This invention relates generally to a medical instrument for use in anterior cervical spine surgery and more specifically to a distraction instrument used in the treatment and correction of pathologies in the cervical spine. 
The spinal column consists of more than 20 discrete bones which are joined together in a highly complex arrangement which houses and protects critical elements of the nervous system and serves as a structural framework around which there are innumerable peripheral nerves and circulatory bodies in close proximity.  In spite of these complicating features, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction permitting human beings a high degree of physical dexterity.  In general these bones are coupled sequentially to one another by tri-joint complexes which consists of an anterior intervertebral disc and the two posterior facet joints.  The anterior intervertebral discs of adjacent bones are cushioning cartilage spacers. 
The bones and connective tissue of the neck, or cervical spine, are particularly complex in that they are smaller, are the most flexible in the spinal column, and are adjacent to the most critical circulatory, respiratory, and digestive tissues in the body.  Long term degeneration of the cartilage spacers and/or trauma, can cause adjacent bones of the cervical spine to collapse together and/or become axially displaced (that is, become segmentally offset).  These and other failure modes can cause significant pain as well as catastrophic neurological problems. 
The traditional course of surgical treatment can include the implantation of a plate which seats against the anterior surfaces of adjacent bones, and the fixation of the plate thereto with bone screws immobilizes the two bones in the distracted and aligned position. 
Concerns for the overall safety of this method of treatment, however, include the risk of screw pull out and/or breakage, or plate breakage, each of which are accompanied by the severe risk of esophegeal perforation, which can be fatal if not treated immediately and aggressively.  The surgical effectiveness of this treatment is enhanced dramatically if the adjacent bones are able to fuse together across the distracted space, thus forming a single bony element which can support the neck without needing to load the metal plate (thus risking breakage or other failure mode).  The problem is that the distraction increases the distance between the fusing bones, and the need to create the bone bridge requires that the bones be close enough to grow together.  These competing factors have made the use of porous material as a spacer material desirable.  This spacer physically braces the bones at the desired positions, permitting the bones to fuse across the entire assembly. 
Similar techniques have been employed in other spinal infirmities, including collapsed disc spaces in the thoraco-lumbar regions of the spine.  The present invention is therefore applicable to the treatment of any spinal disorder in which the space between vertebral bones needs to be surgically separated (the bones distracted), and then fused to one another, however, for the purposes of this invention disclosure, only the application to the cervical spine shall be described hereinbelow. 
Before the implant may be inserted into the space, however, the height of the disc space (the distance between the opposing end plates of the adjacent bones) must be restored.  Restoring the appropriate height and orientation of the vertebral bones and the intervertebral space is critical, and when done properly (that is, through a series of sequentially larger trial spacers) is also instructive for the purposes of determining the appropriate size of the implant to be provided. 
It shall be understood that the use of an anterior cervical plate in conjunction with the foregoing will be the preferred method of surgical treatment, however, its use should not be considered a requirement in the practice of the present invention, which should be viewed as a wholly separate and independent surgical process. 
It is, therefore, an object of the present invention to provide a new and novel treatment for cervical disc pathology, as well as for the treatment of spinal pathologies in general. 
It is, correspondingly, another object of the present invention to provide an intervertebral distraction tool that more accurately and easily separates collapsed intervertebral spaces and restores the proper anatomical orientation of cervical bones. 
It is further an object of the present invention to provide an implantable intervertebral spacer device, and insertion instrument, which permits more anatomically appropriate and rapidly osteogenic fusion across the intervertebral space. 
Other objects of the present invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter. 
The preceding objects of the invention are achieved by the present invention, which provides an intervertebral distraction tool and an intervertebral spacer device and insertion instrument. 
A preferred embodiment of an intervertebral distraction tool (also referred to herein as a trial) of the present invention has a proximal end defined by an elongate shaft, and a distal end defined by a clamshell head.  The head has upper and lower halves, each having a curvate outer surface and a flat inner surface.  The distal side of the head is hinged so that the head opens and closes from the proximal side of the head.  Preferably, the hinge is a separating hinge that allows the halves to not only angulate with respect to one another about the hinge axis, but also to vertically separate from one another at the hinge.  Preferably, the proximal side of the head has a tapered lip that facilitates insertion of the distal end of the distraction separator (described below) in between the halves.  The head further has a pair of posts and corresponding bores into which the posts compress fit when the head is closed.  The force of the compression fit holds the head closed, so that the head can be held closed during the insertion of the head into the target intervertebral space.  When the distraction separator is actuated to open the head, the compression force is overcome to allow the head to open. 
