in fig1 an embodiment of a total disc replacement device 1 according to the invention is represented which comprises a first and a second plate - shaped apposition member 2 , 3 and an elastic spacer 4 disposed between the first and second apposition member 2 , 3 . the first and second apposition members 2 , 3 and the elastic spacer 4 are being cut by a central axis 13 extending along the longitudinal axis of the vertebra . the first and second apposition members 2 , 3 each comprise an apposition surface 7 , 9 and an intermediate 8 , 10 whereby said apposition surfaces 7 , 9 are apt to abut the end plates of the adjoining vertebral bodies and whereby said intermediate surfaces 8 , 10 are arranged facing each other . furthermore , the elastic spacer 4 comprises a first and a second surface which are essentially parallel and abut an intermediate surface 8 , 10 of the apposition members 2 , 3 each . in order to limit the mutual translation of the apposition members 2 , 3 transverse to the central axis 13 the intermediate surfaces 8 , 10 of the first and second apposition members 2 , 3 are provided with first and second constraint means 22 , 23 . the first constraint means 22 is provided at the intermediate surface 8 of said first apposition member 2 while the second constraint means 23 is provided at the intermediate surface 10 of said second apposition member 3 in a manner that said second rigid constraint means 23 interfere with said first constraint means 22 and being configured such that a gap 21 is provided transversely as well as parallel to the central axis 13 between said first and second constraint means 22 , 23 in the unloaded state of the total disc replacement device 1 . the elastic spacer 4 completely fills the gap 21 such allowing a damping effect in case of relative motion of said first and second apposition member 2 , 3 in three orthogonal directions . as shown in fig2 and 3 the first constraint means 22 are configured as a projection having the form of a central pin 14 protruding over the intermediate surface 8 of the first apposition member 2 while the second constraint means 23 comprise first and second curved protrusions 15 , 16 at the intermediate surface 10 of the second apposition member 3 , said first and second curved protrusions 15 , 16 enclosing a recess partially surrounding said central pin 14 such forming a rigid spatial limitation . the central pivot pin 14 has a certain range of motion parallel and transverse to the central axis 13 within said spatial limitation . as shown in fig3 said gap 21 has a width w 1 parallel to a first transverse axis 24 being orthogonal to the central axis 13 and a width w 2 ≠ wi parallel to a second transverse axis 25 being orthogonal to the central axis 13 and the first transverse axis 24 such allowing relative motions of the first and second apposition member 2 , 3 being differently in the antero - posterior direction and in the lateral direction . furthermore , said central pin 14 has a convex tip further allowing a tilting motion between said first and second apposition member 2 , 3 when the elastic spacer 4 is compressed parallel to the central axis 13 until the intermediate surface 10 of the second apposition member 3 abuts the tip of the central pin 14 . said first and second curved protrusion 15 , 16 being provided with a concavely curved interior wall facing the central axis 13 which forms a boundary line of the cross - sectional area of each of said first and second protrusion 15 , 16 orthogonal to the central axis 13 , said boundary line being a section of an ellipse . furthermore , said first and second curved protrusions 15 , 16 have a cross - sectional area orthogonal to the central axis 13 such that the two crosssectional areas are centrally symmetric to the point of intersection where the central axis 13 cuts the intermediate surface 10 of said second apposition member ( 3 ). furthermore , said total disc replacement device 1 comprises form fitting retention means 6 disposed between each of said first and second apposition members 2 , 3 and said elastic spacer 6 such allowing a firm bonding of the elastic spacer 4 to the first and second apposition plates 2 , 3 . said retention means 6 consist of a peripheral frame 18 disposed near the periphery of the cross - sectional areas orthogonal to said central axis 13 of said first and second apposition member 2 , 3 and projecting out over said intermediate surfaces 8 , 10 of said first and second apposition members 2 , 3 . an undercut 19 is provided at said peripheral frame 18 which is encompassed by said elastic spacer 4 . furthermore , said peripheral frame 18 is provided with perforations 20 