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Wear Simulation of Artificial Hip Joints: Effect of Materials
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 18 (2019) 3867–3875
www.materialstoday.com/proceedings
ICMPC-2019
Wear Simulation of Artificial Hip Joints: Effect of Materials Vivek Jangida, Abhishek Kumar Singha, Abhishek Mishraa* a
Department of Mechanical Engineering, National Institute of Technology Delhi, Sector A-7, NarelaAbhishek Mishra, Delhi – 110040, India
Abstract In present work, the artificial hip joint implant is simulated in finite element code ANSYS 18.0. The finite element analysis is done to evaluate the stresses and sliding distance for different set of materials. A bipolar model is used which consists of backing, acetabular cup and femoral head. The wear is calculated using the modified Archard’s wear law. The results are obtained for different body weights. The results show that wear is least when Co-Cr alloy is used for backing and femoral head, and ultra high molecular weight polyethylene (UHMWPE) is used for the acetabular cup. Also, comparative study is done to observe the effects of nodal force and normal pressure applied on backing of the implant. Simulation results indicate that wear volume obtained for nodal force as load condition is more than obtained for normal pressure as load condition. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: Artificial hip joint implant; bipolar model; wear simulation; effect of materials
1. Introduction Hip joint is a ball and socket synovial joint formed between acetabulum and femur in the pelvis. Hip joint is having high capability of transmitting static as well as dynamic loads. Hip joint is one of the most important joints in the human body. It allows us to perform a number of activities i.e. standing, walking, running, jumping, sitting, climbing up-stairs, going down-stairs, bending. Need of the artificial hip joint implant came into existence because of the need to replace the diseased joint. The diseased joint involves the fractured joint, absence of the synovial fluid in the joint, damaged acetabular cup and damaged femoral head [1]. Synovial fluid acts as lubricant between femoral head and acetabulum. Total hip replacement (THR) is done to patients suffering from diseased or deformed hip joint, in order to relieve the joint pain [2].
* Corresponding author. Tel.: +91-11-33861212; fax: +91-11-27787503. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
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THR is a surgical procedure in which the diseased hip joint is replaced surgically by artificial hip joint which is made of biocompatible material having shape and functionality similar to that of natural hip joint of human body [2]. Successful THR involves correct operative technique used for the fixation of the parts, choice of the biocompatible materials and design of the parts of the artificial hip joint implants ensuring safety. On other hand, it is impossible to avoid the failure of the artificial replaced part over short or long period of time when exposed to various day-to-day activities [3]. Components of the artificial hip joint implant are shown in the Figure 1.
Figure 1. Anatomy of artificial hip joint implant. A number of experiments for recording the forces acting on hip joints during different activities have been performed as found in literature [4-7]. The investigations revealed that the stresses induced in the bones and the implants were directly influenced by magnitude and direction of forces acting on the implant. The magnitude and direction of the forces vary with kind of activities performed by human beings. An analytical model for estimating the natural biological variations in muscles forces was developed [8]. The effects of these variations were observed on hip forces. The anatomical parameters of north Indian cadaveric hip joint of 54 people belonging to age group of 50-70 years were measured with the help of vernier caliper as found in literature [9]. Computational model to calculate wear of hip joint and to measure the temperature of surrounding zone was developed and, heat and contact problems were solved numerically using finite element code ANSYS [10]. The effect of stresses and environment on hip implant was studied [11]. They identified that stress dependent electrochemical mechanism was one of the key mechanisms in governing the degradation of surface in fretting and crevice corrosion of biomedical implants of joints of human body. A geometrical method for measuring the wear of hip joint was developed which evaluates wear directly from the worn out surface [12]. Finite element model for wear-fatigue analysis of modular hip joints using Co–28Cr– 6Mo/DMLS Ti–6Al–4V and Co–28Cr–6Mo/forged Ti–6Al–4V as the material of head and steam was developed in ABAQUS [13]. The wear resistance and hardness of DMLS Ti–6Al–4V were obtained superior compared to those of forged Ti–6Al–4V. A study was carried out on challenges related to biomaterials of artificial hip joints. It was investigated that hard-on-hard, and hard-on-soft hip bearings are quite effective for having low degrees of wear [14]. The polycrystalline diamond (PCD)-on-PCD has low wear progression [15]. Finite element analysis of the artificial hip joint movement during human activities using finite element code ABAQUS revealed that possibility of prosthetic impingement depends on the activities performed by human beings [16]. Artificial hip joint wear was evaluated using coordinate measuring machine (CMM) technique by increasing the number of scanning points on the retrieved ex-vivo implants and were compared with the in-vivo radiographic measurements (RSA) [17]. It was observed that the results obtained by using CMM and RSA measurements were close to each other but the wears obtained by applying the mathematical formulae were overestimated. In present work study and simulation of
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artificial hip joint implant was performed to evaluate wear for different material combinations based on material properties obtained from literature. Wear was observed under the effect of loading conditions i.e. applying nodal force and normal pressure on the backing of the artificial hip joint implant. 2. Finite Element Analysis of Artificial Hip Joints 2.1. Modeling of Artificial Hip Joint A geometric model of artificial hip joint implant was developed using the dimensions as mentioned in Table 1 based on the range obtained from various literature [9, 16, 18]. No clearance was considered in between the parts in contact of the artificial hip joint implant. Figure 1 shows the geometric model of the artificial hip joint implant made using CAD software. Table 1. Parts of the artificial hip joint with dimensions. Dimensions Part Inner radius Outer radius Backing Acetabular cup Femoral head
23 mm 16 mm 0 mm
26 mm 23 mm 16 mm
2.2. Materials Selection The requirement for material selection for total hip replacement is bio-compatibility. The material is chosen such that the debris formed due to wear of the artificial hip joint implant parts should not cause any harm to the tissues of human body [19]. The material should be corrosion resistant as well. Most commonly used metallic biomaterials for artificial implants include titanium, cobalt, magnesium and stainless steel based alloys [20]. Hydroxyapatite is considered as one of the ceramic materials used for coating the metallic biomaterials. Common bio-compatible materials for artificial hip joint implants with their properties are given in Table 2 used in present study. Properties
Table 2. Properties of bio-compatible materials [21, 22, 23]. Co-Cr alloy Ti6Al4V Cortical Bone
Modulus of elasticity (GPa) Density (gm/cc) Poisson’s Ratio
225 8.3 -
110 4.5 0.33
12.4 1.7 0.22
UHMWPE 0.725 0.95 0.45
2.3. Mesh A bipolar model was used in the analysis part consisting femoral head, acetabular cup and backing [24]. Meshing of model was done in commercial code ANSYS 18.0 using tetrahedral 10 node (Tet10) elements. The element names were SOLID187 for solid parts of hip joint assembly. CONTA174 and TARGE170 elements were used to define contact surfaces. Mesh physics preference was chosen as mechanical, size function was adaptive. Meshed model of artificial hip joint implant using above mentioned elements is shown in Figure 2. 2.4. Boundary and Load Conditions Finite element analysis of artificial hip joint implant was done in the “Stand-up” condition with different body weights. The maximum load applied on hip joint during stand-up condition was 190% of body weight [7]. Contacts behavior of the hip joint parts was assumed as symmetric. Augmented Lagrange method was used as the contact algorithm. Ramped loads were applied on the top node in negative z-direction of backing within time duration of one second. The bottom surface of femoral head was constrained with all degrees of freedom.
