Abstract:
A motion restoring prosthesis to be interposed between the ends of two bones adjoining a mammalian appendage or spinal joint is formed of two components with the components having inner cooperating articulating surfaces and outer bone engaging surfaces. At least one of the components has an inner section made of relatively hard, stiff material defining one or the articulating surfaces and an outer section made of a softer material defining the bone engaging surface. The softer material having a hardness/stiffness characteristic compatible with the bone to reduce stress shielding and more evenly distribute the forces from the articulating surfaces to the associate bone interface.

Description:
RELATED APPPLICATION 
       [0001]    This application is based on and claims priority of U.S. Provisional Application No. 60/676,744 (“&#39;744 application”) for a Motion Restoring Intervertebral Device filed on May 2, 2005, the contents of the provisional application are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]    This application relates to (a) motion restoring prosthesis for placement between the ends of two bones adjoining a mammalian appendage or spinal joint, such as an artificial intervertebral disc or hip/knee/shoulder joint and (b) intervertebral spacer for accommodating fusion between the separated vertebrae. 
       BACKGROUND OF THE INVENTION  
       [0003]    With respect to motion restoring intervertebral devices see German Patent No. 2,263,842 and the devices disclosed therein. These include ball and socket prostheses with single and/or dual articulations and with and without motion dampening elements. The clinical use of the device containing a ball supported by two opposing sockets extending from generally flat plates, was discussed in a Hoffmann-Daimler&#39;s article appearing in Intervertebral Disk Displacement, Vol. 112, No. 4, Aug. 1974. Hansen et al., U.S. Pat. No. 5,676,701 (“&#39;701 patent”) discloses a low wear artificial spinal disc having opposing convex and concave contoured surfaces with a full 360 degree circumference. This allows for rotation but no translation. Yuichiro and Koichi, U.S. Pat. No. 5,899,941 discloses a similar device, but where the articulating surface is generally non-conforming to allow rotation and translational movement. This is similar to the device illustrated in U.S. Pat. No. 6,113,637 by Gill et al. This artificial spinal disc incorporates a ball and trough type articulation with a substantially flat portion. Both the Yuichiro and Gill et al. patents describe devices which rely on point contact, a potential source of excessive wear debris. 
         [0004]    Also, see the prior art discussed in the &#39;701 patent and U.S. Pat. No. 5,071,437 (“&#39;437 patent”). These prior art intervertebral devices generally rely on an elastomeric core to provide the articulation, which core is sandwiched between rigid plates abutting the respective vertebral bodies (“&#39;437 patent”) or hard metal ball and socket components interposed between adjacent vertebrae. 
         [0005]    The use of a hard/stiff material, generally ceramic or a cobalt chromium alloy, to provide the articulating surfaces decreases the wear and increases the life expectancy of the artificial disc. However, the use of the same material or rigid plates, as in the &#39;437 patent, abutting the faces of the separated vertebrae, results in stress shielding, i.e., a nonuniform distribution of the forces transmitted from the articulating surfaces to the abutted vertebrae bone faces. This nonuniform loading can result in so called hard spots on the bone face where the load is being transmitted and soft spots where little or no load is being transmitted, with the consequence that the soft spots tend to result in bone absorption, thereby loosening the implant and causing eventual failure. 
         [0006]    There is a need to provide a softer/less stiff material to interface with the vertebrae bone faces and a hard material to serve as the articulating surfaces. Since a softer material, capable of supporting the articulating layer, may not be conducive for bone attachment, there is a need to modify the vertebrae engaging or buttressing surface of the softer material to facilitate bone attachment. There is a further need to inhibit any slight motion, e.g., macromotion, between the material forming the articulation surface and the material forming the vertebrae engaging or bone buttressing surface to reduce or eliminate any wear at the interface of these materials. Such wear, given time, will cause separation at the interface and shorten the useful life of the prosthesis. 
         [0007]    The above problems are also associated with artificial limb replacement joints, except that some joint replacements such as hip joints may, as a practical matter, have a need to relieve the stress imparted only to one of the bone ends, e.g., the bone portion forming the acetabulum. Artificial hip joints typically include a metal cup which houses the acetabular bearing portion, which metal cup is secured, via cement, for example, directly to the bone. See, for example, U.S. Pat. Nos. 6,368,354 (“&#39;354 patent”) and 6,966,932 (“932 patent”). As discussed above, the metal to bone contact provides a nonuniform stress loading to the exposed bone face with a significant potential of causing the implant to loosen from the underlying implant. It is noted that both of the above patents illustrate an interlocking arrangement between several sections of the acetabular component. However, the interface between the outer metal shell and the bone end or face forming the acetabulum does not resolve the nonuniform loading on the bone face. As with the artificial intervertebral joint there is a need to provide stress shielding in artificial hip replacements as well as in other artificial appendage joints. 
         [0008]    With respect to item (b) above, a spacer or disc replacement member, made of a material such as Ti, has been used to maintain the vertebrae encompassing a failed natural disc in a separated condition during the fusion process. Currently, spacers formed of a softer material, such as PEEK, have been used for this purpose to provide better stress transfer to the bone during the fusion process. However, PEEK is not particularly bone attachment friendly. There is a need to provide an artificial disc replacement member which accommodates both even stress transfer and bone-on growth attachment. 
       SUMMARY OF THE INVENTION  
       [0009]    In a broad sense a motion restoring prosthesis adapted to be placed between the ends of two bones adjoining a mammalian appendage or spinal joint, in accordance with the present invention, includes two components with each component defining an outer surface for attachment to a respective bone end and an articulating surface for engaging the articulating surface of the other component. At least one of the components has an inner and outer section. The inner section, defining the articulating surface, is made of a primary relatively hard/stiff material such as ceramic, cobalt chromium, or other hard, stiff bio-compatible material for inhibiting the wear between the articulating surfaces. The outer section is made of a secondary material (sometimes hereinafter referred to as the “bone buttressing material”) that is softer and less stiff than the primary material (sometimes referred to as the “articulating material”). Preferably the secondary material is formed of a biocompatible polymer having a hardness/stiffness characteristic closer to that of the bone to be buttressed. Polyetheretherketone (“PEEK”) or carbon fiber reinforced PEEK (“CFRPEEK”) are clear choices. These polymers reduce stress shielding by more evenly distributing the forces or load from the articulating surfaces to the respective bone interface end thereby enhance the life of the prosthesis. 
         [0010]    Preferably the flexural moduli of the primary and secondary materials are, respectfully, within the range of about 80± and about 60 or less on the Giga Pascal Scale (GPa), which scale provides a measure of elasticity versus pressure. Most preferably the secondary material has a GPa range of about 4 to 30 or about 12-25 while cortical bone has a GPa of about 17±5-8. 
         [0011]    Optionally, the bone buttressing surface of each of the outer sections are roughened and/or made uneven to enhance the attachment of bone thereto. Further, it is desirable for the bone buttressing surface to have a very thin coating or layer of a bone-on growth attachment friendly material, such as titanium, calcium phosphate or hydroxyapatite thereon, which does not materially increase the stiffness of the bone buttressing surface of the secondary material to detract from the stress shielding characteristic thereof, while enhancing the attachment of the bone thereto. Preferably the thin coating of such material is within the range of about 0.5 to 15.0 or more microns and most preferably within the range of about 0.50 to 3.0 microns. 
         [0012]    In a narrower sense, and particularly for creating artificial spinal joints, both components are formed with inner and outer sections. The inner sections are formed of the primary material and preferably define a ball and socket type articulating surface as described in the &#39;744 application. The outer sections are formed of the softer secondary material which define the bone buttressing surfaces for attachment to the respective bone faces of the separated vertebral bodies. 
