Abstract:
A spinal fusion device for implantation between adjacent vertebrae is formed in the approximate shape of a hollow cube. The device has an upper section and a lower section relatively movable with regard to each other. The sidewalls of the upper section and the lower section terminate in complementary inclined planes so that as the sections move away from each other the height of the device lessens and as the sections move toward each other the height of the device increases. A distractor is placed within the hollow body and contacts the upper and lower sections by a link which is used to adjust the height of the device.

Description:
FIELD OF THE INVENTION 
   This invention relates to the field of orthopedic and neuro-surgery and, more particularly, to implants to be placed between vertebrae in the spine. 
   BACKGROUND OF THE INVENTION 
   Spinal stabilization is one approach to alleviating chronic back pain caused by displaced disk material or excessive movement of individual vertebrae. Conventional stabilization techniques include fusing two or more vertebrae together to circumvent or immobilize the area of excessive movement. Normally, the vertebral disk material which separates the vertebrae is removed and bone graft material is inserted in the space for interbody fusion. In addition to or, in place of, the bone graft material, a spinal implant may be inserted in the intervertebral space. 
   The conventional surgical approach for stabilization has been posteriorly for ease of access to the spine and to avoid interfering with internal organs and tissue. Usually the implant site is prepared to maintain natural lordosis and to accept a certain sized implant within certain pressure limits. This requires considerable time and skill by the surgeon. 
   DESCRIPTION OF THE PRIOR ART 
   U.S. Pat. No. 6,562,074 to Gerber et al issued May 13, 2003 discloses a spinal insert which can be manipulated to adjust the height of the implant through links connected to the upper and lower plates. 
   U.S. Pat. No. 6,120,506 issued Sep. 19, 2000 to Kohrs et al discloses a lordotic implant and a tap for use in preparing the vertebrae. The implant is designed to be inserted between the non-parallel end plates of adjacent vertebrae and maintain the natural lordotic angle of the spine. This is done through the use of a threaded tapered plug inserted in a tapped hole in the direction required by the lordosis of the spine. The implant is hollow and has radial apertures for accommodating bone graft material. 
   U.S. Pat. No. 6,015,436 issued Jan. 18, 2000 to Shoenhoeffer discloses a tubular spinal implant. The implant is hollow and has radial apertures for interbody fusion through bone growth material. The device is placed between adjacent vertebrae with the opposite ends of the tube contacting the opposing vertebrae. The opposite ends are threaded together to form the hollow tube. 
   SUMMARY OF THE INVENTION 
   The spinal fusion device is particularly suited for posterior lumbar implantation. The implant has a main body having upper and a lower sections with mating sidewalls relatively movable along an inclined ramp. The sections form a hollow cube-shaped structure with a common open side. The main body is inserted in an extended thin mode between adjacent vertebrae and a distractor is inserted through the common open side. The distractor is connected to one of the sections by a link which causes one section to move along the inclined ramp of the other section for increasing the height of the implant and engaging the opposing surfaces of adjacent vertebrae. The adjacent vertebrae are forced apart as the height of the implant increases. The spinal fusion device may be used unilaterally or bilaterally. 
   Accordingly, it is an objective of the instant invention to teach a posterior surgical approach for placement of an adjustable spinal implant for interbody fusion allowing the implant to be inserted through a small incision and increased in size in situ. 
   It is another objective of the instant invention to teach a spinal implant which allows the surgeon to provide for lordosis intraoperatively and to distract through the implant. 
   It is a further objective of the instant invention to teach a spinal implant having increased contact area in the disk space. 
   It is yet another objective of the instant invention to teach an implant facilitating interbody fusion through bone graft or an ingrowth-type implant. 
   Other objectives and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of the spinal fusion implant of this invention in the insertion mode; 
       FIG. 2  is a side view of the spinal fusion device in the increased height mode; 
       FIG. 3  is a perspective view of the upper section of the implant; and 
       FIG. 4  is a perspective view of the lower section of the implant. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The spinal fusion device  10  is inserted in the intervertebral space in the insertion mode, shown in  FIG. 1 , to replace damaged, missing or excised disk material. This extended position allows the implant to be inserted in a small intervertebral space without the necessity of excising structurally sound bone. The upper section  11  has a top surface  12  for engaging the end plate of a vertebra and the lower section  13  has a bottom surface  14  for engaging the end plate of an adjacent vertebra. The top surface  12  and the bottom surface  14  are planar to provide a large contact area with each vertebra. Each contact surface has a roughened finish to provide better purchase on the end plates of the vertebrae. As shown, the top and bottom surfaces have a series of lands and grooves  15 ,  16 ,  17  and  18  though other stippled treatment may be employed. Of course, the device may be rotated about its longitudinal axis 180 degrees so that the upper section becomes the lower section and vice versa. 
