The Effect of Kinematic Conditions and Synovial Fluid Composition on the Frictional Behaviour of Materials for Artificial Joints

ma e ials A icle The E ec o Kinema ic Condi ions and Syno ial Fluid Composi ion on he F ic ional Beha iou o Ma e ials o A i icial Join s Da id Neˇcas * ID , Ma in V bka, I an Kˇ upka and Ma in Ha l Facul y o Mechanical Enginee ing, Ins i u e o Machine and Indus ial Design, B no Uni e si y o Technology, Technická2896/2, 616 69 B no, Czech Republic; [email p o ec ed] (M.V.); [email p o ec ed].cz (I.K.); [email p o ec ed] (M.H.) *Co espondence: [email p o ec ed]; Tel.: +420-541-143-239 Recei ed: 23 Ap il 2018; Accep ed: 8 May 2018; Published: 10 May 2018   Abs ac : The pape in oduces an expe imen al in es iga ion o ic ional beha iou o ma e ials used o join eplacemen s. The measu emen s we e pe o med using a ball-on-disc ibome e , while ou ma e ial combina ions we e es ed; me al-on-me al, ce amic-on-ce amic, me al-on-polye hylene, and ce amic-on-polye hylene, espec i ely. The con ac was lub ica ed by pu e saline and a ious p o ein solu ions. The expe imen s we e ealized a wo mean speeds equal o 5.7 mm/s and 22 mm/s and wo slide- o- oll a ios, − 150% and 150%. I was ound ha he implan ma e ial is he undamen al pa ame e a ec ing ic ion. In gene al, he me al pai exhibi ed app oxima ely wo imes highe ic ion compa ed o he ce amic. In pa icula , he ic ion in he case o he me al a ied be ween 0.3 and 0.6 while he ce amic pai exhibi ed ic ion wi hin he ange om 0.15 o 0.3 a he end o he es . The lowes ic ion was obse ed o polye hylene while i dec eased o 0.05 unde some condi ions. I can be also concluded ha adding p o eins o he lub ican has a posi i e impac on ic ion in he case o ha d-on-ha d pai s. Fo ha d-on-so pai s, no subs an ial in luence o p o eins was obse ed. The e ec o kinema ic condi ions was ound o be negligible in mos cases. Keywo ds: join eplacemen ; ic ion; ma e ial; p o eins; kinema ic condi ions 1. In oduc ion To al join a h oplas y is an e icien and well-es ablished su gical p ocedu e imp o ing he li e o pa ien s su e ing om join diseases. Acco ding o he Heal h a a Glance 2015 epo [ 1 ], 161 ope a ions pe 100,000 inhabi an s we e pe o med in O ganisa ion o Economic Co-ope a ion and De elopmen (OECD) coun ies in 2013. Howe e , despi e he apid imp o emen o he implan ed ma e ials in he las ew decades [ 2 ], limi ed longe i y is s ill ecognized as he main d awback o he eplacemen s. I should be highligh ed ha ailu e o he implan is accompanied by he need o a e ising ope a ion, which is an economic bu den and subs an ially de e io a es he com o o he pa ien . As he numbe o young ac i e people wi h a i icial join s s ill inc eases [ 3 ], i is necessa y o ensu e a su icien se ice-li e o implan s o a oid any incon enience associa ed wi h i s imp ope unc ion. Since he main cause o he ailu e is os eolysis [ 4 ] ollowed by asep ic loosening, he ibological pe o mance o implan s has o be unde s ood in he con ex o he u he de elopmen o eplacemen s. Se e al aspec s ha e o be aken in o accoun conside ing ibological p ocesses wi hin a i icial join con ac . Ma aki and Cann [ 5 ] employed a ball-on-disc con igu a ion when e alua ing he coe icien o ic ion (CoF) as a unc ion o mean speed. Me al-on-me al con ac was in es iga ed, while se e al es lub ican s we e es ed. In gene al, CoF was kep be ween 0.1 and 0.4 while he es Ma e ials 2018,11, 767; doi:10.3390/ma11050767 www.mdpi.com/jou nal/ma e ials Ma e ials 2018,11, 767 2 o 12 luid had a signi ican impac on esul s. I was ound ha bo ine se um (BS) and simple p o ein solu ions lead o a educ ion o ic ion in a slow-speed egime. I was also highligh ed ha he choice o bu e solu ion a ec s he lub ica ion condi ions. Finally, i was concluded ha ic ion as well as lub ica ing ilm hickness a e ime and sliding-dis ance dependen , indica ing he impo ance o he adso bed p o ein laye on he su aces. One o he i s s udies which was ocused on he in luence o lub ican composi ion on ic ion be ween polyme and me al su ac was unde aken by Yao e al. [ 6 ]. A pin-on-disk ibome e was used o he e alua ion o CoF, while a ious es luids we e used as he lub ican s; pa icula ly deionized wa e , dilu ed (25%, 50%, 75%) and undilu ed bo ine cal se um (BCS), bo ine syno ial luid (SF) and human pe ip os he ic SF. A signi ican a iance o CoF esul s was obse ed, a ibu ed mainly o he subs an ial changes o p o ein concen a ions and heological p ope ies o he luids. The lowes ic ion ( ≈ 0.013) was exhibi ed by human SF ollowed by deionized wa e . Rema kably, highe ic ion was de ec ed o bo ine SF and undilu ed BCS ( ≈ 0.024). The highes CoF was de ec ed o 25% BCS, which was a ound 0.028. Scholes and Unswo h [ 7 ] demons a ed he impo ance o lub ican composi ion in ela ion o he ma e ial combina ion o he implan s. The au ho s es ed a ious model luids inding ha he highes ic ion can be de ec ed in he case o me al-me al implan s. No signi ican di e ence be ween he ce amic-ce amic and me al-polye hylene pai was obse ed. Al hough he e was a apid in luence o he model luid on he CoF, he e ec o p o eins could no be gene alized, since bo h dec ease and inc ease o ic ion could be obse ed dependen on implan ma e ial. The ole o indi idual cons i uen s con ained in SF was demons a ed by Sawae e al. [ 8 ]. The con ac be ween an ul a-high molecula weigh polye hylene (UHMWPE) pin sliding agains me al/ce amic disc was s udied, while a clea posi i e e ec o hyalu onic acid added o saline solu ion on ic ion was de ec ed. Mo eo e , i was well epo ed ha he ic ional beha iou o he con ac pai is subs an ially in luenced by he su ace condi ions. Widme e al. [ 9 ], in es iga ed he ce amic-polye hylene sliding pai , inding ha he oxygen-plasma based ea men o polye hylene (PE) led o imp o ed abili ies o p o ein adso p ion on o ubbing su aces, hus educing bo h s a ic and dynamic ic ion. Subsequen ly, i was poin ed ou ha con o ma ional changes ha e o be aken in o accoun [ 10 ]. The au ho s showed ha hyd ophilic na u e o he su aces suppo s adso p ion o he p o eins in a na i e s a e, o ming hicke lub