Receiving part for receiving a rod for coupling the rod to a bone anchoring element and a bone anchoring device

A receiving part for coupling a rod to a bone anchoring element includes: a receiving part body with a rod receiving portion and a head receiving portion having an open end for introducing a head of a bone anchoring element, and a locking ring, wherein the head receiving portion includes a plurality of flexible wall portions, the flexible wall portions and the locking ring being configured to engage each other at circumferentially distinct pressure areas having positions corresponding to circumferentially separated projections on at least one of the flexible wall portions or the locking ring, and wherein the locking ring exerts a first force on at least one of the flexible wall portions at a corresponding pressure area, and exerts a second force less than the first force on the at least one flexible wall portion in a region circumferentially adjacent to the corresponding pressure area.

BACKGROUND

1. Field of the Invention

The invention relates to a receiving part for receiving a rod for coupling the rod to a bone anchoring element, and a bone anchoring device with such a receiving part. The receiving part includes a receiving part body with a rod receiving portion and a head receiving portion for receiving the head of the bone anchoring element and a locking ring for locking the head in the head receiving portion. The head can be clamped by compressing a plurality of flexible wall sections with the locking ring, where the clamping force is generated in a wall section at a circumferentially distinct pressure area.

2. Description of Related Art

U.S. Pat. No. 5,733,285 describes a polyaxial colletted locking mechanism for use with an orthopaedic apparatus including a screw having a curvate head and a coupling element. The coupling element has a tapered and colletted portion having an interior chamber in which the curvate head is initially polyaxially disposed. A locking collar is disposed around the tapered and colletted portion such that translation thereof in the direction of the expanding taper causes the interior volume to contract onto the curvate head and lock it therein.

WO 2007/038350 A2 discloses an apparatus for connecting a bone anchor to a support rod, the apparatus including a connector body and a cap. The connector body has a socket for insertion, angulation and removal of a bone anchor. A sleeve is provided which is configured to fit over the connector body for locking the bone anchor in the socket.

SUMMARY

It is an object of the invention to provide an improved receiving part for receiving a rod for coupling the rod to a bone anchoring element, and a bone anchoring device with such a receiving part, which allows for safe handling during surgery and safe fixation of the bone anchoring element and the rod.

A receiving part according to embodiments of the invention is improved with respect to clamping and locking of a head of a bone anchoring element. Exerting pressure by the locking ring at positions where a head receiving portion has slits may not contribute to an effective clamping of the head. Therefore, the bone anchoring device according to embodiments of the present invention is designed such that the pressure force exerted by the locking ring onto the head receiving portion is concentrated at distinct pressure areas separated from each other in a circumferential direction. Hence, the clamping force can be precisely applied, which improves the safety of the fixation.

Furthermore, the receiving part according to embodiments of the invention can have a pre-locking function, where the locking ring is latched with respect to the receiving part body in a position in which the head is inserted but is not yet locked, so that the head is prevented from removal from the receiving part.

With the bone anchoring device according to embodiments of the invention, a modular system can be provided that allows for combining various anchoring elements with any suitable receiving part on demand, depending on the specific clinical requirements. This reduces the costs associated with polyaxial screws, reduces inventory, and gives a surgeon a substantial or wider choice of implants.

DETAILED DESCRIPTION

A first embodiment of a bone anchoring device is shown inFIGS. 1 to 5c. The bone anchoring device includes a bone anchoring element1in the form of a bone screw having a threaded shaft2and a head3with a spherically-shaped outer surface portion. The head3has a recess4for engagement with a driver or tool. The bone anchoring device further includes a. receiving part body5for receiving a rod100to connect the rod100to the bone anchoring element1. For locking the head3in the receiving part body5, a locking ring6is provided that engages the receiving part body5. Furthermore, a fixation device in the form of; for example, an inner screw7is provided for fixing the rod100in the receiving part body5.

FIG. 3cis a cross-sectional view where the section is being taken in a plane containing a rod axis, andFIG. 3dis a cross-sectional view where the section is taken perpendicular to the rod axis. As shown in particular inFIGS. 1 to 3e, the receiving part body5includes a rod receiving portion9which is substantially cylindrical and has a first end9aand a second end9bopposite the first end9a, and a central axis or cylinder axis C. A coaxial first bore10is provided at the second end9b. A diameter of the first bore10is smaller than a diameter of the head3of the bone anchoring element1. A coaxial second bore11extends from the first end9ato a distance from the second end9b. A diameter of the second bore11is greater than that of the first bore10. A substantially U-shaped recess12extends in the rod receiving portion9from the first end9ain the direction of the second end9b, a diameter of the recess12being slightly larger than a diameter of the rod100to allow for placing of the rod100in the recess12and for guiding the rod100therein. By means of the recess12two free legs12a,12bare formed, on which an internal thread13is provided. The internal thread13can be a metric thread, flat thread, a negative angle thread, a saw-tooth thread, or any other thread form. If the fixation device is in the form of the inner screw7, a thread form such as a flat thread or a negative angle thread can be used which prevents or reduces splaying of the legs12a,12bwhen the inner screw7is tightened. A depth of the recess12is such that the rod100and the inner screw7can be inserted between the legs12a,12b. It shall be noted that the diameter of the coaxial bore11can vary along the central axis C.

As can be seen in particular inFIGS. 3aand3ecut-outs15a,15bare provided in the rod receiving portion9on either end of the channel formed by the recess12.

At the side of the second end9b, the receiving part body5further includes a head receiving portion17providing an accommodation space for the head3of the bone anchoring element1. The head receiving portion17is substantially cylindrically-shaped, with an outer diameter which may be smaller than a greatest outer diameter of the rod receiving portion9, such that the head receiving portion17may be recessed with respect to the rod receiving portion9, as can be best seen, for example, inFIGS. 3band3c. An internal hollow section18in the head receiving portion17forms a seat for the head3of the bone anchoring element1. The hollow section18is open via the opening19to a free end17bof the head receiving portion17. In the embodiment shown, the hollow section18is spherically shaped to accommodate the spherical head3. However, the hollow section18generally may be adapted to any other shape of the head3, or can be shaped otherwise so as to allow locking of the head3in the head receiving portion17.

A plurality of slits20are provided in the head receiving portion17. The slits20extend from the free end17bto a distance from the second end9bof the rod receiving portion9. Generally, the slits20extend over a region which includes a largest inner diameter of the hollow section18. The slits20render the head receiving portion17flexible, so that the head receiving portion17can be compressed to clamp and finally lock the head3in the internal hollow section18by friction. The head receiving portion17is configured to allow the insertion of the head3by expanding the head receiving portion17, and to clamp and finally lock the head3by compressing the head receiving portion17. Some of the slits20extend further, for example, into the rod receiving portion9, forming slits20aas shown, for example, inFIGS. 3aand3b. By means of this, the insertion of the head3can be further facilitated.

By way of the slits20,20a, flexible wall sections17aof the head receiving portion17are formed. On each of the flexible wall sections17a, a pressure area is formed in which pressure generated by cooperation of the locking ring6with the head receiving portion17is applied to the head3. The pressure area is a distinct area seen in a circumferential direction within the flexible wall sections17a. In the first embodiment, the pressure area is formed by a teardrop-shaped projection21at the outer surface of each flexible wall section17a. The teardrop-shaped projection21is arranged at a center of a corresponding flexible wall section17ain an axial direction and in a circumferential direction. The teardrop-shaped projection21is oriented such that its height and width increases towards the free end17b. By means of this, an outer diameter of the head receiving portion increases towards the free end17bin regions of the teardrop-shaped projections, as can be seen best inFIG. 3d. Hence, an outwardly tapered pressure area is provided at each of the flexible wall sections17a.

The locking ring6will now be described with reference toFIGS. 4ato4d.FIG. 4ashows a perspective view of the locking ring6. The locking ring6is substantially cylindrical and has an upper end6aand a lower end6b. In a mounted state, the upper end6ais oriented closer to the first end9aof the rod receiving portion9, and the lower end6bis oriented towards the free end17bof the head receiving portion17. Adjacent the lower end6b, the locking ring6is substantially hollow with a bore having a cylindrically-shaped inner cylindrical surface61, a diameter of which is slightly smaller than the outer diameter of the head receiving portion17in the regions of the outermost areas of the teardrop-shaped projections21. By means of this, the head receiving portion17can be compressed to clamp and to finally lock the head3when the locking ring6is around the head receiving portion17in a locking position. The lower end6bof the locking ring6may have a rounded edge as shown, for example, inFIG. 4d, which shows a cross-sectional view of the locking ring6along line B-B ofFIG. 4c. Adjacent the cylindrical surface61, the locking ring6has a circular rim62including the upper end6a. Between the circular rim62and the hollow cylindrical surface61an annular abutment surface63is formed. A height of the circular rim62is such that when the locking ring6is in its lowermost position with respect to the receiving part5, the rim62bridges the gap between the abutment surface63and the second end9b, as shown, in particular inFIG. 5b. This may serve, for example, for preventing tissue growing into the gap between the abutment surface63of the locking ring6and the second end9bof the receiving part9. The locking ring6further has two projections64, as shown in particular inFIG. 4aandFIG. 4dwhich project from the abutment surface63towards the upper end6a. The projections64are offset from each other by 180° and may have a concave upper surface64afor facilitating receiving the rod100.

All parts of the bone anchoring device may be made of a bio-compatible material, for example, of titanium or stainless steel, or of a bio-compatible alloy, such as Nitinol, or of a bio-compatible plastic material such as, for example, polyetheretherketone (PEEK). The parts can all be made of the same or of different materials.

The assembly and use of the bone anchoring device will now be explained with reference toFIGS. 5ato5c. The locking ring6is mounted from the free end17bof the head receiving portion17by compressing the flexible wall sections17a. The locking ring6is moved into a first position, which is an insertion position, shown inFIG. 5a. In this position, the annular abutment surface63of the locking ring6abuts against the lower end9bof the rod receiving portion9. Between the inner cylindrical surface61and the outer surface of the head receiving portion17, there is a gap which allows the expansion of the flexible wall sections17ato some extent. In this condition, the head3can be inserted from the free end17bby expanding the hollow internal section18. Once the head3is inserted into the hollow internal section18, the locking ring6is prevented from falling out, since the outer diameter of the head receiving portion17in the area of the highest point of the teardrop-shaped projections21is larger than the inner diameter of the cylindrical surface61. The projections64of the locking ring are aligned with the U-shaped recess12. As can be seen inFIG. 5a, in this first position, the projections64project above a bottom of the U-shaped recess12.

In this condition, between the receiving part body5, the locking ring6and the anchoring element1, which in some embodiments may be preassembled, the anchoring element1can be inserted into a bone part or a vertebra. The recess4of the head can be accessed with a driver or tool through the first bore10. In the condition shown inFIG. 5a, the receiving part5is still pivotable relative to the head3of the bone anchoring element1. Usually, at least two bone anchoring devices are used and are connected to a stabilization rod100. After insertion of each of the bone anchoring devices, the receiving part bodies5are rotated and/or pivoted to be adjusted to receive the rod100. Once a correct or desired position of the bone anchoring devices with respect to the rod is achieved, the inner screw7is screwed between the legs12a,12bof each bone anchoring device until it presses onto the rod100. The rod100is thereby shifted towards the bottom of the U-shaped recess12, thereby engaging the upper surface64aof the projections64and shifting the locking ring6down (e.g., towards the free end17b).

When the locking ring6is shifted towards the free end17bof the head receiving portion17, the cylindrical inner surface61engages the outwardly tapering surfaces of the teardrop-shaped projections21, thereby creating an increasing pressure onto the flexible wall sections17a.When the locking ring6has been fully moved downwards, the pressure exerted by the locking ring6onto the flexible wall sections17ais such that the head3is finally locked in the hollow internal section18.

As can be seen inFIG. 5c, which shows a cross-sectional view of the bone anchoring device with inserted rod and tightened inner screw7in a cross-section containing the rod axis, the locking ring6does not engage the flexible wall section17aother than at the distinct pressure areas corresponding to the teardrop-shaped projections21. In other areas, there may be a gap between parallel surfaces of the inner cylindrical surface61and the flexible wall sections17a. Hence, pressure is applied only at the distinct pressure areas, which are evenly distributed in a circumferential direction. Pressure is not exerted at positions which do not contribute to or which may otherwise contribute less to clamping the head3, such as at or around the slits20. Therefore, efficiency of applying the clamping force is increased or improved.

A second embodiment of the bone anchoring device will now be described with reference toFIGS. 6 to 10c. Parts and portions which are identical or similar to the parts and portions of the first embodiment are described by the same reference numerals, and the descriptions thereof will not be repeated. The bone anchoring device of the second embodiment differs from the bone anchoring device of the first embodiment in the designs of the pressure area and of the locking ring.

As can be seen in particular inFIGS. 6 and 8ato8d, a receiving part body5∝ has at its outer surface of a rod receiving portion9an engagement portion for a locking ring6′, in the form of a circumferential groove22and a cylindrical portion23with a slightly reduced diameter compared to a substantially cylindrical portion on an opposite side of the groove22. The groove22acts as a stop for the locking ring6′ in a second position, which is a pre-locking position described below.

The head receiving portion17includes pressure areas which are realized by one or more ball bearings21′ provided in each flexible wall section17a, respectively. The head receiving portion17has a first cylindrical section17cadjacent to the second end9bof the rod receiving portion9with a first diameter, and a second cylindrical portion17dwith a second diameter greater than the first diameter and which may include the ball bearings21′. A third portion17ewith a diameter that may be essentially the same as that of the second portion17dis provided at the free end17b. Between the second portion17dand the third portion17e, a groove24may be provided that serves for engagement with a portion of the locking ring6′. It shall be noted that the grooves22and24may each have an inclined lower edge which is inclined towards the free end17b, to facilitate disengagement of the locking ring6′ when moving the locking ring6′ downwards. The second portion17dis arranged substantially at a position around a largest diameter of the hollow internal section18, which corresponds to a largest outer diameter of the head3when the head3is inserted into the hollow interior section18. Slits20′ preferably extend from the free end17bonly as far as an upper end of the second portion17d, as shown, for example, inFIGS. 8band8c.

Each of the flexible wall sections17aincludes a distinct pressure area in the form of a ball bearing21′ at its center in a circumferential direction. Each ball bearing assembly21′ may include a ball26and a recess25having a substantially circular cross section which is sized and shaped to accommodate the ball26therein. The ball26can rotate in the recess25, where a portion of the ball26projects out of the second cylindrical portion17d. After the locking ring6′ is mounted, the balls26cannot fall out of the recesses25. As best seen inFIGS. 6 and 10ato10cthe receiving part body5′ includes in a circumferential direction equidistantly spaced balls26that cooperate with the locking ring6′.

The locking ring6′ will now be described with respect toFIGS. 9ato9d. The locking ring6′ is substantially cylindrical and has an upper end6aand a lower end6b. Near the lower end6b, an inwardly projecting circular edge610is provided that is configured to cooperate with the third portion17eadjacent the free end17bof the head receiving portion17. The inwardly projecting edge610provides a cylindrical inner surface. Furthermore, the inwardly projecting edge610is configured to engage the groove24of the head receiving portion17.

Adjacent the inwardly projecting edge610, the locking ring6′ has a hollow cylindrical portion611, a height of which is such that the hollow area can receive the portions of the balls26protruding out of the recesses25, as can be seen in particular inFIG. 10a. Hence, an inner diameter of the portion611is larger than the inner diameter of the inwardly projecting edge610. Adjacent the hollow cylindrical portion611on a side opposite the projecting edge610, there is a hollow cylindrical portion612, an inner diameter of which may be smaller than the inner diameter of the hollow cylindrical portion611and larger than the inner diameter of the inwardly projecting edge610. The inner diameter of the hollow cylindrical portion612is such that, as can be seen inFIG. 10bwhen the locking ring6′ is around the head receiving portion17such that the hollow cylindrical portion612is positioned around the balls26, the inner surface of the hollow cylindrical portion612can slide along the balls26of the ball bearings21′.

An upper portion of the locking ring6′ adjacent to the upper end6aincludes upwardly extending wall portions613, which are separated from each other by slits614. The upwardly extending wall portions613are arranged at an outer circumference of an inner circumferential shoulder615of the locking ring6′, and render the upper portion of the locking ring6′ flexible. A number and size of the slits614and a thickness of the wall portions613are configured such that a desired flexibility is obtained. At their free ends the wall portions613include engagement sections613a, which are shaped so as to engage the groove22provided on the outer surface of the rod receiving portion9.

The locking ring6′ is sized in such a way with respect to the head receiving portion17that the head receiving portion17can expand within the locking ring6′ to allow the introduction of the head3when the locking ring6′ is in the first position as shown inFIG. 10a.

