Abstract:
An intervertebral implant with a central axis, a top part, a bottom part, a joint comprising a joint part and a joint shell, and a joining means. The top part has a top apposed surface that is suitable for placing it on a vertebra situated above it; The bottom part has a bottom apposed surface, that is suitable for placing it on a vertebra situated below it. One of the two parts is operatively associated with a convex joint part and the other part to a matching joint shell. The joint part and the joint shell are mounted against one another in a sliding manner such that the top part and the bottom part can rotate relative to one another at least about one axis of rotation. The joining means holds the top part and the bottom part together without impairing the capability of the joint to pivot while allowing a clearance between the joint part and the joint shell.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a continuation of International Patent Publication PCT/CH2003/000495, filed Jul. 22, 2003, which is incorporated by reference herein in its entirety.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to an intervertebral implant and, more particularly, to an artificial intervertebral implant.  
       BACKGROUND OF THE INVENTION  
       [0003]     Intervertebral disks often become damaged creating discomfort for a patient. It is known that partial or total removal of a damaged intervertebral disk may alleviate some of this discomfort but may limit the natural function of the spine. Thus, nowadays following the removal of a damaged, natural intervertebral disk or a damaged nucleus pulposus of a natural intervertebral disk, an implant or prosthesis is placed in the intervertebral space between the two vertebra adjacent to the site of removal. The aim of implanting such devices is to bring about an as natural state as possible, particularly duplicating the original height of the intervertebral disk and consequently restoring the original distance between the two adjacent bodies of the vertebra. Furthermore, movements of adjacent bodies of the vertebra relative to one another should be able to be carried out with minimal hindrance of their natural function. For this purpose the retention of the ability to move when bending forward or backward, i.e. the flexion and the extension of the bodies of the vertebra as well as the lateral bending of the intervertebral bodies within the natural limits, is desirable. It is also desirable that the natural ligaments and muscles along the spine are left substantially intact, to further stabilize the movements of a mechanical replacement for a intervertebral disk.  
         [0004]     Such an intervertebral implant is known from U.S. Pat. No. 5,556,431 to Büttner. This known implant comprises a bottom base plate and a top cover plate, the exterior surfaces of which can be placed on the adjacent bodies of the vertebra, as well as a joint provided between the cover plates. This joint consists of a hemispherical first joint part and a matching joint shell as the second joint part, so that the cover plates can pivot polyaxially relative to one another. A disadvantage of this known intervertebral implant is that although the two cover plates are connected with a joint part each, the joint parts are not held together. This requires the implant to be assembled by the surgeon creating a possibility for erroneous assembly. In-situ the joint parts and consequently the entire implant must be held together using a special instrument. This creates a possibility that the implant will fall apart prior to and during implantation, risking damage to surrounding tissue and risking loss of parts.  
         [0005]     From U.S. Pat. No. 5,895,428 to Berry an intervertebral implant is known, that comprises joining means to hold the joint parts together. A disadvantage however, of this known intervertebral implant is that during a relative movement of the two joint parts the joining means slide on one another without any clearance and no lubricating film of the body&#39;s own fluids can be formed on them. A lubricating film of the body&#39;s own fluids may advantageously reduce wear and erosion between the sliding surfaces.  
       SUMMARY OF THE INVENTION  
       [0006]     Thus it is desirable to produce a device for artificial replacement of an intervertebral disc, that has a joint that can execute a rotary movement, can be pre-assembled and where the joint parts can be held together with a clearance by joining means so that a lubricating film of the body&#39;s own fluids can form between the sliding joint surfaces. Further it would be desirable that such clearance be within a physiologically required range so as not to hinder the movement of the joint parts.  
         [0007]     It would be further desirable that the joining means holding the parts together could be achieved using geometrically simple elements to reduce costs of manufacture. And still further it is desirable to have a joint with only a single pair of articular surfaces.  
