Artificial intervertebral disc having a universal joint

An artificial intervertebral implant including a first baseplate having a top surface, a bottom surface, an aperture extending therethrough and a strap attached to the bottom surface of the first baseplate and underlying the aperture. The implant further includes a second baseplate juxtaposed with the first baseplate. The second baseplate includes a top surface with a cavity exposed therein. An articulating element is attached to a pair of opposing sidewalls of the cavity for retaining the strap within the cavity.

BACKGROUND OF THE INVENTION

The bones and connective tissue of an adult human spinal column consist of more than twenty discrete bones coupled sequentially to one another by a tri-joint complex, which consists of an anterior disc and two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. These more than twenty bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine up to the base of the skull, includes the first seven vertebrae. The intermediate twelve bones are the thoracic vertebrae, and connect to the lower spine comprising the five lumbar vertebrae. The base of the spine comprises the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic spine, which are in turn smaller than those of the lumbar region. The sacral region connects laterally to the pelvis.

The spinal column is highly complex in that it includes these more than twenty bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. In spite of these complications, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction.

Genetic or developmental irregularities, trauma, chronic stress, tumors, and degenerative wear are a few of the causes that can result in spinal pathologies for which surgical intervention may be necessary. A variety of systems have been disclosed in the art that achieve immobilization and/or fusion of adjacent bones by implanting artificial assemblies in or on the spinal column. The region of the back that needs to be immobilized, as well as the individual variations in anatomy, determine the appropriate surgical protocol and implantation assembly. With respect to the failure of the intervertebral disc, the interbody fusion cage has generated substantial interest because it can be implanted laparoscopically into the anterior of the spine, thus reducing operating room time, patient recovery time, and scarification. Referring now toFIGS. 6aand6b, in which a side perspective view of an intervertebral body cage and an anterior perspective view of a post implantation spinal column are shown, respectively, a more complete description of these devices of the prior art is herein provided. These cages101generally comprise tubular metal body102having an external surface threading103. They are inserted transverse to the axis of the spine104, into preformed cylindrical holes at the junction of adjacent vertebral bodies (inFIG. 6bthe pair of cages101are inserted between the fifth lumbar vertebra (L5) and the top of the sacrum (S1)). Two cages101are generally inserted side by side with the external surface threading103tapping into the lower surface of the vertebral bone above (L5), and the upper surface of the vertebral bone (S1) below. The cages101include holes105through which the adjacent bones are to grow. Additional materials, for example autogenous bone graft materials, may be inserted into the hollow interior106of the cage101to incite or accelerate the growth of the bone into the cage. End caps (not shown) are often utilized to hold the bone graft material within the cage101.

These cages of the prior art have enjoyed medical success in promoting fusion and grossly approximating proper disc height. It is, however, important to note that the fusion of the adjacent bones is an incomplete solution to the underlying pathology as it does not cure the ailment, but rather simply masks the pathology under a stabilizing bridge of bone. This bone fusion limits the overall flexibility of the spinal column and artificially constrains the normal motion of the patient. This constraint can cause collateral injury to the patient's spine as additional stresses of motion, normally borne by the now-fused joint, are transferred onto the nearby facet joints and intervertebral discs. It would therefore, be a considerable advance in the art to provide an implant assembly which does not promote fusion, but, rather, which mimics the biomechanical action of the natural disc cartilage, thereby permitting continued normal motion and stress distribution.

It is, therefore, an object of the invention to provide an intervertebral spacer that stabilizes the spine without promoting a bone fusion across the intervertebral space.

It is further an object of the present invention to provide an implant device that stabilizes the spine while still permitting normal motion.

It is further an object of the present invention to provide a device for implantation into the intervertebral space that does not promote the abnormal distribution of biomechanical stresses on the patient's spine.

It is further an object of the present invention to provide an artificial intervertebral disc that provides limited rotation of the baseplates transverse to the axis of the spine.

It is further an object of the present invention to provide an artificial disc that provides limited angular rotation of the baseplates relative to a centroid of motion centrally located within the intervertebral space.

It is further an object of the present invention to provide an artificial intervertebral disc that supports compression loads.

It is further an object of the present invention to provide an artificial intervertebral disc that permits the baseplates to axially float toward and away from each other.

It is further an object of the invention to provide an artificial intervertebral disc that supports tension loads.

It is further an object of the present invention to provide an artificial intervertebral disc that prevents lateral translation of the baseplates relative to one another.

