Expandable spinal implant

An expandable spinal implant assembly is configured to be inserted between two adjacent vertebral bodies. The expandable spinal implant assembly includes a substantially hollow first body with a first superior endplate with a first top inside face and a first inferior endplate with a first bottom inside face. The first superior endplate and the first inferior endplate are connected together by a lateral wall. At least one strut is arranged within said first body, connecting the first top inside face with the first bottom inside face, said strut including a first threaded through bore. The assembly further includes a substantially hollow second body with a second superior endplate and a second inferior endplate, said second body inserted within said first body. A central screw with a first end including a ball-head and a drive, said central screw further having a threaded shaft is engaged within said first threaded through bore.

TECHNICAL FIELD

The invention relates to an expandable spinal implant assembly.

BACKGROUND ART

Low back pain is a common disease which may be caused by herniated discs, compressed nerve roots, degenerative discs or joint disease.

If a patient suffers severe low back pain and does not respond to conservative treatment, spinal fusion is an option to eliminate the pain. Spinal fusion is a surgical technique wherein two or more vertebrae are joined together. Spinal fusion interventions are also performed to correct back deformities.

With spinal fusion, often an intervertebral spacer or device is placed between the involved vertebrae after removal of the intervertebral disc. The intervertebral device corrects the spine alignment and restores the disc height.

Common intervertebral devices are made from titanium alloys or PEEK (polyetheretherketone) polymer. Often these devices comprise pockets that can be filled with bone graft material or artificial graft substitute. The fusion itself takes place when the bone of the vertebral endplates grows into and through the intervertebral device. Finally both vertebrae are grown together. Often, a pedicle system provides additional posterior stabilisation. Intervertebral fusion devices can be implanted with various approaches, for example with an anterior, posterior or lateral approach.

Over the past years minimally invasive techniques have been introduced. One advantage of the minimal invasive techniques is a reduction of soft tissue trauma resulting in a faster recovery. Other complications are reduced as well. In minimally invasive techniques the implant is brought into position between the vertebral bodies through a small incision with small instruments. However, the intervertebral device must still have a sufficient large foot-print to withstand the forces between the vertebrae before complete fusion has taken place. If a device has a too small foot-print it will sink into or break through an endplate of a vertebra, and the initially restored height is lost.

Combining advantages of the minimally invasive surgery approaches with intervertebral devices with large footprint affording a good support would require a device which may be brought into place through a small incision and which in a second step may be expanded to a larger size.

SUMMARY OF THE INVENTION

It is the object of the invention to create an expandable spinal implant assembly which may be implanted in a first, collapsed configuration with small dimensions and which may be transformed into a second, expanded configuration with a larger footprint by simple means. Further, the expandable spinal implant assembly should be easily collapsible in the event that the expandable spinal assembly has to be removed.

The solution of the invention is specified by the features of claim1. According to the invention the expandable spinal implant assembly for insertion between two adjacent vertebral bodies comprises a substantially hollow first body with a first superior endplate having a first top inside face. The hollow first body further comprises a first inferior endplate with a first bottom inside face. Said first superior endplate and said first inferior endplate are connected together on at least one side by a lateral wall. At least one strut is arranged within said substantially hollow first body and connects the first top inside face of said superior endplate with the first bottom inside face of said inferior endplate. The strut comprises a first threaded through bore with a central axis. A central screw with a first end comprising a ball-head and a drive as well as having a threaded shaft is engaged within said first threaded through bore.

The expandable spinal assembly further comprises a substantially hollow second body with a second superior endplate and with a second inferior endplate. The substantially hollow second body is at least partially inserted within said substantially hollow first body. At least one track is arranged on a second top inside face of the second superior endplate and/or on a second bottom inside face of said second inferior endplate. The at least one track is oriented at an acute angle relative to the central axis of said first threaded through bore when said second substantially hollow body is inserted into said first substantially hollow body. The ball-head of said central screw is engaged into said at least one track.

With the arrangement of the ball-head within the at least one track, the expandable spinal implant assembly may be moved from a first, collapsed configuration into a second, expanded configuration having a larger footprint than said first configuration. I.e. the simple rotation movement of the central screw is transformed into a translation movement of the second substantially hollow body relative to the first substantially hollow body. Hence, a very simple yet effective mechanism for the expansion of the spinal implant assembly is established.

Both said first body and said second body are “substantially hollow”. In the present application, “substantially hollow” is understood as a body having a defined shape which comprises a void space between the superior endplate and the inferior endplate, wherein said void may comprise at least one element spanning between said superior endplate and said inferior endplate, e.g. in the form of the at least one strut. Said superior endplate and said inferior endplate are connected together by at least one wall on a side face of said first implant body or said second implant body. At least one side face of both said first substantially hollow body and said second substantially hollow body does not comprise a wall such as to allow access to the central screw from the outside of the expandable spinal implant.

The “endplates” of the first body are those faces of the expandable spinal implant assembly which are positioned to bear against the vertebral bodies once the expandable spinal implant assembly is implanted.

The “endplates” of the second body are those faces which will at least partially be arranged next to the inside faces of the endplates of the first body.

The expandable spinal assembly preferably has a generally rectangular shape. More preferably, said superior and said inferior endplates of both said hollow implant bodies are generally oval in shape, wherein the front face is preferably straight.

The expandable spinal assembly hence has six faces. The upper and the lower faces are those faces which will bear against the endplates of the upper and the lower vertebral bodies, respectively, once implanted. Further, the expandable spinal assembly has an anterior and a posterior face. The posterior face is on the side of the expandable spinal implant assembly which will face towards the spinous process of the vertebra once implanted. Consequently, the anterior face is the side which will be oriented towards the torso of a patient. Additionally, the expandable spinal assembly comprises a front face and a back face. The front face is located on the side of the implant which is oriented towards a surgeon during an implantation of the expandable spinal assembly via a lateral approach. The back face is located on the side of the implant which is opposite the front face.

The strut preferably only partially spans along one dimension of the first body within said void space between the first superior endplate and the first inferior endplate. Preferably, the strut is arranged to be essentially parallel to the front side of said expandable spinal implant assembly.

The drive of the central screw is intended to be engaged with a surgical instrument, such as a screw driver. Hence, the drive preferably is in the form of a slot, Philips drive, hex socket, Torx drive or the like. The drive hence serves to transmit torsional moment from a surgical instrument to the central screw such as to entail a rotational movement of the central screw.

The at least one track is oriented at an acute angle relative to the central axis of said first threaded through-bore. An “acute angle” as used in the present application is understood to be an angle of less than 90° but more than 0°. By providing a track having an acute angle relative to the central axis of the first threaded through bore, a translational movement of the central screw along said central axis is transformed in a translational movement of the substantially hollow second body relative to the substantially hollow first body by the engagement of the ball head of the central screw into said at least one track. Preferably, said first body and/or said second body comprise means to guide said translation in a direction which is perpendicular to said central axis of the first threaded through bore.

Preferably, said acute angle of said at least one track relative to said central axis of the first through bore is such that the at least one track is inclined in an anterior to posterior direction as seen from the front side. Hence, a translation of the central screw towards the rear side will result in an increase of the footprint of the expandable spinal implant assembly, as the second body is pushed out of the void between the endplates of the first body towards the expanded configuration.

Alternatively, the acute angle of said at least one track relative to said central axis of the first through bore is such that the at least one track is inclined in a posterior to anterior direction as seen from the front side. In this variant, the expansion of the expandable spinal implant assembly is achieved by a translation of the central screw towards the front side.

By varying the acute angle of the at least one track relative to the central axis of the first threaded through bore and/or the thread pitch of said first threaded through bore and said central screw, the translation speed of said second body relative to said first body may be varied.

Preferably, the expandable spinal assembly includes a superior track located on said second top inside face of said second superior endplate and an inferior track located on said second lower inside face of said second inferior endplate, said superior track and said inferior track being arranged symmetrically to each other and forming a cylindrical channel.

In this embodiment of the present invention, the ball head of the central screw is engaged in both said superior track and said inferior track. Preferably, said superior track and said inferior track have the same centre and radius, e.g. they both form an arc of the same cylindrical channel. This allows an easy manufacture of said superior and said inferior tracks, as they may be made using a drill. Preferably, the radius of said cylindrical channel is marginally larger than a maximal radius of the ball head.

Preferably, the acute angle of said at least one track relative to said central axis of the first threaded through bore is between 5° and 45°, more preferably between 10° and 30°. By using angles within said range, an optimal translational speed of said second body relative to said first body may be achieved.

Further, the force with which the expandable spinal implant assembly expands is related to the angulation of the acute angle. The lower the angle is, the larger the exerted force of the expandable spinal implant assembly will be.

The first body preferably comprises at least one slot and said second body preferably comprises at least one protrusion. The at least one protrusion is configured to engage with said at least one slot. Further, said protrusion substantially extends to said first superior endplate and to said first inferior endplate.

