Bone fixation device, particularly of the sacrum, for spinal osteosynthesis

The present invention relates generally to a fastening device, in particular the sacrum, usable in osteosynthesis of the spine.

It is common practice for osteosynthesis of the spine using fasteners taking roots in the sacrum. It is thus possible to proceed, between floors attached to reduction of movements that can be contractions, distractions or vertebrae recalls having slipped to the front of the spine. The latter condition is known as the "spondylolisthesis".

In the case of spondylolisthesis reduction, distraction is often practiced. Partial reduction is obtained by retightening the fibers of the ligament and disc unit of the patient. The reduction is complemented by the use of equipment according to the surgical technique associated with it. This operation is performed, except in exceptional cases, bilaterally. Throughout the following description, we will describe one side of the treatment.

Most plates or mounting blocks to the sacrum known in the prior art have certain disadvantages. One of these drawbacks is that the orientation of the bone anchor screw is imposed, giving the product an obvious lack of flexibility of use, taking account of the sacrum configuration changes and its neighborhood of one patient to another.

Another disadvantage of the plates or mounting blocks known sacrum lies in the risk of unscrewing of the anchoring screw under the effect in particular of the relative micro-movements occurring between the hardware and the sacrum. This then results in the failure of the sacral fixation.

Moreover, osteosynthesis of the spine are achieved through hardware for attaching to often very rigid way, different functional units of the spine. This results in a sudden change in the distribution of effort, and thus states of stresses and deformations resulting, at the suspension drives and underlying fixing. This change leads to more or less long-term disc degeneration. This condition is known as the "hinge syndrome".

Also known from EP-A-0625337 a device according to the preamble of claim 1 which allows to vary the inclination of the anchoring screw with respect to an elongate connecting element, as well as the spacing between these screws.

This known dispostif is however disadvantageous in that the elongate connecting means consist of several components, namely an elongate member shaped bowl and a plurality of mounting blocks distributed along the elongate member and each receiving a head screws and, in the lower part, the corresponding region of the elongate member.

As a result, the handling and installation of this device by the surgeon are extremely tedious, especially with the need to slide on the elongated member a series of mounting blocks, and keep them there in position.

The present invention therefore aims to overcome this disadvantage.

It proposes for this purpose a bone fixation device as defined in claim 1.

Preferred, but not limiting, of the device according to the invention are defined in the dependent claims.

Other aspects, objects and advantages of the invention appear better on reading the following detailed description of preferred embodiments of the latter, given by way of example and with reference to the accompanying drawings, wherein:<ul><li>Figure 1 is a median longitudinal section view of a fastener according to the invention,</li><li>Figure 2 is a front view of the Figure 1 element,</li><li>Figure 3 is a side view of the element as well as three bone anchorage screws and associated plugs,</li><li>Figure 4 is a front view of the element provided with screws and plugs,</li><li>Figure 5 is an elevational side view of a plug adapted to cooperate with the element of Figures 1 and 2 and with a bone anchor screw ,,</li><li>Figure 6 is a side elevational view of the bone anchor screw,</li><li>Figure 7 is a cross-sectional view of the attachment element, a bone anchor screw and the associated cap, after mounting,</li><li>FIG 8 is a partially elevational and partially in section of a screw auxiliary view bone anchor intended to cooperate with another part of the fastener of Figures 1 and 2,</li><li>Figures 9 to 12 are views similar cross section to Figure 7, four alternative embodiments of the invention,</li><li>Figure 13 is a cross-sectional view of various components of a fifth alternative embodiment of the invention, and</li><li>Figure 14 is a cross-sectional view of the fifth variant in the assembled state.</li></ul>

Note the preliminary view that, from one figure to another, elements or same or similar parts are designated as far as possible by the same reference symbols and are not described each time.

Referring now to Figures 1 and 2, there is shown a fastener to the sacrum, generally designated by reference 1, which consists of three parts, namely an eyelet upper 4, a sacral plate 5 and rod 6, by example cylindrical, connecting the lug to the plate. The element 1 as a whole has, in profile, a curvature in lordosis illustrated in Figure 1.

In the plate 5 is formed a series of holes 7 aligned in the longitudinal direction of the element 1, for bone anchoring screw 2.

