Does the addition of vitamin E to conventional UHMWPE improve the wear performance of hip acetabular cups? Micro-Raman characterization of differently processed polyethylene acetabular cups worn on a hip joint simulator

In knee replacements, vitamin E-doped ultra-high molecular weight polyethylene (UHMWPE) shows a better wear behavior than standard UHMWPE. Therefore, different sets of polyethylene (PE) acetabular cups, i.e. standard UHMWPE and cross-linked polyethylene irradiated with 50 kGy and 75 kGy, were compared, at a molecular level, with vitamin E-doped UHMWPE to evaluate their wear performance after being tested on a hip joint simulator for five million cycles. Unworn control and worn acetabular cups were analyzed by micro-Raman spectroscopy to gain insight into the effects of wear on the microstructure and phase composition of PE. Macroscopic wear was evaluated through mass loss measurements. The data showed that the samples could be divided into two groups: 1) standard and vitamin E-doped cups (mass loss of about 100 mg) and 2) the cross-linked cups (mass loss of about 30-40 mg). Micro-Raman spectroscopy disclosed different wear mechanisms in the four sets of acetabular cups, which were related to surface topography data. The vitamin E-doped samples did not show a better wear behavior than the cross-linked ones in terms of either mass loss or morphology changes. However, they showed lower variation at the morphological level (lower changes in phase composition) than the UHMWPE cups, thus confirming a certain protecting role of vitamin E against microstructural changes induced by wear testing.

Fractal Characterization and Classification Characteristics of the Artificial Joint Wear Particles / 医用生物力学

Objective To study the morphology and fractal characterization of UHMWPE wear particles by simulation experiment on knee joint, analyze the classification characteristics of wear particles and discuss the correlation between fractal dimension and wear state. Methods The knee joint simulator was used to realize the knee joint wear motion. The forged CoCrMo alloy and UHMWPE were selected as artificial joint prosthesis materials. The wear particle extraction was based on ISO 17853. The automatic extraction and fractal identification system of wear particles were used to investigate fractal characteristics of wear particles. Three kinds of models were established to classify and recognize wear particles, by using the meshing method, cluster analysis and genetic simulated annealing algorithm, respectively. Results The fractal characteristics of UHMWPE single wear particles was very obvious. The fractal dimension calculated by radar fractal method decreased, during the transition from the larger size of strip debris to the smaller size of roundness debris, and the fractal dimension D of spherical debris was close to zero. The weighted sum of squared error values of fractal dimension for wear particle population classification was the smallest by the model of genetic simulated annealing algorithm, and the clustering feature was very obvious. When the wear cycles were low, the large fractal dimension of strip, needle and fibrous abrasive debris with larger fractal dimension had the largest proportion, and the main abrasion modes were the ploughing and spalling wears. With the extension of wear period, the proportion of wear particles with large fractal dimension decreased, and the proportion of flaky, blocky and near-spherical wear particles with low fractal dimension increased clearly. The wear mechanism changed to the fatigue and adhesive wear, and the wear state transited to the composite wear period. During the stable wear period, the proportion of all kinds of abrasive grains changed little. Due to the increase in the number of small particles, the fractal dimension decreased in stable wear state. Conclusions Based on the improved radar graph method, the fractal dimension of wear particles with different profiles could be obtained by automatic extraction and fractal identification system of wear particles. The research findings can be used in shape extraction, fractal dimension calculation and parameter statistics, as well as providing a new digital analysis tool for identification and diagnosis for wear particles of artificial prosthesis.

Análisis numérico sobre esfuerzos y áreas de contacto en una PTR Scorpio II® Stryker®. Base para el diseño de PTR personalizada al fenotipo Mexicano / Numerical analysis on stresses and contact areas in a TKR Scorpio II® of Stryker®. Basis to design of customized TKR in accordance to Mexican Phenotype

