitudinal axis of the bone anchor in the region of the anchor head is non-parallel or non-coaxial and thus oriented at an angle with respect to a central longitudinal axis of the receiving opening, as can regularly be the case when using a system according to the invention.

[0029] If both the receiving opening and the anchor head have conical or conical or frustoconical basic shapes, it may be particularly preferred that these basic shapes have different taperities, i.e. the changes in diameter are different for the different conical basic shapes.

[0030] The invention also relates to a method for osteosynthesis using a system according to the invention, in which the bone plate is applied to a bone in the area of ​​a bone fracture and at least one, preferably both, of the two bone segments separated by the bone fracture are connected to the bone plate by means of one or more bone anchors, thereby supporting the bone fracture via the bone plate. Connecting a bone segment to a bone anchor is achieved by inserting and fixing the bone anchor in the bone segment and, preferably at least temporarily simultaneously, by forming a positive-locking and / or friction-locking connection between the anchor head of the bone anchor and the bone plate.

[0031] The invention is explained in more detail below with reference to embodiments illustrated in the drawings. The drawings show, in some cases in simplified form: Fig. 1: the processing of a workpiece by means of hammering; Fig. 2: a system according to the invention for osteosynthesis in use; Fig. 3: a permissible angular range with respect to the alignment of a bone plate and a bone anchor in the system; Fig. 4: the bone plate of the system in a top view; Fig. 5: The bone anchor of the system in a side view; Fig. 6: the bone anchor of the system in a top view; Fig. 7: the thread forms of an internal thread of a receiving opening of the bone plate and of an external thread of the bone anchor of the system; Fig. 8: Alternative thread forms of an internal thread of a receiving opening of a bone plate and of an external thread of a bone anchor of a system according to the invention for osteosynthesis; Fig. 9 the internal thread of the bone plate according to the Fig. 8 in detail; and Fig. 10: the external thread of the bone anchor according to the Fig. 8 in detail.

[0032] The Fig. Figure 2 shows a system according to the invention for osteosynthesis in use, i.e., for stabilizing two bone segments 1b of a bone 1 separated by a bone fracture 1a until the bone fracture 1a has healed. The system is intended to transmit forces F A in the longitudinal direction of bone 1, as well as forces F B in the transverse direction of bone 1, which cause bending, as well as torsional moments M Tto support the longitudinal direction of the bone 1. For this purpose, a bone plate 2 of the system rests against the two bone segments 1b and spans the bone fracture 1a. The bone plate 2 is firmly connected to each of the two bone segments 1b via at least one bone anchor 3. The bone anchors 3 are firmly anchored in the respective bone segment 1b, which can preferably be achieved by having a threaded shaft 5 formed in the area of ​​an anchor shaft 4 of each bone anchor 3 (see figure). Fig. 5), is screwed into a pre-drilled hole in the respective bone section 1b, whereby the shaft thread 5 cuts into the bone material of the respective bone section 1b.

[0033] The bone anchors 3 are each firmly connected to the bone plate 2 via a positive-locking and / or friction-locking connection. This connection is achieved through the interaction of an external thread 7, acting as the connecting structure of each bone anchor 3 and formed in the area of ​​an anchor head 6 of each bone anchor 3, with an internal thread 9, acting as the connecting structure of the bone plate 2 and formed in the area of ​​each of a plurality of receiving openings 8 of the bone plate 2. However, it is designed that the external threads 7 of the bone anchors 3 are not complementary to the internal threads 9 of the bone plate 2 in their respective initial states due to differing thread parameters, so that screwing the external thread 7 of a bone anchor 3 into an internal thread 9 of the bone plate 2 causes a defined plastic deformation (essentially exclusively) of the material of the bone plate 2.This allows for a particularly secure connection between the bone anchors 3 and the bone plate 2, whereby the loosening torque required to unscrew the external thread 7 of the anchor head 6 of a bone anchor 3 from the corresponding internal thread 9 of the bone plate 2 is comparatively high, effectively preventing unintentional loosening.

[0034] Both the bone plate 2 and all of the bone anchors 3 consist entirely of resorbable magnesium alloys of the same chemical composition (e.g., Magnesium WE43). To ensure that when the external threads 7 of the anchor heads 6 of the bone anchors 3 are screwed into the corresponding internal threads 9 of the bone plate 2, essentially only the magnesium alloy of the bone plate 2 is deformed, the magnesium alloy of the bone anchors 3 is designed to have a greater hardness (e.g., approximately 20% greater) than the magnesium alloy of the bone plate 2.This is achieved despite the use of the same magnesium alloy for the bone plate 2 on the one hand and the bone anchors 3 on the other, by processing the magnesium alloy blanks used for the production of the bone anchors 3 by hammering, thereby achieving a corresponding increase in hardness, while the magnesium alloy blank from which the bone plate 2 was produced was not processed by hammering.

