Laterally insertable artificial vertebral disk replacement implant with crossbar spacer

[0001] This application claims priority under 35 USC 119 to U.S. Patent Applications No. 60/517,791, filed Nov. 5, 2003 and entitled “ARTIFICIAL VERTEBRAL DISK REPLACEMENT IMPLANT WITH CROSSBAR SPACER AND LATERAL IMPLANT METHOD,” and U.S. No. 60/517,973, filed Nov. 6, 2003 and entitled “ARTIFICIAL VERTEBRAL DISK REPLACEMENT IMPLANT WITH CROSSBAR SPACER AND LATERAL IMPLANT METHOD,” both of which are hereby incorporated by reference.

[0002] This application is related to U.S. Provisional Application No. 60/422,039, filed Oct. 29, 2002, entitled “ARTIFICIAL VERTEBRAL DISK REPLACEMENT IMPLANT WITH TRANSLATING PIVOT POINT AND METHOD,” U.S. patent application Ser. No. 10/684,668, filed Oct. 14, 2003, entitled “ARTIFICIAL VERTEBRAL DISK REPLACEMENT IMPLANT WITH CROSSBAR SPACER AND METHOD,” U.S. patent application Ser. No. 10/684,669, filed Oct. 14, 2003, entitled “ARTIFICIAL VERTEBRAL DISK REPLACEMENT IMPLANT WITH TRANSLATING PIVOT POINT AND METHOD,” U.S. Provisional Application No. 60/422,011, filed Oct. 29, 2002, entitled “TOOLS FOR IMPLANTING AN ARTIFICIAL VERTEBRAL DISK AND METHOD,” U.S. patent application Ser. No. 10/685,134, filed Oct. 14, 2003, entitled “TOOLS FOR IMPLANTING AN ARTIFICIAL VERTEBRAL DISK AND METHOD,” U.S. Provisional Application No. 60/422,022, filed Oct. 29, 2002, entitled “ARTIFICIAL VERTEBRAL DISK REPLACEMENT IMPLANT WITH A SPACER AND METHOD,” U.S. Provisional Application No. 60/422,021, filed Oct. 29, 2002, entitled “ARTIFICIAL VERTEBRAL DISK REPLACEMENT IMPLANT WITH CROSSBAR SPACER AND METHOD,” U.S. patent application Ser. No. 10/685,011, filed Oct. 14, 2003, entitled “ARTIFICIAL VERTEBRAL DISK REPLACEMENT IMPLANT WITH SPACER AND METHOD,” U.S. patent application Ser. No. 10/981,863, filed Nov. 5, 2004 entitled “LATERALLY INSERTABLE ARTIFICIAL VERTEBRAL DISK REPLACEMENT IMPLANT WITH TRANSLATING PIVOT POINT,” U.S. patent application Ser. No. 10/981,807, filed Nov. 5, 2004, entitled “METHOD OF LATERALLY INSERTING AN ARTIFICIAL VERTEBRAL DISK REPLACEMENT IMPLANT WITH TRANSLATING PIVOT POINT,” U.S. patent application Ser. No. 10/981,952, filed Nov. 5, 2004, entitled “METHOD OF LATERALLY INSERTING ARTIFICIAL VERTEBRAL DISK REPLACEMENT IMPLANT WITH A CROSSBAR SPACER,” U.S. patent application Ser. No. 10/981,923, filed Nov. 5, 2004, entitled “LATERALLY INSERTABLE ARTIFICIAL VERTEBRAL DISK REPLACEMENT IMPLANT WITH CURVED SPACER,” U.S. patent application Ser. No. 10/981,945, filed Nov. 5, 2004, entitled “METHOD OF LATERALLY INSERTING AN ARTIFICIAL VERTEBRAL DISK REPLACEMENT IMPLANT WITH CURVED SPACER,” all of which are incorporated herein by reference.

[0003] The field of art of this disclosure is directed to an artificial vertebral disk replacement and method.

