Elastic element for the use in a stabilization device for bones and vertebrae and method for the manufacture of such elastic element

A stabilization device for bones or vertebrae includes a substantially cylindrical elastic element. The elastic element has a first end and a second end opposite to the first end. An elastic section extends between the first end and the second end. The elastic section includes at least first and second helical coils. The first and second helical coils are arranged coaxially so that the first helical coil extends at least in a portion between the second helical coil. The elastic element may form, for example, a portion of a rod, bone anchoring element, or plate.

BACKGROUND

The present invention relates to an elastic element for use in a bone anchoring element, a connecting element, a rod, and a stabilization device and a method for manufacturing the same.

It is known to use fixation and stabilization devices to fix fractures and stabilize spinal columns. These fixation and stabilization devices commonly comprise at least two bone anchoring elements or bone screws. Each of the bone anchoring elements is anchored in a bone or vertebra and is connected by a rigid plate or a rod. These types of fixation and stabilization devices generally do not allow any movement of the bones or vertebrae relative to each other.

In some instances, however, it is desirable to stabilize the bones or vertebrae so that the bones or vertebrae can carry out limited, controlled motion relative to each other. This is known as dynamic stabilization. Dynamic stabilization devices commonly comprise an elastic element instead of a rigid plate or rod that connects each of the bone anchoring elements.

One example of a dynamic stabilization device for vertebra is disclosed in United States Patent Application Publication No. 2003/0109880 A1. The dynamic stabilization device comprises first and second screws that are each anchored in a vertebra. Each of the screws has a receiving member for insertion of a spring which thereby connects the screws. The spring is provided in the form of a helical spring having closely neighboring coils like a tension spring. The spring is fixed in the receiving members by clamping screws. In this arrangement, however, because the spring is flexible, the spring can evade the pressure of the clamping screw and therefore become unfixed from the bone screw. Furthermore, both the elasticity and the flexural strength of the spring depend on the length of the spring. Thus, in applications requiring a spring with a short length, the elasticity and flexural strength of the spring is relatively small.

Another example of a dynamic stabilization device for a joint such as a wrist or knee joint is disclosed in U.S. Pat. No. 6,162,223. The dynamic stabilization device comprises a rod having a proximal rod section and a distal rod section connected to bone pins. The proximal rod section and the distal rod section are connected to each other by a flexible spring. The proximal rod section, the distal rod section, and the flexible spring are arranged outside of the body. The proximal rod section and the distal rod section are not fixedly connected to the flexible spring, but can move freely along a bore therein. In this arrangement, the flexible spring must be formed to have a diameter larger than a diameter of the rod. Additionally, the flexible spring must be large in order to have a high flexural strength. This dynamic stabilization device therefore has a complicated and voluminous structure, which prevents the dynamic stabilization device from being used inside the body on spinal columns.

BRIEF SUMMARY

The invention relates to an elastic element for use in a stabilization device for bones or vertebrae. The elastic element comprises a substantially cylindrical member having a first end, a second end opposite to the first end, and an elastic section between the first end and the second end. The elastic section includes at least first and second helical coils. The first and second helical coils are arranged coaxially so that the first helical coil extends at least in a portion between the second helical coil.

The invention further relates to a stabilization device for bones or vertebrae comprising a substantially cylindrical elastic element. The elastic element has a first end and a second end opposite to the first end. At least one of the first and second ends has threads. An elastic section extends between the first end and the second end. The elastic section includes at least first and second helical coils. The first and second helical coils are arranged coaxially so that the first helical coil extends at least in a portion between the second helical coil.

The invention still further relates to a method of manufacturing an elastic element for a stabilization device for bones or vertebrae. The method includes providing a substantially cylindrical body and forming first and second helical recess in the cylindrical body from an outside so that the first helical recesses are formed at least in a portion between the second helical recesses.

Various embodiments of the invention are illustrated inFIGS. 1-11and described herein. Elements of the various embodiments that are substantially identical will be referred to with the reference numerals.

FIGS. 1-2bshow an elastic element1according to a first embodiment of the invention. The elastic element1may be made, for example, from a bio-compatible material, such as titanium. Examples of other bio-compatible materials include stainless steel, titanium alloys, nickel-titanium alloys, nitinol, chrome alloy, cobalt chrome alloys, shape memory alloys, materials with super elastic properties, carbon reinforced composites, silicone, polyurethane, polyester, polyether, polyalkene, polyethylene, polyamide, poly(vinyl) fluoride, polyetheretherketone (PEEK), polytetrafluoroethylene (PTFE) and shape memory materials or alloys, such as nickel titanium or nitinol. As shown inFIG. 1, the elastic element1is a substantially hollow cylindrical member with an outer wall and a continuous coaxial bore2. The coaxial bore2extends from a first end9to a second end9′ of the elastic element1and has a diameter D1. A first helical recess3is formed in the outer wall in a direction of a central axis M of the cylindrical member. The first helical recess3has a height H and opens into the coaxial bore2in a radial direction. The first helical recess3is formed at a predetermined angle a and extends over a predetermined length L of the outer wall. A second helical recess4is formed in the outer wall in-between the first helical recess3in the direction of the central axis M of the cylindrical member. The second helical recess4is formed at substantially the same angle a and extends over substantially the same length L of the outer wall as the first helical recess3. The second helical recess4opens into the coaxial bore2in the radial direction.

First and second internal threads5,5′ are formed at the first and second ends9,9′, respectively, of the elastic element1. The first and second internal threads5,5′ extend over a predetermined length in an axial direction. The first and second internal threads5,5′ do not overlap or extend into the first and second helical recesses3,4formed in the outer wall. The elastic element1has an outer diameter, which is selected according to the desired use thereof. The length L of the first and second helical recesses3,4in the direction of the central axis M of the cylindrical member, the height H of the first and second recesses3,4, the angle a of the helices along which the first and second helical recesses3,4are formed, and the diameter D1of the coaxial bore2is selected to provide a desired stiffness to the elastic element1with respect to axial forces Fax, bending forces FBand torsional forces FTacting on the elastic element1.

As shown inFIGS. 2a-2b, a double helical spring or elastic section6consisting of a first helical coil7and a second helical coil8is formed by the first and second helical recesses3,4. Coils of the first helical coil7extend between coils of the second helical coil8. The first and second helical coils7,8are substantially identical and have substantially the same angle a. The coils of the first helical coil7are rotated approximately 180 degrees with respect to the coils of the second helical coil8around the central axis M, which is common to both the first and second helical coils6,7, so that the first and second helical recesses3,4oppose each other. The coils of the first helical coil7therefore run midway between the coils of the second helical coil8and vice versa. It will be appreciated by those skilled in the art that the elastic member1may additionally comprise more than two of the helical coils, wherein coils of each of the helical coils extend in-between coils of adjacent helical coils.

In order to obtain optimal elastic properties in an elastic element (not shown) with a single helical spring (not shown) having a predetermined length, the angle of the helices of the single helical spring (not shown) must be formed to have at least one whole turn. In the double helical spring6shown inFIGS. 2a-2b, however, the first helical coil7and the second helical coil8require less than one whole turn to obtain optimal elastic properties even though the double helical spring6has the same predetermined length as the single helical spring (not shown). Unlike the angle of the helices of the single helical spring (not shown), the angle a of the helices of the double helical spring6may therefore be increased to increase the flexural strength of the elastic element1. Additionally, the elastic element1may be formed, for example, to have an ovular cross-section or to be wasted such that the elastic element1has a flexural strength which is dependent on direction. The elastic element1therefore has a high flexural strength and a short length such that the elastic element1may be handled easily while at the same time providing a high operational reliability. Additionally, the elastic element1may be combined with other elements in various different ways to be a dynamic stabilization device for vertebrae or bones.

