Bilateral lamina implant

A reinforcing implant for the lamina of a vertebra includes a transverse support, one anchoring device for the right-hand side and one anchoring device for the left-hand side of the lamina of the vertebra. An expansion element with a guide device and cheek elements is provided, the cheek elements being mounted to be longitudinally movable along the guide device. Bearing surfaces for the lamina are formed on outer faces of the cheek elements directed away from each other, and a return stop for the cheek elements is provided. At least one of the cheek elements is provided with an aligning device for modifying the orientation of its bearing surface relative to the guide device. In this way, a secure anchoring of the reinforcing implant on the resected surfaces produced during a laminectomy can be achieved, with precise adaptation of the reinforcing implant to the actual anatomical conditions following the laminectomy.

FIELD OF THE INVENTION

Background of the Invention

The invention relates to a bilateral reinforcing implant for the lamina of a vertebra, comprising a transverse support and one anchoring device for the left-hand side and one anchoring device for the right-hand side of the lamina of the vertebra.

The spinal column forms a central structural element of the human skeleton. It comprises a multiplicity of vertebrae arranged one above another in order to transmit loads and interconnected in an articulated manner in order to permit movements. The vertebrae of the spinal column are not identical and instead differ in shape depending on where they are located in the spinal column. They do however have some features in common. Thus, each vertebra has a solid vertebral body with two osseous projections (pedicles) which extend laterally and to the rear and which, in their rear part, are in turn connected by an osseous arch. In the connection area, this osseous arch is shaped as a broad plate (lamina) and has, at its center, a rearwardly protruding spinous process. The spinous process and two further transverse processes on the side surfaces of the pedicles form articulation points for muscles and ligaments. In the area where the pedicles merge into the broad lamina, an upper and a lower articulating process are arranged on each side of the vertebra. These each form part of a facet joint with an adjacent upper or lower vertebra. Moreover, for the load transmission of the vertebrae, an intervertebral disk is arranged in each case between the vertebral bodies of adjacent vertebrae and fills the space between the relatively flat cover surfaces of the adjacent vertebral bodies. The area bounded by the rear side of the vertebral body and by the osseous arch (vertebral arch) forms a hollow space in which nerve fibers running parallel to the spinal column are accommodated.

Diseases or injuries can lead to many different forms of back pain. Common causes are, in particular, defects of the intervertebral disk or of the facet joints, or the nerve fibers extending through the hollow space becoming pinched or trapped. In the latter case in particular, it is known that this pressure is often generated by osseous growths forming in the area of the hollow space. In order to combat the pain, the pressure must be reduced and, for this purpose, the growth has to be removed. For this purpose, an access route to the hollow space is usually created through the rear side of the vertebral arch, that is to say generally through the lamina, and the growths causing the problems are removed from there by means of suitable instruments known per se. When access is made on only one side of the vertebra, that is to say in one area of the lamina to the side of the spinous process, this is referred to as a unilateral approach, whereas, when access is made on both sides of the spinous process and the latter is then removed, this is referred to as a bilateral approach. In this procedure called a laminectomy, the opening created in the lamina is in most cases not closed again after the operation. It has been shown that this may prove problematic in the long term as regards mechanical stability and also as regards the rate of complications.

SUMMARY OF THE INVENTION

The object of the invention is to restore the mechanical stability of the vertebra and thus reduce or eliminate the potential problems.

The solution according to the invention lies in the features broadly disclosed herein. Advantageous developments of the invention are the subject matter of the detailed disclosure.

In a bilateral reinforcing implant for the lamina of a vertebra, comprising a transverse support and one anchoring device for the right-hand side and one anchoring device for the left-hand side of the lamina of the vertebra, an expansion element with a guide device and with cheek elements is provided according to the invention, which cheek elements are mounted so as to be longitudinally movable along the guide device, wherein bearing surfaces for the lamina are formed on outer faces of the cheek elements directed away from each other, and at least one of the cheek elements is provided with an aligning device for modifying the orientation of its bearing surface relative to the expansion element.

The invention is based on the concept of using the expansion element acting on the cheek elements in order to securely anchor the reinforcing implant on the resected surfaces produced during the laminectomy. This can be done with elastic widening of the lamina or of the vertebral arch, in order to further increase the fastening reliability in this way. Moreover, in combination with the outward bearing surfaces of the cheek elements, this has the advantage that a collapse of the vertebral arch, as could unfortunately occur hitherto, is rendered impossible. On the contrary, under the pressure hitherto leading to the collapse, the reinforcing implant is only pressed more firmly into its seat and is thus able to meet its purpose. Although the elastic widening in itself can ensure a stable fit in the long term, it is additionally possible to provide a return stop for the cheek elements, in order thereby to further increase the long-term fastening reliability.

