it has been discovered that a prosthetic intervertebral disc fabricated from biocompatible and bioresorbable fibers can be surgically implanted into the intervertebral space so as to provide normal joint motion and strength . this prosthetic intervertebral disc also acts as a scaffold for regenerating disc tissue whose ingrowth is encouraged by the physical characteristics of the implanted device . following implantation , tissue ingrowth , regeneration , and finally resorption of the scaffold , natural intervertebratal tissue remains . fig1 shows the normal positioning of an intervertebral disc 100 in the human intervertebral space 110 between the vertebral bodies 120 and 130 . an exemplary prosthetic intervertebral disc 200 is shown in fig2 . the disc 200 is a generally porous , dry volume matrix which extends circumferentially in about a central axis 10 . as used herein , the term &# 34 ; volume matrix &# 34 ; refers to a porous array characterized by relatively comparable ( but not necessarily equal ) outer dimensions in three orthogonal directions ( as contrasted with a sheet matrix which would have relatively comparable dimensions in two orthogonal directions but relatively small dimensions in a third orthogonal direction ). in the preferred form , prior to implantation , the prosthetic intervertebral disc 200 has the shape of a cylindrical pad , extending circumferentially about the axis 10 , and comprising a relatively high compliance central region 12 disposed about a relatively low compliance peripheral region 14 . in fig2 the separation of regions 14 and 16 is indicated generally by broken line 17 , although the transition is normally gradual . in the preferred form , the top and bottom ( as shown ) surfaces of disc 200 are concave so that disc 200 has maximum height a at its peripheral edge of approximately 8 mm and a maximum radial dimension c of approximately 35 mm . fig3 shows a sectional view along line 3 -- 3 of the prosthetic disc 200 shown in fig2 . fig4 shows an additional embodiment 220 of the present invention which is similar in composition to the prosthetic disc 200 depicted in fig2 . the prosthetic intervertebral disc 220 is similar to disc 200 , but includes convex top and bottom surfaces and further includes a mesh member 20 extending from its lateral surface . the mesh member 20 is composed of a biocompatible , bioresorbable material . following implantation , the mesh member 20 may be sutured to adjacent tissue to anchor the disc 220 in place . the mesh member 20 may function in this capacity until sufficient tissue ingrowth occurs to provide that function . since the anchor function of mesh member 20 is only temporary , the mesh member 20 may be a # 1 mesh screen composed of absorbable suture materials such as polyglyconate , dexon , or polydioxane ( pds ) woven into a mesh . alternatively , nonabsorbable suture materials such as expanded polytetrafluoroethylene ( ptfe ) may be used . fig5 a and 5b show yet another embodiment 230 which is similar to that of fig1 but having concave top and flat bottom surfaces . other combinations might also be used . in alternative forms of the invention , still other shapes than full cylinders may be used . for example , it is not required that the full 360 ° ( about axis 10 ) pad be used if partial disc replacement is undertaken . for angular segment type discs , the cylindrical form may subtend any angle between zero and 360 degrees about axis 10 . it is however important that the matrix have characteristics so that when implanted , at least the top and bottom surfaces substantially assume the shape or contour of a natural intervertebral disc . the various embodiments of the invention may have certain densities of collagen fibers and dispersions of gag molecules and cross - links that permit accommodation of differing stress levels , rates of ingrowth , and resiliency . differing densities may be obtained in vivo where a device having uniform density is implanted , and body loading causes non - uniform compression of the device . alternatively , the prosthetic disc may be initially configured with non - uniform construction of a type so that the in vivo configuration provides the desired spatial densities and dispersions necessary for the desired function . the prosthetic intervertebral disc may be fabricated of any biocompatible , bioresorbable fibers such as a natural material , an analog thereof or a synthetic material . the fibers are preferably polymeric in structure so that they can provide mechanical strength , protection , and lubrication while encouraging tissue ingrowth . such polymeric fibers include , for example , collagen , reticulin , elastin , cellulose , and biosynthetic analogs thereof . these fibers may be ordered in substantially circumferentially - extending or substantially radially - extending orientations , with the density of fibers being substantially uniform throughout the matrix . alternatively , the matrix fibers may be unordered . in either the ordered or unordered configuration , the density of the fibers may be non - uniform . in the non - uniform configuration , relatively high densities of fibers may be established at anticipated points of high stress . in an alternative aspect of the invention , the intrafibrillary ( i . e ., the space within the fiber ) and interfibrillary ( the space between the fibers ) space is relatively high , a condition which promotes ingrowth of regenerated disc tissue . for example , the density of the intervertebral disc may be in the range of about 10 - 25 g matrix / cm 3 alternatively , the intrafibrillary and interfibrillary space may be relatively low , a condition which provides superior cushioning , lubrication , and mechanical support for the intervertebral space , and which retards tissue and cell ingrowth , thereby diminishing the rate of scaffold resorption ( e . g ., density is in the range of about 2 - 10 g matrix / cm 3 ). the temporary stability of the shape of the structure when in vivo , and the rate of disc resorption , are both attributed to the effective cross - link formation between at least one portion of the fibers . the cross - linking reagents used with the above - noted fiber materials may be any biocompatible , bifunctional reagents which interacts with amino , carboxyl , or hydroxyl groups on a single fiber forming intramolecular cross - links , or on multiple fibers or on the fibers and the gags , resulting in covalent bond formation between adjacent molecules ( intermolecular cross - links ). useful cross - linking reagents include aldehydes , hexamethylene diisocyanate , bisimidates , polyglycerol polyglycidyl ether , acyl azide , and carbodiimides . the cross - linked device maintains a sufficient degree of hydrophilicity and elasticity which simulates the properties of the natural intervertebral disc , i . e ., ability to sustain mechanical stress and to protect and lubricate articular surfaces . in addition , the structure provides an ideal environment for cell infiltration and extracellular matrix synthesis and deposition , resulting in regeneration of natural disc tissue . gags may be dispersed throughout the fibers . alternatively , they may act as intermolecular cross - links between fibers . these gags typically include at least one of the group of molecules consisting of chondroitin 4 - sulfate , chondroitin 6 - sulfat , keratin sulfate , dermatan sulfate , heparan sulfate , heparin , and hyaluronic acid . the dispersion of gag cross - links is preferably uniform , but may be more concentrated at anticipated points of high stress , typically at the peripheral region 14 , and less concentrated in the central region 12 ( fig2 ). in such configurations , the gag concentration may be in the range of about 0 - 25 % in the distal region 14 , and in the range of about 0 - 10 % in the central region 12 . however , when uniform , the dispersion of gags throughout the prosthetic intervertebral disc may be , for example , in the range of about 1 - 15 %. intermolecular cross - links can also be established through a dehydrothermal process ( heat and vacuum ) which results in peptide bond formation between an epsilon amino group of lysine or hydroxylysine and a carboxyl group of aspartic or glutamic acid . the cross - linked disc has a relatively high thermal stability at between about 55 °- 85 ° c ., and preferably at between about 65 °- 75 ° c . for sufficient in vivo stability . this may be achieved through manipulation of the cross - linking conditions , including reagent concentration , temperature , ph , and time ( see example 1 ). in a one embodiment the prosthetic intervertebral disc is constructed mainly of type i collagen fibers without gag cross - links . type i collagen fibers may be obtained from the achilles tendons of animals . however , the fibers may also be obtained from animal skin or from the skin or tendon of humans . the tissues are treated with a series of mechanical and chemical means to either totally remove the non - collagenous materials or reduce them to a minimal level . in the preferred processing steps , the tendon or skin is mechanically disintegrated into fine pieces useful for further processing . the disintegration may be achieved by grinding the tissue at liquid nitrogen temperature , or by cutting the tissue into small pieces with a sharp knife . in certain applications , the tendons are mechanically disintegrated along the fiber direction in order to maintain the length of the fibers for mechanical strength . salt extraction of tendon at neutral ph removes a small portion of the collagen molecules that are newly synthesized and have not yet been incorporated into the stable fibrils . salt also removes some glycoproteins and proteoglycans that are associated with collagen through electrostatic interactions . other salts such as kcl can be used as a substitute for nacl . lipids that are associated with the cell membranes or collagenous matrices may be removed by first extracting with detergents such as triton x - 100 ( sigma chemical co ., st . louis , mo . ), followed by extracting with ether - ethanol mixtures . the concentration of triton x - 100 is usually about 2 - 4 %, but is preferably about 3 %. the preferred mixture of ether - ethanol is usually at about a 1 : 1 ratio ( v / v ). the period of extraction is usually from 8 hours to 96 hours , and is preferably from about 24 to 48 hours . further extraction may be accomplished by matrix swelling conducted at two extreme phs . both acidic and basic swelling weakens the non - covalent intermolecular interactions , thus facilitating the release of non - covalently attached glycoproteins , gags , and other non - collagenous molecules through the open pores of the collagenous matrices . the swelling of the collagenous matrix at alkaline ph is performed by treating the collagen at high ph with ca ( oh ) 2 , naoh , or the like , for a period of about 8 - 96 hours . alkali extraction in the presence of triple - helical stabilizing salts such as ( ch 3 ) ncl or nh 3 so 4 reduces the potential risk of denaturation of the collagen . alkali treatment dissociates the non - cross - linked glycoproteins and gags from the collagen matrices . the alkali also removes the residual lipids through saponification . acid swelling may be conducted at a low ph in the presence of acetic acid , hcl , or similar acids . like the alkali treatment , the swelling removes non - cross - linked glycoproteins and gags . the non - triple helical portions of the molecule ( telopeptides ) are involved in intermolecular cross - linking formation . they are weak antigens and are susceptible to attack by proteases such as pepsin and trypsin . prolonged digestion with such proteases dissociates the fibrils ( fibers ) into individual molecules . however , if the digestion process is properly controlled such that maximal telopeptides are removed without complete dissociation , the immunogenic properties of the fibrils can be reduced to a minimal level without compromising the mechanical strength . for example , to isolate molecular collagen , the digestion of skin or tendon with pepsin is usually conducted at an enzyme : collagen ratio of about 1 : 10 for about 24 - 96 hours at below room temperature . in comparison , fibrils may be obtained by limited pepsin digestion achieved at a ratio of about 1 : 100 ( enzyme : collagen ) for about 24 - 96 hours at 4 ° c . collagen fibers obtained according to this methodiology are then used to fabricate the prosthetic intervertebral disc of the present invention . however , it must be appreciated that collagen obtained from other sources , such as biosyntheticallyproduced collagen or analogs thereof , may also be used in the construction of the prosthetic intervertebral disc . one method of fabrication includes molding the collagen fibers into a predetermined shape using , for example , the mold forms described below in conjunction with fig6 and 7 . the fibers may be placed randomly in the mold , or may be oriented in specific directions to achieve a intervertebral disc having specific structure characteristics . other components such as gags which may participate in the cross - linking reaction , can be mixed in with the fibers in a random or non - random fashion before the structure is subjected to various cross - linking procedures including chemical methods and / or dehydrothermal methods . by following the processes described in the examples set forth hereinbelow , a prosthetic intervertebral disc of the form shown in fig2 or 3 may be constructed having the characteristics listed below in table 1 . the prosthetic discs were evaluated in vivo and in vitro to determine ability to function physically , or to serve as a regeneration template for the fibrochondrocytes expected to serve as precursor cells for the subsequent fibrocartilaginous matrix . these studies demonstrate that the prosthetic disc allows for , and induces fibrochondrocyte infiltration and disc regeneration through the prosthetic material . the following non - limiting examples describe methods of fabrication and in vivo use of the prosthetic intervertebral disc of the present invention . a mold useful for fabricating the prosthetic intervertebral disc is made of implantable stainless steel or biocompatible plastics such as polypropylene , delrin , or combination of these materials . exemplary molds 300 and 300 &# 39 ; are composed of three pieces 302 , 304 , and 306 as shown in fig6 and 7 . by way of example for the disc - shaped intervertebral disc illustrated in fig5 a and 5b , the mold 300 of fig6 is used . piece 302 is disc - like and has a diameter substantially equal to that of the desired intervertebral disc . piece 302 is perforated to allow liquid to pass through under pressure . the inner surface 303 of piece 302 has the desired shape of one side of the intervertebral disc - to - be - formed . piece 304 is a hollow cylinder which has the same inner dimension as piece 302 . piece 306 is a cylindrical piston which has an outer diameter slightly less than the inner diameter of piece 304 . the &# 34 ; top &# 34 ;, or crown , surface 308 of piston 306 has the desired shape of one side of the intervertebral disc - to - be - formed . for an intervertebral disc having flat top and bottom surfaces , the mold 500 of fig7 is used where pieces 502 and 504 are the same as pieces 302 and 304 in fig6 and piece 506 is similar to piece 306 in fig6 but has a flat crown surface 508 . during fabrication of the prosthetic disc 230 , mold piece 302 is first assembled within piece 304 , as shown in fig6 . the constituent fibers ( in a fluid ) are placed against the surface 303 of piece 302 . the crown surface 308 of piston 306 is then driven toward surface 303 along a compression axis until the fibers are compressed , the fluid is driven out through piece 302 , and the desired axial dimension of the compressed fiber array is attained . the mold is then frozen in preparation for cross - linking . bovine , porcine , or sheep achilles tendon is obtained from usda - approved slaughter houses . the preferred age of the animals is between 12 - 18 months . the tissue is kept cold during the purification process except where specified to minimize bacteria contamination and tissue degradation . the adhering tissues of carefully selected tendons are first scrapped off mechanically . the tendons are then minced or cut into fine pieces and washed in excess quantities ( about 10 volumes ) of cold water to remove residual blood proteins and water soluble materials . the washed tendons are extracted in ten volumes of 5 % nacl , 0 . 01 m tris , ph 7 . 4 , for 24 (+/- 4 ) hours to remove salt soluble materials . the salt - extracted tendons are repeatedly washed in about 10 volumes of water to remove the salt . to extract lipid , the material is extracted in 3 % triton x - 100 for 24 (+/- 2 ) hours . the detergent is removed by extensive washing with water . the material is then extracted in 3 - 4 volumes of ether - ethanol ( 1 : 1 vol / vol ) for 24 (+/- 2 ) hours to further minimize the lipid content . the lipid extracted material is extensively washed in water to remove the ether and ethanol . the material is then subjected to two extreme ph extractions to remove non - collagenous materials . alkaline extraction is conducted with 3 - 4 volumes of 0 . 2 m naoh at ph 12 . 5 - 13 . 5 at room temperature in the presence of 1 . 0 m ( ch 3 ) nc1 for 24 (+/- 2 ) hours with mild agitation . following alkaline extraction , the ph is neutralized with hcl , and the material is washed with water . the ph is then adjusted to 2 . 5 - 3 . 0 by adding concentrated acetic acid to a final concentration of 0 . 5 m . the acid extraction is continued for 24 (+/- 2 ) hours with agitation . the acid swollen tendon is then subjected to a limited proteolytic digestion with pepsin ( enzyme : collagen = 1 : 100 ) for 24 (+/-) 2 hours . the pepsin and resulting telopeptides are removed through dialysis . the swollen fibrillar material is then coacervated by adjusting the ph to its isoionic point with 1 m naoh or hcl or by adjusting the ionic strength to 0 . 7 with nacl . the aggregated collagen fibers are harvested by filtration , and the filtered material extensively washed with cold phosphate buffered saline solution . the highly purified type i collagen may be stored at - 20 ° to -&# 39 ;° c . until used . a ) the collagen content of the highly purified type i collagen fibrils from example 2 is determined either by gravimetric methods or by determining the hydroxyproline content assuming a 13 . 5 % by weight of hydroxyproline in type i collagen the amount of purified material needed to fabricate a given density of a prosthetic intervertebral disc device is then determined and weighed out . b ) a solution of fibrillar collagen is carefully fit into a mold of desired , specified dimensions ( see example i and fig6 for a description of molds ). collagen fibers are laid down in random manner or in an oriented manner . in the oriented manner , circumferential orientation of the fibers is produced by rotation of the piston about its principal axis as the material is compressed in the mold ; radial orientation is produced by manual painting of the collagen fibers in a linear , radially directed fashion . c ) the fibers are frozen at - 20 ° c ., turned out of the mold , and thawed at room temperature . d ) the fibers are then resuspended in phosphate buffered saline , put back into the mold in the desired orientation ( s ), and compressed with the piston . e ) the compressed fibers are then refrozen at - 20 ° c . and then thawed at room temperature . f ) the resulting structure is cross - linked by soaking in a 0 . 2 % glutaraldehyde solution , ph 7 . 6 , for 24 (+/- 0 . 5 ) hours . each glutaraldehydecross - linked prosthetic disc is subsequently rinsed repeatedly in 500 ml of phosphate buffered saline ( pbs ) solution ph 7 . 4 , for 4 , 8 , 24 and 48 hours . f ) the structure is immersed in an aqueous solution of 0 . 5 m sodium nitrite , 0 . 3 m hcl , and nacl ( om , 0 . 34 m , 1 . 0 m , or 1 . 34 m ) for 3 minutes at 4 ° c . h ) the lyophilized matrix is subjected to dehydrothermal cross - linking by vacuum and heat . the vacuum is first applied to reduce the residual water content to a minimal level . some structural water ( about 3 %) may still be associated with collagen triple - helix as part of the structure stabilizing factor . the heat is increasing in steps to 110 ° c . (+/- 5 ° ), and continually applied at 110 ° c . under vacuum for 24 (+/- 2 ) hours . b ) the collagen material is dispersed in 0 . 01 m hcl at ph 2 . 0 - 2 . 5 . predetermined amounts of various gags are weighed and dissolved in water . for example , for a given density of 0 . 25 g / cm 2 , the collagen content will be 0 . 244 g , the hyaluronic acid content will be 0 . 003 g , and the chondroitin sulfate content will be 0 . 003 g for a 2 . 5 % gag content . the gag solution is mixed in with the collagen solution and placed in the mold in the desired orientation as described in example 2 . d ) ( same as in example 3 except that the fibers laid down are not compressed . f ) the molded collagen is cross - linked in 5 % polyglycerol polyglycidyl ether in 50 % ethanol and 0 . 1 m na 2 co 3 at ph 10 . 0 for 24 (+/- 2 ) hours . the cross - linked device is rinsed for 4 , 8 , 24 and 48 hours , each with 500 ml of pbs , ph 7 . 4 . f ) the molded collagen is cross - linked in presence of 1 - ethyl - 3 -( 3 - dimethylaminopropyl ) carbodiimide ( 10 mg / g matrix ) in 0 . 9 % nacl , ph 4 . 7 at room temperature for 24 (+/- 2 ) hours . the addition of carbodiimide is made every 3 - 4 hours , and the ph is adjusted to 4 . 7 after each addition of carbodiimide . e ) for attachment purposes , a mesh of absorbable polyglyconate suture material , matched to the size of the mold , is laid in the dispersed collagen such that it protrudes from the structure &# 39 ; s periphery to form a skirt which may extend over the vertebral body . this mesh provides both immediate attachment sites and long term fibrous ingrowth . intervertebral discs are aseptically harvested from mature goats or dogs , trimmed of all adherent tissue , and placed into gey &# 39 ; s balanced saline solution . each disc is bisected in the coronal plane and 3 mm full - thickness circular defects are made in each half . the defects are filled with a 3 mm diameter plug of one of two prototypes of a complex collagen - based matrix . the discs are placed in six well culture plates containing 6 ml of dulbecco &# 39 ; s modified eagle &# 39 ; s medium supplemented with 10 % fetal bovine serum , sodium ascorbate , and 0 . 1 % penicillin / streptomycin . cultures are maintained at 37 ° c . in a humidified atmosphere of 10 % c0 2 / 90 % air , fed three times per week , and placed in fresh culture wells every week to prevent the formation of explant cell cultures . at intervals of one , four , and six weeks after initiation of culture , three discs from each group are removed , fixed , and evaluated with serial sections and staining . new collagen and glycosaminoglycan formation is evidenced histologically using alcian blue and masson &# 39 ; s trichrome stains . the results demonstrate increasing cellular migration and invasion over time . there is no apparent toxicity from the material . the depth of cellular penetration into the scaffold appears to be limited by the density of the prosthetic complex . the cervical vertebral disc of a mature goat was primarily excised and surgically replaced by a prosthestic disc . the goat returned to full cage activities within a day after surgery . serial radiographs have documented preservation of the intervertebral joint space . the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof . the present embodiments are therefore to be considered in all respects as illustrative and not restrictive , the scope of the invention being indicated by the appended claims rather than by the foregoing description , and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein .