Articulating Spinal Implant

1. Field of the Invention

The invention relates generally to a spinal implant for use in intervertebral disc replacement; and more specifically relates to an articulating implant that fuses to adjacent vertebrae by bone ingrowth, thus restoring proper intervertebral spacing, and eliminating nerve root and/or spinal cord compression, while preserving spinal flexibility.

2. Description of Related Art

The spinal column is formed from a number of vertebrae, which in their normal state are separated from one another by cartilaginous intervertebral discs. These discs form a cushion between adjacent vertebrae, resisting compression along the support axis of the spinal column, but permitting limited movement between the vertebrae to provide the characteristic flexibility of the healthy spine. Injury, disease or other degenerative disorders may cause one or more intervertebral discs to shrink, collapse, deteriorate or become displaced, herniated or otherwise damaged. This can lead to compression of adjacent nerve root(s) or the spinal cord causing chronic and often disabling pain, and in advanced circumstances, irreversible paralysis of upper and/or lower limbs.

A number of devices and methods have been suggested for the replacement of damaged or dislocated intervertebral discs. One common approach is to permanently stabilize or "fuse" the adjacent vertebrae to maintain the proper intervertebral spacing and eliminate relative movement between the vertebrae. Various methods of vertebral stabilization have been developed. For example, autogenous grafts of dowel-shaped sections of bone have been implanted between the vertebrae to cause bone growth across the intervertebral space, thereby fusing the adjacent vertebrae into one bone mass. This procedure disadvantageously requires the harvest of donor bone for the graft from other parts of the patient's body, typically requiring a separate surgical procedure and resultant increases in complications and expense. An alternative source is cadaver bone, with potential complications of transmissible diseases, impaired graft incorporation, collapse or displacement. A further development to this method of vertebral stabilization involves the implantation of a perforated cylindrical bone basket between adjacent vertebrae. Bone fragments produced in preparing the vertebrae for the implantation are inserted into the bone basket to promote bone growth into, through and around the basket.

Vertebral stabilization by fusion of adjacent vertebrae has proven successful in permanently preserving intervertebral spacing, but has been found to present a number of disadvantages. Fusion of adjacent vertebrae necessarily eliminates a portion of the spine's normal range of motion, thereby reducing the subject's spinal flexibility. Additionally, fusion of the vertebrae increases the stresses imposed on adjacent mobile portions of the spinal column, often hastening the subsequent breakdown of normal joint surfaces above and below the fused vertebrae.

It has also been proposed to replace an injured intervertebral disc with a prosthesis which is "jointed" to permit relative movement between vertebrae. Previously-known devices of this type generally have been found to suffer from inadequate attachment between the prosthesis and the vertebrae. The intended movement between the components of previously-known jointed prostheses can cause relative motion between the prosthesis and adjacent bone surface(s). Because such motion would disrupt bone ingrowth, jointed prostheses have generally been considered incompatible with attachment by bone ingrowth. In addition, because the joint elements of these devices typically must occupy a substantial vertical extent in order to achieve the desired range of motion, and yet must fit within the intervertebral space, attachment of such devices generally has been by use of flat plates or surfaces provided on either side of the joint elements as points of fixation to the vertebrae. This attachment may be accomplished by compressive or friction fits, spiked projections, screws or pins, complemented in some instances with tissue ingrowth into porous surfaces. These mechanisms of attachment, however, may lack the degree and strength of fixation desired. Moreover, several such devices have used attachment flanges which extend beyond the surfaces of the vertebrae to which the device is attached. This has been found undesirable, as the extending flanges may interfere with or injure adjacent tissue. For example, it has been reported that flanges extending into immediately adjacent delicate esophageal area may interfere with swallowing and speech, or cause perforation and potentially fatal infection. An additional drawback to the use of screw and pin connections is the potential for such connectors to dislodge and cause injury.

