Abstract:
An intervertebral implant includes an upper surface configured for engagement with a first vertebral body, and a lower surface configured for engagement with a second vertebral body. A wall extends between the upper surface and the lower surface, and forms a chamber for containing osteogenic material. At least a portion of the wall is collapsible from a first position associated with a first volume of the chamber to a second position associated with a second volume of the chamber. The second volume is less than the first volume.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to implants for placement into bone recesses, and more specifically to interbodies for dynamically transmitting loads while promoting fusion between bones. 
     BACKGROUND OF THE INVENTION 
     In spinal fusion, two or more vertebrae are joined by additional bone material placed between the vertebrae. Once fusion is complete, the bone material immobilizes the vertebrae. Spinal fusion is used primarily to treat pain caused by abnormal motion of the vertebrae. Anterior lumbar interbody fusion (ALIF) is a spinal fusion technique that can be used for treating degenerative discs from an anterior approach. The anterior approach allows access to the interbody space with minimal damage to the posterior musculature, while allowing full decompression of the diseased disc. During an ALIF procedure, an interbody device is inserted within the intervertebral body space. This interbody is generally composed of PEEK or titanium with a central opening for bone graft material, which is typically an autograft or allograft material. The objective of interbody fusion is to fuse the central graft material to the cranial and caudal endplates, creating a rigid boney union between motion segments. 
     Known interbody designs have a propensity to stress-shield the graft material. That is, the interbodies, or the fasteners used to anchor the interbodies, absorb axial loads during settling of the implant. This has the effect of shielding the graft material from axial loads. Some interbody designs are configured to expand in an axial direction after being implanted to increase the height of the disc space to a desired spacing. This also stress-shields the graft material, and actually removes load from the graft material because the height of graft space expands. 
     SUMMARY OF THE INVENTION 
     In a first exemplary embodiment of the invention, an intervertebral implant includes an upper surface configured for engagement with a first vertebral body, and a lower surface configured for engagement with a second vertebral body. A wall extends between the upper surface and the lower surface. A chamber, which is enclosed within the wall, includes an upper end opening through the upper surface, and a lower end opening through the lower surface. The wall includes a collapsible section between the upper surface and the lower surface. The collapsible section is collapsible from a first position associated with a first volume of the chamber to a second position associated with a second volume of the chamber. The second volume is less than the first volume. 
     In a second exemplary embodiment of the invention, an intervertebral implant includes an upper plate configured for engagement with a first vertebral body, and a lower plate configured for engagement with a second vertebral body. A chamber extends between the upper plate and the lower plate. The chamber contains an osteogenic material under a hydrostatic pressure in the chamber. The upper plate is axially movable toward the lower plate to reduce the volume of the chamber and increase the hydrostatic pressure on the osteogenic material in the chamber. 
     In a third exemplary embodiment of the invention, an intervertebral implant includes a body formed of a shape memory material. The body is deformable in response to temperature from a pre-insertion configuration to a post-insertion configuration. In addition, the body forms a chamber and contains an osteogenic material in the chamber. The chamber has a first volume and exerts a first hydrostatic pressure on the osteogenic material in the pre-insertion configuration. The chamber has a second volume and exerts a second hydrostatic pressure on the osteogenic material in the post-insertion configuration. The second volume is less than the first volume, and the second hydrostatic pressure is greater than the first hydrostatic pressure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following description will be more clearly understood in conjunction with the drawing figures, of which: 
         FIG. 1  is a perspective view of a first exemplary embodiment of an interbody in accordance with the present invention; 
         FIG. 2  is an exploded perspective view of the interbody shown in  FIG. 1 ; 
         FIG. 3  is a bottom view of a superior component of the interbody shown in  FIG. 1 ; 
         FIG. 4  is a top view of an inferior component of the interbody shown in  FIG. 1 ; 
         FIG. 5  is a perspective view of a second exemplary embodiment of an interbody in accordance with the present invention; 
         FIG. 6  is a perspective view of a third exemplary embodiment of an interbody in accordance with the present invention; 
         FIG. 7  is an exploded perspective view of the interbody shown in  FIG. 6 ; 
         FIG. 8A  is an exploded side view of the interbody of  FIG. 6  with a portion cut away to expose an interior component in a first condition; 
         FIG. 8B  is an assembled side view of the interbody of  FIG. 6  with a portion cut away to expose an interior component in a second condition; 
         FIG. 9  is a perspective view of a fourth exemplary embodiment of an interbody with biologic material in accordance with the present invention; 
         FIG. 10A  is a schematic cross-sectional view of the interbody of  FIG. 6 , shown in a first condition; 
         FIG. 10B  is a schematic cross-sectional view of the interbody of  FIG. 6 , shown in a second condition; 
         FIG. 11A  is a schematic cross-sectional view of a fifth exemplary embodiment of an interbody in accordance with the present invention, shown in a first condition; and 
         FIG. 11B  is a schematic cross-sectional view of a fifth exemplary embodiment of an interbody in accordance with the present invention, shown in a second condition. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention. 
