Bone anchoring device

0001The present invention relates to a bone anchoring device, and more particularly to a polyaxial bone anchoring device comprising a shank and a head formed as separate parts.

0002<patcit id="pcit0001" dnum="US6835196B2"><text>US 6,835,196 B2</text></patcit> describes an anchoring element for connecting a screw to a rod. The shank of the screw and the head are formed as separate parts. The head is shaped as a segment of a sphere which is pivotably held in a receiving portion. In one example the end portion of the shank has a cylindrical shape and the head is provided with a slit and has a cylindrical bore into which the cylindrical end portion of the shank is inserted.

0003<patcit id="pcit0002" dnum="US20050049588A1"><text>US 2005/0049588 A1</text></patcit> discloses a polyaxial bone screw for connection to a spinal fixation rod. At one end the shank of the bone screw has a frusto-conical capture structure diverging in diameter in a direction away from the threaded end of the shank. A retainer ring has a central bore with a conical shape which is compatible with the conical surface of the shank for receiving the shank. The angular orientation of the shank relative to a receiving part is fixed by the spinal fixation rod acting on an end portion of the shank.

0004<patcit id="pcit0003" dnum="US20060036252A1"><text>US 2006/0036252 A1</text></patcit> discloses a polyaxial screw which has a threaded shaft for securing the screw to bone. One end of the shaft has a reverse tapered head tapering toward and away from the shaft from an enlarged median. A spherical shaped swivel, having an opening smaller than the median, is snapped over the head to rotatably engage the shaft.

0005It is the object of the invention to provide a bone anchoring device with a shank and a head formed as separate parts which allows convenient and reliable handling during assembly of the shank and the head and provides secure locking of the shank in the head after assembly. Further, a modular system having variability shall be provided.

0006The object is attained by a bone anchoring device according to claim 1.

0007Further developments of the invention are specified in the dependent claims.

0008Since the shank and the head are formed as separate parts, in use of the bone anchoring device, an appropriate shank can be selected depending on the application and can be connected to the head for forming the bone anchoring device. In particular, shanks of different lengths can be provided in combination with the head such that the costs for stock-keeping can be reduced.

0009The cylindrical shape of the end portion of the shank cooperating with the cylindrical bore in the head allows convenient and reliable insertion of the shank into the head. Easy insertion is further improved by at least one slit provided in the head which imparts a certain flexibility to the head. Engagement of a projection with a corresponding groove during assembly leads to the result that the shank is securely retained in the head in an assembled condition.

0010Further features and advantages of the invention will arise from the description of embodiments with reference to the enclosed figures, of which: <dl id="dl0001"><dt>Fig. 1</dt><dd>is a perspective view of a bone anchoring device according to a first embodiment in an assembled condition;</dd><dt>Fig. 2</dt><dd>is an exploded view of the bone anchoring device of <figref idref="f0001">Fig. 1</figref>;</dd><dt>Fig. 3</dt><dd>is a sectional view of the bone anchoring device of <figref idref="f0001">Fig. 1</figref>;</dd><dt>Fig. 4a to 4c</dt><dd>are illustrations of the shank (<figref idref="f0004">Fig. 4a</figref>), the head (<figref idref="f0004">Fig. 4b</figref>), and a snap ring (<figref idref="f0004">Fig. 4c</figref>) of the first embodiment;</dd><dt>Fig. 5</dt><dd>is a perspective view for illustrating the assembly of the bone anchoring device according to the first embodiment;</dd><dt>Fig. 6</dt><dd>is a perspective view of the shank and the head of a second embodiment;</dd><dt>Fig. 7</dt><dd>is an exploded view of the embodiment of <figref idref="f0006">Fig. 6</figref>;</dd><dt>Fig. 8</dt><dd>is a sectional view of the upper part of the embodiment shown in <figref idref="f0006">Fig. 6</figref> in an assembled state;</dd><dt>Fig. 9</dt><dd>is an exploded sectional view of the shank and the head of the second embodiment;</dd></dl><dl id="dl0002"><dt>Fig. 10</dt><dd>is an exploded view of a third embodiment; and</dd><dt>Fig. 11</dt><dd>is a sectional view of the upper part of the third embodiment in an assembled state.</dd></dl>

First Embodiment

0011A first embodiment will be described with reference to <figref idref="f0001 f0002 f0003 f0004">Figs. 1 to 4</figref>. As can be seen in the exploded view of <figref idref="f0002">Fig. 2</figref>, the bone anchoring element 1 of the first embodiment comprises a shank 2, a head 3 provided separately from the shank 2, a receiving part 4 and a pressure element 5. Further, in the first embodiment a ring 7 is provided which will be described in detail below.

0012As can be seen in <figref idref="f0004">Fig. 4a</figref>, the shank 2 comprises a tip 20, a first portion 21 which is provided with a bone thread 22, and a second portion 23 adjacent to the free end 27 opposite to the tip 20 which has a cylindrical surface. The second portion 23 has a substantially smooth surface. Further, a circumferential groove 24 is provided in the outer surface of the second portion 23. The free end 27 of the second portion 23 has a beveled shape such that the diameter tapers towards the free end 27.

0013As can be seen in <figref idref="f0003">Fig. 3</figref>, the shank 2 is further provided with a coaxial bore 25. The bore 25 is open at the free end 27 and extends to a predetermined distance from the tip 20. Lateral through-holes 26 can be provided in the first portion 21 which extend from the bore 25 to the outer surface of the first portion. The through-holes 26 can be arranged in the base of the bone thread 22 between the thread crests. The bore 25 and the through-holes 26 allow, for example, introduction of bone cement to enhance the fixation of the shank 2 in the bone. The free end 27 of the shank further comprises a recess for engagement with a screwing-in tool.

0014The head 3 has a substantially sleeve-like shape with a first end 32 and a second end 33. The outer surface 30 is shaped as a segment of a sphere. A cylindrical bore 31 is provided which extends through the head from the first end 32 to the second end 33. A circumferential groove 34 is provided approximately at the center of the cylindrical bore 31. As can be seen in <figref idref="f0003">Figs. 3</figref> and <figref idref="f0004">4b</figref>, the groove 34 has a substantially rectangular cross-section.

0015Further, a longitudinal slit 35 is provided in the head 3 which extends from the first end 32 to the second end 33. Although only one slit 35 which extends from the first end 32 to the second end 33 is shown in the embodiment, the slit does not have to extend continuously from the first end 32 to the second end 33. One slit extending from the first end 32 or the second end 33 to a predetermined distance from the respective other end may be provided. As an alternative, for example, several slits which alternately extend from the first end 32 to a predetermined distance from the second end 33 and from the second end 33 to a predetermined distance from the first end 32 may be provided. The slit in the head 3 serves to provide elasticity to the head 3 in order to allow insertion even of a slightly oversized shank, as will be described below, and to provide secure clamping after final locking with a securing element.

0016The ring 7 is provided as a snap ring which has a substantially annular shape with a clearance 70, as can be seen in <figref idref="f0004">Fig. 4c</figref>. The ring 7 has a substantially rectangular cross-section and is shaped such that it fits in the groove 24 on the second portion 23 of the shank 2 and in the groove 34 in the cylindrical bore 31 of the head 3. Due to the clearance 70 the ring 7 has certain flexibility, i.e. its diameter can be enlarged and decreased by a certain amount.

0017As can be seen in <figref idref="f0002">Figs. 2</figref> and <figref idref="f0003">3</figref>, the receiving part 4 has a substantially cylindrical shape with a first end 40 and a second end 41 and a longitudinal axis Z. A coaxial bore 42 extends through the receiving part 4 from the first end 40 to the second end 41. The shape of the longitudinal bore 42 tapers towards the first end 40 such that a seat 43 for accommodating the outer surface 30 of the head 3 is formed. The seat can have, for example, a spherical shape or a conical shape or another tapering shape. Further, a substantially U-shaped recess 44 is formed in the receiving part 4 adjacent to the second end 41 such that two free legs 45 and 46 are formed which form a channel extending substantially perpendicular to the bore 42 for receiving a rod 60. On the inner surface of the free legs 45, 46 adjacent to the second end 41 an inner thread 47 is provided for screwing-in a securing element 61 for securing the rod 60 in the U-shaped recess. In the embodiment the inner thread 47 is designed as a flat thread having the thread flanks arranged perpendicular to the longitudinal axis. However, the thread can have any other suited thread shape.

