Preparation device for preparing an intervertebral disc compartment

The invention relates to a preparation device for preparing an intervertebral disc compartment (14) which is delimited by a first and a second vertebra (16, 18). Such a preparation is carried out prior to the insertion of an intervertebral disc prosthesis (110), in order to guarantee a correct positioning of the intervertebral disc prosthesis between the vertebrae (16, 18). The preparation device (10) comprises a reference frame (22), which is capable of being fastened above an operating table (12), and a first fixing element (58) which is capable of being rigidly connected to the first vertebra (16) are capable of being fastened to the reference frame (22) in varying first positions. A second fixing element (68) is capable of being rigidly connected to the second vertebra (18) and capable of being fastened to the reference frame (22) in varying second positions. A material-abrading tool (78) is capable of being fastened to the reference frame (22), preferably in varying positions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a preparation device for preparing an intervertebral disc compartment which is delimited by a first and a second vertebra. Such a preparation is carried out prior to the insertion of an intervertebral disc prosthesis, in order to ensure a correct positioning of the intervertebral disc prosthesis between the vertebrae.

2. Description of the Prior Art

US 2002/0161446 A1 discloses a material-abrading tool which is capable of being fastened to a reference frame in various angular positions relative to the vertical. Prior go the preparation of the intervertebral disc compartment, a radiograph of the segment of the vertebral column in question is taken from the side. On the basis of this radiograph, the setting angle of the tool on the reference frame with respect to the vertical is determined. Then the reference frame is fastened laterally to the operating table, so that it extends over and beyond the patient. Via a ventral access canal the cervical vertebrae in question are exposed, the intervertebral disc that is present is removed, and the wedge-shaped tool is introduced into the exposed intervertebral disc compartment at the setting angle determined in advance. With the aid of the tool the vertebrae delimiting the intervertebral disc compartment are machined in such a manner that plane abutment faces arise for the intervertebral disc prosthesis.

However, it has become evident that in the course of intervertebral disc operations that are carried out using such known preparation devices complaints occur relatively frequently, the cause of which is to be found in incorrect loadings of the segment of the vertebral column in question.

SUMMARY OF THE INVENTION

For this reason, an object of the invention is to specify a preparation device with which the risk of later complaints of such a type is reduced.

This object is achieved by means of a preparation device for preparing an intervertebral disc compartment, which is delimited by a first and a second vertebra, for the insertion of an intervertebral disc prosthesis, with:a) a reference frame which is capable of being fastened above an operating table;b) a first fixing element which is capable of being rigidly connected to the first vertebra and capable of being fastened to the reference frame in varying first positions;c) a second fixing element which is capable of being rigidly connected to the second vertebra and capable of being fastened to the reference frame in varying second positions;d) optionally a material-abrading tool which is capable of being fastened to the reference frame in varying positions.

The invention is based on the perception that an intervertebral disc prosthesis can only alleviate complaints with respect to the vertebral column efficaciously, or eliminate them completely, when the anatomical specifications of the segment of the vertebral column in question are taken into account as far as possible. These include, in particular, the consideration that the intervertebral disc prosthesis is positioned in such a manner that the center of motion, which is predetermined by the vertebrae adjoining the intervertebral disc compartment, is retained as far as possible. The center of motion corresponds to the swivel axis or swivel point about which the two vertebrae are able to swivel relative to one another. More particularly, the center of motion is determined by the ventral intervertebral disc system and also by two dorsal joint parts of the spinal canal.

The muscular and ligamentous apparatus surrounding the segment of the vertebral column in question is matched to the center of motion.

However, if an intervertebral disc prosthesis, the joint of which is not optimally matched to the anatomically predetermined center of motion, is inserted into the intervertebral disc compartment, this muscular and ligamentous apparatus is loaded unnecessarily, which generally gives rise to complaints in the patient.

In the case of the device known from US 2002/0161446 A1 which was mentioned at the outset it is not possible to obtain the anatomically predetermined center of motion by means of the implanted intervertebral disc prosthesis. This is related, on the one hand, to the fact that the radiograph taken pre-operatively, on the basis of which the setting angle for the tool is determined, registers the segment of the vertebral column in the form that it takes in the affected patient. In this state, however, the vertebrae may have been displaced by reason of highly diverse causes—for example, osseous hypertrophies or intervertebral disc deformations—in such a way that the position of the vertebrae differs considerably from that of a healthy vertebral column. If the setting angle of the tool is determined on the basis of such a radiograph, the intervertebral disc prosthesis is ultimately implanted in such a way that it merely replaces a part of the affected vertebral column, but without transferring the latter back into a healthy position.

