bioinert ceramic biomaterials -advanced applications

8/10/2019 Bioinert Ceramic Biomaterials -Advanced Applications

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Woodhead Publishing Limited, 2014

173

7Bioinert ceramic biomaterials:

advanced applications

DOI:10.1533/9781782422662.173

Abstract:First-generation, inert ceramics exhibit excellent mechanicalstrength, corrosion and wear resistance. This chapter reviews the fundamentalproperties that make alumina, zirconia, titania and pyrolytic carbon thematerials of choice for the production of numerous load-bearing implants. Theshortcomings of these materials, namely their relative brittleness and limitedability to be integrated with soft and hard tissues in vivo, are also discussed as a

limiting factor for their clinical application.

Key words:bioinert ceramics, alumina, zirconia, leucite, bioinert refractorypolycrystalline compounds.

7.1 Introduction

Ceramics are inorganic materials that are generally formed by a high-temperaturesynthesis process. They are comprised of non-directional ionic and covalent bonds,and are typically crystalline in nature (Bauer et al., 2013). The first generation of

ceramics was hard, stabile and inert under a wide range of environmental conditions,often displaying superior properties to metallic biomaterials.

Metal-based alumina, zirconia and titania are amongst the most highly studiedceramics. The high compressive strength, low friction, corrosion and wear resistanceof alumina, zirconia, titania and pyrolytic carbon makes these ceramics an excellentmaterial for the fabrication of load-bearing implants. The high abrasive strength ofthese ceramics renders them highly suitable for use as bearing balls in artificial

joints. When compared to metallic ball heads, ceramic heads lead to less long-termwear when used in conjunction with a polyethylene cup, reduce aseptic loosening,and therefore exhibit a reduced degree of osteolysis within the peri-implant space(Ihle et al., 2011). Despite these advantages, ceramics suffer from the disadvantagethat they can be brittle (due to the nature of ionic bonds), which somewhat limitstheir use in clinical applications (Bauer et al., 2013). However, their excellent osteo-conductive properties also make ceramics a coating of choice for the encapsulationof metallic load-bearing implants, such as titanium and stainless steel.

7.2 Hardness, high compressive strength and wear

resistance of bioinert refractory polycrystallinecompounds

Alumina (Al2O

3) and zirconia (ZrO

2) oxides represent two examples of the most

widely used inert ceramics, with applications in the construction of orthopaedic


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174 New functional biomaterials for medicine and healthcare


joint replacement and load-bearing implants, for example prosthesis, implantcoatings and dental implants. These are discussed in the following sections,together with leucite-containing ceramics.

7.2.1 Alumina

Alumina-based ceramics are typically prepared from very fine-grainedpolycrystalline -Al

2O

3 using hot isostatic pressing, followed by sintering at

temperatures of 1600 to 1800 C. The compressive strength of alumina is notablyhigher than that of zirconia at 4250 and 2000 MPa for -Al

2O

3 and tetragonal

zirconia stabilised with yttria, respectively. The Youngs modulus and hardnessfor the former was greater, at 400 GPa and 2400 HV, respectively, compared to208 GPa and 1130 HV. However, for zirconia, the bending strength was found to

be higher for the zirconium oxide (1000 MPa) compared to 595 MPa for -Al2O

3.

In comparison, the bending and compressive strengths of dense hydroxyapatiteceramics are 20 to 80 MPa and 100 to 900 MPa, respectively, with a Youngsmodulus of 70 to 120 GPa and hardness of 500 to 800 HV.

The mechanical properties, strength and corrosion resistance in crystallinematerials is known to depend on the grain size within the material. Studies havedemonstrated that for alumina to maintain its favourable mechanical and wear

properties, the grain size should be kept below 4m, since an increase in the grain

size to more than 7m has been shown to reduce the mechanical strength of theceramic by close to 20% (Bauer et al., 2013). Addition of magnesium oxide to the-Al

2O

3facilitates the preservation of the fine-grained structure during the sintering

process, contributing to the increased strength, resistance to dynamic and impactforces, fracture toughness and subcritical crack growth resistance. Figure 7.1 showsdifferences in microstructure and alumina doped with Ca, Mn and Cr (Pabbruwe etal. 2004). However, the use of such sintering agents should be kept to a minimum,since their excessive use may result in precipitation at the grain boundary and aconsequent loss of fatigue resistance. In addition, implant materials that have a

Youngs modulus that is relatively compared to that of natural bone tissue havebeen associated with aseptic loosening of the implant, with this behaviour beingtypically observed in metallic implants, such as those based on titanium.

