Advanced Machining of Joint Implant UHMWPE Inserts

Ci a ion: Piska, M.; U banco a, K. Ad anced Machining o Join Implan UHMWPE Inse s. Machines 2022,10, 1008. h ps://doi.o g/10.3390/ machines10111008 Academic Edi o : Yuwen Sun Recei ed: 16 Sep embe 2022 Accep ed: 28 Oc obe 2022 Published: 1 No embe 2022 Publishe ’s No e: MDPI s ays neu al wi h ega d o ju isdic ional claims in published maps and ins i u ional a il- ia ions. Copy igh : © 2022 by he au ho s. Licensee MDPI, Basel, Swi ze land. This a icle is an open access a icle dis ibu ed unde he e ms and condi ions o he C ea i e Commons A ibu ion (CC BY) license (h ps:// c ea i ecommons.o g/licenses/by/ 4.0/). machines A icle Ad anced Machining o Join Implan UHMWPE Inse s Mi osla Piska * and Ka e ina U banco a Facul y o Mechanical Enginee ing, B no Uni e si y o Technology, FME IMT, 2 Technicka, 616 69 B no, Czech Republic *Co espondence: [email p o ec ed].cz; Tel.: +420-5-4114-2555 Abs ac : The mode n o hopaedic implan s o applica ions in hips, knees, shoulde s, and spines a e composed o ha d me al alloys o ce amics and a ibological sub-componen ha is made o so ma- e ials, wi h good ic ional p ope ies—e.g., UHMWPE (Ul a High Molecule Weigh Polye hylene). The UHMWPE implan s need o be machined in o hei inal shape a e he polyme iza ion and consolida ion in o a blank p o ile o nea ne shaped implan . Thus, machining is a c ucial echnology ha can gene a e an accu a e and p ecise shape o he implan ha should comply wi h he join s’ unc ion. Howe e , he machining echnology can a ec he opog aphy and in eg i y o he su ace, ansmi ed s esses, and esis ance o wea . Technology, cu ing ools, and cu ing condi ions can ha e an impac on he physical and mechanical p ope ies o he en i e implan and i s longe i y. This pape shows an e ec i e and compe i i e echnology o acqui ing high-quali y inse shape, dimensions, and su ace, needed especially o cus omized implan s. Keywo ds: machining; UHMWPE; implan ; su ace in eg i y; ibology 1. In oduc ion Cus omized pa ien -ma ched echnology join eplacemen p esen s a ision o be e medical ca e ha e lec s oday’s knowledge o ma e ials and compu e aided echnolo- gies [ 1 ]. The echnology can minimize he loss o human bone compa ed o he s anda d su ge y, and he use o uni o med p oduc ion se ies p olongs he li e o implan s and im- p o es he implan s’ sus ainabili y. The design o he join is based on a h ee-dimensional image o a join made by magne ic esonance imaging, which allows he su geons o iden- i y he ex en o a epai needed o he enginee s o p oduce a ue model o he join , all new pa s, and co ec sizes. This has been p o en success ul in he p oduc ion o he me al knee componen s made o Ti-6Al-4V ELI wi h he elec on beam mel ing machine A cam Q10+. The excellen ma e ial s uc u e was based on he use o globula Ti-6Al-4V ELI pa s o size 50.65 ± 22.46 µ m (mean ± s anda d de ia ion) in diame e , which we e mel ed in a acuum chambe [ 2 ] in an op imized way o sin e ing. This ma e ial is di icul o machine [ 3 ], bu exhibi s unique s uc u es and p ope ies needed o emo al and ibial join p os hesis [ 4 ] in a wide ange o mo phologies. The e y dense, solid s uc u es can be used o ibological, highly loaded applica ions and he spongious s uc u es o a e y good ancho ing o he bone and bone ing ow h [ 5 , 6 ]. The ma e ial quali y, su ace in eg i y, mechanical p ope ies, machinabili y o he alloy by g inding, polishing and umbling, and, inally, a high esis ance o he implan o a igue and wea a e c ucial [ 6 ]. Such ad an ages open no el echnologies o join eplacemen s [ 7 ]. Howe e , he p oblem a ose wi h he p oduc ion o he “so ” pa ha makes he ibological con a pa . The pa o indi idual knee design should be machined om a solid block o ma e ial. The basic machining echnologies a e u ning and milling (each can be used as oughing o inishing). The e a e many ways o machine hese su aces. Ne e heless, p oblems such as low igidi y o he p oduc , poo he mal p ope ies o he ma e ial, high mel iscosi ies, and s icking o he ma e ial o he cu ing edge (p oduc ion o buil -up edges) should be aced. The UHMWPE ma e ial may be damaged by excessi e hea , eed a e, cu ing o ce, and ool mic o-geome y. The shapes and dimensions o he cus omized implan s a y b oadly in Machines 2022,10, 1008. h ps://doi.o g/10.3390/machines10111008 h ps://www.mdpi.com/jou nal/machines Machines 2022,10, 1008 2 o 19 humans, which complica es he machining echnology. No s anda d p og ams like hose used o uni ied implan shapes and dimensions can be used epea edly, so each join mus be designed and p oduced indi idually. Howe e , his esul s in a longe implan li e and be e com o o pa ien s. Many p oduce s do no publish he de ails abou p ecise machining o he UHMWPE pa s, bu d y o c yogenic machining, acuum as ening o he wo kpieces, e y sha p cu ing ools, and low cu ing condi ions a e gene ally known and ecommended. The o a ional pa s a e u ned, bu he shaped pa s (as he emo al join inse s) can be milled wi h ball milling cu e s only due o complex geome ies o he unc ional su aces. This echnology p e ails o he s anda dized join pa s p oduced in a ious sizes and se ies. The main ad an age o he echnology is o sa is y cu en needs in si u a he join su ge y in he hospi al. Howe e , he machining should be s udied mo e, because he quali y o he unc ional pa su aces a ec s he o al success o he a h oplas y and longe i y o he implan s. 2. Ma e ials and Me hods 2.1. Ma e ial and Design o Join Inse s The e a e mo e han one hund ed design solu ions o knee implan s all o e he wo ld, bu he e is a s ong de elopmen in cus omized p oduc s, indi idual o each human o animal. Basically, he solu ions a e p incipally simila , and mos o hem con ain an inse o gua an ee a low ic ion in e ace be ween he emo al and ibia pa s made om me al alloys o ce amics [ 8 , 9 ]. The use o UHMWPE o he inse seems o be p e ailing [ 10 – 12 ] and longs anding [ 13 ] in su gical p ac ice. The ma e ial was polyme ized in he 1950s and he i s UHMWPE ib es we e comme cialized in he la e 1970s. UHMWPE is a ype o polyole in ha is composed o e y long chains o polye hylene wi h a e y high pe cen age o pa allel o ien a ion and a high le el o c ys allini y. The ex emely long polyme chains enable load ans e by s eng hening in e molecula in e ac ions. The ma e ials su e om gamma adia ion, which a ec s hei molecula s abili y and mechanical p ope ies like o he ma e ials o medical applica ions [ 14 ], bu so a hei ibological p ope ies and esis ance o a igue a e unique [ 15 ] and he e a e many echnologies o p ocessing i acco ding o he ad anced s anda ds [ 16 – 19 ]. This speci ica ion [ 16 ] co e s UHMWPE powde and ab ica ed o ms o use in su gical implan s. UHMWPE powde shall be o i gin polyme manu ac u ed om a homopolyme o e hylene. Tes s o iscosi y numbe , elonga ion s ess, ash con en , ex aneous ma e , and ace elemen s shall be pe o med o UHMWPE powde s, while es s o densi y, ash con en , ensile s eng h, yield s eng h, elonga ion, and impac s eng h shall be pe o med o ab ica ed o ms. The documen [ 17 ] speci ies he equi emen s and co esponding es me hods o moulding ma e ials in powde o m made om UHMWPE o use in he manu ac u e o su gical implan s. The s anda d [ 18 ] speci ies he equi emen s and co esponding es me hods o moulded o ms, e.g., shee s and ods, made om UHMWPE o use in he manu ac u e o su gical implan s. This documen is no applicable o di ec -moulded, i adia ed, o inished p oduc s, o p oduc s manu ac u ed om polye hylene blended wi h addi i es o by blending di e en o ms o polye hylene. The documen [ 19 ] speci ies a es me hod o in es iga ing he oxida i e s abili y o UHMWPE ma e ials as a unc ion o he p ocessing and s e iliza ion me hod. The e a e plen y o o he applica ions in a wide ange o con ac ing su aces— o ming, manipula ion o ma e ials, guiding pads, suppo ing uni s, e c. The e a e some ypical p oblems wi h machining hem o he speci ied shapes, dimensions [ 20 ], and high-quali y su aces [ 21 , 22 ]. I has been equen ly con i med ha his inse pa su e s ei he om he high speci ic loading and/o speci ic ma e ial p ope ies, so i s in ensi e wea in knee implan applica ions is e y common–Figu es 1and 2. The o igins come gene ally om he ma e ial composi ion [ 10 ], echnological p ocessing, applica ion, and use [ 23 – 29 ]. Obse a ions o wea damage in [ 23 ] we e pe o med on e ie ed wo n po ous coa ed ana omic ibial componen s. The noncon o ming a icula ing su aces on hin polye hylene componen s would be a highe isk o damage han mo e con o ming su aces on hicke componen s. Machines 2022,10, 1008 3 o 19 Mo eo e , he high cyclic loads a e mos esponsible o he deg ada ion in p ope ies o he ma e ial nea he a icula ing su ace. Acco ding o [ 24 ], he wea o ma e ials causes di e en consequences depending on he ype o he sys em in which i occu s. In biological sys ems, pa icles p oduced by wea elici an en i onmen al esponse, e mina ing he li e o he bea ing sys em as in hip and knee join p os heses. I is shown ha polye hylene wea p oduces sub-mic ome e pa icles, which a e known o con ibu e o os eolysis and implan loosening. The mechanisms esponsible o p oduc ion o polye hylene wea pa icles a e mainly ploughing and ab asion by ha d aspe i ies and/o wea pa icles, as well as delamina ion wea . The po en ial o os eolysis induced by wea deb is is highligh ed by he wo k [ 25 ] also. These esul s sugges ha ea ly knees may ha e simila wea mechanisms and indica e ha simila olumes o biologically ac i e mic ome e and sub-mic ome e UHMWPE pa icles we e p oduced, which may ha e impo an implica ions in he longe - e m ou come o o al knee a h oplas ies. Ano he wea mechanism—delamina ion, in which a hin su ace shee o polye hylene sepa a es om he deepe laye s—was con i med in [ 26 ] and some ea lie wo ks also. I has been con i med ha he delamina ion is no only a p oblem o ca bon- ein o ced UHMWPE, bu he mechanism o damage can be ound in no mal UHMWPE ma e ials. The same mechanisms and many o he s, such as ab asion, e osion, adhesion, e c. a e also desc ibed ho oughly in [ 27 ]. The book [ 28 ] sugges s a new app oach o explo ing he e ec o applied load and su ace de ec s on he a igue wea beha iou o polyme s. I discusses e ec s o su ace c acks unde di e en s a ic and cyclic loading pa ame e s on wea , and p esen s an in elligen algo i hm o map he ela ionship be ween wea a e and ele an ac o s. Machines2022,10,xFORPEERREVIEW3o 19   composi ion[10], echnologicalp ocessing,applica ion,anduse[23–29].Obse a ionso  wea damagein[23]we epe o medon e ie edwo npo ouscoa edana omic ibial componen s.Thenoncon o minga icula ingsu aceson hinpolye hylenecomponen s wouldbea highe  isko damage hanmo econ o mingsu aceson hicke componen s. Mo eo e , hehighcyclicloadsa emos  esponsible o  hedeg ada ioninp ope ieso  hema e ialnea  hea icula ingsu ace.Acco ding o[24], hewea o ma e ialscauses di e en consequencesdependingon he ypeo  hesys eminwhichi occu s.Inbiolog‐ icalsys ems,pa iclesp oducedbywea elici anen i onmen al esponse, e mina ing heli eo  hebea ingsys emasinhipandkneejoin p os heses.I isshown ha polye h‐ ylenewea p oducessub‐mic ome e pa icles,whicha eknown ocon ibu e oos eol‐ ysisandimplan loosening.Themechanisms esponsible o p oduc iono polye hylene wea pa iclesa emainlyploughingandab asionbyha daspe i iesand/o wea pa i‐ cles,aswellasdelamina ionwea .Thepo en ial o os eolysisinducedbywea deb isis highligh edby hewo k[25]also.These esul ssugges  ha ea lykneesmayha esimila  wea mechanismsandindica e ha simila  olumeso biologicallyac i emic ome e and sub‐mic ome e UHMWPEpa icleswe ep oduced,whichmayha eimpo an implica‐ ionsin helonge ‐ e mou comeo  o alkneea h oplas ies.Ano he wea mechanism— delamina ion,inwhicha hinsu aceshee o polye hylenesepa a es om hedeepe  laye s—wascon i medin[26]andsomeea lie wo ksalso.I hasbeencon i med ha  he delamina ionisno onlyap oblemo ca bon‐ ein o cedUHMWPE,bu  hemechanism o damagecanbe oundinno malUHMWPEma e ials.Thesamemechanismsandmany o he s,suchasab asion,e osion,adhesion,e c.a ealsodesc ibed ho oughlyin[27].The book[28]sugges sanewapp oach oexplo ing hee ec o appliedloadandsu ace de ec son he a iguewea beha iou o polyme s.I discussese ec so su acec acks unde di e en s a icandcyclicloadingpa ame e sonwea ,andp esen sanin elligen  algo i hm omap he ela ionshipbe weenwea  a eand ele an  ac o s. Thepin‐on‐discwea  es s(pinso AISIF138s ainlesss eelo ASTMF136 i anium alloyagains aUHMWPEdisc,wi hbo inese um)we es udiedin[29].Thelowe  alues o  ic ioncoe icien we emeasu edwhena hin ilmo polyme was ans e ed o he me allicsu aces,bu  he iscoelas icbeha iou o UHMWPEwas esponsible o  hein‐ c easein ic ioncoe icien a e a es ingpe iod. Apa  ommanyo he  es s, hepa ame e ssuchas helocalizedHe ziancon ac  s esses ha de elopas wocu edjoin su acescomeincon ac (andde o msligh ly) unde  heimposedloadsandsliding eloci yp esen  hec ucialp oblem.Thead anced designo acus omizedjoin implan should espec  heindi idualhumano animalana ‐ omyandallbodyp edisposi ions,andmakeab oad la con ac wi h egula ly ansmi ‐ edloadingind y ic ioncondi ions.  Figu e1.TheUHMWPEinse  o  hehumankneejoin :(a)newone,(b) hedamagedsu acea e  se e alyea so use. Figu e 1. The UHMWPE inse o he human knee join : ( a ) new one, ( b ) he damaged su ace a e se e al yea s o use. Machines2022,10,xFORPEERREVIEW4o 19    Figu e2.Ade ailo  hewo nsu aceo  heUHMWPEinse —Adeepca i ya ec ing hequali y o  he ibologicalcon ac . Fo  hesegoals, hedesigns a swi h3Dcompu e  omog aphy(CT)o magne ic esonance(MR),andcompu e digi al econs uc iono  eno a iono  hede ec edjoin  su aces ollows.A e  hep oduc iono  he emo aland ibiame allicpa s ia,e.g.,elec‐ onbeam echnology(EBM),apiece om henon‐me allicma e ialshouldbeins alled a  hein e acebe ween heme allicimplan s. Thispieceo ma e ialshouldgua an eepe ec con ac o  hesu aces o eachape ‐ ec sliding i  ha p oducesminimalcon ac p essu esandalowcoe icien o  ic ion o  along imewi hou anylub ica ion.Thep ecisep o ilescanbeob ained,e.g.,byscanning he inishedme alliccomponen sandanalyseso  heimplan c oss‐sec ionsinpe pendic‐ ula planes—seeFigu e3. UHMWPEisasemic ys allinepolyme consis ingo anamo phousma ix ein‐ o cedbyas i e ,c ys allinelamellaedomain(hal o i ss uc u e)[8–10].UHMWPE composi escanbeenginee edinawide angea amic o/nanome e ‐leng hscalebyblend‐ ingpolyme powde  esinwi hmic o‐o nanopa iclesand ib esbe o econsolida ion. UHMWPE ib escanbewo en,kni ed,o manu ac u edin oshee s o mingcomplex s uc u esandaniso opicp ope ies.UHMWPEcanplay he oleo ma ixand ib ein ab oad angeo composi ema e ials,dependingupon heapplica ion.Polyme s,which includeallplas ics,consis o chainso buildingblockscalledmonome s.Thesechains g owbyaddingnewmoleculeson o hei ends—Figu e4.  Figu e3.Digi alc oss‐sec ionso a emo alkneeimplan , hede ini iono  adiiinsagi al(a,b)and ans e seplanes(c,d) o  igh kinema icso  headjacen pa o  hejoin – heUHMWPEinse . Figu e 2. A de ail o he wo n su ace o he UHMWPE inse —A deep ca i y a ec ing he quali y o he ibological con ac . Machines 2022,10, 1008 4 o 19 The pin-on-disc wea es s (pins o AISI F138 s ainless s eel o ASTM F136 i anium alloy agains a UHMWPE disc, wi h bo ine se um) we e s udied in [ 29 ]. The lowe alues o ic ion coe icien we e measu ed when a hin ilm o polyme was ans e ed o he me allic su aces, bu he iscoelas ic beha iou o UHMWPE was esponsible o he inc ease in ic ion coe icien a e a es ing pe iod. Apa om many o he es s, he pa ame e s such as he localized He zian con ac s esses ha de elop as wo cu ed join su aces come in con ac (and de o m sligh ly) unde he imposed loads and sliding eloci y p esen he c ucial p oblem. The ad anced de- sign o a cus omized join implan should espec he indi idual human o animal ana omy and all body p edisposi ions, and make a b oad la con ac wi h egula ly ansmi ed loading in d y ic ion condi ions. Fo hese goals, he design s a s wi h 3D compu e omog aphy (CT) o magne ic esonance (MR), and compu e digi al econs uc ion o eno a ion o he de ec ed join su aces ollows. A e he p oduc ion o he emo al and ibia me allic pa s ia, e.g., elec on beam echnology (EBM), a piece om he non-me allic ma e ial should be ins alled a he in e ace be ween he me allic implan s. This piece o ma e ial should gua an ee pe ec con ac o he su aces o each a pe ec sliding i ha p oduces minimal con ac p essu es and a low coe icien o ic ion o a long ime wi hou any lub ica ion. The p ecise p o iles can be ob ained, e.g., by scanning he inished me allic componen s and analyses o he implan c oss-sec ions in pe pendicula planes—see Figu e 3. Machines2022,10,xFORPEERREVIEW4o 19    Figu e2.Ade ailo  hewo nsu aceo  heUHMWPEinse —Adeepca i ya ec ing hequali y o  he ibologicalcon ac . Fo  hesegoals, hedesigns a swi h3Dcompu e  omog aphy(CT)o magne ic esonance(MR),andcompu e digi al econs uc iono  eno a iono  hede ec edjoin  su aces ollows.A e  hep oduc iono  he emo aland ibiame allicpa s ia,e.g.,elec‐ onbeam echnology(EBM),apiece om henon‐me allicma e ialshouldbeins alled a  hein e acebe ween heme allicimplan s. Thispieceo ma e ialshouldgua an eepe ec con ac o  hesu aces o eachape ‐ ec sliding i  ha p oducesminimalcon ac p essu esandalowcoe icien o  ic ion o  along imewi hou anylub ica ion.Thep ecisep o ilescanbeob ained,e.g.,byscanning he inishedme alliccomponen sandanalyseso  heimplan c oss‐sec ionsinpe pendic‐ ula planes—seeFigu e3. UHMWPEisasemic ys allinepolyme consis ingo anamo phousma ix ein‐ o cedbyas i e ,c ys allinelamellaedomain(hal o i ss uc u e)[8–10].UHMWPE composi escanbeenginee edinawide angea amic o/nanome e ‐leng hscalebyblend‐ ingpolyme powde  esinwi hmic o‐o nanopa iclesand ib esbe o econsolida ion. UHMWPE ib escanbewo en,kni ed,o manu ac u edin oshee s o mingcomplex s uc u esandaniso opicp ope ies.UHMWPEcanplay he oleo ma ixand ib ein ab oad angeo composi ema e ials,dependingupon heapplica ion.Polyme s,which includeallplas ics,consis o chainso buildingblockscalledmonome s.Thesechains g owbyaddingnewmoleculeson o hei ends—Figu e4.  Figu e3.Digi alc oss‐sec ionso a emo alkneeimplan , hede ini iono  adiiinsagi al(a,b)and ans e seplanes(c,d) o  igh kinema icso  headjacen pa o  hejoin – heUHMWPEinse . Figu e 3. Digi al c oss-sec ions o a emo al knee implan , he de ini ion o adii in sagi al ( a,b ) and ans e se planes (c,d) o igh kinema ics o he adjacen pa o he join – he UHMWPE inse . UHMWPE is a semic ys alline polyme consis ing o an amo phous ma ix ein o ced by a s i e , c ys alline lamellae domain (hal o i s s uc u e) [ 8 – 10 ]. UHMWPE composi es can be enginee ed in a wide ange a a mic o/nanome e -leng h scale by blending polyme powde esin wi h mic o- o nanopa icles and ib es be o e consolida ion. UHMWPE ib es can be wo en, kni ed, o manu ac u ed in o shee s o ming complex s uc u es and aniso opic p ope ies. UHMWPE can play he ole o ma ix and ib e in a b oad ange o composi e ma e ials, depending upon he applica ion. Polyme s, which include all plas ics, consis o chains o building blocks called monome s. These chains g ow by adding new molecules on o hei ends—Figu e 4. Machines 2022,10, 1008 5 o 19 Machines2022,10,xFORPEERREVIEW5o 19   Once o med,polyme scanbeshapedin o3Dobjec susinginjec ionmouldingand machinedins anda dwaysby u ning,milling,d illing,e c.Howe e ,polyme sa edi ‐ icul  ocu mainlydue o hei  iscoelas icp ope ies,low he malconduc i i y,high elas ici y,ando he ma e ialcha ac e is ics—Table1.All hesep ope iesa ec  hechip o ma ionandwo kpiecede o ma ion,and esul inde e io a iono su ace inish ia he o ma iono buil ‐upedges,chips icking,andpoo su acep oduc ion.Aldwelle al.[22] ecognizesix ypeso chips o medinpolyme machining,including hedecisi e oleo  shea ing,c acking,and oolmic o‐geome y.I hasbeen ound he e ha wo kpiecep e‐ cooling oc yogenic empe a u es(100–150K)inc easeswo kpieces i nessandimp o es su acequali yin hemachiningo UHMWPE.Tomee  unc ional equi emen sand o becompe i i einaneconomiccon ex ,i isnecessa y o educe hecos so implan p o‐ duc ion.Machinings a egyshould espec  hesu ace obemachined,soballendmilling cu e swe e he i s choice.Machining imedependson hepa geome y,i sp ecision, andcu ingcondi ions o  hecalcula ed oolpa hs.Theop imiza ionp oblemcanbe sol ed iaop imiza ionwi h heobjec i eso  educingcu ing ime,cu ingcos s,su ace quali y,o maximal empe a u e eachedwhenmachining.Themanu ac u edsu ace quali ys ems om hecu ingcondi ions.The oolpa hsa ecompu edacco ding o he su acecu es, ool adius, eedpe  ee h,and adialandaxialdep hso cu s.  Figu e4.Selec edbasics uc u alcha ac e is icso polyme sa e syn hesis—(a)linea polyme  chain,(b)b anchpolyme ne wo ks uc u es. Table1.Mechanicalp ope ieso selec edma e ialsusedinmedicalapplica ions(adap ed om [8–12]). PMMA PEEK PUR PP UHMWPE Mola mass[g/mol]1.3×104 −2.2×105 1.4×104 −1.0×105 7×103 −1.2×104 1×105 −6×105 3×106 −6×106 Densi y[kg/m3]1180 1320 15–600 900–910 941–965 C ys allini y[%]0 16–47 0–13 60–75 39–75 Flow empe a u e T [°C]1.6 334 141–150 176 135 Tensilemodulus[MPa]1800–3100 3000–4000 6 1325 655–1077 Commen :PMMA—polyme hylme hac yla e,PEEK—polye he e he ke one,PUR—polyu e hane, PP—polyp opylene,UHMWPE—ul ahighmolecula weigh polye hylene. TheUHMWPEma e ialwaspu chased om heB i ishcompanyO hoplas ics.I  wasex uded om ineg anulesbyGUR®2024,manu ac u edby hecompanyCelanese. Themoduluso elas ici yo  hema e ialwasapp oxima ely830MPaandi sdensi ywas 930kg/m3.Thesemi‐ inishedp oduc had heshapeo ap ismwi hdimensionso 80× 100–500mm.Sixblocksampleso 20mmin hicknesswe ecu o usingaBOMARB no bandsaw,and hebandsawPILANA1640×13×0.65/10zC125WSTANDARDa a cu ingspeedo 20m/minanda eedspeedo 50mm/minwi hou cooling. Figu e 4. Selec ed basic s uc u al cha ac e is ics o polyme s a e syn hesis—( a ) linea polyme chain, (b) b anch polyme ne wo k s uc u es. Once o med, polyme s can be shaped in o 3D objec s using injec ion moulding and machined in s anda d ways by u ning, milling, d illing, e c. Howe e , polyme s a e di icul o cu mainly due o hei iscoelas ic p ope ies, low he mal conduc i i y, high elas ici y, and o he ma e ial cha ac e is ics—Table 1. All hese p ope ies a ec he chip o ma ion and wo kpiece de o ma ion, and esul in de e io a ion o su ace inish ia he o ma ion o buil -up edges, chip s icking, and poo su ace p oduc ion. Aldwell e al. [ 22 ] ecognize six ypes o chips o med in polyme machining, including he decisi e ole o shea ing, c acking, and ool mic o-geome y. I has been ound he e ha wo kpiece p e- cooling o c yogenic empe a u es (100–150 K) inc eases wo kpiece s i ness and imp o es su ace quali y in he machining o UHMWPE. To mee unc ional equi emen s and o be compe i i e in an economic con ex , i is necessa y o educe he cos s o implan p oduc ion. Machining s a egy should espec he su ace o be machined, so ball end milling cu e s we e he i s choice. Machining ime depends on he pa geome y, i s p ecision, and cu ing condi ions o he calcula ed ool pa hs. The op imiza ion p oblem can be sol ed ia op imiza ion wi h he objec i es o educing cu ing ime, cu ing cos s, su ace quali y, o maximal empe a u e eached when machining. The manu ac u ed su ace quali y s ems om he cu ing condi ions. The ool pa hs a e compu ed acco ding o he su ace cu es, ool adius, eed pe ee h, and adial and axial dep hs o cu s. Table 1. Mechanical p ope ies o selec ed ma e ials used in medical applica ions (adap ed om [ 8 – 12 ]). PMMA PEEK PUR PP UHMWPE Mola mass [g/mol] 1.3 ×104 −2.2 ×105 1.4 ×104 −1.0 ×105 7×103 −1.2 ×104 1×105 −6×105 3×106 −6×106 Densi y [kg/m3]1180 1320 15–600 900–910 941–965 C ys allini y [%] 0 16–47 0–13 60–75 39–75 Flow empe a u e T [◦C] 1.6 334 141–150 176 135 Tensile modulus [MPa] 1800–3100 3000–4000 6 1325 655–1077 Commen : PMMA—polyme hylme hac yla e, PEEK—polye he e he ke one, PUR—polyu e hane, PP— polyp opylene, UHMWPE—ul a high molecula weigh polye hylene. The UHMWPE ma e ial was pu chased om he B i ish company O hoplas ics. I was ex uded om ine g anules by GUR ® 2024, manu ac u ed by he company Celanese. The modulus o elas ici y o he ma e ial was app oxima ely 830 MPa and i s densi y was 930 kg/m 3 . The semi- inished p oduc had he shape o a p ism wi h dimensions o 80 ×100–500 mm . Six block samples o 20 mm in hickness we e cu o using a BOMAR B no band saw, and he band saw PILANA 1640 × 13 × 0.65 /10z C125 W STANDARD a a cu ing speed o 20 m/min and a eed speed o 50 mm/min wi hou cooling. Machines 2022,10, 1008 6 o 19 2.2. Machining Tes s The samples we e subsequen ly machined wi h ace milling cu e Oc omil and six indexable cu ing inse s OFEX05T305FN-M05 PCD20 D125 mm om SECO Tools. The cu ing speed was 300 m/min and eed a e 315 mm/min a an axial dep h o cu 1 mm, wi hou any cooling, espec ing he cu ing condi ions men ioned in [ 22 , 23 ] and ou p elim- ina y es s. The ec angula pla es we e di ided in o ou a eas by g oo es 10 mm×5 mm along he samples, so 24 combina ions o cu ing speeds and eed a es could be es ed by milling in he p epa ed a eas. The spindle was inclined a 45 ◦ in all machining es s. Each a ea was es ed wice, so eal cu ing speeds 70.3, 113.0, 175.8, and 282.6 m/min, adial s epo e 0.35 mm, and eed a es 315, 450, and 630 mm/min we e se , gi ing he eed 0.012–0.094 mm/ oo h a he milling machine FNK 25, TOS Ku im—Figu e 5a. Due o he use o a ounded cu ing edge, he mean chip hickness co esponding o he angle φmax was ecalcula ed om he maximal chip c oss sec ion and adius o he milling cu e R acco ding o he Equa ion (1) and Figu e 5b–d: hm= ADmax R.ϕmax , (1) when he non-de o med chip c oss sec ions a e equalled by he ela ion (2) ADmax =Am(2) In addi ion, a special a angemen o clamping ( la con ac o a acuum as ening) should be a anged, o he wise he UHMWPE la samples end o buckle. The speci ic cu ing o ce was calcula ed as a a io o he mean cu ing o ce and he mean chip c oss sec ion—see Equa ion (3): kc= Fc Am(3) Machines2022,10,xFORPEERREVIEW6o 19   2.2.MachiningTes s Thesampleswe esubsequen lymachinedwi h acemillingcu e Oc omilandsix indexablecu inginse sOFEX05T305FN‐M05PCD20D125mm omSECOTools.The cu ingspeedwas300m/minand eed a e315mm/mina anaxialdep ho cu 1mm, wi hou anycooling, espec ing hecu ingcondi ionsmen ionedin[22,23]andou p e‐ limina y es s.The ec angula pla eswe edi idedin o ou a easbyg oo es10mm×5 mmalong hesamples,so24combina ionso cu ingspeedsand eed a escouldbe es ed bymillingin hep epa eda eas.Thespindlewasinclineda 45°inallmachining es s. Eacha eawas es ed wice,so ealcu ingspeeds70.3,113.0,175.8,and282.6m/min, a‐ dials epo e 0.35mm,and eed a es315,450,and630mm/minwe ese ,gi ing he eed 0.012–0.094mm/ oo ha  hemillingmachineFNK25,TOSKu im–Figu e5a.Due o he useo a oundedcu ingedge, hemeanchip hicknessco esponding o heangleϕmax was ecalcula ed om hemaximalchipc osssec ionand adiuso  hemillingcu e R acco ding o heEqua ion(1)andFigu e5b–d: ℎ𝑚𝐴𝐷𝑚𝑎𝑥 𝑅.𝜑𝑚𝑎𝑥 ,(1) when henon‐de o medchipc osssec ionsa eequalledby he ela ion(2) 𝐴   𝐴 (2) Inaddi ion,aspeciala angemen o clamping( la con ac o a acuum as ening) shouldbea anged,o he wise heUHMWPE la samples end obuckle. Thespeci iccu ing o cewascalcula edasa a ioo  hemeancu ing o ceand hemean chipc osssec ion—seeEqua ion(3): 𝑘𝑐𝐹𝑐 𝐴 𝑚 (3)  Figu e5.Machiningexpe imen alse ‐up,(a)ano e iewo  he es a angemen ,(b)unde o med chipc oss‐sec ionADi,(c)s epo e whenmilling o  eedpe  oo h z,(d)a ecalcula iono  he a ‐ iablechip hicknesshialong hecu ingedge o henominalmean alueo chip hicknesshm. Thedownd ymillingwaspe o medwi haspecialballendmonoli hiccu e SECO JS730200D3B.3Z6‐HXT,ISOK10,co e edwi hPVDAlTi(N)coa ing( hickness2m),jus  de eloped o  hemachiningo aluminaalloyswi hex asha pcu ingedges o pa s usedinae onau icalenginee ing—Figu es6and7.AliconaIFG‐5(B uke Alicona,G az, Aus ia)wasused o  heanalysiso  hecu ing oolgeome yandsu ace opog aphyo  machinedsamples,aswellas hescanningelec onmic oscopeMIRA‐3(TESCANB no, CzechRepublic).Thepiezo‐elec icdynamome e Kis le 9575B, heKis le cha geampli‐ ie sType2825Aand heDynoWa eso wa e o uni e salda aacquisi ion(Kis le ,Win‐ e hu ,Swi ze land)we eusedwo kinga  hesampling equencyo 1000Hz, helow‐ pass il e sa 100–450Hz,and helong imecons an pe each o cecomponen induced incu ingo  he ibological es ing.D ymachiningisalways hes ingen  ule o im‐ plan p oduc ion oa oidsu acecon amina ionwi hanemulsion,soakingo  heliquid Figu e 5. Machining expe imen al se -up, ( a ) an o e iew o he es a angemen , ( b ) unde o med chip c oss-sec ion A D i, ( c ) s epo e when milling o eed pe oo h z , ( d ) a ecalcula ion o he a iable chip hickness hialong he cu ing edge o he nominal mean alue o chip hickness hm. The down d y milling was pe o med wi h a special ball end monoli hic cu e SECO JS730200D3B.3Z6-HXT, ISO K10, co e ed wi h PVD AlTi(N) coa ing ( hickness 2 µ m), jus de eloped o he machining o alumina alloys wi h ex a sha p cu ing edges o pa s used in ae onau ical enginee ing—Figu es 6and 7. Alicona IF G-5 (B uke Alicona, G az, Aus ia) was used o he analysis o he cu ing ool geome y and su ace opog aphy o machined samples, as well as he scanning elec on mic oscope MIRA-3 (TESCAN B no, Czech Republic). The piezo-elec ic dynamome e Kis le 9575B, he Kis le cha ge ampli ie s Type 2825A and he DynoWa e so wa e o uni e sal da a acquisi ion (Kis le , Win e hu , Swi ze land) we e used wo king a he sampling equency o 1000 Hz, he low- pass il e s a 100–450 Hz, and he long ime cons an pe each o ce componen induced in cu ing o he ibological es ing. D y machining is always he s ingen ule o implan Machines 2022,10, 1008 7 o 19 p oduc ion o a oid su ace con amina ion wi h an emulsion, soaking o he liquid in o he plas ics, e c. Each combina ion o cu ing condi ions was es ed a minimum o six imes o ob ain s a is ically signi ican esul s (p< 0.05) o measu emen o o ces, su ace quali y and ibological es ing. The da a unde wen Gaussian dis ibu ion, p esen ed by a e ages and s anda d de ia ions. Machines2022,10,xFORPEERREVIEW7o 19   in o heplas ics,e c.Eachcombina iono cu ingcondi ionswas es edaminimumo six imes oob ains a is icallysigni ican  esul s(p<0.05) o measu emen o  o ces,su ace quali yand ibological es ing.Theda aunde wen Gaussiandis ibu ion,p esen edby a e agesands anda dde ia ions.  Figu e6.Ballendmillingcu e SECOJS730200D3B.3Z6‐HXT.(a)ano e iew,(b)a on al iew, (c)cu ingedgegeome yin heo hogonalplane.  Figu e7.Theballendmillingcu e andi sdimensions.DMM=20mm,OAL=121mm,APMXS= 62mm,DC=20mm,Z=6,(a)ske ch,(b) he akeplaneA, he lankplaneA, heo hogonalplane Po,(c) hesu ace oughnessa  heo hogonal akeplanePo. NewCu ingTool Thenex  esea chwasbasedon he e e ences[20,22],ou expe ience,andexcellen  esul so singlepoin diamond u ning[30]used o  hemachiningo polyme s o op ics. Apa  omi suniqueha dness,diamondhasse e al imes(2–10×)highe  he malcon‐ duc i i y hano he cu ingma e ials(cemen edca bides,ce amics, ools eels),so he de o ma ionhea  omcu ingcanbe ansmi ede ec i ely h ough hecu ingedge, supp essingas ickye ec andbu  o ma ionon hechip– oolin e ace.Un o una ely, he u ningope a ionsp esen acon inuouspolyme chip lowa highcu ingspeedsand i isno applicable o asymme icalo mo ecomplica edshapesusedinimplan designs. Thus,dec easing hecu ingspeedwassuppo edinou  es s,bu a ec ed he imepe  machining,sowedecided omodi y hecu ingedgeand e lec  heapp oxima eEqua‐ ion(4) o  hea e age oughness[31]: 𝑅𝑎  32. 𝑓  𝑟  (4) Thegene alo e iewo  henewcu ingblade(aso o inne sca ing)canbeseen inFigu e8.Thediame e o  he oolwas30mmand he ip adiuswas0.4–0.6m.The Figu e 6. Ball end milling cu e SECO JS730200D3B.3Z6-HXT. ( a ) an o e iew, ( b ) a on al iew, (c) cu ing edge geome y in he o hogonal plane. Machines2022,10,xFORPEERREVIEW7o 19   in o heplas ics,e c.Eachcombina iono cu ingcondi ionswas es edaminimumo six imes oob ains a is icallysigni ican  esul s(p<0.05) o measu emen o  o ces,su ace quali yand ibological es ing.Theda aunde wen Gaussiandis ibu ion,p esen edby a e agesands anda dde ia ions.  Figu e6.Ballendmillingcu e SECOJS730200D3B.3Z6‐HXT.(a)ano e iew,(b)a on al iew, (c)cu ingedgegeome yin heo hogonalplane.  Figu e7.Theballendmillingcu e andi sdimensions.DMM=20mm,OAL=121mm,APMXS= 62mm,DC=20mm,Z=6,(a)ske ch,(b) he akeplaneA, he lankplaneA, heo hogonalplane Po,(c) hesu ace oughnessa  heo hogonal akeplanePo. NewCu ingTool Thenex  esea chwasbasedon he e e ences[20,22],ou expe ience,andexcellen  esul so singlepoin diamond u ning[30]used o  hemachiningo polyme s o op ics. Apa  omi suniqueha dness,diamondhasse e al imes(2–10×)highe  he malcon‐ duc i i y hano he cu ingma e ials(cemen edca bides,ce amics, ools eels),so he de o ma ionhea  omcu ingcanbe ansmi ede ec i ely h ough hecu ingedge, supp essingas ickye ec andbu  o ma ionon hechip– oolin e ace.Un o una ely, he u ningope a ionsp esen acon inuouspolyme chip lowa highcu ingspeedsand i isno applicable o asymme icalo mo ecomplica edshapesusedinimplan designs. Thus,dec easing hecu ingspeedwassuppo edinou  es s,bu a ec ed he imepe  machining,sowedecided omodi y hecu ingedgeand e lec  heapp oxima eEqua‐ ion(4) o  hea e age oughness[31]: 𝑅𝑎  32. 𝑓  𝑟  (4) Thegene alo e iewo  henewcu ingblade(aso o inne sca ing)canbeseen inFigu e8.Thediame e o  he oolwas30mmand he ip adiuswas0.4–0.6m.The Figu e 7. The ball end milling cu e and i s dimensions. DMM = 20 mm, OAL = 121 mm, APMXS = 62 mm , DC = 20 mm, Z = 6, ( a ) ske ch, ( b ) he ake plane A γ , he lank plane A α , he o hogonal plane Po, (c) he su ace oughness a he o hogonal ake plane Po. New Cu ing Tool The nex esea ch was based on he e e ences [ 20 , 22 ], ou expe ience, and excellen esul s o single poin diamond u ning [ 30 ] used o he machining o polyme s o op ics. Apa om i s unique ha dness, diamond has se e al imes (2–10 × ) highe he mal conduc i i y han o he cu ing ma e ials (cemen ed ca bides, ce amics, ool s eels), so he de o ma ion hea om cu ing can be ansmi ed e ec i ely h ough he cu ing edge, supp essing a s icky e ec and bu o ma ion on he chip– ool in e ace. Un o una ely, he u ning ope a ions p esen a con inuous polyme chip low a high cu ing speeds and i is no applicable o asymme ical o mo e complica ed shapes used in implan designs. Thus, dec easing he cu ing speed was suppo ed in ou es s, bu a ec ed he ime pe machining, so we decided o modi y he cu ing edge and e lec he app oxima e Equa ion (4) o he a e age oughness [31]: Ra =32. 2 ε (4) The gene al o e iew o he new cu ing blade (a so o inne sca ing) can be seen in Figu e 8. The diame e o he ool was 30 mm and he ip adius was 0.4–0.6 µ m. The Machines 2022,10, 1008 8 o 19 p o o ype was made o he ha dened s eel DINX155C VMo121 (62 ± 1 HRC). The cu ing speed was 50 m/min, axial dep h o cu 0.005 mm, and adial wid h o cu ( he s epo e ) 0.010 mm. Machines2022,10,xFORPEERREVIEW8o 19   p o o ypewasmadeo  heha deneds eelDINX155C VMo121(62±1HRC).Thecu ing speedwas50m/min,axialdep ho cu 0.005mm,and adialwid ho cu ( hes epo e ) 0.010mm.  Figu e8.Thecylind icalcu ingblade o  inishing hemachinedsu acebylongi udinalsca ing. (a)cu ingend,(b)de ailo  hecu ingedge. 2.3.T ibologicalTes s Se e al esea chwo ksha ebeendealingwi h hepin‐on‐disc echniquesande ec  o sliding eloci yon he ic ionandwea o UHMWPE o usein o ala i icialjoin s [32]and ibologicalbeha iou o ul a‐highmolecula weigh polye hylene(UHMWPE) unde d y ecip oca ingsliding[33]. The ibological es sin hiswo kuseda ealcus omizedimplan  omTi6Al4VELI (g oundand umbled oRa=0.03m)whichwasused o  heanalysiso  ic ion—Figu e 9.Theimplan  as ened o heholde wasp essedby o ceso 20,25,100,and300N,and henwasd i enalong heUHMWPEsu acea speedso 200,800,1400,and2000 mm/min.Eachcombina iono  he h us loadingand eedspeedwasmeasu edsix imes, and hea e agecoe icien o  ic ionwascalcula eda  i s app oxima ionas he a ioo  angen ialandno mal o ce(5): 𝜇       .(5)  Figu e9.Theexpe imen alse up o  es ing he ic ion.(a) hemachineFV25CNC/Heidenhain iTNC530, hedynamome e Kis le 9575B,and hesampleclampedin o ice,(b)cus omisedjoin  implan in hemechanicalholde . Figu e 8. The cylind ical cu ing blade o inishing he machined su ace by longi udinal sca ing. (a) cu ing end, (b) de ail o he cu ing edge. 2.3. T ibological Tes s Se e al esea ch wo ks ha e been dealing wi h he pin-on-disc echniques and e ec o sliding eloci y on he ic ion and wea o UHMWPE o use in o al a i icial join s [ 32 ] and ibological beha iou o ul a-high molecula weigh polye hylene (UHMWPE) unde d y ecip oca ing sliding [33]. The ibological es s in his wo k used a eal cus omized implan om Ti6Al4V ELI (g ound and umbled o Ra = 0.03 µ m) which was used o he analysis o ic ion—Figu e 9. The implan as ened o he holde was p essed by o ces o 20, 25, 100, and 300 N, and hen was d i en along he UHMWPE su ace a speeds o 200, 800, 1400, and 2000 mm/min. Each combina ion o he h us loading and eed speed was measu ed six imes, and he a e age coe icien o ic ion was calcula ed a i s app oxima ion as he a io o angen ial and no mal o ce (5): µ= F Fn = Fx Fz. (5) Machines2022,10,xFORPEERREVIEW8o 19   p o o ypewasmadeo  heha deneds eelDINX155C VMo121(62±1HRC).Thecu ing speedwas50m/min,axialdep ho cu 0.005mm,and adialwid ho cu ( hes epo e ) 0.010mm.  Figu e8.Thecylind icalcu ingblade o  inishing hemachinedsu acebylongi udinalsca ing. (a)cu ingend,(b)de ailo  hecu ingedge. 2.3.T ibologicalTes s Se e al esea chwo ksha ebeendealingwi h hepin‐on‐disc echniquesande ec  o sliding eloci yon he ic ionandwea o UHMWPE o usein o ala i icialjoin s [32]and ibologicalbeha iou o ul a‐highmolecula weigh polye hylene(UHMWPE) unde d y ecip oca ingsliding[33]. The ibological es sin hiswo kuseda ealcus omizedimplan  omTi6Al4VELI (g oundand umbled oRa=0.03m)whichwasused o  heanalysiso  ic ion—Figu e 9.Theimplan  as ened o heholde wasp essedby o ceso 20,25,100,and300N,and henwasd i enalong heUHMWPEsu acea speedso 200,800,1400,and2000 mm/min.Eachcombina iono  he h us loadingand eedspeedwasmeasu edsix imes, and hea e agecoe icien o  ic ionwascalcula eda  i s app oxima ionas he a ioo  angen ialandno mal o ce(5): 𝜇       .(5)  Figu e9.Theexpe imen alse up o  es ing he ic ion.(a) hemachineFV25CNC/Heidenhain iTNC530, hedynamome e Kis le 9575B,and hesampleclampedin o ice,(b)cus omisedjoin  implan in hemechanicalholde . Figu e 9. The expe imen al se up o es ing he ic ion. ( a ) he machine FV 25CNC/Heidenhain iTNC 530, he dynamome e Kis le 9575B, and he sample clamped in o ice, ( b ) cus omised join implan in he mechanical holde . Machines 2022,10, 1008 9 o 19 3. Resul s 3.1. Resul s o Machining Tes s The o ce decomposi ion, o ce a ios, and ime se ies we e e y simila in all cu ing condi ions—see Figu e 10a. Fo he e alua ions, he s abilized pa o he ime se ies was selec ed o de ine he a e age s a is ical pa ame e s. The dominan componen was he passi e pe pendicula o ce Fcn, which was nea ly wice as high compa ed o he cu ing o ce Fc. On he con a y, he speci ic milling o ce exhibi ed a e y ypical cou se, known o mos known ma e ials and echnologies—see Figu e 10b. Machines2022,10,xFORPEERREVIEW9o 19   3.Resul s 3.1.Resul so MachiningTes s The o cedecomposi ion, o ce a ios,and imese ieswe e e ysimila inallcu ing condi ions—seeFigu e10a.Fo  hee alua ions, hes abilizedpa o  he imese ieswas selec ed ode ine hea e ages a is icalpa ame e s.Thedominan componen was he passi epe pendicula  o ceFcn,whichwasnea ly wiceashighcompa ed o hecu ing o ceFc.On hecon a y, hespeci icmilling o ceexhibi eda e y ypicalcou se,known o mos knownma e ialsand echnologies—seeFigu e10b.  b)  Figu e10.A ypical imese ieso cu ing o ceswhenmillingUHMWPE.(a)Fc‐cu ing o ce,Fcn‐ pe pendicula cu ing o ce,Fp‐passi e o ce,(b)speci iccu ing o ceasa unc iono chip hick‐ ness. Su ace opog aphyin heballendmillingp ocesswas es ed i s wi h ans e se andlongi udinals eps—Figu e11a,bu  hencon inuedinonlylongi udinals eps—Fig‐ u e11b.Thecu ingspeedand imeo millingexhibi edasigni ican e ec on hemo ‐ phologyo chipswhenmillingUHMWPE,and equen lysome aceso chipmel ingand highadhesioncanbeseen—Figu e12.The ypicalphenomenaa chipp oduc ionwe e e ysimila .  Figu e11.Su ace opog aphyinballendmillingp ocess:(a)millingwi h ans e seandlongi u‐ dinals epswi h adials epo e sae×ae,(b)millingsolelyinlongi udinals epsae. Figu e 10. A ypical ime se ies o cu ing o ces when milling UHMWPE. ( a ) Fc-cu ing o ce, Fcn- pe pendicula cu ing o ce, Fp-passi e o ce, ( b ) speci ic cu ing o ce as a unc ion o chip hickness. Su ace opog aphy in he ball end milling p ocess was es ed i s wi h ans e se and longi udinal s eps—Figu e 11a, bu hen con inued in only longi udinal s eps—Figu e 11b. The cu ing speed and ime o milling exhibi ed a signi ican e ec on he mo phology o chips when milling UHMWPE, and equen ly some aces o chip mel ing and high adhe- sion can be seen—Figu e 12. The ypical phenomena a chip p oduc ion we e e y simila . Su ace opog aphy con i med he signi ican e ec o eed speed and cu ing speed on su ace quali y: o he bes quali y, only he slowes cu ing and lowes eeds gi e he bes esul s—Figu e 13. Howe e , he in eg i y o he machined su aces wasn ´ op imal. Some semi-de ached pa icles (Figu es 14 and 15) o p o ube an ib es (Figu e 16) a ec ed he solid su ace and possibly could con ibu e o he p oduc ion o deb is on he su aces in sliding con ac . The p oduc ion o he aspe i ies s ems om he kinema ics o he milling echnology (Figu e 17) in gene al, bu is enhanced by he plas ic low o he polyme ib es also. Machines2022,10,xFORPEERREVIEW9o 19   3.Resul s 3.1.Resul so MachiningTes s The o cedecomposi ion, o ce a ios,and imese ieswe e e ysimila inallcu ing condi ions—seeFigu e10a.Fo  hee alua ions, hes abilizedpa o  he imese ieswas selec ed ode ine hea e ages a is icalpa ame e s.Thedominan componen was he passi epe pendicula  o ceFcn,whichwasnea ly wiceashighcompa ed o hecu ing o ceFc.On hecon a y, hespeci icmilling o ceexhibi eda e y ypicalcou se,known o mos knownma e ialsand echnologies—seeFigu e10b.  b)  Figu e10.A ypical imese ieso cu ing o ceswhenmillingUHMWPE.(a)Fc‐cu ing o ce,Fcn‐ pe pendicula cu ing o ce,Fp‐passi e o ce,(b)speci iccu ing o ceasa unc iono chip hick‐ ness. Su ace opog aphyin heballendmillingp ocesswas es ed i s wi h ans e se andlongi udinals eps—Figu e11a,bu  hencon inuedinonlylongi udinals eps—Fig‐ u e11b.Thecu ingspeedand imeo millingexhibi edasigni ican e ec on hemo ‐ phologyo chipswhenmillingUHMWPE,and equen lysome aceso chipmel ingand highadhesioncanbeseen—Figu e12.The ypicalphenomenaa chipp oduc ionwe e e ysimila .  Figu e11.Su ace opog aphyinballendmillingp ocess:(a)millingwi h ans e seandlongi u‐ dinals epswi h adials epo e sae×ae,(b)millingsolelyinlongi udinals epsae. Figu e 11. Su ace opog aphy in ball end milling p ocess: ( a ) milling wi h ans e se and longi udi- nal s eps wi h adial s epo e s ae×ae, (b) milling solely in longi udinal s eps ae. Machines 2022,10, 1008 16 o 19 Machines2022,10,xFORPEERREVIEW15o 19    Figu e23.Acompa isono  hesu aces.(a)be o ei oning(b)a e linea i oningwi h hep ehea ed i oncylinde . Table2.A e agesu ace oughnessa e machiningand he mali oning. Ra[μm]Rz[μm]Sa[μm] Su acea e milling2.49–2.6118.68–19.843.63–3.68 I oning( la con ac )1.29–1.369.21–9.721.37–1.42 I oning(cylind icalcon ac )1.70–1.8612.29–13.453.27–3.41 I oning(ballcon ac )1.93–2.0214.17–14.842.58–2.72 Thenewcu ing oolpe o meda lowcu ingspeedswi hexcellen su ace ough‐ ness, eeo  hebu s—Figu e24,exhibi ingRa<0.1m.The eisoneimpo an condi ion o  eachingsuchagoodsu ace— omake he oughingand inishingcondi iona one clamping,wi hou any eleasingo  hemachinedpa  om heclampingandCNCma‐ chine.The oolli eexp essedin he o almachineda eawasabou 12,000–14,000mm2pe  ool.   Figu e24.Scanningelec onmic oscopyo  he esul s:(a) inishedUHMWPEsu ace‐ eeo bu s, smoo h( c=5m/min,ap=0.01mm,ae=0.2mm),(b)su ace oughnesscha ac e is ics,(c)apiece o a e y inechip. Figu e 24. Scanning elec on mic oscopy o he esul s: ( a ) inished UHMWPE su ace - ee o bu s, smoo h ( c = 5 m/min, a p = 0.01 mm, a e = 0.2 mm), ( b ) su ace oughness cha ac e is ics, ( c ) a piece o a e y ine chip. 3.2. Resul s o T ibological Tes s The esul s—see Table 3and Figu e 25—con i med excellen ibological beha iou o he pai o su aces and ex emely low ic ion was ound: 0.07–0.11. A sligh e ec o no mal o ces on he coe icien o ic ion was p o ed, bu mo e signi ican eed speed was con i med. Then, he mean alue and s anda d de ia ions (SD) we e calcula ed om all o ce da a a a cons an eed speed. A sligh ise o he coe icien o ic ion wi h he inc easing eed speed was ound also. Possibly, he highe speeds gene a ed mo e hea and he su ace s a ed o be plas ically de o med, which esul ed in a s icking e ec o he con ac egions. Machines2022,10,xFORPEERREVIEW16o 19   3.2.Resul so T ibologicalTes s The esul s—seeTable3andFigu e25—con i medexcellen  ibologicalbeha iou  o  hepai o su acesandex emelylow ic ionwas ound:0.07–0.11.Asligh e ec o  no mal o ceson hecoe icien o  ic ionwasp o ed,bu mo esigni ican  eedspeed wascon i med.Then, hemean alueands anda dde ia ions(SD)we ecalcula ed om all o ceda aa acons an  eedspeed.Asligh  iseo  hecoe icien o  ic ionwi h he inc easing eedspeedwas oundalso.Possibly, hehighe speedsgene a edmo ehea  and hesu aces a ed obeplas icallyde o med,which esul edinas ickinge ec o  hecon ac  egions. Table3.Coe icien so  ic ionas unc iono no mal o ceand eedspeed. Th us Fo ce [N] FeedSpeed[mm/min] 20080014002000 200.0740.0800.0820.080 250.0850.0890.0930.094 1000.0800.0960.1030.094 3000.0880.0980.0970.099 Mean±SD0.082±0.0060.091±0.0080.093±0.0090.092±0.008   Figu e25.The ibological esul s o  hecoe icien o  ic ion.(a) hee ec o no mal o ceand ansla i espeed,and(b) hee ec o  he ansla i espeedoni s alue o all es edno mal o ces (a e age alues± s anda dde ia ions). 4.Discussion Fi s , hema e ialexhibi eda e ycommoncou seo speci iccu ing o cea milling, likemanyo he  echnicalma e ials.The hinne  helaye sepa a edbychip emo al, he highe speci ic esis anceo  hede o medma e ialcanbeexpec ed.On hecon a y, es ‐ ingo  hecu ingspeedmagni udescon i med ha  hebes su acequali ywasachie ed, su p isingly,a  helowes cu ingspeedand helowes  eedspeed.Theuseo highcu ing speedsand eedwi h hecemen edca bide ool esul edin heaccumula iono plas ic ene gya  heplaceo chip emo al,wi halimi edhea  lux.Thema e ialhea edupand s a ed omel ,wi hallde e io a inge ec sonchip o ma ionandquali yo  hema‐ chinedsu ace,e ena mildcu ingcondi ions.Thec ucialp oblemis hep incipleo  milling,whichin e up scu ingac ionandini ia es hep oduc iono  he“ ishscales”a  allcu ingcondi ionsusedin hiswo k.Thedis ibu iono  helocali egula i iesdepends oncu ingcondi ions,andcanbe educedpa iallywi hlow eedpe  oo h, oolo e s eps, andlowcu ingspeed.Howe e ,i a ec s hep oduc i i yo  hemachiningope a ion. Thesamep oblemso su acein eg i y,ma e ialpile‐ups,ando he  echnologicalphe‐ nomenao su acequali yha ebeen oundin[22]and[30]also.Shin okuandNa i a,in hei s udyo inclinedplanemilling echnology[22],concluded ha  hebes ‐quali yma‐ chining esul sha ebeen ounda a eed a ein he angeo 120–240mm/minandacu ‐ Figu e 25. The ibological esul s o he coe icien o ic ion. ( a ) he e ec o no mal o ce and ansla i e speed, and ( b ) he e ec o he ansla i e speed on i s alue o all es ed no mal o ces (a e age alues ±s anda d de ia ions). Machines 2022,10, 1008 17 o 19 Table 3. Coe icien s o ic ion as unc ion o no mal o ce and eed speed. Th us Fo ce [N] Feed Speed [mm/min] 200 800 1400 2000 20 0.074 0.080 0.082 0.080 25 0.085 0.089 0.093 0.094 100 0.080 0.096 0.103 0.094 300 0.088 0.098 0.097 0.099 Mean ±SD 0.082 ±0.006 0.091 ±0.008 0.093 ±0.009 0.092 ±0.008 4. Discussion Fi s , he ma e ial exhibi ed a e y common cou se o speci ic cu ing o ce a milling, like many o he echnical ma e ials. The hinne he laye sepa a ed by chip emo al, he highe speci ic esis ance o he de o med ma e ial can be expec ed. On he con a y, es ing o he cu ing speed magni udes con i med ha he bes su ace quali y was achie ed, su p isingly, a he lowes cu ing speed and he lowes eed speed. The use o high cu ing speeds and eed wi h he cemen ed ca bide ool esul ed in he accumula ion o plas ic ene gy a he place o chip emo al, wi h a limi ed hea lux. The ma e ial hea ed up and s a ed o mel , wi h all de e io a ing e ec s on chip o ma ion and quali y o he machined su ace, e en a mild cu ing condi ions. The c ucial p oblem is he p inciple o milling, which in e up s cu ing ac ion and ini ia es he p oduc ion o he “ ish scales” a all cu ing condi ions used in his wo k. The dis ibu ion o he local i egula i ies depends on cu ing condi ions, and can be educed pa ially wi h low eed pe oo h, ool o e s eps, and low cu ing speed. Howe e , i a ec s he p oduc i i y o he machining ope a ion. The same p oblems o su ace in eg i y, ma e ial pile-ups, and o he echnological phenomena o su ace quali y ha e been ound in [ 22 ] and [ 30 ] also. Shin oku and Na i a, in hei s udy o inclined plane milling echnology [ 22 ], concluded ha he bes -quali y machining esul s ha e been ound a a eed a e in he ange o 120–240 mm/min and a cu ing speed o 50 m/min. The ange o cu ing condi ions is compa able o he milling pa ame e s used in ou esea ch (315 mm/min, 70 m/min). In ha wo k, i was also con i med ha he eed a e has a highe impac on he esul ing quali y han he cu ing speed, bu his is logical due o Equa ion (2). The high a io o passi e o ces ound in [ 22 ] cons i u es a new inding, because he ac is known ypically o g inding echnology ( he use o a nega i e ake geome y o g inding g ains). Howe e , in he milling o plas ics, e y sha p cu ing edges a e used no mally, so he highe alues o Fcn han Fc can be explained by he isco-elas ic beha iou o he ma e ial, he in ensi e plas ic low o he machined ma e ial, and an ex eme ploughing e ec o he cu ing ool when he chip is o med. In o he wo ds, he ene gy needed o he e ia y plas ic de o ma ion [ 31 ] was highe compa ed o he ene gy consumed o he p ima y plas ic de o ma ion. The s uc u e o he UHMWPE, he low modulus o elas ici y, he low he mal conduc i i y, and he low empe a u e o plas ic low suppo he occu ence o his phenomenon. The he mo-mechanical pos -p ocessing con i med ha i may be a way o smoo hing o he su aces, bu he e-mel ing and in ensi e plas ic de o ma ion may ha e an e ec on he polye hylene chains wi h unp edic ed a e e ec s. The ac ha mechanical milling can cause a phase ans o ma ion o UHMWPE can be ound in [34] also. P e-cooling o he ma e ial, o c yogenic machining, is a e y good echnology o implan p oduc ion [ 35 , 36 ], e i ied especially on he Ti-6Al-4V alloy, bu e y ad anced equipmen and echnology is needed. The u ning echnology using diamond ools and high cu ing speeds [ 22 ] is di icul o apply o he gene al shapes and ad anced ool ajec o ies needed o eal a i icial join s. Ou inno a ion, based on he use o ex emely sha p ools and he echnology o sca ing, seems o be mo e ealis ic o se ial use. A special design o a ounded ool simila o inne sca ing o welded ubes was designed (pa en pending) and has been es ed on. Machines 2022,10, 1008 18 o 19 The ibological es con i med he esul o he ex a-low ic ion coe icien agains he p o o ype knee implan (made o Ti6Al4V ELI by he echnology EBM) and he ic ion de- penden on he mo ion speed, no he no mal o ce, bu no e y d ama ically. I con i med a e y small e ec o speed on ic ion and wea , as has been ound in he pin-on-disc es [ 32 , 33 ]. The highe speeds can gene a e some ic ional hea , bu hey a e a enough in hei magni udes compa ed o a ange used in no mal walking. 5. Conclusions Fo he high longe i y o he implan , i is necessa y o achie e he bes possible su ace in eg i y in o de o a oid any possible s ess ise s in he ma e ial. This esea ch wo k con i ms ha UHMWPE is a e y good ma e ial o ibological applica ions in implan inse ions, bu i is di icul o machine. To ge an accep able su ace quali y, a e y low cu ing speed and a combina ion o milling ( o oughing wi h cemen ed ca bide ball milling ool) and sca ing o he aspe i ies ( o inishing wi h a ha dened s eel cu ing ool) o he unc ional su aces a e ecommended, wi h no cooling. This has a se ious impac in p ac ice because no c yogenic appa a us is needed. Howe e , a igid machine wi h a e y sha p cu ing edge and p ese a ion o echnological sequence ( oughing ollowed wi h inishing, wi hou any elease o he inse om a clamping de ice and no in e up ion o he cu ing ac ion a he su ace) should be kep . I he ma e ial is consolida ed well, he inished su aces a e machining a e smoo h, glossy and wi hou any de ec o in eg i y. All he esul s a e impo an o inalize he whole assembly o cus omized implan s and ex end he li espan o he inse , hus maximizing he e-ope a ion pe iod o pa ien s o elimina ing he necessi y o eplace he inse al oge he . Au ho Con ibu ions: Concep ualiza ion, M.P. and K.U.; me hodology, M.P.; alida ion, M.P. and K.U.; o mal analysis, K.U.; in es iga ion, M.P.; esou ces, K.U.; da a cu a ion, M.P.; w i ing—o iginal d a p epa a ion, M.P.; p ojec adminis a ion, M.P.; unding acquisi ion, M.P. All au ho s ha e ead and ag eed o he published e sion o he manusc ip . Funding: This esea ch was unded by he PID g an FV40313, Applica ion o he New Su ace T ea men Technologies in Me al Packaging Indus y, and by he B no Uni e si y o Technology, Facul y o Mechanical Enginee ing, Speci ic esea ch “Mode n echnologies o p ocessing ad anced ma e ials used o in e disciplina y applica ions”, FSI-S-22-7957. Da a A ailabili y S a emen : No applicable. Acknowledgmen s: A special hanks go o he company As a Mo o s, s. .o., B no, Czech Republic, ep esen ed by Jan Kep da, o his kind help in he echnology o d ag umbling, and o Phillip Jones o his English co ec ions. Con lic s o In e es : The au ho s decla e no con lic o in e es . Re e ences 1. Wo ld Heal h S a is ics 2021: Moni o ing Heal h o he SDGs, Sus ainable De elopmen Goals; Wo ld Heal h O ganiza ion: Gene a, Swi ze land, 2021; ISBN 978-92-4-002705-3. 2. Píška, M.; Buˇcko á, K. On he analysis o Ti6Al4V-ELI powde ma e ial, elec on beam echnology and machining on quali y o machined implan su aces. In P oceedings o he Eu opean Ad anced Ma e ial Cong ess, S ockholm, Sweden, 20–23 Augus 2018; VBRI P ess: S ockholm, Sweden, 2018; pp. 257–258, ISBN 978-91-88252-12-8. 3. Píška, M.; Buˇcko á, K. On he machining o Ti-6Al-4V ELI alloy made wi h EBM echnology o implan s. In P oceedings o he UTIS 9 h In e na ional Cong ess on Machining Cong ess P oceedings, An alya, Tü kiye, 8–11 No embe 2018; UTIS Royal Seginus: An alya, Tü kiye, 2018; pp. 242–247. 4. Píška, M.; Buˇcko á, K. Ad anced knee implan s o he hi d millenium. J. Ma e . Sci. Eng. 2019,8, 20–21. 5. Píška, M.; Buˇcko á, K. Analysis o machined elec on beam ea ed Ti6Al4V-ELI implan su aces. Ad . Ma e . Le . 2019 ,10, 381–385. [C ossRe ] 6. Píška, M.; Buˇcko á, K. On he SLM and EBM o Ti-6Al-4V ELI Alloy o Ad anced Knee A h oplas y; Bone Resea ch Socie y and B i ish O hopaedic Resea ch Socie y: Ca di , Wales, 2019; p. 92. 7. Píška, M.; Buˇcko á, K. A Se o Applica ion o Join Implan . Czech Pa en No 309024, 2021. 8. Ka i, K.S. Bioma e ials in o al join eplacemen . Colloids Su . B Bioin e aces 2004,9, 133–142. [C ossRe ] [PubMed] Machines 2022,10, 1008 19 o 19 9. Ku z, S.M. UHMWPE Bioma e ials Handbook-Ul a-High Molecula Weigh Polye hylene in To al Join Replacemen and Medical De ices, 3 d ed.; D exel Uni e si y: Philadelphia, PA, USA; Else ie : Ams e dam, The Ne he lands, 2015. 10. Mali o, G.L.; A e alo, S.; Adam, K.; S ephen, S.; Anuj, B.; Lisa, P. Ma e ial p ope ies o ul a-high molecula weigh polye hylene: Compa ison o ension, comp ession, nanomechanics and mic os uc u e ac oss clinical o mula ions. J. Mech. Beha . Biomed. Ma e . 2018,83, 9–19. [C ossRe ] [PubMed] 11. Panayo o , I.V.; O i, V.; Cuisinie , F.; Yachouh, J. Polye he e he ke one (PEEK) o medical applica ions. J. Ma e . Sci. Ma e . Med. 2016,27, 118. [C ossRe ] 12. Mckeen, L.W. Fa igue and T ibological P ope ies o Plas ics and Elas ome s, 3 d ed.; Else ie : Ams e dam, The Ne he lands, 2016. 13. Za iba , F.P. Medical-g ade ul a-high molecula weigh polye hylene: Pas , cu en and u u e. Ma e . Sci. Technol. 2018 ,34, 1940–1953. [C ossRe ] 14. Ka a zyna, G.; Sylwes e , G. The e ec o gamma adia ion on molecula s abili y and mechanical p ope ies o biodeg adable polyu e hanes o medical applica ions. Polym. Deg ad. S ab. 2003,79, 465–474. 15. Ducháˇcek, V. Polyme y: Vý oba, Vlas nos i, Zp aco ání, Použi í, 2nd ed.; VSCHT: P ague, Czech Republic, 2006; ISBN 80-708-0617-6. (In Czech) 16. ASTM F648-21, 10 h ed.; S anda d Speci ica ion o Ul a-High-Molecula -Weigh Polye hylene Powde and Fab ica ed Fo m o Su gical Implan s; ASTM in e na ional: Wes Conshohocken, PA, USA, 2021. 17. ISO 5834-2:2019; DIN ISO 5834-1 Implan s o Su ge y-Ul a-High-Molecula -Weigh Polye hylene-Pa 1: Moulded Fo ms. ISO: Gene a, Swi ze land, 2019. 18. ISO 5834-2:2019; DIN ISO 5834-2 Implan s o Su ge y-Ul a-High-Molecula -Weigh Polye hylene-Pa 2: Moulded Fo ms. ISO: Gene a, Swi ze land, 2019. 19. ISO 5834-1:2019; DIN ISO 5834-1 Implan s o Su ge y-Ul a-High-Molecula -Weigh Polye hylene-Pa 3: Powde Fo m. ISO: Gene a, Swi ze land, 2019. 20. Axin e, D.; Guo, Y.; Liao, Z.; Shih, A.J.; M’Saoubi, R.; Sugi a, N. Machining o biocompa ible ma e ials—Recen ad ances. CIRP Ann. 2019,68, 629–652. [C ossRe ] 21. Aldwell, B.; Hanley, R.; O’Donnell, G.E. Cha ac e ising he Machining o Biomedical G ade Polyme s. P oc. Ins . Mech. Eng. Pa B J. Eng. Manu . 2014,228, 1237–1251. [C ossRe ] 22. Kousuke, S.; Na i a, H. S udy on Ball End Milling o Inclined Su aces o Ul a High Molecula Weigh Polye hylene. In . J. Au om. Technol. 2017,11, 948–957. 23. W igh , T.M.; Rimnac, C.M.; S ulbe g, S.D.; Min z, L.; Tsao, A.K.; Klein, R.W.; McC ae, C. Wea o Polye hylene in To al Join Replacemen s Obse a ions F om Re ie ed PCA Knee Implan s. Clin. O hop. Rela . Res. 1992,214, 124–134. [C ossRe ] 24. Mohsen, M.; Suh, N. Wea Pa icles o Polye hylene in Biological Sys ems. T ibol. T ans. 1996,39, 843–848. 25. Howling, G.I.; Ba ne , P.I.; Tippe , J.L.; S one, M.H.; Fishe , J.; Ingham, E. Quan i a i e cha ac e iza ion o polye hylene deb is isola ed om pe ip os he ic issue in ea ly ailu e knee implan s and ea ly and la e ailu e Cha nley hip implan s. J. Biomed. Ma e . Res. 2001,58, 415–420. [C ossRe ] [PubMed] 26. Tulp, N.J.A. Polye hylene delamina ion in he PCA o al knee Ma e ial analysis in wo ailed cases. Ac a O hop. Scand. 2009 ,63, 263–266. 27. E ans, D.C.; Lancas e , J.K. The Wea o Polyme s; Wea ; Else ie : Ams e dam, The Ne he lands, 1979; Volume 13, pp. 85–139. 28. Abdelba y, A. Wea o Polyme s and Composi es, 1s ed.; Else ie : Ams e dam, The Ne he lands, 2014. 29. Wilches, L.V.; U ibe, J.A.; To o, A. Wea o ma e ials used o a i icial join s in o al hip eplacemen s. Wea 2008 ,265, 143–149. [C ossRe ] 30. Mish a, V.; Singh, G.; Yada , C.B.; Ka a , V.; P akash, C.; Singh, S. P ecision machining o biopolyme s: A b ie e iew o he li e a u e and case s o y on diamond u ning. J. The moplas . Compos. Ma e . 2021,34, 557–578. [C ossRe ] 31. Shaw, M.C. Me al Cu ing P inciples, 2nd ed.; Ox o d Uni e si y P ess: Ox o d, UK, 2005; ISBN 0-19-514206-3. 32. Fishe , J.; Dowson, D.; Hamdzah, H.; Lee, H.L. The e ec o sliding eloci y on he ic ion and wea o UHMWPE o use in o al a i icial join s. Wea 1994,175, 219–225. [C ossRe ] 33. Song, J.; Liu, P.; C emens, M.; Bonu i, P. E ec s o machining on ibological beha io o ul a high molecula weigh polye hylene (UHMWPE) unde d y ecip oca ing sliding. Wea 1999,225–229, 716–723. [C ossRe ] 34. Gab iel, M.C.; Mendes, L.B.; Ca alho, B.d.; Pinhei o, L.A.; Capocchi, J.D.T.; Kubaski, E.T.; Cin ho, O.M. High-ene gy mechanical milling o ul a-high molecula weigh polye hylene (UHMWPE). In P oceedings o he Se en h In e na ional La in Ame ican Con e ence on Powde Technology, A ibaia, SP, B azil, 8–10 No embe 2010; pp. 342–346. 35. Pe ei a, O.; Gonzáles, H.; Calleja, A.; Rod igues, A.; U bikaín, G.; López de Lacalle, L.N. Manu ac u ing o human knee by c yogenic machining: Walking owa ds cleane p ocesses. P ocedia Manu . 2019,41, 257–263. [C ossRe ] 36. Gómez-Escude o, G.; Jimeno Bei ia, A.; Ma ínez de Pissón Ca uncho, G.; López de Lacalle, L.N.; González-Ba io, H.; Pe ei a Ne o, O.; Calleja-Ochoa, A. A eliable clean p ocess o i e-axis milling o knee p os heses. In . J. Ad . Manu . Technol. 2021 ,115, 1605–1620. [C ossRe ]