finite element based design of hip joint prosthesis

FINITE ELEMENT BASED DESIGN OF HIP JOINT PROSTHESIS Thesis submitted in partial fulfillment of the requirements for the degree of Master of Technology In Biomedical Engineering

By Vicky Varghese 209bm1016

Department of Biotechnology & Medical Engineering National Institute of Technology Rourkela-769008, Orissa, India 2011

FINITE ELEMENT BASED DESIGN OF HIP JOINT PROSTHESIS Thesis submitted in partial fulfillment of the requirements for the degree of Master of Technology In Biomedical Engineering By Vicky Varghese 209bm1016 Under the guidance of Prof. Tarapada Roy And Prof. Kunal Pal

Department of Biotechnology & Medical Engineering National Institute of Technology Rourkela-769008, Orissa, India

ACKNOWLEDGMENTS I wish to express my sincere gratitude to my thesis supervisor Prof. Tarapada Roy. For his excellent guidance, encouragement and support during the course of my project work. He has helped me gain knowledge in the field of mechanical engineering and also has taught me the importance of simplicity and keeping a cool head during troubles and difficulties. His constant talks of encouragement has always helped me to overcome the different hardships which I faced during the project I also express my heartfelt gratitude to my thesis co-supervisor Prof. Kunal Pal, for his invaluable advice and guidance throughout the course of my research work. He is my role model for sincerity, punctuality and discipline .I thank him for providing me a wonderful opportunity to work on an interdisciplinary project. I also would like to thank my friends Koteswarao and Shince V Joseph who have helped my migration from Biomedical department to mechanical department a memorable experience. I also would like to thank my mallu gang member , my swimming team members, my lab mates and my roommate Sadanand Jinna who were always a constant source of inspiration and helped me during various ups and downs of the project I would like to thank my parents who were always very much supportive for my project and have helped me enjoy each and every success and failure during the project Last but not the least I express my sincere thanks to the NESCAFE coffee shop girls especially Rajitha and Sonia for providing a constant supply of coffee during good and bad times in the project.

Table of Contents

List of Figures…………………………………………………………………………….………..I List of Tables …………………………………………………………………….………….…II Abstract……..……………………………………………………………….……….…………..III

1.Introduction……………………………………………….………………………………….…1 1.1 Objective of the Research ………………………….…………………………4 2. Literature Review…………………………………………………………………………........5 2.1 Biomaterials in Hip Joint Prosthesis……………………………….…………6 2.1.1 Metals ……………………………………………..……………….8 2.1.2 Ceramics…………………………………………………………….9 2.1.3 Polymer……………………………................................................10 2.1.4 Composites………………………………..………………….……11 2.1.4.1 Functionally Graded Composites……………………….11 2.1.4.2 Polymer-Ceramic Composites…………………..….…..11 2.1.4.3 Composites with biological macromolecules ………….12 2.2 Finite Element analysis…………………………………….….……13 3. Materials and Methods……………………………………………………………….……...23 4. Results and discussion…………………………………………………………………….. 27 5. Conclusion……………………………………………………………………….………….40 5.1 Future Work ………………………………………………..…….41 6. References …………………………………………………………………………………42

List Of Figures

1. Biomaterials used in Hip joint replacement.............................................................................4 2. 2-D model of the implant…………………………………………..…………………………24 3. Solid 92 Geometry …………………………………………………………………….……. 25 4. 3D model and the meshed model…………………………………………………………..….26 5. The models for the static and dynamic analysis with the forces acting on them………………………………………………………………………..…..…..28 6. The static analysis of 316 L SS model, von mises stress, displacement in x, y and z direction………………………………………………………….29 7. The static analysis of Ti-6Al-4V model, von mises stress, displacement in x,y and z direction...................................................................................30 8. The static analysis of Co-Cr model, von mises stress, displacement in x,y and z direction……………………………………………………...30 9. The static analysis of Magnesium alloy AZ 91, von mises stress, displacement in x,y and z direction ……………………………..……31 10. The static analysis CF-PA 12 model, von mises stress, displacement in x,y and z direction ……………………………………………..………31 11. The static analysis CF-PEEK model, von mises stress, displacement in x,y and z direction………………………………………..…………….32 12. The static analysis bone model, von mises stress displacement in x,y and z direction ………………………………………..……………32 13. The graph of the various forces acting on the hip joint prosthesis during one gait cycle….. 34 14. The variation of Von Mises stress during dynamic analysis by the metals and bone……… 35 15. The variation of Von Mises stress during dynamic analysis by the composite and bone….. 35 16. The variation of x-component of displacement during dynamic analysis by the metals and bone ……………………………………………….36 17.The variation of x-component of displacement during dynamic analysis by the composite and bone…………………………………………... 36 18. The variation of y-component of displacement during dynamic analysis by the metals and bone ……………………………………………….37 19.The variation of y-component of displacement during dynamic analysis by the composite and bone…………………………………………... 37 20. The variation of z-component of displacement during dynamic analysis by the metals and bone ……………………………………………….38 21.The variation of z-component of displacement during dynamic analysis by the composites and bone …………………………………………38

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List of Tables

1. Various Important Factors For The Selection Of Material For Biomedical Application….. 3 2: Mechanical Properties Of Alloys In Total Joint Replacement ……………………………. 9 3. Mechanical Properties Of Ceramics Used In THR …………………………………………10 4 .Mechanical Properties Of Composites ………………………………………..……………13 5. Material Properties Of Metals Used For The Implant……………………………………... 25 6. Material Properties Of Composites Used For The Implants ……………………………….26 7 .The Von Mises Stress And Yield Strength Of The Metals……………………………….. 33 8. The Von Mises Stress Developed In Composite Materials ………………………………..33

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Abstract

The work deals with the proper design of hip join prosthesis using finite element method. Hip joint is one of the most important weight bearing and shock absorbing structure of the human body. The longevity and functionality of the implant greatly depends on the design of the implant. The shape of the implant is one of most easily recognizable feature in the design of the implant. In the present study, Cobalt-chromium (Co-Cr), Low-carbon stainless steel of type 316(316L SS), titanium alloy (Ti6Al4V), magnesium alloy (AZ91), carbon fiber reinforced PEEK (CF-PEEK) and carbon fiber reinforced PA -12 (CF-PA 12) have been used for implant material which are biocompatible. ANSYS finite element package has been used for modeling and analysis of the implant. The implant has been analyzed under static as well as dynamic loading. Based on those analysis, some important results have been obtained and presented in the results and discussion sections Keywords: Hip joint, implant, finite element modeling, finite element analysis and dynamic loading

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Chapter 1 Introduction 1|Page

