Failure of total hip arthroplasties. Numerical modeling of debris formation through plastic strains

The life span of a total hip prosthesis is one of the main points on which the long-term success of arthroplasties depends. It is, by now, widely recognized that hip arthroplasty failure is mainly due to the aseptic loosening resulting from the presence of wear debris forming at the contact interface between the femoral head and the cup of the acetabulum. The size of these particles varies from a few micrometers to some tens of micrometers or more. The main aim of this study was therefore to investigate the formation of debris in the microscopic size range. For this purpose, a numerical study was carried out on various mechanisms leading to plastic deformations, which can lead to damage and wear in material. Numerical analyses were performed with a laboratory software program LMGC90, on the evolution of the plastic strains involved in various wear mechanisms on the microscopic scale.     

ER Stress Mediates TiAl6V4 Particle-Induced Peri-Implant Osteolysis by Promoting RANKL Expression in Fibroblasts

Wear particle-induced osteolysis is a major cause of aseptic loosening, which is one of the most common reasons for total hip arthroplasty (THA) failure. Previous studies have shown that the synovial fibroblasts present in the periprosthetic membrane are important targets of wear debris during osteolysis. However, the interaction mechanisms between the wear debris and fibroblasts remain largely unknown. In the present study, we investigated the effect of ER (endoplasmic reticulum) stress induced by TiAl₆V₄ particles (TiPs) in human synovial fibroblasts and calvarial resorption animal models. The expression of ER stress markers, including IRE1-α, GRP78/Bip and CHOP, were determined by western blot in fibroblasts that had been treated with TiPs for various times and concentration. To address whether ER stress was involved in the expression of RANKL, the effects of ER stress blockers (including 4-PBA and TUDCA) on the expression of RANKL in TiPs-treated fibroblasts were examined by real-time PCR, western blot and ELISA. Osteoclastogenesis was assessed by tartrate resistant acid phosphatase (TRAP) staining. Our study demonstrated that ER stress markers were markedly upregulated in TiPs-treated fibroblasts. Blocking ER stress significantly reduced the TiPs-induced expression of RANKL both in vitro and in vivo. Moreover, the inhibition of ER stress ameliorated wear particle-induced osteolysis in animal models. Taken together, these results suggested that the expression of RANKL induced by TiPs was mediated by ER stress in fibroblasts. Therefore, down regulating the ER stress of fibroblasts represents a potential therapeutic approach for wear particle-induced periprosthetic osteolysis.     

The Morse taper junction in modular revision hip replacement--a biomechanical and retrieval analysis.

All biomaterials used for total joint surgery are subjected to wear mechanisms. Morse taper junctions of modular hip revision implants are predilection sites for both fretting and crevice corrosion, dissociation and breakage of the components. The aim of this study is to quantify wear and study metallurgical changes of Morse taper junctions of in-vitro and in-vivo loaded modular revision stems. Three modular revision stems (MRP-Titan, Peter Brehm GmbH, Germany) were loaded by a servohydraulic testing machine. The loads and conditions used exceeded by far the values required by ISO-standard 7206. The tests were performed with maximum axial loads of 3,500 N to 4,000 N over 10-12 x 10(6) cycles at 2 Hz. Additionally, the female part of the taper junctions were coated with blood and bone debris. The free length of the implant was set to 200 mm. One other MRP stem was investigated after retrieval following 5.5 years of in-vivo use. All contact surfaces of the modular elements were assessed by visual inspection, optical microscopy and scanning electron microscopy (SEM). The degree of plastic deformation of the male part of the morse taper junction was determined by contouroscopy. None of the morse taper junctions broke or failed mechanically. Corrosion and wear affected all tapers, especially at the medial side. The retrieved implant showed no cracks and the amount of debris measured only one third of that for the stems tested in-vitro. The present retrieval and laboratory investigations have proven, that the morse taper junctions of the MRP-titanium stem are stable and resistant to relevant wear mechanisms. The longevity of the junctions for clinical use is given. If an optimal taper design is selected, the advantages of modular femoral components in total hip revision arthroplasty will outweigh the possible risks.     