         [0008]     An intervertebral implant herein described includes a top part, a bottom part, a joint and a joining means. The top part comprises a top apposed surface suitable for placement against a vertebra situated above and adjacent the top part. The bottom part comprises a bottom apposed surface suitable for placement against a vertebra situated below and adjacent the bottom part. The joint preferably connects the top part and the bottom part and comprises a convex joint part with an articular surface and a joint shell with an articular surface. The joint part and the joint shell are mounted in a sliding manner. Either the top part or the bottom part is operatively associated with the convex joint part and the other part is operatively associated with the joint shell such that the top part and the bottom part can rotate relative to one another about at least one axis of rotation. The joining means preferably holds the top part and the bottom part together without impairing the capability of the joint to pivot and whilst permitting an axial clearance between the joint part and the joint shell.  
         [0009]     In a further embodiment of the implant the joining means comprises an anchoring means that can be joined with the top part and is loosely mounted between the bottom part and the joint. The convex joint part may comprise a hollow space, that is open on the virtual apex of the convex joint part and in which the anchoring element can be axially displaced until it comes to a rest on the wall of the hollow space after overcoming a given axial clearance. The hollow space may be cylindrical. The hollow space may contract along the axis of rotation of the joint.  
         [0010]     In another embodiment of the implant the convex joint part comprises a spherical first articular surface and the joint shell has a second articular surface, matching the first articular surface.  
         [0011]     In a further embodiment of the implant, the convex joint part and the joint shell may be made from a metal/plastics material pair.  
         [0012]     In another embodiment of the implant the articulating surfaces of the joint part and the joint shell are coated with some friction reducing substance.  
         [0013]     In a further embodiment of the implant the top and bottom apposed surfaces are coated with titanium.  
         [0014]     In another embodiment the top apposed surface and the bottom apposed surface are provided with macroscopic structures. The macroscopic structures may be protuberances. The protuberances may be pyramidal. Some protuberances may take the form of a wedge-shaped ridge or saw-tooth like serrations that are situated in a straight line. The protuberances may be coated at least partly with a coating of some substance that integrates with bone. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The features of the invention will be better understood by reference to the accompanying drawings which illustrate presently preferred embodiments of the invention. It will nevertheless be understood that the invention should not be limited to such preferred embodiments and that the features may be used singularly or in combinations and that modification and alterations of the illustrated and described devices and methods are contemplated. In the drawings:  
         [0016]      FIG. 1  is a section through an embodiment of the device of the present invention;  
         [0017]      FIG. 2  is a dorsal view of the device of  FIG. 1 ;  
         [0018]      FIG. 3  is a perspective view of the device of  FIGS. 1 and 2 ;  
         [0019]      FIG. 4  is an exploded view of the device of  FIGS. 1, 2  and  3 ;  
         [0020]      FIG. 5  is a perspective view of the top part of an alternative embodiment of the device;  
         [0021]      FIG. 6  is a lateral view of the top part of the embodiment of the device of  FIG. 5 ;  
         [0022]      FIG. 7  is a lateral view of the top part of a further embodiment of the device; and  
         [0023]      FIG. 8  is a dorsal view of the top part of the device of  FIG. 7 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     The embodiment of the intervertebral implant  1  illustrated in  FIGS. 1-4  comprises a central axis, a top part  3 , a bottom part  4 , a joint  9  and an anchoring element  15 . The top part  3  comprises a top apposed surface  5  that intersects a central axis  2  wherein the top apposed surface  5  is adapted to be placed adjacent and preferably in contact with a vertebra located above the site of a removed intervertebral disk. The bottom part  4  comprises a bottom apposed surface  7  that intersects a central axis  2  wherein the bottom apposed surface  7  is adapted to be placed adjacent and preferably in contact with a vertebra located above the site of a removed intervertebral disk. The joint  9  intersects the central axis and is provided between the top part  3  and bottom part  4 , for the articulated joining of the top part  3  and bottom part  4 . The joint  9  has a two-part construction comprising a convex joint part  12  and a joint shell  13 . The convex joint part  12  is shaped as a partial spherical segment and comprises a second articular surface B. The convex joint part  12  is joined with the bottom part  4 . The joint shell  13  matches the convex joint part  12  and has a first articular surface A. The joint shell  13  is preferably joined with the top part  3 . By virtue of the partial spherical shape of the articular surfaces both parts can be pivoted polyaxially relative to one another. The anchoring element  15  holds the top part  3  and the bottom part  4  together and will be discussed in greater detail infra.  