1. Introduction Hip joint is a diarthrosis or synovial joint between the femoral spherical head and the convex-shaped acetabulum in the pelvis. It is wrapped in a capsule that contains a biological lubricant, the synovial fluid (SF), which also acts as a shock-absorber (Nordin M 2001). The hip joint can transmit high dynamic loads (7–8 times the body weight) and accommodate a wide range of movements because of the presence of the SF and the ball-in-socket geometry. The hip joint can be affected, more often in aged people, by chronic pain due to diseases such as osteoarthritis, rheumatoid arthritis, bone tumors or traumas. In these cases, the best clinical solution is the total hip arthroplasty, a surgical procedure that replaces the unhealthy hip joint with an implant, preserving the synovial capsule. There are about 200,000 and 80,000 interventions/year are performed in the USA and in the UK, respectively, and they are estimated to increase of about 170% by 2030 (O.K. Kurtz Steven 2007). In orthopedic surgery, hip arthroplasty is considered as one of the greatest achievements in the last decades, but from an engineering point-of-view hip implants need further developments because they tend to have a limited service life of about 15 years, which is not satisfactory for patients under 60 years of age (Brown 2006). In hip joint replacement operations, the head of the femur is sectioned off, and the soft marrow is removed to create a hollow intra medullar cavity through the centre of the femur shaft. An artificial implant (mainly comprising of a long stem and a head) is then glued into the femoral cavity. The implant head fits into the acetabular socket of the hip bone. The critical mechanical property requirements of the implant material include (but are not limited to) high specific bending stiffness, high bending stiffness comparable to that of the surrounding cortical bone, biocompatibility, corrosion resistance and high endurance limit. The region where two contacting surfaces have dissimilar mechanical properties, such as implant–cement, bone–cement and implant–bone interfaces are the most susceptible regions to failure in standard Total Hip Replacement (THR). The phenomenon of bone stress shielding can be problematic at implant–bone interfaces because of the large difference in mechanical stiffness between the metallic stem and the often osteoporotic and osteopenic host femur. Bone resorption due to stress shielding and osteoporosis may cause micromotion. Micromotion along with particle debris (formed during osteolysis) may cause implant loosening. This has motivated the 2|Page

development of hip stems from other materials which are more closely stiffness-matched with the host femur. The finite element method (FEM) is also referred as finite element Analysis (FEA) is a computational technique used to obtain approximate solution of boundary value problems in engineering(Hutton 2005). The advantage of Finite Element Analysis (FEA) for total hip replacement studies is that the different implant configuration and design can be tested on computer rather than by physical testing and eliminate and/or minimize the time and cost involved for in vivo and in vitro experimentation The purpose of FEA in the field of orthopedic biomechanics is to predict the mechanical behavior of bones, develop and improve the design of implants. Biomaterials used for total hip joint replacement (THR) have different wear rate and liberate particle of different size which have different biological reactivity. The various factor which are important for selection of materials for biomedical application are listed in the table 1 Table 1 various important factors for the selection of material for biomedical application Factors

1st level material properties 2nd level material properties

Specific functional requirements

Processing and fabrication

Description Biological characteristics Chemical composition

Physical characteristics

Mechanical characteristics

Density

Elastic modulus, Poisson’s ratio, Yield strength, Tensile strength Adhesion Surface topology Hardness, Shear module, Shear strength, Flexural modules, Flexural strength Biofunctionality(non- Form ( solid, porous, Stiffness or rigidness, thrombogenic, Cell coating, film, powder Fracture toughness, adhesion etc), etc), coefficient of fatigue strength, creep Bioinert, thermal expansion, resistance, adhesion Bioactive,Biostability, elastic conductivity, strength, impact Biodegradability colour, refractive strength, proof stress, index, opacity abrasion resistance Reproducibility, quality, sterilizability, packaging, secondary processability

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The different biomaterials which are used in hip joint prosthesis are as shown in the fig 1

Materials

Metal

Ceramic

Stainless steel 316L

Carbon Alumina

Cobalt- based alloys

Titanium- based materials

Zirconia

Polymer

PMMA UHMWPE/ HDPE Polysulfone PTFE

Composite

Polymerbased

Biomimetic

Calcium phosphate Bioglasses

Fig 1): Biomaterials used in Hip joint replacement In order to design a hip prosthesis adequately, the following important issues must be considered: (1) stress shielding; (2) stem/bone interface stresses; and (3) prosthesis strength.

1.1 Objective of the Research In this present study, three dimensional finite elements have been used for modeling and analysis of the hip joint prosthesis. Six different types of materials have been considered for prosthesis material in order to compare their performance under loading conditions. The implant has been analyzed under static as well as dynamic loading.

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Chapter 2 Literature Review

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2. Literature Review Some of the important work that has been carried out in this field are reported as below. The section is divided into the different biomaterials which are used in hip joint prosthesis and also about the different finite element analysis which has been done in the field of hip joint prosthesis

2.1

Biomaterials in Hip joint Prosthesis

This section deals with the important works which have been carried out in the field of biomaterials for hip joint prosthesis. Biomaterials used for total hip joint replacement (THR) have different wear rate and liberate particle of different size which have different biological reactivity. (Bougherara, Zdero et al. 2011) did the biomechanical study of novel carbon fiber hip implant and standard metallic hip implants. Carbon–fiber composite stems are able to reduce stress gradients at the bone–stem interface and are biocompatible and its cross-weave surface texture permits good bony on growth. CF/PA12 stems have a similar flexural modulus as human cortical bone. It was found that the Composite was less mechanically stiff compared to metallic stems, thereby yielding much lower stresses and much higher strains for the same applied load. (H Bougherara 2010) to design a novel hybrid total knee replacement, it had improved stress transfer to the bone in the distal femur. Carbon fiber-reinforced polyamide 12 lining layers were used in place of the standard material. The new design has a higher stress transfer to the host femoral bone compared to the standard TKR which helps in decreasing the stress shielding and hence minimizes the consequence of bone loss and device loosening in the hybrid design. Only static analysis was carried out for the FE analysis. (I. Sridhar 2010 ) used carbon and aramid fiber reinforced polymer matrix(such as polyether eher ketone-PEEK) composite material for hip joint prosthesis. The advantage of the material is that they possesses superior biomechanical properties such as better fatigue strength, chemical resistance, environmental stability and resistance to sterilization by ϒ radiation and the stiffness of the implant can be tailored according to the requirement. The optimal ply winding sequence of femoral stem to obtain orthotropic moduli similar to that of natural bone is achieved 6|Page

using a MATLABR. Stress analysis was carried out using ABAQUS/CAE commercial finite element analysis software.

The optimal ply sequence was found to be [-7515

/6015/4515/3015/6515]5 . CF/PEEK produced similar Von Mises Stress patterns as those of the intact femur, while metallic stem experienced high stress at bone stem interface while the surrounding bones experienced lower stress which produces stress shielding which causes bone resorption. (Rahaman, Huang et al. 2010) did the in vivo studies on the use of a composite of Al2O3 and niobium (Nb) for potential use as an alternative femoral head material. The composite is fabricated by hot pressing. The flexural strength of Al2O3–Nb laminates in four-point loading was 720 ± 40 MPa, compared with a value of 460 ± 110 MPa for Al2O3. The interfacial shear strength between Al2O3 and Nb, measured by a double-notched specimen test, was 290 ± 15 MPa. The composite femoral head combined the low wear of an Al2O3 articulating surface with the safety of a ductile metal femoral head which could reduce the risk of catastrophic brittle failure of Al2O3 femoral heads in vivo. (Cilingir 2010) found that ceramics are a good alternative to metal because of its better wear resistance for bearing couple material. Contact pressure distribution was significantly affected by a reduction in radial clearance. The major concern regarding the use of ceramic is the stress shielding compared with metallic resurfacing prosthesis. The best design for ceramic resurfacing prosthesis was to have a alumina implant with a metal backing. (Kane, Yue et al. 2010) investigated the effects of zirconia fiber reinforcement on the fatigue life of acrylic bone cements. PMMA cement with sintered straight or variable diameter fibers (VDFs) was added to test the reversed uniaxial fatigue until failure. The cement reinforced with 15 and 20 vol% straight zirconia fibers had the mean fatigue life increased by

40-fold, on average, compared to

a commercial benchmark (Osteobond™). The a cement reinforced with 10 vol% VDFs had a higher mean fatigue life the same cement reinforced with 10 vol% straight fibers. (Fujihara, Huang et al. 2004) for orthopedic bone plate application studied the fabrication and characterization of carbon/PEEK fabric composite. It was found that the unidirectional laminated composites suffer the delamination fracture. The micro-braiding technique contains matrix and reinforcement fibers and can be fixed into the complicated contour of orthopedic fixations. It was found that the composite had a 55-59 % better bending stiffness, 40-63 % yield bending moment and 54-77 % maximum bending moment. (Staiger, Pietak et al. 2006) 7|Page

magnesium-based implants have the potential to serve as osteoconductive, biocompatible, and degradable implants for load-bearing applications because it is a lightweight metal with mechanical properties similar to natural bone. The elastic modulus and compressive yield strength are closer to those of natural bone than the other commonly used metallic implants. It is naturally found in bone tissue and is essential to human metabolism. For orthopedic applications, alkali-heat treatments, is used to induce a biomimetic precipitation of calcium phosphate at the implant surface.The different biomaterials which are used in hip joint prosthesis are described as below

