Abstract:
A cervical disk prosthesis includes a ball and socket joint interposed between opposing plates for attaching to adjacent vertebrae. The bearing plate has a dimension in the frontal plane larger than a corresponding dimension of the upper plate for providing a limit stop for the upper plate, and by its outside face, a bearing surface for a vertebra that is greater than that presented by the second plate. The disk prosthesis has a trapezoidal form in the horizontal plane with the upper and smaller plate having a parallelepiped shape for defining flexion and extension mobility. One of the plates carries an elastic ring about the perimeter for making a shock absorbing contact with the opposing plate.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application incorporates by reference and claims priority to French Application having Registration Number 04 00087 filed on Jan. 7, 2004 for “Discal Prosthesis For Vertebrae”.  
       FIELD OF INVENTION  
       [0002]     The present invention generally relates to spinal implants for use in intervertebral disk replacement, and more particularly to an articulating disk prostheses and insertion device for artificially replacing the fibro-cartilaginous disk that connects vertebrae of a spinal column.  
       BACKGROUND  
       [0003]     An intervertebral disk comprising a deformable element, known as nucleus pulposus, surrounded by a number of elastic fibrous layers, can undergo alterations such as compression, deformation, slippage or wear and, more generally, degeneration associated with mechanical stresses applied to it. This may result in anatomical and functional destruction of the disk and of the vertebral segment. This alteration of the disk changes the mechanical behavior of the disk and leads to a reduction in the height of the intersomatic space, which results in a disturbance of articular function as a whole. This produces instability that, in particular, creates an osteoarthritic reaction that is the source of pain and of osteophytic processes.  
         [0004]     It is well known to replace a defective disk with an artificial disk, designed so as to attempt to reproduce the kinematics of a natural movement. For example, U.S. Pat. No. 5,562,738 describes a disk prosthesis including first and second plates made of a metallic material such as titanium for attachment to adjacent vertebrae. A ball-and-socket joint between the plates comprises a first insert mounted on one of the plates and a spherical cap cooperating with the spherical cupola of a second insert mounted on the other plate. The inserts are made of a biocompatible ceramic material with improved tribological characteristics, specifically with respect to its wear resistance.  
         [0005]     However, such a disk prosthesis does not allow the vertebrae to regain their natural mobility. Indeed, such prosthesis has limited clearances in certain planes that depend on asymmetric forms of embodiments of a ball-and-socket joint. The ball-and-socket joint has a form that is relatively difficult to handle and is sensitive to breakage or cracking, thus reducing the life span of the prosthesis.  
         [0006]     In order to develop a prosthesis for reproducing the kinematics of the natural movement, patent FR 2 730 159 describes a prosthesis involving an intermediate core with two spherical faces oriented in the same direction but having different radii. Such an intermediate nucleus is designed to slide over a convex surface belonging to a lower plate. The core also has a convex upper surface on which the upper plate slides. The core thus has mobility in the horizontal plane, so that it is capable of moving to one side when the plates approach from the opposing side. Such a prosthesis, however, has the disadvantage of ejecting the core toward the outside of the prosthesis.  
         [0007]     In order to prevent ejection of the core from the prosthesis, French Patent FR 2 659 226 describes a prosthesis whose upper plate has a concave face that slides on a polyethylene core in the form of a spherical cap, fixed securely in a hollow of a lower plate. Lower and upper titanium plates are each equipped with bosses for anchoring in a respective vertebra to avoid risk of separation. Double baffle flanges limit the angular clearance of the lower and upper plates using complementary sections on the circumference of the upper plate and on the circumference of the lower plate. In practice, the double baffle flanges are intended to overlap at the moment of maximum angular shifting between the plates, which can lead to an undesirable nesting with friction, even blockage. Moreover, such prosthesis requires a relatively traumatizing implantation in the vertebral plates and does not make it possible to ensure the optimum transfer of the thrust of the upper vertebra on the lower vertebra. Further, the contact interface between a polyethylene core and a titanium plate degrades over time, thus undesirably modifying the mobility of the prosthesis.  
         [0008]     The present invention remedies such drawbacks by providing an intervertebral disk prosthesis for vertebrae that faithfully reproduces the natural movements of the disk and ensures an optimum transmission of the thrust of upper vertebra on lower vertebra while offering control of the relative angular clearance between the vertebrae.  
