Patent Description:
In orthophedic surgery, bone anchors, such as bone screws, are used in a wide field of applications. For example, bone screws form part of polyaxial bone anchors that couple a rod to the pedicles of vertebrae or that form part of bone plate assemblies. The bone screws are required to have a sufficient mechanical performance in view of loosening or inadvertent pull-out as well as in view of resistance against bending. Moreover, it is desirable that the bone screws allow for ingrowth of bone and tissue.

<CIT> describes an orthopedic screw having a thread with two parts, a distal part and a proximal part, each having a different thread configuration. The distal section has a distal screw thread with an outer dimension and pitch suitable for entry into cancellous bone, while the proximal section has a double thread having a first screw thread having the same pitch and formed on the same helix as that of said distal screw thread and a second screw thread having the same pitch but a smaller outer diameter than that of said distal screw thread and formed on a different helix. The screw is used in bone having a harder cortical outer section and a softer cancellous inner section.

<CIT> describes a bone screw that includes a main body having a proximal end and a distal end, which main body extends along a longitudinal axis. The main body has an externally threaded surface that includes at least one helically extending thread with two flank surfaces. The thread includes at least one aperture that extends along an aperture axis through the thread between the flank surfaces. The apertures provide space for the ingrowth of bone material.

<CIT> describes a bone screw comprising a screw body with a first helical thread and a second helical thread. The first helical thread comprises notches that are provided in the crest of the thread and are separated by non-notched portions.

It is the object of the invention to provide an alternative and/or improved bone anchor that has improved mechanical performance.

The object is solved by a bone anchor according to claim <NUM>. Further developments are given in the dependent claims.

The bone anchor includes a shank configured to be anchored in bone, wherein the the shank comprises a first end configured to be inserted first and an opposite second end and a longitudinal axis extending through the first end and the second end, a core and a thread configured to engage bone. The thread comprises a number of turns of a helix around the core, the number of turns defining an axial length of the thread. A thread shape of the thread is defined at least by a lower flank facing towards the first end and an upper flank facing towards the second end. The shank further comprises an additional thread structure with a modified thread shape along a portion of the core which portion is shorter than the axial length of the thread, wherein the additional thread structure comprises a recess that is at least partially helix-shaped, wherein the recess is configured to render the additional thread structure at least partially flexible.

By means of the additional thread structure mechanical stress or tension may be generated between the flanks of the additional thread structure when the additional thread structure engages the bone. Preferably, the additional thread structure is provided at a position of the shank that remains in the cortical bone once the bone anchor has been fully inserted into bone. In the cortical bone, the additional thread structure may provide for an increased holding force and/or increased resistance against screwing-back, loosening or backing-out of the bone anchor. The thread may be a single or a multiple thread, in particular a double thread.

According to a specific embodiment, the additional thread structure may include a lower flank that is separated from the upper flank by the at least partially helix-shaped slit. Thereby, the lower flank may be slightly flexible towards the upper flank. This may create the tension between the upper flank and the lower flank and the surrounding bone. In a further development, the lower flank of the additional thread structure may include a plurality of transverse slits that render the lower flank segmental. Such a segmental lower flank may enhance the flexibility and/or the segments may act like barbs that can reduce the risk of loosening of the bone anchor or provide resistance against screwing-out.

In a further specific embodiment, the additional thread structure comprises a modified thread shape that is thicker compared to the thread shape of the thread. Such a thicker thread may result in additional compression of the surrounding bone when the bone anchor is inserted.

According to a further embodiment, the at least partially helix-shaped recess of the additional thread structure forms an undercut that separates the upper and lower flank of the additional thread structure at least partially from the core. Thereby, a slight flexibility of the additional thread structure may be achieved which may result in a tension created between the flanks of the additional thread structure and the bone.

According to a further embodiment, the additional thread structure comprises a separate helix in-between the helical turns of the thread. Moreover, the separate helix may comprise a section that is offset towards the second end of the shank. Thus, a ramp-like structure is created that may result in an additional compression of the surrounding material during screwing-in and may account for an improved holding force of the bone anchor and reduces the risk of loosening or pull-out of the bone anchor.

According to a still further embodiment, the at least partially helix-shaped recess is comprised of at least one aperture that extend from the upper flank through the thread turn to the lower flank and which are arranged inside the crest of the thread. Such aperture may improve the ingrowth of bone material.

In an example, the at least partially helix-shaped recess comprises at least one a helical groove in the upper and/or the lower flank that extends along a portion of the helix of the thread. This may also improve the ingrowth of bone material.

Still further embodiments emerge by combination of the various additional thread structures. Compared to the thread shape of the thread, the additional thread structure includes complex deviations from this shape. Such deviations may include recesses and/or an addition of material. When using a subtractive manufacturing technology, in particular the manufacturing of undercuts becomes difficult. Therefore, the bone anchor is advantageously manufactured by an additive manufacturing method, such as laser sintering or laser melting, electron beam melting or any other suitable three-dimensional printing technique. This allows to manufacture arbitrary complex shapes. As a result, a suitable additional thread structure may be designed according to particular needs of a patient and may also be easily manufactured on demand.

A particular field of application of the bone anchor is orthopedic surgery, more particularly spine surgery. The bone anchor can be, for example, part of a monoaxial or polyaxial pedicle screw that is configured to connect the vertebra to a spinal rod. However, the bone anchor can also be used in other fields of spine and orthopedic surgery, for example, in connection with additional fixation of interbody cages, bone plates for osteosynthesis or fixation of joint replacements.

Further features and advantages of the invention will become apparent from the description of embodiments by means of the accompanying drawings.

Referring to <FIG>, a first embodiment of the bone anchor <NUM> is shown as a part of a polyaxial bone anchoring device. The bone anchor <NUM> comprises a shank <NUM> to be anchored in bone and a head <NUM> at a free end of the shank <NUM>. The head <NUM> may comprise a recess <NUM> configured for engagement with screwing-in tool. A receiving part <NUM> of the polyaxial bone anchoring device is configured to receive the head <NUM> of the bone anchor and a rod <NUM> in such a manner that the head <NUM> can pivot within the receiving part to assume various angular positions with respect to the receiving part <NUM> and the inserted rod <NUM>. The polyaxial bone anchoring device further includes a pressure member <NUM> for exerting pressure onto the head <NUM> to lock the head <NUM> within the receiving part <NUM>. A locking device <NUM> which may be a set screw is configured to cooperate with the receiving part <NUM> and to press onto the rod <NUM> to fix the rod which in turn transfers the pressure onto the pressure member <NUM> to lock the head <NUM> in the receiving part <NUM>.

The receiving part <NUM> comprises a first or upper end 5a and an opposite second or lower end 5b and a passage <NUM> extending from the first end 5a to the second end 5b. Adjacent to the second 5a, a seat <NUM> that may have a substantially spherical shape is provided for pivotably receiving the head <NUM> therein. The rod <NUM> can be inserted into a channel <NUM> formed by a substantially U-shaped recess extending from the first end 5a to a distance from the second end 5b. Adjacent to the first end 5a, an internal thread <NUM> may be provided in the receiving part <NUM> so that the locking device <NUM> can be screwed into the receiving part <NUM>. The pressure member <NUM> is adapted to move within the passage <NUM> of the receiving part and comprises a substantially spherical recess <NUM> on its side facing the inserted head <NUM> that is configured to contact the spherical outer surface portion of the head <NUM>. On the opposite side, a substantially cylinder segment-shaped recess <NUM> is formed for providing a support surface for the rod <NUM>. A coaxial hole <NUM> provides access for a tool to the head <NUM> when the head <NUM> is within the receiving part.

It shall be noted, that the bone anchor <NUM> is not limited to be used with the polyaxial bone anchoring device described above. It can be combined with various other receiving parts of different design and various other pressure members, locking devices or rods as those shown in the Figures.

As further shown in <FIG>, the shank <NUM> has a first end 2a that is configured to enter the bone first and an opposite second end 2b. At the first end 2a, a tip may be formed that may be blunt or sharp. Adjacent to the second end 2b of the shank <NUM>, a neck 2c with a reduced outer diameter may be provided that continues into the head <NUM>. The head <NUM> preferably has a spherical outer surface portion, in particular, the head <NUM> may have the shape of spherical segment with a substantially flat end surface 3a. The entire bone anchor <NUM> may optionally be cannulated. It comprises a channel <NUM> extending fully from the free end surface 3a of the head <NUM> through the shank <NUM> up to the tip. The channel <NUM> is preferably coaxial with the shank axis S and has a circular cross-section. An inner diameter of the channel may be substantially constant over the length or may vary along the length and/or the cross-section may be other than circular.

The shank <NUM> comprises a core <NUM> and a thread <NUM> winding in a helix around the core <NUM> in a plurality of turns. A central longitudinal axis S of the shank <NUM>, the shank axis, is also the screw axis of the thread <NUM>. The core <NUM> may be cylindrical along most of the length of the shank <NUM>. In greater detail, the core <NUM> may be cylindrical between the second end 2b and a distance from the first end 2a with an outer diameter or core diameter that is the same or that may be greater than the outer diameter of the neck 2c. Also, the core <NUM> may taper in a region towards the first end 2a. In this embodiment, the thread <NUM> is a double thread comprising two thread entries and therefore a first helix <NUM><NUM> and a second helix <NUM><NUM> that are winding in an alternating manner around core <NUM>. The helices <NUM><NUM> and <NUM><NUM> have the same pitch P and the second helix <NUM><NUM> runs in the middle between adjacent turns of the first helix <NUM><NUM>. Moreover, the thread entries of the double thread are offset by <NUM>°. The thread pitch P and geometry of the thread <NUM> may be such that there is gap between the thread turns on the core <NUM>. Generally, the thread shape and the thread pitch is such that the thread <NUM> is adapted to engage bone. A specific thread shape, the pitch, the number of threads, etc. are parameters that may depend on the type of bone which the anchor is to be inserted and on the purpose of the bone anchor. The shank may also have thread free portions, i.e. the thread <NUM> may be present only in a portion or portions of the shank.

An axial length of the thread <NUM> preferably extends from the first end 2a of the shank <NUM> to the second end 2b. In a first axial thread section s1 adjacent to or close to the first end 2a and preferably extending up to or beyond the middle of the shank <NUM>, the thread <NUM> comprises a plurality of turns. The shape of the thread <NUM> is substantially defined at least by a lower flank 20a facing towards the first end 2a and an upper flank 20b facing towards the second end of the shank <NUM> as present in the first thread section s1. Between the lower flank 20a and the upper flank 20b and, a substantially flat crest 20c of the thread <NUM> may be formed. In greater detail, parameters such as the shape of the flanks, the angle that the flanks form with each other, the thickness of the thread in the axial direction, the cross-section of the thread may all contribute to define the thread shape. Generally, the thread shape of the thread <NUM> may have any shape that is configured to engage bone. For example, it can have a V-shape with sharp, flat or rounded crest. Thus, for the following, the thread shape of the thread <NUM> in the thread section s1 is referred to as regular thread shape. In addition, in the first thread section s1, the thread <NUM> may run out towards the first end 2a. It shall be noted that the thread <NUM> may include cutting structures (not shown), for example axial grooves, close to the first end or at a distance therefrom which are disregarded in the definition of the regular shape of the thread <NUM>.

In a second axial thread section s2 that is located closer to the second end 2b than to the first end 2a of the shank <NUM>, an additional thread structure is provided. Preferably, the second axial thread section s2 is at such a position that the additional thread structure is configured to engage the cortical bone when the bone anchor is fully inserted in bone. Thereby, the additional thread structure may increase the holding force that holds the bone anchor in the bone and/or may increase a resistance against loosening and/or pull-out of the bone anchor. In the embodiment, the additional thread structure comprises a modified thread <NUM> that is modified with respect to the regular thread shape as present in the first section s1. Preferably the modified thread <NUM> comprises at least one turn of the helix.