El desgaste de los insertos de Polietileno de Ultra-Alto Peso Molecular (UHMWPE pos sus siglas en inglés) continúa afectando la longevidad de las prótesis totales de rodilla (PTR) junto con el aflojamiento aséptico, y ambos constituyen las dos principales causas de falla de las prótesis. Considerando esto, es necesario encontrar soluciones adecuadas para evitar el desgaste excesivo y hasta la ruptura de los insertos de polietileno. En este trabajo se realizó el estudio mediante simulación numérica de una PTR Scorpio II® Stryker®, la cual se retiró por desgaste del inserto de UHMWPE en el Hospital 1° de Octubre del ISSSTE en México. Se utilizaron las hipótesis de Bartel et al. (1995) y Chillag et al. (1991) para la validación del método numérico utilizado, las cuales establecen que el desgaste del polietileno puede reducirse utilizando insertos tibiales de mayor espesor, lo cual disminuye las presiones de contacto. Los análisis se realizaron mediante MEF variando el espesor del inserto de 6, 8, 10, 12 y 14 mm, suponiendo cargas axiales de tipo cuasi-estático en la articulación a cero grados de flexión, para 1.33 veces el peso de un individuo de 75 kg (736 N) empleando el ciclo normalizado de marcha. Los resultados obtenidos muestran similitud con los reportados por Bei et al. (2004) y Deen et al. (2006). Después de validar el método, se desarrolló el modelo de MEF de la PTR y se determinaron las curvas de esfuerzo y de áreas de contacto del inserto de UHMWPE, con lo que se obtuvo información importante para modificar el diseño y obtener una prótesis de geometría conforme en los planos coronal y sagital del inserto femoral y el inserto de polietileno, de acuerdo con el fenotipo mexicano.

Wear of UHMWPE inserts continues affecting the longevity of total knee replacements (TKR) together with septic loosening, and both constitute two main causes of prosthesis failure. It is necessary to find appropriate solutions to avoid excessive wear and failure of polyethylene inserts. In this work a study was carried out by means of numeric simulation of a Scorpio II® Stryker® TKR, which was retired due to wear of UHMWPE in the Hospital 1° de Octubre of ISSSTE in Mexico city. Hypotheses of Bartel et al. (1995) and Chillag et al. (1991) were used, which settle down that wear of polyethylene can decrease using thicker tibial inserts, which can be reduced contact pressures. Analyses of this work was carried out by means of FEM varying insert thickness of 6, 8, 10, 12 and 14 mm, considered quasi-static axial loads actuating on the articulation with zero degrees of flexion and loads equivalent to 1.33 times of bodyweight of a subject of 75 kg (736 N) was considered. Normalized gait cycle was employed and results obtained are similar to those reported by Bei et al. (2004) and Deen et al. (2006). After validating the method, a model of study case of TKR in FEM was developed and the curves of stress and contact areas of UHMWPE were determined, with which important information was obtained to modify the design, as well as to obtain a prosthesis of optimal conformity in both coronal and sagital planes of the femoral and UHMWPE inserts, in agreement with characteristics of the Mexican phenotype.

Improvement of Lubrication Properties for New Surface Structure with Regular Patterning for the Frictional Surfaces of the Artificial Joints: Evaluation of Lubrication Properties by the Pendulum Type Hip Joint Simulator / 대한정형외과학회잡지

PURPOSE: A new surface structure with a regular patterning of the frictional surfaces of the artificial joints were designed to improve the wear problems. MATERIALS AND METHODS: The lubrication properties for the cup (ultra high molecular weight polyethylene, UHMWPE) vs. ball (Co-Cr alloy) were examined to evaluate the coefficient of friction, wear and surface roughness. The surface structure is a dent pattern, which has diameter of 0.3-0.8 mm, a pitch of 1.2 mm. In this study, such surface patterns were applied to artificial hip joints, and the effects of regular patterning on the frictional surface were examined using a pendulum type hip joint simulator. RESULTS: The coefficient of friction between the cup and a patterned ball showed the same value between the cup and the non-patterned ball. However, the amount of wear of the cup, which was in contact with the patterned head, was only 45% that of the cup, which was in contact with the non-patterned head, and the surface roughness of the cup with the non-patterned head was three times greater than that of the cup with a patterned head. CONCLUSION: These results demonstrate that the lubrication properties can be improved by patterning the frictional surface of the artificial joint. Consequently, patterning on the frictional surfaces can exted the life of an artificial joint.