Hence, the at least one slot and at least one protrusion interact with each other in a zip like manner. As said at least one slot substantially extends to the first superior endplate and the first inferior endplate, i.e. said at least one slot is flush with said first endplates, the expandable spinal implant assembly provides a substantially flat support for the vertebral bodies of the two adjacent vertebrae both in said first, collapsed configuration and in said second, expanded configuration.

In a preferred embodiment, said first body and said second body each comprise a multitude of slots and protrusions, respectively. Most preferably, the number of protrusions matches the number of slots.

Preferably, the strut comprises a second threaded through bore adjacent said first threaded through bore for engagement with a coupling core of an insertion instrument. This allows to couple said expandable spinal implant assembly with an insertion instrument by means of e.g. a threaded coupling core.

Preferably, a first front side of said first implant body comprises a recess in said first superior endplate and said first inferior endplate. Said recess is preferably arranged parallel to a second central axis of said second through bore for forming a connection means with the insertion instrument.

By providing said recess, the expandable spinal implant assembly may be connected with the insertion instrument in an angle stable manner, i.e. any rotation of the expandable spinal implant assembly around an axis of the insertion instrument is prevented by the engagement of the insertion instrument with said recess.

The first top inside face and the first bottom inside face are preferably spaced from each other by a first distance D1and are substantially parallel to each other. The second superior endplate and the second inferior endplate are arranged substantially parallel to each and are spaced from each other by a second distance D2which is smaller than said first distance D1.

Hence, the second body has a height which is smaller than the distance between the first top inside face and the first bottom inside face of the first body. This allows the second body to be at least partially inserted into the first body. Preferably, the difference between the first distance D1and the second distance D2is selected such that the second superior endplate and the second inferior endplate engage with the first top inside face and the first bottom inside face, respectively, while allowing a gliding motion of the endplates relative to the inside faces. For example, the first distance D1may be 0.2 mm bigger than the second distance D2.

The ball head of said central screw is preferably a cylindrical head, a conical head or a double conical head. These shapes of the ball head provide a good interaction with the at least one track, especially in cases where the at least one track is provided in the form of an arc of a cylindrical channel.

The second body preferably comprises at least one second track arranged on a second top inside face of said second superior endplate and/or on a second bottom inside face of said second inferior endplate, said at least one second track being oriented parallel relative to the central axis of said first through bore.

Preferably, said at least one second track is arranged such as to be co-axial with the central axis of the first threaded though bore when said expandable spinal implant assembly is in the first, collapsed configuration. Hence, the central screw may be easily inserted and removed when the expandable spinal implant assembly is in the first, collapsed configuration.

The strut preferably comprises a first portion connecting the first top inside face of the superior endplate with the first bottom inside face of the inferior endplate and a second cylindrical portion extending therefrom. The second body preferably comprises a guiding bore for receiving said second cylindrical portion.

This allows a further guiding of a translation of the first body relative to the second body when said expandable spinal implant assembly is moved from the first, unexpanded configuration to the second, expanded configuration or vice versa.

In this embodiment, the first threaded through bore is preferably located on said second cylindrical portion.

Said ball head of said central screw has a second diameter and said elongated shaft of said central screw has a third diameter, wherein the ratio between said second diameter and said third diameter preferably is at least 110:100, more preferably at least 130:100.

The expandable spinal implant assembly has a first footprint in the first, unexpanded configuration and a second footprint in the second, expanded configuration, wherein the ratio between said first footprint and said second footprint preferably is at least 100:125.

In the present application, the term “footprint” is understood as the area of a vertebra covered by said expandable spinal implant assembly, i.e. an area which corresponds to the product of the first length L1and the first width W1or the second width W2, respectively.

Hence, in this preferred embodiment, the footprint in said second, expanded configuration is at least 25% larger than in said first, unexpanded configuration. It is to be noted that said increase in footprint is mediated uniquely by an increase of the width of the expandable spinal implant assembly, while the first length L1does not vary between said first, unexpanded configuration and said second, expanded configuration.

Said spinal implant assembly has a first length L1and said central screw (70) has a third length L3, wherein the ratio between said first length L1and said third length L3is smaller than 100:80, preferably smaller than 100:70.

With this ratio, it may be ensured that the central screw will not protrude from the expandable spinal implant assembly in the second, expanded configuration, while the third length L3is sufficient to provide enough translational movement of the central screw within said first threaded through bore and hence of the ball head within said at least one track to ensure an efficient expansion of the expandable spinal implant assembly from the first, unexpanded configuration to the second, expanded configuration.

Preferably, said first element, said second element and said central screw are made of titanium, a titanium allow, stainless steel or a biocompatible polymer, preferably polyetheretherketone (PEEK).