It is further an object of the present invention to provide an artificial intervertebral disc that provides a centroid of motion centrally located within the intervertebral space.

It is further an object of the present invention to provide artificial intervertebral disc baseplates having outwardly facing surfaces that conform to the concave surface of adjacent vertebral bodies.

Other objects of the present invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter.

SUMMARY OF THE INVENTION

The proceeding objects are achieved by the present invention, which is an artificial intervertebral disc or intervertebral spacer device having a pair of support members (e.g., spaced-apart baseplates), each with an outwardly-facing surface. Because the artificial disc of the present invention is to be positioned between the facing endplates of adjacent vertebral bodies, the baseplates are arranged in a substantially parallel planer alignment (or slightly offset relative to one another in accordance with proper lordotic angulation) with the outwardly-facing surfaces facing away from one another. The baseplates are to mate with the vertebral bodies so as not to rotate relative thereto, but rather to permit the spinal segments to bend (in some embodiments, actually compress) relative to one another in manners that mimic the natural motion of the spinal segment. This natural motion is permitted by the performance of a ball-and-socket-type joint using a spherical member disposed between the secured baseplates, and the securing of the baseplates to the vertebral bone may be achieved through the use of a vertebral body contact element attached to the outwardly-facing surface of each baseplate.

Preferably, vertebral body contact elements include, but are not limited to, one or more of the following: a convex mesh, a convex solid dome and one or more spikes, as disclosed in U.S. patent application Ser. No. 10/256,160, the disclosure of which is hereby incorporated by reference herein.

The ball and socket joint of the present invention permit rotation between the two elements by capturing a strap integrally formed with one of the baseplates within a groove of the other baseplate. The strap, preferably, has an inner surface having a curvature which is substantially equal to the curvature of a ball also disposed between the two baseplates, thereby permitting rotation and angulation of the strap about a central point of the ball. This further permits angulational movement and rotational movement of one baseplate relative to the other baseplate.

The groove of the other baseplate, i.e., second baseplate, has a wider dimension than the strap so as to permit the strap to move freely about the central point of the ball at least with a desired angulation and rotation range. Additionally, the groove has a depth, which, in conjunction with the space between the first baseplate and the ball, limits the ability of the strap to come into contact with a bottom surface of the groove, even during axial movement of the two baseplates.

In one preferred embodiment, the ends of the groove are angled relative thereto so as to reduce wear and tear between the strap and groove as the strap angulates and rotates about the central point of the ball within the groove.

In one embodiment of the present invention, the artificial intervertebral disc includes a first baseplate having a top surface, a bottom surface and an aperture extending therebetween. The first baseplate further includes a strap having a top surface, a bottom surface, a first end and a second end. The ends of the strap are remote from one another and are attached to the bottom surface of the first baseplate such that a portion of the strap underlies the aperture.

The artificial intervertebral disc of the present invention also includes a second baseplate having a top surface, a bottom surface and a cavity exposed at the top surface of the second baseplate. The cavity preferably includes a groove having a first sidewall and a second sidewall, with the sidewalls being remote from each other. A spherical element having a central point is disposed within the aperture of the first baseplate and overlies the strap. The spherical element is preferably attached to the first sidewall and second sidewall of the second baseplate such that the strap is positioned and captured within the groove, thereby permitting the first baseplate and the second baseplate to move in an angulational direction and a rotational direction relative to one another with the strap translating about the central point of the spherical element.

The first sidewall and second sidewall may each include an indent such that the spherical element is attached to the first sidewall and second sidewall at respective indents. Additionally, the first sidewall and second sidewall may have a plurality of ends that are angled, such that during rotational movement of the first baseplate or second baseplate the strap has an increased range of motion within the groove.

The artificial intervertebral implant of the present invention may also include a cover having a bottom surface. The cover is preferably designed to be at least partially disposed within the aperture such that the cover overlays the spherical element, thereby capturing the spherical element between the cover and strap.

In certain embodiments of the present invention, the cover may include a cap and post with the top surface of the first baseplate further including a recess circumferentially extending about the aperture such that the post of the cover is compression fit within the aperture and the cap of the cover is compression fit within the recess.

The groove of the second baseplate may have a bottom surface and the spherical element may have an apex. Additionally, a distance between the bottom surface of the groove to the apex of the spherical element is preferably greater than a distance between the bottom surface of the cover to the bottom surface of the strap. More preferably, the distance between the top surface of the strap to the bottom surface of the cover is greater than a diameter of the spherical element, such that the combination of the two permits the first baseplate and the second baseplate to move in an axial direction relative to one another.

