Abstract:
A method and apparatus for fixation and correction of spondylolisthesis anteriorly includes a disk cage which is inserted into the space between adjacent vertebrae, a drill guide for guiding and aligning a drill bit to angle an opening anteriorly between adjacent vertebrae, a distractor for temporarily aligning the vertebrae to position a disk cage and an elongated hollow screw positioned in said drilled opening through one vertebra, said disk cage and into said adjacent vertebra.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method and apparatus for fixation and reduction of spondylolisthesis, e.g., misalignment of the vertebrae comprising the spinal column. More specifically, the present invention relates to a disk cage positioned in the disk space between affected vertebra, and a screw through said vertebra, disk cage and other vertebra and a method utilizing that apparatus. 
     2. Description of the Related Art 
     Treatments for conditions involving subluxation of one vertebrae upon another, resulting in misalignment of the spinal column, involve the use of screws which extend through a plate and which are tightened to draw the misaligned vertebrae back into alignment. U.S. Pat. No. 5,531,747 to Ray (hereinafter &#34;Ray&#34;) is an example of a plate and screw system for treating spondylolisthesis. Other approaches include insertion of an implant into the disk space between misaligned vertebrae such as shown in U.S. Pat. No. 5,601,556 to Pisharodi (hereinafter &#34;Pisharodi&#34;). The Ray approach has its limitations as discussed in Pisharodi. That patent has a disadvantage in requiring removal of the disk and having multiple steps, that is, two implants are inserted into the disk space only to be substituted later with monolithic implants, Schafer et al., in U.S. Pat. No. 5,800,547 discloses an angled block which is implanted into an intervertebral space. The block includes multiple openings through which securing pins extend after they are pushed into position by a camming mechanism inserted into another opening in the device. 
     There is, therefore, a need for an improved method of treatment of conditions involving misalignment of the vertebrae of the spinal column, or spondylolisthesis, and it is a principal object of the present invention to provide such a method and an apparatus for use in connection with that method 
     The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is &#34;prior art&#34; with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 C.F.R. §1.56(a) exists. 
     SUMMARY OF THE INVENTION 
     The invention provides a means for correcting and fixing spondylolisthesis anteriorly. It involves using a distracting tool against adjacent vertebra to temporarily re-align the vertebrae and to open the disk space to allow insertion of a disk cage therewithin. The disk cage is then rotated to align an angled guide hole to the adjacent vertebrae and to space the vertebrae as desired. A drill guide is placed into the disk cage to provide a guide for accurately drilling a hole through one vertebra, through the angled guide hole of the disk cage and into the adjacent vertebra. A hollow, threaded screw is then threaded into the drill hole thus formed, connecting said vertebrae together through the disk cage. The disk cage and screw may be further bonded to the fused vertebrae by application of bone ingrowth medium into the disk cage and hollow screw to allow bone and fibrous fusions to form. The implant cross-links the disk cage and screw together to stabilize the spinal motion segment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A detailed description of the invention is hereafter described with specific reference being made to the drawings in which: 
     FIG. 1 is a perspective view of the disk cage of the invention; 
     FIG. 2 is a cross-sectional view of the disk cage through line 2--2 of FIG. 1; 
     FIG. 3 is a cross-sectional view of the disk cage through line 3--3 of FIG. 1; 
     FIG. 4 is an enlarged partial view of a corner of the cage of the invention; 
     FIG. 5 is a perspective view of the end of an insertion device for the disk cage; 
     FIG. 6 is a cross-sectional view of the insertion device of FIG. 5 inserted into a disk cage; 
     FIG. 7 is a perspective view of the screw alignment device of the invention; 
     FIG. 8 is a perspective view of a distracting tool for aligning adjacent vertebrae; 
     FIG. 9 shows the distracting tool being used to lift and distract one vertebra from an adjacent vertebra; 
     FIG. 10 shows the vertebrae properly aligned from FIG. 9 and reaming of the disk space; 
     FIG. 11 shows a threaded, hollow, screw for use in securing adjacent vertebrae together through a disk cage; 
     FIG. 12 shows the disk cage being inserted and rotated to align the screw holes; 
     FIG. 13 shows a screw alignment device attached to a disk cage to guide a drill bit; and 
     FIG. 14 shows the vertebrae held in proper alignment via a disk cage and a screw passing through a vertebra, disk cage and into the adjacent disk. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the Figures, it will be seen that a typical misalignment, or spondylolisthesis of vertebrae occurs when one vertebra slides forward relative to another as best shown in FIG. 9. Typically, the L5 vertebra is pushed anteriorly from the desired position. This causes severe pain which can be alleviated by either fixation of the vertebrae or with correction of the alignment and fixation. 
