Abstract:
The invention relates to a cage-type intervertebral implant that is made up of a dished side wall ( 1 ), a cambered side wall ( 2 ), a front part ( 3 ), a rear part ( 4 ) and at least one intermediate wall ( 5, 6 ), thus comprising at least two cavities ( 7, 8, 9 ). An upper and a lower cage surface ( 10, 11 ) include a first lordosis angle (α 1 ) in the direction front part—rear part and a second lordosis angle (α 2 ) perpendicular thereto, said cage surfaces ( 10′, 11 ) intersecting outside the cage. The cage structure is characterized by a double-wedge geometry (double-wedge-shaped cage) that is defined by the two lordosis angles (α 1 ) and (α 2 ) and that advantageously adapts itself to the anatomical conditions in the intervertebral area. The cage is further characterized by a high moment of tilt that effectively counteracts a tipping of the cage. The method used for producing the cage structure is essentially characterized by working the cage material by means of a high-pressure water jet, said cold-cutting technique having proved to be the most economical.

Description:
FIELD OF THE INVENTION 
     The invention relates to a cage-type intervertebral implant, and also to a method for its production. 
     It relates to a vertebral column implant and its method of production, the implant being used for insertion between two vertebrae of the vertebral column. It serves as a means of fusion (arthrodesis) of the two vertebral bodies through which the original intervertebral disk height is restored and also the neural foreamen return to their original size. 
     BACKGROUND OF THE INVENTION 
     The individual vertebrae of the vertebral column have a vertebral body, a vertebral arch, a spinous process, two transverse processes, and two upper and two lower articular processes. The vertebrae are connected to the abutting intervertebral disks (disci intervertebralis) and give rise to the vertebral body (corpus vertebrae). The intervertebral disk consists of liquid-rich fibrous cartilage and connects the individual vertebral bodies with each other. The size of the intervertebral disks increases from top to bottom, corresponding to the loads arising in the human body. The intervertebral disks serve as elastic buffers and reislently damp impacts. 
     It is known that the intervertebral disks can become displaced, or that the inner gelatinous core (nucleus pulposus) can emerge through cracks in the cartilaginous outer ring (annulus fibrosus), which is similar to connective tissue. The intervertebral disk can then partially enter the inververtebral foramina (foramina intervertebralia) or into the spinal canal. Furthermore, this prolapse can be dorsal, medial, or lateral. Such prolapses most frequently occur at the L4-L5-S1 and C6-C7 vertebrae. If such prolapses are not treated, irreversible pressure damage of nerve roots, foramina or transverse lesions, result. If physiotherapy according to the symptoms, e.g., remedial exercises or massage, show no promise of success, the intervertebral disk (discuss intervertebralis) has to be surgically removed. There now exist the possibility of implantation of such an implant (cage), by means of which an arthrodesis between the two vertebral bodies can take place. 
     An intervertebral implant is known from EP 0916323-A1 which has a bean-shaped structure and can be inserted between two vertebrae. The implant has a wedge shape, conferred by a different height of the two longitudinal sidewalls. The walls surrounding the implant are provided with rows of holes in order to promote the ingrowth of bone tissue. 
     It is disadvantageous that the implant has a wedge shape in only one direction, and is expensive to manufacture because of the many laterally formed holes. 
     Furthermore, cage structures are known under the designation “Brantigan cage” structures are known which have many teeth on their cage surfaces in order to prevent an undesired displacement of the cage. Made of polyether ether ketone (PEEK), as so-called PEEK moldings, they have inadequate strength, which can lead to breakage of the cage structure under load. A single thread is provided to receive instruments, resulting in unsatisfactory instrument manipulation. 
     A relatively small moment of tilt is conferred by the cuboidal geometry, with disadvantageous effects. 
     SUMMARY OF THE INVENTION 
     The invention has as its object to provide a cage-type intervertebral implant which is characterized by a double wedge geometry defined by two lordosis angles which ensures an improved instrument manipulation. 
     A further object of the invention consists of the production of such an implant. 
     According to the invention, this object is attained with an implant according to the wording of patent claim  1  and by a method of production of the same according to the wording of patent claim  23 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described herein below using the accompanying drawings: 
     FIG. 1 shows a perspective view of a cage 
     FIG. 2 shows a plan view of the cage according to FIG. 1 
     FIGS. 3A-3D show sectional views of FIG. 2 
     FIGS. 4A-4C show sectional views of FIG. 2 
     FIG. 5 is a sectional view of FIG. 2 along the developed radium line ′a-a″. 
     FIGS. 6A-6B show side views of the rear portion with differently arranged guide elements 
     FIG. 7 is a plan view of an embodiment example of a cage with a partition and oblique rear portion with openings. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a cage  100  in a perspective view, consisting of a dished side wall  1 , a cambered side wall  2 , a front part  3  and a rear part  4 . The side walls  1  and  2  are connected by intermediate walls  5  and  6 , so that the interior of the cage is divided into cavities  7 ,  8  and  9 . A first, inner, radius of curvature R 1  is then allocated to the internal sidewall  1 , and a second, outer radius of curvature R 2  is allocated to the outer side wall  2 . 
     The side walls  1 ,  2 , the intermediate walls  5 ,  6 , the front part  3  and the rear part  4  have upper, or lower, respective boundaries which define an upper or a lower cage surface. 
     The dished side wall  1  has rounded openings  13 ,  14  and  15 , which are placed about in the middle of the cavities  7 ,  8  and  9 , and are conducive to the formation of bone substance. The cambered side wall  2  can likewise have such openings (not shown). 
     The cage surfaces  10 ,  11  have, in the region of the front part  3 , the part  4  and the intermediate walls  5 ,  6 , tabular raised portions  24 ,  25 ,  26 ,  27  which run substantially parallel to the cage surfaces and who properties are described hereinafter. 
     The front part  3  is rounded, and connects the sidewalls  1 ,  2  of the cage by means of an equal wall thickness. On the front side it has bevels  23 ,  23 ′ which facilitate the introduction and positioning of the cage in the intervertebral region. 
     The rear part  4  is of a rectangular form and connects the sidewalls  1 ,  2  of the cage, likewise by means of an equal wall thickness. It has a bore  20  on the rear side which is provided with an internal thread and is intended for instrument attachment. Guide elements  21  and  22  are arranged on both sides of the bore  20 , and are constituted here as, e.g., ribs, but can also consist of openings in the form of a half cylinder. The guide elements serve to guide instrument introduction, and prevent any improper rotary movement of the cage when the instrument is removed. As soon as the cage is situated in its final position between the two vertebrae, which among other things is the case when the axis of the instrument is perpendicular to the dorsal plane of the patient, the instrument can be detached from the cage. It has become apparent that this possibility of control has turned out to be very helpful and useful. 
     The transition of the guide elements, or ribs,  21 ,  22  to the surface of the rear part  4  is rounded off on both sides of the ribs, in order to avoid possible notch effects, which is of importance in embodiments in plastic or composite materials. Holes  31 ,  32  or  33  are provided in the rear part  4  and in the front part  3 , to receive a marker of a high density metal. Tantalum balls and/or pins are particularly suitable for this purpose. The pins are arranged in bores which are arranged either perpendicular or parallel to the bore  20 . The position of the cage can thereby be observed and assessed during the operation by means of an image intensifier. 
     FIG. 2 shows a plan view of the cage  100  according to FIG. 1, with data on the position of the sections A-A′ through D-D′ and E-E′ through G-G′. The course of a radius line a-a′ which runs through the middle of the cage can likewise be seen. 
     FIGS. 3A-3D show sectional illustrations of the cage, with section positions according to FIG.  2 . FIG. 3A shows a section A-A′ through the rear portion  4 . The hole  20  to receive an instrument, and the holes  31  and  32  for the markers, are visible. It can furthermore be recognized that the height at the outer side of the rear part is greater than that at the inner side. Thus the upper surface of the rear portion no longer runs parallel to the lower surface, as portions of the cage surfaces  10  and  11 . The two surfaces form a lordosis angle α 2 , which is 0.1-4°, but preferably 2°. This lordosis angle is shown on an exaggerated scale in FIG. 3A, easier recognition. It is shown by α 2 / 2  at the lower side of the rear part. 
     In the case that the height at the outer side of the rear part is smaller than that at the inner side, there results an opposed slant of the cage, or a wedge shape formed by the case surfaces  10  and  11 , with the point of the wedge facing in the reverse direction. If the lordosis angle α 2  in the two described cases is identically zero, the cage surfaces  10  and  11  are then parallel, as a special case or borderline case, which of course represents a less preferred design of the cage. 
     The raised parts  24 ,  24 ′ are affixed to the parts  10 ,  11  of the cage surfaces, and here are constituted parallel to the cage surfaces, although this by no means obligatory. 
     FIGS. 3B and 3C show a section B-B′, or C-C′, through the intermediate walls  5  or  6 , with the raised parts  25 ,  25 ′, or  26 ,  26 ′, which are positioned on the parts of the cage surfaces  10 ,  11 . These raised parts again run substantially parallel to the cage surfaces which likewise enable the lordosis angle α 2  to be perceived. 
     FIG. 3D shows a section D-D′ through the front part  3 . The bore  33  for the marker can be seen. The raised parts  27 ,  27 ′ can also be seen, which are affixed to the  10 ,  11  of the cage surfaces of the front part  3 . Again, these raised parts run substantially parallel to the cage surfaces, which likewise enable the lordosis angle α 2  to be recognized. 
     The raised parts  24 ,  25 ,  26  and  27 , which all project from the cage surfaces  10 ,  11 , but are only 0.3-0.8 mm, serve to anchor the cage after the successful operation, and help to prevent a migration of the cage. 
     FIGS. 4A-4C show sectional diagrams of the cage, with positions of the cross sections according to FIG.  2 . There can be seen the sidewalls  1 ,  2 ; the upper and lower cage surfaces  10 ,  11 ; and the half lordosis angle α 2 , which is only shown in FIG. 4A on one side. 
     FIG. 5 shows a sectional diagram of FIG. 2 along the developed radius line a-a′. The hole  33  for the marker, the bevels  23 ,  23 ′, and the raised portions  27 ,  27 ′ can be seen in the front part  3 ; the hole  20  and the raised portions  24 ,  24 ′ can be seen in the rear part  4 . The intermediate walls  5 ,  6  respectively have the raised parts  25 ,  25 ′ or  26 ,  26 ′. 
     It can further be seen that the height of the front part  3  is greater than that of the rear part  4 . Thus the case surfaces  10  and  11  no longer run parallel. The two surfaces form a so-called lordosis angle α 1 , which is 2-8°, but preferably 3°, 5°, or 7°. This non-parallelism conditioned by the lordosis angle α 1  is shown on an exaggerated scale to FIG. 5, to be more easily visible. 
     The cage structure can of course be modified within wide limits within the scope of this invention. Thus, for example, the number of the intermediate walls  5 ,  6 , or that of the cavities  7 ,  8 ,  9 , or that of the cavities  7 ,  8 ,  9  is not limited to  2  or  3 . Cage structures with one or more intermediate walls are possible. 
     FIGS. 6A and 6B each show a side view of the rear part with different guide elements  21  and  22  arranged around the hole  20 . 
     In FIG. 6A, the guide elements are arranged approximately parallel to the cage surfaces  10 ,  11 , while those in FIG. 6B have an angle of about 45° to the cage surfaces. However, this angle can assume any value from 0 to 90°. The guide elements  21 ,  22  in their turn of course do not necessarily have to be arranged parallel to one another; they can also have a V-shaped arrangement. 
     Possible materials are plastics, carbon fiber reinforced plastics and metals or metal alloys. Plastics such as polyether ether ketone (PEEK), polyether ketone ether ether ketone (PEKEEK) and polysulfone (PS) are preferably used, and particularly preferred as composite materials, carbon fiber reinforced composites of polyether ether ketone (CFK/PEEK) and polyether ketone ether ketone ketone (PEKEEK), which are also known under the names of ULTRAPEK and OSTAPEK. 
     As metals or metal alloys, titanium and its alloys are preferably used, such as e.g., the titanium alloy Ti6-Al4-V according to ISO standard 5832-3. 
     The metallic cage can have a hydroxyapatite ceramic (HAK) coating or a tricalcium phosphate (TCP) coating, which advantageously affect the long-term properties of the implant. 
     The curved shape of the cage gives this an advantageous high tilting moment M, which effectively counteracts a tipping of the cage. In comparison with the moment of tilet of a cuboidal cage with equal middle cross section, equal length, and comparable cage structure, it has been found that the cage structures according to the invention exceed this by a factor of at least 1.30. For a cage according to FIG. 1, the factor is 1.58. 
     The advantages of the cage structure according to the invention result from the double-wedge geometry, which is defined by the two lordosis angles α 1  and α 2 , and that advantageously adapts itself to the anatomical condition in the intervertebral area. 
     The designation “double wedge-shaped cage” or “DWS cage” is therefore used for such a cage. 
     The raised portions positioned on the cage surfaces effectively prevent a migration of the cage during the healing process after a successful operation. 
     Cage structures of the described kind are distinguished by high strength attained in spite of a small proportion of material. The formation of bone material is thereby strongly accelerated. 
