Abstract:
A spinal facet fixation device includes a pair of jaws hinged on a common base. A threaded actuator bears against a cam surface on the jaws to draw the points of the jaws together. A surgeon applies the opposed points to respective facets of vertebral elements, and then tightens the actuator to draw the facets together.

Description:
[0001]     This is a continuation of application Ser. No. 10/230140, filed Aug. 29, 2002. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates to the field of orthopedic spinal fusion surgery and particularly to the process of interarticular facet fixation or fusion, serving to stabilize adjacent vertebral elements, thereby facilitating the development of bony union between them and thus long term spinal stability.  
         [0003]     Of all animals possessing a backbone, human beings are the only creatures who remain upright for significant periods of time. From an evolutionary standpoint, this erect posture has conferred a number of strategic benefits, not the least of which is freeing the upper limbs for purposes other than locomotion. From an anthropologic standpoint, it is also evident that this unique evolutionary adaption is a relatively recent change and as such has not benefitted from natural selection as much as have backbones held in the horizontal attitude. As a result, the stresses acting upon the human backbone (or “vertebral column”), are unique in many senses, and result in a variety of problems or disease states that are peculiar to the human species.  
         [0004]     The human vertebral column is essentially a tower of bones held upright by fibrous bands called ligaments and contractile elements called muscles. There are seven bones in the neck or cervical region, twelve in the chest or thoracic region, and five in the low back or lumbar region. There are also five bones in the pelvis or sacral region which are normally fused together and form the back part of the pelvis. This column of bones is critical for protecting the delicate spinal cord and nerves, and for providing structural support for the entire body.  
         [0005]     Between the vertebral bones themselves exist soft tissue structures—discs—composed of fibrous tissue and cartilage which are compressible and at as shock absorbers for sudden downward forces on the upright column. More importantly, the discs allow the bones to move independently of each other to permit functional mobility of the column of spinal vertebrae. Unfortunately, the repetitive forces which act on these intervertebral discs during repetitive day-to-day activities of bending, lifting and twisting cause them to break down or degenerate over time.  
         [0006]     Presumably because of humans&#39; posture, their intervertebral discs have a high propensity to degenerate. Overt trauma, or covert trauma occurring in the course of repetitive activities disproportionately affect more highly mobile areas of the spine. Disruption of a disc&#39;s internal architecture leads to bulging, herniation or protrusion of pieces of the disc and eventual disc space collapse. Resulting mechanical and even chemical irritation of surrounding neural elements (spinal cord and nerves) cause pain, attended by varying degrees of disability. In addition, loss of disc space height relaxes tension on the longitudinal spinal ligaments, thereby contributing to varying degrees of spinal instability.  
         [0007]     While various types of spinal fusion operations have been developed, most procedures involving the articular facets have focused either on the passive grafting of bone between facet surfaces denuded of their synovium, or mechanical fixation of the facet joint with a simple screw. In the former case, additional instrumented fixation of the spine is required to prevent dislodgement of the bone grafts from between the articular surfaces of the joint and in the latter case, the procedure is largely adjunctive since its long term success is usually dependent upon bony union occurring elsewhere between the adjacent vertebral elements being fused, i.e., interbody or inter-transverse postero-lateral fusions.  
       SUMMARY OF THE INVENTION  
       [0008]     An object of this invention to provide for a facet fixation device that can be utilized either directly in a stand alone facet fusion procedure or as an adjunctive fixator to be utilized when other forms of spinal fusion are employed, e.g., as back up for an anterior fusion. It is also the object of this invention to provide for deployment of the device either radiographically or through endoscopically assisted minimally invasive approaches.  
         [0009]     To achieve these objectives, the invention provides a device having opposable jaws bearing teeth or pointed tips to grasp, clasp, crimp or hold the articular surfaces of a single facet joint, thereby immobilizing the joint. The resultant inhibition of mobility serves to facilitate bony union or fusion of the involved spinal elements either directly at the facet joint or at some other chose point between the involved vertebral segments.  
         [0010]     The clasping action of the opposable jaws is achieved by a screw- or ratchet-type mechanism that allows for varying degrees of opposition while simultaneously inhibiting unwarranted or undesirable separation or expansion of component elements. In the simplest version, a metal structure approximating the function of a staple can be crimped together to achieve fixation of the facet joint.  
         [0011]     In primary facet fusion, the device is to be applied after a wafer of bone has been placed between the articular surfaces of the facet joint suitably prepared by decortication. As an adjunctive fixator, the device may be applied radiographically or endoscopically to an intact facet joint thereby inhibiting movement at the joint site until fusion is achieved elsewhere.  
         [0012]     In either scenario, the salient feature of the device is the opposable nature of the component elements that function as jaws to bite and hold the separate articular components of the facet joint thereby serving to immobilize them. The jaws of the device in turn have pointed tips or teeth that engage the cortical surfaces of the joint as the jaws are mechanically closed. Unidirectional closure of the fixation device is achieved either through a screw or a ratchet mechanism which prevents opening of the jaws or disengagement of the teeth once the desired degree of crimping has been achieved. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     In the accompanying drawings,  
         [0014]      FIG. 1  is a top plan view of a spinal facet fixation device embodying the invention;  
         [0015]      FIG. 2  is a front elevation thereof; and  
         [0016]      FIG. 3  is a side elevation thereof.  
         [0017]      FIG. 4  is a top plan view of a second form of the invention;  
         [0018]      FIG. 5  is a front elevation thereof; and  
         [0019]      FIG. 6  is a side elevation thereof.  
         [0020]      FIG. 7  is a top plan view of a third form of the invention;  
         [0021]      FIG. 8  is a front elevation thereof; and  
         [0022]      FIG. 9  is a side elevation thereof.  
         [0023]      FIGS. 10 and 11  show a three-finger version of the device shown in  FIGS. 1-3 .  
         [0024]      FIGS. 12 and 13  show the device of  FIG. 1  being applied to hold superior and inferior articular facets together.  
         [0025]      FIGS. 14 and 15  show, respectively, a spinal facet staple, and the staple being crimped over a pair of spinal facets. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]     The spinal facet fixation device shown in  FIGS. 1-3  comprises a base member  10  having a threaded post  12  affixed at its center and extending perpendicularly therefrom. A nut  16  having an integral or captive washer  18  thereon is threaded onto the post.  
         [0027]     A pair of pivot pins  20 , 22  are affixed to the bottom of the base, equally offset from the center. The ends of the pins fit within holes (not shown) formed in respective jaws  24 , 26 . Each of the jaws has one or more curved fingers  30 , each terminating at a pointed tip  32 .  
         [0028]     The upper surface of each jaw has an upwardly protruding cam  34  ( FIG. 3 ) designed to bear against the washer. When the nut  18  is turned clockwise, it advances down the post, and the washer  16 , bearing against the cams  34  on either side, forces the jaws to pivot downward, bringing their pointed tips  32  closer together.  
         [0029]     To draw a facet joint together, a surgeon places the pointed tips of the jaws against neighboring facets ( FIG. 12 ), and then tightens the nut, whereupon the jaws draw the facets more closely together ( FIG. 13 ) and retain them thus.  
         [0030]     An alternative form of the invention is shown in  FIGS. 4-6 . Here, the jaws have inwardly extending cam followers  34 ′ rather than the upwardly protruding cam surface  34  of the first embodiment. The post  12 ′ in this instance has a rounded head  13  at its bottom; its threaded shaft extends through a threaded collar  15  on the base. The threads may be left-handed, if desired, in which case the head moves upward when the nut us turned clockwise, raising the cam followers and levering the fingers downward to grasp the facets. Other variations on the details of the actuating mechanism will occur to those of ordinary skill.  
         [0031]     A third form of the invention is shown in  FIGS. 7-9 . In this case, the jaws are hinged on along an axis by a single pin  20 ″. The threaded actuator and cams have been replaced by a ratchet segment  40  having raked teeth which permit the jaws to be drawn together, but does not permit them to spread apart thereafter. One end of the segment is fixed to the jaw  24 ; the other passes through a slot  42  on the jaw  26  so that its teeth catch against the side of the slot. This type of device is closed with a tool such as forceps.  
         [0032]     The number of fingers on each jaw may be varied, depending on the intended application. In the examples illustrated in  FIGS. 1-9 , each jaw each had two fingers. As an exemplary variation,  FIGS. 10 and 11  show the device of  FIGS. 1- 3 , modified to have only one finger on one of the jaws, two fingers on the other. The exact shape of the fingers, and the geometry of their tips, may also be selected according to preference and intended use.  
         [0033]     The invention can also be practiced with a spinal facet staple  50 , illustrated in  FIGS. 14 and 15 . The staple has a center portion  52  extending between opposed arms  54 , each of which has an elbow  56  subtending an obtuse angle A. The obtuse angles face one another so that the tips  58  are directed along axes which, if extended, would intersect. In use, the staple is placed with a suitable crimping tool (not shown) so that the tips engage neighboring spinal facets. Then the tool is manipulated to apply sufficient inward force to the elbows to crimp (permanently deform) the center portion as illustrated in  FIG. 15 , drawing the facets toward one another, and holding them together after the tool is released.  
         [0034]     Since the invention is subject to modifications and variations, it is intended that the foregoing description and the accompanying drawings shall be interpreted as only illustrative of the invention defined by the following claims.