as can be seen from fig1 to 6 , the spacer 1 according to one embodiment comprises a first cylindrical tubular member 2 and a second cylindrical tubular member 3 . the outer diameter of the second tubular member 3 is chosen to allow a sliding guidance of the second tubular member 3 in the first tubular member 2 . the first tubular member 2 has a first end 4 , an opposite second end 5 and a hollow interior surrounded by the wall of the tube . in the wall at a predetermined distance from the first end 4 an access opening 6 is provided which extends over approximately half of the length of the first tubular member 2 . in the embodiment shown the access opening is a rectangular opening . the length of the long side of the rectangle is approximately half of the length of the first tubular member 2 and the length of the short side is approximately a quarter of the length of the first tubular member 2 . the dimension of the access opening 6 is such that natural bone graft can be filled in and removed from the opening during or after surgery . similarly the second tubular member has a first end 7 and an opposite second end 8 , a hollow interior and wall defined by the tube . at a distance from the second end an access opening 9 is provided in the wall of the second tubular member . the access opening 9 extends over approximately half of the length of the second tubular member 3 . in the embodiment shown , the access opening 9 has a rectangular shape with the same dimensions as the access opening 6 of the first tubular member . in an assembled state as shown in fig2 , the second tubular member 3 is inserted in the first tubular member 2 in such a way that the access opening 6 of the first tubular member and the access opening 9 of the second tubular member are aligned in a radial direction . since the access opening of each of the tubular members is provided at the distance from the first end and the second end , respectively , the access openings are separated from the first or the second end , respectively , by a part of the wall and therefore have a closed rim . the position of the second tubular member 3 relative to the first tubular member 2 in a direction of the longitudinal axis l of the spacer can be fixed by a locking structure . the locking structure is formed by a plurality of threaded openings 10 aligned in an axial direction in the wall of the first tubular member 2 . the threaded openings 10 are arranged at one side of the access opening 6 and close to the access opening in such a way that the threaded openings are accessible by the surgeon during surgery from the same side as the access opening . further , the locking structure comprises a plurality of spherical segment - shaped recesses 11 which are arranged in a line in the direction of the longitudinal axis l of the spacer in the outer surface of the wall of the second tubular member 3 . the recesses 11 are formed side by side and have a depth less than the radius thereof . the line of recesses 11 is arranged on one side of the access opening 9 of the second tubular member 3 and close to the access opening 9 in such a way that in the assembled state as shown in fig2 and 3 the line of threaded openings 10 matches with the line of the recesses 11 . the locking structure further comprises locking screws 12 which can be screwed into the threaded openings 10 . the locking screw 12 has on one side a spherical segment - shaped projection 13 which fits into the spherical segment - shaped recesses 11 . on the opposite side , the locking screw 12 has a recess for engagement with a screwing - in tool . the line of recesses 11 extends at least over the same length as the line of threaded openings 10 . an unthreaded opening 10 a can be provided in the line of threaded openings for engagement with a holding tool ( not shown ). as can be seen in fig2 , 3 and 12 , the first tubular member 2 comprises an elongate opening 15 which extends from a distance from the first end 4 to a distance from the second end 5 . the elongate opening 15 has a length which is approximately the same length as the access opening 6 and has a width which is considerably smaller than the width of the access opening 6 in such a way that a bone graft cannot pass through the elongate opening 15 . further , the first tubular member 2 comprises a plurality of diamond - shaped openings 16 which are arranged in a region adjacent to the second end 5 . the diamond - shaped openings 16 have a size which is smaller than the size of the access opening 9 in such a way that bone graft cannot pass through the diamond - shaped openings . the diamond - shaped openings 16 are arranged in circumferential rows wherein one row is offset from the adjacent row by half a diamond height in axial direction . the row of diamond - shaped openings 16 which is closest to the second end 5 is cut so that the openings are open to the second end 5 forming prongs 17 for engagement with the adjacent vertebra . as can be seen in particular from fig1 and 4 , the first tubular member 2 additionally comprises a plurality of circular openings 18 in a region starting at the first end 4 . the circular openings 18 have a size which does not allow introduction and removal of bone graft . the diamond - shaped openings 16 and the circular openings 18 serve for allowing in - growth of blood vessels and tissue . similarly , the second tubular member 3 comprises adjacent to its second end 8 a plurality of circular openings 19 and adjacent to the first end 7 a plurality of diamond - shaped openings . the circular openings 19 and the diamond - shaped openings 20 are sized so that bone graft cannot be introduced or removed through these openings . similar to the first tubular member 2 the row of diamond - shaped openings 20 which is nearest to the first end 7 of the second tubular member is cut so as to form prongs 21 for engagement with the vertebral body . as can be seen in fig1 and 4 , a pin 25 is provided which can be fixed for example to the first tubular member 2 and guided in an elongate slit 28 ( fig4 ) provided in the second tubular member 3 to prevent rotation and forming a stop to prevent separation of the first and the second tubular member . the spacer further comprises a third tubular member 22 the axial length of which is approximately the axial length of the region of the second tubular member in which the diamond - shaped openings 20 are located . the third tubular member 22 also comprises diamond - shaped openings 23 in its wall and prongs 24 at both free ends . the third tubular member 22 is fixedly connected to the second tubular member by means of pins 26 as shown in the schematic exploded representation in fig7 and 8 , or with screws , rivets or the like . the third tubular member 22 can have an oval - shaped cross section as shown in fig7 or a circular cross section as shown in fig8 . the third tubular member and the end section of the second tubular member form a double cage structure which enhances the area of contact with the vertebra compared to the area of contact of only the second tubular member . since the outer diameter of the second tubular member is smaller than that of the first tubular