     FIGS. 1-4 show a disk cage 20 having one or more angled guide holes 22. As shown, the disk cage 20 preferably is substantially hollow, having an opening 24 therethrough, as well as numerous side openings 26 which increase the porosity of the device such that bone ingrowth may occur. As later described, opening 24 is required to receive tool 80 and does not need to pass through cage 20. As shown, disk cage 20 includes an upper surface 28 and a lower surface 30 through which the angled guide holes 22 are positioned. In addition, surfaces 28, 30 may include threads 32 or other surface treatment to cause the surfaces to grip better to bone. FIG. 4 shows a corner of cage 20 showing that the threads 32 may stop at the intersection of the sides 34, 36 at a notch 38. Cage 20 includes a ventral end 23 and a dorsal end 25. The opening 24 which functions as the tool 80 receiving mechanism is in the ventral end 23. 
     In order to use the disk cage, the disk 18 must be reamed out to accommodate the size of the disk cage 20. FIGS. 8-10 show an instrument 40 used to both lift the adjacent vertebrae apart for reaming and to reduce the misalignment. Instrument 40 includes a pair of linked arms 42, 44 having free ends 46, 48 which each include a sharp projection 50, 52 which may hold to the bone of a vertebra. Instrument 40 further includes a rod 54 attached to the linked arms 42, 44 through opening 56 and to a slide member 60 which allows the slide member 60 to move up and down along the length of the linked arms. Instrument 40 includes a depth adjustment mechanism 62 which fixes the position of the slide member 60 to the linked arms 42, 44 at the position desired. A knob 64 may be held to control the instrument during adjustment. The slide member 60 includes a pair of spaced push members 66, 68 projecting opposite to the sharp projections 50, 52. 
     FIG. 9 shows the instrument 40 being used to increase the gap between adjacent vertebra 14, 16 and to correct the spondylolisthesis. As shown, the instrument is inserted on either side of the disk 18 and the sharp projections 50, 52 dig into the bone of vertebra 16. The slide member 60 is adjusted down the linked arms 42, 44 by manipulating the adjustment mechanism 62 until the push members 66, 68 are firmly against the vertebra 14. The instrument 40 may then be lifted up, causing the disk space to increase and for the spondylolisthesis to be reduced. While held up, the surgeon uses a reaming tool 70 as shown in FIG. 10 to prepare the disk space for a disk cage 20. The unique configuration of instrument 40 allows the surgeon to use a reamer while the instrument distracts the vertebrae. 
     Once the disk space is prepared, the disk cage 20 is inserted. FIGS. 5, 6 and 12 show that insertion may utilize a tool 80 which includes a head 82 that may temporarily lock into opening 24 in the ventral end 23 of the disk cage 20. The tool 80 allows the surgeon to obtain leverage in inserting disk cage 20. Tool head 82 may include spring loaded, `recessed bearings 84 which may lock into side openings 26 as is common in socket wrenches. The disk cage 20 as shown has a narrower diameter across sides 34, 36 than across upper and lower surfaces 28, 30. When so configured, the disk cage may be inserted into the disk space with a side 34, 36 up, making insertion easier. The surgeon may then simply rotate the tool 80 to twist the disk cage 20 such that the upper and lower surfaces 28, 30 engage the bone of the vertebrae and the angled guide openings 22 are positioned as shown in FIGS. 13 and 14. This allows the tool 80 to be used as a distracting tool, but the surgeon determines whether the twist is needed or whether the disk cage 20 may simply be inserted without a need for rotation. 
     Once in place, the disk cage is also used in conjunction with a drill guide 90 as shown in FIGS. 7 and 13 to guide the formation of a hole 92 through the vertebra 14, 16. Drill guide 90 includes a head 94 configured as head 82 to fasten to disk cage 20 and an angled guide tube 96 which is angled such that it will guide a drill bit 98 accurately through angled guide holes 22 of the disk cage 20. Hole 92 as drilled may be threaded or unthreaded. Drill guide 90 is then removed and a long, hollow screw 100 is threaded into hole 92 to fix the vertebrae 14, 16 together via the disk cage 20. 
     As shown in FIG. 11, screw 100 is preferably hollow and includes threads 102 to engage with the bone. A variety of bone growth inducing substances may be added to the disk cage 20 and screw 100 to foster bone ingrowth therethrough to make the fusion stronger. The screw 100 as shown has a hollow bore 104 and side openings 106 for bone ingrowth. 
     As best shown in FIG. 14, the combination of the disk cage 20 and the screw 100 fuses the vertebrae together with very few parts in a stable, solid fusion that keeps the correction of the spondylolisthesis as desired. The tools and instruments described function to ease the insertion of the combined system which reduces and fixes the corrected alignment of the vertebrae. 
     While this invention may be embodied in many different forms, there are shown in the drawings and described in detail herein specific preferred embodiments of the invention. The present disclosure is an exemplification of the principles of the invention and is not intended to limit the invention to the particular embodiments illustrated. 
     This completes the description of the preferred and alternate embodiments of the invention. Those skilled in the art may recognize other equivalents to the specific embodiment described herein which equivalents are intended to be encompassed by the claims attached hereto.