Abstract:
Provided is an implant comprising: a platform; a deformable pad affixed to the platform and adapted to seat two or more bearings; and the two or more bearings. The deformable pad can have a number of neutral positions matching the number of bearings, with the deformability of the pad adapted to apply force on the bearings any time the bearings are displaced laterally, with the force being in the direction of the corresponding neutral position.

Description:
The present invention relates to an implant that can be used for replacing a spinal disk or a joint between two bones. The implant allows cushioned, dynamic motion of the adjoining vertebrae or bones. 
   Disc implants  1  are used to replace damaged spinal disks and provide the function of stabilizing the adjacent vertebrae  2  (see  FIG. 1 ). The most traditional implants have provided a framework for bone growth that serves to fuse the adjacent vertebrae. There have been attempts to design implants that allow more dynamic, post-surgery movement, such as better accommodating the compression indicated by the arrows in  FIG. 1 . One attempt has used a deformable material sandwiched between two titanium platforms that fuses to the adjacent vertebrae. Other attempts use articulating pivot or ball socket type constructs. These articulating pivot or ball socket type constructs have not provided the dynamism necessary, due either to mechanical or construct failure or poor physiologic bone implant incorporation. Further efforts to make more dynamic implants are reviewed in “Lumbar Artificial Disc Surgery for Chronic Back Pain,” by Jack Zigler, at http://www.spinehealth.com/research/discupdate/artificial/artificial01.html (recited as updated Mar. 15, 2004). 
   The current invention is believed to provide dynamic movement by allowing the construct components to integrate functionally into a coupled spine The spinal implant of the invention is designed to allow the transmission of physiologic stress at the bone-implant interface, while allowing for eventual rigid bone incorporation of the implant. A depressible pad allows for coupled movement with the spinal segments superiorly and inferiorly by allowing for complex motion in multiple planes. The depressible pad allows for dissipation of axial loads while a graded contoured  FIG. 8  design allows for synchronization of movement across the midline with sagittal (midline) and rotational conformity. Symmetric design with graded eccentric contours allows for constrained and limited eccentric motion away from the midline. 
   SUMMARY OF THE INVENTION 
   In one embodiment, the invention provides an implant comprising: a platform; a deformable pad affixed to the platform and adapted to seat two or more bearings; and the two or more bearings. The deformable pad can have a number of neutral positions matching the number of bearings, with the deformability of the pad adapted to apply force on the bearings any time the bearings are displaced laterally, with the force being in the direction of the corresponding neutral position. 
   In another embodiment, the invention provides a method of replacing a spinal disk while providing rotation and flexing of adjacent vertebrae, the method comprising: placing between the vertebrae a bearing bed comprising a platform and a deformable pad affixed to the platform and adapted to seat two or more bearings, the placing made with the titanium platform seated against a first of the two vertebrae; and seating the two or more bearings in the bearing bed. 
   In a further embodiment, the invention provides a method of replacing a joint between two articulating bones comprising: placing between the bones a bearing bed comprising a platform and a deformable pad affixed to the platform and adapted to seat two or more bearings, the placing made with the platform seated against a first of the two bones; placing between the bones a second bearing bed comprising a platform and a deformable pad affixed to the platform and adapted to seat two or more bearings, the placing made with the platform seated against a second of the two bones; and seating the two or more bearings in the bearing bed such that the bearings rest in both bearing beds. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  displays two vertebrae with an artificial implant serving as the disk. 
       FIGS. 2A and 2B  show a perspective and side view of an implant of the invention. 
       FIG. 3  shows a top view of the illustrative implant. 
       FIG. 4  shows an analogous sandwich structure for the implant. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As shown with the exemplary embodiment of  FIGS. 2A and 2B , provided is a platform  20 , deformable pad  30  and, for example, two bearings  40 , the combination providing implant  10 . The platform is placed against a first vertebrae, and the bearings against a second vertebrae. Because the bearings can move within certain boundaries and can rotate, they allow the supported vertebrae to move relative to one another. The deformability of the deformable pad provides that the bearings will be pressed against and seated against the second vertebrae. 
   By use of a difference in compressibility in the deformable pad, optionally in combination with indentations in the deformable pad, the bearings are energy biased to a neutral position. The neutral positions are defined in an x-y axis as illustrated in  FIG. 3 , where the cross marks indicate the neutral positions. This is the position favored when the implant (when used as a spinal implant) is at the neutral axis. When forces on the implant displace the bearings away from the neutral position, the deformable pad is compressed so as to push the bearings towards their neutral position. Thus, as illustrated, displacement D generates a force F of a magnitude related to the size of the displacement. Of course, when force loaded in this way the bearings are typically offset from the neutral position, but nonetheless the pad provides force to bias towards the neutral position as forces shift. Where two bearings are used, the differential compressibility defines a  FIG. 8  that bounds the movement of the bearings in the deformable pad. In  FIG. 3 , pad  30  has top surface  31 . 
   In exemplary embodiment of  FIG. 2 , the bearings can be, for example, 9-15 mm in diameter, distance d can be, for example, 24-30 cm. In certain embodiments, the bearing diameter is from one of the following lower values to one of the following higer values, or between the values. The lower values are 9, 10, 11, 12, 13 or 14 mm. The higher values are 10, 11, 12, 13, 14 or 15 mm. The axis defining d is typically installed along the left lateral side to right lateral side axis of the patient. The depth of the device, anterior to posterior as placed in a patient, can be for example 18-24 cm. These dimensions will vary with, among other things, age, spinal level, or physiology. 
   The arrows in  FIGS. 2A and 2B  illustrate how the implant can dynamically accommodate side-to-side compression with complementary expansion. Thus,  FIGS. 2A and 2B  illustrate how coupled motion asymmetric load will cause the contralateral side to rise in the y-axis and rotate in the z-axis and translate in the x-axis as the ipsilateral side is depressed and loaded. 
   In certain embodiments there is a second combination of deformable pad and platform placed on the other side of the bearings from the first. One can picture a mirror image of the first deformable pad and platform in  FIG. 2A  (though strict symmetry is not required). Hence, we have a sandwich structure of: first platform, first deformable pad, bearings, second deformable pad and second platform. This embodiment can be used for spinal implants, or to replace a joint between two articulating bones, such as a knee or elbow. For example,  FIG. 4  shows a mirror image second deformable pad  132  and platform  122  placed, as described above, relative to the first deformable pad  131  and platform  121 . The pads and platforms of  FIG. 4  are arrayed as in  FIG. 2A , or as the mirror image of the corresponding component of  FIG. 2A . The dyanamicly mobile bearings  140  are illustrated in the same position as in  FIG. 2A . 
   The platform is made of a material that promotes fusion to bone, for instance, as is known in the art, titanium, tantalum, carbon fiber or the like. It will be recognized that two or more materials can be joined to form the platform, with the bone fusion function provided by one or more of the materials and placed appropriately for promoting bone fusion. When used as a disc implant, the platform is typically placed against the lower of the two vertebrae. 
   The base of the platform can be textured to provide one or more of (i) reducing slipping along the bone or (ii) providing a framework for bone growth (which promotes fusion of the platform with the bone) or (iii) providing structures that can be impressed into the bone. 
   The deformable pad can be constructed of a suitable polymer. For example, the deformable pad can be constructed of ultra high molecular weight polyethylene (Spine, Inc., Raynham, Mass.), surgical polyurethane (Zimmer Spine, Minneapolis, Minn.) or the like. As needed, these polymers can be augmented with elastomeric polymers and monomers. The deformable pad can have varying compressibility C such that the value of C is highest at the preferred seating locations for the bearings. Lower compressibility away from this preferred seating acts, when a given bearing moves out of the preferred seating to create pressure pushing the bearing back to a more preferred location. While there can be some indentation or socket depth at the preferred seating location, such depth preferably does not diminish the action of the deformable and resilient pad to push on the bearings and keep them snugly placed against the second vertebrae (or, in some embodiments, a second deformable pad). 
   A gradient of compressibility can be produced, for example, with a three-dimensional polymer printer. Those of skill will recognize how to vary compressibility in an elastomeric polymer with variation in crosslinker, elastomeric monomer, filler or the like. The printing can be of monomers or incomplete polymers, with the final polymer cured after the three-dimensional printing. Or, a stepwise gradient can be constructed by forming a first portion of the pad with least compressible polymer with a insert void, and filling the void with successive inserts of increasing compressibility (analogous to the inserts of nesting dolls). The inserts can be annealed with a compatible monomer mix. Or, an insert void can be cut from the first cured product having the least compressible polymer, and the void filled with a next successive compressible polymer. The process of making and filling voids can be repeated to provide the gradient in compressibility. 
   The bearings are sized and provided in a number such that, from the combination, the second vertebrae is appropriately supported. Where two bearings are used, the bearings will typically be somewhat larger than is needed with three or more bearings. It will be recognized that over time, the bone at the second vertebrae will reshape somewhat consistent with the shape and movement of the bearings. The bearings resist adhering to the second vertebrae due to shape and movement, but can also be constructed of a material selected to resist such adherence. The bearings can be made, for example, of a suitable composite, titanium, or other material suitable for use in a rotatable internal prosthesis. 
   DEFINITIONS 
   The following terms shall have, for the purposes of this application, the respective meanings set forth below. 
   Laterally Offset 
   The spine of a patient has an anterior side, right lateral side (from the perspective of the patient), a posterior side, and a left lateral side. Bearings are “laterally offset” if two or more are such that their centers of mass would be on opposite sides of a hypothetical anterior to posterior line bisecting at the midpoint of the spine, with 25% or less of any of these two bearings overlaps the hypothetical line. 
   Publications and references, including but not limited to patents and patent applications, cited in this specification are herein incorporated by reference in their entirety in the entire portion cited as if each individual publication or reference were specifically and individually indicated to be incorporated by reference herein as being fully set forth. Any patent application to which this application claims priority is also incorporated by reference herein in the manner described above for publications and references. 
   While this invention has been described with an emphasis upon preferred embodiments, it will be obvious to those of ordinary skill in the art that variations in the preferred devices and methods may be used and that it is intended that the invention may be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications encompassed within the spirit and scope of the invention as defined by the claims that follow.