Two projections64which may each have a concave upper surface portion64aare provided 180° offset from each other, like for the locking ring6of the first embodiment. A curvature of the upper surface64amay correspond to a curvature of the rod100. The projections64have a height such that they project above a bottom of the U-shaped recess12and extend into the cut-outs15a,15bwhen the locking ring6′ is in positions in which the head3is not yet locked, as shown inFIGS. 10aand10b. The height of the upwardly extending wall portions613are such that, when the locking ring6′ is in a first position, as shown inFIG. 10ain which the inner circumferential shoulder615of the locking ring6′ abuts against the second end9bof the rod receiving portion9, portions of the upwardly extending wall portions613including the engagement portions613aextend above the circumferential groove22. On its outside, the locking ring6′ may be tapered towards its lower end6bto reduce an outer dimension of the bone anchoring device.

The function and use of the bone anchoring device according to the second embodiment will now be explained with reference toFIGS. 10ato10c. As shown inFIG. 10a, a first position of the locking ring6′, which is the insertion position in which the locking ring6′ is latched with respect to the receiving part body5′, is defined in such a way that the inwardly projecting edge610engages or is adjacent to the groove24at the outer surface of the head receiving portion17. In this condition, the head3can be inserted through the opening19into the hollow internal section18of the head receiving portion17. Since the inner diameter of the inwardly projecting edge610is larger than the outer diameter of the groove24, an expansion of the head receiving portion17when the head3is introduced is possible. In the first position, the locking ring6′ may additionally be held by a clamping force between the rod receiving portion9and the flexible wall portions613of the locking ring6′, which may be bent slightly outwards, as can be seen in particular inFIG. 10a.

When the locking ring6′ is in the first position, the head receiving portion17is not compressed. In this position, the locking ring is prevented from moving further upwards towards the first end9aof the rod receiving portion9, since the locking ring6′ abuts with the shoulder615against the second end9bof the rod receiving portion9, and/or with the inwardly projecting edge610against an upper wall of the groove24. This holds the locking ring6′ in place. The inclined lower edge of the groove24prevents an inadvertent downward movement of the locking ring6′, but allows for downward movement upon exertion of an additional downwardly directed force on the locking ring6′. In the first position, the head3can freely pivot in the hollow internal section18. The head receiving portion17is not compressed and the balls26may not be touched by the locking ring6′, since the balls26project into the hollow cylindrical portion611.

A second position in which the locking ring6′ is latched with respect to the receiving part body5′ is shown inFIG. 10b. The second position is a pre-locking position in which the head3is prevented from removal from the hollow internal section18and optionally may be held in a preliminary angular position by a slight friction force exerted by the flexible wall sections17a. In the second position, the locking ring6′ has been shifted towards the free end17bof the head receiving portion17until the engagement portions613aof the upwardly extending wall portions613resiliently snap into the groove22provided at the rod receiving portion9. The free upper edge of the engagement portions613amay abut against an upper wall of the groove22, as shown inFIG. 10b, thereby preventing upward movement of the locking ring6′ out of the pre-locking position back towards the insertion position. On the other hand, the inclined lower edge of the groove22prevents an inadvertent downward movement of the locking ring6′ further towards the free end17b, but allows such downward movement upon exertion of an additional axial force on the locking ring6′.

To reach the second position from the first position, the locking ring6′ is shifted downwards. While the locking ring is shifted downwards, the hollow cylindrical portion612slides along the rotating balls26, thereby exerting pressure onto the flexible wall section17aat the positions of the balls26. Hence, jamming between the two cylindrical surfaces17dof the head receiving portion17and the hollow cylindrical portion612of the locking ring6′ is prevented or reduced, while simultaneously pressure is exerted onto the balls26which project out from the surface17d. The balls26define distinct pressure areas to exert pressure onto the head3to clamp and eventually lock the head3.

In the pre-locking position, the bone anchoring element1cannot be removed from the receiving part5′. Hence, an accidental or inadvertent removal of the head3is not possible in this position. However, an angulation of the bone anchoring element1is still possible by overcoming a friction force between the head3and the receiving part5′, for example, manually.

A third position, which is a locking position, is shown inFIG. 10c. The third position is defined as a position in which the head3is finally locked within the receiving portion17. The inwardly projecting edge610compresses the third portion17eadjacent the free end17bof the head receiving portion17in this position. The combination of the pressure exerted via the pressure areas around the balls26and the pressure exerted via the inwardly projecting edge610firmly locks the head3within the receiving part body5′. The third position is reached by a further downward movement of the locking ring6′ upon the action of the rod100pressing the upper surface64aof the projections64. The rod100is pressed downward when tightening the fixation screw7.