FIG. 3shows an elastic element11according to a second embodiment of the invention. The elastic element11is a substantially hollow cylindrical member having first and second helical recesses13,14formed in an outer wall thereof to form a double helical spring or elastic section. The double helical spring is formed in a similar fashion to the first embodiment. The elastic element11of the second embodiment differs from the first embodiment in that the elastic element11has a coaxial bore12that extends partially through the cylindrical member. The coaxial bore12extends from a first end17over the length L of the double helical spring and is coaxial with the central axis M of the cylindrical member. Internal threads15are provided in the coaxial bore12adjacent to the first end17. At a second end17′, which opposes the first end17, the elastic element11is provided with a cylindrical projection16. The cylindrical projection16has external threads. Alternatively, the coaxial bore12may have a diameter smaller than an outer diameter of the cylindrical projection16and may extend through the entire cylindrical member.

FIGS. 4a-4bshow an elastic element21according to a third embodiment of the invention. The elastic element21is a substantially cylindrical member having first and second helical recesses24,25formed in an outer wall thereof to form a double helical spring or elastic section consisting of a first helical coil26and a second helical coil27. The double helical spring is formed similar to the first and second embodiments. The elastic element21of the third embodiment differs from the first and second embodiments in that the elastic element21does not have a bore coaxial with the central axis M of the cylindrical member. The elastic member21has a first end22and a second end22′. The first and second ends22,22′ have first and second cylindrical projections23,23′, respectively. The first and second cylindrical projections23,23′ have external threads.

An elastic element according to a fourth embodiment is shown inFIG. 5a.FIG. 5bis a sectional view of the elastic element ofFIG. 5a. The elastic element30according to the fourth embodiment differs from the elastic element according to the first embodiment in that the pitch a of the recesses31,32which form the double helical coil is not constant but varies over the length L of the double helical coil of the elastic element30. The pitch a varies in such a way that the distance L of the recesses31,32increases from the free ends of the elastic element30towards the middle. Accordingly, the bending stiffness of the elastic element30varies and increases with increasing distance L of the recesses31,32. By varying the pitch of the recesses along the central axis of the elastic element, it is possible to achieve a particular stiffness at a particular position. Similar to the elastic element of the first embodiment, the elastic element30according to the fourth embodiment has coaxial bore34having an inner diameter D1and inner threads33,33′ extending a predetermined length from the free end, respectively.FIG. 6shows sectional view of an elastic element according to a fifth embodiment.

The elastic element according to the fifth embodiment differs from the elastic element30according to the fourth embodiment in that the inner diameter D1of the continuous coaxial bore42is not constant but varies of the length L′ of the elastic element40. The inner diameter D1of the bore42varies in such a way that it decreases from the free ends towards the middle of the elastic element40. Accordingly, the final stiffness of the elastic element40varies and increases with decreasing inner diameter D1. By varying the inner diameter of the coaxial bore, the stiffness of the elastic element40can be varied at different positions.

Similar to the fourth embodiment the elastic element40includes a section with an inner thread41,41′ having a predetermined length adjacent to each of its free end, respectively.

InFIG. 7aan elastic element according to a sixth embodiment is shown.FIG. 7bis a sectional view of the elastic element ofFIG. 7a.

The elastic element35according to the sixth embodiment differs from that of the fifth embodiment in that the outer diameter D2of the elastic element35is not constant but varies over the length L′ of the elastic element35. The outer diameter D2varies in such a way that it increases from the free ends towards the middle of the elastic element35. Accordingly, the bending stiffness of the elastic element35varies and increases with increasing outer diameter. Therefore, a position with a desired bending stiffness can be obtained by adjusting the outer diameter of the elastic element.

Similar to the fourth embodiment the elastic element35has adjacent to its free ends a section with an inner thread36,36′ of a predetermined length, respectively, and a continuous coaxial bore37with an inner diameter D1. Recesses38and39to form the double helical coil are formed like in the other embodiments.

In the fourth to sixth embodiments of the instant invention, the bending stiffness of the elastic element increases from the free ends towards the middle of the elastic element. However, by appropriate selection of the pitch a of the recesses, the outer diameter D2of the elastic element and the inner diameter D1of the coaxial bore, the bending stiffness can be adjusted to have a desired stiffness at a particular position along the length L, L′ of the double helical coil of the elastic element.