A novel low wearing differential hardness, ceramic-on-metal hip joint prosthesis

Osteolysis and loosening of artificial joints caused by polyethylene wear debris has prompted renewed interest in alternative bearing materials for hip prosthesis designs. Lower wearing metal-on-metal (MOM) and ceramic-on-ceramic prostheses are being used more extensively, and there is considerable interest in further improving on their performance. This study investigated the wear properties and debris morphology of a novel differential hardness ceramic-on-metal (COM) prosthesis, in comparison with MOM articulations in a physiological anatomical hip joint simulator.The COM pairings were found to have wear rates approximately 100-fold lower than the MOM pairings. The MOM pairings showed a higher "bedding in" wear rate (3.09+/-0.46mm(3)/10(6) cycles) in the first million cycles, which then reduced to a steady state wear rate of 1.23+/-0.5mm(3)/10(6) cycles. The wear rate of the COM pairings over the duration of the test was approximately 0.01mm(3)/10(6) cycles with very little wear detected on the surface of the prosthesis components.The wear particles from both articulations were oval to round in shape and in the nanometer size range. After one million cycles the mean maximum diameter of the MOM and COM wear particles were 30+/-2.25 and 17.57+/-1.37nm, respectively. After five million cycles the wear particles were statistically significantly smaller than at one million cycles, 13.9+/-0.72nm for the MOM pairings and 6.11+/-0.40nm for the COM pairings.The wear rates of the MOM prostheses were representative of clinical values. The use of differential hardness COM pairings dramatically reduced the wear rate compared to MOM hip prostheses. The wear particles from the MOM articulation were similar to particles found in retrieved tissues from around MOM prostheses. The extremely low wearing differential hardness COM bearings presented in this study produced far smaller volumetric particle loads compared to MOM prostheses currently used clinically.     

Mixing and matching in ceramic-on-metal hip arthroplasty: An in-vitro hip simulator study

The clinical success of second-generation metal-on-metal hip replacement and the good tribological performance of alumina ceramic revived an interest in hip articulation as a solution to reduce wear.This study was aimed at characterizing the wear behaviour of new hybrid ceramic-on-metal bearings. In particular, this study investigated the wear behaviour of ceramic-on-metal hip components (three different diameters configurations: 28, 32 and 36 mm), not specifically proposed to be coupled, in order to compare them with ceramic-on-ceramic, which is considered to be the gold standard for wear resistance. For this purpose, the weight loss over a standard wear simulation was monitored. Moreover, scanning electronic microscope observations were used to verify if any carbides removal, for the metallic components, triggered wears debris production promoting abrasive third-body wear.After five million cycles, our results showed significantly greater wear-in ceramic-on-metal compared with ceramic-on-ceramic, and significant greater wear for the 32-mm diameter compared with the 36-mm one. Our findings showed an increase in wear for the proposed hybrid specimens with respect to that of the ceramic-on-ceramic ones confirming that even in the case of ceramic-on-metal bearings, mixing and matching could not prove effective wear behaviour, not even comparable with that of the ceramic-on-ceramic gold standard. Wear patterns and roundness tolerances certainly discourage the coupling of components not specifically intended to be coupled. Unsuitable geometrical conformity could, in fact, result in a poor dynamic behaviour and lead to clinical failure.     

Microstructure analysis and wear behavior of titanium cermet femoral head with hard TiC layer

Titanium cermet was successfully synthesized and formed a thin gradient titanium carbide coating on the surface of Ti6Al4V alloy by using a novel sequential carburization under high temperature, while the titanium cermet femoral head was produced. The titanium cermet phase and surface topography were characterized with X-ray diffraction (XRD) and backscattered electron imaging (BSE). And then the wear behavior of titanium cermet femoral head was investigated by using CUMT II artificial joint hip simulator. The surface characterization indicates that carbon effectively diffused into the titanium alloys and formed a hard TiC layer on the Ti6Al4V alloys surface with a micro-porous structure. The artificial hip joint experimental results show that titanium cermet femoral head could not only improve the wear resistance of artificial femoral head, but also decrease the wear of UHMWPE joint cup. In addition, the carburized titanium alloy femoral head could effectively control the UHMWPE debris distribution, and increase the size of UHMWPE debris. All of the results suggest that titanium cermet is a prospective femoral head material in artificial joint.     