Thus, it can be seen that a need yet exists for a spinal implant effective in permanently maintaining intervertebral spacing to prevent nerve or spinal cord compression while preserving as much of the natural range of motion between the affected vertebrae as possible. A need further exists for such a device which is capable of forming a permanent, strong attachment to the vertebrae and yet does not protrude beyond the external surfaces to which it is attached. Still another need exists for a method of replacing a damaged or displaced disc, which method maintains intervertebral spacing to prevent nerve and spinal cord compression, while preserving the natural relative motion between the vertebrae. It is to the provision of a device and method meeting these and other needs that the present invention is primarily directed.

Briefly described, in a preferred form, the present invention comprises a spinal implant including a first element having first connection means for engaging a first vertebra. The first connection means includes a first fusion chamber having at least one opening therein for facilitating bone ingrowth into the first fusion chamber to fuse the first element to the first vertebra. The implant also includes a second element having second connection means for engaging a second vertebra. The second connection means includes a second fusion chamber having at least one opening therein for facilitating bone ingrowth into the second fusion chamber to fuse the second element to the second vertebra. The implant also includes internal articulation means, coupling the first element to the second element, for allowing relative movement between the first element and the second element. The term "internal articulation means," is intended to mean a device that permits relative motion between components of the device, rather than between the device and an external structure such as adjacent bone or other tissue. The first and second elements are preferably hemicylindrical components, each having a radiused outer wall perforated with one or more openings provided therein to allow bone ingrowth into the fusion chambers, and having abutting joint surfaces forming the internal articulation means. In a further preferred embodiment, the joint surfaces are formed as engaging concave and convex surfaces to create a ball-and-socket type joint, which allows relative pivotal motion between the vertebrae, but resists compression therebetween. The hemicylindrical first and second elements join to form a generally cylindrical element which can be implanted using methods similar to those followed in the implantation of previously-known non-articulating vertebral fusion implants. Temporary stabilizing means can be provided for rigidly coupling the first and second elements by interposition of bioreabsorbable elements to permit implantation and enable bone ingrowth into the fusion chambers and fusion of the first and second elements to adjacent bone during an initial stabilization period, after which said temporary means biodegrades permitting articulation between the first and second elements.

The present invention can be further described as comprising a spinal implant having a first articulation surface, a second articulation surface engaging at least a portion of the first articulation surface along a support axis generally parallel to the spinal column, first connection means for connecting the first articulation surface to a first vertebra, and second connection means for connecting the second articulation surface to a second vertebra. The first and second articulation surfaces resist axial compression between the first and second elements in the direction of the support axis, but allow relative pivotal motion between the first and second elements. At least one, and preferably both, of the first connection means and the second connection means comprise a fusion chamber having at least one opening therein for facilitating bone ingrowth, thereby permanently and securely affixing the implant in place.

In another preferred form, the present invention comprises a method of maintaining an intervertebral space between a first vertebra and a second vertebra. The method includes the steps of removing a section of the first vertebra to form a cavity therein, mounting a first element of a spinal implant within the cavity formed in the first vertebra, and connecting a second element of the spinal implant to the second vertebra. The first element includes a first articulation surface and a first fusion chamber having at least one opening therein for facilitating bone ingrowth from the first vertebra. The second element includes a second articulation surface, wherein the first and second articulation surfaces adjoin to form a joint allowing relative pivotal movement between the first and second elements but preventing relative compression between the first and second elements. In a further preferred embodiment, the method may also include the removal of a section of the second vertebrae to form a cavity for receiving the second element. The second element may also include a second fusion chamber for facilitating bone ingrowth from the second vertebra. Bone fragments formed by the removal of vertebral sections to accommodate implantation or bone growth stimulating compounds or devices may be inserted into the fusion chamber(s) to enhance bone ingrowth.

These and other objects, features and advantages of the present invention will be more readily understood with reference to the following detailed description, read in conjunction with the accompanying drawing figures.

FIG. 1 shows a perspective view of a spinal implant according to one preferred form of the present invention.

FIG. 2 shows a cross-sectional side view of the spinal implant of FIG. 1.

FIG. 3 shows a cross-sectional end view of the spinal implant of FIG. 1.

FIG. 4a is an end view of a spinal implant in-situ according to the present invention.

FIG. 4b is a side view of a spinal implant in-situ according to the present invention.

FIG. 5 is a cross-sectional end view of another embodiment of the present invention.

Referring now in detail to the drawing figures wherein like reference numbers represent like parts throughout, preferred forms of the present invention will now be described. As seen best in FIGS. 1-3, the device of the present invention comprises a spinal implant 10, generally comprising a first element 20 and a second element 22. The first element 20 is coupled to the second element 22 by an internal articulation means 24 for allowing relative pivotal movement between the first and second elements.

FIGS. 4A and 4B show the spinal implant 10 of the present invention installed in situ, between a first vertebra 12 and a second vertebra 14. The implant 10 is shown implanted into an anterior aspect of the vertebral body, between the fifth cervical and sixth cervical vertebrae. The device and method of the present invention, however, are applicable to anterior and posterior approaches to the lumbar vertebrae; as well as to anterior implantation between cervical and thoracic vertebrae.

As seen best in FIGS. 2-3, the first element 20 of the implant 10 comprises first connection means, in the form of a first fusion chamber 30, for engaging the first vertebra 12. At least one opening 32 is formed in the first fusion chamber 30, to facilitate bone growth into, through and around the first fusion chamber 30 from the first vertebra 12, to fuse the first element to the first vertebra 12. Preferably, a plurality of openings 32 are provided to further promote bone ingrowth. Fusion by bone ingrowth has been found to provide a more permanent, rigid connection between the implant 10 and the skeletal structure than provided by other known connection means.

The second element 22 of the implant 10 comprises a second connection means for engaging the second vertebra 14. The second connection means preferably comprises a second fusion chamber 34, substantially similar in construction to the above-described first fusion chamber 30, and comprising at least one opening 36 formed therein for facilitating bone growth into, through and around the second fusion chamber 34 from the second vertebra 14. Alternatively, the second connection means can comprise an adhesive connection, screw connection, pin connection, or any other effective alternative connection means. However, the provision of a second fusion chamber 30 is preferred, as it provides a more permanent and secure coupling than other known attachment means. Although the first element 20 and first vertebra 12 are depicted in the figures as above the second element 22 and second vertebra 14, it will be understood that the present invention equally comprehends the reverse configuration wherein the first element, as described herein, is attached to a first vertebra lower on the spinal column than the second vertebra to which is attached the second element. The first and second elements can be fabricated from biocompatible materials including, without limitation, titanium, surgical alloys, stainless steel, chrome-molybdenum alloy, cobalt chromium alloy, zirconium oxide ceramic, non-absorbable polymers and other anticipated biocompatible metallic or polymeric materials.

Referring now to FIGS. 2-3, the articulation means 24 of the present invention preferably comprises a first articulation surface 40 provided on the first element 20, and an abutting second articulation surface 42 provided on the second element 22. The first and second articulation surfaces 40, 42 are preferably fabricated from or coated with low-friction, wear and impact-resistant, biocompatible materials, such as, for example, titanium, stainless steel, surgical alloys, chrome molybdenum alloys, cobalt chromium alloy, zirconium oxide ceramic, non-absorbable polymers and other anticipated bio-compatible metallic or polymeric materials. The articulation means 24 resists axial compression between the first and second elements 20, 22, but allows relative pivotal movement therebetween. Thus, when implanted, as shown in FIG. 4, the articulation means 24 resists axial compression between first and second vertebra 12, 14 along a support axis 44 extending generally along the spinal column, as shown in FIG. 4, but permits pivotal movement between vertebrae. The term "pivotal," is intended to comprehend either or both of a rotational or twisting motion about the support axis 44 (for example, rotation between cervical vertebrae by turning the head to the right or left), and/or a tilting motion angularly inclined in any direction relative to the support axis 44 (for example, nodding the head forward or backward and/or tilting the head downward to the right or left).