2.1.1Metals Metals have been the primary material used for THR due to superior mechanical properties. Originally steel was used to make femoral component of hip joint that was replaced by cobalt-chromium-molybdenum alloy (VitalliumTM). Most commonly, for femoral part stainless steel, Co–Cr alloys, or Ti alloys are the metals of choice and for the cup component alumina or zirconia ceramic, polytertrafluoroethylene (PTFE) or Co–Cr alloy are usually used (Joa˜o F.Mano 2004). However there are many nuisance associated with metals like release of dangerous particles from wear debris, detrimental effect on the bone remodelling process due to stress shielding and also loosening of the implant tissue interface. Metals like Cr and Ni used some cases are reported to cause oxidative damage to cell signaling molecules and also induce osteolysis (Sargeant and Goswami 2007). It has been shown that the degree of stress shielding is directly related to the difference in stiffness of bone and implant material (Huiskes and Chao 1983; Black 1987). Titanium alloys are favorable materials for orthopedic implants due to their good mechanical properties. However, titanium does not bond directly to bone resulting in loosening of the implant. Undesirable movements at the implant-tissue interface results in failure cracks of the implant. The application of bioactive coatings to titanium-based alloys enhance the adhesion of Ti-based implants to the existing bone, resulting in significantly better implant lifetimes than can be achieved with materials in use today. An ideal bioactive coating would bond tightly both to the bone and the metal. Some ceramic coatings are known to be bioactive and have been tested on Ti implants. Since the modulus of the Ti alloys is lower than that of the Co–Cr–Mo alloys, they have been more for THR components. The elastic modules of the Ti alloys have been engineered to be more suitable

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by heat treatments resulting in microstructures that have a reduced elastic modulus. Metallic femoral head articulating inside a polymeric (PTFE or UHMWPE) acetabular cup has been one of the most favorable THR element structure (Wang, Liu et al. 2009). Clinical results show that excessive wear and wear debris is the primary cause of failure of UHMWPE or metal implants. Thus, the use of materials with lower modulus and strength such as polymers appear to be more useful for use as bone biomaterials Table 2 : Mechanical properties of alloys in total joint replacement (Katti 2004) Alloy Cp Ti (pure titanium) Ti–Zr Co–Cr alloys Co–Cr–Mo Ti–6Al–4V Ti–6Al–7Nb Ti–12Mo–6Zr–2Fe Stainless steel 316 L Ti–35Nb–5Ta–7Zr (TNZT)

Microstructure {α} Cast {α’/β} {Austenite hcp} {α ‘/β}

(fcc)

{α’/β} {Metastable β} {Austenite} {Metastable β}

Tensile strength( MPa) 785 900 655–1896 + 600–1795

Modulus ( GPa) 105 210–253 200–230

960–970

110

1024 1060–1100 465–950 590

105 74-85 200 55

2.1.2. Ceramic Ceramics are most compatible bone substitute biomaterials used for joint and joint surface replacement. Since 1970s, when ceramic was introduced, ceramic components are being used for total hip arthroplasty (THA). Compared with metallic components, ceramic femoral heads for THA have the potential advantage of lower wear rates in articulations with acetabular liners. Alumina due to their excellent biocompatibility, stability in physiological environment and high strength are used frequently in THA. Conventional ceramics such as alumina were evaluated due to their excellent properties of high strength, good biocompatibility and stability in physiological environments (Cao and Hench 1996) However, alumina materials have poor chemical bonding with tissues, as a result use of it as a potential bone substituent is limited. Other biocompatible ceramics like Zirconia, yttria, Bioglass, C-( Graphite) and Calcium phosphate ceramics like HAP (Ca10(PO4)6(OH)2 ) and Tricalcium phosphate (TCP) (Ca3(PO4)2) 9|Page

are widely used for hard tissue replacement because of their osteoconductive properties (J.D. Helmer 1969; L.L. Hench 1993; Katti 2004).Alumina and Zirconia are primarily used in the fabrication of femoral heads. For hip replacement metallic femoral stem is used in conjugation with alumina femoral and an acetabular cup made from UHMWPE for the opposing articulating surface. As compared to conjugation of metal with UHMWPE, the wear rates for alumina on UHMWPE conjugation is reported to be 20 times less than that of metal- UHMWPE conjugation. Ceramic components are used increasingly in total hip arthroplasty (THA). However, the use of ceramic components is also associated with unique risks, including sudden fracture and intolerable bearing noise or squeaking. Alumina and titanium dioxide have been used as nano ceramics separately or in nano composites with polymers such as polylactic acid or polymethlyl methacrylate. The nano ceramic formulations promote selectively enhanced functions of osteoblasts (bone-forming cells. Ceramic particles are also weakly genotoxic and non-cytotoxic on human fibroblasts in contrast to Co Cr particles, which show dose-dependent cytotoxicity and genotoxicity (Tsaousi, Jones et al. 2010) Table 3 Mechanical properties of ceramics used in THR (F.O. Schmitt 1988; S. Ramakrishna 2001) Ceramic Zirconia Alumina Bioglass C –( graphite) C-( Vitreous) HAP C- ( LTI pyrolitic) AW glass- Ceramic

UCS (MPa) 2000 4000 1000 138 172 600 900 1080

UTS ( MPa) 820 300

50

Modulus ( GPa) 220 380 75 25 31 117 28 118

2.1.3 Polymers Polymers of very high stiffness and strength are used for orthopaedic surgery. For orthopedic applications, common polymers used are: acrylic, nylon, silicone, polyurethane, ultra high molecular weight polyethylene (UHMWPE), and polypropylene (PP). Highly stable polymeric systems such as PTFE, UHMWPE or poly(etheretherketone) (PEEK) have been investigated due to their excellent mechanical properties. Polyethylene wear particles are 10 | P a g e

reported to enhance osteolysis. Methacryloyloxyethyl phosphorylcholine (MPC) over metal surface provides high lubricity and also reduce wear (Kyomoto, Iwasaki et al. 2007). Biodegradable

polymers

both

natural

and

synthetic

like

poly(dioxanone)(PDO),

poly(trimethylene carbonate) (PTC) copolymers. PLA and PGA, polycaprolactone (PCL), polyanhydrides (PA), polyorthoesters are being subjected to THR research (T.H. Barrows 1986) A shift from biostable to biodegradable biomaterials for mediacl application is seen in recent time

and biodegradable polymers have extensive application in tissue engineering. Due to

versatility polymeric material have replace other class of biomaterial and also used in combination with metal and ceramic (Nair and Laurencin 2007)

2.1.4. Composites: 2.1.4.1 Functionally graded composites The graded composition of functionally graded composites marks them with two different properties at two ends of the composites. Ceramic- metals composites are prepared by powder metallurgy methods, ceramic part provides biocompatibility and metals part marks a good mechanical property to the composite graft e.g. HAP/titanium composites (F. Watari 1997), fluoroapatite/ TCP (H. Wong 2002), HAP/Zirconia composites.

