Disk prosthesis for cervical vertebrae

A disk prosthesis for cervical vertebrae. The prosthesis includes a spherical cap formed on a first insert, while a spherical cup is formed on a second inset, two inserts made of ceramic material, one of the inserts being mounted on a first plate while the other insert is mounted on a second plate in such a manner that the center of rotation of the joint is substantially centered relative to the edges of the plates, a spherical cup possessing a contact surface area that is not less than that of the spherical cap and being connected via an annular molding to the base of the insert, and a plate provided with the insert having the spherical cap includes an annular setback to leave clearance for the annular molding.

The invention relates to a disk prosthesis for cervical vertebrae, intended to replace the fiber and cartilage disk that interconnects cervical vertebrae in the spinal column.

BACKGROUND OF THE INVENTION

It is known that an intervertebral disk can be subject to damage, such as compression, deformation, displacement or wear, and more generally degeneration associated with the mechanical stresses that are applied thereto and that lead to anatomical and functional destruction of the disk and of the vertebral segment. This damage to the disk alters its mechanical behavior and leads to a reduction in the height of the intersomatic gap, which leads to all of the functions of the joint being disturbed. This gives rise to instability which induces, in particular, an osteoarthritic reaction which is painful and gives rise to osterophytic processes.

Proposals have therefore been made to replace the defective disk with an artificial disk, and various types of embodiment have been envisaged. Thus, a disk prosthesis is known, e.g. from patent FR 2 718 635, for cervical vertebrae where the prosthesis comprises first and second plates for fixing to adjacent cervical vertebrae. That prosthesis also has a ball joint interposed between the two plates that are mounted in a superposed position. The joint comprises a spherical cap made of a synthetic material, such as polyethylene, mounted on one of the plates and designed to co-operate with a spherical cup provided in the other plate and made of a metal, such as titanium, for example.

Although such a cervical prosthesis makes it possible to restore an appropriate height to the intersomatic gap, the ball joint of that prosthesis is subject to high levels of friction, making it sensitive to wear, and as a consequence of the wear that prosthesis does not give full satisfaction firstly because of its relative instability, particularly in bending movements.

A disk prosthesis for lumbar vertebrae is also known, in particular from patent U.S. Pat. No. 5,562,738, which comprises first and second plates for fixing to adjacent vertebrae, and made of a metal, such as titanium. Between the plates, a ball joint is interposed that comprises a first insert mounted on one of the plates and constituted by a spherical cap co-operating with a spherical cup of a second insert mounted on the other plate. The inserts are made of a biocompatible ceramic material having improved tribological characteristics, in particular concerning resistance to wear.

Nevertheless, such a disk prosthesis for lumbar vertebrae is unsuitable for replacing the disk of cervical vertebrae insofar as such a prosthesis does not enable cervical vertebrae to recover their natural mobility. It also turns out that the ball joint presents a shape that is quite difficult to get right and that is sensitive to breaking or cracking phenomena that reduce the lifetime of the prosthesis.

OBJECT AND SUMMARY OF THE INVENTION

The object of the present invention is thus to remedy the drawbacks of the state of the art by proposing a disk prosthesis for cervical vertebrae that is designed to present relative long lifetime by being practically insensitive to phenomena of wear and breaking, and while being suitable for providing physiological mobility between the two cervical vertebrae to which it is fitted.

To achieve this object, the prosthesis of the invention is of the type comprising:first and second plates designed to be fixed to adjacent cervical vertebrae; anda ball joint interposed between the two plates mounted in a superposed position, the joint being constituted by a spherical cap co-operating with a spherical cup.

According to the invention:the spherical cap is provided on a first insert, while the spherical cup is provided on a second insert;each insert is made of a ceramic material and possesses a base of circular right cross-section;one of the inserts is mounted on the first plate while the other insert is mounted on the second plate in such a manner that the center of rotation of the joint lies substantially centered relative to the edges of the plates so as to be centered in the sagittal plane and in the frontal plane of the vertebrae;the spherical cup possesses a contact area that is not less than the contact area of the spherical cap and is connected via an annular molding to the base of the insert; andthe plate provided with the insert having the spherical cap has an annular setback to leave clearance for the annular molding of the spherical cup during movements of the plates.

MORE DETAILED DESCRIPTION

As can be seen more clearly inFIGS. 1to4, the subject matter of the invention is a disk prosthesis1for implanting as a replacement for a disk between two adjacent cervical vertebrae. The cervical prosthesis1of the invention comprises a first plate2, referred to as a top plate in the example shown, and a second plate3referred to as a bottom plate. The plates2and3are designed to be fixed to adjacent cervical vertebrae and each has a respective outer face21,31of dimensions that are substantially similar and adapted to fit approximately to the outlines of the surfaces of the associated joint. Each plate2and3also has a respective inner face22and32that extends to face the other interface. The general shape of each of the plates2and3is that of a rectangular parallelepiped processing a respective posterior edge2a,3a extending at a distance from a respective anterior edge2b,3b. The anterior edge2b,3b of each plate2,3is connected to the corresponding posterior edge2a,3a via two side edges2c,3c that are opposite each other. The edges2a,2b,2c, and3a,3b,3c are preferably straight in profile and they are interconnected by rounded corners. The posterior edge2a,3a of each plate preferably presents a profile that is convex in a transverse plane T, while the anterior edge2B,3b presents a profile that is concave.

