Abstract:
An intervertebral fusion implant for fusion of two adjacent vertebrae, comprising a base piece and a cover piece, which are each designed to bear on a facing end plate of one of the adjacent vertebrae, wherein the cover piece is adjustable in height with respect to the base piece, wherein a ratchet mechanism, provided between base piece and cover piece, effects a stepped adjustment of height and secures an adopted height against reversal. In this way, a defined expansion can be achieved that is dependent on the number of actuations. A precise control of the state of expansion is permitted, specifically in a purely haptic manner. Moreover, a locking of the adopted state of expansion is achieved with the ratchet mechanism.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a nonprovisional of U.S. Provisional Application No. 61/738,253, filed Dec. 17, 2012, which claims priority to German Application No. 20 2012 011 958.3, filed Dec. 14, 2012, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to an intervertebral cage that is expandable in steps, with a bottom piece and a cover piece for bearing on end plates of an upper and lower adjacent vertebral body, respectively. 
       BACKGROUND OF THE INVENTION 
       [0003]    The intervertebral disks of the spinal column suffer degeneration as a result of wear or of pathological changes. If conservative treatment by medication and/or physiotherapy is ineffective, surgical treatment is sometimes indicated. In this connection, it is known for a movable or immovable implant to be inserted into the intervertebral space containing the degenerated intervertebral disk. These implants take over the support function of the degenerated intervertebral disk and to this extent restore a stable bearing between the adjacent vertebrae. Immovable implants are also referred to as “cages”. 
         [0004]    Various surgical techniques are known for implanting the cages. A traditional surgical technique involves a ventral access route, in order thereby to avoid the danger of damaging the spinal cord in the vertebral column. However, this advantage is obtained at the price of a very long access route through the abdominal cavity or thoracic cavity of the patient. Since this can cause complications, an alternative access route has become established, namely from the dorsal direction. Although the latter affords the advantage of a short route, there is the danger of collision with or damage to the spinal cord. To minimize this danger, the operation is usually performed by minimally invasive surgery. Approaches of this kind directly from the dorsal direction or more from the side are known as PLIF (posterior lumbar intervertebral fusion) or TLIF (transforaminal lumbar interbody fusion), in which the intervertebral disk is exposed from the posterior or lateral direction, respectively. Because of the small transverse incisions used in an approach by minimally invasive surgery, the size of the cage is of course greatly restricted. 
         [0005]    For treatment using the PLIF or TLIF technique, very small cages are known. They afford the advantage of being able to be implanted by minimally invasive surgery thanks to their small size. However, an inherent disadvantage of their small size is that the support function is limited because of the small dimensions and is sometimes inadequate. Although a larger size of the cages would improve the support function, this is impractical because of the limits of minimally invasive surgery. 
       SUMMARY OF THE INVENTION 
       [0006]    The invention addresses the problem of improving a cage of the type mentioned at the outset in such a way that a better supporting action can be achieved while still using a small access cross section of the kind customary in minimally invasive surgery. 
         [0007]    A solution according to the invention lies in the features broadly described herein. Advantageous developments are the subject matter of the detailed embodiments described below. 
         [0008]    In an intervertebral fusion implant for fusion of two adjacent vertebrae, comprising a base piece and a cover piece for bearing on end plates of lower and upper adjacent vertebrae, wherein the cover piece is adjustable in height with respect to the base piece, provision is made for a ratchet mechanism between base piece and cover piece, which ratchet mechanism effects a stepped adjustment of height and secures an adopted height against reversal. The ratchet mechanism is preferably designed such that the base piece has a bottom wall, for bearing on the lower vertebra, and a vertical wall rising from this bottom wall, and the cover piece has a cover wall, for bearing on the upper vertebra, and a side wall projecting downward from this cover wall, said vertical wall and side wall being parts of the ratchet mechanism. Here, the ratchet mechanism expediently has a plurality of locking lugs on the side wall and a locking spring on the vertical wall, or vice versa. 
         [0009]    The invention is based on the concept of using a fusion implant that is adjustable in height such that, at the time of implantation, it can be introduced in the compact state through the minimally invasive access route into the intervertebral space and can be expanded there in height. This is done by actuating a height adjustment mechanism, which is designed as a ratchet mechanism. In this way, a defined expansion can be achieved that depends on the number of actuations. This allows the surgeon precise control of the state of expansion, specifically by purely haptic means. The invention is therefore free of limitations of the kind that would arise with purely visual control of the state of expansion (if appropriate by X-ray). 
         [0010]    Moreover, the expansion state that has been set can be locked using the ratchet mechanism. The form fit of the ratchet mechanism provides protection against inadvertent return movement, even under very high loading forces. This results in excellent stability, specifically also immediately after the operation, until complete fusion of the two adjacent vertebral bodies is finally achieved by bone growth. 