ica ing ilm leading o lowe ic ion be ween he componen s. An ex ensi e in es iga ion o he e ec o model luid composi ion on CoF dependen on kinema ic condi ions was p o ided in ou p e ious s udy [ 11 ]. Recip oca ing pin-on-pla e (me al-UHMWPE) es was conduc ed, showing ha he kinema ic condi ions ha e a c ucial impac on he con o ma ional changes o p o eins, hus in luencing ic ional beha iou o he con ac pai . A a slow speed egime (10 mm/s), a e y sligh inc ease o ic ion wi h inc easing p o ein concen a ion was obse ed o bo h albumin and γ -globulin solu ion. The highes ic ion occu ed when a mix u e o he men ioned p o eins was used as he es lub ican . An inc ease o sliding speed o 50 mm/s caused jus a negligible change o ic ion. Independen o sliding speed, CoF was subs an ially highe when compa ing p o ein solu ions ( ≈ 0.17–0.27) wi h phospha e-bu e ed saline (PBS) wi hou any cons i uen s. In ha case, ic ion was lowe han 0.05. These esul s a e in good ag eemen wi h he obse a ion p o ided p e iously [ 12 , 13 ]. In he s udy men ioned, he e ec o albumin unde going con o ma ional changes on he ic ion be ween he UHMWPE pin and s ainless s eel pla e was e alua ed. A subs an ial e ec o he applied load on he esul s was obse ed; while a low loads, pos - ic ion deg aded albumin added o PBS led o a dec ease o ic ion compa ed o esh p o ein. The opposi e beha iou occu ed a highe loads. Based on he abo e e e ences [ 11 , 12 ], i can be concluded ha ic ion be ween he ubbing su aces is a ec ed by bo h kinema ic and loading condi ions. The e ec o eplacemen geome y has o be aken in o accoun as well. Dowson e al. [ 14 ] conduc ed a p edic i e nume ical analysis o ic ion and wea o me al-me al implan , inding ha he ic ional o que inc eases wi h inc easing nominal diame e ; howe e , i is a ec ed jus a li le by he ini ial clea ance be ween he su aces. An expe imen al s udy o he e ec o diame ic clea ance Ma e ials 2018,11, 767 3 o 12 oge he wi h he ole o model luid was unde aken by B ocke e al. [ 15 ]. Nominal diame e o he implan was 54.6 mm, while h ee di e en clea ances we e conside ed; 53 µ m, 94 µ m, and 194 µ m, espec i ely. The lowes ic ion ac o was de ec ed o he medium size o he clea ance. Lowe ing he clea ance led o a negligible inc ease o ic ion; howe e , in he case o he highes clea ance, a subs an ial inc ease o ic ion o almos double was de ec ed. Rega ding he model luid, 25% and 100% BS we e es ed, wi h he inding ha lowe ic ion could be eached in he case o highe p o ein concen a ion, in gene al, which is in a good ag eemen wi h one o he p e ious s udies [ 6 ]. Finally, an expe imen al in es iga ion o he ic ion coe icien using a pendulum hip join simula o ocusing on he e ec o implan ma e ial and diame e was conduc ed by V bka e al. [ 16 ]. This applied 25% BS o a o al p o ein concen a ion o 22.4 mg/mL, es ing 28 mm and 36 mm me al-UHMWPE, ce amic-UHMWPE and ce amic-ce amic pai s. The highes CoF ( ≈ 0.16) was de ec ed in he case o he me al-UHMWPE pai wi h no clea e ec o nominal diame e . Ce amic-UHMWPE exhibi ed lowe ic ion ( ≈ 0.14), while a la ge diame e led o a u he dec ease ( ≈ 0.13). The lowes ic ion (0.12) was de ec ed in he ce amic-ce amic pai wi h he same e ec o diame e as in he p e ious case. Based on he abo e e e ences, i is appa en ha he ibological beha iou o a i icial join s is in luenced by se e al aspec s, such as composi ion o he model SF, implan ma e ial, geome y, kinema ic and loading condi ions. Howe e , mos o he au ho s ocused on one pa icula ma e ial combina ion o speci ic composi ion o he es lub ican . The e o e, he aim o he p esen pape is o p o ide a de ailed analysis o ic ional beha iou conside ing a ious implan ma e ials, kinema ic condi ions, and he composi ion o model syno ial luid. Acco ding o he au ho s’ bes knowledge, such an ex ensi e in es iga ion has no been pe o med be o e. 2. Ma e ials and Me hods The measu emen s o he CoF we e conduc ed u ilizing a comme cial ball-on-disc es de ice Mini T ac ion Machine (MTM, PCS Ins umen s, London, Uni ed Kingdom). Bo h he componen s, he ball and he disc, could be d i en independen ly, hus enabling he applica ion o a ious kinema ic condi ions. The mean dynamic (some imes also called kine ic o sliding) ic ion coe icien [ 17 ] is e alua ed based on he a io o ic ion and no mal o ce measu ed by he load cell. CoF was e alua ed wi h he acquisi ion equency o 0.1 Hz. A con ac pai consis ed o me al (cobal -ch omium- molybdenum—CoC Mo) and alumina ce amic balls o diame e 19.05 mm sliding agains 46 mm- diame e discs o me al, ce amic, and highly-c osslinked PE (HXPE), espec i ely. The hickness o he discs was 6 mm. A schema ic illus a ion o he measu emen con igu a ion is displayed in Figu e 1. Ini ial su ace opog aphy o he es specimens was checked using he phase-shi ing in e e ome y me hod (Con ou GT-X8, B uke , Bille ica, MA, USA). Fi e poin s which we e expec ed o be in he con ac d ag o each sample we e measu ed, while he su ace oughness esul s a e shown in Table 1. To cla i y he e ec o kinema ic condi ions, he expe imen s we e ealized unde wo di e en mean speeds, 5.7 mm/s and 22 mm/s. Mean speed ep esen s he a e age alue o he ball and he disc speed unde a ious olling/sliding condi ions. In he ollowing ex , he e ms posi i e and nega i e sliding a e used. In he case o nega i e sliding, he ball o a es as e han he disc. By con as , unde posi i e sliding he disc is as e . The cou se o he speeds o he indi idual componen s conside ing nega i e and posi i e sliding is demons a ed using he ed a ows in he bo om pa o Figu e 1. Rega ding he human body, i can be assumed ha he hip join ope a es unde pu e nega i e sliding condi ions, which means ha he cup is s a iona y, while he head o a es. Conside ing he ball-on-disc con igu a ion, pu e sliding ep esen s bi se e e condi ions and so signi ican wea o he samples appa en ly in luencing he esul s o CoF could be expec ed. The e o e, he expe imen s we e ealized unde pa ial sliding o 150%. To a oid inaccu acies coming om he slippage o he componen s, he es s we e ealized unde bo h posi i e (150%) and nega