Influence of hip joint simulator design and mechanics on the wear and creep of metal-on-polyethylene bearings

Tribomechanical Properties of PVA/Nomex® Composite Hydrogels for Articular Cartilage Repair

Due to the increasing prevalence of articular cartilage diseases and limitations faced by current therapeutic methodologies, there is an unmet need for new materials to replace damaged cartilage. In this work, poly(vinyl alcohol) (PVA) hydrogels were reinforced with different amounts of Nomex® (known for its high mechanical toughness, flexibility, and resilience) and sterilized by gamma irradiation. Samples were studied concerning morphology, chemical structure, thermal behavior, water content, wettability, mechanical properties, and rheological and tribological behavior. Overall, it was found that the incorporation of aramid nanostructures improved the hydrogel's mechanical performance, likely due to the reinforcement's intrinsic strength and hydrogen bonding to PVA chains. Additionally, the sterilization of the materials also led to superior mechanical properties, possibly related to the increased crosslinking density through the hydrogen bonding caused by the irradiation. The water content, wettability, and tribological performance of PVA hydrogels were not compromised by either the reinforcement or the sterilization process. The best-performing composite, containing 1.5% wt. of Nomex®, did not induce cytotoxicity in human chondrocytes. Plugs of this hydrogel were inserted in porcine femoral heads and tested in an anatomical hip simulator. No significant changes were observed in the hydrogel or cartilage, demonstrating the material's potential to be used in cartilage replacement.

How does bicycling affect the longevity of Total Hip Arthroplasty? A finite element wear analysis

As the number of young and active individuals undergoing Total Hip Arthroplasty (THA) are increasing yearly, there is a need for hip prostheses to have increased longevity. Current investigations into the longevity of these prostheses only include walking as the patient's activity as there is limited data on the amount and intensity of other activity performed by the patient. To further understand the evolution of wear and increase the longevity of these implants, the impact of different activities on the hip prosthesis needs to be investigated. In this study, a finite element model and wear algorithm was developed to simulate both walking and bicycling over a 5-year period. The XLPE acetabular cup volumetric wear rate was found to be 33 mm³/yr while the femoral head taper wear rates were between 0.01 - 0.39 mm³/yr. The results showed that by adding bicycling of up to 80 km per week with normal walking activity, the XLPE mean volumetric wear rate increased by 67% and the metallic mean volumetric wear rate by 11%. However, the patient may gain further health benefits from this additional activity. Assistive electric bikes may also be used to further reduce the loads on the hip joint, allowing for lower amounts of wear.

Development and Validation of a Finite Element Model of Wear in UHMWPE Liner Using Experimental Data From Hip Simulator Studies

The wear of acetabular liner is one of the key factors determining osseointegration and long-term performance of total hip joint replacement implants. The experimental measurements of wear in total hip replacement components are time and cost-intensive. While addressing this aspect, a finite element model of a hip joint bearing consisting of zirconia-toughened alumina femoral head and ultrahigh molecular weight polyethylene liner was developed to predict the dynamic wear response of the liner. The Archard-Lancaster equation, consisting of surface contact pressure, wear rate, and sliding distance, was employed to predict the wear of the acetabular liner. The contact pressure and wear at the articulating surface were found to decrease over time. A new computational method involving three-dimensional point clouds from the finite element analyzed results were used to construct wear maps. The model was able to predict the linear wear, over 2 × 106 cycles with relative errors ranging from 9% to 36% when compared to the published results. The increasing error percentage occurring primarily from the use of a constant wear rate was reduced to a maximum of 17% by introducing a correction factor. The volumetric rate was predicted with a maximum relative error of 7% with the implementation of the correction factor. When the model was implemented to study acetabular liners of diameters ranging from 28 to 36 mm, the linear wear was seen to decrease with an increase in femoral head diameter, which is in agreement with the clinical data. This study emphasizes the need to develop more such FEA-based computational studies to reliably predict and correlate with experimentally measured temporal evolution of wear of load-bearing articulating joints.

Computational method for bearing surface wear prediction in total hip replacements

Total hip replacement (THR) is a revolutionary treatment when a hip joint becomes severely damaged. Wear is known as one of the main reasons for THR failure. Current experimental techniques to investigate the wear at the bearing surfaces of THRs are time-consuming, complicated and expensive. In this study, an in-house fretting wear algorithm has been further developed to investigate the wear damage that occurs on bearing surfaces of THRs and its consequence on the longevity of the implants. A 3D finite element model has been created with a 36 mm diameter Cobalt-Chromium femoral head and a 4 mm thick cross-linked polyethylene bearing liner. A gait loading cycle was used to simulate walking for up to 5 million cycles (Mc). The wear algorithm extracts relative displacements and contact shear stresses from the finite element package to predict the linear and volumetric wear rates. This method is shown to have modelled the evolution of wear effectively and found it to be similar to those from experimental analyses. The linear and volumetric wear per million cycles predicted in this study were 0.0375mm/Mc and 33.6mm³/Mc which are comparable to those measured in-vivo THRs. The wear patterns obtained from this study are also comparable to the wear patterns shown on available conventional polyethylene liners. This method can be used to further aid in the design and clinical technique to reduce wear rate in THRs.

Finite element analysis of polyethylene wear in total hip replacement: A literature review

Evaluation and prediction of wear play a key role in product design and material selection of total hip replacements, because wear debris is one of the main causes of loosening and failure. Multifactorial clinical or laboratory studies are high cost and require unfeasible timeframes for implant development. Simulation using finite element methods is an efficient and inexpensive alternative to predict wear and pre-screen various parameters. This article presents a comprehensive literature review of the state-of-the-art finite element modelling techniques that have been applied to evaluate wear in polyethylene hip replacement components. A number of knowledge gaps are identified including the need to develop appropriate wear coefficients and the analysis of daily living activities.

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.