A device for setting a spline

The present invention relates to a device for setting damaged vertebras in a spline according to the preamble of claim 1.

BACKGROUND OF THE INVENTION

It is known to stabilize a spline having damaged vertebras by implanting a stabilizing system in the vertebras on either side of a damaged vertebra. For this, the spline is first set and then stabilized. It is further generally known to turn so-called pedicel screws into the pedicels of vertebras and to connect the pedicels screws to each other through a more or less rigid connecting system. For example, this can be obtained by using a so-called distracting or compressing bar which is inserted in a receiving slot provided in the heads of the pedicel screws. The bar is usually provided with a thread, a grooving or some other surface roughness such that it can be fixed in the head of the pedicel screws by means of an appropriate locking means. EP 0 348 272 discloses a pedicel screw having a receiving slot which is provided with a thread to receive a locking screw. The locking screw fixes the distracting or compressing bar in the slot. U.S. patent 4,763,644 discloses a pedicel screw which head is provided with an outer thread onto which a cap shaped nut can be screwed. By means of a pin portion inserted into the receiving slot, the cap nut urges the bar against the bottom of the receiving slot. French patent 26 24 270 discloses a cap nut including a locking screw to fix the distracting or compressing bar in the receiving slot. Cap nuts have the advantage of holding together the legs of the slotted pedicel screw head. As mentioned above, there is the danger that the legs spread apart, thus loosening the locking screw when a solid screw is screwed into the receiving slot. Furthermore, fixing of the distracting bar is endangered. In the process of the operation, the pedicel screws are first secured. The position of the pedicel screws or, respectively, the heads thereof is optional to some extent, in particular, when the vertebras are more or less displaced. According to the prior art it is required to prebend the distracting or compressing bar to be received in the receiving slots of the pedicel screws. Since the distracting and compressing bars must be stiff by nature, the process described requires the surgeon to apply substantial efforts, and after all the process is complicate.

From WO91/16020 it has become known to provide a pedicel screw with a spherical head. The spherical head is accommodated by a swivel head having a spherical receiving portion and a receiving slot for receiving a cross bar or rod. The rod is pressed into the slot by a nut threaded on an outer thread of the swivel head in that a pressing portion is urged against the rod. The rod in turn is pressed against the end of the receiving slot without interfering the ball head of the pedicel screw.

The object of the present invention is to provide a device for setting a spine having damaged vertebras, in which the need for bending a distracting or compressing bar is lessened when the bar has to be received and fixed in receiving slots of anchoring elements secured to the vertebras when displaced vertebras are existent.

SUMMARY OF THE INVENTION

According to the invention, this object is solved by the features of claim 1.

As known, the receiving slot is formed in a separate swivel head including an inner end which is in the shape of a ball receiving portion. The ball receiving portion has an aperture through which the shaft portion of the anchoring element extends, wherein the diameter of the shaft portion adjacent the aperture is smaller than that of the aperture. A ball head is provided at the end of the shaft portion, the ball head having a diameter larger than that of the aperture. According to the invention, the ball head extends up to the terminal region of the receiving slot.

The anchoring element on the one hand and the swivel head on the other can take any angular position with respect to each other. Accordingly, the bar must be bent less and this is true for the sagittal as well as the lateral setting direction.

When the anchoring element is represented by a pedicel screw, the latter must be not completely screwed into the vertebra. Rather, the inserted bar can be fixed by means of the cap nut after an initial screwing operation. Subsequently the pedicel screw can be rotated about its shaft to either pull a dislocated vertebra rearwards or sidewards when the instrumentation is performed from anterior. After repositioning a luxated vertebra (trauma, degenerative change or olisthesis) the bar can be compressed or distracted to obtain a further lordosizing or kyphosizing, eliminating the need of bending the bar. Thereafter, a cap nut is tightened, whereby an axial fixing portion thereof secures the bar against the ball head to fix all parts in the axial, rotational and angular direction as well. As mentioned before, a pedicel screw may be used as the anchoring element. Alternatively, a hook can be provided which acts beyond an extension of the vertebra, for example. The ball head of the anchoring element can be formed separately with respect to the anchoring element, however, an integral structure of ball head and anchoring element is preferred.

In a preferred embodiment the axial fixing portion as well is integrally formed with the cap nut.

In particular, the swivel head is shaped to have minimized outer dimensions, if possible, while maintaining sufficient stability and eliminating distinct projections or edges.

According to a further aspect of the invention, the end region of the swivel head facing the anchoring element is tapered, the said tapered section being joined by a radial shoulder and a cylindrical portion extending away from the shoulder, wherein the outer diameter of the nut approximately corresponds to the outer diameter of the shoulder.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanying drawings which show: <dl id="dl0001"><dt>Fig. 1</dt><dd>a side view of a hook provided for the device according to the invention;</dd><dt>Fig. 2</dt><dd>a section of a swivel head of the device according to the invention;</dd><dt>Fig. 3</dt><dd>the assembly of the hook of Fig. 1 and the swivel head of Fig. 2 in combination with a distracting bar;</dd><dt>Fig. 4</dt><dd>a pedicel screw for a device similar to the embodiment of Fig. 3.</dd></dl>

DESCRIPTION OF THE PREFERRED EMBODIMENT

A hook 10, a so-called laminar hook for example, comprises a hook portion 12, a ball head 14 and a shaft portion 16 located between the hook portion 12 and the ball head 14. A swivel head 20 as shown in Fig. 2 comprises a dead end bore 22 which is ball-shaped at the bottom 24 Diametrically opposed slots 38 extend from the free end which slots end at a distance from the bottom of the dead end bore as indicated at 26. The bottom of the bore 22 is provided with a central aperture 28 tapering outwardly.

The portions 30 which are left between the slots 38 of the swivel head 20 are provided with an outer thread 32. Extending from the aperture 28, the swiwel head 20 is formed to include a conical section 34 ending in a radial shoulder 36. The thread 32 is terminated at the radial shoulder 36.

As Fig. 3 shows, the ball head 14 of the hook 10 is received in the ball receiving portion 24 of the swiwel head 20. Since the shaft portion 16 has a smaller diameter than that of the aperture 28, the hook can perform a pivoting motion up to a maximum of 30°. This is indicated in Fig. 3 by dashed lines.

When the hook portion 12 cannot be inserted through the aperture 28 from the free end of the bore 22, the hook portion 12 and the ball head 14 including the shaft portion 16 can be shaped in two parts which are screwed to each other, for example.

Fig. 3 further shows a distracting or compressing bar 40, extending through the slots 38. A cap nut 42 is screwed on the outer thread 32 of the swiwel head 20. An axial fixing portion 44 integrally formed on the nut 42 includes a rounded end 46 suited to engage the bar 40. Since the diameter of the ball 14 is larger than the distance between the ball receiving portion and the end 26 of the slot 38, the bar 40 is urged towards the ball head 14 when the nut is screwed on the thread 32 and when the fixing portion 44 is urged to press against the bar 40 from outside. Before the fixing step is performed, the ball head 14 and the bar 40 can be positioned in different angular positions. It is thus not necessary to bend the bar 40 when different angular positions due to the position of each vertebra have to be negotiated while inserting the bar 40.

Fig. 4 shows a pedicel screw 50 comprising a threaded shaft 52 followed by a threadless shaft portion 54 ending in an integral ball head 56. The dimension of the shaft 52 is selected to be placed through the aperture 28 of the swiwel head 20, while the ball head 56 is received in the ball receiving portion 24. Alike the hook 10, the pedicel screw 50 can take any angular position within limits.

The shaft portion 54 is preferably provided with tool engaging surfaces to rotate the pedicel screw 50, while the ball head 56 sits in the ball receiving portion 24. Accordingly, it is possible to relocate a luxated vertebra.

It is further noted that the nut 42 can be provided with tool engaging surfaces, for example a pair of diametrically opposite flats.