FIG. 8aillustrates a first example of an application of the elastic element1. As shown inFIG. 8a, the elastic element1may form a portion of a rod50, which may be used, for example, to connect pedicle screws (not shown) at a spinal column (not shown). The rod50in the illustrated embodiment consists of the elastic element1and first and second end portions51,51′. The first and second end portions51,51′ each have a cylindrical projection (not shown) with external threads (not shown) that cooperates with the first and second internal threads5,5′, respectively, of the elastic element1, shown inFIG. 1. Alternatively, an external nut (not shown) or other attachment member may be used to fix the elastic element1to the first and second end portions51,51′. In the illustrated embodiment, the first and second end portions51,51′ and the elastic element1have approximately the same outer diameter. The first and second end portions51,51′ have a length that may be selected independently from the length L of the elastic element1, which is shown inFIG. 1. The length of the first and second end portions51,51′ and the length L of the elastic element1selected depends on a desired end application. Because the rod50is formed with the elastic element1, the rod50can absorb compression forces, extension forces, bending forces and torsional forces to a predetermined extent by means of the elastic properties of the elastic element1.

FIG. 8billustrates a second example of an application of the elastic element1. As shown inFIG. 8b, the elastic element1may form a portion of a bone anchoring element, such as a polyaxial bone screw60. The polyaxial bone screw60in the illustrated embodiment includes a screw61with a shaft62and a head63. The shaft62has a tip (not shown) and includes bone threads64for screwing into a bone (not shown) and. A cylindrical projection (not shown) extends from the shaft62on a side opposite from the tip (not shown) and has external threads (not shown) that cooperate with the internal threads5of the elastic element1, which are shown inFIG. 1. As shown inFIG. 8b, the head63has a cylindrical section65adjacent thereto. A cylindrical projection (not shown) extends from the cylindrical section65and has external threads (not shown) that cooperate with the internal threads5′ of the spring element1, which are shown inFIG. 1.

As shown inFIG. 8b, the screw61is pivotally held in a receiving member66in an unloaded state. The receiving member66is substantially cylindrical and has a first receiving member bore67and a second receiving member bore68. The first receiving member bore67is provided at a first end of the receiving member66. The first receiving member bore67is substantially axially symmetrical and has a diameter larger than a diameter of the shaft62but smaller than a diameter of the head63. The second receiving member bore68is substantially coaxial and opens at a second end of the receiving member66opposite the first end. The second receiving member bore68has a diameter large enough that the shaft62of the screw61may be guided through the second end and the second receiving member bore68until the head63abuts an edge of the first receiving member bore67. The receiving member66has a substantially U-shaped recess69, which extends from the second end towards the first end. The substantially U-shaped recess69forms first and second legs70,70′ with free ends. In a region adjacent to the free ends, the first and second legs70,70′ have internal threads, which cooperate with corresponding external threads of a securing element71that fixes a rod72in the receiving member66.

A pressure element73that is provided for fixation of the head63in the receiving member66has a concave recess74on a side facing the head63. The concave recess74has a radius substantially identical to a radius of the head63. The pressure element73has an outer diameter selected so that the pressure element73can be inserted into the receiving member66and can slide towards the head63. The pressure element73has a coaxial pressure element bore75for providing access to a tool receiving recess (not shown) in the head63.

During assembly, the cylindrical projection (not shown) of the shaft62is screwed into the internal threads5of the elastic element1and the cylindrical projection (not shown) of the cylindrical section65of the head63is screwed into the internal threads5′ of the elastic element1to form the screw61. The shaft62of the screw61is then inserted into the second end of the receiving member66and guided through the second receiving member bore68until the head63abuts the edge of the first receiving member bore67. The pressure element73is inserted into the second receiving member bore68so that the concave recess74is positioned adjacent to the head63. The screw61is screwed into a bone (not shown) or vertebra (not shown). The rod72is inserted into the receiving member66and is arranged between the first and second legs70,70′. The angular position of the screw61relative to the receiving member66is then adjusted and fixed with the securing element71.

Because the screw61is formed with the elastic element1, the screw61may be diverted from the angular position by a limited extent. Additionally, if the elastic element1protrudes at least partially above a surface of the bone (not shown), the elastic element1can absorb compression forces, extension forces, bending forces and torsional forces because of the elastic properties of the elastic element1. If the elastic element1does not at least partially protrude above the surface of the bone (not shown), the screw61can still slightly yield, when the bone (not shown) or vertebra (not shown) moves such that the occurrence of unfavorable stress is avoided.

