referring to fig1 through 6 , in the preferred embodiment the hydrogel prosthetic nucleus of the present invention , generally denoted as 10 , conforms when hydrated to its ewc , generally to the shape of the natural nucleus . alternately , the hydrogel can be constrained in a polymer jacket . such is taught in u . s . pat . nos . 5 , 674 , 295 and 6 , 132 , 465 . the prosthetic nucleus is implanted within cavity 11 the disc 12 of the vertebrae 14 and is surrounded by the natural annulus 16 . vertebral end plates 20 and 22 , as shown in fig1 , cover the superior and inferior faces of nucleus 10 respectively . the implant is inserted through an opening 62 in annulus 12 . referring to fig3 , there is shown the prosthetic nucleus of the present invention , including metal particles 30 dispersed uniformly throughout . uniformly is used in a relative sense not to indicate that the particles are exactly spaced within the hydrogel . referring to fig4 , there is shown the prosthetic disc nucleus of the present invention with a radiopaque foil strip 40 located therein . referring to fig5 , there is shown a polymeric implant of the present invention having a radiopaque or radiovisible coil 50 located therein . referring to fig6 , there is shown a prosthetic disc nucleus 10 of the present invention having radiopaque layers of metal particles or foil particles 60 located therein . while three layers are shown in fig6 , one layer or two layers or more than three layers may be utilized . as shown in fig6 , the three layers 60 having the radiopaque powder or particles extend across the entire distance between the inner walls of cavity 11 of annulus 16 of disc 12 . these layers are sandwiched between two layers having no radiopaque particles . this structure results from the method of manufacturing nucleus 10 as set forth in example 2 below . hydrogels useful in the practice of the invention include lightly cross - linked biocompatible homopolymers and copolymers of hydrophilic monomers such as 2 - hydroxylalkyl acrylates and methacrylates , e . g ., 2 - hydroxyethyl methacrylate ( hema ); n - vinyl monomers , for example , n - vinyl - 2 - pyyrolidone ( n - vp ); ethylenically unsaturated acids , for example , methacrylic acid ( ma ) and ethylenically unsaturated bases such as 2 -( diethylamino ) ethyl methacrylate ( deaema ). the copolymers may further include residues from non - hydrophilic monomers such as alkyl methacrylates , for example , methyl methacrylate ( mma ), and the like . the cross - linked polymers are formed , by known methods , in the presence of cross - linking agents , such as ethyleneglycol dimethacrylate and methylenebis ( acrylamide ), and initiators such as 2 , 2 - azobis ( isobutyronitrile ), benzoyl peroxide , and the like , and radiation such as uv and γ - ray . methods for the preparation of these polymers and copolymers is well known to the art . the ewc of these hydrogels can vary , e . g ., from about 38 % for polymacon ™ ( poly hema ) to about 79 % for lidofilcon ™ b ( a copolymer of n - vp and mma ) under ambient conditions . another type of hydrogel , useful in the practice of the invention , is illustrated by hypan ™ and poly ( vinyl alcohol ) ( pva ) hydrogels . these hydrogels , unlike the aforementioned hydrogels , are not cross - linked . their insolubility in aqueous media is due to their partially crystalline structures . hypan ™ is a partially hydrolyzed polyacrylonitrile . it has a multiblock copolymer ( mbc ) structure comprising hard crystalline nitrile blocks , which provide the hydrogel with good mechanical properties , and soft amorphous hydrophilic blocks to provide the hydrogel with good water binding capability . the methods of preparing hypan ™ hydrogels of different water contents and mechanical properties have been disclosed in the u . s . pat . nos . 4 , 337 , 327 , 4 , 370 , 451 , 4 , 331 , 783 , 4 , 369 , 294 , 4 , 420 , 589 , 4 , 379 , 874 and 4 , 631 , 188 . the pre - nuclear forms of this material , for use in this invention , can be prepared by melt processing using solvents such as dmf and dmso , as melting aids or by solution processing . other types of polymers useful in the practice of the invention include medical grade polyurethanes and materials formed by crosslinking protein precursors . these materials may or may not form hydrogels in their final form but are still useful as materials to form prosthetic nucleus replacements . such materials are shown in u . s . pat . nos . 5 , 888 , 220 , 6 , 189 , 048 , 6 , 183 , 518 and in publication u . s . 20020049498 a1 , the teachings of which are incorporated herein by reference . a preferred hydrogel for use in the practice of this invention is highly hydrolyzed crystalline poly ( vinyl alcohol ) ( pva ). the amount of hydrolyzation may be between 95 and 100 percent depending on the desired ewc which will be from about 60 % to about 90 %. generally , the final hydrogel water content increases with decreasing hydrolyzation of the initial pva which results in decreased crystallinity . partially crystalline pva hydrogels may be prepared , from commercially available pva powders , by any of the methods disclosed in the u . s . pat . no . 4 , 663 , 358 , the teachings of which are incorporated herein by reference . typically , 10 - 15 % pva powder is mixed with a solvent , such as water , dimethyl sulfoxide ( dmso ), ethylene glycol and mixtures thereof . a preferred solvent is 15 % water in dmso . the mixture is then heated at a temperature of about 100 to about 120 ° c ., until a viscous solution is formed . the solution is then poured or injected into a tubular metal , glass or plastic mold and allowed to cool to below − 10 ° c ., preferably about − 20 ° c . the solution is maintained at the temperature for several hours , preferably about 20 hours , during which time crystallization and , therefore , gelation of the pva occurs . the shaped gel is soaked with several portions of water which are periodically replaced , over a period of at least two days , until all the organic solvent in the gel has been replaced by water . the hydrated gel can then be partially or completely dehydrated for implantation . the hydrogels thus prepared have ewc &# 39 ; s between 60 - 90 % and compressive strengths of at least 1 mnm − 2 , preferably about 4 mnm − 2 , when subject to the same constraints as the natural nucleus in an intervertebral disc . in general , any polymer that can be used for biomedical purposes can be used as long as the polymer exhibits the desired stiffness characteristics . completion of the solvent exchange is determined by known methods . for instance , when the solvent is dmso its removal , from the gel , is determined as follows : 50 μl of a 0 . 01 n kmno 4 are added to 50 ml aliquots of the water which has been separated from the gels . the presence of dmso , in the water , will be indicated by disappearance of the characteristic pink color of the kmno 4 . when the dmso has been completely removed , the pink color will not disappear . this method has a detection limit of 0 . 3 ppm , for dmso , when compared to a blank and 0 . 3 ppm aqueous dmso standard . in general , any hydrogel that can be used for biomedical purposes can be used as long as the hydrogel exhibits an ewc from about 30 to 90 % and a compressive strength of at least about 1 mnm − 2 , preferably 4 mnm − 2 , when subjected to the constraints of the annulus and end plates of the disc . a rod or tube made from these materials , in a dehydrated form , i . e ., as xerogels , can be prepared either by cast molding or lathe cutting . in cast molding , the liquid monomer mixture , with initiator , is poured into a mold of predetermined shape and size , and cured . if desired , the casting mixture may include water , or another aqueous medium . under those circumstances the resultant rod or tube will be partially hydrated , i . e ., a hydrogel . in the case of lathe cutting , the xerogel can be prepared , in a similar manner to the above , in the form of a block or rod which is larger than needed to form the prosthetic nucleus . the xerogel is then cut to the shape and size required for implantation into the disc cavity . in both cases , the hydrogel expansion factor , due to polymer swelling upon hydration , has to be taken into account in designing the mold or in cutting the block , rod or tube . the exact size of the prosthetic nucleus , at its ewc , can be varied for different individuals . a typical size of an adult nucleus is about 2 cm in the semi - minor axis , about 4 cm in the semi - major axis and about 1 . 2 cm in thickness . polymers curable within the body can also be used to replace the natural nucleus and strengthen the annulus which is made of cartilage . natural cartilage is a non - vascular structure found in various parts of the body . articular cartilage tends to exist as a finely granular matrix forming a thick incrustation on the surfaces of joints . the natural elasticity of articular cartilage enables it to break the force of concussions , while its smoothness affords ease and freedom of movement . preferred biomaterials , therefore , are intended to mimic many of the physical - chemical characteristics of natural cartilage . biomaterials can be provided as one component systems , or as two or more component systems that can be mixed prior to or during delivery , or at the site of repair . generally such biomaterials are flowable in their uncured form , meaning they are of sufficient viscosity to allow their delivery through a cannula of on the order of about 2 mm to about 6 mm inner diameter , and preferably of about 3 mm to about 5 mm inner diameter . such biomaterials are also curable , meaning that they can be cured or otherwise modified , in situ , at the tissue site , in order to undergo a phase or chemical change sufficient to retain a desired position and configuration . when cured , preferred materials can be homogenous ( i . e ., providing the same chemical - physical parameters throughout ), or they can be heterogenous . an example of a heterogenous biomaterial for use as a disc replacement is a biomaterial that mimics the natural disc by providing a more rigid outer envelope ( akin to the annulus ) and a more liquid interior core ( akin to the nucleus ). in an alternative embodiment , biomaterials can be used that provide implants having varying regions of varying or different physical - chemical properties . with disc replacement , for instance , biomaterials can be used to provide a more rigid , annulus - like outer region , and a more fluid , nucleus - like core . such di - or higher phasic cured materials can be prepared by the use of a single biomaterial , e . g ., one that undergoes varying states of cure , or by using a plurality of biomaterials . examples of suitable biomaterials includes , but are not limited to , polyurethane polymers . the in situ cured polymer may comprise a thermosetting polyurethane polymer based on a suitable combination of isocyanates , long chain polyols and short chain ( low molecular weight ) extenders and / or crosslinkers . suitable components are commercially available and are each preferably used in the highest possible grade , e . g ., reagent or preferably analytical grade or higher . examples of suitable isocyanates include 4 , 4 ′- diphenyl methane diisocyanate (“ mdi ”), and 4 , 2 ′- diphenylmethane diisocyanate (“ tdi ”). examples of suitable long chain polyols include tetrahydrofuran polymers such as poly ( tetramethylene oxide ) (“ ptmo ”). particularly preferred are combinations of ptmo &# 39 ; s having molecular weights of 250 and 1000 , in ratios of between about 1 to 1 and about 1 to 3 parts , respectively . examples of suitable extenders / crosslinkers include 1 , 4 - butanediol and trimethylol propane , and blends thereof , preferably used at a ratio of between about 1 to 1 and about 1 to 7 parts , respectively . such performance can be evaluated using procedures commonly accepted for the evaluation of natural tissue and joints , as well as the elevation of biomaterials . in particular , the in situ cured polymer forms , exhibit mechanical properties that approximate those of the natural tissue that they are intended to replace . for instance , for load bearing applications , preferred cured composites exhibit a load bearing strength of between about 50 and about 200 psi ( pounds per square inch ), and preferably between about 100 and about 150 psi . such composites also exhibit a shear stress of between about 10 and 100 psi , and preferably between about 30 and 50 psi , as such units are typically determined in the evaluation of natural tissue and joints . biomaterials provided as a plurality of components , e . g ., a two - part polyurethane system , can be mixed with the radiopaque or radiovisible metal powder at the time of use using suitable mixing techniques , such as those commonly used for the delivery of two - parts adhesive formulations . more preferably , the metal powder can be added during melt processing of the polymer . a suitable mixing device involves , for instance , a static mixer having a hollow tube having a segmented , helical vein running through its lumen . a two - part polyurethane system can be mixed by forcing the respective components through a lumen , under pressure . the hydrogels and polymers of the present invention have a much higher structural integrity than the natural nucleus , i . e ., they are deformed with greater difficulty under a mechanical compressive load ( shaped gel vs . loose gel ). that is because , unlike the loose gel of the natural nucleus , the shaped gel has shape memory due to the cross - linking or strong hydrogen bonding in the polymer matrix . however , it would still have extensive lateral bulging under high compressive load if there were no boundaries to constrain the deformation . because use of the prevent invention does not involve removal of the disc annulus and / or end - plates , the lateral bulging of the hydrogel nucleus will be restricted by the restoring forces of the stretch fibers . also , due to its superior structural integrity , the hydrogel nucleus will not herniate or bulge through the previously herniated areas or the incision which was made to remove the degenerated nucleus . if a hydrogel is used , since the natural nucleus is also primarily , a hydrogel , the implanted prosthetic nucleus can easily restore all the biomechanical functions of the nucleus which had been removed . unlike the prior art prosthetic discs , the hydrogel nucleus of the present invention will restore the viscoelastic behavior of the disc due to the water binding capability of the prosthetic hydrogel . the implantation of a prosthetic nucleus 10 can be performed in conjunction with a discectomy or chemonuclealysis . because the properties of the prosthetic nucleus of the present invention are similar to those of the nucleus material , the herniated nucleus can be partially or totally replaced by the hydrogel prosthetic nucleus . due to the small size of the prosthetic it can be implanted into the disc by means of a posterior lateral approach , thereby significantly reducing the difficulty and the risk of the operation . the volume of a hydrogel nucleus of about 80 % ewc will be reduced by about 80 % ( to about 20 % of its original volume ) when dehydrated . consequently , the surgeon does not need to jack apart the vertebrae adjacent to a damaged disc as required by , for example , the device disclosed in u . s . pat . no . 4 , 772 , 287 . the height of the dehydrated prosthetic nucleus , when inserted , is smaller than the disc space . furthermore , the rigidity of the dehydrated prosthetic nucleus will help the surgeons to manipulate the prosthetic nucleus during the operation . after implantation , the hydrogel nucleus of the present invention swells in the body to a predetermined height which is enough to maintain the space between the vertebral body . the swelling process normally takes several hours to two days depending on the size of the prosthetic nucleus and type of hydrogel . the preferred method for making the radiopaque hydrogel or polymer involves incorporating a metallic element into the structure of the implant . the metallic element is in a form that will allow it move with the polymeric structure as the implant changes dimensions and / or geometry . this property is important because it minimizes any internal stress amplification that could be caused by incorporating the metallic component into the dynamic ( i . e . non - fusion ) spine implant . three embodiments of the invention are described . the first embodiment involves incorporating radiopaque materials such as a metal powder , for example , gold or tungsten , into the polymer while it is a liquid either due to the use of solvents , heat if the polymer can be processed in the melt , or is in a pre - polymer form prior to a curing step . the metal powder has a nominal diameter of 10 - 100 μm , with a preferred maximum size of 75 μm . the powder is incorporated into the liquid polymer solution / melt preferably in a concentration between 0 . 02 and 0 . 5 g per cc of polymer , with a preferred concentration of 0 . 1 g / cc . the powder may be evenly dispersed throughout the entire implant , or incorporated into the implant in specific areas only . for example , by combining the use of liquid - phase polymer that contains no metal powder in a mold with liquid polymer that does contain metal powder it is possible to form radiovisible areas of an implant in a variety of geometries ( e . g . lines , discs , spheres ). an embodiment of the layered polymeric implant has one or more planes in the implant comprised of the radiovisible polymer . it is possible to incorporate 1 to 5 or more bands of powder - filled polymer in any plane across an implant that also has polymer regions where no metal powder exits using techniques that prevent the powder from migrating until the liquid - phase polymer has formed a solid , as discussed below or creating an even dispersion of metal powder throughout the entire implant . a second embodiment involves the use of a metal foil 0 . 001 to 0 . 1 mm thick for example gold , tantalum or tungsten foil . the foil can be placed into the liquid polymer in the form of sheets or strips , or it can be chopped into small pieces and incorporated into the implant in the manner described for metal powders . small pieces of foil may provide an advantage for some polymer systems with a lower viscosity in the liquid phase because each piece may have less mass than a metal particle , which would result in less tendency for migration through the liquid - phase polymer . in addition , the geometry of a piece of foil with multiple irregular folds may also have less tendency to migrate through liquid - phase polymer than , for example , a smoother spherical particle . examples include suspending 1 to 5 or more strips of metal foil in any plane in the implant , or incorporating 1 to 5 or more bands of polymer that contain small pieces of chopped foil that may or may not be “ crinkled ” as described above for powder , and creating an even dispersion of pieces of chopped metal foil that may or may not be “ crinkled ” throughout the implant . a third embodiment involves the use of a metal wire or coil , preferably 0 . 01 to 1 . 0 mm in length , for example , gold wire , tungsten wire or platinum wire . the metal wire is suspended in the liquid - phase polymer in such a manner that the wire will be completely encapsulated by solid polymer at the end of the manufacturing process . alternatively , the wire can be formed into a coil or other shape that may provide better radiographic information about the implant and have less of an ability to migrate through the solid polymer . examples include suspending 1 to 5 or more pieces of wire in the liquid - phase polymer in such a way as they will not be exposed to the surface of the implant , or incorporating 1 to 5 or more bands of polymer that contain small pieces of chopped wire or creating an even dispersion of pieces of chopped metal wire throughout the implant using techniques as described below . a pva solution was formed by mixing 15 g of pva powder ( kuraray 117 or equivalent ), having a molecular weight about 78000 and about 99 . 7 % hydrolysed ( cat . no . 15129 , polysciences inc ., warrington , pa . ), with 85 ml of a solvent comprising 15 % water in dmso . the mixture was heated at about 110 ° c . until a homogenous viscous solution formed . 0 . 1 gram of gold powder ( maximum diameter 75 μm ) per cc of liquid - phase of pva solution of example i were mixed . the two ingredients were combined in the following manner to create a metal - filled polymer solution . a plunger from a 5 cc first syringe was removed and the first syringe was slowly filled half way with pva solution , 0 . 5 g of the gold powder was poured into the syringe . the first syringe was then completely filled with pva solution , and the plunger replaced . a two - way luer connector was screwed onto the tip of the first syringe and the connector was primed with pva solution from the syringe . the first syringe and a second syringe of equal size were connected using the connector . the gold powder solution from the first syringe was squeezed into the empty syringe . this was repeated until the solution was uniformly mixed . a third syringe was filled with pva solution without any gold . 5 cc of the pva solution of the third syringe was injected into a nucleus mold having a total volume of about 20 cc and a diameter of about 1 . 5 cm . the mold was cooled down in freezer ( 4 ° c .) for about 20 minutes . 1 cc of gold powder - filled pva solution was injected on top of the cooled solution in the mold . 9 cc of the pva solution was slowly injected into the mold , on top of the gold powder solution . the mold was again cooled down in the freezer for about 15 minutes . about 5 cc of gold powder - filled solution was again injected on top of the cooled solution in the mold to form a second radiopaque layer . the mold was then completely filled with the pva solution from the third syringe . about 20 cc of the gold solution was prepared as described in example ii using two 20 cc syringes and 2 grams of gold powder . the metal filled polymer solution was slowly injected into a 20 cc implant mold ( for a # 5 size implant ), filling the mold completely . the mold reservoir was capped so that the metal - filled pva solution will not leak out of the mold if the mold is inverted . the mold was pressurized and placed in a turbula mixer which was placed into a programmable freezer . the mixer used must be able to both rotate and tip the mold during gelation of the metal filled pva solution to keep the powder in the solution uniformly distributed . the mixer can rotate the mold about a central axis , tilt the mold back and forth through a 90 ° arc about an axis perpendicular to the central axis . this kept the metal particles uniformly suspended in the solution until it gelled . the pva solution prepared for the third syringe of example ii without the gold powder was used in this example . a strip of gold metal foil was suspended in an empty mold using a thin monofilament of nylon so that the strip is positioned in approximately the center of the mold . the nylon filament was smooth and non - porous . the mold was slowly filled with the pva solution . the mold was then placed in the freezer and after completing the process , the monofilament was pulled out of the implant leaving the strip of metal foil intact . the gold foil of example iv was cut into small pieces using a food processor or other convenient method to create metal flakes . 0 . 03 g of metal flakes per cc of the liquid phase polymer of example i . 20 cc of solution were placed in a mold and the solution was then frozen as above to form the implant . a gold coil was suspended in an empty 20 cc mold using a thin monofilament so that the coil is positioned in approximately the center of the mold . the filament was smooth and non - porous . a barb fitting through a small loop at the top of the metal coil is a convenient way to attach the filament and the coil . slowly fill the molds with the pva solution of example i . after completing the gelation process by freezing , the monofilament was pulled out of the implant leaving the gold coil intact . after gelation , the implants were sterilized as described in my ion treated hydrogel copending application , u . s . ser . no . 10 / 020 , 389 and then packaged . the implants are formed into 10 different sizes by volume with a size range of 1 . 1 to 5 . 2 cc for future implantation . although the invention herein has been described with reference to particular embodiments , it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention . it is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims .