Patent Publication Number: US-2010114104-A1

Title: System for aligning a meterial-abrading tool relative to an intervertebral-disc compartment

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This applications claims benefit of U.S. provisional application U.S. Ser. No. 60/696,887 filed Jul. 6, 2005. The full disclosure of this earlier application is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a system for aligning a material-abrading tool, for example a milling cutter, relative to an intervertebral-disc compartment into which an intervertebral-disc prosthesis is to be inserted in the course of an intervertebral-disc operation. 
     2. Description of the State of the Art 
     From US 2002/0161446A1 a device is known in which a material-abrading tool is guided on a reference frame. The reference frame is fastened laterally to an operating table and extends over and beyond the patient. The intervertebral-disc compartment is firstly measured out with the aid of imaging methods, with the vertical (i.e. the direction of the gravity force) being a reference direction. For this purpose the intervertebral-disc compartment is X-rayed from the side, so that the lordosis of the vertebral column relative to the vertical direction can be surveyed. 
     With the aid of a plumb bob or similar, the reference frame is aligned above the stationary operating table. The vertical consequently represents the common reference system for the surveying of the intervertebral-disc compartment, on the one hand, and for the alignment of the reference frame, on the other hand. 
     However, it turned out that with the use of such reference frames, even after intervertebral-disc operations during which no complications occurred, complaints occur time and again that are to be ascribed to incorrect loadings of the vertebral column. 
     SUMMARY OF THE INVENTION 
     For this reason, it is an object of the present invention to provide a system for aligning a material-abrading tool relative to an intervertebral-disc compartment, with which the risk of complaints of such a type is reduced. 
     According to the invention, this object is achieved by means of a system comprising:
         a) an operating table having a longitudinal axis, wherein said table is configured for being tilted about the longitudinal axis,   b) an image-recording device for recording lateral images of the intervertebral-disc compartment,   c) a reference frame,
           which is arranged above the operating table,   on which the tool is capable of being fastened in a defined relative position with respect to the reference frame, and   which is configured for being tilted about a tilt axis which runs parallel to the longitudinal axis of the operating table, and   
           d) an alignment element which is rigidly connected to the reference frame and is arranged such that it is detectable, together with the intervertebral-disc compartment, by the image-recording device.       

     The inventor has discovered that the vertebral column is frequently not only curved towards the front or towards the rear, but may also be twisted in itself along its longitudinal direction. Such a twisting may have pathological causes, but it may also arise in the course of the exposing of a ventral access if tissue connected to the vertebrae is displaced to one side. If such a twisting of individual vertebrae or of several vertebrae is left out of account, the material-abrading tool will be set incorrectly, so that the intervertebral-disc prosthesis is also later received in the intervertebral-disc compartment in a position that is twisted about the longitudinal axis of the adjacent vertebrae in relation to the nominal position. 
     This fact is taken into account by virtue of the fact that the reference frame is not aligned parallel to the surface of the operating table but is aligned parallel to the vertebral plane of the vertebrae adjacent the intervertebral-disc compartment. The alignment is carried out based on the laterally recorded images. The images or, what is equivalent, the image-recording device thus constitute(s) the common reference system both for the alignment of the vertebral column by tilting of the operating table and for the alignment of the reference frame. In order to orient the intervertebral-disc compartment in relation to this reference system, the operating table is, for example, tilted in such a way that the two transverse processes of a vertebra delimiting the intervertebral-disc compartment substantially overlap one another in the images recorded from the side. 
     An alignment of the reference frame in the images is possible, for example, if the reference frame is rigidly connected to an alignment element that is detected by the images. The reference frame is then tilted about an axis parallel to the longitudinal axis of the operating table until the alignment element appears on the recorded images in the form of a predetermined pattern. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawing in which: 
         FIG. 1  in a greatly schematised front view of a system according to the invention prior to the tilting of the operating table; 
         FIG. 2  the system shown in  FIG. 1  after the tilting of the operating table; 
         FIG. 3  the system shown in  FIG. 1  after the aligning of the reference frame in relation to a fluoroscope; 
         FIG. 4  a simplified perspective representation of a part of an alignment element; 
         FIG. 5  a longitudinal section through the part shown in  FIG. 4 ; 
         FIG. 6   a  an image recorded by the fluoroscope of the alignment element shown in  FIGS. 4 and 5  if said element is aligned with the direction of propagation of the X-rays; 
         FIG. 6   b  the image recorded by the fluoroscope of the alignment element shown in  FIGS. 4 and 5  if said element is not aligned with the direction of propagation of the X-rays. 
     
    
    
     DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS 
       FIG. 1  shows, in a considerably schematised front view, a system  10  for aligning a material-abrading tool relative to an intervertebral-disc compartment in a configuration prior to the start of an intervertebral-disc operation. 
     The system  10  comprises an operating table  12  which is capable of being tilted about a longitudinal axis indicated by  14 . On the longitudinal sides of the operating table  12  a reference frame  20  is fastened above the operating table  12  via height-adjustable lateral supports  16 ,  18 . Similar height-adjustable lateral supports are arranged further down towards the rear side of the operating table  12  (not discernible in  FIG. 1 ). The reference frame  20  is configured for fastening a material-abrading tool, for example a milling cutter, that is needed during the intervertebral-disc operation, and for positioning this tool exactly in relation to the vertebral column. 
     Along the longitudinal sides of the operating table  12  a fluoroscope  22  is indicated which comprises an X-ray source  24  arranged on one longitudinal side of the operating table  12  and an image recorder  26  arranged on the opposite longitudinal side. The coverage area of the fluoroscope  22  is indicated by dotted lines  28 . The fluoroscope  22  as such does not have to be aligned. 
     Rigidly fastened to the reference frame  20  is an alignment element  30  which extends into the coverage area of the fluoroscope  22 . 
     Indicated by  32  is a patient who is lying on his back on the operating table  12 ; the outline of his head is indicated by  34 . A vertebra  36  which delimits an intervertebral-disc compartment of the patient  32 , into which an intervertebral-disc prosthesis is to be surgically inserted, has a vertebral plane indicated by  38  which normally extends parallel to the back of the patient  32  and therefore also parallel to the reclining surface of the operating table  12 . Occasionally, however, the vertebral plane  38  is twisted by a few degrees about the longitudinal axis of the vertebral column. The tilt angle a is indicated in  FIG. 1  relative to a horizontal plane  40 . 
     The twisting of the vertebra  36  (and possibly of further neighboring vertebrae) shown in  FIG. 1  may occur as a result of anatomical changes to the vertebral column of the patient  32 . However, such a twisting may also arise only during the surgical intervention. In the course of preparing a ventral access to the intervertebral-disc compartment it may happen—as a result of the displacement of surrounding tissue, for example—that the vertebra  36  is twisted about the longitudinal axis of the vertebral column, and the vertebral plane  38  consequently tilts relative to the horizontal plane  40 . 
     If in the case of a vertebra that has been twisted in such a manner an intervertebral-disc prosthesis were now to be inserted vertically from above into the intervertebral-disc compartment which has been exposed with the aid of the material-abrading tool, the intervertebral-disc prosthesis would not be positioned at the anatomically correct place. As a consequence of this, incorrect loadings and, associated therewith, complaints with respect to the vertebral column may occur. 
     The system  10  permits the material-abrading tool to be aligned correctly in relation to the twisted vertebra  36 : 
     To this end, the operating table  12  is tilted about the tilt axis  14  until the vertebral plane  38  extends perpendicular to the image plane of the image recorded in the picture at number  26 . In this rotary position of the vertebra  36  the transverse process  42  of the vertebra  36  pointing towards the X-ray source  24  overlaps the transverse process  44  pointing towards the image recorder  26 , so that the latter transverse process is more or less completely overlapped by the anterior transverse process  42 . 
     The state after the tilting of the operating table  12  is shown in  FIG. 2 . The vertebral plane  38  is now located in a defined position with respect to the fluoroscope  22  or the image that has been recorded thereby. As already mentioned, this defined position is distinguished in the present exemplary embodiment by the fact that the two transverse processes  42 ,  44  are aligned with one another, so that substantially only the transverse process  42  pointing towards the X-ray source  24  can be discerned in the recorded image. Since the reference frame  20  was firstly tilted together with the operating table  12  via the lateral supports  16 ,  18 , the intervertebral-disc prosthesis would still be inserted incorrectly if a material-abrading tool were to be mounted on the now inclined reference frame  20 . 
     For this reason, in a second step the reference frame  20  is tilted in such a way that it is likewise transferred into a defined position with respect to the fluoroscope  22 . For this purpose, in the exemplary embodiment shown, an alignment element  30  which is rigidly connected to the reference frame  20  comprises on its end a U-shaped bracket, through the shanks of which there extend two coaxial bores  46 ,  48 . This is shown in a simplified manner in the perspective representation of  FIG. 4 . The alignment element  30  is arranged in such a way that the bores extend into the coverage area  28  of the fluoroscope  22 . 
     With the aid of the length-adjustable lateral supports  16 ,  18  the reference frame  20  is now tilted about an axis parallel to the tilt axis  14  in such a way that an axis of symmetry  50  extending through the bores  46 ,  48  extends perpendicular to the image plane of the image recorded by the fluoroscope  22 . In the image this state can be discerned from the fact that the bores  46 ,  48  are imaged as a single circular disc  51 , as shown in  FIG. 6   a . If the X-rays generated by the X-ray source  24  do not pass through the two bores  46 ,  48  parallel to the axis of symmetry  50  but inclined relative to it, no circular disc appears in the image. Instead, a segment  52  will be observed having the shape of disk which is reduced by a crescent-shaped shadow region  54  (see  FIG. 6   b ). This shape arises for the reason that some of the X-rays that have passed through the first bore  46  cannot pass through the second bore  48 , on account of their inclination relative to the axis of symmetry  50 . 
     The reference frame  20  is now adjusted by adjustment of the lateral supports  16 ,  18  until the shadow region  54  disappears. The reference frame  20  is now mounted in a defined position with respect to the fluoroscope  22 . Since the X-rays extend both parallel to the vertebral plane  38  and parallel to the plane of symmetry  50  of the bores  46 ,  48  of the alignment element  30 , the vertebral plane  38  is now in a known angular position with respect to the reference frame  20 . 
     Of course, for the alignment of the tool it is most expedient if the reference plane defined by the reference frame  20  extends parallel to the vertebral plane  38 . This can be obtained in straightforward manner by the alignment element  30  being arranged exactly at a right angle relative to the reference frame  20 . 
     The reference frame  20  now offers a reference system for the subsequent surgical steps. In  FIG. 3  a material-abrading tool  56  with a milling head  58  is illustrated which has been aligned in relation to the reference frame  20  with the aid of devices that are known as such in the art and are therefore not represented in any detail. The tool  56  can now be moved in the transverse direction on the reference frame  20  via a slide or similar, in order to expose the intervertebral-disc compartment for the accommodation of the intervertebral-disc prosthesis. 
     In this connection it is also to be noted that in the case of the exemplary embodiment described above the X-rays do not necessarily have to pass through the bores  46 ,  48  of the alignment element  30  and through the vertebra  36  parallel to the horizontal plane  40 . Such an exact alignment of the fluoroscope  22  would also be relatively difficult for practical reasons. Since both the vertebral plane  38  and the reference frame are aligned with respect to the fluoroscope, the absolute positioning of the fluoroscope  22  is irrelevant for the relative position between the reference frame  20  and the vertebral plane  38 . For the correct positioning of the intervertebral-disc prosthesis, however, all that matters is that the reference frame is correctly aligned in relation to the vertebral plane  38 .