i e ( − 150%) sliding. The slide- o- oll a io (SRR) which de e mines he le el o slip, can be easily de e mined ollowing he equa ion SRR = 2(u D− u B )/(u D + u B ), whe e u d is he speed o he disc and u b is he speed o he ball. Combined olling/sliding condi ions no mally occu in he knee join ; howe e , i should be highligh ed ha he Ma e ials 2018,11, 767 4 o 12 p esen s udy is no ocused on one pa icula join ype, bu on he gene al assessmen o ic ional beha iou o ma e ials used o join implan s. Applied load was 0.5 N in he case o he me al and 0.4 in he case o he ce amic balls, leading o he ollowing con ac p essu es: me al-on-me al (264.7 MPa), me al-HXPE (9.9 MPa), ce amic-ce amic (280.5 MPa), ce amic-HXPE (9.5 MPa). Ma e ials 2018, 11, x FOR PEER REVIEW 4 o 12 howe e , i should be highligh ed ha he p esen s udy is no ocused on one pa icula join ype, bu on he gene al assessmen o ic ional beha iou o ma e ials used o join implan s. Applied load was 0.5 N in he case o he me al and 0.4 in he case o he ce amic balls, leading o he ollowing con ac p essu es: me al-on-me al (264.7 MPa), me al-HXPE (9.9 MPa), ce amic-ce amic (280.5 MPa), ce amic-HXPE (9.5 MPa). Figu e 1. Schema ic illus a ion o he measu emen con igu a ion. Table 1. Su ace opog aphy pa ame e s o he es samples. Pa ame e Ball Disc - Me al Ce amic Me al Ce amic HXPE A e age su ace oughness Ra (μm) 0.087 0.035 0.008 0.998 0.841 S anda d de ia ion SD (μm) ±0.00587 ±0.00414 ±0.00039 ±0.10707 ±0.13631 The main in e es o he cu en s udy was in he e ec o he composi ion o model luid. The e o e, i e es lub ican s we e p epa ed while he esul s we e compa ed wi h hose o pu e PBS. The model luids con ained he dominan p o eins o SF; albumin and γ-globulin, espec i ely. Lub ican s we e p epa ed 24 h p io o he es s and we e s o ed in a e ige a o o enable a comple e solu ion o he p o eins in PBS. Bo ine se um albumin (9048-46-8, Biomol GmbH, Hambu g, Ge many) and γ-globulin om human blood (G4386, Sigma Ald ich, Da ms ad , Ge many) we e added o PBS in a ious concen a ions, while he simple p o ein solu ions we e in es iga ed as well. A summa y o he es luids is p o ided in Table 2. Despi e he abili y o he es de ice o con ol he empe a u e o he po , he expe imen s we e ealized unde ambien labo a o y empe a u e o 22 °C, since i was p e iously e e enced in li e a u e ha he inc ease o body empe a u e does no in luence he esul s subs an ially [18]. The expe imen s we e conduc ed unde ully looded condi ions o a oid any e ec s coming om s a a ion o he con ac . Eigh y expe imen s we e ealized in o al, while Table 3 p o ides he comple e o e iew o he pe o med es s o be e o ien a ion. The de ailed in o ma ion abou he speeds o he ball and he disc du ing a ious es s is gi en in Table 4. Random es s we e conduc ed h ee imes o check he epea abili y o he esul s. Since he e was excellen da a ag eemen in e ms o bo h he ends and he absolu e alues o CoF (s anda d de ia ion no la ge han 0.04), he es o he expe imen s we e pe o med only once. Figu e 1. Schema ic illus a ion o he measu emen con igu a ion. Table 1. Su ace opog aphy pa ame e s o he es samples. Pa ame e Ball Disc - Me al Ce amic Me al Ce amic HXPE A e age su ace oughness Ra (µm) 0.087 0.035 0.008 0.998 0.841 S anda d de ia ion SD (µm) ±0.00587 ±0.00414 ±0.00039 ±0.10707 ±0.13631 The main in e es o he cu en s udy was in he e ec o he composi ion o model luid. The e o e, i e es lub ican s we e p epa ed while he esul s we e compa ed wi h hose o pu e PBS. The model luids con ained he dominan p o eins o SF; albumin and γ -globulin, espec i ely. Lub ican s we e p epa ed 24 h p io o he es s and we e s o ed in a e ige a o o enable a comple e solu ion o he p o eins in PBS. Bo ine se um albumin (9048-46-8, Biomol GmbH, Hambu g, Ge many) and γ -globulin om human blood (G4386, Sigma Ald ich, Da ms ad , Ge many) we e added o PBS in a ious concen a ions, while he simple p o ein solu ions we e in es iga ed as well. A summa y o he es luids is p o ided in Table 2. Despi e he abili y o he es de ice o con ol he empe a u e o he po , he expe imen s we e ealized unde ambien labo a o y empe a u e o 22 ◦ C, since i was p e iously e e enced in li e a u e ha he inc ease o body empe a u e does no in luence he esul s subs an ially [ 18 ]. The expe imen s we e conduc ed unde ully looded condi ions o a oid any e ec s coming om s a a ion o he con ac . Eigh y expe imen s we e ealized in o al, while Table 3p o ides he comple e o e iew o he pe o med es s o be e o ien a ion. The de ailed in o ma ion abou he speeds o he ball and he disc du ing a ious es s is gi en in Table 4. Random es s we e conduc ed h ee imes o check he epea abili y o he esul s. Since he e was excellen da a ag eemen in e ms o bo h he ends and he absolu e alues o CoF (s anda d de ia ion no la ge han 0.04), he es o he expe imen s we e pe o med only once. Ma e ials 2018,11, 767 5 o 12 Table 2. Summa y o he applied model luids. Model Syno ial Fluid (MSF) Volume (mL) P o ein Con en (mg/mL) To al P o ein Concen a ion (mg/mL) - - Albumin γ-globulin - Alb70+Glob35 10 7 3.5 10.5 Alb35+Glob70 10 3.5 7 10.5 Alb140+Glob70 10 14 7 21 Albumin70 10 7 - 7 γ-globulin35 10 - 3.5 3.5 PBS 10 - - - Table 3. O e iew o he pe o med expe imen s. Expe imen Se Ma e ial Combina ion Mean Speed (mm/s) Slide-To-Roll Ra io (%) Model Syno ial Fluid 1Me al-on-Me al (MoM) 5.7; 22 150; −150 Alb70+Glob35; Alb35+Glob70; Alb140+Glob70; PBS 2 Me al-on-highly c osslinked polye hylene (HXPE) (MoP) 5.7; 22 150; −150 Alb70+Glob35; Alb35+Glob70; Alb140+Glob70; PBS 3Ce amic-on-Ce amic (CoC) 5.7; 22 150; −150 Alb70+Glob35; Alb35+Glob70; Alb140+Glob70; PBS 4Ce amic-on-HXPE (CoP) 5.7; 22 150; −150 Alb70+Glob35; Alb35+Glob70; Alb140+Glob70; PBS 5 MoM 22 150; −150 Albumin70; γ-globulin35 6 MoP 22 150; −150 Albumin70; γ-globulin35 7 CoC 22 150; −150 Albumin70; γ-globulin35 8 CoP 22 150; −150 Albumin70; γ-globulin35 Table 4. A de ailed o e iew o he kinema ic condi ions. Slide-To-Roll Ra io (%) Mean Speed (mm/s) Ball Speed (mm/s) Disc Speed (mm/s) −150 5.7 9.975 1.425 −150 22 38.5 5.5 150 5.7 1.425 9.975 150 22 5.5 38.5 