Microwear phenomena of ultrahigh molecular weight polyethylene cups and debris morphology related to gamma radiation dose in simulator study

Microscopic asperity contact and deformation of ultrahigh molecular weight polyethylene bearing surfaces

The effect of the roughness and topography of ultrahigh molecular weight polyethylene (UHMWPE) bearing surfaces on the microscopic contact mechanics with a metallic counterface was investigated in the present study. Both simple sinusoidal roughness forms, with a wide range of amplitudes and wavelengths, and real surface topographies, measured before and after wear testing in a simple pin-on-plate machine, were considered in the theoretical analysis. The finite difference method was used to solve the microscopic contact between the rough UHMWPE bearing surface and a smooth hard counterface. The fast Fourier transform (FFT) was used to cope with the large number of mesh points required to represent the surface topography of the UHMWPE bearing surface. It was found that only isolated asperity contacts occurred under physiological loading, and the real contact area was only a small fraction of the nominal contact area. Consequently, the average contact pressure experienced at the articulating surfaces was significantly higher than the nominal contact pressure. Furthermore, it was shown that the majority of asperities on the worn UHMWPE pin were deformed in the elastic region, and consideration of the plastic deformation only resulted in a negligible increase in the predicted asperity contact area. Microscopic asperity contact and deformation mechanisms may play an important role in the understanding of the wear mechanisms of UHMWPE bearing surfaces.

In vivo biological response to highly cross-linked and vitamin e-doped polyethylene--a particle-Induced osteolysis animal study

Polyethylene particle-induced osteolysis is the primary limitation in the long-term success of total joint replacement with conventional ultra high molecular weight polyethylene (UHMWPE). Highly cross-linked polyethylene (HXLPE) and vitamin E-doped cross-linked polyethylene (VE-HXLPE) have been developed to increase the wear resistance of joint surfaces. However, very few studies have reported on the incidence of particle-induced osteolysis for these novel materials. The aim of this study was to use a particle-induced osteolysis animal model to compare the in vivo biological response to different polymer particles. Three commercially available polymers (UHMWPE, HXLPE, and VE-HXLPE) were compared. Osseous properties including the bone volume relative to the tissue volume (BV/TV), trabecular thickness (Tb. Th), and bone mineral density (BMD) were examined using micro computed tomography. Histological analysis was used to observe tissue inflammation in each group. This study demonstrated that the osseous properties and noticeable inflammatory reactions were obviously decreased in the HXLPE group. When compared with the sham group, a decrease of 12.7% was found in BV/TV, 9.6% in BMD and 8.3% in Tb.Th for the HXLPE group. The heightened inflammatory response in the HXLPE group could be due to its smaller size and greater amount of implanted particles. Vitamin E diffused in vivo may not affect the inflammatory and osteolytic responses in this model. The morphological size and total cumulative amount of implanted particles could be critical factors in determining the biological response.

Characterization of polyethylene wear particle: The impact of methodology

Due to the prevalence of problems caused by wear particles, the reduced durability of total joint replacements is well documented. The characterization of wear debris enables the size and morphology of these wear particles to be measured and provides an assessment of the biological response in vivo. However, the impact of different methodologies of particle analysis is not yet clear. Hence, the aim of this investigation was to analyze the influence of different particle characterization methods performed by three research centers within the scope of a "round robin test". To obtain knowledge about possible pitfalls, single steps of the particle characterization process (storage, pore size of the filter, coating durations by gold sputtering and scanning electron microscopy (SEM) magnification) were analyzed. The round robin test showed significant differences between the research groups, especially for the morphology of the particles. The SEM magnification was identified as having the greatest influence on the size and shape of the particles, followed by the storage conditions of the wear particle containing lubricant. Gold sputter coating and filter pore size also exhibit significant effects. However, even though they are statistically significant, it should be emphasized that the differences are small. In conclusion, particle characterization is a complex analytical method with a multiplicity of influencing factors. It becomes apparent that a comparison of wear particle results between different research groups is challenging.