Hive debris counts in honeybee colonies: a method to estimate the size of small populations and rate of growth of the miteVarroa jacobsoni Oud. (Mesostigmata: Varroidae)

Fifteen honeybee colonies (Apis mellifera L.) infested with the ectoparasiteVarroa jacobsoni Oud. were monitored for the number of mites falling to the bottom of the hive. Mites in the debris were counted periodically on the plastic sheet on which they were collected. Two months later, colonies were treated with an acaricide to determine mite population. A high positive correlation was found between the number of mites collected in the hive debris over different periods and the final population size. Based on this correlation, it was possible to use hive debris counts to predict the degree of infestation. Furthermore, counting fallen mites over a period of two months, followed by an acaricide treatment, might be a useful method of estimating the rate of growth ofV. jacobsoni in honeybee colonies.     

Effect of cup inclination on predicted contact stress-induced volumetric wear in total hip replacement

In order to increase the lifetime of the total hip endoprosthesis, it is necessary to understand mechanisms leading to its failure. In this work, we address volumetric wear of the artificial cup, in particular the effect of its inclination with respect to the vertical. Volumetric wear was calculated by using mathematical models for resultant hip force, contact stress and penetration of the prosthesis head into the cup. Relevance of the dependence of volumetric wear on inclination of the cup (its abduction angle ?_A) was assessed by the results of 95 hips with implanted endoprosthesis. Geometrical parameters obtained from standard antero-posterior radiographs were taken as input data. Volumetric wear decreases with increasing cup abduction angle ?_A. The correlation within the population of 95 hips was statistically significant (P = 0.006). Large cup abduction angle minimises predicted volumetric wear but may increase the risk for dislocation of the artificial head from the cup in the one-legged stance. Cup abduction angle and direction of the resultant hip force may compensate each other to achieve optimal position of the cup with respect to wear and dislocation in the one-legged stance for a particular patient.     

Effects of implant design parameters on fluid convection, potentiating third-body debris ingress into the bearing surface during THA impingement/subluxation

Aseptic loosening from polyethylene wear debris is the leading cause of failure for metal-on-polyethylene total hip implants. Third-body debris ingress to the bearing space results in femoral head roughening and acceleration of polyethylene wear. How third-body particles manage to enter the bearing space between the closely conforming articulating surfaces of the joint is not well understood. We hypothesize that one such mechanism is from convective fluid transport during subluxation of the total hip joint. To test this hypothesis, a three-dimensional (3D) computational fluid dynamics (CFD) model was developed and validated, to quantify fluid ingress into the bearing space during a leg-cross subluxation event. The results indicated that extra-articular joint fluid could be drawn nearly to the pole of the cup with even very small separations of the femoral head (<0.60mm). Debris suspended near the equator of the cup at the site of maximum fluid velocity just before the subluxation began could be transported to within 11 degrees from the cup pole. Larger head diameters resulted in increased fluid velocity at all sites around the entrance to the gap compared to smaller head sizes, with fluid velocity being greatest along the anterosuperolateral cup edge, for all head sizes. Fluid pathlines indicated that suspended debris would reach similar angular positions in the bearing space regardless of head size. Increased inset of the femoral head into the acetabular cup resulted both in higher fluid velocity and in transport of third-body debris further into the bearing space.     

Effect of taphonomic processes on dental microwear

Taphonomic processes have the potential to affect microscopic wear on teeth and to modify the wear patterns so as to confound dietary reconstructions based on dental microwear which was formed during the lifetime of an animal. This study describes a series of experiments which were conducted to simulate various taphonomic agents and to record their effect on dental microwear. Three types of experiment were carried out in order to explain anomalous microscopic wear that had been found on the dentition of several hominoid specimens from the 15 M.a. site of Pasalar in Turkey. The effect of two different acids-citric and hydrochloric acid-on dental microwear was investigated. Modification to microscopic wear caused by alkali (carbonatite ash) was examined in the second set of experiments. Lastly, the effect of abrasion by three different size classes of sediment from the site of Pasalar-quartz pebbles (grain size varied from 2,000-11,000 microm), coarse sand (grain size ranged from 500-1,000 microm), and medium-sized sand (grain diameters were between 250 and 500 microm)-was investigated. Results confirm previous findings that the taphonomic modification of dental microwear is readily identifiable and causes the obliteration rather than secondary alteration of microwear features. The experiments show that both citric and hydrochloric acid affect dental microwear but to varying degrees, whereas alkali did not cause any modification of microscopic features. The different size classes of sediment also had different effects on the dental microwear. The largest size sediment (quartz pebbles) polished the enamel and removed finer microwear features. The coarse sand, however, did not have any effect on the microwear. The greatest amount of abrasion was caused by the smallest sediment particles -the medium-sized sand. Several hominoid dental specimens from Pasalar display similar microscopic wear to the two types of acid erosion and the abrasion caused by the medium-sized sands.     

Who is where in the Plastisphere,and why does it matter?

To fully understand how plastic is affecting the ocean, we need to understand how marine life interacts directly with it. Besides their ecological relevance, microbes can affect the distribution, degradation and transfer of plastics to the rest of the marine food web. From amplicon sequencing and scanning electron microscopy, we know that a diverse array of microorganisms rapidly associate with plastic marine debris in the form of biofouling and biofilms, also known as the “Plastisphere.” However, observation of multiple microbial interactions in situ, at small spatial scales in the Plastisphere, has been a challenge. In this issue of Molecular Ecology Resources, Schlundt et al. apply the combination labelling and spectral imaging – fluorescence in situ hybridization to study microbial communities on plastic marine debris. The images demonstrate the colocalization of abundant bacterial groups on plastic marine debris at a relatively high taxonomic and spatial resolution while also visualizing biofouling of eukaryotes, such as diatoms and bryozoans. This modern imaging technology provides new possibilities to address questions regarding the ecology of marine microbes on plastic marine debris and describe more specific impacts of plastic pollution in the marine food webs.     

Grain size effect on the plastic deformation of nanocrystalline silver

The plastic deformation of nanocrystalline Ag, with columnar grains, has been studied by molecular dynamics simulations. The nanocrystalline systems show two types of deformation mechanisms. One is the split of grain boundary that occurs before the activation of the dislocation in nanocrystalline Ag, and almost no dislocation debris and twins are left in the grains. Moreover, split of grain boundary is shown between the grains of nanocrystalline Ag. Another mechanism consists in partial dislocations dominating the process of plastic deformation. Plenty of stacking faults and twins remain in the grains of nanocrystalline Ag. It is revealed that different grain aspect ratios have induced the difference in deformation mechanisms of nanocrystalline Ag. When the grain aspect ratio is less than or equal to 1, the process of plastic deformation is dominated by partial dislocations. Otherwise, the process is dominated by split of grain boundary. The grain aspect ratio is the height in z direction to length in x direction ratio, which was found to noticeably impact yield strength, grain coarsening, indicating that the observed behaviour should have contributed to the plastic deformation significantly.     

Does a micro-grooved trunnion stem surface finish improve fixation and reduce fretting wear at the taper junction of total hip replacements? A finite element evaluation

The generation of particulate debris at the taper junction of total hip replacements (THRs), can cause failure of the artificial hip. The taper surfaces of femoral heads and trunnions of femoral stems are generally machined to a certain roughness to enhance fixation. However, the effect of the surface roughness of these surfaces on the fixation, wear and consequently clinical outcomes of the design is largely unknown. In this study, we asked whether a micro-grooved trunnion surface finish (1) improves the fixation and (2) reduces the wear rate at the taper junction of THRs. We used 3D finite element (FE) models of THRs to, firstly, investigate the effect of initial fixation of a Cobalt-Chromium femoral head with a smooth taper surface mated with a Titanium (1) micro-grooved and (2) smooth, trunnion surface finishes. Secondly, we used a computational FE wear model to compare the wear evolution between the models, which was then validated against wear measurements of the taper surface of explanted femoral heads. The fixation at the taper junction was found to be better for the smooth couplings. Over a 7 million load cycle analysis in-silico, the linear wear depth and the total material loss was around 3.2 and 1.4 times higher for the femoral heads mated with micro-grooved trunnions. It was therefore concluded that smooth taper and trunnion surfaces will provide better fixation at the taper junction and reduce the volumetric wear rates.     

