US20090030521A1 - Elastomeric intervertebral disc prosthesis - Google Patents

Abstract

An intervertebral disc prosthesis includes upper and lower rigid endplates and an elastomeric core structure located between the endplates and attached thereto, wherein the elastomeric core structure has a centroid positioned posterior to a centroid of at least one of the endplates.

Description

CROSS REFERENCE TO RELATED APPLICATIONS
• This application is a division of U.S. application Ser. No. 11/334,423 filed Jan. 19, 2006. This application also claims the benefit of U.S. Provisional Application No. 60/644,527, filed Jan. 19, 2005, the entire disclosure of which is incorporated herein by reference, and the benefit of U.S. Provisional Application No. 60/693,430, filed Jun. 24, 2005, the entire disclosure of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• This invention relates to intervertebral disc prostheses and more particularly to intervertebral disc prostheses having rigid endplates and an elastomeric core.
• 2. Background Art
• Low back pain is a very common pathological condition, affecting approximately 80% of the general population at some time. Although most of patients experience the painful symptoms only occasionally and recover fully, approximately 10% of these patients come to experience chronic and disabling low back pain in spite of various medical treatments.
• The most common cause of chronic disabling low back pain is degeneration of one or more of the intervertebral discs that are positioned between the vertebrae of the spine and permit the various natural movements of the spinal column. Such degenerative disc disease (DDD) may become intractable to non-surgical treatment and have to be treated by surgical intervention. Spinal fusion has been a traditional and generally effective treatment method for chronic disabling low back pain that is not responding to non-operative treatments. More recently, alternative treatments involving replacement of the entire disc or its nucleus have been developed for treatment of discogenic pain.
• The first generation of prostheses for replacement of degenerated intervertebral discs has generally incorporated mutually sliding surfaces of relatively hard materials to provide for the required intervertebral motion in flexion, extension, lateral bending and torsion. Although such prostheses have been found to be helpful, improvements in shock absorption and replication of the natural motion of the intact intervertebral disc have been sought.
• Accordingly, subsequently developed prostheses have incorporated elastomeric members in order to provide for the required motion and shock absorption. Such prostheses typically include relatively hard endplates for contacting the endplates of adjacent vertebrae and fixing the prosthesis thereto, together with an elastomeric disc core, positioned between the hard endplates and fastened thereto.
• However, in conventional designs of such intervertebral disc prostheses, the bone-contacting members, i.e., rigid endplates, typically have a and shape and size in a horizontal plane that conforms generally to the shape and size of the vertebral endplate; and the elastomeric element, positioned between the prosthesis endplates, also typically has a similar and shape and size. When such a prosthesis is subjected to stresses induced by bending of the spinal column, e.g., flexion, the elastomeric material at the periphery of the prosthesis may be compressed between the hard endplates and caused to bulge outwardly. Such deformation of the elastomeric component in repeated flexion may lead to eventual failure of the prosthesis. In some known prostheses, the outer periphery of the elastomeric core is provided with a concavity of the lateral wall to reduce the fixation stress in the peripheral region where the elastomer interfaces with the rigid, e.g., metal, endplates. However, even such a structure may be subject to eventual failure.
• The present invention has been devised in view of the above background.
SUMMARY OF THE INVENTION
• According to the invention, an intervertebral prosthesis is provided having generally rigid endplates for fixation to the upper and lower vertebrae of a spinal motion segment and an elastomeric core fastened between the endplates wherein at least an antero-posterior dimension of the interface between the core and at least one of the endplates is less than the antero-posterior dimension of the endplate. The lateral dimension of the interface between the core and at least one of the endplates may also be made smaller than the lateral dimension of the endplate.
• Accordingly, it is an object of the invention to provide an intervertebral disc prosthesis having rigid endplates and an elastomeric core.
• A further object is to provide such an intervertebral disc prosthesis wherein stress between the elastomeric core and the rigid endplates is reduced.
• A further object is to provide an intervertebral disc prosthesis which is less prone to failure in use.
• A further object is to provide an intervertebral disc prosthesis wherein the resistance to motions in flexion-extension, lateral bending, and torsion may be readily controlled.