The cervical prosthesis1of the invention also has a ball joint4interposed between the two plates2and3which are mounted in the superposed configuration. The ball joint4is constituted by a first insert5presenting a spherical cap6and by a second insert7presenting a spherical cup8that co-operates with the spherical cap6. Each insert5,7is designated to be mounted in a housing11, preferably a blind housing (which refers to a bore with an end which is closed, as shown in FIG.1), formed from the inside face22,32of each plate2and3. Each insert5,7is generally circularly symmetrical in shape and possesses a respective base12,13of circular right cross-section, with one of its ends being shaped to present the spherical cap6or the spherical cup8. The right cross-section of the base12,13of each insert5,7is constant, or preferably tapering from the spherical cap6or the spherical cup8. In this preferred embodiment, each reception housing11is complementary in shape to the insert5,7so as to enable the inserts5,7to be assembled conically in the plates2,3. Naturally, any other type of assembly could be envisaged for the insert, e.g. by means of adhesive or by means of crimping. In an embodiment, provision might be made to fit a damping element against the end of the blind housing11so as to be interposed between the corresponding insert and the plate in order to camp the axial forces acting on the prosthesis.

In the preferred embodiment, the insert5provided with the spherical cap6is mounted on the top plate2, while the insert7provided with the spherical cup8is mounted on the bottom plate3. This disposition enables the ball joint4to absorb the forces to which it is subjected better.

The spherical cap6is defined by a contact surface having a radius of curvature that is equal to the radius of curvature of the contact surface that defines the spherical cup8so as to form a ball joint. The spherical cap6is connected to the base12of the first insert5via a connecting curve14while the spherical cup8is connected to the base13of the second insert7via a connecting curve forming an annular molding15. In accordance with an advantageous characteristic of the invention, the spherical cup8possesses a contact surface area that is not less than that of the spherical cap6so as to obtain good mechanical behavior between the spherical cap6and the spherical cup8. In other words, the base13of the second insert7provided with the spherical cup8has a circular right cross-section that is greater in area than the right cross-section of the base12of the first insert5that is provided with the spherical cap6, because of the presence of the annular molding15.

In accordance with another advantageous characteristic of the invention, the plate2provided with the first insert5presenting the spherical cap6has an annular setback18surrounding the housing11so as to provide clearance for the annular molding15of the spherical cup5during movements of the plates2,3. Naturally, the first insert5is mounted on the plate2in such a manner as to project from the inner face22so as to enable it to co-operate with the spherical cup8. Similarly, the second insert7is mounted so as to project relative to the inner face32of the plate so as to obtain sufficient angular clearance between the plates without them coming into contact with each other.

In accordance with an advantageous characteristic of the invention, the inserts5,7are made of a ceramic material. The inserts5and7are preferably made of ceramic materials of different hardnesses. For example, the second insert7provided with the spherical cup8can be made of zirconium oxide (ZrO2) while the first insert5provided with the spherical cap6is made of aluminum oxide (Al2O3).

In accordance with another advantageous characteristic of the invention, the inserts5,7are mounted on the plates2and3in such a manner that the center of rotation of the joint4is substantially centered relative firstly to the side edges2c and3c of the plates so as to be centered in the sagittal or antero-posterior plane S, and secondly relative to the anterior and posterior edges2a,3a;2b,3b of the plates so as to be centered in the frontal plane F of the vertebrae. Such a centered disposition for the center of rotation of the joint4enables the prosthesis1to reproduce the natural movements of the intervertebral disk of the cervical vertebrae.

By appropriate dimensioning of the joint4and of the position of its center of rotation, as defined above, the forces applied to the surfaces that are in contact are limited. It should be observed that the surfaces in contact corresponding to the spherical cap6and to the spherical cup8allow angular displacement equal to or less than 10°, such that rubbing always occurs between the inserts5,7, i.e. between surfaces that are made of ceramic material. This reduces the wear on the inserts. Amplitude of movement is limited by causing the plates2,3to come into contact with each other. The engagement of the spherical cap6in the spherical cup8makes it possible to obtain stability for the joint4while providing it with suitable three-dimensional mobility that is practically identical to that provided by a natural disk.

The plates2,3can advantageously be made of titanium and the contact surfaces with the vertical plates of the vertebrae, i.e. the outer faces21,31are preferably covered in hydroxy apatite or in surface effect titanium, for example, so as to improve anchoring between the prosthesis and the adjacent bone.

It should be observed that plates2,3can be provided that are of different shapes adapted to possible different morphotypes of vertical body. Thus, provision can be made for one or both of the plates2,3to present different heights so as to enable them to match the height of the intersomatic gap that is to be reestablished. Furthermore, as shown in the example ofFIG. 1, provision can be made for the outer faces21,31of the plates2and3to present a plane profile optionally fitted with anchor elements20for anchoring in the vertebrae. In the example shown inFIGS. 1 and 4, each outer face21,31possesses anchoring notches20. In the example shown inFIGS. 5,6, the outer face21of the first plate2has notches as anchoring elements20that are parallel to one another and to the front plane F. It could be envisaged that the top plate2has a profile that is convex in the sagittal plane, as shown in FIG.5. In accordance with another characteristic of the invention, at least one of the outer faces21,31of the plates2,3is provided with two anchoring studs (not shown) that are long enough to pass through each of the plates of the vertebrae so as to prevent the cage from sliding.

As can be seen more clearly inFIGS. 1 and 4, each plate2,3is provided on its posterior edges2a,3a with two positioning holes22for the endpieces of a tool that serves to hold both plates simultaneously. It should be observed that in this position, as shown inFIG. 1the plates2,3form an angle in the sagittal plane S to facilitate insertion of the cage into the intersomatic gap. It can be seen that the height h of the prosthesis1at its front face as defined by the anterior edges2b,3b is smaller than its height H of its rear portion as defined by the posterior edges2a,3a. In accordance with a preferred characteristic the positioning holes22in any one plate converge on each other so as to facilitate withdrawal of the positioning tool.

The invention is not limited to the examples described and shown, since various modifications can be made thereto without going beyond its ambit.