Load sharing interbody fusion device

Abstract

The present invention is a load sharing intervertebral fusion device that allows for reconstruction of the proper disc space between two vertebral bodies while allowing bone material packed within the fusion device to share loading and stress for enhanced healing. The device includes two sections, an upper and lower section, separated by a bioresorbable spacer. The upper section slides relative to the lower section with the bioresorbable material being placed in load therebetween. The upper section and lower sections are effectively held apart, creating an initial fixed spacer for implantation of the intervertebral disc space. Openings in the implant construct allow for bone graft and bone substitutes to be placed within the implant to allow for fusion through the implant construct. After implantation and over a period of time based on bone resorbtion and remodeling, the spacer resorbs, thereby maintaining load on the bone graft material.

What is claimed is: 
     
         1 . An interbody fusion device comprising:
 body means for spacing apart opposing vertebral surfaces when said body means is implanted therebetween; and   bone graft load sharing means for sharing loads, placed on said body means by the opposing vertebral surfaces, between said body means and bone graft material contained by said body means, wherein said bone graft load sharing means includes height reducing means for decreasing a height of said body means over time and loading both said body means and the bone graft material over time.   
     
     
         2 . The interbody fusion device of  claim 1 , wherein said height reducing means includes a male portion of said body means in sliding and mating engagement with a female portion of said body means defining a chamber therebetween for containing a bioresorbable material therein. 
     
     
         3 . The interbody fusion device of  claim 2 , wherein said body means includes at least two body members, each of said body members including one of said male and female portions, said body members being interconnected by said female and male portions allowing axial movement relative to each of said body portions;
 wherein each of said body members includes end plate surfaces opposite either of said female and male portions for engaging an opposing end plate of a vertebral body;   wherein said bone graft load sharing means further includes at least one bone graft chamber for containing bone graft material therein, said chamber including openings in each of said end plate surfaces allowing for contact of the bone graft material contained therein with the vertebral end plates, said bone graft chamber defining a column extending axially through said body means whereby bone graft material fully packed into said bone graft chamber takes on load when said body means is disposed between the opposing vertebral end plates; and   wherein said body means includes at least one port extending from an external surface of said body member containing said chamber, which contains the bioresorbable material for allowing body fluids to contact and cause resorbtion of said bioresorbable material and effect a continuous decrease in height of said body means effecting a continuous load on the bone graft material contained within said bone graft chambers.   
     
     
         4 . The interbody fusion device of  claim 1 , wherein said height reducing means includes a collapsible chamber within said body means, said collapsible chamber including an open condition for containing bioresorbable material therein and a collapsed condition in which said body means reduces height thereof. 
     
     
         5 . The interbody fusion device of  claim 4 , wherein an insert of resorbable material is disposed in said chamber to initially support said chamber in said open condition. 
     
     
         6 . The interbody fusion device of  claim 1 , wherein said female portion includes wall portions surrounding a floor portion defining an axially extending recess and said male portion end an end surface, said end surface, wall portions, and floor defining said chamber for containing the bioresorbable material. 
     
     
         7 . The interbody fusion device of  claim 1 , wherein said body means includes multiple body portions, each of said body portions including bone graft load sharing means. 
     
     
         8 . A method of spacing apart opposing vertebral end plates by:
 inserting a spacer between opposing vertebral end plates; and   controlling reduction in height of the spacer during a healing process of a reconstructed vertebral space defined by the end plates.   
     
     
         9 . The method of  claim 8 , wherein said controlling step is further defined as collapsing an internal chamber of the spacer to decrease the height of the spacer. 
     
     
         10 . The method of  claim 9 , including the steps of packing the internal chamber with bioresorbable material and bioresorbing the bioresorbable material once the spacer is inserted between the opposing vertebrae. 
     
     
         11 . The method of  claim 10 , including accessing body fluid into and out of the internal chamber to cause resorbtion of the bioresorbable material and loss of the bioresorbable material from the internal chamber to collapse the internal chamber and decrease the height of the spacer. 
     
     
         12 . The method of  claim 11 , including containing a column of bone graft material in the spacer under load sharing conditions with the spacer. 
     
     
         13 . The method of  claim 12 , including completely collapsing the internal chamber if the bone graft material fails to space apart the opposing vertebral end plates and prevent impingement of proximal nerves. 
     
     
         14 . An interbody fusion device comprising:
 a first portion including a first side having a first end plate surface for engaging an end plate of a first vertebral body and a second opposite side having male portion extending therefrom, said male portion including an end surface; and   a second portion including a second end plate surface for engaging an opposing end plate of a second vertebral body, said second portion including a cavity defined by a wall portion surrounding a floor portion for slidingly receiving said male portion therein and entrapping a bioresorbable material between said end surface and said floor portion.   
     
     
         15 . An interbody fusion device comprising:
 piston means for entrapping a bioresorbable material therein and having a predetermined height which reduces as the bioresorbable material resorbs; and   vertebral end plate gripping means for fixedly engaging opposing vertebral end plates when said device is disposed therebetween.   
     
     
         16 . An interbody fusion device comprising:
 a male member including a first end plate surface for engaging an end plate of a first vertebral body and a lower surface; and   a female member including a second end plate surface for engaging an opposing end plate for a second vertebral body, said female member including a cavity having wall portions surrounding a floor portion for slidingly receiving said lower surface therein and entrapping a biosorbable material between said lower surface and said floor portion.   
     
     
         17 . A method of spacing apart opposing vertebral surfaces by:
 entrapping a bioresorbable material within a piston body having a predetermined height;   fixedly engaging the piston between opposing vertebral end plates; and   reducing the height of the piston as the bioresorbable material resorbs.   
     
     
         18 . A method of fusing together opposing intervertebral surfaces by:
 implanting a body between opposing vertebral surfaces;   maintaining spacing between the opposing vertebral surfaces as the opposing vertebral surfaces place a load on the body; and   sharing the load between the body and bone graft material contained by the body.   
     
     
         19 . A method of remodeling vertebral bone by:
 containing a column of bone graft material in a body member having a reducible height while exposing ends of the column;   inserting the body member between opposed vertebral end plates; and   decreasing height of the body member over time to load the bone graft material over time.