POLYAXIAL PEDICLE SCREW WITH A HEAD IN THE SHAPE OF A BALL SEGMENT

Abstract

Polyaxial pedicle screw with a head in the shape of a ball segment and with a threaded screw shank, wherein a hollow ball segment with a recess for receiving a rod-shaped connector is mounted in the head, in order to permit pivoting movements and rotation movements of the connector, wherein moreover, in the direction of the longitudinal axis of the threaded screw shank, the head in the shape of a ball segment has a cutout with an inner thread in which the rod-shaped connector can be moved in and out and, by means of the inner thread in the cutout, a grub screw can be introduced for blocking the hollow ball segment. According to the invention, a plastic buffer body is fitted in the hollow ball segment and is connected to a ram, wherein the ram has a head which engages in an associated annular groove in the plastic buffer body. Moreover, the plastic buffer body has a circumferential guide ring and a spherical guide stop at one end, wherein an opening for the ram is formed lying opposite the guide stop, and the edge of the opening bears on a shell, which in turn bears on the inner side of the hollow ball segment, in such a way that the plastic buffer body with its guide stop is fixed with pretensioning in the head shaped as a ball segment.

Claims

1. A polyaxial pedicle screw with a head (1) in the shape of a ball segment and with a threaded screw shank (10), wherein a hollow ball segment (3) with a recess for receiving a rod-shaped connector (7) is mounted in the head, in order to permit pivoting movements and rotation movements of the connector (7), wherein moreover, in the direction of the longitudinal axis of the threaded screw shank (10), the head (1) in the shape of a ball segment has a cutout (8) with an inner thread in which the rod-shaped connector (7) can be moved in and out and, by means of the inner thread in the cutout (8), a grub screw or similar means (9) can be introduced for blocking the hollow ball segment (3), characterized in that an elastic plastic buffer body (5) is fitted in the hollow ball segment (3) and is connected to a ram (6), wherein the ram (6) has a head (61) which engages in an associated annular groove in the plastic buffer body or which is at least in part surrounded by the plastic buffer body, the plastic buffer body (5) moreover has a circumferential guide ring (51) and a spherical guide stop (52) at one end, wherein an opening for the ram (6) is formed lying opposite the guide stop (52), and the edge (53) of the opening bears on a shell (4), which in turn bears on the inner side of the hollow ball segment (3), in such a way that the plastic buffer body (5) with its guide stop (52) is fixed under pretension in the head (1) shaped as a ball segment.

2. The polyaxial pedicle screw according to claim 1, characterized in that the ram (6) has a section for fastening the rod-shaped connector (7).

3. The polyaxial pedicle screw according to claim 1, characterized in that grooves or cutouts (54) are formed in the guide ring (51) and/or guide slot (52) for fluid passage.

4. The polyaxial pedicle screw according to claim 1, characterized in that via the design of the plastic buffer body (5) regarding geometry and/or material, radial, tensile and compressive forces and lifting and pivoting movements of the ram (6) may be predefined that can be achieved in this respect.

5. The polyaxial pedicle screw according to claim 1, characterized in that the plastic buffer body (5) is made of polycarbonate urethane (PCU).

6. The polyaxial pedicle screw according to claim 1, characterized in that the hollow ball segment (3) is secured within the head (1) shaped as a ball segment by means of a clamping ring (2).

7. The polyaxial pedicle screw according to claim 1, characterized in that the hollow ball segment (3) is made of a titanium material or a titanium alloy.

8. The polyaxial pedicle screw according to claim 1, characterized in that the shell (4) is made of a titanium alloy or a ceramic material.

9. The polyaxial pedicle screw according to claim 1, characterized in that the threaded screw shank (10) has a self-tapping thread.

10. The polyaxial pedicle screw according to claim 1, characterized in that the guide stop (52) has an access to the head (61) of the ram (6).

11. The polyaxial pedicle screw according to claim 1, characterized in that the head (1) in the shape as a ball segment is formed to be divided, in particular divided in two.

12. The polyaxial pedicle screw according to claim 11, characterized in that the head (1) in the shape of a ball segment is composed of two half-shells (100; 101).

13. The polyaxial pedicle screw according to claim 12, characterized in that the half-shells (100; 101) are joined by locking means and counterlocking means (103; 104) in a form-fit manner.

Description

[0025] The invention will be explained below in more detail by means of an exemplary embodiment and with the aid of Figures.

[0026] Shown are in:

[0027] FIGS. 1a and 1b a perspective view and a sectional view of the polyaxial pedicle screw according to the invention with the connector, ram and plastic buffer body being indicated;

[0028] FIGS. 2a and 2b a perspective view and a sectional view similar to that of FIGS. 1a and 1b, but with a grub screw being introduced in the cutout with the inner thread in the head shaped as a ball segment;

[0029] FIGS. 3a and 3b various perspective views with a polyaxial pivoting of 20 between the longitudinal axis of the threaded screw shank and the connector;

[0030] FIG. 4 a representation of the arrangement of the connector pivoted into the cutout for the purpose of screwing in the implant in an unimpeded manner;

[0031] FIG. 5 a perspective representation of the plastic buffer body with a ram;

[0032] FIG. 6 a detailed representation of the pedicle screw in a sectional view;

[0033] FIG. 7 a representation of the inventive pedicle screw with the head in the shape of a ball segment, which head is divided into two half-shells or half-shell segments, wherein the segments or half-shells are configured to be connectable by locking means and counterlocking means, in a separate position, and

[0034] FIG. 8 a representation similar to that of FIG. 7 but with the position of the half-shells being coherent.

[0035] The polyaxial pedicle screw according to the invention as shown in the Figures comprises a head 1 shaped as a ball segment, which head merges into a threaded screw shank 10 in its base portion.