The second cheek element is preferably likewise provided with an aligning device. In this way, the second cheek element can also be modified in terms of the orientation of its outer face relative to the guide device. This permits a more precise adaptation of the reinforcing implant to the actual anatomical conditions following the laminectomy. In particular, it allows the bearing surfaces of the cheek elements to be oriented in a wedge shape relative to each other while still fastening the implant as a whole in a manner free from tilting.

The return stop is advantageously designed as a clamping device acting between the cheek elements and the guide device. With such clamping, the expansion position achieved can be easily fixed after the expansion. This avoids slipping of the cheek elements. Should the safety demands as regards undesired movements of the cheek elements be higher, then the return stop can preferably be provided with locking elements, which are arranged between the cheek elements and the guide device. The locking means expediently comprise a ribbing and catches engaging in the latter. With this locking action, achieved by the ribbing in combination with the catch, a secure form-fit connection is made. This affords the advantage of achieving a sufficiently secure hold of the reinforcing implant even in the case of very active patients with corresponding loading of the spinal column.

It has proven useful if the expansion element has at least one area protruding laterally beyond the cheek elements. Thus, independently of the actual expansion position, it is ensured that the reinforcing implant with its cheek elements can be inserted into the opening created by the laminectomy only to such an extent that the bearing surface of the guide device facing the cheek elements bears on the lamina. This prevents the reinforcing implant from being inserted too far and with undesired consequences in terms of the nerves extending through the hollow space being subjected to pain or even in terms of these nerves being damaged or injured.

The guide device is preferably designed as a non-circular bar. Here, non-circular is understood as meaning that the bar does not have a circular cross section. By virtue of this non-circular design, the cheek elements guided on the guide bar are prevented from undesired twisting through a form fit. The bar is advantageously V-shaped, preferably forming a V angle of 10° to 20° at the center. A V angle of 20° has proven particularly useful. In this way, the guide bar fits particularly favorably into the anatomy of the rear of the vertebra. In particular, it barely juts out at all in this way, such that irritation of surrounding tissue is reduced to a minimum.

Provision can also be made that the guide device comprises a fork-shaped rail and a slide guided in the fork interspace. The guide is in this way enclosed and protected against damage, and irritation of the surrounding tissue is avoided. Advantageously, the slide is guided on the rail in such a way that the side ridges engage with a form fit in a pair of grooves. The arrangement will in most cases be such that a pair of grooves facing each other are arranged on the fork-shaped rail, while corresponding and complementary side ridges are arranged on opposite lateral sides of the slide and engage with a form fit in the grooves and guide the slide in a longitudinally movable manner therein. However, the reverse arrangement is also conceivable.

In principle, the slide can be moved by an externally applied movement. However, provision can also be made that a drive device for the slide is provided, which drive device bears on the rail. This drive device can in particular be a screw spindle or a toothed rack. In the latter case, an edge of the rail facing the slide is designed as a toothed rack, while a pinion meshing with the toothed rack is arranged on the slide. By rotation of the pinion, the slide then moves along the rail. It is not absolutely necessary that the pinion is arranged permanently on the slide, and instead it may suffice for the pinion to be temporarily provided for adjustment on the slide. A bore for bearing the pinion is preferably provided on the slide. Practical handling can be made even easier if the drive device is self-locking. This is understood as meaning that, without application of an adjustment force, the drive device does not automatically move under the influence of a force acting on the slide; this means in particular that, when the drive device is not actuated, the expansion element cannot be pressed together, and instead such a movement is blocked by the self-locking. In this way, the position reached upon the adjustment can be secured additionally to the securing by a return stop, in particular a clamping device.

The bearing surfaces on the cheek elements preferably have pointed protuberances (spikes). Proven shapes of such spikes are, for example, conical tips, pyramids, prismatic or V-shaped elevations. Secure primary fixation can be achieved in this way. In order to additionally achieve a rapid and reliable secondary fixation, the bearing surfaces are preferably provided with a coating that promotes bone growth. This can in particular be hydroxyapatite or other osteo-inductive substances.

The bearing surfaces are preferably arranged on the two cheek elements in such a way as to be flush with one another. This is understood as meaning that they do not have a horizontal or vertical offset as seen in the direction of the adjustment path of the expansion element. This avoids an asymmetrical force being applied to the reinforcing implant, with the result that there are no undesired torques acting on the reinforcing implant and seeking to turn it from its intended position.

