Interbody fusion system with intervertebral implant retention assembly

#The present invention relates generally to an interbody fusion system having an intervertebral retention assembly, and more particularly to intervertebral implants, and intervertebral implant assemblies incorporating a biomechanical implant construct inclusive of stabilizing instrumentation (an anterior, posterior or lateral stabilizer), constructed and arranged to provide superior and inferior vertebral fixation by multiple fixation elements via a single through hole, which through hole can accommodate a fixation element retainer component (hereinafter referred to as a screw retainer component, e.g. a third piece of instrumentation which acts as a retention device to prevent separation of a loosened fixation element from the stabilizing anterior, posterior or lateral instrumentation.

#Intervertebral spacer implants, either alone or further provided as an assembly, inclusive of a retention mechanism to help alleviate expulsion and movement of the implant when placed in the spine, are well known. Such implant assemblies are advantageous in providing an implant that is easier to insert in the spine, and which resists expulsion subsequent to implantation. Intervertebral spacer implant assemblies which include a spacer and a plate, where the plate comprises a supplemental or alternative retention mechanism having one or more holes in the anterior end of the plate that are directed toward the superior, inferior or both endplates of adjacent vertebrae are also known in the art. Such implants are used to stabilize and immobilize the spinal segments in the treatment of single or multi-level degenerative disc disease, spinal stenosis, and failed previous fusions, as well as other spine conditions.

#The problem with many of these implants resides in preservation of the highest degree of mobility possible, while avoiding backing out or loosening of the implant assembly fastening elements, such as bone screws and the like.

#FIG. 1 illustrates a cross-sectional view of a biomechanical implant construct, in accordance with the instant invention, inserted within the intervertebral space, and fixated to both the superior and inferior vertebral bodies adjacent thereto;

#FIG. 2A illustrates a top-view of a biomechanical implant construct as positioned in FIG. 1, illustrating the bone fixation elements positioned in the adjacent superior and inferior vertebral bodies, and in locking engagement with the retention member;

#FIG. 2B illustrates a top-view of a biomechanical implant construct as positioned in FIG. 1, illustrating through holes positioned so as to enable two bone fixation elements to be separately positioned within one or both of the adjacent superior or inferior vertebral bodies, and in locking engagement with the retention member;

#Now referring to FIG. 1, the intervertebral implant assembly, generally referred to by numeral 10, includes an implant member 12, and a retention member 14, illustrated as, albeit not limited to, a bone plate. Both plate 14 and implant 12 may be formed of PEEK (poly(ether ether ketone)), titanium, titanium alloy, stainless steel, allograft bone or any other suitable, biocompatible material. It is contemplated that implant 12 may be formed of bone, or an artificial material other than bone which may be harder and/or stronger than bone, such as plastic or ceramic materials. It is further contemplated that the implant material will have the same, more or less elasticity than bone. The implant 12 may include, or be treated with, a bone growth promoting material such as, but not limited to, bone morphogenic protein, hydroxyapatite, and genes coding for the production of bone. Implant 12 may be a source for osteogenesis, be at least in part bioabsorbable, and be treated with, or applied in conjunction with a substance, as known in the art, to inhibit scar formation and/or promote fusion of the bones. Preferably retention member 14 is formed of metal or metal alloy and the spacer is formed of PEEK or other polymer, or alternatively bone or ceramic or radiolucent biocompatible material. Bone fixation elements 16, illustrated, albeit not limited to bone screws, may be formed of titanium, titanium alloy or stainless steel. The bone fixation elements 16 may be in the form of a screw, a staple or nail, a fluted cannulated screw or nail with collapsible/deployable spurs which are deployed and thereby embedded within the bone upon insertion of a secondary nail or screw within the cannulated space, or similar functionally equivalent elements. Furthermore, the implant 12 may include one or more openings (FIG. 3B, 42) designed to receive bone graft material. Subsequent to insertion of the intervertebral implant assembly 10 within the intervertebral space, bone fixation elements, e.g. screws 16, are driven into the inferior 20 and superior 18 adjacent vertebral bodies, via access through hole 17. Upon being fixated within said access hole 17, the elements 16 are urged into locking engagement with the curvilinear seat of anterior instrumentation retention member 14, and each other, so as to reduce or eliminate the possibility of loosening. Due to the single access hole design, the screws essentially reside in a single plane common to the superior and inferior vertebral bodies, and have adjustable angularity at the surgeon's discretion.

#It is contemplated by the invention that the access through hole 17 may include a single exit aperture or dual diverging exit apertures. The seat of each aperture, whether integral with the assembly, or formed within a metallic insert which is subsequently inserted into the assembly, may include a spherical or semi-spherical dished and curvilinear seat for each fastener. This feature will provide for variation in screw trajectory and increased face contact between the fixation elements and the plate or seat. In a preferred albeit non-limiting embodiment, the fastener will be formed with a spherical or semi-spherical head with a substantially flat upper surface. Such construction will allow the spherical surface of the second fastener, e.g. screw, to engage the top surface of the first screw regardless of trajectory of both screws. Such construction insures repeatable and reliable head-to-head contact independent of fastener trajectories.

#In a particular embodiment of this invention the stabilizing bone plate useful in an interbody fusion system for stabilizing a biomechanical spinal implant will be comprised of a bone plate having a left side surface and a right side surface, a top surface and a bottom surface, a first upper surface and a second lower surface, with a distance between said first upper surface and said second lower surface defining a thickness of said plate, a horizontal middle plane between the first upper surface and the second lower surface and a plurality of boreholes suitable for accommodating longitudinal fixation elements extending through said thickness of said plate, wherein each of said boreholes has an elongated slot located at or near said horizontal middle plane, and surrounded by arcuate and dished curvilinear sidewalls which form an engagement seat extending from said elongated slot to said first upper surface for receiving at least two bone fixation elements therein, each of said bone fixation elements having a screw head adapted to engage or be engaged by an engaging surface of each screw head to urge the screw heads into a convergent and interference relationship with one another and with said engagement seat upon tightening, thereby resulting in a locking engagement of said bone plate engagement seat and said fixation elements and a diverging outlet portion extending from said elongated slot toward said second lower surface, said divergent outlet being adapted to provide a range of angulation of each said fixation elements, whereby each of said fixation elements is individually positionable within both an inferior and superior vertebral body associated with said interbody fusion system.