DYNAMIC BONE ANCHOR AND METHOD OF MANUFACTURING THE SAME

[0001] The present application claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 61/733,769, filed Dec. 5, 2012, the contents of which are hereby incorporated by reference in their entirety, and claims priority from European Patent Application EP 12 195 758.3, filed Dec. 5, 2012, the contents of which are hereby incorporated by reference in their entirety.

[0002] 1. Field

[0003] The invention relates to a dynamic bone anchor and a method of manufacturing a dynamic bone anchor. The dynamic bone anchor comprises an anchor member for anchoring in a bone or a vertebra and a longitudinal core member provided in the anchor member a portion of which is movable with respect to the anchor member. The core member is made at least partially of a material comprising a Ni—Ti based shape memory alloy having superelastic properties. The method of manufacturing such a dynamic bone anchor makes use of the shape memory effect of the material of the core member. The dynamic bone anchor is particularly applicable in the field of dynamic bone fixation or dynamic stabilization of the spinal column.

[0004] 1. Description of Related Art

[0005] A dynamic bone anchor is known, for example, from U.S. 2005/0154390 A1, The shaft of the bone anchor comprises an elastic or flexible section,

[0006] A further dynamic bone fixation element is known from U.S. 2009/0157123 A1. The dynamic bone fixation element includes a bone engaging component and a load carrier engaging component. The bone engaging component includes a plurality of threads for engaging a patient's bone and a lumen. The load carrier engaging component includes a head portion for engaging a load carrier and a shaft portion that at least partially extends into the lumen. The distal end of the shaft portion is coupled to the lumen and at least a portion of an outer surface of the shaft portion is spaced away from at least a portion of an inner surface of the lumen via a gap so that the head portion can move with respect to the bone engaging component. The load carrier engaging component may be made from a high strength material, for example, a strong metal or metal alloy such as CoCrMo, CoCrMoC, CoCrNi or CoCrWNi. In a particularly preferred embodiment, the bone engaging component is made from titanium or a titanium alloy, while the load carrier engaging portion is made from cobalt chrome (CoCr).

[0007] It is the object of the invention to provide further improved dynamic bone anchor that allows that the head of the bone anchor can perform a limited motion after anchoring the bone anchor into a bone or a vertebrae. Further, a method of manufacturing such a dynamic bone anchor shall be provided.

[0008] The object is solved by a dynamic bone anchor according to claim 1 and by a method of manufacturing the dynamic bone anchor according to claim 15. Further developments are given in the dependent claims.

[0009] With the dynamic bone anchor, bone parts or vertebrae to be fixed or stabilized are able to carry out a controlled limited motion relative to each other. The core member provided in the anchor member of the dynamic bone anchor is preferably made of Ni—Ti based shape memory alloy that is in the superelastic metallurgical state under the conditions in which the bone anchor is used in a patient.

[0010] Superelasticity or sometimes called pseudoelasticity involves the creation of stress-induced martensite which simultaneously undergoes strain when it is formed to release the applied stress. When the applied stress is removed, the thermally unstable martensite reverts to austenite, and the strain returns to zero. This behaviour provides a high elasticity to the material.

[0011] Due to the superelastic behaviour of the core member the degree of possible movement of the core member relative to the anchor member is increased compared to materials without superelasticity. The plateau in the stress-strain diagram of a Ni—Ti based shaped memory alloy shows a substantially constant stress exhibited when stress-induced martensite starts forming. This provides for an overload protection, for example during the screwing in of the dynamic bone anchor. In addition, the whole bone anchor can be designed with a relatively short length compared to bone anchors of the same type made of other materials.

[0012] The head of the bone anchor can perform rotational and/or translational motions with respect to the central axis of the bone anchor.

[0013] The dynamic bone anchor can be provided as a modular system, wherein a core member can be selectively combined with anchor members of different shape, different length or other different properties, such as different thread types, barbs etc. This has the advantage that the characteristics of the core member that substantially define the dynamic characteristics of the whole anchor, are known for many different combinations of core members with anchor members.

[0014] By providing core members that are different with respect to their contour, length or other properties, and combining them with suitable anchor members, various dynamic properties can be achieved.

[0015] With the method of manufacturing the bone anchor that makes use of the shape memory effect of the Ni—Ti shape memory alloy of the core member, the core member can be easily connected to anchor members of different length or other different properties. The achieved press-fit fit connection between the core member and the anchor member has a higher strength than conventionally generated press-fit connections.