[0004] The spinal column is a biomechanical structure composed primarily of ligaments, muscles, vertebrae and intervertebral disks. The biomechanical functions of the spine include: (1) support of the body, which involves the transfer of the weight and the bending movements of the head, trunk and arms to the pelvis and legs, (2) complex physiological motion between these parts, and (3) protection of the spinal cord and nerve roots.

[0005] As the present society ages, it is anticipated that there will be an increase in adverse spinal conditions which are characteristic of aging. For example, with aging comes an increase in spinal stenosis (including, but not limited to, central canal and lateral stenosis), and facet joint degeneration. Spinal stenosis typically results from the thickening of the bones that make up the spinal column and is characterized by a reduction in the available space for the passage of blood vessels and nerves. Facet joint degeneration results from the constant load borne by the facet joints, and the eventual wear that results. Pain associated with both conditions can be relieved by medication and/or surgery.

[0006] In addition to spinal stenosis and facet joint degeneration, the incidence of damage to the intervertebral disks is also common. The primary purpose of the intervertebral disk is to act as a shock absorber. The disk is constructed of an inner gel-like structure, the nucleus pulposus (the nucleus), and an outer rigid structure comprised of collagen fibers, the annulus fibrosus (the annulus). At birth, the disk is 80% water which then gradually diminishes with time, thereby becoming stiff. With age, disks may degenerate, and bulge, thin, herniate, or ossify. Additionally, damage to disks may occur as a result disease, trauma or injury to the spine.

[0007] The damage to disks may call for a range of restorative procedures. If the damage is not extensive, repair may be indicated, whereas extensive damage may indicate full replacement. Regarding the evolution of restoration of damage to intervertebral disks, rigid fixation procedures resulting in fusion are still the most commonly performed surgical intervention. However, trends suggest a move away from such procedures. Currently, areas evolving to address the shortcomings of fusion for remediation of disk damage include technologies and procedures that preserve or repair the annulus, that replace or repair the nucleus, and that advance implants for total disk replacement. The trend away from fusion is driven both by issues concerning the quality of life for those suffering from damaged intervertebral disks, as well as responsible health care management. These issues drive the desire for procedures that are minimally invasive, can be tolerated by patients of all ages, especially seniors, and can be performed preferably on an out-patient basis.

[0008] Most recently, there has been an increased interest in total disk replacement technology. A number of artificial disks are beginning to appear in the medical device marketplace. These artificial disks vary greatly in shape, design and functionality. With these devices are available tools and methods for insertion of the devices between the vertebrae.

[0009] Accordingly, there is a need in the art for innovation in technologies and methods that advance the art in the area of minimally invasive intervertebral disk replacement. This not only enhances the quality of life for those suffering from the condition, but is responsive to the current needs of health care management.

[0025] The following description is presented to enable any person skilled in the art to make and use the implant of the present invention. Various modifications to the embodiments described will be readily apparent to those skilled in the art, and the principles defined herein can be applied to other embodiments and applications without departing from the spirit and scope of what is disclosed and defined by the appended claims. Thus, what is disclosed is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. To the extent necessary to achieve a complete understanding of what is disclosed herein, the specification and drawings of all patents and patent applications cited in this application are incorporated herein by reference.

[0026] FIG. 1A shows a front view of an embodiment of the implant 100. The designations, “A” for anterior, “P” for posterior, “RL” for right lateral, and “LL” for left lateral are given in the drawings for spatial directional orientation. These designations give the relationship of all faces or ends of the implant with respect to the superior perspective; i.e. looking down the axis of the spine. As shown in the Figures, the implant 100 has an anterior side A, a posterior side P, and two lateral sides, LL and RL which extend between the anterior side A and the posterior side P. The anterior side A faces the anterior direction when the implant 100 is inserted into the spine. The posterior side P faces the posterior direction when the implant 100 is inserted into the spine. The right lateral side RL faces the lateral direction away from the right side of the spine, whereas the left lateral side LL faces the lateral direction away from the left side of the spine when the implant 100 is inserted therein. In one embodiment, as shown in FIG. 1C, the implant 100 has a perimeter shape which is configured to correspond to the perimeter shape of the vertebral disks, whereby the anterior side A is curved and the posterior side P is parallel to the sagittal plane. In another embodiment, the implant 100 has a perimeter shape which does not contour to the shape of the vertebral disks. As will be appreciated by those of ordinary skill in the art, the perimeter shape of the upper end plate 110 and the lower end plate 120 can be the same or different from one another.

[0027] The implant 100 preferably includes an upper end plate 110 that is configured to mate with an upper vertebral body. The implant 100 preferably includes a lower end plate 120 that is configured to mate with a lower vertebral body. The implant 100 also includes a spacer 130 positioned between the upper end plate 110 and the lower end plate 120. The spacer 130 separates the upper end plate 110 from the lower end plate 120 and also facilitates pivotal and/or rotational as well as twisting movement of the upper end plate 110 and the lower end plate 120 relative to each other and the spacer 130. The spacer 130 is preferably in the form of a crossbar as discussed in more detail below.

[0028] The upper end plate 110 has a first outer surface 112 which comes into contact and mates with the underside of the upper vertebral body. The implant 100 includes a first keel 114, as shown in FIGS. 1A-1C, which preferably extends away from the first outer surface 112 in a direction substantially perpendicular to the first outer surface 112. In one embodiment, when the implant 100 is inserted between the vertebral bodies, the first keel 114 preferably extends into a keel receiving channel which is cut into the underside of the upper vertebral body to anchor the upper end plate 110 thereto. The first keel 114 extends longitudinally across the first outer surface 112 between the left lateral side LL and the right lateral side RL, as shown in FIG. 1C. The first keel 114 is thus oriented to be substantially perpendicular to the sagittal plane of the spine, which is known to one skilled in the art as the plane which traverses from the posterior toward the anterior of the patient or patient's spine. As shown in FIG. 1C, it is preferred that the right lateral side RL and left lateral side LL of the implant 100 are parallel to the sagittal plane. In a preferred embodiment, the first keel 114 is oriented to be substantially parallel to the coronal plane of the spine which is known in the art as the plane that is parallel to the patient's shoulders. In another embodiment, the first keel 114 extends longitudinally only partially across the first outer surface 112. It should be noted that although one keel 114 is shown laterally across the first outer surface 112 in FIG. 1C, more than one keel 114 is alternatively disposed on the first outer surface 112, as shown in FIG. 4. It is contemplated that the plurality of keels 114, as shown in FIG. 4, are parallel to each other and are perpendicular to the sagittal plane of the spine, although not necessarily. Although the implant of the present invention is shown and described herein as having one or more keels laterally oriented between the right and left lateral sides, the implant described herein can alternatively have keels which are oriented between the anterior and posterior sides as described in U.S. patent application Ser. No. 10/684,668 which is incorporated by reference.

[0029] It is preferred that the first keel 114 of the upper end plate 110 includes a plurality of teeth 115. In one embodiment, the teeth 115 of the keel 114 are angled and point towards the left lateral face of the implant 100, as shown in the example in FIG. 1A. The left angled teeth 115 allow the upper end plate 110, as shown in FIG. 1A, to be easily inserted between the vertebral bodies from a left lateral approach. This is so, because the angled configuration of the teeth 115 provide little frictional resistance against the underside of the vertebral body when inserted into the spine. The angled configuration of the teeth 115 also prevents the upper end plate 110 from becoming dislodged or unintentionally slipping out of the spine after being inserted therein. In another embodiment, the teeth 115 of the keel 114 are angled and point towards the right lateral face RL of the implant 100, opposite of that shown in the embodiment in FIG. 1A. This configuration allows the upper end plate 110 to be easily inserted into the spine from a right lateral side approach.

