DISC IMPLANT

FIELD OF THE INVENTION

The invention relates to an intervertebral disc implant for the total replacement of an intervertebral disc of the cervical spine and to a method for restoring function to a damaged func ^¬ tional spinal unit using such an intervertebral disc implant.

BACKGROUND OF THE INVENTION

Cervical disc prostheses have been in clinical use for total disc replacement in the cervical spine for more than 10 years. Motion preservation or restoration after cervical discectomy by the use of a disc prosthesis is believed to be advantageous com ^¬ pared to fusion with respect to certain biomechanical considera ^¬ tions. Fusion of a cervical spinal motion segments leads to significant increase of intradiscal pressure at the adjacent segments. Therefore, fusion might be a trigger for accelerated adjacent segment degeneration. Fusion after cervical discectomy also decreases or at least changes the range of motion of the cervical spine to a greater extent than arthroplasty. Typically, patients are found to be back to their daily activities quicker after arthroplasty than after fusion.

It is generally accepted that a cervical disc prosthesis should replicate the kinematics of a natural cervical disc as closely as possible. The disc prosthesis should be able to follow the natural motion of the respective motion segment after its im ^¬ plantation. If the biomechanical design of a disc prosthesis is inadequate, then the motion segment may not move naturally but is forced to follow the biomechanics of the prosthesis. Such un ^¬ natural motion may cause increased stress to the facet joints and painful facet degeneration, further it may cause increased stress forces at the contact area of the prosthesis with the ad ^¬ jacent vertebral bodies endplates thus leading to implant migra ^¬ tion into the vertebral bodies or even implant displacement.

Further, it must be presumed that biomechanically inadequate disc prostheses also change the kinematics at the adjacent mo- tion segments and therefore bear a similar risk for adjacent segment degeneration than conventional fusion.

Accordingly, before designing a cervical disc prosthesis, the kinematics of a natural cervical motion segment must be fully understood. Several independent motion properties are found in a cervical motion segment. There is flexion-extension motion which is coupled to mild anterior-posterior translation. Translation is mainly found in the cranial segments and gradually decreases from C3/4 to C6/7 (the C 2 /3-motion-segment is mostly excluded in the respective studies in the literature, most probably be ^¬ cause the indications for total cervical arthroplasty generally include the segments from C3/4 to C6/7) . This motion pattern is determined by a center of rotation (COR) which is found approx ^¬ imately at the level of the upper endplate of the C7-vertebra at C6/7 and which gradually moves caudally for the cranial motion segments being approximately 12mm below the upper C4-endplate for the C3/4-segment . In addition, there is coupled motion for side-bending and rotation between C3 and C7 : every side-bending at these cervical motion segments also leads to ipsilateral ro ^¬ tation, and rotation leads to ipsilateral side-bending. This mo ^¬ tion pattern is defined by a center of rotation or "COR" which is entirely independent from the previously described COR for flexion-extension and is found superior to the lower endplate of the upper vertebra of the respective motion segment. This coupled side-bending/rotation is facilitated around a longitud ^¬ inal anterior-posterior axis through the respective COR following an oblique direction approximately crossing the anterior edge of the lower endplate of the upper vertebra and finally crossing the posterior edge of the upper endplate of the same vertebra .

In order to allow natural motion a cervical disc prosthesis must closely replicate the above mentioned biomechanical properties and must definitely allow motion through two independent CORs . Moreover, the prosthesis COR for flexion-extension must be variable in order to replicate the different kinematics for the up ^¬ per and the lower cervical spine. Finally, inter-individual differences for the respective patient's COR should be taken in account, and also the fact that candidates for cervical disc surgery often present degenerative changes at the entire cer ^¬ vical spine also affecting kinematics of their motion segments.