Ceramics based on alumina are used in clinical applications such as dental bridgesand implants, joint prosthesis, bone screws, alveolar ridge and maxillofacialreconstructions (Cruz-Pardos et al., 2012; Huet et al., 2011). Under loadconditions that are typical for load-bearing applications (12 kN), alumina-basedimplants are expected to perform for 30 years, with their lifetime being limited bythe material properties and the environment in which the ceramics are required to

operate (Ritter et al., 1979). The fracture mechanics theory used to make thisprojection assumes that fatigue failure is controlled by the slow crack growthof pre-existing flaws in the structure when placed in biological environments.For example, the long-term strength of glass-infiltrated alumina- and various


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Bioinert ceramic biomaterials: advanced applications 175


zirconia-ceramics for applications in an oral cavity environment differed significantly

(Tinschert et al., 2007). The study found that under moist environmental conditions,the glass-infiltrated alumina- and some zirconia-ceramics have a high susceptibilityto subcritical crack growth, while zirconia ceramics containing 0.25 wt% aluminaoxide demonstrated the highest initial and most favourable long-term strength.

7.1 Differences in microstructure of the internal surface of aluminatubes (1.3 mm outer diameter, 0.6 mm inner diameter, 15 mm length)doped with Ca, Mn, or Cr: (a) pure alumina; (b) 0.5 mol% Ca-doped

alumina; (c) 5.0 mol% Ca-doped alumina; (d) 0.5 mol% Mn-dopedalumina; (e) 5.0 mol% Mn-doped alumina; (f) 0.5 mol% Cr-dopedalumina; and (g) 5.0 mol% Cr-doped alumina. The scale bars are 5m.The doping significantly altered tissue ingrowth, differentiation andosteogenesis within a porous implant when implanted into femoralmedullary canals of female rats for 16 weeks (Pabbruwe et al., 2004).


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7.2.2 Zirconia

As with Al2O

3 ceramics, zirconia is biocompatible, exhibiting good corrosion

resistance under normal physiological conditions. As mentioned previously,

zirconia has higher bending strength and fracture toughness compared to alumina,making it highly suitable for load-bearing implant applications. Furthermore, thestress shielding between the bone and the zirconia is reduced compared to that ofalumina, owing to the notably lower tensile modulus of the former material.Monoclinic at room temperature, zirconia crystallises to form a tetragonal phaseat temperatures between 1000 and 1170 C. Subsequent heating to a temperatureof 2370 C causes the zirconia to transform from a tetragonal to cubic phase.During the sintering process, zirconia also undergoes a volumetric expansion of 3to 5%. As the material cools, the mechanical stresses within the structure form.

This mechanical stress has been linked to the formation of cracks in ceramics thatare based on pure zirconia. Sintering agents may be introduced to stabilise the

phases during these temperature changes. Introduction of magnesium oxide,calcium oxide and yttrium oxide results in the formation of a partially-stabilisedzirconia microstructure, that is, it is comprised primarily of cubic zirconia withminor monoclinic and tetragonal zirconia precipitates. For example, incorporationof CuO into zirconia ceramics resulted in a significant reduction of friction in asliding test against Al

2O

3balls, with the coefficient of friction reducing from 0.8

to values between approximately 0.2 and 0.3 (Pasaribu et al., 2003). The flexural

strength of partially-stabilised zirconia has been reported to be further affected bythe fabrication methodology and the properties of the powders used (Adolfssonand Shen, 2012). The introduction of low amounts of Y

2O

3 (23 %mol) into a

tetragonal zirconia polycrystal yields a ceramic structure with typical grain size ofunder 1m and narrow size distribution.

As a result of their superior mechanical properties arising from its highcrystallinity, yttrium oxide stabilised tetragonal zirconia polycrystals are attractivecandidates for various load-bearing dental applications (Crisp et al., 2012; Tarumiet al., 2012). However, the same degree of crystallinity renders yttria-stabilisedtetragonal zirconia polycrystals highly opaque, which is undesirable for dentalrestoration from an aesthetic point of view. Clinically speaking, the stability of anyttria partially-stabilised tetragonal zirconia implant will be highly dependent onthe adhesive bond strength that exists between the zirconia-based core and theresins used for dental restorations (Jevnikar et al., 2012). In contrast to silica-

based ceramics, chemically-inert zirconia ceramics exhibit resistance to etchingwith acids, such as hydrofluoric acid and functionalisation. A host of methodshave been trialled to establish a long-term bond with this substrate, including

mechanical and chemical roughening of the surface to create micro-scale retention,for example particle abrasion, tailoring bonding cements, for example zinc-phosphate, composite resin, glass ionomer and resin-modified glass ionomercements, and deposition of thin layers, such as alumina coatings.


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The addition allows for the stabilisation of the tetragonal phase at room

temperature, with the size of the grain and the amount of yttrium oxide presentaffecting the proportion of the tetragonal phase retained. The retention of themetastable tetragonal phase is favourable, since it limits the extent of crack

propagation. Specifically, the application of stress and resultant crack formation

7.2 Schematic representation depicting how the tetragonal tomonoclinic (tm) transformation of ZrO

2increases fracture

toughness. When a section containing metastable tetragonal zirconia

(t-ZrO2) is subjected to a remote macroscopic tensile stress, the stressintensification due to the presence of the crack tip is sufficient totransform some t-zirconia grains to the monoclinic form. Since thistransformation entails a volumetric expansion, which is constrainedby the surrounding materials, the net result is compressive stressacting on the surfaces of the crack, whose propagation is thushindered (Lughi and Sergo, 2010).


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induces the metastable tetragonal zirconia grains located at the crack tip totransform to the stable monoclinic phase (Fig. 7.2). The expansion associated withthe phase transformation works to offset the compression stress onto the materials,thus enhancing the mechanical roughness of the ceramics. In order to expand the

desirable crack limiting characteristics of zirconia materials, these ceramicparticles have been dispersed into the Al

2O

3bulk ceramics to ensure mechanical

pre-stressing of the resultant ceramics as the material loses its temperature afterthe sintering treatment. The resultant materials are characterised by a favourableflexing strength of over 900 MPa and a hardness of 1500 HV. However, it is notclear if the improvement in mechanical properties is associated with the microcrackformation. Furthermore, the wear resistance of alumina-containing zirconia

particles has been shown to be less than that of pure Al2O

3. In order to overcome

this limitation, the zirconia particle size was reduced to a nanoscale.Adding SrO and Cr

2O

3to this nanoparticle zirconia-toughened alumina allows

the formation of an alumina matrix composite ceramic (Bahraminasab et al.,2012). The mechanical properties of the composite are even more superior,exhibiting bending and compressive strengths of 1150 and 4700 MPa, respectively,Youngs modulus of 350 GPa and hardness of 1975 HV. The enhanced mechanicalstrength allows for the development of more reliable, thinner walled componentswith a comparable load-bearing capacity. In orthopaedic applications, thesematerials could facilitate the fabrication of large ball heads to decrease the

incidence of implant dislocation.In vivostudies using a sheep model showed astable osseo-integration of porous coated alumina matrix composite ceramicmonoblock cups (Schreiner et al., 2012). Clinical studies of the alumina matrixcomposite/alumina matrix composite-bearing surfaces showed no evidence ofabnormal wear, osteolysis or implants migration (Lazennec et al., 2012).

7.2.3 Leucite

Leucite (KAlSi2O

6) is a mineral composed of potassium and aluminium

tectosilicate, with the crystals conforming to a polygonal shape and ranging insize from 1 to 5 micrometres (Fig. 7.3). At ambient temperatures, leucitecrystallises to tetragonal structure, which is transformed to a cubic structure at625 C. A volumetric expansion of over 1% takes place during the displacive

phase transformation, which is where a structural change is associated with thecoordinated movement of atoms relative to their neighbouring atoms. Leucite is amajor crystalline phase in ceramics that are used for dental veneers, termedfeldspathic porcelains, where the amorphous phase is a feldspar-derivedglass. Since the coefficient of thermal expansion of the glass phase is significantly

lower than that of the underlying metallic implant, it is not suitable for veneeringapplications on its own. The incorporation of leucite notably increases thecoefficient of thermal expansion. In terms of fabrication, leucite-containingceramics can be prepared via the incongruent melting of naturally-occurring


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feldspars at temperatures between 1150 and 1530 C (Bauer et al., 2013). As withother ceramics, microcracking within leucite-containing ceramics occurs as aresult of the mismatch in the coefficient of thermal expansion between the

crystalline and amorphous phases, causing a decoupling of crystals fromthe ceramic matrix. The thermal behaviour and the resultant mechanicalproperties and stability of leucite-based ceramics are dependent on the amountof the crystalline phase present. Typically, due to the presence of an extensiveamorphous phase, leucite-based materials have been found to exhibit inferiormechanical properties compared to that of other biologically relevant inertceramics.

7.3 Techniques for the fabrication of bioinert ceramicimplants

The fabrication methodologies employed to manufacture finished implantproducts are vast, with the choice of method being dependent on the material and

7.3 Surface crystallisation of leucite in a SiO2-Al

2O

3-K

2O-Na

2O glass.

Dendritic crystals grow like petals of a flower. SEM, etched sample(3% Hydrofluoric acid, 10 s), Au sputtered (Hland et al., 2007).


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operation environment requirements. For instance, ceramic-based dental implantscan be fabricated by heat pressing, slip casting, computer-aided hard machining ofcompletely sintered ceramic units, or by sequential soft machining of partiallysintered porous ceramic units and sintering (Denry and Holloway, 2010). Vacuum-

aided sintering is commonly used for deposition of ceramic veneers, mostcommonly leucite-based feldspathic porcelains (1525 vol% leucite) onto metallicdental implants. The technique is used to minimise the porosity of the ceramicveneer, as well as to reduce the presence of defects (cracks, inclusions) that mayevolve into microcracks under operating pressure. Within the porcelain, theleucite particles have been shown to be partially encircled by the microcracksformed as a result of a thermal expansion mismatch between the leucite crystalline

phase and the surrounding glass matrix, with the magnitude of the stress at theparticlematrix interface independent of the leucite particle size (Mackert et al.,2001). The firing/cooling cycles have been reported to influence the leuciteconcentration within the ceramic veneer, thus affecting the mechanical propertiesof the product. Slow cooling has been demonstrated to increase the proportion ofcrystalline leucite within the porcelain (Mackert and Evans, 1991). The ratio ofcrystalline leucite to amorphous feldspar glass affects the coefficient of thermalexpansion of the resultant ceramic, and is fine-tuned to ensure that the thermalcontraction of the veneer is less than that of the underlying metal unit (Mackertand Williams, 1996). In addition to tempering, ion-exchange treatments of

feldspathic porcelains (via application of ion exchanging agents to the porcelainsurface) have been demonstrated to create a surface less prone to crack initiation(Anusavice et al., 1992).

Heat pressing is an easy and inexpensive technique that has mostly been usedto produce ceramics based on leucite and lithium disilicate with a reinforcingcrystalline phase. The ceramics produced in this fashion contain a significantlyhigher proportion of crystalline component (3545 vol% leucite), resulting inan enhanced bending strength and fracture toughness compared to that ofveneers (Denry and Holloway, 2010). The observed improvement in the

mechanical performance of heat pressed ceramics can be partially attributedto the dispersion of fine leucite crystals throughout the ceramic matrix and thedifference in tangential compressive stresses around the crystal phase as aconsequence of cooling. According to Guazzato et al. (2004), the highly localisedresidual stresses can arise in a region with thermal expansion anisotropy in

polycrystalline materials with elongated grains and/or thermal expansion orelastic mismatch in polyphase materials and/or in transforming materials. Theseresidual stresses induce the microcrack toughening of the material, wheremicrocracks emerge along the lowest energy path, such as the lower modulus and

toughness glassy phase in a glass-ceramic. However, the negative aspect of themicrocrack evolution is the potential decoupling of the crystalline phase from the

porcelain matrix, which can result in an untimely loss of strength and factureresistance.


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While the typical porosity of heat pressed leucite-based ceramics isapproximately 10%, a significantly lower porosity (~1%) can be attained usinglithium disilicate (Li

2Si

2O

5)-based ceramics (65 vol%). The heat pressing-induced

crystallisation of lithium disilicate yields a highly interlocked crystal phase

with nanosized dimensions, with lithium metasilicate (Li2SiO3) and cristobalite(SiO

2) intermediates forming prior to the formation of lithium disilicate crystals.

As with leucite-based ceramics, the formation of tangential compressivestresses at the boundary between Li

2Si

2O

5crystals and surrounding amorphous

phase is associated with improved crack deflection and mechanical stability(Borom et al., 1975). The high proportion of the Li

2Si

2O

5 to glassy phase and

applied heat and pressure lead to crystal alignment, with the mechanical fracturetoughness anisotropy. Specifically, the susceptibility to crack formation and

propagation is significantly reduced in the direction perpendicular to crystalalignment.

As with other processes, the addition of a sintering aid into the ceramicsprecursor allows for property tunability of the resultant material. For example, hotpressing was used to produce fully dense ceramics from magnesium aluminatespinel (MgAl

2O

4) with a good optical transparency (>80% throughout the visible

spectrum). The determining parameter in attaining the high transparency was theaddition of 0.25 wt% LiF agent prior to hot pressing at 1600 C under vacuum and20 MPa uni-axial load, followed by hot isostatic pressing at 1850 C under

200 MPa Ar atmosphere (Sutorik et al., 2012). Other variables that affect thetransparency of the resultant MgAl2O

4ceramics include heating rate, temperature,

powder treatment and pressure at which the processing takes place. Recently,spark plasma sintering has been suggested as a good method for the production oftransparent MgAl

2O

4 ceramics, which was found to be a superior technique

compared to both hot pressing and hot isostatic pressing methodologies (Fu et al.,2013). Within a few minutes, the technique promises to achieve an almost fulldensification, resulting in a restricted grain growth and uniform grain-sizedistribution compared to that obtained using hot pressing.

Another approach used to produce dense ceramics is sequential dry pressingand sintering, where an over-sized specimen (1020% larger than the finalimplant) is first fabricated using a computer-designed die and then sintered at atemperature of about 1550 C. This production method is commonly used on high

purity Al2O

3-based ceramics and results in a high degree of crystallinity and a

small distribution of grain size around a mean of 4 m, the bending strength ofapproximately 600 MPa, and good in vivostability for many years of use. Sincethe high crystallinity of the bulk material renders it opaque and not aesthetically

pleasing for dental applications, the resultant product often undergoes finishing

coating using a translucent porcelain veneer. The methodology is also suitable forfabrication of fine stabilised zirconia powders (Laberty-Robert et al., 2003). Uni-axial dry pressing and sintering (14001550 C, 2 hrs) of yttria partially stabilisedtetragonal zirconia yielded ceramics with a mean grain size of 0.26 to 0.57 m


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disilicate (Li2Si2O

5) crystals, are characterised by a lower strength of 130 MPa.

Both soft and hard machining processes incur a notable amount of wastage of theceramic material, and involve a ceramic preform containing a high degree ofdetail regarding the implant uniformity, compaction density and robustness to

enable computer machining.A number of additive production methodologies have been developed to

fabricate complex ceramic structures in a more time and economically efficientmanner and increase the accuracy with which the implant is manufactured. Themethodologies are based on the availability of small modular elements that can beassembled quickly yet reliably. Stereolithography is a cost-effective method thatinvolves laser polymerisation of an UV curable monomeric system followed byremoval of organic particles (Chartier et al., 2002). Robocasting is anothercomputer-aided design/computer-aided manufacturing process, where ceramic

paste is deposited in a layer-by-layer sequence to build up core and fixed partialdenture structures (Silva et al., 2011). Selective laser sintering is a type of solidfreeform fabrication in which a ceramic part is generated in layers from powderusing a computer-controlled laser/scanning apparatus and power feed system(Bourell et al., 1992). Three-dimensional printing allows for manufacture oftooling and functional prototype parts directly from computer models via thedeposition of powdered material in layers and the selective binding of the

powder by ink-jet printing of a binder material (zkol et al., 2012; Sachs et al.,

1992).

7.4 Conclusion

Inert ceramics are characterised by a favourable combination of mechanicalstrength, corrosion and wear resistance, making them well suited for a wide rangeof load-bearing applications, most notably as integral components of artificial

joints and in dentistry. Indeed, alumina ceramics are frequently used for thefabrication of femoral joint heads to be used in conjunction with an ultra-high

density polyethylene acetabular liner in hip arthroplasty. From a mechanical wearpoint of view, a ceramicceramic pairing would be even more beneficial, since itwould reduce the incidence of the aseptic loosening associated with polymerwear. However, limited in vivosoft and hard tissue integration of alumina andother bio-inert ceramics leads to loosening of all-ceramics-based acetabularsockets in the pelvic bone. Along with brittleness, such poor osseo-integrationhinders the potentially beneficial clinical applications of these ceramics. Tocircumvent these issues, several bio-activation strategies have already beenreported. Among them, sodium hydroxide treatment and surface immobilisation

of biological agents have already been demonstrated to successfully improve celladhesion, proliferation and secretion of osteocalcin in the absence of thedeteriorating effect on the short- and long-term strength behaviour. However,more research into the surface modification of ceramics is required to ensure that


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sufficient levels of cementbone interaction take place if these materials are to beused clinically. This has resulted in second- and third-generation ceramicmaterials, which are discussed in Chapter 8.

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bioinert ceramic biomaterials -advanced applications

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