At least one fixing tongue is advantageously arranged on the cheek elements. It is expediently designed such that, in the implanted state, it bears on an outer surface of the pars of the vertebra. The angle of the fixing tongue in relation to the cheek element can preferably be modified in order to achieve a good bearing contact in accordance with the individual anatomy. This can be achieved in a practical and effective manner by a flexible design of the fixing tongue, preferably with a reduced material thickness in the area of the transition between fixing tongue and cheek element.

The fixing tongue advantageously has a receiving opening for a fastening device. The receiving opening is expediently designed for the polyaxial mounting of a pars screw. Polyaxial is understood as meaning that the screw, with its head, has a secure planar contact in the area of the receiving opening not only in a central position, but also at angle deviations of up to 15° in each direction. In this way, even with a different anatomy of the vertebra, the pars screw can always be inserted in an orientation favorable to fastening. The fastening reliability improves as a result. The receiving opening is preferably oblong and has a plurality of defined receiving positions for the fastening device. The defined receiving positions make it possible to provide different positions for the fastening device (in particular a screw) in relation to the fixing tongue. For this purpose, several dividing lugs are expediently provided, such that a pars screw is mounted with a form fit in each receiving position, which is not the case in a purely oblong hole. In this way, the pars screw can be arranged not only with a translational degree of freedom but also with two rotational degrees of freedom in relation to the fixing tongue, which permits reliable fastening even in difficult anatomical situations.

Receiving couplings for a spreading instrument are expediently provided on the expansion element. These can in particular be locating holes designed as blind bores. By means of these locating holes, the spreading instrument can be coupled quickly and easily to the expansion element. For this purpose, spreading arms of the spreading instrument are preferably provided with complementary carriers for the receiving coupling. A kind of quick-action coupling is thus formed which, in a particularly simple and convenient manner, permits a rapid connection between spreading instrument and implant.

The spreading instrument is expediently designed as a spreading forceps comprising a shaft and a handle. With a spreading forceps, the expansion element can be actuated comfortably and in a manner almost transparent to the operating surgeon. A transmission mechanism is expediently provided between shaft and handle and increases the course of an actuating movement introduced on the handle. For this purpose, the transmission mechanism is expediently provided with a double-action, L-shaped lever, of which the pivot point divides the lever in the ratio of at least 2:1.

It has proven useful if the transmission mechanism is arranged in the shaft. Such an arrangement of the transmission mechanism saves space and thus also permits use in confined conditions without damage to the surrounding tissue. The transmission mechanism is advantageously designed such that the spreaders execute a linear movement. Thus, compared to a conventional rotatory spreading movement, it is possible to achieve more exact guiding of the elements of the expansion element, in particular of the slide. The danger of jamming, as would arise in the case of spreaders guided in an arc of a circle, can be countered in this way.

A drive instrument is preferably provided, which comprises a shaft with a coupling head at one end and with a handle at the other end, and also an angle marking facing the side. With such a drive instrument, the reinforcing implant can be easily and safely actuated even at its intended implantation site and can be brought into a desired clamping position. In particular, with the angle marking, it is possible to actuate a drive device (for example with a toothed rack) over a defined course, simply by means of a defined angle difference occurring upon actuation of the drive instrument. This heightens the reproducible nature of the spreading procedure, and it also allows less experienced operating surgeons to determine the expansion course in a reproducible manner. Angle marking and handle are advantageously combined with each other. This permits a particularly space-saving construction of the drive instrument.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1depicts a first illustrative embodiment of a reinforcing implant according to the invention, which is designated in its entirety by reference number1and is a so-called fork variant. The reinforcing implant has, as guide device, a fork-shaped slide bracket2, in which a carriage3is guided in a longitudinally movable manner. Cheek elements4,4′ are arranged on the slide bracket2and on the carriage3. On their outer faces, i.e. the faces43directed away from each other, they have a bearing surface for a lamina of the vertebra.

The fork-shaped slide bracket2and the carriage3cooperate in such a way that the carriage3is guided in a longitudinally movable manner on the slide bracket2. In this way, an expansion element1is formed that can adjust the distance between the two outer faces43of the two cheek elements4to different sizes. This adjustability makes it possible, by means of the expansion element and through movement of the carriage3along its guide on the fork-shaped slide bracket2, to create a reinforcing implant that can bridge gaps of different widths in the lamina of a vertebral body that have been produced by resection.