Autophagy mediated CoCrMo particle-induced peri-implant osteolysis by promoting osteoblast apoptosis

Wear particle-induced osteolysis is the leading cause of aseptic loosening, which is the most common reason for THA (total hip arthroplasty) failure and revision surgery. Although existing studies suggest that osteoblast apoptosis induced by wear debris is involved in aseptic loosening, the underlying mechanism linking wear particles to osteoblast apoptosis remains almost totally unknown. In the present study, we investigated the effect of autophagy on osteoblast apoptosis induced by CoCrMo metal particles (CoPs) in vitro and in a calvarial resorption animal model. Our study demonstrated that CoPs stimulated autophagy in osteoblasts and PIO (particle-induced osteolysis) animal models. Both autophagy inhibitor 3-MA (3-methyladenine) and siRNA of Atg5 could dramatically reduce CoPs-induced apoptosis in osteoblasts. Further, inhibition of autophagy with 3-MA ameliorated the severity of osteolysis in PIO animal models. Moreover, 3-MA also prevented osteoblast apoptosis in an antiautophagic way when tested in PIO model. Collectively, these results suggest that autophagy plays a key role in CoPs-induced osteolysis and that targeting autophagy-related pathways may represent a potential therapeutic approach for treating particle-induced peri-implant osteolysis.     

Local head roughening as a factor contributing to variability of total hip wear: a finite element analysis

Large inter-patient variability in wear rate and wear direction have been a ubiquitous attribute of total hip arthroplasty (THA) cohorts. Since patients at the high end of the wear spectrum are of particular concern for osteolysis and loosening, it is important to understand why some individuals experience wear at a rate far in excess of their cohort average. An established computational model of polyethylene wear was used to test the hypothesis that, other factors being equal, clinically typical variability in regions of localized femoral head roughening could account for much of the variability observed clinically in both wear magnitude and wear direction. The model implemented the Archard abrasive/adhesive wear relationship, which incorporates contact stress, sliding distance, and (implicitly) bearing surface tribology. Systematic trials were conducted to explore the influences of head roughening severity, roughened area size, and roughened area location. The results showed that, given the postulated wear factor elevations, head roughening variability (conservatively) typical of retrieval specimens led to approximately a 30 degrees variation in wear direction, and approximately a 7-fold variation in volumetric wear rate. Since these data show that randomness in head scratching can account for otherwise-difficult-to-explain variations in wear direction and wear rate, third-body debris may be a key factor causing excessive wear in the most problematic subset of the THA population.     

Accumulation and fragmentation of plastic debris in global environments

One of the most ubiquitous and long-lasting recent changes to the surface of our planet is the accumulation and fragmentation of plastics. Within just a few decades since mass production of plastic products commenced in the 1950s, plastic debris has accumulated in terrestrial environments, in the open ocean, on shorelines of even the most remote islands and in the deep sea. Annual clean-up operations, costing millions of pounds sterling, are now organized in many countries and on every continent. Here we document global plastics production and the accumulation of plastic waste. While plastics typically constitute approximately 10 per cent of discarded waste, they represent a much greater proportion of the debris accumulating on shorelines.Mega- and macro-plastics have accumulated in the highest densities in the Northern Hemisphere, adjacent to urban centres, in enclosed seas and at water convergences (fronts). We report lower densities on remote island shores, on the continental shelf seabed and the lowest densities (but still a documented presence) in the deep sea and Southern Ocean. The longevity of plastic is estimated to be hundreds to thousands of years, but is likely to be far longer in deep sea and non-surface polar environments. Plastic debris poses considerable threat by choking and starving wildlife, distributing non-native and potentially harmful organisms, absorbing toxic chemicals and degrading to micro-plastics that may subsequently be ingested. Well-established annual surveys on coasts and at sea have shown that trends in mega- and macro-plastic accumulation rates are no longer uniformly increasing: rather stable, increasing and decreasing trends have all been reported. The average size of plastic particles in the environment seems to be decreasing, and the abundance and global distribution of micro-plastic fragments have increased over the last few decades. However, the environmental consequences of such microscopic debris are still poorly understood.     