FIG. 8cillustrates a third example of an application of the elastic element1. As shown inFIG. 8c, the elastic element1may form a portion of a bone anchoring element, such as a monoaxial screw80. The monoaxial screw80in the illustrated embodiment consists of a head formed as a receiving member81and a shaft86. The receiving member81has a substantially U-shaped recess83formed at a first end thereof. First and second legs84,84′ are formed by the U-shaped recess83. The first and second legs84,84′ receive a rod82therebetween. Internal threads (not shown) that correspond to external threads on securing member85are formed on inside surfaces of the first and second legs84,84′. The rod82is clamped between a bottom surface of the U-shaped recess83and the securing member85when the securing member85is engaged with the internal threads (not shown). A cylindrical projection (not shown) extends from a second end of the receiving member81opposite from the first end. The cylindrical projection (not shown) has external threads (not shown) that correspond to the internal threads5′ of the elastic element1, which are shown inFIG. 1. The shaft86is similar to the shaft62previously described and has a cylindrical projection (not shown) extending therefrom with external threads (not shown) that corresponds to the internal threads5of the elastic element1, which are shown inFIG. 1.

During assembly, the cylindrical projection (not shown) of the shaft86is screwed into the internal threads5of the elastic element1and the cylindrical projection (not shown) of the receiving member81is screwed into the internal threads5′ of the elastic element1to form the monoaxial screw80. The monoaxial screw80is screwed into a bone (not shown) or vertebra (not shown). The U-shaped recess83is aligned and the rod82is inserted into the receiving member81and is arranged between the first and second legs84,84″. The rod82is then fixed by the securing member85.

FIGS. 8d-8eillustrate a fourth example of an application of the elastic element1. As shown inFIG. 8d, the elastic element1may form a portion of a connecting element90. The connecting element90in the illustrated embodiment consists of a rod91and a plate92. The rod91has a cylindrical projection (not shown) with external threads that correspond to the internal threads5of the elastic element1, which are shown inFIG. 1. A cylindrical projection (not shown) extends from the plate92and has external threads (not shown) corresponding to the internal threads5′ of the elastic element1, which are shown inFIG. 1. As shown inFIG. 8d, the plate92has a first section93and a second section93′ connected by a bridge94. The first and second sections93,93′ are substantially circular from a top view. The bridge94has a width B smaller than a diameter D of the first and second sections93,93′. The first and second sections93,93′ each have a screw receiving bore95,95′, respectively, formed coaxially with the first and second sections93,93′. The screw receiving bores95,95′ have a shape adapted for the reception of countersunk screws (not shown). As shown inFIG. 8e, a first side96of the plate92has a convex curvature and a second side97of the plate92has a concave curvature for abutting a surface of a bone (not shown). Due to the different curvatures of the first and second sides96,97, the plate92tapers towards lateral edges98,98′. The plate92is, therefore, stable and compact.

Modifications of the rod50, the polyaxial bone screw60, the monoaxial screw80, and the connecting element90, shown inFIGS. 8a-8eare also possible. For example, the elastic element1in the rod50, the polyaxial bone screw60, the monoaxial screw80, and the connecting element90is illustrated as being a separate element that requires connection therewith. Alternatively, the elastic element1may be integrally formed with the polyaxial bone screw60, the monoaxial screw80, and the connecting element90or press-fit thereto.

FIG. 9illustrates a fifth example of an application of the elastic element1. As shown inFIG. 9, the elastic element1may form a portion of a stabilization device100that is used, for example, in spinal columns. The stabilization device100in the illustrated embodiment consists of first and second bone anchoring elements101,101′, respectively, connected by a rod103. Each of the first and second bone anchoring elements101,101′ has a screw102,102′, respectively, formed with an elastic element1. The rod103is also formed with an elastic element1. Each of the screws102,102′ is screwed into a vertebra104,104′ so that a dynamic stabilization is established between the vertebrae104,104′ and the stabilization device100. Because the rod103and the screws102,102′ are made of several elements, the stabilization device100has various properties by the combination of only a few basic elements. The stabilization device100is not limited to the embodiment illustrated and depending on a desired application thereof, it is possible, for example, to provide only the rod103with the elastic element1.

