Two-part prosthetic nucleus replacement for surgical reconstruction of intervertebral discs
A prosthetic nucleus replacement comprises a modular two-part body formed into an oval disk. The top part has a domed surface with a crest and is made of a high molecular weight polyethylene or ceramic. The bottom part is made of biocompatible metal like titanium and locks into an underlying vertebrae with a peg or brace molding that extends down into a socket. The prosthetic nucleus replacement is surgically implanted into the hollowed out intervertebral space through a flap cut in the natural annulus fibrosis. The lower vertebra is prepared to receive the peg by clearing the material covering the top of the bone matrix. Bone cement is used around the peg to ensure a tight fit and immobile attachment of the disc to the lower vertebrae.
BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to surgical methods and devices to treat back and leg pain and in particular to the surgical insertion of prosthetic nucleus replacement within the annulus fibrosis. The device replaces a portion of a damaged spinal intervertebral disc to restore function.
 2. Description of Related Art
 In the spine, the principal function of the disco-vertebral joint is to transmit compressive loads and still allow flexibility. Adjacent vertebrae are joined by a triple-joint complex. The anterior complex or column is formed by the vertebral bodies which are shaped like flattened cylinders with discoid shaped or ovoid shaped intervertebral discs sandwiched between each vertebral body. Facet joints in the rear of each vertebra have a smooth cartilage surface, lubricating joint fluid, and a covering capsule. The facet joints restrict the disc to small degrees of flexion and extension, limit rotation, and protect against translational shear stress. The disc itself comprises two principle parts, the nucleus pulposus at the core, and the annulus fibrosis, which is a multilayer bias-ply wrapping that surrounds the nucleus. The nucleus starts early in life as eighty percent water, and slowly desiccates with age.
 A damaged disc can cause nerve dysfunction and debilitating pain in the back, legs and arms. Typical treatments that provide relief and allow patients to function again include back braces, medical treatment, physical therapy and surgery to remove the disc. A conventional surgical solution removes the bad disc and promotes new bone growth in the space to fuse the adjacent vertebrae together.
 Several different prosthetic intervertebral disc devices are described by Casey K. Lee, et al., in “Prosthetic Intervertebral Disc,” Chapter 96, The Adult Spine: Principles and Practice, Raven Press, Ltd., New York, © 1991. The conclusion of Lee, et al., is that “An appropriately designed and fabricated prosthetic intervertebral disc may provide an improved alternative to currently available surgical approaches to low back disorders.” Lee, et al., describe their work at the orthopedic research laboratories at the New Jersey Medical School “to produce a prosthetic intervertebral disc design that has biomechanical characteristics similar to the natural disc.” One result has been the manufacture of a unit with a nucleus, annulus, and end plates that are molded under heat and fused into a single prosthetic disc. However, success of such a device depends on solid bone attachment. Most prior concepts have been excessively complex and never used.
 A prosthetic nucleus replacement can be surgically implanted within the annulus fibrosis. The natural attachments of the annulus would therefore be able to produce the requisite tensile strength of the repaired site. The prosthetic nucleus replacement would be subject primarily to compressive forces.
 Such a prosthetic nucleus replacement is described by the present inventor, Kevin Lawson, in U.S. Pat. No. 6,146,422, issued Nov. 14, 2000. But this device is composed of one homogeneous material and thus its top and bottom sides exhibit identical material characteristics. The described construction can also be unnecessarily expensive to produce.
 In general, the replacement nucleus top must be biocompatible, exhibit a low coefficient of friction, have a smooth surface, be resilient, and if possible radiolucent. It should help produce clear easy to read x-ray, CAT, and/or MRI medical images, e.g., to enable post-operative evaluations that are non-invasive.
 The bottom of the replacement nucleus must also be biocompatible, but it should stay put. So a high coefficient of friction is desirable, and maybe even cementable to the bone of the underlying vertebrae.
 No prior art replacement nucleus meets these apparently conflicting criteria.
SUMMARY OF THE INVENTION
 An object of the present invention is to provide a prosthetic nucleus replacement that is useful and functional.
 Another object of the present invention is to provide a prosthetic nucleus replacement that allows for medical images of the spine to be taken for non-invasive postoperative assessments.
 Briefly, a prosthetic nucleus replacement embodiment of the present invention comprises a modular two-part body formed into an oval disk. The top part has a domed surface with a crest and is made of a high molecular weight polyethylene or ceramic. The bottom part is made of biocompatible metal like titanium and locks into an underlying vertebrae with a peg or brace molding that extends down into a socket or recess in the bone. The prosthetic nucleus replacement is surgically implanted into the hollowed out intervertebral space through a flap cut in the natural annulus fibrosis. The lower vertebra is prepared to receive the peg by clearing the material covering the top of the bone matrix. Bone cement may be used around the peg to ensure a tight fit and immobile attachment of the disc to the lower vertebrae as necessary.
 An advantage of the present invention is that a prosthetic nucleus replacement is provided that supports the normal compressive loads experienced by natural vertebrae.