[0039] As shown in FIGS. 1 and 2, the dynamic bone anchor according to a first embodiment comprises an anchor member 1 in the form of a screw member, a core member 2 and a head 3. The core member can be inserted into the anchor member 1 and connected thereto and the head 3 can be connected to the core member.

[0040] As further shown in FIG. 3, the anchor member 1 comprises a first end 11, an opposite second end 12 and a longitudinal axis L extending through the first end 11 second end 12. The longitudinal axis L forms the central axis of the bone anchor. Adjacent to the first end 11, the anchor member 1 comprises a tubular section 13 so that the anchor member 1 is open at the first end 11. The tubular section 13 extends to a distance from second end 12 and has an inner diameter d₁ and a length adapted to accommodate a portion of the core member 2 as described below. An end surface 13a of the tubular section provides a stop for the insertion of the core member. The second end 12 of the anchor member 1 is formed as a tip portion. At least a portion of an outer surface of the anchor member is provided with a bone engagement structure 14 that is configured to engage the bone or a vertebra, when the anchor member is used. In the embodiment shown, a bone thread is provided as the bone engagement structure. The bone thread extends over substantially the length of the anchor member but it may also be present only at a portion of the outer surface of the anchor member.

[0041] The anchor member 1 is made of a material that has a higher modulus of elasticity compared to a modulus of elasticity of the material of the core member 2, meaning that the anchor member is made of a stiffer material compared to the material of the core member. Preferably, the anchor member 1 is made of titanium or stainless steel. The anchor member 1 can also be made of a biocompatible polymer material, provided that the dimension, such as the length and wall thickness of the anchor member are such that the anchor member does not have a flexible behaviour when it is inserted into the bone.

[0042] As shown in FIGS. 4 and 5, the core member 2 is a longitudinal member comprising a first end 21 and an opposite second end 22 and a substantially rod-shaped central portion 23. The central portion 23 has a circular cross-section with an outer diameter d₂ that is smaller than the inner diameter d₁ of the tubular section 13 of the anchor member 1. Adjacent to the first end 21, there is a first connection portion 21a and adjacent to the second end 22, there is a second portion 22a. As can be seen in particular in FIG. 5, the connection portions 21a, 22a have the outer contour of a square with rounded edges. The distance d₃ from one flat side to the opposite flat side of the square is slightly greater than the outer diameter d₂ of the central portion 23 of the core member 2 such that the connection portion 22a is connected in a press-fit manner in the tubular section 13 as explained below. The connection portion 22a has a length in an axial direction adapted to provide sufficient fixation within in the tubular section 13. The connection portion 21a at the first end 21 serves for connection with the head 3 and has a shape similar to that of the second connection portion 22a. Between the central portion 23 and the connection portions 21a, 22a, there is a transition portion 21b, 22b, respectively, with increasing outer diameter towards the respective connection portion 21a, 22a. The total length of the core member 2 is such that when the core member 2 is inserted into the anchor member 1 and the second end 22 of the core member 2 abuts against the end surface 13a of the tubular section 13 of the anchor member, the core member 2 projects out of the open first end 11 of the anchor member with at least the first connection portion 21a and the first transition portion 21b.

[0043] The core member 2 is made of a material that is based on a nickel-titanium based shape memory alloy, preferably from Nitinol. The material exhibits superelasticity. Superelasticity is present in the austenitic metallurgical state. In particular, superelasticity is present in a temperature range sligthly above the stress-free martensite to austenite transition temperature. This should be the temperature range of use including the body temperature. More preferably, the core member 2 is made of a nickel-titanium based shape memory alloy of the ELI (extra low interstitial) type, in particular Nitinol of the ELI type. Such a material is of high purity and comprises in particular less oxygen compared to other Nitinol alloys that are not of the ELI type. Such a material is of high purity and comprises in particular less oxygen compared to other Nitinol alloys that are not of the ELI type. For example, the oxygen content is less than 0.025 wt%, preferably equal to or less than 0.010 wt % and more preferably equal to or less than 0.005 wt %. It has a fatigue strength limit that can be up to two times higher than that of other shape memory alloys being not of the ELI type.

