Wear estimation at the contact surfaces of oval shaped hip implants using finite element analysis

Abstract

The hip joint is one of the most essential joints for transmitting weights to the lower abdomen during day-to-day activities. Loosening of hip implants is mostly caused by wear. The wear assessment during the design stage offers a clear sense of the implant’s life expectancy, and modest adjustments in the design may also greatly enhance the implant’s life expectancy, lowering the probability of revision surgery. Linear wear is estimated at the contact surfaces of the femoral head to the acetabular cup and the acetabular cup to the backing cup in this study. In this work, oval-shaped hip implant is considered with a femoral head diameter of 28 mm, an acetabular cup thickness of 4 mm, and a backing cup thickness of 2 mm. The Archard’s law used to estimate the linear wear rate. It is observed that when the acetabular cup is made of UHWMPE and the stem, femoral head, and backing cup are made of CoCr, the least overall deformation is 0.394 mm during walking loads. When the stem is considered Ti−6Al−4 V and the acetabular cup of UHWMPE, the minimum wear between the femoral head and acetabular cup is 0.063 mm/year. During a typical standing posture, an acetabular cup-to-backing cup wear rate of 0.007 mm/year is estimated. Overall, the CoCr material combination had the lowest wear rate in the four activities considered for this work. These implants designs can be 3D-printed and further can be tested in a hip simulator under the same loading conditions.

Keywords:

• Hip implants
• finite element analysis
• wear
• material. Optimisation
• Biomaterials

1. Introduction

The hip joint is among the most essential load-bearing joint surfaces, enabling both strength and multiaxial movement while transferring multiple times body weight (BW) during daily activities like walking, standing, bending and sprinting (Hua et al., 2022). The human body is composed of roughly 270 bones at the moment of birth, and this number will be lowered to 206 bones by the time of maturity when some of the bones are merged together (Clarke, 2008). When it comes to the significance of the hip, it may be characterised by the fact that it allows human mobility and sustains the full body weight without causing distress (Hampton et al., 1980). The inability of the artificial joint to function properly is among the most prevalent reasons for complete joint revision. Any severe relative movement involving joint partners is classified as joint instability, which is often characterised by harm to implant components or adjacent soft tissue (Herrmann et al., 2012). However, because of the joint’s load-carrying capabilities in six degrees of freedom (flexion, extension, abduction, adduction, internal rotation, and external rotation), it is susceptible to degeneration and local loading, resulting in discomfort (K. N. Chethan et al., 2014). Artificial joints are frequently used to replace degraded hip joints, also known as total hip arthroplasty to avoid such outcomes. Total hip joint replacement implant consists of stem, femoral head, acetabular cup and backing cup. Figure 1 shows the components associated in THR. Total hip replacements are often patient-specific and reliant on the requirements of the individual patient in terms of shape and sizes. The range of motion and physical function are critical considerations, particularly in younger patients who have high expectations for their life quality after THA surgery (Morlock et al., 2011). When a wide-ranging series of patients from various experimental studies were analysed, the findings revealed that younger individuals are more prevalent among older patients undergoing THR to restore function to a clinically relevant level (Foucher, 2016; Lunn et al., 2019). Furthermore, a substantial reduction in the average age of patients who need hip implants has prompted studies to prolong the durability of THRs in attempt to improve quality of life for active younger patients (K. N. Chethan, Ogulcan, et al., 2020).

Figure 1. Components used in hip implants (Bhawe et al., 2022).

Implants were traditionally held in place by pressure, friction or screws. Due to the insufficiency of these methods, new and improved methods have been developed. Now, there are two common types of femoral stem fixation: cemented femoral fixation and cementless femoral fixation (Babaniamansour, 2017) Designing an Optimized Novel Femoral Stem. A femoral head is attached by press fitting, then an acetabular cup and backing cup are attached (Bhawe et al., 2022). Although a press-fit fixation approach eliminates the need for screws or cups with screw holes, it can result in acetabular fractures or incompetence. However, for large-diameter metal-on-metal arthroplasty procedures, using an implant with no screw holes provides a wide surface area for liner support and prevents debris from entering through screw holes (K. N. Chethan et al., 2021; Pickering et al., 2009). The acetabulum and the lateral condyle of the knee joint are connected by the femoral head (K. N. Chethan, Zuber, et al., 2019). The femur bone is composed of two layers: cortical bone, which is tougher and more rigid on the outside, and cancellous bone, which is soft and spongy on the inside. An implant’s femoral component experiences bending and torsional loads due to the increasing strain on the femur caused by an individual’s increased weight and physical activity. If these loads are applied repeatedly and in a variety of ways, fatigue fractures or deformations in hip joint implant can occur (Delikanli & Kayacan, 2019). The finite element approach is commonly applied to analyse the numerous biomedical implants (K. N. Chethan et al., 2021).