• According to one of its principal aspects, the present invention provides an intervertebral disc prosthesis for implanting between adjacent vertebrae in a spinal motion segment. The prosthesis comprises an upper rigid prosthesis endplate for fixation to an adjacent upper vertebra, and having a periphery, an antero-posterior dimension, and a transverse dimension; a lower rigid prosthesis endplate for fixation to an adjacent lower vertebra, and having a periphery, an antero-posterior dimension, and a transverse dimension; and an elastomeric core structure located between the prosthesis endplates and attached to the endplates. The elastomeric core structure includes at least one core member and has a total cross-sectional area in a horizontal plane and durometer hardness sufficient to provide sufficient compressive strength to support physiological axial loads.
• According to one preferred feature, the elastomeric core structure has at least an average antero-posterior dimension, sufficiently less than at least one of the upper prosthesis endplate antero-posterior dimension and the lower prosthesis endplate antero-posterior dimension, such that the elastomer core does not protrude beyond the periphery of one of the prosthesis endplates during normal flexion and extension of the spinal motion segment.
• According to another preferred feature, the core member has an antero-posterior dimension not greater than three times an axial height dimension of the core member.
• According to yet another preferred feature, the core member has at least an average antero-posterior dimension not greater than three times an axial height dimension of said core member.
• According to another preferred feature, the core member has a minimum antero-posterior dimension in a horizontal plane located axially between the endplates, the minimum antero-posterior dimension being not greater than three times an axial height dimension of the said core member.
• According to still another of its principal aspects, the present invention provides an intervertebral disc prosthesis for implanting between adjacent vertebrae in a spinal motion segment, comprising an upper rigid prosthesis endplate for fixation to an adjacent upper vertebra, and having a periphery, an antero-posterior dimension, and a transverse dimension; a lower rigid prosthesis endplate for fixation to an adjacent lower vertebra, and having a periphery, an antero-posterior dimension, and a transverse dimension; and an elastomeric core structure located between the prosthesis endplates and attached to the endplates, the elastomeric core structure including a first elastomeric core member and a second elastomeric core member disposed outside an outer periphery of the first elastomeric core member, with the first elastomeric core member having durometer hardness greater than said second elastomeric core member.
• Further objects, aspects, and advantages of the invention will be apparent from the description of the invention which follows.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a schematic lateral view of an intervertebral disc of the prior art installed between adjacent vertebral bodies showing a median sagittal plane cross-sectional view of the prosthesis.
• FIG. 2 is a schematic lateral view of another type of intervertebral disc of the prior art installed between adjacent vertebral bodies showing a median sagittal plane cross-sectional view of the prosthesis.
• FIG. 3 is a sagittal sectional view of an embodiment of the invention.
• FIG. 4 is a horizontal section taken through a central plane within the elastomeric core of an embodiment of the invention.
• FIG. 5 is a horizontal section taken through a central plane within the elastomeric core of another embodiment of the invention, showing an elastomeric core having a generally elliptical cross-section.
• FIG. 6A is a horizontal section taken through a central plane within the elastomeric core of another embodiment of the invention, showing an elastomeric core having a generally peanut shell-shaped cross-section positioned generally centrally in an antero-posterior dimension of the prosthesis.
• FIG. 6B is a horizontal section taken through a central plane within the elastomeric core of another embodiment of the invention, showing an elastomeric core having a generally peanut shell-shaped cross-section such as illustrated inFIG. 6A, positioned somewhat more posteriorly in an antero-posterior dimension of the prosthesis.
• FIG. 7 is a horizontal section taken through a central plane within the elastomeric core of another embodiment of the invention, showing an elastomeric core having a generally circular cross-section.
• FIG. 8 is a horizontal section taken through a central plane within the elastomeric core of another embodiment of the invention, showing an elastomeric core comprising two elastomeric elements positioned laterally symmetrically with respect to a median sagittal plane.
• FIG. 9 is a horizontal section taken through a central plane within the elastomeric core of another embodiment of the invention, showing a central elastomeric element having a generally elliptical cross-section and having a relatively hard durometer surrounded by a peripheral elastomeric element having a somewhat softer durometer.
• FIG. 10 is a horizontal section taken through a central plane within the elastomeric core of another embodiment of the invention, showing a central elastomeric element having a generally peanut shell-shaped cross-section and having a relatively hard durometer surrounded by a peripheral elastomeric element having a somewhat softer durometer.