 Another advantage of the present invention is that a prosthetic nucleus replacement is provided that fixes well to the inferior vertebrae it sits upon.
 A further advantage of the present invention is that a prosthetic nucleus replacement is provided that slides easily under the superior vertebrae it supports.
 The above and still further objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed description of specific embodiments thereof, especially when taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
 FIG. 1 is a diagram representing the spine of a patient with a prosthetic nucleus replacement embodiment of the present invention;
 FIGS. 2A and 2B are an on-edge view and a bottom view of a prosthetic nucleus replacement embodiment of the present invention similar to that shown in FIG. 1; and
 FIGS. 3A-F are lateral cross sectional diagrams of a variety of implementations of spinal nucleus replacement prosthesis embodiments of the present invention, similar to those shown in FIGS. 1, 2A, and 2B.
DETAILED DESCRIPTION OF THE INVENTION
 FIG. 1 illustrates a prosthetic spinal nucleus replacement embodiment of the present invention, referred to herein by the general reference numeral 100. A human spine 102 commonly comprises a series of vertebrae 104-108 interdigitated with a corresponding series of discs 110-113. Each natural disc comprises a nucleus pulposus surrounded and contained by a corresponding annulus fibrosis. Natural nucleus pulposus have jelly-like structures that can absorb and dampen compressive shock loads. Natural annulus fibrosis structures comprise multiple layers of bias-ply filaments set at forty-degree angles that resemble the construction of an automobile bias-ply tire carcass.
 Disc 112, between vertebra 106 and 107, is assumed in FIG. 1 to be degenerated. The spinal nucleus replacement prosthesis 114 is surgically embedded in the inter-vertebral space between vertebra 106 and 107, and inside an annulus fibrosis 116.
 Prosthetic nucleus replacement embodiments of the present invention comprise a two-part modular assembly that resembles a flattened oval disk. The superior, or top part is domed and is made of a biocompatible material that slides easily and articulates well with the superior vertebrae 106. The inferior base part is made of a different biocompatible material that can be fixed readily to the bone of the inferior vertebrae 107. For example, a porous material for bone in-growth, or a textured material for cementing. The possible biocompatible materials include ceramics, polymers and plastics, titanium, stainless steel, tantalum, chrome cobalt alloys, etc. Ultra-high molecular-weight polyethylene is presently preferred so that metal radiograph markers may be strategically placed in the nucleus replacement prosthesis 114.
 In general, prosthetic nuclei of the present invention are implanted using a straight anterior or anterior lateral approach with incision of the anterior longitudinal ligaments of the annulus. Just before use, and after the implant site has been evaluated, a prosthetic annulus is assembled from two modular parts. E.g., a top dome and a base made of dissimilar materials. A flap technique is used for the incision of the annulus, and such tissues are closed back up with conventional sutures or suture anchors to the bone. The endplate cartilage of the superior vertebrae is preserved for permanent articulation with the implanted nucleus prosthetic. The endplate cartilage of the inferior vertebrae is curetted down to bone. The bone is prepared to receive a peg embedded in the implanted nucleus prosthetic. Such pinning and also cement are used to permanently immobilize this interface. The whole assembly is carefully centered as far posterior as possible to help reestablish natural kinematics of flex-extension and lateral bending.
 As shown in FIGS. 2A and 2B, a nucleus replacement prosthesis 200 has a top half 202 that is domed and has a crest. This interlocks for modular assembly with a base half 204. The outside diameter of the nucleus replacement prosthesis 200 can vary, e.g., in the range of twenty to thirty-six millimeters. The overall height can also vary, e.g., in the range of eight to sixteen millimeters. The actual dimensions required depend on the size of the patient and the exact site to receive the implant. Such required sizes are discernable from patient radiographs, CT-scans, and MRI-scans.
 A peg 206 extends down from the base 204. The peg 206 is preferably two to four millimeters long, and is primarily used to pin the nucleus replacement prosthesis 200 to the adjacent underlying vertebrae, e.g., vertebrae 107 in FIG. 1. A pair of metal radiograph markers 208 and 210 are placed so that radiographs can be used to determine the position of the nucleus replacement prosthesis 200.
 The prosthetic nucleus replacement 200 is surgically implanted into the hollowed out intervertebral space through a flap cut in the natural annulus fibrosis. Such “hollowing out” is commonly called a diskectomy. The lower vertebra is prepared to receive the peg 206 by clearing the material covering the top of the bone matrix. Bone cement can be used around the peg 206 to ensure a tight fit and immobile attachment of the disc to the lower vertebrae. Alternatively, a non-cement method can be used to promote and receive bone growth that will eventually immobilize the base 204.
 FIGS. 3A-F show a variety of implementations of spinal nucleus replacement prosthesis 114 and 200. In FIG. 3A, a spinal nucleus replacement prosthesis 300 has a top dome 302 of ceramic that interlocks with a titanium base 304. A porous titanium bone-in-growth peg 306 is used to help immobilize the whole to an underlying vertebrae.
 In FIG. 3B, a spinal nucleus replacement prosthesis 310 has a top dome 312 of polished titanium that interlocks with a textured ceramic base 314. A tapered peg 316 and perimeter teeth 318 and 319 are used to help immobilize the whole to an underlying vertebrae.
 In FIG. 3C, a spinal nucleus replacement prosthesis 320 comprises a ceramic or polyethylene dome 322 that is jacketed on the bottom by a metal base 324. Titanium or chrome-cobalt can be used for the base 324. A pointed peg 326 is used to help immobilize the whole to an underlying vertebrae.
 In FIG. 3D, a spinal nucleus replacement prosthesis 330 includes a modular dome 332 interlocked onto a base 334 with a peg 336. Each part is made in a variety of sizes and shapes so the surgeon can select a best fit for each situation.
 In FIGS. 3E and 3F, a spinal nucleus replacement prosthesis 340 comprises a dome top 342 and a base 344 of dissimilar biocompatible materials. An anti-pivot keel 346 is used to anchor and positively prevent swiveling of the whole.
 A surgical method embodiment of the present invention for correcting a degenerated nucleus pulposus by the implantation of a prosthetic in a human spine begins with a flap-technique incision of an annulus fibrosis corresponding to an affected area of a spine. Then a diskectomy of a degenerated nucleus pulposus is done in the affected area. The cartilage is cut down to the bone of an inferior vertebrae adjacent to the affected area and the bone is prepared for anchoring to a modular annulus base. A solid ellipsoidal body is then assembled from interlocking ones of a modular top dome of a first biocompatible material and the modular annulus base of a second biocompatible material. The assembly provides for replacement of a natural nucleus pulposus. The assembled solid ellipsoidal body is inserted into the affected area through an incision in the annulus fibrosis. The solid ellipsoidal body is immobilized with respect to the inferior vertebrae, e.g., using bone cement. And, then the incision in the annulus fibrosis is closed. The result is a permanent articulation between the solid ellipsoidal body and a superior vertebrae after surgery.
 The present inventor's previous U.S. Pat. No. 6,146,422, issued Nov. 14, 2000, is incorporated herein by reference.
 Although particular embodiments of the present invention have been described and illustrated, such was not intended to limit the invention. Modifications and changes will no doubt become apparent to those skilled in the art, and it was intended that the invention only be limited by the scope of the appended claims.
1. A prosthetic nucleus replacement for implanting within an annulus fibrosis in one part of a human spine, comprising:
- a top dome comprised substantially of a first biocompatible material, and having a convex upper side for contacting and articulating with an end-plate cartilage of a supported superior vertebrae;
- a base mechanically fixed to and underneath the top dome and comprised substantially of a second biocompatible material, and providing for an immobile foundation on an underlying inferior vertebrae; and
- a brace molding extending from a bottom side of the base and providing for an immobilization to said underlying inferior vertebrae;
- wherein, said first biocompatible material has a relatively low coefficient of friction and slides easily when in contact with natural cartilage; and
- wherein, said second biocompatible material has a relatively high coefficient of friction and fixes well to the bone of said inferior vertebrae.
2. The prosthetic nucleus replacement of claim 1, wherein:
- the base has a porous bottom surface providing for a bone-growth fixation with said inferior vertebrae.
3. The prosthetic nucleus replacement of claim 1, wherein:
- the base has a textured bottom surface providing for a cement fixation with said inferior vertebrae.
4. The prosthetic nucleus replacement of claim 1, wherein:
- the top dome comprises ultra-high molecular-weight polyethylene.
5. The prosthetic nucleus replacement of claim 1, further comprising:
- a radiograph marker placed within the top dome, and that can assist a physician in determining the in stiu orientation of said prosthetic nucleus replacement by non-invasive medical imaging.
6. A method for surgically correcting a degenerated nucleus pulposus by the implantation of a prosthetic in a human spine, the method comprising:
- a flap technique incision of an annulus fibrosis corresponding to an affected area of a spine;
- a diskectomy of a degenerated nucleus pulposus in said affected area;
- curetting of cartilage down to the bone of an inferior vertebrae adjacent to said affected area and preparing said bone to receive a pin;
- assembling a solid ellipsoidal body from interlocking ones of a modular top dome of a first biocompatible material and a modular base of a second biocompatible material, wherein the product provides for replacement of a natural nucleus pulposus;
- inserting said solid ellipsoidal body into said affected area through an incision in said annulus fibrosis;
- immobilizing said solid ellipsoidal body with respect to said inferior vertebrae; and
- repairing incision in said annulus fibrosis;
- wherein, a permanent articulation between said solid ellipsoidal body and a superior vertebrae exists after surgery.
Filed: Mar 13, 2002
Publication Date: Sep 18, 2003
Inventor: Kevin Jon Lawson (Redding, CA)
Application Number: 10097711
International Classification: A61F002/44;