Characteristics of highly cross-linked polyethylene wear debris in vivo

Despite the widespread implementation of highly cross-linked polyethylene (HXLPE) liners to reduce the clinical incidence of osteolysis, it is not known if the improved wear resistance will outweigh the inflammatory potential of HXLPE wear debris generated in vivo. Thus, we asked: What are the differences in size, shape, number, and biological activity of polyethylene wear particles obtained from primary total hip arthroplasty revision surgery of conventional polyethylene (CPE) versus remelted or annealed HXLPE liners? Pseudocapsular tissue samples were collected from revision surgery of CPE and HXLPE (annealed and remelted) liners, and digested using nitric acid. The isolated polyethylene wear particles were evaluated using scanning electron microscopy. Tissues from both HXLPE cohorts contained an increased percentage of submicron particles compared to the CPE cohort. However, the total number of particles was lower for both HXLPE cohorts, as a result there was no significant difference in the volume fraction distribution and specific biological activity (SBA; the relative biological activity per unit volume) between cohorts. In contrast, based on the decreased size and number of HXLPE wear debris there was a significant decrease in total particle volume (mm(3)/g of tissue). Accordingly, when the SBA was normalized by total particle volume (mm(3)/gm tissue) or by component wear volume rate (mm(3)/year), functional biological activity of the HXLPE wear debris was significantly decreased compared to the CPE cohort. Indications for this study are that the osteolytic potential of wear debris generated by HXLPE liners in vivo is significantly reduced by improvements in polyethylene wear resistance.

Do tissues from THA revision of highly crosslinked UHMWPE liners contain wear debris and associated inflammation?

BACKGROUND

Polyethylene wear debris is a major contributor to inflammation and the development of implant loosening, a leading cause of THA revisions. To reduce wear debris, highly crosslinked ultrahigh-molecular-weight polyethylene (UHMWPE) was introduced to improve wear properties of bearing surfaces. As highly crosslinked UHMWPE revision tissues are only now becoming available, it is possible to examine the presence and association of wear debris with inflammation in early implant loosening.

QUESTIONS/PURPOSES

We asked: (1) Does the presence of UHMWPE wear debris in THA revision tissues correlate with innate and/or adaptive immune cell numbers? (2) Does the immune cell response differ between conventional and highly crosslinked UHMWPE cohorts?

METHODS

We collected tissue samples from revision surgery of nine conventional and nine highly crosslinked UHMWPE liners. Polarized light microscopy was used to determine 0.5- to 2-μm UHMWPE particle number/mm2, and immunohistochemistry was performed to determine macrophage, T cell, and neutrophil number/mm2.

RESULTS

For the conventional cohort, correlations were observed between wear debris and the magnitude of individual patient macrophage (ρ=0.70) and T cell responses (ρ=0.71) and between numbers of macrophages and T cells (ρ=0.77) in periprosthetic tissues. In comparison, the highly crosslinked UHMWPE cohort showed a correlation between wear debris and the magnitude of macrophage responses (ρ=0.57) and between macrophage and T cell numbers (ρ=0.68). Although macrophages and T cells were present in both cohorts, the highly crosslinked UHMWPE cohort had lower numbers, which may be associated with shorter implantation times.

CONCLUSIONS

The presence of wear debris and inflammation in highly crosslinked UHMWPE revision tissues may contribute to early implant loosening.

Crosslinked polyethylene in knee arthroplasty: a simulator study evaluating the positive influence on the tribocontact area in the fixed-bearing knee

BACKGROUND

Crosslinked polyethylene (XPE) was developed to reduce the wear rate in hip as well as knee arthroplasty. The crosslinking process reduces the mechanical properties of ultra-high-molecular-weight polyethylene (UHMWPE), particularly its fatigue strength. UHMWPE fatigue occurs more frequently in the knee than in the hip joint due to its changing tribocontact areas (TCAs) combined with high weight bearing. This is why XPE is still controversially discussed for use in total knee arthroplasty. Therefore, the potential advantage of using XPE in the knee was analysed in a simulator study with a focus on potential fatigue wear mechanisms.

METHODS

Three different kinds of XPE and one conventional UHMWPE were tested over 5 million cycles in fixed-bearing knee designs. The TCAs were examined by replicas, and their extent was measured. The wear mechanism was analysed by scanning electron microscopy.

RESULTS

The extent of the TCAs was less than 5% for all XPEs, whereas 35% for the conventional UHMWPE. Fatigue wear mechanisms were not observed.

CONCLUSION

The measured small extent of the TCAs as a predictor of a low wear rate without any fatigue wear mechanism shows a possible advantage for the use of XPE even in knee arthroplasty.

"Severe" wear challenge to 36 mm mechanically enhanced highly crosslinked polyethylene hip liners

Our purpose was to compare the wear performance of mechanically enhanced 5Mrad highly crosslinked polyethylene (MEP, ArComXL) hip liners to (control) 3Mrad UHMWPE liners (ArCom) in 36 mm head size. As a more severe synergy of clinically relevant test models, we contrasted wear with custom roughened Co-Cr surfaces (Ra 500 nm) to the standard pristine Co-Cr heads (Ra < 20 nm) using a severe microseparation test mode in our hip simulator. We adopted a previously published model to estimate potential biological activity. On new Co-Cr heads, the MEP liners showed a 47% reduction in volumetric wear a 13% reduction in wear particle size and a 27% reduction in Functional Biological Activity (FBA) compared to our control. On rough Co-Cr heads, the MEP liners showed little advantage in terms of volumetric wear compared with the control. However, the MEP liners overall showed a 38% reduction in FBA compared to the control owing to a larger volume fraction of larger particles. Thus overall the MEP liners appeared to offer advantages in terms of reduced FBA indices.

Characterization of a highly cross-linked ultrahigh molecular-weight polyethylene in clinical use in total hip arthroplasty

This article reports on a commercially available extensively cross-linked ultrahigh molecular-weight polyethylene (HXPE) produced by subjecting molded GUR 1050 ultrahigh molecular-weight polyethylene (UHMWPE) to 100 +/- 10 kGy of electron beam radiation followed by melt annealing and sterilization by gas plasma. When compared to contemporary conventional molded GUR 1050 UHMWPE sterilized by 37 kGy of gamma radiation, the HXPE material has enhanced wear properties, has no detectable free radicals, and is resistant to oxidation and oxidative-related material property changes. The relative wear improvement of the HXPE is maintained in the presence of bone cement or alumina particles. The HXPE produced greater than 90% fewer wear particles in all size ranges and statistically significantly (P < .0001) smaller average-size particles than did the conventional UHMWPE.

Dose effects of cross-linking polyethylene for total knee arthroplasty on wear performance and mechanical properties

Wear performance and mechanical properties of cross-linking polyethylene (XLPE) tibial inserts were investigated using a knee simulator, scanning electron microscopy (SEM), and a small punch test (SPT). Ultrahigh molecular weight PE made from GUR1050 resin was irradiated at doses ranging from 0 to 200 kGy and then machined into tibial inserts followed by annealing. The knee simulator was run for up to four million cycles. As the radiation dose increased from 0 to 100 kGy, the wear rate decreased dramatically, yielding 95% wear reduction at 100 kGy. The microwear features observed by SEM supported the dose-dependent wear reduction. The SPT for XLPE after the simulation test showed that, as the radiation dose increased from 0 to 200 kGy, the ultimate displacement decreased dose-dependently, while the ultimate load increased from 0 to 75 kGy and decreased from 75 to 200 kGy. The resulting toughness of the PE increased to its maximum at a dose of 50 kGy and then decreased with higher doses up to 200 kGy. PE cross-linked with radiation doses from 25 to 75 kGy had greater toughness than virgin, nonirradiated PE. However, PE irradiated with 100 kGy or more had lower toughness than virgin PE. These data suggest that a certain amount of irradiation enhances both wear performance and toughness of PE tibial inserts. Although a certain amount of cross-linking would be effective for clinical application of PE tibial inserts, an optimal radiation dose should be much smaller than that used in current XLPE in total hip arthroplasty.

Surface micro-analyses of long-term worn retrieved “Osteal™” alumina ceramic total hip replacement

We analyzed wear pattern of long-term retrieved alumina-alumina hip prostheses from Osteal, which were implanted for 15-19 years. A comparison was carried out with our previous study of 17-year Biolox alumina-on-alumina hip prostheses, (Shishido et al., J Biomed Mater Res B 2003;67:638-647) and all-alumina total hip replacement run under microseparation simulator tests. Of particular interest was the occurrence of stripe wear in these first generation alumina ceramic bearings. Two balls of Osteal revealed only one stripe wear as did the respective liners on their rim areas. In these latter balls, the stripes were shallower than those previously observed in Biolox implants. A microscopic analysis of the bearing surface was carried out using scanning electron microscopy and fluorescence microprobe spectroscopy. On average, the Osteal retrievals had one grade lower wear than Biolox retrievals. Fluorescence microprobe maps showed that Biolox ball surfaces had higher compressive stress than the Osteal likely due to severe impingement and microseparation promoted by the bulky implant design.

Effect of low protein concentration lubricants in hip simulators

BACKGROUND

Proteins play an important role as boundary lubricants in vivo and in vitro. Hip simulator studies have tested various protein concentrations of lubricants. Several groups reported that nonphysiological pits were created with low protein concentrations. Our study showed details of wear findings with low protein concentrations. This may be the first mapping of the run-in wear morphology on ultra high molecular weight polyethylene (UHMWPE) cups.

