US20100234903A1 - Bone anchoring element with thread that can be unscrewed - Google Patents

Abstract

A bone anchoring element (1) for anchoring an external device in a bone is described. A head (2) capable of connection to the external device is provided with a shaft (3) that can be anchored in a bone. A predetermined section of the shaft (3) has a bone thread (5). Moreover, the shaft (3) also has at least one bone thread-free surface (6,7; 11,12; 16,17; 21,22,23) that extends from a first end to a second end of the predetermined section essentially along the direction of the shaft axis. The bone anchoring element (1), thus provided, is pressed or soft-hammered into a hole (8, 28) previously generated in the bone and fixed in the bone by rotation by a predetermined angle. If the bone anchoring element has to be removed this can be achieved by unscrewing it in the standard fashion. The disclosure also describes methods for using the bone anchoring element.

Description

FIELD OF THE INVENTION
• The present invention relates to a bone anchoring element for the anchoring of a mechanical device in a bone.
BACKGROUND OF THE INVENTION
• A bone anchoring element is described inDE 42 34 118 in the form of a pedicle screw. It comprises a shaft and a head that can be connected to a rod for interconnection of a plurality of pedicle screws. The shaft is provided with a thread that allows the screw to be screwed into a bone.
• DE 43 07 576 describes a bone anchoring element with a polyaxial connection between the head of a bone screw and a rod.
• Such bone anchoring elements are inserted into the bone by screwing-in which is a relatively time-consuming and strength-requiring process. Moreover, a classical screwing-in process may be associated with high pressure forces acting on the bone, which is undesirable in the case of older or pre-damaged bones.
• For this reason, the classical process of screwing-in is less well-suited for certain clinical requirements, especially in pediatric surgery, surgery at the cervical spine, and neurosurgery.
• Therefore, it is desirable to provide a bone anchoring element, in particular for use in pediatric surgery, surgery at the cervical spine, and neurosurgery, which can be inserted into the bone more rapidly, more easily, and with less force so as to avoid exerting damaging forces on the bone, while providing for secure attachment.
SUMMARY OF THE INVENTION
• The present invention provides a bone anchoring element for anchoring an external device in a bone. The bone anchoring element comprises a head (2) that can be connected to the external device and a shaft (3) that can be anchored in a bone. The shaft comprises a bone thread (5) in a predetermined section and at least one bone thread-free surface (6,7;11,12;16,17;21,22,23) that extends from a first end to a second end of the predetermined section essentially along the direction of the shaft axis.
• By providing an area that bears no bone thread it is possible to press or insert the bone anchoring element into an attachment hole that was previously generated in the bone and then anchor the bone anchoring element by rotating it by a predetermined angle without involving a process of screwing-in. If the bone anchoring element has to be removed this can be achieved by unscrewing it in the standard fashion.
• Preferred embodiments of a bone anchoring element in accord with the invention have one or more of the following features:
• • the distance between the bone thread-free surface (6,7;11,12;16,17;21,22,23) and the shaft axis is smaller than the distance between the crest of the bone thread (5) and the shaft axis;
  • a multitude of bone thread-free surfaces (6,7;11,12;16,17;21,22,23) is provided, preferably two or three;
  • the multitude of bone thread-free surfaces (6,7;11,12;16,17;21,22,23) are arranged at equal distances from each other in a circumferential direction;
  • at least one of the bone thread-free surfaces is provided as a planar surface (6,7;21,22,23);
  • at least one of the bone thread-free surfaces (11,12) is provided to be concave in shape;
  • the bone thread-free surfaces (6,7;21,22,23) extend parallel to the shaft axis;
  • the bone thread-free surfaces (16,17) extend from the first end to the second end of the predetermined section in the form of a helical section;
  • the predetermined section extends to the end of the shaft (3) that is opposite to the head (2);
  • a tip (4) is provided at the end of the shaft (3) that is opposite to the head (2);
  • at least one of the bone thread-free surfaces (6,7;11,12;16,17;21,22,23) is roughened or coated in order to improve the in-growth of bone;
  • the shaft (3) is made from a shape memory alloy;
  • the head (2) is provided as receiving member (31) for connecting to a rod (30); and
  • the head (2) is connected to the shaft (3) either firmly or articulated.