2.1.4.2 Polymer-ceramic composites Ceramic and polymer combination biomaterial have superior mechanical properties and biocompatibility with tissues and are better than solitary polymer or ceramic used for THR(N. Galego 2000) . Polymers has very low module as compare to bone, high weight % of filler component is used. HAP has been the material of choice for ceramic filler component and high % of HAP is been used with polymers of low module eg. PLA [poly(lactic acid) ], PHA [poly(_hydroxyalkanoates)] . Composites of (PHA) with HAP have shown ultimate strength, elastic modulus and elongation at break similar to bone and are being investigated as potential materials for THR(N. Galego 2000). Biodegradable polymers ( starch- based) and ethylene vinyl alcohol (EVOH) filled with 10-30% HAP has shown high module (about 7GPa) (44) has prospective for THR.

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Coupling agents like zirconate, titanate and silanes have been used

for EVOH- HAP,

HAP-Ca and PLA-HAP combination composites (Y.E. Greish 2001; T. Kasuga and . 2003). However, composites have poor adhesive properties and bone repairing materials like bioglass and bioceramics are used to enhance adhesion to tissues. HAP along with bioceramics and bioglasses have been studied extensively as bone repairing material and is used as a coating for implanted prostheses to enhance direct adhesion to bone tissue (A. Ravaglioli 1992). Apart from that number of polymer ceramic composites are used that are shown in table4 with their mechanical properties as compare to bone

2.1.4.3 Composites with biological macromolecules Bone is a nanocomposite of HAP and type I collagen. Composites like HAP in combination with soluble collagen, gelatine, polyacryl acid, phosphoprotein, bone Gla protein etc is used to mimic bone structure (M. Kikuchi 1999; M.C. Chang 2001; S.C. Liou 2003) HAP– gelatin composites are being currently studied for potential bone replacement materials (S.C. Liou 2003). Other biomimetic routes include in situ mineralization of HAP in the presence of polymeric macromolecules such as calcium binding polyacrylic acids of high molecular weight (K.S. Katti 2001). Molecular level association of polymers with HAP represents a significant mechanical properties exhibits a core-shell configuration(S.C. Liou 2003) . Composites of HAP with bone phosphoprotein, bone Gla protein and collagen (Heywood et al., 1990) have also been attempted. Mechanical properties in the above composite systems are still inadequate for the potential use of these composites for total bone replacement. The

biomimetic composite

obtained has excellent biocompatibility and biointegrative activities, equivalent to autogenous bone and are much better than other artificial bone materials (M. Kikuchi 1999). Hydroxyapatite and collagen composites (HAP/Col) have the potential in mimicking and replacing skeletal bones. The combination of HAP with natural cell cytoskeleton proteins has been studied extensively as a favorable composite for bone tissue engineering due to their natural biological resemblance and properties (Alina Sionkowska 2010).

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Table 4 Mechanical properties of composites

Materials

UTS (MPa)

Functionally graded: HAP/Yttria, 0–40% yttria content w/w PHB/HAP, 30% w/w P(βHB-co-8–24% βHV)/HAP, 30%w/w Chemically coupled HAP/PE, 7–40 vol% filler Nano HAP, 30–70 to 60 w/w PAAC/in situ nano HAP, 40–70 w/w Starch-EVOH (SEVA) blend/HAP, 10–30% w/w Starch-EVOH (SEVA) /10% HAP w/w with 1% coupling agents (zirconate, titanate and silane)

Bending strength: 160–200

2.2

67 62–23

Elastic modulus Elongation ( GPa) Break ( %) 100–160

2.52 2.75–0.47

18.34–20.67 0.88–4.29

35.8–78.4

at References (X. Wang 2002)

2.65 2.25–5.42

(N. Galego 2000) (N. Galego 2000)

>500 to 2.6

(Wang, Liu et al. 2009)

20–60

(Bending) 2.3–6.2 Elongation 1– (X. Wang 2002) 2.8 1–1.8 2–6 (K. Kato 1997)

42.3–30.2

1.8–7.0

14.7–0.6% strain

(M.C. 2001)

Chang

43.3–49.9

3.75–4.3

1.33–1.99

(M.C. 2001)

Chang

Finite element analysis

According to the study conducted by (S.A. Asgari 2004) the finite element analysis was conducted using 10 noded tetrahedral element. Two types of materials, bone and implant were assigned to the models. Von Mises Stresses were calculated for intact bone, stem less and stemmed structure. Intact bone was considered as control solution for evaluation of two structures. From the analysis it was found that the stem less implant was a perfect fit to the values of strain, but there were certain problems associated with the stem less implants. The femoral neck region of the model had a sudden change in the stress value which might become a 13 | P a g e

source of problem post-operatively and might fail. The sub-trochanteria area also produced high stress concentration at the lateral side which might result in implant displacement or in bone atrophy and consequently bone loss in that area. (Mattei, Di Puccio et al. 2011) describes the main lubrication and wear models of hip implants. Lubrication and wear models have been used to investigate the effect of geometric and material parameters on the lubrication regime and on wear trends, One of the most significant findings that comes out from this study is that, although lubrication and wear are two different aspects of the same tribological scenario, they are modeled completely neglecting each other. Lubrication models do not consider the 3D topography of the articulating surfaces as well as the asperity contact and surface evolution caused by mixed and boundary regimes; on the other hand wear models simulate only dry contact. These limitations underline the difficulty of a realistic theoretical description of the hip implant tribological behavior, increased by the complex model solution, which need to cross use several numerical approaches. In a work by (Habiba Bougherara 2010) the FE analysis of two piece modular and single piece mono block revision hip implants. Analysis was carried out using hip implant alone and hip implant implanted in the femur model. From the analysis it was found that there was a significant amount of stress absorption by the modular implant than in mono block, which suggested that the host femur carried more of the load when the mono block device was employed. A stress concentration on the modular implant occurred at the modular junction which may be susceptible to failure, however no comparable potential failure points were identified on the mono block prosthesis.(S. Gross 2001) studied the use of a hollow stemmed hip implant for reducing the effect of stress shielding. Finite element analysis was used to compare the stresses produced in the various regions of hip implants. The parallel and tapered hollow stems produced an increase in both proximal bone and cement stresses relative to the solid stem. The results showed that the reduced proximal stress shielding can be obtained with stems with hollow sections compared with solid stems. The advantage of hollow stem lies in the greater controllability of rigidity while maintaining an acceptable anatomical fit. The outer shape of the stem can be chosen according to the anatomical criteria and the inner dimensions then adjusted to optimize bone stresses.

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(Chaodi Li 2002) used numerical method to predict the progressive failure of a thick laminated composite femoral component for total hip arthroplasty. Using the CT scan data and IDEAS software the 3-D model of the femur was created. Carbon fiber reinforced polyetheretheketone(C/PEEK) laminated composite was used to fabricate the prosthesis. The model was meshed using high order 20 noded quadratic brick elements and 15 node wedge elements (element types C3D20 and C3D15 in FE software ABAQUS). The critical region limiting fatigue life was identified, using SED to occur in the neck region of the implant. The ply orientation and the stacking sequence has a great significance in the stress distribution (David Bennett 2008) performed the finite element analysis (FEA) of six hip stem design. Forces ranging from 2.5-7 kN was applied. The cross-section which comprised of a circle in the medial end and a square at the lateral end was found to provide suitable design characteristics. Assuming that the implant was made of metal and polyethylene liner the fatigue and wear analyses were performed. CoCrMo, Ti and SS and bone cement was used for the analysis. From the fatigue life studies it was found that Co Cr Mo did not fail under wear conditions until around 20 years, while the SS and Ti64 failed in 11 years (Anthony L. Sabatini 2008) conducted the finite element analysis for stems with various cross sections. Von Mises stress and displacement was recorded at designated locations. Co-CrMo, stainless steel SS 316L and titanium alloy Ti-6Al-4V was used to perform the analysis. Ti6Al-4V exhibited lower stress compared to other two materials. Stem cross section of trapezoid, oval, circular and elliptical were tested for stress distribution. Even distribution of stress was found in elliptical and circular cross section.(Oguz Kayabasi 2006) conducted the FE analysis of four stem shape to design and optimum stem shape. Using the average body weight the static analysis was conducted and dynamic analysis was conducted using a load of walking condition. The stem with a notched geometry produced a better fatigue resistant structure. Ti-6Al-4V was found to be more stable in the analysis (Oguz Kayabasi 2007) conducted the parametric modeling of the newly designed hip implant under body weight load condition. Ti-6Al-4V and cobalt-chromium was used as the material for the hip implant. The stem shape was optimized by numerical shape optimization method. Implicit static analysis was conducted with ANSYS software for femur-bone cement interface, and the bone-cement-implant interface. Bone cement was considered to be a 15 | P a g e

viscoelastic material model. The model consisted of 72,458 elements: 48,205 for the femur, 13,760 for the bone cement and 10,493 for the implant. These were modeled with SOLID45 of ANSYS element library, defined by four nodes each having three degrees of freedom. Fatigue calculations were carried out for both the materials using Goodman, Soderberg, and Gerber fatigue theories and it was found that the best stem shape for fatigue under static and dynamic loading condition was fabricated of Ti-6Al-4V. (Santanu Majumdera 2007) simulated the sideways fall which is the main cause for hip fracture in elders. Finite element model of the pelvis-femur complex was developed using the computed tomography scan. MIMICS was used to produce the solid model after proper thresholding and segmentation. The output was taken in DXF format and was transferred to IGS format with the help of Mechanical Desktop (version 6.0, Auto Desk Inc.) with the help of ANSYS-LS-DYNA software the IGS data was translated and three dimensional (3D) FE model of human pelvis-femur-soft tissue complex was generated and analyzed for various side way falls and accident like conditions (Mohammed Rafiq Abdul-Kadir 2008) did a finite element model of the construct to predict micro motion and instability of femoral stem. The model was correlated with an in vitro micromotion experiment carried out on four cadaver femurs. Using AMIRA software (Mercury Computer Systems, Inc., San Diegao, CA) the 3D model of the hip was constructed. Using Marc.Mentat (MSC.Software, Santa Ana, CA) software the computer model was converted to solid linear tetrahedral element. In this study Marc finite element software package was used. The model was loaded with 3kN at the shoulder of the stem to match the experiment condition. From the study it was found that for achieving good primary fixation and stability of the implant an optimal level of interference fit to be around 50µm.To predict the wear appearing in hip joint (James Shih-Shyn Wu 2003) proposed a computer algorithm using FE analysis based on Archard’s wear law , contact features and analogue wear process. The femoral head was made of stainless steel with an outer diameter of 22mm and the surface roughness was between 0.004 and 0.005µm. The acetabular cup was made of UHMWPE with an interior diameter of 50mm. The wear coefficient of 0.8x10-6 mm3/Nm was taken between hip joint and acetabular cup. Hexahedron brick element was used to mesh the femoral head and the acetabular cup. FORTRAN with 64-but data precision was used to run the program. From the analysis it was found that for a femoral head with a radius of 22 mm had the average yearly wear depth of 0.111

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(mm per year ) and the average wear volume is 42 (mm3 per year) which matched with the clinical measurement and experiment data. (A. Zafer Senalp 2007) did the finite element analysis of four different stem shape of varying curvature. Using ANSYS finite element codes the static, dynamic and the fatigue behavior of the design was analyses. For CAD modeling the stem shape Pro /Engineer was used. ANSYS Workbench software was used for studying the fatigue behavior of the stem shapes. Ti6Al-4V and cobalt-chromium metal materials were used for the stem design and the results were compared with the commonly used Charnley model. The model was meshed using SOLID187 which has a quadratic displacement and is well suited to modeling irregular meshes. The element is defined by 10 nodes having three degrees of freedom at each node. From the analysis it was found that Ti-6Al-4V had the better stability under static and dynamic loading conditions and the stem with notched geometry was the best design for both static and dynamic loading conditions. (E. Pyburn 2004) used different femoral cross section geometries in the presence of bone cement for finite element analysis. It was found that the more force is transferred through the implant with stiffer implant material. It was found that linear recesses and broad lateral areas help transfer more of the load in a compressive manner and reduce implant failure (Achour, Tabeti et al. 2010) used Abaqus 6.5-1 code to model the femoral part. Crack tip was meshed using focused mesh. The plastic and cement layer was defined as isotropic and linearly elastic and bone material was defined as orthotropic and elastic. It was found that opening and shearing propagates

the interfacial crack (cement/bone). In the distal zone the

cement/implant crack propagates by shearing. (Dopico-González, New et al. 2010) used probabilistic finite element analysis to study the effect of implant geometry and femoral characteristics on uncemented hip implant. Mimics was used to generate the geometries of the model. 4- noded tetrahedral element was used to generate the mesh and ANSYS 11.0 was used for the analysis. From the study it was found that the maximum nodal micromotion for the IPS implant was less than 30 μm which helps in osseointegration to take place.(Anderson, Ellis et al. 2010) analyzed the finite element models of hips with rigid bone material assumption and simplified geometry. The bone-cartilage geometry was subject specific, spherical and rotational conchoids. It was found that the subject-specific bone geometry with smoothed articulating cartilage predicted evenly distributed patterns of 17 | P a g e

contact and underestimated pressures. Higher pressures were obtained in models with rigid bones than the subject-specific model with deformable bones. (Jhurani and Fred Higgs Iii 2010) did the finite element analysis of artificial hip joint made of metallic femoral head and ultra-high molecular weight polyethylene (UHMPE) acetabular cup. The wear particle which is generated in the joint was simulated within the lubrication regime. Mathematica programming environment was used to develop the code. The femoral head with acetabular cup for a constant load support by the joint was used for the first part of the code and the second part of the code was developed to simulate the motion of the wear particles within the synovial fluid. It was found that less than 20% of the particles actually collide with the femoral head or cup. (Bouziane, Bachir Bouiadjra et al. 2010) studied the micro cavities in the cement mantle of total hip arthroplasty using the three dimensional finite element method. It was found from the results that the higher stress was obtained at the micro cavity present at the proximal zone of the prosthesis. When the micro cavity was present at the proximal or the distal end the static loading generated higher stress than the dynamic loading and when the microcavity was present at the medial zone then the dynamic loading generated higher amount of stress than the static loading. (Jun and Choi 2010) developed a software system to design a patient specific hip implant. To construct the hip implant the parameters which are required are anatomical femoral axis, femoral shaft isthmus, femoral head center/radius, neck shaft angle, femoral neck, anteversion, head offset length, and canal flare index (CFI). The program was developed in C++ (Pal, Gupta et al. 2010) studied the load transfer and failure mechanisms of short-stem femoral resurfacing component. In this study the stem length was reduced by approximately 50% as compared to the current long-stem design. The loading conditions were normal walking and stair climbing. Shortening the stem length helped in better physiological stress distribution and better bone apposition, and reduced strain concentration in the cancellous bone around the femoral neck–component junction and the bone resorption was considerably less .The short-stem design offers better prospects than the long-stem resurfacing component.(Yang, Zhang et al. 2010) studied the loosening of model cemented joint replacement under cyclic loading condition and the fatigue strength of the bone/cement interface was characterized. Using Digital Image Correlation the loosening of the bone/cement interface was quantified and the specimens were 18 | P a g e

