CA2006750A1 - Reinforced bone cement, method of production thereof and reinforcing fiber bundles therefor - Google Patents

Abstract

The distribution of short, fine, rough surface titanium reinforcing fibers homogeneously throughout acrylic surgical bone cement is accomplished by adding the fibers in the form of bundles of several hundred fibers with the fibers being bonded to each other with an adhesive binder that is soluble in the liquid monomer component of the bone cement. The fiber bundles are either premixed with the powdery polymer bead component of the bone cement followed by addition of the liquid monomer or the polymer and liquid are premixed into a viscous liquid and the fiber bundles are gradually added with stirring to the viscous liquid. In either case, as the adhesive binder dissolves in the liquid monomer the individual fibers are freed and the stirring mixes the fibers homogeneously throughout the cement mass.

Description

2~0~7~

UP~-100 REINFORCED E~ON~ CEM~NT~ IIOD C)F PRODIJCTION
THEF~o~ AND R~:IN~OR~ING PIB13R BuNr~Es T~lr~RE:~OR

ck~round of the Invention (1) Fi~ld of Islverlt:ion This invent;on rel~es to a fiber ceinforced bone cement useful for adh~ring or grouting bone to the s~rface of a riyid fiU~S~rate~ such ~s a prosthesis. More par~icularly, the invention relates t~ a ~ib~c reinforced hone cemen~ wh;ch can be easlly used by a physician during bone repair procedur~s while providin~ lmproved tc~ughness ancl fatigue strength: to a kit and to a method fo~ p~ep~lng the reinforced bone cement; and to a novel ~lber bundle for ~se in preparing the reinorced bone cemen~.

~2) P~oblems Solved and Prior Art In jolnt ~urge~y i~ à3 common practice today to an~hor components oE repl~cement joint~ by using as ~one cement a two-~omponcnt resin which polyme~izes during the operation at normal ~emperature~ and whi~h, on accQunt of its pla~t~c properties leads to an interlocking of the prosthesi3 component in ~he bony sheath. Because o~ its phy~ic~l properties, the bone ce~nent ~llrin~s onto the prosthesis resulting in a closed metal-to-cement conta~t.
The bone ce~nents commonly used ~e polyme~hylmeth-acryla~e (PMM~) consisting of p~wdery bead polymers which are super~icially dissolved b~ liquid monomers and embeddecl during the polymeri~ation process. During mixing the polymer 15 ~l~()fi~;?SO

Immersed in the monomer~ . The ~MMA 2~eads are supe~f ic;ally dissolved and embedded in a composite manr~er.
Despite th~3ir widespread use PMM~ and rel~te~ bone cements tend to reprcsen~ the "~eak link" in prosthesis ixati~n.
S Tll~ long term success o~ a to~al joint prosthesis depen~s on the ~ontinue~ ~oction ~nd i~ter~ction o each of the components o~ the prostheti~:: s~tem~ In a c:em~nted total hip prosthesis, ~or instance, stres~ transer erom the pelvis to the femur is a ~unction of ~he mate~ia~s be~ween the ~wo bones (e.g.
b~ne-P~MA-m~tal-u~MWpEl -metal-PMMA-bone) an~ the interfaces between the materials, as illustrated in Figure 1 for a hip prosthesi5 (l-cortical bone; 2,2a-spon~eous hone, 3,3a-acrylic cement; 4,4a-metal prosthesis; Sa-U~MWP~; where subscrlpt "a"
designates acetabular compon~nts ~nd the non-subso~ipted numbers ~p~esent the feinoral comp~nents). The weakest of the m~erials i5 the PMMA, with the lowest ~actur~ toughness and ultlmate strength.
The common mode o~ Eailure o~ total joint pros~heses is aseptic loosenin~. X-raY examination~ of pa~ients with loosened prostheses oeten reveal a radiolucent .line in the bulk of the cem~nt, indicating that the ~ement ha~ fractured. Becau~e the yeometry of the pros~hesis i~ complex, the state oE stress i~
also hi~llly compl~x, and the re~sons for ce~ent ~ailure are not ~lear. For example, it ha~ been postula~ed that tl-e inte~rity 0 the metal stem~pMM~ in~erace is the critical link in the performance o the pro~thesi8~ however, tlle cause and e~ect -1 UHMWPE: Ultra-lligh Molecular wei~ht Polyethylene is ~,sed as a lubricant mate~ial, generally to prevent met~ om articu~ting on metal, and ls u ed, ~or ex~mpl~, ~s part of ~he a~etabular component of a tot~l hip pros~hesis, or the tibial Component of a ~o~al knee pro~thesis.

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relatlon~hip between th~ me~al p~sth~gls/PMMA ln~erface Lailure and ~ement ~actur~ is not we~.l understood although the ~racture mechanics of t~e two phenomena are most llkely linked. T~l~
improvement o~ ~he fracture characterlstics o~ the hone cement, however, is a problem that has ~eceived some attention in recent years, ~ h~ comyosition of the PMM~ used ~or ~otal joint surgeries today is substanti~lly the same a~ th~t used 20 years agO; very little has been done to improve the ~ateri~l itsel~
~he acoeptable success rate of cemented pro~heses was achieved using existing cements, ho~ever, in a predomlnantly elde~l~
patient population and with imp~oved surgical handlil)g technlques. The 90~ success rate at ten years is goo~, ~ut should be im~ro~ed. cement ~ailures do occ~r, and generally le~
lS to revision surgery. F~rt~ermore, younger p~tiento now receiving total ~olnt ceplacement~ have a greater 1ife ~xpectancy than the de ign expec~ations of the total joint pros~hesis. Impxovem~nt of the bone ~ement, exclusively, m3y not solve every pro~lem associated wi~h total joi.nt replac~msnts. Bu~, by making improvements in each ~omponent of a ~o~ oint pro~thesis, including the cement, the success cate of prostheses will improve, and ~echani.cal ~Ailures can be virtually eli~inated.
Increasin~ the longevity of P~MA by improving the reslstance t~ fa;lure o~ the polym~r has received s~me, a~beit surpris1nyly l;ttle, attentl~n ;n th~ bioe~gineering literature in the past ten years. One suggested me~hod o~ improve~ent ~as to ormulate a new bone cement, ba~ed on n-butyl methacrylate r rather than the methyl methacrylat~ monome~. It has been repocted that th~ material show~d a higher ductil~ty, a higher apparent eracture toughness, and a greater fatigue lie.

X~)~6750 However, the actual fracture toughne.ss dete~min~d ~y separate impac~ ~est~ sllowed no improvemen~ o~ the new cement with ~espect to PMMA cementsO An even more detrimental re~ult was that the new ?olymer hsd only halE the modul~s and hale the ultimate S ten~le stren~th of PMMA.
Anotlle~ ~et~od o~ attemptin~ to improve ~MMA was the addition o~ a reineorcing phase, gene~ally sho~t fibers or whi~kers. Early work was done by ~n~ell, et al., Ann. Biome~.
~X~ 3- 1~75~ pp. 225-229 with car~on ~ibers approxim~tely 6 mm in length, 1, 2, 3 and 10~ fiber content by weight ~proximately 0.67, 1.33, 1.96 and 5.87% fiber content by v~lume, wi~h me~ured inceeases of 100% in the average Young's modulus ~or the reinforced PMM~. They also report~d a dec:rease in peak ~uring temperature of the reinforced PMMA. They ~ound the reinorced cement viscous an~ di~icult to mix, and the~ ~lteLed the ~atio of powder polymer to llquid monom~r ~o tacilitat~ mixing o~ the rein~orced cement. Pilliar, et al ., J, ~iomed. Mater. ~es., Vol 10, 1~76, pp. 89~-~06); Fati~ue of Filamentary Component Materials ASTM STP 636, ed5. ReiEsnifler, et al., ASTM lg77 pp 206-227; used carbon iber~ (~ mm length, 7 micrometers dia~ete~) with a 2~ volume content, They measured a 50~
improvement in tension-tenSion ~ati~ue limit, improved impact performance, ~n~ ~ 36~ increase in ultimate tensile strength.
However ~ lt was implied that the relnforced PMMA had poor ~5 intrusion characteris~ics due to inc~eased viscosity, and poor fiber distrib~tion. Wrigl-t, et al., J. Mater. Sci. Let., 14, 1979, pp. 503-50S, did preliminary stuflies u~ing PMM~ rein~orced with chopped aramid Eibers. PMMA reinfor~ed wit~ 5.17~ by volume (7~ by ~eight) ~x~ibit~d a 74~ ~nc~se in fracture toughn~ss ~o over the plain PMM~. Tlley were not ~ble ~o p~oduce reinEorced ~f o PMM~ w.ith a iber ~ontent grea~er than 5~ by volume he~use of m.;xin~ and handlinq dl~ficultie6. Beaumon~, J. Mater. ~i., 12, 1977. pp. 1845-1852 included glass ~eads in the PMMA mass ~nd measured a 103 decrease in crack propa~ation velocity, usin~ 30 volume content of the beads.
yery ~ew investiqations involv¢d the use of m~tAl ~ibers to ~elnforce PMMA. Taitsmctll and s~ha, J. B~ne ~oint Surg.t Vol 5~-A, No. 3, Apr lg77, pp. 41~-~25, used larg~
dl~me~er (0.5 to 1.0 mm) stalnle~s s~eel and ~ita].lium wires a~ a rein~oroin~ phase. They embedded 1, 2 or 3 wires in their PMM~
specimens, They re~a~ted up to an ao8 lnCLeASe in tenslle strength o~ the PMM~, with three embedded vitallium wires, but noted that clinical applications o~ the~ wire rein~or~ed cement were limited. ~ait~man and Saha's u~ o~ rein~orcing wireS is analoyous to rein~orcing bars embedded in structural cotl~rete~
and not a homogeneous, ibe~ composite materlal. Fishbane and Pond, Clin, Orthop., No. 128. 1977, pp. 1~4-1~9, reinorced industri~l grade PMMA and PMMA bone cement with stainles~ steel whiskers ~0~5-1.0 mm length and 6S micron dia~eter; 3-6 mm leng~h and 90 microns diamete~). Th~y de~ermined that the addition ~f ~i~ers up to 6.5~ by volume l~p~oved the campres5ive st~ength by nearly 100% or the industrial PMM~, but only ~% for the sur~i~al grade PMMA. The compee~sive streng~h of PMM~ is ~ot a critical property for t~le longevity of the cement ln v~o. Thes~
authors po~tulate that ttle ~ea30n or the decreased perfor~nce of the ~urgiaal PMM~ was: n, ~ ~due ~o ~he limitatlons imposed by the Isuryical) me~hacrylate preparation technique."
Schnur an~ Lee, J. Biom~. Mater. ~es-, Vol. 17, 1~83, pp. ~73~91, used ~it~nium ~Ti) sheet, ~lre, me~ and powder as a reinforcing phase ~ith the purpose of increasing the modulus of ~oo~so P~A to the modulus o~ cor~ical bone. A 16~ volume fra~tion of 1 ~m diam~er wires ~a total of 25 wl~es) inc~ e~ the modulus o~ the PM~A by 3808, and the maximum compressive stress by 75~.
The concept i5 again similar to th~ reinforcins bars embedded in conc~ete.
Tile more recent wo~ Ln re~nfor~i~g PMMA bone cement a~
reported in the litera~ure, has involved either ~arbon, graphlte or ar~mid ~ib~rs, Robinson, et al., J. Biomed. Mater. Res,, Yol.
15, 1981, pp. 2~3-205, te8ted both regular PMMA and low vis~si~y PMMA cement (~vailable from Zimmer ~o., Warsaw, Indiana~
reinforced with 2~ volume of ~rbon fibers ~1.5 mm in length, 10 miC~ons diameter). soth r~inforced cements exhibited an increase ln apparent fr~cture toughne~ (notched bending strength tests~
of approximately 32~ over their plain counterparts. Surfa~e frac~ography re~ealed no ~viden~e of iber ~ractu~e, indicating that the increases in ~tou~hness" was due p~incip~lly to fiber pull out. In other work with ¢arbon fib~r reinforced PMM~ an order of magnitude decrease in crack propa~ation velocity was attributed to the carbon fiber reino~cement of b~th the regula~
and low viscoaity ~emen~s.
Saha and Pal, J. Biomechanics, Vol. 17, No. 7, 1984, pp. 467-478, tested pMMh rei~forced with ca~bon fi~ers, 0.67~ by vo~ume (10 by w~ight; 6 mm length, ~ ~lc~on~ diam~ter1 and PMMA
reinfo~ced with ar~mid ~ibers (DUpont Kevlar-29), 1.61 and 3.82 2S by volume (2 and 4~ by ~eight; 12-13 mm leny~h, unspeci~ied diameter). The r~inforced PMMA showed an increase in ~he ulti~ate compressive strength of 20,S~ ~or the carbon ~ibers, and 19.5~ and 28.1~ for the 1.61 and 3.82% volume ~ aramid ~ibers, respectlvel~. Two importAnt consequences ~f the addition of Ei~ers to PMMA were proposed; ~he peak temp~r~ture o~ the ~ ~uo~i~7r~

reinforc~d PMMA w~c~ lower t~ he plain ~MMA, ~nd ~he additlon o~ ~ibel:s changed the w~k~bilil:y o~ the cemerl~. Th~y recogn~ed that uniÇorm di~persion o~ Eibee.s w~ not achieve~. Saha and Pal studied a machine mixing technique for distcibuting ~be ~ibers, The1r claim ~hat machine mixed ~pecimens w~re stronger than non-machine mixed specimens is misleading. They used a difEerent ~hape~ fiber fo~ their machine ~i~ed specimens. It is the supe~ior s~lape o ~he fit~er which is pre8~e-1 to ac~ount ~or the increase in s~reng~h. MaGhine mlxing w~s never sllown to imp~ove the p~operties of rein~orce~ PM~A.
Ek~trand, J. BiOmed. ~ater. Res., vol. 2~, 1987, pp. 1065-1080, ~ab~icat~d carbon Eiber reinEo~sed PMMA ~y using clinically irrelevan~, indus~rial ~abricatiotl te~hniq-les with fiber content as high a~ 16.38t by volume (~0% by weight), Recent work by Pourdeyhimi, et al., ~nn~ Biomed. ~ng ., 14, 198~, pp. 27~-294, studied the ef~ect Oe the fiber con~en~ of the fractut~e toughne~s of hand-mixedl reinforced, den~al PMMA.
~hey ltsed a~amid fibers from O.~Z to 5.17% by volume ~1 eo 7~ by weight), and gr~phite from 0.67 ~o 5.87~ by ~olume ~1 to 10% by ~0 weight) . FO~ e~h ~ype of fiber rein~orGed ~ement, the f~a~ture tough~ess increased with increased iber cont~nt. ~he aramid fiber specimens ~howed a greater ~crea~e ~han the carbon fiber specimens oE the same wei~ht percent, presumably because the energy dis8ipa~ed in the microme~hanisms o fail~re 15 great~r ~5 for the aramid ~ibers than for the carbon ~ibers. They were not ~ble to produce a uniform distribu~ion of th~ fibe~3.
U.S. Patent 4,064,5~6 to Pletcher, et al. dis~loses a graphitc: ~iber rein~orc~d bone cement of the acryl1c: type stated to have mechanical pro~e~r ties m~e ne~rly matched to those o~
30 bone an~ ther~al c~rin-~ characte~istics re8ulting in ~ low~r ~ ~0~6'~5C7 ~

exothe~mio temperatur~ reac~ion during t;urlng. The ~one t,ement composition is a dispersion o~ from 2 to 1~ by wei~ht o~ very fine high ~o~ulus graphlte ~ibers ~)aving a diameter below SO
m~crons and between 0.1 to 15 mm in average length in a solution o~ biocompatible polymer dissolved in a re3c~ive ~onomer.
Fletcher reports only an inc~ease in the modulus of the bone cem~nt, ~hich is not o~ p~imary concern to a r~inforced bone cement, and indeed can be detrimental to the pcosthesis system.
There was a decreast~ in compressive strength, and more negatively, a decrease in ~lexural .strength for the ceported oomposite U.S. Patent 4,239,113 to Gros6, et al. dlscloses an acrylic bas~d bone ct,~ment f illed with between 15 snd 75~ by weight of inoryanic ~ater~al comprised o~ a~out ~o to 99~ by 1~ weight o~ a bio-active gl~s ceramic powdeL and Abou~ 1 to 10~ by weight of vitreous mineral, e.g. gla~sr Eibers haviny a len~th below about 20 mm. The particle slze o~ the powd~ is ~ro~ 10 to 200 micrometers. Fiber diameter~ are not disclosed. Imp~ove-ments in impact strength, and comE~r~ssion stren~th were reported.
However, a si~nificant decrea~e in the bending st~ength and an increase in the mo~lus of elasticity were also reported.
F~rther, there ~re no examples given as to ~he clinic~1 usef~lne~s o~ this cement. ~ioactive ~lass degrades with ~ime, and hence the integrity of the reln~oroed bont~ cement wlll also degrade with ~i~e, l'~e controlled expe~imentation shows t~at there is no mechanic~l improvement d~ue to the fiber rein~orcing phase alone. Any i~prove~ent iS d~e to the combinatioll of Bioactive glass and iber in conc~t. Since the Bioactive ~lass degrades with time, the propertie~ oe the r~in~orced cem~nt propo8ed by Gross, et aJ.. will also degrade w1th time.

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~ avi~on, in U.S. Pat~n~ 4,735,625, ~ports the inven~ion o~ a ~in~o~aed bone cement ormed using d 60ck-like mesh o~ a fiber-like mater ial to r~inforce the cement in the vicini~y of the prosthesis. The volume of "reinEorced" ~on~
cement is limited; critical area~ ~re no~ rein~orce~. Draenert, in U~S. Pa~ent 4,365,357, pre~ents ~n invention si~ilar to Davidson~s, but using a mesh of polymeric ~iber~. The invention is rest~icted to use in repairing ~one de~ec~s, and not as a bone cement ln the sense d~s~r ibed for ~o~al joint arthroplasty.
~raenert, in U.S. Patent ~,~18,glO, describes a bons cement ~ixt~re where a second phase o~ Eibers i~ added. The fibers, however, are made up of the ~a~e polymeric material as the ~one cem~nt. Draenert in~ludes a graph of ~he per~ormanc~ of ~he new material versus existing ~ement~. The inventor states that the fibe~ i8 only e~ectiv~ because of the s~pe of the prepolym~r po~der. Therefore, the improvement i5 ~ue to the use of a dif~e~ent cement, and not to the ~ddition of the ~ibers.
~ he u~e of duGtile, high 6trength, larger diameter, fibers to rcinforce the b~ittle matrix ma-teri~1, PMM~I has certaln advantages over reinforcement ~ith brit~le ~ibers, such as carbon ~r ~e~lar. Cooper and Piggo~t in Advances in Rese~rch on the Strength and ~racture of ~ate~ials, ed. Toplin, Pergammo~
press, 19~8, pp. S57-601, showed that Eracture properties of composite materials i~prove as the diamet~r of ~he ~ibers increase. However, high ~trongth ceramic ibers, including carbon fibers, are dlf~iGult and expensive to produce with - diameters greater than 10 micromet~r~. ~uctile f ibers also increa~e the amo~nt o~ energy dis~ipated ~uring c~ack growth, especially in ~hort ~iber composites. The u~e o~ carbon fibers, 2(~0~

there~ore, .~i~its the extent to wl~ich the mecl-ani.~al p~op~rtie5 of PMM~ c~n be lmproYed.
Wi~h this ~ackground ln ~ind, the presen~ inve~tors have been engage~ in an ~xtensive re~earch pro~ram designed to ~tudy th~ mode o~ failure oL su~glc~l bone ceme~t~s based on acrylic polymers and mean~ ~o~ improving the fracture tou~hness, tensile strength, modulu5, and oth~r physi~o-mechani properties, while ta~ing into ~onsiderati~n the reqUi~ement that the su~gical bone cements mu~t b~ easily mixed by ~he surgeon under clinical condition~ (e.~. in the operating room) and must cemain workable for suE~icien~ periods o~ time to ~e applied without extensive use of force.
As a re~ult oÇ thl~ re6earc~ it bec~me ~pparent that ~he use o~ sh~rt metal ~ibe~r~ as rein~or~ment ~or acrylic based surgical bone cements would oEfer the mo~t signiicant i~prvve-monts in the strength charaçteristic~. The shvrt metal fibers ~rovide the followin~ advantage when used to rein~or~e the acrylic ba~ed surgical bo~e cement: the fibe~ are ductile and still provide fracture energy dis~ipation mechani3ms tha~ are 20 not available o~ other type~ of ~einorcing ~iber~, inçluding carbon or g~aphi~e, glass or ceramic, or polymeric rein~orcing ~lbers; the diame~ers oE tho metal Eibers can be widely varied by al~ering the d~a~ing conditions durin~ the manuactur~ng process;
metal flbers have gen~ally high ~trengtll ~nd high fracture toughness, It i~ generally ag~eed tha~ as the ~uantity of reinEorcing fibe~.~ increa~es ~o do the m~chanical ~trength propor~ies. How~ver, as the elber conten~ in~re~5es it become~
incre~singly diL~icult and 0ventually not pr~tlcal or pos~ible to ef~ct homogene~u~ di6tribution of the fibe~s througllout the 1~

~)06~ 0 cem~nt mas~ And in ~dition tho vi.gcosity ~f th~ mas~ increa8~
and its work~bility by the surgeon during surgery dec~eases, Any p~acti~ally useful surgical bone cement must be c~able of being easily mixed by t~e su~geon in a clinical ~etting, i,e. during surge~y, and must remain suficiently ~lo~able and workable to be applied to the bone surace or cavity and/or ~o the p~osthesis or other implant device.

Summ3ry o Invention a~d ob~ects Acc~rdingl~, it i~ an objec~ of the invention to p~ovide a ~urgical bone cement with improvad mechanical properties, in~luding fs~c~ure toughness and ~atigue strength~
the~eby imp~oving long term prognosis o to~al joint replacements and other sur~ical bone repair ~reatments involving bone cemen~s.
1$ It is another object o~ the inventlon to provi~e a fiber rein~o~ced surgi~al bone ~ement ~he~ein mixing oE the iber ~ein~o~cement into the cement matrix can be easily per~o~med by the ~urgeon unde~ clinical conditions.
Stlll other ob~ect6 of the inventioll ~re ~O ~rovide a novel fiber reinfos~ing material which can be easily and homo~en~ou~ly incorpora~ed into ~ two co~p~nen~ bone ce~ent including biocompatible polymer beads or powde~ and bio~ompatible rea~tive li~uid monome~s; a bone cement treatment kit for suryicRl bone repair which kit include~ the fiber reino~cing ~5 material, biocompatible polymer and reactive liquid monomer; and method for uniformly and homogeneou~ly incorporatin~ shor~
fine fiber reinforcing material into a two ~mponent acrylic bas~d surgical bone cement.
The abov~ and other objects o the invention which will become mose apparent a~ter ~eading the followin~ detailed ~()6~

descriptlon and pre~erred embodiments in conjunc~ion ~i~h the acco~pan~ing drawings are accomplished, accordiny to a Eirst ~spect o~ the invention, by an a~rylic bas~d surgical bon~ cement rein~orced with shor~ fine Eibers which are rando~ly but homogeneously distributed throughout the cement matrix, the fibers having an ~verage length of ~rom about 0. 5 to 10 ~illimeters and an average diame~er of ~rom about 5 to 100 micron$. pre~era~ly the fi~e¢s are inco~porated in an a~ount up to about 20~ by volume of the cem~nt compositi~n.
In accordance with a fur~her a~pect of the invention, a ~ib~ reinforcing ~aterial 1~ provided whicl- ~acilitates it homogeneous in~orporation into an~ throughout the acrylic based bone cement. The ~iber r~infor~ing material is formed as a bundle of sho~t fine fibers bonded together by an adhesive ~5 material which is soluble in the liq~id monomer co~ponent oE the a~rylic bone cement . pref erably, ~he ~ i~er bundles have a length-to-diameter ratio in the range o~ from about lO:l to 1:10.
The ~i~er rein~orcing mate~iaI can be provided as a component ~E a b~ne cement treatment ki~ ~or surglcal bone ~epair or other treatment of a bone disease or bone ~eeect requiring ~pplication of a ~urgical bone cement. ~he kit lncludes ~
biocompatible polymer, generally ln the ~orm of powder or ~ead~, a liquid reactive monomer, and a plucali~y of bundles of reinforcing me~al fibers ~herein the ~ibers in each bundle are bonded to ea~h o~her with an adhesive material which i8 soluble in the liquid reactive monomer. When the three components are combined with ~tirrin~, the adhe~ive material will dissolve in the liq~id ~o~omer to thereby allow and promote the individual fibers of the hundle~ to be homogeneously dlstrib~ted througho~t 30 the bone cement~

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Accordlng to anotller aspect oE the inven~i~n th~re is provided a ~e~hod for prepa~ing ~ E~ber-rein~or~e~ hone ~e~en~
for surgic~l application. In one ~mbl3diment of this aspect, the adhesiv~-bonded L~in~orcing fiber bundles are irst mixed with S the powdery or granular polymer to ~orm ~ fl~st mixture and then the Elrst Inixture is further mixed wlth the reactive ].iquld mvnomer, whereby the adhesive material bondlng the individu~l metal ~ibers of th~ fiber bun~l~s disg~lves in the liquid monome~
to thereby pr~mote homogeneous ~istrib~ti.on of the reinforcing fibers throughout the bone ~eme~t, In anothc~ and preferred embodim~nt o this a~pe~t, ~he pow~e~y polymer and li~uid monomer are premixed until a viscous liquid is ~orme~, the fiber bun~les are gradually added to the viscous liquid mixture, and t~ resulting mixture stirred to di~tribute the ~iber bUndles through~ut the resulting ~ixtu~e, while allowing the adhesive component to dissolve in the liquid monomer whereby the 1nd.ividual ~iber-~ of the Eiber ~un~les are released into and ran~omly distribute throughout the bone cement.

Brief Description of the Drawings Figure 1 i~ a schematic of a total joint pro~thesis, s~owing the relationships of ~he various materi~l~, both natur~l ~bone) and implanted ~m~tal, cement, e~c . );
Figu~e 2 is a schemati~ o a 3 point bending, frac~ure toughness specimen;
Fi~ure 3 i~ a schematic side elevation view, partially broken, of an embodiment of a fiber bundl~ with a rectangular cross-se~tion accordin~ to this f.nvention:
Figure 4 i5 a ~chematic si~e elevation view, partially 2[)~)6~

broken, o ano~her em~odiment o~ a fiber bundle with a circular cross-section according to the inv~ntion;
Pigure 5 is a schematic view, in cross-section, accordin~ to one embo~imellt of the inVen~ion, sho~ing the distribution o~ discrete fiber bun~les in the v18co~6 liquld ce~ent;
Fi~ure 6 i~ an enlar~ed s~hematic view, in ~ros~-~ection, o~ a portion o~ the fiber reinorce~ bone c~ement of Figure 5 a~te~ the adhesive binder of the bundleS has dis~olved and the lndividual fihers are r~ndomly and homogeneously distributed throughout the cement ~a6s;
Fig~re 7 is a bar gr~ph plotting the average fr~cture toughness of reinforced acrylic bone ce~en~s according to the invention ~s a ~unction of ~iber ~ength and ~ontent for 12 micron diame~er reinforcing titanium fiber~ and for a non-reit-forc~d control;
Figure 8 is a ba~ g~aph plotting the averay~ ra~ture touqhness of reinforce~ acrylic bone çement ~ccording to the inVQntion as a function of f iber content for 22 micron diameter titanium Eibe~s and a non-reinfo~ced control;
Figure 9 is a graph plotting the avera~e fract~re tou~hness o rein~o~ed low viscosity acr~lio bone cemen~
according to ~he invention for 22 micron diameter titanium fi~ers and non-reinorced control;
Yi~ures 10 an~ 11 are each microphoto~r~p~s (magnifica~ion 75X) o~ cross-se~tionR of the c~red reinorced bonc cement ~mple o~t~ined according to Fxa~ple l, illuminated ~rom both above and below the samp.le or from only below the ~ample, re~pectively; and ~0~6~

Figure 12 is a microphotograph ~magnifica~lon 75X) of a cross-se~tion of the cu~ed reinforced bone ~ement sample obtained according to Example ~, illumina~ed from above the sample, and showing th~ fiber cross se~tions as white dots.

Detailed Descrl~tlon of Invention an~ pre~erre~ EmbodLments ~ n this invention, the terms " (~eth) ac~ylate~ anq ~poly~meth)acrylate" include ~he monome~s and polymers, respec-tively, of methacrylic acid es~er~ and acrylic acid ester~, ~nd the ~olym~rs also include the co-polymers Qf ~he compounds named.
The preferred bone cement material to which ~he iber reinfor~emen~ is added lncludes ~ solid ~in~ly divided powdesy or granular polym~r componen~ and A liquid reactive or polymerizable monomer component which is also a solvent or sw~lling agent for the polyme~ component. The polymer an~
monom~e componen~s can b~ ba~ed on the acrylic, e.g.
~m~th)acrylate syatem, how~ver, other polymeric sys~ems can:also be used. For convenience, the cement sys~em may at time.s be 20 broadly re~erred to as ~n acrylic polymer, or a~ based on PMMA
(polymethylmethacrylate), th~ preferred polymer component.
Mo~e generally, the polymer component of the composition Cah be any methyl(meth)acrylate polyme~ such as methyl~meth)acrylate homopolymers and c~polymers of methyl~
(meth)acryla~e with alpha,b~ta-ethylenically unsaturated compounds such as vinyl acetate, alkyl (e.g. C2-C~) (meth)acryla~es and multi-functional acryli.c monomers such as alkylene dimeth~rylate and alkylene ~lacrylate~ and t~acryl~tes, Th~se poly~ers gen~ally have a m~lecular w~ight ~0~7SO

between 500,000 and 2,000,000. Methylmethacrylate homopoly~ers and copolymers ~e preeerred.
~ `he reactive monomer componcnt is p~eerably methyl acrylate or metllyl methacrylate although ~he C2~C4 alkyl (meth)-a~ylates, sU~h as ~thyl~meth)a~yla~e, propyl(meth)acrylate or (n-, or iso-~butyl (meth)a~rylate, ~n also be used.
These bone cemel-t ~aterials, w~ich are the~selves w~
known and ~o~mercially aYail~ble, are ~s~ally provided wi~h 2 parts by weig~lt oi. the ~lnely divided polymec and 1 part by w~ight of liquid monomer. a].though higher or lowe~ ratios can also be used, and are characterized as being self-polymerizable subst~nces whl~h are mixe~ ~ogether with a polyme~ization ~atalyst, such as dibenzoyl pesoxide, and polymeriz~tion accelerator, such ~s dimethyl-p-toluidine~ immediately p~ior to the operation to form a YiSCous liquid or pas~y mass. The pas~y mass 1~ introduced into the appropriate body cavity and/or to the surgical implant device, and wlll harden in sit~, at room temperature ~via an exothermic zeaction) within a ew minutes.
More ~p~ciflcally, Curing of bone cement composition:is 20 typically ac~omplished by any suitable initiatOr ~ys~m such as from about 0.1 to abou~ 3% by ~ight~ pre~erably about 0.6% o~ a conventional free radical $nitiator. ~he initi~tor can be a p~roxy compount~ or an ~zo conlpound. Fo~ purpo~es o~ biocompata-bility benzoyl peroxide is a ~ery suit~ble free radic~l initi~tor, Th~ curing temperature 1.~ generally reduced to room tempera~Ure, e,g. about ~5 to 30~C by i.n~lusion in the forlnul~tion of an ac~ivator fo~ ~he peroxide catalyst which cau.es more rapid dec~mpt~sition o~ the peroxi~e to for~ ~ree radicals. Suitable peroxide catalysts include ben2~yl peroxide, 2,4-dichloroben~oyl peroxide and 4-chl~robenzoy~ pecoxide, ~ ;~00~;~750 --' ~ctivatorS or a~C~ ators Eor these catalysts lnclude N,N-dialkyl aniline~ or N,~-dlalkyl ~oluidines generally employed in amounts ranging ~rom ahout 0.1 ~o 1% based on the weight of monomer present. A pre~erred activa~or is N,N-di(2-hydroxyethyl~-p-toluidine. In o~der to p~ovide longer ~helE liFe fo~ th~ compo~i~ion~ of ~he invention, the composition may be stored in a closed con~ainer at cold temper~u~e. St~bllizers, such as hydroquinone or Ghlorophyll may also be added to the monomer compound. Bone cement~ containing both activator and peroxido are provided as two-part compositionS in which the ac~ivator and mono~er and peroxide an~ polymer component are packaged in separate contal~e~s. The proportlons by wei~ht o~
polymer and liquid monomer can ranye f~om a~out 4:1 to 1:~, pre~erably 3:1 to 1:1.5, such a~ 2:1, 1.5:1, 1:1 or 1:1~5.
While it i5 k~own, a~ indica~ed b~ the literature and pA~ent art discussed above, to inco~yOrate f iber rein~o~aing material into the bone cement, ln actual practice it has proven ~i~eicult to incorporate the short, fine fibers, in signiicant amounts, uni~ormly and h~ogeneously throug~out the visco~s cement mass~
The primary hindr~noe to a uniEorm fibe~ dis~ribution is that the ~i~ers tcnd to clum~ together~ That is, ~ny given Elbcr will no~ move freely when it is surrounded by o~he~ ~ibers, presumably due ~o friction and other fiber su~face effects~ The ~5 high vlscosity o~ the ~one cement, which begins to cure immediately upon mix~n~ the cement components, Eurthe~
exacerbates this p~o~l~m which iB al~o made more acute by virtue of the pcactiaal requirement that the mixing takes plac~ in the operating room by the surgeon (or hi~ or her a95ist~nt~) where l~rge scale high power mixing equipment is neither available no~

~C~()fi'~

appropri~t~. Usually mixlng is ac;com~lished t4tally tnanually, e.g. using ~ bowl ~nd spA~ula, althou~h ~mall, low power ~:lixers or blender ~ay he a~ailable.
The present invention provides ~ ~olution to iber cl~mping an~ suhstantially avold~ ~e resulting ~iber distributi~n p~oblem by allow~.n~ ~he reinfo~ing fibers to move freely with ~espec~ ~o one ~not~e~. This is a~ompli~hed by providing the fibers in small/ di~crete bundles with g~eatly expand~d, low a~pec~ ~a~io. An embodimen~ o~ SUCh ~ f~ber bundle, witt) a rec~angula~ ~os~-sectlon is shown at 10 in ~ig~re 3. A cylindrical ~iher bundle, i.e. circula~ cro5s-s~tion, according to the invention is ~hown yenerally at 20 in Figure 4. Th~ fi~ers 30 in ea~h ~undle are bonded ~ogether with an adhe~i~e 50 which is soluble in the liquifl mono~er oomponent lS of the ~on~ cement. The ~esu].ting fiber bundle~ or rnugge~s~ are é~sy to work with and can be easily distributed throu~ho~t th~
pasty aem~nt ma6s while still in the liquified (although vis~ou~) tate. The bundles effec~ively reduce ~he a~pec~ ratio ~length/diamete~ oE ~he individual ~lber.s in a mass o~ p~wder or liquid. Hi~h aspect ratio particles (long ~hin ~iber~ in~rea~e th~ viscosity of a fluid, while lo~ aspec~ ratio ~rtiqles ~like the bundle~, do not increase th~ visco5ity greatl~t but may provide insu~ ient r~inforcem~nt, The low aspect rativ bundles are close~ to spherical particles than individual ibers.
The ~undiing process, u~ing a coar~e analo~y~ take~ fibers ~nd ~undles them into ~ough spheres. High afipect ratio ibers al o tend to tangle and ~ill not dispers~ uniformly.
As the ~iber bundle~ are ~ticred into th~ uefied oement mass th~ ~lue binding the fi~er6 to~eth~r i~ di~sol~ed Ln the liquid monomer ~omponent~ the bundles start to break up, an~

~o~ o the lndividual fibers of each bun~le are distributed randomly through~u~ ~he cement ma~s.
Thus, in the first instance~ and re~e~ring to ~igure 5, ~he use oL fibe~ bundlea lOO ~llowa the bundled fiber3 ~o b¢
S easily dist~ibuted th~oughout the ~em~nt mass 120~ Se~ondly, the use o a glue for ~he ~iber bundles which di~solv~.~ in t~le li~uld m~nom~r and continued stirrin~ t~en allows the individ~al fibers 1~0, of each bundle~ e.~. b~ndles 100' and 100", to be randomly and homogeneously di~trib~ted throughout the bone c~men~ a~
10 schematically ill~strated in Figure 6 be~ore it i~ fully cured, In ~he peesent inv~ntion, the reinforcing f~ber~ are pre~era~ly formed o~ ductile, high atreng~h n~etal materials, rather than th~ more bri~le ca~bon or ceramic fiber5 or hi~h strength synthetic ~ibe~, such a~ the aromatic~ polye~te~s or lS polyamides, e.g~ Kevl~r0. However, the adv~ntage of the present invention, in te~ms of l:he more hon~eneous dlstrib~tion o~ the reinforcing fibers throughou~ the bone ~ement, etc., can ~lso be achieved with carbon, yraphite, ceramic, vit~eou~, or high-st~en~th synthetic polyme~ iber ma~erials~ Mixt~r~s of 20 di f ~erent ~ype~ Oe fibers can al~o b~ ~S~d.
The use of metal Eiber5 is preferred in view o~ thqir hlgh s~reng~h, ductlllty~ and e~se of manufa~tUre in various diameters ~sin~ conventional~ metal iber drawin~ te~hnolog~.
The ~reEerred metal ~aterial i~ titanium although any 25 other biocompa~ible~ ductile, biologically inert and sterilizable ~etal can he u~ed. Other appropriate Metals include, for ~xample, co~alt-ch~omi~m alloys, tan~alum, nio~iu~, alloys of ~hese metals with each other ~nd othes rne~als, an~ th~ lik~.
Titaniu~ is the preferred metal sin~e lts biocompata~ility i~
well docum~nted. Other met~l ~lbe~ uch a~ stainles~ ~teel, 1~

~6)06r75iil) may al90 be used ~or ce~t~in application~, but ~e~use ~ the pos$ibility of ~rev1~e ço~rosion t~ use o~ sta~nles~ ~t~el i~
not generally reco~n1ended.
The size o~ the ~eln~oro;ng fibel~s i~ im~ortant to assure the desire~ improvements in s~rength of the reinfo~ced bone cemen~ wi~hout an ~naoceptabl~ decrease in workability o~
~he mixed unCured ~emen~ composition resulting fro~ reductions in ~lowabili~y (i.~. inccease in viscosity). In o~er to hav~ a minimum impact on lowability, the ~ihers ~hould be ~s short as possible; ~isco-~it~ inçreases with increasing fiber leng~h.
tEo~YeveL~ l~nger fi.bers provide increased strenyth a~ ~ompared ~o an equival~nt amount of shoL ter fibers.
Aac:ordingly, fibe~ h~ving l~n~ths in the r~ng~ o~ f~:c)m about 0.5 to ~bou~ lO millimeters, pre~erably ~rom about l to lS about 8 ~llllmeters, su~h a~ l~S, ~, 3, 4, 5 or 8 ~illimeter$, are used in this invention. ~urt~ermor~, within thi~ range, the fiber size ~hould b~ preferably ~elected to be accomm~dated within the anticipated thi~kness of the cu~e~ bone ~ement, which, depending on the partic~lar patient and type o~ ~urgery, etc. may ~ange from only a few mlllimeters to sev~ral or tens o~
mLllimeters. ~s sho~n by th~ examples tha~ follow l.5 mm long metal fi~ers ~rodu~e substantially the same degree o~ fractur~
toughne~s results as the 5.0 mm long Fibe~s~
E'iber thickneR~ ~diam~3ters~ shoul~ be sel~cte~d within the ~z~nge of ~rom about 5 to abo~l~ loO mi.c~or s, pre~erably f~om about 8 to about 80 microns, especially pre~erably from about 12 to 6~ ml~rons, such as 1~, 20, 30, 40 or 50 micron3. Beluw about 5 microns the ~:otal 6~r~ace area of the ~iber~ becc~mes unduly high, :leadlrl~ to exces5ive sdsorption o~ the liquid monom~r, and the strength o~ the reinforced c~emellt i.~ not su~ficLently : ~, ~617 increased. Abo~e ~bou~ 100 mlc~ons ~he ~lexib~lity of the ~ib~s i5 so~ewha~ diminis~le~, and there exists ~he po~s1bility of puncturing sur~ical gloves in ~ clinical situation~
Within th~ ~bove range~ ~ lengths and diam~ters fiber5 having l~ngtl~-to-diameter ~l~d) ra~ios in the range of from about ~0:1 to about 1000:1, pre~era~ly from ~bout 40:1 to a~out 600:1, are preferred.
To ~orm the individual fib~r~ into bundles, a large number, e.~. ~rom abou~ SO to 500, pr~erably 100 to ~0~, of the continuou$ leng~h fibers are gathered together, pref~r~bly in longltudinal alignment or ~ubstanti~1 ~lignm~ht, and the r~ulting ~i~er arc~y is ~a~zsed throu~h ~ ~upply ~f binder o~
glue, ~c~ording to techniques well known in the art. A~ter the binder has cured or a~ le~st set ~o a stage where it is no long~r tac~y the bonded fiber array is cut into individual ~undles o~
the desired length~
Typically, ~he metAl Eibers will be d~awn ~om ~ods or wires or bundle~ wires ~hrouy~l a ~ingle die orifice, the drawn ~iber being stretched and drawn to ~he ~redeter~inqd f iber diameter and after drawing the fibers from one or more drA~ing machines are ~athered by a gathering deviae. Prqferably, in ocder to draw the met~l fit:~er~ to the desired diameter~, pa~ti~ularly diameters o~ 25 microns or le ~, it is convenient to start wlth a bundle of wires, ~ Og . ~rom abo~lt 10 to abou~ 100 or 25 more wires, ~ightly enca~ed wlthln a met~l tube, such that drawing the tube will ~imultaneously draw the individual f iber~, usually ~hrough ~everal pa ~e~, ulltil tl~ de~ired diameter is rea~hed. q~hen, the metal tube is chemically etched a~way, e.g.
di~solved, using an et~hanl: liquid ~hiah will not dl~;~olve ttle 30 metal fibers. The ~hus ~reed ~netal ~ibers will ~Isually have a ~o~

hexagonal-like cross~sec~ion a~ ~ result ~f the tight paakin~ in the tube. ~he metal tube ~ay be copper, steel, stainless .~teel, or ~t~e~ ea~ily dr~hle metal ~hlch can ~e dissolved, e.g. in an aeid bath, whi~h will not dissolve t~le r~info~cing m~tal fi~ers.
S Thi~ metal ~iber drawing technoloyy i5 well knowrl in ~he art.
BeEor~ or after gathering into a fiber pack~ o~ a~ray the fiberS are passed through a supply o~ ~dhesive material w~llc~
will bind oc glue t~g~ther the individu~1 fibers in each fiber package. The adhesive may be in ~he form o~ a ~inel~ divided powder in which case the p~wd~r adhe~ing to the fibers will be caused to melt, either due ~o the hi~h temperature of th~ fibers or by external application oE heat, and the molten adhesive can 10w a~ound and ooat the individual~ ~ibers as well a~ in~o the interioe oE the flber pa~k~ge in ~he case wher~ the fibers are lS ~losely packed when ~hey pas~ through the adhesive powder.
Alt~rn~tively, ~he adhesive m~y be in the liq~id or molten ~orm ~hen it co~es int~ ~u~tact with the ~i~er p~ckage.
IE ~he adhesive is applied to the f ibers ~e~ore the fiber~ are gathered togethe~ to the final dimension o~ the ~undl~
2~ the iber ~r~ay will then be pas~ed th~ough one or ~ e;gathering devices, whlch ~ay simply be a single apertu~¢ or ~ube or several apertures or tubes of continuously decreasing diameter, wi~h the la~t aperture having the diameter ~e~i~ed ~o~ the fiber bundle.
Th~ gatherers nlay also functlon as ~dhesive metering device& ~o -~ipe of~ excessiv~ adhesive as the ~iber array passes through and contaats the edge o the aperture, The sp~ed of travel ~f the flbers ~hrough the adhe~lve supply asld th~ough the g~therers ~ill be such that the adhesive is not fully set or cu~ed until after the fiber array passe~ t~rough the las~ gathere~. q'he aperture~
~r tubes through which the ~lber pdckages are ~thes.~d may have 2~0~;t7~0 any de~ired geometrical shape, bu~ will usually ~e cir~lar, squ~re or re~tan3ul~r i~ ~LOsS--~eCtion. Simil~rly, ~he fiber bundle product$ may have any desir~d c~os~-~ec~ional conEiguration, ~u~ cir~ular~ rectangul~r, s~uar~, etc.
Furthe~more, even wi~h apertU~eS ~aving circular cros~-sec~ions the iber bundle~ ~ay ~enerally ~I.atten to a Inore r~ctan~ular cross-sectivn duriny the curin~ proces~.
~ urthe~ore, be~ore or after the adhesive is fully cu~ed, but p~efe~bly a~ter the adhesi~e is no lon~er taaky to the touch, the fiber array is cut by any appropridte autting devi~e in~o ~iber bundles. The fi4er bundles, depending on the nun~er and diame~er of the fibers in the bundle will itsel~ have ~ diameter in the r~nge v~ from about 1 to about 10 milli~eters, preferably from ~bou~ 2 to B mm. Furth~rmore, to assur~ uniform dis~ribution of the fiber bundles in the cemen~ mass wi~h Tninimum tanglin~ and viscosi~Y inçrease and ~or e~se of h~ndling, the a3pect ra~lo, i,~. leng~h-to-diamete~ d~ ratlos of ~he fiber bundles will range f~orn about 10:1 to 1:10, preera~1y 3:1 to 1:~, more pre~erably 2:1 t~ 1:2. As used herein ~length-to-2~ diarneter (l/d) ratios" for fiber bundles whic~ do not have a~ircular cross-~ection a~e dete~mined on the basis of the longest edge dlmension o the cross-sec~ional configu~ation Fo~
instance~ ~or a fiber ~undle o length (1) and with a rect~ngular cross-sec~ion, su~h ~s sho~n in ~i~ur~ 3, o wldth w and heiyht h, where w ~ h, the a~ect ratio (leng~h-to-di~meter) ratio is taken as l~w. This d2finition is consis~ent with gene~al prin~iples of bulk viscosity measuremen~s. In short, ~he contr;ibution to increased viscosity in the bone c~ment ~ due to ~00~7~) the l~r~el dil7~en~1Orl. Since, ir, the illu6trated c~se of igD 3, the ~eight ~o~ depth) h of the fib~ bundle 1~ ~mall with ~e~pect to th~ len~th 1 and wid~h w, the ~ntributlon ~o lnarease in vis~osity (e.g~ from the "dra~ oE ~he fiber bundle th~ough the cement ma~rix) will be negligible.
l`he a~hesi~ binder for :le ~ibc~ bundl~ y be selected ~rom any biocompat ,le m~ter1~1 whi~h is sol~ble in t~e liquid monomer componen~ o~ the bvne ~emen~. The adhesive material should also l;~e comp~tible with and not adversely a~fect 10 ~he ~uring or ~reng ~h of the ~one cemen'c, ~ he pr}~l~erl:~d adhesi~e bl~er materia~ elected ~o~ th~ (me~h)acrylate compounds~ and may be the same a~ o~
di~erent ~rom t~l~ polymer OL liquid mon~mer componen~ o~ t~e bone cem~nt.
lS Particul -ty good results are o~t~ined with poly(butyl~
metha~rylate) adhesive aolnpo . ition~ ~nd wi.th poly(methylmeth-acrylate) adhe~ivq c:ompo5itlons.
The ~mvunt o~ ~he adhesive ~inder fo~ the f ib~ bllndl~s is no~: partic:ularly c:ri~ic~l bu~ generally may be th~ minimum 20 amounl: requ~red ~o ~lold th~ fib~rs in the fi~e~ bundle~ during h~ndling. For exampl.e, it i~ not nece~sary t~at all of th~
f i~ers are ur,iormly or even tot~lly cove~ed hy the adheF:ive binde~ . However, ~he amount oE a~lhesi~, e bir~der should also ~e sufficient to allow the bundle~ to be dis~ribu~ed thLouyhout the 2~ m~xed bone ~emen~ poly~er and monom~r b~fore the adhesive is subst~nti~lly dis601ved in the monome~ ~omponent and a ~i~nif ican1: propo~tio~l o the indi~idual f ibers are ~lea~ed fr~om 'che bundle into the c:ement mixtu~. u~ually, an amoun~ of adhesive binder ~anging ~rom a~out 1 t~ ahou~ 20~ bX ~eigh~ o ~306t75~

the metal ~ibe~, yre~erably from ~bout ~ ~o ~0~ by weight, is 6uficient~
~e me~hod ~f ini~ing ~he ~ib~r bundles wi~h the c~ponen~s of ~he bone cement i~ ~lso impor~ant. For exampl~, it has ~een ~ound ~hat mixing th~ fibe~ bundle~ with liquid m~nomer prio~ t4 comt~ininy tt~e polymer powder and liq~id ee~ults in a ~ongealed mass o~ damp ~ibers and it is virtually imposslble to mix use~ul amounts of the damp ~ibers ~i~h ~he polyme~ powder.
It is presumed that the reason ~or the ~lumping of the fibers released from the fiber bundl~s is due ~o a lar~e p~oportion o~
the liquid monomer being adsorbed ~o th~ ~ibe~ ~urfaçe~, especially a~ter ~he adhesive binder is dissolved. The adsorbed monomer is then not availa~le du~ing the initlal phas~ of the polyme~i~ati~n proce~s. FO~ exa~ple, ~or a 6ingle s~and~rd commercially avail~ble pa~kage o P~MA bone cemant contalning about 40 gr~m. pQlymethylmethacrylate polymer and a~oll~ 20 ~ram~
of methylmeth~cryl~te ~on~me~ at ~ ~ v/v flber lo~dLng with fibe~s h~ving a di~m~er of 12 micronB, the total ~urface a~ea OÇ
the f ibers available to ~d~o~b t~le ll~uid monomer i8 ~rom ~bout ~0 0.5 to 3.0 m2-There~re, in aaco~dance with one em~odiment ~f ~he invention, e.~p~clally for the additlon of smaller vol~me~ o~
~ibers to th~ bone cement the f lhe~ bundleS ~re pre-mix~d wit,h the powdery polymer befoLe mixing wi~h the liquid monom~r.
For small amoun~s oE iber~, ~or examplc, up to abo~t 5& by volume ~v~v), especially up ~o about 3% v~vr ~uch a~ 1~ o~
2~ v/v, the fiber bundles may firet be mlxed with ~he powdery polym~r componen~, This t~chni~ue will avoi~ the clump~ng p~oblem described above, however, when the liquid monome~ is 3Q added to ~he fiher/poly~er mix~ure, ~he Yisco~i~y o the X~)Ç;7S~ ~

resulting mixt~re inareases ve~y rapidly ~king thq m~ e ~iicult to work, wi~h, especlally with more than about ~~3~ v/v O~ fi~e~s. Above ahout 5~, for example at above 6~ v~v, esp~ci~lly a~ about ~ V~V, i~ is g~ne~ally no longer pos6ible to e~ectively hand mix the liquid monomer With the ~ib~r/pol~mer ~ixture. When the resulting mixture is ~llowed ~o polyme~ize sm~ll rein~orc~d cement "nu~gets," rather than the desired coiles ive mass, are ~or~ed.
By fo~miny the flbers into bundles the ini~
l~ avail~ble s~r~a~e area ls ~reatly redu~e~ there~y minimiZing ~he amount Oe ad~orbed liquid mo~omer unaYaila~le ~or polymerizatlon.
Thi~ advantage r ho~ever, is offset or larger volume~ of the added f ib~ bundle~, ~ .9 . above about 2-3g v/v, especially above s~ v/v~ pa~icl~larly ~or ~ eKs wi~h smaller dlameter~, e.g~
b~low ab~ut 15-20 micr~ns. For larger diame~e~ f i~ers ~ e .g ~ ~t leas~ 20 mi~on~, especially At le~st 30 microns, or example 4 ~o lO0 ~icrons, the ~otal availabl~ sur~ace acea ~an be ~uf ~iciently reduced to ret~in ths ~dvant~ge of the f Ibe~
bundles, even a~ higher to~al iber volumes~ ~or example, up to 20 3~ v/v or higher, such a~ ~p to about S~ v/v.
~ nother u~eful app~o~ch ~ in~e~se the monomer available for poly~erizatio~ is ~o compensate dire~tly ~o~ the monom~ a~sorbed on~o the metal ~ibe~ by incr~asing the amount o the monome~ relative to the ~mo~n~ of polymer. For example, in stan~ard c~mmercially available acrylic bone cement~ the polymer powder to monome~ uid ~atio, ~y weightt is 2:1.
~n~ea~lng the a~ount o~ mono~er will also increase the ti~e a~ailabl~ to mix th~ eiber~ and monomer, thereby provLding thQ
additional advanta~e o~ distri~uting the ~i~ers more uni~or~ly th~ou~hout the cement ma66. still ~urther it is expeoted that ~I~Ofi~

~he inc~ased monomer con~ent and in~eas~d working time ~ill al~o l~ad to an incLe~se in the wei~ht a~erage ~ole~ul~r welght ~f the resulting polyme~. There~ore, in accordance wlth this embodimen~ of the invention the bone ~ement co~po~ition i8 Eo~med by mixin~ th~ polymer powAer alld liqui~ monOmer at a po~y~e~:monom~r wei~ ratio o~ ~rom ~..5 1 ~0 1:1.5, ~or example, about 1.1:1, 1 1 or 1:1.1.
Ac~ording to ~till ano~her embodiment of the invention th~ mixing and handling p~ope~les o~ th~ reineorced bone ce~ent i~ facilitated using a low visco~i~y ~one cement (LV~ uch as that commercially av~ilable ~rom Zlmme~, In~. of Warsaw, Indiana, Generally, the LVC exhiblt lo~ v~ ty le.~. from a~out 10 t~
1000 ~ps) due to the smaller ~iz~ of the finely divided polyme~
pow~er, a~ compared ~o the viscosities of rom a~out 1000 to ~000 15 Cp8 ~or more conventional a~rylic ~ PMM~ hone ~ements. The use of ~C ~ype bone cemen~s will in~re~se the ~nlEo~mity vf ~ber distribu~ion hy increasing the available mixiny time a~ well as deg~ee of mixing.
~y inc~easin~ the ~onomer contes~t and/or b~ using a lo~
2~ vis~05ity bone ce~ent it iz possible to ~ni~ormly incorporate higher total vol~m~s of the reinforcin~ ~ihers, for example, ~om 3 to 10% vfv, especially 3 to ~ v/v.
However, a~ter e~ten~ive t~ial ~n~ ecco~ experi-mel~tation it h~6 been ~ound that the most e~icient app~oach and 2S p~efer~ed method of introdu~in~ ~he fiber~ into ~he bone cement, es~ecially for ~ibe~ amount~ abov~ ~hout ~ v/v, is ~o first mix the powder and liguid monomer componen~6 o~ the bone camen~, without th* Eiber bundles, a~ in conventional practi~e E~ non-reino~ced bone cement. A~ter the po~ery oc granular polymar co~ponent is di~solved $n the li~u,id monomer and ~oly~r ~2~tion ~o~ o has begun (for exa~p1e, about 30 to 60 seconds, e.g. ~bou~ 45 ~econd~, after mix~ng is completed) the Eiber ~undles are ~radually add~d to the initial dis~olve~ polymer mi~r~ over ~
peri~d o from about 15 to ~5 seconds, such as 30 second~. After S all the fiber bundles are added to the dlsso~ved polyme~ mixture, hand mixing is ~on~inued for an ~dditional per~od o~ ~ime to assure h~mogeneity, Addit~.inal h~nd mixing ~or ~ro~ abvut 15 to bO se~onds, such as ~a, 3~, 4a or ~5 seconds, is gener~lly su~ficien~ to a~hieve ur,ifor~ fiber dis~ri~utlon. The resulting mass ~ay then be c~ntrifu~ed or subjected ~o a vacuum ~ccordins to common current ~a~ti~e, usually a~ter being fed into a cement gun. Cen~rifuging or va~um application is used to remo~e air b~bbles or v~idq present in the cement mass. The ~ement ~ass, wi~ or wi~hout the air remov~l, is ready ~or i.njection into the body and~o~ onto the pr~thesis.
In oraer to delay the onset of polymeriza~ion afte~ the initial mixing o~ polymer powder and, li~uld monomer and ~hereby incr~ase ~vailable mixing time, it i.~ prefe~re~ to chill the liquid m~nomer before mixing it wi~h the pslyme~ p~wder~
Chilling for about 15 minu~eR or more in an i~e water bath i~
usually suffi~ient to s~.ow the p~lymerization rate to allow the ~olym~r and liquid monOme~ to b~ mixed by hand until a substant1ally homo~eneous viscous liquid ~ formed~ usually mixing ~y hand for a~o~t 4S second ac~4mplishes thi9 resu1~.
The inven~ion will now be described by the foll~wing ~llustrative n~n-limiting example~.
In the ex~mpl~s, a three point ben~ geoTnet~y i~ us~3d ~or the fraoture toughness lKI~ ) tes~inSI. 8ince the~e are n~
AS~M stan~d~ Çor the fractu~e toughness testing o po.Lymer~, a geom~try was adap~d from AST~-E 359-~3~ th~ stan~ar~ for ~ z~n~s~

~acture toughne~s ~e~t~n~ of metals. T~owever~ PMMA ~cts as ~
brittle ma~e~;~1 du-ing rapicl Eractu~e, and does no~ ~orm a large pl~stlc æon~, ~herefore, t~e spe~imen dimensions dictated for met~ls ~hould p~c.vide a v~'ld mea~ure ~ XIc for PMMA. The spe~imen geome~cy i~ sçl~e~"a~ically illus~rated in ~lgu~e 2.
starter notch is ~achined with a diamond ~aw, and then a groove is ~U~ into the tip of the not~h wi~h a scalpel. ~ fati~ue crack is grown from the s~ar~e~ notch in order to ~reate a sh~rp crack in the ~pe~imen prior ~o the ~Ic teqting. The specimen~ are 0 prefatigued un~er lo~d contr~lled Ç~tigue using an Ins~ron 112 ~crew-ty~e tes~in~ machine or under stroke c~ntrolled ~ati~ue usin~ an Instron 1331 servo~hy~raulic ~estin~ machine. The lat~e~ is congidered to enahle more s~bl~ fati~ue rrack g~owth and, there~ore, provide more consistent tests.

l~xample 1 A serie~ of fract~re toughness te~ts wer~ per~ormed Ll~in9 commercially available ~owmedic~ s;~plex P PM~ bone ~emen~
with no ~lberq ~control~ by volume ~Run A), 2~ ~y volume ~Run B~, 5~ by ~olume ~RLIn ~ or 10~ by v~lum~ (Run D) of ti~anium fibe~s.
The tit~nium ~iber~ ~re ~rov~de~ a~ fl~ ctangular bundles (obtalned er~m N.V~ Bekaert S.A~, Zwenegen, Bel~ium) a~proxi~atel~ 0~9 m~ h~gh, and a. 3 ~m in depth containin~ ~everal hun~red individual Ti f~bers, with the ~ib~r6 in each bundle being ~ither ~.~ mm or 5.Q mm in l~ngth and 12 micron~ in diameter, the fi~er~ ln ~ach bundle beln~ glue~ tog~ther by a poly~ bu~ylme~ha~rylate~ 9lue.
~ h~ ~i fibe~.s have a r~ug~ ~ur~ac~ w~i~h contributes to 7~

the ~dhesion b~tween the ~i eeinfor~ln~ ~iher~ and the PMMA ~one cement.
In th~ example, t~e fiber ~undles are first added to tl~e polymethylmeth~crylate polymer powd~r co~ponent by simply 5 mixing (e.g. "foldin~n) the bundles into the polymer powde~ with a fla~ Sp~tula. ~he 11quid metha~rylate monomer is similarly mixed with ~he p~lymer/fiber premixture for about 60 se~ond5, For ~un C the viscosity bec:~me ve~y h~.gh and mixing of ~he result~ng monome~/polymer~fiber mixture was ex~remely di~Eicult.
10 ~or Run D test ~pecimens could not be produced ~ince the ~einforced cen~ent polym~rized into ~mall "nu~getj" rat~er ~han a aohesive mass, presumably due to the exces~iv~ adso~ption o liqui~ monomer to the fiber suc~a~es, and the una~allability of ~he adsorbed monomer fo~ the ea~ly porti~n of th~ polymerization 15 pro~e~s.
EAch Q~ the cont~ol~and r~infor~ed hone cements of R~h~
A, B and C for ea~h of the 1.5 ~nd 5.0 mm Ti fibe~s we~e injected using a standard cement g~n into Te10n and UliMWPE molds In~o the s~ape illustrated in Figure 2~ Prio~ to moldin~ the ~ontrol ("control" re~erg t~ ~pecim~ns made without f~bers, i.e. p~re bone cement spe~imens) composition ~n~ ~he compositions o Runs A
an~ B ~ere ~ent~iuged ~o~ one minute ~t 25~0 rpm (Da~on model.
HN-SII IEC #~04 rotor with a rotating r~dius of 9.2 cm) t~ eeduce the poroslty inherent in bone cement. The ~omposition of Run C
25 was not centrif~g~3d, hc~wever, SinGe it has b~n reported in the literature that standard f racture toughn~ss tests do not re1ect the lmprovements in PMMA due to oentri~ugation. ('IStandard'' fracture ~oughn~ss test~ are de~i~ned to orce a crack ~o propagate ~rom a predetermined point in a speci~ic direction, The si~e of crack tip pla~ti~ zones In PMM~, the extent o 3~

influence of a~ inclusion on the stre~s ~ield o~ a body, the statistic~ of di~tributlon o~ inclu io~s, ~nd the velocity oi pro~a~ating ~racks in mo~ T fracture toughneS5 t~t~t make it hiyhly unlikely that ~he inclusions will afect ~he ~endency for the crac~ ~o p~opaga~e in ~andard tests.
The ~ra~t~r~ ~oughn~ss d~ta for the cantrol and reinforced PMMA spe~mens ar~ plotted in Figure 7. There ls an increase ~n Eracture toughness with increase4 ~iber cvntent.
The number over each b~r i8 ~he pe~cent~ge inç~eas~ in frac~ure 10 toughn~ss relatlve to th~ fracture toughness o~ the control o~
the respective sample. ~t 5~ ~ol. fiber content, the ~ract~tre toughne~s of the 1.~/12 and the 5.0jl2 6peclmens ar~e rtearly eq~al. The in~rease in fracture ~oughne~s for the 5~ 4pecime~s is nearly sO~. yet it seems that it should be even grea~er when lS extrapolated f~om the 1 to 2% data. It is presumed th~t amoun~
oÇ monomer ad~o~bed into the ~5~) ~iber sucfaces w~9 excessive, and the reinforced PMMA did not polyme~ize as completely as it did ior the 1 or 2~ specimen~.
The ~igue d~t~ collected ~urin~ the pre~atiguing o~
20 the fracture ~oughness specimens provide~ an indioation of ~h~
fatigue per~or~ance o~ the rein~or~d PMMA. Al~hough not providing ~u~nti~ative f~tAgu~ ~ata, ~ince starter notches may be of di~ferent len~th6~ such that ini~ial stress inten~ities m~y differ erom speci~en to speci~en, and ~he ~atigua cracks may gro~
~5 ~o ~lightly di~feren~ l~ngth~, ~he d~t~, neverth~le6s, is us~ful to demons~rate ~ome trends. Contr~s~ing the pla~n PMMA ~atig~ed in ~t~oke con~rol ~ith the 5~ ~inforced PMMA ~o~ both fiber diamete~s) ~how~ that ~he plain P~M~ i~ loaded cyclically with a peak ampllt~de o~ approxima~ely 30 lb~, and requi~e8, on the 30 average, approximately 4600 cyçle8 to grow a ~dtigUe c~ack to an 3~.

200~r~

~ep~able 5Ize. ~he 1.5 mm Ti f1~r r~inEorced PMMA i~ l~aded with a peak a~plitude of 35 lbs.~ and requi~es, on ~h~ average, a80Q aycles, Thus, ~he additlon o~ the reinEorcing fibers de~reases t~e ~rack velo~ity d~ring the ~rack pro~agati~n phase S o~ f at ig ue, Load da~a were al~o recorded as a unction o time ~u~ing the st~oke ~yc1e ~or the control and fo~ ~4inforced P~MA
specimen~. ~he load distinc~ly ~ec~e~5es ~s a cra~k propagates throu~h the ~on~l specimen~ since the spe~imen compli~nc~
increases and the di~placemæ~ remains const~nt as the ~rack ~rows~ The load trace ~or the reinf4r~e~ P~MA, conver~ely, rqmains nearly constant aæ the ~rack ~rows, indicating that the reinforced PMMA speci~en is able to m~int~in its ¢ompliance, and ~arry a Signi~cant lo~d. ~h~ ability o~ the rein~orced PMMA
specimen t~ ~aintain ~ constant loa~ ~s a crack propa~ates represents a significant improve~ent in th~ f~tigue properties over plain PMMA.

Exampl~ 2 2~ In this exa~ple, isobu~ylmeth~crylate bonded Ti ~ib~r ~undles pcepaLqd ~ro~n rou~h ~urface Ti ~ibe~ 22 microns in diameter and 1. ~ microns ln length were used to r~inforce ~lo~medi~a Silnplex P ~MMA ~urgical b~ne ~em~nt.
~he polym~r powder and liquld monomer ~cooled ln an ice ba~h ~or 25 minutes pri~r to use~ ~re combine~, and ~h~ mixt~ra is stirred, by hand, ~or 4S ~econd~ un~il th~ mixture become~ a viscous liquid. The Ti ~ib~r bundle~ are added gradually du~ing the next 30 ~econds un~il 5% by volume i8 in~orporated ln~o the viscous mixture and mixing i~ ~ontinue~ ~c an ad~ ional 30 ~rond~. The re~ulting mixture i5 t~n~err~d ~o a cement gun 06~7~

and is ~hen inj~cted into molds to ~o~m sp~cLmens for fr~c~u~e tou~hne~s ~ests. The ~e~ults ~re sho~n in bar graph ~orm in Figu~e ~ T~ere was a 56~ increa~e in toughnRss as com~ared to th~ con~rol ver~us a 46% in~rease in to~ghn~ss foL th~ reinforced PM~A o~ Run C o~ Example 1 ~Ti fibers = 12 microns~ 1.5 mm length; 5% v/v). Figuce 10 is ~ mi~rophoto~raph ~75X) o~ a cros~
sec~ion of the ~ured sample illumina~e~ f~o~ ~bove and below such that the random distri~ution of fi~er o~lentation o~ th~ flber strands (seen as dark lines) e~bedded in the cured cement ~ee~
as a grayish backg~ound) can be observed and ~lsa illuminat~d from above su~h that ~he ~iber cross sectlons can also be ob5e~ved ~as white dot~ at the end~ of the ~i~e~s)~ Fi~u~e 11 ls a similar ~nicrophotograPh bu~ ~aken with lightlng only eom below such that fiber cro~ ~ections are not lllumin~ted, ~xample 3 ~h~ p~ooeaure of ~xample 2 was repeated except that in pla~¢ of the t~owmedloa Simplex P s~gLcal bone cement Zimmerls LVC ~urgical bone cement was u~ed. The fracture toughn~6s was 86% greater than the ~ontrol ~Ex~mple 1) and 50~ grea~er th~n th~ control o~ Example 2 (~ee the bar graph plot in Figure 9).
Figure 12 is a microphotograph ~75X) with llghting from aboYe tO
illumlnate f iber CI:OS& sec~i~ns (~en as white dol;s on the gr~yi~h ¢ement backg~o~n~), It is seen ~rom the photograph that the ~iber dispersion is ~airly uniform~ without Cl~nps of ~iber~
and witho~t large sections where f ibers are absent~

Claims (29)

1. Claim 1. A fiber reinforcing material for incorpor-ation into a two component bone cement including particulate polymer component and a polymer?za? liquid monomer component and which reinforcing material can be readily incorporated by manual mixing into the bon? c?ment by a surgeon during a surgical operation, said reinforcin? material comprising a bundle of reinforcing fibers he? together by an adhesive material which is soluble in the liquid monomer component of the two component bone cement, the fibers of said bundle having a diameter in the range of from about 5 to about 100 microns and a length in the range of from about 0.5 to about 10 millimeters, and the length-to-diameter ratio of said bundle being in the range of from about 10:1 to about 1:10.
2. Claim 2. The fiber reinforcing material of claim 1 wherein the reinforcing fibers comprise metallic fibers.
3. Claim 3. The fiber reinforcing material of claim 2 wherein the metal fibers, comprise titanium fibers.
4. Claim 4. The fiber reinforcing material of claim 1 wherein the adhesive material comprises an acrylic adhesive.
5. Claim 5. The fiber reinforcing material of claim 4 wherein the acrylic adhesive comprises an acrylate or methacrylate adhesive.
6. Claim 6. The fiber reinforcing material of claim 1 wherein the reinforcing fibers have a diameter of from about 10 to about 80 microns and a length of from about 1 to about 8 millimeters.
7. Claim 7. The fiber reinforcing material of claim 1 which comprises from about 50 to about 500 fibers per bundle.
8. Claim 8. The fiber reinforcing material of claim 1 which comprises from about 100 to about 300 fibers per bundle.
9. Claim 9. The fiber reinforcing material of claim 1 wherein the length-to-diameter ratio is from about 3:1 to about 1:3.
10. Claim 10. A bone cement treatment kit for bone repair or other treatment of a bone defect, said kit comprising (1) finely divided acrylic polymer and (2) polymerizable liquid acrylic monomer for preparation of bone cement useful in bone repair or other bone treatment, and (3) a reinforcing material comprising bundles of reinforcing fibers wherein the fibers of said bundles are bonded to each other by an adhesive material which is soluble in said liquid monomer (2), whereby when said components (1), (2) and (3) are combined the adhesive material dissolves in said liquid monomer and allows said fibers to be homogeneously distributed throughout said bone cement by manual mixing.
11. Claim 11. The kit of claim 10 wherein said fibers are comprised of metal and have a diameter in the range of from about 10 to about 100 microns and a length in the range of from about 0.5 to about 10 millimeters.
12. Claim 12. The kit of claim 11 wherein the metal reinforcing fibers comprise titanium fibers.
13. Claim 13. The kit of claim 12 wherein said bundles of reinforcing fibers have a length-to-diameter ratio of from about 10:1 to about 1:10.
14. Claim 14. The kit of claim 10 wherein said bundles of reinforcing fibers have a length-to-diameter ratio of from about 10:1 to about 1:10.
15. Claim 15. The kit of claim 10 wherein said prepolymer (1) comprises a methacrylate or acrylate polymer, said liquid monomer (2) comprises a polymerizable acrylate, methacrylate or methylmethacrylate monomer, and the adhesive material (3) comprises an acrylate or methacrylate based adhesive.
16. Claim 16. The kit of claim 10 wherein each of components (1), (2) and (3) are separately packaged.
17. Claim 17. The kit of claim 10 wherein components (1) and (3) are premixed and component (2) is packaged separately.
18. Claim 18. The kit of claim 16 wherein said package containing said prepolymer (1) further comprises a polymerization initiator.
19. Claim 19. A method for forming a fiber reinforced bone cement for bone repair or over treatment of bone, said method comprising first mixing a powdery polymer component of said bone cement and a plurality of reinforcing fiber bundles, each of said bundles comprising a plurality of short reinforcing fibers bonded together by an adhesive material, and thereafter mixing said fiber-polymer mixture with a polymerizable liquid monomer component of said bone cement which can dissolve said adhesive material and thereby promote uniform and homogeneous distribution of said reinforcing fibers throughout said bone cement.
20. Claim 20. The method of claim 19 which further comprises chilling the polymerizable liquid monomer prior to mixing the liquid monomer with said fiber-polymer mixture.
21. Claim 21. The method of claim 19 wherein the amount of fibers mixed with said powdery polymer comprises up to about 5%
    by volume, based on the total volume of the bone cement.
22. Claim 22. The method of claim 19 wherein said fiber bundles comprise from about 50 to about 500 reinforcing fibers, per bundle, said reinforcing fibers having a diameter of from about 5 to about 100 microns and a length of from about 0.5 to about 10 millimeters, said bundles having a length-to-diameter ratio of from about 10:1 to about 1:10.
23. Claim 23. The method of claim 22 wherein said reinforcing fibers comprise titanium fibers.
24. Claim 24. A method of forming a fiber reinforced bone cement for bone repair or other treatment of bone, said method comprising premixing a powdery or granular polymer component of said bone cement and a liquid polymerizable monomer component of said bone cement which is a solvent for said polymer component to form a viscous liquid mixture, gradually adding to said liquid mixture a plurality of bundles of short reinforcing fibers, the fibers of said bundles being glued together by an adhesive which is soluble in said liquid monomer and stirring the resulting mixture to distribute the fiber bundles throughout the mixture, while allowing the adhesive component to dissolve in said liquid monomer whereby the individual fibers of said fiber bundles are released into and randomly distributed throughout the bone cement.
25. Claim 25. The method of claim 24 wherein the fiber bundles are added in an amount to provide from about 1 to about 10% by volume of the bone cement.
26. Claim 26. The method of claim 24 wherein the fiber bundles are added in an amount to provide from about 3 to about 10% by volume of the bone cement.
27. Claim 27 . The method of claim 24 which further comprises chilling the liquid monomer prior to premixing with the powdery polymer.
28. Claim 28. The method of claim 24 wherein said fiber bundles comprise from about 50 to about 500 reinforcing fibers, per bundle, said reinforcing fibers having a diameter of from about 5 to about 100 microns and a length of from about 0.5 to about 10 millimeters, said bundles having a length-to-diameter ratio of from about 10:1 to about 1:10.
29. Claim 29. The method of claim 28 wherein said reinforcing fibers comprise titanium fibers.