• FIG. 11 is a horizontal section taken through a central plane within the elastomeric core of another embodiment of the invention, showing an elastomeric core comprising two elastomeric elements positioned laterally symmetrically with respect to a median sagittal plane and having a relatively hard durometer surrounded by a peripheral elastomeric element having a somewhat softer durometer.
• FIG. 12 is a median sagittal plane cross-sectional view of a prosthesis of the invention implanted between two vertebrae of a spinal motion segment.
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS
• The present invention provides an intervertebral prosthesis that utilizes flexible elastomeric materials confined between hard, typically metallic, endplate components in order to secure mechanical properties that can adequately substitute for the properties of a natural intervertebral disc. The intervertebral prosthesis of the invention is can be configured with adequate degrees of freedom to control the motion of a spinal motion segment in flexion-extension, lateral bending, and torsion when implanted to replace a damaged or degenerated spinal disc in a spinal column of a human patient. The invention will be explained and discussed in connection with the accompanying drawings.
• FIGS. 1 and 2 are partial sagittal sectional views of prior art illustrative examples wherein, as depicted inFIG. 1 the singleelastomeric core100 extends to the periphery of thehard endplates101 and102, and inFIG. 2 a multiple-durometerelastomeric core200 is utilized, with the harder or reinforcedelastomer201 placed at the periphery of thesofter elastomer202. In both cases, the elastomer at the peripheral regions is highly compressed when the spinal motion segment is moved in flexion-extension and in lateral bending. Such prostheses, when exercised by such repeated compression, have been shown to exhibit failure due to both bulging and to impingement of thehard endplates101 and102 onto the bulging elastomer. Since the elastomeric core and hard endplate peripheries coincide, the centroids of area of the respective components in horizontal planes will coincide with aline103.
• FIG. 3 illustrates a typical configuration of the elements of a preferred embodiment of the invention. The preferred embodiment is illustrated as a sagittal plane (midline vertical plane) section with the elastomericintervertebral disc prosthesis300 implanted between adjacentvertebral bodies301 and302. Thedisc prosthesis300 comprises a first or upperrigid plate303, a second or lowerrigid plate304, and flexibleelastomeric core305 interposed between, and fixedly assembled to, the two rigid plates. For the disc prosthesis, the upper and lowerrigid plates303 and304 are generally similar to each other, and thecore305 is symmetrically placed about the midline vertical plane.Rigid plates303 and304 are provided for fixation of the prosthesis to the vertebral bone and are made of biocompatible material, preferably a metallic material such as Ti6Al4V. Conventional metal fabrication methods may be used to fabricate the rigid plates. Theelastomeric core305 is preferably made of polyurethane and is fixedly connected to the rigid endplates by mechanical or adhesive means.Width309 is the maximum sagittal width ofendplates303 and304.Width307 is the minimum width ofelastomer305 in the sagittal plane.Axial thickness308 is the thickness of theelastomeric core305 at the anterior limit L ofwidth307.Plane306 is the horizontal plane for the section view ofFIG. 4. As clearly illustrated, according to the invention,dimension307 is significantly reduced relative todimension309, thereby providing for the endplateanterior margins310 to converge in deep flexion without severe compression of the elastomeric core.
• FIG. 4 is a cross sectional view in the horizontal plane ofdisc prosthesis300 at the level of plane306 (containing the limit position L) ofFIG. 3. Theperiphery401 ofendplate303 is configured to be smaller but closely match theperiphery402 of thevertebral body endplate302 since a large portion of the natural disc annulus is maintained during the surgical implantation of the disc prosthesis.Width307 is the minimum width ofelastomer305 in the sagittal plane.Width403 is the minimum width ofelastomer305 in the coronal plane. Lateral flexion stiffness of the normal lumbar disc is about double that of the anterior flexion, and it is desirable to maximize torsional stiffness. Thus,width403 is advantageously equal or greater than 1.4times width307.Peripheral shape404 ofelastomer305 is different from bothperipheries401 and402.Shape404 is a typical elastomeric shape for cervical disc applications where there are low torsional requirements for the proper functioning of the disc.Peripheral shape405 is shown for purposes of comparison to shape404 and depicts the elastomer shape of an alternate embodiment where higher torsional stiffness is desired, such as for the lumbar region.
• InFIGS. 3 and 4,line311 indicates a coronal plane containing the centroid C_cof the cross-sectional area ofcore305 inplane306.Line312 indicates a coronal plane containing the centroids C_p(which are coincident in this example) of projected areas of the core contacting surfaces ofendplates303 and304 ontoplane306. Such coronal planes shall be referred to hereinafter as centroidal planes. The posterior placement of centroid C_crelative to projected centroids C_pallows the disc flexion axis to be closer to the normal anatomical center of rotation.