[0044] The head will be described with reference to FIGS. 6 and 7. The head 3 comprises a first end 31, an opposite second end 32 and a spherical-segment shaped portion 33 adjacent to the first end 31. At the free end surface of the first end 31, an engagement portion 34 for engagement with a driver is provided. Adjacent to the spherical-segment shaped portion 33, there is a cylindrical neck portion 35 with a cylindrical recess 36 for accommodating the first connection portion 21a of the core member 2. The length and the inner diameter of the recess 36 is such that the first connection portion 21a can be accommodated therein with a press-fit connection.

[0045] In FIGS. 8a to 9b steps of manufacturing the dynamic bone anchor according to the first embodiment are shown. First, the core member 2 may be pre-assembled with the head 3, in that the first connection portion 21a is connected to the cylindrical recess 36 of the head 3 in a press-fit manner. The second connection portion 22a may be previously brougth into its final shape as shown in FIG. 4 and 5. Then, at least the second connection portion 22a of the core member is cooled down below the martensite finish temperature M_r so that a phase transition of the material from austenite into martensite takes place.

[0046] As shown in FIG. 8a a modular system may be provided comprising at least two anchor members 1, 1′, that differ, for example, in their length of the shaft The core member 2 may be selectively introduced in the tubular section 13 of one of the at least two anchor members 1, 1′,

[0047] Referring to FIG. 9a, the pre-assembled core member 2 with the head 3 is introduced into the tubular section 13 of one of the anchor 1, 1′, until the second end 22 of the core member abuts against the bottom surface 13a of the tubular section 13. Thereby, the second connection portion 22a of the core member is deformed for example such, that the flat sides are impressed so that they have a smaller distance form each other than in the original shape, as depicted in FIG. 8b. By means of this, the second connection portion 22a can be introduced into the tubular section 13 of the anchor member 1. Due to the ability of the martensite phase to deform, the insertion can be achieved with low force and little abrasion.

[0048] In a next step, heating the second connection portion 22a above the austenite finish temperature A_r effects a phase transition from martensite to austenite and a change of shape of the second connection portion 22a back to its original shape with the rounded square contour as shown in FIG. 9h due to the shape memory effect. Hence, the manufacturing process uses the shape memory behaviour of the core member 2. By means of this procedure and the square shape of the connection portion, a particularly strong press-fit connection can be achieved through a distortion-fit connection using the shape memory effect, which is stronger than a press-fit connection based on conventional machining techniques.

[0049] It should be noted that the connection between the core member 2 and the head 3 can also be made in the same manner.

[0050] Referring to FIG. 10, in the assembled state, there is a gap 37 between the second end 32 of the head and the first end 11 of the anchor member. Also, there is a gap 38 between the central portion 23 of the core member 2 and the wall of the tubular section 13. This allows the head 3 to perform translational movement with respect to the anchor member 1 in a direction substantially perpendicular to the anchor axis L. The extent of deflection out of the central axis of the bone anchor depends on the elasticity of the material of the core member 2 and also of the size of the gaps 37, 38, that means on the thickness and the length of the core member 2. A translation movement takes place when the deflection of the core member is mostly in the region of the first connection portion 21a.

[0051] Referring to FIG. 11, also a rotational movement of the center point of the head 3 around the anchor axis L is possible. For the rotational movement, the deflection of the core member takes place almost over the whole length of the central portion 23 and the first connection portion 21a.

[0052] Due to the superelasticity of the material of the core member 2 a deflection of the core member 2 out of the anchor axis is possible with a core member that is shorter compared to a core member made of another metallic material.

[0053] In use, the dynamic bone anchor is inserted into a bone part or a vertebra. Because the core member 2 is in the austenitic metallurgical state and in the conditions of use, the core member 2 has superelastic characteristics. In the stress-strain diagram of the bone anchor shown in FIG. 12, the stress-strain plateau is shown. Because of the plateau, the force acting onto the screw head 3 during screwing in remains constant over a certain range and an overloading of the anchor head 3 may not occur.

[0054] In the anchored state, the head is capable of performing a limited motion. The motion is constrained by the abutment of the core member against the inner surface of the tubular section 13 of the anchor member 1.

