CA2201607C

CA2201607C - Prosthetic spinal disc nucleus

Info

Publication number: CA2201607C
Application number: CA002201607A
Authority: CA
Inventors: Charles D. Ray; Eugene A. Dickhudt; Philip J. Ledoux; Beth A. Frutiger
Original assignee: RayMedica Inc
Current assignee: RayMedica LLC
Priority date: 1994-10-17
Filing date: 1995-10-17
Publication date: 2001-02-13
Anticipated expiration: 2015-10-17
Also published as: DE69532856D1; JP2005058781A; JPH10507386A; EP0786963A4; EP0786963B1; US5674295A; DE69532856T2; EP0786963A1; WO1996011642A1; JP2007283121A; CA2201607A1; ES2216021T3

Abstract

An elongated, pillow shaped prosthetic spinal disc nucleus body (10) for implantation into a human intervertebral spinal disc (32), made of a hydrogel core (12) and a flexible constraining jacket (14) surrounding the hydrogel material core (12) that permits the hydrogel core (12) to expand and contract. The hydrogel core (12) has a length approximating the sagittal diameter of a nucleus of the human disc, a width less than the length, and a height less than the length or width. The hydrogel core will expand (See fig. I) and contract in a desired fashion as it imbibes and expels fluids in response to various loads placed upon the spinal tract. The constraining jacket (14) is porous to allow fluids to pass through to the hydrogel core (12), but prevents the hydrogel (12) from escaping, thus fostering the natural physiology of the human invertebral disc. By implanting two prosthetic spinal disc nucleus bodies (10) side-by-side into a damaged disc (32) of a human spine, both height and motion can be maintained.

Description

~ ~ O 9 6 ~ 7 PRO~l~;l`lC SPINAL DISC NUCLEUS
BACKGROUND OF THF INVFl~TION
The present invention concerns a prosthetic spinal disc nucleus.
More particularly it relates to an impl~nt~hle c~ps~ or pillow-shaped prosthetic5 discs nllcleu~ having the abiliq to stimul~te the ~ u~pLion of the natural physiology of a de~ dted human disc.
The ve, t~rdLe spine is the axis of the skeleton upon which all of the body parts "hang". In h~-m~n~, the normal spine has seven cervical, twelve thoracic and five lumbar se~ t~. The lumbar spine sits upon the sacrum, 10 which then ~tt~-~hlos to the pelvis, in turn ~u~Jv Led by the hips and leg bones.
The bony vertebral bodies of the spine are se~Led by intervertebral discs, which act as joints but allow known degrees of flexion, extension, lateral bending, and axial rotation.
The intervertebral disc primarily serves as a mf~h~nic~l cushion 15 between vertebral bones, pelmiLLillg controlled motions within vertebral segmPnt~ of the axial skeleton. The normal disc is a unique, mixed structure, comprised of three component tissues: the nucleus pulposus ("nucleus"), the anulus fibrosus ("anulus") and two vertebral end-plates. The two vertebral end-plates are composed of thin cartilage overlying a thin layer of hard, cortical 20 bone which attaches to the spongy, richly vascular, cancellous bone of the vertebral body. The end plates thus act to attach ~djacent vertebrae to the disc.
In other words, a transitional zone is created by the end plates between the m~ hle disc and the bony vertebrae.
The anulus of the disc is a tough, outer fibrous ring which binds 25 ~ogether adjacent vertebrae. This fibrous portion, which is much like a l~min~tlod automobile tire, is generally about 10 to 15 millimefPrs in height and about 15 to 20 millimeters in thicknes~ The fiber layers of the anulus consist of fifteen to twenty overlapping multiple plies, and are inserted into the superior WO 96/11642 ~ 7 PCT/US95/13172 and inferior ~el~b.dl bodies at roughly a 40 degree angle in both directions.
This configuration particularly resists torsion, as about half of the angulated fibers will tighten when the ve ~,de rotate in either direction, relative to each other. The 1A.... ...........-;nAt~d plies are less firmly ~ h~ to each other.
Immersed within the anulus, positioned much like the liquid core of a golf ball, is the nucleu~. The healthy nucleus is largely a gel-like substance having a high water content, and like air in a tire, serves to keep the anulus tight yet flexible. The nucle~ls-gel moves slightly within the anulus when forceis exerted on the ~dj~ nt vertebrae while bending, lifting, etc.
The m~cleus and the inner portion of the anulus have no direct blood supply. In fact, the principal nutritional source for the central disc arises from circulation within the vc. ~ldl body. Microscopic, villous-like fingerlingsof nuclear and anular tissue penetrate the ~,e.Lel).~l end plates and allow fluids to pass from the blood across the cell membrane of the fingerlings and then inward to the nuclear tissue. These fluids are primarily body water and the ~mAllest molecular weight nutrients and electrolytes.
The natural physiology of the nucleus promotes these fluids being brought into and released from the nucleus by cyclic loading. When fluid is forced out of the nucleus, it passes again through the end plates and then back into the richly vascular vertebral bodies. This cyclic loading amounts to daily variations in applied ~l~S~Ul`~ on the vertebral column (body weight and muscle pull) c~n~ing the nucleus to expel fluids, followed by periods of relaxation andrest, resulting in fluid absorption or swelling by the nucleus. Thus, the nucleus changes volume under loaded and non-loaded conditions. Further, the tiEhtening and loosening effect stimulates normal anulus collagen fibers to remain healthy or to f~en~ldte when torn, a process found in all normal lig~mf~nts related to body joints. Notably, the ability of the nucleus to release and imbibe fluids allows the spine to alter its height and flexibility through ~ wo 96/11642 2 2 ID 1 6 0 7 PCT/US95/13172 periods of loading or r~ tiom Normal load cycling is thus an effective nurl~Pllc and inner anulus tissue fluid pump, not only bringing in fresh nutrients, but ~lhd~s more i~ y~ removing the ~r~uml~l~t~, potentially autotoxic by-products of metabolism.
The spinal disc may be displaced or damaged due to trauma or a disease process. A disc herniation occurs when the anulus fibers are we~l~Pn~ or torn and the inner tissue of the nllcle~ls becomes permanently bulged, f~i.cten~, or extruded out of its normal, internal anular confines. The mass of a herniated or "slipped" nucl~llc tissue can co~ rès5 a spinal nervet resulting in leg pain, loss of muscle control, or even paralysis. Alternatively,with discal de.~eneration, the m~clel~s loses its water binding ability and defl~t~s, as though the air had been let out of a tire. Subsequently, the height of the nucleus dec;,~ses c~ in~ the anulus to buclde in areas where the l~",in~
plies are loosely bonded. As these overlapping l~min~teA plies of the anulus begin to buckle and separate, either ci,culllfclcnlial or radial anular t~ars may occur, potentially resl-lting in pcrsistent and disabling back p~un. Adj~cent ancillary spinal facet joints will also be forced into an overriding position, which may crcate additional back p~un.
Whenever the nuclear tissue is herniated or removed by surgery, the disc space will narrow and may lose much of its normal stability. In many cases, to alleviate pain from degenerated or herniated discs, the nucleus is removed and the two adjacent vertebrae surgically fused together. While this tre~tm~nt alleviates the pain, all discal motion is lost in the fused s~ment.
Ultim~t~ly, this procedure places greater stresses on the discs adjacent to the ~used St~gm~nt as they c~ cnsate for the lack of motion, ~elh~s leading to ~re,lldlurè degenc.dlion of those ~ nt discs. A more desirable solution would involve replacing in part or as a whole the damaged disc with a suitable wo 96/11642 2 2 0 9 ~ 0 7 PCT/US95/13172~

pçu~ .P-~ic having the ability to comr'^~n~ the norrnal height and motion of a disc while mimicking the natural physiology of the disc.
The nutrition-flushing cycle of a natural disc is i~ t for a prosthetic spinal disç nuçle~ls to be sucr~s~ful. Vascular circulation and nerveS supply to the disc is limited to the outer layers of the anulus, never penetrating more than a few millim~tto.rs or about five of the plies. Most of the nutrition of the inner anulus and nucleus is provided by diffusion through the end plates of the vertebral bodies and by the illl~l L~1t ~ulllpillg action between the partially loaded and fully loaded conditions of the disc. If the nutritional cycle is 10 im~ded, a variety of degelle,d~ e changes may occur. Nutrition to the inner disc slowly ceases, resulting in intradiscal build-up of acids and toxins, and other ch~nges This is followed by nuclear and anular fiber degeneration, shrinkage of the nucleus, s~.gment~l laxity, spur formation, disc space collapse, and perhaps spontaneous fusion. Additionally, significantly disabling back pain 15 may develop.
Degenerated, painfully disabling interspinal discs are a major economic and social problem for patients, their f~milies, employers and public at large. Any significant means to correct these conditions without further destruction or fusion of the disc may therefore serve an important role. Other 20 means to replace the function of a degenerated disc have major problems such as complex surgical procedures, which may require opening of the abdomen to install a large device that replaces the entire disc. Therefore, a substantial need exists for an easily implantable, prosthetic spinal disc nucleus having the ability to mimic the natural physiology of a human disc while restoring and maintaining 25 the normal size of the disc space.

~ wo 96/11642 ~ ~ ~) 9 ~ ~ 7 PCT/US95/13172 SU~IAl~Y OF l~F rlWF~TION
The invention provides an elongaled, pillow-shaped prosthetic spinal disc nuclPII~ body ~or i.~ snl~l;o~ deep inside a human disc. The ~n,s~ ;c body is ~...pos~d of a hydrogel core, and a flexible cons~ ing S jacket ~ulloundillg the hydrogel core. These cG...poll~nts reestablish near normal disc height and norrnal anulus po~ition and function. Ad~litionslly, the pr~ .c~;c body will expand and contract in a primarily vertical direction, providing n~es~ly sul,l)o-L to the discal area, tightPning and loosening the anulus in a normal, healthy manner. The co---~one--~s also work in concert to 10 simulate the natural physiology of a human disc. In response to the removal and exertion of co...p.~s~ive loads, the ~rv~ el;c body will imbibe and expel fluids to l.~o-.-ole the natural cyclic pumping of the discal area.
The hydrogel core has a length appro~hing the sagittal ~ meter of the nucleus of a natural disc, a width which is less than the length and is 15 subst~nti~lly constant over the length, and a height which is less than the length and width. The hydrogel core can imbibe and expel fluids. In a p-~felled emboclim~nt, the hydrogel core has a water content of a~)l),o,~imately 25-655~
when fully hydrated. When imbibing and expelling fluids, the hydrogel core will expand and contract in the vertical dimension.
The hydrogel core of the present invention is surrounded by a constraining jacket. The constraining jacket is made of a flexible material which allows the hydrogel core to expand and contract in the vertical direction, while1imi~ing simultaneous de~l.--~Llion in the horizontal direction of the frontal plane. The constraining jacket is porous and allows fluids to pass through as 25 they are imbibed and expelled by the hydrogel core.
Once constructed, the prosthetic spinal disc nucleus body can be placed into the damaged disc space. According to a ~l~fell~d embodiment, the prosthetic spinal disc nucleus body is implanted in pairs.

WO 96/11642 ~ 2 ~ 1 ~ 0 7 PCT/US95/13172 B~TFF DF-~CRIPTION OF ~TF DRAWINGS
FIG. 1 is a ~ ~ e view of an elol-g~ prosthetic spinal disc n~lcleus body, including a ;ul~w~y view showing a portion of a hydrogel material core, in accordance with the present invention.
SFIG. 2 is a side section~l view of the p/cfe~ d prosthetic spinal disc nucleus body along the line of 2-2 of FIG. 1.
FIG. 3 is a frontal s~tion~l view of the p-e~.led prosth~tic spinal disc mlcleus body along the line 3-3 of FIG. 1.
FIGS. 4-6 illustrate steps of fabricating the prosthetic spinal disc 10nucleus body of FIG. 1.
FIG. 7 is a perspective view of a spinal segment including a degenerated discal area.
FIG. 8 is a posterior view of a human spine showing two flaps that have been cut through an anulus.
15FIG. 9 is a top, sectional view of a human disc space having two prosthetic spinal disc nucleus bodies implanted.
FIG. 10 is a perspective view of an alternative embodiment of the prosthetic spinal disc nucleus body which includes a tine assembly.

~ wo 96/11642 ~ 2 0 'I ~ 7 PCT/US95/13172 DFTATT Fn D~SCRIPI'ION OF lHE p~FFERRED E3MBODTMF.I~TS
A ~-~rc.l~d embo~ t of the pr~s~l.P~ ic spinal disc nllclel~C body 10 is shown in FIG. l. The prosthPtic spinal disc nl~clP~u~ body l0 is comprisedof a hydrogel core 12 and a co1.c~ nil-g jacket 14. The prostheti~ spinal disc nu~lP,1l~ body l0 has an anterior end 16 and a posterior end 18. The co11~Lldi1ling jacket 14 is secured around the hydrogel core 12 by an anterior closure 20 located at the anterior end 16 and a posterior closure 22 located at the posterior end 18.
As shown in FIGS. 2 and 3, the hydrogel core 12 is fabricated to assume a pillow shape. Along the lon~itu~lin~l ~or sagittal) plane (as shown in FIG. 2), the hydrogel core 12 has an obround configuration whereas the frontal plane (as shown in FIG. 3) is oval.
The hydrogel core 12 is form~ ted as a ~ Lur~ of hydrogel polyacrylonitrile. AlLt;."aLi~ely, the hydrogel core 12 can be any hydrophilic acrylate derivative with a unique multiblock copolymer structure or any other hydrogel material having the ability to imbibe and expel fluids while --~in~;ning its structure under various stresses. For example, the hydrogel core can be ~ormulated as a mixture of polyvinyl alcohol and water. Much like a normal human nucleus, the hydrogel core 12 will swell as it absorbs fluids. The hydrogel core 12 has a time constant of swelling which is highly similar to thatof the natural nucleus and will thus experience a 5-30% and preferably a 15-20% volume change depçnf~ing on load over the course of 2-8 (preferably 4-8) hours. When fully hydrated, the hydrogel core 12 will have a water content of between 25-65%. The hydrogel material 12 of the p-~fe -~d embodiment is manufactured under the trade name Hypan0 by Hymedix International, Inc.
Completely surrounding the hydrogel core 12 is the constraining jacket 14. The constraining jacket 14 is ~1e~1~bly a closed tube made of a tightly woven high molecular weight, high tenacity polymeric fabric. Further, WO 96/11642 2 2 ~ 9 ~ 0 7 PCT/US95/13172~

the constraining jacket 14 is flexible. In a ~-cre -~d embodiment, high mol~Pcul~r weight polyethylene is used as the weave material for the constraining jacket 14. However, polyester or any other high mol~.~ r weight, high tenacity material can be employed. For example, carbon fiber yarns, ceramic S fibers, mPt~llic fibers, etc. are all accept~l-le.
The ~ Çel.l;d woven construction of the constraining jacket 14 creates a plurality of small op~ning~ 24. These op~ningS are large enough to allow bodily fluids to interact with the hydrogel core 12, which is maintained within the constraining jacket 14. However, the opçning~ 24 are small enough 10 to prevent the hydrogel 12 from escaping. Preferably, the opening~ 24 have anaverage ~i~metPr of about 10 micrometers, ~lthough other llimencions are acceptable. While the consLIdining jacket 14 is described as having a weave configuration, any other configuration having a se~ x;....eable or porous attribute can be used.
By employing a flexible material for the constraining jacket 14, the hydrogel core 12 is allowed to expand and contract in a controlled fashion as it imbibes and expels fluids. When the hydrogel core 12 swells as a result of an influx of water, the constraining jacket 14 has sufficient flexibility to allow the hydrogel core 12 to expand. The strength and flexibility characteristics of 20 the material used for the constraining jacket 14 are such that the pillow shape of the hydrogel 12 will always be maintained. By i,.,p~ ling a uniform constraining force on the surface of the hydrogel core 12, the constraining jacket 14 prevents undesired deÇo..,.ation of the prosthetic spinal disc nucleus body 10.
However, for the prosthetic spinal disc nuclells body 10 to function as would a 25 natural nucleus, some desired changes in the shape and size of the hydrogel core 12 must take place as loads are increased and decreased.
As fluids are imbibed, the woven constraining jacket 14 works in conjunction with the oval cross sectional shape of the hydrogel core 12 to WO 96/1164~ 2 ~ 0 7 PCT/USg5113172 .

g control ~-r~n~ion of the hyd~gel core 12. The prosthetic spinal disc nllcle~
body 10 initially ~c~llmes an oval shape in its fronW plane (as shown in FIG

3). The nucleus body 10 will ...~in~ this shape and act as a c~lchiQn against various loads placed upon it. As these loads are decreased (eg. when the patient5 r~clines), the hydrogel core 12 imbibes sullou.lding fluids and ~ndS~ The constraining jacket 14 ensures that this ~Apal~sion is only in the forrn of the hydrogel core 12 becollling more circular in fronW cross section. In other words, the conct~ining jacket 14 allows the hydrogel core 12 to expand in the y-direction (vertically), but prevents a simlllt~neous PYp~n~ion in the x-direction 10 (horizontally). Further, while limited horizontal contT~ction will preferablyoccur, the vertical expansion proceeds at a l,lo~llionately greater rate than the ho. ;'~ contraction. Tl.~.~for~, the smaller the load placed upon the prosthetic spinal disc nucleus body 10, the closer the body 10 is to a circular frontal cross section. To help achieve this unique effect, the l rc~rc~llt;d 15 constraining jacket 14 is s.~ ti~lly inelastic. To prevent the hydrogel core 12 from escaping, the constraining jacket 14 has a burst strength which is greater than the swelling pressure of the hydrogel core 12 when fully hydrated.
FIGS. 4-6 illustrate the manufacturing of the prosthetic spinal disc nucleus body 10. First, the hydrogel core 12 is formulated. An a~rop.iately 20 sized volume of hydrogel material is dehydrated, resulting in an undersized, subst~nti~lly cylindrical gel capsule. This dehydrated hydrogel material 12 is then inserted into the constraining jacket 14.
As shown in FIG. 4, the constraining jacket 14 is preferably tubular in shape with openings at both the anterior end 16 and the posterior end25 18. The dehydrated hydrogel material 12 is placed within the constraining jacket 14 and cen~.~ belween the anterior end 16 and the posterior end 18.
The ends of the constraining jacket 14 are then secured by forming the anterior closure (not shown) and the posterior closure 22.

WO 96/11642 2 ~ 7 PCT/IJS95/1317 -1~

In the centered posi*on, the hydr~gel material core 12 will have a length smaller than tnat of the con~ ing jacket 14, res~l*n~ in excess outer layer material 26 at both the anterior end 16 and the posterior end 18. The excess outer layer material 26 at both the anterior end 16 and the posterior endS 18 is closed to prevent the hydrogel material 12 from escaping or leaking fromthe confinçs of the constraining jacket 14. As shown in FIGS. 5 and 6, to form the posterior closure 22, the excess outer layer material 26 is preferably folded or tucked and then closed. The fold is created by pinching two opl)osil1g sides of the excess material 26 centrally towards one another, a~ro~ g a "figure 10 8" form. The two r~m~ining free ends are fl~ nf~d against one another, resulting in an "H-shaped" fold as shown in FIG. 5.
The fold is then closed by sewing a dense, bar-tack stitch 28 across the folded section at a position near the hydrvgel core 12. The bar-tack stitch 28 material is preferably the same high tenacity polymeric material, such15 as high molecular weight polyethylene, as is used for the constraining jacket 14.
By employing the same material for both the constraining jacket 14 and the bar-tack stitch 28, the biocol-lpalibility of the entire prosthetic spinal disc nucleus body 10 is ensured. The rem~ining excess material 26 is removed by a thermal cut made at a point distal to the bar-tack stitch 28. This thermal cut fuses the20 potentially fraying ends of the jacket, distal to the stitched portion 28.
While FIGS. 5 and 6 only show the posterior closure 22 on the posterior end 18, the excess material 26 on the anterior end 18 is folded and sealed in a similar fashion to form the anterior closure 20. Notably, it is not always n~es~ry to fold the excess outer layer material 26, where the anterior 25 end 16 and the posterior end 18 are simply sealed by the dense, bar-tack stitch 28 without folding the material 26. Further, while the constraining jacket 14 has been described as having two openings, it may instead be manufactured with a single opçnin~ either on an end or side, through which the hydrogel core 12 is inserted.
To aid in ensuring proper pl~remPnt of the prosthPtir spinal disc nucleus body 10 within the inter~ l disc space and to review the stability 5 of the ~r~s~ l;c disc body 10 during patient follow-ups, a radiopaque wire 30 is placed inside the constraining jacket 14, at either the anterior end 16 or the posterior end 18, or both or lon~it l~iin~lly along the length of the constraining jacket 14. The radiopaque wire 30 is visible in x-ray applications and is preferably made of a platinum-iridium material, but can be any other material 10 having a radiopaque and biologically inert characteristics. The wire 30 is placed within the excess material 26 at the anterior end 16 or the posterior end 18 andis secured by the bar-tack stitch 28. Al~ll,ali~ely, a radiopaque thread can be woven into the constraining jacket 14 or a radiopaque material can be added to the hydrogel core 12.
In its final form, the prosthetic spinal disc nuclel-c body 10 will have lengths of about 15 to 25 millimPtPrs and an outer rii~mPpr of about 6 to 15 millimçtçrs. The pl~fe~red disc body 10 is 25 millimçters in length and 10 millimeters in outer ~ met~Pr. These limPncions conform with the approximate length of the sagittal ~ mpter and a~r(~imate height of an adult human disc 20 nucleus space, respectively. It is realized that not all human discs are of the same size. Therefore, the prosthetic spinal disc nucleus body 10 alternatively is constructed to assume ~limPn~ions of 20 millimPters in length and 10 millimetPrs in outer ~ meter; 25 millimeters in length and 7 millimet~Prs in outer fli~mPt~Pr; and 20 millimPters in length and 7 millimetPrs in outer ~ mPtPr.
25 Notably, other sizes are possible. The ~ ul~liale prûsthetic disc for a particular patient is dete~ ed by various diagnostic procedures prior to and during surgery. Basically, tl e ~)r~lly ~iimçn~ioned prûsthesis is a functiûn ofthe patient's size and spinal level. By providing prosthetic spinal disc nucleus ~ 2 ~ 7 WO 96111642 PCT/US95/13172~

bodies 10 with varying ~ .C;ons~ the space ~ui~ lenL~ reflect~d by any spinal segl.-ent, human or ~nim~l, are s~ti~fiP~l Following closure of the consL,~ini,lg jacket 14 about the hydrogel core 12, the prosth~Ptic spinal disc nu~l~Puc body 10 is rehydrated and then S subjected to co,ll~,essi~e loads or "contlitio~ln. The conditioning amounts toa series of at least three co"l~l~s~ e loads being applied across the length of the prosthPtic body 10. The m~nitutle of in vivo con~le:~ive loads will vary from patient to patient and is a function of the patient's size and spinal level. Forexample, published li~.alur~ has stated that the normal sitting or standing 10 co",l,r~ssi~/e load on the discal area is 1.8 multiplied by the patient's body weight. Further, the m~ximllm ~.,-pr~ e load placed upon the lumbar discal area during usual, daily activities is 3.6 multiplied by the patient's body weight.
The conditioning, the~er~,le, will consist of a series of col~ ,ssi~e loads being placed upon the prosthetic body 10 equivalent to a minimum of 1.8 multiplied 15 by the typical body weight up to a maximum of 3.6 multiplied by the typical body weight. Following conditioning, the hydrogel core 12 will consistently return to its desired shape and size following the application and removal of compressive loads.
As a further benefit, the hydrogel 12 and its manufacturing 20 process place volume expansion constraints on the hydrogel 12. Even if the hydrogel 12 were unconstrained (eg. if the constraining jacket 14 ruptures), following conditioning the hydrogel 12 will not expand to more than about twice its volume after conditioning. Thus, a continuous, llnlimited, potentially hazardous swelling of the hydrogel 12 will not occur should the constraining 25 jacket 14 be disrupted. This internalized constraint will also prevent possible over expansion of the hydrogel core 12 if the prosthetic discal body 10 is continually unlo~tl~ in the disc space or if the prosthetic discal body 10 were ~ ~ ~ 9 ~ ~ 7 to be displaced into another body cavity such as the _pinal canal or abdomen.

The conditioning renders the prosthetic spinal disc nucleus body 10 to a partially fl~ttened or oval shape. For example, a ~.lc.slLeLic body 10 S originally having a ~ mP~ter of about 10 milli,.,..t~,-" will have a height of about 7 milli".. ~ and a width of about 14 mill;,~ e-~ following conditioning.
Similarly, conditioning will alter a prosthetic bcdy 10 having an original metPr of about 7 millim~ters to one having a height of about 5 millimeters and a width of about 12 millim-otPrs. The con-iiti-ned prosthetic spinal disc 10 nucleus body 10 is then inserted into a r~ il,g tube to ~ ir,t;.in this oval shape up until implantation. The lel;.;llitl,~ tube is preferably made of implantable grade st~inless steel, but can be any other surgically safe material such as polyethylene. The prosthecic 10 and its r~ p tube may be packaged, ,u".~.u.~ded by sterile water, saline or physiological solution ~Ringer's). The 15 entire surgical package is sterilized in a tray, via g~mm~ steam or other type of sterilization. Once conditioned"~ ine~, and sterilized, the prosthetic spinaldisc nucleus body 10 is ready for implantation into the human disc space.
As shown in FIGS. 7, 8 and 9, the final prosthetic spinal disc nucleus body 10 is preferably inserted in pairs into a damaged disc space 32.
20 The disc space 32 se~.ai~les two ~dj~c~nt verte'crae 33. Proper positioning is achieved by first pelrc.,~ g a bilateral laminotomy in a targeted lamina area 35. A pair of flaps 34a and 34b are created in the anulus 36 and any excess material, such as the nucleus 38, ne~ecc~ry to create room for the prosthetic spinal disc nucleus body 10 is removed. The flaps 34a and 34b preferably have 25 a height less than the height ~limen~ion of the prosthetic spinal disc nucleus body 10. In a plerelled embo~limP-nt the flaps 34a and 34b have a length of about 12 millimeters and a height of about 6 millimeters for use with a prosthetic body WO 96/11642 ~ 7 PCT/US95/13172 having a height of 7 millimeters. The ve.leblde 33 ~dj~ nt to the damaged disc 32 are slightly se~hd~ed to allow for the impl~nt~tion.
This slight s~pard~ion is achieved by inserting an inflatable jack through one of the flaps 34a or 34b and jacking apart the ~ rpnt velLeblde 33.
S Once se~ ion s--fficient to insert a mlcl~lls body 10 is achieved, the flap 34a or 34b not occupied by the jack has a l,r~.sll,Glic spinal disc nucleus body 10 inserted via a tapered holding tube. The jack is then ci~fl~t~ and removed, and a second prosthetic spinal disc nucleus body 10 is placed through the rem~ining flap 34a or 34b. To promote an increase in hydration, saline or similar fluid 10 is injected or flushed into the nucleus area 38. When p~ ly implanted, the anterior end 16 of each prosthetic spinal disc mlçlells body 10 will be adjacentto and inside of the anterior end of the anulus 36; the posterior end 18 will bePnt to and inside of the posterior end of the anulus 36. By i~ h lhlg the flaps 34a and 34b with a height llim~n~iQn smaller than that of the body lQ and 15 closing the flaps 34a and 34b after implant, a positive fixation within the anulus 36 is provided and likewise the retropulsion of the discal nucleus 10 from the anulus 36 is prevented.
Following implantation, the prosthetic spinal disc nucleus bodies 10 function as intervertebral spacer, a cushion and fluid pump. As previously 20 described, the prosthetic spinal disc nucleus bc~dy 10 has an oval shaped frontal cross section. As fluids are imbibed (via the absence, or removal, of loads uponthe spinal tract), the hydrogel core 12 begins to swell. The constraining jacket14 forces the hydrogel core 12 to become more circular in frontal cross section by allowing the frontal height to expand while preventing an increase in width.
25 ~n.~t~d, as the height expands, the width preferably will slightly contract. In this regard, the height of the hydrogel core 12 changes at a proportionately greater rate than the width changes. This controlled swelling effectively pushes ~ 9~7 or further ~ A~es the v~ bl~.c 33 ~jw.nt to the disc space apart, as would a normal nuelells Conversely, as loads on the spinal tract increase, the prosthetic spinal disc nuc~ c 'oody 10 cl~shionc the ~dj~rent ~ de 33 and slowly cont.,~r~i in the frontal plane as the Lydl`ogel core 12 releases or "pUlllp5 out"
fluids and thus flushes out the ~ccllmlll~t~i acids or aulolo~ins cQ~ cl therein.
During this pulllpil~g action, the co~ r,~ jacket 14 forces a vertical contraction while preventing a horizontal contraction. Notably, some expansion in the ho,i~on~l plane will occur. The height of the prosthetic spinal disc 10 nucleus body 10 contracts at a proportionately greater rate than the width expands. The h~drvgel core 12 thus becomes more oval shaped in cross section and loses volume as co~lp,e~c;on~l lodds are placed upon the discal area.
Notably, the con~tr~ining jacket 14 of the present invention indPpendently absorbs the force/~,~s~ of the hydl~Jgel core 12 as it expands and colllld~
15 Thus, the anulus 36 is not ~ uiled to ~ o~l the force/pressure from the hydrogel core 12.
In an alternative embodiment shown in FIG. 10, to assist in preventing the retropulsion of the prosthetic spinal disc nucleus body 10 after impl~nt the prosthetic body lO can be provided with a tine assembly 40 located 20 on the external surface of the prosthetic body 10. When properly oriented, the tine assembly 40 will promote the simple implantation of the prosthetic body into the disc space, but greatly inhibits removal or spontaneous retropulsion.
The tine assembly 40 provides an ~ddition~l fixation of the ~l-JSlh~liC spinal disc nurleus within the disc space.
The tine assembly 40 is ~ c~lP~1 to the posterior end 18 of the prosthetic spinal disc nllcle~ls body 10 and projects away from the external face of the constraining jacket 14. Each individual tine 42 on the tine assembly 40 has an approximately triangular shape, including a base 44 and an end 46. The WO 96/11642 ~ ;2 ID 1 6 0 7 PCTMS95/13172~

base 44 of each tine 42 is int~.~lly ~tt~h~ to a frame 48 of the tine assembly 40. Each tine 42 projects laterally away from the tine assembly frame 48 in an angular f~chinn In other words, when the tine assembly 40 is plo~lly oriented on the p~s~ ic spinal disc nuclGuC body 10, each individual tine 42 p~jec~
away from the cor,sl,dinillg jacket 14 in a direction l~U vv~l with respect to the anterior end 16 and oulw~r~ with respect to the posterior end 18.
The tine assembly 40 is plGr~ldbly made of the same high molçcul~r weight, high tenacity polymeric material, such as polyethylene, as is used for the constraining jacket 14. By employing a material of this type, the 10 tine assembly 40, and therefore each individual tine 42, will have the desired strength and flexibility characteristics required for proper implantation of theprosthetic spinal disc nucleus body 10. Prior to and during implant, the tine 42material has sufficient flexibility to allow each tine 42 to fold down against the eYtern~l surface of the constraining jacket 14. When implanted, the tine 42 15 material has a reciliPncy which forces each tine 42 to assume the angular position shown in FIG. 10. In this exr~nded position, each tine 42 has a strength characteristic which will prevent the retropulsion of the prosthetic spinal disc nucleus body 10 from its final implantation position and provides a positive fixation within the anulus.
The tine assembly 40 has been described as preferably having individual tine bodies 42 extending from the frame 48. Each tine 42 is equally spaced from one another, providing uniform support to the prosthetic spinal discnucleus body 10 when placed within the anulus. However, any number or configuration of tines 42 can be used which also provide a solid fixation within25 the anulus and prevent retropulsion.
During manufacture, once the anterior closure 20 and the posterior closure 22 have been formed, the tine assembly 40 is ~tt~rhed to the prosthetic spinal disc nucleus body 10. The tine assembly 40 is slid over the -wo 96/11642 2 2 0 ~ 7 PCT/USg5/13172 posterior end 18 and seculo~ to the c~l.cl.^;.-;l-g jacket 14 by frictional or A1 rA~nin~ or sewing, which may include a hook and loop configuration, or adhesive.
An ~kiitic~n~l means for lcl~ling expulsion is the lJotr~ use 5 of tapered collars s~col~ ;ly Att~h~l to the collsL.~inillg jacket 14 by way of sewing or spin ent~n~l~mP-nt Such collars would collapse against the jacket 14 on insertion of the p-~sll~el;c spinal disc nUc~eus body 10 and flare on atL~ L~d removal or forceful expulsion from the anular confines.
By providing a small, pillow-shaped body having the distinct 10 ability to imbibe and expel fluids, the discal n~lcle~-s body of the present invention: a) Ic;SlOl~S the height of the ~1,^,m~i disc space, b) .Gslores and tightens the natural anulus to stop further degenG.dLion and permit its healing,c) lGsLo~es the normal load-unload cycling and thus flushes out toxic by-products, bringing in fresh nutrients to the nu~ s and anulus, d) allows a near 15 normal range of motion, e) relieves the movement-in~lced discogenic pain of the vertebral s~ , and f) allows the use of a minim~1, posterior surgical procedure that provides both cost and m~Aic~l benefits. The device of the present invention can be implanted with a high degree of celL~ 'Ly that the required ~lim~n~ions p.esenLed by the damaged disc space will be m~int7~in~d 20 following insertion of the discal nucleus device.
Although the present invention has been described with reference to p-~re--cd embodiments, workers skilled in the art will recognize that changesmay be made in form and detail without departing from the spirit and scope of the invention. For ~ le, other methods of sealing the ends of the 25 constraining jacket exist such as heat, ultr~cound, crimp ring seals or spin entanglement. Additionally, more than a single layer of material may be used to ..,~ ;n the inleglily of the hyd,ogel core. In other words, a plurality of constraining jackets can ~ul~Jund the hydrogel material which act in concert to WO 96/11642 2 ~ 7 6 ~ 7 PCTIUS95/13172~

allow fluids to be ill~bibed and e~pPlled while ~ ;n~ g the pillow shape of the l~ydl~cl core.
The hydrogel itself can have an outer "skin" formed by ion i..~plAnl~lion which causes outer layer polymerization and fimctiQn~ as the 5 consLIdining jacket or as an inte.~osed I-emb.~ c b~lween the gel mass and the jacket. ~ltern~tively, while the above-described eYI~An~ion and contraction of the hydr~gel core is achieved via the use of a constraining jacket, other means exist for limiting expansion and contr~tion in the width of the hydrogel core without the use of a se~dle consl,~ g jacket. For example, a truss can be 10 embedded along the sides of the hyd~ugel core. The truss is perpendicular to the width of the hydrogel core and effectively creates an anisotropic scenario in which the hyd~gel core is allowed to expand solely in height when imbibing fluids. Similarly, the hydrogel core will contract only in height when expellingfluids. Other tine or circumferential collar configurations exist which act to 15 prevent retropulsion of the discal body and can be located at any other position along the discal body, including the anterior end. Finally, the prosthetic spinal disc nucleus body can be used in all areas of the spine, and can be implanted in~nim~l~, such as in the disc space of a dog or the ankle of a horse.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A prosthetic spinal disc nucleus for implantation into a human disc space having a nucleus region defined by an anulus and adjacent vertebral end plates, the prosthetic spinal disc nucleus comprising:
a hydrogel core configured to expand from a dehydrated state to a hydrated state, wherein the hydrogel core is configured to have a predetermined shape in the hydrated state; and a flexible, substantially inelastic constraining jacket surrounding the hydrogel core, the constraining jacket having a volume less than a volume of the nucleus region and being configured to allow passage of fluid;
wherein following implant and hydration, the constraining jacket maintains the hydrogel core independent of the anulus.

2. The prosthetic spinal disc nucleus of claim 1, wherein following final implantation and hydration, the prosthetic spinal disc nucleus is subjected to placement and removal of loads, and further wherein the hydrogel core is configured to deform and to reform in response to the replacement and removal of loads, respectively.

3. The prosthetic spinal disc nucleus of claim 2, wherein the hydrogel core is configured to reform to the predetermined shape.

4. The prosthetic spinal disc nucleus of claim 1, wherein upon implant, the constraining jacket is configured to force the hydrogel core to expand to the predetermined shape.

5. The prosthetic spinal disc nucleus of claim 1, wherein the hydrogel core defines a height corresponding with a height of the nucleus region as defined by the adjacent vertebral end plates, and further wherein upon implant, the constraining jacket is configured to direct the hydrogel core to expand in height after reaching a horizontal limit of the constraining jacket.

6. The prosthetic spinal disc nucleus of claim 1, wherein the constraining jacket is sufficiently flexible to conform to a shape of the hydrogel core and sufficiently inelastic to constrain expansion of the hydrogel core independent of the anulus.

7. The prosthetic spinal disc nucleus of claim 6, wherein the hydrogel core defines a width and a height, and further wherein the constraining jacket is configured to allow the hydrogel core to decrease in height in response to placement of loads on the disc space and to assist in forcing the hydrogel core to increase height following removal of loads.

8. The prosthetic spinal disc nucleus of claim 1, wherein the hydrogel core has a water content of 25-65 percent when fully hydrated.

9. The prosthetic spinal disc nucleus of claim 1, wherein the hydrogel core is subjected to a plurality of compressive forces prior to implant to render the hydrogel core to the predetermined shape.

10. The prosthetic spinal disc nucleus of claim 9, wherein the hydrogel core has an initial volume following the plurality of compressive forces and an expanded volume when fluids are imbibed, and further wherein a fully expanded volume of the hydrogel core is less than twice the initial volume of the hydrogel core.

11. The prosthetic spinal disc nucleus of claim 1, wherein the hydrogel core defines a height and a width, and further wherein the hydrogel core and the constraining jacket cooperate such that as the hydrogel core imbibes fluids, the height changes at a greater rate than the width changes.

12. The prosthetic spinal disc nucleus of claim 1, wherein the constraining jacket is a woven tube.

13. The prosthetic spinal disc nucleus of claim 1, further including a radiopaque material disposed within the constraining jacket.

14. A prosthetic spinal disc nucleus for implantation into a human disc space having a nucleus region defined by an anulus and adjacent vertebral end plates, the prosthetic spinal disc nucleus comprising:
a hydrogel core configured to expand from a dehydrated state to a hydrated state, wherein the hydrogel core is subjected to a plurality of compressive cycles prior to implantation such that the hydrogel core is configured to return to a predetermined shape following removal of a load;
and a flexible, substantially inelastic constraining jacket surrounding the hydrogel core, the constraining jacket having a volume less than a volume of the nucleus region and being configured to allow passage of fluid.

15. The prosthetic spinal disc nucleus of claim 14, wherein following implant and hydration, the constraining jacket maintains the hydrogel core independent of the anulus.

16. A method of manufacturing a prosthetic spinal disc nucleus for implantation into a human disc having a nucleus region defined by an anulus and adjacent vertebral end plates, the method including:
forming a hydrogel core configured to expand from a dehydrated state to a hydrated state;
forming a flexible, substantially inelastic constraining jacket having a volume less than a volume of the nucleus region, the constraining jacket being configured to allow passage of fluid;
securing the hydrogel core within the constraining jacket; and subjecting the hydrogel core to a plurality of compressive loads prior to implant.