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Figure 2. Meshed model of artificial hip joint implant. 3. Results and Discussions 3.1. Simulation for Different Materials for Nodal Force Condition In this section, nodal force was applied on top node of backing in negative z-direction. Six combinations were considered for analysis. The cases indicated in Table 3 were classified under two categories: hard-on-hard and hardon-soft parts of hip joint implants. Cases 3, 4 and 6 were considered as hard-on-hard and cases 1, 2 and 5 were considered as hard-on-soft. Materials selected for different cases of artificial hip joint implants parts are given in Table 3. Table 3. Materials selected for different parts. Cases Material of backing Material of acetabular Material of femoral head cup Case 1 Case 2 Case 3 Case 4 Case 5 Case 6
Ti6Al4V Co-Cr alloy Ti6Al4V Ti6Al4V Cortical bone Ti6Al4V
UHMWPE UHMWPE Co-Cr alloy Cortical bone UHMWPE Ti6Al4V
Ti6Al4V Co-Cr alloy Ti6Al4V Ti6Al4V Cortical bone Ti6Al4V
The distribution of contact stress for case 1 is shown in Figure 3. The maximum stress value was 4.241 MPa at the central region of acetabular cup at top surface, and the minimum stress value was -0.134 MPa at the circumference of outer and inner surfaces of acetabular cup. It indicates that the central region of acetabular cup remains under tension whereas circumferential region of acetabular cup is under compression in standing position. Results indicate that the range of contact stresses vary from case to case whereas the pattern of contact stress was same for all cases considered. The variation of contact stress for different cases is shown in Figure 4. It can be observed that the contact stress was relatively high when hard-on-hard hip joint parts were used. Whereas the contact stress was relatively low when hard-on-soft hip joint parts were used. For all the cases, contact stress varies linearly with respect to body weight and increase in stress is proportional to increase in body weight. Maximum contact stress was observed for case 3 and minimum contact stress was for case 2 as indicated in Table 3. Contact stress obtained for the set of materials used in cases 1 and 2 are more accurate when compared to contact stress obtained when cortical bone was selected as material.
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(a) Inner surface view (b) Outer surface view Figure 3. Example figure of distribution of contact stress in surfaces of acetabular cup for case 1 (Max. contact stress 4.241 MPa and min. contact stress -0.134 MPa). Figure 5 shows the variation of sliding distance with respect to body weight. The sliding distance is relatively more for hard-on-soft hip joint parts, whereas it is relatively less for hard-on-hard hip joint parts. Sliding distance for cases 1, 4 and 5 varies linearly with respect to body weight, whereas in cases 2, 3 and 6 it is non-linear. Sliding distance obtained for cases 1 and 2 is similar with the sliding distance obtained when cortical bone used as material.
Figure 4. Contact stress with respect to body weight.
Figure 5. Sliding distance with respect to body weight.
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3.2. Wear Analysis of Artificial Hip Joint Modified Archard’s wear law was used for evaluation of wear volume of acetabular cup [25]. It gives wear rate per cycle. It is basically used for the evaluation of the wear in abrasion and adhesion. The actual measurement of hip joint implant wear is a challenge. According to the modified Archard’s wear law: V=KwσcASL (1) where, V = Volume wear (mm3) Kw = Wear coefficient (mm3/N-m) σc = Contact stress in MPa A = Contact area (mm2) SL = Sliding distance (mm) Kw is the exponential function of surface roughness (Ra) [26] Kw = 0.235 * 10-4 * Ra2.03 (2) Considering 10,000 cycles per day, surface roughness value of femoral as 0.7 µm, radius of femoral as 16 mm, the contact area as 1608.5 mm2, the wear volume per year was calculated using: V=KwσcASL * 10000 * 365 * 10-3 (mm3/year) (3) Variation of wear volume for different cases selected is shown in Figure 6. The results show that in stand-up condition maximum wear occur in case 5 i.e. when the material of backing and femoral head was cortical bone, and that of acetabular cup was UHMWPE. The wear volume was least when the material of backing and femoral head was Co-Cr alloy and that of backing was UHMWPE. It was observed that wear volume is relatively lesser for hardon-soft parts compared to hard-on-hard parts of artificial hip joint implants. It indicates that for all the cases except case 5, wear volume obtained is lesser; which means materials used in those five cases have more durability compared to cortical bone.
Figure 6. Variation of wear volume with respect to body weight.
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3.3. Comparative Study between Nodal Force and Normal Pressure Condition In this section, case 1 was considered for study. Normal pressure was applied on backing surface of artificial hip joint implant. The pressure was obtained by considering the actual applied load on implant and the outer surface area of backing. Figure 7 shows the contact stress distribution under normal pressure condition. The stress obtained was comparatively less than those which were obtained under nodal force condition for the same case 1. Though the maximum contact stress was induced at the central region of outer surface of acetabular cup and the minimum contact stress was obtained at circumferential region of outer and inner surfaces of acetabular cup. Some regions of relatively high contact stresses were also observed at the inner circumferential surface of acetabular cup. Figure 8 shows the comparison between the wear volumes obtained under normal pressure condition and nodal force condition for set of materials used in case 1.
