Wear Behaviours and Oxidation Effects on Different UHMWPE Acetabular Cups Using a Hip Joint Simulator.

Given the long-term problem of polyethylene wear, medical interest in the new improved cross-linked polyethylene (XLPE), with or without the adding of vitamin E, has risen. The main aim of this study is to gain further insights into the mutual effects of radiation cross-linking and addition of vitamin E on the wear performance of ultra-high-molecular-weight polyethylene (UHMWPE). We tested four different batches of polyethylene (namely, a standard one, a vitamin E-stabilized, and two cross-linked) in a hip joint simulator for five million cycles where bovine calf serum was used as lubricant. The acetabular cups were then analyzed using a confocal profilometer to characterize the surface topography. Moreover; the cups were analyzed by using Fourier Transformed Infrared Spectroscopy and Differential Scanning Calorimetry in order to assess the chemical characteristics of the pristine materials. Comparing the different cups' configuration, mass loss was found to be higher for standard polyethylene than for the other combinations. Mass loss negatively correlated to the cross-link density of the polyethylenes. None of the tested formulations showed evidence of oxidative degradation. We found no correlation between roughness parameters and wear. Furthermore, we found significantly differences in the wear behavior of all the acetabular cups. XLPEs exhibited lower weight loss, which has potential for reduced wear and decreased osteolysis. However, surface topography revealed smoother surfaces of the standard and vitamin E stabilized polyethylene than on the cross-linked samples. This observation suggests incipient crack generations on the rough and scratched surfaces of the cross-linked polyethylene liners.

On the roughness measurement on knee prostheses.

The majority of total knee replacements currently implanted present an articulation composed of two metal parts, femoral and tibial components, between which there is a polyethylene insert serving as a bearing surface. The finishing surface of the metal components is a very important factor in minimizing the polyethylene wear rate and, later, the gradual production of metal and plastic debris. Considering the role of surface roughness on volumetric wear rates, the purpose of this study is to develop a protocol for the roughness characterization of total knee prosthesis (TKP) metal components, taking into consideration a limited number of points on each surface. Six mobile TKP of different sizes (three size 2 and three size 6) were tested on a knee joint simulator to compare the wear behavior of each group. After 2 million cycles the weight loss by the polyethylene inserts was measured with gravimetric method and the surface roughness of the metallic components was assessed in terms of average surface roughness, Ra, skewness, Rsk, and total roughness, Rt. Roughness measurement involved 29 points on each femoral condyle and 26 points on each metal tibial plate. The data collected has shown an increased roughness upon wear testing for both the investigated TKP sizes. No statistical differences were observed between the two groups for both the parameters Ra, Rsk, and Rt. The surface of all metallic components became more negatively skewed, indicating diminishing peaks. The various parameters were correlated to the volumetric loss using a linear regression analysis.

In vitro effects on mobile polyethylene insert under highly demanding daily activities: stair climbing.

PURPOSE: Wear and survival of total joint replacements do not depend on the duration of the implant in situ, but rather on the amount of its use, i.e. the patient's activity level. With this in mind, the present study was driven by two questions: (1) How does total knee replacement (TKR) respond to the simulation of daily highly demanding activities? (2) Are certain activities to be advised against or, on the contrary, useful to implanted patients, in order to reduce wear of TKR and its related problems? METHODS: One set of the same total knee prosthesis (TKP), equal in design and size, was tested on a three-plus-one knee joint simulator for two million cycles using a highly demanding daily load waveform, replicating a stair-climbing movement. The results were compared with a set of TKP previously tested with the ISO level walking task. A digital microscope was used to characterise the superficial structure of all the TKPs. Gravimetric and micro-Raman spectroscopic analyses were carried out on the polyethylene inserts. Visual comparison with in vivo explants was carried out. RESULTS: The average volumetric mass loss after two million cycles was 44 ± 6 mm(3). Microscope examinations showed some deep scratches along the flexion/extension movements for all the components. Also, the metallic backside surface showed intense non-linear scratches and the polyethylene counterface was characterised by some craters. A decrease in crystallinity, induced by mechanical stress was observed on all polyethylene components and was quantitatively confirmed by the orthorhombic fraction αo value. CONCLUSIONS: The results of this study demonstrated that the forces and motion sustained by the knee are highly activity-dependent. Moreover, this test confirmed that under more severe conditions, the material properties change according to a different wear mechanism and a decrease in crystallinity occurs. Loading characteristics for specific activities should be considered for the design of functional and robust TKRs.

A new protocol from real joint motion data for wear simulation in total knee arthroplasty: stair climbing.

In its normal lifespan, a knee prosthesis must bear highly demanding loading conditions, going beyond the sole activity of level walking required by ISO standard 14243. We have developed a protocol for in vitro wear simulation of stair climbing on a displacement controlled knee simulator. The flexion/extension angle, intra/extra rotation angle, and antero/posterior translation were obtained in patients by three-dimensional video-fluoroscopy. Axial load data were collected by gait analysis. Kinematics and load data revealed a good consistence across patients, in spite of the different prosthesis size. The protocol was then implemented and tested on a displacement controlled knee wear simulator, showing an accurate reproduction of stair climbing waveforms with a relative error lower than 5%.