[0055] A first application of the bone anchor together with a stabilization device is shown in FIG. 13. The bone anchor according to the first embodiment is coupled to a receiving part 4 to form a polyaxial bone anchor. The receiving part 4 is substantially cylindrical and comprises a top end 41, a bottom end 42 and a coaxial bore 43 extending from the top end to a distance from the bottom end. The bore 43 narrows towards the bottom end 42 and provides an opening 44 near the bottom end. Near the opening, a seat 45 is provided for pivotably receiving the head 3. A U-shaped recess extends from the top end 41 to a distance from the top end for receiving a stabilization rod 5. By means of the U-shaped recess two free legs 46, 47 are provided which have an internal thread 48 for cooperating with a locking member such as a set screw 6. Furthermore, a pressure member 7 is provided that exerts pressure onto the head 3 such that the head 3 can be locked in a certain angular position by tightening the locking member. The bone anchor may be used with other designs of receiving parts and polyaxial bone screws. Also, the head 3 of the core member 2 may be designed such that it comprises a section for receiving a rod and for receiving a locking member to fix the rod as known from other monoaxial bone screws. In use, at least two polyaxial bone anchors are used and connected via the rod 5. Once the anchor members 1, 1′ are inserted into the bone parts or adjacent vertebrae, the heads 3, 3′ can perform a limited motion with respect to the anchor members 1, 1, respectively. Once the head 3 is locked in the receiving part 4, the bone anchor provides for a dynamic stabilization that allows small movements of the bone parts with respect to each other or small movements of a motion segment of the spinal column.

[0056] A second example of application is shown in FIG. 14, wherein the bone anchors according to the first embodiment are used together with a bone plate 9 comprising holes 9a, 9a′ with seat portions 9b, 9b′ for receiving the heads 3, 3′ of two bone anchors 1, 1′, respectively. The two bone anchors 1, 1′ are inserted in adjacent bone parts 101, 101′ and the bone plate 9 bridges at least a portion of a fracture site 102. In a specific application, a distance between the center axes C of two holes 9a, 9a′ that accommodate the heads 3, 3′ of the bone anchors is slightly smaller than the distance between the longitudinal axes L of the anchor members 1, 1′. Because the core members 2, 2′ with the heads 3, 3′ can perform a limited motion in a direction transverse to the longitudinal axis L, the bone parts 101, 101′ can be drawn together at the fracture site 102 as shown by the arrows in FIG. 14.

[0057] Referring to FIGS. 15 to 17, a second embodiment of the dynamic bone anchor is described. The dynamic bone anchor according to the second embodiment differs from the dynamic bone anchor according to the first embodiment in that the anchor member 1″ is formed completely as a tubular member. That means, the anchor member 1″ is open at the first end 11 and at the second end 12. The core 2″ includes a tip 24 between the second connection portion 22a and the second end 22. The second connection portion 22a is configured to be fixed in the previously described manner via a distortion-fit connection using the shape memory effect to the portion adjacent to the second end 12 of the anchor member 1″. The tip 24 projects out of the open second end 12 of the anchor 1″. It may be a tip with a smooth surface or may have further features, such as a self tapping structure, barbs or a roughened surface, etc. All other parts of the dynamic bone anchor according to the second embodiment are similar to the first embodiment and the description thereof is not repeated.

[0058] Referring to FIGS. 18 to 20, a third embodiment of the dynamic bone anchor will be described. The bone anchor according to third embodiment differs from the bone anchor according to the second embodiment in that the core member 2″′ comprises a head 3″′ similar to the head 3 of the first embodiment at the first end 21, wherein the head 3′″ is formed integrally with the central portion 23 of the core member 2′″. Hence, the core member 2″′ with head 3″′ is a monolithic piece and the whole bone anchor comprises only two pieces. The head 3″′ is in its outer contour similar to the head 3 described before.

[0059] Further adaptations or modifications of the dynamic bone anchor described in the embodiments can be accomplished by one of ordinary skill in the art without departing from the scope of the invention. For example, the head may have any other shape suitable for connecting it to other stabilization devices such as bone plates, receiving parts for accommodating stabilization rods etc. The head may even be omitted if the free end of the core is suitable for connection to another device. In such a case, the free end of the core member may comprise an engagement portion for a driver. In both cases, with or without a head or a head portion, the drive portion of the bone anchor for engagement with a tool is at the movable end of the core member.

[0060] Any kinds of tips may be provided. The tips shown in the embodiment may even be omitted. For example, the hollow tubular anchor member according to the second and third embodiment may have prongs at the second end.

[0061] The bone engagement structure may be a bone thread any type or may be accomplished by barbs or may even be only a roughened surface.

[0062] The embodiments may also be combined among each other, only as an example for such combination, the anchor member of the first embodiment may be provided with the core member having an integrally formed head as in the third embodiment.

[0063] While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is instead intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof.