变 Biomaterials ELSEVIER Biomaterials20()1-25 Review Zirconia as a ceramic biomaterial C.Piconi".*G.Maccaurob Reccived 19 March 1997:accepted 18 December 1997 Abstract Zirconia ceramics have severa advantages over other ceramic materials,due to the transformation toughening mechanism ;Mechanical properties Stability Biocompatibility Wear,Radioactivity 1.Introduction Thename te meohe Aumbe same using zirconia as a ceramic biomaterial. Zargon (golden in colour)which in turn comes from the The R&D on zirconia as a biomaterial was started in d)and Gun (Colour).Zirco in the reaction product obtained after heating some of zirconia to manufacture ball heads for Total Hip gems,and was used for a long time blended with rare or ce In the early stages of the development,several solid ceramics are used to manufacture parts operating in solutions (ZrO2-MgO,ZrO2-CaO,ZrOz-Y2O3)were aggressive environments,like extrusion dyes,valves tested for biomedical applications(Table 1).But in the for com on engines,low co ollowing years the rese d to be more foundries.Zirconia blades are used to cut Kevlar,mag netic tapes,cigarette filters (because of their reduced ia ocramics uable as solide e almost all the manufactu arers that are introducing into the market zirconia ball heads (Table 2).More than 30000 TZP ball heads has been implanted [4],and only ailures were reported [5]up to now Pabiod by eveed
Biomaterials 20 (1999) 1 —25 Review Zirconia as a ceramic biomaterial C. Piconi!,*, G. Maccauro" !ENEA, New Technologies Dpt., New Materials Div., Roma, Italia "Institute of Orthopaedics, Universita` Cattolica del S. Cuore, Roma, Italia Received 19 March 1997; accepted 18 December 1997 Abstract Zirconia ceramics have several advantages over other ceramic materials, due to the transformation toughening mechanisms operating in their microstructure that can give to components made out of them, very interesting mechanical properties. The research on the use of zirconia ceramics as biomaterials started about twenty years ago, and now zirconia (Y-YZP) is in clinical use in THR, but developments are in progress for application in other medical devices. Recent developments have concentrated on the chemistry of precursors, in forming and sintering processes, and on surface finish of components. Today’s main applications of zirconia ceramics is in THR ball heads. This review takes into account the main results achieved up to now, and is focused on the role that microstructural characteristics play on the TZP ceramics behaviour in ball heads, namely mechanical properties and their stability, wear of the UHMWPE paired to TZP, and their influence on biocompatibility. ( 1998 Published by Elsevier Science Ltd. All rights reserved Keywords: Zirconia; Mechanical properties; Stability; Biocompatibility; Wear; Radioactivity 1. Introduction Zircon has been known as a gem from ancient times. The name of the metal, zirconium, comes from the Arabic Zargon (golden in colour) which in turn comes from the two Persian words Zar (Gold) and Gun (Colour). Zirconia, the metal dioxide (ZrO2 ), was identified as such in 1789 by the German chemist Martin Heinrich Klaproth in the reaction product obtained after heating some gems, and was used for a long time blended with rare earth oxides as pigment for ceramics. Although low-quality zirconia is used as an abrasive in huge quantities, tough, wear resistant, refractory zirconia ceramics are used to manufacture parts operating in aggressive environments, like extrusion dyes, valves and port liners for combustion engines, low corrosion, thermal shock resistant refractory liners or valve parts in foundries. Zirconia blades are used to cut Kevlar, magnetic tapes, cigarette filters (because of their reduced wear). High temperature ionic conductivity makes zirconia ceramics suitable as solid electrolytes in fuel cells and in oxygen sensors. Good chemical and dimensional * Correspondence address: ENEA-INN-NUMA, CR Casaccia 049, Via Anguillarese 301, 00060 Roma, Italy. Fax:#39 6 3048 4928; e-mail: piconi@infosl.casaccia.enea.it stability, mechanical strength and toughness, coupled with a Young’s modulus in the same order of magnitude of stainless steel alloys was the origin of the interest in using zirconia as a ceramic biomaterial. The R&D on zirconia as a biomaterial was started in the late sixties. The first paper concerning biomedical application of zirconia was published in 1969 by Helmer and Driskell [1], while the first paper concerning the use of zirconia to manufacture ball heads for Total Hip Replacements (THR), which is the current main application of this ceramic biomaterial, was introduced by Christel et al. [2]. In the early stages of the development, several solid solutions (ZrO2 —MgO, ZrO2 —CaO, ZrO2 —Y2 O3 ) were tested for biomedical applications (Table 1). But in the following years the research efforts appeared to be more focused on zirconia—yttria ceramics, characterised by fine grained microstructures known as Tetragonal Zirconia Polycrystals (TZP). Nowadays, TZP ceramics, whose minimal requirements as implants for surgery are now described by the standard ISO 13356 [3], are the materials selected by almost all the manufacturers that are introducing into the market zirconia ball heads (Table 2). More than 300 000 TZP ball heads has been implanted [4], and only two failures were reported [5] up to now. 0142-9612/98/$—See front matter ( 1998 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 6 1 2 ( 9 8 ) 0 0 0 1 0 - 6
2 C.Piconi.G.Maccauro/Biomaterials 20(1999)1-25 Property Units Alumina Mg-PSZ TZP Chemical 9%A0, 450-700 1200 engrth 2000 re toughn css K 7-10 1x10- ardness 1200 1200 with monoclinic and tetragonal zirconia precipitates as Producers of zirconia ball heads for THR the minor phase.These precipitates may exist at grain Producer Country boundaries or within the cubic matrix grains.In 1972 Garvie anc holson [8]sh wed that the mechanica Astrome USA of the Ceramte Germany matrix.The development of zirconia as an engineering material was marked by Garvie et al.[9],who in their paper owed how to make the best of tion in PS Z improving mecn observed that tetrago nal metastable p dispersed within the cubic matrix were able to be trans forme into the monoclinic phase v e m e by a cra 2.Microstructural properties associated with ex nsion due to the tion acts in opposition to the stress field that promotes Zirconia is a well-known polymorph that occurs in hree forms:monoclinic(M),cubic(C)and tetragonal(T) es Because the propa ransforms into tetragonal and then into cubic phase at due to the volume nsion.A schematic representation 2370C.During cooling.a T-M transformation takes of this phenomenon is given in Fig.1.The development ol suc tetrago I metastable may ob ing plac on ol s of Mgo to ZrO imat matrix of a tetra nate cracks in pure zirconia ceramics that,after sintering gonal metastable phase.during controlled cooling and in the range 1500-1700C,break into pieces at roo ageing. s1n92 that n and coworkers [6 also be ob ined in the ZrC aoe时adin trm ed a ure with a tetra of Cao onal phase only.called TZP.This result was reported The addition of'stabilising'oxides,like CaO.Mgo. first by Rieth et al[]and by Gupta et al.[11] TZP materials,containing approximately 2-3 %m0 (PSZt d by tetragonal ra fraction of T-phase retained at room temperature is
Table 1 Characteristics of some ceramics for biomedical applications Property Units Alumina Mg—PSZ TZP Chemical composition 99.9% Al2 O3 ZrO2 ZrO2 #MgO #8%10 mol% MgO #3 mol% Y2 O3 Density g cm~3 *3.97 5.74—6 '6 Porosity % (0.1 — (0.1 Bending strength MPa '500 450—700 900—1200 Compression strengrth MPa 4100 2000 2000 Young modulus GPa 380 200 210 Fracture toughness KIC MPa m~1 4 7—15 7!10 Thermal expansion coeff. K~1 8]10~6 7—10]10~6 11]10~6 Thermal conductivity W mK~1 30 2 2 Hardness HV 0.1 2200 1200 1200 Table 2 Producers of zirconia ball heads for THR Producer Country Astromet USA Ceraver France Ceramtec Germany Norton France Kyocera Japan Metoxit Switzerland Morgan Matroc United Kingdom NGK Japan SCT France Xylon USA 2. Microstructural properties Zirconia is a well-known polymorph that occurs in three forms: monoclinic (M), cubic (C) and tetragonal (T). Pure zirconia is monoclinic at room temperature. This phase is stable up to 1170°C. Above this temperature it transforms into tetragonal and then into cubic phase at 2370°C. During cooling, a T—M transformation takes place in a temperature range of about 100°C below 1070°C. The phase transformation taking place while cooling is associated with a volume expansion of approximately 3—4%. Stresses generated by the expansion originate cracks in pure zirconia ceramics that, after sintering in the range 1500—1700°C, break into pieces at room temperature. It was in 1929 that Ruff and coworkers [6] showed the feasibility of the stabilisation of C-phase to room temperature by adding to zirconia small amounts of CaO. The addition of ‘stabilising’ oxides, like CaO, MgO, CeO2 , Y2 O3 , to pure zirconia allows to generate multiphase materials known as Partially Stabilized Zirconia (PSZ) whose microstructure at room temperature generally consists [7] of cubic zirconia as the major phase, with monoclinic and tetragonal zirconia precipitates as the minor phase. These precipitates may exist at grain boundaries or within the cubic matrix grains. In 1972 Garvie and Nicholson [8] showed that the mechanical strength of PSZ was improved by an homogeneous and fine distribution of the monoclinic phase within the cubic matrix. The development of zirconia as an engineering material was marked by Garvie et al. [9], who in their paper ‘Ceramic Steel?’ showed how to make the best of T—M phase transformation in PSZ improving mechanical strength and toughness of zirconia ceramics. They observed that tetragonal metastable precipitates finely dispersed within the cubic matrix were able to be transformed into the monoclinic phase when the constraint exerted on them by the matrix was relieved, i.e. by a crack advancing in the material. In that case, the stress field associated with expansion due to the phase transformation acts in opposition to the stress field that promotes the propagation of the crack. An enhancement in toughness is obtained, because the energy associated with crack propagation is dissipated both in the T—M transformation and in overcoming the compression stresses due to the volume expansion. A schematic representation of this phenomenon is given in Fig. 1. The development of such tetragonal metastable precipitates may be obtained by the addition of some 8% mol of MgO to ZrO2 . This allows the formation a fully cubic microstructure at 1800°C, and the nucleation within the matrix of a tetragonal metastable phase, during controlled cooling and ageing. PSZ can also be obtained in the ZrO2 —Y2 O3 system (Fig. 2). However in this system it is also possible to obtain ceramics formed at room temperature with a tetragonal phase only, called TZP. This result was reported first by Rieth et al. [10], and by Gupta et al. [11]. TZP materials, containing approximately 2—3% mol Y2 O3 , are completely constituted by tetragonal grains with sizes of the order of hundreds of nanometers. The fraction of T-phase retained at room temperature is 2 C. Piconi, G. Maccauro / Biomaterials 20 (1999) 1—25
C.Piconi.G.Maccauro/Biomaterials 20(1999)1-25 process zone 88 O ● ● 0 crack 0kTi ● 6oo 02 Y:O content (mol%) untransformed transformed transforming particle particle particle Fig.1.Repr of stre and in c ing the matrix transforming toughned roonia seramics.Mat Sci Tech Tetragonal PhaseVolume Fraction 2500 2000 50 sz TZP 500 Y:O content (mol%) mas+Cas s vs.ytt system M 92120 2 Y:O content (mol%) onia part of zirconia-yttria phase diagram.Commer ant to onal grains.A critical grain size exists.linked to the yttria concentration,above which spontaneous T-M transt characteristic of transformatiot dependent on the size of grains,on the yttria content,on toughened zirconia ceramics is the formation of compres sive layers on their surface [13].Surface tetragonal grains an transform to
Fig. 1. Representation of stress-induced transformation toughening process. Energy of the advancing crack is dissipated in phase transformation and in overcoming the matrix constraint by transforming grains (Reprint with permission from Butler EP, Transformation toughned zirconia ceramics. Mat Sci Tech 1985;1:417—32.). Fig. 2. High zirconia part of zirconia—yttria phase diagram. Commercial PSZ and ZTP composition and processing temperatures are indicated by shaded regions (Reprint with permission from Scott HG, Phase relationship in zirconia—yttria systems. J Mater Sci 1975; 10:1527—35.). Fig. 3. Retention of tetragonal phase. Critical grain size against Yttria content in tetragonal zirconia (Reprint whith permission from Lange FF, Transformation toughenining, Part 3—Experimental observations in the ZrO2 —Y2 O3 system. J Mater Sci 1982;17:240—6.). Fig. 4. Fracture toughness vs. yttria content (Reprint whith permission from Lange FF, Transformation toughenining, Part 3—Experimental observations in the ZrO2 —Y2 O3 system. J Mater Sci 1982;17:240—6.). dependent on the size of grains, on the yttria content, on the grade of constraint exerted on them by the matrix. Mechanical properties of TZP ceramics (Figs. 3 and 4) depend on such parameters. It is very important to consider the metastable nature of the tetragonal grains. A critical grain size exists, linked to the yttria concentration, above which spontaneous T—M transformation of grains takes place, whereas this transformation would be inhibited in a too fine grained structure [12]. An interesting characteristic of transformation toughened zirconia ceramics is the formation of compressive layers on their surface [13]. Surface tetragonal grains are not constrained by the matrix, and can transform to monoclinic spontaneously or due to abrasive processes C. Piconi, G. Maccauro / Biomaterials 20 (1999) 1—25 3
4 C.Piconi.G.Maccauro/Biomaterials 20(1999)1-25 that can induce compressive stresses at a depth of several a loss of stability of the tetragonal phase moreo microns under the surface. mullite (3AlO.2Sio.)pockets were detected in the The surface phase transition and the consequent sur aluminosilicate glass,which leads to a loss of stability of face hardening may have a relevant role in improving the prop in T-M surface transformation may originate surface cracking.followed by ejection of grains from the surface with catastrophic effects on mech- anica 3.Mechanical properties our and joi cially Mg- as ceran There is no doubt that zirco ni vourable results.But R&D on this material for biomedi- ical properties better than other ceramic hiomaterials ie cal applications appears to have to be stopped in the alumina [18].as shown in Table 1.Comparison among fac Young's moduli,strength and hardness of some bio- materials,including ceramics,are shown in Table 3 by a residu with in they 3.1.Results of compression tests on TZP ball heads UHMWPE sockets that are currently coupled with zir- ive Load (UCL)of ball heads is into acp ME-PSZ at higher )720 6-5 stand The initation ment of the metastable tetragonal precipitates,that empera It can be erved fro all heads UCL t that using erancan mm can wi Difficulties in obtaining Ms-PSZ free of heads UCL depends on design and material character SiO2,Al2O3 and other impurities [14].increase in SiO istics both of the ball head and of the metallic spigot:the contents due to the wear ing media during powde angle mismatch betw een bore and taper,and the surface ma yhave contrib TZP the magr an d position ma materials.In ceramics containing MgO.magnesi nalysis on different design [20-221 of ceramic hall cates like enstatite (MgSiO3)and forsterite (Mg2SiO) heads has shown that two main stress concentrations are may form at grain boundaries [14],lowering the Mgo ocalized in the inner surface of the ceramic bore,one d prom oting the bending fress)at the top of the cavity,an one (hoop aland its stabilit anichal prope d -me ape The mago Nevertheless.Mg-PSZ ball heads were used in the USA he metal-ceramic contact area.the roug ness of [16]and Australia.Also TZP precursors can contair he surfaces and the friction coefficient of the two which i as a liquid phase fo rming ac ests per ormed on TZP ball heads [23]show that to served that in the und aries scavenge yttrium ions from TZP grains,leading to of mem ormdi Property Units Ti 6Al 4V 316SS CoCr Alloy TZP Alumina Young's modulus 210 380
Table 3 Properties of some materials for biomedical applications Property Units Ti 6Al 4V 316 SS CoCr Alloy TZP Alumina Young’s modulus GPa 110 200 230 210 380 Strength MPa 800 650 700 900—1200 '500 Hardness HV 100 190 300 1200 2200 that can induce compressive stresses at a depth of several microns under the surface. The surface phase transition and the consequent surface hardening may have a relevant role in improving the mechanical and wear properties of zirconia parts, the thickness of the transformed layer being one of the limit conditions. Progresses in T—M surface transformation may originate surface cracking, followed by ejection of grains from the surface with catastrophic effects on mechanical behaviour and joint wear. Several PSZ were tested as ceramic biomaterials, especially Mg—PSZ, which was extensively tested with favourable results. But R&D on this material for biomedical applications appears to have to be stopped in the early 1990s. Several reasons can account for this fact: Mg—PSZ are characterised by a residual porosity as is normal in materials with grain sizes in the range 30—40 lm. This can influence negatively the wear rate of UHMWPE sockets that are currently coupled with zirconia ball heads. Also technological aspects may have been taken into account. Mg—PSZ sinter at higher temperatures than TZP (1800°C vs. 1400°C), implying the need of special furnaces. The precipitation and development of the metastable tetragonal precipitates, that occurs during cooling, requires a strict control of the cooling cycle in terms of temperature and time, especially in the ageing step that takes place at about 1100°C, during which the precipitation of T-phase occurs. Difficulties in obtaining Mg—PSZ precursors free of SiO2 , Al2 O3 and other impurities [14], increase in SiO2 contents due to the wear of milling media during powder processing before firing [15] may have contributed to shift the interest of ball head manufacturers towards TZP materials. In ceramics containing MgO, magnesia silicates like enstatite (MgSiO3 ) and forsterite (Mg2 SiO4 ) may form at grain boundaries [14], lowering the MgO contents in the grains and promoting the formation of the monoclinic phase, reducing the mechanichal properties of the material and its stability in a wet environment. Nevertheless, Mg—PSZ ball heads were used in the USA [16] and Australia. Also TZP precursors can contain silica, which is sometimes used as a liquid phase forming additive to achieve full density at temperatures lower than 1500°C limiting grain growth. Lin et al. [17] observed that aluminosilicate glasses in the grain boundaries scavenge yttrium ions from TZP grains, leading to a loss of stability of the tetragonal phase. Moreover, mullite (3Al2 O3 )2SiO2 ) pockets were detected in the aluminosilicate glass, which leads to a loss of stability of the material in a wet environment. The use of such additives is hence to be avoided in TZP as ceramic biomaterials. 3. Mechanical properties There is no doubt that zirconia ceramics have mechanical properties better than other ceramic biomaterials, i.e. alumina [18], as shown in Table 1. Comparison among Young’s moduli, strength and hardness of some biomaterials, including ceramics, are shown in Table 3. 3.1. Results of compression tests on TZP ball heads Ultimate Compressive Load (UCL) of ball heads is tested following the ISO 7206-5 standard [19]. The test procedure consists of the application of static loads to the ball head inserted in a metallic spigot until fracture, and it may be considered a useful tool to compare different designs. It can be observed from UCL tests that using TZP ceramics, ball heads of 022.22 mm can withstand static loads ranging several times the physiologic ones. Ball heads UCL depends on design and material characteristics both of the ball head and of the metallic spigot: the angle mismatch between bore and taper, and the surface roughness controls the magnitude and position of maximum stress in the ceramic ball head. Finite elements analysis on different designs [20—22] of ceramic ball heads has shown that two main stress concentrations are localized in the inner surface of the ceramic bore, one (bending stress) at the top of the cavity, and one (hoop stress) at the ceramic—metal taper interface. The magnitude of such stresses is dependent on the position, the metal—ceramic contact area, the roughness of the surfaces and the friction coefficient of the two counterfaces. Tests performed on TZP ball heads [23] show that to minimize the concentration of stresses it is necessary to maintain a gap *2 mm between the spigot and the top of the conical cavity, and maximize the extension of the 4 C. Piconi, G. Maccauro / Biomaterials 20 (1999) 1—25
C Piconi G.Maccauro/Biomaterials 20 (1999)1-25 contact area,taki 3.2.Stability of the tetragonal phase peeynenedii0hecoaicaheore Mechanical r perties of zirconia relate to its fine In standard prostheses design,taper to ceramic bore coupling is made with a tolerance of some 0 to -5'on structure during the lifetime of TZP components is the e taper and o to on the conical bore [24].An key point to attain the expected of tay tha nowr extended to the complete taper surface due to the metallic taper strain following loading.Drouin and Cales [23]. monoclinic phase.This behaviour is well known in the Da ore observing a decrease in bovin the pnc of r were summarised by There is ex erimental evidence r25.261 that UCL of Swab [331 in the followi zirconia ball heads is 2-2.5 times higher than the UCL of (1)The most critical temperature range is 200-300C alumina bal heads of thes (2)The t al.[2 rve s ar nd density,and an 1 toughne UCL of alumina ball he ads of 02 s due to the but itmust be remarked that the failure by these authors are far below the ones of currently manu racking of the material factured ball heads -M transition arts on the surface and progress es variations in spigo to the material bulk and/or increase in conc mismatch of the an and in the tanet inser tion of stabilising oxide reduce the transformation tion depth all play an important role on the results rate. obtained by this test.A summary of results obtained by (6)T-M transformation is enhanced in water or in va- several authors [23, ]is reported in Table4 ed to on the incou s T-M published [164]. zirconium hydroxides [31,32,34]or yttrium hydroxides Taper material Taper type Ball head Neck length Remarks (mm) 30 28 35 mm (S) +35mm( mm (S Ti6Al4V 10/12 200 28 0 3.5 mm (L) 540107%54 28 12/14 HV:o:310 Ti6l4VA 288 0 +3mm四
Table 4 UCL of TZP ball heads on different tapers Ref. Taper material Taper type Taper roughness Ball head Neck length UCL Remarks (lm) diameter (kn) (mm) !3.5 mm (S) 110 30 28 0 105 #3.5 mm (L) 85 !3.5 mm (S) 140 22 Ti6Al4V 10/12 200 28 0 130 #3.5 mm (L) 110 3 22.22 0 78 30 22.22 0 98 27 Ti6l4VA 8/10 Not specified 22.22 Not specified 45 10/12 34 Ti alloy 4.6 80 HV10 : 352 28 Ti alloy 48.4 93 HV10 : 320 CoCr alloy 12/14 2.7 28 L 44 HV10 : 435 CoCr alloy 2.7 47 HV10 : 644 NiCrMo alloy 60.3 108 HV10 : 310 32 0 122$16 29 Ti6l4VA 4° Not specified 28 0 97$11 28 #3 mm (L) 84$6 28 !3 mm (S) 133$13 contact area, taking care of the rise of hoop stresses in the rim portion of the ball head when the taper is not completely inserted into the conical bore. In standard prostheses design, taper to ceramic bore coupling is made with a tolerance of some 0 to !5@ on the taper and 0 to #5@ on the conical bore [24]. Angle mismatch is selected in such a way that contact takes place first in the upper part of the ceramic bore, and is extended to the complete taper surface due to the metallic taper strain following loading. Drouin and Cale´ s [23], reported that the angle mismatch (10@) can be more than doubled in TZP ball heads before observing a decrease in ball heads UCL. There is experimental evidence [25, 26] that UCL of zirconia ball heads is 2—2.5 times higher than the UCL of alumina ball heads of the same diameter and neck length. Also Tateishi et al. [27, 28] observed UCL of TZP ball heads of 022.2 mm on Ti6Al4V spigots almost double the UCL of alumina ball heads of 028 mm on CoCr spigots, but it must be remarked that the failure loads reported by these authors are far below the ones of currently manufactured ball heads. It is clear that variations in the spigot material and roughness, in the roughness of the ceramic bore, in the mismatch of the bore/taper angle, and in the taper insertion depth all play an important role on the results obtained by this test. A summary of results , obtained by several authors [23, 28—30] is reported in Table 4. A comprehensive summary of the main parameters of the taper influencing the head-Trummion assembly was recently published [164]. 3.2. Stability of the tetragonal phase Mechanical properties of zirconia relate to its fine grained, metastable microstructure. The stability of this structure during the lifetime of TZP components is the key point to attain the expected performances. Mechanical property degradation in zirconia, known as ‘ageing’, is due to the progressive spontaneous transformation of the metastable tetragonal phase into the monoclinic phase. This behaviour is well known in the temperature range above 200°C in the presence of water vapour [31, 32]. The main steps of TZP ageing were summarised by Swab [33] in the following way: (1) The most critical temperature range is 200—300°C. (2) The effects of ageing are the reduction in stength, toughness and density, and an increase in monoclinic phase content. (3) Degradation of mechanical properties is due to the T—M transition, taking place with micro and macrocracking of the material. (4) T—M transition starts on the surface and progress es into the material bulk. (5) Reduction in grain size and/or increase in concentration of stabilising oxide reduce the transformation rate. (6) T—M transformation is enhanced in water or in vapour. The models proposed to explain the spontaneous T—M transformation in TZP are based on the formation of zirconium hydroxides [31, 32, 34] or yttrium hydroxides C. Piconi, G. Maccauro / Biomaterials 20 (1999) 1—25 5