         [0013]    Again, it is preferable that the softer outer sections partially encapsulate the inner sections, e.g., by an injection molding process, to substantially prevent any motion including macromotion between the inner and outer sections. 
         [0014]    With respect to item (b) an artificial intervertebral disc or spacer, for accommodating fusion, is preferably formed of a material such as PEEK or CRFPEEK having a flexure modulus comparable to that of cortical bone. The spacer is provided with a roughened and/or uneven surface and subsequently provided with a very thin coating of a material such as Ti which enhances bone-on growth without adversely affecting the bone friendly surface characteristic of the PEEK or CFRPEEK as discussed above. 
         [0015]    The invention may best be understood in reference to the following description taken in conjunction with the appended drawings where like components and parts thereof are given the same or primed numerals in the several figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0016]      FIG. 1  is a side elevational view of a motion restoring prosthesis for replacing a natural spinal disc in accordance with the present invention; 
           [0017]      FIG. 2  is a cross-sectional view of the prosthesis of  FIG. 1  taken along lines  2 - 2  of  FIG. 1 ; 
           [0018]      FIG. 3  is a cross-sectional view of the prosthesis of  FIG. 1  taken along lines  3 - 3  of  FIG. 2 ; 
           [0019]      FIGS. 4 and 5  are perspective and cross-sectional views, respectfully, of a slightly modified artificial spinal disc in which the bone buttressing surface of the secondary material are coated with a coating of bone-on growth attachment friendly material; 
           [0020]      FIG. 6  is a cross-sectional view taken along lines  6 - 6  of  FIG. 5 ; 
           [0021]      FIG. 7  is a simple diagrammatic view of a hip joint prosthesis illustrating a dual section acetabular component in accordance with this invention; 
           [0022]      FIGS. 8 ,  9 , and  10  are simple diagrammatic views showing the several parts which make up the prosthesis of  FIG. 7  in a partially unassembled and assembled condition; 
           [0023]      FIG. 11  is a diagrammatic view of the femoral component inserted in the femoral bone; 
           [0024]      FIGS. 12 ,  13 , and  14  are perspective, front and rear elevational views, respectively, of an acetabular component of the hip joint prosthesis of  FIG. 7 ; 
           [0025]      FIG. 15  is a cross-sectional view of the acetabular component of  FIGS. 11-13  showing the inner and outer cup sections; 
           [0026]      FIGS. 16 and 17  are cross-sectional and perspective views of an acetabular cup showing an outside uneven and roughened surface with a thin layer of coating of a bone-on growth friendly attachment material thereon; 
           [0027]      FIG. 18  is a side elevational view of the acetabular component of  FIG. 15  and; 
           [0028]      FIG. 19  is a cross-sectional view of a mold suitable for partially encapsulating the primary (articulating) material with the softer secondary (bone buttressing material) for one of the components of a motion restoring vertebral prosthesis of  FIGS. 1-6 . 
           [0029]      FIG. 20  is a perspective view of an artificial intervertebral spacer formed of a material, e.g., PEEK, and coated with a bone-on growth friendly material, e.g., Ti, for accommodating relatively uniform stress transfer to the vertebral bone faces while promoting bone-on growth attachment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    Referring now to  FIGS. 1-3 , a motion restoring intervertebral prosthesis, in accordance with this invention, comprises first and second components  10  and  12  with the components having inner sections  10   a  and  12   a  (formed of a primary material) and outer sections  10   b  and  12   b  (formed of a secondary material), respectively. The sections are integrally formed, preferably by an injection molding process, so that the secondary material partially encloses or encapsulates the primary material (except for the articulating surfaces) as will be explained with respect to  FIG. 17 . Briefly, outwardly extending protrusions  10   c  and  12   c  of the primary material are overlapped by an inwardly extending shoulders  10   d  and  12   d  of the secondary material. The inner sections  10   a  and  12   a  define cooperating articulating surfaces  10   e  and  12   e  in the form of a ball and modified socket to provide controlled rotation in the frontal plane and controlled rotation with translation in the sagittal plane as is described in some detail in the &#39;744 application (and in the companion PCT application no. ______ based on the provisional application and filed herewith) in connection with  FIGS. 3 and 4 . The top outer surfaces  10   f  and  12   f  form the bone (vertebral) buttressing surfaces. 
