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EVALUATION OF ARCHARD’S WEAR AMONG HIP IMPLANT’S STEM-HEAD BEARING COUPLE USING FINITE ELEMENT ANALYSIS
Nikhil Pandey1, R.P. Kori2 and Jyoti Vimal3
1,2 & 3Department of Mechanical Engineering, M.I.T.S. Gwalior, M.P., India, 474005 Abstract - In this research article, the finite element analysis of hip implant prosthesis was performed to investigate the wear behavior of hip implants' head-neck junction. The materials employed in the study include Ti6Al4V, Co-Cr-Mo, 316L Stainless steel for the stem, and Alumina for the femoral head. The study utilized modified Archard’s wear theory to calculate head-neck junctions' linear and volumetric wear rate for two million life cycles.
The finite element analysis showed results in forms of equivalent von-mises stresses and total deformation for overall hip implant prosthesis. The values of contact pressures and sliding distances were utilized to evaluate the linear and volumetric wear rates of head-neck junctions for two million life cycles. When considering the overall mechanical stability and lower wear rates, the Co-Cr-Mo-Alumina material combination was found to be the optimal configuration for the hip implant's head-neck junction.
Keywords: Total hip arthroplasty, finite element analysis, biomaterials, hip implants, Archard’s wear.
1. INTRODUCTION
The applications of biomaterials in the medical field have increased significantly over the last few decades. These biomaterials have replaced many body tissues such as teeth, ligaments, bones, tendons, and many more organs. Still, various aspects of biomaterials are expected to heal and re-functionalize the diseased or damaged body parts.
Biotherapy and biomedical engineering vastly employ biomaterials to fabricate implants and medical devices. In the present scenario and according to the human lifestyle, the implanted biomaterial should possess strong biological, wear, and mechanical properties for prolonged survival [1].
Hip arthroplasty or hip replacement consists of replacing the damaged or diseased hip joint with an artificial hip joint made with feasible biomaterial [2]. There are two ways of performing hip replacement surgery: total hip arthroplasty or total hip replacement and Hemi replacement arthroplasty. The primary aim of total hip arthroplasty is to relieve the pain of the patients facing arthritis. In some cases, it can be adopted to treat fractured or damaged hip joints as well. The major difference between total hip replacement surgery and Hemi replacement surgery is the replacement of the acetabulum and femoral head, both in total hip arthroplasty, while replacing only the femoral head in Hemi replacement surgery [3]. As per the analysis, hip replacement finds a
significant space in orthopedic surgeries.
However, the patient-specific satisfaction rate varies significantly. In the last 25 years, it has been reported that 58% of total hip surgeries have taken place, and it is a pretty costly procedure. It is only recommended when initial treatments and cures do not seem to work efficiently to improve hip functionalization [4].
A thorough study of the literature survey reveals that the acetabular cup and the femoral head junction were the primary bearing couple investigated prominently. The head-neck junctions are not addressed significantly in most of Archard’s wear finite element studies. The femoral head and neck junction also play a crucial role in affecting the service life of the hip implant. Furthermore, it has been evinced from the literature survey that the higher load-bearing gait activities such as stairs up, stairs down, and jogging were hardly examined. Moreover, the patient weight-specific studies are also lacking in the literature. Hence, this study proposes exploring the finite element wear behavior of hip implant’s head-neck junction with higher loading gait activities (cycling, walking, stairs up, stairs down, and jogging) considering an average 75 Kg weight patient. Furthermore, considering mechanical and wear feasibility, the study would aim to find out the optimal material combination for head-neck junction among Ti6Al4V-Alumina, Co-Cr-Mo- Alumina, and 316L Stainless Steel- Alumina.
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Fig. 1 Hip prosthesis and total hip arthroplasty [5].
2. METHODOLOGY
2.1 Modeling of the Hip Implant
The hip implant was modelled in Ansys Spaceclaim 19.2 3D modelling software.
The design dimensions of dimensions of the hip implant were chosen as shown in figure taken from previous study[6].
Fig. 2 a) Disassembled hip implant; b) Assembled hip implant; c) Hip implants
with modeling parameters [6].
2.2 Meshing of the hip implant
The meshing of the hip implant was performed by finite element analysis software Ansys Workbench 19.2. The meshing employed tetrahedral meshing elements with element size of 1 mm. The below figure shows the meshing of hip implant with tetrahedral meshing
elements. The meshing generated total 165436 nodes and 90217 elements.
Fig. 3 Meshing of the hip implant 2.3 Forces and boundary conditions The hip implant was fixed at the bottom while the forces for jogging gait activity was applied at the upper face of the femoral head as shown in figure[31].
Fig. 4 Force and fixed support applied for jogging gait activity
2.4 Materials properties
The materials used for the hip implant stem were Ti6Al4V, Co-Cr-Mo, 316L Stainless steel and Alumina for the femoral head. The material properties for the simulation process have been given in table 1.
Table 1 Mechanical properties of employed materials[7]–[15].
Name of the material
Density
(g/cm3) Elastic modulus
(GPa)
Yield strength
(MPa)
Tensile strength
(MPa)
compressive strength
(MPa)
Fatigue limit (MPa)
Poisson ratio
Ti6Al4V 4.43 114 896-
1034 900–
1172 450–1850 620-
725 0.33
Co-Cr-Mo 8.3 210 500-
1500 900-
1540 480–600 496-
896 0.30
316L SS 8.0 193 170-310 515–620 170 241-
820
0.28
Alumina 3.98 420 - 282–551 4400 269 0.21
2.5 Evaluation of Archard’s wear
The linear and volumetric wear was investigated using the Archard’s wear law as given in previous studies. The material
properties for evaluation of linear wear among stem-head interface namely wear coefficients and coefficient of friction have been given in table 2.
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Table 2 Wear properties of employed head-neck bearing couple [16].
Wear coefficient
(mm3/Nm) Friction coefficient (COF)
Ti6Al4V-Alumina 58.2 × 10−8 0.41
Co-Cr-Mo-Alumina 2.4 × 10−8 0.27
316L stainless steel-
alumina 33.5 × 10−8 0.62
In the current study, we have utilized the principle of modified Archard's wear law to estimate the wear rate of the head-neck junction of the hip implant prosthesis for two million life cycles. According to the modified Archard's wear theory, the wear volume of two mating components can be estimated in terms of linear and volumetric wear rates. The linear and volumetric rate can be calculated based on Archard's wear law modified, as shown in equations (4.1) and (4.2).
Lw = Kw× P × S Eqn (1).
Where Lw is the linear wear rate in mm, Kw is the wear coefficient between mating materials in mm3/Nmm, P denotes the contact pressure in MPa, and S represents the sliding distance in mm.
Vw = Lw × A Eqn (2).
Where Vw is the volumetric wear rate in mm3, and A represents the surface area between bearing couples.
In the current study, bearing couple's wear coefficients (Kw), contact stresses (P), sliding distances (S), and the contact surface area (A) will be used to calculate
the linear and volumetric wear rate.
3. RESULTS AND DISCUSSION
This finite element analysis examined the wear behavior of the hip implant’s head- neck junction. The study utilized modified Archard’s wear theory to evaluate the head-neck junction’s linear wear rate followed by volumetric wear rate. The study employed effective materials combinations: Ti6Al4V-Alumina, Co-Cr- Mo-Alumina, and 316L Stainless Steel- Alumina for head-neck junction.
Meanwhile, the study incorporated jogging gait activities for wear behavior investigation. The equivalent von-mises results showed the efficacy of the Ti6Al4V-Alumina material combination, as it resulted in the minimum stress generation on the implant.
Similarly, the values of total deformations were lowest from the Ti6Al4V-Alumina material combination.
Compared to other employed combinations, these lower values of stresses and deformations from the Ti6Al4V-Alumina material combination reduced the risk of stress-shielding in the implant. While, in terms of wear behavior, Co-Cr-Mo-Alumina showed superiority over others, as it provided the lowest amount of linear and volumetric wear.
The head material alumina suffers a negligible amount of stress, while the stem bears significant loads (Ti6Al4V, Co- Cr-Mo, and 316L Stainless steel), hence could be ignored. As far as the concern of survival up to two million life cycles, Ti6Al4V and Co-Cr-Mo can easily survive, as their yield strengths range up to 1034 and 1500 MPa, respectively. The 316L Stainless Steel-Alumina material combination could survive for two million cycles only for cycling gait activity, as its yield strength limits up to 310MPa.
