Patent Publication Number: US-11642231-B2

Title: Intervertebral disc and insertion methods therefor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/243,547, filed Jan. 9, 2019, which is a continuation of U.S. patent application Ser. No. 15/700,776, filed Sep. 11, 2017, which is continuation of U.S. patent application Ser. No. 14/956,844, filed Dec. 2, 2015, which is a continuation of U.S. patent application Ser. No. 14/746,347, filed Jun. 22, 2015, now U.S. Pat. No. 9,226,837, which is a continuation of U.S. patent application Ser. No. 14/153,514, filed Jan. 13, 2014, now U.S. Pat. No. 9,095,451, which is a divisional of U.S. patent application Ser. No. 11/439,808, filed May 24, 2006, now U.S. Pat. No. 8,777,959, which claims the benefit of the filing dates of U.S. Provisional Patent Application Nos. 60/790,415, filed Apr. 7, 2006, 60/721,053, filed Sep. 27, 2005, 60/701,306, filed Jul. 21, 2005 and 60/685,295, filed May 27, 2005, the disclosures of which are hereby incorporated by reference herein. 
     The present application relates to U.S. Pat. No. 6,908,484, entitled “Cervical Disc Replacement” and filed on Mar. 6, 2003; U.S. Pat. No. 6,994,728, entitled “Cervical Disc Replacement Method” and filed on Feb. 11, 2004; United States Patent Application Publication No. 2004/0176851, entitled “Cervical Disc Replacement” and filed on Feb. 11, 2004; U.S. Pat. No. 6,994,729, entitled “Cervical Disc Replacement” and filed on Feb. 11, 2004; U.S. Pat. No. 6,997,955, entitled “Cervical Disc Replacement” and filed on Feb. 11, 2004; U.S. Pat. No. 6,972,037, entitled “Cervical Disc Replacement” and filed on Feb. 11, 2004; U.S. Pat. No. 6,972,038, entitled “Cervical Disc Replacement” and filed on Feb. 11, 2004; U.S. Pat. No. 6,997,954, entitled “Cervical Disc Replacement Method” and filed on Feb. 11, 2004; United States Patent Application Publication No. 2005/0240272, entitled “Cervical Disc Replacement” and filed on May 9, 2005; United States Patent Application Publication No. 2005/0240271, entitled “Cervical Disc Replacement” and filed on May 9, 2005; United States Patent Application Publication No. 2005/0240270, entitled “Cervical Disc Replacement” and filed on May 9, 2005; U.S. Pat. No. 6,896,676, entitled “Instrumentation And Methods For Use In Implanting A Cervical Disc Replacement Device” and filed on Oct. 17, 2003; United States Patent Application Publication No. 2004/0176773, entitled “Instrumentation And Methods For Use In Implanting A Cervical Disc Replacement Device” and filed on Feb. 18, 2004; United States Patent Application Publication No. 2004/0176843, entitled “Instrumentation And Methods For Use In Implanting A Cervical Disc Replacement Device” and filed on Feb. 18, 2004; United States Patent Application Publication No. 2004/0176778, entitled “Instrumentation And Methods For Use In Implanting A Cervical Disc Replacement Device” and filed on Feb. 18, 2004; United States Patent Application Publication No. 2004/0176777, entitled “Instrumentation And Methods For Use In Implanting A Cervical Disc Replacement Device” and filed on Feb. 18, 2004; United States Patent Application Publication No. 2004/0176852, entitled “Instrumentation And Methods For Use In Implanting A Cervical Disc Replacement Device” and filed on Feb. 18, 2004; United States Patent Application Publication No. 2004/0176774, entitled “Instrumentation And Methods For Use In Implanting A Cervical Disc Replacement Device” and filed on Feb. 18, 2004; United States Patent Application Publication No. 2004/0176772, entitled “Instrumentation And Methods For Use In Implanting A Cervical Disc Replacement Device” and filed on Feb. 18, 2004; United States Patent Application Publication No. 2004/0220590, entitled “Instrumentation And Methods For Use In Implanting A Cervical Disc Replacement Device” and filed on Feb. 18, 2004; United States Patent Application Publication No. 2005/0071013, entitled “Instrumentation And Methods For Use In Implanting A Cervical Disc Replacement Device” and filed on Nov. 19, 2004; and United States Patent Application Publication No. 2004/0193272, entitled “Instrumentation And Methods For Use In Implanting A Cervical Disc Replacement Device” and filed on Feb. 19, 2004, the disclosures of which are hereby incorporated by reference herein. 
     The present application also relates to U.S. Pat. No. 6,607,559, entitled “Trial Intervertebral Distraction Spacers” and filed on Jul. 16, 2001; U.S. patent application Ser. No. 10/436,039, entitled “Trial Intervertebral Spacers” and filed May 12, 2003; U.S. patent Ser. No. 10/128,619, entitled “Intervertebral Spacer Having A Flexible Wire Mesh Vertebral Body Contact Element” and filed Apr. 23, 2002; U.S. patent application Ser. No. 11/073,987, entitled Intervertebral Spacer Having A Flexible Wire Mesh Vertebral Body Contact Element; U.S. patent application Ser. No. 10/140,153, entitled “Artificial Intervertebral Disc Having A Flexible Wire Mesh Vertebral Body Contact Element” and filed May 7, 2002; U.S. patent application Ser. No. 10/151,280, entitled “Tension Bearing Artificial Disc Providing A Centroid Of Motion Centrally Located Within An Intervertebral Space” and filed May 20, 2002; U.S. patent application Ser. No. 10/175,417, entitled “Artificial Intervertebral Disc Utilizing A Ball Joint Coupling” and filed Jun. 19, 2002; U.S. patent application Ser. No. 10/256,160, entitled “Artificial Intervertebral Disc” and filed Sep. 26, 2002; U.S. patent application Ser. No. 10/294,983, entitled “Artificial Intervertebral Disc Having A Captured Ball And Socket Joint With A Solid Ball And Retaining Cap” and filed Nov. 14, 2002; U.S. patent application Ser. No. 10/294,982, entitled “Artificial Intervertebral Disc” and filed Nov. 14, 2002; U.S. patent application Ser. No. 10/294,981, entitled “Artificial Intervertebral Disc Having A Captured Ball And Socket Joint With A Solid Ball And Compression Locking Post” and filed Nov. 14, 2002; U.S. patent application Ser. No. 10/642,523, entitled “Axially Compressible Artificial Intervertebral Disc Having Limited Rotation Using A Captured Ball and Socket” and filed Aug. 15, 2003; U.S. patent application Ser. No. 10/642,522, entitled Artificial Intervertebral Disc Having A Circumferentially Buried Wire Mesh Endplate Attachment Device and filed Aug. 15, 2003; U.S. patent application Ser. No. 11/073,987, entitled “Intervertebral Spacer Device Having A Circumferentially Buried Wire Mesh Endplate Attachment Device” and filed Aug. 15, 2003; U.S. patent application Ser. No. 10/642,526, entitled “Circumferentially Buried Wired Mesh Endplate Attachment Device For Use With An Orthopedic Device” and filed Aug. 15, 2003; U.S. patent application Ser. No. 10/294,984, entitled “Artificial Intervertebral Disc Having Limited Rotation Using A Captured Ball And Socket Joint With A Retaining Cap And A Solid Ball Having A Protrusion” and filed Nov. 14, 2002; U.S. patent application Ser. No. 10/294,985, entitled “Artificial Intervertebral Disc Having Limited Rotation Using A Captured Ball and Socket Joint With A Compression” and filed Ser. No. 10/294,985; U.S. patent application Ser. No. 10/294,980, entitled “Artificial Intervertebral Disc Having Limited Rotation Using A Captured Ball And Socket Joint With A Solid Ball, A Retaining Cap, And An Interference Pin” and filed Nov. 14, 2002; U.S. patent application Ser. No. 10/294,986, entitled “Artificial Intervertebral Disc Having Limited Rotation Using A Captured Ball and Socket Joint With A Solid Ball, A Compression Locking Post, And An Interference Pin” and filed Nov. 14, 2002; U.S. patent application Ser. No. 10/282,356, entitled “Artificial Intervertebral Disc” and filed Sep. 26, 2002; U.S. patent application Ser. No. 10/784,646, entitled Artificial Intervertebral Disc Trial Having A Controllably Separable Distal End” and filed Feb. 23, 2004; U.S. patent application Ser. No. 10/309,585, entitled “Static Trials And Related Instruments and Methods For Use In Implanting An Artificial Intervertebral Disc” and filed Dec. 4, 2002; U.S. patent application Ser. No. 10/784,637, entitled “Instrumentation For Properly Seating An Artificial Disc In An Intervertebral Space” and filed Feb. 23, 2004; U.S. patent application Ser. No. 10/783,153, entitled “Parallel Distractor And Related Methods For Use In Implanting An Artificial Intervertebral Disc” and filed Feb. 20, 2004, the disclosures of which are hereby incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is directed to a spinal joint replacement implant and more particularly to a cervical intervertebral disc implant having saddle shaped articulating surfaces and to methods of inserting the cervical intervertebral disc implant. 
     As is well known to those skilled in the art, the structure of the intervertebral disc disposed between the cervical bones in the human spine comprises a peripheral fibrous shroud (the annulus) which circumscribes a spheroid of flexibly deformable material (the nucleus). The nucleus comprises a hydrophilic, elastomeric cartilaginous substance that cushions and supports the separation between the bones while also permitting articulation of the two vertebral bones relative to one another to the extent such articulation is allowed by the other soft tissue and bony structures surrounding the disc. The additional bony structures that define pathways of motion in various modes include the posterior joints (the facets) and the lateral intervertebral joints (the unco-vertebral joints). Soft tissue components, such as ligaments and tendons, constrain the overall segmental motion as well. 
     Traumatic, genetic, and long term wearing phenomena contribute to the degeneration of the nucleus in the human spine. This degeneration of this critical disc material, from the hydrated, elastomeric material that supports the separation and flexibility of the vertebral bones, to a flattened and inflexible state, has profound effects on the mobility (instability and limited ranges of appropriate motion) of the segment, and can cause significant pain to the individual suffering from the condition. Although the specific causes of pain in patients suffering from degenerative disc disease of the cervical spine have not been definitively established, it has been recognized that pain may be the result of neurological implications (nerve fibers being compressed) and/or the subsequent degeneration of the surrounding tissues (the arthritic degeneration of the facet joints) as a result of their being overloaded. 
     Traditionally, the treatment of choice for physicians caring for patients who suffer from significant degeneration of the cervical intervertebral disc is to remove some, or all, of the damaged disc. In instances in which a sufficient portion of the intervertebral disc material is removed, or in which much of the necessary spacing between the vertebrae has been lost (significant subsidence), restoration of the intervertebral separation is required. 
     Unfortunately, until the advent of spine arthroplasty devices, the only methods known to surgeons to maintain the necessary disc height necessitated the immobilization of the segment Immobilization is generally achieved by attaching metal plates to the anterior or posterior elements of the cervical spine, and the insertion of some osteoconductive material (autograft, allograft, or other porous material) between the adjacent vertebrae of the segment. This immobilization and insertion of osteoconductive material has been utilized in pursuit of a fusion of the bones, which is a procedure carried out on tens of thousands of pain suffering patients per year. 
     This sacrifice of mobility at the immobilized, or fused, segment, however, is not without consequences. It was traditionally held that the patient&#39;s surrounding joint segments would accommodate any additional articulation demanded of them during normal motion by virtue of the fused segment&#39;s immobility. While this is true over the short-term (provided only one, or at most two, segments have been fused), the effects of this increased range of articulation demanded of these adjacent segments has recently become a concern. Specifically, an increase in the frequency of returning patients who suffer from degeneration at adjacent levels has been reported. 
     Whether this increase in adjacent level deterioration is truly associated with rigid fusion, or if it is simply a matter of the individual patient&#39;s predisposition to degeneration is unknown. Either way, however, it is clear that a progressive fusion of a long sequence of vertebrae is undesirable from the perspective of the patient&#39;s quality of life as well as from the perspective of pushing a patient to undergo multiple operative procedures. 
     While spine arthroplasty has been developing in theory over the past several decades, and has even seen a number of early attempts in the lumbar spine show promising results, it is only recently that arthroplasty of the spine has become a truly realizable promise. The field of spine arthroplasty has several classes of devices. The most popular among these are: (a) the nucleus replacements, which are characterized by a flexible container filled with an elastomeric material that can mimic the healthy nucleus; and (b) the total disc replacements, which are designed with rigid baseplates that house a mechanical articulating structure that attempts to mimic and promote the healthy segmental motion. 
     Among these solutions, the total disc replacements have begun to be regarded as the most probable long-term treatments for patients having moderate to severe lumbar disc degeneration. In the cervical spine, it is likely that these mechanical solutions will also become the treatment of choice. At present, there are two devices being tested clinically in humans for the indication of cervical disc degeneration. The first of these is the Bryan disc, disclosed in part in U.S. Pat. No. 6,001,130. The Bryan disc is comprised of a resilient nucleus body disposed in between concaval-covex upper and lower elements that retain the nucleus between adjacent vertebral bodies in the spine. The concaval-convex elements are L-shaped supports that have anterior wings that accept bones screws for securing to the adjacent vertebral bodies. 
     The second of these devices being clinically tested is the Bristol disc, disclosed substantially in U.S. Pat. No. 6,113,637. The Bristol disc is comprised of two L-shaped elements, with corresponding ones of the legs of each element being interposed between the vertebrae and in opposition to one another. The other of the two legs are disposed outside of the intervertebral space and include screw holes through which the elements may be secured to the corresponding vertebra; the superior element being secured to the upper vertebral body and the inferior element being attached to the lower vertebral body. The opposing portions of each of the elements comprise the articulating surfaces that include an elliptical channel formed in the lower element and a convex hemispherical structure disposed in the channel. 
     As is evident from the above descriptions, the centers of rotation for both of these devices, which are being clinically tested in human subjects, is disposed at some point in the disc space. More particularly with respect to the Bryan disc, the center of rotation is maintained at a central portion of the nucleus, and hence in the center of the disc space. The Bristol disc, as a function of its elongated channel (its elongated axis being oriented along the anterior to posterior direction), has a moving center of rotation which is at all times maintained within the disc space at the rotational center of the hemispherical ball (near the top of the upper element). 
     Thus, there remains a need for improved intervertebral discs, as well as new and improved methods for safely and efficiently implanting intervertebral discs. 
     SUMMARY OF THE INVENTION 
     Disclosed herein are intervertebral discs or implants, surgical instruments and procedures in accordance with certain preferred embodiments of the present invention. It is contemplated, however, that the implants, instruments and procedures may be slightly modified, and/or used in whole or in part and with or without other instruments or procedures, and still fall within the scope of the present invention. Although the present invention may discuss a series of steps in a procedure, the steps can be accomplished in a different order, or be used individually, or in subgroupings of any order, or in conjunction with other methods, without deviating from the scope of the invention. 
     In certain preferred embodiments of the present invention, a method of inserting an intervertebral disc into a disc space includes accessing a spinal segment having a first vertebral body, a second vertebral body and a disc space between the first and second vertebral bodies, securing a first pin to the first vertebral body and a second pin to the second vertebral body, and using the first and second pins for distracting the disc space. The method preferably includes providing an inserter holding the intervertebral disc, engaging the inserter with the first and second pins, and advancing at least a portion of the inserter toward the disc space for inserting the intervertebral disc into the disc space, wherein the first and second pins align and guide the inserter toward the disc space. 
     In certain preferred embodiments, the inserter desirably includes an inserter head having an upper channel and a lower channel During the advancing step, the first pin is preferably in contact with the upper channel and the second pin is preferably in contact with the lower channel. The channels may taper inwardly toward one another for urging the first and second pins away from one another as the inserter advances toward the disc space (preferably to more fully open the disc space as the inserter advances toward the disc space). In certain preferred embodiments, the inserter head has a distal end adapted to contact vertebral bone and a proximal end, and the upper and lower channels taper inwardly toward one another between the proximal and distal ends of the inserter head. As a result, the channels are closer together near the distal end of the inserter than near the proximal end of the inserter. In preferred embodiments, the inserter head includes distally extending arms for securing an intervertebral disc implant. Each of the distally extending arms may include an inwardly extending projection engageable with the intervertebral disc implant. 
     In other preferred embodiments of the present invention, a method of inserting an intervertebral disc implant into a disc space includes accessing a spinal segment having a first vertebral body, a second vertebral body and a disc space between the first and second vertebral bodies, securing a first pin to the first vertebral body and a second pin to the second vertebral body, and using the first and second pins for distracting the disc space. The method may include engaging a chisel guide having a distal head with the first and second pins, and advancing the chisel guide toward the disc space for inserting the distal head of the chisel guide into the disc space, whereby the first and second pins align and guide the chisel guide as the chisel guide advances toward the disc space. The method may also include coupling a chisel having one or more cutting blades with the chisel guide and advancing the one or more cutting blades toward the first and second vertebral bodies for forming channels in one or more endplates of the first and second vertebral bodies. The distal head of the chisel guide preferably has a top surface with at least one groove formed therein for guiding the one or more chisel blades toward the disc space. The bottom surface of the head may also have at least one groove for guiding the chisel. 
     The method may also include providing an inserter holding an intervertebral disc implant, and after forming channels in the one or more endplates of the first and second vertebral bodies, disengaging the chisel guide from the first and second pins and engaging the inserter with the first and second pins. The inserter is preferably advanced toward the disc space for inserting the intervertebral disc implant into the disc space, whereby the first and second pins align and guide the inserter as the inserter advances toward the disc space. 
     In other preferred embodiments of the present invention, a kit includes a plurality of two-part intervertebral disc implants having different sizes, and a plurality of implant dispensers, each implant dispenser holding together the two parts of one of the two-part intervertebral disc implants so that it can be manipulated as a single implantable unit. Each implant dispenser preferably has indicia corresponding to the size of the intervertebral disc implant held by the implant dispenser. The indicia on the implant dispenser may include a color code or text indicating the size of the intervertebral disc implant held by the implant dispenser. 
     In particular preferred embodiments, each intervertebral disc implant has a top element including a bone engaging surface and an articulating surface and a bottom element including a bone engaging surface and an articulating surface. The implant dispenser desirably holds the articulating surfaces of the top and bottom elements in contact with one another. 
     The implant dispensers may be flexible. In preferred embodiments, an implant dispenser includes a first arm engaging a top element of the intervertebral disc implant, a second arm engaging a bottom element of the intervertebral disc implant, and a connecting element for interconnecting the first and second arms. The connecting element is preferably flexible for enabling the first and second arms to move away from one another for releasing the intervertebral disc. 
     The kit may also include a plurality of inserters, the inserters being adapted to couple with the intervertebral disc implants while the intervertebral disc implants are held in the implant dispensers, so that the intervertebral disc implants can be transferred from the implant dispensers to the inserters. Each inserter preferably has indicia corresponding to the size of a corresponding one of the intervertebral disc implants. The indicia on the inserter may include a color code or text. The intervertebral disc implants are preferably transferable from the implant dispensers to the inserters while being maintained as a single implantable unit. In certain preferred embodiments, an implant inserter will couple directly to the intervertebral disc implant while the disc implant is held by an implant dispenser. 
     In other preferred embodiments of the present invention, a template for marking score lines on a spinal segment includes a shaft having a proximal end and a distal end, and a template marker provided at the distal end of the shaft. The template marker preferably includes a cruciform-shaped structure having a first vertical arm and a second vertical arm that extends away from the first arm, the first and second vertical arms being aligned with one another along a first axis. The cruciform-shaped structure also preferably includes a first lateral arm and a second lateral arm extending away from the first lateral arm, the first and second lateral arms being aligned with one another along a second axis, whereby distal surfaces of the first and second lateral arms form a concave curved surface that conforms to an anterior surface of a disc between superior and inferior vertebral bodies. 
     The template may include a central pin or a plurality of pins provided at the distal end of the lateral arms for being inserted into the natural disc for stabilizing the template adjacent the disc space, and the vertical arms and the lateral arms spread outwardly from the distal end of the shaft. The first vertical arm desirably includes a first distally extending tack for engaging an anterior surface of the first vertebral body and the second vertical arm desirably includes a second distally extending tack for engaging an anterior surface of the second vertebral body. 
     In certain preferred embodiments of the present invention, each of the top and bottom elements of the implant has an anterior wall that preferably connects the anterior ends of the protrusions on the element. The anterior wall preferably serves as a vertebral body stop to prevent over-insertion of the implant and/or posterior migration of the implant. The anterior wall preferably serves as an engageable feature for engagement with instruments, including but not limited to tamps, extraction or repositioning instruments. The anterior wall in some embodiments may have a curved posterior face to sit flush against a curved anterior endplate face. At least the posterior surface of the wall may be coated with an osteoconductive material to facilitate long-term fixation to the endplate surface. 
     In certain preferred embodiments of the present invention, the intervertebral disc implants includes a top element and a bottom element. Each implant part may have protrusions with outwardly laterally facing surfaces. One or more of the outwardly laterally facing surfaces may have a vertically extending channel, or groove, or depression, or like feature for engagement with instruments, including but not limited to insertion, extraction or repositioning instruments. Preferably, the surface of this feature can be coated with an osteoconductive material to facilitate long-term fixation to the endplate bone. 
     In certain preferred embodiments, the intervertebral disc implant, or the instruments, may alternatively or additionally incorporate any or all of the features discussed previously, disclosed herein, or discussed in U.S. patents and/or patent applications incorporated by reference herein. Preferably, the configuration of the bearing surfaces of the intervertebral disc implant in this preferred embodiment may be substantially similar to those of the other bearing surface configurations discussed previously, disclosed herein, or incorporated by reference herein. 
     Prior to insertion of the intervertebral disc implant disclosed herein, a surgeon preferably performs a cervical anterior exposure and initial natural disc removal (e.g., discectomy). After simple exposure and initial natural disc removal, the surgeon may introduce a guide, such as a reference pin drill guide that enables the surgeon to anchor a pair of alignment or reference pins (e.g., Caspar pins) into the adjacent vertebral bones, preferably along the midline of the bones, and at predetermined vertical distances from the endplate edges. 
     The present application discloses the use of reference or alignment pins for properly aligning tooling and/or implants with bone. The reference or alignment pins shown herein are merely representative examples of certain preferred embodiments of the present invention. It is contemplated that other reference or alignment tools and techniques may be used for properly aligning tools and/or implants with bone, and that these other reference or alignment tools and techniques are within the scope of the present invention. 
     With the reference pins in place, the surgeon may apply distraction to the disc space by using a distraction tool, such as a standard Caspar distractor, and then complete the discectomy and distraction. Once the disc space is cleared and restored to a desired height, the surgeon may choose to remove the distraction tools and advance a guide, such as a burr or drill guide along the reference pins and into the disc space. The burr or drill guide preferably engages the reference pins as the burr/drill guide is advanced toward the disc space. Thus, the reference pins serve to provide proper alignment of the burr/drill guide relative to the disc space. In certain preferred embodiments, the burr/drill guide includes a distal head that fits within the disc space. The burr/drill guide preferably permits the surgeon to introduce a burr or drill bit through each of four holes in the guide and burr or drill pilot grooves or holes at predetermined locations in the endplates. As will be described in more detail below, the pilot grooves are used to form protrusion channels for the protrusions of the intervertebral disc. 
     In certain preferred embodiments of the present invention, in order to cut protrusion channels in the endplates, a chisel guide may be utilized. The chisel guide preferably includes a distal head that is insertable into the disc space. The distal head preferably has grooves formed in top and bottom surfaces of the distal head for guiding a chisel for cutting protrusion channels. The chisel guide preferably has alignment openings for sliding over the reference pins. The reference pins preferably align and direct the chisel guide into the disc space. Chisels may then be advanced along the sides of the chisel guide for cutting the protrusion channels. In certain preferred embodiments of the present invention, a first pair of chisels (e.g., roughening chisels) is advanced along the sides of chisel guide to cut channels. Preferably, the first pair of chisels cuts channels that are approximately 1 mm wide. A second pair of larger chisels (e.g., finishing chisels) can be used to widen the protrusion channels, preferably to about 2 mm. In other preferred embodiments of the present invention, a first pair of chisels is approximately 1 mm wide and 1.5 mm high, and a second pair of chisels (e.g., the finishing chisels) are 1.5 mm wide and 2.5 mm high. 