0018Further, as shown in <figref idref="f0002">Fig. 2</figref> the pressure element 5 has a substantially cylindrical shape and is dimensioned such that it can be inserted in the coaxial bore 42 of the receiving part 4 and is slidably guided therein. On its upper side the pressure element 5 has a recess 52 shaped as a segment of a hollow cylinder the diameter of which substantially corresponds to the diameter of the bottom of the U-shaped recess 44 of the receiving part 4. On the lower side 51 of the pressure element 5 a recess (not shown) shaped as a segment of a hollow sphere is provided the diameter of which substantially corresponds to the outer diameter of the head 3. A bore 53 is provided in the pressure element 5 coaxially to its cylinder axis in order to allow guiding through of a screwing-in tool. In use, the pressure element 5 is placed in the longitudinal bore 42 such that the recess 52 shaped as a segment of a hollow cylinder is aligned with the U-shaped recess 44, as can be seen in <figref idref="f0001">Fig. 1</figref>.

0019The operation of the bone anchoring device 1 of the first embodiment will now be described with reference to <figref idref="f0005">Fig. 5</figref>.

0020In operation, first the shank 2 can be screwed into a bone or vertebra by engagement with a screwing-in tool.

0021The receiving part 4, the head 3, the ring 7 and the pressure element 5 can be provided in a preassembled state. In the pre-assembled state the ring 7 is inserted in the cylindrical bore 31 of the head 3 such that it rests in the ring-shaped groove 34. Further, the head 3 is placed in the longitudinal bore 42 of the receiving part 4 such that it rests on the seat 43 and remains pivotable with respect to the receiving part 4. The pressure element 5 is placed in the longitudinal bore 42 of the receiving part 4 such that the recess on the lower side 51 faces the outer surface 30 of the head 3 and such that the recess 52 shaped as a segment of a hollow cylinder is aligned with the U-shaped recess 44. In this assembled state, due to the flexibility provided by the clearance 70, the ring 7 can still be expanded such that its diameter is enlarged. Further, due to the slit 35, the head 3 can still be expanded such that its diameter is enlarged.

0022The assembly of receiving part 4, head 3, ring 7 and pressure element 5 can then be connected to the shank 2 by guiding the second portion 23 of the shank 2 into the cylindrical bore 31 of the head 3. The beveled shape of the upper free end 27 of the shank 2 facilitates insertion. During insertion the head 3 and the ring 7 are slightly expanded by the second portion 23 of the shank until the ring 7 snaps into the groove 24 of the second portion 23 of the shank. In this condition the ring 7 rests in the groove 24 of the end portion of the shank 2 and in the groove 34 of the head 3. As a result, the shank 2 is prevented from retracting again from the head 3 by form-locking engagement of the ring 7 in the groove 34 in the head 3.

0023The cylindrical shape of the second portion 23 of the shank 2 allows easy and convenient insertion of the shank 2 into the head 3. The projection formed by the ring 7 arranged in the groove 24 on the shank 2 allows secure fixation of the shank 2 to the head 3 by form-locking.

0024After insertion of the shank 2 in the head 3, the angular orientation of the shank 2 relative to the cylinder axis Z of the receiving part 4 can still be varied. In operation then a rod can be placed in the U-shaped recess 44 such that it rests in the recess 52 shaped as a segment of a hollow cylinder of the pressure element 5. A securing element can be screwed-in between the legs 45, 46 by cooperating with the inner thread 47. The securing element exerts pressure on the rod and the rod in turn exerts pressure on the pressure element 5. As a result, the pressure element 5 presses on the head 3 and the angular orientation of the head 3 relative to the receiving part 4 is thereby fixed. Further, since the head 3 is pressed against the seat 43 which is formed in a tapered shape, the head 3 is slightly compressed and the connection of the shank 2 to the head 3 is further strengthened.

0025The slit in the head 3 provides certain elasticity to the head 3. Thus, a shank 2 having a slightly over-dimensioned second portion 23 can be connected to the head 3. Further, this elasticity provides secure clamping of the shank 2 in the head 3, when the securing element is fastened.