Density of the New Zealand flatworm, a predator of earthworms, in allotments in Edinburgh between 1993 and 1995

The density of the New Zealand flatworm, Artioposthia triangulata, found between 1993 and 1995 on the surface of the soil under various types of debris scattered in four allotments, was 1–2 m^?2. The type of debris did not markedly affect the density of the flatworm and the distribution of the flatworm within the allotment studied was significantly clustered. Fluctuation in numbers of specimens under different sets of compost‐filled plastic sacks varied in a similar manner to one another. The flatworm was most abundant in the upper 100 mm of soil and its egg capsules were most numerous between 100 and 200 mm below the surface. The flatworm was as abundant under the soil surface shelters as it was beneath shelters buried below the soil surface.     

Periprosthetic osteolysis and its association with RANKL expression

Extensive osteolysis adjacent to orthopedic implants is often associated with wear particles of prosthetic material. The activation of the RANKL/RANK/OPG system is considered to be a likely cause of periprosthetic osteolysis leading to implant failure. The aim of this study was to examine the possible correlation between the clinical extent of osteolysis, the number of wear particles and expression of the osteoclastic mediator RANKL (receptor activator of nuclear factor kappa B ligand) in the tissues around aseptically loosened cemented and non-cemented total hip replacements. Periprosthetic tissues were harvested from 59 patients undergoing revision of hip replacement for aseptic loosening. We observed RANKL-positive cells in 23 of our 59 patients, their presence was noted predominantly in tissues with a loosened cemented endoprosthesis. We have found that RANKL is present only in tissues with a large amount of wear debris and predominantly in cases involving loosened cemented implants.     

Sexual dimorphism,moult and body condition of seabirds killed by longline vessels around the Prince Edward Islands, 1996–97

Ryan, P.G. 1999. Sexual dimorphism, moult and body condition of seabirds killed by longline vessels around the Prince Edward Islands, 1996–97. Ostrich 70 (3&4): 187–192.

A total of 393 seabirds from nine species killed by longline fishing in South African waters around the Prince Edward Islands during summer 1996–97 were sexed, measured, and their body fat levels and primary moults scored. Males of Whitechinned Petrels Procellaria aequinoctialis and Greyheaded and (Indian Ocean) Yellownosed Mollymawks Thalassarche chrysostoma and T. [chlororhynchosj bassi averaged 1–9% larger than females. Measures of bill depth were the most dimorphic characters in all three species. I recommend that if only a single measure is taken to estimate sex of adults, bill depth at the base be the standard measure for petrels, and bill depth at the nail be the standard measure for mollymawks. Discriminant functions are described for predicting the sex of unknown individuals; the proportion of misclassified birds ranged from 2–10%. With the exception of giant petrels Macronectes spp., only two young birds were actively moulting their primary flight feathers. Analysis of wear patterns among adult Greyheaded Mollymawk primaries conforms with studies of moult in this species at South Georgia. Fat scores did not differ between sexes. They tended to decrease during the breeding season, but trends were masked by great inter-individual variation. There was no significant relationship between fat scores and the presence of ingested plastics. Plastic debris was found in the stomachs of 90 birds from five species. Incidence in Whitechinned Petrel stomachs (37%) was less than that reported from birds collected off South Africa in the early 1980s (57%), but this is probably due to the predominance of breeding adults. The frequency and size of plastic loads decreased through the breeding season, which is consistent with the hypothesis that inter-generational transfer is important in the dynamics of ingested debris.     

Comparisons of the Colonisation by Invertebrates of Three Species of Wood,Alder Leaves,and Plastic “Leaves” in a Temperate Stream

Small woody debris in streams is abundant, and may be a food source or may provide a substrate on which other food sources such as biofilm may develop, both of which may be significant to invertebrates in times of food scarcity. We examined patterns of invertebrate colonisation of small woody debris (veneers of red alder, Douglas‐fir, and western red cedar), red alder leaves, and plastic (as an inert substrate to mimic leaves). Invertebrate colonisation was high on alder leaves, but low on wood substrates and plastic, controlling for the available surface area. Detritivorous invertebrates had significantly higher colonisation rates of alder leaves versus the other four substrates, whereas predators and collectors did not (consistent with their use of these as substrates and not food). All wood decreased in mass by <15% and leaves by ∼50% over the 75 days of the experiment. For all taxa tested, there was no significant difference in their colonisation of the wood veneers versus the plastic sheets. These results suggest that wood was not directly used by these invertebrates as a food source, or that there could be similar biofilm development on the surfaces of these substrates. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)