In one preferred embodiment of the present invention, the bottom surface of the cover and top surface of the strap have a radius of curvature substantially equal to a radius of curvature of the spherical element, such that the strap and the cover pivot about the central point of the spherical element as the first baseplate moves relative to the second baseplate in both an angulational direction and a rotational direction.

In one aspect of the present invention a distance between the top surface of the strap and the bottom surface of the cover is greater than a length of the articulating element, such that the first baseplate and the second baseplate may move in an axial direction relative to one another.

The bottom surface of the cover and the top surface of the strap may have a radius of curvature substantially equal to a radius of curvature of the articulating element, such that the strap and the cover may translate about the articulating element.

In another aspect of the present invention the aperture may be partially defined by the strap and not be included within the first baseplate.

In another aspect of the present invention the articulating element may be stationary relative to a first element, with the strap being captured between the articulating element and the first element.

DETAILED DESCRIPTION

The present invention will now be described with reference to the accompanying figures. The embodiments described herein are meant to be illustrative of the present invention and in no way should be thought of as limiting the present invention.

As shown inFIG. 1, an artificial intervertebral disc1, according to the present invention, preferably includes an upper baseplate10, a lower baseplate12, a ball14and a cover16. Upper baseplate10is provided with a top surface20and a bottom surface22. Disposed within the boundary of top surface20is a recess24. Recess24includes a circular skirt26positioned adjacent top surface20and defining the outer boundary of recess24. Recess24further includes a shoulder28defining a lower limit of the recess. An aperture30is disposed adjacent shoulder28and extends from the shoulder to bottom surface22of upper baseplate10.

As best shown inFIG. 2aperture30is defined by circumferential wall32which extends adjacent and between shoulder28and bottom surface22. Also as shown inFIG. 2, upper baseplate10includes a strap34. Strap34preferably includes a substantially semispherical inner surface36and a substantial semispherical outer surface38. Inner surface36and outer surface38are attached to one another through edges40and40′ extending between the two surfaces and defining remote sides of strap34. Inner surface36and outer surface38have ends remote from one another and preferably include a first chamfered end42and a second chamfered end44. Chamfered ends42,44extend from bottom surface22of upper baseplate10downward toward lower baseplate12and connect strap34to upper baseplate10. Strap34may be integral with upper baseplate10. As will be described below, aperture30as well as semispherical inner surface36of strap34preferably have a radius which is at least slightly larger than the radius of ball14.

As illustrated inFIGS. 1 and 2, lower baseplate12preferably includes a top surface50and a bottom surface52. Top surface50preferably includes a cavity54exposed near a central portion of lower baseplate12. Cavity54preferably includes a groove56and a pair of indents58,59disposed on opposite sidewalls60,61positioned about groove56. Groove56preferably has a generally semicircular shape—when viewing from the direction X—with opposite sidewalls60,61positioned adjacent to indents58,59, respectively, and extending in the Y direction. Groove56is preferably larger in size than strap34, so that when the artificial intervertebral disc1is assembled and the strap is disposed within the bounds of groove56, as will be described below, strap34does not touch the bottom or sidewalls60,61of groove56. Although groove56is shown as having a semicircular shape—viewed from the direction X—the shape of groove56is not essential to the present invention so long as it is large enough such that strap34does not touch the bottom of groove56when the artificial intervertebral disc1is assembled. For clarity of illustration, it is to be understood that, as described below, the sizing and shaping of strap34and groove56are such that when the ball14is secured to lower baseplate12, the strap34is freely movable about ball14in the space between sidewalls60,61of groove56. As previously alluded to, indents58,59are disposed on opposite sidewalls60,61respectively and are preferably semispherical in shape to complementarily support ball14, as will be described below.

Ball14is sized so as to be able to fit within aperture30and be supported by strap34. In a method of assembly, ball14is placed into aperture30through recess24of top surface20.

As best illustrated inFIGS. 1 and 2, cover16preferably includes a top surface66and a bottom surface68. Cover16further includes a circumferential edge70extending between top surface66and bottom surface68. Top surface66, bottom surface68and edge70define a cap portion72of cover16. Cover16further includes a cylindrical post74having a circumferential skirt76adjacent to and extending down from bottom surface68. Post74preferably further includes a concave bottom surface78, the concavity of which may extend into cap portion72of cover16. The radius of curvature of concave bottom surface78(best shown inFIG. 2) is preferably configured to approximate the curvature of ball14. In a preferred embodiment, cylindrical post74has a diameter that is slightly smaller than the diameter of aperture30extending through upper baseplate10.