     Once the protrusion channels have been cut, the implant may be mounted to an insertion tool (e.g., to the distal tip of an insertion tool) and inserted into the disc space. The insertion tool preferably includes upper and lower guide slots or openings that permit the insertion tool to slide along the reference pins. The guide slots are preferably ramped so that the disc space is distracted (to preferably approximately 2 mm wider than the height of the implant) to ensure easy insertion of the implant. In other preferred embodiments, the reference pins may also be engaged by a distraction tool to distract the disc space during insertion, e.g., if such distraction is necessary. This additional distraction may ensure that the device is implanted easily without requiring excessive impacting. 
     Once the intervertebral disc implant has been inserted into the disc space, a tamping instrument may be used to adjust the final position of the disc components relative to one another and/or relative to the vertebral bones. Should the surgeon want to remove the device intra-operatively, or in the case of a revision, a proximal feature of the device (e.g., an anterior wall) may be engaged by an instrument (e.g., an extraction instrument) to pull the device free from the disc space. 
     In other preferred procedures, after simple exposure and initial disc removal, the surgeon may introduce a guide, such as a reference pin grill guide, that permits the surgeon to drill guide holes in superior and inferior vertebral bodies (preferably parallel to one another) for the placement of the pair of reference pins. A second guide, such as a sleeve or reference pin driver guide may be used to ensure that the reference pins are placed in the pre-drilled holes so that the pins are parallel, and are driven into the adjacent vertebral bones preferably along the midline of the bones, and at predetermined distances from the endplates. 
     With the reference pins in place, the surgeon may apply distraction to the disc space, e.g., by means of a distraction tool, and then complete the discectomy and distraction. The surgeon should preferably remove any anterior or posterior osteophytes that may interfere with the ultimate placement of the implant. 
     It should be noted that features and methods and functionalities of the present invention, including but not limited to features and methods and functionalities for engaging one tool (or parts thereof) with one or more other tools (or parts thereof) or with the implants (or parts thereof), and vice-versa; for addressing, avoiding, manipulating, or engaging the patient&#39;s anatomy; for aligning one or more tools with anatomic or non-anatomic reference points; and for aligning the tools and implants with one another and/or a treatment space; are not and should not be limited to those embodied in and achieved by the structures and methods of the specific embodiments described and shown, but rather the structures and methods of the specific embodiments described and shown are merely examples of structures and methods that can achieve certain features and methods and functionalities of the present invention. 
     These and other preferred embodiments of the present invention will be described in more detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows a perspective view of an intervertebral disc implant, in accordance with certain preferred embodiments of the present invention. 
         FIGS.  2 A- 2 H  show a top element of the intervertebral disc implant shown in  FIG.  1   . 
         FIGS.  3 A- 3 H  show a bottom element of the intervertebral disc implant shown in  FIG.  1   . 
         FIGS.  4 A- 4 H  show other views of the intervertebral disc implant shown in  FIG.  1   . 
         FIG.  5    shows a perspective view of the top and bottom elements of the intervertebral disc implant shown in  FIG.  1   . 
         FIG.  6 A  shows an anterior end view of the intervertebral disc implant shown in  FIG.  1   . 
         FIG.  6 B  shows a side elevational view of the intervertebral disc implant shown in  FIG.  1   . 
         FIGS.  7 A- 7 D  show a template, in accordance with certain preferred embodiments of the present invention. 
         FIGS.  8 A- 8 D  show a template marker, in accordance with other preferred embodiments of the present invention. 
         FIGS.  9 A- 9 B  show the template marker of  FIG.  8 A  being attached to a template handle, in accordance with certain preferred embodiments of the present invention. 
         FIGS.  10 A- 10 D  show the template marker and the template handle shown in  FIGS.  9 A- 9 B . 
         FIGS.  11 A- 11 D  show a reference pin drill guide, in accordance with certain preferred embodiments of the present invention. 
         FIG.  12    shows a drill bit used with the reference pin drill guide shown in  FIGS.  11 A- 11 D . 
         FIGS.  13 A- 13 B  show the reference pin drill guide of  FIGS.  11 A- 11 D  inserted into an intervertebral disc space, in accordance with certain preferred embodiments of the present invention. 
         FIGS.  14 A- 14 C  show a reference pin insertion guide, in accordance with certain preferred embodiments of the present invention. 
         FIGS.  15 A- 15 C  show a reference pin, in accordance with certain preferred embodiments of the present invention. 
         FIG.  16    shows a reference pin, in accordance with another preferred embodiment of the present invention. 
         FIGS.  17 A- 17 C  show a reference pin driver, in accordance with certain preferred embodiments of the present invention. 
         FIGS.  18 A- 18 B  show a sleeve used with the reference pin insertion guide of  FIGS.  14 A- 14 C , in accordance with certain preferred embodiments of the present invention. 
         FIGS.  19 A- 19 C  show a distractor, in accordance with certain preferred embodiments of the present invention. 
         FIGS.  20 A- 20 D  show a drill guide, in accordance with certain preferred embodiments of the present invention. 
         FIGS.  21 A- 21 D  show a chisel guide, in accordance with certain preferred embodiments of the present invention. 
         FIGS.  22 A- 22 D  show a chisel used in cooperation with the chisel guide of  FIGS.  21 A- 21 D . 
         FIG.  23    shows the chisel of  FIGS.  22 A- 22 D , coupled with the chisel guide of  FIGS.  21 A- 21 D . 
         FIGS.  24 A- 24 B  show a mallet, in accordance with certain preferred embodiments of the present invention. 
         FIGS.  25 A- 25 D  show a sizer, in accordance with certain preferred embodiments of the present invention. 
         FIGS.  26 A- 26 E  show the sizer of  FIGS.  25 A- 25 D , coupled with a sizer handle, in accordance with certain preferred embodiments of the present invention. 
         FIGS.  27 A- 27 D  show a trial, in accordance with certain preferred embodiments of the present invention. 
         FIGS.  28 A- 28 F  show an implant dispenser, in accordance with certain preferred embodiments of the present invention. 
         FIGS.  29 A- 29 E  show the implant dispenser of  FIGS.  28 A- 28 F , coupled with the intervertebral disc implant shown in  FIG.  1   . 
         FIGS.  30 A- 30 F- 1    show an inserter head for inserting an intervertebral disc into a disc space, in accordance with certain preferred embodiments of the present invention. 
         FIG.  31    shows the inserter head of  FIG.  30 A  and an exploded view of an inserter handle, in accordance with certain preferred embodiments of the present invention. 
         FIGS.  32 A- 32 B  show the inserter head and inserter handle of  FIG.  31    assembled together. 
         FIGS.  33 A- 33 B  show an intervertebral disc implant being transferred from an implant dispenser to an inserter head, in accordance with certain preferred embodiments of the present invention. 
         FIGS.  34 A- 34 B  show an intervertebral disc implant, coupled with an inserter head, in accordance with certain preferred embodiments of the present invention. 
         FIGS.  35 A- 35 B  show an intervertebral disc implant being disengaged from a distal end of an inserter head, in accordance with certain preferred embodiments of the present invention. 
         FIGS.  36 A- 36 B  show a tamp, in accordance with certain preferred embodiments of the present invention. 
         FIGS.  37 A- 37 D  show an extractor, in accordance with certain preferred embodiments of the present invention. 
         FIGS.  38 - 74    show a method of inserting an intervertebral disc implant, in accordance with certain preferred embodiments of the present invention. 
         FIG.  75 A  shows a side view of the intervertebral disc implant shown in  FIG.  72   . 
         FIG.  75 B  shows an anterior end view of the intervertebral disc implant shown in  FIG.  73   . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG.  1   , in certain preferred embodiments of the present invention, an intervertebral disc implant  100  includes a top element  102  and a bottom element  104 . As will be described in more detail below, the top and bottom elements  102 ,  104  have opposing articulating surfaces that engage one another. The intervertebral disc implant is adapted to be inserted into a disc space between adjacent vertebrae. 
     Referring to  FIGS.  2 A- 2 H , the top element  102  includes a first bone engaging surface  106  having protrusions  108 A,  108 B and a second articulating surface  110 . Referring to  FIGS.  2 C and  2 D , the top element has a posterior end  112  and an anterior end  114 . As shown in  FIGS.  2 A and  2 C , the two protrusions  108  are interconnected by an anterior wall  116  that extends along the anterior end  114  of the top element. The anterior wall preferably serves as a vertebral body stop to prevent over insertion of the intervertebral disc implant and/or posterior migration of the implant. The anterior wall of the top element  102  preferably provides an engagement surface to be engaged by instruments, including but not limited to tamps and extraction or repositioning instruments. In certain preferred embodiments, the anterior wall may have a curved posterior face adapted to sit flush against a curved anterior face of a vertebral body. In certain preferred embodiments, one or more surfaces of the anterior wall may be coated with an osteoconductive material to facilitate long-term fixation to an endplate surface. 
     Referring to  FIGS.  2 E and  2 H , the articulating surface  110  preferably defines a convex curve extending between the sides  118 ,  120  of the top element  102 . Referring to  FIGS.  2 F and  2 G , the articulating surface  110  also defines a concave curve or surface extending between the posterior and anterior ends  112 ,  114  of the top element  102 . In certain preferred embodiments, the articulating surface  110  defines a toroidal saddle-shaped surface. 
     Referring to  FIG.  2 C , each protrusion  108  preferably has an engagement feature, or depression  121  formed in an outer surface thereof. In certain preferred embodiments, the depressions  121  are vertically extending. In other preferred embodiments, the protrusions may have one or more holes extending at least partially or completely therethrough. The holes may receive or be suitable for receiving a bone-growth inducing material. As will be described in more detail below, the depressions  121  facilitate engagement of the top element with instruments, and specifically preferably facilitate securing and handling of the top element  102  during an intervertebral disc insertion operation. The depressions  121  on the two protrusions  108  are preferably in alignment with one another. In other words, the depressions  121  are preferably at the same distance between the posterior end  112  and the anterior end  114  of the top element  102 . 
     As shown in  FIGS.  2 A,  2 C, and  2 F , each protrusion  108  preferably includes teeth  122  having sloping surfaces  124  (e.g., having a low point nearer to the posterior end  112  of the top element  102  and a high point nearer to the anterior end  114  of the top element  102 ) that facilitate insertion of the posterior end  112  of the top element  102 . Referring to  FIG.  2 F , the sloping surfaces  124  of the teeth  122  facilitate insertion of the implant in a direction indicated by arrow D 1 . The vertical surfaces  126  of the teeth  122  hinder or prevent dislodgement of the implant in the direction indicated by arrow D 2 . 
     Referring to  FIG.  2 H , the teeth  122  on protrusions  108  preferably also include laterally sloping surfaces  126  that slope downwardly from apexes close to axis A 1  to the lateral sides  118 ,  120  of the top element  102 . Thus, the sloping surfaces  126  slope away from axis A 1 . 
     Referring to  FIGS.  3 A- 3 H , the intervertebral disc implant preferably includes a bottom element  104  having a first bone engaging surface  128  and a second articulating surface  130  that engages the articulating surface  110  of the top element  102  ( FIG.  2 A ). The bottom element  104  includes a posterior end  132 , an anterior end  134 , and lateral sides  136 ,  138 . Referring to  FIGS.  3 A and  3 C , the first bone engaging surface  128  includes first and second protrusions  140 A,  140 B. Each protrusion preferably has an engagement feature or depression  142  formed in an outer surface thereof. In certain preferred embodiments, the depressions  142  are vertically extending. In other preferred embodiments, the protrusions may have one or more holes extending at least partially or completely therethrough. The holes may receive or be suitable for receiving a bone-growth inducing material. As will be described in more detail below, the depressions  142  facilitate engagement of the bottom element with instruments, and specifically preferably facilitate securing and handling of the bottom element  104  during an intervertebral disc insertion operation. The depressions  142  on the two protrusions  140 A,  140 B are preferably in alignment with one another. In other words, the depressions  142  are preferably at the same distance between the posterior end  132  and the anterior end  134  of the bottom element  104 . Referring to  FIGS.  3 F and  3 G , each protrusion  140 A,  140 B preferably also includes teeth  144  having sloping surfaces  146  having a low point nearer to the posterior end  132  of the bottom element  104  and a high point nearer to the anterior end  134  of the bottom element  104 . Similar to the sloping surfaces of the teeth of the top element  102  described above, the sloping surfaces  146  on the teeth  144  facilitate insertion of the bottom element  104  in the direction indicated by arrow D 3 . The vertical surfaces  147  of the teeth  144  hinder or prevent dislodgement of the implant in the direction indicated by arrow D 4  ( FIG.  3   ). 