Wear of Retrieved UHMWPE Hip Liners

After the gamma-irradiation sterilization, the most widely used orthopaedic grade polymer bearing liner material for the total joint replacement, ultra-high molecular weight polyethylene (UHMWPE), degrades through the progressive in vivo oxidation. The oxidative degradation makes UHMWPE brittle and leads to reduction of its mechanical properties. In this study, the effect of the in vivo post-irradiation ageing time on the wear of UHMWPE was investigated. Twelve retrieved polyethylene hip liners implanted for 3-16 years and then stored in the air for 1.5-8 years were used. Two types of the pin-on-disk wear testing were conducted. The uni-directional repeat pass rotating and the linear reciprocating wear testing were done with stainless steel disks against stationary polyethylene pins under 4MPa at 1Hz with bovine serum lubrication. Wear of the retrieved polyethylene hip liners does not have significant correlation with the in vivo or total ageing time. The linear reciprocal sliding motion generated a more pronounced wear than the uni-directional repeat pass sliding motion. This indicates that the kinematic motion significantly affects the wear of aged UHMWPE, having a brittle, white band region.

Stress Analysis of Laxity and Conformity using Geometry Modeling Program in Total Knee Replacement Arthroplasty / 대한정형외과연구학회지

The wear or deformation of UHMWPE contact surfaces, one of the major causes of the failure in total knee replacement arthroplasty (TKRA), results from the stress concentration. Thus the kinematics of knee implant should be considered. The objective of this study was to investigate the stress distribution and kinematics of UHMWPE in TKRA. Geometry analysis of UHMWPE was performed using FEM program(ABAQUS 5.8, 1998). 3D auto meshing preprocessor, which made it possible to perform rapid geometry modeling, was developed for the generation of computer model including the effect of the nonlinear soft tissue. For the linear analysis, 2 steps of loading conditions (i) 550N axial compression, (ii) 550N axial compression with 150N shear in AP direction were applied. The compression test using Fuji pressure sensitive film was utilized to validate this finite element analysis. Increasing the femoral frontal radius and closing the frontal clearance reduced the maximum contact stress in inverse proportion to the contact area. Increasing the tibial sagittal radius increased the AP laxity but little affected the conformity.

The Effects of Polyethylene(UHMWPE) Particles on MG63 Osteoblastlike Cells / 대한정형외과연구학회지

We performed an in vitro study to determine the effects of ultra high molecular weight polyethylene (UHMWPE) particles on the cell proliferation, matrix synthesis(type I collagen mRNA), and cytokines production(interleukin-lbeta and prostaglandin E2) of MG63 osteoblastlike cells. UHMWPE wear particles were isolated from osteolysis tissue of 8 patients during revision hip arthroplasties. Sub-micron UHMWPE powders were also isolated from fabricated virgin UHMWPE powders. Group I (control culture) contained no UHMWPE particles. In group II andIII, the UHMWPE wear particles and the UHMWPE powders were added to cultures of MG63 osteoblastlike cells with the different concentration of 0.2mg/ml, 0.02mg/ml, 0.002mg/ml, and 0.0002mg/ml. The average diameter of the retrieved UHMWPE particles was 0.4micrometa(ranged, 0.1 to 1.4 micrometa), and that of the fabricated UHMWPE powders was 0.6micrometa(ranged, 0.1 to 2.3micrometa). In group II and III, the UHMWPE particles induced an increase in osteoblastlike cell growth(p0.05). The release of IL-lbeta was higher in group II than in group III(p<0.05). These data support the hypothesis that direct suppression and cytokines release of MG63 osteoblastlike cells by UHMWPE particles may play a role in particle-mediated osteolysis.

Wear Properties of Surface Modified Ultra-high Molecular Weight Polyethylene / 대한정형외과연구학회지

The wear debris of ultra-high molecular weight polyethylene (UHMWPE) used as a bearing material in total joint arthroplasty evokes a serious adverse biological reaction (osteolysis). In this study, therefore, the wear resistance of UHMWPE is enhanced by attaching a hydrophilic graft on the UHMWPE surface and by improving surface hardness without deteriorating the mechanical properties, which may result in the formation of lubrication film and the increased resistance to penetration, scratching or deformation during articulation. This was done by ion-implantation technique. The results showed that the wettability and the surface hardness of the modified UHMWPE by the above treatments increased due to the increased cross-linking density and hydrophilic graft attachment on the surface. The wear test was performed by a wear testing machine of pin-on-disk type. The modified UHMWPE revealed about 30~50% less volumetric wear than the control specimen. It was also shown that the volumetric wear depended not only on the optimal treatment condition but on the applied load and the sliding speed.