The fork-shaped slide bracket2has a top surface20, an underside21, beveled ends22,23, and substantially plane lateral surfaces24,25. Guide surfaces26,27, along which the carriage3is movable, are formed in the fork interspace parallel to the lateral surfaces24,25. For more precise and more secure guiding, each of the guide surfaces26,27is formed with a guide groove28extending parallel to the underside21. This is open toward the end23and leads, in the transition area, into the bevel of said end.

The carriage3is plate-shaped in its basic configuration and has two ends32,33and two lateral surfaces34,35. A longitudinal ridge38arranged on each of the two lateral surfaces34,35has a shape matching the guide grooves28and is dimensioned such that it is received in the latter with a form fit and minimal play. Near the end33, the carriage3is also provided with a through-opening36, in which a clamping device5is arranged for positioning and securing one of the cheek elements4. The other cheek element4′ is arranged at the other end of the expansion element1in the area of the end22and is secured by means of a similar type of clamping element5′. In the illustrative embodiment shown inFIG. 1, the clamping element5′ of the associated cheek element4′ is arranged in the area of a bridge of the fork-shaped slide bracket2.

The cheek elements4,4′ are constructed with mirror symmetry to each other (they can also be designed identically to each other). On their outer faces43directed away from the respective other cheek element4′,4, they each have a plurality of spikes44for anchoring in the resected surfaces of the lamina. The outer faces43with the spikes44are preferably provided with a coating, such as hydroxyapatite, that promotes bone growth. On their top surface40directed toward the underside21,31of the fork-shaped slide bracket2and of the carriage3, respectively, the cheek elements4,4′ are preferably provided with a further spike46, generally a spike46of smaller dimensions. This spike46is designed to cooperate with the underside21,31, which preferably has a ribbing, in such a way that, under the action of a clamping force applied by the clamping device5, an extremely firm and preferably form-fit connection is established between the cheek elements4,4′ and the fork-shaped slide bracket2and the carriage3. By means of the clamping device5,5′ the cheek elements4,4′ are drawn with their top surface40, and the spike46arranged on the latter, against the ribbing on the underside21,31. For this purpose, the clamping device5,5′ has a screw, which is designated in its entirety by reference number50and which comprises a screw head51and a shank52with an external thread. The head51has a greater diameter than the through-opening36in the carriage3, such that the shank52can engage through said opening into a corresponding mating thread53in the cheek element4,4′. By tightening of the screw50, the cheek element4,4′ is thus drawn against the underside31of the carriage3and in a corresponding manner against the underside21of the fork-shaped slide bracket2. This results in a force-fit and also a form-fit connection.

The cheek element4,4′ arranged on the carriage3has, on its outer face43, a greater dimension than the distance between the lateral surfaces34,35of the carriage3. In this way, the cheek element4,4′ protrudes beyond the area of the carriage3and is thus drawn by the clamping device5not only against the underside31of the carriage3but also against the underside21of the fork-shaped slide bracket2. The effect of the clamping force thus fixes the relative position between the carriage3and the fork-shaped slide bracket2, that is to say the respective position of the expansion element1. In this way, it is not only possible to provide a form-fit connection to secure the cheek element4,4′ against undesired relative twisting about the axis of the screw50of the clamping element5, it is also possible to prevent an undesired return movement of the carriage3by the effect of a compressing force acting on the cheek elements4,4′ through the form-fit engagement of the spikes46in the corresponding ribbing on the underside21of the slide bracket2. In this way, securing against twisting and also at the same time against return movement is achieved by a simple construction.