         [0011]    A double ratchet mechanism is advantageously provided, preferably one on each lateral face of the implant. On the one hand, this provides secure support, even under loads that are not exerted centrally. On the other hand, a double ratchet mechanism of this kind makes it possible to give both sides different heights, which results in a defined tilting of the cover piece with respect to the base piece. In this way, an adaptation to curvatures in the spinal column can be made in the respective intervertebral space. For this purpose, provision can also be made that the two ratchet mechanisms are offset in height with respect to each other, preferably by an offset of their respective locking lugs. 
         [0012]    Two vertical walls are preferably provided laterally on the bottom wall, and the bottom wall is elastic. This permits a flexible, reversible spreading open of the two vertical walls, such that the expansion movement is made easier. At the same time, by purposeful spreading open of the two side walls, a resetting possibility can be made available for the cover piece. This permits testing of the fusion implant and resetting in the event of overexpansion. In an alternative variant, only the vertical walls are elastic, while the bottom wall is stiff. Bulging of the bottom wall is thus avoided. 
         [0013]    The cover piece is preferably made of a less elastic material than the bottom piece. For example, the cover piece is composed of a cobalt/chromium alloy, while the base piece is composed of a more elastic titanium alloy. In this way, it is possible for a greater force to be taken up, especially at the locking lugs of the ratchet mechanism. 
         [0014]    In order to prevent accidental overexpansion, a catch mechanism is preferably provided which limits the adjustment movement of the cover piece. The catch mechanism is expediently formed by a pawl that interacts with a shoulder projection. Harmful overexpansion is thus prevented in a simple and efficient manner. In an alternative embodiment, the catch mechanism is formed by windows in the vertical walls and by a transverse pin which projects laterally from the side walls and engages in the windows. The transverse pin is preferably arranged at the bottom on the side walls. In this way, the locking teeth can run uninterrupted on the side walls, which improves the force transmission. 
         [0015]    To permit fine adjustment in the case of double ratchet mechanisms, the locking lugs on one of the two ratchet mechanisms are preferably designed such that they have a spacing which is an integral multiple of the spacing of the locking lugs on the other of the two ratchet mechanisms. For example, the spacing on one ratchet mechanism is twice as great as the spacing on the other one. On the one hand, a rapid adjustment can thus be achieved by the greater spacing. On the other hand, a finer adjustment can be obtained using the ratchet mechanism with the smaller spacing, which is a considerable advantage in practice, in particular as regards adjustment of an angle of tilt between cover piece and base piece. 
         [0016]    A suitable size of the spacing of the locking lugs from one another is approximately 1.5 to 3.0 times their elevation (which is defined by the distance between tooth tip and tooth bottom). 
         [0017]    The locking lugs are advantageously designed asymmetrically, preferably in a sawtooth shape. This allows a configuration which is such that only a low force is needed for the expansion, by virtue of favorable flank angles, whereas high retention forces are applied against return movement. It has proven suitable if the angle of a supporting flank is in the range of 5 to 35°, preferably approximately 10°. The supporting flank is understood as being the one that bears the load and thus counteracts a return movement. The sawtooth shape is preferably designed such that a tooth angle in the range of 45° to 70°, preferably of approximately 60°, is obtained. In a particularly expedient embodiment, the locking lugs overlap one another in an imbricated manner, wherein the counter-flank is preferably oriented in the same direction as, but flatter than, the supporting flank. This configuration permits a particularly favorable adjustment of the force relationships between the actuating force during the expansion and the retention force with respect to return movement. This can be further increased if the locking spring engages with a form fit in the locking lugs, wherein the shape of the locking lugs is preferably adapted to the locking spring such that a self-securing effect is achieved. This provides maximum securing against undesired return movement, which is ultimately defined only by the load limits of the material with respect to fracture or shear. 
         [0018]    To maximize the self-securing effect, the interacting surfaces on locking lug and locking spring are not parallel, but instead oriented at an acute angle to each other. At an acute angle is understood here as meaning that the respective surfaces diverge from each other in the direction toward the bottom of the locking teeth. This configuration has the effect that, under a load, the locking spring draws further between the locking teeth. An undesired escape of the locking spring from the locking teeth is thus safely avoided. The securing effect is thus maximized. 
         [0019]    An inner hollow space is preferably formed between cover piece and base piece. This permits introduction of bone substance, such as grafts or chips, in order thereby to promote inward growth of bone substance for stiffening. In order to promote introduction of bone substance, and in particular also inward growth from the direction of the two adjacent vertebrae, at least one aperture is preferably provided on the cover plate and/or bottom wall and is connected to the hollow space. 