A method of manufacturing the elastic element1by wire electrical discharge machining (EDM) is shown inFIGS. 10a-10c. As shown inFIG. 10a, a first bore110is formed in a solid cylinder112of a biocompatible material, such as titanium, perpendicular to a central axis M′ of the cylinder112. The first bore110extends through the whole cylinder112. A second bore111is formed coaxial with the central axis M′ of the cylinder112so that the cylinder112is made hollow. The order of forming the first and second bores110,111is arbitrary and may be varied according to a desired manufacturing process. A wire113for wire EDM is guided through the first bore110in a direction indicated by arrow P.

As shown inFIG. 10b, wire EDM is performed by moving the cylinder112in a direction indicated by arrow X along the central axis M′. The cylinder112is moved at a constant feed rate relative to the wire113and is simultaneously rotated around the central axis M′ in a direction indicated by arrow R with a constant angular velocity. Only relative movement of the wire113relative to the cylinder112is relevant. Accordingly, either the wire113or the cylinder112may be fixed during the wire EDM. As the cylinder112is rotated, first and second helical recesses114,115are formed.

As shown inFIG. 10c, after the first and second helical recesses114,115have been formed over a predetermined length of the cylinder112along the central axis M′, the rotation of the cylinder112is stopped.FIG. 10cshows the elastic element1shortly before completion of the wire EDM. The wire EDM thereby simultaneously forms in the outer wall of the cylinder112, first and second helical recesses114,115having approximately identical angles, which open in a radial direction into the second bore111.

As shown inFIG. 11, a first run-out120may be formed at a beginning of the wire EDM and at a second run-out120′ may be formed at an end of the wire EDM. The first and second run-outs120and120′ have a configuration by which load peaks can be minimized in the material at a transition from the elastic section to the rigid section during operation. The first and second run-outs120,120′ may have, for example, a semi-circular configuration. The first and second run-outs120,120′ advantageously may be made in one common manufacturing step. Additionally, unlike during the manufacture of a single helical spring (not shown), during the manufacture of the elastic element1, switching between each axis of the wire EDM machine is not necessary. Internal threads are then formed along the central axis M′ in end sections of the second bore111adjacent to the first and second ends.

Alternatively, the elastic element1may be milled. A first helical recess is milled along a first helix of a central axis of a solid cylinder formed of a bio-compatible material, such as titanium, having a predetermined outer diameter. The first helical recess is formed collinear with the central axis of the cylinder by a side mill. A second helical recess is milled along a second helix of the central axis such that coils of the second helical recess run between coils of the first helix. A bore is formed along the central axis of the cylinder over the whole length of the cylinder so that the first and second helical recesses communicate with the bore. The first and second helical recesses have first and second run-outs, respectively. The first and second run-outs of the first and second helical recesses at a transition between the first and second helices and end sections of the cylinder have a large influence on the stability of the elastic element1. The first and second run-outs of the first and second helixes at both of the end sections are reworked by an end mill so that sharp edges on an internal surface of the bore are removed. The first and second run-outs are milled by the end mill at an angle that is tangential relative to a helical line. The part is then chamfered on an inside and on an outside. Internal threads are then formed along the central axis in the end sections of the bore adjacent to first and second ends of the cylinder.

Further alternative methods for manufacturing the elastic element1are, for example, laser milling or hydro milling. These methods are performed similar to the wire EDM method, but instead of simultaneously forming the first and second helical recesses by a wire, a laser beam or a water beam is used. Additionally, instead of forming at least one of the internal threads, a cylindrical projection with external threads may be formed at a beginning of any one of the manufacturing methods by a lathe. In this instance, the bore has a diameter smaller than a diameter of the cylindrical projection. The spring element1may also be formed without the bore.

The embodiments described above and shown herein are illustrative and not restrictive. The scope of the invention is indicated by the claims, including all equivalents, rather than by the foregoing description and attached drawings. The invention may be embodied in other specific forms without departing from the spirit and scope of the invention.