[0030] The lower end plate 120 of the present implant 100 includes a second outer surface 122, as shown in FIGS. 1A and 1B. The second outer surface 122 preferably comes into contact with and mates with the top-side of the lower vertebral body. The implant 100 includes a second keel 124, as shown in FIGS. 1A and 1B, which preferably extends away from the second outer surface 122 in a direction substantially perpendicular to the second outer surface 122. In one embodiment, when the implant 100 is inserted between the vertebral bodies, the second keel 124 preferably extends into a keel receiving channel in the top-side of the lower vertebral body to anchor the lower end plate 120 thereto. The second keel 124 extends longitudinally across the second outer surface 122 between the left lateral side LL and the right lateral side RL, as shown in FIG. 3. In another embodiment, the second keel 124 extends longitudinally only partially across the second outer surface 122. The second keel 124 is thus oriented to be substantially perpendicular to the sagittal plane of the spine. In a preferred embodiment, the second keel 124 is oriented to be substantially parallel to the coronal plane of the spine. It should be noted that although one keel 124 is shown laterally across the second outer surface 122 in FIG. 3, more than one keel 124 is alternatively disposed on the second outer surface 122, as shown in FIG. 4. It is contemplated that the plurality of keels 124, as shown in FIG. 4, are parallel to each other and are perpendicular to the sagittal plane of the spine.

[0031] It is preferred that the second keel 124 of the lower end plate 120 includes a plurality of teeth 125. In one embodiment, the teeth 125 of the keel 124 are angled and face towards the left lateral face of the implant 100, as shown in the example in FIG. 1A. The left angled teeth 125 allow the lower end plate 120, as shown in FIG. 1A, to be easily inserted between the vertebral bodies from a left lateral approach. This is so, because the angled configuration of the teeth 125 provide little frictional resistance against the top-side of the vertebral body when inserted into the spine. The angled configuration of the teeth 125 also prevents the lower end plate 120 from becoming dislodged or unintentionally slipping out of the spine after being inserted therein. In another embodiment, the teeth 125 of the keel 124 are angled and point towards the right lateral face RL of the implant 100, opposite of that shown in the embodiment in FIG. 1A. This configuration allows the lower end plate 120 to be easily inserted into the spine from a right lateral side approach.

[0032] In the embodiment shown in FIG. 1A, the first and second keels 114,124 preferably include ports 148, 152 therethrough. The ports 148, 152 facilitate bone in-growth, wherein bone from the vertebral bodies can grow thorough the ports 148,152 to aid in securing the first and second keels 114, 124, and thus the implant 100 to the spine. In one embodiment the outer surfaces of the first and second keels 114,124 and the first and second outer surfaces 112, 122 of the implant 100 are roughened in order to promote bone in-growths into the defined roughened surfaces of the implant 100. In one embodiment, the first and second keels 114,124, and the first and second outer surfaces 112, 122 of implant 100 are coated with one or more materials that promote bone growth. Such materials include, but are not limited to, bone morphogenic protein (BMP) and hyaluronic acid. Other substances which promote bone growth relative to and into the keel, keel ports, and other external surfaces of the implant 100 are contemplated.

[0033] The first and second keels 114, 124 preferably extend between the vertebral bodies to anchor the implant 100 to the spine. The lateral orientation of the first keel 114 and the second keel 124 allow the implant 100 to be inserted into the spine using a lateral approach as opposed to an anterior or posterior approach, which is advantageous, because the spinal nerves in the spinal cavity are minimally undisturbed when the implant 100 is inserted laterally. In comparison to a posterior insertion approach in which the spinal nerves can be substantially disturbed, the spinal nerves are bypassed and relatively undisturbed when the implant 100 is inserted laterally between the vertebral bodies from the side of the spine. Although an anterior insertion approach has its benefits, the lateral insertion approach can allow the present implant 100, and associated implantation tools, to be inserted into the spine with less disturbance of the patient's internal organs. This can translate into less time and risk associated with preparing the spine for insertion as well as inserting the implant itself into the spine. Further, the laterally oriented first and second keels 114, 124 offer substantial stability to the vertebral bodies during extension, flexion and lateral bending of the spine.

[0034] As shown in FIGS. 1D and 1E, the upper end plate 110 includes a first inner surface 116. It should be noted that the upper end plate 110 is shown oriented upside down in FIGS. 1D and 1E, whereby the inner surface 116 is shown facing upwards. The first inner surface 116 receives and engages the spacer 130 of the implant and opposes an inner surface 126 (FIG. 1B) of the second end plate 120. In one embodiment, the first inner surface 116 is designed to form a planar surface that is parallel with the first outer surface 112. In another embodiment, the first inner surface 116 is designed to form a planar surface that is non-parallel with the first outer surface 112. In particular, as shown in FIGS. 1B and 1E, the anterior side A of the end plate 110 has a larger thickness (i.e. distance between the first outer side 110 and first inner side 116) than the thickness of the posterior side P.

[0035] As shown in FIGS. 1D and 1E, the first inner surface 116 of the upper end plate 110 preferably includes a channel, also referred to as a socket, 150 therein. As will be discussed below, the spacer includes a spacer beam, wherein at least a portion of the spacer beam can be seated in the channel 150 to allow the first and/or second end plates of the assembled implant 100 to pivot or rotate relative to each other. As shown in FIGS. 1D and 1E, the channel 150 is preferably concave and extends lengthwise between the left lateral side LL and the right lateral side RL of the upper end plate 110. Alternatively, the channel 150 extends lengthwise between the anterior side A and the posterior side P of the upper end plate 110. In one embodiment, as shown in FIG. 1E, the upper end plate 110 includes a ridge 117 formed in the first inner surface 116, whereby the ridge 117 is surrounded by the first inner surface 116, which is raised relative thereto. In one embodiment, the channel 150 receives a portion of the spacer beam. In another embodiment, the spacer beam fits within the channel 150.

[0036] As shown in FIGS. 1F and 1G, the lower end plate 120 includes a second inner surface 126. The second inner surface 126 receives and engages the spacer 130 of the implant and opposes the inner surface 126 (FIG. 1E) of the first end plate 110. In one embodiment, the second inner surface 126 is designed to form a planar surface that is parallel to the second outer surface 122. In another embodiment, the second inner surface 126 is designed to form a planar surface that is non-parallel to the first outer surface 122. In particular, as shown in FIGS. 1G and 1E, the anterior side A of the end plate 120 has a smaller thickness (i.e. distance between the second outer side 120 and second inner side 126) than the thickness of the posterior side P.

[0037] As shown in FIGS. 1G and 1H, the second inner surface 126 of the lower end plate 120 preferably includes a channel, also referred to as a socket, 160 therein. As will be discussed below, the spacer 130 includes a spacer beam which can be placed into the channel 160 in order to allow the first and/or second end plates of the assembled implant to pivot or rotate relative to each other. As shown in FIGS. 1G and 1H, the channel 160 is preferably concave and extends lengthwise between the anterior side A and the posterior side P of the lower end plate 120. Alternatively, the channel 160 extends lengthwise between the left lateral side LL and the right lateral side RL of the lower end plate 120. In one embodiment, as shown in FIG. 1G, the lower end plate 120 includes a ridge 127 formed in the second inner surface 126, whereby the ridge 127 is surrounded by the second inner surface 126, which is raised relative thereto. In one embodiment, the channel 160 receives a portion of the spacer beam. In another embodiment, the spacer beam fits within the channel 160.

[0038] In one embodiment, the first outer surface 112 of the first end plate 110 is substantially parallel to the second outer surface 122 of the second end plate 120 when the implant 100 is assembled and is in a neutral position (i.e., the position where the first end plate 110 has not rotated relative to the second end plate 120). Alternatively, the first outer surface 112 of the first end plate 110 is non-parallel to the planar surface of the second outer surface 122 of the second end plate 120 when the implant 100 is assembled and in the neutral position. The non-parallel orientation of the first end plate 110 and the second end plate 120 allows the plates to pivot a greater degree with respect to each other. Additionally, other factors such as the height of the spacer 130 and the position of the keel receiving channels can be adjusted in order to increase the degree by which the first end plate 110 and the second end plate 120 can pivot relative to each other and the spacer 130.

[0039] FIG. 1H illustrates a cross-section of the implant 100 taken along the lines H-H of FIG. 1A. As shown in FIG. 1H, the first inner surface 116 of the first plate 110 substantially opposes the second inner surface 126 of the second plate 120. In addition, FIG. 1H illustrates the first channel 150 located in the first inner surface 116 which extends laterally between the left lateral side LL and the right lateral side RL. Additionally, the second channel 160 is located in the second inner surface 126 and extends between the anterior side A and the posterior side P, as shown in FIG. 1H.

[0040] In one embodiment, the planar surfaces which correspond to the first and second outer surfaces 112, 122 of the implant lie parallel or substantially parallel to the axial plane of the body when the implant 100 is inserted between adjacent vertebrae. In one embodiment, the planar surfaces which correspond to the first and second inner surfaces 116, 126 of the first and second end plates 110, 120 lie parallel, or substantially parallel, to the axial plane of the body when the implant is implanted. In one embodiment, the outer surfaces 112, 122 and the inner surfaces 116, 126 both lie parallel or substantially parallel to the axial plane of the body when the implant is implanted. In each of the embodiments, either or both keels 114, 124 are perpendicular to the sagittal plane. Preferably, the keels 114, 124 are also parallel to the coronal plane of the body.

[0041] FIG. 2A illustrates a top view of the preferred crossbar spacer 130 of the present invention. The crossbar spacer 130 preferably has a first spacer beam 210 and a second spacer beam 220 as shown in FIG. 2A, whereby the first spacer beam 210 is oriented perpendicular to the second spacer beam 220. The first spacer beam 210 has a first end 212 and a second opposed end 214 as well as a midpoint 216 therebetween. The second spacer beam 220 has a first end 222 and a second opposed end 224 as well as a midpoint 226 therebetween. As shown in the figures, the spacer beams 210, 220 are preferably circular in cross section, although other appropriate shapes are contemplated.

[0042] In the embodiment shown in FIG. 2A, the midpoint 216 of the first spacer beam 210 is not aligned with the midpoint 226 of the second spacer beam 220. Instead, the midpoint 216 of the first spacer beam is proximal to the first end 222 and distal to the second end 224 of the second spacer beam 220. In this configuration, the first and second beams 210, 220 substantially form a cross or “T” shape.

[0043] in another embodiment, the first beam 210 can be positioned transversely along the length of the second beam 220 such that the midpoint 216 of the first beam 210 and the midpoint 226 of the second beam 220 are aligned with one another. Where both beams 210, 220 are positioned at the respective midpoints 216, 226, the crossbar spacer 130 substantially forms a plus sign, “+”.

[0044] FIG. 2B shows a side view of the crossbar spacer 130 of the implant 100. As is apparent from FIG. 2B, the first beam 210 is oriented such that the center 213 of the first beam 210 is off-set from the center 223 (FIG. 2C) of the second beam 220. Thus, the first beam 210 is located along a plane which is parallel and adjacent to a plane along which the second beam 220 is oriented. Similarly, as shown in FIG. 2C, the second beam 220 is oriented such that the center 223 of the first beam 220 is off-set from the center 213 of the first beam 210. The off-set positioning of the first beam 210 and the second beam 220 allows the spacer 130 to be positioned between, and in contact with, the upper end plate 110 and the lower end plate at the same time. Additionally, the off-set positioning of the beams 210, 220 allow the upper and lower end plates 110, 120 to pivot about the beams of the spacer 130 to accommodate flexion, extension, twisting and/or lateral bending of the spine.

[0045] In constructing the crossbar 130, the first beam 210 can be formed integrally with the second beam 220 to be unitary in construction. Alternatively, the first beam 210 can be adhered to the second beam 220 using any other suitable method (e.g. spot welding). It should be noted that the first beam 210 and the second beam 220, as well as the spacer 130 as a whole, are sufficiently constructed to withstand the load forces applied by the end plates 110, 120 in the neutral position as well as during flexion, extension and/or lateral bending movements. The spacer 130 can be formed by extrusion, injection, compression molding, machining or any other appropriate techniques.

[0046] Viewing FIGS. 1A, 1B and 3, an assembled embodiment of the implant 100 is depicted. The upper end plate 110 is configured to mate with the first vertebra and the lower end plate 120 is configured to mate with a second vertebra. A crossbar 130 that sits between the first end plate 110 and the second end plate 120 is also provided. As is evident from the figures, the upper beam 210 of the crossbar 130 is placed in the channel 150 of the upper end plate 110 such that the upper beam 210 is approximately parallel with the keels 114, 124. As can be seen from FIGS. 1H and 4, the “T”-shaped spacer 130 sits between the upper and lower end plates 110, 120, wherein the first beam 210 is preferably received in the channel 150 (FIG. 1H) and the second beam 220 is preferably received in the channel 160 (FIG. 1H). As shown in FIG. 4, the first beam 210 is positioned proximal to the posterior side P rather than the anterior side A of the implant 100, whereby the first beam 210 is oriented parallel to the keels 114, 124 and perpendicular to the sagittal plane. In another embodiment, the upper beam 210 can be positioned midway between the posterior and the anterior faces of the implant 100 in the embodiment that the crossbar spacer 130 has a “+” shape. As shown in FIG. 4, the lower beam 220 is placed in the channel 160 of the lower end plate 120 such that the lower beam 220 is approximately perpendicular to the keels 114, 124 and parallel with the sagittal plane.

[0047] As stated above, the crossbar spacer facilitates pivotal or rotational movement of the first end plate 110 and the second end plate 120, relative to each other. In particular, the interface between the first beam 210 of the spacer 130 and the channel 150 allows the upper end plate 110 and/or lower end plate 120 to pivot or rotate about the first beam 210 when the patient moves backwards (extension) and forwards (flexion). Additionally, the interface between the second beam 210 of the spacer 130 and the channel 160 allows the lower end plate 120 and/or upper end plate 110 to pivot or rotate about the second beam 210 when the patient bends side to side (laterally). The patient's weight as well as gravity hold the spacer 130 securely seated within the channels 150, 160. As stated above, the implant 100 includes channels 150, 160 in one embodiment, although not necessarily.

[0048] In one embodiment, there is a loose fit between the spacer 130 and the upper and lower end plates 110,120. In particular, there is a loose fit between the upper beam 210 and the upper channel 150 and also between the lower beam 220 and the lower channel 160. This loose fit allows for a twisting motion of the upper and/or lower end plates, and thus vertebral bodies, about a vertical axis through the center of the spine.

[0049] FIG. 4 shows a side view of an implant 100 inserted between two vertebral bodies 410, 420. The implant 100 is shown inserted between the two vertebrae 410, 420 with two first keels 114 extending from the first end plate 110, and two second keels 124 extending from the second end plate 120. The first and second keels 114,124 are approximately perpendicular to the sagittal plane of the spine, and preferably straddle the point of articulation of the spacer 130. A gap is present between the first end plate 110 and the second end plate 120 at the anterior A face of implant 100 is preferably greater than at the posterior end P face of implant 100. The larger gap at the anterior face vs. the posterior face of implant 100 allows forward (flexion) movement to be facilitated to a greater degree than backward (extension) movement. Thus, in one example of a forward bending movement of up to 10° can be achieved while a backward bending movement of 5° can be achieved. Other angles are contemplated within the scope of the present invention.

[0050] In one embodiment, the implant can be made of medical grade titanium, stainless steel or cobalt chrome. The material has appropriate physical and mechanical properties and is suitable for carrying and spreading the physical load between the spinous process. Other materials that have appropriate structural strength and that are suitable for implantation into a patient can also be used. One class of materials contemplated for use in implant 100 is the class of biocompatible polymers. Copolymers, blends and composites of polymers are also contemplated for fabrication of parts of the disclosed device. A copolymer is a polymer derived from more than one species of monomer. A polymer composite is a heterogeneous combination of two or more materials, wherein the constituents are not miscible, and therefore exhibit an interface between one another. A polymer blend is a macroscopically homogeneous mixture of two or more different species of polymer.

[0051] One group of biocompatible polymers are the polyarylesterketones which has several members, which include polyetheretherketone (PEEK), and polyetherketoneketone (PEKK). PEEK has proven as a durable material for implants, as well as meeting criteria of biocompatibility. Medical grade PEEK is available from Victrex Corporation under the product name PEEK-OPTIMA. Medical grade PEKK is available from Oxford Performance Materials under the name OXPEKK, and also from CoorsTek under the name BioPEKK. Other materials that can be used include polyetherketone (PEK), polyetherketoneether-ketoneketone (PEKEKK), and polyetheretherketoneketone (PEEKK), and, generally, a polyaryletheretherketone. Further, other polyketones can be used as well as other thermoplastics.

[0052] Reference to appropriate polymers that can be used in the spacer can be made to the following documents, all of which are incorporated herein by reference. These documents include: PCT Publication WO 02/02158 A1, dated Jan. 10, 2002, entitled “Bio-Compatible Polymeric Materials;” PCT Publication WO 02/00275 A1, dated Jan. 3, 2002, entitled “Bio-Compatible Polymeric Materials;” and, PCT Publication WO 02/00270 A1, dated Jan. 3, 2002, entitled “Bio-Compatible Polymeric Materials.”

[0053] Still another interesting group of biocompatible polymers are polyalkyl biocompatible polymers, such as polyethylenes, polypropylenes, and the like. These medical grade biocompatible polymers are also available as reinforced polymer materials. To reinforce a polymeric material, fillers, are added to a polymer, copolymer, polymer blend, or polymer composite. Fillers are added to modify properties, such as mechanical, optical, and thermal properties. In this case, fillers, such as carbon fibers, are added to reinforce the polymers mechanically to enhance strength for certain uses, such as load bearing devices.

[0054] For example, other grades of PEEK are also available and contemplated, such as 30% glass-filled or 30% carbon-filled, provided such materials are cleared for use in implantable devices by the FDA, or other regulatory body. Glass-filled PEEK reduces the expansion rate and increases the flexural modulus of PEEK relative to that which is unfilled. The resulting product is known to be ideal for improved strength, stiffness, or stability. Carbon-filled PEEK is known to enhance the compressive strength and stiffness of PEEK and lower its expansion rate. Carbon-filled PEEK offers wear resistance and load carrying capability.

[0055] Alternatively, the spacer 130 can be made out of a polymer, and more specifically, the polymer is a thermoplastic with the other components made of the materials specified above. Still more specifically, the material is PEEK 450G, which is an unfilled PEEK approved for medical implantation available from Victrex of Lancashire, Great Britain. (Victrex is located at www.matweb.com or see Boedeker www.boedeker.com). Other sources of this material include Gharda located in Panoli, India (www.ghardapolymers.com). Further in this embodiment, the PEEK has the following additional approximate properties:

[0056]

	Property	Value
	Density	1.3	g/cc
	Rockwell M	99
	Rockwell R	126
	Tensile Strength	97	Mpa
	Modulus of Elasticity	3.5	Gpa
	Flexural Modulus	4.1	Gpa

[0057] FIG. 5 is a block diagram showing the basic steps of the method of laterally inserting the implant 100. First the spine is exposed through a lateral access 610. The intervertebral disk is then removed laterally 620, if necessary. Following, the implant 100 is inserted laterally 630 between the adjacent vertebral bodies. Finally, the wound is closed 640. Additionally, the method includes preparing the spine for the implant by cutting channels into the vertebral bodies to accept the keels of the plates. In one embodiment, the method includes assembling the implant by inserting the crossbar spacer between the upper and lower end plates prior to installation. In one embodiment, the upper and lower end plates can be attached individually to the vertebral bodies and then assembled with the spacer to form the entire implant assembly thereafter.

[0058] What has been disclosed herein has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit what is disclosed to the precise forms described. Many modifications and variations will be apparent to the practitioner skilled in the art. What is disclosed was chosen and described in order to best explain the principles and practical application of the embodiments described herein, thereby enabling others skilled in the art to understand the various embodiments and various modifications that are suited to the particular use contemplated. It is intended that the scope of what is disclosed be defined by the following claims and their equivalence.