         [0025]     In one embodiment of the implant the top part  3  and bottom part  4  are coated with titanium on the apposed surfaces.  
         [0026]     The top part  3  and the joint shell  13  are made of two parts in the embodiment illustrated in  FIG. 1  so that the top part  3  and joint shell  13  can be manufactured, for example from a combination of materials. A range of metal/plastic combinations are known to those of skill in the art and provide the advantage of allowing relatively low friction sliding between the articular surfaces, damping of axial load impacts on the joint and being biocompatible and proven to perform as a joint implant. An exemplary pairing is titanium or a cobalt-chromium alloy top part  3  and a highly cross-linked polyethylene (X-UHMWPE) joint shell.  
         [0027]     In another embodiment the articular surfaces A, B are coated with a substance that results in a reduced coefficient of friction between the articular surfaces A, B. Some exemplary substances include titanium carbide and amorphous carbon (ADLC) although other such substance will be known to one of skill in the art.  
         [0028]     The dimensions of the articular surfaces depend upon the application. In a preferred embodiment the radius of the first articular surface A of the convex spherical joint part is between about 3 mm and about 25 mm, more preferably between about 4 mm and about 20 mm.  
         [0029]     The convex joint part  12  is substantially concentric with the central axis  2  and converges towards the bottom surface  6  of the top part  3 . The joint shell  13  is also substantially concentric with the central axis  2 , while the opening of the joint shell  13  is directed towards the top surface  8  of the bottom part  4 . The anchoring element  15 , by which the two parts  3 ,  4  are held together, is substantially coaxial with the central axis  2  and comprises a pin  16 , that with its slot end  25  passes through the joint shell  13  at approximately a virtual apex  30  on its surface and is fastened to the top part  3  and has a circular disc-shaped extension or plate  26  with a greater diameter, said extension provided on the front end  27  of the pin  16 . The convex joint part  12  has an opening  28  that commences from its articular surface and is coaxial with the central axis  2  and terminates in a cylindrical hollow space  29  with a greater diameter in the interior of the convex joint part  12 . The hollow space  29  is open approximately on the virtual apex  30  of the convex joint part  12  and is configured such that the anchoring element  15  can be axially displaced until it comes to rest on the wall of the hollow space  12  after overcoming the axial clearance X. Configuring joining means as illustrated may further result in low manufacturing costs. The geometry of the anchoring element  15 , as well as of the opening  28  and that of the hollow space  29  is so chosen, that the pin  16  and the extension  26  in the opening  28  and in the hollow space  29 , respectively, are arranged in a displaceable manner. The diameter of the opening  28  is smaller than the diameter of the disc-shaped plate  26 , so that the convex joint part  12  and the joint shell  13  and consequently the two parts  3 ,  4  fastened on them, are axially held together without hindering the movement of the joint  9 . The mobility of the pin  16  in the opening  28  and of the plate  26  in the hollow space  29  make a polyaxial pivoting of the two parts  3 ,  4  relative to one another possible. The length of the pin  16  is so dimensioned, that for any articulation of the joint  9  the extension  26  on the front end  27  of the pin  16  comes to a rest only after overcoming the clearance X at the transition between the opening  28  and the hollow space  29 . In one embodiment the clearance X is at least about 0.005 mm, preferably at least about 0.05 mm. This will result in the advantage, that a lubricating film with the thickness X of the body&#39;s own fluids can be formed between the sliding surfaces. Furthermore, the articular surfaces can be protected from getting damaged by means of additional temporary fixing means.  
         [0030]     In another embodiment, the clearance X is a maximum of about 0.5 mm and the cylindrical hollow space  29  has a diameter D between about 6 mm and about 20 mm, and a height H of the hollow space, measured parallel to the central axis between about 0.5 mm and about 8 mm.  
         [0031]     In a further embodiment, the cylindrical hollow space  29  has a contraction that is coaxial with the central axis and terminates in the articular surface A of the convex joint part, the contraction is cylindrical and coaxial with the central axis, and the contraction has a diameter d, while the ratio of d:D is between about 30% and about 75%.  