One of the most essential requirements for materials used in THR is they should be biocompatible and have a low wear rate in order to minimise the number of revisions required owing to aseptic loosening of implants (K. N. Chethan, Zuber, et al.,). Nowadays, implants are made up of Ti−6Al−4 V, Co-Cr and UHWMPE (K. N. Chethan et al., 2021). Because of differences in individual anatomy, femoral heads can range in size from 22 millimetres to 54 millimetre (K. N. Chethan, Shyamasunder Bhat, et al., ; K. N. Chethan, Zuber, et al., 2020). Femoral stems appear in a range of geometrical shapes, including straight, tapered, short length and anatomical. The different stem designs used are circular, ellipse, oval and trapezoidal (K. N. Chethan, Zuber, et al., 2020).

Metal-on-polyethylene (MoP), metal-on-metal (MoM), ceramic-on-ceramic (CoC), and ceramic-on-polyethylene (CoP) bearings are the four basic types of bearings evaluated as well as utilised in THA. Metal-on-polyethylene (MoP) and ceramic-on-polyethylene (CoP) bearings are examples of hard-on-soft bearings, whereas hard-on-hard bearings include metal-on-metal (MoM), ceramic-on-ceramic (CoC), and ceramic-on-metal (CoM) (K. N. Chethan, Shyamasunder Bhat, et al.,). Ceramic heads with metallic inserts (CoM) have lately been launched as a mixed composition (Merola & Affatato, 2019). MOMs and COCs have garnered increased attention due to their ability to minimise wear rates of arthroplasties, especially in younger patients with an active lifestyle (Agarwal et al., 2005).

Delikanli et al (Delikanli & Kayacan, 2019)., suggested revascularisation of medullary tissue by lightening the implant. The lightened and roughened implants have asserted to be successful in integrating with the bone in the implantation location, according to animal research using a titanium mesh acetabular cup that had been implanted in a human body for 27 years, Koch et al., investigated the histological discoveries of the bone—implant junction in the cup. The findings revealed that there was adaptive bone remodelling at the interface between the inner solid core of the acetabular cup and the surrounding bone (Koch et al., 2018).

In the present study, oval stem profile with femoral head size 28 mm, acetabular cup of 4 mm and backing cup of 2 mm is considered. Different widely used materials are used for the analysis. Deformation, stresses and wear rates at the junction of acetabular cup of femoral head to acetabular cup and acetabular cup to backing cups are calculated for various activities like walking, running, knee bend etc. ANSYS R−19 is used to do finite element analysis on the models. ASTM F2996–13 standards have been used to specify boundary conditions (ASTM, 2015). The materials used in the study are Ti−6Al−4 V, CoCr alloy and UHWMPE. These materials are evaluated with different combinations for stem, femoral head, acetabular cup and backing cup to find the best suited materials with least wear rate per million cycles which can be used for hip stems. This work will significantly help to evaluate the life of the current design used for the THR.

2. Materials and methods

Several distinct designs are currently available for usage in THA (Colic & Sedmak, 2016; Jun & Choi, 2010; Kim & Yoo, 2016). They all have their own set of benefits and drawbacks. A stem with an oval cross-section is considered for the analysis in the current work (K. N. Chethan, Zuber, et al., ; K. N. Chethan et al., 2021). The femoral head size of 28 mm, acetabular cup of 4 mm and backing cup of 2 mm is considered. Modelling is carried out using CATIA V−6. CoCr alloy, Ti−6Al−4 V, and UHWMPE are the materials that were employed in the current work due to their mechanical properties. The length of stem is considered as 180 mm. Figure 2 shows the cross-sectional frontal view of the assembled hip implant.

Figure 2. Frontal view of assembled hip implant.

Titanium and its alloys are prominent metallic implant biomaterials that are utilised in total hip arthroplasty. In the medical industry, titanium alloys, such as Ti−6Al−4 V, are the most frequently used materials for stems and acetabular cementless hip replacements (Hu & Yoon, 2018). Titanium and its alloys have high corrosion resistance, great mechanical strength and a relatively low density (Hu & Yoon, 2018). This metal is widely used in many applications in the medical sector because of its outstanding biocompatibility (Shrestha, 2017). Because of its strength properties, durability, and chemical inertness, UHMWPE has been adopted for acetabular bearings for nearly 50 years. Recent advances have resulted in a material that is extremely wear-resistant and may last more than 15 years (Wang, 2013)(Encyclopedia of Tribology). Alloys of Co-Cr have excellent strength, corrosion, and wear properties, making them a popular implant material choice. Since its Young’s modulus is greater than titanium alloys’ and its wear resistance is greater, it is mainly used as a cement type femoral stem material (Hu & Yoon, 2018). The mechanical materials properties are shown in Table 1.

Table 1. Mechanical properties of the materials used for hip implant (Chen et al., 2018; K. N. Chethan, Shyamasunder Bhat, et al.,)

SI	Materials	Young’s Modulus [GPa]	Density [gm/cm^3]	Poisson’s ratio	Ultimate tensile Strength [Mpa]
1.	CoCr	200	8.5	0.30	1503
2.	Ti−6Al−4V	114	4.5	0.31	930
3.	UHWMPE	0.963	0.949	0.31	48

In this work, femoral head and backing cup material is kept constant as CoCr while femoral head material is varied as Ti alloy and CoCr alloy in combination with acetabular cup material as UHWMPE and CoCr. The general body weight of a person undergoing total hip replacement considered for this analysis is 767N which is approximately 78.26 kg.

In addition to the force acting on the implant, this study also considers the effects of the moment induced on the implant to get accurate results. The forces and moments acting on the backing cup are acquired by analysing prior data for activities that a person undergoing THR would be engaged in on a daily basis. Activities such as walking, knee bending, sitting and standing are considered when calculating the wear on the hip implant.

2.1. Meshing and boundary conditions

For the current work, unstructured mesh is selected for the meshing (Gutmann et al., 2023). A mesh grid independence research is performed in order to determine the final mesh size for all of the analysis. A mesh convergence study was conducted to determine the ideal mesh size for the study (K. N. Chethan, Ogulcan, et al., 2020). Mesh size was varied from 5 mm with an interval of 1 mm each. There was no substantial difference in the von Mises stresses when the mesh size was less than 1 mm. So 1 mm mesh size is finalised for the complete analysis. The mesh grid independence study is shown in Figure 3.

Figure 3. Mesh grid independence study.

The bottom of the stem is used as a fixed support for the analysis. Boundary conditions are adopted in accordance with ASTM F2996–13 (ASTM, 2015). An analysis of static structural behavior is performed on the models in the ANSYS R−19 to determine the von Mises stress, von Mises strain and total deformation (K. N. Chethan, Zuber, et al.,). In this analysis, sliding distance and pressure between two contact surfaces are determined with a contact tool, and a friction coefficient between them is considered.

2.2. Contact interaction behavior

Contact behavior between all the parts is defined as “frictional”. The frictional coefficient for CoCr and CoCr is taken as 0.19, for Ti and CoCr, it is 0.17 and between CoCr and UHWMPE, it is 0.16.

2.2.1. Wear estimation

The wear rates were estimated using Archard’s law of wear given by

(1) $\mathrm{V}=\mathrm{Kw}\ast \mathrm{S}\ast \mathrm{Pn}$(1)

In equation 1, V represents the volume of wear debris, Kw represents the wear coefficient, S represents the sliding distance, and Pn is the normal contact pressure. This equation can also be expressed as (K. N. Chethan, Ogulcan, et al., 2020; Wu et al., 2003):

(2) $\mathrm{dV}=\mathrm{Adh}=\mathrm{Kw}\times \mathit{\sigma }\times \mathrm{A}\times \mathrm{ds}$(2)

Which can also by written as

$\mathrm{dh}=\mathrm{Kw}\times \mathit{\sigma }\times \mathrm{ds}$

where dh (mm/cycle) represents wear depth, σ (MPa) represents maximum normal contact pressure, and ds (m) represents sliding distance (K. N. Chethan, Ogulcan, et al., 2020). In estimating the life span of the hip implants, it is considered as one million cycles per year. So, the wear rate calculated for one cycle is converted into million cycles to find the wear rate per year. Wear rate is estimated between femoral head to acetabular cup and acetabular cup to backing cup. These two junctions in the hip implant experiences higher wear rate which leads to aseptic loosening resulting in hip replacement surgery.

3. Results

A preliminary research is carried out on the model to determine the optimal material combination for the stem, femoral head, acetabular cup and backing cup. It can be observed from Table 2 that the least total deformation 0.394 mm is exhibited in the walking loading condition with the material combination of acetabular cup as UHWMPE and stem, femoral head and backing cup as CoCr. The corresponding von Mises stress and strain for this material combination is 407.99MPa and 0.002 mm/mm, respectively.

Table 2. Static study of a hip implant performed in accordance with ASTM F2996–13 standards with different materials combination

Activities	Stem	Femoral Head & backing cup	Acetabular Cup	Total Deformation (mm)	von Mises Stress (MPa)	Strain (mm/mm)
Walking	Ti	CoCr	UHWMPE	0.688	405.22	0.003
	CoCr		UHWMPE	0.394	407.99	0.002
	Ti		CoCr	0.693	405.99	0.003
	CoCr		CoCr	0.395	408.75	0.002
Kneebend	Ti		UHWMPE	2.268	1229.1	0.012
	CoCr		UHWMPE	1.298	1237.5	0.007
	Ti		CoCr	2.261	1229	0.012
	CoCr		CoCr	1.291	1237.5	0.007
Sit-down	Ti		UHWMPE	1.979	1086.9	0.010
	CoCr		UHWMPE	1.133	1094.4	0.006
	Ti		CoCr	1.973	1086.9	0.010
	CoCr		CoCr	1.126	1094.4	0.006
Stand-up	Ti		UHWMPE	1.115	698.51	0.006
	CoCr		UHWMPE	0.638	703.26	0.003
	Ti		CoCr	1.111	698.51	0.006
	CoCr		CoCr	0.634	703.26	0.003

Among four activities that were studied under varied loading conditions (walking, knee bending, sit down, and stand up), the one with the least total deformation, von Mises stress and elastic strain was shown in Table 2.

The greatest deformation recorded under walking loading conditions is 0.688 mm when the stem is made of Ti−6Al−4 V, the acetabular cup is made of UHWMPE, and the femoral head and backing cup are made of CoCr.

As a general trend, it can be concluded that the total deformation is minimal when the stem, femoral head and backing cup are composed of CoCr and the acetabular cup is made of CoCr or UHWMPE.

Walking shows least deformation, stresses and strain compared to the other three activities. The patient’s body weight (BW) varies from 280% to 870% with different activities. Wwalking has the least load acting on the hip implants. Figure 4 shows the deformation, stresses and strain acting on the hip implant during waking condition for an individual of 767N weight.

Figure 4. Total deformation, von Mises stress, and strain for walking cycle.

3.1. Wear studies using Archard’s wear law

The second part of the study focuses on estimating wear between the femoral head to acetabular cup and acetabular cup to backing cup. CoCr femoral heads with polyethylene cups were found to exhibit wear behaviors in prior research, and it was reported that the wear intensity distributed more evenly along the contact of acetabular cups, and that may have the surface to preserve conforming contact geometry (HUNG & JS-S, 2002). Figure 5 shows the wear pattern between femoral head to acetabular cup during different activities.

Figure 5. Sliding distance and pressure between femoral head to acetabular cup.

Minimum wear between the femoral head and acetabular cup found to be 0.063 mm/year when the stem is made of Ti−6Al−4 V, the acetabular cup is made of UHWMPE during walking conditions. Maximum wear was found to be 0.84 mm/year during knee bend loading condition. These results are relatively closer with the available literatures. Also, these results were more realistic as moment is considered in this study along with the loading conditions. This actually mimics the normal load distribution in THR patients. Table 3 shows the detailed wear rate per year with different activities and different materials combinations considered for the current study.

Table 3. Sliding distance, contact pressure and wear rate per year with different activities

Activities				Femoral Head to Acetabular Cup
	Stem	Femoral Head & backing cup	Acetabular Cup	Sliding Distance	Pressure	Wear Per cycle	Wear per Year
Walking	Ti	CoCr	UHWMPE	1.230E^−06	0.320	6.314E^−08	0.063
	CoCr	CoCr	UHWMPE	1.227E^−06	0.327	6.434E^−08	0.064
	Ti	CoCr	CoCr	4.164E-^06	0.858	6.790E^−07	0.679
	CoCr	CoCr	CoCr	3.781E^−06	0.918	6.601E^−07	0.660
Knee bend	Ti	CoCr	UHWMPE	4.18E^−06	1.257	8.4238E^−07	0.842
	CoCr	CoCr	UHWMPE	4.200E^−06	1.258	8.459E^−07	0.845
	Ti	CoCr	CoCr	7.135E^−07	6.215	8.425E^−07	0.842
	CoCr	CoCr	CoCr	4.014E^−07	6.556	5.000E^−07	0.500
Sit down	Ti	CoCr	UHWMPE	3.351E^−06	0.848	4.548E^−07	0.454
	CoCr	CoCr	UHWMPE	3.34E^−06	0.850	4.541E^−07	0.454
	Ti	CoCr	CoCr	6.246E^−07	4.240	5.032E^−07	0.503
	CoCr	CoCr	CoCr	3.522E^−07	4.586	3.069E^−07	0.306
Stand up	Ti	CoCr	UHWMPE	1.974E^−06	0.344	1.089E^−07	0.108
	CoCr	CoCr	UHWMPE	1.971E^−06	0.346	1.092E^−07	0.109
	Ti	CoCr	CoCr	3.477E^−07	1.649	1.089E^−07	0.108
	CoCr	CoCr	CoCr	1.967E^−07	1.837	6.868E^−08	0.068

In the second part, it also tried to estimate the wear between the acetabular cup and backing cup. This interface has a significant effect in the loosening of hip implants which is one of the main causes for the revision surgery. The active younger patients have more wear due to their intense movement. In this work, four different activities are considered along with moment and the linear wear rate is estimated between these two contact surfaces. In general, for all four activities considered for the study, CoCr material combination shown least wear rate over the other combinations of materials made up of. A least wear of 0.007 mm/year is estimated during normal stand-up position. Whereas the maximum linear wear rate found 0.848 mm/year when acetabular cup is made of UHWMPE for the complete CoCr implant. Figure 6 shows the different a pattern for the activities considered for the study.

Figure 6. Sliding distance and pressure between acetabular cup to backing cup.

In knee bend activities, a maximum linear wear rate found to be 8.48 mm/year, this is, however, matching with previously available literature. Table 4 shows the linear wear rate per year for all the four activities along with the different widely used materials.

Table 4. Sliding distance, contact pressure and wear rate per year with different activities

Activities				Acetabular Cup to Backing Cup
	Stem	Femoral Head & backing cup	Acetabular Cup	Sliding Distance	Pressure	Wear Per cycle	Wear per Year
Walking	Ti	CoCr	UHWMPE	1.406E^−06	0.382	8.617E^−08	0.086
	CoCr	CoCr	UHWMPE	1.405E^−06	0.380	8.558E^−08	0.085
	Ti	CoCr	CoCr	1.622E^−06	0.123	3.813E^−08	0.038
	CoCr	CoCr	CoCr	1.19E^−06	0.126	2.865E^−08	0.028
Knee bend	Ti	CoCr	UHWMPE	4.687E^−06	1.132	8.489E^−07	0.848
	CoCr	CoCr	UHWMPE	4.684E^−06	1.132	8.489E^−07	0.848
	Ti	CoCr	CoCr	7.929E^−07	0.666	1.004E^−07	0.100
	CoCr	CoCr	CoCr	4.536E^−07	0.486	4.196E^−08	0.041
Sit down	Ti	CoCr	UHWMPE	3.820E^−06	1.053	6.441E^−07	0.644
	CoCr	CoCr	UHWMPE	3.817E^−06	1.054	6.441E^−07	0.644
	Ti	CoCr	CoCr	6.919E^−07	0.532	7.006E^−08	0.070
	CoCr	CoCr	CoCr	3.959E^−07	0.355	2.675E^−08	0.026
Stand up	Ti	CoCr	UHWMPE	2.21E^−06	0.697	2.466E^−07	0.246
	CoCr	CoCr	UHWMPE	2.207E^−06	0.697	2.463E^−07	0.246
	Ti	CoCr	CoCr	3.906E^−07	0.275	2.046E^−08	0.020
	CoCr	CoCr	CoCr	2.238E^−07	0.182	7.745E^−09	0.007

4. Discussion

In current study, linear wear rate is estimated between the femoral head and acetabular cup, and between the acetabular cup and backing cup, using various material combinations which are widely available nowadays. This was attained by considering the variations in force and moment acting on the backing cup when a typical patient undergoing total hip replacement performs movements such as walking, bending the knee, standing up and sitting down. When all four components are considered of CoCr, the overall deformation and linear wear rate per year found to be least with comparison of other material combinations. When the stem is made of Ti−6Al−4 V, the acetabular cup is made of ultra-high-molecular-weight polyethylene, and the femoral head and backing cup are made of CoCr alloy, the wear rate per year is the least between the femoral head and acetabular cup, whereas it is the least between the acetabular cup and backing cup when all four components are made of CoCr.

According to Jiang et al (Jiang, 2007), four distinct structural models were studied, which were constructed of ultra-high molecular weight polyethylene (UHMWPE), CoCr alloy, 316 L stainless steel, and Ti−6Al−4 V alloy.

Because of their excellent mechanical qualities and biocompatibility, CoCr and Ti−6Al−4 V materials are increasingly being employed as stem materials. This research used CATIA V−6 to model stem designs with three distinct profiles. The designs included circular, elliptical, oval and trapezoidal shapes with three distinct profiles. Chethan et al (K. N. Chethan, Zuber, et al.,). found that CoCr was the best material to use in the design of the stem.

When an implant is subjected to a constant cyclic load associated with normal daily activities, von Mises stress has a significant effect on the implant’s life. Aseptic loosening is a significant issue with implants (K. N. Chethan, Zuber, et al., 2020; Shi et al., 2003).

The acetabular cup is most often replaced in THA, possibly owing to the valgus neck position of the widely cemented Poldi endoprosthesis (Št’astný et al., 2014). A major improvement in revision THA was achieved in 1980 when oval-shaped cups were introduced into clinical practice. Restoring the anatomical rotation center with these cups necessitated less severe bone excision (Št’astný et al., 2014). Sir John Charnley performed the first complete hip arthroplasty in 1958 using a low-friction UHMWPE-on-metal design. This design dealt with the issue of increased wear and shortened joint life (Das & Chakraborti, 2018).

The wear of the Ultra-High Molecular Weight Polyethylene (UHMWPE) acetabular cup is the most important component in determining the lifespan of metal on polyethylene (MoP) hip replacements (Zeman et al., 2018). It is well established that when prosthetic hip joints are exposed to high levels of stress, the UHMWPE cup subsurface wear adds to the amount of wear debris. Given that surface stress has an effect on the fatigue process, it is becoming extremely critical to decrease contact stress at the cup in order to lower the volume of UHMWPE cup wear debris (Kandemir et al., 2018; Korhonen et al., 2005; Monif, 2012; Rixrath et al., 2008; The et al., 2008).

According to the subject’s body weight, contact pressures during common activities such as walking, standing and sitting range between 3 and 11 MPa (Hodge et al., 1986; Kottan et al., 2022; David et al., 1998; Saputra et al., 2013). When a 1 mm or 3 mm metal backing shell is employed to support the CoCr femur head, the stresses at the UHMWPE cup fall from 27 to 26 MPa and to 25.8 MPa, respectively (Monif, 2012).

When a femoral head with a diameter of 32 mm was employed instead of a diameter of 22 mm, stress in the UHMWPE cup dropped by 33%, 25% and 28%, respectively. Lowered stress at the UHMWPE cup was associated with a metal backing thickness of 1 mm; however, there were no statistically significant changes in stress with increasing the metal backing thickness (K N et al.,). Griza et al (Griza et al., 2009). used a stem of length 165 mm and exerted a force in accordance with ASTM F 745 guidelines in their study. The lateral section of the stem was found to have the highest von Mises stresses, which was 210 MPa. Among the profiles studied by Chethan et al (K. N. Chethan, Zuber, et al., 2020), profile two having femoral head diameter of 36 mm, liner thickness of 2 mm with shell thickness of 1 mm displayed the least von Mises stress of 139 MPa. Additionally, the distortion of the profile two-shaped stem is the smallest at 0.042 mm. Increasing the diameter of the femur head, regardless of the material of the femur head, leads to a considerable decrease in maximum Von Mises stress on the cup made of UHWMPE. As an example, in the case of the titanium alloy femoral head, the stress on the cup drops from 21 to 14 MPa when a 32-mm-diameter femur head is employed instead of a 22 mm diameter (Monif, 2012). The results presented in this work matches with the previous works (B. N. S. Chethan et al., 2022).

4.1. Limitations of current work

The study’s first limitation is that it only considers linear elastic loading conditions when evaluating the hip implant wear. In order to prevent inaccurate wear rate figures, the dominant contribution of sliding distance or contact pressure should be examined. Furthermore, considering the rate of corrosion in the finite element approach is problematic. As a result, it is not included the corrosion in our model since, according to many academics, corrosion isn’t the primary cause of debris creation when compared to wear (K. N. Chethan, Ogulcan, et al., 2020). This work provides a clear understanding of the wear at the junctions of the femoral head and acetabular cup, as well as acetabular cup and backing cup, by taking into account the force and moment acting on the backing cup, and also taking into consideration more realistic loading conditions such as walking, knee bending, sitting and standing up; however, there is just a single designs addressed in this study, ignoring the varying lengths of implants, as well as the different diameters of femoral heads, cups, and backing cup thicknesses. Finally, employing a hip simulator, all of the above-mentioned studies may be carried out in real time (K. N. Chethan et al., 2021).

5. Conclusions

Along with the force and moment considered in this work, there are several factors which affects the wear rate in the contact surfaces. The surface finish in the contacting region has a significant effect on the wear debris generation. In previous literature, it tries to find the wear rate in the contacting surfaces considering different loads acting on it. But in all those works, moment is neglected. Moment acting on hip implants along with the loads has a significant effect on the implants. In this work, different loading conditions are considered along with moment acting on it. Hence, this works carries novelty, also the linear wear is close with the real conditions in which these implants are used. Oval-shaped cross-section stem is considered for this study. Linear wear is estimated at the contact surfaces of the femoral head to the acetabular cup and the acetabular cup to the backing cup. A constant femoral head diameter of 28 mm, an acetabular cup thickness of 4 mm, and a backing cup thickness of 2 mm. The Archard’s law used to estimate the linear wear rate. Acetabular cup is made of UHWMPE and the stem, femoral head, and backing cup are made of CoCr, the least overall deformation is 0.394 mm during walking loads. When the stem is considered Ti−6Al−4 V and the acetabular cup of UHWMPE, the minimum wear between the femoral head and acetabular cup is 0.063 mm/year. During a typical standing posture, an acetabular cup-to-backing cup wear rate of 0.007 mm/year is estimated. It is required to consider other activities and also different body weights loading conditions to know how this design responds to these conditions. Before concluding this work, it is necessary to evaluate the obtained results with experimental studies using hip simulator.

Submission declaration

This paper is not being considered for publication elsewhere.

Author contributions

Conceptualisation, Chethan K N; methodology, Sawan Shetty, Satish Shenoy B; software, Numa Shaikh, Chethan K N; validation, Chethan K N, Satish Shenoy; formal analysis, Numa Shaikh; investigation, Numa Shaikh, Sawan Shetty, Shaymasunder Bhat N, Chethan K N; resources, Numa Shaikh, Satish Shenoy B, Shaymasunder Bhat N; data curation, Numa Shaikh; writing—original draft preparation, Numa Shaikh.; writing—review and editing, Chethan K N, Satish Shenoy; visualisation, Numa Shaikh.; supervision, Satish Shenoy B, Shaymasunder Bhat N, Chethan K N; project administration, Chethan K N.; All authors have read and agreed to the published version of the manuscript.

Acknowledgments

The authors thank the Department of Aeronautical and Automobile Engineering, Manipal Institute of Technology, Manipal Academy, Manipal for providing the high computational facility to carry out this research.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The original contributions presented in the study are included in the article and supplementary materials.

Additional information

Funding

This research was non-funded.

Notes on contributors

Chethan K N

Chethan K N is working as an Associate Professor in the Department of Aeronautical and Automobile Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, Karnataka, INDIA. He holds B. E. (Mechanical Engineering), M.Tech (Manufacturing Engineering & Technology), and Ph.D. (Computational Biomechanics) degrees. Author has worked as post-doctoral research associate in New Jersey institute of Technology, USA. He has a more than 10 years of teaching and research experience. His area of interest includes Biomechanics of the hip joint, Finite element analysis of biomedical implants, Composite materials, and Manufacturing of medical implants. The author has published more than 35 articles in reputed journals related to finite element analysis of hip implants, mechanical characterisation of materials, and composite materials.