         [0020]    A blocking element can also be provided, which can be pushed into the hollow space. In this way, after implantation and expansion to the desired height, the cover piece can be fixed in its position. An additional securing effect is thus obtained. A plurality of blocking elements with different heights are preferably provided, such that a blocking element can be pushed in that is of the right fit for the expansion. Preferably, at least one additional aperture is provided on the blocking element and is flush with the aperture on the cover plate and/or bottom wall. Thus, growth of bone substance into the hollow space can be promoted even when the blocking element has been pushed in. Moreover, the cover piece and/or base piece can preferably have openings on their end faces. On the one hand, this further promotes inward growth, and, on the other hand, bone substance can be introduced into the hollow space through this end opening even after implantation. 
         [0021]    The cover piece and/or base piece is advantageously rounded at its end faces, in particular at its corners. This makes it easier to push the fusion implant into the intervertebral space. 
         [0022]    The top face of the cover piece and/or the bottom face of the base piece is preferably structured. This is understood to mean that it has regular elevations or depressions. These are preferably fine grooves extending longitudinally, that is to say in the direction from end face to end face, and/or furrows extending transversely, that is to say from side wall to side wall. The grooves have a spacing of at least 0.2 mm to at most 1 mm, while the furrows have a width of at least 2 and preferably not more than 5 mm. The fine grooves act like teeth and hold the intervertebral fusion implant in place. The furrows protect it from migration, particularly under the effect of a force, as often occurs when the patient stoops or bends down. Moreover, they initially secure the intervertebral fusion implant against slipping. Advantageously, the top face and/or bottom face are provided with a coating of titanium plasma spray, which promotes inward growth of bone substance. The titanium plasma spray is preferably finely particulate with a grain size of at most 90 μm, such that it does not even close fine structures such as said grooves. Preferably, the edge areas toward the end faces are smooth or are provided only with the grooves and not with the furrows. This ensures a greater load-bearing capacity for the cortical vertebral margin. 
         [0023]    The overall configuration of the intervertebral fusion implant is preferably generally box-shaped. This is favorable from the point of view of production technology and simplifies introduction of the implant in a straight line into the intervertebral space. However, it may also be desirable that the implantation does not take place in a straight line but instead in an arc shape. For this purpose, the intervertebral fusion implant is preferably curved in the shape of an arc of a circle. This simplifies implantation, particularly in the case of an access route obliquely from behind by means of the TLIF (transforaminal lumbar interbody fusion) technique. The intervertebral fusion implant can also be wedge-shaped, in such a way that one end wall is lower than the other. Wedge angles in the range of approximately 3° to 15° are preferred. This can allow better adaptation to a curvature of the spinal column, in particular to a lordosis. 
         [0024]    It may be expedient to return the already expanded intervertebral fusion implant to the compact size in which it is provided for implantation. This can be done after an extracorporeal test of functionality or following an (over)expansion performed for test purposes. In order to be able to reset the intervertebral fusion implant, a release mechanism for the ratchet mechanism is provided. In a simple case, it can be in the form of seats that are arranged at the free end of the vertical wall and/or side wall and are provided for a laterally acting spreading element. This spreads the vertical walls open, such that the locking teeth move out of the area of engagement of the locking spring, and the cover piece can be moved back again relative to the base piece. In a preferred embodiment, seats are designed as holes at the upper end of the vertical wall, preferably in the area of the pawl. This permits a particularly simple resetting option, albeit only in a state outside the intervertebral space. 
         [0025]    A particularly suitable spreading element is a forceps-like instrument which has grippers that engage in the seats. 
         [0026]    The invention further extends to an arrangement composed of an intervertebral fusion implant and of an actuation instrument, which is designed to actuate the height adjustment. Preferably, the actuation instrument is designed such that it has a cross section which at most is as great as that of the intervertebral fusion implant. This ensures that the expansion using the actuation instrument can take place through the same small minimally invasive access route used for the actual insertion of the intervertebral fusion implant. A second separate access route is therefore not needed. This therefore reduces the work involved and also places less strain on the patient. 
         [0027]    The actuation instrument can be designed as a draw-key expander with two spreading jaws. The draw key is preferably arranged in the front area on the spreading jaws. By moving the draw key, an expansion of the spreading jaws can be effected, which spreading jaws in turn act from the inside on the cover piece or the base piece and thus expand these. 
         [0028]    For the movement of the draw key, a tensioning member is preferably provided, which is routed rearward between the spreading jaws. Here, rearward means being routed outward away from the implant in the direction of the surgeon. This allows the surgeon comfortable actuation from the outside via the minimally invasive access. The routing of the tensioning member between the spreading jaws permits a compact structure. Moreover, this permits good protection of the tensioning member against mechanical impairment. The tensioning member is advantageously designed as an actuation spindle. This permits a precise adjustment and also allows reversible actuation by rotation in the opposite direction. 