[0035] In the context of implementing a system according to the invention, it is not usually possible to guarantee that the bone anchors 3 are screwed in with their central longitudinal axis 10 aligned exactly coaxially with the central longitudinal axis 11 of the corresponding receiving openings 8 of the bone plate 2. To nevertheless ensure sufficient load-bearing capacity of the threaded connections formed between the bone anchors 3 and the bone plate 2, the receiving openings (including the internal threads 9) of the bone plate 2 and the anchor heads 6 (including the external threads 7) of the bone anchors 3 have specific, mutually adapted basic shapes that ensure sufficient load-bearing capacity at least up to a deviation of a maximum of 15° from the coaxial alignment (see Figure 1). Fig. 3) ensure. According to the Fig. 7. For this purpose, a partially spherical basic shape of the anchor heads of the bone anchors can be combined with a conical basic shape of the receiving openings of the bone plate. In the alternative design according to the Fig. In contrast, both the basic shape of the anchor heads of the bone anchors on the one hand and the basic shape of the receiving openings of the bone plate on the other hand are conical, but these are designed with different conicities.

[0036] In order to specifically influence the degradation behavior of the components of a system according to the invention, these are at least partially provided with a coating, for example an oxide layer produced by means of PEO (cf. Fig. 5). REFERENCE MARK LIST 1 bone 1a Bone fracture 1b Bone section 2 bone plates 3 bone anchors 4 Anchor shaft of the bone anchor 5 shaft threads 6 Anchor head of the bone anchor 7 External thread of the anchor head 8. Bone plate receiving opening 9 internal threads of the receiving opening 10. Central longitudinal axis of the bone anchor 11 Central longitudinal axis of the receiving opening 12 coating A tool B workpiece C workpiece longitudinal axis

Claims

System for osteosynthesis comprising a bone plate (2) having at least one receiving opening (8) and a bone anchor (3) comprising an anchor head (6), wherein the anchor head (6) is configured for fixation in the receiving opening (8) by forming a positive and / or frictional connection with the bone plate (2), wherein the bone plate (2) is formed at least in a section comprising the receiving opening (8) and the bone anchor (3) is formed at least in a section comprising the anchor head (6) each from a magnesium alloy, wherein the magnesium alloy of the bone anchor (3) on the one hand and the magnesium alloy of the bone plate (2) on the other hand have different hardnesses, characterized in thatthat • the magnesium alloy with greater hardness has a material structure achieved at least in one step by cold forming and • the bone plate (2) has a coating (12) on a base body made of the respective magnesium alloy at least in a section encompassing the receiving opening (8) and / or the bone anchor (3) has a coating (12) on a base body made of the respective magnesium alloy at least in a section encompassing the anchor head (6). System according to claim 1, characterized in that the magnesium alloy of the bone anchor (3) has a greater hardness than the magnesium alloy of the bone plate (2). System according to claim 1 or 2, characterized in that the magnesium alloy with the greater hardness has a material structure achieved by hammering. System according to one of the preceding claims, characterized in that the magnesium alloys of the bone plate (2) on the one hand and of the bone anchor (3) on the other hand have the same chemical composition. System according to one of the preceding claims, characterized in that the magnesium alloy of the bone plate (2) and / or the magnesium alloy of the bone anchor (3) is / are resorbable. System according to one of the preceding claims, characterized in that the receiving opening (8) in an initial state does not have a connection structure which is designed to be complementary to a connection structure of the anchor head (6) or the anchor head (6) in an initial state does not have a connection structure which is designed to be complementary to a connection structure of the receiving opening (8). System according to one of the preceding claims, characterized in that the connection structure of the anchor head (6) and / or the connection structure of the receiving opening comprises or is a thread (7). System according to claims 6 and 7, characterized in that the receiving opening (8) is unthreaded in the initial state or is provided with a thread (9) that is incomplementary to the thread (7) of the anchor head (6), or the anchor head (6) is unthreaded in the initial state or is provided with a thread (7) that is incomplementary to the thread (9) of the receiving opening (8). System according to one of the preceding claims, characterized in that the bone anchor (3) has an anchor shaft (4) with a shaft thread (5). System according to claim 9, characterized in that the thread (7) of the anchor head (6) and the shaft thread (5) have different thread parameters. System according to one of the preceding claims, characterized in that the coating (12) comprises or is an oxide layer and / or a phosphate layer. System according to one of the preceding claims, characterized in that the receiving opening (8) and / or the anchor head (6) has / have a conical basic shape. System according to claim 12, characterized in that the receiving opening (8) and the anchor head (6) have conical basic shapes with different taperities.

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