In order to actuate the expansion element1, a spreading instrument7is provided (seeFIG. 5) which moves the carriage3along the track defined by the grooves28, and relative to the fork-shaped slide bracket2, in such a way that the distance between the cheek elements4,4′ increases. To be able to do this from the direction of the top, the top surfaces20,30of the fork-shaped slide bracket2and carriage3, respectively, are provided, near their ends22,32, respectively, with at least two receiving openings29,39, respectively, designed as locating bores. In the embodiment shown inFIG. 1, two receiving openings29are arranged on the two fork-forming outer elements of the fork-shaped slide bracket2, and one receiving opening39is arranged on the carriage3. The receiving openings29are designed as locating holes for carriers arranged on a tip of the spreading instrument7. These are designed as cylindrical stubs79, of which two are arranged rigidly with respect to a shaft70of the instrument7and one is arranged on a longitudinally movable pusher member78of the instrument7. At the end of the shaft70remote from the stubs79, the instrument7has an actuating grip71, which is mounted pivotably on the shaft70. The actuating grip71acts, via a first linking rod72, on an L-shaped pivot lever73whose grip-side end is connected via a pivot bearing to the linking rod72and whose front end acts, via a sliding rail bearing75, on the pusher member78in such a way that the latter is moved linearly, specifically such that it is movable transversely with respect to the axis of the stubs79. In this way, a pivoting movement of the actuating grip71is converted into a linear movement of the pusher member78, in which the stub79′ arranged on the pusher member78moves away from the stubs79″ arranged fixedly on the shaft70. The arrangement of the stub79′ relative to the stubs79″ is chosen such that, in a starting position (without manual force acting on the actuating grip71), the stubs79′,79″ are all in a line (seeFIG. 6a) and, when the actuating grip71is moved, the movable stub79′ arranged centrally between the two stubs79″ fixed on the shaft moves out of the line.

Proceeding from the starting position, with the stubs79″ engaging in the receiving openings29designed as locating bores on the fork-shaped slide bracket2, and with the stub79′ engaging in the receiving opening39on the carriage3, increasing movement of the actuating grip71results in the carriage3being moved away from the fork-shaped slide bracket2(seeFIG. 6b), as a result of which the distance between the cheek elements4,4′ increases and the expansion element1is thus spread. By actuation of the clamping device5, the spread position (seeFIG. 6c) is fixed and secured against return movement, and therefore the actuating instrument7can be removed.

Not only does the form-fit coupling by means of the stubs79in the openings29,39function as a quick-action coupling allowing rapid and reliable coupling or separation even without visual monitoring, it also permits, through the form-fit engagement, a positive feedback of force in the sense that the operating surgeon can sense directly from the actuating grip71the forces that are produced by the spreading of the cheek elements4,4′. He thus acquires precise feedback as regards the pressing force exerted by the cheek elements4,4′ on the lamina of the vertebra9. In this way, the operating surgeon can suitably adjust the force applied during implantation.

FIG. 2shows an alternative embodiment, which mainly differs from the embodiment shown inFIG. 1in that an integrated drive device8is provided for spreading the expansion element1. For this purpose, the carriage3is provided with a receiving bore83as an abutment for a drive instrument80and, furthermore, a toothed configuration82is formed on the edge between the top surface20and the guide surface27. This acts as a stationary part of a drive device8, of which the movable part is formed by the drive instrument80(seeFIG. 7a) inserted into the receiving bore83. The drive instrument80comprises a shaft81with a grip84and, formed integrally on the shaft at the other end, a toothed wheel85, which is provided with a pin stump86. For actuation, the drive instrument80is fitted with its pin stump86into the drive bore83of the carriage3, as a result of which the toothed wheel85comes into engagement with the toothed configuration82on the fork-shaped slide bracket2. The drive device8is actuated by means of the drive instrument80being rotated from a starting position by way of the grip84, as a result of which the toothed wheel85meshing with the toothed configuration82moves with the carriage3along the toothed configuration82, and the cheek element4arranged on the carriage3moves away from the cheek element4′ arranged on the fork-shaped slide bracket2. When the desired distance is reached, then, as in the first embodiment shown inFIG. 1, the clamping device5is actuated by tightening of the clamping screw50, and the carriage is thus fixed in the position that it has reached. The drive instrument80can then be removed.

As in the first embodiment, the attainment of the desired position can be defined as being when a defined adjustment force is achieved. However, there is also the possibility of using a defined adjustment travel as the criterion, which can be set simply by an actuating angle of the drive tool8. In this respect, the grip84functions as an angle index. When the drive instrument80is inserted in a normal position, and if an adjustment travel corresponding to a rotation of 270° is provided, it is thus actuated by rotation until the grip84reaches a three-quarter turn corresponding to 270°. In this way, the desired adjustment is achieved, and therefore also the desired distance between the two cheek elements4,4′. A reproducible adjustment of the implant can thus be achieved even under different conditions of use.

It will be noted that the second embodiment, shown inFIG. 2, does not necessarily have to be adjusted by means of the drive device and the associated drive instrument80, and instead a receiving coupling for a gripping tool can likewise be provided, that is to say, as in the first embodiment, with locating bores29on the fork-shaped slide bracket2′ and a corresponding locating hole39on the carriage3′.

