(PMMA) Based Bone Cement - Tribology in Industry

F. ŽIVIĆ, M. BABIĆ, G. FAVARO, M. CAUNII, N. GRUJOVIĆ,S. MITROVIĆMicroindentation of PolymethylMethacrylate (PMMA) Based BoneCementRESEARCHCharacterization of polymethyl methacrylate (PMMA) based bone cement subjected to cyclicalloading using microindentation technique is presented in this paper. Indentation techniquerepresents flexible mechanical testing due to its simplicity, minimal specimen preparation andshort time needed for tests. The mechanical response of bone cement samples was studied.Realised microindentation enabled determination of the indentation testing hardness HIT andindentation modulus EIT of the observed bone cement. Analysis of optical photographs of theimprints showed that this technique can be effectively used for characterization of bonecements.Keywords: Biomaterials, Polymethyl methacrylate (PMMA), Bone cement, Microindentation..1. INTRODUCTIONPolymers are large molecules synthesized fromsmaller molecules, called monomers. Polymers formedical applications can be divided into threegeneral fields:1. Polymers for artificial joints2. Bioabsorbable polymers for surgicalapplications3. Adhesives for medical applicationsAn acrylate polymer, commonly known as acrylicsor polyacrylates, belongs to a group of polymerswhich could be referred to generally as plastics.They are noted for their transparency and resistanceto breakage and elasticity. Polyacrylates are basedon acrylic acid, methacrylic acid, and their esters.Among them, polymethyl methacrylate (PMMA) islargely used for biomedical applications. Theclinical history of polyacrylates began when it wasunexpectedly discovered that the fragments ofPMMA plastic aircraft canopies stayed in the bodyof the wounded without any adverse chronicFatima Živić 1 , Miroslav Babić 1 , Gregory Favaro 2 ,Mihaela Caunii 2 , Nenad Grujović 1 , SlobodanMitrović 11 Faculty of Mechanical Engineering, Sestre Janjic 6,Kragujevac, Serbia2CSM Instruments, SwitzerlandE-mail: zivic@kg.ac.rsreactions [1]. PMMA has been used as selfpolymerisingbone cement in orthopaedics since the1960s. Acrylic bone cement is used to fill theirregular space between prosthesis and bone duringtotal hip replacements in order to keep theprosthesis in place (Fig. 1). The primary functionsof bone cement, when used to anchor artificialjoints, are to secure the orthopedic implants to boneand transfer mechanical loads from the implant tothe bone. Approximately 50% of all orthopedicimplants utilize bone cement to achieve implantfixation [2].Figure 1. Schematic view of total hipreplacement [3]146Tribology in industry, Volume 33, No. 4, 2011.