METHODS

The UHMWPE cups used were holding cobalt chrome (CoCr) balls. This study was run on orbital-type hip simulators at up to 1.0 million cycles (Mc). The lubricant was bovine calf serum (0, 3, 5, 7, 10, and 1 mg/ml protein concentrations). Two volumes of lubricant were used (40 and 16 ml). Volumetric wear rates were calculated and the cup surfaces were observed using reflected light microscopy (RLM) and scanning electron microscopy (SEM).

RESULTS

The 40-ml volume showed a lower wear rate than the 160-ml volume. The RLM findings showed that the machine marks gradually disappeared up to 1.Mc, but there were no obvious pits on the polyethylene surface. The SEM findings showed nonphysiological wear phenomena in the 0-mg/ml protein concentration and physiological wear phenomena in the low protein concentrations. In the main bearing wear area, many nodules were observed with fibrils. In the peripheral bearing wear area, mainly ripples were observed. At the nonbearing wear area, folds were observed.

CONCLUSIONS

Our study showed the microwear findings with low-protein serum: First, folds were formed, followed by ripples, and finally nodules accompanied by fibrils. During the run-in wear phase, nodules and fibrils had already appeared in the main bearing wear area. Thus, we need to investigate more details of the wear process when folds or ripples change to nodules at the main wear area in the run-in wear.

Wear mode and wear mechanism of retrieved acetabular cups

Nineteen ultra-high-molecular-weighted polyethylene (UHMWPE) cups were retrieved at revision surgery. The implant period was on average 9.9 (0.25-20.7) years. The cups were classified into the high-wear group (wear rate above 140 mm(3)/year), intermediate-wear group (wear rate 80-140 mm(3)/year) and low-wear group (wear rate below 80 mm(3)/year). The wear rate was measured by a fluid displacement method. The cups were studied using scanning electron microscopy. Ripples mainly appeared on the cups in the low-wear group, and nodules and fibrils mainly appeared in the high-wear group. Folding was observed in all cups, but folding with numerous fibrils was conspicuous in the high-wear group. The cup retrieved at 3 months already exhibited folding. From these results, the wear response appeared to be a three-step process: (1) foldings are generated, (2) ripples are formed on the surface, and (3) fibrils are produced from the surface and delaminated, leading to the formation of wear debris.

Metrology to quantify wear and creep of polyethylene tibial knee inserts

Assessment of damage on articular surfaces of ultrahigh molecular weight polyethylene tibial knee inserts primarily has been limited to qualitative methods, such as visual observation and classification of features such as pitting, delamination, and subsurface cracking. Semiquantitative methods also have been proposed to determine the linear penetration and volume of the scar that forms on articular surfaces of tibial knee inserts. The current authors report a new metrologic method that uses a coordinate measuring machine to quantify the dimensions of this scar. The articular surface of the insert is digitized with the coordinate measuring machine before and after regular intervals of testing on a knee simulator. The volume and linear penetration of the scar are calculated by mathematically taking the difference between the digitized surface maps of the worn and unworn articular surfaces. Three conventional polyethylene tibial knee inserts of a posterior cruciate-sparing design were subjected to five million cycles of normal gait on a displacement-driven knee wear simulator in bovine serum. A metrologic method was used to calculate creep and wear contributions to the scar formation on each tibial plateau. Weight loss of the inserts was determined gravimetrically with the appropriate correction for fluid absorption. The total average wear volume was 43 +/- 9 and 41 +/- 4 mm3 measured by the metrologic and gravimetric methods, respectively. The wear rate averaged 8.3 +/- 0.9 and 8.5 +/- 1.6 mm3 per million cycles measured by the metrologic and gravimetric methods, respectively. These comparisons reflected strong agreement between the metrologic and gravimetric methods.

The nature and dissemination of UHMWPE wear debris retrieved from periprosthetic tissue of THR

The role of wear debris in provoking joint replacement failure through bone resorption is now supported by much research. This study presents the analysis of 104 tissue samples using laser diffraction wear particle analysis in conjunction with standard histologic methods. The number and volume distributions were correlated to a range of joint and patient parameters. The median particle diameter by number was 0.69 microm. No particles smaller than 0.113 microm were resolved. No variation in terms of particle distribution was found among joint types. The ability of particles to migrate away from their point of origin was found to be inversely proportional to their size. The numbers of particles per gram of tissue found in various regions around the prosthesis varied little. Further, the numbers of particles in tissue samples shown to have a chronic foreign-body reaction was > 1 x 10(9) particles/gram.