     Referring to  FIGS.  3 E and  3 H , the teeth  144  preferably also include laterally sloping surfaces  148  that slope downwardly toward axis A 2  ( FIG.  3 H ). More specifically, the sloping lateral surfaces have apexes or high points closer to the lateral sides  136 ,  138  and low points that are closer to axis A 2 . 
     Referring to  FIGS.  3 A and  3 C , the bottom element  108  also includes an anterior wall  150  that extends between the protrusions  140 A,  140 B. The anterior wall preferably serves as a vertebral body stop to prevent over insertion of the intervertebral disc and/or posterior migration of the implant. The anterior wall preferably provides an engagement surface that can be engaged by instruments, including but not limited to tamps and extraction or repositioning instruments. In certain preferred embodiments, the anterior wall may have a curved posterior face adapted to sit flush against a curved anterior face of a vertebral body. In certain preferred embodiments, one or more surfaces of the anterior wall may be coated with an osteoconductive material to facilitate long-term fixation to an endplate surface. 
     Referring to  FIGS.  3 F and  3 G , the articulating surface  130  preferably defines a convex curve or surface extending between the posterior  132  and anterior ends  134  of the bottom element  104 . Referring to  FIGS.  3 E and  3 H , the articulating surface  130  preferably defines a concave curve or surface extending between the lateral sides  136 ,  138  of the bottom element  104 . As will be described in more detail herein, the articulating surface  130  preferably defines a toroidal saddle-shaped surface that engages the articulating surface of the top element  102  ( FIG.  2 G ). 
       FIGS.  4 A and  4 B  show the top element  102  of  FIG.  2 A  being coupled with the bottom element  104  of  FIG.  3 A . Referring to  FIG.  4 B , each of the top and bottom elements  102 ,  104  desirably has a respective anterior wall  116 ,  150  that extends between protrusions. The anterior walls  116 ,  150  preferably extend along the anterior ends  114 ,  134  of the respective top and bottom elements. 
       FIG.  4 D  shows top element  102  including posterior end  112 , anterior end  114 , and lateral sides  118 ,  120 . The top element  102  includes first bone engaging surface  106  and protrusions  108 A,  108 B having depressions  121  formed in outer surfaces thereof. The top element  102  includes anterior wall  116  extending between protrusions  108 A,  108 B. 
     Referring to  FIG.  4 C , bottom element  104  has a posterior end  132 , anterior end  134 , and lateral sides  136 ,  138 . The bottom element  104  includes bone engaging surface  128  and protrusions  140 A,  140 B. The protrusions include depressions  142  formed in outer surfaces thereof. The bottom element  104  also includes anterior wall  150  extending between protrusions  140 A,  140 B. 
     Referring to  FIGS.  4 E and  4 H , the opposing articulating surfaces  110 ,  130  of the top element  102  and the bottom element  104  are adapted to engage one another. The teeth  122  on the top element  102  slope downwardly toward the posterior end  112  thereof. Similarly, the teeth  144  on the bottom element  104  slope downwardly toward the posterior end  132  thereof. 
     Referring to  FIGS.  4 F and  4 G , the teeth  122  on the top element  102  have lateral sloping surfaces  126  that slope downwardly toward the sides  118 ,  120 . In contrast, the teeth  144  on the bottom element  104  include lateral sloping surfaces  148  that slope inwardly toward axis A 3  ( FIG.  4 F ). As a result, the lateral sloping surfaces  126  of the teeth  122  on the top element  102  slope in a different direction than the lateral sloping surfaces  148  of the teeth  144  on the bottom element  104 . Thus, the apex of the teeth  122  on the top element  102  is closer to axis A 3  than the apex of the teeth  144  on the bottom element  104 . It has been observed that stacking two implants in successive disc spaces may result in cracking of vertebral bone between the implants because the apexes on the teeth of the two implants are in alignment. The present invention seeks to avoid this cracking problem by offsetting the apexes of the teeth on the top element  102  from the apexes of the teeth on the bottom element  104 . Although the present invention is not limited by any particular theory of operation, it is believed that providing teeth having off-set apexes enables two or more intervertebral disc implants to be inserted into two or more successive disc spaces, while minimizing the likelihood of cracking the vertebral bodies between the disc spaces. 
     Referring to  FIG.  5   , prior to insertion into an intervertebral space, the articulating surface  110  of the top element  102  opposes the articulating surface  130  of the bottom element  104 . In preferred embodiments, the articulating surface  110  of the top element  102  defines a toroidal saddle-shaped surface including a concave surface extending between proximal and anterior ends thereof and a convex surface extending between the sides of the top element  102 . The articulating surface  130  of the bottom element  104  also includes a toroidal saddle-shaped surface having a convex surface extending between the posterior and anterior ends and a concave surface extending between the sides of the bottom element  104 . 
     The articulating surfaces may be similar to the articulating surfaces disclosed in commonly assigned U.S. Pat. No. 6,997,955. In certain preferred embodiments of the present invention, the longitudinally inwardly directed articulation surface of the top element  102  forms a constant radii saddle-shaped articulation surface. More particularly, the saddle surface is defined by a concave arc that is swept perpendicular to and along a convex arc. The articulation surface has a cross-section in one plane that forms a concave arc, and a cross-section in another plane (perpendicular to that plane) that forms a convex arc. The concave arc has a respective constant radius of curvature about an axis perpendicular to the one plane. The convex arc has a respective constant radius of curvature about an axis perpendicular to the other plane. 
     In a preferred embodiment, the concave arc has a constant radius of curvature A about an axis perpendicular to the anterior-posterior plane, and the convex arc has a constant radius of curvature B about an axis perpendicular to the lateral plane. Preferably, radius A is less than radius B. 
     The longitudinally inwardly directed articulation surface of the bottom element  104  also preferably forms a constant radii saddle-shaped articulation surface. More particularly, the saddle surface is defined by a convex arc that is swept perpendicular to and along a concave arc. The articulation surface has a cross-section in one plane that forms a convex arc, and a cross-section in another plane (perpendicular to that plane) that forms a concave arc. The convex arc has a respective constant radius of curvature about an axis perpendicular to the one plane. The concave arc has a respective constant radius of curvature about an axis perpendicular to the other plane. 
     In a preferred embodiment, the convex arc has a constant radius of curvature C about an axis perpendicular to the anterior-posterior plane, and the concave arc has a constant radius of curvature D about an axis perpendicular to the lateral plane. Preferably, radius C is less than radius D. 
     The constant radii saddle shaped articulation surfaces are configured and sized to be nestable against one another and articulatable against one another, to enable adjacent vertebral bones (against which the top and bottom elements are respectively disposed in the intervertebral space) to articulate in flexion, extension, and lateral bending. More particularly, the intervertebral disc of the present invention is assembled by disposing the top and bottom elements so that the vertebral body contact surfaces are directed away from one another, and the articulation surfaces are nested against one another such that the concave arcs accommodate the convex arcs. 
     Accordingly, movement of the adjacent vertebral bones relative to one another is permitted by the movement of the top and bottom elements relative to one another. In flexion and extension, the concave arcs of the top element  102  ride on the convex arcs of the bottom element  104  about a center of rotation below the articulation surfaces. In lateral bending, the concave arcs of the bottom element  104  ride on the convex arcs of the top element  102  about a center of rotation above the articulation surfaces. During these articulations, the elements are maintained at constant relative distraction positions, i.e., the elements do not move in directions that are directed away from one another (for example, do not move in opposing axial directions from one another (e.g., along a longitudinal axis of the spine)). Accordingly, in certain preferred embodiments, the present invention provides a pair of articulation surfaces that have a center of rotation above the surfaces in one mode of motion (e.g., lateral bending), and below the surfaces in another (e.g., flexion/extension), consistent in these regards with a natural cervical intervertebral joint. Preferably, the articulation surfaces are sized and configured so that the respective ranges of angles through which flexion/extension and lateral bending can be experienced are equal to or greater than the respective normal physiologic ranges for such movements in the cervical spine. 
     It is preferable that, in addition to the flexion, extension, and lateral bending motions described above, the adjacent vertebral bones be permitted by the intervertebral disc implant to axially rotate relative to one another (e.g., about the longitudinal axis of the spinal column) through a small range of angles without moving in opposite (or otherwise directed away from one another) directions (e.g., along the longitudinal axis) within that range, and then to engage in such opposite (or otherwise directed away from one another) movement once that range is exceeded. Preferably, the articulation surfaces are accordingly configured and sized to permit such movements. Because of the differing radii of the opposing articulation surfaces, the top and bottom elements are able to axially rotate relative to one another about the longitudinal axis of the spinal column through a range of angles without causing the vertebral body contact surfaces to move away from one another along the longitudinal axis. Once the axial rotation exceeds that range, the articulation surfaces interfere with one another as the concave arcs move toward positions in which they would be parallel to one another, and the distance between the vertebral body contact surfaces increases with continued axial rotation as the concave arcs ride up against their oppositely directed slopes. Thus, the articulation surfaces are configurable according to the present invention to permit normal physiologic axial rotational motion of the adjacent vertebral bones about the longitudinal axis through a range of angles without abnormal immediate axially opposite (or otherwise directed away from one another) movement, and to permit such axially opposite (or otherwise directed away from one another) movement when under normal physiologic conditions it should occur, that is, outside that range of angles. 
     The articulation surfaces preferably maintain contact over a range of normal physiologic articulating movement between the adjacent vertebral bones. That is, through flexion, extension, lateral bending, and axial rotation, the articulation surfaces are in contact with one another. Preferably, the surface area dimensions of the articulation surfaces are selected in view of the selected radii of curvature to prevent the edges of the saddle surfaces (particularly the edges of the concave arcs) from hitting any surrounding anatomic structures, or other portions of the opposing upper or lower element, before the limit of the normal physiologic range of an attempted articulation is reached. 
       FIGS.  6 A and  6 B  show, according to a preferred embodiment of the present invention, an intervertebral disc implant  100  including top element  102  and bottom element  104 . The articulating surface of the top element  102  preferably engages the articulating surface of the bottom element  104 . The articulating surface of the top element  102  preferably defines a convex surface extending between lateral sides  118 ,  120  thereof. The articulating surface of the bottom element  104  defines a concave surface extending between the lateral sides  136 ,  138  thereof. Each of the top and bottom elements  102 ,  104  include respective anterior walls  116 ,  150  that prevent over insertion and/or posterior migration of the intervertebral disc implant  100 . The teeth  122  on the protrusions of the top element  102  include laterally sloping surfaces  124  that slope downwardly toward the sides  118 ,  120 . In contrast, the teeth  144  on the protrusions of the bottom element  104  include laterally sloping surfaces  148  that preferably slope downwardly toward a central region of the bottom element  104 . The opposite sloping configuration of the teeth on the respective top and bottom elements  102 ,  104  preferably permits stacking of two intervertebral disc implants in two successive disc spaces, while minimizing the likelihood of cracking the vertebral bone between the adjacent disc spaces. In other preferred embodiments, the opposite sloping configuration of the teeth enable three or more intervertebral discs to be stacked atop one another over three or more successive disc spaces. In still other preferred embodiments, the teeth of the top and bottom elements may slope laterally in the same direction. 