Axial compression, as well as lateral translation normal to the support axis 44, is resisted between the first and second vertebrae 12, 14, by providing the first internal articulation surface 40 with a void, such as a concave surface 46, which receives a protuberance, such as a convex surface 48, projecting from the second internal articulation surface 42. This "ball-and-socket" arrangement allows relative rotation about the support axis 44 between the first and second vertebrae 12, 14. The articulation means 24 can be provided with one or more unshown stops to limit the range of rotational movement allowed. The articulation means 24 preferably further comprises one or more angularly offset bevels 50 formed in the first internal articulation surface 40 and/or the second internal articulation surface 42, to allow relative tilting movement between the first and second vertebrae 12, 14 in one or more directions. According to the embodiment shown in the figures, the first and second internal articulation surfaces 40, 42 are each provided with an angularly offset bevel 50, in a generally pyramidal configuration, as seen best in FIG. 1, thereby enabling tilting movement in all directions (360.degree.). A generally conical configuration is also possible and, likewise, would permit both rotational movement and 360.degree. tilting movement. In addition to the pyramidal and conical configurations of the articulation surfaces 40, 42, other configurations are possible. For example, the first articulation surface 40 can comprise a generally U-shaped or V-shaped channel extending along the length of the implant 10, and the second articulation surface 42 can comprise a correspondingly-shaped lengthwise ridge. Such an arrangement will generally be less preferred, as it would permit only tilting motion in one plane. Such an arrangement may, however, be desirable for patients with conditions rendering a more full range of motion undesirable. The natural range of motion of the spine may be approximated by providing bevels 50 of approximately 5.degree. around the periphery of each of the first and second articulation surfaces 40, 42, thereby allowing approximately 10.degree. of tilt in all directions between adjacent vertebrae. As depicted in the figures, the pivot point or axis of the articulation means 24 is generally centrally located on the first and second articulation surfaces 40, 42, and will be aligned with the spine's normal axis of rotation when implanted as shown in FIG. 4. This location, however, can be selectively varied to position the center of rotation of the articulation means 24 centrally, anteriorly, posteriorly, to the left, to the right, or eccentrically (off-center in both the anterior/posterior direction and the left/right direction) of the spine's normal axis of rotation, in order to achieve proper alignment of the spine, thereby restoring optimal sagittal and coronal spinal balance and alignment.

As shown best by FIG. 1, the first and second elements 20, 22 of the implant 10 of the present invention preferably comprise generally hemicylindrical elements adjoining to form a generally cylindrical body. When in their assembled configuration, the first element 20 and the second element 22 abut one another with their respective first and second articulating surfaces 40, 42 adjacent and engaging one another, as described above. The first element 20 preferably further comprises a first radiused outer wall 60. The one or more openings 32 for facilitating bone ingrowth are provided in this first radiused outer wall 60, and communicate with a first fusion chamber 62 formed between the first radiused outer wall 60 and the first articulating surface 40. Similarly, the second element 22 preferably comprises a second radiused outer wall 70, defining one or more openings 36 for facilitating bone ingrowth. The openings 36 communicate with a second fusion chamber 72 formed between the second radiused outer wall 70 and the second articulating surface 34. The first and second radiused outer walls 60, 70 can be provided with threads 80 to facilitate advancing the implant 10 into the intervertebral space during implantation, and to help secure the implant 10 in position once implanted. The threads 80 on each of the first and second radiused outer walls 60, 70 are preferably aligned to form continuous threads when the first and second elements 20, 22 are engaged. In some instances, it may be desirable to provide self-tapping threads 80, and/or to configure the threads 80 to direct bone fragments generated by implantation into the openings 32, as taught by U.S. Pat. No. 5,489,308 to Kuslich, et al. In other less preferred embodiments, the threads 80 can be replaced with a contoured outer surface comprising smooth, splined, flanged, spiked or beaded surface features. The implant 10 can further comprise one or more support flanges 85 in the first and/or second elements 20, 22, for additional strength.

