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

Abstract

Disclosed is an osteosynthesis system comprising at least one bone plate (2) which has at least one receiving opening (8), and a bone anchor (3) which has an anchor head, the anchor head being designed to be fixed in the receiving opening by forming a form-fit and / or friction-locked connection with the bone plate (2). The bone plate (2), at least in a portion comprising the receiving opening, and the bone anchor (3), at least in a portion comprising the anchor head, are each made of a magnesium alloy, the magnesium alloy of the bone anchor (3) having a different hardness to the magnesium alloy of the bone plate (2).

Description

[0001] DESCRIPTION

[0002] System for osteosynthesis with bone plate and bone anchor made of magnesium alloys

[0003] 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 designed 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.

[0004] Such a system is known from WO 00 / 66012 A1. Fixation can be achieved through the interaction of threads on the anchor head and the bone plate (as the internal thread of the receiving opening). These threads are not complementary, so that the material of one of the connecting partners is deformed when the threaded connection is formed. This results in a high loosening torque for the threaded connection, which can prevent unintentional loosening.

[0005] To achieve targeted deformation of the material of one connecting partner in such an osteosynthesis system, the bone plate and the bone anchor are typically made of different materials, which differ particularly in terms of hardness. A thread made of a harder material can thus cut precisely into the softer material of the other connecting partner. In this regard, it is known to use different titanium alloys, for example, Grade 4 and Grade 5, for the connecting partners.

[0006] The invention is based on the object of improving such a system for osteosynthesis.

[0007] This object is achieved by a system according to patent claim 1. Advantageous embodiments thereof are the subject matter of the further patent claims and / or emerge from the following description of the invention.

[0008] 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 designed for fixation in the receiving opening by forming a positive and / or frictional connection with the bone plate, it is provided that the bone plate, at least in a section comprising the receiving opening, and the bone anchor, at least in a section comprising the anchor head, are each made of a magnesium alloy, wherein the magnesium alloy of the bone anchor, on the one hand, and the magnesium alloy of the bone plate, on the other hand, have different hardnesses. It is preferably provided that the magnesium alloy of the bone anchor has a greater hardness than the magnesium alloy of the bone plate.

[0009] A "bone plate" is understood to mean a component intended and designed to be attached to a bone. The component can, in particular, be flat (flat or curved) so that the length and width of the component are each greater, in particular at least two, five, or ten times greater, than the height of the component.

[0010] A “bone anchor” is a component that is intended and designed for fixation within a bone.

[0011] The hardness of the magnesium materials can be determined in particular as Vickers hardness (HV) according to DIN EN ISO 6507-1:2018 to -4:2018.

[0012] Magnesium and magnesium alloys, particularly magnesium-rare earth alloys, magnesium-calcium-zinc alloys, and magnesium-aluminum alloys with or without the addition of yttrium, exhibit favorable biocompatibility, making them ideal for use in osteosynthesis systems. At the same time, magnesium alloys exhibit sufficient mechanical properties (particularly with regard to strength) to adequately support the forces typically exerted on the components of such a system during use. Another particularly relevant advantage of magnesium alloys is their resorbability, meaning that at least the magnesium component dissolves over a longer period of time through interaction with the body's own substances.This makes it possible to avoid having to remove an implanted osteosynthesis system after the bone fracture has healed. Accordingly, it is preferable that the magnesium alloy of the bone plate and / or the magnesium alloy of the bone anchor be resorbable.

[0013] If a system according to the invention comprises several bone plates and / or bone anchors, it can preferably be provided that all of these components are partially or completely made of magnesium alloys. 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.

[0014] According to a preferred embodiment of a system according to the invention, it can be provided that the magnesium alloy with the greater hardness has a material structure achieved by cold forming. Cold forming represents an advantageous possibility for increasing the hardness of a magnesium alloy. To achieve the inventive hardness difference between the magnesium alloys, it can then further preferably be provided that the magnesium alloy with the lower hardness does not have a material structure achieved by cold forming. In principle, however, different hardnesses can be achieved in magnesium alloys through different cold forming processes, so that the magnesium alloy with the lower hardness can also have a material structure achieved by cold forming.

[0015] A particularly pronounced increase in the hardness of a magnesium alloy can be achieved by hammering, so that the magnesium alloy with the greater hardness can preferably have a material structure achieved by hammering. The magnesium alloy with the lower hardness, on the other hand, can preferably not have a material structure achieved by hammering.