     It has been found that this property can be described by a Cage Mass Index (CMI), which is defined according to Equation (1), 
     
       
         CMI=Volume of cage material/Volume of cage  (1) 
       
     
     namely as the ratio of the cage material volume to the total cage volume. The results are: 
     (a) for CFK/PEEK, CFK/PEKEKK, CFK/PS less that 0.25, preferably 0.22, and 
     (b) for titanium or Ti alloys, less that 0.20, preferably 0.17, whereby the variations dependent on the on the cage sizes are taken into account and result in only unimportant difference. 
     The process for the production of such a cage is described hereinafter. It is divided into of four process steps, as follows: 
     1. Water Jet Cutting 
     In a first step, a blank of cage material is machined in a first direction by means of a high pressure water jet. This known and economical cold cutting process is as a rule operated with an abrasive addition at 3,000 bar (U. W. Hunziker-Jost, Swiss Precision Manufacturing Technique, p. 81-86, C. Hanser Verlag, Munich (1991)). 
     The blank is clamped so that the water jet is directed perpendicularly to the later cage surface. The contours of the sidewalls  1 ,  2 , of the front part  3 , of the rear part  4 , of the at least one intermediate wall  5 ,  6 , of the at least two cavities  7 ,  8 ,  9 , and of the guide elements  21 ,  22  are cut with high precision . The cut edges display little fraying. With material thicknesses of 10 mm, cutting speeds are attained of up to 100 mm/min for metals and up to 300 mm/min for composite materials 
     2. Milling 
     The cage blank cut from the blank in this manner is now clamped again in a second step, and in fact in a second direction, essentially perpendicular to the first direction, in which the cage blank is further machined with a miller. The surfaces milled are the cage faces  10 ,  11  corresponding to the lordosis angle α 1 , the wedge-shaped raised portions  24 ,  25 ,  26 ,  27  corresponding to the second lordosis angle α 2 , the beveled surfaces  23 ,  23 ′ of the front part  3 , the at least one opening of the side walls  1 , and the first hole  20 . Likewise in this step, the hole  20  is provided with an internal thread, which is intended to receive an instrument. Small-calibered milling inserts are used here on a CNC-steered automatic milling machine. 
     If openings are likewise to be provided in the outer sidewall  2 , the cage blank has to be newly clamped once more. 
     3. Affix Markers 
     In a third step, the markers are mounted on the cage blank; later, during the operation and thereafter, they make it possible to assess the position of the cage by means of an image intensifier. Second holes  31 ,  32 ,  33  are installed for the markers in the rear portion and the markers in the front portion, and the markers are inserted into them as tantalum balls and/or pins. 
     4. Finishing 
     In a fourth step the last operations, summarized as finishing, take place, namely trovalization in order to deburr or round off the partially sharp edges. Then follows the marking of the cage, which can be done by means of a laser marking device. The cage is next subjected to a cleaning process, which includes, for example, multi-step ultrasonic cleaning. The packaging of the cage likewise belongs to these finishing operations. 
     An important process step is cutting with a high-pressure water jet. An advantageous cutting process was thereby selected, which has proven to be particularly economical. 
     The examples described hereinafter give an insight into the diversity of the cage design, and their enumeration is not to be considered as final in any way. 
     FIG. 7 shows in plan view, as an embodiment example, a cage with one intermediate wall and an oblique rear part with openings. 
     Side walls  1 ,  2 , front part  3  with raised portion and bevel  23 , intermediate wall  5  with raised part, and the cavities  7 ,  9 , correspond to the cage elements described in FIG.  1 . The rear part  4  with a part raised with respect to the cage surface has here, however, a rhomboidal constitution. While the guide elements  21 ,  22  are constituted as recesses here, but are arranged on the rear part surface  4 ′ as in FIG. 1, the direction of the hole for receiving an instrument is shown at the edge of the cavity  7  by the position of the axis  30  for the hole. The hole is provided with an M4 internal thread. 
     The cage surfaces form a lordosis angle α 1  of 3° in the front part—rear part, and a lordosis angle α 2  of 2° in the direction of the centers for the curvature radii of the curved side walls  1  and  2 . Thus the height of the intermediate wall with raised part is 8.1 mm on the outer side and 7.8 mm on the inner side. The side wall  1  has two lateral openings which are situated about in the middle of the cavities  7  and  9 . The inner radius of curvature R 1  is 11 mm, and the outer radius of curvature R 2  is 19 mm, the respective centers being 1.1 mm apart. The cage was made of CFK/PEEK, a BYJET water jet cutting apparatus (Bystronic Laser AG, CH-3362 Niederönz) being used in the first process step. 
     As a further embodiment example, the cage structure according to FIG. 6 was manufactured from a titanium alloy, Ti6-Al4-V according to ISO Standard 5832-3, a BYJET water jet cutting apparatus likewise being used in the first process step.