WO2011012705A1

WO2011012705A1 - Implantable vertebral prosthesis

Info

Publication number: WO2011012705A1
Application number: PCT/EP2010/061141
Authority: WO
Inventors: Michaël BRUNEAU
Original assignee: Universite Libre De Bruxelles
Priority date: 2009-07-30
Filing date: 2010-07-30
Publication date: 2011-02-03

Abstract

Implantable vertebral prosthesis (10), comprising a first member (11) fixedly attachable to a first vertebra (C1) and a second member (12) fixedly attachable to a second vertebra (C2), the second member (12) comprising a guide (122) arranged for guiding the first member (11) relative to the second member (12) along an arcuate path (123). The arcuate path (123) is so formed, that when the vertebral prosthesis (10) is in use and the first member (11) is attached to the first vertebra (C1) and the second member (12) is attached to the second vertebra (C2), movement of the first member (11) along the guide (122) effects substantially a physiological rotation of the first vertebra relative to the second vertebra. The first member (11) is secured to the second member (12) so as to limit displacement of the first member (11) relative to the second member (12) perpendicularly to the transverse plane to an amount of not more than the physiological flexion and extension of the first vertebra (C1) relative to the second vertebra (C2) in a sagittal plane of the patient when in use.

Description

IMPLANTABLE VERTEBRAL PROSTHESIS

[0001] The present invention is related to an implantable vertebral prosthesis. Particularly, the invention is related to a prosthesis for replacing damaged articulations of the atlas and axis region.

[0002] The first two cervical vertebrae Cl and C2, also referred to as respectively atlas and axis are extremely mobile and care for an important part of the skull's mobility relative to the spine. In particular, the range of axial rotation between Cl and C2 can amount to about 50°. No other vertebrae allow such a large range of axial rotation.

[0003] Severe and possibly irrecoverable injury at the occipito-cervical junctions and in particular fracture of the transverse ligament of the atlas, or a bone fracture of the atlas and/or axis, such as due to a traumatism, vertebral degeneration, tumour, etc. can cause an instability at the C1-C2 level. Such instability most often concerns the flexion-extension movement. At present, the above instability is obviated by complete immobilisation of the C1-C2 articulation. As a result, the mobility between the two vertebrae is reduced for all possible movements, not only those affected by the pathology (e.g. flexion- extension) , but also those which are not or only partially affected by the degenerations (e.g. axial rotation and lateral bending). Present treatments hence reduce the mobility more than is actually needed. This results in a severely reduced comfort of the patient.

[0004] From EP 1795136 and US 2006/079896, implantable vertebral prostheses are known for alleviating the load born by an intervertebral disc. The prostheses comprise a guide which guides the flexion-extension movement between two adjacent vertebrae. The guide maintains the intervertebral space intact, but substantially prevents axial rotation and lateral bending. Such prostheses can be suitable for the vertebrae below C2, where the physiological principal movement is in flexion- extension, and where the physiological movements in axial rotation and lateral bending are very small compared to the ones that can be effected physiologically by C1-C2. For the above reasons, such prostheses would form no solution to a treatment of the abovementioned pathologies at the C1-C2 level, as they would substantially suppress the principal movement allowed by C1-C2, namely axial rotation, in particular since axial rotation most often is not at all affected by such pathologies.

[0005] It is an aim of the present invention to increase the comfort of patients affected by the abovementioned pathologies, and in particular of the C1-C2 articulations, by providing a prosthesis enabling them to experience an increased mobility of the spine compared with state-of-the-art treatments.

[0006] The present invention therefore aims at providing an implantable vertebral prosthesis, capable of stabilising the unstable movement (s) (e.g. flexion- extension) and at the same time allowing the unaffected vertebral movements to be effected in a physiological or near-physiological way (e.g. axial rotation and possibly also lateral bending) . [0007] According to the present invention, there is provided an implantable vertebral prosthesis as set out in the appended claims.

[0008] Vertebral prostheses according to the invention are arranged to stabilise an unstable movement between two vertebrae, while at the same time allowing the other, stable, vertebral (relative) movements. For example, vertebral prostheses of the invention are capable of stabilising the flexion-extension movement, while allowing (axial) rotation and possibly lateral bending of such vertebrae. Hence, on the one hand the unstable movement is stabilised, which can mean, depending on the case, limited in range or completely fixated. On the other hand, other movements, and in particular axial rotation, can be maintained as physiologically occurring.

[0009] Vertebral prostheses of the invention hence allow an improved mobility of the patient compared to present treatments.