In a method of assembly, ball14is placed within aperture30so as to be supported by strap34of upper baseplate10. Subsequently, cover16is placed within recess24of upper baseplate10with cylindrical post74preferably being compression-fit or locked within aperture30. Additionally, in a preferred embodiment cap portion72may also be compression fit to upper baseplate10by edge70of cover16being engaged with skirt26of the upper baseplate.

As best shown inFIGS. 4A and 4B, strap34preferably has a width extending from edge40to edge40′ that is smaller than the width of groove56defined by sidewalls60and61. This configuration allows strap34and upper baseplate10to rotate around a central point of ball14about an axis parallel to axis Z (FIG. 1) (angulational and rotational motion). Such a relative rotation in the transverse plane is limited to some extent by the limited space between sidewalls60and61of groove56and edges40and40′ of strap34.

In one preferred embodiment, as shown inFIG. 5, sidewalls60and61of groove56are angled at their respective ends80,81,82, and83relative to one another to accommodate desired rotation and angulation ranges and/or limit rotation to within a desired range of angles, without inviting excess wear or line contact endured by edges40and40′ of strap34against sidewalls60and61. That is, if sidewalls60and61were not angled, the edges40and40′ will dig into the sidewalls, causing undesirable wear characteristics over multiple articulations of the device; whereas if the sidewalls60and61are angled to align with the edges40and40′ of strap34during the maximum desired axial rotation range, edges40and40′ will hit flush against sidewalls60and61, minimizing wear debris and improving the wear characteristics of the device.

Rotation (or articulation) of upper baseplate10about an axis perpendicular to axis Z, (lateral bending articulation and flexion-extension articulations) relative to lower baseplate20can be limited by the distance between bottom surface22of upper baseplate10and top surface50of lower baseplate12. In other words, such articulation will be stopped when the two surfaces22and50come to meet each other. This distance can be determined by properly designing the size of ball14as well as the position (depth) of indents58and59on sidewalls60and61, respectively in lower baseplate20and the dimensions of groove56in the lower baseplate, which will be further described below.

Top surface20of upper baseplate10and bottom surface52of lower baseplate20are preferably designed to be convex in shape to match the concave shape of endplates of adjoining vertebral bones. Similarly, the top surface66of cover16preferably has a convex design and is a smooth extension of top surface20of upper baseplate10as best shown inFIGS. 1 and 3.

To assemble the artificial intervertebral disc1of the present invention, as previously mentioned, ball14is placed through recess24of upper baseplate10and into aperture30so as to be supported by strap34. With ball14resting on semispherical inner surface36of strap34, the strap is placed within groove56of lower baseplate12, with portions14A, and14B of ball14, contacting respective indents58and59as best illustrated inFIGS. 4A and 4B. Portions14A and14B of ball14are then fixed to respective indents58and59by, for example, welding or an adhesive, whereby the ball is fixed to lower baseplate12, and strap34is retained in groove56by ball14. This also prevents upper baseplate10from disengaging from lower baseplate12. Cover16is next disposed within recess24of upper baseplate10. Preferably, cover16is secured to upper baseplate10by a compression lock, threading, an adhesive or the like.

After the assembling is finished, artificial intervertebral disc1can be implanted between the adjoining endplates of vertebral bones. Strap34and therefore upper baseplate10, can articulate and rotate about a center of ball14in universal directions relative to lower baseplate12. The distance between upper baseplate10and lower baseplate12limits the articulation about an axis perpendicular to axis Z. Moreover, upper baseplate10can move toward and away from (along axis Z) lower baseplate12with such a translation being limited by the space between cover16and ball14as well as the distance between ball14and the bottom surface of groove56. Angulational and rotation (rotation about an axis perpendicular to the axis Z) are limited by the difference between the width of strap34and the width of groove56and, preferably, opposing walls60and61of groove56being angled relative to one another to accommodate desired motion ranges, and/or limit motion to within a desired range of angles, without inviting excess wear or line contact of the edges40and40′ against the sidewalls60and61.

Alternatively, although not shown in the drawings, edges40and40′ of strap34and/or sidewalls60and61of groove56are not necessarily flat, but can be curved (concave/convex) in shape, which may result in a smoother contact between the strap and the groove.