• The invention also provides a method for using a bone anchoring element for anchoring an external device in a bone, the method comprising providing a bone anchoring element as described herein, having a first cross-sectional shape forming a hole in the bone having a second cross-section with dimensions equal to or slightly less than the dimensions of the first cross-sectional shape, inserting the bone anchoring element into the hole, and rotating the bone anchoring element around its longitudinal axis to a predetermined angle to engage the bone thread in the bone. A plurality of bone anchoring elements can be connected to a rod for stabilization of vertebrae or bones. Optionally the method includes a step of removing the bone anchoring element. The step of removing comprises a step of unscrewing the bone anchoring element in the standard fashion.
• Additional features and characteristics of the present invention are evident from the description of embodiments on the basis of the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 shows a perspective view of a bone anchoring element according to a first embodiment of the present invention.
• FIG. 2 shows a plan view looking at the tip of the bone anchoring element ofFIG. 1.
• FIG. 3 shows a front elevational view of the bone anchoring element ofFIG. 1 (i.e., generally in the direction of the arrow A inFIG. 2).
• FIG. 4 shows a side elevational view of the bone anchoring element ofFIG. 1 (i.e., generally in the direction of the arrow B inFIG. 2).
• FIG. 5 shows a schematic representation of the function of the bone anchoring element according to the first embodiment.
• FIG. 6 shows a plan view looking at the tip of a bone anchoring element according to a second embodiment of the present invention.
• FIG. 7 shows a perspective view of a bone anchoring element according to a third embodiment of the present invention.
• FIG. 8 shows a perspective view of a bone anchoring element according to a fourth embodiment of the present invention.
• FIG. 9 shows a plan view looking at the tip of the bone anchoring element ofFIG. 8.
• FIG. 10 shows a schematic representation of the function of the bone anchoring element according to the fourth embodiment;
• FIG. 11 shows a schematic representation of a monoaxial connection between a bone anchoring element and a rod;
• FIG. 12 shows a schematic representation of a polyaxial connection between a bone anchoring element and a rod;
• FIG. 13 shows a second embodiment of the invention;
• FIG. 14 shows a third embodiment of the invention; and
• FIG. 15 shows a further example of application of the bone anchoring element according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
• In accord with the present invention, bone anchoring elements are provided that can be inserted into a bone without screwing the threaded length of the shaft into the bone to the desired depth. A first embodiment of a bone anchoring element in accord with the present invention is described with reference toFIGS. 1 through 5.
• As is best seen inFIG. 1, the bone anchoring element1 comprises ahead2 and ashaft3.Head2 is provided to be spherical segment-shaped and comprises means for engagement with a rotating tool for turning the bone anchoring element. Such means can comprise a slot, cross slot, hexagonal recess, or outer hexagonal geometry, or similar device (not shown).
• Theshaft3 comprises a tip4 at its end opposite to the head. Abone thread5 extends over the entire length of the shaft.
• Two opposite sides of theshaft3 are provided with twoplanar surfaces6,7, which are parallel with respect to each other and to the shaft axis (longitudinal axis of shaft3) and extend over substantially the entire length of the shaft. The distance betweensurfaces6,7 is smaller than the core diameter ofbone thread5, such thatsurfaces6,7interrupt bone thread5.
• As is best seen inFIG. 2, the cross-section of theshaft3 thus becomes approximately rectangular, but has curved narrow sides that are formed by the thread portion. A first dimension of the cross-section is determined by the distance betweensurfaces6,7 and a second dimension orthogonal to the first direction is determined by the outer diameter ofbone thread5.
• The bone anchoring element1 typically is made of steel, titanium or any other body-compatible and sufficiently stable material.
• In order to insert anchoring element1, first a corresponding oblong hole8 is generated in the bone, the hole having a cross-sectional shape as is best illustrated by the cross-hatched area inFIG. 5. This can be accomplished, for instance, by drilling twin holes followed by removal of the separating wall with a rasp. Alternatively, the oblong hole can be generated with a transverse cutter or profile-forming cutter. The profile of oblong hole8 corresponds approximately to the cross-section ofshaft3. However, alternatively, it also can be slightly smaller.
• Subsequently,shaft3 of bone anchoring element1 is inserted into oblong hole8 by pressing or soft-hammering such that the longitudinal axis of a sectional plane ofshaft3 perpendicular to the shaft axis is approximately identical to the longitudinal axis t of oblong hole8. In this arrangement, the bone anchoring element1 is seated in oblong hole8 only so firmly that it can still be removed by pulling.