         [0029]    In a well proven embodiment, the draw key is arranged at the front end of the actuation spindle, and an adjustment wheel is mounted at the rear outer end. This permits a comfortable reversible actuation from the outside. 
         [0030]    It may in principle also be sufficient for tactile feedback to give the surgeon an idea of the extent of the height adjustment. However, it may also be advantageous if the actuation instrument has a display mechanism, which indicates the attained height adjustment of the intervertebral fusion implant. For this purpose, the display mechanism has a presentation proportional to the attained height adjustment. This makes it easier for the surgeon to determine the expansion that has already been attained. 
         [0031]    Alternatively, an actuation instrument can also be provided as a rotary lever plate. It comprises a guide part and a lever part guided pivotably in the latter, wherein seats for either the base piece or the cover piece are provided on both. It is expediently configured such that the base piece is mounted at a front end of the guide part, while the cover piece is mounted on the lever part so as to rotate therewith. For this purpose, a screw connection is expediently provided at the front end of the lever part and engages in a corresponding mating thread of an opening in the end face of the cover piece. 
         [0032]    The lever part preferably has a toggle for actuation at its rear end. It is mounted in the guide part, which for this purpose is designed such that the lever part is mounted movably in the radial direction. This means that, by pivoting the toggle in the radial direction, the lever part correspondingly rotates. 
         [0033]    The lever part also preferably has an axial bore through which a fastening spindle is fitted. It serves for the easily releasable fastening of the cover piece on the lever part. It has proven useful to provide, on the guide part, a bell with a recess for the toggle. Here, the recess is shaped such that, in the radial direction, it has a greater width than corresponds to a thickness of the toggle. The toggle is thus movable in the radial direction in the bell and can thus be moved to and fro. This reciprocating movement transfers to the cover piece firmly screwed on the front end, which cover piece accordingly tilts and thus migrates upward alternately on the two ratchet mechanisms by in each case one further locking tooth. A height adjustment is effected in this way. Moreover, if tilting is desired, a tilt can be adopted by means of an uneven number of actuation procedures. 
         [0034]    A further alternative actuation instrument is designed as a parallel expander with cross lever. The cross lever forms a reverse kinematics system, as a result of which the expander spreads open when pressure is applied to the handle. This kinematics system is more expedient in use than simple forceps without a cross lever, where the handle would have to be spread open. Moreover, the parallel expander preferably has an adjustable limit stop device. With this, a maximum expansion can be fixed, which is then not exceeded. This is particularly advantageous in implantation situations where access conditions are difficult, since the surgeon, during the expansion procedure, no longer has to manually control the position attained. A rotary wheel with a plurality of predefined and inscribed positions is preferably provided. With this, the limit stop device is adjusted accordingly. The desired degree of expansion can therefore be easily adjusted. 
         [0035]    The spreading jaws are preferably designed differently as regards the retention of base piece and cover piece. Devices for form-fit connection are preferably arranged on only one of the spreading jaws, preferably in the form of projections engaging in openings in the base piece. By means of the different design, a fail-safe coding is obtained in the sense that the intervertebral fusion implant can only be correctly received with one orientation by the actuation instrument. Errors caused by the implant being received the wrong way round are thus ruled out. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    The invention is explained in more detail below with reference to the attached drawing and on the basis of advantageous illustrative embodiments. In the drawing: 
           [0037]      FIG. 1  shows a view of an illustrative embodiment of an intervertebral fusion implant in the implanted state on vertebral bodies; 
           [0038]      FIG. 2  shows perspective views of base piece and cover piece as per the embodiment shown in  FIG. 1 ; 
           [0039]      FIG. 3  shows schematic views of the action of a ratchet mechanism; 
           [0040]      FIG. 4  shows an enlarged view of teeth of the ratchet mechanism as per  FIG. 3 ; 
           [0041]      FIG. 5  shows an example of a catch mechanism in a variant of the illustrative embodiment; 
           [0042]      FIG. 6  shows views of a release mechanism in the locked position and the unlocked position; 
           [0043]      FIG. 7  shows a perspective view and a cross-sectional view of a securing element in the dismantled state and the assembled state; 
           [0044]      FIG. 8  shows a perspective view of a grip head for an actuation instrument; 
           [0045]      FIG. 9  shows cross-sectional views of the actuation instrument as per  FIG. 8  in the relaxed state and partially expanded state; 
           [0046]      FIG. 10  shows a plan view and cross-sectional details of the actuation instrument; 
           [0047]      FIG. 11  shows a perspective view of a second embodiment; 