subjected to cyclic shear of 10 million cycles. It was found that with the increase in surface roughness of the bone and the degree of cement inter digitation the fatigue strength increased. (Rothstock, Uhlenbrock et al. 2010) developed the elastic and plastic finite element (FE) models of the proximal femur to study the influence of interference, bone quality and friction on the micromotion during walking and stair-climbing. It was found that at an interference of 30 μm the plastic deformation starts and at 150 μm interference the amount of plastified bone at the interface increases up to 90%. It was found that for a stable situation a 60 μm press-fit and a force of 4 kN allowed bony ingrowth for both constitutive laws (elastic, plastic) for walking and stair climbing.(Waanders, Janssen et al. 2010) investigated how the mechanical response of the cement–bone interface at various load levels in terms of plastic displacement and crack formation. The deformation modes of the cement were ‘only creep’, ‘only damage’ or ‘creep and damage’. It was found that the fatigue cracks which arose at the contact interface and subsequently progressed further into the bulk cement was the major time dependent plastic displacement found at the cement–bone interface. It was found that the increase in stresses in the bone at the interface was due to the fatigue cracking of the cement, while cement creep did not appear to have a considerable effect on bone stresses (Meireles, Completo et al. 2010) quantified the strain shielding effect on the distal femur after patellofemoral arthroplasty because it leads to a reduction of density in bone surrounding the implant. Strain shielding is a mechanical effect occurring in structures combining stiff with more flexible material. The studies were carried out at three activities of daily living: level walking, stair climbing and deep bending at different angles of knee flexion. It was found that with the increase in angle of knee flexion and applied load the occurrence of strain shielding was more significant. (Bah, Nair et al. 2009) used a mesh morphing technique to automate the model generation process. From slices obtained from CT scan data the femur geometry was generated. First a set of points describing the femur in 3D space, appropriate for CAD analysis, was created and the outline curves were exported into I-DEAS to form a 3D volume and then imported into ANSYS 11 ICEM CFD for the analysis. The bone interface region was meshed with 53,743 nodes and 232,404 elements. The rest of the femur was made of 80,522 nodes and 398,605 elements and the implant had 17,937 nodes and 78,482 elements. (Helgason, Pálsson et al. 2009) 19 | P a g e

studied the risk of failure of femoral prosthesis with direct skeletal attachment using finite element analysis. Hollow cylindrical shaft was used to model the femur and the implant was assumed to have a hollow conical shape. The average inner and outer diameter of the femoral shaft were estimated from the CT data using a threshold of HU = 200 and the outer and inner radii were found to be R = 16 mm and r = 8 mm, respectively. Meshing was done using 10 noded tetrahedral solid element. It was found that the implant system had three times greater chance of failure than the intact femur and it was also concluded that the porous coated implant can be beneficial for Osseo integrated fixation than the normal implant. (Dopico-González, New et al. 2009) used probabilistic methods to control the behavior of the implanted femur to determine the performance of the construct. Meshes for both bone and implant were generated using 10 noded tetrahedral elements in ANSYS and the material property was assumed to be elastic, linear and isotropic and the bone was considered homogeneous. The parameters that were considered for the probabilistic modeling are the magnitude of the applied load (P) and the angle between the load and the z-axis (ANGZ), the Young's modulus of the bone (EXB) and the Young's modulus of the prosthesis (EXP). Monte Carlo simulation techniques were applied to provide simulations. From the study it was found that the parameters that most affect the value of the maximum strain are the bone stiffness, followed by the load and the prosthesis stiffness.(Afsharpoya, Barton et al. 2009) investigated the Charnley and C-Stem model for the fixation failure and loosening in cemented total hip prosthesis. Strain developed on the femoral surface was compared in 3 D finite element model and in an in vitro experimental simulation. It was found that proximal de bonding of the cement/bone interface and distal de bonding of the implant/cement interface combined with the heel-strike position applies a high retroversion torque to the femoral stem which increases the strain transfer to the cement that may ultimately lead to the breakdown of the cement mantle leading on to osteolysis and loosening of the prostheses. (Yang, Wei et al. 2009) developed a hip prosthesis model using FEM studies on the already available prosthesis with certain amount of modification the prosthesis was made with a hollow stem and the bore depth is extended to the distal end of the stem. Since the proximal stem has a larger cross-sectional area and higher rigidity the wall thickness is 3 mm below a depth of 80 mm, and gradually becomes 2 mm in the upward direction. In the lower part of the proximal 20 | P a g e

stem region, the cross-sectional area is smaller because of high stress-critical region. The crosssectional area was larger at higher part of proximal stem region because of the stress-safe region. The maximum stress of this final hollow design stem was 371.2 MPa (von Mises stress) and 383.5 MPa (maximum tensile stress).(Mathias, Leahy et al. 1998) studied the finite element analysis on a hip prosthesis with two holes in the shoulders which are used to engage stem introducer. From the FE analysis it was found that there was no unacceptable stress level around the holes. All the three prosthesis satisfied the cyclic mechanical testing of 5 million loading cycles at a peak load of 2.3kN with no evidence of damage. (Gross and Abel 2001) to reduce the effect of stress shielding, a study on the use of hollowed stemmed hip implant was done. The stresses at the proximal femur were calculated for hollowed stem and solid stem with different values of elastic modulus. Stress shielding was reduced in hollow stem. It was found that there in all tapered stem there was an increase in proximal stress in the bone (El'Sheikh, MacDonald et al. 2003) used finite element analysis to optimize the material and design for hip joint prosthesis. The loading condition in static analysis on the head of the femur was taken as 8.7 times the body weight (BW=70 kg) while stumbling which is resolved into Fx=2188.86N, Fy=-669.56N and Fz=5472.1N. It was found that along the medial and lateral side of the hip prosthesis the axial stress was higher in dynamic loading than in static loading. At the distal direction the curves diverges. From the studies it was concluded that dynamic load condition analysis is required to get a close to reality results (El-Sheikh, MacDonald et al. 2002) used the finite element analysis for studying the physiological loading condition on hip joint prosthesis. The fatigue strength was used as the selection criteria for material. The meshing of the cancellous bone was done using 4- noded isoparametric tetrahedral elements and 8 noded isoparametric brick element was used to mesh the rest of the model. It was found that the higher prosthesis stress and lower cement stress is caused due to increasing prosthesis stiffness. At the proximal region maximum cement stresses occur which may cause cracks to propagate in the distal direction.(Kluess, Martin et al. 2007) studied the dislocation of the hip replacement based on the head size. Experimental and numerical studies were carried out on cobalt chromium heads with head diameter of 28,32,36 and 40 mm with four different ultra-high molecular weight (UHMW) polyethylene liners, each liner was 7 mm thick and the 2mm of head inset of acetabular component was used. ABAWUS 21 | P a g e

V 6.4 was used for the finite element modeling. It was found that there was a decrease in contact stresses with the increase in femoral head size. (Bouiadjra, Belarbi et al. 2007) computed the stress intensity factor along the crack front to analyze the fracture behavior of cement of reconstructed acetabulum. It was found that the sliding and tearing effect of the crack located at the free edge of the cement mantle was less risky than the crack propagation by opening effect. When the crack is inclined at 45º the sliding effect is more dominant (Li, Granger et al. 2002) developed the 3 D FE model of the femur using I-DEAS. The prosthesis was fabricated using a carbon fiber-reinforced polyetherehterketone (C/PEEK). The meshing was done using 20 noded quadratic brick elements and 15 noded wedge element. The finite element modeling of the femur/implant was done using FE modeling package I-DEAS. Form the analysis it was found that the fatigue life was limited due to the critical region present at the neck of the implant.

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Chapter 3 Material and Method 23 | P a g e

3. Materials and Methods The two dimensional (2-D) model of the hip implant is shown in Fig 3 was created using CAD software AUTODESK MECHANICAL 2008. The dimensions required for the construction of the model was obtained from the literature (Oguz Kayabasi 2006). To reduce the stress concentration and to increase the high fatigue life of prosthesis, stem shape was considered to be smooth (Oguz Kayabasi 2007; Anthony L. Sabatini 2008). The CAD model was then modeled in ANSYS 13.0.