0026It has been described above that the shank 2 is first screwed into the bone or vertebra before connecting it to the head 3. Alternatively the shank 2 can first be connected to the head 3 and the receiving part, such that the anchoring element is in a pre-assembled state, and thereafter the shank 2 can be screwed into the bone or vertebra by guiding a screwing-in tool through the longitudinal bore 42 of the receiving part 4 and through the bore 53 of the pressure element 5 into the structure in the longitudinal bore 25 of the shank 2. Thus, variability is ensured.

Second Embodiment

0027A second embodiment will be described with reference to <figref idref="f0006 f0007 f0008 f0009">Figs. 6 to 9</figref>. The second embodiment differs from the first embodiment in the structure of the shank and the head. The receiving part 4 and the pressure element 5 are identical to those of the first embodiment and will not be described again. The receiving part 4 and the pressure element 5 are omitted in <figref idref="f0006 f0007 f0008 f0009">Figs. 6 to 9</figref>.

0028The shank 102 according to the second embodiment comprises a first portion 121 which is provided with a bone thread 122 and a second portion 123 having a cylindrical outer surface with a first diameter wherein the surface is substantially smooth. As can be seen in <figref idref="f0008">Figs. 8</figref> and <figref idref="f0009">9</figref>, the shank 102 does not have a longitudinal bore as the shank in the first embodiment. A recess 126 for engagement with a screwing-in tool is provided at the free end 127. As in the first embodiment the free end 127 of the shank 102 has a beveled shape to facilitate insertion of the shank 102 into the head 103.

0029A circumferentially extending projection 107 is formed on the second portion 123. The projection 107 is formed integrally with the second portion 123.

0030Similar to the first embodiment, the head 103 has a substantially sleeve-like shape. The outer surface 130 is shaped as a segment of a sphere and cylindrical bore 131 is provided. A circumferential groove 134 is provided approximately at the center of the cylindrical bore 131.

0031Further, a plurality of longitudinal slits 135 are provided in the head 103 which alternately extend from the first end 132 to a predetermined distance from the second end 133 and from the second end 133 to a predetermined distance from the first end 132. This structure of the slits 135 provides a uniform flexibility to the head 103 in the region of the first end 132 and in the region of the second end 133.

0032Operation of the second embodiment is similar to the first embodiment. When the free end 127 of the shank 102 is inserted into the cylindrical bore 131 of the head 103, the diameter of the head 103 becomes enlarged by introduction of the circular projection 107. When the projection 107 reaches the ring-shaped groove 134 in the cylindrical bore 131, the head 103 elastically contracts and the projection 107 rests in the groove 134 such that the shank 102 is prevented from retracting from the cylindrical bore 131. After assembly of the complete bone anchoring device the pressure transmitted via the pressure element 5 further enhances the fixation of the shank 102 in the head 103 similar to the first embodiment.

0033According to the first embodiment, a projection projecting from the cylindrical outer surface of the second portion 23 of the shank 2 is formed by the ring 7 which is placed in a groove 24 in the shank 2. Similarly in the second embodiment a projection projecting from the cylindrical outer surface of the second portion 123 of the shank 102 is formed by the circular projection 107 which is integrally formed with the second portion 123. In both cases, easy and convenient insertion of the shank 2, 102 into the cylindrical bore 31, 131 of the head 3, 103 is allowed due to the cylindrical shape of the outer surface of the second portion 23, 123. Further, in both cases the shank 2, 102 is secured against unintended retraction by means of the projection 7, 107 projecting from the outer surface of the second portion 23, 123 engaging with the groove 34, 134 provided in the head 3, 103. The slits 35, 135 provide flexibility to the respective head 3, 103 which further facilitates insertion.

Third embodiment

0034A third embodiment is shown in <figref idref="f0010">Figs. 10</figref> and <figref idref="f0011">11</figref>. The third embodiment differs from the first and second embodiments in the structure of the head and the pressure element. The further structure is similar to that of the first and second embodiments and will not be described again.

0035The bone anchoring devices of the first and second embodiments provides a so-called polyaxial connection of the shank 2, 102 and the receiving part 4. In contrast, the bone anchoring device according to the third embodiment provides a monoaxial connection between a shank 202 and the receiving part 4. The shank 202 is constructed similar to the shank 2 in the first embodiment with a first portion 221, a cylindrical second portion 223 adjacent to a free end 227, and with a groove 224, but without a longitudinal bore and without through-holes.

