FIELD OF THE INVENTION 
     The present invention relates to an intravertebral implant, having an approximately cylindrical hollow body which has two ends, four side faces, and one or more openings, as well as to an instrument for implanting such an intravertebral implant and a method for expanding such an intravertebral implant. 
     BACKGROUND OF THE INVENTION 
     Between the individual vertebral bodies of the spinal column, the intravertebral disks are located, better known as simply “disks”. The disks comprise an outer ring of connective tissue and cartilage and an inner gelatinous core, and they serve as elastic buffers between the vertebral bodies. The inner gelatinous core can shift, however, or prolapse through cracks in the outer ring. This disorder is known as prolapse or herniation of the disks. In severe cases, nucleotomy or diskotomy is necessary; that is, the disk has to be removed. After that the vertebral bodies have to be fused; that is, the spacing between them must be maintained by suitable provisions; otherwise, they would slip into the gap that was previously filled by the disk. This would cause compressions in this region of the spinal column; the spinal cord would be compressed and stretched in the vicinity of the affected vertebral bodies. The consequence would be pain and neurological defects. 
     Fusion of the vertebral bodies can be done for example by means of intravertebral implants. 
     One known intravertebral implant is known by the term PLIF (for posterolateral interbody fusion), in which cagelike hollow bodies (intersomatic cages) are inserted between the vertebral bodies. To that end, in the region previously occupied by the disk, a space into which the hollow body is driven is created by roughening the adjacent vertebral bodies or removing their periosteum and the cortical substance. A disadvantage of this is that in this method additional stabilization is always necessary, for instance internal fixation by means of four pedicle screws connected to two rods. Even then, the hollow bodies can slip because they grow into the implant only gradually. This is due to the fact that the cortical substance, which is jointly responsible for bone grown, is involved. Especially, the implants can be forced away toward the rear, until they have grown firmly into place. 
     Another possibility known from the prior art is to implant screwable hollow cylinders for lumbar interbody fusion. These implants are hollow cylinders with a male thread. This means that a thread has to be cut into the vertebral body adjacent to the disk space. The hollow cylinders are then screwed into that thread. In this method, it is true that the implants can no longer slip, but the method is extremely complex, which also entails the risk of complications. 
     The necessity of injuring the periosteum and the cortical substance of the vertebral bodies is also felt to be a disadvantage, because these structures are hard and solid and normally have a stabilizing effect. The bone growth and thus the fusion are slowed down by the injury to the cortical substance. 
     One disadvantage common to both these known intravertebral implants is their fixed dimensions. However, the spacing to be maintained between the affected vertebral bodies is different for each patient. Hence numerous versions with different dimensions are needed. Nevertheless, it cannot be avoided that the intravertebral implants are not seated optimally, for instance being seated too firmly or too loosely. 
     The object of the invention is therefore to furnish an adjustable intravertebral implant of the above type, with which the above-described disadvantages are avoided and with which in the simplest possible way, fast and durable fusion of vertebral bodies is made possible. 
     This object is attained in that the hollow body can be expanded on at least two opposed sides and locked in the expanded position by an adjusting element that can be introduced into the hollow body, and the expansion is done on the end at which the adjusting element can be introduced. 
     The object of the invention is also to furnish a fast, simple method for expanding an intravertebral implant. 
     The method according to the invention contemplates the following method steps: 
     (a) prewidening the hollow body; 
     (b) expanding the hollow body. 
     The object of the invention is also to furnish instruments for implanting an intravertebral implant. 
     Instruments according to the invention are defined by the characteristics recited in claims  17 ,  22 ,  23  and  24 . 
     The intravertebral implant of the invention has fixation elements which secure it on the vertebral bodies so that it will not change position. These elements anchor the intravertebral implant to the vertebral bodies. The expansion can be done to a variable extent, so that variable dimensions are available. That is, the diameter of the intravertebral implant and hence the spacing of the vertebral bodies from one another are adjustable. Thus the correct size and in particular the correct diameter for every patient is always available. Moreover, the intravertebral implant is always precisely correctly seated, namely not too firmly and not too loosely. The play is approximately 1 to 2 mm. Substantially fewer intravertebral implants of different dimensions are needed for the varying vertebral body spacings of the patients. The widening elements are also used to retract the fixation elements into the “insertion position”, so that the hollow body can be driven in without hindrance between the vertebral bodies. Only after that is the expansion done, which thus brings about the fixation of the fixation elements on the vertebral bodies. 
     The expansion of the intravertebral implant can be done in various ways. One possibility is to provide spreader tongues, which are cutout from the hollow body and can be bent outward and which have fixation elements on their outer surface. Another possibility is for the hollow body itself to have a slot of a certain width, which thus divides it into two halves that can be pressed together or expanded. In that case, the play is not defined by the diameter of the hollow body itself. 
     It is especially advantageous for the intravertebral implant of the present invention to be designed with a somewhat crowned or bellied form. Thus it is especially well adapted to the contours of the space, formed by the surfaces of the vertebral bodies, that was previously filled by the disk. 
     The expansion of the intravertebral implant according to the present invention is preferably done in two steps. Since the expandable regions are first contracted, it is prewidened in a first step, until the fixation elements touch the vertebral bodies. In a second step, it is then widened to the final extent and thus fixed on the vertebral body. The fixation elements have advantageously have a relief the inside of which can be engaged by a retrieval instrument, so that they can optionally be pulled back in again after the expansion. 
     The instruments necessary for the implantation include an implant holder, with which the intravertebral implant of the present invention can be guided easily and securely until its final fixation. This implant holder is preferably hollow and can receive other instruments within it as well, such as a distractor for the prewidening or an insertion instrument for introducing the adjusting element. Hence it is assured that the location and seating of the intravertebral implant can be monitored at all times during the insertion process. 
     The subject of the present invention is also an instrument for measuring the spacing between two vertebral bodies. It is thus possible to find out what size of implant is needed or how much the implant has to be expanded. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the present invention will be described in further detail below in conjunction with the accompanying drawings. Shown are: 
     FIG. 1, which is a perspective view of a first exemplary embodiment; 
     FIG. 2, which is a side of the exemplary embodiment shown in FIG. 1; 
     FIG. 3, which is a view in the direction III—III of FIG. 2; 
     FIG. 4, which is a longitudinal section through the exemplary embodiment shown in FIG. 1, in which the upper half of the drawing shows the spread-open state, while the lower half shows the insertion state which is not yet spread open; 
     FIGS. 5-8 which show a second exemplary embodiment, functionally identical to the first, seen in FIGS. 1-4; 
     FIG. 9, which is a schematic illustration of two implanted exemplary embodiments of FIG. 5; 
     FIGS. 10-12, which show three instruments for implanting the exemplary embodiments shown in FIGS. 1 and 5; 
     FIG. 13, which is an exemplary embodiment of an implant holder according to the present invention for the implants shown in FIGS. 1 and 5; 
     FIG. 14, which is an exemplary embodiment of a distractor; 
     FIG. 15, which is an illustration of the distractor shown in FIG. 14, in the implant holder shown in FIG. 13; 
     FIGS. 16 and 17, which show two further instruments; 
     FIGS. 18 and 19, which show enlarged details of region XVIII of FIG.  15  and XV of FIG. 16, which in the upper half of the drawings show the implant with the instrument before actuation and the lower half shows the instrument or element after actuation; 
     FIG. 20, which is an enlarged detail of region XX of FIG. 17, in which the upper half shows the implant after actuation and the lower half shows the implant with the instrument before actuation; 
     FIG. 21, which is a further instrument for implantation; 
     FIG. 22, which is a plan view of a third exemplary embodiment of an intravertebral implant according to the present invention; 
     FIG. 23, which is a section taken along the line XXIII—XXIII of FIG. 22 in the insertion state; 
     FIG. 24, which is a view in the direction of the arrows XIV—XIV of FIG. 23; 
     FIG. 25, which is a view as in FIG. 23 but in the spread-open state; 
     FIG. 26, which is a view in the direction of the arrows XVI—XVI of FIG. 25 in the spread-open state, without the screw  77 ; 
     FIGS. 27-31, which are views of a fourth exemplary embodiment, functionally identical to the third exemplary embodiment of FIGS. 22-26; 
     FIG. 32, which is an insertion instrument (impactor) for the exemplary embodiments of FIGS. 22-31; 
     FIGS.  33 — 34 , which shows details of the mounting in the exemplary embodiment of FIGS. 27-31, corresponding to the detail XXXIII of FIG. 32 (FIG. 34 is rotated 90° compared to FIG.  33 ); 
     FIG. 35, which is a view in the direction of the arrows XXXV—XXXV of FIG. 34; 
     FIG. 36, which is a schematic view of an instrument for measuring the spacing between two vertebral bodies; 
     FIG. 37, which is an enlargement of detail XXXVII of FIG.  36 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The exemplary embodiments described below are parts made of titanium or optionally also of carbon. Titanium is tissue-compatible and has a high modulus of elasticity, so that it is an extreme highly suitable material for the intravertebral implants of the present invention. 