(a) Inner surface view (b) Outer surface view Figure 7. Contact stress distribution on surfaces of acetabular cup under normal pressure condition (Max. contact stress 0.845 MPa and min. contact stress 0.103 MPa). From the results obtained for the standing condition, it can be concluded that the wear volume for two different loading conditions as mentioned above have significant difference. For the same materials and body weight of 860 N the wear volume under nodal force condition was 3.885mm3/year, whereas the wear volume under normal pressure condition was 0.48654mm3/year. Under the nodal force condition, the rate of increase of wear volume was more than that of under normal pressure condition with respect to increase in body weight. Also, for the nodal force condition variation of wear volume is linear, whereas it is non-linear for normal pressure condition for the set of materials used in case 1.
Figure 8. Wear volume for case 1
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4. Conclusions Simulations of artificial hip joint implant based on different material combinations were performed and results were compared in present study. It can be concluded that for femoral head and backing, Co-Cr alloy in combination with UHMWPE material for acetabular cup gives least wear volume for different body weights as discussed in results. The other suitable material for the femoral head and backing is Titanium alloy as it has slightly higher wear volume than Co-Cr alloy when used in combination with UHMWPE material for acetabular cup. It was observed that wear volume obtained in nodal force condition was significantly higher than that of in normal pressure condition for the same case i.e. material for acetabular cup and backing was Titanium alloy and that of femoral head was UHMWPE. The present work presents the comparative numerical study which is useful for selection of alloys for artificial hip joint implants for better performance. Acknowledgements Authors would like to thank the National Institute of Technology Delhi for providing the facilities required. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
M. Nordin, V.H. Frankel (Eds.), Basic biomechanics of the musculoskeletal system, Lippincott Williams & Wilkins, 2001. Z.M. Jin, J. Fisher, E.Ingham, Handbook of Lubrication and Tribology, CRC Press, 2006. D. Dowson, C. M. McNie, A. A. J. Goldsmith, Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science 214.1 (2000) 75-86. G. Bergmann, F. Graichen, A. Rohlmann, Journal of biomechanics 26.8 (1993) 969-990. G. Bergmann, H. Kniggendorf, F. Graichen, A. Rohlmann, Influence of shoes and heel strike on the loading of the hip joint, Journal of biomechanics 28.7 (1995) 817-827. G. Bergmann, F. Graichen, A. Rohlmann, H. Linke, Hip joint forces during load carrying, Clinical orthopaedics and related research 335 (1997) 190-201. G. Bergmann, G. Deuretzbacher, M. Heller, F. Graichen, A. Rohlmann, J. Strauss, G.N. Duda, Hip contact forces and gait patterns from routine activities, Journal of biomechanics 34.7 (2001) 859-71. D.E. Hurwitz, K.C. Foucher, T.P. Andriacchi, A new parametric approach for modeling hip forces during gait, Journal of biomechanics 36.1 (2003) 113-119. R. Chauhan, S. Paul, B.K. Dhaon, Anatomical parameters of North Indian hip joints: cadaveric study, J Anat Soc India 51.1 (2002) 39-42. J.C. Fialho, P.R. Fernandes, L.Eça, J. Folgado, Computational hip joint simulator for wear and heat generation, Journal of Biomechanics 40.11 (2007) 2358-2366. A. Chandra, J.J. Ryu, P. Karra P, P. Shrotriya, V. Tvergaard, M. Gaisser, T. Weik, Life expectancy of modular Ti6Al4V hip implants: influence of stress and environment, Journal of the mechanical behavior of biomedical materials 4.8 (2011) 1990-2001. X.Q. Hu, C.M. Han, R.J.K. Wood, A geometric method to measure the wear of hip joint, Wear 271.11 (2011) 2775-81. T. Zhang, N.M. Harrison, P.F. McDonnell, P.E. McHugh, S.B. Leen, A finite element methodology for wear–fatigue analysis for modular hip implants, Tribology International. 