On the other hand, particularly in the course of operations in the cervical vertebral segment of the vertebral column, it frequently happens that the relatively small cervical vertebrae are displaced in the course of the exposure of the ventral access canal. These displacements are not taken into account in the course of the removal of material by the tool and the subsequent implanting of the intervertebral disc prosthesis, since the establishment of the setting angle is undertaken solely on the basis of the radiograph taken pre-operatively.

With the aid of the alignable fixing elements according to the invention, on the other hand, it is possible firstly to transfer the vertebrae delimiting the intervertebral disc compartment into a position such as corresponds to the healthy segment of the vertebral column of the patient being operated on in the given case.

For the purpose of determining this position, various methods may be employed.

If still older radiographs of the as yet unaffected vertebral column of the patients are available, the position of the vertebrae can be determined on the basis of these radiographs. In addition, there is the possibility of inferring from the position of the adjacent vertebrae the position of the vertebrae delimiting the intervertebral disc compartment such as could be observed in the case of a healthy vertebral column. A further possibility consists in distracting the vertebrae during the operation with the aid of the fixing elements. Provided that the surrounding ligamentous apparatus has been preserved, the vertebrae place themselves in a position such as might be observed in the case of a healthy vertebral column.

Conventional intervertebral disc prostheses consist of two supporting plates which each bear a part of a ball-and-socket joint. The supporting plates have flat external surfaces by which they are supported on flat-milled abutment faces of the vertebrae. The position of the abutment faces consequently determines the position of the intervertebral disc prosthesis between the two vertebrae. A part of the ball-and-socket joint is exchangeably fastened to the supporting plate, in order to take account of varying spacings between the vertebrae.

However, with such known intervertebral disc prostheses the center of motion can be obtained reliably only when one is prepared to abrade relatively large amounts of the healthy vertebral material. In order to avoid this, the use is recommended of intervertebral disc prostheses in which both parts of the ball-and-socket joint are exchangeable. This allows the spacing of the spherical ball surfaces from the abutment faces to be adapted to the height of the intervertebral disc compartment and to the position of the center of motion. Intervertebral disc prostheses of such a type are described in the international patent application entitled “Intervertebral disc prosthesis” filed on the same day by the applicant. The full content of this other application is hereby incorporated by reference.

A further advantage with the use of the fixing elements consists in the fact that the forces required for the alignment are supplied externally, and not via adjacent vertebrae. In addition, an alignment extracorporeally is possible with the aid of fixing elements that have been introduced percutaneously, even when a dorsal or ventral access canal for the intervertebral disc prosthesis is to be created only at a later time.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 and 2show a preparation device according to the invention in a greatly schematised side view and top view, respectively. The preparation device is denoted in its entirety by10and is fastened to an operating table12. The device10serves for preparing an intervertebral disc compartment14, which is formed between a first vertebra16and a second vertebra18of a patient20, for the insertion of an inter-vertebral disc prosthesis. In the embodiment that is represented inFIGS. 1 and 2, the vertebrae16,18are cervical vertebrae which are shown, for the sake of simplicity, without dorsal vertebral processes.

The preparation device10has the task of moving the vertebrae16,18into an anatomically correct position before the prosthesis is inserted. The anatomically correct position is the position that the vertebrae16,18would have if the patient's vertebral column was still healthy. Before or after the vertebrae have been brought into their anatomically correct position, the intervertebral disc compartment14can be machined with the aid of a material-abrading tool in such a way that the intervertebral disc prosthesis can be inserted into the intervertebral disc compartment in optimal position.

The preparation device10has a reference frame22which is constructed above the patient20who lies on his/her back on the operating table12. The reference frame22has two arcuate brackets24,26each having a circular cross-section. The two brackets24,26are connected to one another via rails28,30, that also have a circular cross-section. This connection is made via joint elements32a,32b,32c,32d, which inFIGS. 1 and 2are represented in schematised manner fixable as ball-and-socket joints. As can best be discerned inFIG. 1, the joint elements32a,32b,32c,32dallow the brackets24,26to be swivelled relative to the rails28,30. InFIG. 1the swivelling capability is indicated by arrows34,36.

The brackets24,26are so wide that in the angled position shown inFIG. 1they extend over the head38and the chest40of the patient20. The rails28,30can thus be lowered very close to the neck of the patient20without appreciably restricting the access canal to the cervical region for the surgeon.

The brackets24,26are engaged by first supports42a,42band second supports42c,42d, respectively, which align the reference frame22in relation to the operating table12. For this purpose, the supports42a,42b,42c,42dare received in height-adjustable manner in sockets44a,44band44c,44d, respectively, which are firmly connected to the operating table12. The displaceability of the supports42a,42bin the vertical direction is indicated inFIG. 1by arrows47a,47b.

The supports42a,42band42c,42dare fastened to the brackets24,26, respectively, via second joint elements46a,46band46c,46d, respectively. The second joint elements46a,46b,46c,46dare designed in such a manner that the brackets24,26can be fixed in varying angular positions. The swivelling capability of the brackets24,26relative to the supports42a,42b,42c,42dis indicated inFIG. 1by arrows48a,48c.

Furthermore, the second joint elements46a,46b,46c,46denable the entire reference frame22to be tilted about a longitudinal axis that extends parallel to the longitudinal direction of the operating table12. The tiltability is assisted by the fact that the second joint elements46a,46b,46c,46denclose the tubular brackets24,26, so that the brackets24,26in the second joint elements46a,46b,46c,46dcan be pushed back and forth also in the aforementioned longitudinal direction. Such a tilting can be generated, for example, by the supports42a,42csituated on one side of the operating table12being drawn out of, or pushed further into, the sockets44aand44c, respectively. In case sockets44a,44b,44c,44dwhich are already available on the operating table do not enable adjustability in height, the supports42a,42b,42c,42das such may also be of length-adjustable design, for example with the aid of a threaded pinion.

With its ends a first slide50encompasses the rails28, in such a manner that it is capable of being moved on the rails28,30in the longitudinal direction indicated by an arrow52and capable of being locked at arbitrary longitudinal positions with the aid of locking screws54a,54bor similar fixing means. The first slide50bears a first clamping element56which is firmly connected to the first slide50but which may also be capable of being fixed in varying positions along the longitudinal direction of the first slide50. The first clamping element56has the task of being able to fix a first fixing screw58in varying angular positions within a swivel plane that is perpendicular to a reference plane defined by the rails28,30and that extends parallel to the rails28,30. For this purpose the clamping element56may include, for example, a cylinder, which is supported so as to be capable of swivelling, with a bore into which the first fixing screw58can be introduced and fixed.

A second slide60is likewise capable of being moved in longitudinally displaceable manner on the slides28,30in a direction indicated by an arrow62and capable of being fixed on the rails28,30in an arbitrary longitudinal position with the aid of locking screws64a,64b. The second slide60bears a second clamping element66which is constructed so as to be analogous to the first clamping element56and which serves for fixing a second fixing screw68.

Between the two slides50,60a third slide70is arranged which likewise is capable of being moved on the rails28,30in a longitudinal direction indicated by72and capable of being fixed at the desired position with the aid of locking screws74a,74b. The third slide70bears a third clamping element76which serves for receiving a material-abrading tool78. The material-abrading tool78has a milling head80and is likewise capable of being fastened to the third clamping element76in varying angular positions. The third clamping element76may, in addition, be moved in a guide groove82in a transverse direction indicated by an arrow84.

In the following, the function of the preparation device10will be elucidated in more detail with reference toFIGS. 3 to 10, which show the two vertebrae16,18in an enlarged side view in different subsequent states during the preparation process.

Firstly the patient is aligned and fixed on the operating table12in a manner known as such. After the mounting of the reference frame22above the patient20, the surgeon prepares a ventral access canal to the vertebrae16,18, which is indicated by90inFIG. 2. In principle a dorsal access canal is suitable as well if the patient lies on his/her chest. Spatulas that are required for this purpose may be fastened to the rails28,30.

In a next step, the reference frame22may be (optionally) tilted about its longitudinal axis, as described above. As a result of such a tilting, a twisting of the vertebral column can be taken into account such as may arise as a result of pathological changes to the bone tissue but also as a result of the exposure of the ventral access canal90. In order to be able to detect such a twisting, the patient20is X-rayed from the side. The operating table12is now tilted about its longitudinal axis in such a way that the transverse processes (not shown inFIGS. 1 and 2) of the vertebrae16,18are aligned. Subsequently the reference frame22is likewise tilted until no double edges can be detected under fluoroscopy in bores provided in an alignment element22which is fastened to the rail30. In this way, the reference plane defined by the rails28,30extends exactly parallel to the transverse processes of the vertebrae16,18. Further particulars relating to this type of alignment are described in an international patent application entitled “System for aligning a material-abrading tool relative to an intervertebral disc compartment”, which was filed by the applicant on the same date as the pre-sent application. The full disclosure of this patent application is incorporated herein by reference.