[0010] Vertebral prostheses according to the invention therefore comprise a first member fixedly attachable to a first vertebra and a second member fixedly attachable to a second vertebra. The second member comprises a guide arranged for guiding movement of the first member relative to the second member along an arcuate path.

[0011] According to a first aspect, the prosthesis is so formed that, in use (i.e. when the vertebral prosthesis is implanted) , the guide is arranged at a location where the arcuate path (or the guide) spans a portion of the perimeter of the second vertebra, as considered in a transverse plane of the patient. In addition, the arcuate path is so shaped, that when the vertebral prosthesis is in use and the first member is attached to the first vertebra and the second member is attached to the second vertebra, movement of the first member relative to the guide (i.e. along the arcuate path) effects (substantially) a physiological axial rotation of the first vertebra relative to the second vertebra. Such rotation can amount to at least a fraction (at least 40%, 50%, 60%) of the range in physiological axial rotation occurring in a healthy person.

[0012] Axial rotation refers to the rotation about an axis defined by the intersection of a sagittal and a frontal plane of the patient.

[0013] Furthermore, according to a second aspect of the invention, the first member is secured to the second member so as to limit or totally suppress displacement of the first member relative to the second member in a direction perpendicular to the plane of the arcuate path

(i.e. perpendicular to the transverse plane when the prosthesis is implanted) . Such displacement is limited to between zero displacement and not more than the physiological range in flexion and extension of the first vertebra relative to the second vertebra in a sagittal plane (as occurring in a healthy person) .

[0014] The physiological range in flexion and extension is about 10°, in particular for C1-C2.

[0015] The second aspect can achieve stabilisation of the possibly unstable flexion and extension movement. It possibly allows flexion and extension movement, although confined to a range as indicated. Therefore, the vertebral prosthesis can act in substitution of the damaged tissues, which would otherwise care for confinement.

[0016] Simultaneously, the first aspect of the prosthesis allows axial rotation between the vertebrae, being a movement mostly unaffected by the indicated pathologies, to be performed in a natural (physiological) way. This is achieved by configuring the guide in such a way that it guides the first member along a path or track, which is suitably arcuately shaped.

[0017] As the vertebral prosthesis allows such rotation to correspond to the physiological one, load constraints on the prosthesis and on the neighbouring vertebrae can be reduced. Consequently, there is less risk of failure of the prosthesis when in use.

[0018] The first member can be so secured to the second member that the flexion and extension of the first vertebra relative to the second vertebra is limited to a fraction of the physiological range of flexion and extension of a healthy person, such as to not more than 80%, 60%, 40% of that range, by limiting said displacement of the first member relative to the second member.

[0019] Preferably, displacement of the first member relative to the second member in a direction perpendicular to the plane of the arcuate path (corresponding in use to the transverse plane) amounts to maximally 5 mm, more preferably maximally 3 mm, most preferably maximally 2 mm. As a result, flexion and extension of the first vertebra relative to the second vertebra in a sagittal plane is limited.

[0020] The first member can be so secured to the second member that no relative displacement is allowed between the two in a direction perpendicular to the transverse plane. In other words, they are arranged at a fixed distance from each other. This means that such a prosthesis would completely suppress flexion and extension movement when in use.

[0021] Preferably, motion along the arcuate path is defined by instantaneous centres of rotation, which are located, in use, internal to the vertebral body of the second vertebra. [0022] Preferably, the guide has the form of the arcuate path.

[0023] Preferably, the first member is (laterally) tiltable relative to the second member. When the prosthesis is in use, such tilting corresponds to lateral bending of the first vertebra relative to the second vertebra. More preferably, the implantable vertebral prosthesis comprises engaging means for the first member and the second member to engage with each other for restricting said (lateral) tilting movement of the first member relative to the second member to at most 10°, preferably at most 5°. More preferably, the first member can tilt laterally relative to the second member over an angle between 2° and 10°, preferably between 2° and 5°.

[0024] Alternatively, the first member is not tiltable (laterally) relative to the second member.

[0025] Preferably, the vertebral prosthesis comprises at least two elements (sliders, rollers, etc.), movable along the arcuate path (they are in engagement with the guide) . The movable elements are arranged at a

(preferably fixed) distance from each other and are

(preferably rigidly) connected to the first member.

[0026] The provision of multiple movable elements provides for increased security in case one element, or the connection between one element and the first member fails. Moreover, when the at least two movable elements are in engagement with (are secured to) the guide at a fixed distance from each other, said tilting movement can be limited.