However, the 316L Stainless Steel- Alumina material combinations showed promising results in terms of wear- resisting capabilities.
The below figure shows the values of contact pressure, sliding distances, equivalent von-mises stresses and total deformation for Ti6Al4V-Alumina stem- head interface.
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Fig. 5 a) contact status; b) contact pressure; c) sliding distance; d) equivalent von- mises stresses; and e) total deformation for Ti6Al4V-Alumina material combination
under jogging gait activity.
The below figure shows the values of contact pressure, sliding distances, equivalent von- mises stresses and total deformation for CoCrMo-Alumina stem-head interface.
Fig. 6 a) contact status; b) contact pressure; c) sliding distance; d) equivalent von- mises stresses; and e) total deformation for Co-Cr-Mo-Alumina material combination
under jogging gait activity.
The below figure shows the values of contact pressure, sliding distances, equivalent von- mises stresses and total deformation for 316L Stainless steel-Alumina stem-head interface.
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Fig. 7 a) contact status; b) contact pressure; c) sliding distance; d) equivalent von- mises stresses; and e) total deformation for 316L Stainless Steel-Alumina material
combination under jogging gait activity.
Table 3 illustrates the overall summary of the hip prosthesis’s finite element investigation. It concludes the values of equivalent von-mises stresses, total deformations, contact pressures, and
sliding distances for jogging gait activity considering all material combinations (Ti6Al4V-Alumina, Co-Cr-Mo-Alumina, and 316L Stainless steel-Alumina).
Table 3 Results summary for different material combinations in jogging gait activities.
Material
combination Equivalent von-
mises stresses (MPa) Total
deformation(mm) Contact
pressure(MPa) Sliding distance (mm)
Ti6Al4V-Alumina 894.74 1.7975 82.083 1.1438e-002
Co-Cr-Mo-Alumina 913.51 0.98183 61.962 7.4676e-003
316L SS-Alumina 924.04 1.0673 71.466 6.5385e-003
Table 4 shows the values of linear and volumetric wear rates calculated for hip implants stem-head interface calculated according to the Archard’s wear law.
Table 4 Linear and volumetric wear rates for different materials combinations under various gait activities.
Material
combination Wear coefficient
(Kw) (mm3 /Nmm)
Contact pressure (P) (MPa)
Sliding distance (S) (mm)
Linear wear (Lw = Kw×P×S)
(mm/ 2×106 cycles)
Surface area of head-neck junction (A)
(mm2)
Volumetric wear (Vw = Lw ×A) (mm3/2×106
cycles) Ti6Al4V-Alumina 58.2 × 10−8 82.083 0.011438 1.092839272 1420.5925 1552.479274 Co-Cr-Mo-
Alumina 2.4 × 10−8 61.962 0.007468 0.0222111463
7 1420.5925 31.55298795
316L SS-Alumina 33.5 × 10−8 71.466 0.006539 0.3131018366 1420.5925 444.7901208 As seen from the results, there are
negligible differences in the values of equivalent von-mises stresses for all studied material combinations. Hence,
considering the wear-resisting capability of the Co-Cr-Mo-Alumina material combination, it can be inferred as the best
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suitable combination to employ in the hip implant’s head-neck junction.
4. CONCLUSION
This investigation examined the Archard’s wear of stem-head interface of hip implants by finite element analysis. The jogging gait activity was used and different stem-head material combinations were employed namely Ti6Al4V-Alumina, Co-Cr-Mo-Alumina, Stainless Steel-Alumina. It is necessary to have lower values of equivalent von-mises stresses in the implant to avoid the stress-shielding effect. The Ti6Al4V- Alumina material combination showed the lowest stresses while not showing favorable results in wear investigation (showed highest linear and volumetric wear rate). The Co-Cr-Mo-Alumina material combination showed negligible stress differences compared to Ti6Al4V- Alumina and 316L Stainless Steel- Alumina while offering the lowest linear and volumetric wear rates. Considering the wear as a significant criterion of hip implant failure, we can conclude that Co- Cr-Mo-Alumina is the optimum material combination for hip implant prostheses.
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