A further aspect of the present application is to provide a kit comprising at least two expandable spinal implant assemblies according to the present invention. The at least two expandable spinal implant assemblies differ in at least one of the following: the first length L1, a first width W1in the unexpanded configuration, a second with W2in the expanded configuration, the first distance D1, the second distance D2, a first inclination angle and/or a second inclination angle of said first upper endplate relative to said first lower endplate or of said second upper endplate relative to said second lower endplate, respectively.

Such a kit allows a surgeon to choose the best fitting expandable spinal implant assembly for a patient.

A further aspect of the present application is to provide a method for spinal fusion using an expandable spinal implant assembly according to the present invention. Said method comprises as a first step the removal of an intervertebral disc between two adjacent vertebrae. Various methods on how to remove a vertebral disc are known to a person having skill in the art. In a second step, an expandable spinal implant assembly according to the present invention is placed between said two adjacent vertebrae in the first, collapsed configuration. Said first, collapsed configuration has a first footprint. Afterwards, the expandable spinal implant assembly is expanded to the second, expanded configuration by rotation of the central screw, e.g. by means of a screw driver or power tool. The second, expanded configuration has a second footprint which is larger than said first footprint.

Preferably, the placement of said expandable spinal implant assembly is performed using an insertion instrument which is coupled to said expandable spinal implant assembly by means of a threaded tip of a coupling core of said insertion instrument threadably engaged with a second through bore of said expandable spinal implant assembly.

Other advantageous embodiments and combinations of features come out from the detailed description below and the totality of the claims.

In the figures, the same components are given the same reference symbols.

PREFERRED EMBODIMENTS

FIGS. 1aand 1bshow a perspective representation of a first embodiment of an expandable spinal implant assembly1. InFIG. 1a, the expandable spinal implant assembly1is in a first, collapsed configuration. In this configuration the expandable spinal implant assembly1has a first footprint surface area defined by a first width W1and a first length L1.

FIG. 1bshows the expandable spinal implant assembly1in a second, expanded configuration. In said second configuration, the expandable spinal implant assembly1has a second footprint surface area defined by a second width W2and the first length L1. The second width W2is larger than said first width W1.

The expandable spinal implant assembly comprises a substantially hollow first body10. The first body10includes a first superior endplate11and a first inferior endplate12. Said first endplates11,12are connected together by a first side plate28. Between said two first endplates11,12and said first side plate28the substantially hollow first body10comprises a first void27.

Further, the expandable spinal implant assembly10comprises a substantially hollow second body40. Said second body40includes a second superior endplate41and a second inferior endplate42. Both said second endplates41,42are connected together by a second side plate48. Between said two second endplates41,42and said second side plate48the substantially hollow second body40comprises a second void51.

The expansion of the expandable spinal implant assembly1is caused by a translation of the first body10relative to the second body40. This translation results in an increase of the overall width of the expandable spinal implant assembly1from the first width W1to the second width W2.

Translation of the two bodies10,40relative to each other is caused by a translation of a central screw70in a direction which is substantially perpendicular to the translation direction of the second body relative to the first body. A ball head71of said central screw70is engaged in a cylindrical channel defined by two tracks55,56arranged opposite each other on the inside of said second body40and being configured to have an acute angle relative to said central screw70. When said central screw70is rotated a threaded engagement of said central screw70will cause a translation of the central screw70along said two tracks55,56. As the two tracks55,56include an acute angle in relation to the central screw70, the translation of the central screw70within the two cylindrical channels55,56will cause a translational movement of said substantially hollow second body40relative to said substantially hollow first body10.

FIGS. 2aand 2bshow different perspective views of said first body10. The first body10comprises a first superior endplate11and a first inferior endplate12. The first body10has a first anterior end13, a first posterior end14, a first front side15and a first rear side16. The first superior endplate11and the first inferior endplate12define a first inner top face25and a first inner bottom face26, respectively. A strut17is arranged between said first inner top face25and said first inner bottom face26, spanning from said first posterior end14to said first anterior end13. The strut has a first thickness T1which is oriented in the direction from the first front side15to the first rear side16. The ratio between the first thickness T1and the first length L1of the expandable implant assembly1, which corresponds to the length of the first body10, is preferably larger than 1:3, preferably between 1:7 and 1:15.

The first superior end plate11and the first inferior endplate12are connected by the strut17and by the first side plate28, which spans between said first endplates11,12along said posterior end14of the substantially hollow first body10.