[0036] Into the head 1 shaped as a ball segment, a hollow ball segment 3 is inserted which comprises a recess for receiving a rod-shaped connector 7. The hollow ball segment 3 is pivotable within the head 1 shaped as a ball segment. Corresponding pivoting and rotating movements of the connector 7 may be performed in this respect.

[0037] In the direction of the longitudinal axis of the threaded screw shank 10, the head 1 shaped as a ball segment has a cutout 8 with an inner thread.

[0038] During the screwing in of the pedicle screw, the connector together with the hollow ball segment is brought into such a position that the connector comes to rest within the cutout 8.

[0039] After screwing the pedicle screw into the bone material, in particular vertebral body material, the connector may be positioned in the location as then required, wherein a grub screw 9 permits the hollow ball segment 3 to be blocked via an inner thread in the cutout 8, wherein the connector 7 as well is positionally fixed. The rod remains pivotable relative to the hollow ball even after fixing of the hollow ball segment 3.

[0040] In the hollow ball segment 3, a plastic buffer body 5 is situated which is in communication with a ram 6, wherein the ram 6 has a head 61 which engages in an associated annular groove in the plastic buffer body or is overmolded by the plastic buffer body in the head portion 61.

[0041] The plastic buffer body 5 has a circumferential guide ring 51 and a spherical guide stop 52 at one end. An opening for the ram 6 is formed in the plastic buffer body 5 lying substantially opposite the guide stop 52.

[0042] The edge 53 of the opening of the plastic buffer body 5 is supported on a shell 4, which in turn bears on the inner side of the hollow ball segment 3, and namely in such a way that the plastic buffer body 5 with its guide stop 52 is fixed under pretension in the head shaped as a ball segment. Due to the elastic properties of the plastic buffer body 5, the movability of the ram and thus of the connector 7 is maintained so that physiological movements may be performed by an optional change of length and simultaneous tilting of the connector in relation to the fixed screw. Due to that, the arrangement can follow the existing movements in the spinal column such as flexion, extension, lateral tilt and rotation.

[0043] Due to the construction according to the invention, the plastic buffer body 5 is only subjected to compressive load so that possible wear is reduced.

[0044] Grooves or cutouts 54 are formed in the guide ring 51 and/or guide slot 52 for fluid passage. Entered fluids may thus overflow with respect to the cutout 54. Moreover, the action of the plastic buffer body 5 may be influenced in a targeted manner.

[0045] Via the design configuration of the plastic buffer body 5, there is the option of individually setting the tensile and compressive side and thus to vary both maximum strokes and forces and the force progression. This allows changes of length and movement abilities of the connector to be defined in a targeted manner. By varying the configuration of the plastic buffer body 5, also with regard to the material selection, an initially linear and subsequently progressive force progression may be realized.

[0046] A targeted pretension when inserting the plastic buffer body in the hollow space of the ball anticipates a setting or creeping of the material so that the necessary long-term stability and a reproducible behavior are guaranteed.

[0047] The hollow ball segment 3 is secured in the head 1 shaped as a ball segment by means of a clamping ring 2.

[0048] In a preferred configuration, the plastic buffer body 5 has an access to the head 61 of the ram 6 in the area of the guide stop 52 in order to introduce a tool in a recess present therein.

[0049] The ball-shaped head of the pedicle screw fixes all of the components that are necessary for the solution according to the invention. The clamping ring 2 holds these components inside the head of the pedicle screw. The clamping ring 2 may be fixed e.g. by dowel pins, but may also be fastened in a material-fit manner by means of laser welding, for example.

[0050] The hollow ball segment 3 virtually represents an inner sleeve and forms the front half of the entire ball closing the open part of the head in the shape of a ball segment and clamps the remaining components inside the head.

[0051] During the operation in using the pedicle screw, the inner sleeve allows the angle between the rod and the pedicle screw to be adjusted in the range from about 20 in any arbitrary direction, whereby the desired polyaxial function is guaranteed. Incidentally, the fixing within the framework of this mobility is not subject to stress. Moreover, the inner sleeve forms a counter-bearing for the movement of the supporting shell 4 and the plastic buffer body 5. As already explained, the inner sleeve 3 is fixed by means of the grub screw 9 or a similar means at the end of the implantation. After fixing the hollow ball segment 3 by the mentioned grub screw 9, the plastic buffer body with the connected connector 7 and shell 4 remains pivotable within the hollow ball segment 3 and thus compensates for misalignments which are still existing.

[0052] The shell 4 closes the remaining opening and constitutes a counter-bearing for the plastic buffer body 5 which is brought to compression. When the entire arrangement is moved postoperatively, it will serve as a sliding partner for the inner sleeve, i.e. for the hollow ball segment 3.

[0053] The actual element of motion is formed by the plastic buffer body 5, and namely in case of tension and compression on the ram or the connector 7.

[0054] According to one exemplary embodiment of the invention, the created geometry in conjunction with the employed material polycarbonate urethane allows the system to be shortened by about 2.2 mm and lengthened by about 2.8 mm.

[0055] Due to the configuration of the ram 6 with head 61, the plastic buffer body 5 is only subjected to compressive load.

[0056] The connector 7 functions as a connecting element to the next pedicle screw of an arbitrary system. For the purpose of implantation, the rod is pivotable upward into the cutout 8 with all of the components connected thereto so as to enable the already mentioned screwing in of the pedicle screw.

[0057] FIGS. 7 and 8 show a further development of the invention with a pedicle screw which has a head 1 realized in the shape of a ball segment and composed of two half-shells or half-shell segments 100; 101.

[0058] The two half-shells 100; 101 have complementary locking means 103 and counterlocking means 104 so as to join them in a form-fit and/or force-fit manner. This form-fit and/or force-fit joining preferably is made using the principle of a known bayonet lock.