Preferably, the surgeon may propose several models of elements 1, including different lengths of rod 6, and in the plate 5, different numbers of holes for bone anchor screw.

As shown in Figure 3, the plate 5 can receive, in a version with three holes 7, two screw 2 bone anchors and, in a version of five holes (not shown), three screws 2 bone anchor.

The holes 7 communicate with each other through a through groove 9 of oblong section, as can be seen in Figure 2. This groove makes it possible to easily pass a screw 2 from one position to another during 'reduction operation. Each hole has, in the region of its bottom, a spherical cup 7a adapted to cooperate with a complementary lower part of the head 21 of the screw 2, said head 21 being in its spherical together and having a clamping cavity 211 by example type six hex head screws.

This conformation of each hole 7 and the associated screw head 21 makes an angular movement of the screw in a solid angle through which the half angle at the top is for example of the order of 30 °.

Furthermore, each screw being stabilized in its housing during installation, two remote screws are suitable for maintaining bone distraction without requiring the use of a distractor instrument.

As particularly shown in Figures 1 and 7, each hole 7 has internally, away from the cup 7a, a thread 8 allowing it to receive a plug 3 shown in detail in Figure 5. This stopper has on its outer face a clamping footprint 302, preferably identical to that of the screw head 21 so as to be able to use the same clamping tool, and a housing in spherical cup 301 also cooperates with the spherical crown of the head of screw 21. the stopper comprises also at its periphery a thread 304 for co-operating with the thread 8 of the hole 7. the cap 3 is intended, after screwing and tightening, to ensure the locking of the screw 2 in the angular position determined by the surgeon.

The plate 5, the screw 2 and the cap 3 after assembly are illustrated in particular in Figure 7. The screw cap 3 provides a non-return function to prevent accidental unscrewing of the screw anchor 2 associated, particularly in the conditions set forth in preamble of the present application.

In addition, thanks to the device of the present invention, spondylolisthesis can be reduced by implanting in the pedicle of the vertebra in question is an intermediate screw fixed to the intermediate connecting rod 6, a screw disposed in the eyelet 4.

In the first case, the intermediate shaft 6 can thus be used to perform a return vertebra. To this end, and as shown in Figure 8, one can associate a screw top-loading, generally designated by the reference 10 known per se, which has been implanted beforehand in the vertebra in question.

One can also attach the vertebra to seek to the intermediate shaft 6 by means of another type of device, not shown but known per se, wherein the screw is offset from the axis of the stem 6 but connected thereto by a holding svstem.

In the second case, it is the upper element 4 of the grommet 1, which is used to receive a bone anchoring screw 2 into the vertebra to be treated, the screw 2 being preferably of the same type as that described above. For this purpose, a hole 7 identical to that described with reference to the plate 5 is formed in said grommet, to enable again a angulation of the screw 2 and a non-return function allowing through a plug 3 avoid that it loosens.

In order not to cause the hinge described in the preamble syndrome can be avoided to ensure strict locking screw 2 inserted into the upper ring 4. then retains in this case a relative mobility of the screw 2 over the eyelet 4.

In this way, the position of the vertebra in which is anchored the screws 2 inserted into the eyelet 4 is fixed, but the functionality of the intervertebral disc underlying is partly maintained. This creates a stiffness gradient in the attachment that has the effect of mitigating the sudden change of stress state at the overlying disc.

This relative mobility of the screw 2 in the eyelet 4 can be achieved in different ways.

A first solution, not illustrated, is to take the concept illustrated in Figure 7 using a screw cap 3 and by providing in the hole 7 of the eyelet a shoulder preventing the plug 3 to achieve a screw locking position 2 in angular position.

Alternatively, also not illustrated, such a shoulder may be formed on the screw cap 3.

In addition, or alternatively, can be reduced to render negligible friction between the plug 3 and the head 21 of the screw 2 by applying a thin layer with a low coefficient of friction coating, eg ceramic or diamond like carbon, on one or more of the different surfaces in contact, namely the screw 21 head, the cup 7a of the hole 7 of the eyelet 4, and the cup 301 of the screw cap 3.

For example, such a coating may be applied only on the head 21 of the screw 2.

Now with reference to Figures 9 to 14 will be described a number of embodiments for mitigating the rigidity of the assembly of the screw 2 and the eyelet 4 in which it is introduced, in order again to avoid the hinge syndrome.

And Figure 9 describes a solution in which contact between the rigid parts to adopt relative movement is prevented by the use of a first O-ring 401, lower, and a second O-ring 402, above. Both rings are preferably made of a damping material such as silicone, and are illustrated in Figure 9 in the deformed state after clamping.

So when the rings 401, 402 are pressurized and deformed when tightening, they provide rigidity attenuated connection between the screw 2 and the eyelet 4.

Advantageously provides the surgeon with rings of different shades of hardness, while maintaining the same dimensions, which enables it to modulate the stiffness of the assembly without having to play on the tightening torque of the screw cap 3, that which will prove very difficult to implement reliably.

In addition, to minimize friction between the rings 401, 402 and the head 21 of the screw, it may be advantageous to carry on the screw surface treatment by ion bombardment with nitrogen, which improves the tribological characteristics of the screw head.

Still referring to Figure 9, it is observed that the plug 3 has again a spherical cup 303, intended only to allow angulation of the screw 2, but without support on the blocking head 21 of said screw.

Figure 10 depicts a variant in which the cap 31 differs from that of Figure 5 in that its cup, indicated at 311, is no longer complementary to the spherical head 21 of the screw 2, but is flared, and that a 411 ball of damping material is interposed between the cup 311 and said head 21 and pressurized and deformed during tightening.

here is ensured even attenuation of the assembly rigidity.

11 depicts a variant in which the screw cap 32 has a flat work surface, and wherein the direct contact between the plug 32 and the screw head 21 is removed by use of an O-ring 412 carried in a damping material, pressurized and deformed during tightening.

Figure 12 depicts a variant in which the plug, designated by the reference 33, provided on its working face a conical recess 331, and wherein contact between the plug 33 and the screw head 21 is suppressed by the injection, during the laying operation, a thixotropic fluid 413 providing a damping function.

In addition, in the three embodiments of Figures 10, 11 and 12 of the contact friction between the spherical head 21 and the cup 7a of the hole 7 formed in the eyelet 4 can also be reduced by the use of a coating as cited above. In this regard, the coating used in the variant of Figure 12 will be chosen so that the contact between the base of the spherical head 21 and the cup 7a seals the thixotropic fluid pressurized during screwing.

Figures 13 and 14 describe another embodiment wherein the eyelet is changed. Specifically, the grommet 4 has a generally cylindrical cup 421 which the distance portion of the bottom is threaded into 8 to receive the screw cap 3, identical to that of Figure 5.

Between the cup 421 and the screw head 21 is interposed a ring 422. It can be either a rigid ring, titanium alloy made, for example coated with a ceramic or titanium alloys treated by ion bombardment , or even a bulk ceramic, or even of an O-ring similar to the ring 401 used in the embodiment of Figure 9, carried again by a damping material, for example silicone.

In the case where the ring 422 is rigid, it may be advantageous to provide the radius of curvature of the inner spherical bearing slightly less than the radius of the screw head 21, so as to ensure for tensioning of said head screw during tightening. In this case, the version of Figures 13 and 14 may advantageously be used also when a total blocking of the screw is desired.

According to another variant, the ring 422 may be split, with again a constraint formatting the head 21 and a locking possibility.

Naturally, the parts such as the element 1, the screw 2, the caps 3 are made of any biocompatible material of suitable mechanical properties, such as a titanium alloy.

Below we give to complete a few additional comments.

First the form and the item 1 dimensions are chosen so as to enable to resist torsional stresses generated by the support plane between the base part 5 shaped plate and the adjacent bone.

This form also limits the translation movements longitudinqale on intermediate screw.

Next, the attenuation means of the rigidté described above provide such attenuation both in the sagittal plane and in the transverse plane.

Finally, the invention allows to propose an osteosynthesis system with a high modularity: so, for example, different versions of the ring 422 can be provided to the practitioner to manage the front contact zone (vertebra side), while the a damping means described above can be offered with different degrees of stiffness to handle the damping rear portion (cap side). In practice, it may well cover the range between the flexible connections and rigid connections.

In this regard, it will be observed that the stiffness attenuation means described above can be used both at the screw of the eyelet 4 and at the level of the plate screw 5.

Of course, the present invention is not limited to the embodiments described and shown, but the skilled person can apply any variation or modification within the spirit.