The strut17is located substantially centrally between said first front side15and said first rear side16. The strut17comprises a first threaded through bore18. The first threaded through bore18is intended for engagement with said central screw70.

FIGS. 3aand 3bshow the second body40in different perspective views. The second body40comprises a second superior endplate41and a second inferior endplate42located at a second top side48and a second bottom side49, respectively. Further, the second body40has a second anterior end43, a second posterior end44, a second front side45and a second rear side46. The second superior endplate41and the second inferior endplate42are parallel to each other and include a second void51between each other. The second superior endplate41and the second inferior endplate42are spaced from each other by a second distance D2.

The second endplates41,42are joined together at the second anterior end43by a second side plate59. The second side plate59is dimensioned such as to extend over the second superior endplate41and the second inferior endplate42, such as to form two protrusions47.1,47.2on said second top side48and said second bottom side49. Said protrusions47.1,47.2are dimensioned to substantially extend to the level of the first superior endplate11and the first inferior endplate12, respectively, of the first body10when said second body40is at least partially inserted into said first body10.

The second superior endplate41and the second inferior endplate42comprise a guiding recess53extending from the second posterior end44to the second side plate59. Said guiding recess53has a third width W3, wherein said third width W3is substantially equal to the first thickness T1of the strut17of the first implant body10.

Further, a cylindrical channel54with a first diameter A1is defined by a superior track55located on a surface of a second top inside face57of the second superior endplate41facing towards the second void51and by an inferior track56located on a surface of a second bottom inside face58of the second inferior endplate42facing towards the second void51. The tracks55,56extend from said second front side45towards said guiding recess53. The tracks55,56form an acute angle with the second front side54. Once the second body40is at least partially inserted into the first body, the tracks55,56will include an acute angle relative to the axis of the first trough bore18.

FIG. 4shows the central screw70in greater detail. The central screw70has a first end71with a ball-head72. The ball-head72includes a drive73and has a second diameter A2. The second diameter A2is substantially equal to the first diameter A1of the cylindrical channel54of the second body40. The central screw70furthermore comprises an elongated shaft74extending from said first end71to a second end75. The elongated shaft74is a threaded shaft having an outer screw thread76and a third diameter A3. The central screw70has a third length L3. The third length L3spans the entire elongated shaft74as well as the ball-head72. In a preferred embodiment, the ratio between the second diameter A2and the third diameter A3is larger than 110:100, preferably larger than 130:100. With a large ratio between the second diameter A2and the third diameter A3the transfer of translational forces between the central screw70and the cylindrical channel54is enhanced.

Preferably, the ratio between the first length L1of the expandable spinal implant assembly1and the third length L3of the central screw70is smaller than 100:80, preferably smaller than 100:70.

FIGS. 5ato 5cshow the interaction of all individual components of the expandable implant assembly1in perspective views. The first body10and the second body40are shown in a partial cross-sectional view.

FIG. 5adepicts the expandable implant assembly1in a first, collapsed configuration. The elongated shaft74of the central screw70is engaged into first threaded through bore18of the strut17of the first body10. Simultaneously, the ball-head72of the central screw70is engaged within the superior track55and the inferior track56of the second body40. The strut17is arranged within the guiding recess53. This restricts the movement of the bodies10,40relative to each other to the direction of the recess53and prevents any motion of the bodies10,40relative to each other in any other direction. Further, the interaction of said first top inside face25with the second superior endplate41and the interaction of the first bottom inside face26with the second inferior endplate42prevents any vertical or angular motion of the two bodies10,40relative to each other.

FIG. 5bshows the expandable implant assembly1in a second, expanded configuration. Upon rotation of the central screw70, e.g. by means of an instrument coupled to the drive73, a linear motion will be imparted on said central screw70by the interaction of the outer thread76on the elongated shaft74with the first threaded through bore18. The engagement of the ball head72within the tracks55,56leads to a transfer of this motion onto the second body40. As the tracks55,56form an angle with the second front side54as well as with the axis of the first through bore18and the second body40is blocked of moving in the translational direction of the central screw70relative to the first body10by the engagement of the guiding recess53with said strut17, the linear motion of the ball head72within the first threaded through bore18is transformed into a translational motion of the substantially hollow bodies10,40relative to each other in a direction which is perpendicular to the axis of said through bore18by the engagement of the ball head72within said tracks55,56. As a result, by rotating the central screw70the second body40is pushed relative to the first body10towards the second, expanded configuration.