• Subsequently, using a rotating tool engaged onto or into the head, bone anchoring element1 is rotated by a pre-determined angle, preferably 90° for this embodiment, as depicted by the dashed line inFIG. 5. As a result, the teeth ofbone thread5 now engage the bone and bone anchoring element1 is firmly seated and fixed in the bone.
• By rotating it further to an angle of 180° or by rotating back to the initial position, if needed, bone anchoring element1 can be loosened and/or removed from the bone.
• In the course of time, the bone grows into the empty spaces and, thus provides additional stability to bone anchoring element1.
• In certain cases, it may be necessary to remove bone anchoring element1 from the bone after an extended period of time, when bone anchoring element1 is firmly surrounded by bone, e.g. if the anchored device is to be removed. At this time, simply rotating the bone anchoring element 90° to its original insertion position and pulling out the element will no longer be possible. In this case,bone thread5 serves for the purpose of allowing bone anchoring element1 to be removed from the bone by unscrewing with a rotating tool like a conventional bone screw.
• FIG. 6 shows a second embodiment of a bone anchoring element in accord with the present invention. In place ofplanar surfaces6,7, as illustrated for the first embodiment,shaft3 of bone anchoring element1 comprises twoopposite surfaces11,12, which are curved in the direction of the shaft axis such that the cross-section ofshaft3 has opposing concave longitudinal sides. In all other regards, the design and the use of the bone anchoring element1 are identical to the first embodiment.
• Due to theconcave surfaces11,12 the in-growth of bony material is improved which can provide for more stability of the attachment of bone anchoring element1 in the bone.
• FIG. 7 shows a third embodiment according to the present invention. Here, in place ofsurfaces6,7 of the first embodiment or surfaces11,12 of the second embodiment,shaft3 of bone anchoring element1 comprises twoopposite surfaces16,17 that are twisted around the shaft axis by a predetermined angle a, such that they form a helix section. In this embodiment, at any cross section of the shaft perpendicular to the shaft axis thesurfaces16,17 will show a straight line. In all other regards, the design and the use of the bone anchoring element,1, are identical to the first embodiment.
• Although the predetermined angle a is shown inFIG. 7 as being approx. 90° for illustration purposes, the angle is preferably in the range of 90°±45°. The slight twisting ofsurfaces16,17 also improves the anchoring of bone anchoring element1 in the bone.
• Similar tosurfaces11,12 of the second embodiment, alternatively, thesurfaces16,17 of the third embodiment can also be concave.
• A bone anchoring element according to a fourth embodiment according to the present invention is described with reference toFIGS. 8 through 10. In place of the twoparallel surfaces6,7,shaft3 of bone anchoring element1 comprises threeplanar surfaces21,22,23 which are off-set from each other by about 120° each, such that the cross-section ofshaft3 becomes approximately triangular. The distance between the bone thread-free surfaces,21,22,23, and the shaft axis is smaller than the core radius ofbone thread5 such that surfaces21,22,23 interruptbone thread5. In all other regards, the design and the use of the bone anchoring element,1, are identical to the first embodiment.
• In order to insert the bone anchoring element1, according to the fourth embodiment, first a correspondingtriangular hole28 is generated in the bone, as is best seen inFIG. 10 where it is illustrated by the cross hatched area. This can be accomplished for instance by drilling a hole followed by shaping with a rasp. Alternatively,triangular hole28 can also be generated with a transverse cutter or profile-forming cutter. The cross-section oftriangular hole28 corresponds approximately to the cross-section ofshaft3 or is slightly smaller than the cross-section.
• Subsequently,shaft3 of bone anchoring element1 is inserted intotriangular hole28 by pressing or soft-hammering such that the triangular sides of the cross-section throughshaft3 correspond approximately to the triangular sides oftriangular hole28. In this arrangement, the bone anchoring element1 is seated intriangular hole28 firmly to such an extent that it can still be removed by pulling.
• Subsequently, bone anchoring element1 is rotated by a rotating tool engaging on or in the head by a pre-determined angle, preferably 60° (depicted by the dashed line inFIG. 10). As a result, the teeth ofbone thread5 engage the bone and bone anchoring element1 attains a firm seat.
• By rotating further to an angle of 120° or rotating back to the initial position, bone anchoring element1 can be loosened, if needed, and removed from the bone.
• As discussed above, bone anchoring element1 of the fourth embodiment can be removed from the bone after an extended period of time, when it is firmly surrounded by bone, by unscrewing it with a rotating tool much like a conventional bone screw.