penetrating the peripheral frame 18 transversely to said central axis 13 and being filled with the material of said elastic spacer 4 . fig4 and 5 show posteriorly inserted total disc replacement devices 1 ′, 1 ″ that are both provided with constraint means 22 , 23 and form fitting retaining means 6 as illustrated in fig1 - 3 . the total disc replacement devices 1 ′, 1 ″ therefore essentially differ only in their lateral and antero - posterior dimensions from the total disc replacement device 1 shown in fig1 - 3 . due to the small access to the intervertebral space these lateral and antero - lateral dimensions must be selected such that the total disc replacement devices 1 ′, 1 ″&# 39 ; must be implanted in pairs . since one crucial aspect to apply the surgical techniques as described above are the intact anterior longitudinal ligaments and the posterior longitudinal ligaments . these ligaments and the residual elements of the annulus fibrosis provide segmental stability to the functional spine unit or spinal motion segment . from posterior lumbar interbody fusion surgery it is known that all these structures remain intact . therefore , posteriorly inserted total disc replacement devices 1 ′, 1 ′″ can be designed and applied with motion preserving elements , i . e . an elastic spacer 4 as well . said pair of total disc replacement devices 1 ′, 1 ″ are inserted through the posterior median incision as described above under variant 2 . the devices must be passed through the spinal canal during insertion . furthermore , the elastic spacer 4 allows even a slight malpositioning of the total disc replacement devices 1 ′, 1 ″ since there are no predefined centers of rotation as being provided by implants having e . g . a spherical ball - and - socket joint . the embodiment shown in fig6 - 8 differs from the embodiment shown in fig1 - 3 only in the shape of the cross - sectional area orthogonal to said central axis 13 of the first and second apposition member 2 , 3 . fig6 - 8 illustrate an embodiment also comprising a first apposition member 2 , a second apposition member 3 and an elastic spacer 4 therebetween . opposite said elastic spacer 4 the first and second apposition member 2 , 3 comprise a first apposition surface 7 , respectively a second apposition surface 9 , whereby said second apposition surface 9 is configured for abutting the base plate of a first intervertebral body contacting the total disc replacement device 1 on top and said first apposition surface 7 is configured for abutting the cover plate of a second intervertebral body contacting the total disc replacement device 1 at the bottom . each of the first and second apposition surfaces 7 , 9 is disposed transversely to the central axis 13 . when viewed parallel to said central axis 13 said first and second apposition members 2 , 3 have an elongated shape with a major axis 27 and a transverse minor axis 28 , whereby said central axis 13 , major axis 27 and transverse minor axis 28 intersect each other . said central axis 13 and said transverse minor axis 28 further define a middle plane 26 . furthermore , said first and second apposition member 2 , 3 have a cross - sectional area orthogonal to said central axis 13 which is essentially elliptical and comprises two concavities lying on different sides of said middle plane 26 and on the same side of said major axis 27 . the two concavities are disposed symmetrically to said middle plane 26 such that one of said two concavities is arranged in a first quadrant of a circle the centre of which coincides with the point of intersection of the major axis 27 , the transverse minor axis 28 and the central axis 13 and the circumference of which is tangent to the periphery of said cross - sectional area at the principal vertices . the second of said two concavities is arranged in a clockwise succeeding , second quadrant of said circle . furthermore , the two concavities have an essentially semielliptical shape and have a depth t measured parallel to said transverse minor axis 28 amounting to about 5 % of the maximum width b of said first and second apposition members 2 , 3 .
an intervertebral implant for implantation between an upper vertebra and a lower vertebra having a central axis . the implant may have a first member with a top surface for contacting at least a portion of the upper vertebra and a bottom surface as well as a second member with a top surface and a bottom surface for contacting at least a portion of the lower vertebra . an elastic spacer may be disposed between the first member and the second member . contraints may be employed to to limit the amount of lateral movement between the first and second members .