[0027] Preferably, the guide is configured to guide the first member relative to the second member along multiple arcuate paths. Said multiple arcuate paths are enveloped by (or interposed between) an inner arcuate path and an outer arcuate path. As a result, better conformance of the relative motion of the first member to the physiological axial rotation of the first vertebra relative to the second vertebra can be achieved for each patient.

[0028] The inner and outer arcuate paths both have a concave side, which is located at the side of the second vertebra .

[0029] The inner and outer arcuate paths are preferably located in a same plane, more preferably in the transverse plane. In use, the inner and outer arcuate paths span a same portion of perimeter of the second vertebra.

[0030] The inner and outer arcuate paths are suitably spaced apart. The interspacing can be constant. The interspacing alternatively increases towards the ends of the inner and outer arcuate paths.

[0031] Preferably, the guide comprises a sleeve-like case having the form of the arcuate path. The vertebral prosthesis in such case comprises a movable element provided in the sleeve-like case and connected to the first member. Preferably, the sleeve-like case comprises an opening for connection of the movable element to the first member. The movable element can be slidably provided in the sleeve-like case. The movable element is preferably rigidly connected to the first member.

[0032] Preferably, the internal cross-section of the sleeve-like case is larger than the cross-section of the movable element (s) therein. In addition, or alternatively, the internal cross-section of the sleeve-like case increases towards the ends of the guide (the arcuate path) . This enables motion of the first member along multiple arcuate paths.

[0033] Preferably, the movable element is a slider. Preferably, the slider is ball-shaped. More preferably, it is oval-shaped. [0034] Preferably, the sleeve-like case is so formed as to prevent the movable element from leaving the case. As a result, the first member is secured to the second member to (substantially) prevent said displacement in the direction perpendicular to the plane of the arcuate path (transverse plane). This prevents or limits flexion and extension movement.

[0035] Preferably, the first member and the second member are arcuately shaped in order to fit around the posterior of (the arch of) a vertebra.

[0036] Preferably, the first member and the second member are so formed as to be attachable to the Cl and C2 vertebrae .

[0037] Preferably, the first member and/or the second member is additionally attachable to respectively a third vertebra and/or a fourth vertebra.

[0038] Preferably, the guide and the elements movable along the arcuate path are wrapped in a sealed membrane for preventing ingrowth of biological tissue.

[0039] Preferably, the first member is rotatable relative to the second member, by movement along the arcuate path, over an angle of at least 25°, with angles of at least 30° being more preferred and angles of at least 40° being most preferred. As a result, axial rotation of the first vertebra relative to the second vertebra in a transverse plane can be effected over an angle respectively of at least 25°, of at least 30° and of at least 40°.

Brief Description of the Drawings

[0040] Figure 1 represents a vertebral prosthesis according to the invention implanted at the Cl and C2 vertebrae .

[0041] Figure 2 represents schematically an envelop of arcuate paths along which the first member is movable. [0042] Figures 3-5 represent perspective views of the implantable vertebral prosthesis as in figure 1.

[0043] Figure 6 represents the vertebral prosthesis of figure 1 schematically.

[0044] Figure 7 represents the fastening of the first/second member of the prosthesis to implanted screws.

Detailed Description of the Invention

[0045] A primary aim of the present invention is the stabilisation of an unstable intervertebral (i.e. between two or more vertebrae) movement or junction, while simultaneously not affecting, or affecting only in a minor way the other movements. Such other movements are possibly allowed or guided in such a way as would be physiological.

[0046] Even though stabilisation of the C1-C2 articulation is primarily aimed at by prostheses of the present invention, it will be clear that other, similar vertebral articulations can be treated by such prostheses. Prostheses of the invention are thus not limited in application to the Cl and C2 vertebrae only.

[0047] According to an aspect of the present invention there is thus provided a vertebral prosthesis, described with the aid of a non-limiting embodiment 10 as illustrated in figure 1. In figure 1, the vertebral prosthesis is shown in an implanted configuration in order to stabilise the movement between the Cl and C2 vertebrae.

[0048] A vertebral prosthesis 10 according to the invention comprises two main parts: a first member 11 and a second member 12. The first member 11 is so configured that it can be fixed to a first vertebra, for example Cl as shown in fig. 1. The second member 12 is so configured that it can be fixed to a second vertebra, for example C2 as shown in fig. 1. The fixation can be performed by suitable means as are known in the art, for example by screws 111 and 121. The fixation results in each of the members 11 and

12 being non-movably (rigidly) connected to the corresponding vertebra. Once fixed, the first member 11 will move integrally with the first vertebra (Cl) and the second member will move integrally with the second vertebra

(C2) .