• Similar tosurfaces11,12 of the second embodiment, surfaces21,22,23 of the fourth embodiment also can be provided to be concave in shape and/or twisted with respect to each other likesurfaces16,17 of the third embodiment.
• The bone anchoring element of the invention, such as described in the first four embodiments, is ideally suited for the insertion into the pedicle of a vertebral bone, but also is suited for all other bones in which a mechanical device is to be anchored. Two types of a connection between a bone anchoring element and a rod are described in the following as examples for a connection of the bone anchoring element to an external device.
• FIG. 11 is a schematic representation of a monoaxial connection between a bone anchoring element1 and arod30.
• As is well known to those skilled in the art, thehead2 of bone anchoring element1 is provided as the receivingmember31 forrod30 and comprises aU-shaped recess32 by which twofree legs33,34 are formed. The width ofU-shaped recess32 corresponds approximately to the diameter ofrod30. Aninternal thread35 is provided on the inside offree legs33,34.
• After the insertion of bone anchoring element1 into the bone,rod30 is inserted intoU-shaped recess32. Subsequently, ascrew36 is screwed intointernal thread35 far enough forrod30 to be fixed betweenscrew36 and the base ofU-shaped recess32.
• Any other type of monoaxial connection betweenshaft3 and the receiving member known to those skilled in the art is also useful in the practice of the present invention. Moreover, the design of the receiving member for the rod is not limited to the embodiment shown. For instance, in one modification, the receiving member does not comprise an internal thread. In this case, the receiving member is connected to a threaded rod and fixed by nuts engaging on the sides. Again, any receiving member design known to those skilled in the art is useful in the practice of the present invention.
• FIG. 12 is a schematic representation of a polyaxial connection between a bone anchoring element1 and arod40.
• To provide for the connection, an essentially cylinder-shaped receivingmember41 with alongitudinal bore42 is provided. As is well known to those skilled in the art, the diameter oflongitudinal bore42 is slightly larger than that of the spherical segment-shapedhead2 of bone anchoring element1. Longitudinal bore42 extends from a first end of receivingmember41 to a second end opposite to the first end. A spherical segment-shapedsection43, with a diameter of the shape that is approximately identical to that of spherical segment-shapedhead2 of bone anchoring element1, is provided between the second end of receivingmember41 andlongitudinal bore42.
• In addition, receivingmember41 comprises a U-shaped recess (not shown), whose width corresponds approximately to the diameter ofrod40. This U-shaped recess forms twolegs44,45 in receivingmember41. The inside of thelegs44,45 is provided with aninternal thread46 and the outside with anexternal thread47.
• In addition, a sleeve-shapedpressure element50 is provided. At one end of sleeve-shaped pressure element50 a spherical segment-shapedrecess51 is provided which has approximately the same diameter of the sphere as spherical segment-shapedhead2 of bone anchoring element1, and at the other end a cylinder segment-shapedrecess52 is provided which has approximately the same diameter asrod40.
• Moreover, for the purpose of fixing the rod and the head, aninternal screw56, which can be screwed intointernal thread46, and anut57, which can be screwed ontoexternal thread47, are provided.
• In operation,shaft3 of bone anchoring element1 is inserted into receivingmember41 proceeding from the first end of receivingmember41 untilhead2 of bone anchoring element1 is held within spherical segment-shapedsection43. Subsequently, bone anchoring element1 is introduced into the bone. For this purpose, bone anchoring element1 is pressed or hammered into a hole in the bone that was previously generated, as described above.
• Subsequently, the bone anchoring element1 is rotated by a rotating tool an appropriate amount to fix it in the bone and proceeding from the first end of receivingmember41 sleeve-shapedpressure element50 is slid intolongitudinal bore42 such that spherical segment-shapedrecess51 comes to rest onhead2 of bone anchoring element1. Thenrod30 is inserted into the U-shaped recess such that it comes to rest in cylinder-segment shapedrecess52 ofpressure element50.
• Alternatively, receiving member and sleeve-shapedpressure element50 are preassembled and anchoring element1 is rotated by a rotating tool, which is guided through a coaxial bore in sleeve-shapedpressure element50.
• Subsequently, screw56 is screwed intointernal thread46 such thathead2 of bone anchoring element1 is fixed bypressure element50 and, simultaneously,rod40 is fixed byscrew56. Finally,nut57 is screwed ontoexternal thread47 to secure the fixation.
• The polyaxial connection between the head and the receiving member described above is presented only as an example. Again, any receiving member and polyaxial connection design known to those skilled in the art is useful in the practice of the present invention. Thesection43 of the receiving member can be of a shape other than a spherical segment, for example otherwise rounded or conical.
• The spherical segment-shaped head illustrated in the first through fourth embodiment is particularly well-suited for the polyaxial connection between the bone anchoring element and the external device described above. However, the bone anchoring element can be provided with a head of any other shape that allows the external device to be anchored to be connected and/or fixed in a monoaxial or polyaxial fashion. This includes, for example, the provision of a simple countersunk or fillister head for fixing a plate to a bone.
• Alternatively, the shaft of the bone anchoring element can be without tip. Moreover, it can be slightly conical in shape to ease its insertion into the bone.
• The thread can extend only over a predetermined longitudinal section of the shaft rather than over the entire length of the shaft.
• The number of bone thread-free surfaces is not limited to two or three. Alternatively, only one surface, or any number of surfaces greater than three, can be provided. The number is limited only by the size of circumference of the shaft.
• The distance of the surfaces from the shaft axis does not necessarily have to be smaller than the core radius ofbone thread5. It can also be selected to be identical to or larger than the core radius. In any case, though, is has to be smaller than the distance between the crest of the bone thread and the shaft axis.
• The surfaces corresponding toconcave surfaces11,12 do not have to be curved across their entire width. It is sufficient if merely a part of the surface is curved. Alternatively, there can be a v-shaped groove in the surface or any other geometric configuration that provides for additional bone growth around the shaft.
• In order to improve the in-growth of bone, the surfaces can be roughened and/or coated, e.g. with hydroxyapatite.
• Moreover, the bone anchoring element can be provided with a longitudinal bore which extends through its head and is open or closed at the tip, and bores which branch off from the longitudinal bore in a lateral direction and open on the lateral walls of the shaft, said bores allowing for a medication or bone cement to be injected into the attachment bore.
• In a further embodiment, the bone anchoring element can be made from a shape memory alloy, e.g., nitinol™. In this case, the bone anchoring element may be provided, e.g., such that, at room temperature, it assumes the shape corresponding toFIG. 1, in which the bone thread-free surfaces are planar and parallel to each other, and, at body temperature, assumes the shape corresponding toFIG. 7, in which the surfaces are twisted with respect to each other.
• During insertion at room temperature, the parallel surfaces allow for easy insertion of the bone anchoring element. Once inside the body, the shape of the bone anchoring element changes such that the surfaces become twisted and additional stability in the bone can be achieved.
• The bone anchoring element made from the shape memory alloy also can be formed such that the teeth of the bone thread are smaller at room temperature than at body temperature. As a result, the teeth of the bone thread grow once they are inside the body such that additional stability in the bone can be achieved.
• In a second embodiment shown inFIG. 13 thebone anchoring element100 comprises a shaft, but lacks a head. The shaft has asection104 which is formed like theshaft3 of the bone anchoring elements of the previously described embodiments withbone thread sections105 and bone thread-free sections106. Adjacent to section103 the bone anchoring element comprises a cylindrical section without bone thread. Such bone anchoring element is used e.g. in place of a usual Schanz screw or pin screw.
• FIG. 14 shows an example of application of thebone anchoring element100 in a Fixateur externe to be used for example for stabilizing fractures oflong bones150. The bone ancoring element is anchored with section103 in the long bone and connected to a receivingelement108 for receiving arod109.
• In a third embodiment shown inFIG. 15 the bone anchoring element110 is formed as a marrow nail to be anchored in along bone111 such as femur or humerus. The bone anchoring element comprises a shaft having asection114 which is is formed like theshaft3 of the bone anchoring elements of the first and second embodiment with bone thread sections and bone thread-free sections. The shaft further has a bone threadfree section115 and asection116 through which abone screw117 is screwed for fixation. In the example shown there is no external device to be connected.
• The invention has been described in detail, including the preferred embodiments thereof. However, it will be appreciated that those skilled in the art may make modifications and improvements within the spirit and scope of the invention.