     Referring to  FIG.  6 B , the articulating surface of the top element  102  defines a concave surface extending between posterior  112  and anterior  114  ends thereof. The articulating surface of the bottom element  104  defines a convex surface extending between the posterior  132  and anterior  134  ends of the bottom element  104 . The teeth  122  on the protrusions of the top element  102  include sloping surfaces  124  that slope downwardly toward the posterior end  112  of the top element  102 . The teeth  144  on the protrusions of the bottom element  104  have sloping surfaces  146  that slope downwardly toward the posterior end  132  of the bottom element  104 . As a result, the sloping surfaces  124 ,  146  of the respective teeth  122 ,  144  slope in the same direction, i.e., toward the posterior ends of the top and bottom elements  102 ,  104 . The respective sloping surfaces  124 ,  146  facilitate insertion of the implant  100  into a disc space. The respective vertical surfaces  122 ,  144  hinder or prevent expulsion or migration of the implant from the disc space after it has been inserted. 
     Referring to  FIGS.  7 A- 7 D  in certain preferred embodiments of the present invention, a template  154  has a distal end  156  and a proximal end  158 . The template  154  includes a shaft  160  extending between the distal and proximal ends and a handle  162  secured to a proximal end of the shaft. The template includes a template marker  164 . As shown in  FIG.  7 D , the template marker  164  has a cruciform-like structure with a first vertically extending arm  166 , a second vertically extending arm  168 , a first lateral arm  170  and a second lateral arm  172 . The upper and lower ends of the respective first and second vertically extending arms  166 ,  168  preferably have apexes that may be used for aligning scoring of the anterior faces of the adjacent vertebral bodies. The score marks may later be used for aligning other tools and/or the intervertebral disc. The template marker  164  includes a central pin  174 , a first tack  176  on the first vertical arm  166  and a second tack  178  on the second vertical arm  168 . The central pin  174  is adapted to engage a natural disc and the tacks  176 ,  178  are adapted to engage bone, such as vertebral bone. The central pin  174  may also be replaced or supplanted by a plurality of pins positioned on the lateral arms  170 ,  172 . Referring to  FIG.  7 D , the lateral arms  170 ,  172  preferably define a distal surface  180  that is curved for matching the curve of the anterior surface of a natural intervertebral disc. 
       FIGS.  8 A- 8 D  show a template marker for a template, in accordance with another preferred embodiment of the present invention. The template marker  164 ′ is substantially similar to the template marker shown in  FIG.  7 D . However, the template marker  164 ′ shown in  FIGS.  8 A- 8 D  includes first and second engagement features or projections  182 A′ and  182 B′ projecting from top and bottom surfaces of adapter shaft  184 ′. 
     Referring to  FIGS.  9 A and  9 B , the template marker  164 ′ may be attached to a distal end  156 ′ of a template handle  162 ′. The adapter shaft  184 ′ of the template marker  164 ′ is inserted into an opening at the distal end  156 ′ of the template handle  162 ′. The projections  182 A′,  182 B′ on the adapter shaft  184 ′ are inserted into opposing grooves  186 ′ formed in the template handle  162 ′. 
       FIGS.  10 A- 10 B  show the template marker  164 ′ secured to the distal end  156 ′ of the template handle  162 ′. Referring to  FIGS.  10 A- 10 C , after the projections  182 A′ and  182 B′ on the template marker  164 ′ have been received within the grooves at the distal end of the template handle  162 ′, a rotatable handle  188 ′ is rotated for advancing shaft  190 ′ relative to outer shaft  192 ′ so as to lock the template marker  164 ′ to the distal end  156 ′ of the template handle  162 ′. In certain preferred embodiments, the template handle  162 ′ is rotated to seat the projections  182 A′,  182 B′ in the grooves  186 ′ and the rotatable handle  188 ′ is rotated to hold the projections  182 A′,  182 B′ forward in the grooves  186 ′. 
       FIGS.  11 A- 11 B  show a reference pin drill guide  194  having a distal end  196 , a proximal end  198 , a shaft  200  extending between the distal and proximal ends and a handle  202  at the proximal end of the shaft  200 . The distal end  196  of the reference pin drill guide includes a main body  204  having an upper end  206  and a lower end  208 . The main body includes a first opening  210  extending therethrough adjacent upper end  206  and a second opening  212  extending therethrough adjacent lower end  208 . The main body includes a head  214  that projects from a distal side thereof. The head includes a tapered nose  216 , a top surface  218 , and a bottom surface  220 . The head  214  also includes a first vertebral body stop  222  projecting upwardly from top surface  218  and a second vertebral body stop  224  projecting below second surface  220 . 
     Referring to  FIGS.  11 A and  11 C , the shaft  200  of the reference pin drill guide  194  is preferably angled or curved so that the working end of the tool may be observed by a surgeon. As shown in  FIGS.  11 A- 11 C , a distal end  226  of a drill bit  228  may be passed through openings  210 ,  212  for forming holes in the vertebral bone. As will be described in more detail below, the threaded ends of reference pins may be inserted into the holes. Referring to  FIG.  11 A , the main body  204  preferably includes an upper alignment flange  230  projecting from upper end  206  thereof and a lower alignment flange  232  projecting from lower end  208  of main body  204 , for use in aligning the flanges with alignment marks previously scored on the vertebral bones. 
     The drill bit  228  includes a distal end  226  and a proximal end  234  adapted to be secured by a drill. The drill bit  228  includes a shoulder  236  that limits advancement of the drill bit through the openings  210 ,  212  of the main body  204 . 
     Referring to  FIGS.  13 A and  13 B , in order to make holes in the vertebral bone for alignment or reference pins, the head  214  of the reference pin drill guide  194  is inserted into the disc space between the vertebral bodies. The head is advanced until the vertebral body stops abut the anterior faces of the respective vertebral bodies. The distal end of the drill bit  228  is then inserted through the openings  210 ,  212  in the main body  204 . The distal end of the drill bit  228  is advanced into bone to form the openings for the reference pins. 
     Referring to  FIGS.  14 A- 14 C , in certain preferred embodiments of the present invention, a reference pin insertion guide  236  has a distal end  238 , a proximal end  240 , a shaft  242  extending between the distal and proximal ends and a handle  244  secured to the distal end of the shaft  242 . The reference pin insertion guide  236  includes alignment guide body  246  having an upper end including a first opening  248  and a lower end including a second opening  250 . The distal end also includes a head  252  insertable into an intervertebral disc space. The head  252  has a tapered nose  254 , a top surface  256  terminating at a first vertebral body stop  258  and a bottom surface  260  terminating at a second vertebral body stop  262 . The first and second vertebral body stops  258 ,  262  preferably prevent over insertion of the tool into an intervertebral disc space. 
       FIGS.  15 A- 15 C  show an alignment or reference pin  264 , in accordance with certain preferred embodiments of the present invention. Referring to  FIGS.  15 A and  15 B , the reference pin  264  has a distal end  266  and a proximal end  268 . Referring to  FIG.  15 B , the distal end  266  includes a threaded portion  270  that is threadable into vertebral bone. The distal end of reference pin  264  also preferably includes a flange  272  that limits insertion of the reference pin. A proximal side of the flange  272  includes a feature or hex nut  274  engageable by a driver. 
       FIG.  16    shows a rescue reference pin  264 ′, in accordance with another preferred embodiment of the present invention. The rescue pin  264 ′ is substantially similar to the reference pin  264  shown in  FIG.  15 B  and includes a threaded shaft  276 ′ located on a posterior side of flange  272 ′. The rescue pin  264 ′ is preferably used if the reference pin  264  shown in  FIG.  15 B  does not remain anchored to bone or pulls out of the bone. The rescue pin  264 ′ preferably has larger diameter threading (than the reference pin  264 ) at the leading end thereof. 
     Referring to  FIGS.  17 A- 17 C , in certain preferred embodiments of the present invention, a reference pin driver  280  is utilized for driving reference pins into bone. The reference pin driver  280  includes a distal end  282  having an opening adapted to receive a reference pin  264  (or a rescue pin  264 ′) and a proximal end  284  including a handle  286 . The reference pin driver  280  also includes a shaft  288  that extends between the distal end  282  and the proximal end  284 . The shaft  288  preferably has an opening including at least one hexagonal surface that matches the hexagonal nut  274  on the reference pin  264  ( FIG.  15 A ). 
       FIGS.  18 A and  18 B  show, for use in preferred embodiments of the invention, a sleeve  290  insertable into the openings in the reference pin drill guide  194 . The sleeve  290  includes a distal end  292  having an opening  294  and a proximal end  296  including a stop flange  298 . Referring to  FIG.  18 B , the distal end  292  of sleeve  290  includes opening  294  extending therethrough and larger opening  296  at the distal-most end. The enlarged opening  296  preferably has a circular counterbore that freely slides over the hex nut on the reference pin  264  described above. The smaller opening  294  preferably forms a sliding fit with an outer surface of the reference pin  264 . The sleeve  290  preferably stabilizes the reference pin insertion guide  236  after the first reference pin has been inserted into bone and during insertion of the second reference pin. 
       FIGS.  19 A- 19 C  shows a distraction instrument  300  including support element  302  and first and second distracting arms  304 ,  306  that travel over the support element  302 . Each distracting arm  304 ,  306  has a curved section  308  and openings  310  at distal ends of the arms  304 ,  306 . The distracting element  300  also includes adjustment element  312  that interacts with support body  302  and arms  304 ,  306  for adjusting the distance between the arms. As will be described in more detail herein, after reference pins  264  are inserted into vertebral bone, the distractor arms  304 ,  306  are slid over the reference pins. Once the distractor element  300  is coupled with the reference pins  264 , the adjusting element  312  may be operated for separating the distractor arms  304 ,  305  so as to distract adjacent vertebrae and allow for removal of disc material. 
     Referring to  FIGS.  20 A- 20 D , in accordance with certain preferred embodiments of the present invention, a drill guide  314  has a distal end  316 , a proximal end  318 , a shaft  320  extending between the distal and proximal ends and a handle  322  adjacent the distal end  318 . The drill guide includes a main body  324  attached to the distal end of the shaft  320 . Referring to  FIG.  20 D , the main body  324  includes first and second openings  326 ,  328  for engaging the reference pins. The main body  324  also includes four drill guide openings  330 A- 330 D for guiding a distal end  332  of a drill bit  334 . The four drill guide openings  330 A- 330 D are positioned to coincide with the protrusions of an intervertebral disc implant. Referring to  FIGS.  20 A and  20 C , the main body  324  also includes a head  336  insertable into an intervertebral disc space. The head  336  includes a tapered nose  338 , a top surface  340 , a bottom surface  342 , and first and second vertebral body stops  344 ,  346  extending above and below the top and bottom surfaces  340 ,  342 . As will be described in more detail herein, the alignment openings  326 ,  328  are slid over the reference pins and the tool is advanced until the head  336  is positioned in the intervertebral disc space. The distal end  332  of the drill bit  334  is then passed in series through the drill alignment openings  330 A- 330 D for at least partially forming protrusion openings for the protrusions of an intervertebral disc implant. 
     Referring to  FIGS.  21 A- 21 D , in accordance with certain preferred embodiments of the present invention, a chisel guide  350  includes a distal end  352 , a proximal end  354  with a handle  356  and a shaft  360  extending from the proximal end toward the distal end. The shaft  360  includes on one side a left track  362  having an opening  364  at the proximal end  354  and extending toward the distal end  352 , and, on an opposite site, a right track  362 ′ having an opening  364 ′ at the proximal end  354  and extending toward the distal end  352 . The chisel guide  350  includes a head  366  at the distal end  352 . Referring to  FIGS.  21 C and  21 D , the head  366  includes alignment grooves  368  formed in top and bottom surfaces thereof. The alignment grooves are adjacent the left and right tracks of the shaft  360 . 
     Referring to  FIGS.  21 B and  21 D , the chisel guide also includes first and second reference pins openings  370 ,  372 . In operation, the reference pin openings  370 ,  372  are slid over the reference pins described above, and the head  366  is inserted into an intervertebral disc space. 
     Referring to  FIGS.  22 A- 22 D , in certain preferred embodiments of the present invention, a chisel  374  has a distal end  376  with first and second cutting blades  378 ,  380 , and a proximal end  382  having a handle  384  and a striking surface  386 . The chisel  374  includes a shaft  388  extending between the distal and proximal ends of the tool. The shaft  388  includes projections extending therefrom for guiding the chisel in the tracks of the chisel guide shown in  FIGS.  21 A- 21 D . Referring to  FIGS.  22 B and  22 D , the projections  390  guide the chisel along the track while maintaining alignment of the chisel. Referring to  FIG.  22 C , the distal end of the chisel includes an opening extending between cutting blades  378 ,  380 . The opening  394  allows the cutting blades  378 ,  380  to slide over the grooves  368  formed in the top and bottom surfaces of the head  366  ( FIG.  21 D ). 