Often in surgery an implant must be removed. Therefore any implant that can expand must be collapsible as well.FIG. 5cshows the collapsing of the expandable spinal implant assembly1from the second, expanded configuration towards the first, collapsed configuration. Upon unscrewing of the central screw70, the ball head72is moved towards the first front side15of the first implant body10. The ball-head72located in the tracks56,57will exert a force upon said substantially second hollow body pulling said second body40back into the first void27of said first body10.

FIGS. 6ato 6dshow different perspective views of a first body10according to a second embodiment of the expandable spinal implant assembly1.

The first superior endplate11and the first inferior endplate12are arranged relative to each other under a first inclination angle29. The first inclination angle29is chosen such as to match the lordotic curve of the natural spine. The inclination angle may thus vary from 3° to 20°, preferably from 8° to 10°.

In reference to the first superior endplate11and the first inferior endplate12at the first anterior end13, the first endplates11,12each comprise multiple slots20a-20z, extending through the endplate in an anterior to posterior direction. These slots20a-20zdefine fingers21a-21z. In a preferred embodiment the first body10comprises at least three fingers21a-21z, defined by at least two slots20a-20z. In essence, there is always one slot20a-20zless than the number of fingers21a-21z. A second length L2of the slots20a-20zin anterior to posterior direction is at least equal to the difference between the second width W2of the expandable spinal implant assembly1in the expanded configuration and the first width W1of the expandable spinal implant assembly1in the collapsed configuration.

FIG. 6bis a top view of the first body10where the dimensions and the arrangement of the slots20a-20zis more clearly visible. As depicted in this figure, the slots20a-20zare defined by a second length L2and a fourth width W4. Further, the first body10comprises one recess22in both said first endplates11,12. Said recess22is intended for engagement with an insertion instrument. In a preferred embodiment, the first body10is symmetrically configured in relation to central plane ‘P1’. Due to this symmetry, the first body10may be used in two orientations, i.e. the first top endplate11and the first inferior endplate12may be flipped.

The first endplates11,12comprise teeth23or another rough structure for primary fixation over friction with the vertebral bodies once implanted. Further, one or more pockets24.1,24.2are provided in each of said first endplates11,12to allow for bone-graft placement and bone in-growth. The pockets24.1,24.2connect the first void51with the space adjacent said first endplates11,12.

FIG. 6cshow the substantially hollow first body10according to the second embodiment of the expandable spinal implant assembly1from the first posterior end14. In contrast to the first embodiment as shown inFIGS. 1 to 5, the first side plate28is replaced by four support struts28.1,28.2,28.3,28.4. Each support strut28.1,28.2,28.3,28.4transfers loads exerted on the first superior endplate11to the first inferior endplate12and vice versa. In a preferred embodiment, the first body10comprises four support struts28.1,28.2,28.3,28.4which are substantially evenly divided along the posterior end14. Alternatively, the first body10may comprise either more or less support struts28.1,28.2,28.3,28.4. The position, shape and size of the support struts28.1,28.2,28.3,28.4may also vary. The support struts28.1,28.2,28.3,28.4are configured for an optimal load transfer, but are preferably as small as possible to provide as much space as possible for bone in and over-growth, which finally causes the spinal vertebral bodies to fuse. The position, shape and size of the support struts28.1,28.2,28.3,28.4may also vary depending on the chosen manufacturing technique, such as for example milling, wire EDM for titanium implants, or milling and injection moulding for PEEK implants.

The ratio between the first thickness T1of the strut and the first length L1of the expandable spinal implant assembly1preferably is larger than 1:3 and more preferably lies in a range of between 1:7 and 1:15. The strut17is located substantially centrally between said first front side15and said first rear side16. In addition to the first threaded through bore18, the strut17comprises a second threaded through bore19. The first threaded through bore is intended for engagement with the central screw70, while the second threaded through bore19is intended for engagement and coupling with an insertion instrument.

FIGS. 7ato 7cshow different perspective views of the substantially hollow second body40according to the second embodiment of the present invention. The second superior endplate41and the second inferior endplate42are substantially parallel and spaced to each other by the second distance D2. At the second anterior end43, multiple elongated protrusions47a-47zextend from said second superior endplate41and said second inferior endplate42. Each protrusion47a-47zhas a third length L3and fifth width W5, wherein the third length L3and the fifth width W5are substantially equal to the dimensions of the slots20a-20zof said first body10, which are defined by the fourth width W4and the second length L2. The protrusions47a-47zform the second top side48and the second bottom side49and extend to the level of the first superior endplate11and the first inferior endplate12of the first body10. The second top side48and the second bottom side49may be oriented under a second inclination angle50relative to each other. The second inclination angle50is chosen to match the lordotic curve of the natural spine. The second inclination angle50may vary from 3° to 20°, preferably from 8° to 10°. The second inclination angle50is equal to the first inclination angle29of the substantially hollow first body10.