Some drawbacks associated with PMMA-basedbone cements are: local tissue damage due tochemical reactions during polymerization andstrong exothermic setting reaction, the highshrinkage of the cement after polymerization, thestiffness mismatch between bone and the cement,toxic effect of the monomer, inability to bonddirectly to bone causing loosening at the interfaceand brittle nature [2]. Loose cement particles alsomediate osteolysis of the bone and are highlyunwanted. Special problems occur at the interfacesdue to the different elastic module of the materials(110 GPa for titanium, 2.2 GPa for PMMA, and 20GPa for bone) [3].The role of the cement is directly related to themechanical properties of the cement, especially theresistance to fracture of the cement in the mantle atthe cement-prosthesis interface or the cement-boneinterface. Method of cement mixing is veryimportant since porosity is one of the crucialcharacteristics that determine its resistance tofracture. Cement that is inadequately mixedexhibits a high degree of porosity. High porositymeans that number of pores is present acting asstress raisers and initiating spots for cracks, furtherpromoting early fatigue failure. Number of types ofbioactive bone cements has been investigated toovercome above listed problems.PMMA is one of the polymeric materials also usedas matrix for carbon fibers composites [1]. Carbonfiber has been recognized, recently, as a materialwith many exciting applications in medicine, sincethey, when used as a reinforcing materialsignificantly enhance the mechanical properties ofthe observed system. Orientation and fiber contentcan be varied in such a way to provide the implantwith mechanical property needed for properfunctioning. One common feature that implant mustsatisfy is its resistance to fatigue failure and to beresistant to attacks from the physiologicalenvironment [1]. Many new approaches have beentried to enhance mechanical properties of PMMAbone cements [4,5]. Recent methods includeaddition of reinforcing particle or fiber in order toincrease cement stiffness, strength and toughness[4]. Common approach considers improvement offatigue characteristics, by reinforcement of PMMAwith carbon fibers, hydroxyapatite particles,stainless steel fibers, titanium fibers or zirconiaparticles or fibers, etc.A recognized issue is that it is not always possibleto distinguish the medical-grade polymers from theconventional polymers. They are selected based onclean condition or trace element analysis ormechanical properties. Processing involve cleanroom conditions and special care not to contaminatethe materials.The ready bone cement is a compound consisting of90% of polymethylmetacrylate, (PMMA), whichexists in an amorphous state and is completelytransparent. The rest are mainly crystals of bariumsulfate or Zirconium oxide that make the resultingproduct radio-opaque. The microscopic structure ofbone cement is made by two substances gluedtogether. One part consists of the small particles ofpre-polymerized PMMA (PolyMethylMetaAcrylate),so called “pearls”. These pearls are supplied as awhite powder. The other substance is a liquidmonomer of MMA (MethylMetacrylate). Bothsubstances are mixed together at the operation tablewith added catalyst that starts the polymerization ofthe monomer fluid. Amorphous polymers such asPMMA are brittle, hard plastics at roomtemperature [6]. It is isotropic and significantlybioinert material. PMMA is attacked by mineralacids but is resistant to alkalis, water and mostaqueous inorganic salt solutions [6]. PMMA is alsosubject to both elastic and viscoelastic (creep)deformation under load [7]. Considering previouslystated, it is very important to properly definemechanical characteristics of PMMA bonecements, to be able to further enhance theirfeatures.Characterization of bone cement to cyclical loadingis extremely significant and is a subject of manyongoing studies [8-14]. For instance, if total hipreplacement is considered, many daily activitiesinvolve many cycles of different alternating loadingpatterns (walking, etc.). Studies showed that one ofthe main reasons of cement failure mechanism isrelated to fatigue failure and fatigue crackpropagation [8]. Indentation represents flexiblemechanical testing due to its simplicity, minimalspecimen preparation. It allows selection of loadsand tip geometry. Forces can be applied from kilonewtonsdown to nano-newtons, but alsodisplacements down to nanometer. Indentationresponse is related to specific aspects of materialsin question (porous structure, biomaterials, etc.) andinterpretation of results requires specific knowledgeabout indentation mechanics and physics of theobserved material [15]. The hardness of a materialis usually defined as the resistance to localdeformation [15]. Until development ofinstrumentation to accurately measure imprintdimensions (contact area) after unloading, hardnesstests were limited to macro scales. However, sinceTribology in industry, Volume 33, No. 4, 2011. 147

great advancements in this area, micro and nanoscale optical measurements are widely availablethus leading to expansion of indentationinterpretation. Devices with depth sensingpossibilities, such as CSM Nano Indentation Tester,enable determination of hardness, elastic modulus,plastic stress-strain behavior and/or creep behaviordirectly using the tester, without the need tomeasure contact impressions.2. PRINCIPLE OF INSTRUMENTEDINDENTATION TESTING (IIT)The Nano Indentation Tester uses an alreadyestablished method where an indenter tip with aknown geometry is driven into a specific site of thematerial to be tested, by applying an increasingnormal load. When reaching a pre-set maximumvalue, the normal load is reduced until partial orcomplete relaxation occurs. This procedure isperformed repetitively; at each stage of theexperiment the position of the indenter relative tothe sample surface is precisely monitored with adifferential capacitive sensor. For eachloading/unloading cycle, the applied load value isplotted with respect to the corresponding positionof the indenter. The resulting load/displacementcurves (Fig. 2) provide data specific to themechanical nature of the material underexamination. Established models are used tocalculate quantitative hardness and elastic modulusvalues for such data.that describes the upper portion of the unloadingcurve by a power law relationship:where,F - is the test force,Fmax - is the maximum applied force,h - is the indentation depth under applied test force,hp - is the permanent indentation depth after theremoval of the test force,hmax - is the maximum indentation depth at Fmax,m - is a power law constant exponent.The power law exponent m is determined by a leastsquares fitting procedure and is a function of theindenter geometry.The contact stiffness S is given by the derivative atpeak load:And the tangent depth, hr, is thus given by:Where h r is the point of intersection of the tangentc to curve b at F max with the indentation depth-axis.The contact depth (depth of the contact of theindenter with the test piece at Fmax), h c , is then:where ε depends on the power law exponent m.The Indentation Testing Hardness H IT isdetermined from the maximum load, F max , dividedby the projected contact area A p at the contactdepth h c :Figure 2. Typical Load/displacement curveEvaluation of elastic modulus and hardness usinginstrumented indentation is realised by the methoddeveloped and proposed by Oliver and Pharr. It isthe most common approach to determine hardnessand modulus by interpretation of load - penetrationdepth (F - h) behavior during indentation (Fig. 1).Oliver and Pharr developed Power Law MethodWhere hc is the depth of the contact of the indenterwith the test piece at F max . A p (h c ) is the projectedarea of contact of the indenter at distance h c fromthe tip. A p is a function of the contact depth h c andis determined by a calibration of the indenter tip.The Vickers Hardness HV is defined by:148Tribology in industry, Volume 33, No. 4, 2011.