A third embodiment of the bone anchoring device will now be described with reference toFIGS. 11 to 15c. Parts and portions which are the same or similar to those of the first and/or second embodiment are designated with same reference numerals and the descriptions thereof will not be repeated. The third embodiment of the bone anchoring device differs from the first and second embodiments in the designs of the pressure areas. In the third embodiment, pressure areas21″ are defined as circumferentially distinct projections which are located at an inner side of the locking ring6″ (see, e.g.,FIG. 15c).

As can be seen in particular inFIGS. 13ato13d, the receiving part body5″ has a head receiving portion17similar to the head receiving portion17of the second embodiment. A second cylindrical portion17d′ is provided at an axial position of the head receiving portion17which corresponds substantially to a largest outer diameter of the head3when the head3is inserted in the head receiving portion17. The slits20extend similarly as in the first and second embodiments from the free end17bto a distance from the second end9bof rod receiving portion9, and over the second cylindrical portion17d′.

The locking ring6″ differs from the locking ring6′ according to the second embodiment in that the locking ring6″ has pressure areas in the form of circumferentially distinct projections620which are configured to be at an axial position corresponding to the position of the second cylindrical portion17d′ when the locking ring is in a third position as shown inFIG. 15c.The projections620project toward an inside of the locking ring6″. A number of projections620corresponds to a number of flexible wall segments17a, and a distance between projections620corresponds to a distance, for example, between centers of the flexible wall sections17a, so that when the locking ring6″ is mounted, each projection620can engage one flexible wall section17a.The projections620have a height in an axial direction which is approximately the same as a height of the second cylindrical portion17d′. The width of the projections620in a circumferential direction is smaller than a width of the flexible wall sections17a, thereby creating distinct pressure areas for each flexible wall section17a. The projections620can be generated by providing cut-outs of a projecting ring with inclined cutting surfaces620a, for example, as shown inFIG. 14aandFIG. 14d. However, other shapes may be possible to generate distinct pressure areas with respect to the flexible wall sections17a.

Assembly and use of the bone anchoring device according to the third embodiment is similar to that of the second embodiment. As shown inFIG. 15ain a first position, an inwardly projecting edge610of the locking ring6″ engages a groove24at the head receiving portion17. The head3can be introduced from the free end17bwhen the locking ring6″ is in this position.

In a second position, which is a pre-locking position, as shown inFIG. 15bengagement portions613aengage a groove22at a rod receiving portion9of the receiving part5″. In this position the projections620of the locking ring6″ may begin to press against the second cylindrical portion17d′ of the head receiving portion17.

In the third position, as shown inFIG. 15c, the projections620are positioned substantially at the centers of the respective second cylindrical portions17d′and compress the flexible wall sections17ato clamp the head3. In addition, the inwardly projecting edge610engages a third cylindrical portion17e. The combination of pressure from the projections620and the inwardly projecting edge610causes the head3to be firmly locked within the hollow internal section18.

Individual features from one of the embodiments described can be combined or exchanged with other features from other embodiments. For example, it is possible to have projections that form a distinct pressure areas on the flexible wall portions and on the locking ring alternately in a circumferential direction. The locking ring6and the receiving part5of the first embodiment can also be configured to have a pre-locking function, and alternatively, the locking rings and the receiving parts of the second and third embodiments can be designed without a pre-locking function.

Further modifications of the embodiments shown are also possible. For example, the head of the bone anchoring element can have various other shapes, such as, for example, a cylindrical shape, whereby a mono-axial bone screw is provided, allowing rotation of the bone anchoring element with respect to the receiving part body around a single axis.

The projections forming the pressure areas can have any shape. Preferably, they may have a rounded shape to facilitate movement of the locking ring.

The head receiving portion can have an inclined open end or can be otherwise asymmetric to allow a greater angulation of the head in one direction.

For the bone anchoring element, various different kinds of bone anchoring elements can be used and combined with the receiving part. These anchoring elements may include, for example, screws with different lengths, screws with different diameters, cannulated screws, screws with different thread forms, nails, hooks, anchoring elements where the head and the shaft are separate parts, etc.

The rod receiving portion may also have various different shapes. For example, the recess can be configured to allow the rod to be introduced from a side instead of being introduced from the top. The recess can also be closed instead of U-shaped. Various kinds of locking devices, including locking device with two or more parts, outer nuts, outer caps, bayonet locking devices, or various others types of locking devices may be utilized.

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