3. Resul s and Discussion 3.1. The E ec o Ma e ial and Kinema ic Condi ions Ini ially, he expe imen s we e pe o med wi h pu e PBS o all he ma e ial combina ions, conside ing bo h posi i e and nega i e sliding a lowe (5.7 mm/s) and highe (22 mm/s) speed. The esul s showed ha independen o kinema ic condi ions, he MoM combina ion exhibi s he highes ic ion; howe e , his e ec was signi ican mainly unde nega i e sliding, when he ic ion a he end o he expe imen eached almos 0.6. By con as , he lowes ic ion was de ec ed o MoP and CoP, while in he case o CoP, he ic ion le el was less han 0.1, pa icula ly a ound 0.05 o he dominan pe iod o he expe imen s, which is in a good co ela ion wi h p e ious obse a ions [ 11 , 19 , 20 ]. CoC ic ional beha iou was ound o be somewhe e be ween, while he ic ion eached abou 0.25. Ne e heless, i should be aken in o accoun ha he su ace oughness o Ma e ials 2018,11, 767 6 o 12 he employed ce amic disc was an o de o magni ude highe han he su ace o he ball, which could a ec he esul s. Based on he p e ious indings, i migh be expec ed ha i ealis ic oughness o he disc could be achie ed, he ic ion coe icien would be lowe . Howe e , wi hou he expe imen al esul s, he po en ial le el o CoF educ ion can be only es ima ed. Ne e heless, i should be emphasized ha he p esen s udy is a he compa a i e, no p edic ing he ic ion wi hin eal implan s. Wi h he excep ion o MoM unde posi i e sliding condi ions, he e was no a subs an ial e ec o speed. Conside ing he e ec o ime, only in he case o MoM was he de elopmen o ic ion wi h ime obse ed. Fo he es o he ma e ial combina ions, CoF was s able wi hou any luc ua ions o e he en i e expe imen . All he esul s can be seen in Figu e 2. I should be no ed ha e en he acquisi ion equency was 0.1 Hz; o be e cla i y o he esul s, he plo ed do s ep esen a e age CoF in pa icula ime s eps (1 s, 15 s, 30 s . . . 300 s). The inse igu es a e shown o illus a e he kinema ic condi ions. The ed a ows ep esen he speed cou se. Ma e ials 2018, 11, x FOR PEER REVIEW 6 o 12 could be achie ed, he ic ion coe icien would be lowe . Howe e , wi hou he expe imen al esul s, he po en ial le el o CoF educ ion can be only es ima ed. Ne e heless, i should be emphasized ha he p esen s udy is a he compa a i e, no p edic ing he ic ion wi hin eal implan s. Wi h he excep ion o MoM unde posi i e sliding condi ions, he e was no a subs an ial e ec o speed. Conside ing he e ec o ime, only in he case o MoM was he de elopmen o ic ion wi h ime obse ed. Fo he es o he ma e ial combina ions, CoF was s able wi hou any luc ua ions o e he en i e expe imen . All he esul s can be seen in Figu e 2. I should be no ed ha e en he acquisi ion equency was 0.1 Hz; o be e cla i y o he esul s, he plo ed do s ep esen a e age CoF in pa icula ime s eps (1 s, 15 s, 30 s … 300 s). The inse igu es a e shown o illus a e he kinema ic condi ions. The ed a ows ep esen he speed cou se. Figu e 2. De elopmen o coe icien o ic ion (CoF) as a unc ion o ime o ou ma e ial combina ions a lowe and highe mean speed unde nega i e (a) and posi i e (b) sliding condi ions. Con ac is lub ica ed by phospha e-bu e ed saline (PBS). Adding he p o eins in o he saline solu ion in ela i ely low concen a ion (10.5 mg/mL in o al) led o a sligh change o CoF e olu ion, see Figu es 3 and 4 Compa ed o he pu e PBS esul s, ime-dependen beha iou was obse ed o MoM and also o CoC con ac pai s, while he maximum a he end o he es was om 0.3 o 0.4 o me al. Fo ce amic, he ic ion le el was abou 0.2, while he e y same beha iou was obse ed in li e a u e, whe e he con ac o a ce amic ball sliding agains a ce amic pla e lub ica ed by BS was analyzed [21]. To de e mine he in luence o luid composi ion, i can be seen ha e en i he concen a ion o albumin and γ-globulin was swi ched, see Figu e 3 s. Figu e 4, almos he same esul s could be ob ained o mos o he ma e ials and kinema ic condi ions. Only in he case o MoP did highe con en o γ-globulin lead o a dec ease o ic ion, as can be seen in Figu e 4a. Figu e 3. De elopmen o CoF as a unc ion o ime o ou ma e ial combina ions a lowe and highe mean speed unde nega i e (a) and posi i e (b) sliding condi ions. Con ac is lub ica ed by model luid Alb70+Glob35. Figu e 2. De elopmen o coe icien o ic ion (CoF) as a unc ion o ime o ou ma e ial combina ions a lowe and highe mean speed unde nega i e ( a ) and posi i e ( b ) sliding condi ions. Con ac is lub ica ed by phospha e-bu e ed saline (PBS). Adding he p o eins in o he saline solu ion in ela i ely low concen a ion (10.5 mg/mL in o al) led o a sligh change o CoF e olu ion, see Figu es 3and 4Compa ed o he pu e PBS esul s, ime-dependen beha iou was obse ed o MoM and also o CoC con ac pai s, while he maximum a he end o he es was om 0.3 o 0.4 o me al. Fo ce amic, he ic ion le el was abou 0.2, while he e y same beha iou was obse ed in li e a u e, whe e he con ac o a ce amic ball sliding agains a ce amic pla e lub ica ed by BS was analyzed [ 21 ]. To de e mine he in luence o luid composi ion, i can be seen ha e en i he concen a ion o albumin and γ -globulin was swi ched, see Figu e 3 s. Figu e 4, almos he same esul s could be ob ained o mos o he ma e ials and kinema ic condi ions. Only in he case o MoP did highe con en o γ -globulin lead o a dec ease o ic ion, as can be seen in Figu e 4a. Ma e ials 2018, 11, x FOR PEER REVIEW 6 o 12 could be achie ed, he ic ion coe icien would be lowe . Howe e , wi hou he expe imen al esul s, he po en ial le el o CoF educ ion can be only es ima ed. Ne e heless, i should be emphasized ha he p esen s udy is a he compa a i e, no p edic ing he ic ion wi hin eal implan s. Wi h he excep ion o MoM unde posi i e sliding condi ions, he e was no a subs an ial e ec o speed. Conside ing he e ec o ime, only in he case o MoM was he de elopmen o ic ion wi h ime obse ed. Fo he es o he ma e ial combina ions, CoF was s able wi hou any luc ua ions o e he en i e expe imen . All he esul s can be seen in Figu e 2. I should be no ed ha e en he acquisi ion equency was 0.1 Hz; o be e cla i y o he esul s, he plo ed do s ep esen a e age CoF in pa icula ime s eps (1 