6 C.Piconi.G.Maccauro Biomaterials 20 (1999)1-25 [35]promoting phase transition for local stress concen- a maximum transformation of 72%at 37C in a tim tration or variation of the yttrium/zirconium ratio. ranging from 7 to 30 yr,encompa sing a THR expected It is worthwhile to remark that the strength degrada- lifetime.But it must be remarked that such results were ceramics characterized at the start ol an M-p nt o occurred in all the mate a low Weibull modulus (m rials but one,where strength remained the same after the 700 MPa measured in three-point bending tests.Bending teatment.This vanan behavious to the di erenc he samples tes was control like yttr th on. 46 10n rain tested 6.Table 5 contains a summary of the results of ageing tests reported by several authors. tests were performed in saline at 37,50,95C for 36 The strength degrada ion in wet environments of zir months,and in an autoclave at 121C for st bending tests were performed on 8 mm gauge ples Modulus Of Rupture (MOR)of Mg-PSZ samples main- Alumina was used as a control.Samples tested at 37C tained for 1000h in a boiling saline solution. On the n vitro and in vivo did not show significant dillerences monoclini 产微 nd,the e in the s The dev opment o the surfac ely 2 d 5 m Bending strength variation of TZP samples implanted with the T-M surface transformation,an increase in the pending strength of samples was but the start ths increase in the bending strength was observed after ransition after 3 vr The corresponding increase in be months,associated with%M-phase formation on the nding strength was about 10% samples xperimental reported hase was 69 mol%in samples aged at anc 391 0,41 whil 121 after 500 h only.being 80 molafter 1000 h. for 100d did not induce significant variations in the Zr-OH bonds were identified by FTIR in samples strength of TZP samples.Also,Ichikawa et al [42]did aged for 960 h in water at 121C.This suggests that the not observe variatic in TZI ohase trans on in th material tested depends on mech he Wista s ageing in line,or subcu- pro et al.[341.the Conflicting results were reported by Drummond [43. tion of Zr-OH bonds being the transition initiator 44]and by Thomson and Rawlings [45].Drummo ond Nev ertheless,no microcracks were observed by SEM estudy on ageing of TZ [49 aterial samples 1920 h in saline a the e authors to time interval 140-304 d,and no significant correlatior dict that the bending stength of the material will main- with the testing environment was lound.Also contro tain for 80 yr a value higher than 800 MPa. n air, d s e ults of ag in vater for three ye ove the the T-M ntimal com 03 atio to the different from tetragonal,probably cubic.This fact Sato and Shimada model [31.32]to be 25.2 kcal mol- makes the samples tested not representative of TZP for On this basis authors can calculate that the flexural implants strength of the material maintained for 50yr in water a omson and R lings [45]reported the M-phase as more than a ate for
[35] promoting phase transition for local stress concentration or variation of the yttrium/zirconium ratio. It is worthwhile to remark that the strength degradation rate is not the same for all TZP ceramics. As it was reported by Swab [33], in the ten materials tested in presence of water vapour at low temperature, different levels of strength degradation occurred in all the materials but one, where strength remained the same after the treatment. This variability in ageing behaviour is related to the differences in equilibria of microstructural parameters like yttria concentation and distribution, grain size, flaw population and distribution in the samples tested [36]. Table 5 contains a summary of the results of ageing tests reported by several authors. The strength degradation in wet environments of zirconia was studied from the early phases of the development of zirconia for biomedical applications [37]. Garvie et al. [38] reported a reduction up to 14% of the Modulus Of Rupture (MOR) of Mg—PSZ samples maintained for 1000 h in a boiling saline solution. On the other hand, the content of monoclinic phase in the surface of the specimens of the same material implanted in paraspinal muscles of rabbits, although rather high (32%), did not show significant variations. Bending strength variation of TZP samples implanted in the marrow cavity and in the paraspinal muscles of NZW rabbits or maintained in a saline solution at 37°C for 12 months was investigated by Kumar et al. [39]. An increase in the bending strength was observed after six months, associated with 2% M-phase formation on the surfaces of samples. Experimental data reported by Schwartz [26] and by Christel [40, 41] are in agreement with those of Kumar’s [39]. Christel [40, 41] showed that gamma sterilization or ageing in Ringer’s solution for 100 d did not induce significant variations in the strength of TZP samples. Also, Ichikawa et al. [42] did not observe variation in the bending strength of TZP samples after 12 months ageing in air, saline, or subcutaneous tissues of Wistar rats. Conflicting results were reported by Drummond [43, 44] and by Thomson and Rawlings [45]. Drummond performed an extensive study on ageing of TZP [43]. Reduction in MOR of about 20% was observed in TZP samples after ageing for 730 d in Ringer’s, saline solutions or distilled water at 37°C. Reduction takes place in the time interval 140—304 d, and no significant correlation with the testing environment was found. Also control specimens, maintained in air, showed similar behaviour. The samples tested contained 5.5—8.5 wt% Y2 O3 , slightly above the optimal composition, and contained phases different from tetragonal, probably cubic. This fact makes the samples tested not representative of TZP for implants. Thomson and Rawlings [45] reported the M-phase as reaching 10% after 18 months ageing in Ringer’s solution. They calculated that the M-phase might reach a maximum transformation of 72% at 37°C in a time ranging from 7 to 30 yr, encompassing a THR expected lifetime. But it must be remarked that such results were obtained on TZP ceramics characterized at the start of the test by an M-phase content of approximately 5%, and by a rather high defect population, indicated by a low Weibull modulus (m"6.5) and MOR below 700 MPa measured in three-point bending tests. Bending strength shows little variations during the test, showing that material strength in the samples tested was controlled by defects more than by phase transitions. Shimizu et al. [46] tested TZP samples (grain size 0.25 lm, density 6 g cm~3) in vitro and in vivo. In vitro tests were performed in saline at 37, 50, 95°C for 36 months, and in an autoclave at 121°C for 960 h. Samples were tested in vivo in subcutaneous tissue and in the tibial marrow of JW rabbits for 30 months. Three-point bending tests were performed on 8 mm gauge samples. Alumina was used as a control. Samples tested at 37°C in vitro and in vivo did not show significant differences. The development of the monoclinic phase on the surface of the samples was only observed 90 d after the beginning of the test, reaching approximately 2 and 5 mol% after 12 and 30 months, respectively (Fig. 5). In correspondence with the T—M surface transformation, an increase in the bending strength of samples was observed, but the starting value was recovered after 30 months. In samples tested at 50°C, 16 mol% of the surface underwent a T—M transition after 3 yr. The corresponding increase in bending strength was about 10%. The monoclinic phase was 69 mol% in samples aged at 95°C after 27 months, while in samples aged in an autoclave at 121°C the monoclinic phase was about 50 mol% after 500 h only, being 80 mol% after 1000 h. Zr—OH bonds were identified by FTIR in samples aged for 960 h in water at 121°C. This suggests that the phase transition in the material tested depends on mechanisms similar to the ones proposed by Sato and Shimada [31, 32] or by Yoshimura et al. [34], the formation of Zr—OH bonds being the transition initiator. Nevertheless, no microcracks were observed by SEM in identical material samples aged 1920 h in saline at 121°C. The activation energy of the transition process in the material tested was calculated to be about 21.5 kcal mol~1. This result allowed the authors to predict that the bending stength of the material will maintain for 80 yr a value higher than 800 MPa. Results of ageing tests in water for three years were recently reported [47]. The activation energy of the T—M transformation process was calculated according to the Sato and Shimada model [31, 32] to be 25.2 kcal mol~1. On this basis authors can calculate that the flexural strength of the material maintained for 50 yr in water at 37°C will be more than adequate for orthopaedic or dental implants. Recently Chevalier et al. [4] reported the results of a study on the T—M transformation kinetics 6 C. Piconi, G. Maccauro / Biomaterials 20 (1999) 1—25
C Piconi G.Maccauro/Biomaterials 20 (1999)1-25 Medium TC) Time %MOR variation Remarks [2 TZP Ringer's Rn七0 [3 Ca-PSZ Ringer's 37 6o TiOzand Fe:O, [3 Mg-PSZ Saline 二舒品 ound:b:polished 00 m porosity [38] TZP Ringer's 37 Subcutis Fracture toughness K [39,40 Ringer 温 +31% [41 HCI up to 12m Noiatio T-phase>90% -P 66%mol YO at test start Water [43 Mg-PSZ Air ( 37 Air seal [4 Rineer's 37 19m -16.4 %rnet [4 TZP 30m [28 TZPA 58oPeen TZP B ohaat e [30 TZP Ringer's 37 783d No change Time units:h-hour,d-day,w-week.m-month
Table 5 Summary of some results of ageing tests on zirconia ceramics Ref. Material Medium T (°C) Time % MOR variation Remarks [25] TZP Ringer’s 37 6 w Roughly#10% 12 w after 52 weeks 24 w 52 w [36] Ca—PSZ Ringer’s 37 1 w !16.1 ZrO2 #4%CaO, 2 w !17.4 1% SiO2 , 1% Al2 O3 . 4 w !18.5 Presence of Rabbit — 3 m !25.8 TiO2 and Fe2 O3 dorsa [37] Mg—PSZ Saline !6.5 a a: ground; b: polished 100 1000 h !13.7 b Samples characteristics: 7 d Grain size: 50 lm porosity: Rabbit 1 m — 2% muscles 3 m M-phase 12—30% 6 m [38] TZP Ringer’s 37 3 m 0 M-phase increase was less 6 m #19.5 than 2% in all samples at 12 m 12 m #22 Bone 3 m 0 marrow 6 m #17 12 m #9.8 Subcutis 3 m 0 6 m #22 12 m #5 Fracture toughness KIC: [39, 40] Y—PSZ Ringer’s 37 1 d — !7.4% 7 d !6.6% 50 d !6.6% 100 d #3.1% [41] ZrO2 # HCl sol. 37 up to 12 m No T-phase'90% 3% Y2 O3 variations Subcutis [42] Y—PSZ Ringer’s 37 140 d 0 6.6% mol Y2 O3 at test start 304 d !12.9 453 d !22 Saline 140 d 0 304 d !19 453 d !19.5 Water 140 d !1.7 304 d !15.5 453 d !17.3 [43] Mg—PSZ Air (*) 37 6 m !1 MOR for crosshead speed 12 m !4.9 0.1 mm min~1 18 m !2.5 * Autoclaved at 121°C in 6 m !8 water prior to ageing Water (*) 12 m !3.6 18 m !2.5 Air seal 6 m 0 12 m 0 18 m !3 [44] TZP Ringer’s 37 19 m !16.4 5% M phase at test start 14% M phase at test end [45] TZP Bone 30 m !5 Average increase M phase marrow 2 mol% per year [28] TZP A Steam 140 24 h !15 5 vol% M phase at test start 48 h !21 '80 vol% M phase at test end 120 h !25 TZP B 24 h !6.5 11 vol% M phase at test start 48 h !6.5 60 vol% M phase at test end 120 h !11.5 [50] TZP Ringer’s 37 783 d No change Time units: h—hour, d—day, w—week, m—month. C. Piconi, G. Maccauro / Biomaterials 20 (1999) 1—25 7
8 C.Piconi.G.Maccauro/Biomaterials 20(1999)1-25 :Bone marrow 10 0 3 Aging Time(Years) llead Batch N umbe Fig.6.TZP ball heads ultimate ion load after i in TZP obtained from coprecipitated powders.These ow to pred a 2 9945376-30 yr ageing period at 37C order of the one measured by Shimizu et al.[46]. from patients after 22,24,27,39 months.The results obtained are very relevant as the ball heads were sub- tion plays a rol 10n n TZP mate only to the action of the body environment, ing the stab ing of the introduced in zirconi th cess,i.e.coprecipitation of yttrium and zirconium salt A different approach to the introduction of stabilizing nding test were obtained from the ball retrieved after 22 months.The UCL of the retrieved ball heads was within h r g ar of this the each producti hydrothermal stability was inv do test bars obtained from the [29].Samples obtained by coprecipitated and coated 22 months implanted ball head.as well as results of tests performed on test bars machined from current produc in an aut in the presence o tion TZP I heads age ngers solution up tode devel the dingyr [23 folow a quite different evolution:in cop ecipitated sam The effects of the combination of stress and a we t increase n M-phase content voaeo after 2 Th success ve er ution o hea nm,were ma aintained in Ringer h amoun 之% nder sta ars to b s allowed Ringers solution tor ch the ton of the conical bore in the ball head.Compression tests did not The thickness of the monoclinic layer after 120h ap show significant variations in UCL.Wha prox y 120 m in ZP m appled loac der.This esult was achieved UCL mea the te sintered to full density at a relatively low temperature did not show variations in the contents of the monoclinic without glassy additives,resulting in a TZP completely phase up to 12 months.Ageing of samples of the same tetragonal with grains less tha 0.5 um in size [48,49] mat ial for 2 yr was pe orm by 30 THR the on m es ol rats and rab s,and in the ra
Fig. 5. Tetragonal to monoclinic transformation of TZP in vivo and in saline (Reprint with permission from Shimizu K, Oka M, Kumar P et al., Time-dependent changes in the mechanical properties of zirconia ceramic. J Biomed Mat Res 1993;27:729—34.). Fig. 6. TZP ball heads ultimate compression load after clinical use, in comparison to the acceptance values of production batches (Reprint with permission from Cale´ s B, Stefani Y, Mechanichal properties and surface analysis of retrieved zirconia femoral hip joint heads after an implantation time of two to of two to three years. J Mat Sci Mater Med 1994;5:376—80.). in TZP obtained from coprecipitated powders. These results allow to predict a 25 yr ageing period at 37°C to reach 20% monoclinic content in their samples. The activation energy measured (log kJ mol~1) is of the same order of the one measured by Shimizu et al. [46]. Not only the yttria content but also the yttria distibution plays a role on T—M phase transition in TZP materials. The stabilizing oxide is introduced in zirconia during the early steps of the powder manufacturing process, i.e. coprecipitation of yttrium and zirconium salts. A different approach to the introduction of stabilizing oxide in ceramic powders consists a coating zirconia grains with yttria, thus obtaining an yttria gradient in the material. The effects of this yttria distribution on TZP hydrothermal stability was investigated by Richter et al. [29]. Samples obtained by coprecipitated and coated powders following the same preparation and sintering schedule were treated in an autoclave in the presence of water vapour at 140°C up to 120 h. The development of M-phase in the samples made out of the two materials follow a quite different evolution: in ‘coprecipitated’ samples one can observe a fast increase in M-phase content, reaching 80 vol% after 24 h. The successive evolution of the transformation is slower, the amount of M-phase reaching 90 vol% after 120 h of treatment. In ‘coated’ samples, the evolution of the M-phase appears to be progressive, reaching 60 vol% after 120 h of treatment. The thickness of the monoclinic layer after 120 h approximately 120 lm in TZP made out of coprecipitated powder, and around 5 lm when made out of coated powder. This result was achieved using precursors sintered to full density at a relatively low temperature without glassy additives, resulting in a TZP completely tetragonal with grains less than 0.5 lm in size [48, 49]. Cale´ s et al. [50] reported the first results on mechanichal behaviour of THR zirconia ball heads after clinical use. Tests were performed on four ball heads retrieved from patients after 22, 24, 27, 39 months. The results obtained are very relevant as the ball heads were subjected not only to the action of the body environment, but also to the physiological cyclic loading. Three out of the retrieved ball heads were subjected to static compression tests, while bar samples for the bending test were obtained from the ball retrieved after 22 months. The UCL of the retrieved ball heads was within the acceptance values characteristic of each production batch (Fig. 6). The experimental values obtained from the bending tests performed on test bars obtained from the 22 months implanted ball head, as well as results of tests performed on test bars machined from current production TZP ball heads aged in Ringer’s solution and in animals at 37°C for 2 yr [23] did not show significant differences, the bending strength remaining unchanged. The effects of the combination of stress and a wet environment on TZP stability were also reported [5]. TZP ball heads, 032 mm, were maintained in Ringer’s solution for 3, 6 and 12 months under static loads of 10, 20 and 30 kN fitted in Ti6Al4V tapers. An axial bore in tapers allowed Ringer’s solution to reach the top of the conical bore in the ball head. Compression tests did not show significant variations in UCL. Whatever the time in Ringer’s solution and the applied load, the average UCL is (129.5$6.5) kN, which corresponds to the average UCL measured before the test (132 kN). XRD analysis did not show variations in the contents of the monoclinic phase up to 12 months. Ageing of samples of the same material for 2 yr was performed by implanting then in muscles of rats and rabbits, and in the femur of rabbits and sheeps. Fracture toughness measured by the microindentation do not show significant variations, KIC 8 C. Piconi, G. Maccauro / Biomaterials 20 (1999) 1—25
C.Piconi.G.Maccauro/Biomaterials 20(1999)1-25 9 4.Wear and inner bearing surfaces bet virgin and retrie 4.1.Zirconia on zirconia TZP ball heads,nor in density or hardness were observed after 18 months of clinical use.Similarly no difference There is clear experimental evidence that the wear rate were obs of the couple zirconia/zirconia is too high to diase this n of 10 in the predominant tetragonal structure [52.53]was [56.571.show the disastrous amounts of wear of this observed. ceramic couple,up to 5000 times the wear of the The above results presented by different authors,con alumina/alumina one.Recently TZP/TZP wear was the of this [33]reporte e improvements degradation of Ps in wetenvironments depends on the studies)The TZPTZP couple was investi material microstructure,and can be controlled by acting into account the effects of environment,sliding speed on the materal manufacturing process and on the pre e that e.tha metho rd Iso 647 able to mai tor 61).These authors onfirmed the sults baned in wet environments for exp ected implant lifetimes.but general conclusions about the stability of TZP must be Sliding of a pair made of low thermal conductivity as this behaviour speculiar to each material and cturing techn eads to an increase in surface temperature.For ology the yns the wet environment this process may lead to cracking 3.3.Impact tests grain pullout and catastrophic abrasive wear.Neverthe less,the work recently published by Chevalier et al.[165 opens again the res earch in his nel In pin-on- in this field g rde magnitude lower than the wear rate of alumina/alumina Tateishi and Yunoki [28].Bodies growing in weigh pair.These results were not replicated using bovine serum werdeepoL22 m as lubricant 028 m lumina ball heads (on Cocr Zirconia/UHMWPE got)failed un der some 15Jimpact.The role exerted by the spigot The wear of the couple UHMWPE/zirconia was studied by many authors.The results obtained are sum 6.Data are scattered over several or Wear rates of UHMWPE against zirconia five times less than against alumina were observed in ring on disc 3.4.Fatigue resistance tests carried out in conformity to ISO 6474[59].due to h muc nner grain siz onia than alumina(8x from 5 to 10 kN at 30 Hz are reported by Tateishiet al. number of cycles,for alumina surface roughness passing from R=0.06 um to R.=0.22 um.Low residual poro ure.More interest soin zirconi e induced UHM wea ing orted by Ca L39 s tnan a Differen creases from 1s to 90kN.and shows a tendency to Surface zhness and p increase to infinity for loads less than 28 kN.It was finishing ma produce different wear rates.It was hy observed experimentally that TZP 22.2 mm ball head value below 0m202 to50 million cycles with load pothesized that the existence of a threshold changes can influence only a little the wear rate [65]
ranging (9$1) MPa m~1@2 whatever the site or time of implantation. Neither differences in the finish of outer and inner bearing surfaces between virgin and retrieved TZP ball heads, nor in density or hardness were observed after 18 months of clinical use. Similarly no differences were observed in the average bending strength of TZP bars aged for 300 d in SBF at 37°C and 60°C. In these samples the formation of monoclinic phase ((1%) in the predominant tetragonal structure [52, 53] was observed. The above results presented by different authors, con- firm the conclusions of the work by Swab [33] reported at the beginning of this section. The extent of strength degradation of TZPs in wet environments depends on the material microstructure, and can be controlled by acting on the material manufacturing process and on the precursors selected for ceramic manufacture. One can observe that there is experimental evidence that TZP ceramic is able to maintain good mechanical properties in wet environments for expected implant lifetimes, but general conclusions about the stability of TZP must be avoided, as this behaviour is peculiar to each material and of its manufacturing technology. 3.3. Impact tests Impact test constitutes a useful assay to evaluate the ability of a component to dissipate shock energy, i.e. its toughness. There is very limited information in this field: up to now the only results presented on this topic are due to Tateishi and Yunoki [28]. Bodies growing in weight were dropped from a 0.5 m height onto a ball head inserted in its spigot. 022.2 mm TZP ball heads (on Ti alloy spigot) failed under an impact of some 78 J, while 028 mm Alumina ball heads (on CoCr spigot) failed under some 15 J impact. The role exerted by the spigot material due to the differences in elastic properties of the two alloys and its influence on the results reported was not clearified. 3.4. Fatigue resistance Tests in Pseudo Extra Cellular Fluid (PECF) and in saline solution, with loads cycled from 1 to 12 kN and from 5 to 10 kN at 30 Hz are reported by Tateishi et al. [27, 28]. Tests were performed up to 10 million cycles on 022.2 mm TZP ball heads without failure. More interesting results are reported by Cale` s [54]. The number of cycles-to-rupture increase as the maximum load decreases from 15 to 90 kN, and shows a tendency to increase to infinity for loads less than 28 kN. It was observed experimentally that TZP 022.2 mm ball heads can withstand up to 50 million cycles with load cycled from 2.8 to 28 kN. 4. Wear 4.1. Zirconia on zirconia There is clear experimental evidence that the wear rate of the couple zirconia/zirconia is too high to use this ceramic couple in prosthetic joints. Early studies performed by Murakami and Ohtsuki [55], Sudanese et al. [56, 57], show the disastrous amounts of wear of this ceramic couple, up to 5000 times the wear of the alumina/alumina one. Recently TZP/TZP wear was the object of new interest, probably due to the improvements in TZP ceramics processing (reported after the previous studies). The TZP/TZP couple was investigated taking into account the effects of environment, sliding speed, and load on wear properties, using the ball on ring (pin on disk) method [58], by the ring on disk test in conformity to the standard ISO 6474 [59, 60], and on hip simulator [61]. These authors confirmed the results obtained previously. Sliding of a pair made of low thermal conductivity materials leads to an increase in surface temperature. For zirconia/zirconia pair the temperature may rise up to more than 100°C [58], enhancing the T—M phase transition in the wet environment. This process may lead to cracking, grain pullout and catastrophic abrasive wear. Nevertheless, the work recently published by Chevalier et al. [165] opens again the research in this field. In pin-on-disc tests performed using water as a lubricant, they observed zirconia/zirconia or zirconia/alumina wear rates one order of magnitude lower than the wear rate of alumina/alumina pair. These results were not replicated using bovine serum as lubricant. 4.2. Zirconia/UHMWPE The wear of the couple UHMWPE/zirconia was studied by many authors. The results obtained are summarized in Table 6. Data are scattered over several orders of magnitude. Wear rates of UHMWPE against zirconia five times less than against alumina were observed in ring on disc tests carried out in conformity to ISO 6474 [59], due to the much finer grain size of zirconia than alumina (8]) [26]. Other authors [62] found an increase of some 65—70% in UHMWPE volume loss, depending on the number of cycles, for alumina surface roughness passing from R! "0.06 lm to R! "0.22 lm. Low residual porosity in zirconia surface induced UHMWPE wear 40—50% less than alumina ceramics [63, 64]. Different finishing processes can have a big influence on wear. Surface roughness and porosity obtained from sample finishing may produce different wear rates. It was hypothesized that the existence of a threshold value below which surface roughness changes can influence only a little the wear rate [65]. C. Piconi, G. Maccauro / Biomaterials 20 (1999) 1—25 9
C.Piconi.G.Maccauro Biomaterials 20 (1999)1-25 9二 un -4:ww 日z 自着昌消8日美弱酯器的33的色品云色总消的9名示8为9：9州8 939明g 朋 自用 入a
Table 6 Summary of results of UHMWPE wear tests—(1) WRR: Wear Rate Ratio; UHMWPE/zirconia as unit Ref. Method Medium Load Stress Speed Materials Roughness UHMWPE Wear unit WRR Notes (N) (MPa) (m/s) R! (lm) wear (1) [25] Ring on disc Ringer’s — — — Ti6Al4V — 0.019 mm3 h~1 10.6 CoCr 0.015 8.3 Alumina 0.0096 5.3 Zirconia 0.0018 [66] Pin on flat Bovine serum 223 3.5 0.05 Av. Ti6Al4V 0.008 0.56 mg 2.4 Wear measured after CoCr 0.008—0.016 0.05 0.2 1 million cycles Alumina 0.005—0.006 0.29 1.4 Zirconia — 0.21 [60] Pin on disc — 2—3 0.06 Ti6Al4V 0.018 29.84 mm3 Nm~1]10~6 2.8 Unidirectional motion SS316L 0.021 23.90 2.2 Alumina 0.009 18.28 1.7 Bovine serum Zirconia 0.005 10.78 Pin on flat — 3.45 0.05 Av. Ti6Al4V 0.018 2.81 5.2 Reciprocating motion SS316L 0.021 1.88 3.5 Alumina 0.009 1.25 2.3 Zirconia 0.005 0.54 [79] Pin on flat PECF — 3.54 0.05 Av. Ti6Al4V#N2 0.01 0.35 mg 0.92—1.4 Reciprocating motion PSZ 2 (0.01 0.25 Zr(OH) precipitates in PSZ 3 (0.01 0.38 PECF were detected [61] Pin on flat Ringer’s 57 — 0.025 Av. Alumina 0.02 3.9 mm3 Nm~1]10~5 2.0 Reciprocating motion ZTA, 5% 3.1 1.6 ZTA, 20% 2.1 1.1 TZP 1.9 [64, 65] Pin on disc Ringer’s #30% — 3.45 0.025 Alumina A 0.008—0.030 2.15 mm3 Nm~1]10~5 0.7 Unidirectional motion calf serum Alumina B 0.008—0.030 2.25 0.7 Roughness of ceramic Alumina B2 0.008—0.030 2.25 0.69 discs are not Mg—PSZ 0.008—0.030 2.65 0.8 representative of the Si3N4 2.9 0.9 one of ball heads Ti6Al7Nb TIN 2.1 0.7 Ti6Al7Nb ODH 1.35 0.4 CoCr 2.8 0.9 TZP 3.25 [74] Ring on disc Saline#calf — 5.6 0.314 Ti6Al4V-N impl. 0.022 max 7.5 mm3 3.4 Reciprocating motion serum 3:1 CoCr 0.003—0.008 4.2 1.9 Speed 0.025—0.1 m s~1 Alumina 0.003—0.008 2.2 1 p: 5.6 and 9.4 Mpa Zirconia 0.003—0.008 2.2 c sterilized PE 10 C. Piconi, G. Maccauro / Biomaterials 20 (1999) 1 —25