FIG. 3shows a third embodiment which, in contrast to the first two embodiments, does not have a fork-shaped slide bracket, but instead a one-sided slide bracket2′. The guide groove28′ has an upwardly directed undercut, in which a hook continuation of the longitudinal ridge38′ engages and thus holds the carriage3on the one-sided slide bracket2′. This embodiment is designated as “monorail”.

A fourth embodiment, shown inFIG. 4, mainly differs from the embodiments shown inFIGS. 1 to 3in that it is in a skeleton construction. The slide piece is in this case designed as a straight bar or V-shaped bar (the latter is shown inFIG. 4). It functions as a support for two sliding carriages3″, which are fitted onto and are guided in a longitudinally movable manner on the bar2″. Moreover, the bar2″ has a non-circular cross section, in the illustrative embodiment shown an oval cross section (seeFIG. 4d), and thus secures the sliding carriages3″ against twisting with respect to the bar2″. For this purpose, the sliding carriages3″ each have a through-opening congruent with respect to the cross-sectional shape of the bar2″. This through-opening is part of a clamp, which is actuated by a clamping device5″. This comprises a screw50which is mounted with its head51on one side of the clamp and cooperates with a mating thread53″ arranged on the other side of the clamp. By tightening of the screw50, the clamp is actuated and the respective sliding carriage3″ is thereby fixed relative to the bar2″. It will be noted that the non-circular cross section is not essential, and instead the cross section can also be round (seeFIG. 4c).

In order to actuate the expansion element thus formed, receiving couplings for the gripping tool7are once again formed on the sliding carriages3″. For this purpose, as in the aforementioned illustrative embodiments, the sliding carriage3″ in each case has a locating bore39, but, in contrast to the aforementioned embodiments, the bar2″ does not need to have any locating bore. The spreading tool7is fitted with one of its stationary stubs79″ into one of the locating bores39and with its movable stub79′ into the other locating bore39, as a result of which, when the spreading tool is actuated, the two sliding carriages3″ are moved away from each other along the guide defined by the bar2″. When the desired position is reached, it is secured by actuation of the clamping device5″, and the spreading tool7can be removed.

As in the embodiments described above, the cheek elements4,4′ are held by means of the clamping device.

In order to better fix the implant on the vertebra, a fixing tongue37is provided, which is arranged jutting out from the sliding carriages3″. It will be noted that such a fixing tongue37can also be provided in the other embodiments; it is explained in more detail below in respect of the embodiment shown inFIG. 4, and this explanation applies analogously to the other embodiments. The fixing tongue37is provided with a reduced material thickness370in the area of the transition to the sliding carriage3″. By virtue of this reduced material thickness370, the fixing tongue37can be bent in order to modify its angle position relative to the sliding carriage3″. This permits adaptation to the particular anatomical conditions of the vertebra, in order to achieve optimized bearing of the fixing tongue37on the vertebra, or more precisely on the pars thereof. At its free end, the fixing tongue37is provided with a fastening opening371. The latter is preferably elongate and, along its two long sides, is divided into three areas by two projections372. The edge of the fastening opening371is shaped obliquely in order to provide, together with the projections372, a conical contact surface. A pars screw377with a round receiving head can thus be mounted in a total of three positions in the fastening openings371: an upper position, a middle position between the pairs of projections372, and a lower position. The pars screw377is thus mounted in one of the positions in the fastening opening371of the fixing tongue37such that it can assume different axes (polyaxial) in two directions and through ±15°. The dimensions of the fastening openings371are chosen such that the different positions each lie 2 mm apart, that is to say an overall adjustment of 4 mm is permitted by the three positions.

FIGS. 8a, bshow an example of the arrangement of the implant in a vertebra9.FIG. 8ais a rear view of the vertebra9with a lamina93, where a spinous process92and adjoining areas to the right and left of the latter have been removed by resection (indicated by broken lines). The implant is inserted into the opening thus formed in the lamina93and, as has been described above, the expansion element1is moved apart such that the cheek elements4,4′ of the implant come to bear on the sectioned surfaces (shown by hatching inFIG. 8a) that have been produced during the resection of the lamina93and are braced against these surfaces. The implant thus forms a bridge connecting the two ends of the lamina93. The spikes46on the outer faces43of the cheek elements4,4′ engage in the sectioned surfaces on the lamina93and thus provide primary fastening of the implant. In order to further protect the implant against dislocation and against undesired twisting, further anchoring is provided on both sides by the fixing tongue with the pars screw377, specifically in the pars91of the vertebra9. The latter is symbolized for illustrative purposes in the partially cutaway view inFIG. 8c.