The protrusions47a-47zcomprise teeth or another rough structure for primary fixation over friction with the vertebral bodies.

FIG. 7cshows a detailed view of the second body40from the second posterior end44. The second void51comprises angled slope52. The angled slope52is arranged in a posterior recess61which spans from said second superior endplate41to said second inferior endplate42. The angled slope52is arranged at a third inclination angle62relative to the second posterior end44.

The guiding recess53extends from the second superior endplate41to the second inferior endplate42and has a sixth width W6, wherein said sixth with W6is substantially equal to the first thickness T1of the strut17of the first body10.

The cylindrical channel54has the first diameter A1and extends from the second front side45towards the guiding recess53. As for the first embodiment, the cylindrical channel54of the second embodiment is defined by the superior track55and the inferior track56. The superior track55and the inferior track56are circular in shape and share the identical centre and radius. Alternatively at least one of the tracks55,56may have a quadratic or triangular shape.

The second body40is symmetrically configured in relation to a second central plane P2. Due to this symmetry the second body40may be used in two orientations, i.e. flipped in relation to the second central plane P2.

The second body40comprises multiple pockets60a-60zfor bone-graft placement and bone in-growth. Said pockets60a-60zare located at the second anterior end43as well as the second superior endplate41and the second inferior endplate42. The pockets60a-60zat least partially connect the second void51with the outside of the expandable spinal assembly1.

FIG. 8ashows the central screw70in greater detail. The central screw70comprises a first end71with a ball-head72. The ball-head72includes a drive73. Said ball-head72has a second diameter A2which is substantially equal to the first diameter A1of the cylindrical channel54of the second body40. The central screw70furthermore comprises an elongated shaft74extending from said first end71to a second end75. The elongated shaft74is a threaded shaft having an outer screw thread76extending from said ball-head72to said second end75. In an alternative embodiment the said outer screw thread76is a screw thread with a double or triple lead to facilitate a larger translation per turn. The elongated shaft74has a third diameter A3. The central screw70has a third length L3. The third length L3spans the entire elongated shaft74as well as the ball-head72. In a preferred embodiment, the ratio between the second diameter A2and the third diameter A3is larger than 110:100, preferably larger than 130:100. With a large ratio between the second diameter A2and the third diameter A3the transfer of translational forces between the central screw70and the cylindrical channel54is enhanced.

Preferably, the ratio between the first length L1of the expandable spinal implant assembly1and the third length L3of the central screw70is smaller than 100:80, preferably smaller than 100:70.

The second end75preferably comprises a conical tip77. The conical tip77has a fourth angle81which is substantially equal to the third inclination angle62of said angled slope52of the substantially hollow second body40. Said conical tip77is intended to engage with said angled slope52.

FIGS. 8bto 8ddepict alternative designs of the central screw70, namely comprising a cylindrical head78, a conical head79or a double conical head80.

FIG. 9adepicts the expandable implant assembly1according to the second embodiment in a first, collapsed state. The second body40is engaged within the first body10. As such, the strut17is received within the guiding recess53. The protrusions47a-47zof the second body40are arranged within the slots21a-21zof the first body10. The protrusions47a-47zand the fingers20a-20zthereby form a flat plane defining a top face of the expandable spinal implant assembly1. Note that the reference numerals47a-47z,20a-20zand21a-21zhave been omitted onFIGS. 9ato 9cfor clarity reasons.

The elongated shaft74of the central screw70is engaged into the first threaded through bore18of the first body10. Simultaneously, the ball head72of the central screw70is engaged and captured into the superior track55and the inferior track56forming the cylindrical channel54of the second body40. The conical tip77of the central screw70engages against the angled slope52of the second body40.

When a turning motion is imparted on the central screw70, said central screw70is translated towards the first rear side16of the substantially hollow body10, as the outer screw thread76is engaged with the first threaded through bore18. This translation leads to a movement of the ball head72within the tracks55,56of the cylindrical channel54. As these tracks55,56include an acute angle in relation to the central axis of the first threaded through bore18, the motion of the ball head72along said tracks55,56imparts a motion of the second body40relative to the first body10. As the strut17is received within the guiding recess53this movement is guided in a linear anterior to posterior direction.

Simultaneously, the conical tip77of the central screw70presses against the angled slope52hence supporting the force transfer from the linear translation of the central screw70to the motion of the bodies10,40relative to each other.