           [0048]      FIG. 12  shows a two-part actuation instrument for an intervertebral fusion prosthesis as per a third embodiment; 
           [0049]      FIG. 13  shows views of the second actuation instrument in a partially assembled state and a fully assembled state; 
           [0050]      FIG. 14  shows a perspective view of a further illustrative embodiment of the intervertebral fusion implant; 
           [0051]      FIG. 15  shows a sectional view of  FIG. 14 ; 
           [0052]      FIG. 16  shows an enlarged detail from  FIG. 14 ; 
           [0053]      FIG. 17  shows a side view of a variant of  FIG. 14 ; 
           [0054]      FIG. 18  shows a plan view of a fourth embodiment of the actuation instrument; 
           [0055]      FIG. 19  shows an enlarged detail from  FIG. 18 ; and 
           [0056]      FIG. 20  shows another enlarged detail from  FIG. 18 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0057]    An intervertebral fusion implant, designated in its entirety by reference number  1 , is provided for implantation in an intervertebral space  91  between two immediately adjacent vertebral bodies  9 ,  9 ′. In a physiologically intact vertebral column, an intervertebral disk  90  is located in the intervertebral space between the vertebrae. This intervertebral disk  90  may undergo degeneration as a result of disease or wear, with the result that it has to be at least partially resected. In order to achieve sufficient support of the intervertebral space  91 , despite the loss of intervertebral disk material, and to thereby prevent collapse of the vertebral column, the intervertebral fusion implant  1  is inserted into the intervertebral space  91 . It provides a supporting action and thus facilitates fusion of the two adjacent vertebrae  9 ,  9 ′ in a natural way through bone growth. 
         [0058]    The intervertebral fusion implant  1  comprises a base piece  2 , which bears on an upper end plate  92  of the lower adjacent vertebra  9 . Moreover, it comprises a cover piece  3 , which bears on a lower end plate  93  of the upper adjacent vertebra  9 ′. In order to ensure a secure bearing of the base piece  2  on the upper end plate  92 , on the one hand, and of the cover piece  3  on the lower end plate  93  of the upper vertebra  9 ′, it is necessary to suitably adjust the height of the intervertebral fusion implant  1 . It has to be expanded until the base piece  2  and the cover piece  3  bear firmly on the respective end plates  92 ,  93 . 
         [0059]    To permit a movement of base piece  2  and cover piece  3  away from each other in the sense of an expansion, a ratchet mechanism  4  is provided. The latter comprises a multiplicity of locking teeth  43  on the cover piece, which interact with a locking spring  42  on the base piece  2 . 
         [0060]    In order to keep the intervertebral fusion implant  1  compact, and thus be able to implant it through an access prepared by minimally invasive surgery, while at the same time allowing it to exert a sufficient supporting action in the intervertebral space  91 , the intervertebral fusion implant  1  is adjustable in height. For this purpose, it has a base piece  2  and a cover piece  3 , which is arranged to be movable in height relative to the base piece  2 . The base piece  2  is substantially trough-shaped and comprises a bottom wall  22 , which functions as a contact face on the end plate  92  of the lower of the adjacent vertebrae  9 , and an upwardly projecting vertical wall  21  on each of the two lateral faces. The base piece  2  therefore has an overall U-shape in cross section. At the upper end of the vertical wall  21 , the locking spring  42  is arranged as a part of the ratchet mechanism  4 . Moreover, approximately at the middle of an upper end of the vertical wall  21  there is a projection  52 , which forms a part of a catch mechanism  5 . In the bottom wall  22 , there are a plurality of openings  24  via which a hollow space  10 , in the interior of the trough-shaped configuration between the side walls  21 , is connected to the space underneath the bottom wall  22 . This promotes the growth of bone into the intervertebral fusion implant  1 . 
         [0061]    Moreover, the depicted illustrative embodiment of the intervertebral fusion implant  1  comprises a cover piece  3 . The latter has a generally box-shaped configuration, with a substantially plane top face  33 , which functions as a contact face for the lower end plate  93  of the upper adjacent vertebra  9 ′. A side wall  31  is arranged projecting downward on each of the lateral faces. On their outward surface, these side walls  31  carry a plurality of locking teeth  43 , which form a part of the ratchet mechanism  4  and interact with the locking spring  42  on the base piece  2 . In the illustrative embodiment shown, an end wall  30  is provided on an end face. It is optional and can also be omitted. Moreover, a bearing shoulder  53  is also formed approximately at the middle on the side walls  31 . Its dimensions are chosen such that it interacts with the projection  52  on the base piece  2 . The arrangement of the bearing shoulder  53  is chosen here such that it comes to bear on the projection  52  in the position of maximum extension of the cover piece  3  relative to the base piece  2 . A limit stop is thus obtained. The projection  52  and the bearing shoulder  53  thus interact as a catch mechanism  5 . A plurality of apertures  34  are formed on the top face  33 , in order to allow bone substance to grow into the interior of the cover piece. 