member , the third tubular member can be used to enlarge the contact area and to adapt the shape of the ring of prongs to the contour of the end surface of the vertebral body which shall be engaged . as can be seen from fig1 to 6 , a fourth tubular member 27 which is similar to the third tubular member 22 is provided inside the first tubular member 2 at its free second end 5 and is attached to the first tubular member 2 to form a double cage structure . the fourth tubular member 27 also comprises diamond - shaped openings and prongs for attachment to the bone . in an assembled state the elongate opening 15 covers at least one of the circular openings 19 or the diamond - shaped openings 20 of the second tubular member 3 . the elongate opening of the first tubular member and one of the circular or diamond - shaped openings of the second tubular member form an example of a length adjusting structure which allows expansion and compression of the two tubular members relative to each other when being engaged with a tool . in the embodiment shown , the access opening , the locking structure and the length adjusting structure are arranged in the assembled state within approximately one half or less of the circumference of the spacer . a length adjustment tool for telescoping the second tubular member 3 relative to the first tubular member 2 is shown in fig1 and 13 . the tool 30 comprises a tube 31 with a handle 32 at one end and a fork - shaped section 33 at the other end . a rotation axis 34 bridges the fork - shaped section 33 at the free end . a scissor - like structure 35 is pivotably connected with the fork - shaped section 33 . the scissor - like structure 35 can pivot around the axis 34 . it comprises arms 35 a and 35 b which can be moved between a folded position and an unfolded position . one of the arms is structured to engage with the elongate opening 15 of the first tubular member 2 and the other arm is structured to engage with one of the circular or diamond - shaped openings of the second tubular member 3 . the tool further comprises a bar with a handle 37 at its free end . the bar is accommodated in the tube 31 and extends into the fork - shaped section 33 . by rotating the bar with the handle 37 , the scissor - like structure 35 is moved so that the arms 35 a and 35 b open or close , respectively . in use , first the spacer is filled with bone graft before inserting it between the vertebrae . the spacer has its minimum length . then , the spacer is inserted between two adjacent vertebrae , for example after removal of a vertebra to bridge the space . then , the length of the spacer is adjusted by telescoping the second tubular member out of the first tubular member . the movement is a sliding movement without rotation . the tool shown in fig1 and 1 . 3 is used in such a way that the scissor - like structure is inserted into the elongate hole 15 in the first tubular member with one arm 35 a and engaging one of the holes 19 or 20 of the second tubular member with the other arm 35 b . by rotating the bar the scissor - like structure pivots and the second tubular member is telescoped out of the first tubular member . when the desired length is achieved , the locking screws 12 are inserted into the threaded openings 10 until the spherical segment - shaped projection engages with one of the spherical segment shaped recesses 11 . since the access openings of the first tubular member and the second tubular member , respectively , are aligned , the spacer has still openings large enough to insert bone graft . then , additional bone graft is inserted to fill out completely the interior of the spacer . since the locking structure and the elongate opening 15 which serves for expansion or compression are located on the same side , the spacer can be quickly adjusted to its desired axial length and fixed in this position . since the access openings of the first and the second tubular member are on the same side close to the locking structure , it is possible to quickly fill in bone graft during surgery . the spacer can be coated for enhancing in - growth of blood vessels or tissue . the spacer can be coated with different materials on the outer surface and the inner surface of the tubular members , respectively . the first and the second tubular member can be coated with different materials . modifications of the embodiment shown are possible . the access opening needs not to have a rectangular shape . it can have any shape provided that it is large enough to allow the insertion of bone graft . more than one elongate opening 15 can be provided . the circular openings and the diamond - shaped openings can be omitted or can have any other shape , provided that they are small enough not to allow removal or insertion of bone graft . in this case , instead of an opening of the second tubular member for engagement with the tool , another surface structure can be provided on the second tubular member . the locking structure also can be modified . the spherical - shaped recesses 11 can be provided at the inner wall of the first tubular member and the threaded openings can be provided in the wall of the second tubular member . the locking screw is then screwed from the hollow interior into the threaded openings . the tubular members are shown with a circular cross section . however , any other shape of the cross section is possible . for example , the shape of the cross section of the tubular members can be adapted to the contour of the end surface of a vertebral body . the third tubular member and the fourth tubular member also can have a cross section which is different from a circular cross section or an oval - shaped cross section . further tubular members can be attached to the end section of the first and / or the second tubular member to provide a multi - cage structure . alternatively , the third and fourth tubular member can be omitted . while particular forms of the disclosure have been illustrated and described , it will be apparent that various modifications can be made without departing from the spirit and scope of the disclosure . accordingly , it is not intended that the disclosure be limited , except as by the appended claims .
a spacer for insertion between two vertebrae includes a variable axial length and comprises a first member having a hollow interior and a tubular wall and a second member having a hollow interior and a tubular wall the second member being slidable within the first member in an axial direction for adjusting an overall length . at least the first member or the second member comprises an access opening in its wall , the opening having a size which is adapted for inserting bone grafts through the opening into the interior . the spacer further comprises a length adjustment structure being engageable with an expansion tool for telescoping the second member relative to the first member and a locking structure for fixing the axial length . the access opening , the length adjustment structure and the locking structure are arranged so as to be accessible from the same side of the spacer . the spacer allows in situ filling of bone grafts into its interior and an improved handling .