HOOK FOR THE OSTEOSYNTHESIS OF THE SPINAL COLUMN

The present invention relates to an implant for an osteosynthesis device, in particular for the spine, the implant comprising a bone-anchoring device surmounted by a fixing head constituted by two upwardly extending members or lateral branches forming an open U-shaped channel and designed to receive a link rod in order to hold it in place by clamping, via a tapped nut suitable for being screwed onto corresponding threaded portions made on the walls of the lateral branches of the fixing head.

It is generally known to use hooks or pedicular screws as bone-anchoring devices. Numerous devices relate to pedicular screws which have the advantage of providing effective fixing for the implant since a pedicular screw is anchored by being screwed into a pedicle.

Nevertheless, it is not always possible to use pedicular screws, particularly for a vertebra at the end of the spine, or because of regulations in certain countries which allow pedicular screws to be used only under very restrictive circumstances, use thereof being considered difficult and therefore dangerous for the patient.

Thus, in some cases, it is necessary to make use of hooks.

The hooks that are normally used are shaped so as to hook onto a projecting portion of the bones of the spine, such as the foot of a pedicle, or else to fix on the vertebral arch, either on the infralamary side or on the supralamary side.

Nevertheless, it must be observed that using hooks gives rise to numerous drawbacks since a single hook cannot on its own provide effective mechanical fixing as can be done by a screw. Hooks can act effectively only as an abutment when used in association with another hook so as to obtain a clamping effect in which case it becomes almost as effective as a screw.

A hook comprises a hook body having an inner bearing face and an outer fixing face provided with means for fixing to a rod for interconnecting anchoring points. It is the interconnecting rod that serves to hold the hooks on two successive vertebrae.

Such an assembly has the effect of securing two successive vertebrae together by moving them towards each other, but it does not provide any means for acting on either of them separately. In addition, it is necessary to find correct bearing points for both hooks simultaneously, and lateral stabilization is insufficient.

Anchoring devices using hooks have already been proposed that include devices for improving stability. Thus, international patent application WO-A-91/01691 describes a device comprising two facing hooks situated on either side of the vertebra, interconnected by a clamping screw for moving them towards each other on request, each hook having a fixing head in which a rod for linking together and preventing relative movement of the assembly is engaged by clamping nuts on each of said hooks.

That embodiment suffers from the major drawback of requiring clamping action to be taken on two hooks and of relying on the link rod to obtain effective clamping, which amounts to saying that prior thereto, the device remains unstable.

The same clamp system is to be found in documents FR 2,695,550 or FR 2,718,994.

Also known, e.g. from patent FR 2,695,550, is a self-contained implant having its own effective fixing means so that it does not require the link rod whose purpose is to link together two or more successive vertebrae. That patent describes a pedicular clamp having a hook secured to a fixing head constituted by two lateral branches forming an open U-shaped channel and designed to receive a link rod in order to prevent it from moving relative thereto by means of a tapped nut suitable for screwing onto corresponding threaded portions made on the lateral branches of the fixing head. That implant also has a counterhook directed towards the said hook and connected thereto by means of an elongate member, finger or threaded rod. The hook and the counterhook are designed to clamp onto opposite sides of a vetrebra independently of the link rod.

That implant suffers from a major drawback relating to the possibility of relative movement between the hook and the counterhook. It also turns out that the operation of adjusting the spacing between the hook and the counterhook with that kind of implant is not an operation than is simple to carry out successfully.

The invention thus seeks to remedy the above-specified drawbacks by proposing an implant for an osteosynthesis device implementing a hook and a counterhook which are independent of the link rod and which offer the advantages firstly of making it easy to adjust the spacing between the hook and the counterhook, and also to maintain them in a relative position, and secondly of preventing the hook and the counterhook from moving relative to each other in a manner that is safe and effective.

The invention thus provides a solution serving firstly to provide primary stabilization of bone anchoring by means of hooks, by improving the bone anchoring as from the preliminary step of placing the implant on the spine, during the step of reducing deformation of the spine and throughout the subsequent duration of the spine being held in place, and secondly of providing the anchoring with stability that is reliable and permanent so as to avoid loss of reduction in the short or medium term.

To achieve this object, the implant comprises:

a hook secured to a fixing head constituted by two lateral branches in an open U-shaped channel designed to receive a link rod for the purpose of preventing movement relative thereto by means of a tapped nut suitable for being screwed onto corresponding threaded portions made on the lateral branches of the fixing head; and

a counterhook directed towards the hook and freely connected to said hook to enable the hook and the counterhook to clamp onto at least one vertebra, independently of the link rod.

According to the invention, the implant includes:

an elongate member connected at one end to the counterhook and arranged to slide in a bore formed in the top potion of the hook and while lying substantially along a center plane of the hook;

a position control device for holding the hook and the counterhook in position relative to each other; and

an interconnection arrangement between the hook and the counterhook preventing relative motion of the hook and counterhook.

Various other characteristics appear from the following description given with reference to the accompanying drawings which show embodiments and implementations of the invention as non-limiting examples.

The implant shown inFIGS. 1 and 2comprises a reference hook1and a counterhook9. The hook1is secured to a fixing head2which is constituted in conventional manner by two upwardly extending members or lateral branches3,4forming an open U-shaped channel and designed to receive between them a link rod5, for the purpose of holding it fixed relative thereto by means of a tapped nut6suitable for being screwed onto corresponding threaded portions7made in the example shown on the outside walls forming portions of cylinders of the lateral branches3,4of the fixing head2.

In the embodiment shown inFIGS. 1 and 2, the nut6has a shoe8in its diametral zone, which shoe8is mounted so as to rotate freely and is designed to cooperate with the link rod5by clamping thereagainst.

The implant has free connection means between the hook1and the counterhook9enabling them to be moved freely towards each other or away from each other. The counterhook9faces towards the hook1and co-operates freely therewith by means of an elongate member or a finger10. The finger10extends one of the ends9aof the counterhook9and it slides in a central bore, a cylindrical hole, or a cylindrical drilling11formed in the top portion1a of the hook1and situated substantially in the same plane as the hook1.

Thus, sliding of the finger10of the counterhook9in the bore11of the hook1enables the hook1and the counterhook9to clamp a vertebra12to a greater or lesser extent by acting on opposite sides of the vertebra12, and to do so independently of the link rod5. The assembly comprising the hook1and the counterhook9is held in place on the vertebra12by means13belonging thereto. The implant of the invention thus includes means13adapted to maintain the hook1in position relative to the counterhook9.

As shown inFIGS. 1 and 2, the axis Y-Y′ of the bore11in the hook1which receives the finger10of the counterhook9, extends perpendicularly to the common vertical axis X-X′ of the fixing head2and of the hook1, and lies in a plane that is identical to the plane of the hook1, thus putting the hook and the counterhook9face to face.

In an alternative embodiment shown inFIG. 6, the longitudinal axis A—A of the central bore11is aligned perpendicular to the vertical axis B—B of the fixing head2. However, the central longitudinal axis A—A of the central; bore11is aligned obliquely to the central longitudinal axis Z—Z of the hook1such that the counterhook9and the hook1are in a mutually offset relation to one another.

In another embodiment of the invention shown inFIG. 8, the counterhook9is pivotably connected to the finger10, thereby enabling it to hinge relative to the axis Y-Y′.

In another embodiment of the invention, the fixing head2is rotatably mounted to the hook1.

The means13for holding the hook1and the counterhook9in place are formed by an elastically deformable element constituted by a pin15, in the example shown inFIGS. 1 and 2.

This pin has a branch16that is fixed in a housing17whose longitudinal axis extends substantially perpendicularly to the plane of the hook1in the top portion1athereof. The other branch18of the pin15is elastically deformable and is received freely in a cylindrical hole19that is substantially parallel to the first housing17and that is formed through the top portion1aof the hook1. The holes11and19intersect and they are substantially perpendicular. In this way, during clamping by moving the counterhook9towards the hook1, the resilent branch18cooperates like a pawl with notches20formed along the finger10, e.g. transversely, or else it can release the hooks by being lowered. It must be understood that the pin15allows the hook1and the counterhook9to move towards each other but prevents them from moving apart unless action is exerted specifically on the elastically deformable element which thus acts as a non-return system.

In the embodiment shown inFIG. 3, the means (13A) for holding the hook1and the counterhook9in place on a given vertebra12are constituted by an elastically deformable element formed by a pin21having one branch22fixed in a housing23whose axis extends substantially perpendicularly to the plane of the hook1through the top portion1athereof; its other branch24is elastically deformable and extends freely and parallel to the first branch outside the hook1so as to co-operate with the finger10of the counterhook9which is substantially perpendicular thereto. In this way, during clamping in which the counterhook9is moved towards the hook1, the resilient branch cooperates like a pawl with the notches20formed transversely on the finger10, or it can release the finger on being moved down.

In the embodiment shown inFIGS. 4 and 5, the means (13B) for holding the hook1and the counterhook9in place on a given vertebra12are constituted by an elastically deformable blade25fixed parallel to the plane of the hook1, parallel to the finger10of the counterhook9, and having its free end25acurved towards the finger10. In this way, during clamping by moving the counterhook9towards the hook1, the free end25cooperates like a pawl with the notches20formed transversely along the finger10, or else it can allow the finger to be released when traction is applied to the blade25.

According to a characteristic common to all of the embodiments described above and visible in particular inFIG. 2, the implant has an interconnection arrangement locking the hook1and the counterhook9in a final position so as to ensure that they cannot move relative to each other. In a preferred embodiment of the invention, clamping the link rod5simultaneously causes the link rod to lock the finger10. The bore11is made so as to intersect the bottom of the U-shape defined by the lateral branches3and4of the fixing head2for receiving the link rod5so that when said rod5is clamped by the nut6it simultaneously provides final clamping of said rod5and of the finger10relative to the hook1. The hook1and the counterhook9are thus definitively prevented from moving relative to each other after previously being put into place and held in position with the help of the elastically deformable element.

In an alternative embodiment of the counterhook of the above-mentioned type, the counterhook can include a traditional pedicular screw so as to reinforce its fixing relative to a given vertebra.

It should be observed that the hook1and the counterhook9can be designed to clamp around a single vertebra or a plurality of vertebrae.

The implant of the invention thus makes it easy to adjust the spacing between the hook1and the counterhook9merely by moving the hook and the counterhook relative to each other so as to clamp onto one or more vertebrae. The hook1and the counterhook9are held in this stable position throughout the duration of the preliminary step of putting the implant into place and reducing the deformation of the spine. Definitive and reliable prevention of relative movement between the hook and the counterhook is performed during the later step of definitively fixing the implant.