Vertebral implant

An implant for insertion between a pair of spaced vertebrae has a center element and a pair of end elements fitted to the center element and each adapted to engage a respective one of the vertebrae. The center and end elements have formations for varying the spacing between the end elements. Respective brackets fixed to the end elements are each fixable to the respective vertebra. The formations include screwthreads engaged between the center and end elements and the end elements are formed as caps engaged over ends of the center element. Each bracket is generally L-shaped and has a long leg attached to the respective vertebrae and an arcuate short leg attached to the respective element.

FIELD OF THE INVENTION
 The present invention relates to vertebral implant. More particularly this
 invention concerns such an implant positioned between two vertebrae,
 normally in the place of a removed vertebra or intervertebral disk.
 BACKGROUND OF THE INVENTION
 When a vertebra is broken or crushed it is frequently necessary to ablate
 the body of the crushed or broken vertebra or vertebrae. In order,
 however, to prevent the spinal column from collapsing with damage to the
 fragile spinal cord running in the vertebral foramen forward of the
 vertebral body, it is necessary to employ an implanted spacer. This device
 is braced vertically between the bodies of the adjacent vertebra and holds
 them apart at the desired spacing. It may even serve to distract two
 vertebrae which have become too closely spaced due to crushing of a
 vertebra or disk.
 In U.S. Pat. No. 5,571,192 a spinal implant is described for engagement in
 a space left by ablation of a vertebral body between a pair of adjacent
 vertebrae. It has a tubular center element extending along an axis and a
 pair of end elements. The center element is formed with upper and lower
 screwthreads of opposite hand and with a plurality of radially
 throughgoing apertures. The upper and lower tubular end elements are each
 formed with a plurality of radially throughgoing apertures, each have a
 circular-section inner end threaded onto a respective one of the
 screwthreads, and each have an outer end adapted to bear on a respective
 one of the adjacent vertebrae.
 Thus such an implant can be set in an area where the body or bodies or one
 or more vertebra have been ablated. The length of the implant is then
 increased by rotating the center element to force out the end elements and
 bring their outer ends into solid engagement with the confronting
 vertebral surfaces. The screwthreads offer sufficient mechanical advantage
 so that the system can even be used to distract the vertebrae, as is
 frequently necessary in the event of a crushing injury. The tubular
 elements of the implant can be filled with bone cement and/or bone
 fragments to ensure that the implant becomes anchored in place in living
 bone. Since the outer elements surround the screwthreads of the inner
 element, once installed the screwthreads will be largely covered so that
 their sharp edges do not impair healing.
 Such an arrangement serves very well to maintain the axial spacing between
 the vertebrae it is installed between, but must normally be used in
 conjunction with a separate bone plate and or external dorsal appliance
 secured by bone screws to the flanking vertebrae. This extra equipment is
 needed to prevent any relative lateral shifting of the vertebrae and to
 prevent the implant from moving out of position.
 Not only is the plate or appliance difficult to install, but it is
 extremely inconvenient for the patient whose wound must often be left
 open. In addition the plate or appliance often block access to the implant
 so if same needs to be adjusted, for instance lengthened, it must be
 removed and reinstalled.
 OBJECTS OF THE INVENTION
 It is therefore an object of the present invention to provide an improved
 intervertebral implant.
 Another object is the provision of such an improved intervertebral implant
 which overcomes the above-given disadvantages, that is which eliminates
 the need for substantial additional hardware.
 SUMMARY OF THE INVENTION
 An implant for insertion between a pair of spaced vertebrae has according
 to the invention a center element and a pair of end elements fitted to the
 center element and each adapted to engage a respective one of the
 vertebrae. The center and end elements have formations for varying the
 spacing between the end elements. According to the invention respective
 brackets fixed to the end elements are each fixable to the respective
 vertebra.
 Thus it is possible to secure the implant in place by fixing its end
 brackets to the respective vertebrae. This leaves the center element
 exposed for adjustment. Thus during the operation the core assembly formed
 by the center and end elements can be set in place, then the end elements
 are secured to the respective vertebrae, and finally the center element is
 turned to separate the end elements and perform the desired amount of
 distraction of the flanking vertebrae. The brackets leave the center of
 the assembly completely exposed for adjustment or filling with bone
 fragments.
 In accordance with the invention the formations includes screwthreads
 engaged between the center and end elements. The end elements are formed
 as caps engaged over ends of the center element.
 According to the invention each bracket is generally L-shaped and has a
 long leg attached to the respective vertebrae and an arcuate short leg
 attached to the respective element. The long leg is also arcuate in
 section and each long leg is formed with at least one elongated slot. A
 bone screw is engaged through the slot with the respective vertebra.
 Furthermore each slot is formed with seats for a head of the bone screw
 and each short leg is formed with two holes and the respective end element
 is formed aligned therewith with two holes. Respective bolts each engage
 through a respective one of the holes of the short leg and into the
 respective hole of the respective end element.
 Each short leg is split according to the invention between the respective
 holes into a pair of halves. A respective screw is engaged between each
 pair of halves for pulling same together and clamping the bolts in the
 holes of the respective end element. Each bracket is formed with a slot
 between the halves and with a rounded hole at an end of the slot and the
 long leg is formed with an elongated slot to each side of and generally
 parallel to the slot.
 The elements are at least partially tubular and are each formed with an
 array of holes spaced so that the holes of the center element overlap with
 the holes of the end elements. This facilitates bone growth through the
 implant to lock it in place and in effect incorporate it in the patient's
 spinal column.