         [0062]    For implantation, the cover piece  3  is fitted into the hollow space  10  of the trough-shaped base piece  2 . The implant is thus in the state of minimal expansion, as is shown in the uppermost of the three views arranged above one another in  FIG. 3   a . In this state, the implant is introduced into the intervertebral space  91  until it more or less reaches the position shown in  FIG. 1   b . In a next step, it can then be expanded until it is set to the height of the intervertebral space  91 . This is done by the cover piece  3  being moved upward by means of an actuation instrument  8 , wherein the locking spring  42  successively engages with different locking teeth  43  on the lateral faces  31  of the cover piece  3  and thereby secures the attained position of expansion. This expansion procedure is depicted in the three views in  FIG. 3   a.    
         [0063]    The locking teeth  43  are sawtooth-shaped. Each tooth  43  has two flanks  45 ,  47 . Here, the shorter flank  45  is the load-bearing flank, which comes into engagement with the locking spring  42  in such a way as to transmit force. This load-bearing flank  45  is shaped facing downward and is thus oriented in the same direction as the other, non-load-bearing flank  47 . This results in a certain degree of imbrication of the locking teeth  43  when viewed from the side. In the illustrative embodiment shown in  FIG. 4 , the angles α and β are chosen such that the angle α is about 60° and the angle β is 8°. The top face  46  of the locking spring  42  is inclined by said angle β. Thus, together with the downwardly facing orientation of the load-bearing flank  45 , this means that a loading force F results in an inwardly directed lateral force L, which draws the locking spring  42  into the teeth  43  and thus secures it against undesired escape from the locking teeth  43 . A self-securing effect is thus obtained in the sense that the securing force L increases the greater the loading force F. 
         [0064]    In order to prevent a situation in which the cover piece is moved too far up during the expansion and the locking spring  42  thus leaves the area of the locking teeth  43 , a catch mechanism  5 ′ as shown in  FIG. 5  is alternatively provided. It comprises a projection  52 ′ on each locking spring  42 , specifically at the middle thereof. A downwardly facing stirrup  53 ′ is arranged on the cover piece  3 , underneath the locking teeth  43 , its dimensions being chosen such as to enclose the projection  52 ′. When the maximum expansion is reached, the stirrup  53 ′ engages under the projection  52 ′ and prevents a further expansion movement. 
         [0065]    For the purpose of height adjustment, the expansion movement generally takes place uniformly on both sides of the ratchet mechanism  4 , as is also depicted in  FIG. 3   a . However, it may also be desirable for the adjustment to be non-uniform in order to adopt a defined tilt angle. A desired tilt angle can be adopted in a defined manner depending on whether the adjustment on one side takes place across one or two extra locking teeth  43 . For this purpose, the surgeon simply has to count off the number of “clicks”, i.e. the number of locking procedures in the ratchet mechanism  4  made by the engagement of the locking spring  42  in the different locking teeth  43 . For a finer adjustment of the tilt, more finely stepped teeth  43 ′ can also be provided, twice as many teeth  43 ′ of half the size of the teeth  43 . Preferably, only one of the two ratchet mechanisms is provided with the finer teeth, as is shown in  FIG. 5 . Optionally, the start points of the teeth  43 ,  43 ′ are mutually offset between the two ratchet mechanisms. 
         [0066]    It may be necessary to return the cover piece  3  from its expanded position to a less expanded position or to a position of no expansion at all, as is used for the implantation. For this purpose, a resetting mechanism  6  is expediently provided. This comprises an instrument with forceps-like grippers  61  which, with corresponding gripper tips  62  at their front end, engage in receiving openings  60  formed for this purpose on the top face of the vertical walls  21 . By spreading the grippers  61  in a forceps-like movement, the two vertical walls  21  lying opposite each other are moved away from each other, as a result of which the locking spring  42  on the respective vertical wall  21  is no longer in engagement with the locking teeth  43  on the cover piece  3 . The ratchet mechanism  4  is thereby released. The cover piece  3  can be freely moved downward and in this way reset. For this purpose, the material of the base piece  2  is preferably chosen such that it is able to deform elastically, in particular the bottom wall  22 . This is shown in  FIG. 6 . 