s, 15 s, 30 s … 300 s). The inse igu es a e shown o illus a e he kinema ic condi ions. The ed a ows ep esen he speed cou se. Figu e 2. De elopmen o coe icien o ic ion (CoF) as a unc ion o ime o ou ma e ial combina ions a lowe and highe mean speed unde nega i e (a) and posi i e (b) sliding condi ions. Con ac is lub ica ed by phospha e-bu e ed saline (PBS). Adding he p o eins in o he saline solu ion in ela i ely low concen a ion (10.5 mg/mL in o al) led o a sligh change o CoF e olu ion, see Figu es 3 and 4 Compa ed o he pu e PBS esul s, ime-dependen beha iou was obse ed o MoM and also o CoC con ac pai s, while he maximum a he end o he es was om 0.3 o 0.4 o me al. Fo ce amic, he ic ion le el was abou 0.2, while he e y same beha iou was obse ed in li e a u e, whe e he con ac o a ce amic ball sliding agains a ce amic pla e lub ica ed by BS was analyzed [21]. To de e mine he in luence o luid composi ion, i can be seen ha e en i he concen a ion o albumin and γ-globulin was swi ched, see Figu e 3 s. Figu e 4, almos he same esul s could be ob ained o mos o he ma e ials and kinema ic condi ions. Only in he case o MoP did highe con en o γ-globulin lead o a dec ease o ic ion, as can be seen in Figu e 4a. Figu e 3. De elopmen o CoF as a unc ion o ime o ou ma e ial combina ions a lowe and highe mean speed unde nega i e (a) and posi i e (b) sliding condi ions. Con ac is lub ica ed by model luid Alb70+Glob35. Figu e 3. De elopmen o CoF as a unc ion o ime o ou ma e ial combina ions a lowe and highe mean speed unde nega i e ( a ) and posi i e ( b ) sliding condi ions. Con ac is lub ica ed by model luid Alb70+Glob35. Ma e ials 2018,11, 767 7 o 12 Ma e ials 2018, 11, x FOR PEER REVIEW 7 o 12 Figu e 4. De elopmen o CoF as a unc ion o ime o ou ma e ial combina ions a lowe and highe mean speed unde nega i e (a) and posi i e (b) sliding condi ions. Con ac is lub ica ed by model luid Alb35+Glob70. An inc ease o p o ein concen a ion o 21 mg/mL led o a e y sligh inc ease o ic ion o ce amic and me al pai s. In he case o MoP and CoP, he ic ion emained he same o dec eased a li le bi . Howe e , since he change is negligible, see Figu es 3 and 4 s. Figu e 5, i can be concluded ha he p o ein con en does no play an impo an ole ega ding hese so-called ha d-on-so bea ing pai s. Finally, ocusing on he pa icula ma e ial combina ions, i can be seen ha he beha iou co esponds well o he pu e PBS esul s, while he highes ic ion is exhibi ed by MoM, ollowed by CoC, MoP and CoP (Figu es 2–5). The highes ic ion in he case o me al-me al con ac was epo ed also by Scholes and Unswo h [7]. Ne e heless, i should be no ed ha he esul s do no comple ely co espond o obse a ions p o ided by V bka e al. [16], who de e mined he ic ion coe icien o a ious implan pai s. In he e e ence, he au ho s measu ed he lowes ic ion when conside ing a ce amic head sliding agains a ce amic cup. Howe e , i mus be no ed ha di e en expe imen al condi ions we e employed; in pa icula , BS was employed as he es lub ican , and he expe imen s we e ealized in di e en geome ical con igu a ions. Mo eo e , as men ioned abo e, he oughness o ce amic discs used in he p esen s udy was an o de o magni ude highe han is he oughness o eal ace abula cups, which can also play a signi ican ole. Figu e 5. De elopmen o CoF as a unc ion o ime o ou ma e ial combina ions a lowe and highe mean speed unde nega i e (a) and posi i e (b) sliding condi ions. Con ac is lub ica ed by model luid Alb140+Glob70. 3.2. The E ec o Model Syno ial Fluid Composi ion To clea ly iden i y he e ec o model SF composi ion on ic ional beha iou , he addi ional es s we e pe o med a highe speed conside ing he simple p o ein solu ions o albumin and γ-globulin as well. The esul s o indi idual ma e ial combina ions a e displayed in Figu es 6–9, espec i ely. Fo MoM con ac , i can be seen ha he esul s o pu e PBS a e in luenced by he posi i i y/nega i i y o sliding. Unde nega i e sliding, he le el o ic ion is conside ably highe (a ound 0.6 a maximum) o PBS compa ed o p o ein solu ion (a ound 0.4). In e ms o indi idual luids, albumin-con aining lub ican exhibi ed he lowes ic ion, app oxima ely 0.37. Fo he es o he lub ican s, he e is no clea impac on lub ica ion, while he ic ion is kep be ween 0.4 and 0.43 Figu e 4. De elopmen o CoF as a unc ion o ime o ou ma e ial combina ions a lowe and highe mean speed unde nega i e ( a ) and posi i e ( b ) sliding condi ions. Con ac is lub ica ed by model luid Alb35+Glob70. An inc ease o p o ein concen a ion o 21 mg/mL led o a e y sligh inc ease o ic ion o ce amic and me al pai s. In he case o MoP and CoP, he ic ion emained he same o dec eased a li le bi . Howe e , since he change is negligible, see Figu es 3and 4 s. Figu e 5, i can be concluded ha he p o ein con en does no play an impo an ole ega ding hese so-called ha d-on-so bea ing pai s. Finally, ocusing on he pa icula ma e ial combina ions, i can be seen ha he beha iou co esponds well o he pu e PBS esul s, while he highes ic ion is exhibi ed by MoM, ollowed by CoC, MoP and CoP (Figu es 2–5). The highes ic ion in he case o me al-me al con ac was epo ed also by Scholes and Unswo h [ 7 ]. Ne e heless, i should be no ed ha he esul s do no comple ely co espond o obse a ions p o ided by V bka e al. [ 16 ], who de e mined he ic ion coe icien o a ious implan pai s. In he e e ence, he au ho s measu ed he lowes ic ion when conside ing a ce amic head sliding agains a ce amic cup. Howe e , i mus be no ed ha di e en expe imen al condi ions we e employed; in pa icula , BS was employed as he es lub ican , and he expe imen s we e ealized in di e en geome ical con igu a ions. Mo eo e , as men ioned abo e, he oughness o ce amic discs used in he p esen s udy was an o de o magni ude highe han is he oughness o eal ace abula cups, which can also play a signi ican ole. Ma e ials 2018, 11, x FOR PEER REVIEW 7 o 12 Figu e 4. De elopmen o CoF as a unc ion o ime o ou ma e ial combina ions a lowe and highe mean speed unde nega i e (a) and posi i e (b) sliding condi ions. Con ac is lub ica ed by model luid Alb35+Glob70. An inc ease o p o ein concen a ion o 21 mg/mL led o a e y sligh inc ease o ic ion o ce amic and me al pai s. In he case o MoP and CoP, he ic ion emained he same o dec eased a li le bi . Howe e , since he change is negligible, see Figu es 3 and 4 s. Figu e 5, i can be concluded ha he p o ein con en does no play an impo an ole ega ding hese so-called ha d-on-so bea ing pai s. Finally, ocusing on he pa icula ma e ial combina ions, i can be seen ha he beha iou co esponds well o he pu e PBS esul s, while he highes ic ion is exhibi ed by MoM, ollowed by CoC, MoP and CoP (Figu es 2–5). The highes ic ion in he case o me al-me al con ac was epo ed also by Scholes and Unswo h [7]. Ne e heless, i should be no ed ha he esul s do no comple ely co espond o obse a ions p o ided by V bka e al. [16], who de e mined he ic ion coe icien o a ious implan pai s. In he e e ence, he au ho s measu ed he lowes ic ion when conside ing a ce amic head sliding agains a ce amic cup. Howe e , i mus be no ed ha di e en expe imen al condi ions we e employed; in pa icula , BS was employed as he es lub ican , and he expe imen s we e ealized in di e en geome ical con igu a ions. Mo eo e , as men ioned abo e, he oughness o ce amic discs used in he p esen s udy was an o de o magni ude highe han is he oughness o eal ace abula cups, which can also play a signi ican ole. Figu e 5. De elopmen o CoF as a unc ion o ime o ou ma e ial combina ions a lowe and highe mean speed unde nega i e (a) and posi i e (b) sliding condi ions. Con ac is lub ica ed by model luid Alb140+Glob70. 3.2. The E ec o Model Syno ial Fluid Composi ion To clea ly iden i y he e ec o model SF composi ion on ic ional beha iou , he addi ional es s we e pe o med a highe speed conside ing he simple p o ein solu ions o albumin and γ-globulin as well. The esul s o indi idual ma e ial combina ions a e displayed in Figu es 6–9, espec i ely. Fo MoM con ac , i can be seen ha he esul s o pu e PBS a e in luenced by he posi i i y/nega i i y o sliding. Unde nega i e sliding, he le el o ic ion is conside ably highe (a ound 0.6 a maximum) o PBS compa ed o p o ein solu ion (a ound 0.4). In e ms o indi idual luids, albumin-con aining lub ican exhibi ed he lowes ic ion, app oxima ely 0.37. Fo he es o he lub ican s, he e is no clea impac on lub ica ion, while he ic ion is kep be ween 0.4 and 0.43 Figu e 5. De elopmen o CoF as a unc ion o ime o ou ma e ial combina ions a lowe and highe mean speed unde nega i e ( a ) and posi i e ( b ) sliding condi ions. Con ac is lub ica ed by model luid Alb140+Glob70. 3.2. The E ec o Model Syno ial Fluid Composi ion To clea ly iden i y he e ec o model SF composi ion on ic ional beha iou , he addi ional es s we e pe o med a highe speed conside ing he simple p o ein solu ions o albumin and γ -globulin as well. The esul s o indi idual ma e ial combina ions a e displayed in Figu es 6–9, espec i ely. Fo MoM con ac , i can be seen ha he esul s o pu e PBS a e in luenced by he posi i i y/nega i i y o sliding. Unde nega i e sliding, he le el o ic ion is conside ably highe (a ound 0.6 a maximum) o PBS compa ed o p o ein solu ion (a ound 0.4). In e ms o indi idual luids, albumin-con aining lub ican exhibi ed he lowes ic ion, app oxima ely 0.37. Fo he es o he lub ican s, he e is Ma e ials 2018,11, 767 8 o 12 no clea impac on lub ica ion, while he ic ion is kep be ween 0.4 and 0.43 a he end o he es , see Figu e 6a. Conside ing he posi i e sliding condi ions, he CoF de elopmen is sligh ly di e en . The highes ic ion o e he dominan pa o he expe imen was exhibi ed by he solu ion wi h ela i ely high p o ein con en . Howe e , a e 75 s, he ic ion was s abilized a abou 0.4 wi hou any u he ime-dependen change. The cou se o ic ion o saline solu ion is linea ly inc easing ( he same beha iou as unde nega i e sliding), eaching 0.45 a he end. Fo he o he p o ein solu ions, he beha iou is almos he same, wi h he excep ion o lub ican con aining 70 mg/mL o albumin and 35 mg/mL o γ -globulin, which exhibi s he lowes ic ion. This can be a ibu ed o he ac ion o he p o ein ilm. I was obse ed in li e a u e ha , conside ing he same model luid as in he case o he p esen s udy, a me al head o med much hicke p o ein ilm unde posi i e sliding, which can lead o a dec ease o ic ion be ween he su aces [22]. Ma e ials 2018, 11, x FOR PEER REVIEW 8 o 12 a he end o he es , see Figu e 6a. Conside ing he posi i e sliding condi ions, he CoF de elopmen is sligh ly di e en . The highes ic ion o e he dominan pa o he expe imen was exhibi ed by he solu ion wi h ela i ely high p o ein con en . Howe e , a e 75 s, he ic ion was s abilized a abou 0.4 wi hou any u he ime-dependen change. The cou se o ic ion o saline solu ion is linea ly inc easing ( he same beha iou as unde nega i e sliding), eaching 0.45 a he end. Fo he o he p o ein solu ions, he beha iou is almos he same, wi h he excep ion o lub ican con aining 70 mg/mL o albumin and 35 mg/mL o γ-globulin, which exhibi s he lowes ic ion. This can be a ibu ed o he ac ion o he p o ein ilm. I was obse ed in li e a u e ha , conside ing he same model luid as in he case o he p esen s udy, a me al head o med much hicke p o ein ilm unde posi i e sliding, which can lead o a dec ease o ic ion be ween he su aces [22]. Figu e 6. De elopmen o CoF as a unc ion o ime o me al-on-me al pai a highe mean speed unde nega i e (a) and posi i e (b) sliding condi ions o a ious model luids. I is appa en ha he MoM combina ion exhibi s an inc easing endency o ic ion, in gene al. By con as , when he disc is o polye hylene, ic ion is dec easing o cons an , as is shown in Figu e 7. In ha case, he ic ion le el is associa ed wi h he kinema ic condi ions. When he ball o a es as e han he disc, he lowes ic ion was exhibi ed by PBS, and he lub ican wi h he highe con en o γ-globulin. The highes ic ion was obse ed o lub ican wi h lowe p o ein concen a ion (10.5 mg/mL) and he dominan p esence o albumin. The endency was also qui e uns able in ha case. Ne e heless, i has o be aken in o accoun ha he di e ence be ween minimum and maximum ic ion is jus 0.05; he e o e, i migh be assumed ha he e ec o he lub ican is negligible. When he disc was as e , he lowes ic ion was eached, once he lub ican wi h highe p o ein con en (21 mg/mL) was employed. In addi ion, bo h he simple solu ion exhibi ed ela i ely low ic ion o a ound 0.11. Wha is in co ela ion wi h p e ious indings is he ic ion le el o saline solu ion. In a ecen s udy [23], a polye hylene disc conduc ed ecip oca ing mo ion agains a me al pin (pu e posi i e sliding), while he ic ion was om 0.12 o 0.16, which co esponds o he p esen esul s, see Figu e 7b. Figu e 7. De elopmen o CoF as a unc ion o ime o me al-on-polye hylene pai a highe mean speed unde nega i e (a) and posi i e (b) sliding condi ions o a ious model luids. Figu e 6. De elopmen o CoF as a unc ion o ime o me al-on-me al pai a highe mean speed unde nega i e (a) and posi i e (b) sliding condi ions o a ious model luids. I is appa en ha he MoM combina ion exhibi s an inc easing endency o ic ion, in gene al. By con as , when he disc is o polye hylene, ic ion is dec easing o cons an , as is shown in Figu e 7. In ha case, he ic ion le el is associa ed wi h he kinema ic condi ions. When he ball o a es as e han he disc, he lowes ic ion was exhibi ed by PBS, and he lub ican wi h he highe con en o γ -globulin. The highes ic ion was obse ed o lub ican wi h lowe p o ein concen a ion (10.5 mg/mL) and he dominan p esence o albumin. The endency was also qui e uns able in ha case. Ne e heless, i has o be aken in o accoun ha he di e ence be ween minimum and maximum ic ion is jus 0.05; he e o e, i migh be assumed ha he e ec o he lub ican is negligible. When he disc was as e , he lowes ic ion was eached, once he lub ican wi h highe p o ein con en (21 mg/mL) was employed. In addi ion, bo h he simple solu ion exhibi ed ela i ely low ic ion o a ound 0.11. Wha is in co ela ion wi h p e ious indings is he ic ion le el o saline solu ion. In a ecen s udy [ 23 ], a polye hylene disc conduc ed ecip oca ing mo ion agains a me al pin (pu e posi i e sliding), while he ic ion was om 0.12 o 0.16, which co esponds o he p esen esul s, see Figu e 7b. Ma e ials 2018, 11, x FOR PEER REVIEW 8 o 12 a he end o he es , see Figu e 6a. Conside ing he posi i e sliding condi ions, he CoF de elopmen is sligh ly di e en . The highes ic ion o e he dominan pa o he expe imen was exhibi ed by he solu ion wi h ela i ely high p o ein con en . Howe e , a e 75 s, he ic ion was s abilized a abou 0.4 wi hou any u he ime-dependen change. The cou se o ic ion o saline solu ion is linea ly inc easing ( he same beha iou as unde nega i e sliding), eaching 0.45 a he end. Fo he o he p o ein solu ions, he beha iou is almos he same, wi h he excep ion o lub ican con aining 70 mg/mL o albumin and 35 mg/mL o γ-globulin, which exhibi s he lowes ic ion. This can be a ibu ed o he ac ion o he p o ein ilm. I was obse ed in li e a u e ha , conside ing he same model luid as in he case o he p esen s udy, a me al head o med much hicke p o ein ilm unde posi i e sliding, which can lead o a dec ease o ic ion be ween he su aces [22]. Figu e 6. De elopmen o CoF as a unc ion o ime o me al-on-me al pai a highe mean speed unde nega i e (a) and posi i e (b) sliding condi ions o a ious model luids. I is appa en ha he MoM combina ion exhibi s an inc easing endency o ic ion, in gene al. By con as , when he disc is o polye hylene, ic ion is dec easing o cons an , as is shown in Figu e 7. In ha case, he ic ion le el is associa ed wi h he kinema ic condi ions. When he ball o a es as e han he disc, he lowes ic ion was exhibi ed by PBS, and he lub ican wi h he highe con en o γ-globulin. The highes ic ion was obse ed o lub ican wi h lowe p o ein concen a ion (10.5 mg/mL) and he dominan p esence o albumin. The endency was also qui e uns able in ha case. Ne e heless, i has o be aken in o accoun ha he di e ence be ween minimum and maximum ic ion is jus 0.05; he e o e, i migh be assumed ha he e ec o he lub ican is negligible. When he disc was as e , he lowes ic ion was eached, once he lub ican wi h highe p o ein con en (21 mg/mL) was employed. In addi ion, bo h he simple solu ion exhibi ed ela i ely low ic ion o a ound 0.11. Wha is in co ela ion wi h p e ious indings is he ic ion le el o saline solu ion. In a ecen s udy [23], a polye hylene disc conduc ed ecip oca ing mo ion agains a me al pin (pu e posi i e sliding), while he ic ion was om 0.12 o 0.16, which co esponds o he p esen esul s, see Figu e 7b. Figu e 7. De elopmen o CoF as a unc ion o ime o me al-on-polye hylene pai a highe mean speed unde nega i e (a) and posi i e (b) sliding condi ions o a ious model luids. Figu e 7. De elopmen o CoF as a unc ion o ime o me al-on-polye hylene pai a highe mean speed unde nega i e (a) and posi i e (b) sliding condi ions o a ious model luids. Ma e ials 2018,11, 767 9 o 12 Conside ing he ce amic-ce amic con ac , he ic ion is inc easing unde nega i e sliding as in he case o he me al-me al pai . Mo eo e , he e is a good co ela ion o he e ec o p o eins, while adding he p o eins o he saline solu ion led o a dec ease o ic ion, especially du ing he i s pa o he es , see Figu e 8a. The same beha iou can be obse ed e en o posi i e sliding, while he e ec o he lub ican is mo e subs an ial; anyway, he a iance in ic ion is s ill limi ed. In his pa icula case, he ic ion is ela i ely s able o all he es luids, as is displayed in Figu e 8b. Simila ic ion wi h ilm hickness de elopmen can be ound, as men ioned o he me al sliding pai . Using he luo escen mic oscopy echnique, we e alua ed he cen al ilm hickness in ce amic-on-glass con ac , while i was ound ha he lub ican laye o alumina ce amic unde posi i e sliding was much hicke compa ed o nega i e sliding [24]. Ma e ials 2018, 11, x FOR PEER REVIEW 9 o 12 Conside ing he ce amic-ce amic con ac , he ic ion is inc easing unde nega i e sliding as in he case o he me al-me al pai . Mo eo e , he e is a good co ela ion o he e ec o p o eins, while adding he p o eins o he saline solu ion led o a dec ease o ic ion, especially du ing he i s pa o he es , see Figu e 8a. The same beha iou can be obse ed e en o posi i e sliding, while he e ec o he lub ican is mo e