A preferred embodiment of a distraction separator of the present invention has an elongate shaft having a longitudinal bore.  The distal end of the shaft has a decreasing taper defined by upper and lower surfaces that are angled toward one another, terminating at a distal face of the separator.  The longitudinal bore accommodates the elongate shaft of the trial so that the separator can be moved longitudinally relative to the shaft.  The longitudinal movement is preferably effected by simply pushing the proximal end of the separator toward the head of the trial.  Upon forward movement of the separator, the tapered upper and lower surfaces engage the flat inner surfaces of the head, causing the halves to angulate about the hinge axis of the head, thereby opening the head.  Further advancement of the separator in between the halves causes the halves to not only angulate with respect to one another about the hinge axis, but also to vertically separate from one another at the hinge, due to the separating hinge.  Once the head has been opened as desired by the surgeon, extraction of the separator from between the halves and removal of the head from the intervertebral space leaves the distracted space ready for accepting additional trials or for insertion of a spacer of the present invention. 
A preferred embodiment of a spacer device of the present invention has a thickness that is preferably predetermined to be a distance that is to be established between two vertebral discs that are to be fused together.  The spacer further has an overall rectangular shape with rounded corners to limit interference with surrounding tissue during the insertion procedure.  The spacer further has upper and lower surfaces that are curvate to allow them to more easily fit into and remain in the intervertebral space, with each curvate surface seating within the vertebral cavity presented to the surface when the spacer is disposed in the intervertebral space.  The spacer is preferably formed from a material that can withstand compressive forces that are present within an intervertebral space, and further is preferably formed of a porous material that facilitates bone growth thereinto for a successful fusion procedure.  Further preferably, the upper and lower surfaces are rough surfaces that will stimulate bone growth into the porous material.  The spacer further has a plurality of linear grooves to facilitate insertion of the spacer into the intervertebral space.  Each of the grooves has a depth, preferably is rounded, and preferably has a smooth surface. 
A preferred embodiment of a spacer insertion tool of the present invention has a scissor-style body having upper and lower arms hinged about a hinge rod, each of the arms having a distal end having a head, each of the arms having a proximal end having a gripping handle.  Each of the heads has an inner surface having a pair of linear protrusions that are spaced and dimensioned to fit within the linear grooves of the spacer when the heads are closed about the spacer.  That is, when the arms are actuated by a surgeon gripping and bringing together the handles, the arms hinge about the hinge rod and therefore the heads are brought to bear on the spacer such that each of the protrusions fits into a respective one of the grooves, and the spacer is held between the heads by the continual compression of the spacer in this manner.  Accordingly, the surgeon, while holding the spacer with the arms, can insert the spacer into the intervertebral space that has been distracted. 
It should be noted that when the spacer is held with the arms, the upper and lower curvate surfaces are preferably not engaged by the heads.  That is, the dimensions of the protrusions and/or the grooves are such that spaces are present between the upper and lower curvate surfaces and the inner surfaces of the heads.  This feature is primarily provided so that when the spacer is released from the insertion tool, the upper and lower surfaces are not altered during the removal of the heads from the spacer. 
In order to facilitate removal of the heads from the spacer so that the spacer can be left in the intervertebral space, the insertion tool is configured such that the arms can be moved longitudinally relative to a bracing rod.  In the illustrated embodiment, this feature is provided inasmuch as the insertion tool is provided with a bore, preferably though the hinge rod, and the bracing rod is disposed through the bore. 
In operation to remove the heads from the spacer while holding the spacer in the intervertebral space, the distal end of the bracing rod is held against the spacer by the surgeon while the surgeon pulls the handles.  The pulling of the handles while the spacer is held in the intervertebral space causes the protrusions to slide out of the grooves until the spacer is released from the heads and is compressed between the vertebral discs by the natural compression force present in the spine.  Accordingly, the upper and lower surfaces of the spacer, being curvate, easily fit into and remain in the intervertebral space, with each curvate surface seating within the vertebral cavity presented to the surface when the spacer is disposed in the intervertebral space.  Further, the surfaces being rough, stimulate bone growth, and being porous, provide for bone growth into the material to facilitate fusion.