In a further preferred embodiment, the present invention may further comprise the provision of one or more stabilizing means for temporarily rigidly coupling the first element 20 to the second element 22 to prevent relative movement therebetween. For example, it is preferred that the first and second elements 20, 22 be held rigidly in place during installation of the implant 10 into the intervertebral space. In addition, the first and second elements 20, 22 should remain rigidly coupled for a sufficient length of time after implantation to permit sufficient bone ingrowth into the fusion chambers to prevent relative motion between the implant 10 and the vertebrae 12, 14 during normal activities of the patient. Typically, approximately eight to twenty weeks of bone ingrowth will be sufficient, before the first and second elements 20, 22 are uncoupled and permitted to articulate relative to one another. This temporary stabilization of the first and second elements is accomplished by the present invention, without the requirement of a second surgical procedure, through the use of medium-term stabilizing means formed from bioreabsorbable material. Examples of bioreabsorbable materials include polyglycolate polymers or analogues, lactides, polydioxanone, polyglyconate, lactide/glycolide copolymers. By appropriate selection of the material(s) of construction, the length of time required to biodegrade the stabilizing means can be effectively controlled. After the stabilizing means are dissolved and absorbed by the body, the first and second elements of the implant 10 are uncoupled, allowing articulation. In one example embodiment, depicted in FIG. 3, the stabilizing means can comprise one or more biodegradable shims 82 wedged between the first element 20 and the second element 22, to prevent relative motion therebetween. In another example embodiment, the first and second elements 20, 22 are placed in their coupled configuration, as shown in FIGS. 2-3, with the spaces between the articulating surfaces 40, 42 then being injected or filled with a biodegradable polymer, which provides the medium term temporary stabilizing means to couple the elements in position. Care will be taken to avoid filling the fusion chambers 30, 34 and the openings 32, 36 to the fusion chambers with the polymer, which could inhibit bone ingrowth. The threads 82 will also remain exposed to assist in implantation of the device. In another example embodiment, depicted in FIG. 2, the stabilizing means comprise a removable and/or bioreabsorbable endcap 90, which releasably engages the trailing end 92 of the implant 10 to couple the first and second elements 20, 22. The endcap can comprise one or more clips 94 for engaging the implant 10, and one or more keyways 96 for engaging a wrench, driver or other actuation device used to advance the implant 10 into the intervertebral space. A second removable and/or absorbable endcap can be installed on the leading end 93 of the implant 10. The endcaps may additionally function to retain the bone fragments within the chambers 62, 72 of the fusion chambers 30, 34, as will be described in greater detail below, until sufficient bone ingrowth and fusion has occurred.

Another embodiment of the present invention is depicted by FIG. 5, which shows an implant 10, generally comprising first and second elements 20, 22, substantially as described above, and further comprising an intermediate articulation element, which in the depicted embodiment comprises a generally hemispherical bowl-shaped cap 90 interposed between the concave surface 46 of the first element 20 and the convex surface 48 of the second element 22. The cap 90 is preferably fabricated from a wear-resistant, low friction material of construction, which may be somewhat resilient to absorb impact loading. The provision of the cap 90 allows articulation at the interface between the top surface of the cap and the concave surface 46 of the first element 20, and between the bottom surface of the cap and the convex surface 48 of the second element 22, resulting in a bipolar articulation. It is believed that this arrangement may reduce frictional wear on the articulation surfaces by distributing wear over two articulation interfaces. The engagement of the concave lower surface of the cap 90 with the convex surface 48 of the second element 22, and the engagement of the convex upper surface of the cap 90 with the concave surface 46 of the first element 20, prevents the cap 90 from becoming dislodged when the implant 10 is installed.