         [0067]    In order to achieve additional securing of the attained height adjustment after successful expansion, a separate blocking element  70  can be provided. It is preferably shaped such that it can be pushed with a good fit into the hollow space  10  between cover piece  3  and base piece  2 , which hollow space  10  has been enlarged by the expansion. It will be appreciated that different blocking elements  70  have to be used depending on the expansion status. The blocking element  70  preferably has a central aperture  74 , which is flush with the apertures  34  on the cover piece  3  and the apertures  24  on the base piece  2 . In this way, the hollow space  10  can be more easily reached and accessible for the inward growth of bone substance. In the inserted state, the blocking element  70  secures the cover piece  3  against a reverse movement back into the base piece  2 , as is shown in  FIG. 7   b.    
         [0068]    For the height adjustment by means of expansion, an actuation instrument is provided. It is designed as a draw-key expander  8  with two spreading jaws  81 . They are provided at their front end with seats  84 , which engage with a form fit in the apertures  34 . At the front end, the spreading jaws  81  are indented in a V-shape, and a wedge-shaped draw key  82  is fitted into this indent. The draw key  82  is connected to a tensioning member  80 , which extends along the longitudinal axis of the actuation instrument  8 , over the entire length of the latter (see  FIG. 10 ). It begins with its front end on the draw key  82  and terminates, at the rear end of the actuation instrument  8 , in a handwheel  88 , in which it is mounted via a spindle thread  89 . When the handwheel  88  is turned, the tensioning member  80  is moved rearward by means of the spindle thread  89 , as a result of which the draw key  82  retreats into the V-shaped depression between the two spreading jaws  81 . The spreading jaws  81  are thereby driven apart from each other (see  FIG. 9 , where the top view shows the non-expanded initial state and the lower view shows an expanded state). Depending on the pitch of the spindle thread  89 , the surgeon can obtain a desired spreading action by means of a defined number of rotations of the handwheel  88 , as a result of which the cover piece  3  is adjusted in height in relation to the base piece  2 . For additional control of the attained state of expansion, a display mechanism  85  is provided. It comprises a scale  86  arranged on the spreading jaws  81 , and an indicator pin  87  arranged on the tensioning member  80 . The indicator pin  87  is drawn rearward together with the tensioning member during the spreading procedure, as a result of which its position relative to the scale  86  changes, and the state of expansion attained can thus be read off. 
         [0069]    A second alternative illustrative embodiment of the intervertebral fusion implant  1 ′ is shown in  FIG. 11 . Although it is substantially the same as the one in  FIG. 1 , it differs in that it is not designed rectilinearly in a box shape but is instead substantially curved in the shape of an arc of a circle. As regards structure and function, the above description applies by analogy. 
         [0070]    In a further embodiment, which is suitable in particular for a transverse implantation (as is indicated in  FIG. 1   a  by a broken line in the area of the intervertebral disk  90 ), a further embodiment of the intervertebral implant is provided. In the area of its end face  30 , it has an opening in which a part of the actuation instrument can be screwed (see opening  38  indicated by broken lines in  FIG. 2   a ). The actuation instrument  7  provided therefor is shown in  FIGS. 12 and 13 . The actuation instrument comprises a lever part  71  and a guide part  73 . Moreover, a threaded rod  72  is provided, which is to be fitted into an axial bore  75 ′ arranged along the longitudinal axis of the lever part  71 . At its rear end, the lever part  71  has a toggle  75  protruding radially to the side. At its front end, the lever part  71  has a gripping jaw  71 ′, which is designed to grip the bottom wall  22  of the base piece  2 . At its front end, the guide piece  73  has a second gripping jaw  73 ′, which is correspondingly designed to grip the cover wall  33  of the cover piece  3 . At the rear end, the guide piece  73  has a guide bell  77 , which is open on one side at the front and, at its rear, closed end, has a central handle  78 . On its front edge surrounding the open end of the bell, it has a recess  79 . The recess  79  is dimensioned such that, seen in the radial direction, it has a greater width than the toggle  75  of the lever piece  71 . 
         [0071]    For assembly, the threaded rod  72  is pushed through the axial bore  75 ′ of the lever piece  71  and, with a thread  72 ′ at its front end, is fastened in a screw thread of the opening  38  on the end face  30  of the cover piece  3 . It is screwed tightly using a knurled head  76  at the rear end of the threaded rod  72 . The lever part  71 , with the cover piece  3  fastened thereon, is then mounted on the guide piece  73 , specifically in such a way that its toggle  75  engages in the recess  79 . The base piece  2  is fitted onto the front end of the actuation piece  73 . In this way, the implant is fitted onto the instrument  7  in its assembly position, i.e. non-expanded. 