In order to achieve motion through two independent CORs, a three piece prosthesis with two endplates and an inlay offers greater flexibility with respect to its biomechanical design than a two piece construct. Such a three piece prosthesis with two inde ^¬ pendent gliding pairs - upper prosthesis-endplate versus upper surface of inlay, and lower surface of inlay versus lower pros ^¬ thesis endplate potentially bears the property of including two independent motion patterns in a single prosthesis. Neverthe ^¬ less, hardly any of the presently available three piece pros ^¬ thesis fulfill the above mentioned biomechanical requirements. With exception of the Bryan-prosthesis, which closely replicates natural motion, most other three piece prostheses lack adequate biomechanical properties. None of the three pieces prostheses types disclosed in patent US 2010/0137992 Al demonstrates a COR for side-bending/rotation above the implant as described above, neither a sufficiently variable COR for flexion/extension as de ^¬ scribed above. The two piece prosthesis which is disclosed in patent US 2010/0137992 Al shows different diameters for

flexion/extension and side-bending/rotation, however the COR for side-bending is below the implant and therefore may not provide natural motion. The drawings of patent US 2005/0228497 Al show a disc prosthesis with several embodiments: some of the figures demonstrate unphysiological posterior translation in flexion and anterior translation in extension; other figures disclose saddle-like articulating surfaces allowing near- physiological translation with flexion/extension, but according to these drawings any rotation would cause cranio-caudal distraction of the implant which is entirely unphysiologic .

A brief overview of spinal anatomy and terminology will be bene ^¬ ficial in explaining one or more aspects of the inventions de ^¬ scribed herein. Figure 1A shows a functional spinal unit from a lateral or sagittal view having a bony superior or upper vertebral body 1 having a vertebral endplate 2 connected to a bony in ^¬ ferior or lower vertebral body 4 via an intervertebral disc 3 comprised of an outer ring of fibrous collagen material (the an- ulus) surrounding an inner amorphous mass of material, the nuc- leus pulposus. Also shown are the posterior elements including the spinous process 5, pedicle 6, facet joint 7 and transverse process 8. Figure IB shows a vertebral body 1, 4 from a trans ^¬ verse plane view or an axial cross-section along the cranial caudal axis. The front or anterior portion of the vertebral body 1, 4 is curved and the posterior portion is relatively flat. Further posterior lie the facet joints 7 and other posterior elements and various ligaments (not shown) .

SUMMARY OF THE INVENTION

One or more embodiments of the invention disclosed herein provide improved two part intervertebral or cervical disc im ^¬ plants for total cervical disc replacement which have two separ ^¬ ate independent CORs for flexion/extension and side- bending/rotation and the COR for flexion/extension is operable to be widely variable and further, the range of motion for side- bending and coupled rotation is hardly restricted by the flex ^¬ ion/extension angle after implantation. The implant or prosthetic designs disclose a convex curvature at the upper

prosthesis-endplate which is congruent to a both concave-convex saddle-like gliding surface at the lower prosthesis-endplate therefore facilitating anterior posterior translation and distribution of compressive forces over a greater contact area and avoiding point-like contact of the two articulating surfaces. The edge-design of one or more of the disclosed prostheses allow unrestricted motion for flexion/extension, side-bending and rotation within the physiological range of motion of a cervical motion segment, independent from the angle of prosthesis-im ^¬ plantation and independent from the momentary position of its endplates, therefore avoiding the risk of implant-loosening caused by high shear forces created from edge-contact, and therefore replicating natural conditions, where motion is mainly controlled and restricted by the uncovertebral joints, the facet joints and the posterior longitudinal ligament. Thus, the outer edges or the periphery of the members do not interfere with the motion of the implant or contact each other when positioned between opposing vertebral bodies.

In one embodiment provided herein an intervertebral disc implant may include a first or superior member including a first articu ^¬ lating surface formed in the shape of a convex projection and a second opposing surface configured to engage a first vertebral endplate and a second or inferior member including a second articulating surface, this surface further defines an extension having a saddle shaped projection with convex profile along a first axis and concave profile along a second axis perpendicular to said first axis and a second opposing surface configured to contact a second opposing vertebral endplate. In use, the first articulating surface rides along the convex portion of the second articulating surface in an arcuate path along the first axis thereby allowing multiple centers of rotation between said members and allowing travel along the first axis.

Each member may include a baseplate and/or and anchor from which the bearing surface extends .

The convex projection on the first articulating surface can op ^¬ tionally be convex along more than one axis, in three dimensions and/or asymmetrical about two or more axes.

In some embodiments one or more kinematic conditions may apply: the rotation between the convex projection and the saddle shaped projection is not limited; translation between the convex projection and the saddle shaped projection along the first axis is curvilinear; translation between the convex projection and the saddle shaped projection along the second axis is limited;

translation between the convex projection and the saddle shaped projection along the second axis is prevented; translation along the medial lateral axis is limited; wherein translation along the medial lateral axis is prevented.

Also presented is method of restoring function to a damaged functional spinal unit using such an intervertebral disc implant which involves:

- establishing along a first endplate of an upper vertebral body a first articulating surface formed in the shape of a convex projection, said first articulating surface having a second opposing surface configured to contact a first vertebral endplate; - establishing along a second endplate of an opposing lower vertebral body a second articulating surface having a sadlle shaped projection with convex profile defining an anterior-posterior axis (first axis) and concave profile defining a medial-lateral axis (second axis) , said second articulating surface having a second opposing surface configured to contact a second opposing vertebral endplate of the lower vertebral body;

- mating said two articulating surfaces such that the first ar ^¬ ticulating surface rides along the convex portion of the second articulating surface in an arcuate path along the anterior-pos ^¬ terior axis thereby allowing multiple centers of rotation between said members and limiting translation along said anterior-posterior axis.

Another method described herein for restoring function to a damaged functional spinal unit using such an intervertebral disc implant including two opposing vertebral body endplates involves the following steps:

- implanting along a first endplate of an upper vertebral body a first or superior member including a first articulating surface, said surface defining an extension with a convex profile and wherein said articulating surface has second opposing surface, opposite said articulating surface, configured to engage a first vertebral endplate of the upper vertebral body;

- implanting along a second endplate of a lower vertebral body a second or inferior member including a second articulating surface, said surface defining an extension having a saddle shaped projection with convex profile along a first axis and concave profile along a second axis perpendicular to said first axis and a second opposing surface, opposite said articulating surface, configured to contact a second opposing vertebral endplate of the lower vertebral body;

- engaging said articulating surfaces such that the first artic ^¬ ulating surface is operable to translate along the convex por ^¬ tion of the second articulating surface in an arcuate path along the first axis thereby allowing multiple centers of rotation between said members and allowing travel along the first axis.

Finally, these methods may further includes a step wherein in ^¬ serting the second articulating along a surgical approach that is not collinear to a patient's anterior posterior axis and then mating said surface it with the first articulating surface and thereafter rotating said second surface such that said convex profile second surface is collinear with the anterior-posterior axis of the second endplate.

BRIEF DESCRIPTION OF THE DRAWINGS

Figs. 1A and IB show a sagittal view of a functional spinal unit and an axial view of a vertebral body.

Fig. 2 shows a side-view of a concept of a intervertebral disc implant positioned between opposing vertebral bodies, the pos ^¬ terior elements are not shown.

Figs. 3A-3C show side-views of an intervertebral disc implant inserted between adjacent vertebral bodies undergoing flexion, extension, and translation following a short radius like in the C6/7-segment .

Figs. 4A-4C show side-views of an intervertebral disc implant inserted between adjacent vertebral bodies undergoing flexion, extension, and translation following a wide radius like in thee C3/4-segment .

Figs. 5A-5C show frontal views of an intervertebral disc implant inserted between adjacent vertebral bodies undergoing side-bend ^¬ ing .

Fig. 6A-6C show side-views of an intervertebral disc implant malpositioned between adjacent vertebral bodies undergoing flex ^¬ ion, extension, and translation following a short radius like in the C6/7-segment .

Fig. 7A-7C show side-views of an intervertebral disc implant malpositioned between adjacent vertebral bodies undergoing flex ^¬ ion, extension, and translation following a wide radius like in the C3/4-segment .

Fig. 8A-8C show frontal views of an intervertebral disc implant malpositioned between adjacent vertebral bodies undergoing side- bending .

Fig. 9 shows a perspective view of a lower or inferior member of the intervertebral disc implant.

Fig. 10 shows a top view of the inferior member of the intervertebral disc implant according to Fig. 9 and cross-sectional views along the cutting lines A-A, B-B and C-C of Fig. 10.

Fig. 11 shows a frontal view of the inferior member of the intervertebral disc implant in Fig. 9 and includes a reference circular cross-section of a torus including 60 degree section for determining the geometry of the concavity of the saddle shaped projection.

Fig. 12A-12C shows a perspective view of the portion of a torus from which the shape of the "saddle-like" surface of the saddle shaped projection of the inferior member of the intervertebral disc implant can be derived.

Fig. 13A shows a perspective view of an superior member of the intervertebral disc member.

Fig. 13B shows a frontal view of the superior member in Fig. 13A.

Fig. 13C shows a cross-sectional view of the superior member in Fig. 13A and includes a reference cross-section of a sphere for determining the geometry of the convex projection.

Fig. 14-17 shows various fixation modalities of the superior member and the inferior member of the intervertebral disc im ^¬ plant in frontal, side, top and bottom views. DETAILED DESCIPTION

Implant devices and methods disclosed herein provide improved two part intervertebral/cervical disc prostheses for total cer ^¬ vical disc replacement which have two separate independent cen ^¬ ters of rotation or "CORs" for flexion/extension and side- bending/rotation and the COR for flexion/extension is operable to be widely variable and further, the range of motion for side- bending and coupled rotation is hardly restricted by the flex ^¬ ion/extension angle after implantation.

As disclosed herein the two parts, partners, or members of one or more aspects of the invention may form a coupling, joint, or pair of articulating, sliding or bearing surfaces operable to replace a damaged intervertebral disc and/or posterior elements of a functional spinal unit.

One or more embodiments of the intervertebral disc implant de ^¬ scribed herein provides for dorsoventral motion (flexion/extension) and laterolateral motion (side-bending) and rotation independently from each other over independent centers of rota ^¬ tion with amplitudes that differ in magnitude and follow a cra- nially convex curve with a variable radius for dorsoventral motion (flexion/extension), but follow a caudally convex curve with a defined radius for laterolateral motion (side-bending) .

The dorsoventral motion (flexion/extension) over the variable center of rotation, which results in a variable extent of dor ^¬ soventral translation together with dorsoventral rotation (flexion/extension) , may be facilitated by the articulation of the cranial (superior) member with the convex projection within the saddle shaped projection of the caudal (inferior) member in a manner that allows dorsoventral rotation (flexion/extension) of the cranial (superior) member, and/or rolling and/or gliding over the convex articulation of the caudal (inferior) member.

The independent laterolateral motion (side-bending) may be fa ^¬ cilitated by the articulation of the cranial (superior) member with the convex projection within the hyperbolic paraboloid "saddle-like" convex-concave articulation of the caudal (in- ferior) member in a manner that allows laterolateral rotation (side-bending) over a defined center of rotation located above the articulation of the two members independent from the moment ^¬ ary flexion/extension-angle or the momentary dorsoventral trans ^¬ lation amplitude.

The independent transversal rotation (rotation) is facilitated by the articulation of the cranial (superior) member with the convex projection within the saddle shaped projection of the caudal (inferior) member in a manner that the cranial (superior) member may always rotate around a fictitious vertical axis inde ^¬ pendent from the momentary flexion/extension-angle or the mo ^¬ mentary dorsoventral translation amplitude or the momentary side-bending-angle, and the cranial (superior) member also may always rotate around a fictitious axis that is perpendicular to the endplate of the cranial (superior) member independent from the momentary flexion/extension-angle or the momentary dorsov ^¬ entral translation amplitude or the momentary side-bending- angle .

The different motion-angles, curves and amplitudes, together with the independent ability for coupled laterolateral motion (side-bending) and rotation of a cervical two part disc pros ^¬ thesis, according to certain embodiments of the intervention, mimic the natural kinematics of a cervical intervertebral disc.

Turning to the figures, Fig. 2 illustrates one embodiment of one or more aspects of the invention in its implanted position between two opposing vertebral bodies 1, 4. The intervertebral disc implant 20 is shown from a side-view or sagittal plane per ^¬ spective as two paired articulating sliding members, where the upper or superior member 21 on its upper surface has a vertebral engagement portion or baseplate 22 and is adapted for a firm as ^¬ sembly to the upper vertebral body 1. Extending from the base ^¬ plate 22 is a convex projection 23 that has a preferably

spherically convex curvature defining a bearing surface. The lower or inferior member 25 has on its lower surface a vertebral engagement portion or baseplate 26 and is adapted for a firm as ^¬ sembly to the lower vertebral body 4. The opposing upper surface of the baseplate 26 has a sadlle shaped projection 27 which is convex in its longitudinal anterior-posterior extension along a first axis Al with a diameter approximately double (though other ratios are possible) the diameter of the preferably spherically convex projection 23 of the superior member 21, and which is concave in its transversal lateral extent along a second axis A2 with approximately the same diameter (or within l-5mm) as the spherically convex projection 23 of the superior member 21. The superior member 21 is also shown mated with and partially concealed by the concavity (along the anterior posterior axis or first axis Al in the frontal plane) contours of the inferior member 25 as depicted by the dotted line 28.

Either member may be implanted along either the superior or inferior endplate of the respective vertebral body and are thus interchangeable; references made herein with such terms as "up ^¬ per," "lower," "superior," "inferior," "first," and "second" should be construed accordingly in all embodiments describe herein. Also, although both bearing surfaces have been described in terms of circles and spheres other convexities and concavit ^¬ ies and other shapes that are less symmetrical, contain one or more flat surfaces or have varying radii of curvatures such as an elongated, shallow or pitched "D" or curvilinear "hour glass" concavities along one or both axes are contemplated. Although most of the paired bearing surfaces extensions of various embod ^¬ iments of the intervertebral disc implant include separate and distinct baseplates for contacting the opposing vertebral end- plates, one or both baseplates can be eliminated and the oppos ^¬ ing nonbearing surface of the projections can contact directly contact the endplate and affixed to it or held in place via friction or other means.

In order to better understand how the various intervertebral disc implant described herein function, a series of figures will be presented depicting how each member of the paired bearing surfaces in several implanted embodiments articulate relative to the other members and how natural physiologic motion of the spinal segment is preserved. The following series will show flexion, extension, and lateral bending with and without translation. Also shown will be implantations wherein the device is seemingly "malpositioned" yet it will be apparent the relatively natural physiologic motion is preserved between the segments. Finally, due to the manner in which the bearing surfaces are shaped and mated the device does not limit or restrict rotation (when not implanted the surfaces may be rotated 360 degrees) , the posterior elements and ligaments do however control the de ^¬ gree when the device is implanted. Medial-lateral translation is limited or even prevented (except for minute play between the surfaces) but anterior-posterior translation is on limited in its arcurate or curvilinear path or trajectory (perpendicular) along the anterior posterior axis. In some embodiments the inferior member include a flanged, rim, protrusion or stop surface that prevents further translation in either of both portions of the saddle shaped projection. The stop surface can be located at any point along the bearing surface such as the extreme ends of the baseplate or that area corresponding the edge of the end- plate. Other locations could be chosen to prevent the tendency of slipped discs (limit anterior translation) or to lessen forces on the facet joins (limit posterior translation) .

Figs. 3A-3C depict a vertebral segment with an intervertebral disc implant 20 according to one aspect of the invention. In the sequence, shown from a sagittal, or side view, the convex pro ^¬ jection 23 of the superior member 21 does not translate (or very minutely) along the anterior posterior axis of the saddle shaped projection 27 of the inferior member 25 which might be typical of the spinal motion found in cervical C6/7 vertebrae perhaps because of the smaller radius. Figs. 3A-3C show the vertebral segment undergoing flexion, in a neutral position, and under extension respectively.

Figs. 4A-4C depict a vertebral segment with an intervertebral disc implant 20 according to another aspect of the invention. In this sequence the convex projection 23 of the superior member 21 translates along the anterior posterior axis of the saddle shaped projection 27 of the inferior member 25 which might be typical of the spinal motion found in cervical C3/4 vertebrae perhaps because of the greater radius. Figs. 4A-4C show the ver ^¬ tebral segment undergoing flexion, in a neutral position, and under extension respectively. As in the above sequence the COR for all positions shown is different because there has been rolling or sliding linear translation along the anterior posterior axis of the saddle shaped projection 27 of the inferior member 25 and the point of contact between the two bearing sur ^¬ faces has changed. Also, in each position the bearing surfaces are free to rotate as in the above sequence. Finally, Figs. 5A- 5C show a intervertebral disc implant 20 from a frontal view im ^¬ planted within a segment undergoing lateral bending. Here it can be seen that the COR for side-bending is independent of that of extension, neutral, and flexion.

In the next sequence of figures, a similar device as depicted in Figs. 3, 4 and 5 is implanted under less than ideal conditions as a consequence of the surgery itself or because of unfavorable or degenerated anatomy. Fortunately, the bearing surfaces of the paired implant will provide a spatial and kinematic relationship between the two vertebral bodies 1, 4 that permits load bearing and natural movement. Consequently, though seemingly "malposi- tioned" the intervertebral disc implant 20 still mimics the nat ^¬ ural physiologic movement of the spinal segment. Figs. 6A-6C show an intervertebral disc implant 20 implanted with a segment such the coupling of the bearing surfaces creates a lordotic condition perhaps because the superior member 21 was placed in too posteriorly. As depicted, this sequence does not involve translation along the saddle shaped projection 27 of the inferior member 25. As can be seen in Fig. 6B, the so-called neutral position appears to be in light extension and in Fig. 6C the vertebral segment appears to be hyper extended but still func ^¬ tional and load bearing. In Figs. 7A-7C, the same intervertebral disc implant 20 is depicted undergoing translation in the same lordotically malpositioned implanted environment. The interver ^¬ tebral disc implant 20 is shown in flexion, neutral, and exten ^¬ sion respectively. Finally, Figs. 8A-8C show the same

intervertebral disc implant 20 from a frontal view implanted within a segment. The vertebral endplate 2 of the upper verteb ^¬ ral body 1 is malpositioned (not parallel and canted to the left) . However, as can be seen in right bending, neutral, and left bending sequence, relative physiologic movement of the seg ^¬ ment is preserved.

In one embodiment an intervertebral disc implant 20 according to one or more aspects of the invention is adapted especially for the use in the cervical spine includes a superior member 21 and an inferior member 25. Each member can include a baseplate 22, 26 with a projection 23, 27 on a first upper surface and a second lower opposing surface which is adapted for placement against an endplate 2 of the vertebral body 1, 4. Alternatively, the baseplate 22, 26 can simply be formed as the opposing side of the bearing surface of the projection 23, 27. Each projection 23, 27 or bearing surface of the paired members 21, 25 compris ^¬ ing the intervertebral disc implant 20 cooperates with the op ^¬ posing bearing surface when both members 21, 25 are implanted between opposing vertebral bodies 1, 4. The saddle shaped pro ^¬ jection 27 of the inferior member 25 is generally shaped to form a saddle-like structure that defines a convex-concave surface area. One such embodiment is depicted in Figs. 9 and 10 and in ^¬ cludes a baseplate 26 and saddle shaped projection 27 having a saddle like surface 30. The surface 30 area is convex in the sagittal plane along axis Al with a diameter Dl of approximately 12mm (plus or minus 5mm according to the different sizes of the implant) and represents a 45-degrees (or between 20 and 60 de ^¬ grees) sector of a circle CI in the sagittal plane with its "center" below. The surface 30 area is concave in the frontal section along axis A2 with a diameter D2 of approximately 6mm (plus or minus 5mm according to the different sizes of the im ^¬ plant) and represents a 60-degrees (or between 40 and 80 de ^¬ grees) sector of a circle in the frontal plane with its "center" above. The surface 30 area both results from a 60-degrees rota ^¬ tion of the 45-degrees-convex-sagittal-plane-circle-sector in the frontal plane with a radius of approximately 6mm (plus or minus 5mm according to the different sizes of the implant) and its rotation- center approx. 6mm (plus or minus 5mm according to the different sizes of the implant) above the mid-sagittal sec ^¬ tion. The surface 30 can be further characterized as providing a 45-degrees rotation of the 60-degrees-concave-frontal-plane- circle sector in the sagittal plane with a radius of approxim ^¬ ately 12mm (plus or minus 5mm according to the different sizes of the implant) and its rotation-center approximately 12mm (plus or minus 5mm according to the different sizes of the implant) below the mid-frontal section thus creating a saddle-like sur ^¬ face 30 with a sagittal-plane convexity being lowest in the mid- sagittal plane, and a frontal-plane concavity being highest in the mid-frontal plane and is enclosed by a margin defined by a transversal semayt-plane cutting off the 45-degrees-convex- sagittal-plane-circle-sector . Figs. 11 and 12A-12C show some of the geometric reference shapes for defining the concave and con ^¬ vex surface of the saddle shaped projection 27 of the inferior member 25 including a circle, torus, and saddle.