         [0032]     As it is shown in  FIG. 4 , the top part  3  comprises a ventral side surface  31 , a dorsal side surface  32 , a first top lateral side surface  33  and a second top lateral side surfaces  34 . The bottom part  4  comprises a ventral side surface  41 , a dorsal side surface  42 , a first bottom lateral side surface  43  and a second bottom lateral side surfaces  44 .  
         [0033]     Furthermore, the top apposed surface  5  and the bottom apposed surface  7  have a convex construction and are provided with protuberances  18 . Between the top lateral side surfaces  33 ,  34  and between the bottom lateral side surfaces  43 ,  44  there is a central plane  35 , that in this case forms also the plane of symmetry for the intervertebral implant  1 . Furthermore, the top and bottom apposed surfaces  5 ,  7  include two parallel grooves  39 , which are approximately symmetrical about the central plane  35  and terminate at least in the ventral side surfaces  31 ,  41  and are suitable to accommodate, for example, the arms of a tractioning instrument (not illustrated). In this case the protuberances  18  are constructed as pyramid-like protuberances  38 . In one embodiment the pyramid-shaped protuberances have a volume between about 0.12 mm 3  and about 1.4 mm 3 . In a further embodiment convex construction of the top and bottom apposed surfaces  5 ,  7  are adapted to suit the natural cover and base surfaces, respectively, of the adjacent bodies of the vertebra.  
         [0034]     In addition to the pyramid-like protuberances  38  illustrated in  FIG. 4 , the protuberances  18 , as illustrated in  FIGS. 5 and 6 , may comprise a wedge-shaped rib  36  on each of the top and bottom apposed surfaces  5 ,  7 . The wedge-shaped rib  36  is approximately symmetrical about the central plane  35 . The ribs are taller than the pyramid-like protuberances  38  and are situated on the top and bottom apposed surface  5 ,  7  and are substantially parallel to the central plane  35  ( FIG. 4 ). So that the intervertebral implant  1  could be more easier introduced into the intervertebral space, the height of the ribs  36  generally decreases towards the dorsal side surface  32 .  
         [0035]     A further embodiment of protuberances  18  is illustrated in  FIGS. 7 and 8 . The serrations  37 , which are taller than the pyramid-shaped protuberances  38 , are constructed partly with saw-tooth shapes, while the steeper flank is facing the ventral lateral surface  31 . In this case the serrations  37  are situated substantially parallel to the central plane  35  ( FIG. 4 ) and extend from the ventral lateral surface  31  up to the dorsal lateral surface  32 . In another embodiment the saw-tooth serrations  37  are arranged approximately symmetrically about the central plane  35 . The advantage of the serrations is, that they can more easily penetrate into the end plates of the adjacent bodies of the vertebra.  
         [0036]     In FIGS.  5  to  8  only the top part  3  is illustrated in the form of an example. The same construction of the protuberances  18  with pyramid-shaped protuberances  38 , a rib  36  ( FIGS. 5 and 6 ) or serrations  37  ( FIGS. 7 and 8 ) can be applied in an analogous manner to the bottom part  4 .  
         [0037]     By virtue of these macroscopic structures ( FIGS. 5-8 ) the advantages achievable are, that on the one hand, torques about an axis of rotation, intersecting the apposed surfaces, can be better transferred from the bodies of the vertebra to the intervertebral implant, and on the other hand the surface area, to which the bones can adhere, is increased.  
         [0038]     In another preferred embodiment the protuberances are coated at least partially with a substance that fully integrates with bone or may even be replace with new, natural bone tissue. Examples of such substance include hydroxylapatite and bi-phased hydroxylapatite-tricalcium phosphate mixtures.  
         [0039]     While the foregoing description and drawings represent the preferred embodiments of the present invention, it will be understood that various additions, modifications and substitutions may be made therein without departing from the spirit and scope of the present invention as defined in the accompanying claims. In particular, it will be clear to those skilled in the art that the present invention may be embodied in other specific forms, structures, arrangements, proportions, and with other elements, materials, and components, without departing from the spirit or essential characteristics thereof. One skilled in the art will appreciate that the invention may be used with many modifications of structure, arrangement, proportions, materials, and components and otherwise, used in the practice of the invention, which are particularly adapted to specific environments and operative requirements without departing from the principles of the present invention. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and not limited to the foregoing description.