0036As can be seen in <figref idref="f0010">Fig. 10</figref>, in the third embodiment the head and the pressure element are not constructed as separate parts but are formed in one piece as a head 203. The head 203 has a substantially cylindrical shape with a cylindrical outer surface 230 dimensioned such that it can be inserted in the longitudinal bore 42 of the receiving part 4. At a first end 203a the outer diameter of the head 203 tapers in a spherical shape. The opposite second end 203b is formed similar to the upper side 50 of the pressure element 5 in the first embodiment, i.e. a cylindrically-shaped recess 252 is formed at the second end 203b. A plurality of slits 235 extend from the first end 203a to a predetermined distance from the second end 203b. The slits 235 serve to provide flexibility to the head 203. Again, only one slit 235 may be provided alternatively.

0037The head 203 has a cylindrical bore 231 and a ring-shaped groove 234 similar to the head 3 in the first embodiment.

0038Assembly of the third embodiment is similar to the first embodiment. In operation, the head 203 is inserted in the longitudinal bore 42 of the receiving part 4. The snap ring 7 is placed in the groove 224 on the second portion 223. Then, the cylindrical second portion 223 of the shank 202 is inserted into the cylindrical bore 231 of the head 203 and the head 203 slightly widens due to the flexibility provided by the slits 235. The shank 202 is inserted until the snap ring 7 faces the ring-shaped groove 234 provided in the head 203 and then, due to the flexibility of the head 203, the snap ring 7 engages with the groove 234. As a result, the shank 202 is prevented from retracting again from the head 203.

0039After insertion of the rod 60 the securing element 61 is inserted for locking the rod 60 in the receiving part 4. During tightening of the securing element 61 pressure acts on the second end 203b of the head 203. The head 203 is pressed towards the first end 40 of the receiving part 4. Due to the tapering shape of the longitudinal bore 42 and to the slits 235 provided in the head 203, the head 203 is compressed and the shank 202 becomes securely locked in the head 203.

0040In an alternative way of operation the snap ring can first be placed in the ring-shaped groove 234 of the head 203 and thereafter the shank 202 can be inserted.

Further Modifications

0041In a further modification of the polyaxial bone anchoring device the shank is formed similarly to the shank 2 in the first embodiment with a groove 24 provided in the cylindrical outer surface of the second section 23. In contrast to the first embodiment the ring 7 is omitted and, instead of the ring-shaped groove 34 in the cylindrical bore 31 of the head 3, a ring-shaped projection with a reduced diameter as compared to the cylindrical bore 31 is integrally formed in the cylindrical bore 31. When the shank 2 is inserted in the cylindrical bore 31, this ring-shaped projection engages with the groove 24 of the shank 2.

0042As a result, a similar connection as that described with respect to the first and the second embodiments can be achieved. This structure can also be applied to the monoaxial connection shown in the third embodiment.

0043The ring 7 in the first and third embodiment can be considered to form a projection provided on the cylindrical inner surface of the bore 31, 231. The projection formed by the ring 7 engages with the groove 24, 224 provided on the shank 2, 202.

0044The structure of the projection 107 of the second embodiment cooperating with a groove provided in the head can be applied to the monoaxial connection of the third embodiment.

0045Although embodiments have been described in which locking of the rod and the angular orientation of the shank in the polyaxial bone anchoring device is performed by one single securing element, it is also possible to provide two securing elements for independently fixing the rod and the head. Alternatively, the securing element can be formed by an outer nut which is screwed on an outer thread provided on the free legs of the receiving part.

0046In the embodiments above the pressure element has a groove in the upper surface. However, it is also possible to provide a pressure element with a flat upper surface.

0047Further modifications are possible. For example the features of the head according to the second embodiment can be combined with the shank according to the first embodiment and vice versa.

0048Different types of shanks with or without coaxial bores and with different types of bone threads can be provided such that a modular system is formed. In such a system, also hooks may be provided. Further, different types of heads may be provided which can be combined with the shanks depending on the respective application. In this way, a modular system can be provided which offers variability in application.