65 (2013) 113-127. G. Pezzotti, K..Yamamoto, Artificial hip joints: The biomaterials challenge, Journal of the mechanical behavior of biomedical materials 31 (2014) 3-20. M.S. Uddin, L.C. Zhang, Predicting the wear of hard-on-hard hip joint prostheses, Wear 301.1 (2013) 192-200. E. Saputra, I.B. Anwar, J. Jamari, E.van der Heide, Finite element analysis of artificial hip joint movement during human activities, Procedia engineering 68 (2013) 102-108. M.S. Uddin, C.Y.E. Mak, S.A. Callary, Evaluating hip implant wear measurements by CMM technique, Wear 364 (2016) 193-200. Y. Jun, K. Choi, Design of patient-specific hip implants based on the 3D geometry of the human femur, Advances in Engineering Software 41.4 (2010) 537-547. E. Ingham, J. Fisher, Biological reactions to wear debris in total joint replacement, Proceedings of the Institution of Mechanical Engineers Part H: Journal of Engineering in Medicine 214.1 (2000) 21-37. W.S.W. Harun, R.I.M. Asri, J. Alias, F.H. Zulkifli, K. Kadirgama, S.A. Ghani, J.H.M. Shariffuddin, A comprehensive review of hydroxyapatite-based coatings adhesion on metallic biomaterials, Ceramics International 44.2 (2017) 1250-1268. K.S. Katti, Biomaterials in total joint replacement, Colloids and Surfaces B: Biointerfaces 39.3 (2004) 133-142. S. Zameer, M. Haneef, Fatigue Life Estimation of Artificial Hip Joint Model Using Finite Element Method, Materials Today: Proceedings 2.4-5 (2015) 2137-2145.
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S.M. Kurtz, UHMWPE biomaterials handbook: ultra high molecular weight polyethylene in total joint replacement and medical devices, second ed., Academic Press, Burlington, Massachusetts, 2009. Q. Chang , S. Liu, C. Guan, F. Yu, S. Wu, C. Jiang, Bipolar hip arthroplasty, The Journal of arthroplasty 26.8 (2011) 1455-1459. J.S.S. Wu, J.P. Hung, C.S. Shu, J.H. Chen , The computer simulation of wear behavior appearing in total hip prosthesis, Computer Methods and Programs in Biomedicine 70.1 (2003) 81-91. S.H. Teoh, W.H. Chan, R. Thampuran, An elasto-plastic finite element model for polyethylene wear in total hip arthroplasty, Journal of Biomechanics 35.3 (2002) 323-330.
ScienceDirect Materials Today: Proceedings 18 (2019) 3867–3875
www.materialstoday.com/proceedings
ICMPC-2019
Wear Simulation of Artificial Hip Joints: Effect of Materials Vivek Jangida, Abhishek Kumar Singha, Abhishek Mishraa* a
Department of Mechanical Engineering, National Institute of Technology Delhi, Sector A-7, NarelaAbhishek Mishra, Delhi – 110040, India
Abstract In present work, the artificial hip joint implant is simulated in finite element code ANSYS 18.0. The finite element analysis is done to evaluate the stresses and sliding distance for different set of materials. A bipolar model is used which consists of backing, acetabular cup and femoral head. The wear is calculated using the modified Archard’s wear law. The results are obtained for different body weights. The results show that wear is least when Co-Cr alloy is used for backing and femoral head, and ultra high molecular weight polyethylene (UHMWPE) is used for the acetabular cup. Also, comparative study is done to observe the effects of nodal force and normal pressure applied on backing of the implant. Simulation results indicate that wear volume obtained for nodal force as load condition is more than obtained for normal pressure as load condition. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: Artificial hip joint implant; bipolar model; wear simulation; effect of materials
1. Introduction Hip joint is a ball and socket synovial joint formed between acetabulum and femur in the pelvis. Hip joint is having high capability of transmitting static as well as dynamic loads. Hip joint is one of the most important joints in the human body. It allows us to perform a number of activities i.e. standing, walking, running, jumping, sitting, climbing up-stairs, going down-stairs, bending. Need of the artificial hip joint implant came into existence because of the need to replace the diseased joint. The diseased joint involves the fractured joint, absence of the synovial fluid in the joint, damaged acetabular cup and damaged femoral head [1]. Synovial fluid acts as lubricant between femoral head and acetabulum. Total hip replacement (THR) is done to patients suffering from diseased or deformed hip joint, in order to relieve the joint pain [2].