[0016] Hammering, also known as circular die forging or circular swaging, is characterized in that two or more tools (A), which are arranged circumferentially of a workpiece (B), exert forming strokes directed radially towards a workpiece center, while the workpiece (B) rotates relative to the tools (A) about the workpiece center, as shown schematically in Fig. 1. A relative movement between the workpiece (B) and the tools (A) along a longitudinal axis (C) of the workpiece (B) enables continuous or discontinuous forming even 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 realized by (only) moving the workpiece (B) longitudinally axially.Alternatively or additionally, a corresponding movement of the tools (A) can also be provided. The strokes performed by the tools (A) can, in particular, be relatively short (e.g., between 0.25 mm and 3 mm, in particular between 0.3 and 1.5 mm) and relatively high-frequency (e.g., 1,000 strokes per minute and more). Furthermore, it can preferably be provided that the tools (A) surround the workpiece (B) almost completely, i.e., with only minimal distances between the tools (A), circumferentially.

[0017] 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 from which the magnesium alloys are mainly composed (i.e. in each case with a proportion of at least 0.1 wt.%; since elements whose respective proportion is below 0.1 wt.% are regarded as impurities according to the invention), are on the one hand the same and are also present in each case in the same proportions (“equal” according to the invention means with regard 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 by different processing and / or treatments of the magnesium alloys.

[0018] The use of magnesium alloys of 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 particularly advantageous feature is that, if necessary, only one magnesium alloy (defined in certain areas by its chemical composition) needs to demonstrate chemical, toxicological, and biological safety or the required biocompatibility according to DIN EN ISO 10993:2016, and the interaction of different materials does not need to be addressed with regard to certification as a medical device.

[0019] According to a preferred embodiment, the magnesium alloy of the bone plate and / or the magnesium alloy of the bone anchor can be a Mg-Y-RE-Zr alloy, and particularly preferably a Mg-Y-Nd-Zr alloy, also known as a WE43 alloy. In particular, the rare earth elements (RE) Dy, Y, Nd, and Gd have low toxicity when used as alloying elements in magnesium alloys and exhibit advantageous mechanical and corrosion properties when used clinically in corresponding alloys as implant material.

[0020] The magnesium alloy of the bone plate and / or the magnesium alloy of the bone anchor can advantageously also be a Mg-Y-Nd alloy with or without the addition of Zr. In a preferred embodiment, such a magnesium alloy has an yttrium content of between 3 wt% and 5 wt% and an Nd content of between 2 wt% and 4 wt%.

[0021] The magnesium alloy of the bone plate and / or the magnesium alloy of the bone anchor can 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 can preferably each be less than 1 wt.%, less than 2 wt.%, or less than 5 wt.%.

[0022] According to a preferred embodiment of a system according to the invention, it can be provided that the receiving opening of the bone plate, in an initial state (i.e., before initial connection to the bone anchor), does not have a connecting structure that is complementary to a connecting structure of the anchor head. Alternatively, it can be provided that the anchor head, in an initial state, does not have a connecting structure that is complementary to a connecting structure of the receiving opening. Connecting the bone plate and the bone anchor therefore causes plastic deformation.

[0023] Preferably, it can be provided that the connecting structure of the anchor head, if provided, comprises or is 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 connecting structure complementary to the connecting structure or the thread of the anchor head, it can preferably be provided that it is thread-free in the initial state or is provided with a thread, in particular an internal thread, that is incomplementary to the thread of the anchor head due to different thread parameters.

[0024] It can also be provided that the connecting 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 connecting structure complementary to the connecting structure or the thread of the receiving opening, it can preferably be provided that it is thread-free in the initial state or is provided with a thread, in particular an external thread, that is incomplementary to the thread of the receiving opening due to different thread parameters.

[0025] 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 which is intended to engage or cut into bone material when the anchor shaft is (possibly only partially) positioned within an opening in a bone. This makes it possible to create a connection between the bone anchor or the bone screw and the bone which is easy to effect and, at the same time, securely held. Furthermore, it can then preferably also be provided that the thread of the anchor head and the shaft thread have different thread parameters at least in sections (in particular different thread leads and / or thread pitches and / or flank angles and / or core or outer diameters), whereby the stability of the fixation in the bone material and the necessary torque when screwing into the bone material can be reduced or increased.can be controlled.