         [0072]    The expansion takes place by means of the cover piece  3  being successively tilted relative to the base piece  2 , such that the locking spring  42  always jumps one locking tooth  43  further on one side. This alternate tilting takes place by means of the toggle  75  being pivoted to and fro in the recess  79  from one radial end position to the other radial end position, as shown in  FIG. 13   c  by the double arrow. In this way, the cover piece  3  is successively tilted and thus at the same time expanded. The above description applies accordingly for the further description. 
         [0073]    Another alternative embodiment of the intervertebral fusion implant is shown in  FIGS. 14 to 17 . It is based on the embodiment shown in  FIG. 2 , and similar components are provided with the same reference signs and therefore, to this extent, require no explanation. This embodiment has an alternative catch mechanism  5 ′. This comprises a transverse pin  52 ′, which is arranged at the bottom on the side walls  31  and protrudes outward (see also the detailed view in  FIG. 16 ). The transverse pin  52 ′ engages in a window-like cutout  53 ′, which is formed in the vertical walls  21 . By having a shape extending in the vertical direction, they permit a height adjustment of the cover piece  2 , specifically until the transverse pin  52 ′ finally abuts against the upper end of the window-like cutout  53 ′. A further movement is thus prevented. Arranging the transverse pin  52 ′ at the bottom on the side walls  31  has the advantage that the teeth  43  can run uninterrupted on the side walls. The arrangement of teeth is therefore stiffer and stronger than the interrupted design in the alternative embodiment shown in  FIG. 5 . 
         [0074]    The embodiment shown in  FIG. 14  is also provided with a structuring. On its top face  33  and also on its bottom face  22 , it has furrows  29 , which extend in the transverse direction from one side to the other. They are approximately 2 mm wide and serve for better anchoring of the intervertebral fusion implant on the end plates  92 ,  93  of the vertebrae  9 . The edge area of the top face  33  and of the bottom face  22 , which edge area later comes to lie in the anterior or posterior area, remains free of the furrows  29 , so as not to adversely affect the load-bearing capacity in these areas. A large number of fine grooves  28  are formed in the longitudinal direction and extend from one end face to the other. They have a spacing of approximately 1 mm and likewise serve for better anchoring. Moreover, the top face  33  and also the bottom face  22  are provided with a coating of titanium spray  27 . This is finely particulate and, as a result, does not block the grooves  28 . This coating promotes growth of bone substance onto and into the implant. It will be noted that the structuring or the coating is not limited only to this embodiment but instead can also be provided just as well in the other embodiments. 
         [0075]    As can be seen from  FIG. 17 , the intervertebral fusion implant can have a wedge shape with a wedge angle γ. It is therefore better suited for implantation between vertebrae that are arranged in a more strongly curved section of the spinal column. It will be noted that the wedge shape is not limited only to this embodiment but instead can also be provided just as well in the other embodiments. 
         [0076]    A further embodiment of an actuation instrument  8 ′ is shown in  FIGS. 18 to 20 . In terms of its front area, designed to receive the intervertebral fusion implant, it substantially corresponds to the instrument shown in  FIG. 10 . Similar components bear the same reference signs and therefore to this extent require no explanation. The instrument  8 ′ is designed as a parallel expander. It is actuated like forceps, wherein the spreading jaws  81 ″,  81 ′″ are moved rectilinearly away from each other. A particular feature is that the instrument  8 ′ has, firstly, a cross lever  89 ′, with which a reverse kinematics system is formed. Its function is such that, when the instrument  8 ′ is pressed together in the rear area of the handle, the spreading jaws  81 ″,  81 ′″ in the front area move away from each other. This greatly simplifies handling. Secondly, a mechanism with a draw key  82  is provided for expansion, as is also the case in the instrument shown in  FIGS. 9 and 10 . 
         [0077]    Moreover, the actuation instrument  8 ′ has a limit stop device  87 ′. The latter limits the adjustment path of the draw key  82 ′ and, consequently, the expansion stroke. For this purpose, a rotary wheel  88 ′ is provided at the rear end of the instrument  8 ′ and can be adjusted to predefined positions. Depending on the position, it forms a limit stop  87 ′ that protrudes to a greater or lesser extent and that is provided for the pulling member  80 , which is actuated via a trigger lever  80 ′. To obtain expansion of the implant after it has been inserted, the surgeon only needs to actuate the trigger lever  80 ′, specifically until the rear end of the pulling member  80  bears on the limit stop  87  that is adjustable via the rotary wheel  88 ′. The implant is thus expanded in the desired manner. Markings  86 ′ arranged on the rotary wheel indicate the respective adjustable expansion positions. By adjusting the handwheel  88 ′ to the desired expansion position, it is automatically ensured that the actuation instrument  8 ′ stops when this position is reached. The danger of overexpansion, with the resulting risks to the patient, is thus eliminated.