* Corresponding author. Tel.: +91-11-33861212; fax: +91-11-27787503. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
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THR is a surgical procedure in which the diseased hip joint is replaced surgically by artificial hip joint which is made of biocompatible material having shape and functionality similar to that of natural hip joint of human body [2]. Successful THR involves correct operative technique used for the fixation of the parts, choice of the biocompatible materials and design of the parts of the artificial hip joint implants ensuring safety. On other hand, it is impossible to avoid the failure of the artificial replaced part over short or long period of time when exposed to various day-to-day activities [3]. Components of the artificial hip joint implant are shown in the Figure 1.
Figure 1. Anatomy of artificial hip joint implant. A number of experiments for recording the forces acting on hip joints during different activities have been performed as found in literature [4-7]. The investigations revealed that the stresses induced in the bones and the implants were directly influenced by magnitude and direction of forces acting on the implant. The magnitude and direction of the forces vary with kind of activities performed by human beings. An analytical model for estimating the natural biological variations in muscles forces was developed [8]. The effects of these variations were observed on hip forces. The anatomical parameters of north Indian cadaveric hip joint of 54 people belonging to age group of 50-70 years were measured with the help of vernier caliper as found in literature [9]. Computational model to calculate wear of hip joint and to measure the temperature of surrounding zone was developed and, heat and contact problems were solved numerically using finite element code ANSYS [10]. The effect of stresses and environment on hip implant was studied [11]. They identified that stress dependent electrochemical mechanism was one of the key mechanisms in governing the degradation of surface in fretting and crevice corrosion of biomedical implants of joints of human body. A geometrical method for measuring the wear of hip joint was developed which evaluates wear directly from the worn out surface [12]. Finite element model for wear-fatigue analysis of modular hip joints using Co–28Cr– 6Mo/DMLS Ti–6Al–4V and Co–28Cr–6Mo/forged Ti–6Al–4V as the material of head and steam was developed in ABAQUS [13]. The wear resistance and hardness of DMLS Ti–6Al–4V were obtained superior compared to those of forged Ti–6Al–4V. A study was carried out on challenges related to biomaterials of artificial hip joints. It was investigated that hard-on-hard, and hard-on-soft hip bearings are quite effective for having low degrees of wear [14]. The polycrystalline diamond (PCD)-on-PCD has low wear progression [15]. Finite element analysis of the artificial hip joint movement during human activities using finite element code ABAQUS revealed that possibility of prosthetic impingement depends on the activities performed by human beings [16]. Artificial hip joint wear was evaluated using coordinate measuring machine (CMM) technique by increasing the number of scanning points on the retrieved ex-vivo implants and were compared with the in-vivo radiographic measurements (RSA) [17]. It was observed that the results obtained by using CMM and RSA measurements were close to each other but the wears obtained by applying the mathematical formulae were overestimated. In present work study and simulation of
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artificial hip joint implant was performed to evaluate wear for different material combinations based on material properties obtained from literature. Wear was observed under the effect of loading conditions i.e. applying nodal force and normal pressure on the backing of the artificial hip joint implant. 2. Finite Element Analysis of Artificial Hip Joints 2.1. Modeling of Artificial Hip Joint A geometric model of artificial hip joint implant was developed using the dimensions as mentioned in Table 1 based on the range obtained from various literature [9, 16, 18]. No clearance was considered in between the parts in contact of the artificial hip joint implant. Figure 1 shows the geometric model of the artificial hip joint implant made using CAD software. Table 1. Parts of the artificial hip joint with dimensions. Dimensions Part Inner radius Outer radius Backing Acetabular cup Femoral head
23 mm 16 mm 0 mm
26 mm 23 mm 16 mm