[0026] According to a preferred embodiment of a system according to the invention, it can be provided that the bone plate has a coating on a base body made of the (respective) magnesium alloy, at least in a section comprising the receiving opening, and / or the bone anchor has a coating on a base body made of the (respective) magnesium alloy, at least in a section comprising the anchor head. Such a coating can be used, in particular, to influence and, in particular, to slow down the corrosion and thus possibly 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 the 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 usually transmitted, can be kept stable for as long as possible.

[0027] Preferably, it can be provided that the coating comprises or is an oxide layer, in particular a magnesium oxide layer. Such an oxide layer can particularly preferably be produced or have been produced by means of 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 made of an oxide ceramic (cf. WO 2015 / 090267 A1). Native barrier layer formers, such as magnesium, are particularly suitable as valve metals. The surface layer can be produced 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 function as counterelectrodes.When using direct current, the component(s) to be coated are anodically polarized. With alternating current or bipolar pulse currents, the component(s) to be coated are merely "electrodes," with the surface layer only forming in those current segments in which the component(s) are the anode. With symmetrical pulse or alternating currents (i.e., the same proportion of current in both directions), both the components used as the "electrode" and the "counter electrode" can be alternately anodically connected and thus provided with a surface layer. During PEO, 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.This involves locally penetrating the passive layer of the component to be coated, triggering plasma-chemical solid-state reactions, the spark discharges. PEO is therefore also referred to as "anodic oxidation under 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 take place at those points of the passive layer that have the smallest local layer thickness and ensure layer thickness growth there, the surface is coated with a very uniform surface layer. In order to permanently break 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, it can be provided that the receiving opening and / or the anchor head has / have a conical or conical or truncated cone-shaped basic shape, whereby advantageous transmissions of forces and moments between the bone plate and the bone anchor can be realized, this applies in particular also in the case of a non-parallel and in particular also non-coaxial alignment of the bone anchor within the receiving opening of the bone plate, ie when a central longitudinal axis of the bone anchor in the region of the anchor head is non-parallel or non-coaxial and thus aligned 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 truncated cone-shaped basic shapes, it can be particularly preferably provided that these basic shapes have different conicities, i.e. the changes in diameter are different for the various conical basic shapes. 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 region of a bone fracture and at least one, preferably both, of the two bone sections separated by the bone fracture are connected to the bone plate by means of the bone anchor or bone anchors, and the bone fracture is thereby supported via the bone plate.The connection of a bone section to a bone anchor is achieved by inserting and fixing the bone anchor in the bone section and, preferably at least temporarily simultaneously, by forming the positive and / or frictional connection between the anchor head of the bone anchor and the bone plate.

[0030] The invention is explained in more detail below with reference to exemplary embodiments shown in the drawings. The drawings show, partly in simplified form:

[0031] Fig. 1 : machining of a workpiece by hammering;

[0032] Fig. 2: an osteosynthesis system according to the invention in use;

[0033] Fig. 3: a permissible angular range regarding the orientation of a bone plate and a bone anchor in the system;

[0034] Fig. 4: the bone plate of the system in a top view;

[0035] Fig. 5: the bone anchor of the system in a side view;

[0036] Fig. 6: the bone anchor of the system in a top view;

[0037] Fig. 7: the thread shapes of an internal thread of a receiving opening of the bone plate and an external thread of the bone anchor of the system;

[0038] Fig. 8: Alternative thread shapes of an internal thread of a receiving opening of a bone plate and an external thread of a bone anchor of an osteosynthesis system according to the invention;

[0039] Fig. 9 shows the internal thread of the bone plate according to Fig. 8 in detail; and

[0040] Fig. 10: the external thread of the bone anchor according to Fig. 8 in detail. Fig. 2 shows a system according to the invention for osteosynthesis in use, i.e. in the stabilization of two bone sections 1b of a bone 1 separated by a bone fracture 1a until the bone fracture 1a heals. The system is intended to support forces FA in the longitudinal direction of the bone 1, forces FB in the transverse direction of the bone 1, which cause bending, as well as torsional moments MT about the longitudinal direction of the bone 1. For this purpose, a bone plate 2 of the system lies against the two bone sections 1b and spans the bone fracture 1a. The bone plate 2 is firmly connected to each of the two bone sections 1b via at least one bone anchor 3.In this case, the bone anchors 3 are, on the one hand, firmly anchored in the respective bone section 1b, which can preferably be realized in that a shaft thread 5, which is formed in the region of an anchor shaft 4 of each bone anchor 3 (cf. Fig. 5), is screwed into a previously drilled hole in the respective bone section 1b, wherein the shaft thread 5 cuts into the bone material of the respective bone section 1b.