SPECIFIC DESCRIPTION
 FIGS. 1 and 2 show a tubular center element 1 and an end cap or element 2
 identical to those shown in above-cited U.S. Pat. No. 5,571,192. The
 center element 1 has upper and lower external screwthreads 3 of opposite
 hand that mesh when assembled with complementary internal screwthreads of
 the respective elements 2. The upper and lower end elements 2 are each
 formed at their confronting inner ends with a thickened outwardly
 projecting collar and at one or more locations on this collar with a
 thickened region which in turn is formed with a radially throughgoing
 threaded bore 5. A set screw can be threaded into this bore 5 to lock each
 of the end elements 2 on the center element 1 against relative angular
 movement. The center element 1 is formed with a thickened center region 6
 that is clear of screwthreads and that is formed with an array of
 angularly equispaced and radially throughgoing circular holes or bores 7.
 A tool with a simple cylindrical end can be inserted into any of these
 holes 7 to rotate the center element 1 relative to the end elements 2 so
 as to increase or decrease the overall length of the implant along its
 axis A.
 All the elements 1 and 2 are formed with arrays of radially throughgoing
 elongated apertures 4 that are arranged in staggered rows spaced axially
 by a spacing such that each of the holes 4 in an end element 2 will always
 overlap with at least one of the holes 4 of the element 1. As a result if
 the passage formed by the tubular implant is filled with bone cement
 and/or bone fragments before installation, good bone growth around and
 through this implant is insured.
 As shown in FIGS. 9 and 10, the core assembly 1, 2 of the elements 1 and 2
 is fitted between a pair of vertebrae 20 and the element 1 is rotated to
 screw the end elements 2 apart to distract and stabilize the vertebrae 20.
 It is then necessary to laterally stabilize the two vertebrae 20,
 something done hitherto by attaching a plate or complex dorsal appliance
 to them.
 In accordance with the invention each of the end elements 2 is secured to
 the respective vertebra by a respective mounting bracket 8. As shown in
 FIGS. 3 through 8 each such bracket 8 is basically L- or J-shaped with a
 long leg 9 and a short leg 10, both part-cylindrical in shape. The short
 leg 10 is formed with a pair of cylindrical throughgoing holes 11 centered
 on respective axes intended to meet in the assembled structure at the axis
 A and each normally receiving a bolt 21 engaged in a corresponding hole 12
 of the respective end element 2.
 The short leg 10 and most of the long leg 9 are formed with a longitudinal
 split 13 subdividing most of the bracket 8 into two halves 16. At the
 upper end of the split 13 the long leg 9 is formed with a circular hole 15
 to allow the two halves 16 to flex relative to each other without cracking
 the metal of the bracket 8. A hole 14 threaded in one of the halves 16
 traverses the split 13 so that a screw 22 inserted into this hole 14 can
 pull the two halves 16 together. Thus if two bolts 21 fitted to the holes
 11 are engaged in two of the holes 12 and the screw 22 is tightened, this
 action will effectively lock the bracket 8 to the end element 2 by wedging
 the two bolts 21 against the element 2.
 In addition the long leg 9 is formed with a pair of longitudinally
 elongated throughgoing holes or slots 17 having widened seat-forming outer
 edges 18 and undercut inner edges 19. Bone screws or studs 23 seated in
 the vertebrae 20 project through the slots, fitting to the undercut inner
 edges 19, and nuts 24 engaged over them and fitting in the seats 18 thus
 can solidly lock the brackets 8 to the vertebrae 20.
 With this system it is therefore possible to solidly attach each end
 element 2 to the respective vertebra 20. The center element 1 is left
 exposed for adjustment of the distraction, and the entire assembly can, if
 desired, be left in place in the patient.