2.2. Materials Selection The requirement for material selection for total hip replacement is bio-compatibility. The material is chosen such that the debris formed due to wear of the artificial hip joint implant parts should not cause any harm to the tissues of human body [19]. The material should be corrosion resistant as well. Most commonly used metallic biomaterials for artificial implants include titanium, cobalt, magnesium and stainless steel based alloys [20]. Hydroxyapatite is considered as one of the ceramic materials used for coating the metallic biomaterials. Common bio-compatible materials for artificial hip joint implants with their properties are given in Table 2 used in present study. Properties
Table 2. Properties of bio-compatible materials [21, 22, 23]. Co-Cr alloy Ti6Al4V Cortical Bone
Modulus of elasticity (GPa) Density (gm/cc) Poisson’s Ratio
225 8.3 -
110 4.5 0.33
12.4 1.7 0.22
UHMWPE 0.725 0.95 0.45
2.3. Mesh A bipolar model was used in the analysis part consisting femoral head, acetabular cup and backing [24]. Meshing of model was done in commercial code ANSYS 18.0 using tetrahedral 10 node (Tet10) elements. The element names were SOLID187 for solid parts of hip joint assembly. CONTA174 and TARGE170 elements were used to define contact surfaces. Mesh physics preference was chosen as mechanical, size function was adaptive. Meshed model of artificial hip joint implant using above mentioned elements is shown in Figure 2. 2.4. Boundary and Load Conditions Finite element analysis of artificial hip joint implant was done in the “Stand-up” condition with different body weights. The maximum load applied on hip joint during stand-up condition was 190% of body weight [7]. Contacts behavior of the hip joint parts was assumed as symmetric. Augmented Lagrange method was used as the contact algorithm. Ramped loads were applied on the top node in negative z-direction of backing within time duration of one second. The bottom surface of femoral head was constrained with all degrees of freedom.
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Figure 2. Meshed model of artificial hip joint implant. 3. Results and Discussions 3.1. Simulation for Different Materials for Nodal Force Condition In this section, nodal force was applied on top node of backing in negative z-direction. Six combinations were considered for analysis. The cases indicated in Table 3 were classified under two categories: hard-on-hard and hardon-soft parts of hip joint implants. Cases 3, 4 and 6 were considered as hard-on-hard and cases 1, 2 and 5 were considered as hard-on-soft. Materials selected for different cases of artificial hip joint implants parts are given in Table 3. Table 3. Materials selected for different parts. Cases Material of backing Material of acetabular Material of femoral head cup Case 1 Case 2 Case 3 Case 4 Case 5 Case 6
Ti6Al4V Co-Cr alloy Ti6Al4V Ti6Al4V Cortical bone Ti6Al4V
UHMWPE UHMWPE Co-Cr alloy Cortical bone UHMWPE Ti6Al4V
Ti6Al4V Co-Cr alloy Ti6Al4V Ti6Al4V Cortical bone Ti6Al4V
The distribution of contact stress for case 1 is shown in Figure 3. The maximum stress value was 4.241 MPa at the central region of acetabular cup at top surface, and the minimum stress value was -0.134 MPa at the circumference of outer and inner surfaces of acetabular cup. It indicates that the central region of acetabular cup remains under tension whereas circumferential region of acetabular cup is under compression in standing position. Results indicate that the range of contact stresses vary from case to case whereas the pattern of contact stress was same for all cases considered. The variation of contact stress for different cases is shown in Figure 4. It can be observed that the contact stress was relatively high when hard-on-hard hip joint parts were used. Whereas the contact stress was relatively low when hard-on-soft hip joint parts were used. For all the cases, contact stress varies linearly with respect to body weight and increase in stress is proportional to increase in body weight. Maximum contact stress was observed for case 3 and minimum contact stress was for case 2 as indicated in Table 3. Contact stress obtained for the set of materials used in cases 1 and 2 are more accurate when compared to contact stress obtained when cortical bone was selected as material.
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(a) Inner surface view (b) Outer surface view Figure 3. Example figure of distribution of contact stress in surfaces of acetabular cup for case 1 (Max. contact stress 4.241 MPa and min. contact stress -0.134 MPa). Figure 5 shows the variation of sliding distance with respect to body weight. The sliding distance is relatively more for hard-on-soft hip joint parts, whereas it is relatively less for hard-on-hard hip joint parts. Sliding distance for cases 1, 4 and 5 varies linearly with respect to body weight, whereas in cases 2, 3 and 6 it is non-linear. Sliding distance obtained for cases 1 and 2 is similar with the sliding distance obtained when cortical bone used as material.