After the (optional) alignment of the reference frame22relative to the vertebral column of the patient20, the fixing screws58,68are, according to a first embodiment, screwed into the vertebrae16,18via the previously exposed ventral access canal90. The screwing-in operation is indicated inFIG. 3by arrows94,96. In the course of this operation, the fixing screws58,68are screwed in so that they extend—preferably bicortically—through the thick lower plates of the vertebrae16,18which are distal from the intervertebral disc compartment14, as shown inFIG. 4. In this way, the fixing screws58,68are rigidly connected to the first vertebra16and to the second vertebra18, respectively. Since the thick lower plates are readily detectable radiographically, the precise positioning of the fixing screws58,68relative to the vertebrae16,18is comparatively easy.

In a next step, the two vertebrae16,18are brought into their natural correct anatomical position. This is understood to mean a position that the vertebrae16,18occupy if the patient is in an upright, non-stooped posture and no pathological changes whatsoever—such as, for example, bony processes on the lower plates—have appeared. In case the natural anatomical position of the patient20cannot be established on the basis of older radiographs, recourse may also be taken to a biometric survey of the segment of the vertebral column in question. Furthermore, there is the possibility of the surgeon distracting the two fixing screws58,68by hand. In the case of a functioning muscular and ligamentous apparatus, the two vertebrae16,18pass over into their natural anatomical position.

In the course of the transfer of the vertebrae16,18into their natural anatomical position the angle between the two fixing screws58,68will generally have to be changed. The two clamping elements56,66, through which the fixing screws58and68respectively extend, permit such a swivelling of the fixing screws in the forward and rearward directions in a swivel plane that extends perpendicular to the reference plane defined by the rails28,30. In the course of this transfer of the vertebrae16,18into their natural anatomical position it will furthermore generally be necessary to move the two slides50,60on the rails28,30while the fixing elements58,68are swivelled in the clamping elements56and66, respectively.

As soon as the natural anatomical position of the vertebrae16,18has been attained, the fixing screws58,68are fixed with the aid of the clamping elements56,66and with the aid of the locking screws54a,54band64a,64b, respectively.

In the embodiment that is represented here, it is assumed that the angle between the two vertebrae16,18has to be made smaller, so that the two fixing screws58,68are swivelled inwards, as indicated inFIG. 5by arrows98,100.

FIG. 6shows the two vertebrae16,18in their natural anatomical position as established by swivelling the two fixing screws58,68in relation to the stationary reference frame22.

In this fixed natural anatomical position the adjacent thick plates of the vertebrae16,18pointing towards one another are now machined in material-abrading manner with the aid of the material-abrading tool78. This is usually necessary in order to remove pathological bone deformations that extend into the spinal canal and often cause severe pain to the patient. As a side effect flat abutment faces for the intervertebral disc prosthesis to be inserted are prepared by this milling operation.

In this embodiment it is assumed that the lower plate of the first vertebra16pointing towards the second vertebra18is machined first. For this reason, the material-abrading tool78is fixed to the third clamping element76in such a way that the longitudinal axis of the milling head80takes up a predetermined angle relative to the first fixing screw58. This angle may amount to 2°, for example. In order to set the angle between the material-abrading tool78and the first fixing screw58, a wedge-shaped template102can be introduced into the interspace between the first fixing screw58and the material-abrading tool78, as indicated inFIG. 1by an arrow104. If another angle between the material-abrading tool78and the first fixing screw58is to be set, an appropriately shaped different template is to be used.

The spacing between the third slide70and the first slide50is chosen so that when the material-abrading tool78is operated a flat abutment face can be created without an unnecessarily large amount of bone material having to be removed.

FIG. 7shows the position of the milling head80at the start of the milling operation. The angle and the spacing between the first fixing screw58and the milling head80is defined with the aid of the clamping elements56and76, respectively, and the slides50and70, respectively, which are fixed to the rails28,30. By moving the third clamping element70along the guide groove82, or by means of a swivelling of the milling head80in a plane defined by the guide groove82, a flat abutment face, denoted by106inFIG. 8, for a supporting plate of the intervertebral disc prosthesis is now created on the first vertebra16.

This operation is now repeated in like manner for the second vertebra18, as indicated inFIG. 8. The abutment face created thereby on the second vertebra18is denoted by108inFIG. 9.

After conclusion of the machining of the vertebrae16,18with the aid of the material-abrading tool78, the space between the vertebrae16,18is prepared for insertion of the intervertebral disc prosthesis. AsFIG. 9shows, the two abutment faces106,108of the vertebrae16,18facing towards one another are now flat. Given appropriate choice of the template102, the abutment faces106,108include the natural anatomical angle of the intervertebral disc compartment.