   The device  10  has two extreme positions and is adjustable infinitely between those positions, eg., in the insertion mode the extended position of the structure has a height  20  approximately the same as the height of one of the sections and a length approximately twice the length of one section, as shown in  FIG. 1 . In the increased height mode, the expanded position, shown in FIG.  2 , the height  19  is the sum of the height of the individual sections and the length is approximately the same as the length of a section. 
   The fusion device  10  may be made of conventional materials used for surgical implants, such as stainless steel and its many different alloys, titanium, and any other metal with the requisite strength and biologically inert properties. Polymeric materials with adequate strength and biological properties may also be used in the construction of the fusion device. 
   The upper section  11  is formed with an end wall  21  a top surface  12  and depending sidewalls  22  and  23 . The sidewalls terminate in an inclined plane  24  which extends from the end wall  21  to the top surface  12 . The top surface  12  has a large aperture  25  therethrough to provide for bone ingrowth. The top surface  12  has a narrower flange  26  extending beyond the sidewalls  22  and  23 . The flange  26  engages the end wall of the lower section  13  to guide the relative movement of the sections maintaining the upstanding sidewalls and the depending sidewalls in alignment. The end wall  21  has a bore  27  with internal threads  28  to cooperate with the threads  41  on the link  40 . The bore may be a blind bore or extend through the end wall  21 . 
   The bottom surface  14  of the lower section  13  has a large aperture  30 , as shown in  FIG. 4 , to facilitate bone ingrowth after implantation. The lower section  13  is a U-shaped channel with opposed upstanding sidewalls  31  and  32  projecting from the bottom surface. The side walls  31  and  32  have a short end  33  and a long end  34 . The sidewalls  31  and  32  terminate in an inclined plane extending from the short end  33  toward the long end  34 . The upstanding walls each have a vertical extension  35  and  36  beyond the end of the inclined plane. A reduced thickness  37  is formed in the vertical extensions  35  and  36  to accommodate the flange  26  as the upper and lower sections move relative to each other. The movement of the flange through the reduced thickness contributes to the alignment of the upper and lower sections as they move relative to each other. 
   The ends of the inclined planes of the upstanding and depending walls are smooth ramps to provide ease in the relative sliding contact between the surfaces. Other embodiments of the complementary surfaces may provide additional or substitute guidance to maintain the upper and lower sections in alignment during movement of the contacting surfaces of the inclined planes, such as, the ends of the inclined planes may be sloped across the thickness of the side walls or a stepped ramp may be used. 
   A distractor  42  is shown in  FIG. 2 . The distractor  42  is dimensioned to be inserted into the interior cavity between the upper section and the lower section of the spinal infusion device  10 , as shown in  FIG. 1 . A plug  43  is dimensioned to be inserted and closes the opening formed in the lower section by the upstanding sidewalls  31 ,  32  ( FIG. 4 ). The upper surface of the plug has an inclined ramp  44  on each side (only one side shown in  FIG. 2 ) to accommodate the inclined plane  24  of the depending walls  22  and  23  of the upper section. The plug  43  has a larger circumferential end plate  45  dimensioned to extend to the outer periphery of the upper and lower sections to make a smooth outer surface. The upper portion  46  of the end plate  45  engages the end of the flange  26  to act as a stop for relative movement. Extending from the end plug into the cavity of the hollow structure  10  is the body  47  of the distractor  42 . The body is connected to the end plug by two rails (not shown) leaving a central area open for bone ingrowth. The end plug  43  and the body  47  each have a bore  61  and  62 , respectively. These bores are aligned with the bore  27  in the end wall of the upper section  11 , as shown in  FIG. 2 . The bore  61  has a larger countersunk bore  63  in the end plate  45  ( FIG. 1 ). 
   As shown in  FIG. 1 , the spinal fusion device is inserted in the disk space between adjacent vertebrae in the extended position with the top surface in contact with the end plate of one vertebra and the bottom surface in contact with the end plate of an adjacent vertebra. A link  40  traverses the bores  61 ,  62  and is threaded in bore  27 . The surgeon turns the link  40  causing the upper and lower sections to move along the complementary inclined plane to shorten the fusion device and increase the distance between the end plates of the adjacent vertebrae. The adjustment may continue until the flange  26  contacts the end plate  46 . At this time, the link may be removed and replaced by a bolt of sufficient length to tighten the upper and lower sections together. While a threaded link and bore are illustrated for adjusting the device, other mechanisms may be used for generating the force to move the sections. For example, a pneumatic, hydraulic or mechanical puller may be used against the end plate to apply linear force to the link rather than torque. And the end wall may have a nipple rather than a bore. 
   A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiment but only by the scope of the appended claims.