Figure 4. Contact stress with respect to body weight.
Figure 5. Sliding distance with respect to body weight.
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3.2. Wear Analysis of Artificial Hip Joint Modified Archard’s wear law was used for evaluation of wear volume of acetabular cup [25]. It gives wear rate per cycle. It is basically used for the evaluation of the wear in abrasion and adhesion. The actual measurement of hip joint implant wear is a challenge. According to the modified Archard’s wear law: V=KwσcASL (1) where, V = Volume wear (mm3) Kw = Wear coefficient (mm3/N-m) σc = Contact stress in MPa A = Contact area (mm2) SL = Sliding distance (mm) Kw is the exponential function of surface roughness (Ra) [26] Kw = 0.235 * 10-4 * Ra2.03 (2) Considering 10,000 cycles per day, surface roughness value of femoral as 0.7 µm, radius of femoral as 16 mm, the contact area as 1608.5 mm2, the wear volume per year was calculated using: V=KwσcASL * 10000 * 365 * 10-3 (mm3/year) (3) Variation of wear volume for different cases selected is shown in Figure 6. The results show that in stand-up condition maximum wear occur in case 5 i.e. when the material of backing and femoral head was cortical bone, and that of acetabular cup was UHMWPE. The wear volume was least when the material of backing and femoral head was Co-Cr alloy and that of backing was UHMWPE. It was observed that wear volume is relatively lesser for hardon-soft parts compared to hard-on-hard parts of artificial hip joint implants. It indicates that for all the cases except case 5, wear volume obtained is lesser; which means materials used in those five cases have more durability compared to cortical bone.
Figure 6. Variation of wear volume with respect to body weight.
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3.3. Comparative Study between Nodal Force and Normal Pressure Condition In this section, case 1 was considered for study. Normal pressure was applied on backing surface of artificial hip joint implant. The pressure was obtained by considering the actual applied load on implant and the outer surface area of backing. Figure 7 shows the contact stress distribution under normal pressure condition. The stress obtained was comparatively less than those which were obtained under nodal force condition for the same case 1. Though the maximum contact stress was induced at the central region of outer surface of acetabular cup and the minimum contact stress was obtained at circumferential region of outer and inner surfaces of acetabular cup. Some regions of relatively high contact stresses were also observed at the inner circumferential surface of acetabular cup. Figure 8 shows the comparison between the wear volumes obtained under normal pressure condition and nodal force condition for set of materials used in case 1.
(a) Inner surface view (b) Outer surface view Figure 7. Contact stress distribution on surfaces of acetabular cup under normal pressure condition (Max. contact stress 0.845 MPa and min. contact stress 0.103 MPa). From the results obtained for the standing condition, it can be concluded that the wear volume for two different loading conditions as mentioned above have significant difference. For the same materials and body weight of 860 N the wear volume under nodal force condition was 3.885mm3/year, whereas the wear volume under normal pressure condition was 0.48654mm3/year. Under the nodal force condition, the rate of increase of wear volume was more than that of under normal pressure condition with respect to increase in body weight. Also, for the nodal force condition variation of wear volume is linear, whereas it is non-linear for normal pressure condition for the set of materials used in case 1.
Figure 8. Wear volume for case 1
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4. Conclusions Simulations of artificial hip joint implant based on different material combinations were performed and results were compared in present study. It can be concluded that for femoral head and backing, Co-Cr alloy in combination with UHMWPE material for acetabular cup gives least wear volume for different body weights as discussed in results. The other suitable material for the femoral head and backing is Titanium alloy as it has slightly higher wear volume than Co-Cr alloy when used in combination with UHMWPE material for acetabular cup. It was observed that wear volume obtained in nodal force condition was significantly higher than that of in normal pressure condition for the same case i.e. material for acetabular cup and backing was Titanium alloy and that of femoral head was UHMWPE. The present work presents the comparative numerical study which is useful for selection of alloys for artificial hip joint implants for better performance. Acknowledgements Authors would like to thank the National Institute of Technology Delhi for providing the facilities required. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
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