Abstract:
A spinal fusion stimulator comprising an interbody fusion cage or other interbody fixation device adapted to be implanted in the intervertebral disc space of a patient&#39;s spine, the interbody fusion cage in the preferred embodiment having a hollow body with internal and external conductive surfaces. The stimulator includes a constant current generator connected to the interbody fusion cage and set to provide a DC current effective to produce a surface current density of at least 1 μA/cm 2  in the interbody fusion cage when implanted.

Description:
BACKGROUND OF THE INVENTION 
     This application relates generally to spinal fusion devices and methods and, more particularly, to spinal fusion devices and methods employing direct current (DC) bone growth stimulation as an adjunct to interbody fusion. 
     Approximately 100,000 lumbar spine fusion surgeries are performed annually in the U.S. on patients with lower back pain due to degenerative disc disease or spondylolisthesis. Adjacent vertebrae are fused together in such surgical procedures, thereby eliminating motion among the spine levels and significantly reducing lower back pain in most patients. Known spinal fusion methods include posterolateral fusion, also known as inter-transverse process fusion because it involves fusion of the transverse processes of adjacent vertebrae, and interbody fusion, which involves fusion of the bodies of adjacent vertebrae. Traditional interbody fusion involves insertion of autograft or allograft (bone graft from the patient&#39;s own body or from another body, respectively) into the intervertebral disc space as a spacer. 
     Interbody fusion is gaining popularity as an alternative to posterolateral fusion. The introduction of threaded titanium interbody fusion cages has made this technique simpler and more common among orthopedists and neurosurgeons. Such a cage is typically a rigid, hollow cylinder with a pattern of holes through its sidewall to allow bone growth into and through the cage. The cage is typically filled with morselized bone to facilitate fusion by bone growth through the cage, which thereby promotes interbody fusion. A number of interbody fusion devices and techniques have been proposed, as exemplified by those described in the following patents: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Patent No. 
                 Inventor 
                 Issue Date 
               
               
                   
                   
               
             
             
               
                   
                 4,501,269 
                 Bagby 
                 February 26, 1985 
               
               
                   
                 4,961,740 
                 Ray 
                 October 9, 1990 
               
               
                   
                 5,015,247 
                 Michelson 
                 May 14, 1991 
               
               
                   
                 5,489,307 
                 Kuslich et al. 
                 February 6, 1996 
               
               
                   
                 5,489,308 
                 Kuslich et al. 
                 February 6, 1996 
               
               
                   
                   
               
             
          
         
       
     
     Fusion success rates greater than 85% at 1-2 year follow-ups have been reported for titanium interbody fusion cages in one-level fusions, whereas the success rates for such cages in two-level fusions have been reported to be approximately 70-80%. Fusion success rates as low as 70% present a serious and expensive problem, since a large portion of patients require revision surgery, which is costly, painful, and disabling. One proposed solution to the problem is to pack interbody fusion cages with synthetic bone growth factors, particularly bone morphogenic proteins, which exhibit the ability to induce bone and/or cartilage formation. There is a safety concern over the potential formation of ectopic bone on or near the spinal cord with the use of potent growth factor products. It is also believed that bone growth factors remain at the fusion site for only hours or at most a few days before diffusing away. 
     Implantable direct current bone growth stimulation has been established as a useful adjunct for posterolateral fusion as well as for the traditional type of interbody fusion involving bone graft alone as the interbody spacer. Spinal fusion stimulators with titanium cathodes are available from Electro-Biology, Inc., the assignee of the present invention, and have been employed with the titanium cathodes placed in contact with bone graft material and the decorticated spine to deliver direct current to the fusion site. A small direct current (typically 20 or 40 μA divided among multiple cathodes) is delivered constantly for six months to enhance bone fusion. However, physicians are cautioned not to allow any metallic part of the stimulator to contact any metal internal fixation device. Bone graft is packed in and around the cathodes to form a fusion mass in which the cathodes are completely embedded. 
     In spite of all known adjuncts for interbody fusion, there remains a need for devices and methods for speeding interbody fusion and increasing fusion success rates. 
     SUMMARY OF THE INVENTION 
     The present invention meets this need and provides significant advantages over prior art devices and methods with a spinal fusion stimulator adapted to supply a constant DC current directly to an interbody fixation device adapted to be implanted in the intervertebral disc space of a patient&#39;s spine. The interbody fixation device functions as the electronegative cathode in addition to providing immediate stabilization and weight-bearing capability. DC current is supplied at a current level effective to produce a surface current density in said interbody fixation device that is effective to enhance osteogenesis. According to one aspect of this invention, a current density of at least 1 μA/cm 2  is produced in the interbody fixation device when implanted. 
     Another aspect of the invention is a method of stimulating spinal fusion that includes implanting an interbody fixation device in the intervertebral disc space of a patient&#39;s spine, and supplying a constant DC current to the interbody fixation device at a level based on the exposed electrically conductive surface area of the interbody fixation device and a predetermined desired surface current density. 
     A general object of the present invention is to provide an improved method and apparatus for promoting osteogenesis in interbody fusion. 
     Another object of the present invention is to employ direct current bone growth stimulation with a direct connection to an interbody fixation device with sufficient surface current density to significantly improve the efficacy of the interbody fusion. 
     These and other objects and advantages of the present invention will be more apparent upon reading the following detailed description in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the preferred embodiment of a spinal fusion stimulator with interbody fusion cages according to the present invention. 
     FIG. 2 is an anterior view of a spinal segment with the interbody fusion cages of FIG. 1 implanted therein. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. 
     With reference to FIG. 1, an implantable DC generator  10  is preferably connected to a pair of titanium interbody fusion cages  12  and  14  via respective insulated lead wires  16  and  18  and connectors  20  and  22 . DC generator  10  may be of the type commercially available from Electro-Biology, Inc., the assignee of the present invention, in its SPF® line of spinal fusion stimulators, except modified to provide a surface area current density appropriate to the interbody fusion cages as will be described. DC generator  10  includes a platinum or iridium oxide surface  24  that serves as an electropositive anode. Lead wires  16  and  18  are conventional lead wires of the type used with EBI SPF® fusion stimulators. Connectors  20  and  22  are designed to be suitable for attachment to any of several titanium interbody fusion cage devices, including, for example, the BAK™ interbody fusion cage available from Spine-Tech, Inc., the Ray cage available from Surgical Dynamics and the Novus threaded fusion cage (TFC) available from Sofamor Danek. Interbody fusion cages appropriate for the present invention are preferably made of either pure titanium or 6Al—4Va titanium alloy, although other suitable biocompatible electrode materials may also be used such as tantalum, platinum and others disclosed in U.S. Pat. No. 5,458,627 to Baranowski et al., which patent is hereby incorporated by reference. If desired, an interbody fusion cage may be preoperatively prepared for use with the present invention by permanent attachment of a connection terminal thereto, e.g., by welding a stud thereto for subsequent connection to a connector on the end of one of the lead wires from the signal generator. Alternatively, an attachable connector, with spring or screw-clamp mechanism, would create a conductive connection to a fixation device. Other types of fixation devices are also contemplated, as will be described herein. 
     The present invention is suitable for interbody fusion from an anterior approach or a posterior approach. For purposes of illustration, however, an anterior lumbar interbody fusion employing the principles of the present invention is shown in FIG. 2, wherein it can be seen that fusion cages  12  and  14  are inserted side-by-side in the intervertebral space  30  between two adjacent vertebrae  32  and  34  having respective vertebral bodies  36  and  38  and pairs of transverse processes  40  and  42  and  44  and  46 . According to the preferred method of implantation, the interbody fusion cages are first implanted into respective evacuated disc spaces and then the two insulated lead wires are attached thereto. The site is prepared and the fusion cages are selected and packed with graft material and then inserted into the disc spaces in a conventional manner, and the DC generator is implanted at an appropriate subcutaneous or intramuscular site, e.g., on the psoas muscle in the case of an anterior lumbar interbody fusion. 
     According to the present invention, the interbody fusion cages function both as weight-bearing mechanical fixation devices and as electronegative bone stimulation electrodes. Electrification of titanium interbody fusion cages by direct delivery of electric current to the cages can speed the rate of bony incorporation of cages and raise success rates, especially for high-risk patients or in fusions involving two or more levels of the lumbar spine. Direct current technology is relatively cost effective and therefore more available to patients. The treatment is highly localized, which eliminates the safety issues related to ossification of the spinal cord. Direct current will have a very strong effect inside the cage, where bony fusion is most important. 
     Important to the osteogenic efficacy of an electrically stimulated interbody fusion cage is the level of current applied to the cage. There is believed to be a therapeutic window or range of surface current densities within which the current density is high enough to be efficacious, i.e., to promote bone growth, but not so high as to be toxic or cause necrosis. It is presently believed that this therapeutic window extends from a current density not less than 1 μA/cm 2  to a current density of approximately 150 μA/cm 2 . While current densities outside this range may be efficacious to some degree, current densities within this range are believed to provide significantly better results. 
     DC generator  10  is preferably designed to be adjustable so as to have the capability of providing sufficient current to generate current densities in the therapeutic window for various sizes of interbody fusion cages. For example, with a BAK™ cage of a nominal implant size of 11 mm (11 mm in diameter and 20 mm in length), the total exposed electrically conductive surface area of the cage is approximately 21.3 cm 2 , and the corresponding current required to be supplied to the cage to provide a current density of 1 μA/cm 2  is approximately 20 μA. Experimental studies have demonstrated enhanced bony ingrowth in a BAK™ cage of this size with a current level of 40 μA, corresponding to a current density of approximately 1.9 μA/cm 2 , and have demonstrated excellent efficacy with a higher current level of 100 μA, corresponding to a current density of approximately 4.7 μA/cm 2 . Such results were obtained after four months of direct current stimulation of the fusion cage in sheep lumbar fusions. Alternatively, the DC generator may be set to deliver current at a fixed level of, for example, 100 μA to a predetermined range of sizes of fusion cages for which the fixed current level is effective to produce a surface current density in the therapeutic window. Electric current according to the present invention may be delivered for a period of anywhere from one to twelve months, although treatment for a period of six months is most preferred. 
     As alluded to above, interbody fixation devices other than cylindrical fusion cages are contemplated, including cages having a square or other cross-section, and other devices that span the intervertebral space. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.