The convex projection 23 of the superior member 21 is generally shaped to form a convex, preferably spherical, surface area. One such embodiment is depicted in Figs. 13A and 13B and includes a baseplate 22 and convex projection 23 having a convex-like sur ^¬ face. The surface area is convex both in the sagittal and in the frontal plane with a diameter D2 of approximately 6mm (plus or minus 5mm according to the different sizes of the implant) and represents a 90-degrees-sector of a sphere though other sectors between 70 and 110 are possible. The surface is generally con ^¬ gruent to the surface area of the inferior member 25 and its shape can be characterized as that which is enclosed by a margin defined by a transversal semayt-plane cutting off the 90-degrees ball-sector. This reference ball C2 is depicted in Fig. 13C.

In the implant orientation in which the superior member 21 and inferior member 25 are coupled such that the opposing extensions are mated or otherwise contacting each other several advantages will be apparent. The flat design of the margins or periphery of the opposing baseplates 22, 26 and the height of the convex pro ^¬ jection 23 of the superior member 21 allow unrestricted motion within the physiological range of motion of cervical motion seg ^¬ ments for flexion/extension, side-bending and rotation without contact of the edges. Further, the saddle shaped projection 27 of the inferior member 25 together with the convex projection 23 of the superior member 21 allow anterior-posterior sliding of the superior member 21 over the inferior member 25 upon the convexity of the inferior member 25. The design further facilitates anterior-posterior rotation of the superior member 21 upon the inferior member 25 and anterior-posterior rolling of the superior member 21 upon the inferior member 25. Also, facilitated is side-bending (lateral rotation) of the superior member 21 upon the inferior member 25 with the center of rotation above. Also, the design further permits transversal rotation of the superior member 21 upon the inferior member 25 therefore coupled motion for side-bending (lateral rotation) and transversal rotation over a physiological oblique sagittal rotation-axis independent from the momentary extent of flexion/extension and therefore flexion/extension with a widely variable physiological center of rotation independent from the momentary extent of side-bending and/or rotation.

Regarding the material of the intervertebral disc implant 20, as per the intervention, the members 21, 25 are preferably manufac ^¬ tured from well-established materials from implantation tech ^¬ niques like titanium, titanium alloys, cobalt-chrome-alloys, tantalum alloys, carbon-fiber-composites, PEEK, ceramics, poly ^¬ ethylene, or a combination of any of the above, for instance. The articulating surfaces are preferably high gloss polished in order to minimize abrasion following the low-friction principle. It is further intended to cover the outer part of the two end- plates 2 with porous titanium or similar materials or bio-active materials, for instance, in order to promote bone ingrowth at the bone-prosthesis-interface.

In a further preferred design, as per the intervention, it is also intended to include polyethylene or other suitable plastics as a shock-absorbent layer in at least a portion of one of the members 21, 25 along the baseplate 22, 26 or projection 23, 27.