3. EXPERIMENTAL TESTWhere Ac is the developed contact area and can becalculated from the projected contact area A p andthe indenter geometry as:Where α is the angle between the axis of thediamond pyramid and its faces, α = 68° for aVickers indenter (Fig. 3) and α = 65.27° for amodified Berkovich indenter.For Vickers indenter: HV = 0.0945 H ITFor modified Berkovich indenter: HV = 0.0926 H ITBone cement prepared from commercially availabletype of mixture was used as a sample material forindentation tests. PMMA is isotropic, showing nocrystallinity, elasto-viscoplastic solid andamorphous polymer. Several indentations havebeen performed on each sample using the CSMMicro Indentation Tester (Fig. 4). Indentation testlasted for 300 cycles. Tests were performed withmaximum normal load of 15 N, minimum load of 5N and approach speed of 5000 nm/min.Vickers indenter was applied with quadraticloading type. There was no pause at maximum loadbefore unloading. Displacement measurementswere realised using LVDT sensor which is a part ofthe tester. Maximum possible indentation depth waslimited to 200 µm with displacement resolution of0.3 nm.Figure 3. CSM Micro instrumented indentationtesting: a) Optical micrograph of a Vickersindentation into a standard steel reference sample;b) Detail of Vickers indenter mounted abovesampleIndentation Modulus E ITThe reduced modulus of the indentation contact, E r ,is given by:Where β is a geometric factor depending on thediamond shape (circular: β = 1, triangular: β =1.034, square: β = 1.012). The Young’s modulus ofthe sample, E IT , can then be obtained from:Figure 4. a) CSM Micro Indentation Testerb) Schematic view of the main elements of MicroIndentation TesterWhereν i is the Poisson’s ratio of the indenter.ν s is the Poisson’s ratio of the sample.E i is the modulus of the indenter.4. RESULTS AND DISCUSSIONIndentation curve, that is, diagram of the normalforce applied during micro indentation as a functionof the number of cycles is shown in Fig. 5. This isclassical fatigue mode of indentation testing,Tribology in industry, Volume 33, No. 4, 2011. 149

whereas the force is loaded and unloaded constantlyduring indentation without a pause. Material issubjected to constant load/unload pattern and is notleft to rest, such as in case of indentation mode oftesting on creep.Figure 5. Normal force versus penetration depthcurve during micro indentationImprint on bone cement sample after indentationlasting for 300 cycles, is shown in Fig. 6. Typicalpattern produced by Vickers indentation is clearlyseen. Detailed view of the left lower angle is shownin Fig. 6b. The imprint is well shaped with clearlydeveloped edges and no microcracks around angles.This is in consistence with the fact that amorphousmaterials usually present an elastic behavior up tofailure [16]. Permanent deformation occurs due tohigh shear and compressive stresses which are bothpresent in highly localized area.Calculated values of hardness H IT and elasticmodulus E IT of bone cement sample after 300cycles obtained by the indentation tester is shownin Table 1.Figure 6. a) Micrograph of the indentation imprinton bone cement after 300 cycles, b) Detail of theindentation imprint on PMMA after 300 cycles (leftbottom angle)Table 1. Values of hardness H IT and elasticmodulus E IT of bone cement sample after 300cycles obtained by the indentation testerValue for PMMAsample after 300 cyclesHardness, H IT [MPa] 162.84Elastic modulus, E IT [GPa] 2.74Hardness profile and elasticity profile duringmulticycling loading, unloading and reloading areshown in Fig. 7. Values given in Table 1 areautomatically calculated by the tester according toOliver and Pharr approach, by analysing theunloading curves (Fig. 5). It can be clearly seen thatE IT value is rather uniform, whereas hardness hasslightly decreasing trend.Figure 7. Hardness, H IT and elasticity, as a functionof the number of cycles during micro indentationLoosening is recognized as one of the primarysources of total hip replacement failure. One of themain reasons is considered to be fatigue failure ofthe implant-bone cement and bone cement - boneinterface. That is the reason why characterization of150Tribology in industry, Volume 33, No. 4, 2011.

one cement to cyclical loading is extremelysignificant and is a subject of many ongoing studies[8-14]. Also, PMMA has been widely used as amodel for studies of fatigue and fracture inpolymers. Effects of different factors have beeninvestigated, whereas variable amplitude loadingoffer insight in its behavior from aspects of fatigueand crack propagation. The loading curve in Fig. 5consists of both elastic and plastic contribution,while the unloading curve is purely elastic andallows calculations of elastic modulus andhardness. Bone cements have a more complicatedstructure if compared to pure PMMA, withpreviously polymerised beads in a softer matrixwhich cures on implantation, and other componentssuch as particles of barium sulphate or zirconia tomake the cement visible in radiographs. Thismicrostructural complexity means that the cementmay behave very differently from pure PMMA.There are several commercially available bonecement mixtures and they have beencomprehensively studied both in laboratory andclinical practice. However, there are still featuresthat need to be enhanced and indentation tests canbe used for further understanding of its behavior.Clinical failure of the cement occurs over long timeperiods, and this implies that the crack growth rateis very low, perhaps as low as 10 -12 m/cycle. It isclear from Fig. 6 that cracks did not occur in thistesting and that edges are smooth and clearlymarked with well shaped imprint. Therefore, valuesof hardness H IT and elastic modulus E IT of bonecement sample can be taken as valid. Microindentation method can be used for measuring ofelastic modulus and hardness of prepared bonecement mixtures in a short period of time.5. CONCLUSIONCharacterization of bone cement to cyclical loading isextremely significant and is a subject of manyongoing studies. Studies showed that one of the mainreasons of cement failure mechanism is related tofatigue failure and fatigue crack propagation.Indentation represents flexible mechanical testing dueto its simplicity, minimal specimen preparation andshort time needed for tests. Devices with depthsensing possibilities, such as CSM Nano IndentationTester, enable determination of hardness, elasticmodulus, plastic stress-strain behavior and/or creepbehavior directly using the tester, without the need tomeasure contact impressions.Results obtained within this study were fullycomparable with the literature data found for PMMAand commercial bone cements. Micro indentationmethod can be used for measuring of elastic modulusand hardness of prepared bone cement mixtures. Veryimportant feature of CSM Micro indenter is automaticrecording of optical photographs of realised imprints.REFERENCES[1] J. Black, G. Hastings, Ed., Handbook ofBiomaterial Properties, Chapman & Hall, 1998[2] P.A. Revell, Editor, Joint replacementtechnology, Woodhead Publishing Limited, 2008[3] J. D. Enderle, J.D. Bronzino, S. M. Blanchard.,Introduction to biomedical engineering,Elsevier Academic Press, 2005[4] R. J. Kane, W.Y. James, J. Mason, R.K.Roeder, Improved fatigue life of acrylic bonecements reinforced with zirconiafibers, Journal of the Mechanical Behavior ofBiomedical Materials, Vol. 3, (2010) 504-511[5] Y.H. Nien, C. Huang, The mechanical study ofacrylic bone cement reinforced with carbonnanotube, Materials Science and Engineering:B, Vol. 169 (2010) 134-137[6] M. Kutz Editor, Biomedical Engineering andDesign Handbook, Vol. 1, McGraw-Hill, 2009[7] Lu Z, McKellop H., Effects of cement creep onstem subsidence and stresses in the cementmantle of a total hip replacement, Journal ofBiomedical Materials Research, 34 (1997)221–226[8] S. L. Evans, Fatigue of PMMA Bone Cement,Fracture of Nano and Engineering Materialsand Structures, B., Springer, DOI: 10.1007/1-4020-4972-2_133, pp. 271-272, 2006[9] Arola D, Stoffel KA, Yang DT., Fatigue of thecement/bone interface: the surface texture ofbone and loosening, Journal of BiomedicalMaterials Research Part B: Applied Biomaterials,76B: 287–297, Wiley Periodicals, 2005[10] G. Lewis, Fatigue Testing and Performance ofAcrylic Bone-Cement Materials: State-of-the-Art Review, Journal of Biomedical MaterialsResearch Part B: Applied Biomaterials, 66B:457–486, Wiley Periodicals, 2003[11] D. A. Gorham, A. D. Salman, M. J. Pitt, Staticand dynamic failure of PMMA spheres,Powder Technology, Vol. 138, (2003) 229-238[12] B. J. Briscoe, A. Chateauminois, T. C. Lindley,D. Parsonage, Contact damage ofTribology in industry, Volume 33, No. 4, 2011. 151

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