(PMMA) Based Bone Cement - Tribology in Industry

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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.
Page 1: F. ŽIVIĆ, M. BABIĆ, G. FAVARO, M
Page 5 and 6: whereas the force is loaded and unl
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(PMMA) Based Bone Cement - Tribology in Industry

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