Claims (20)

1. A bone anchoring element for anchoring an external device in a bone, the bone anchoring element comprising:

a head section that can be connected to the external device; and

a shaft that can be anchored in the bone and is connected to the head section, having a longitudinal axis;

the shaft comprising:

a bone thread on a predetermined section of the shaft, the bone thread having a crest, and a first end and a second end spaced apart longitudinally along the shaft, and

at least one bone thread-free surface that extends along the direction of the shaft axis from the first end to the second end of the bone thread.

2. The bone anchoring element according toclaim 1, wherein a first distance between the bone thread-free surface and the shaft axis is smaller than a second distance between the crest of the bone thread and the shaft axis.

3. The bone anchoring element according toclaim 1, further comprising a plurality of bone thread-free surfaces.

4. The bone anchoring element according toclaim 3, wherein the shaft further comprises a circumference and wherein the plurality of bone thread-free surfaces are positioned approximately at equal distances from each other around the shaft circumference.

5. The bone anchoring element according toclaim 1, further comprising two or three bone thread-free surfaces.

6. The bone anchoring element according toclaim 1, wherein at least one bone thread-free surface has a geometry such that a cross section of the shaft perpendicular to the longitudinal axis of the shaft shows the surface as a straight line.

7. The bone anchoring element according toclaim 1, wherein at least one bone thread-free surface has a concave shape.

8. The bone anchoring element according toclaim 1, wherein the bone thread-free surface extends parallel to the shaft axis.

9. The bone anchoring element according toclaim 1, wherein the bone thread-free surface extends from the first end to the second end of the bone thread in a helical form.

10. The bone anchoring element according toclaim 1, wherein the shaft has an end opposite to the head section and the predetermined section extends to the end of the shaft.

11. The bone anchoring element according toclaim 1, the shaft further comprising an end opposite to the head section, the end comprising a tip.

12. The bone anchoring element according toclaim 1, wherein at least one bone thread-free surface is roughened or coated in order to improve the in-growth of bone.

13. The bone anchoring element according toclaim 1, wherein the shaft comprises a shape memory alloy.

14. The bone anchoring element according toclaim 1, wherein the head section comprises a receiving member for connecting to a rod.

15. The bone anchoring element according toclaim 1, wherein the head section is fixedly connected to the shaft.

16. The bone anchoring element according toclaim 1, wherein the head section is connected to the shaft in an articulated arrangement.

17-22. (canceled)

23. A bone anchoring element comprising a shaft to be anchored in the bone, the shaft having a longitudinal axis; wherein the shaft comprises a bone thread on a predetermined section of the shaft, the bone thread having a crest and a first end and a second end spaced apart longitudinally along the shaft, and

at least one bone thread free surface that extends along the direction of the shaft axis from the first end to the second end of the bone thread.

24. The bone anchoring element ofclaim 23, wherein the bone anchoring element is formed as a pin.

25. The bone anchoring element ofclaim 23, wherein the bone anchoring element is formed as a marrow nail.

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