[0049] The first and second vertebrae are preferably adjacent, though need not be so.

[0050] The first member 11 and the second member 12 can be fixed to respectively the first and second vertebra at any suitable location around such vertebrae (as considered in a transverse plane of the patient, i.e. a horizontal plane for a patient in an upright position) . Preferably, the location of such fixation is at a posterior or anterior side thereof. The first and second members 11 and 12 hence will span a part of the perimeter of the vertebrae in a transversal plane. Therefore, either one, or both the first member 11 and the second member 12 can be suitably arcuately shaped.

[0051] Most preferred is a fixation at the posterior of the vertebra. If necessary, a part of the spinous processes, or of the vertebral arch can be removed (e.g. by laminectomy) in order to better accommodate either one or both the first and second members. In figure 1 an implanting of the vertebral prosthesis 10 at the posterior side of the vertebrae Cl and C2 is shown.

[0052] It will be clear that vertebral prostheses 10 according to the invention are not arranged to be used as intervertebral disc prosthesis.

[0053] The second member 12 comprises a guide 122. The guide 122 is arranged for guiding the motion of the first member 11 relative to the second member 12 (the guide 122) along a path 123, which is suitably shaped into an arc or curved as will be indicated and which is referred to as an arcuate path. Such motion is forced to occur along the arcuate path 123, which is determined by the arrangement of the guide 122. The guide can therefore be formed into such arcuate path 123.

[0054] When the vertebral prosthesis 10 is implanted

(fixated to the vertebrae) as indicated, the arcuate path

123 is arranged substantially in a transverse plane. The path 123 is arcuately shaped, such that it allows axial rotation of the first vertebra relative to the second vertebra due to the relative movement of the first member 11 along the arcuate path 123 (the guide 122) . By suitably shaping the arcuate path 123 (and hence the guide 122), such relative movement can be made to conform to a physiological axial rotation of the first vertebra relative to the second vertebra.

[0055] The arcuate path 123 is therefore preferably curved towards one side only. This means that the radii of curvature of the arcuate path are preferably located at one side only, namely at the side of the vertebrae. The curvature at points along the arcuate path hence preferably does not change sign.

[0056] The arcuate path preferably has only a single concavity, which is located at the side of the vertebrae.

[0057] As a physiological axial rotation can span an angle of about 50° in the case of the C1-C2 vertebrae, the arcuate path 123 (and thereby the guide 122) is preferably so shaped that it allows axial rotation of the first vertebra relative to the second vertebra over an angle of at least 25°, more preferably at least 30°, most preferably at least 40°.

[0058] The guide 122 is preferably so shaped that the arcuate path 123 spans an angle α (see fig. 2) ranging between 25° and 60°, more preferably between 25° and 50°, even more preferably between 30° and 50°. The angle α refers to the angle enclosed by two normals 203 and 204 to the arcuate path, at both ends thereof, as illustrated in figure 2.

[0059] In other words, the guide 122, and by extension, the arcuate path 123 preferably allow a rotation of the first member 11 relative to the second member 12 over an angle of at least 25°, more preferably at least 30°, most preferably at least 40° (i.e. by motion along the arcuate path) .

[0060] Motion of the first member 11 along the arcuate path 123 preferably corresponds to a sequence of rotations about instantaneous centres (axes) of rotation. Such instantaneous centres of rotation, which are defined by the arcuate path 123, will in use preferably be located internal to the vertebral body (or possibly the intervertebral disc) . For motion along the arcuate path 123 to conform to a physiological axial rotation of the vertebrae, such instantaneous centres of rotation preferably practically coincide with the centre (s) of axial rotation between the first and second vertebrae.

[0061] The arcuate path 123 hence has a concave (hollow) side which is oriented towards the (second) vertebra .

[0062] The arcuate path is possibly planar, i.e. it lies in a plane.

[0063] The guide 122 can be conceived in a number of ways as are known in the art (e.g. rail and rolling element or slider combination) . Preferably, as shown in fig. 1, the guide 122 has the shape of a sleeve. The sleeve comprises an opening or slit 124 all along the arcuate path 123. The slit 124 can be suitably located at the side facing the first member 11. The sleeve's cross-section is thus C- shaped. [0064] The sleeve accepts one or more movable elements, which can be sliders 112, which are preferably rigidly connected to the first member 11, such as by rods 113. The rods 113 extend through the slit 124 and link the sliders 112 to the first member 11. The sliders 112 are arranged to move (slide) in the sleeve of the guide 122. The combination of (sleeve) guide 122 and slider (s) 112 hence ensures that the motion of the first member 11 relative to the second member 12 is effected along the guide 122 and thus along the arcuate path 123.