The modified thread <NUM> forming the additional thread structure comprises an upper flank 21b facing towards the second end 2b of the shank <NUM> and a lower flank 21a facing towards the first end 2a of the shank <NUM>. The upper flank 21b is coincident with the upper flank 20b of the thread <NUM>. In other words, the upper flank 21b of the thread <NUM> has the same shape and size as the upper flank of the thread <NUM> and merges continuously with the upper flank 20b of the thread <NUM> at a transition section s3 of the first section s1 and the second section s2. The lower flank 21a is in the axial direction shifted or offset towards the first end 2a as compared to the lower flank 20a of the regular thread shape of the thread <NUM> in the first section s1. Moreover, the lower flank 21a is separated from the upper flank 21b by a helical slit <NUM>. By means of the slit <NUM> an additional helix <NUM> that comprises the lower flank 21a is formed in the second thread section s2 for each of the helices <NUM><NUM> and helix <NUM><NUM>. The additional helix <NUM> is shifted towards the first end 2a compared to the helix of the thread <NUM>. By means of this, the thickness of the thread in the axial direction is increased compared to the regular thread shape. In the lower flank 21a, transverse slits <NUM> are formed that are open towards the outer free edge of the lower flank 21a. The transverse slits <NUM> form an angle with the shank axis S that may be the same as the flank angle of the lower flank 21a. By means of the slits <NUM>, the additional helix <NUM> is divided into segments 24a. The thickness of the additional helix <NUM> in the axial direction is such that the additional helix <NUM> is flexible to some extent in the axial direction. In greater detail, the additional helix <NUM> is flexible against the remaining portion of the thread <NUM> in the second thread section s2. To achieve a desired flexibility, the number and/or width of the slits <NUM> are selected. The position of the slits <NUM> along the additional helix <NUM> of the first helix <NUM><NUM> and of the second helix <NUM><NUM> of the thread are offset from each other. Thus, the segments 24a are also offset from each other from one turn to a next turn and/or from the first helix <NUM><NUM> to the second helix <NUM><NUM> seen in the axial direction.

In the embodiment, the second section s2 extends along an axial length that is only a little smaller than a half of the shank length. However, the second section is not restricted to such a length. In particular, it can have a length that comprises at minimum a single turn of the additional helix <NUM> and at maximum extends up to a distance from the first end 2a that corresponds to a single turn of the thread <NUM>.

The bone anchor may be made of any bio-compatible material, preferably however of titanium or stainless steel or of any other bio-compatible metal or metal alloy or plastic material. As a bio-compatible alloy, a NiTi alloy, for example Nitinol, may be used. Other materials can also be magnesium or magnesium alloys. Bio-compatible plastic materials for use may be, for example, polyether ether ketone (PEEK) or poly-L-lactide acid (PLLA). The receiving part and other parts of the polyaxial bone anchoring device may be made of the same or a different material.

Preferably, the bone anchor is preferably using an additive manufacturing method. In an additive manufacturing method, the bone anchor is built up layer-by-layer based on three-dimensional data that characterize the shape and size of the bone anchor. A preferred method is, for example, selective laser sintering or selective laser melting or electron beam melting according to which successive layers of a powder material, for example stainless steel or titanium or another body compatible material are sintered or melted at positions corresponding to the cross-section of the bone anchor in the respective layer until the bone anchor is completed. With an additive manufacturing method, the additional thread structure is easily manufactured. Moreover, recesses that form undercuts and complex shapes can be built-up. In the above embodiment, the manufacture of the additional helix <NUM> and the slit <NUM> can be made easily with such an additive method as a subtractive method would be too difficult or even impossible.