• FIGS. 5,6,7,8 present sectional views similar to that ofFIG. 4, providing illustrative examples of alternate embodiments. In these embodiments, the design parameters ofFIG. 3 andFIG. 4 with respect to the endplates and general structure of the prosthesis are held constant while the alternate embodiments relate to the shape ofelastomeric core305.
• FIG. 5 illustrateselastomeric core305 having anelliptical shape501 and positioned so as to have coinciding endplate and elastomercentroidal planes312 and502. The elliptical shape illustrated will provide low flexion stiffness.
• FIG. 6A illustrateselastomeric core305 havingpeanut shell shape601 and positioned so as to have coinciding endplate and elastomercentroidal planes312 and602.
• FIG. 6B illustrateselastomeric core305 havingpeanut shell shape601 and positioned so as to have elastomercentroidal plane602 posterior to endplatecentroidal plane312. Flexion stiffness is similar toFIG. 6A, but with higher lateral and torsional stiffness.
• FIG. 7 illustrateselastomeric core305 having acircular shape701 and positioned so as to have elastomercentroidal plane702 positioned posterior to the endplatecentroidal plane312. In this embodiment, the prosthesis will exhibit low torsion and equal anterior and lateral flexion stiffness.
• FIG. 8 illustrateselastomeric core305 having a two individual columns ofcircular shape801 and802 positioned so as to have elastomercentroidal plane803 positioned posterior to the endplatecentroidal plane312. In this embodiment the prosthesis will exhibit high torsion and moderate lateral flexion stiffness.
• FIGS. 9 to 11 refer to an alternate embodiment of the present invention wherein regions of the core element having different durometers are utilized to achieve desirable performance. Advantageously, lower durometer elastomer is used at the periphery of the elastomeric core where the largest deflections are experienced. For a higher durometer polymer, these larger deflections will result in higher stresses leading to a higher likelihood of debonding from the endplates. The higher durometer regions are introduced at regions of minimal deflection, normally relatively close to the anatomical center of motion. Higher durometer elastomers in such a central position provide for increased axial stiffness, and the peripheral lower durometer elastomers provide additional flexural stiffness allowing for the necessary deflections during normal disc range of motion.
• FIGS. 9,10,11 show a view similar to that ofFIG. 4 with illustrative examples of alternate embodiments. In these embodiments, the design parameters ofFIG. 3 andFIG. 4 with respect to the endplates and general structure of the prosthesis are held constant, and the alternate embodiments relate to the shape and position of higher durometer regions within theelastomeric core305 envelope.
• FIG. 9 illustrateselastomeric core901 having an elliptical shapedhigher durometer region902 and positioned internally of alower durometer region903 so as to have endplatecentroidal plane312 anterior to elastomercentroidal plane904.Elastomer902 provides additional axial and torsional stiffness with nominal contribution to additional flexion stiffness.
• FIG. 10 illustrateselastomeric core905 having a peanut-shell shapedhigher durometer region906 and positioned internally of alower durometer region907 so as to have endplatecentroidal plane312 andelastomer centroidal plane908 somewhat posterior to the endplate centroidal plane.Elastomer906 provides additional axial and lateral stiffness with respect to the example ofFIG. 9 and with minimal contribution to additional flexion stiffness.
• FIG. 11 illustrateselastomeric core909 having a two individual columns constructed of higher durometer elastomer and having acircular shape910 and911 and positioned internally to thelower durometer region912 having a combinedelastomer centroidal plane913 positioned posterior to the endplatecentroidal plane312. In an alternate embodiment, theregions910 and911 are constructed from lower durometer elastomer andregion912 from a higher durometer elastomer.
• FIG. 12 shows a variation of the preferred embodiment ofFIG. 4. It is illustrated in a sagittal plane section with the elastomericintervertebral disc prosthesis920 implanted between adjacentvertebral bodies921 and922. Thedisc prosthesis920 comprises a first or upperrigid plate923, a second or lowerrigid plate924, intermediateelastomeric plates925 and926 fixedly assembled to therigid plates923 and924, and a flexibleelastomeric core927 interposed between and fixedly assembled to the twointermediate plates925 and926. The intermediate plates provide for stress reduction transition between the very low modulus flexible elastomeric core and the extremely stiff hard endplates. Additionally, the transition plates provide for higher mechanical fixation strength to the typically metallic hard endplates as well as a stronger elastomer-to-elastomer adhesive bond to the elastomeric core side. Theperipheral wall928 ofelastomeric core927 advantageously forms a concavity so as to provide for additional fixation area tointermediate plates925 and926.
EXAMPLE
• This example illustrates the determination of a preferred ratio of anterior-posterior dimension to core height.
• A series of experiments was conducted on a polycarbonate polyurethane disc of constant durometer (80 A). The ratio of the anterior-posterior width of the core to its height was varied, and the behavior of the disc as it was made to flex repeatedly to 10 degrees was examined. The results are shown below in Table 1.