[0065] Since the vertebral configuration and/or size can differ between individuals and in order to provide a better conformity of the relative motion between first and second members to the physiological axial rotation between the first and second vertebrae, the guide 122 (and the slider 112 or any other element movable along the guide) can provide for a motion of the first member 11 along multiple arcuate paths 123, which are enveloped by an inner arcuate path 201 and an outer arcuate path 202. This is illustrated in figure 2. The multiple arcuate paths, enveloped by inner arcuate path 201 and outer arcuate path 202 are advantageously arranged in a same plane, which is preferably a transverse plane.

[0066] The inner and outer arcuate paths are advantageously curved in a same direction. They thus have a concave side which is advantageously located at a same side, namely towards the vertebrae.

[0067] Preferably, the interspacing between the inner arcuate path 201 and the outer arcuate path 202 can be constant. Alternatively, it increases from the middle towards the ends of the arcuate path 123 (the guide 122) . This can be accomplished by making either or both the slit opening 124 and the sleeve cross-section of the guide 122 smaller in the middle of the guide compared to the ends, in a same way as represented in figure 2.

[0068] The interspacing between the inner arcuate path 201 and the outer arcuate path 202 is however preferably not larger than 5 mm (in a direction normal to the arcuate path and in the plane thereof) , more preferably not larger than 3 mm.

[0069] As vertebral prostheses of the invention are envisaged to be used primarily in the treatment of instabilities of the flexion-extension articulation of vertebrae, their purpose is also to limit or at least restrain the flexion and extension movement between the first and second vertebrae.

[0070] Therefore, the first member 11 is secured to the second member 12, so as to limit, or possibly substantially suppress, relative movement of the first vertebra relative to the second vertebra in flexion and extension in a sagittal plane (vertical plane for a person standing upright, directed from the front to the back of the person) . This can be performed by securing the first member 11 to the second member 12 such that the former can displace relatively to the latter in a direction perpendicular to the plane of the arcuate path 123

(perpendicular to the transverse plane) over not more than a predetermined distance. In other words, displacement of the first member 11 relative to the second member 12 in a direction perpendicular to the transverse plane when implanted is confined.

[0071] Such securing can be carried out in a number of known ways. Referring to figure 1, the sliders 112 are provided within the sleeve of the guide 122. The slit 124 of the sleeve is made smaller than the size of the sliders 112, hence preventing the latter to "escape" from the sleeve . [0072] A certain "play" can be provided in the movement between the first and the second members in the direction perpendicular to the transverse plane. For the present embodiment, such play can be realised by making the internal cross section of the sleeve somewhat larger

(higher) than the size (height) of the sliders 112.

[0073] Such play however preferably does not exceed

5 mm, preferably it should not exceed 3 mm and most preferably it should not exceed 2 mm in a direction perpendicular to the plane of the arcuate path

(perpendicular to the transverse plane) . This means that the first member 11 can be displaced relative to the second member 12 in such a direction over the distance as indicated. The indicated displacement allows for effecting a flexion-extension movement between the first and second vertebrae, which corresponds to not more than the range that occurs physiologically in a healthy person. Preferably, the maximal allowable displacement corresponds to 80%, 60%, or even 40% of the physiological range in flexion-extension. Hence, even with a degenerated articulation, the vertebral prosthesis can allow for an increased comfort for the patient.

[0074] The slider 112 can hence be allowed motion transverse to the arcuate path 123 in a direction perpendicular to the arcuate path.

[0075] Referring again to figure 1, the cross section of the sleeve can be made somewhat larger than the size of the sliders 112 for allowing motion transverse to the arcuate path.

[0076] The above indicated displacement should be limited to an extent that is preferably decided on a case by case basis, depending on the occurring pathology.

[0077] Possibly, the displacement is totally suppressed. In this case, no such play is present and the first member and the second member are arranged at a fixed distance from each other in a direction perpendicular to the plane of the arcuate path 123.

[0078] The vertebral prosthesis 10 as described up to now hence stabilises the possibly unstable movement between two vertebrae, in particular flexion and extension, while allowing an axial rotation thereof, which moreover conforms with the physiological axial rotation between such vertebrae .

[0079] A third movement of the vertebrae is the lateral bending, which corresponds to a rotation of the first vertebra relative to the second vertebra on an axis defined by the intersection between a transverse plane and a sagittal plane (hence rotation considered in a frontal plane, i.e. directed from left to right of a person, dividing the person in a front and back part) . The lateral bending can be referred to as a lateral tilt. The physiological maximal angle of lateral bending between the Cl and C2 vertebrae is about 5°.

[0080] Vertebral prostheses 10 according to the present invention advantageously also allow such movement - in full or in a limited way. In view of the pathology of the vertebral articulation which the prosthesis is intended to treat, the vertebral prosthesis 10 can be provided with means to limit, in use, the lateral bending between the first and second vertebrae. This can be provided by limiting the (lateral) tilt of the first member 11 relative to the second member 12.

[0081] Such can be achieved by providing a movable element (e.g. a slider) along the guide 122, having a certain length of engagement with the guide 122. Due to this length of engagement, the movable element

(substantially) prevents lateral tilting. Alternatively, multiple sliders 112 in a fixed (rigid) configuration can be provided at a distance from each other and rigidly connected to the first member.

[0082] The maximal lateral tilt of the first member 11 relative to the second member 12 is preferably not larger than 10°, more preferably not larger than 5°, most preferably not larger than 4°. Such maximal lateral tilt can be suitably at least 2°.

[0083] Possibly, the first member is not tiltable laterally relative to the second member, thereby fully preventing lateral bending between the first and the second vertebrae .

[0084] Referring to figure 1, multiple sliders 112 are provided for sliding in the sleeve of the guide 122. All those sliders are rigidly joined to the first member 11 and are spaced apart at a distance from each other. In case the sliders 112 are tightly fitted in the sleeve of the guide 122, tilt (lateral bending) would be completely prevented. Alternatively, the cross-section of the sleeve can be made somewhat larger than the size of the sliders 112 in a direction perpendicular to the plane of the arcuate path, so that the sliders can move in such direction relative to the sleeve. By so doing, some play is introduced and a limited amount of (lateral) tilt of the first member 11 relative to the second member 12 is possible.

[0085] The cross-section of the sleeve can be made to increase towards both the ends of the sleeve 122. This can allow for lateral tilt of the first member 11 relative to the second member 12.

[0086] In the latter case, when in a middle part of the guide, the cross section of the sleeve has same dimensions as the slider (s) 112, it can additionally fully (or substantially) prevent displacement of the first member relative to the second member in a direction perpendicular to the transverse plane (i.e. totally preventing flexion and extension) .

[0087] Hence, the cross-section of the sleeve 122 can be made larger than the size of the slider 112 in any direction transverse to the arcuate path. In case the cross-section of the sleeve 122 is larger in the plane defined by the arcuate path, this allows for multiple arcuate paths to be defined. In case the cross-section of the sleeve 122 is larger perpendicular to such plane, this allows for lateral tilt. A combination of both is possible as well. The cross-section of the sleeve 122 can also increase in size towards the ends thereof.

[0088] Projecting lips 125 on the second member 12 and directed towards the first member 11 can be provided as end stops for the travel along the arcuate path 123. Projecting lips 117 provided at the ends of the first member 11 can be configured for abutment to the lips 125 on the second member 12. Alternatively, the prosthesis can be so designed that the outermost rod 113 or other connecting link cares for abutment against lip 125.

[0089] In such cases, the lips 117 and 125 take up the impact force at the end stop instead of the sliders 112. The sliders 112 are hence protected from such impacts. Furthermore, as the projecting lips 117 are provided closer to the first member 11 compared to the sliders 112, the lever arm between the point of impact at the end stop and the first member 11 is reduced, enabling to lessen the constraints on the first member 11.

[0090] Preferably, multiple sliders 112 can be arranged in a staggered fashion in the guide 122, such as by arranging them alternately in two rows.

[0091] The sliders 112 can be arranged in a lower row and an upper row, as is illustrated in figures 1 and 3- 5 and more schematically in figure 6. The lower row comprises sliders 412 arranged at a larger distance from the first member 11 compared to the sliders 512 of the upper row. The sliders 512 of the upper row and the sliders 412 of the lower row can have different size. Likewise, the link 413, 513 connecting the sliders 412 respectively 512 to the first member 11 can be different (e.g. weaker /stronger ) and can be adapted to the size of the sliders. Such a configuration can provide for increased security. Indeed, should the sliders of the upper row, or the link connecting them to the first member 11 come to a failure (e.g. in case of a sudden movement, accident, etc.), then the sliders of the lower row can still limit the flexion-extension movement between the first and second vertebrae. The robustness of the vertebral prosthesis can therefore be increased.

[0092] The sliders 112 can additionally, or alternatively be staggered in the plane of the arcuate path 123. Hence, sliders 112 can be alternately arranged closer to the convex side of the guide 122 (as seen in relation to the arcuate path and in the plane thereof) and closer to the concave side of the guide 122. Such an arrangement can provide for increased stability.

[0093] Movement of the first member 11 along the guide 122 can hence suitably be a sliding movement, such as performed by one or more sliders 112. Such a movement is preferably damped, for example by providing a damping

(viscous) liquid in the sleeve of the guide 122.

[0094] In case the guide 122 has the shape of a sleeve, the sliders 112 can be ball-shaped, such as having the form of an oval or an egg. As the guide 122 is arcuately shaped, the sliders 112 can be adapted to be less curved at the concave side of the arcuate path (i.e. the anterior or distal side in fig. 1) compared to the curvature at the convex side of the arcuate path (i.e. the posterior or proximal side in fig. 1) .

[0095] In addition, the vertebral prosthesis of the invention can be arranged to restrict or even completely suppress antero-posterior translation of the first vertebra relative to the second vertebra. This can be achieved by limiting possible rotation of the first member about the arcuate path (i.e. rotation in a sagittal plane) to at most 10°, preferably at most 5°. Possibly, the height of the sleeve-like guide 122 can be arranged to be larger than the dimension of the sliders 112, so that rotation thereof is limited due to the rods 113 abutting against the slit 124. Alternatively, the slit 124 can be arranged to have a certain depth (or thickness) , which limits or prevents tilting of the rods therein so that rotation of the sliders 112 and of the first member 11 about the arcuate path is limited or prevented.

[0096] In order to preserve functioning of the vertebral prosthesis over time, the movable elements are preferably embedded within a sealed membrane, so as to be inaccessible to soft biological tissue.

[0097] Implanting of vertebral prostheses according to the invention can be facilitated by first providing means for fixation to the vertebrae (such as screws 111 and 121) and implanting such means to attach them to the vertebrae, independently of the vertebral prosthesis. Thereafter, the first member 11 and the second member 12 can be fastened to the means for fixation by providing the first member 11 and the second member 12 with suitable fastening means. This has the advantage that the size of the first member 11 and of the second member 12 can be adjusted to e.g. the distance between the fixated screws 111 respectively 121. [0098] The fastening (securing) of the first and second members to the screws (or other means for fixation) can be performed as represented in figure 7. Screws 111, 121 are first implanted into the corresponding vertebra. The proximal part 711 of such screws is threaded. Thereafter, the first, respectively second member 11, 12 is fitted over the proximal part 711 of the implanted screws. The first/second member 11, 12 is provided with a recess 712 at the location of fastening to the screws 111, 121, which recess is provided with an internal thread 713. A washer 714, having internal and external thread corresponding to respectively the external thread of the proximal part 711 of the screw and the internal thread 713 of the recess 712 of the first/second member, is inserted for securing the first/second member to the screws.

[0099] In order to provide increased stability, or to avoid excessive loads on one of the vertebrae, which can result in fracture thereof, such as at the locations where the prosthesis is implanted, or even in fracture of the prosthesis itself, either one or both the first member 11 and the second member 12 can be arranged to be attachable to an additional vertebra, different from the first and the second vertebrae.

[0100] Alternatively, vertebral prostheses can comprise a series arrangement of first and second members. In such case, the first member 11 of a first set of the series acts as the first member 11 or as the second member 12 of a successive set of the series. This can provide a solution in cases of serious pathological degenerations of the articulations involving more than two vertebrae.

[0101] As represented in figures 1, 3 and 6, either one or both the first member 11 and the second member 12 can comprise projecting members 116 respectively 126, projecting away from the prosthesis, which provide for an abutment against the adjacent vertebra, thereby limiting flexion and extension movement relative to the adjacent vertebra. In figure 1, the projecting members 116 are arranged for abutment against the occipital part of the skull 3.

[0102] Either one of the first member 11 and second member 12 can be arranged to be fixed (attached) to the (occipital part of the) skull.

[0103] It will be clear from the above description that the position of the first member 11 and the second member 12 can be interchanged.

[0104] It is also noted that prostheses of the invention can not only be used for treatments of unstable flexion-extension movements between two vertebrae. They can for example also be used for the treatment of unstable axial rotation, since the prostheses allow a physiological axial rotation to be effected, which furthermore can be confined (to the dimension of the guide) . In the latter case, the prostheses can allow (and limit) both flexion- extension and lateral bending as indicated hereinabove.

Claims

1. Implantable vertebral prosthesis (10), comprising a first member (11) fixedly attachable to a first vertebra (Cl) and a second member (12) fixedly attachable to a second vertebra (C2), the second member

(12) comprising a guide (122) arranged for guiding the first member (11) relative to the second member (12) along an arcuate path (123), wherein the prosthesis is so formed that, when the vertebral prosthesis is implanted in a patient and the first member (11) is attached to the first vertebra (Cl) and the second member (12) is attached to the second vertebra (C2) :

- the guide (122) is arranged at a location such that the arcuate path (123) spans a portion of the perimeter of the second vertebra (C2), as considered in a transverse plane of the patient and the arcuate path (123) is so formed, that movement of the first member (11) along the arcuate path (123) effects substantially a physiological axial rotation of the first vertebra relative to the second vertebra and

— the first member (11) is secured to the second member (12) so as to limit displacement of the first member (11) relative to the second member (12) in a direction perpendicular to the transverse plane to an amount between zero and the amount corresponding to the physiological flexion and extension of the first vertebra (Cl) relative to the second vertebra (C2) in a sagittal plane of the patient.

2. Implantable vertebral prosthesis (10) as in claim 1, wherein movement of the first member (11) along the arcuate path (123) is defined by instantaneous centres of rotation located, in use, internal to the vertebral body of the second vertebra.

3. Implantable vertebral prosthesis (10) as in claim 1 or 2, wherein the first member (11) is rotatable relative to the second member (12) along the arcuate path (123) over an angle of at least 25°, preferably at least 30°.

4. Implantable vertebral prosthesis (10) as in any one preceding claim, wherein said displacement of the first member (11) relative to the second member (12) in a direction perpendicular to the transverse plane amounts to maximally 5 mm, preferably maximally 3 mm.

5. Implantable vertebral prosthesis (10) as in any one claim 1 to 3, wherein the first member (11) is tiltable relative to the second member (12), the vertebral prosthesis comprising engaging means (112, 122, 113) for the first member (11) and the second member (12) to engage with each other for restricting said tilting movement to at most 10°, preferably at most 5° such that, when the prosthesis is in use, said tilting movement corresponds to lateral bending of the first vertebra relative to the second vertebra.

6. Implantable vertebral prosthesis (10) as in claim 5, wherein the engaging means are formed of at least two elements (112), movable along the arcuate path (123) in engagement with the guide (122), the movable elements (112) being arranged at a fixed distance from each other and rigidly joined to the first member (11), thereby restricting said tilting movement.

7. Implantable vertebral prosthesis (10) as in any one preceding claim, wherein the guide (122) comprises a sleeve-like case having the form of the arcuate path (123) and wherein the prosthesis comprises a movable element (112, 412, 512) provided in the sleeve-like case and connected to the first member (11) .

8. Implantable vertebral prosthesis (10) as in claim 7, wherein the sleeve-like case is so formed as to prevent the movable element (112) from leaving the case, thereby securing the first member (11) to the second member (12) to limit said displacement of the first member (11) relative to the second member (12) in the direction perpendicular to the transverse plane.

9. Implantable vertebral prosthesis (10) as in any one preceding claim, wherein the guide (122) is arranged for guiding the first member (11) relative to the second member (12) along multiple arcuate paths enveloped by an inner arcuate path (201) and an outer arcuate path

(202), in order to better conform the movement of the first member (11) along the guide (122) to the physiological axial rotation of the first vertebra relative to the second vertebra .

10. Implantable vertebral prosthesis (10) as in claim 9, wherein the spacing between the inner arcuate path (201) and the outer arcuate path (202) increases towards the ends thereof.

11. Implantable vertebral prosthesis (10) as in claim 7 or 8 and claim 10, wherein the sleeve-like case comprises an internal cross-section which is larger towards the ends of the guide (122) .

12. Implantable vertebral prosthesis (10) as in any one preceding claim, wherein the arcuate path, or the multiple arcuate paths are arranged in the transverse plane .

13. Implantable vertebral prosthesis (10) as in any one preceding claim, wherein the first member (11) and the second member (12) are so formed as to be attachable to the Cl and C2 vertebrae.

14. Implantable vertebral prosthesis (10) as in any one preceding claim, wherein the first member (11) and/or the second member (11) is additionally attachable to respectively a third vertebra and/or a fourth vertebra.

15. Implantable vertebral prosthesis (10) as in any one preceding claim, wherein the guide and elements movable along the arcuate path are wrapped in a sealed membrane for preventing ingrowth of biological tissue.