In a next step, an intervertebral disc prosthesis110can now be inserted into the prepared interspace between the vertebrae16,18via the previously created ventral access canal, as shown inFIG. 10. The intervertebral disc prosthesis110comprises two supporting plates112,114which rest with their outward-pointing sides against the flat-milled faces106and108, respectively, of the vertebrae16,18. On the insides the supporting plates112,114bear joint parts116,118which together form a ball-and-socket joint.

It is to be understood that many variations of the method explained above are possible and still within the scope of the invention. For example, according to the method described above, the vertebrae16,18are transferred to their correct anatomical position with the help of the screws58,68prior to the milling operation. This is advantageous if prostheses having a fixed design are used. In this case it is necessary to prepare the intervertebral disc compartment by milling such that the prosthesis is received at its optimum position. The intervertebral disc compartment is thus adapted to the prosthesis.

However, in many cases it is not possible or sensible to adapt the shape of the intervertebral disc compartment to the prosthesis to be inserted. Instead, it is often advantageous to adapt the prosthesis to the intervertebral disc compartment. The latter approach is usually preferable because the surgeon should be as free as possible in his determination which parts of the bone tissue have to be removed. If the surgeon finds during the implant surgery that a substantial part of a vertebra has to be removed as a result of bone degenerations, no care should be taken what implications this will have to the position of the prosthesis.

If the prosthesis is adapted to the intervertebral disc compartment, it is sufficient to transfer the vertebrae to their correct anatomical position after the milling operation. For fixing the vertebrae during the milling operation other conventional means may be used, for example brackets.

FIG. 11shows a situation in which a substantial amount of bone material had to be removed by the surgeon using the material-abrading tool78. After the removal of the bone material the screws58,68are now inserted into the vertebrae16,18, and the preparation device10is used for transferring the vertebrae16,18into their anatomically correct position that has been determined beforehand on the basis of biometrical data. However, in this position the compartment between the vertebrae16,18has an unknown shape. For determining the shape, a template120is inserted (seeFIG. 12) into the compartment between the adjacent vertebrae16,18after the adjustment step has been finished. In the embodiment shown it is assumed that the template120has adjustable blades (not shown) so that the template120can have different geometries. The blades are adjusted until they abut along they edges on the milled surfaces of the vertebrae16,18.

Then the template120is retracted, and based on the geometry of the compartment determined with the help of the template120a prosthesis may now be assembled that perfectly fits into the compartment between the vertebrae16,18. A particularly suitable intervertebral disc prosthesis for this purpose is described in the international patent application entitled “Intervertebral disc prosthesis” that has been filed on the same day by the applicant. The full disclosure of this earlier application is incorporated herein by reference. In one embodiment a plurality of different joint parts116,118is provided from which the surgeon selects an appropriate set. This makes it possible to vary the center of curvature of the ball-and-socket joint, and thus to determine the possible movements of the adjacent vertebrae. Ideally the center of curvature of the ball-and-socket joint should be situated as closely as possible to the anatomically predetermined center of motion of the two vertebrae16,18. This position can be determined using fluoroscopes or simulation programs. Alternatively, or in addition, supporting plates112,114of varying thickness may be used to position the center of curvature in order.

Another alternative for positioning the center of curvature of the ball-and-socket joint as closely as possible to the anatomically predetermined center of motion is the use of additional plates, wedges, or wedged-shaped spacer elements122,124as shown inFIG. 13. Instead (or in addition to) of using supporting plates112,114of varying thickness the spacer elements122,124may be inserted between the outward surfaces of the supporting plates112,114of the intervertebral disc prosthesis110and the adjacent vertebrae16,18. The spacer elements122,124are selected from a set of spacer elements having two plane surfaces that are parallel to each other (i.e. spacer elements angle equals 0°) or form a spacer element angle between 1° and 45° therebetween. The spacer elements differ from one another with respect to the distance between the plane surfaces and the wedge angle. The suitable spacer elements122,124are selected on the basis of the shape of the compartment measured with the help of the template120. The intervertebral disc prosthesis110is now, together with the spacer elements122,124, inserted into the compartment, as has been described above with reference toFIG. 10. In this embodiment, the surgeon may either assemble the intervertebral disc prosthesis110immediately prior to its insertion into the intervertebral disc compartment, or even inside the compartment. To this end the surgeon should have direct access to a large variety of differently shaped spacer elements, such as wedge-shaped spacer elements, such that an adjustment to almost any arbitrary shape of the intervertebral disc prosthesis110is possible.