Objectives

Wear debris released from bearing surfaces has been shown to provoke negative immune responses in the recipient. Excessive wear has been linked to early failure of prostheses. Analysis using coordinate measuring machines (CMMs) can provide estimates of total volumetric material loss of explanted prostheses and can help to understand device failure. The accuracy of volumetric testing has been debated, with some investigators stating that only protocols involving hundreds of thousands of measurement points are sufficient. We looked to examine this assumption and to apply the findings to the clinical arena.

Abstract. Introduction. At our national explant retrieval unit, we identified an unusual pattern of backside-deformation on polyethylene (PE) inserts of contemporary total-knee-replacements (TKRs). The PE backside's margins were inferiorly deformed in TKRs with central-locking trays. We reported that this backside-deformation appeared to be linked to tray debonding. Moreover, recent studies have shown high-rate of tray debonding in PS NexGen TKRs. Therefore, we hypothesised that backside deformation on PS inserts may be more than on CR inserts. Methodology. We used peer-reviewed techniques to analyse changes in the bearing (wear rate) and backside surfaces (deformation) of PE inserts using coordinate measuring machines [N=61 NexGen (CR-39 and PS-22) TKRs with non-augmented-trays]. Multiple regression was used to determine which variable had the greatest influence on backside-deformation. The amount of cement cover on trays was quantified as a %of the total surface using Image-J software. Results. There was no statistically significant difference (p=0.238) in median (IQR) wear rate of the CR PEs 18 (12–28) mm. 3. /year and PS PEs 14 (8–20) mm. 3. /year. The PE backside-deformation median (IQR) of PS [297(242–333) µm] was significantly higher (p=0.011), when compared with CR [241(161–259) µm]. Multiple regression modelling showed that duration in-vivo (p=0.037), central-clearance between insert and tray (p<0.001) and constraint (p=0.003) were significantly associated with PE backside-deformation. 27(69%) of CR and 20(91%) PS exhibited ≤10% of cement cover on tray. Conclusion. This explant study showed backside-deformation on PS inserts was more than on CR inserts. Therefore, indicating a high-rate of tibial tray debonding in PS compared to CR NexGen TKRs

Abstract. OBJECTIVES. Dual mobility (DM) total hip replacements (THRs) were introduced to reduce the risk of hip dislocation in at-risk patients. DM THRs have shown good overall survivorship and low rates of dislocation, however, the mechanisms which describe how these bearings function in-vivo are not fully understood. This is partly due to a lack of suitable characterisation methodologies which are appropriate for the novel geometry and function of DM polyethylene liners, whereby both surfaces are subject to articulation. This study aimed to develop a novel semi-quantitative geometric characterisation methodology to assess the wear/deformation of DM liners. METHODS. Three-dimensional coordinate data of the internal and external surfaces of 14 in-vitro tested DM liners was collected using a Legex 322 coordinate measuring machine. Data was input into a custom Matlab script, whereby the unworn reference geometry was determined using a sphere fitting algorithm. The analysis method determined the geometric variance of each point from the reference surface and produced surface deviation heatmaps to visualise areas of wear/deformation. Repeatability of the method was also assessed. RESULTS. Semi-quantitative analysis of the surface deviation heatmaps revealed circumferential damage patterns similar to those reported in the literature. Additionally, the location of the damaged regions corresponded between the internal and external surfaces. Comparing five repeat measurements of the same liner, the maximum geometric variance of each surface varied by 1 µm (standard deviation) suggesting a high repeatability of the method. CONCLUSIONS. This study presents an effective and highly repeatable characterisation methodology to semi-quantitatively assess the wear/deformation of in-vitro tested DM liners. This method is suitable for the analysis of retrieved DM liners whereby no pre-service information is available, which may provide information about the complex in-vivo kinematics and mechanical failure mechanisms of these bearings

Introduction. Total hip replacement failure due to fretting-corrosion remains a clinical concern. We recently described that damage within CoCrMo femoral heads can occur either by mechanically-dominated fretting processes leading to imprinting (via rough trunnions) and surface fretting (via smooth trunnions), or by a chemically-dominated etching process along preferential corrosion sites, termed “column damage”. These corrosion sites occur due to banding of the alloy microstructure. Banding is likely caused during thermo-mechanical processing of the alloy and is characterized by local molybdenum depletion. It was the objective of this study to quantify material loss from femoral heads with severe corrosion, identify the underlying damage modes, and to correlate the damage to the alloy's microstructure. Methods. 105 femoral heads with a Goldberg score 4 were evaluated. Coordinate measuring machine data was used to compute material loss and visualize damage features. Time in situ and stem alloy were identified. Metallographic samples were produced for each case. Grain size and banding were identified using light-microscopy. Mann-Whitney tests were conducted to compare material loss between groups. Results. Heads exhibited imprinting and column damage in 72 and 51 cases, respectively, with an overlap of 36 cases. 18 heads exhibited surface fretting only. All heads with column damage exhibited a banded microstructure. Heads with column damage had higher material loss (p=0.05) than those without. Also, heads with a banded microstructure had higher material loss (p=0.035) than those with a homogenous microstructure. Grain size, carbide content, and time did not correlate with material loss. Conclusion. Column damage is a detrimental damage mode within CoCrMo femoral heads that is directly linked to banding within its microstructure. It appears that banding even affects material loss before column damage is identifiable. These results indicate that implant alloy microstructure must be optimized to minimize the release of fretting-corrosion products and related implant failure