[0041] The bone anchors 3 are furthermore each firmly connected to the bone plate 2 via a positive and / or frictional connection. This connection is achieved by the interaction of an external thread 7, which acts as a connecting structure of each bone anchor 3 and is formed in the region of an anchor head 6 of each bone anchor 3, with an internal thread 9, which acts as a connecting structure of the bone plate 2 and is formed in the region of each of a plurality of receiving openings 8 of the bone plate 2. However, it is provided 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 state due to different 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.As a result, a particularly secure connection between the bone anchors 3 and the bone plate 2 can be realized, wherein in particular 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, so that unintentional loosening is effectively prevented.

[0042] Both the bone plate 2 and all bone anchors 3 are made 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 hand, in that blanks of the magnesium alloy used for the production of the bone anchors 3 were machined by hammering, whereby a corresponding increase in hardness can be realized, while the blank of the magnesium alloy from which the bone plate 2 was made was not machined by hammering.

[0043] When implementing a system according to the invention, it cannot usually be guaranteed that the bone anchors 3 are screwed in with an exact coaxial alignment of their central longitudinal axis 10 with respect to the central longitudinal axis 11 of the associated receiving openings 8 of the bone plate 2. In order 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 thread 9) of the bone plate 2 and the anchor heads 6 (including the external thread 7) of the bone anchors 3 have specific, mutually adapted basic shapes that ensure sufficient load-bearing capacity at least up to a maximum deviation of 15° from the coaxial alignment (cf. Fig. 3). According to Fig. 7, 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 embodiment according to Fig. 8, however, 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.

[0044] In order to specifically influence the degradation behavior of the components of a system according to the invention, these can be provided at least in sections with a coating, for example an oxide layer produced using PEO (see Fig. 5).

[0045] 1 bone

[0046] 1a Bone fracture

[0047] 1b Bone section

[0048] 2 bone plate

[0049] 3 bone anchors

[0050] 4 Anchor shaft of the bone anchor

[0051] 5 shaft threads

[0052] 6 Anchor head of the bone anchor

[0053] 7 External thread of the anchor head

[0054] 8 Opening for the bone plate

[0055] 9 Internal thread of the receiving opening

[0056] 10 Central longitudinal axis of the bone anchor

[0057] 11 Central longitudinal axis of the receiving opening

[0058] 12 Coating

[0059] A tool

[0060] B Workpiece

[0061] C Workpiece longitudinal axis

Claims

PATENT 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-locking and / or friction-locking connection with the bone plate (2), characterized in that the bone plate (2) at least in a section comprising the receiving opening (8) and the bone anchor (3) at least in a section comprising the anchor head (6) are each made of 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. 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 greater hardness has a material structure achieved at least in one step by cold forming. System according to claim 3, characterized in that the magnesium alloy with greater hardness has a material structure achieved by hammering. System according to any one of the preceding claims, characterized in that the magnesium alloys of the bone plate (2) on the one hand and the bone anchor (3) on the other hand have the same chemical composition. System according to any 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 that 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 that is designed to be complementary to a connection structure of the receiving opening (8). A system according to any 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). A system according to claims 7 and 8, 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). A system according to any one of the preceding claims, characterized in that the bone anchor (3) has an anchor shaft (4) with a shaft thread (5). A system according to claim 10, characterized in that the thread (7) of the anchor head (6) and the shaft thread (5) have different thread parameters.A system according to any one of the preceding claims, characterized in that the bone plate (2) has a coating (12) on a base body made of the respective magnesium alloy, at least in a section comprising 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 comprising the anchor head (6). A system according to claim 12, characterized in that the coating (12) comprises or is an oxide layer and / or a phosphate layer. A system according to any one of the preceding claims, characterized in that the receiving opening (8) and / or the anchor head (6) have a conical basic shape. A system according to claim 14, characterized in that the receiving opening (8) and the anchor head (6) have conical basic shapes with different tapers.

Images