       SUMMARY  
       [0009]     A spinal disk prosthesis restores normal physiological function in the spine by preserving intervertebral motion, stability, lordosis, and spacing while protecting vascular, neural, and other spinal structures.  
         [0010]     One embodiment of the present invention provides a prosthesis that may include first and second plates for attaching to adjacent vertebrae, and a ball-and-socket joint interposed between the two plates mounted in stacked form one on top of the other, so that the inside faces of these plates are turned toward one another. The joint may comprise a spherical cap fitting into a spherical cupola. The first plate, herein referred to as a bearing plate, has a dimension in the frontal plane larger than the corresponding dimension of the second plate for providing a limit stop for the second plate, and by its outside face, a bearing surface for a vertebra that is greater than that presented by the second plate.  
         [0011]     The disk prosthesis may further have a substantially trapezoidal form in the horizontal plane, with its large base delimiting the leading edge of the plate while the small base delimits the training edge. The second plate may have a parallelepiped shape in the horizontal plane.  
         [0012]     The plates may be dimensioned in such a way that: E/d=ED 1  for A 1 /A 2 &gt;0.5, with E: distance between the two inner faces of the plates; d: width of the upper plate in the frontal plane; D 1 : length of the upper plate in the sagittal plane; A 1 : maximum angle of clearance between the plates in the frontal plane; and A 2 : maximum angle of clearance between the plates in the sagittal plane.  
         [0013]     The first plate may have an inner face with a flat profile in the frontal plane and an outer face with a convex profile in the frontal plane. One of the plates may include, on its inner face, an elastic ring that makes contact with the inner face of the other plate.  
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0014]     For a fuller understanding of the invention, reference is made to the following detailed description, taken in connection with the accompanying drawings illustrating various embodiments of the present invention, in which:  
         [0015]      FIG. 1  is a front view in the frontal plane of a first exemplified embodiment of prosthesis according to the invention in a median position;  
         [0016]      FIG. 2  is a cutaway view in the frontal plane of the prosthesis according to the invention, in an inclined position;  
         [0017]      FIG. 3  is a side view in the sagittal plane of the prosthesis according to the invention in a median position;  
         [0018]      FIG. 4  is a cutaway view in the sagittal plane of prosthesis according to the invention in an inclined position;  
         [0019]      FIG. 5  is a top perspective in the horizontal or coronal plane of the prosthesis according to the invention;  
         [0020]      FIGS. 6 and 7  are cutaway views in the frontal plane and in the sagittal plane, respectively, of a second exemplified embodiment of prosthesis according to the invention in an inclined position;  
         [0021]      FIGS. 8 and 9  are cutaway views in the frontal plane and in the sagittal plane, respectively, of a third exemplified embodiment of the prosthesis according to the invention in an inclined position;  
         [0022]      FIGS. 10 and 11  are front and side views of an alternate embodiment of the disk prosthesis of  FIG. 1 ;  
         [0023]      FIG. 12  is a cross sectional view taken through lines  12 - 12  of  FIG. 10 ;  
         [0024]      FIG. 12A  is a partial exploded view of a shock absorbing portion of  FIG. 12 ;  
         [0025]      FIGS. 13 and 14  are front and side views of an alternate embodiment of the disk prosthesis of  FIG. 10 ; and  
         [0026]      FIG. 15  is a cross sectional view taken through lines  15 - 15  of  FIG. 13 .  
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0027]     The present invention will now be described more fully with reference to the accompanying drawings in which alternate embodiments of the invention are shown and described. It is to be understood that the invention may be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein. Rather, these embodiments are provided so that this disclosure may be thorough and complete, and will convey the scope of the invention to those skilled in the art.  
         [0028]     With reference initially to  FIGS. 1-4 , one embodiment of the present invention may be described as a disk prosthesis  10  to be implanted in place of a intervertebral disk between two adjacent vertebrae  12 ,  14 . One embodiment of the disk prosthesis  10  herein described, by way of example, includes a first plate  16  illustrated as a lower plate in the illustrated example, and a second plate  18  illustrated as an upper plate. The plates  16 ,  18  are intended to be attached to the adjacent vertebrae  12 ,  14  and are herein described, by way of example, as each having an outer face,  20 ,  22 , respectively, and each having an inner face  24 ,  26 , respectively, extending opposite one another. Embodiments of the first and second plates  16 ,  18 , as herein described by way of example, may be formed from a biometallic material, and optionally have an asteo-conductive coating. By way of example, the plates  16 ,  18  may be made of titanium or a titanium alloy. In one embodiment, the first and second plates  16 ,  18  comprise titanium with an hydroxy-apatite coating.  
         [0029]     With continued reference to  FIG. 1 , and to  FIG. 5 , the first plate  16 , also known as a carrier plate, has a transversal dimension D greater than the corresponding dimension d of the second plate  18 . These dimensions d, D are measured in a transversal direction at the intersection between the horizontal plane T, illustrated with reference again to  FIG. 5 , and the frontal plane F, illustrated with reference again to  FIG. 1 , and are herein considered in terms of well known anatomical planes.  
         [0030]     With reference to  FIGS. 2 and 4 , the inner face  24  of the first plate  16  thus constitutes a limit stop for the second plate  18 , and with the outer face  20  of the first plate  16 , a bearing surface for the vertebra  14  that is larger than the surface of the vertebra  12  contacted by the outer surface  22  presented by the second plate  18 . As illustrated with reference again to  FIG. 5 , the first plate  16 , the bearing plate, has in the horizontal plane T, a substantially trapezoidal shape whose large base defines a leading edge  28 , while a small base defines a trailing edge  30 . The training edge  30  is connected to the leading edge  28  by opposing divergent edges  32 ,  34 . Connecting fillets may preferably connect the leading and trailing edges  28 ,  30  to the divergent edges  32 ,  34 .  
         [0031]     As illustrated with reference again to  FIGS. 1 and 2  for one embodiment herein described by way of example, the first plate  16  has the inner face  24  with a flat profile and the outer face  20  with a convex profile in the frontal plane F. With reference again to  FIG. 5 , the second plate  18  is, in the horizontal plane T, substantially the shape of a parallelepiped, with a leading edge  36  and a trailing edge  38  that are substantially parallel and merged in a stacked position with the leading edge  28  and the trailing edge  30  of the first plate  16 . The leading edge  36  of the second plate  18  is connected to the trailing edge  38  through two linking edges  40 ,  42  substantially parallel to one another. Connecting fillets may connect the leading and trailing edges  36 ,  38  to the linking edges  40 ,  42 . As is clearly shown with reference again to  FIGS. 1, 2 , and  5 , the first plate  16  thus juts out in the horizontal plane T on either side of the linking edges  40 ,  42  of the second plate  18 .  
         [0032]     With continued reference to  FIGS. 1-4 , the disk prosthesis  10  herein described includes a ball-and-socket joint  44  interposed between the two plates  16 ,  18  that are stacked one on top of the other. In the exemplified embodiment illustrated in FIGS.  1  to  5 , the ball-and-socket joint  44  comprises cooperating arcuate surfaces including a first insert  46  that has a spherical cap  48  and a second insert  50  that has a spherical cup  52  that smoothly works with the spherical cap  48 . Each insert  48 ,  50  is mounted in a hole  54 ,  56 , a blind hole for example, through the inner face  24 ,  26  of each plate  16 ,  18 . Each insert  46 ,  50  has a general form of revolution and has, respectively, a base  58 ,  60  of circular transversal cross-section, one of whose ends is arranged so as to have either the spherical cap  48  or the spherical cup  52 . In one embodiment, the inserts  46 ,  50  are made of a ceramic material or of polyethylene.  
         [0033]     As illustrated with reference again to  FIGS. 2 and 4 , by way of example, vent holes  47 ,  51  are provided within the first and second plates  16 ,  18 , respectively, and extend from cavities  17 ,  19  formed within the first and second plates  16 ,  18 . The vent holes  47 ,  51  permit air to escape therethrough when inserting the inserts  46 ,  50  into cavities  17 ,  19  during a manufacturing process.  
         [0034]     It is clear from the preceding description that the first plate  16 , the carrier plate, is wider than the second plate  18  in the transverse direction such as to constitute, on the one hand, the flat limiting surface, the inner surface  24 , for the second plate  18  and, on the other hand, a large bearing surface, the outer face  20 , with the associated vertebra. As is further clear with reference again to  FIGS. 2 and 4 , the prosthesis limit positions, both in the frontal plane F and in the sagittal plane S, are determined by bringing the plates  16 ,  18  into contact with one another.  
         [0035]     As further illustrated with reference again to  FIGS. 1-5 , one embodiment of the disk prosthesis  10  may be dimensioned in such a manner that: E/d=ED 1  for A 1 /A 2 &gt;0.5, wherein E represents the distance between the two inner faces  24 ,  26  of the plates  16 ,  18 ; d represents the width of the second (upper) plate  18  in the frontal plane F; D 1  represents the length of the second (upper) plate  18  in the sagittal plane S; A 1  represents the maximum angle of clearance between the plates  16 ,  18  in the frontal plane F; and A 2  represents the maximum angle of clearance between plates  16 ,  18  in the sagittal plane S. For the example of the disk prosthesis  10 , herein described by way of example, the angles A are about ten degrees or less but may be larger as desired for a particular use without deviating from the teaching of et present invention. Such dimensioning makes it possible to provide a disk prosthesis with controlled clearances that correspond substantially to the natural movements of the spinal disk.  
         [0036]     In the example illustrated with reference to FIGS.  1  to  5 , the spherical cap  48  and the spherical cup  52  are created on the inserts  46 ,  50  attached to the plates  16 ,  18 . Alternatively, and as illustrated with reference to  FIGS. 6 and 7 , another variant embodiment in which the spherical cap  48  is an integral part of the second plate  18 , the upper plate in the example illustrated. For this embodiment, the spherical cap  48  may be made of titanium, while the spherical cup  52  is arranged in an insert  46  made of ceramic or polyethylene.  
         [0037]     Further, in the preceding examples, the spherical cap  48  is carried on the second (upper) plate  18  with the spherical cup  52  carried by the first (lower) plate  16 . Alternatively, and with reference to  FIGS. 8 and 9 , another variant embodiment may include the spherical cup  52  carried by the second (upper) plate  18 , with the spherical cap  48  carried by the first (lower) plate  16 . Yet further and by way of example, one embodiment may include a zirconia-on-alumina articulating surface for the spherical cap  48  and the spherical cup  52  of the ball and socket joint  44 .  
         [0038]     Yet further, and with reference to  FIG. 10-15 , by way of example, either the first plate  16  or the second plate  18  may carry a shock absorbing material  54  for contacting the opposing plate  16 ,  18  at the limits of movement for the articulating plates. As herein presented by way of example, the absorbing material may be an elastic ring  56  extending about and carried in frictional contact within the perimeter portions of the inner faces  24 ,  26  of the plates  16 ,  18 . As herein illustrated by way of example, the elastic ring  62  may be mounted in a recess  58  surrounding the spherical cap  48  and/or the spherical cup  52  and arranged on the inner face  24 ,  26 . In one embodiment, the plates  16 ,  18  may include a notch or recess  58  extending about the perimeter fro receiving the elastic ring  56  therein. Yet further, and as illustrated with reference to  FIG. 12 , the elastic ring  56  may include an inner groove  60  for receiving a tab  62  within a wall of the recess  58  for retaining the elastic ring  56  within the recess  58 .  
         [0039]     As illustrated with reference again to  FIGS. 1 and 4 , one embodiment of the disk prosthesis  10  may comprise positioning cutouts  64 , such as the holes  66  within the anterior surface of the first plate  16  and holes  68  carried within the second plate  18  for receiving a tool for simultaneously holding the plates. Alternatively, and as illustrated with reference to  FIGS. 10, 11 ,  13 , and  14 , the cutouts  64  may comprise a pair of parallel groves  70 ,  72  carried by each of the first and second plates  16 ,  18  respectively, for receiving the tool. By way of further example, and with continued reference to  FIGS. 10-15 , the pairs of parallel groves  70 ,  72  for the first and second plates  16 ,  18  may extend from the anterior to the posterior portions thereof, with the first plate including the grooves  70  along the outer face  20  and the second plate  18  including the grooves  72  along opposing side wall surfaces thereof.  
         [0040]     For the above described embodiments of the disk prosthesis  10 , the outer faces  20 ,  22  for at least one of the first and second plates  16 ,  18  comprises a plurality of teeth  74  for retaining the plates between the adjacent vertebrae  12 ,  14 . As illustrated by way of example with reference again to  FIG. 5 , at least a portion of the plurality of teeth may comprise an anterior slope bias for facilitating an insertion into a space between vertebrae while restricting anterior distraction of the plates  16 ,  18 .  
         [0041]     Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and alternate embodiments are intended to be included within the scope of the appended claims.