In use, the bone anchor <NUM> usually is inserted into a prepared core hole in the bone, for example into the pedicle of a vertebra. The first thread section s1 is configured to be anchored in the softer cancellous bone. The second thread section s2 with the additional thread structure in the form of the modified thread <NUM> is configured to engage the harder cortical bone. During the insertion process and/or the final placement, the additional helix <NUM> is slightly compressed towards the upper flank 21b. Thereby, the upper flank 21b and the lower flank 21a tensioned or pre-loaded against each other and in the bone. This can increase the holding force which holds the bone anchor in the bone. Furthermore, the segments 24a of the additional helix <NUM> can act as barbs that provide additional resistance against loosening, backing-out or against inadvertent screwing-back of the bone anchor.

Referring to <FIG> a bone anchor <NUM>' according to a second embodiment will be described. Parts and portions that are identical or highly similar of the bone anchor of the first embodiment are marked with the same reference numerals and the description thereof will not be repeated. The thread <NUM>' of the first thread section s <NUM> shown in this embodiment has a substantially sharp crest 20c and the thickness of the thread in the axial direction is smaller than that of the thread <NUM> according to the first embodiment. However, it shall be noted that the thread <NUM>' may also have an identical regular thread shape as the thread <NUM> of the first embodiment. Also in this embodiment, the thread <NUM>' that defines the regular thread shape is a double thread with two helices <NUM><NUM>' and <NUM><NUM>'. The additional thread structure comprises a modified thread <NUM>' in the second thread section s2 which lies on the same helix as the thread <NUM>' of the first thread section s1 but which is bifurcated by a slit <NUM>' into an upper helix 21b' and a lower helix 21a'. Moreover, the additional thread structure also comprises a helical recess <NUM>' in the core <NUM> that may have substantially the same helical extension as the slit <NUM>'. As best seen in <FIG>, the helical recess <NUM>' extends under the upper helix 21b' and the lower helix 21a' such that they are separated from the core <NUM>. By means of this, the upper helix 21b' and the lower helix 21a' may be partially resilient with respect to the core <NUM> in the region where the recess <NUM>' is that forms an undercut.

The thickness of the thread <NUM>' formed by the helices 21b' and 21a' in the axial direction is greater than that of the thread <NUM>' of the first thread section s <NUM>. In other words, the helix of the modified thread <NUM>' is thickened compared to the helix of the thread <NUM>'. The shape and size of the slit <NUM>' is selected such that the lower helix 21a' is flexible relative to the upper helix 21b'. In the embodiment, the additional thread structure in the form of the modified thread <NUM>' includes a lower step 26a where the thickened portion begins and that is located in the direction of the first end 2a of the shank and ends with an upper step 26b where the thickened portion stops and that is in the direction of the second end 2b of the shank <NUM>. As depicted in <FIG>, since each of the two helices <NUM><NUM>' and <NUM><NUM>' has the modified thread <NUM>', the step 26a belongs to the beginning of the thickened portion of the helix <NUM><NUM>' and the step 26b belongs to the end of the thickened portion of the helix <NUM><NUM>'. Moreover, the thickened portion of the first helix <NUM><NUM>' is offset from the thickened portion of the second helix <NUM><NUM>' of the double thread. The helical slit <NUM> begins before the lower step 26a in the helical path towards the first end 2a and ends beyond the upper step 26b in the direction in the helical path to the second end 2b. An axial coverage of the modified thread <NUM>' is preferably about half a turn, but it can be less or more up to about one turn.

Adjacent to the neck 2c, a small axial portion of the thread <NUM>' with the regular thread shape may be present, as for example shown in <FIG>.

In use, when the additional thread structure comprising the bifurcated thread <NUM>' enters the bone, preferably the cortical bone region, the upper helix 21b' and the lower helix 21a' are pressed together. This is caused by the increased thickness of the modified thread <NUM>' in the second thread section s2 and the flexibility of the helices 21b' and 21a' due to the slit <NUM>'. In addition, the helices 21b' and 21a' can resiliently flex to some extent with regard to the core <NUM>. Thereby, the two helices 21b' and 21a' may be tensioned or pre-loaded against each other and against the bone which results in an additional holding force of the bone anchor in the bone. Also, the slit <NUM>' and the recess <NUM>' may provide space for ingrowth of bone material that further reduces the risk of loosening.

Referring to <FIG>, a bone anchor <NUM>" according to a third embodiment will be described. Parts and portions of the third embodiment that are identical or highly similar to that of the previous embodiments are marked with the same reference numerals and the description thereof will not be repeated. The thread <NUM>" that defines the regular thread shape may be identical to the thread <NUM>' of the second embodiment. However, it may also be identical to the thread <NUM> of the first embodiment. The additional thread structure comprises two additional helices <NUM><NUM>" and <NUM><NUM>" in the second section s2 that run between the two helices <NUM>"<NUM> and <NUM>"<NUM> of the thread <NUM>". The additional helices <NUM><NUM>" and <NUM><NUM>" have thread entries that are offset from each other by <NUM>°. Each additional helix <NUM><NUM>", <NUM><NUM>" is composed of two differently arranged portions. A first portion 21a" is extending axially in the middle between the helices <NUM><NUM>" and <NUM><NUM>" of the thread <NUM>". A second portion 21b" that is closer to the second end 2b than the first portion 21a" is in the axial direction slightly shifted towards the second end 2b of the shank <NUM>. The second portion 21b" preferably extends along a half turn of each of the helices <NUM><NUM>", <NUM><NUM>". The first portion 21a" and the second portion 21b" taper towards a transition portion 21c" where they join each other. Adjacent to the second portion 21b" a third portion 21c" may be formed in the direction to the second end 2b of the shank <NUM>, that is again located in the middle between the helices <NUM><NUM>" and <NUM><NUM>" like the first portion 21a". The first helix portion 21a" and the second helix portion 21b" of one additional helix <NUM><NUM>" are arranged offset from those of the second additional helix <NUM><NUM>", preferably in such a manner that axially first and second portions alternate from one helix to the next helix.

Furthermore, the additional thread structure includes helix-shaped recesses or undercuts <NUM>" in the core <NUM> that extend under the the additional helices <NUM><NUM>" and <NUM><NUM>" at least in a portion thereof such that the additional helices <NUM><NUM>" and <NUM><NUM>" are partially separated from the core <NUM>. The helix-shaped recesses <NUM>" in the core cover at least a region along the second portion 21b", preferably they extend partially also under the first portion 21a". By means of these recesses <NUM>", the modified thread <NUM>" is flexible to some extent with respect to the core <NUM>.

It should be noted that the second thread section s2 may cover several turns of the thread <NUM>". In such a case, first portions 21a" and second portions 21b" and the corresponding helix-shaped recesses <NUM>" are arranged one after the other in an alternating manner and are joined by thinner transition portions 21c". The thickness of the additional helices may be smaller than the thickness of the helices of the thread <NUM>".

In use, as depicted in particular in <FIG>, when the additional thread structure engages the bone, in particular the cortical bone, the second portions 21b" of the additional helices that form a type of ramp experience a force with a downward component as indicated by the arrows that generates a pre-load that enhances the holding force. Moreover, the resistance against loosening, screwing-out or pull-out may be increased.

Referring to <FIG> a bone anchor <NUM>‴ according to a fourth embodiment will be described. Parts and portions of the bone anchor according to the fourth embodiment that are identical or highly similar to those of the previous embodiments are marked with the same reference numerals and the description thereof will not be repeated. The bone anchor <NUM>‴ according to the fourth embodiment differs from the bone anchor <NUM>" according to the third embodiment in that the additional thread structure comprises a single additional helix <NUM>‴ that runs between two turns <NUM><NUM>‴ and <NUM><NUM>‴ of the thread <NUM>‴ and a recess or undercut <NUM>‴ that separates at least a portion of the additional helix <NUM>‴ from the core <NUM>. The structure of the single additional helix <NUM>‴ is the same as one of the additional helices <NUM><NUM>" and <NUM><NUM>" of the third embodiment.

In use, as shown in <FIG>, when the bone anchor according to the fourth embodiment is inserted into bone and the second section s2 engages the cortical bone, a downward force as depicted with the arrows is generated onto the additional helix <NUM>‴. This may result in a tension or pre-load between the flanks of the additional helix and the bone that may increase the holding force. Moreover, this may increase a resistance against loosening, screwing-back or pull-out.

Referring to <FIG>, a bone anchor 1ʺʺ according to a fifth embodiment of the bone anchor is shown. Parts and portions of the bone anchor 1ʺʺ according to the fifth embodiment that are identical or highly similar to parts and portions of the previous embodiments are marked with the same reference numerals and the description thereof will not be repeated. The bone anchor 1ʺʺ according to the fifth embodiment comprises a thread 20ʺʺ which is a double thread having a first helix <NUM><NUM>ʺʺ and a second helix <NUM><NUM>ʺʺ similar to that of the third embodiment in a first thread section s1 adjacent to the first end 2a of the shank <NUM>. The lower flank 20a and the upper flank 20b form an angle α which is shown in <FIG> only schematically. The shank <NUM> further comprises a second thread section s2 that covers the shank from an end of the first thread section s1 up to the second end 2b and in which an additional thread structure with a modified thread 21ʺʺ is provided. The thread 21ʺʺ is modified such that the lower flank 21aʺʺ and the upper flank 21bʺʺ form a second angle β hat is greater than the first angle α of the thread <NUM>"". Thereby, the thickness of the thread is increased. This also enhances the holding force, particular in cortical bone.

A still further additional thread structure is formed by a plurality of at least partially helix-shaped recesses 27ʺʺ in the first thread section s1, i.e. in the helices <NUM><NUM>ʺʺ and <NUM><NUM>ʺʺ. The recesses 27ʺʺ extend in the axial direction entirely through the thread from the upper flank 20b to the lower flank 20a. The recesses 27ʺʺ are elongate and follow the helical course of the thread <NUM>"". A length of the recesses 27ʺʺ may be about a quarter of a turn. The crest 7c in the region of the recesses 27ʺʺ remains intact. Thus, the recesses <NUM>"" are closed in the radially outward direction. In each of the helices <NUM><NUM>ʺʺ and <NUM><NUM>ʺʺ at least one single recess 27ʺʺ may be formed wherein the positions of the recesses are offset from each other from one helix to the other.

In use, the additional thread structure in the form of the increased thickness of the thread provides for an enhanced holding force. The additional thread structure in the form of the recesses may allow an increased ingrowth of bone material. As the position of the recesses 27ʺʺ is closer to the middle of the shank of the bone anchor, the additional thread structure enhances the holding force, in particular in cancellous bone.

Referring to <FIG> a bone anchor <NUM> according to an example not forming part of the invention will be described. Parts and portions of the bone anchor according to the sixth embodiment that are identical or similar to parts and portions of the bone anchor according to previous embodiments are marked with the same reference numerals and the description thereof will not be repeated. The bone anchor <NUM> according to the sixth embodiment differs in some aspects from the bone anchor according to the previous embodiments. It comprises a thread <NUM> that defines the regular thread shape and which is present in the first thread section s1. In the embodiment shown, the thread <NUM> is a double thread with a first helix <NUM><NUM> a second helix <NUM><NUM>. In the first thread section s1 the upper flank 20b and the lower flank 20a form an angle α. In the second thread section s2 the thread <NUM> is modified to a double thread <NUM> having two helices <NUM><NUM>, <NUM><NUM> which form the additional thread structure. The modified thread <NUM> comprises an angle β between the upper flank 210b and the lower flank 210a that is greater than the angle α of the thread <NUM> in the first section s1. Thus, the thickness of the modified thread <NUM> in the second section s2 is greater than the thickness of the thread <NUM> in the first section s1. Moreover, the crest 20c may be more flat in the second section s2 compared to a more sharp crest in the first section s1.