Fig 2). 2-D model of the implant (all dimensions in mm) The material of construct was chosen as Co-Cr, Ti-6Al-4V, 316 L Stainless Steel (SS), and magnesium alloy AZ91, CF-PEEK, CF-PA 12 and bone. Mechanical properties of the CoCr, Ti-6Al-4V and 316L Stainless Steel (SS) and magnesium alloy AZ91 are listed in table 5. The mechanical properties of the composite and the bone are listed in table 6. A finite element analysis was carried out using ANSYS 13.0 on the three dimensional model of the hip joint. In the present analysis a solid tetrahedral 10node 92 was considered for finite element discretization of the 3 D model as shown in Figure 3 , which provide good accuracy and formed consistent mapped meshes(ANSYS 2010). SOLID92 has a quadratic displacement behavior and is well 24 | P a g e

suited to model irregular meshes (such as produced from various CAD/CAM systems). The element is defined by ten nodes having three degrees of freedom at each node: translations in the nodal x, y, and z directions(ANSYS 2010). The element also has plasticity, creep, swelling, stress stiffening, large deflection, and large strain capabilities. The entire model structure was divided into 6007 elements with 9573 nodes.

Fig 3). Solid 92 Geometry (ANSYS 2010) Table.5 Material Properties of metals used for the implant Material Ti-6Al-4V Cobalt-Chromium 316 L-SS Magnesium AZ91

Young’s modulus (GPa) 110 220 200 41

Poisson ratio (υ) 0.32 0.30 0.30 0.281

Density (kg/mm3) 4.4x10-6 8.5x10-6 7.9x10-6 1.74x10-6

Table 5 depicts the material properties of the metals which are used for designing the implants, Ti-6Al-4V, Cobalt-Chromium and 316 L- Stainless Steel are the most commonly used materials for the design of the hip implants. Magnesium is the new material which is under research for use in implants because of the material properties are near that of the bone (Staiger, Pietak et al. 2006). Table 6 depicts the material properties of the composites and the bone which are used in

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the modeling , composites are the new age materials and has many advantage over the normal metals (K.S. Katti 2001; Fujihara, Huang et al. 2004; Joa˜o F.Mano 2004; I. Sridhar 2010 ). Table.6 Material Properties of composites used for the implants Material Properties

Bone

CF-PEEK

CF-PA

EXX (GPa) EYY (GPa) EZZ (GPa) GXY (GPa) GXZ (GPa) GYZ (GPa) υ XY υ XZ υ YZ

17.9 18.8 22.8 5.71 6.58 7.11 0.26 0.31 0.37

135.3 9.0 9.0 5.2 5.2 1.9 0.34 0.34 0.46

15.4 15.4 3.5 3 3.5 3.2 0.3 0.3 0.3

Fig 4). 3D model and the meshed model Fig 4 shows the volume model which was created using ANSYS 13.0 ,after creating the model the meshing was done using 10 noded tetrahedral solid element . 26 | P a g e

Chapter 4 Results and Discussion 27 | P a g e

4. Results and discussion In this section the results of the static and dynamic analysis are explained. To ensure the safety of the prosthesis, static and dynamic analysis has been done. Static analysis is carried out under body weight dynamic walking load to analyze the suitability of the prosthesis for implantation. Static finite element (FE) analysis was carried out under body weight load and dynamic effect adds up to 10-20% or more loading to the prosthesis (Oguz Kayabasi 2006). For static analysis a load of 3kN (F static) with an angle of 20° is applied on the surface of the implant as shown in the Figure 5 (Oguz Kayabasi 2006). Static loading represents a person of 70kg (El'Sheikh, MacDonald et al. 2003). An abductor muscle load of 1.25kN (F abductor muscle) was applied at an angle of 20° to the proximal area of the greater trochanter. An ilio tibia load of 250 N (F iliotibial-tract) is applied to the bottom of the femur in the longitudinal femur direction. Distal end of the femur was constrained not to move in horizontal direction.

Fig 5). The models for the static and dynamic analysis with the forces acting on them Numerical results for static and dynamic analysis are presented in the following sub sections

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4.1

Static analysis of the model

The static analysis of the hip joint using the seven materials discussed in section 4 has been conducted and the x-component of displacement, y-component of displacement, zcomponent of displacement and von Mises stress have been obtained. If the von Mises stress is less than the tensile strength of the material then the material is said to satisfy the safety condition. Comparing the von Mises stress with the allowable strength of the material used it has been found that all the materials satisfied the safety criteria. The distribution of stresses in the various regions of the hip implant and the displacement are show in the fig 6 to 12 for the seven different materials. If the displacement is more this may cause the displacement or loosening of the hip joint prosthesis.(Anthony L. Sabatini 2008; I. Sridhar 2010 )

Fig 6). The static analysis of 316 L SS model, von mises stress, displacement in x-direction, displacement in y-direction, displacement in z-direction The results of static analysis of 316 L SS model is represented in Fig 6 it is observed that the maximum displacement in the x direction is occurring at the head region which is represented by red color; similarly the maximum displacement in y direction and z direction is also represented in the figure and can be interpreted from the scale. The maximum von mises stress is developed in the region where the abductor muscle load is applied. From the figure it can be seen that the neck region has developed a higher amount of von mises stress of around 140 MPa compared to the rest of the body.

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Fig 7). The static analysis of Ti-6Al-4V model, von mises stress, displacement in x-direction, displacement in y-direction, displacement in z-direction The results of static analysis of Ti-6Al-4V model is represented in Fig 7 it is observed that the x , y and z displacement are maximum at the head of the implant this is mainly because of the load which is being applied at that point . From the figure it can be seen that the neck region has developed a higher amount of von mises stress of around 137 MPa compared to the rest of the body.

Fig 8). The static analysis of Co-Cr model, von mises stress, displacement in x-direction, displacement in y-direction, displacement in z-direction 30 | P a g e

The results of static analysis of Co-Cr model is represented in Fig 8 it is observed that the x , y and z displacement are maximum at the head of the implant this is mainly because of the load which is being applied at that point . From the figure it can be seen that the neck region has developed a higher amount of von mises stress of around 139 MPa compared to the rest of the body.

Fig 9). The static analysis of Magnesium model , von mises stress ,displacement in x-direction , displacement in y-direction, displacement in z-direction The results of static analysis of Magnesium alloy model is represented in Fig 9, from the figure it can be seen that the neck region has developed a higher amount of von mises stress of around 141 MPa compared to the rest of the body.

Fig 10). The static analysis CF-PA 12 model, von mises stress, displacement in x-direction, displacement in y-direction, displacement in z-direction 31 | P a g e

The results of static analysis of CF-PA 12 model is represented in Fig 10 it is observed that the x , y and z displacement are maximum at the head of the implant this is mainly because of the load which is being applied at that point . From the figure it can be seen that the neck region has developed a higher amount of von mises stress of around 132 MPa compared to the rest of the body.

Fig 11). The static analysis CF-PEEK model, von mises stress, displacement in x-direction, displacement in y-direction, displacement in z-direction

The results of static analysis of CF-PEEK model is represented in Fig 11. It is observed from the figure that the neck region has developed a higher amount of von mises stress of around 196 MPa but this region is less than what is observed with other materials

Fig 12). The static analysis bone model, von mises stress, displacement in x-direction, displacement in ydirection, displacement in z-direction 32 | P a g e

The results of static analysis of bone model are represented in Fig 12. It is observed that the x , y and z displacement are maximum at the head of the implant this is mainly because of the load which is being applied at that point . From the figure it can be seen that the neck region has developed a higher amount of von mises stress of around 126 MPa compared to the rest of the body. Table.7 The von Mises Stress and Yield Strength of the metals Material (MPa) Ti-6Al-4V Co-Cr 316 L SS Magnesium AZ91

Von Mises Stress (MPa) 620 627 627 634

Yield Strength 800 720 290 150

From table 7 we can see that von mises stress developed in Ti-6Al-4V and Co-Cr is less than the yield strength of the material. This indicates that the design is safe using these two materials. In 316 L SS and Magnesium AZ91 the yield strength is lower that the von mises stress which indicates that the material will fail under the load condition. The stresses which are developed in the implant can be reduced by changing the design of the hip implant , changes such as removing the extra metal around the region of low stress density. Different alloy of the same material with a higher yield strength can also be used for a better safe design of the implant Table.8 The von Mises Stress developed in composite materials Material Von Mises Stress (MPa) Bone 570 CF PEEK 885 CF PA 12 595 From the table 8 we can see that the von mises stress which is developed in CF-PA 12 is almost near to the value of the stress which is generated in the bone. This shows that the material is safe for use as implant. Since the stress which is developed in CF-PA 12 is almost near to the value of Bone the problem of stress shielding can be reduced which will help in better osseointegration

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4.3

Dynamic analysis of the model

For dynamic analysis a time depended loading as shown in figure was applied on the hip model. The dynamic forces acting on the hip joint was found from (El'Sheikh, MacDonald et al. 2003)

Fig 13). The graph of the various forces acting on the hip joint prosthesis during one gait cycle The dynamic analysis results show the response of the hip joint to dynamic loading condition. Considering the displacement as the model for safety, it can be seen that Magnesium alloy AZ91 has got the maximum displacement; this can lead to extra motion of the implant inside the bone structure and cause the displacement of the prosthesis. Co-Cr and 316 L SS, produced less displacement and hence can be considered to be a safer material for the construction. Young’s modulus determines the displacement of the material and hence the selection of material. From the fig 14 and 15 it can be found that CF PA 12 has the von mises stress which is almost equal to that of the bone , followed by Magnesium alloy AZ 91 , from this observation we can say that CF PA 12 is the ideal material for the construction as said in earlier section.

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700

Von Mises Stress (MPa)

600 500

Ti-6Al-4V 316 L SS Co-Cr magnesium

400 300 200 100 0

0

1

2

3

4

5

time (s)

Fig. 14) The variation of Von Mises stress during dynamic analysis by the metals and bone

1000

Von Mises Stress (MPa)

800

600

400

CF PEEK CF PA Bone

200

0

0

1

2

3

4

5

time (s)

Fig. 15) The variation of Von Mises stress during dynamic analysis by the composite and bone From figure 14 and 15 we can observe that the von mises stress in metals and composites are getting constant after a period of 2 seconds. From figure 14 we can see that the metals have almost the same amount of von mises stresses developed in them. From figure 15 it can be seen that the von mises stresses of bone and CF PA 12 are almost similar which indicates that it will protect the bone from the phenomenon of stress shielding.

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40

Ti-6Al-4V 316 L SS Co-Cr magnesium Bone

Displacement in x direction (mm)

35 30 25 20 15 10 5 0 0

1

2

3

4

5

time (s)

Fig. 16) The variation of x-component of displacement during dynamic analysis by the metals and bone

Displacement in x direction (mm)

70

CF PA CF PEEK Bone

60 50 40 30 20 10 0 0

1

2

3

4

5

time (s)

Fig. 17) The variation of x-component of displacement during dynamic analysis by the composite and bone From figure 16 and 17 it can be seen that the displacement which is produced is following the force applied curve. This shows that the analysis is correct and also it can be observed that the

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displacement produced in bone and CF PA 12 following each other very closely. 30

Ti-6Al-4V 316 L SS Co-Cr Bone magnesium

displacement in y direction (mm)

25

20

15

10

5

0 0

1

2

3

4

5

time (s)

Fig. 18) The variation of y-component of displacement during dynamic analysis by the metals and bone 60 55

CF PA Bone CF PEEK

displacement in y direction (mm)

50 45 40 35 30 25 20 15 10 5 0 -5

0

1

2

3

4

5

time (s)

Fig. 19) The variation of y-component of displacement during dynamic analysis by the composite and bone From figure 18 and 19 it can be seen that the displacement which is produced is following the force applied curve. This shows that the analysis is correct and also it can be observed that the displacement produced in bone and CF PA 12 following each other very closely.

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displacement in z direction (mm)

0

-20

-40

-60

Ti-6Al-4V 316 L SS Co-Cr Bone

-80

-100

0

1

2

3

4

5

Time (s)

Fig. 20) The variation of z-component of displacement during dynamic analysis by the metals and bone

20

displacement in z direction (mm)

0 -20 -40 -60 -80 -100 -120

CF PA CF PEEK Bone

-140 -160 -180 -200

0

1

2

3

4

5

Time (s)

Fig.21) The variation of z-component of displacement during dynamic analysis by the composites and bone From figure 20 and 21 it can be seen that the displacement which is produced is following the force applied curve. This shows that the analysis is correct and also it can be observed that the displacement produced in bone and CF PA 12 following each other very closely.

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From the dynamic analysis we can see that the von mises stresses which are developed in the implant is less than the yield strength of Ti-6Al-4V and Co-Cr alloy which shows that the material will be safe under the loading condition. It is also observed that the von mises stress developed in the Magnesium AZ 91 and 316 L SS is greater than the yield strength of the material and hence the material will fail under the dynamic loading condition. The von mises stresses developed in CF PA 12 is almost similar to the bone which is a good indicator that the material is an ideal candidate for hip joint prosthesis because it will prevent the phenomenon of stress shielding which is a major cause of hip failure. From figure 16 to 21 we can see the displacement of the implant in x, y and z direction using different materials. Here from the graph we can observe that the metal implants have lower amount of displacement compared to the bone and composites. If the amount of displacement is high it may cause the loosening of the implant and also can cause the implant to come out of the joint. This can cause major problem to the patient and also is a major cause for the failure of the implant. From the graph it can be observed that the displacement of metals is less than that of the bone. The reduced displacement will cause the mismatch in the movement of the implant and the bone and can cause the failure of the implant. Composite and bones have an almost similar displacement graph. It can be observed that the displacement produced in bones and CF PA 12 are almost similar which will help in proper flexibility and movement of the implant in the bone and also avoid the complications of failure

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Chapter 5 Conclusion and Future Work

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5. Conclusion The static and dynamic analysis of the hip joint model has been done using finite element method. From the results and discussions the following conclusions can be made 1. The best material for the construction of the hip implant is CF PA 12 because of the value of von misses stress which is generated is almost equivalent to that which is developed in bone 2. CF PA 12 is light weight and is biocompatible and hence is the ideal material for the construction of prosthesis 3. It was found that 316 L SS and magnesium alloy AZ91 failed because of the von mises stress which was developed was more than the yield strength of the material. This can be overcome by changing the design of the prosthesis or incorporating an alloy of better properties 4. Magnesium alloy AZ91 gets corroded in the body environment and is not an good option for load bearing implants, but these can be a good option for implants which requires removal after surgery such as bone plates and nails etc. 5. The CF PEEK is also an option because of it low cost and ease of fabrication. The material properties of the composites can be easily changed by varying the ply angle and composition of the composites. 6. By using composite materials we can avoid the stress shielding and have more life for the implant. 5.1 Future work Much of the research work is needed in the development and analysis of newer materials for the construction of the models, materials such as composites and functionally graded materials are to be studied for the biocompatibility and the mechanical properties. Using MIMICS software the modeling of the hip joint can be done and hence more studies is required in that field of modeling. The models have to be tested for fatigue analysis and also for wear rate measurement. 41 | P a g e

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