1 Introduction

Total hip arthroplasty (THA) is one of the most successful and transformative procedures in orthopaedics, restoring mobility and quality of life for millions worldwide. Among the available bearing options, the metal-on-polyethylene (MoP) configuration—combining a cobalt-chrome femoral head with an ultra-high-molecular-weight polyethylene (UHMWPE) liner—has become the clinical gold standard due to its durability, biocompatibility, and cost-effectiveness. Yet, as implant longevity improves, wear-related complications, particularly polyethylene debris-induced osteolysis and aseptic loosening, have emerged as the predominant causes of late failure.¹,²

The UHMWPE liner remains the weakest link in the MoP system. Repetitive gait loading produces micro-scale wear particles that initiate bone resorption and implant loosening.³,⁴ Under adverse conditions such as subluxation, malposition, or excessive laxity, the femoral head can contact the liner rim, producing localized “edge loading.” These high-stress interactions accelerate fatigue, cracking, and stripe wear, significantly reducing implant lifespan.⁵,⁶

Preclinical wear testing of hip implants typically follows the ISO 14242 series of standards. ISO 14242-1 and ISO 14242-3 simulate normal gait conditions but assume perfect alignment and continuous contact between components—conditions rarely achieved in vivo.⁵ Consequently, these tests may underestimate real-world wear. To address this limitation, ISO 14242-4 introduces controlled micro separation and edge-loading events, replicating physiological rim impacts through lateral offsets and adjusted load profiles.

Understanding how these different ISO protocols influence contact mechanics and wear is vital for designing more durable implants. But despite these advances, a comprehensive computational comparison of the different ISO loading standards has not yet been established. Existing studies largely focus on individual test protocols or employ quasi-static analyses that fail to capture the dynamic interactions occurring during gait.⁷,⁸ The lack of a unified dynamic finite element framework limits our ability to quantify how each testing regimen influences contact mechanics, stress distribution, and wear. Previous studies have investigated edge-loading conditions in hip implants; however, most did not account for the rotational motion of the femoral head. In reality, the femoral head undergoes simultaneous translational and rotational movements during gait, which significantly influence contact mechanics and wear behaviour. The inclusion of such combined motion is a key feature of the ISO 14242-4 hip simulator protocol, which more accurately replicates physiological conditions.⁹,¹⁰ Nevertheless, to the best of the authors’ knowledge, no comprehensive comparative study has yet examined the loading characteristics or stress concentrations in metal-on-polyethylene implants under the ISO 14242-4 standard.

The present study addresses this critical research gap by developing a fully dynamic finite element model of a metal-on-UHMWPE total hip replacement to simulate three distinct ISO testing protocols: normal gait (ISO 14242-1), orbital motion (ISO 14242-3), and micro separation with edge loading (ISO 14242-4).¹¹,¹² Through a direct comparison of contact pressures, contact areas, and predicted wear across these conditions, the study offers new insights into the mechanical implications of incorporating micro separation and edge loading in preclinical assessments.¹³,¹⁴ This work not only establishes a computational framework for evaluating implant behaviour under varied ISO loading regimens but also provides practical guidance for researchers and manufacturers in selecting appropriate testing protocols.¹⁵ By visualizing stress, strain, and deformation patterns under realistic dynamic conditions, the findings contribute to the development of more physiologically representative testing standards and support the design of safer, longer-lasting metal-on-polyethylene hip implants.

2 Materials and methods

The model geometry was based on a ball-in-socket configuration, with the femoral head represented as a sphere of 22 mm diameter and the acetabular cup modelled as a hemispherical liner with diametrical clearance of 0.46 mm and thickness 8 mm (Figure 1A). A 22 mm femoral head was selected to maintain consistency with historical ISO wear-testing benchmarks and to isolate the mechanical influence of loading protocol without confounding effects of head diameter. While larger heads (32–36 mm) are common clinically, the current study focuses on relative differences between ISO standards rather than absolute wear magnitudes.

The acetabular cup, made of Ultra-High Molecular Weight Polyethylene (UHMWPE), was represented using a linear elastic model with a Young's modulus of 500 MPa and a poisons ratio 0.4. The present model captures short-term elastic response and comparative stress localization. Viscoelastic creep and plasticity, which dominate long-term behaviour, were intentionally excluded to preserve numerical stability in multi-million cycle simulations.¹⁶ The femoral head was modelled as a rigid body, assuming CoCr composition. Because of its much higher stiffness relative to UHMWPE, deformation and wear of the metallic head were considered negligible.¹³

The femoral head and acetabular liner were discretized using tetrahedral second-order elements (SOLID187) with an average edge length of approximately 1.5 mm. Contact interactions at the articulating surfaces were defined using CONTA174 and TARGE170 elements, ensuring accurate representation of the head–liner interface. A mesh convergence check was performed, confirming that the selected element size provided stable predictions of contact pressure and stress distribution without imposing excessive computational cost.

The contact between the femoral head and acetabular liner was modelled as asymmetric, with the liner defined as the contact body and the head as the target, since only the polymeric surface undergoes wear. The Augmented Lagrange formulation with nodal-normal detection ensured stable convergence, and a friction coefficient of 0.2 was applied to represent metal-on-UHMWPE articulation.

To simulate the progression of wear, the Archard wear law was adopted as the governing wear model.¹⁶,¹⁷ Archard's law primarily reflects adhesive wear and does not explicitly account for surface fatigue or delamination. However, it remains the most widely adopted wear framework in ISO-based computational studies for relative wear comparison. Since all ISO cases used identical parameters, differences in wear arise purely from kinematic and contact mechanics.¹ In its classical form, Archard's equation is expressed as:(1)V=k∗L∗S

In ANSYS Workbench, the wear simulation was implemented through the TB, WEAR APDL command, which automatically supports the generalized Archard formulation given as: Eq 2(2)h˙=kH∗pm∗vnwhere, h˙ is the wear rate at each point, H is the material hardness, and m and n are the exponents associated with the pressure(P) and sliding velocity(v) dependencies. In this study H, m and n were both set to unity, thereby reducing the generalized equation to the standard local Archard expression (Eq. (2)). The wear coefficient (k) was assigned a literature-reported uncalibrated value of 1.066 ∗ 10⁻⁹mm³/N·mm, while the simulation time period for wear progression was controlled via the TBFIELD command. In the generalized Archard formulation, the coefficients m and n define the sensitivity of wear to contact pressure and sliding velocity. Experimental calibration of these parameters for UHMWPE under combined dynamic micro-separation and edge-loading conditions is currently limited. Accordingly, m and n were set to unity, reducing the formulation to the classical Archard wear law commonly applied in ISO-based computational hip wear studies for relative comparisons.^1,13,16. The hardness term H was similarly normalized to maintain consistency across all loading cases. Under these assumptions, identical wear parameters were applied to all simulations, such that differences in predicted wear are governed solely by variations in kinematics and contact mechanics. Consequently, the results should be interpreted as comparative indicators of wear severity rather than absolute predictors of clinical wear.

The result obtained is compared with the experimental values obtained in previous studies. The pressure, volumetric wear, path followed by femoral head, deformation is analysed and compared.

3 Results and discussion

Analysis of volumetric wear under different standardized and physiological conditions provides a critical benchmark for hip implant performance.²² Under ISO 14242-1 and ISO 14242-3 simulations, wear progressed nearly linearly over five million cycles, with total volumes of 68.98 mm³ and 101 mm³, respectively (Fig. 2A). The simpler sinusoidal motion of ISO 14242-1 yielded lower wear, while the more complex kinematics of ISO 14242-3 substantially increased cumulative wear, highlighting the limitations of oversimplified protocols and the importance of accurately capturing joint motion. The volumetric wear values for the ISO 14242-1 and ISO 14242-3 values are near to the test and studies conducted before on metal on plastic implants under similar conditions.⁴,¹⁷ This validate the accuracy of the model and validation can be done using this and the convergence of model.

Adverse loading conditions, simulated using ISO 14242-4, produced a dramatic acceleration of wear. After five million cycles, ISO 14242-4-1 and ISO 14242-4-3 generated 383 mm³ and 478 mm³ of wear, more than five times the standard ISO 14242-1 values. This kind of large difference is matching with the previous studies of edge loading conducted in metal on ceramic implants.¹⁸ These results demonstrate that edge loading and micro separation induce a fundamentally different, highly destructive wear mechanism, emphasizing the critical impact of realistic testing conditions on predicting long-term implant performance.

Contact pressure analysis mirrored these trends. The highest pressures were observed under ISO 14242-4 with ISO 14242-3 loading, while all other conditions, except ISO 14242-4-1, exhibited similar and lower pressures (Fig. 2B). Standard protocols (ISO 14242-1 and ISO 14242-3) produced consistent, reproducible pressures, validating their use as benchmarks for ideal conditions. ISO 14242-4 protocols showed sharp pressure spikes due to force concentration on a reduced contact area, directly reflecting edge-loading effects and highlighting the mechanical severity of adverse conditions.

The motion of the femoral head for the four cases was visualized by tracking the initial contact point, with data generated and plotted using Python and it revealed a clear distinction in kinematics between standard and adverse loading conditions (Fig. 2 C and D). Under ISO 14242-1 and ISO 14242-3, the trajectories were smooth, contained, and repetitive, reflecting the simplified gait these standards replicate and providing a reliable baseline for ideal implant performance.

ISO 14242-4-1 and ISO 14242-4-3 exhibited pronounced deviations, with the femoral head following an extended, erratic path characteristic of micro separation and edge loading (Fig. 2C and D). Notably, the deviation of the femoral head from ISO 14242-3 to ISO 14242-4-3 was greater than that from ISO 14242-1 to ISO 14242-4-1, consistent with the higher volumetric wear observed in the ISO 14242-4-3 case. This non-concentric motion concentrates load onto a small area, such as the superior rim of the acetabular cup, explaining the sharp increases in contact pressure and volumetric wear observed under adverse conditions.¹⁰,²³

Maximum-stress analysis (Fig. 3A and B), time-series data (Fig. 4A and B), and contour plots (Fig. 4C and D) reveal two distinct stress-response families across the ISO protocols. ISO 14242-1 and ISO 14242-3 produce concentric, symmetric stress fields that decrease from the rim toward the centre, with modest peak von Mises stresses localized at the distal rim. This distribution reflects conforming contact geometry, spreading load over a larger area and promoting uniform stress distribution, which supports progressive, distributed wear without immediate rim damage.

ISO 14242-4 (4-1 and 4-3) introduces medial–lateral displacement, reducing conformity and shifting contact toward the rim. This generates early asymmetry, localized rim stresses, and sharply rising strain energy, particularly from 75 % of the gait cycle(Fig. 4C and D). ISO 14242-4-3 exhibits the highest, most spatially extensive rim stresses. The concentrated stress and elastic energy at the rim explain edge-loading behaviour, leading to accelerated wear, local plasticity, and potential fatigue crack initiation.

The two families imply distinct failure modes: ISO 14242-1/3 promotes distributed adhesive/abrasive wear, whereas ISO 14242-4 drives localized rim damage and catastrophic failure risk. These observations aligned with retrieved-sample studies, where concentrated high strains at a fixed rim location provoked crack nucleation and rim fragmentation, whereas more uniform stresses resulted in generalized wear²,⁴,²⁴

The penetration analysis provides a direct physical link between kinematics, contact pressure, and volumetric wear. Under standard ISO 14242-1 and ISO 14242-3 conditions, box plots show consistently low and tightly clustered femoral head penetration into the cup (Fig. 5A). This shallow, predictable penetration reflects the smooth, concentric motion of the head within the acetabular cup, maintaining a broad contact area. Consequently, load is distributed evenly, producing low peak pressures (∼14.5 MPa) and linear, predictable volumetric wear. Minor variations between ISO 14242-1 and ISO 14242-3 slightly increased wear, highlighting that even small kinematic differences can influence implant performance. These results align closely with previously reported experimental and retrieval studies.²⁴,²⁵

Adverse ISO 14242-4-1 and ISO 14242-4-3 conditions demonstrated a dramatic increase in both mean penetration depth and variability. The maximum penetration, at each section of gait cycle, is almost same and having similar pattern from 35 % to 50 % of the cycle for normal ISO condition and edge loading condition. An abnormal change was observed from 0 to 35 % and 50–100 % (Fig. 5B and C). This “levering-out” motion, observed in the kinematic analysis, reduces concentric articulation, concentrating joint forces on a small rim region and generating extreme pressure spikes (>50 MPa) and elevated strain energy. Volumetric wear in these cases (383–478 mm³) (Fig. 2A) was over five times greater than standard conditions, reflecting localized abrasive wear at the superior rim and confirming findings from retrieved samples.¹⁶,²⁴

Contact visualization further supports these observations. ISO 14242-1 and ISO 14242-3 show broad, centered contact areas throughout the gait cycle, explaining the low, linear wear and moderate pressures (Fig. 6). ISO 14242-4 conditions reveal progressive contact area shrinkage and rim localization, particularly at 75 % of the cycle, physically demonstrating micro-separation and edge loading. The adhesive wear occurring in the cup is demonstrated as the sliding and the wear pattern obtained from the retrieved sample also justifies the same.²,⁴,²⁴ Under ISO 14242-4-1 and 14242-4-3, edge loading drives abrasive wear at the superior rim, whereas standard ISO gait conditions produce wear below and along the rim, consistent with patterns observed in retrieved implants.

Deformation contours reveal two distinct mechanical responses driven by ISO loading kinematics (Fig. 7). ISO 14242-1 and 14242-3 maintain conforming contact: deformation fields are broadly concentric, gradually increasing over the gait cycle, with peak displacement on the distal cup. ISO 14242-4-1 and 14242-4-3 introduce medial–lateral displacement, which reduces conformity and shifts contact toward the rim. This produces early asymmetry, progressive rim-localized displacement, and late-cycle surges in peak von Mises stress and strain energy (rising from ≈187 to 190 mJ in ISO-1/3 to ≈250–259 mJ in ISO-4) (Fig. 3B and 4A).

The altered kinematics in the 4-series induce local bending of the liner and reduced contact area, producing higher localized strains and contact pressures. Since damage initiation is governed by both peak stress and stored elastic energy, these maps indicate that ISO 14242-4, especially 4-3, is far more likely to cause rim plasticity, accelerated edge wear, and notch-driven crack initiation, whereas ISO-1/3 favours distributed wear. These observations support prioritizing 4-series tests for rim stability verification, and motivate reporting time-resolved maps of displacement, contact area, contact pressure, von-Mises stress and strain-energy density when comparing designs.

4 Conclusion

The comparative analysis of ISO 14242 standards for metal-on-plastic hip implants clearly demonstrates the critical differences between standard and adverse loading regimes (Table 1). Under ISO 14242-1 and ISO 14242-3, the implants exhibited predictable kinematics, broad and stable contact patches, low penetration depths, and modest peak pressures (∼14–15 MPa). These conditions resulted in relatively uniform stress distributions and a gradual, linear progression of volumetric wear, reflecting scenarios of idealized implant performance. Such findings confirm that the earlier ISO protocols provide reliable baselines for evaluating wear under normal gait cycles.

ISO 14242-4 series produced distinctly adverse mechanical behaviours. Both ISO14242-4-1 and ISO14242-4-3 loading conditions induced rim and edge contact, characterized by superiorly located, highly localized von Mises stress peaks, dramatic increases in strain energy, and penetration depth variability. These effects translated into pressure spikes exceeding 50 MPa and volumetric wear rates more than five times higher than those observed under standard protocols. The data confirm that micro-separation and edge loading represent fundamentally different and destructive wear mechanisms, directly associated with rim damage, accelerated material removal, and potential implant failure. Collectively, the findings reinforce the necessity of incorporating ISO 14242-4 protocols as worst-case testing scenarios in preclinical evaluation. They also highlight the importance of refining implant design and material selection to mitigate rim loading effects, ensuring durability under both normal and adverse in vivo conditions.

CRediT authorship contribution statement

S. Nithin: Visualization, Software, Methodology, Conceptualization, Investigation, Writing – original draft, Writing-review & editing.

S. Kanagaraj: Supervision, Resources, Conceptualization, Investigation, Visualization.

Ethical statement

This study did not involve human participants, patient data, animal subjects, or clinical interventions. All analyses were conducted using computational finite element modelling based on publicly available standards and previously published biomechanical loading data. Therefore, approval from an institutional ethics committee was not required.

Patient and/or guardian informed consent

Not applicable. This study did not include any human participants, patient records, clinical images, or identifiable patient information; therefore, informed consent from patients or guardians was not required.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.