In a favored design, as per the intervention, the edge of the inferior member 25 has a trapezoid design close to the natural anatomy of the superior endplate of a cervical vertebral body with the longer side of the trapezoid being anteriorly and slightly curved, and the short side of the trapezoid posteriorly and straight. In this design, the outer side of this trapezoid endplate, which is facing the superior endplate of the vertebral body caudal to the respective disc space, has means for a firm fixation inside the vertebral body's endplate, like 1 or 2 keels, for instance, or spikes or a combination of any suitable devices for fixation of a disc prosthesis at a vertebral body's endplate . In a favored design, as per one aspect of the intervention, the edge of the superior member 21 has a slightly curved spherical dome-like design close to the natural anatomy of the inferior endplate of a cervical vertebral body. In this design, the outer side of this dome-shaped endplate, which is facing the inferior endplate of the vertebral body cranial to the respective disc space, has means for a firm fixation inside the vertebral body's endplate, like one or more anchors 24 or keels as shown in Fig 14 which may be blade like or formed from a row of spikes, a full or portion of a circular rim or flange as shown in Fig. 15, or various patterns of spikes or a combination of any suitable devices, such as adhesives, for fixation of an intervertebral disc implant 20 at an endplate 2 of a vertebral body 1, 4.

In a further design, both endplates have a trapezoid design as described above, both with suitable devices or anchors 24 for fixation at the adjacent endplate 2 of the vertebral body 1, 4.

In a further design, both endplates have a slightly curved spherical dome-shaped design as described above, both with suit ^¬ able devices for fixation at the adjacent vertebral bodies' end ^¬ plates .

In a favored design, the outer part of the edge of the endplate of the superior member 21, which faces the inferior endplate 2 of the vertebral body 1 cranial to the respective disc space, is

- irrespective of its shape, whether it is trapezoid or dome- shaped or other - slightly angled with respect to the inner sur ^¬ face of the edge of this sliding member in a manner that a lor ^¬ dotic angle of the outer surfaces of the endplates against each other of approximately 7 degrees (or between 5 andl5 degrees) is created when the inner surfaces of the edges of the two sliding members are parallel and the implant is in neutral position.

In a further design, the outer part of the edge of the endplate of the superior member 21, which faces the inferior endplate 2 of the vertebral body 1 cranial to the respective disc space, is

- irrespective of its shape, whether it is trapezoid or dome- shaped or other - not angled, therefore the outer surfaces of the endplates are parallel against each other when the inner surfaces of the edges of the two sliding members are parallel and the implant is in neutral position.

Certain preferred embodiments adapted especially for a cervical intervertebral disc implant may include the following dimen ^¬ sional considerations for a plate member or articulating member, a maximal width (lateral extension in a frontal section) of 13 to 21mm, including about 13mm, about 15mm, about 17mm, about 19mm, or about 21mm, a maximal depth (dorsoventral extension in a sagittal section) of 12 to 18mm, including about 12mm, about 14mm, about 16mm or about 18mm, and a maximal height of 5 to 13mm, including about 5mm, about 7mm, about 9mm, about 11mm, or about 13mm.

In one method of delivery, the site between the selected verteb ^¬ ral bodies is prepared and at least a portion of the interver ^¬ tebral disc implant is removed. The vertebral bodies may be distracted with instruments or by positioning the patient's neck or spine section along a convex surface. If the angle of the surgical approach is not directly anterior or posterior but rather anterior lateral or posterior lateral or lateral then the inferior member with the convex/concave projection can be delivered with the concave portion oriented perpendicularly to the axis of the implantation trajectory (or surgical approach) .

After it is mated with the superior member with the convex projection it can then be rotated such that the concave surface of the superior member is oriented centrally along the endplate and perpendicular to the longitudinal anterior posterior axis and the convex portion is perpendicular to the transverse lateral axis. Thus either member may be inserted first but the inferior member having the convex/concave projection or extension must be inserted along a precise trajectory to allow proper mating of the bearing surfaces before the inferior surface can be rotated into proper position along the anterior posterior axis. Thereafter, each baseplate can be secured to the corresponding vertebral endplate. As such, translation is facilitated along the posterior lateral axis but restricted or limited along trans ^¬ verse lateral axis but as describe above rotation, flexion, ex ^¬ tension and lateral bending characteristics are preserved.

While various embodiments of the invention have been described with reference to preferred and exemplary embodiments, it will be understood by those skilled in the art that a variety of modifications, additions and deletions are within the scope of the invention, as defined by the following claims.