as seen in fig1 - 5 an intervertebral disk prosthesis has a lower plate 1 and an upper plate 2 having outer surfaces directed outwardly toward the respective flanking ( unillustrated ) vertebrae and each carrying anchor formations 3 . an intermediate sliding plate 13 is provided between the lower plate 1 and the upper plate 2 . at the intermediate sliding plate 13 there is formed a convexity 4 which faces the concave lower inner surface of the upper plate 2 . a guide ridge 5 projecting downward from the intermediate sliding plate 13 is shiftable in a front - to - back or dorso - ventral direction in a similarly extending but upwardly open guide groove 7 formed in the lower plate 1 . the guide ridge 5 has a pair of oppositely laterally directed side surfaces or flanks 8 each having an outwardly projecting lower edge formation 9 . the guide groove 7 similarly has a pair of confronting laterally inwardly directed side surfaces or flanks 6 each having an inwardly projecting upper edge formation 10 . thus the guide ridge 5 is captured in the guide groove 7 and , while it can slide front - to - back in the guide groove 7 , it cannot be pulled upwardly out of the guide groove 7 . in fig1 and 2 the anchor formations 3 are vertically projecting saw teeth . in addition the outer surfaces of the lower plate 1 and the upper plate 2 are formed with vertically open tool guides 17 that allow the implant to be fitted to a four - prong implant tool for installation . with these tool guides 17 it is possible to remove , and if necessary to reinsert , the same intervertebral disk prosthesis without damaging or even destroying it . in this embodiment the intermediate sliding plate 13 and the upper plate 2 are angled . the angles can be similar or different . fig3 shows that the intermediate sliding plate 13 formed on the top surface of the intermediate sliding plate 13 has an upwardly convexity 4 with a smooth upper surface 15 and the upper plate 2 has a concavity 8 with a complementary downwardly concave lower inner surface 16 . the surfaces 15 and 16 , seen in sagittal view , are of same or different shape . in addition the surfaces 15 and 16 are offset from the centers of the plates 1 and 2 dorsally or toward the rear ( the left in fig3 ), and the surrounding annular surfaces 12 and 14 are correspondingly radially formed wider in ventral direction . in sagittal view the upper surface of the upper plate 2 and the lower surface of the lower plate 1 are formed slightly convex toward the unillustrated vertebral bodies , and in plan view they form a virtually round and thus compact intervertebral disk prosthesis . furthermore , it is conceivable to embody the intervertebral disk prosthesis in a kidney - shaped manner or with another preferred shape . with regard to the material selection , it is usual to use the lower plate 1 and the upper plate 2 of titanium , titanium alloys or titanium carbide , alloys of cobalt and chromium or other suitable metals , tantalum or suitable tantalum compounds , suitable ceramic materials , plastics , diamond , carbon or composite materials . often the inner surfaces of the lower plate 1 and the upper plate 2 and the surfaces of the intermediate sliding plate 13 are mirror - polished in order to minimize wear . however , it has proven to be favorable for the intermediate sliding plate 13 including its convexity 4 and its guide ridge 5 to be made from polyethylene , although other elastically deformable materials with similar properties or combinations thereof can also be used . the intermediate sliding plate 13 is centrally formed with a recess in which at least one tantalum ball 19 is held to facilitate x - ray imaging of the installed implant . the ball 19 can be of any other non - radiolucent material , too . it is also within the scope of the invention that the parts of the prosthesis comprise other radiolucent tags for x - ray imaging . furthermore the intermediate sliding plate 13 is formed within the center of its guide ridge 5 with a downwardly open and longitudinally extending slit 18 that facilitates snapping the elastically deformable guide ridge 5 into the guide groove 7 in the much harder lower plate 1 . this slit 18 can be dispensed with when the material of the guide ridge 5 is sufficiently compressible to allow assembly of the implant . fig4 a to 4 f show further possible connections between the guide ridge 5 and the guide groove 7 . fig4 a shows a dovetail guide ridge 5 and a complementary dovetail guide groove 7 . in fig4 b a t - section guide ridge 10 with rounded corners formed on the lower plate 1 engages upward into a complementary t - section guide groove 11 formed in the guide ridge 5 . fig4 c shows a t - section guide ridge 10 with sharp edges engaged in a complementary t - section guide groove 11 of a guide ridge 5 . fig4 d shows additionally that the connection between the guide ridge 5 and the guide groove 7 can also be built the other way round , what is covered by the scope of the invention , too . the guide groove 7 is formed in the intermediate sliding plate 13 and the corresponding guide ridge 5 is integrally formed with or detachably connected to the lower plate 1 . here , in the intermediate sliding plate 13 a pair of the relief slits 18 flanks the guide ridge 5 of the lower plate 1 to allow the intermediate sliding plate 13 to be snapped onto it . fig4 e shows a pair of slits 18 formed in the guide ridge 5 of the intermediate sliding plate 13 while a pair narrow guide rails 20 formed in the guide groove 7 on the lower plate 1 engage upward into complementary guide slots in the guide ridge 5 to ensure good front - to - back guidance as sagittal straight - line motion . fig4 f shows a pair of slits 18 in the guide ridge 5 with respective tantalum balls 19 seated above them . often a rather rounded , semicircular or curved shape of the connection of the guide ridge 5 with the guide groove 7 is useful , since corners and edges are more susceptible to losses of stability of these connections in terms of material technology . fig5 shows the upper inner surface of the lower plate 1 with the guide groove 7 . the surrounding of the guide groove protrudes , which is indicated by the dotted line so that the surrounding encloses the projecting portions of the guide ridge 5 . the figure also shows that the sagittal straight - line motion is limited by a first body stop 21 in dorsal direction or the back and by a second body stop 22 in ventral direction or the front . these body stops 21 , 22 are positioned at or form the ends of the guide groove 7 and they interact with the guide ridge 5 which slides within the guide groove 7 . here it is also possible to form the guide groove 7 with only the first body stop 21 or only the second body stop 22 . then , a slot or an opening arises at the front or the back of the lower plate 1 allowing an easier mounting of the intermediate sliding plate 13 to the lower plate 1 . this opening results in the guide groove 7 being opened to the front which is also advantageous in case the intermediate sliding plate 13 has to be removed . this has to be done when the intermediate sliding plate 13 is damaged or another size is needed . the old intermediate sliding plate 13 can pass through the opening on the front of the lower plate 1 and another new intermediate sliding plate 13 can be inserted between lower plate 1 and upper plate 2 . another inherent advantage is given due to the use of an intermediate sliding plate 13 which is not built of an elastically deformable material , for example other than polyethylene . the intermediate sliding plate 13 can be built of a hard material that leads to other sliding properties between the intermediate sliding plate 13 and the lower plate 1 and / or between the intermediate sliding plate 13 and the upper plate 2 . even the properties of the lower plate 1 and / or the upper plate 2 can be altered to result in different and improved material pairing . for the relative straight - line motion between the intermediate sliding plate 13 and the lower plate 1 , it is sufficient that a minimal clearance is provided between the surfaces of guide ridge 5 and the guide groove 7 . then , of course , also the flat lower surface of the intermediate sliding plate 13 and the flat upper inner surface of the lower plate 1 are shaped adequately to allow this straight - line motion . to close the opening after insertion of an intermediate sliding plate 13 , a covering cap is provided . this covering cap can then be used as a body stop that limits the straight - line motion of the guide ridge 5 within the guide groove 7 on its own . preferably the covering cap is detachable or removable mounted to the opening within the lower plate 1 , whereas the connection is built in a conventional manner , for example by a snap closure or a screwed connection . the foregoing description of the preferred embodiment of the invention has been presented for purposes of illustration and description . it is not intended to be exhaustive or to limit the invention to the precise form disclosed , and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention . the embodiment was chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments as are suited to the particular use contemplated . it is intended that the scope of the invention be defined by the claims appended hereto , and their equivalents . the entirety of each of the above - described documents is incorporated by reference herein .
an intervertebral disk prostheses for the total replacement of an intervertebral disk of the lumbar and cervical spine has an upper plate having upwardly projecting formations anchoring it to an upper vertebra on its upper face and a concavity on its inner face surrounded by an edge . a lower plate is provided with downwardly projecting formations anchoring it to a lower vertebra on its lower face and a flat inner face surrounding a groove extending front - to - back . a middle plate between the upper and lower plate has on its upper face a convexity that is identically or differently shaped to the concavity on the inner face of the upper plate and a ridge extending front - to - back surrounded by a flat lower face of the middle plate . the ridge has flanks and the groove houses the ridge of the middle plate and permits the ridge to slide front - to - back in the groove .