subs an ial; anyway, he a iance in ic ion is s ill limi ed. In his pa icula case, he ic ion is ela i ely s able o all he es luids, as is displayed in Figu e 8b. Simila ic ion wi h ilm hickness de elopmen can be ound, as men ioned o he me al sliding pai . Using he luo escen mic oscopy echnique, we e alua ed he cen al ilm hickness in ce amic-on-glass con ac , while i was ound ha he lub ican laye o alumina ce amic unde posi i e sliding was much hicke compa ed o nega i e sliding [24]. Figu e 8. De elopmen o CoF as a unc ion o ime o ce amic-on-ce amic pai a highe mean speed unde nega i e (a) and posi i e (b) sliding condi ions o a ious model luids. Finally, he expe imen s we e ealized wi h a ce amic ball and polye hylene disc. The e ec o he lub ican was again e y limi ed, as o he MoP pai . In pa icula unde posi i e sliding, he ole o luid is unimpo an , since he di e ence be ween minimum and maximum ic ion is jus abou 0.04, see Figu e 9b. The e o e, i is no sui able o iden i y which lub ican leads o he lowes and he highes ic ion, espec i ely. The only no iceable inding is ha in he case o nega i e sliding, sligh ly highe ic ion (a ound 0.12) was exhibi ed by simple γ-globulin solu ion and by he solu ion wi h a highe con en o p o ein. Figu e 9. De elopmen o CoF as a unc ion o ime o ce amic-on-polye hylene pai a highe mean speed unde nega i e (a) and posi i e (b) sliding condi ions o a ious model luids. As could be expec ed based on he p e ious indings, he p esen esul s clea ly p o e ha he mos impo an ac o in luencing he ic ional beha iou o implan s is he ma e ial o he componen s. Wha is qui e su p ising is he limi ed in luence o model luid composi ion, mainly in he case o MoP and CoP pai s. Ne e heless, i should be aken in o accoun ha in ha d-on-so pai s signi ican ly lowe con ac p essu e occu s; he e o e, his may be an explana ion o ela i ely low sensi i i y o ic ion o es luid. Anyway, since he p e ious obse a ions ocusing on he assessmen o ilm hickness in hip eplacemen s explo ed whe he he e ec o luid composi ion Figu e 8. De elopmen o CoF as a unc ion o ime o ce amic-on-ce amic pai a highe mean speed unde nega i e (a) and posi i e (b) sliding condi ions o a ious model luids. Finally, he expe imen s we e ealized wi h a ce amic ball and polye hylene disc. The e ec o he lub ican was again e y limi ed, as o he MoP pai . In pa icula unde posi i e sliding, he ole o luid is unimpo an , since he di e ence be ween minimum and maximum ic ion is jus abou 0.04, see Figu e 9b. The e o e, i is no sui able o iden i y which lub ican leads o he lowes and he highes ic ion, espec i ely. The only no iceable inding is ha in he case o nega i e sliding, sligh ly highe ic ion (a ound 0.12) was exhibi ed by simple γ -globulin solu ion and by he solu ion wi h a highe con en o p o ein. Ma e ials 2018, 11, x FOR PEER REVIEW 9 o 12 Conside ing he ce amic-ce amic con ac , he ic ion is inc easing unde nega i e sliding as in he case o he me al-me al pai . Mo eo e , he e is a good co ela ion o he e ec o p o eins, while adding he p o eins o he saline solu ion led o a dec ease o ic ion, especially du ing he i s pa o he es , see Figu e 8a. The same beha iou can be obse ed e en o posi i e sliding, while he e ec o he lub ican is mo e subs an ial; anyway, he a iance in ic ion is s ill limi ed. In his pa icula case, he ic ion is ela i ely s able o all he es luids, as is displayed in Figu e 8b. Simila ic ion wi h ilm hickness de elopmen can be ound, as men ioned o he me al sliding pai . Using he luo escen mic oscopy echnique, we e alua ed he cen al ilm hickness in ce amic-on-glass con ac , while i was ound ha he lub ican laye o alumina ce amic unde posi i e sliding was much hicke compa ed o nega i e sliding [24]. Figu e 8. De elopmen o CoF as a unc ion o ime o ce amic-on-ce amic pai a highe mean speed unde nega i e (a) and posi i e (b) sliding condi ions o a ious model luids. Finally, he expe imen s we e ealized wi h a ce amic ball and polye hylene disc. The e ec o he lub ican was again e y limi ed, as o he MoP pai . In pa icula unde posi i e sliding, he ole o luid is unimpo an , since he di e ence be ween minimum and maximum ic ion is jus abou 0.04, see Figu e 9b. The e o e, i is no sui able o iden i y which lub ican leads o he lowes and he highes ic ion, espec i ely. The only no iceable inding is ha in he case o nega i e sliding, sligh ly highe ic ion (a ound 0.12) was exhibi ed by simple γ-globulin solu ion and by he solu ion wi h a highe con en o p o ein. Figu e 9. De elopmen o CoF as a unc ion o ime o ce amic-on-polye hylene pai a highe mean speed unde nega i e (a) and posi i e (b) sliding condi ions o a ious model luids. As could be expec ed based on he p e ious indings, he p esen esul s clea ly p o e ha he mos impo an ac o in luencing he ic ional beha iou o implan s is he ma e ial o he componen s. Wha is qui e su p ising is he limi ed in luence o model luid composi ion, mainly in he case o MoP and CoP pai s. Ne e heless, i should be aken in o accoun ha in ha d-on-so pai s signi ican ly lowe con ac p essu e occu s; he e o e, his may be an explana ion o ela i ely low sensi i i y o ic ion o es luid. Anyway, since he p e ious obse a ions ocusing on he assessmen o ilm hickness in hip eplacemen s explo ed whe he he e ec o luid composi ion Figu e 9. De elopmen o CoF as a unc ion o ime o ce amic-on-polye hylene pai a highe mean speed unde nega i e (a) and posi i e (b) sliding condi ions o a ious model luids. As could be expec ed based on he p e ious indings, he p esen esul s clea ly p o e ha he mos impo an ac o in luencing he ic ional beha iou o implan s is he ma e ial o he componen s. Wha is qui e su p ising is he limi ed in luence o model luid composi ion, mainly in he case o MoP and CoP pai s. Ne e heless, i should be aken in o accoun ha in ha d-on-so pai s signi ican ly lowe con ac p essu e occu s; he e o e, his may be an explana ion o ela i ely low sensi i i y o ic ion o es luid. Anyway, since he p e ious obse a ions ocusing on he assessmen o ilm hickness in hip eplacemen s explo ed whe he he e ec o luid composi ion