     The intravertebral implants shown in FIGS. 1-4 and  5 - 8 , respectively, are functionally identical and are also structurally identical in substantial parts. In the drawings, identical parts are identified by the same reference numerals. 
     The intravertebral implants are approximately 25 mm long, 11-14 mm wide, and 9 to 13 mm high. They have a hollow body  1 , which encloses a hollow chamber  2 . The hollow body  1  is approximately cylindrical and is provided with a dome  3 , four side faces  4 ,  5 ,  6 ,  7 , and a back face  8 . Oval openings  9 ,  10  are formed in the side faces  4 ,  5 , extending over approximately half to two-thirds of the side faces  4 ,  5 . Located on the back face  8  is an approximately centrally disposed bore  11  with a female thread  12 , into which a screw  13  with a male thread  14  is screwed. The head  15  of the screw  13  has a hexagonal socket  16 . The screw forms the aforementioned expander element. 
     Approximately rectangular spreader tongues  18 ,  19 , each approximately 4 mm wide and each defined by a U-shaped slot  22 ,  23 , are located in the two side faces  6 ,  7 . A cramp  24 ,  25  is integrally formed onto the free end of each spreader tongue  18 ,  19 . The cramps  24 ,  25  are triangular in cross section, in such a way that the free edges  26 ,  27  protrude outward. The free edges  26 ,  27  are embodied as sharp, for instance being ground down. The cramps form the aforementioned fixation elements. 
     The exemplary embodiments shown in FIGS. 1 and 5, while being identical in function, have structural differences. The exemplary embodiment in FIG. 1 has two flat side faces  6 ,  7  and two curved side faces  4 ,  5 , while the second exemplary embodiment in FIG. 5 has an entirely curved, “crowned” or “bellied” contour and is approximately square in cross section. The advantage of this contour is schematically shown in FIG.  9 . The faces of the vertebral bodies that enclose the hollow chamber in which the disk was previously located have a slightly concave contour. The more convex contour of the implant corresponds with this in such a way that it contacts the vertebrae over a large surface area and in a sense “mates with” the contours of the hollow chamber. 
     The spreader tongues  18 ,  19  of the first exemplary embodiment (FIG. 1) also have oblong slots  28 , so that the bone tissue can more rapidly penetrate the hollow chamber  2 . In the second exemplary embodiment (FIG. 5) the oblong slots  28  have been dispensed with. Instead, the spreader tongues  18 ,  19  are narrower and are provided with curved spring elements  20 ,  21 , which lend them greater elasticity. The spreader tongues  18 ,  19  of the second exemplary embodiment also have a relief  29 , into which in the spread-open state a retrieval instrument can be inserted, so that when the retrieval instrument is braced against the shoulder of the thread  12  for the screw  13 , the spreader tongues  18 ,  19  can be retracted into the hollow chamber  2  again (for further detail see below). The relief  29  is embodied as round, so that the retrieval instrument cannot slip away to the left or right. 
     In FIGS. 4 and 8, respectively, the mode of operation of these exemplary embodiments is shown. FIGS. 10-12 show schematic exemplary embodiments of the instruments required for them. These are in implant holder  30  (FIG.  10 ), a distractor (FIG.  11 ), and a hexagonal socket wrench  42  (FIG.  12 ). A common feature of all the instruments  30 ,  38 ,  42  is that they have a grip  31 ,  39 ,  43 ; a long shaft  32 ,  40 ,  44 ; and an end  33 ,  41 ,  45 , respectively. In the state (FIG. 4 and 8, bottom) in which the hollow bodies  1  are inserted between vertebral bodies (insertion state), the spreader tongues  18 ,  19  are forced inward into the hollow chamber  2 , so that the free edges  26 ,  27  of the cramps  24 ,  25  do not protrude past the side faces  6 ,  7  of the hollow body  1 . The leading end  33  of the implant holder  30 , which is provided with a male thread  34 , is screwed into the female thread  12  of the bore  11 . The intravertebral implant thus fixed on the implant holder  30  and formed by the hollow body  1  is driven in between two vertebral bodies. Once the intravertebral implant is located at the desired point, the end  33  of the implant holder  30  is unscrewed out of the bore  11  of the hollow body  1  forming the intravertebral implant. Now, with the aid of the distractor  38  (FIG.  11 ), whose end  41  as indicated is approximately elliptical in cross section, the spreader tongues  18 ,  19  are forced gradually outward, and the sharp edges  26 ,  27  of the cramps  24 ,  25  are gradually forced into the cortical substance of the vertebral bodies. Next, with the aid of the screwdriving instrument  42 , the hexagonal socket screw  13  is screwed into the female thread  12  of the bore  11  (FIGS. 4 and 8, bottom). The screw  13  is long enough that viewed in cross section, it protrudes with its leading, conically tapering end  17  past the height of the cramps  24 ,  25  into the hollow chamber  2  of the hollow body  1  (FIGS. 4 and 8, top) and thus on being screwed in forces the spreader tongues  18 ,  19 , and with them the cramps  24 ,  25 , apart. The thickness of the end  17  is approximately equivalent to the diameter of the hollow chamber  2 , so that when the screw is screwed in the spreader tongues  18 ,  19  are locked and the location of the hollow body  1  between the vertebral bodies is fixed. 
     An alternative procedure is for the spreader tongues  18 ,  19  to be forced outward only gently with the aid of the distractor, until the edges  26 ,  27  of the cramps  24 ,  25  rest on the respective vertebral bodies and are pressed only slightly into periosteum of the vertebral bodies. Not until the screw  13  is screwed in are the sharp edges  26 ,  27  of the cramps  24  and  25  pressed to their final extent into the cortical substance of the vertebral bodies. 
     FIG. 13 shows a further exemplary embodiment of an implant holder  45 , with a hollow grip  46  that is penetrated all the way through by a likewise hollow shaft  47 . The free end of the hollow shaft  47  has a male thread  34 . The end of the hollow shaft  47  toward the grip has a female thread  48 . The impact face  46 ′ of the grip  46  has a through opening  49 , which gives access to the hollow shaft. 
     The function of this implant holder  45  is the same as has already been described for the implant holder  30  shown in FIG.  10 . The front end of the implant holder  45  is screwed with its male thread  34  into the female thread  12  of the hollow body  1 . In this way, the implant is held while it is being inserted by the surgeon. Once the implant is seated at the correct point, the leading end of the hollow shaft  45 , with the male thread  34 , stays screwed into the female thread  12  of the bore  11  of the hollow body  1 . The distractor  38 ′ shown in FIG. 14, whose shaft  40  is provided, on its end toward the grip  39 , with a male thread  40 ′, is inserted into the hollow shaft  47  of the implant holder  45 . This situation is shown in FIG.  15 . The thread  40 ′ on the end toward the grip of the distractor  38 ′ engages the thread  48  on the end toward the grip of the hollow shaft  47 . Turning the distractor  38 ′ screws it to the implant holder  45 . This effects a controlled displacement of the distractor  38 ′, so that its tip  41  gradually penetrates the hollow chamber  2  of the hollow body  1  and expands the spreader tongues  18 ,  19 . In this way, the intravertebral implant can be widened in a controlled fashion, and via the implant holder  45  full control over the position and the seat of the implant is simultaneously maintained. 
     The initial and final stages of this process are also shown again, on a larger scale, in FIG. 18; the upper part of the figure shows the situation corresponding to FIG. 15, namely after the insertion and before the distractor  38 ′ is screwed in. The lower part of FIG. 18 shows the situation once the distractor  38 ′ has been screwed all the way into the implant holder  45 . The intravertebral implant is now fully widened. It is understood that the expansion can be terminated in any intermediate stage and the distractor  38 ′ then unscrewed out of the implant holder  45 . The desired final point can be defined for instance by suitably limiting the depth to which the two threads  47 ,  40 ′ can be screwed together. 
     The implant is now “seated”. The distractor  38 ′ is then unscrewed out again and pulled out of the implant holder  45 . Next, the implant holder is unscrewed out of the hollow body  1 . The screw  13  is set against a hexagonal socket wrench  42  (FIG.  16 ), which is equivalent to the instrument shown in FIG.  12  and is therefore identified by the same reference numeral. The screw  13  is screwed into the hollow body  1  by slowly turning the screwdriving instrument  42 . This situation is shown in FIG. 19, bottom part. Once the screw  13  has been screwed all the way in, the screwdriver  42 ′ is set aside. This completes the spreading of the tongues  18 ,  19  that has been done with the aid of the distractor  38 ′, and the intravertebral implant is now fixed. This situation is shown in FIG. 19, top. Naturally it is also possible to expand the implant only partially in the first step and to complete the expansion with the aid of the screw, simply pulled out of the implant holder  30 ′. 
     In FIG. 17, a retrieval instrument  130  is shown which is employed whenever the intravertebral implant of FIG. 5 must be removed, for whatever reason. The retrieval instrument  130  has a grip  131 , a shaft  132 , and a tip  133 . While the drawing in FIG. 17 is purely schematic, the view in FIG. 20 is a detailed illustration of the construction of the tip  133  of the retrieval instrument  130 . In this exemplary embodiment, the tip  133  is adapted in such a way that it is kinked twice, on the order of an offset bend and has both a portion  134  and a portion  135  that is parallel to the shaft  132 . This kink is dimensioned such that after the screw  13  has been unscrewed from the hollow body  1  and the retrieval instrument  130  has been inserted, the portion  135  of the tip  133  engages the relief  29  of the spreader tongues  18 ,  19 . By bracing the retrieval instrument  130  against the shoulder of the thread  12  for the screw  13 , the spreader tongue  18 ,  19  can be bent back into the hollow chamber  2  again. This is shown in FIG. 20, top part. 
     FIG. 21 shows a further exemplary embodiment of a hexagonal socket wrench  110 , comprising a bushing  111  with a grip region  112  and the shaft  113  of the actual screwdriving instrument itself, which is guided in the bushing  111 . 
     Located in the leading end  111 ′ of the bushing  111  are two U-shaped notches  114 ,  114 ′. This creates two tabs  115 ,  116 , which can be spread apart in such a way that the screw  13  is held by its collar during its insertion. The bushing  111  is displaceable on the shaft  113  of the screwdriving instrument. Once the screw  13  is inserted, the bushing  111  is displaced upward on the shaft  113 , so that the tabs  115 ,  116  let go of the screw  13 . 
     A third and fourth exemplary embodiment of an intravertebral implant according to the present invention, which are likewise functionally identical to one another, are shown in FIGS. 22-26 and  27 - 31 , respectively. Identical parts are identified by the same reference numerals. It is approximately 25 mm long and has a diameter of approximately 10-14 mm. This exemplary embodiment again has a hollow chamber  52 , a dome  52 , four side faces  54 ,  55 ,  56 ,  57 , and one back face  58 . Oval openings  59 ,  59 ′,  60 ,  60 ′ are let into all the side faces  54 ,  55 ,  56 ,  57 . The dome  53  has a bore  63  with a female thread  64 . An indentation  83  and a through opening  61  are provided in the back face  58  as well. Thus as can be seen from FIG. 8, bearing faces  67  and  68  that merge with one another are created, and in the compressed state of the hollow body  5  (FIG.  8 ), the bearing face  68  (formed by the indentation  83 ) has a larger diameter and tapers conically, while the bearing face  67  (formed by the bore  61 ) has a smaller diameter and forms the actual through opening  61 . 
     The two opposed side faces  56 ,  57  are divided each by a respective slot  65 ,  66 . Each slot extends from the edge where the back face  58  merges with the corresponding side face  56 ,  57  to the openings  60 ,  60 ′ in the side faces  56 ,  57 . On the thus-created halves of the back face  58 , the bearing faces  67 ,  68  are thereby divided in half as well. 
     In the vicinity of the end facing the dome  53 , two cramps  69 ,  70 ,  71 ,  72  are formed integrally onto each of the side faces  54 ,  55  that extend parallel to the slots  65 ,  66  and are not divided in half. These cramps are again triangular in cross section, and their free edges  73 ,  74 ,  75 ,  76  protrude outward by approximately 2 mm from the side faces  54 ,  55 . These edges are embodied as sharp also, for instance being ground down, and form the aforementioned fixation elements. 
     In FIGS. 25 and 30, a screw  77  with a shaft  78  and a male thread  79  is shown, whose end  82  is screwed into the female thread  64  of the bore  63 . The head  80  has a hexagonal socket  81  and rests with its outer circumferential face on the bearing face  68 . 
     The difference between these exemplary embodiments is again that the third exemplary embodiment (FIGS. 22-26) has four flat side faces  54 - 57 , while the fourth exemplary embodiment again has a completely curved, convex contour and has a “crowned” or “bellied” appearance. 
     The mode of operation of the third and fourth exemplary embodiment is as follows: The situation at the outset can be seen from FIGS. 22 and 23, and FIGS. 27 and 28, respectively. The side faces  54 ,  55  are pressed together, so that the edges of the slots  65 ,  66  rest on one another in narrowed fashion toward the back face  68  (or are spaced apart only slightly), and the cramps  69 ,  70 ,  71 ,  72  are retracted somewhat with regard to an imaginary line extending the parallel portions of the side faces  56 ,  57 . This is the insertion position. As already noted above, in this position the inner bearing face  67  forms a circular-cylindrical passage (see FIGS.  23  and  28 ). 
     For the implantation, the impacter  90  of FIG. 32 is used. It has a handle  91  and a shaft  92  provided with a male thread  93 . An adjusting nut  94  is seated on the shaft  92 . The shaft  92  is penetrated by a threaded spindle  95 , which is provided with a male thread  96  on its end. The threaded spindle may be turned with the grip  91 . It acts as an implant holder; that is, it is screwed into the bore  63  on the dome  53  of the hollow body  51 . A mount  97  is displaceably held on the shaft  92 ; on its upper end it has two grips  98 , and a bushing  99  is formed onto its lower end and encompassingly engages the hollow body  51  in the state in which it is inserted between vertebral bodies. 
     The way in which this encompassing engagement is accomplished is illustrated in FIGS. 33-35, which show the fourth exemplary embodiment. From FIG. 33 it can be seen that the bushing  99  completely encloses the cramps  69 ,  70 ,  71 ,  72  and is flattened toward its end  99 ′, thus assuring a smooth transition between the bushing  99  and the hollow body  51 . The threaded spindle  95  penetrates the hollow chamber  2  and is screwed by its end into the bore  63  on the dome  53  of the hollow body  51 . FIG. 34 shows this state in a view rotated by 90°; it can be seen that the end  99 ′ has recesses  100  which receive the cramps  69 ,  70 ,  71 ,  72 . As a result, the end  99 ′ of the bushing  99  is divided up into two side regions  101  and two narrow regions  102 , which encompass the hollow body  51  and hold it firmly in the insertion position. 
     The intravertebral implant (hollow body  51 ) now thus fixed in its insertion position is then driven in between two vertebral bodies, for instance being hammered in, for which purpose the impact face  91 ′ of the grip  91  is needed. Once the intravertebral implant is positioned, the adjusting nut  94  is loosened, and the mount  97  is grasped by the grips  98  and pulled upward. The threaded spindle  95  is then unscrewed out of the bore  63 . Next, the intravertebral implant (hollow body  1 ) is widened. 
     This is done in such a way that the screw  77  shown in FIG. 9 is placed with its hexagonal socket  81  on the end of the screwdriver instrument  42  (see FIG.  12 ). The screw forms the expansion element. By fine-tuning of the dimensions, care must be taken that the screw  77  is held is in this position by gentle clamping. Then, with the aid of the screwdriver instrument  42 , the screw  77  is guided in the hollow body  51 , and then the end  82  in the screw  77  is then screwed into the female thread  64  of the bore  63 , until the outer circumferential face of the head  40 , by contact with the bearing face  68 , has forced the side faces  54  and  57  apart (or more or less widely apart) into the position shown in FIG. 9, in which the cramps  69 - 72  have dug into the vertebral bodies. The screwdriver instrument  42  is then removed. In this way, variable expansions of the hollow body  51 , which is the intravertebral implant, can thus be achieved. 
     FIG. 36 shows a measuring instrument  120  for measuring the spacing between two vertebral bodies. Before an intravertebral implant is implanted, the spacing between two vertebral bodies must be measured, in order to be able to determine the size of implant required (see the view in FIG.  9 ). The adjustability of the intravertebral implant according to the invention affords a play of a few millimeters. In patients of different body sizes (for instance, women and men), this play is not enough. In these cases, a preselection among various sizes of intravertebral implant must be made. 
     The measuring instrument  120  has two grips  121 ,  122 , which are held apart from one another by a spring  123 . The two grips  121 ,  122  are each joined to a respective shaft  124 ,  125 . The shaft  125  is pivotably connected to a further, shorter shaft  125 ′. Together, the shafts  125 ,  125 ′ have the same length as the single shaft  124 . The shafts  124  and  125 ′ end in two hemispherical parts  126 ,  127 . These parts  126 ,  127  act as measurement probes. In the relaxed state shown in FIG. 36, the hemispherical parts  126 ,  127  have already been expanded. Moving the grips  121 ,  122  toward one another causes the hemispheres  126 ,  127  to approach one another, as indicated by dashed lines in FIG.  36 . The mechanical connection that effects this movement together is shown in greater clarity again in FIG.  37 . By pressing the grips  121 ,  122  together, the shaft  125  is moved forward, that is, in the direction of the hemisphere  126 . In the process it takes the shaft  125 ′, which ends in the hemisphere  127 , along with it. In this process the shaft  1251  rotates in the joint  128 . However, since the shaft  125 ′ is also pivotably connected to the shaft  124  at the joint  129 , it does not move forward with the shaft  125  but instead is moved downward in the direction of the arrow A. 
     With the hemispherical parts  126 ,  127 , one enters the interstice between two vertebral bodies, and then expands them again by relaxing the grips  121 ,  122 . The measured spacing can be read out in various ways. Two of these ways are suggested in FIG. 36. A strip  122 ′ with a scale can be attached to the grip  122 , while the grip  121  is equipped with a pointer  121 ′. The greater the measured spacing, the greater becomes the spacing between the grip  121  and the grip  122 . This spacing can be read off from the scale. Another option is a scale  125 ′ on the end of the shaft  125 . In this case, the extent of forward motion of the shaft  125  compared to the shaft  124  is a standard for the measured spacing between the vertebral bodies. The distance traveled can be read off from the scale  125 ′, via a point  124 ′ permanently marked onto the shaft  124 .
The present invention relates to an adjustable-height intravertebral implant, having an approximately cylindrical hollow body which has one or more openings ( 59, 59', 60, 60 ') and which is variably expandable by adjusting elements, and which has fixation elements for fixing it to vertebral bodies. The present invention also relates to a method for expanding this intravertebral implant as well as to instruments for performing the method.