The present invention further comprises a method of maintaining a desired intervertebral space between a first vertebra 12 and a second vertebra 14. The method preferably comprises removing a section of the first vertebra to form a cavity therein, mounting a first element of an articulating spinal implant into the cavity, and connecting a second element of the implant to the second vertebra. The articulating spinal implant used to carry out this method is preferably substantially similar to the example embodiment described above, having first and second internal articulation surfaces which adjoin to form a joint allowing relative pivotal movement between the first and second elements, but preventing relative axial compression therebetween. At least one of the elements preferably further comprises a fusion chamber, including a chamber and at least one opening communicating with the chamber, for facilitating bone ingrowth into, through and around the fusion chamber from the respective vertebra. In a preferred form, the method of the present invention comprises attachment of both the first and second elements of the implant to the respective vertebra using fusion chambers for bone ingrowth. In this embodiment, a section of the second vertebra is preferably removed to form a cavity therein for receiving the second element of the implant.

The method of the present invention can further comprise the insertion of bone fragments, which may be the fragments formed in the above-described removal of vertebral sections, into the fusion chamber(s) of the implant. The presence of these bone fragments has been shown to stimulate and enhance bone ingrowth into the fusion chamber. In another embodiment, the method of the present invention anticipates the application of means for stimulating bone growth into, through, and/or around the fusion chamber(s). The means for stimulating bone growth can comprise, for example, electrical or magnetic bone growth stimulators, bone morphogenetic protein (BMP), hydroxyl apatite compounds, demineralized bone compound, and/or other bone growth stimulating devices or compounds. It is also anticipated that the introduction of a cyanoacrylate material, commonly referred to as surgical glue, onto the outer surface of the fusion chamber(s) may be used to fix the implant in optimal position awaiting bone ingrowth.

Implantation of the articulating spinal implant according to the present invention may be carried out in a manner generally similar to previously-known methods for the implantation of non-articulating, rigid-fusion implants, with exceptions as set out herein. For example, the method of the present invention may utilize standard surgical approaches to the spine from either an anterior approach for the cervical and thoracic spinal segments, or from an anterior or posterior approach to the lumbar spinal segments. The removal of vertebral sections may be accomplished by drilling or chipping a generally circular bore having a diameter and depth adapted to generally match the implant to be utilized, bridging the first and second vertebrae. The bore is preferably centered over the intervertebral space, approximately midway between the first and second vertebrae. Threads may then be tapped into the bore to facilitate insertion of the implant. Alternatively, the implant can be provided with self-tapping threads. The implant is assembled with the first and second articulating surfaces engaged, and the first and second elements of the implant are rigidly coupled by stabilizing means substantially as described above. The stabilized implant is advanced into the bore to the desired depth. Preferably, the implant is advanced until the trailing end 92 is flush or slightly recessed relative to the outer surface of the vertebrae, thereby eliminating the possibility that outwardly projecting portions of the implant may interfere with or injure adjacent or regional vascular, neural or visceral structures.

In the lumbar area, where greater load bearing is required, two articulating implants can be placed in parallel positions with the lengthwise axis of each adjacent one to the other, in a generally parallel arrangement, allowing flexion/extension movement alone. This arrangement will typically require removal of vertebral material to form two openings, or an extended opening to accommodate both implants. Alternatively, a single, appropriately sized implant could be placed in the interspace utilizing the transabdominal open surgical approach, the transabdominal laparoscopic approach, or the retroperitoneal open anterior approach allowing pivotal motion around the single point of articulation as described above for use in cervical and thoracic intervertebral disc spaces.

Post-operatively, bone ingrowth will occur, fusing the first and/or second elements of the implant to the respective vertebra. After sufficient passage of time, the stabilizing means is dissolved and absorbed into the body, uncoupling the articulation means to permit relative motion between the first and second elements, thus preserving spinal flexibility and minimizing destructive stress upon adjacent intervertebral joints. The implant resists axial compression between the first and second elements, and thereby prevents relative compression between the first and second vertebrae, thus maintaining the desired intervertebral space. Further, the implant is recessed into the recipient vertebral site, eliminating impingement on next adjacent vascular, neural or visceral structures.

While the invention has been disclosed in its preferred forms, it will be apparent to those skilled in the art that many modifications, additions, and deletions can be made therein without departing from the spirit and scope of the invention and its equivalents as set forth in the following claims.