         [0031]    The primary material is formed from a relatively hard, stiff material, such ceramic or a cobalt chromium alloy, to minimize the wear between the articulating surfaces and increase the life expectancy of the prosthesis. 
         [0032]    The secondary or bone buttressing material is softer and less stiff than the primary material to accommodate surface irregularities in the underlying vertebral bone faces and more evenly distribute the forces or load from the articulating surfaces to the respective bone faces thereby inhibiting the formation of the hot and cold spots. In this manner it is believed that the softer secondary material serves to dampen the stresses imparted to the bone faces and increase the life expectancy of the prosthesis. 
         [0033]    The secondary material should have a flexural modulus on the GPa scale of about 60 or less and preferably within the range of 4 to 30 and most preferably about 17±5-8 which is the flexural modulus spectrum of cortical bone. The primary material may be made of a bio-compatible ceramic, e.g., alumina ceramic, which has a GPa flexural modulus of about 158±10% while another suitable primary materials is cobalt chromium alloy (including some molybdenum) which has GPa flexural modulus of about 80. 
         [0034]    PEEK is an excellent implantable bio-compatible material suitable for use as the secondary material with a GPa flexural modulus of about 4 and when reinforced with 20% carbon fiber has a GPa of about 18. The carbon fiber content may be adjusted from 0 to about 80%, but we have found that a mixture of about 30 to 35% of carbon fiber by weight provides a preferred bone-to-implant surface while providing sufficient support for the molded insert of articulating (primary) material and it has a stiffness slightly higher than the average (i.e., about 17 GPa) for cortical bone, but less than 30 GPa, i.e, about 21-24, while being imaging friendly, i.e., translucent to the conventional imaging processes. Protrusions  14 , commonly referred to as keels, may be in the form of headed rods or bolts for enhancing the attachment of the prosthesis to the underlying bone. The heads  14   a  of the rods are preferably embedded in the inner sections as shown. 
         [0035]      FIGS. 4 ,  5 , and  6  illustrate a slightly different design of a motion restoring vertebral prosthesis, with primed numerals being used to refer to corresponding components and portions of the prosthesis of  FIGS. 1-3 . In this embodiment the outer surfaces of the second components, formed of the secondary material, are coated with a very thin layer  16  of a material which promotes the affixation of bone to the bone buttressing surfaces while essentially maintaining the ability of such surfaces to accommodate surface irregularities of the bone face and provide stress softening. We have found that a very thin coating of Ti, calcium phosphate or hydroxyapatite serves this purpose. Such coating or layer can be applied by a conventional vacuum/electronic ionic fusion processes. As pointed out earlier, such a coating should have a thickness within the range of about 0.5 to 15.0 microns and most preferably between about 0.5 to 3.0 microns. It should be noted that such a coating on a prosthesis component made of the described secondary material forming a bone buttressing surface may be useful independently of the material forming the articulating surface. 
         [0036]    A hip joint prosthesis is illustrated in  FIGS. 8-16 . With this type of prosthesis it is feasible, as a general rule, to form only the acetabular component  18  with a relatively soft outer secondary bone buttressing material, such as PEEK or CFRPEEK, secured over the harder and stiffer inner primary articulating material such as ceramic.  FIG. 7 , in diagrammatic format, illustrates a completed hip joint prosthesis in which an acetabular component  18  is fixed to the underlying acetabulum  20 . A femoral component  22  comprises a conventional rod  22   a  inserted into the patient&#39;s femur  24  with the end of the rod carrying a conventional ball  22   b  of primary material positioned on a shaft  22   c.    FIGS. 8-11  illustrate the components in an unassembled and partially assembled form. 
