Finite Element Analysis of Collared Hip Prosthesis Cross-Sections Under Dynamic Loading and Wear Conditions for Durable Orthopedic Implant Design

Abstract

Background/Objective: Traditional hip implant evaluations often overlook patient-specific dynamic loadings. This study investigates the performance of novel collared hip implant designs under walking conditions, focusing on geometric profiles and two common stem materials: Ti-6Al-4V and CoCr alloy. Methods: Patient-specific dynamic forces were applied using commercial finite element analysis, adhering to ISO and ASTM standards. Four cross-sectional profiles—circular, elliptical, oval, and trapezoidal—were initially evaluated for induced stresses and displacements. Subsequently, wear characteristics at implant junctions were analyzed, comparing CoCr (MC 1) and Ti-6Al-4V (MC 2) stems. The study also assessed the impact of using Ultra-High Molecular Weight Polyethylene (UHMWPE) acetabular cups. Results: The elliptical (CS 2) cross-sectional profile demonstrated superior performance. Junction analysis revealed that the CoCr stem (MC 1) exhibited a stem-to-head sliding distance four times higher and contact pressure 5.5 times higher than the Ti-6Al-4V stem (MC 2). Specifically, MC 1 showed 82% higher contact pressure and 89% greater sliding distance at the stem–head junction compared to MC 2. Additionally, utilizing UHMWPE cups effectively eliminated squeaking sounds attributed to CoCr cups due to superior wear resistance. Conclusions: The combination of an elliptical (CS 2) cross-sectional profile with a Ti-6Al-4V stem and UHMWPE acetabular cup offers optimal performance. This configuration significantly reduces wear and contact pressure, suggesting enhanced functionality and durability for hip implants under dynamic loading conditions.

1. Introduction

The hip joint is a synovial joint comprising a ball and socket. It is stabilized by a ligamentous and bony restraint. The hip joint is responsible for balancing forces across the wide range of body motion, which provides the necessary stability for performing everyday tasks such as standing upright, walking smoothly, maintaining balance, standing up from a chair, and bearing weights from a squat position [1]. Stability is essential for allowing a broad span of movements and supporting the forces encountered during everyday activity. This joint is designed for a rotational movement between the femoral head and acetabulum, having negligible discernible translation due to the congruence of the articulating surfaces. The congruency of the articulating surfaces refers to the degree to which the shape of the femoral head and the acetabulum match. This matching shape allows maximum contact between the two surfaces, creating a stable and strong joint that can withstand the forces of everyday life [2]. The hip joint’s absolute limits of motion before bony impingement are defined by the natural stability provided by the acetabulum depth. These are the maximum limits of hip joint movement: flexion (120°), extension (10°), abduction (45°), adduction (25°), internal rotation (15°), and external rotation (35°), which may vary slightly among different patients [3].

Total hip arthroplasty (THA) is a consistently successful and cost-effective orthopedic surgery [4]. It offers dependable results for individuals experiencing end-stage degenerative osteoarthritis of the hip. It specifically leads to comfort in pain, restored functionality, and improved life quality [5]. Compared to total knee arthroplasty (TKA), THA provides more reliable and consistent positive outcomes [6]. THA has also been termed the operation of the century, as it is currently one of the most widely performed surgeries [7]. Data suggests that between 2000 and 2019, THA surgery increased by 177% compared to TKA, which increased by 155%, with a projected annual increase in THA and TKA to be 5.2% and 4.4%, respectively [8]. In India, there has been an increase in the usage of uncemented THR, and dual-mobility THRs have become more popular among surgeons from 2006 to 2019 [9].

The materials used in hip implants for THA play a very crucial part, as they are in contact with human tissues. The materials used are chosen based on their mechanical and biocompatibility features [10,11,12]. Additionally, better wear resistance and mechanical reliability are also considered critical factors in addition to bioreactivity when choosing materials for THA [13,14,15,16,17,18,19]. Several varieties of bearings are commonly employed, namely metal-on-polyethylene (MoP), metal-on-metal (MoM), ceramic-on-ceramic (CoC), and ceramic-on-polyethylene (CoP). Each type has unique advantages and drawbacks to consider before finalizing [20]. The cost of an implant, the patient’s age, the patient’s activity, and other difficulties that may arise during the surgery are all important factors to consider [21].

Stainless steel is a material resistant to oxidation and can be easily machined, formed, and hardened. It was a popular choice for THA, but its poor biocompatibility compared to Co-Cr alloy has decreased use. Co-Cr alloy is a popular choice considering its strength, tolerance to corrosion, and wear properties. These alloys are often used as materials for femoral stems because they have a higher Young’s modulus than titanium alloys [22]. Titanium alloys are highly prized for their low density, high mechanical strength, excellent corrosion resistance, and bone compatibility. However, their application in manufacturing femoral heads is limited by their poor wear resistance [23]. Ultra-High Molecular Weight Polyethylene (UHMWPE) is an engineering polymer commonly used in THA as a material for bearing, along with ceramic or metallic counter surfaces. The material was first introduced in the mid-1960s and has since become THA’s most frequently used bearing material [24,25,26]. Polyether-ether-ketone [PEEK] is a biocompatible polymer with elevated thermal stability, toughness, rigidity, resistance to creep, simplicity in processing, self-lubrication, and resistance to abrasion. During its use as a bearing surface, it releases much lower wear particles [27,28].

Over the years, several researchers have conducted studies on hip implants. Numerous studies use the finite element method to accurately assess complete hip prostheses’ stability. Most of these studies have concentrated on stress distribution in implant and prosthetic designs. An extensive study examined the potential influence of variations in hip implant shape on the variations in stress for static and time-dependent dynamic loading conditions [29,30,31]. The stress distribution effects of square and cylindrical cross-sections were studied by several researchers [32]. The investigation of hip implants featuring porous stems was conducted to evaluate their mechanical and biological responses, as well as their ability to mitigate stress-shielding effects. The results indicate that these implants exhibit a high level of mechanical and biological performance while also demonstrating a reduction in stress-shielding effects [33,34]. However, one of the major observations in the previously analyzed hip implants is that a collar was absent at the end of the stem.

This work is carried out to understand the outcome of the collar provided at the stem under dynamic loading conditions, which provides a novelty to this study. In this study, we have considered the dynamic loading for the walking cycle. Four different cross-sectional profiles of hip implants are utilized in this study by considering four material combinations. The best-suited hip implant is suggested based on the stresses, deflection, and strains induced during the dynamic loading conditions. The present research work supports the Sustainable Development Goals (SDGs) 3 and 9, which ensure good health and wellbeing, and foster industry, innovation, and infrastructure, respectively.

2. Materials and Methods

2.1. Types of Implants

It is imperative to note that the THA utilizes distinct design types, and it is crucial to understand that each design has its own advantages and disadvantages [35,36,37]. Multiple factors are considered in developing and designing hip implants, including neck diameter, stem length, and cross-section. In this study, the implant stem used is as per one of our previously published studies with a collar near the proximal end [38]. An improvisation is that the stems of four different cross sections are considered for this study, using collars at the proximal ends for all the stems. Figure 1 represents the implant used along with all four cross-sectional stems. 3DEXPERIENCE Catia is used to model these implants. The dimensional specifications of hip implants used in this work can be found in Appendix A, Figure A1, Figure A2 and Figure A3.

This work evaluated deflection and von Mises stresses for all the stems using ANSYS 2024 R 2. Quasi-static structural analysis was carried out to evaluate the results.

2.2. Material Properties

Hip implants are surgically placed into the hip joint to replace damaged or diseased hip bone and cartilage. The choice of materials used in hip implants is critical in determining their strength, durability, and ability to integrate with surrounding tissues. The materials must withstand the forces and stresses of daily activities while being biocompatible with the body to prevent adverse reactions.

The study employs three distinct materials, as listed in

Table 1. Stem material properties [39,40,41,42].

Sl No	Material	Young’s Modulus (GPa)	Poisson’s Ratio	Density (gm/cm3)	Ultimate Tensile Strength (MPa)	Yield Strength (MPa)
1	Co Cr Alloy	200	0.30	8.5	1503	612
2	Ti-6Al-4V	114	0.31	4.5	930	880
3	UHMWPE	0.8	0.46	0.949	48	21

, to fabricate the hip implants, which comprise components that include a stem, femoral head, acetabulum, and backing cup. The dimensions of the femoral head (28 mm), acetabular cup (4 mm), and backing cup (2 mm) are per our published work [36].

Based on the materials listed in Table 1, the hip implant is analyzed for two different material combinations, as per

Table 2. Combination of materials used.

Material Combination	Stem	Femoral Head	Acetabular Cup	Backing Cup
MC 1	Co Cr Alloy	Co Cr Alloy	UHMWPE	Co Cr Alloy
MC 2	Ti-6Al-4V	Co Cr Alloy	UHMWPE	Co Cr Alloy

.

2.3. Meshing

In any numerical calculation, the meshing of the model plays a predominant role in defining the accuracy of the numerical solution. The mesh generated should provide accurate results with an optimized computational cost and time. An unstructured mesh is typically employed to analyze hip implants; most commonly, the mesh sizes range from 5 mm to 0.25 mm [43]. Figure 2 below shows the grid independence study used in this work. Figure 2 corresponds to the elliptical stem model, which uses CS 2. It is also noted that the difference in the equivalent stress induced when the element size is decreased from 1.5 mm to 1 mm is 0.78%, which is acceptable. So, 1 mm meshing is considered for all the implants.

The node counts range from 1,092,124 to 1,187,908, and the number of elements ranges from 769,481 to 840,595 for all the models in this work.

2.4. Loads and Boundary Conditions

The loads applied to the implant, along with the boundary conditions, result in the stresses and deflections being induced in the hip implant. This work corresponds to the loading on the hip implant for the walking cycle, a common day-to-day activity. The recorded data shows the force and moment at different points in time [44,45]. The study subject weighed 75 kg, and the load was measured in newtons (N), while the moment was measured in newton meters (Nm). Figure 3 displays the recorded values [46].

The analysis considers the stem’s bottom fixed, as illustrated in Figure 4. The lower bottom of 90 mm length is fixed, and the loads of forces and moments are applied at the acetabular cup as shown in Figure 4. The boundary conditions for this work were set per the ASTM F2996-13 standard [47].

This study assesses the effect of walking on hip implants by thoroughly studying their response to loads. Furthermore, a contact tool is utilized to gauge the sliding distance and pressure between two contacting surfaces, thus determining the friction coefficient along with the extent of wear on a hip implant. The loads were applied incrementally through load stepping to ensure numerical stability and convergence of the nonlinear contact interfaces. The solver employed the ANSYS Static Structural implicit solver with frictional contact defined at the implant junctions during wear analysis.

2.5. Wear Estimation

Wear law, according to Archard, is used to evaluate the rate of wear, which can be determined by Equation (1)

V = Kw × S × Pn

(1)

where V, Kw, S, and Pn represent the wear debris volume, coefficient of wear, sliding distance, and normal contact pressure, respectively.

Equation (1) is alternatively expressed in the form shown in Equation (2)

dV = Adh = Kw × σ × A × ds

(2)

which simplifies to

dh = Kw × σ × ds

(3)

In the above equations, dh (mm/cycle) indicates the depth of wear, σ indicates the maximum contact pressure in MPa, and the sliding distance is indicated as ds (m) [48].

The life of hip implants is evaluated based on the consideration that the hip implants are subjected to 1 million cycles yearly. Correspondingly, the wear rate calculated for a single cycle is extrapolated by converting it into a million cycles to determine the annual wear rate. The wear rate is assessed at the stem and head junction. These particular junctions are prone to higher wear rates, resulting in aseptic loosening, necessitating hip replacement surgery. The coefficient of friction (COF) between the materials, along with the wear coefficient (K_W) for the materials, is given in

Table 3. COF and wear coefficient between the materials [48,49].

Material Combination	COF	Wear Coefficient (mm3/Nm)
CoCr-CoCr	0.2	1.68 × 10−5
CoCr-Ti alloy	0.24	1.31 × 10−5
CoCr-UHMWPE	0.15	3.5 × 10−7

.

3. Results

As specified in the methodology section, this study considers the peak loads and moments observed during the full walking cycle acting on a novel-designed hip implant with a collar near the proximal end of the stem. The results include the von Mises stress, deflection, and strain for stems with four cross-sections and different material combinations. Based on these factors, the best possible cross-section of the hip implant is selected, which is further analyzed for wear rate at the stem-to-head junction.

3.1. FEM Analysis Results for All the Implants During the Walking Cycle

The von Mises stresses, deflection, and strain exerted in all the hip implants used in this study for two material combinations used in this study are indicated in

Table 4. Results of von Mises stress, deflection, and strain rate for the walking cycle.

Cross-Sectional	Material Combination	von Mises Stress in MPa	Deflection in mm	Strain in mm/mm
CS 1 (Circular)	MC 1	82.793	0.0523	0.0006136
	MC 2	81.582	0.0895	0.0008036
CS 2 (Elliptical)	MC 1	68.597	0.04426	0.0006136
	MC 2	67.773	0.07572	0.0006626
CS 3 (Oval)	MC 1	89.496	0.06272	0.0006139
	MC 2	88.382	0.10695	0.0008314
CS 4 (Trapezoidal)	MC 1	292.12	0.18567	0.0015452
	MC 2	286.88	0.31802	0.0026542

.

The von Mises criterion defines yielding as the shear strain energy equaling a critical value. The results for the von Mises stress show that the stem with CS 2 has a lower stress-induced strain than the other cross sections. Overall, the von Mises stress in CS 1, CS 3, and CS 4 is higher than CS 2 by 20.6%, 30.4%, and 325.8%, respectively. On similar lines, the deflections in CS 1, CS 3, and CS 4 are higher than CS 2 by 18.6%, 42.2%, and 319.1%, respectively. The strain rates in CS 1, CS 3, and CS 4 are higher than CS 2 by 21.6%, 25.7%, and 303.7%, respectively. From the results obtained, it can be concluded that overall, the implant with CS 2 has better mechanical properties regarding von Mises stress, deflection, and the strain rate obtained. This is the preliminary study to obtain the best cross-sectional implant for the different material combinations used, and hence, here, the contact regions are assumed to be bonded in nature.

Once the CS 2 cross-sectional hip implant was obtained to have better properties than other cross-sections, further simulations proceeded by considering the appropriate friction coefficient between the implant junctions. Accordingly, Figure 5 show the von Mises stress, deflection, and strain rate for CS 2 for MC 2.

3.2. Wear Rate

As the stem with the CS 2 cross-section is finalized, further analysis is carried out to calculate the wear rate across the stem-to-head junction for the walking cycle, considering all the material combinations. The wear rate for the walking cycle for the stem with CS 2 and two material combinations, MC 1 and MC 2, is reported in this section. The linear wear rate is evaluated per Equation 3, and the desirable wear rate is expected to be low for effective implant functioning. The wear rate calculation is based on one million cycles per year.

Table 5. Calculation of the linear wear rate in the implant.

Material Combination	Junction	Wear Coefficient	Sliding Distance (mm)	Pressure (MPa)	Wear (mm/Cycle)	Wear (mm/Year)
MC 1	Stem–Head	1.68 × 10−5	0.17926	136.44	4.109 × 10−7	0.411
MC 2		1.31 × 10−5	0.01863	24.39	5.94 × 10−9	0.006
MC 1	Head–Acetabular Cup	3.5 × 10−7	1.79 × 10−6	0.47995	3.00 × 10−16	3.00 × 10−10
MC 2		3.5 × 10−7	1.79 × 10−6	0.4794	3.00 × 10−16	3.00 × 10−10
MC 1	Acetabular Cup–Backing Cup	3.5 × 10−7	3.06 × 10−6	0.82066	8.78 × 10−16	8.78 × 10−10
MC 2		3.5 × 10−7	3.06 × 10−6	0.82254	8.82 × 10−16	8.82 × 10−10

below shows the linear wear rate induced at the implant at different junctions.

For the stem-to-head junction, it can be seen from Table 5 that the wear rate per year for the MC 2 material combination is much less than that of the MC 1, which is around 27 times. This is because of the sliding distance and contact pressure values at the stem-to-head junction induced by these two different material combinations. It can also be noted that wear rates do not change significantly for the material combinations for the other junctions.

The sliding distance and the contact pressure for two material combinations for the CS 2 cross-sectional hip implant are depicted in Figure 6 and Figure 7, respectively.

From Figure 6, the sliding distance observed for the MC 1 material combination was approximately four times higher than that observed for the MC 2 material combination. Similarly, from Figure 7, it is seen that the contact pressure is 5.5 times higher for MC 1 than for MC 2.

Figure 8 indicates the sliding distance between the head and the acetabular cup, whereas Figure 9 indicates the contact pressure for the same junction. Figure 10 indicates the sliding distance between the acetabular cup and the backing cup junction, whereas Figure 11 indicates the contact pressure for the same junction.

The contact pressure and sliding distance values observed in the hip implants demonstrate consistent results for both the head-to-acetabular cup and acetabular cup-to-backing cup junctions, regardless of the material combinations. However, at the stem-to-head junction, the contact pressure is 82% higher, and the sliding distance is 89% higher for MC 1 compared to the MC 2 material combination.

4. Discussion

This research work investigates the effects of the collar positioned at the stem when subjected to different loading conditions. The study focused on understanding how this collar influences the overall performance and stability of the structure during such loads. By examining these interactions, this study offers a novel perspective and contributes new insights, enhancing our knowledge of design considerations in similar applications. Additionally, this work evaluates the linear wear rate for all three junctions of a hip implant. The analysis includes two biocompatible material combinations: Co-Cr and Ti-6Al-4V alloys for the stem. The forces, moments, and constraints considered in the simulations were according to ASTM and ISO standards. These loads were applied to the backing cup, and only the walking cycle was considered for the simulation. The initial simulation was carried out by considering four novel designs of stems for which the loads and boundary conditions were added, with all the junctions as bonded contacts. The novelty of the implants here is the use of the collar, which has not been found in many of the previous works [50]. The results showed lower deformations and stress on the implant with an elliptical-shaped stem. The elliptical profile can enhance load distribution along the femoral canal and may improve rotational stability compared with circular designs. Furthermore, improved load transfer characteristics may reduce stress shielding in surrounding bone tissue, which is an important factor in long-term implant fixation. This is further used in the simulation for dynamic walking loading by using appropriate friction coefficients at the three junctions of the hip implants previously mentioned.

The wear debris generation in total hip arthroplasty is significantly affected by design variables, which include a mismatch in taper radius, size of the head considered, offsets of taper in the implants, and the length of the trunnion [51,52]. The position of the acetabular cup significantly affects the outcomes of hip arthroplasty. It influences dislocation, abductor muscle strength, gait, limb lengths, impingement, noise generation, wear, loosening, and cup failure [53]. The top and bottom radii of the trunnion are important variables to consider, as numerous research studies specifically focus on the head and stem of the trunnion junctions [54].

In our previous studies, the collar used two different shapes, namely circular and tapered, and it was seen that the circular-shaped collar produced less wear debris [48]. This is because it provides more flexibility and freedom at the collar junction. The results of this simulation show that, for the head-to-acetabular cup junction, the central region experiences the maximum contact pressure, while the circumferential region experiences the minimum values of sliding distance, which is also in accordance with the trends obtained from the literature [55]. The patients’ weight plays a crucial role in causing wear and tear on the contact surfaces, as indicated by some researchers [56]. However, there have been studies that show an opposite trend where the body mass index, age, and gender did not influence wear rate [57,58].

A higher friction coefficient typically indicates greater resistance to sliding, leading to higher wear rates and stress on the material. Lower deformation in MC 2 suggests better structural integrity maintenance under load than in MC 1. Lower strain in MC 2 indicates less elastic deformation under the same conditions, which is beneficial for material durability. Lower equivalent stress in MC 2 implies more efficient load handling without reaching high-stress levels, reducing the risk of material failure. Lower contact pressure, sliding distance, and frictional stress in MC 2 indicate less mechanical interaction and wear, which is advantageous for longevity. Wear rate is critical in determining material lifespan in joint applications. MC 2 significantly reduces the wear rate for the stem–head junction, making it a superior choice in wear resistance.

MC 2 is the optimal choice for applications necessitating robust and wear-resistant materials, particularly for the stem–head junction in orthopedic implants or similar mechanical systems. It delivers superior structural integrity, reduced stress and strain, and markedly decreased wear rates when compared to MC 1. Titanium alloys possess a lower Young’s modulus (114 GPa) compared with CoCr alloys (200 GPa), which results in improved load sharing and slightly higher elastic deformation under loading. This behavior distributes contact stresses more evenly across the junction surfaces, thereby reducing localized contact pressure and sliding interaction.

Limitations of the Current Work

This study has some limitations. The study only evaluates hip implant wear under linear elastic loading conditions. Furthermore, a finite element method calculation considering the corrosion rate is complex. Hence, our model does not currently incorporate the effect of corrosion, but we plan to address this in future work. Corrosion is not the predominant reason for debris creation compared to wear [48]. Further, only the walking cycle is considered in this study. Future studies shall include other day-to-day activities like cycling, jogging, standing, and stairs up and down. The femur bone can be considered along with the implant so that the wear rate along with the femur can be calculated.

5. Conclusions

Two distinct material combinations were thoroughly evaluated across four design variations in hip implants, all tested under dynamic loading conditions that closely mimic the forces experienced during walking. In the initial phase, it is focused on analyzing the cross-sectional profiles of each design, utilizing two well-established materials for the stem: titanium alloy Ti-6Al-4V and cobalt-chromium (CoCr) alloy. The results from this preliminary study indicated that the hip implant characterized by the cross-sectional two-profile demonstrated better performance, particularly in terms of reduced induced stresses and minimal displacements, when compared to the other variations.

Following this initial analysis, a detailed evaluation of the CS 2 implant, concentrating on the wear that occurs at the critical junction where the stem connects to the head of the implant during walking cycles. This assessment is vital, as it provides valuable insights into the durability and longevity of the implant, factors that are crucial for patient safety and health. The findings revealed that the Ti-6Al-4V material combination for the stem resulted in significantly less wear and tear compared to that observed with the CoCr alloy, highlighting its advantages in real-world applications.

Additionally, the study suggested that replacing the CoCr alloy with Ultra-High Molecular Weight Polyethylene (UHMWPE) for the acetabular cup material in the hip implant could effectively resolve the issue of squeaking noise often associated with implants. UHMWPE is known for its exceptional wear resistance and lower friction characteristics, making it a suitable alternative to CoCr. Nevertheless, it is essential that the overall lifespan of the implant is comprehensively evaluated, particularly through rigorous fatigue life assessments, to ensure its reliability and effectiveness over time.