As a result the second body40is pushed out of the first body10towards the second, expanded configuration, which is shown inFIG. 9b.

FIG. 9cshows the collapsing of the expandable spinal implant assembly1from the expanded configuration towards the collapsed configuration. Upon unscrewing of the central screw70the central screw70travels towards the first front side15of the first body10. The ball head72engaged in tracks55,56will hence pull the second body40back into the first body10.

FIG. 10shows an alternative embodiment of the inventive expandable spinal implant assembly1. In this embodiment, the first body10comprises a cylindrical hole80arranged at one end of the angled slope52. The cylindrical hole80is arranged such that the conical tip77of the central screw70may engage therein. Once the conical tip77is engaged within the cylindrical hole80, a friction fit is established which prevents any rotational motion of the central screw70due to micro motion.

FIGS. 11aand 11bshow an insertion instrument100used in connection with an expandable spinal implant assembly1according to the present invention. The insertion instrument100comprises two central channels101,102. The first central channel101is configured to guide a screwdriver110for actuation of the central screw70. The second central channel102is intended to guide a coupling core115. Coupling core115comprises a threaded tip116for engagement into the second threaded through bore19of the first body10. The insertion instrument100furthermore comprises a nose103. The nose103is configured to engage with recess22of the first body10. By simultaneous engagement of the nose103into the recess22and the engagement of the coupling core115into the second threaded through bore19, the expandable spinal implant assembly1is coupled to the insertion instrument100.

FIGS. 12aand 12bshow the second embodiment of the expandable spinal implant1arranged on a vertebral body121of a target vertebra120in a schematic representation. For reasons of simplicity, only one vertebra120is shown. However, a person having skill in the art recognizes that the expandable spinal implant1would be arranged between two vertebrae in replacement to an intervertebral disc.FIG. 12ashows the expandable spinal implant in the first, collapsed or unexpanded configuration where the expandable spinal implant assembly1has a first footprint on the vertebral body121.FIG. 12bshows the expandable spinal implant assembly1in the second, expanded configuration having an increased footprint on said vertebral body121. This increased footprint is due to the relative motion of the second body40relative to the first body10in a posterior to anterior direction.

FIGS. 13ato 13cshow a third embodiment of the expandable spinal implant assembly1, in which the central screw70is replaced by a dowel90. The dowel90comprises a dowel head91which is engageable in the tracks55,56as well as a saw teeth structure92on an elongate dowel body93. The saw teeth structure92is chosen such that a translation of the dowel90through the first threaded through hole18towards the first rear side16of the first body10is possible, while a reverse motion is prevented by the interaction of the saw teeth structure92with the rim of the first through hole18.

FIGS. 14a-14eshow another alternative embodiment of the expandable spinal implant assembly1. The expansion screw is replaced by an expansion actuating instrument120. The expansion actuating instrument120is used to expand the expandable spinal implant assembly1and is then removed. In order to facilitate the possibility to remove the instrument, the superior track55and an inferior track56of the second body40intersect with a second superior track121and a second inferior track122. Second superior track121and a second inferior track122are oriented parallel to the axis of said first threaded through bore18of the first implant body10. When the expandable spinal implant assembly1has reached the expanded configuration, the expansion actuating instrument120can be removed without causing the expandable spinal implant assembly to collapse by translating the expansion actuating instrument120through the second tracks121,122.

FIGS. 15aand 15bshow a further embodiment of the expandable spinal implant assembly1. The strut17of the first body10comprises a first portion130connecting the first top inside face of the superior endplate11with the first bottom inside face of the inferior endplate12and a second cylindrical portion131extending therefrom. The first threaded through bore18is located in said second cylindrical portion131.

The second body40comprises a guiding bore132which is aligned with the guiding recess53. Both the first body10and the second body40have complementary, essentially V-shaped sides which match together when the expandable spinal implant assembly1is in the first, unexpanded configuration.

The second cylindrical portion131is inserted into the guiding bore132. Hence, in this embodiment, the translation of the first body10relative to the second body40from the first, unexpanded configuration to the second, expanded configuration is additionally guided by the interaction of the second cylindrical portion131of the strut17with said guiding bore132.

The central screw70is engaged with the first threaded through bore18, wherein the ball head72is engaged within the cylindrical channel54arranged within said second body40. Such as to allow the insertion of the central screw70into said expandable spinal implant assembly1, a partial round opening133is provided on the first front side15of the first body10.

Further, the second threaded through bore19is provided on the first front side15of the first body10. The second threaded through bore19allows the coupling of the expandable spinal implant1with an insertion instrument.