     Referring to  FIG.  23   , the chisel guide  350  described in  FIGS.  21 A- 21 D  is slid over the reference pins  264  secured to bone. The chisel  374  of  FIGS.  22 A- 22 D  is then slideably advanced along one of the tracks of the chisel guide for forming protrusion openings in the bone. The same or a second chisel  374  is then slideably advanced along the other of the tracks to form additional protrusion openings in the bone. 
       FIGS.  24 A- 24 B  show a mallet  396  including a handle  398  having a lower end  400  and an upper end  402  with a striking element  404  secured to the upper end. The striking element  404  includes a U-shaped opening  406  formed therein so that the mallet may be used as a slap hammer. 
       FIGS.  25 A- 25 D  show a sizer  408 , in accordance with certain preferred embodiments of the present invention. The sizer  408  includes a main body  410  having a distal end  412 , a top surface  414 , and a bottom surface  416 . The body  410  includes sloping surfaces  418  extending between the distal end  412  and the top and bottom surfaces  414 ,  416  to ease insertion of the sizer between the vertebrae. The sizer  408  also includes an adapter shaft  420  and first and second projections  422 A,  422 B. 
     The sizer also includes vertebral body stops  424  and  426  for limiting insertion of the sizer. Referring to  FIGS.  25 A and  25 B , the sizers are preferably provided in variable heights (e.g., 5-9 mm) and variable base plate widths (e.g., 14 and 16 mm). Sequentially larger sizers are used to determine the desired implant height that will best fit into the disc space without over-tensioning the annulus. In their preferred usage, the sizer that fits snugly into the disc space with mild to moderate resistance to pull-out should indicate the proper height of the disc to be implanted. 
     Referring to  FIGS.  26 A- 26 E , a sizer  408  may be attached to a distal end  430  of a handle  432  by sliding the projections  422 A,  422 B into grooves extending from the distal end  430  of the handle  432 . After the sizer  408  is coupled with the handle  432 , a rotatable element  434  may be rotated for locking the sizer  408  to the handle  432 . In certain preferred embodiments, the handle  432  is rotated to seat the projections  422 A,  422 B in the grooves extending from the distal end  430  of the handle and the rotatable element  434  is rotated to hold the projections  422 A,  422 B forward in the closed ends of the grooves. 
     Referring to  FIGS.  27 A- 27 D , in certain preferred embodiments of the present invention, a trial  436  includes a distal end  438 , a proximal end  440 , a shaft  442  extending between the distal and proximal ends, and a striking surface  444  located at the proximal end  440  of the shaft  442 . The trial also includes a trial implant  446  secured at distal end  438 . The trial implant  446  includes protrusions  448  having teeth  450  that are positioned to coincide with the protrusions of a intervertebral implant. The size of the trial implant  446  is selected based upon the largest sizer that safely fit within the intervertebral disc space. The trial  436  also includes reference pin alignment openings  452 ,  454 . The reference pin alignment openings  452 ,  454  are adapted to slide over the reference pins secured to vertebral bone. 
     Referring to  FIGS.  28 A- 28 F , in accordance with certain preferred embodiments of the present invention, an implant dispenser  456  is adapted to hold the top and bottom elements of an intervertebral disc implant as a single unit with the articulating surfaces held together. Referring to  FIGS.  28 A- 28 C , the implant dispenser  456  is preferably flexible and includes a superior arm  458  for engaging a top element of an intervertebral disc implant and an inferior arm  460  for engaging a bottom element of an intervertebral disc implant. The superior and inferior arms preferably have lateral notches  462  formed therein for receiving the teeth of the top and bottom elements, as will be described in more detail herein. The implant dispenser also preferably includes a central support  464  that enables the superior and inferior arms  458 ,  460  to move away from one another for releasing an intervertebral disc implant. 
     Referring to  FIG.  28 E , the superior arm  458  preferably has indicia such as a text or a symbol  466  indicating that the arm  458  overlies the top element of the implant. The indicia on the superior arm may also preferably include the size  468  of the implant.  FIG.  28 F  shows the inferior arm  460  including indicia such as a text or a symbol  470  indicating that the arm  460  overlies the bottom element of the implant. The inferior element indicia may also include size information  472  as shown. 
       FIGS.  29 A- 29 E  show the intervertebral disc implant of  FIG.  1    secured in the implant dispenser  456  of  FIGS.  28 A- 28 F . The intervertebral disc implant includes top element  102  engaged by superior arm  458  and central element  464 . The bottom element  104  is engaged by inferior arm  460  and central element  464 . The teeth of the implant extend through the notches  462  in the superior and inferior arms. 
       FIGS.  30 A to  30 F- 1    show an inserter head for inserting an intervertebral disc implant. Referring to  FIGS.  30 A- 30 B , the inserter head  474  includes a main body  476  having a distal end  478  and a proximal end  480 . The distal end  478  of the main body  476  includes four spaced arms  482  having inwardly facing surfaces with projections  484  that are adapted to fit within the depressions formed within the protrusions of the top and bottom elements of the intervertebral disc implant. The main body includes a central opening  486  extending from the proximal end  480  to the distal end  478  thereof. Referring to  FIGS.  30 C and  30 D , the main body  476  also includes an upper alignment groove  488  and a lower alignment groove  490 . The alignment grooves  488 ,  490  are adapted to engage the reference pins for guiding advancement of the inserter head. In certain preferred embodiments, the alignment grooves  488 ,  490  taper toward one another so that the grooves are closer to one another at distal ends thereof and farther away from one another at proximal ends thereof. 
       FIGS.  30 E and  30 E- 1    show the projection  484  on one of the four arms  482 . As noted above, the projections  484  on the arms  482  engage the depressions in the protrusions of the top and bottom elements of the intervertebral discs. 
       FIG.  31    shows the inserter head  474  of  FIG.  30 A  prior to assembly with an inserter handle  492 . The inserter handle  492  includes a pusher rod  494  that may be advanced by a rotatable handle  496  coupled with the pusher rod  494 . The handle includes a shaft  496  that is insertable into opening  486  of the inserter head. Referring to  FIGS.  32 A and  32 B , after the inserter head  474  has been coupled with the inserter handle, the element  496  may be rotated for advancing the pusher rod  494 . As will be described in more detail below, advancing the pusher rod  494  will disengage the top and bottom elements of an intervertebral disc with the arms  482  of the inserter head  474 . 
       FIGS.  33 A and  33 B  show the implant dispenser  456  of  FIG.  29 A  holding an intervertebral disc implant  100 . Referring to  FIG.  33 B , the depressions in the protrusions are coupled with the projections in the arms  482  of the inserter head  474 . Referring to  FIGS.  34 A and  34 B , after the implant  100  has been secured to the arms  482  of the inserter head  474 , the implant dispenser may be removed. Referring to  FIG.  34 B , at this point, the implant  100  is held by the arms  482  of the inserter head. Referring to  FIGS.  35 A- 35 B , the pusher rod  494  may be advanced toward the distal end of the inserter head  474  for decoupling the implant  100  from the inserter head  474 . 
     Referring to  FIGS.  36 A and  36 B , in accordance with certain preferred embodiments of the present invention, a tamp  500  includes a distal end  502 , a proximal end  504 , a shaft  506  extending between the distal and proximal ends and a handle  508  adjacent the proximal end  504 . The distal end  502  includes an abutting surface  510  that is adapted to engage the anterior end of an intervertebral disc implant. The tamp  500  also includes a striking surface  512  at the proximal end. A device such as the mallet shown and described above may be impacted against the striking surface  512  for applying force to the striking surface  510  of the tamp. 
       FIGS.  37 A- 37 D  show an extractor  514 , in accordance with certain preferred embodiments of the present invention. The extractor  514  includes a distal end  516  having a hook  518 , a proximal end  520 , and a shaft  522  extending between the distal and proximal ends. The extractor  514  also preferably includes a handle  524  adjacent the proximal end  520  thereof.  FIGS.  37 C and  37 D  show shaft  522  with a hook  518  at a distal end of the shaft. 
     Disclosed herein are implants, surgical instruments and procedures in accordance with certain preferred embodiments of the present invention. It is contemplated, however, that the implants, instruments and procedures may be slightly modified, and/or used in whole or in part and with or without other instruments and procedures, and still fall within the scope of the present invention. Although the present invention may discuss a series of steps in a procedure, the steps may be accomplished in a different order, or may be used individually, or in conjunction with other methods, without deviating from the scope of the present invention. 
     Prior to implanting the intervertebral disc implant, a review of X-rays, MRI or CT-myelogram is preferably conducted to assess the level to be treated for osteophytes and to compare the intervertebral disc height with the adjacent levels. Referring to  FIG.  38   , the patient is preferably positioned in the supine position to provide for an anterior surgical approach to the cervical spine. Steps should preferably be taken to stabilize the patient&#39;s spine in a neutral position and to prevent rotation during the procedure. In certain preferred embodiments, it may be preferable to place a towel or bean bag underneath the patient&#39;s shoulders. Tape, a halter or skeletal traction may be used to prevent rotation. Referring to  FIGS.  38  and  39   , a transverse skin incision is preferably made at the appropriate level to expose the targeted spinal segment including the discs above and below the target spinal segment. Care should be taken to avoid prolonged retraction pressure on vital structures, such as the esophagus. 
     Another step in the intervertebral disc implantation procedure involves identifying and marking a midline on the target segment of the spine. In preferred embodiments, a template, such as the template shown and described above in  FIGS.  7 A- 10 D , is utilized to mark the midline. In certain preferred embodiments, the size and dimensions of the template may vary. The exact template selected may be based upon initial estimation of the appropriate implant size from pre-operative X-rays and/or MRI/CT. The template is preferably attached to the template handle, as described above in reference to  FIGS.  9 A- 9 B and  10 A- 10 D . The template may have different sizes whereby the lateral arms of the template attachments approximate the width options of the implant (i.e., 14 mm and 16 mm). Referring to  FIG.  40    and  FIG.  9 A , the central pin at the distal end of the template is inserted into the middle of the disc so that the upper and lower vertically extending arms are approximately aligned with the midline axis of the vertebral column. The tacks on the upper and lower vertical arms engage the vertebral bone to stabilize the template. The handle at the proximal end of the template handle may be tapped, such as by using a mallet, to push the tacks into the vertebral bone. The template handle may then be disengaged from the template, which remains attached to the vertebral bone. At this stage, fluoroscopy may be used to verify the midline and lateral margins of the disc space. In addition, the spinous processes are preferably centered. 
     Referring to  FIG.  41   , a tool such as a scalpel or an electrocautery tool is preferably utilized to score the midline points on the anterior surfaces of the superior and inferior vertebral bodies. Care is preferably taken to ensure that the midline is well defined for all subsequent endplate preparation and implant insertion steps. Referring to  FIGS.  41  and  42   , after the midline points have been scored, the template handle may be reattached to the template for removing the template from engagement with the target area.  FIG.  42    shows the target area of the spine with an opening formed in the disc space by the central pin of the template, and two smaller openings being formed in the respective superior and inferior vertebral bodies by the tacks of the template. In addition, the scoring of the midline points is evident at the anterior surfaces of the superior and inferior vertebral bodies. 
     Referring to  FIG.  43   , a cutting tool such as a scalpel may be used to dissect a window in the annulus of the targeted disc. The size of the window dissected in the annulus preferably approximates the width of an intervertebral disc implant to be inserted therein. In certain preferred embodiments, radiographic imaging such as fluoroscopy may be used to identify osteophytes that extend anteriorly. Any osteophytes that extend anteriorly are preferably resected back to the vertebral body so that the surfaces of the superior and inferior vertebral bodies are flattened. Moreover, techniques such as radiographic imaging should be used to identify any osteophytes extending downwardly or upwardly into the anterior region of the disc space. Such osteophytes should be resected to the endplates.  FIG.  44    shows the targeted disc space after the initial discectomy has been completed including resection of any anterior osteophytes present in the targeted disc area. 