• 	TABLE 1
  	Ratio of anterior-posterior dimension to core height

  	1:1	2:1	3:1	4:1

  Effect of	Buckled	Neutral	Slight	Bulging and
  10° flexion	inwards		bulging	impingement
  				of polymer
  				on endplate

  
  Test conditions:
• • Elastomer hardness: 80 A durometer
  • Disc height: 5 mm for all samples
  • Anterior-posterior dimension varied to produce varied ratio of core height to anterior-posterior (AP) dimension
• The results suggest that a ratio of 3:1 (anterior-posterior dimension to core height) or less is required to ensure that impingement of the core on the endplates does not occur. Thus, on the basis of this data, a ratio of 2:1 would appear ideal to eliminate bulging and the danger of polymer impingement. However, the mechanical properties of elastomers, coupled with the desire to match the flexural stiffness of a natural disc, dictates maximizing the shape area of the device. Since the intervertebral height is a design envelope limiting factor in practice, for a givenheight308,width307 has a proportionally maximum value of threetimes height308. For ratios of higher than three, impingement and bulging become detrimental to device integrity.
• The invention having been described above in terms of certain embodiments, it will be apparent to those skilled in that that many changes and alterations can be made without departing from the spirit and principles of the invention.

Claims (10)

1-12. (canceled)

13. An intervertebral disc prosthesis for implanting between adjacent vertebrae in a spinal motion segment, comprising:

an upper rigid prosthesis endplate for fixation to an adjacent upper vertebra, and having a periphery, an anteroposterior dimension, and a transverse dimension,

a lower rigid prosthesis endplate for fixation to an adjacent lower vertebra, and having a periphery, an anteroposterior dimension, and a transverse dimension, and

an elastomeric core structure located between said prosthesis endplates and attached to said endplates,

said elastomeric core structure including at least one elastomeric core member and having a total cross-sectional area in a horizontal plane and durometer hardness effective to provide sufficient compressive strength to support physiological axial loads,

wherein said elastomeric core structure has a centroid positioned posterior to a centroid of at least one of said upper prosthesis endplate and said lower prosthesis endplate.

14. The intervertebral disc prosthesis ofclaim 13, wherein said core structure has an antero-posterior dimension not greater than three times an axial height dimension of said core structure.

15. The intervertebral disc prosthesis ofclaim 13, wherein said core member has an antero-posterior dimension not greater than three times an axial height dimension of said core structure.

16. The intervertebral disc prosthesis ofclaim 13, wherein said elastomeric core member has a generally peanut shell-shaped cross section.

17. The intervertebral disc prosthesis ofclaim 13, wherein said elastomeric core structure has at least an average antero-posterior dimension sufficiently less than at least one of said upper prosthesis endplate antero-posterior dimension and said lower prosthesis endplate antero-posterior dimension, such that said elastomeric core structure does not protrude beyond the periphery of said at least one of said prosthesis endplates during normal flexion and extension of said spinal motion segment.

18. The intervertebral disc prosthesis ofclaim 13, wherein said elastomeric core structure includes a first elastomeric core member and a second elastomeric core member disposed outside an outer periphery of said first elastomeric core member, said first elastomeric core member having a durometer hardness greater than that of said second elastomeric core member.

19. The intervertebral disc prosthesis ofclaim 18, wherein said core structure has an antero-posterior dimension not greater than three times an axial height dimension of said core structure.

20. The intervertebral disc prosthesis ofclaim 18, wherein said core structure has a minimum antero-posterior dimension in a horizontal plane located axially between said endplates, said minimum antero-posterior dimension being not greater than three times an axial height dimension of said core member.

21. The intervertebral disc prosthesis ofclaim 18, wherein said first elastomeric core member has a generally peanut shell-shaped cross-section.

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