Evaluation of a new methodology to simulate damage and wear of polyethylene hip replacements subjected to edge loading in hip simulator testing

Importance of dynamics in the finite element prediction of plastic damage of polyethylene acetabular liners under edge loading conditions

After hip replacement, in cases where there is instability at the joint, contact between the femoral head and the acetabular liner can move from the bearing surface to the liner rim, generating edge loading conditions. This has been linked to polyethylene liner fracture and led to the development of a regulatory testing standard (ISO 14242:4) to replicate these conditions. Performing computational modelling alongside simulator testing can provide insight into the complex damage mechanisms present in hard-on-soft bearings under edge loading. The aim of this work was to evaluate the need for inertia and elastoplastic material properties to predict kinematics (likelihood of edge loading) and plastic strain accumulation (as a damage indicator). While a static, rigid model was sufficient to predict kinematics for experimental test planning, the inclusion of inertia, alongside elastoplastic material, was required for prediction of plastic strain behaviour. The delay in device realignment during heel strike, caused by inertia, substantially increased the force experienced during rim loading (e.g. 600 N static rigid, ∼1800 N dynamic elastoplastic, in one case). The accumulation of plastic strain is influenced by factors including cup orientation, swing phase force balance, the moving mass, and the design of the device itself. Evaluation of future liner designs could employ dynamic elastoplastic models to investigate the effect of design feature changes on bearing resilience under edge loading.

Current Preclinical Testing of New Hip Arthroplasty Technologies Does Not Reflect Real-World Loadings: Capturing Patient-Specific and Activity-Related Variation in Hip Contact Forces

BACKGROUND

Total hip arthroplasty (THA) implants are routinely tested for their tribological performance through regulatory preclinical wear testing (eg, ISO-14242). The standardized loading conditions defined in these tests consist of simplified waveforms, which do not specifically represent in vivo loads in different groups of patients. The aim of this study is to investigate, through musculoskeletal modeling, patient-specific and activity-related variation in hip contact forces (HCFs) in a large cohort of THA patients during common activities of daily living (ADLs).

METHODS

A total of 132 THA patients participated in a motion-capture analysis while performing different ADLs, including walk, fast walk, stair ascent, and descent (locomotor); sit to stand, stand to sit, squat, and lunge (nonlocomotor). HCFs were then calculated using the AnyBody Modeling System and qualitatively compared across all activities. The influence of gender on HCFs was analyzed through statistical parametric mapping analysis.

RESULTS

Systematic differences were found in HCF magnitudes and individual components in both locomotor and nonlocomotor ADLs. The qualitative analysis of the ADLs revealed a large range and a large variability in forces experienced at the hip during different activities. Significant differences in the 3-dimensional loading patterns were observed between males and females across most activities.

CONCLUSION

THA patients present a large variability in the forces experienced at the hip joint during their daily life. The interpatient variation might partially explain the heterogeneity observed in implant survival rates. A more extensive preclinical implant testing standard under clinically relevant loading conditions has been advocated to better predict and avoid clinical wear problems.

The effect of femoral head size on edge loading in metal-on-metal hip joint replacement under dynamic separation conditions

Edge loading that occurs in hip joint replacements due to dynamic separation of the joint bearings has been shown to cause severe wear for meal-on-metal bearings. In the present study, the multibody dynamics model for metal-on-metal (MoM) hip joints with a medial-lateral translational mismatch in the centers of rotation of the cup and head has been developed to predict the dynamic separation and contact force of edge loading under gait loading conditions. The effects of larger head diameters (28-55 mm), in combination with the translational mismatch (0-4 mm) and varied cup inclination angles (45°-65°), on edge loading of MoM bearings have been computationally investigated. For the given translational mismatch, increasing head diameters results in negligible effects on the dynamic separation, contact force and severity of edge loading. Increasing head size also leads to increased offset loading torque which has been found to reach at the level that may cause cup loosening under larger translational mismatch at 4 mm. The result highlights the importance of the cup inclination angle of 45° and a lower translational mismatch to avoid severe edge loading.

Hip simulator testing of the taper-trunnion junction and bearing surfaces of contemporary metal-on-cross-linked-polyethylene hip prostheses

Adverse reaction to metal debris released from the taper-trunnion junction of modular metal-on-polyethylene (MoP) total hip replacements (THRs) is an issue of contemporary concern. Therefore, a hip simulator was used to investigate material loss, if any, at both the articulating and taper-trunnion surfaces of five 32-mm metal-on-cross-linked-polyethylene THRs for 5 million cycles (Mc) with a sixth joint serving as a dynamically loaded soak control. Commercially available cobalt-chromium-molybdenum femoral heads articulating against cross-linked polyethylene (XLPE) acetabular liners were mounted on 12/14 titanium (Ti6Al4V) trunnions. Weight loss (mg) was measured gravimetrically and converted into volume loss (mm³ ) for heads, liners, and trunnions at regular intervals. Additionally, posttest volumetric wear measurements of the femoral tapers were obtained using a coordinate measuring machine (CMM). The surface roughness (Sa) of femoral tapers was measured posttest. After 5 Mc, the mean volumetric wear rate for XLPE liners was 2.74 ± 0.74 mm³ /Mc. The CMM measurements confirmed material loss from the femoral taper with the mean volumetric wear rate of 0.045 ± 0.024 mm³ /Mc. The Sa on the worn area of the femoral taper showed a significant increase (p < 0.001) compared with the unworn area. No other long-term hip simulator tests have investigated wear from the taper-trunnion junction of contemporary MoP THRs. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 108B:156-166, 2020.

A novel method to measure rim deformation in UHMWPE acetabular liners

Fluoroscopy studies of total hip replacement (THR) have shown that the femoral head and acetabular cup can separate in vivo, causing edge loading on the rim of the cup. Pre-clinical testing of THR involves ISO standard motion and loading parameters that are representative of a standard walking gait. However, a requirement for more robust testing of THR has been identified and protocols for edge loading in hip simulators have been developed. This technical note describes a method to measure rim wear and deformation on ultra-high molecular weight polyethylene acetabular liners using 2D contacting profilometry and Matlab^® analysis. The method is demonstrated on liners that have been subjected to edge loading in hip simulator tests and that have been retrieved at revision surgery. A quantitative and qualitative evaluation of the rim deformation was performed with good repeatability using the method.