     Referring to  FIG.  45   , after initial preparation of the disc space, reference pins may be attached to the anterior faces of the superior and inferior vertebral bodies. The midline score marks and the tack openings in the superior and inferior vertebral bodies are preferably used for proper placement of the reference pins. Care is preferably taken to ensure proper placement and alignment of the reference pins, which will guide subsequent steps of the procedure. Referring to  FIG.  45   , the reference pin drill guide described in  FIGS.  11 A- 11 D  is inserted into the dissected disc space. Specifically, the head at the distal end of the reference pin drill guide is inserted into the disc space between the vertebral bodies. The head at the distal end is inserted until the vertebral body stops abut against the anterior faces of the superior and inferior vertebral bodies. At this stage, one of the openings in the main body of the reference pin drill guide is preferably in alignment with the superior vertebral body and another one of the openings is in alignment with the inferior vertebral body. The reference pin drill guide is preferably aligned with the midline of the vertebral bodies as marked by the score line markings. The alignment of the reference pin drill guide is preferably checked such as by using fluoroscopy. Referring to  FIG.  46   , with the reference pin drill guide in place, holes are drilled in the superior and inferior vertebral bodies using a drill bit, such as the drill bit shown in  FIG.  12    above.  FIG.  46    shows a hole being drilled in the superior vertebral body.  FIG.  47    shows a hole being drilled in the inferior vertebral body.  FIG.  48    shows the target spinal segment after holes have been drilled in both the superior and inferior vertebral bodies. 
     Referring to  FIG.  49   , after the holes are drilled, the reference pin drill guide is removed from the targeted disc segment. At this stage, a first hole for a first reference pin has been formed in the superior vertebral body and a second hole for a second reference pin has been formed in the inferior vertebral body. The first and second holes in the vertebral bodies are preferably in alignment with the score marks formed previously. 
     Referring to  FIG.  50   , the reference pins may be inserted using the reference pin insertion guide shown and described above in  FIGS.  14 A- 14 C . Preferably, the head at the distal end of the reference pin insertion guide is inserted into the disc space until the vertebral body stops abut against the anterior surfaces of the superior and inferior vertebral bodies. As shown in  FIG.  50   , the reference pin insertion guide preferably has a first opening in alignment with the opening formed in the superior vertebral body and a second opening aligned with the opening formed in the inferior vertebral body. A reference pin, such as the reference pin shown and described above in  FIGS.  15 A- 15 C  is inserted into the first opening of the reference pin insertion guide. The threading at the distal end of the reference pin is threaded into the opening in the superior vertebral body. The reference pin driver shown in  FIG.  17 A- 17 C  may be utilized for threading the reference pin into the vertebral body. The threads on the reference pins may be self-tapping threads. 
     Referring to  FIG.  51   , after the first reference pin is inserted into bone, the second reference pin is passed through the lower opening in the reference pin insertion guide and driven into the inferior vertebral body using the reference pin driver. In other preferred embodiments, a first reference pin may be attached to the inferior vertebral body before attaching a second reference pin to the superior vertebral body. Thus, the insertion of the reference pins can be accomplished in any particular order. 
     Referring to  FIG.  52   , in certain preferred embodiments, after a first reference is inserted into bone, a sleeve such as the sleeve shown and described in  FIGS.  18 A- 18 B  may be slid over the attached reference pin so as to stabilize the reference pin insertion guide. As shown in  FIG.  52   , the sleeve preferably remains in place to prevent movement of the reference pin insertion guide during insertion of the second reference pin into the other vertebral body. 
     In particular preferred embodiments, different sized reference pins may be used. These different sized reference pins may include shafts having different diameters and/or lengths. In certain preferred embodiments, a small and a large set of reference pins is provided in an instrument tray. The smaller of the pair of reference pins should be inserted initially. If the smaller pair of reference pins proves unsatisfactory, the larger pair of reference pins may be utilized. As shown in  FIG.  53   , the reference pins are preferably parallel to each other and in alignment with the midline of the superior and inferior vertebral bodies. 
     Once the reference pins are in place, a surgeon may preferably apply distraction to the disc space by using a distraction tool, such as a distractor as shown and described in  FIGS.  19 A- 19 C . Once distraction has occurred, the disc space may be cleared of any extraneous matter and restored to a desired height. Prior to distraction, the facets of the targeted spinal segment should be reviewed under fluoroscopy in order to monitor facet orientation during distraction. The distractor shown in  FIGS.  19 A- 19 C  is preferably placed over the reference pin shown in  FIG.  53   .  FIG.  54    shows the distractor after it has been placed over the reference pins. If the reference pins loosen at any time during the distraction procedure, the reference pins may be removed and replaced with the larger reference pins provided in the instrument set. 
     The distractor is then utilized to apply distraction to the targeted spinal segment. During the distraction procedure, the facets and the disc space are preferably monitored under fluoroscopy to ensure a complete distraction. The amount of distraction should not exceed the height of the adjacent disc space. As noted above, fluoroscopy should be used to monitor the distraction height so as to prevent over-distraction. As is well known to those skilled in the art, over-distraction may cause nerve and/or facet damage. 
     After the targeted spinal segment has been distracted, the discectomy procedure is completed. In preferred embodiments, the posterior and lateral margins of the disc space are cleared of any extraneous matter. The clearing of the posterior and lateral margins preferably extends to the uncinate processes and all the way back to the nerve root and canal. In certain preferred embodiments, lateral fluoroscopy is utilized to check the anterior aspects of the vertebral body for osteophytes. A cutting tool, such as a burr, may be used to further prepare the endplates of the opposing superior and inferior vertebral bodies. The cutting tool may be utilized to smooth out the curvature of the superior endplate. After the discectomy has been completed, the endplates of the adjacent vertebral bodies are preferably parallel to one another and relatively uniform, thereby preventing undersizing of the implant. 
     In certain preferred embodiments, the decompression of the targeted disc space may be completed by removing any posterior osteophytes or soft tissue material that may inhibit the full distraction of the posterior portion of the targeted disc space. In certain preferred embodiments, it may be necessary to remove the posterior longitudinal ligament (PLL) to achieve optimal restoration of the disc height, decompression and release for post-operative motion. In addition, the posteriolateral corners of the endplates may be resected as needed to provide neural decompression. In certain instances, it may be necessary to remove the posteriolateral uncovertebral joints. The lateral uncovertebral joints are preferably not removed unless they are causing nerve root compression. In addition, in certain preferred embodiments it may be necessary to perform a foraminotomy if there are symptoms of neural/foraminal stenosis. 
     In certain preferred embodiments, another stage of the intervertebral disc implantation method involves initial endplate preparation including drilling pilot grooves in the superior and inferior vertebral bodies. After complete distraction has taken place, the distractor shown in  FIG.  54    is removed. Referring to  FIG.  55   , the protrusion drill guide of  FIGS.  20 A- 20 D  is inserted into the prepared disc space. The openings at the upper and lower ends of the drill guide are aligned with the proximal ends of the reference pins. As shown in  FIG.  55   , the reference pins engage the openings for guiding the drill guide toward the prepared disc space. The head at the distal end of the drill guide is inserted into the prepared disc space until the vertebral body stops abut against the anterior faces of the superior and inferior vertebral bodies. In preferred embodiments, the surgeon should visually check that the vertebral body stops of the drill guide come into full contact with the superior and inferior vertebral bodies. The handle of the drill guide is preferably parallel with the inferior and superior endplates and aligned in the sagittal plane. 
     As shown in  FIG.  55   , a drill bit is utilized to drill four pilot holes for the implant protrusions at precise locations in the vertebral bodies. As shown in  FIG.  55   , the drill guide includes two openings for drilling a pair of pilot holes in the superior vertebral body and two openings for drilling pilot holes in the inferior vertebral body. In certain preferred embodiments, the drill includes a stop to limit the length of the pilot holes. In particular preferred embodiments, there is a stop on the drill so as to ensure that the pilot holes are no more than 10 mm in length. 
     Referring to  FIG.  56   , after the pilot holes have been formed in the superior and inferior vertebral bodies, the drill guide is retracted from the targeted disc space and decoupled from the two reference pins. As shown in  FIG.  56   , two pilot holes are formed in the superior vertebral body and two pilot holes are formed in the inferior vertebral body. The two pilot holes on the left of the adjacent vertebral bodies are preferably in vertical alignment with one another and the two pilot holes on the right of the vertebral bodies are preferably in vertical alignment with one another. 
     In certain preferred embodiments, channels for the protrusions of the intervertebral disc implant are formed in the endplates of the superior and inferior vertebral bodies. Referring to  FIG.  57   , the chisel guide shown and described in  FIGS.  21 A- 21 D  is inserted into the targeted disc space. The alignment openings in the chisel guide are slid over the reference pins. The head at the distal end of the chisel guide is inserted into the targeted disc space until the vertebral body stops come into contact with the anterior surfaces of the vertebral bodies. The handle of the chisel guide is preferably parallel with the superior and inferior endplates and aligned in the sagittal plane. As shown in  FIG.  55   , the chisel described in  FIGS.  22 A- 22 D  is coupled with one of the tracks of the chisel guide and advanced toward the disc space. The cutting blades of the chisel are preferably advanced toward the superior and inferior vertebral bodies while positive pressure is applied to the chisel guide to ensure that it does not back out of the disc space.  FIG.  23    shows the chisel  374  coupled with the chisel guide  350  with the cutting blades at the distal end of the chisel opposing the superior and inferior vertebral bodies. A striking instrument, such as the mallet shown and described in  FIGS.  24 A- 24 B , may be utilized to strike the distal end of the chisel so as to cut channels in the superior and inferior vertebral bodies. 
     Referring to  FIG.  58   , after the first chisel has cut the channels on one side of the chisel guide, a second chisel is preferably utilized in the same manner as described above with respect to the first chisel to cut a second set of channels in the vertebral bodies.  FIG.  59    shows the targeted spinal segment after a set of channels has been cut into the superior and inferior vertebral bodies.  FIG.  60    shows the targeted spinal segment after the channels have been cut in the vertebral bodies and the chisel guide has been removed from the reference pins. 
     In certain preferred embodiments, a sizing operation is conducted to determine the proper size of the intervertebral disc that will be placed into the targeted disc space. Referring to  FIG.  61   , a sizer such as the sizer shown and described above in  FIGS.  25 A- 26 B  may be inserted into the disc space. The sizers may have different heights (e.g., 5-9 mm) and different base plate widths (e.g., 14 mm and 16 mm). In preferred embodiments, a sizer having a height of 5 mm is used initially. The sizer is preferably attached to the handle shown and described above in  FIGS.  26 A- 26 D . The sizer is then advanced into the disc space as shown in  FIG.  62   . The sizer is preferably advanced until the vertebral body stops on the sizer abut against the anterior surfaces of the superior and inferior vertebral bodies. 
     After starting with a sizer having a height of 5 mm, sequentially larger sizers are utilized to determine the desired implant height that will best fit into the disc space without over tensioning the annulus. The correct height for the sizer is preferably determined when the sizer fits snugly into the disc space with mild to moderate resistance to retraction of the sizer. The width of the disc space may also be checked by using a sizer having a different width and inserting the sizer into the disc space. In certain preferred embodiments, the sizer may include alignment openings that engage the reference pins for guiding the sizer as it is advanced toward the disc space. 
     In certain preferred embodiments, a trial is inserted into the disc space to complete preparation of the channels for the protrusions of the intervertebral disc. Referring to  FIG.  63   , a trial such as that shown and described above in  FIGS.  28 A- 28 D  is advanced toward the disc space. As described above, the trial includes alignment openings that are slid over the reference pins for guiding advancement of the trial. The particular size of the trial is selected based upon the corresponding size of the sizer that produced the best fit within the disc space. In certain preferred embodiments, the particular trial selected is based upon the following chart: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Sizer 
                 Trial Size 
               