         [0037]    The present invention, with respect to this type of prosthesis, concerns the structure of the acetabular component  18 , which, as is illustrated in  FIGS. 12-15 , comprises an inner section  18   a  and an outer section  18   b.  Section  18   a,  like section  10   a  of the first component of the device of  FIGS. 1-3 , is formed of a relatively hard/stiff material such as ceramic to form an inner semi-spherical articulating surface  18   c  for cooperating with the femoral ball  22   b.  The inner section  18   a  is mounted within the outer section  18   b,  preferably via an injection molding process in which tongues and grooves  18   d  are preferably formed in the outer surface of the first component prior to the molding step to mechanically lock the first and second components together. The outer or bone buttressing surface  18   e  of the second semi-spherical section  18   b  is preferably formed in the mold with (circular) grooves  18   f  (forming an uneven surface) to aid in locking the bone buttressing surface to the bone. Alternatively (and additionally) the bone buttressing surface may be roughened by blasting the surface with an appropriate abrasive resorbable media, such as kieserite after the molding step to provide a roughened surface  18   g,  as is illustrated in  FIGS. 17 and 18 . This roughened and/or uneven surface is then preferably provided with a thin micron layer  18   h  bone-on growth attachment enhancing material such as Ti. See  FIG. 18 . 
         [0038]      FIG. 19  illustrates a simple two part injection mold comprising parts  26  and  28  with a first or articulating surface forming section  30  (illustrated as ceramic) placed therein. The secondary material, e.g., polymer, such as PEEK or CFRPEEK, is forced in duct  32  while air escapes through duct  34 . The result is a finished or semi-finished component with the secondary material partially encapsulating the primary material. The upper exposed (and perhaps all of the) surface of the upper section may then be roughened and coated with a thin micron layer of Ti, etc. as desired. 
         [0039]      FIG. 20  illustrates a artificial intervertebral spacer  36  for placement in the space between adjacent vertebral bodies which has been vacated by a failed natural disc. The spacer is made of a relatively soft material, e.g., PEEK or CFRPEEK, having a flexural modulus similar to that of the bone forming the vertebral bodies to be fused. 
         [0040]    The GPa flexural modulus of the soft material, as discussed previously, preferably is within the range of about 4-30 and most preferably about 17±8. The top bone buttressing surface  36  is formed with serrated edges or teeth  36   b  (i.e., providing an uneven surface). The spacer is roughened by an apparent blast media or tumbled in an abrasive media to create a micro surface roughening. 
         [0041]    The spacer is then coated with a thin micron layer (i.e., 0.5 to 3-10 microns) of a material e.g., Ti, calcium phosphate or hydroxyaptite, which will promote bone-on growth without materially modifying the stress transfer characteristic between the soft material and the bone. The spacer is formed with holes  36   d  in the side walls and open central portion  36   e  as shown. The bottom bone buttressing surface (not shown) will be similar or identical to the top surface. 
         [0042]    There has been described a novel prosthesis for restoring motion in an appendage or spinal joint which employs a relatively hard-stiff material to form the articulating surfaces while employing a softer less stiff material to form one or both bone buttressing surfaces to inhibit wear at the articulating surfaces while reducing stress shielding at the prosthesis bone interface(s). Preferably the softer material is provided with a thin micron coating of a material which promotes bone attachment without adversely affecting the reduction in stress shield. Additionally, there has been described a novel intervertebral spacer member having the flexural modulus comparable to that of the separated vertebrae while promoting bone attachment to the spacer member. Various modifications to the disclosed invention and perhaps improvements may and probably will occur to those skilled in the art without departing from the spirit and scope of our invention as defined in the appended claims.