Each of the helices in the second thread section s2 comprises a still further additional thread structure formed by at least one groove, preferably by two grooves 270a, 270b that extend along the upper flank 210b and the lower flank 210a, respectively, in a helical path. The outer groove 270a that is on a radial outer position, closer to the crest 210c, may have a greater width in the transverse direction compared to the inner groove 270b that is closer to the core <NUM>. A transverse groove 270c may be provided to radially connect the outer groove 270a to the inner groove 270b at the starting area of the thread <NUM> that is facing towards the first end 2a.

In addition, a plurality of recesses <NUM> extending substantially in the axial direction from the upper flank 20b to the lower flank 20a may be provided. The recesses <NUM> are provided along an axial region between a distance from the second end 2b in the second thread section s2 and a distance from the first end 2a in the first thread section s1. In greater detail, the recesses <NUM> may be arranged in at least one, preferably two or more axial rows offset from each other as can be seen in <FIG>. The shape of the recesses <NUM> may be such that they have a rounded inner part closer to the core <NUM> and a more flat outer part that is located still inside the thread flank. In other words, the recesses <NUM> are closed and do not interrupt the crest 210c. The number of the plurality of recesses <NUM> may be at least two, preferably more than three recesses in one axial row. Also, the axial distance of the recesses <NUM> in one row may be different from that of an other row as depicted in <FIG>.

The bone anchor <NUM> comprises a short channel <NUM> that extends from the free end surface 3a of the head <NUM> to a distance therefrom into the shank.

In use, with the modified thread <NUM>, the holding force can be increased due to the increased thickness of the modified thread <NUM>. By means of the grooves <NUM> the ingrowth of bone material and tissue is promoted. As a result, the holding force of the screw within the bone can be increased and/or the resistance against loosening, screwing-out or pull-out can be increased.

It shall be noted, that all embodiments of the bone anchor are preferably made by an additive manufacturing method as explained with respect to the first embodiment.

Further modifications of the bone anchors may be conceivable. The features of the various bone anchors can be mixed and matched to produce a variety of further embodiments. The shape of the bone anchor is not limited to the detailed shape shown in the embodiments. For example, various designs of the tips of the bone anchor may be conceivable. The head may have other shapes or can be omitted at all. Even the neck portion can be omitted. A suitable drive structure is then formed at the second end of the shank. The additional thread structure can be provided at various positions along the shank. Furthermore, more than one section with an additional thread structure may be provided on the shank and the additional thread structures do not need to be identical on one shank.

It should be noted that the core may also be tapered from the second end up to the first end of the shank or its outer diameter may decrease in steps between the second end and the first end of the shank. In a further embodiment the shank may be fenestrated, i.e. may have one or a plurality of openings that connect the channel with the outside of the bone anchor.

Claim 1:
Bone anchor comprising
a shank (<NUM>) configured to be anchored in bone, the shank (<NUM>) comprising
a first end (2a) configured to be inserted first and an opposite second end (2b) and a longitudinal axis (S) extending through the first end and the second end,
a core (<NUM>) and
a thread (<NUM>, <NUM>', <NUM>", <NUM>‴, 20ʺʺ, <NUM>) configured to engage bone, the thread comprising a number of turns of a helix around the core (<NUM>), the number of turns defining an axial length of the thread, wherein the thread (<NUM>, <NUM>', <NUM>", <NUM>'", <NUM>"") comprises a lower flank (20a) facing towards the first end (2a) and an upper flank (20b) facing towards the second end (2b), the upper flank and the lower flank defining a thread shape;
wherein an additional thread structure (<NUM>, <NUM>', <NUM>", <NUM>'", <NUM>"") with a modified thread shape is provided on a portion (s2) of the core (<NUM>) that is smaller than the axial length of the thread, the additional thread structure comprising a recess (<NUM>, <NUM>', <NUM>', <NUM>", <NUM>‴, <NUM>"") that is at least partially helix-shaped; characterized in that
the recess (<NUM>, <NUM>', <NUM>', <NUM>", <NUM>‴) is configured to render the additional thread structure at least partially flexible.