               
                   
                   
               
             
            
               
                   
                 5-6 mm height 
                 Small, 14 mm 
               
               
                   
                 14 mm × 12 mm baseplate 
               
               
                   
                 7-9 mm height 
                 Large, 14 mm 
               
               
                   
                 14 mm × 12 mm baseplate 
               
               
                   
                 5-6 mm height 
                 Small, 16 mm 
               
               
                   
                 16 mm × 14 mm baseplate 
               
               
                   
                 7-9 mm height 
                 Large, 16 mm 
               
               
                   
                 16 mm × 14 mm baseplate 
               
               
                   
                   
               
            
           
         
       
     
     Referring to  FIG.  64   , the trial is advanced along the reference pins and into the intervertebral space. The protrusions on the trial are preferably aligned with the channels previously formed in the opposing endplates. A mallet may be impacted against the proximal end of the trial for advancing the trial into the disc space. If necessary, a slap hammer may be utilized to remove the trial from the disc space. In preferred embodiments, the trial is inserted at an angle that is parallel to the endplates so as to prevent damage to the endplates and avoid creating bone fragments. One or more trials may be inserted into the disc space until a proper fit is achieved. 
     In certain preferred embodiments, an appropriately sized intervertebral disc implant is then selected and inserted into a targeted disc space. Referring to  FIG.  65   , in certain preferred embodiments, the intervertebral disc implant is provided as a single unit with the top and bottom elements of the implant being held together by an implant dispenser  456 . In preferred embodiments, the implant dispenser is color coded to correspond to the height of the implant. In addition, the implant dispenser is preferably marked with the height of the implant and the width of the top and bottom elements. The outer surface of the implant may also be marked with the height and width of the implant. In particular preferred embodiments, the anterior face of the implant is marked with the height and width of the implant. 
     In preferred embodiments, prior to insertion of the intervertebral disc implant, the size label on the implant is inspected and the size label on the implant dispenser is also inspected to ensure that the correctly sized implant was selected and that the top or superior element of the implant is oriented for proper insertion. In preferred embodiments, an implant is selected having a height and baseplate dimensions that match the corresponding sizer that restored the desired height of the disc space without over-tensioning the annulus or damaging the facets. 
     After an appropriately sized intervertebral disc implant has been selected, an inserter head, such as the inserter head shown and described above in  FIGS.  30 A- 30 B , is selected. The selected inserter head preferably has a height and/or dimensions that match the particular dimensions of the selected implant and selected implant dispenser. Thus, the inserter head may also be color coded to correspond to the height of the implant and the particular dimensions of the implant dispenser. The inserter head may be a single use component that is discarded after the implantation procedure. In certain preferred embodiments, each inserter head may be used for either a 14 mm or 16 mm width implant that is preferably matched to the implant height. 
     Referring to  FIGS.  66  and  67   , the implant and implant dispenser are juxtaposed with the distal end of the inserter head. 
     In certain preferred embodiments, the inserter head is attached to a handle. The attachment may include a threaded attachment whereby a t-bar or handle is rotated to threadably engage the inserter head with the handle. 
     Referring to  FIG.  67   , in certain preferred embodiments, the implant is attached to the inserter head by first matching the superior and inferior labels on the implant dispenser with the inserter head. The four arms of the inserter head are then slid along the outer lateral sides of the implant protrusions. The inwardly extending projections on the arms are preferably engaged with the depressions formed in the outer lateral sides of the protrusions. The implant is preferably secured to the inserter head when the projections are seated in the depressions in the outer lateral sides of the protrusions. Once the implant has been secured to the inserter head, the implant dispenser may be decoupled from the implant. Once secured, the posterior part of the implant preferably extends beyond the ends of the arms of the inserter head. The implant dispenser may then be detached from engagement with the implant. 
     In certain preferred embodiments, the intervertebral disc implant is inserted into a prepared disc space. Referring to  FIGS.  68  and  69   , the inserter head is properly oriented with the disc space. In preferred embodiments, the inserter head includes at least one label or marking that is oriented relative to the superior or inferior vertebral bodies. Preferably, a superior label of the inserter head is oriented on top and an inferior label is oriented on the bottom. The reference pins are then utilized to guide the inserter head toward the disc space. The guide channels formed at the top and bottom of the inserter head preferably engage the reference pins. Referring to  FIG.  69   , as the intervertebral disc is advanced toward the disc space, the implant protrusions are preferably aligned with the protrusion channels previously formed in the endplates. In certain preferred embodiments, fluoroscopy is utilized to check the angle of insertion of the implant. The inserter head is preferably advanced toward the disc space until the four arms of the inserter head come into contact with the anterior surfaces of the vertebral bones. 
     Referring to  FIG.  70   , a T-handle  496  ( FIG.  31   ) may then be rotated for advancing a pusher rod  494  which pushes the implant off the distal end of the inserter head. This procedure is shown and described above with respect to the description of  FIGS.  33 A- 35 B .  FIG.  35 B  shows pusher rod  494  decoupling intervertebral disc  100  from the distal end of inserter head  474 . 
     In certain preferred embodiments, the T-handle may be rotated approximately three or four turns for advancing the intervertebral disc implant  100  into the disc space. The proximal end of the handle for the inserter head may be impacted to ensure that the intervertebral disc implant continues into the disc space as the four arms of the inserter head remain in contact with the vertebral bodies.  FIG.  71    shows further advancement of the intervertebral disc implant into the disc space. In certain preferred embodiments, immediately prior to insertion of the intervertebral disc implant, the disc space may be distracted approximately 2 mm wider than the base plates of the implant to facilitate insertion. The distraction may result from the grooves on the inserter head being angled relative to one another, as described above in certain preferred embodiments. 
     Referring to  FIG.  72   , insertion is completed when the implant is fully disengaged from the inserter head and the posterior faces of the anterior walls of the intervertebral disc&#39;s top and bottom elements are in contact with the anterior surfaces of the superior and inferior vertebral bodies. The anterior/posterior positioning of the implant and the baseplate size are preferably confirmed to be satisfactory using fluoroscopy. Referring to  FIG.  73   , if more posterior positioning of either the top element or the bottom element of the intervertebral disc implant is required, a tamp such as that shown and described above in  FIG.  36 A  may be utilized for adjusting the position of the implant. In preferred embodiments, the tamp may be impacted to adjust the anterior/posterior depth of the top and bottom elements of the implant. The anterior walls of the top and bottom elements serve as vertebral body stops to prevent the implant from being impacted too far posteriorly. After final insertion and adjustments have been completed, the posterior faces of the anterior walls should be flush with the anterior faces of the respective vertebral bodies. 
     Referring to  FIG.  72   , once all relevant tests have been performed to ensure that the intervertebral disc is properly positioned within the disc space, the reference pins may be removed. A biocompatible material, such as a small amount of bone wax, may be applied to the reference pin openings remaining in the anterior surfaces of the superior and inferior vertebral bodies. 
     Referring to  FIGS.  75 A and  75 B , an intraoperative lateral and anterior/posterior image of the implant is preferably obtained to observe its final position. If the implant is not properly positioned, it may be removed such as by using the extractor shown and described above in  FIGS.  37 A- 37 D . In  FIG.  74   , a hook of the extractor is engaged with an anterior wall of a top element of the intervertebral disc implant. Once it has been confirmed that the intervertebral disc is properly positioned within the disc space, a standard surgical closure procedure for anterior spinal surgery may be performed. Prior to discharge from the hospital, a lateral and anterior/posterior X-ray with the patient in the standing and/or sitting position is preferred. 
     Following surgery, in certain preferred embodiments, a goal of post-operative rehabilitation is to return the patient to normal activity as soon as possible without jeopardizing soft and hard tissue healing. Preferably, the patient should wear a soft collar for approximately 1-2 weeks to support healing of the incision. The patient&#39;s rehabilitation program may be modified under the direction of a surgeon to take into consideration the patient&#39;s age, stage of healing, general health, physical condition, life-style, and activity goals. Adherence to a recommended rehabilitation program is highly desirable. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.