     Interbody implants in accordance with preferred embodiments of the invention address a number of interests. One interest is to provide a rigid structure that maintains proper spacing between vertebrae. A second interest is to minimize the pre-implantation height of the interbody, so that the interbody can fit into compressed disc spaces. A third interest is to provide an interbody that provides sufficient space for graft material and promotes fusion of that graft material. Applicants have observed that these three interests frequently compete with one another. Moreover, Applicants have observed that known interbodies fail to balance and satisfy all three interests. Many known interbodies appear to disregard the third interest, namely the interest of promoting fusion of graft material in the implant. This interest is commonly sacrificed in favor of the one of the other competing interests. 
     To promote fusion of the graft material within the interbody, the interbody should allow some load to be maintained on the graft material. A consistent loading on the graft material is important during the fusion process to encourage bone growth in the bone tissue. The importance of maintaining load on graft material is rooted in Wolff&#39;s Law. Under Wolff&#39;s Law, healthy bone will adapt to loads it is placed under, and will remodel itself to become stronger if the loading increases. Conversely, if the loading on a bone is decreased or removed, the bone will gradually become weaker. That is, there is no stimulus for continued remodeling of the bone to maintain bone mass. In the context of spinal fusion, Wolff&#39;s Law holds that applying consistent loading to the graft material promotes fusion. 
     To balance the competing interests described above, the embodiments of the present invention provide structures that dynamically transmit axial load to the graft material during interbody subsidence, while providing a rigid structure to maintain proper disc space height. During subsidence, the bone graft material is confined within the chamber and is compressed under load. As a result, hydrostatic pressure develops in the bone graft material, with pressure bearing on the material from multiple directions, including the axial and radial directions. 
     The preferred interbodies in accordance with the invention include a central chamber filled with osteogenic material. For purposes of this description, “osteogenic material” includes but is not limited to any material that promotes bone growth or healing, including autograft or allograft material, or synthetic graft material. The osteogenic material is maintained under compression to form a solid fusion between the adjacent vertebral bodies. 
     In contrast to interbodies that are designed strictly to expand after insertion into the disk space, preferred interbodies in accordance of the invention include a contraction mechanism that allows the interbodies to contract under load over time, reducing the volume of the chamber containing the osteogenic material. Decreasing the size of the interbody over time promotes fusion of the osteogenic material by applying a constant pressure on the material. The chamber radially encloses the osteogenic material, so that the osteogenic material has no room to expand or migrate during an axial contraction of the interbody. This has the effect of applying a constant pressure both axially and radially around the osteogenic material. Constant application of pressure, or a gradual increase in pressure as the case may be, promotes fusion of the osteogenic material under Wolff&#39;s Law. Because the embodiments of the invention maintain or increase hydrostatic pressure on the osteogenic material, fusion of the bone material is promoted. 
     Contraction mechanisms in accordance with the invention may take one of several forms that allow the interbodies to collapse or shrink with respect to one or more planes of reference. The contraction mechanisms are designed to contract in response to changes in loading on the spine, subsidence of the interbody into the end plates of the vertebrae, resorbing of the osteogenic material, or changes in temperature. As a graft material resorbs into the body, for example, the volume of the material may decrease and no longer be under hydrostatic pressure in the chamber. In such cases, the contraction mechanism allows the interbody to collapse by a controlled amount to reduce the volume of the graft space and maintain constant compression on the graft material. The contraction mechanism can be designed to maintain equilibrium between the osteogenic material&#39;s resistance to compression, and the loads bearing on the interbody. 
     Referring now to  FIGS. 1 and 2 , an implant  10  in accordance with a first exemplary embodiment of the invention is shown. Implant  10  includes an upper plate  20  and a lower plate  30 . Upper plate  20  has an upper surface  22  forming an exterior surface on the implant, and a lower surface  24  forming an interior surface of the implant. Similarly, lower plate  30  has a lower surface  34  forming an exterior surface on the implant, and an upper surface  32  forming an interior surface of the implant. Upper and lower plates  20  and  30  both have ring-shaped bodies that surround hollow interiors. When upper and lower plates  20  and  30  are joined or stacked relative to one another, the hollow interiors align to form a central chamber  50  for containment of an osteogenic material  80 . The superior and inferior end plates adjacent implant  10  form the upper and lower walls of chamber  50 . 
     As noted above, implants in accordance with the present invention include a contraction mechanism that facilitates a controlled rate of implant collapse. Contraction may occur solely in the “axial” direction, represented by axis “A” in  FIG. 2 , the “radial” direction, which is any direction perpendicular to axis “A”, or a contraction on both the axial and radial directions. Implant  10  includes a telescoping contraction mechanism  60  that permits upper plate  20  to collapse axially toward lower plate  30 . A plug or shaft  62  extends from lower surface  24  of upper plate  20 . Lower plate  30  includes a socket  66  that receives the shaft  62  during contraction of implant  10 . Shaft  62  is generally cylindrical and forms a bore  63 . Bore  63  and socket  66  collectively form part of chamber  50  for containing osteogenic material  80 . 
     Shaft  62  is configured to slide telescopically into socket  66  during contraction of implant  10 . As upper and lower plates  20  and  30  collapse into one another, the volume in chamber  50  decreases. In this arrangement, contraction mechanism  60  is operable to reduce the volume of chamber  50  over time and maintain compression on osteogenic material  80 . In the preferred embodiment, implant  10  includes a mechanism for limiting relative rotation between upper and lower plates  20  and  30 . Referring to  FIGS. 2 and 3 , for example, shaft  62  includes a pair of lobes  64 . Lobes  64  mate with a pair of diametrically opposed notches  68  in socket  66 , shown in  FIGS. 2 and 4 . Notches  68  telescopically receive lobes  64  as shaft  62  enters the socket  66 . The sliding engagement between lobes  64  and notches  68  maintains radial alignment between upper and lower plates  20  and  30 , and substantially prevents rotation of one plate relative to the other to stabilize implant  10 . 
     Interbodies in accordance with the invention preferably include surfacing to promote engagement with end plates of vertebral bodies. Referring to  FIG. 2 , for example, upper plate  20  includes a plurality of ridges  23  and lower plate  30  includes a similar plurality of ridges  33 . Upper and lower plates  20 ,  30  are anchored into adjacent vertebrae with a plurality of bone screws  90 . It will be understood, however, that a number of fastener types may be used to anchor the plates, including a variety of screw sizes and configurations. 
     Referring now to  FIG. 5 , an interbody  110  is shown in accordance with an alternative embodiment of the invention. Interbody  110  includes a one-piece body  115  having an upper plate section  120  conjoined with a lower plate section  130 . Body  115  forms a central chamber  150  for containing an osteogenic material. Contraction mechanisms  160  are provided in the walls of body  115  to allow upper plate  120  and lower plate  130  to be collapsible in an axial direction relative to one another. Each contraction mechanism  160  includes a wall section with a large aperture  164  and an elastic member  168  contained in the aperture. Apertures  164  form thinned sections in upper and lower plates  120  and  130  that deflect in response to axial load. In this configuration, upper and lower plates  120 ,  130  are permitted to collapse axially relative to one another when subject to axial loads. Elastic members  168  provide limited resistance to contraction and absorb some of the axial load. Apertures  164  are separated from adjacent fastener holes or other apertures by hinge portions  166  that allow the plates  120 ,  130  to collapse. 
     Elastic members  168  provide a further benefit by absorbing some of the compressive load and protecting against end plate failure. That is, each elastic member  168  counteracts the compressive force and reduces the total net force on the osteogenic material and reaction force on the end plates. Elastic members  168  further allow interbody  110  to self-distract after insertion into the disc space. Distraction may occur by mechanical expansion of the elastic members, or by thermal expansion in the case where the elastic members are formed of shape memory materials. 
     Referring now to  FIGS. 6-8B , an interbody  210  is shown in accordance with another alternative embodiment of the invention. Interbody  210  includes an upper plate  220  telescopically received in a lower plate  230 . Upper and lower plates  220 ,  230  form generally rectangular ring bodies with open center areas that collectively form a chamber  250  for osteogenic material. Interbody  210  further includes contraction mechanisms  260  in the upper and lower plates  220 ,  230 . Lower plate  230  has three hollow sidewalls  232   a ,  232   b ,  232   c , each having a hollow socket  264  with one or more oval-shaped spring members  266  in each socket. A fourth sidewall  232   d , which represents the anterior side of interbody  210  after insertion, forms a large tab  236 . Upper plate  220  has three sidewalls  222   a ,  222   b ,  222   c  with plug extensions  262 . A fourth sidewall  222   d  has a recess  226  that receives tab  236  of lower plate  230 . Plug extensions  262  of sidewalls  222   a ,  222   b ,  222   c  are telescopically received in sidewalls  232   a ,  232   b ,  232   c , respectively. In this arrangement, upper and lower plates  220 ,  230  are permitted to collapse in an axial direction relative to one another. Spring members  266  provide a limited amount of resistance to axial compression so that osteogenic material  280  in chamber  250  may be shielded from some of the axial load during collapse. 
     Interbody  210  is configured to be compressed to a thin profile as shown in  FIG. 8B  to permit the interbody to be inserted into the disc space. After interbody  210  is inserted into the disc space, the interbody is configured to expand or self-distract. Interbody  210  can be compressed by applying axial pressure on upper plate  220  to advance the upper plate into lower plate  230  and compress elastic members  266 . To facilitate compression, elastic members  266  may be formed of shape memory material that is inserted into the disc space at a reduced temperature, and subsequently heated to expand the implant. Elastic members  266  may be expanded in response to body temperature or external heat applied to the elastic members. Interbody  210  self-distracts as elastic members  266  expand. After elastic members  266  are fully expanded, they remain flexible to adjust to changes in load on the interbody. As axial load increases, elastic members  266  flex under load, allowing upper plate  220  to collapse into lower plate. Elastic members  266  absorb some of the load, while allowing some of the load to be applied to osteogenic material in chamber  250 . The height of chamber  250  decreases by an amount corresponding to the amount of collapse. The interior walls of lower plate  230  remain stationary, so that the volume of chamber  250  decreases as upper plate  220  collapses into lower plate  230 . The stationary walls of lower plate  230  confine the osteogenic material and prevent lateral displacement of the material. In this arrangement, collapse of the upper plate  220  into lower plate  230  increases hydrostatic pressure in the graft chamber. The geometry and material of elastic members  266  may be selected to permit a desired range of collapse and increase in hydrostatic pressure. 
     Referring now  FIG. 9 , another exemplary interbody  310  is shown in accordance with the present invention. Interbody  310  includes an annular body  320  that forms an inner wall  340  surrounding a central chamber  350 . Chamber  350  contains an osteogenic material  380 . Interbody  310  further includes a contraction mechanism provided by a shape memory polymer  322  in annular body  320 . Shape memory polymer  322  is designed to contract over time to create hydrostatic pressure in the chamber  350 . Annular body  320  may be formed to contract strictly in response to time, the transfer of heat, or both. 
     Annular body  320  may be designed to contract in the axial direction, radial direction, or a combination of both directions to apply and maintain hydrostatic pressure to osteogenic material  380  in chamber  350 . Referring now  FIGS. 10A and 10B , interbody  310  is configured to contract in both the axial and radial directions over time.  FIG. 10A  shows interbody  310  in an intraoperative state, and  FIG. 10B  shows the same interbody in a post-settling state. Interbody  310  contracts both axially and radially during settling, decreasing the volume of chamber  350 . Osteogenic material  380  is confined between the adjoining vertebral bodies and within annular body  320 . As a result, hydrostatic pressure in chamber  350  increases in response to contraction of interbody  310 . 
     In some circumstances, it may be desirable to provide an interbody that contracts only in the radial direction to apply hydrostatic pressure in the radial direction. Referring now  FIGS. 11A and 11B , another exemplary interbody  410  is shown in accordance with the present invention that contracts only in the radial direction. Interbody  410  includes an annular body  420  containing a shape memory polymer  422 . 
     Although the embodiments described above are discussed with specific examples of contraction mechanisms, including elastic members and shape memory polymers, a number of materials may be used to allow the interbody to change from a desired pre-implantation configuration to a post-implantation configuration. As noted above, the interbody may include a shape memory material, such as a shape memory metal, ceramic or polymer, that is inserted into a disc space or other bone recess in a pre-implantation shape, and then activated into a post-implantation shape. A number of shape memory materials, many of which may be used in accordance with the present invention, are described in International Pub. No. WO 2006/108114, the contents of which is incorporated by reference in its entirety. Interbodies in accordance with the present invention may also include elastomers, mechanical spring members or any other materials that can deform to a desired post-implantation shape. 
     While preferred embodiments of the invention have been shown and described herein, it will be understood that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the spirit of the invention. Accordingly, it is intended that the appended claims cover all such variations as fall within the spirit and scope of the invention.