Objectives. The high revision rates of the DePuy Articular Surface Replacement (ASR) and the DePuy ASR XL (the total hip arthroplasty (THA) version) have led to questions over the viability of metal-on-metal (MoM) hip joints. Some designs of MoM hip joint do, however, have reasonable mid-term performance when implanted in appropriate patients. Investigations into the reasons for implant failure are important to offer help with the choice of implants and direction for future implant designs. One way to assess the performance of explanted hip prostheses is to measure the wear (in terms of material loss) on the joint surfaces. Methods. In this study, a coordinate measuring machine (CMM) was used to measure the wear on five failed cementless Biomet Magnum/ReCap/ Taperloc large head MoM THAs, along with one Biomet ReCap resurfacing joint. Surface roughness measurements were also taken. The reason for revision of these implants was pain and/or adverse reaction to metal debris (ARMD) and/or elevated blood metal ion levels. Results. The mean wear rate of the articulating surfaces of the heads and acetabular components of all six joints tested was found to be 6.1 mm. 3. /year (4.1 to 7.6). The mean wear rate of the femoral head tapers of the five THAs was 0.054 mm. 3. /year (0.021 to 0.128) with a mean maximum wear depth of 5.7 µm (4.3 to 8.5). Conclusion. Although the taper wear was relatively low, the wear from the articulating surfaces was sufficient to provide concern and was potentially large enough to have been the cause of failure of these joints. The authors believe that patients implanted with the ReCap system, whether the resurfacing prosthesis or the THA, should be closely monitored. Cite this article: S. C. Scholes, B. J. Hunt, V. M. Richardson, D. J. Langton, E. Smith, T. J. Joyce. Explant analysis of the Biomet Magnum/ReCap metal-on-metal hip joint. Bone Joint Res 2017;6:113–122. DOI: 10.1302/2046-3758.62.BJR-2016-0130.R2

Little is known about the relationship between head-neck corrosion and its effect on the periprosthetic tissues and distant organs of patients hosting well-functioning devices. The purpose of this study was to investigate in postmortem retrieved specimens the degree and type of taper damage, and the corresponding histologic responses in periprosthetic tissues and distant organs. Fifty postmortem THRs (34 primaries, 16 revisions) retrieved after 0.5 to 26 years were analyzed. Forty-three implants had a CoCrMo stem and seven had a Ti6Al4V stem. All heads were CoCrMo and articulated against polyethylene cups (19 XLPE, 31 UHMWPE). H&E sections of joint pseudocapsules, liver, spleen, kidneys and lymph nodes were graded 1–4 for the intensity of various inflammatory cell infiltrates and tissue characteristics. Corrosion damage of the taper surfaces was assessed using visual scoring and quantitated with an optical coordinate measuring machine. SEM analysis was used to determine the acting corrosion mode. Polyethylene wear was assessed optically. The majority of tapers had minimal to mild damage characterized by local plastic deformation of machining line peaks. Imprinting of the stem topography onto the head taper surface was observed in 18 cases. Column damage on the head taper surface occurred in three cases. All taper surfaces scored moderate or severe exhibited local damage features of fretting and/or pitting corrosion. Moderate or severe corrosion of the head and/or trunnion was present in nine hips. In one asymptomatic patient with bilateral hips, lymphocyte-dominated tissue reactions involving perivascular infiltrates of lymphocytes and plasmacytes were observed. In this patient, mild, focal lymphocytic infiltrates were also present in the liver and kidneys, and there was focal histiocytosis and necrosis of the para-aortic lymph nodes. These two implants, which had been in place for 58.6 and 60.1 months, had severe intergranular corrosion of the CoCrMo trunnion, and column damage and imprinting on the head taper. In the other 41 hips, macrophage responses in the joint pseudocapsule to metallic and/or polyethylene wear particles ranged widely from minimal to marked. Focal necrosis in the pseudocapsules of 12 arthroplasties was related to high concentrations of CoCrMo, TiAl4V, TiO, BaSO4 and polyethylene wear particles. High concentrations of these particles were also detected in para-aortic lymph nodes. Rare to mild macrophages were observed in liver and spleen. This is a comprehensive study of wear and corrosion within well-functioning postmortem retrieved THRs, and the resulting local and distant tissue reactions. One of eight patients with moderate or severe corrosion did have a subclinical inflammatory response dominated by lymphocytes after five years. To what extent such an inflammatory process might progress to become symptomatic is not known. Ionic and particulate products generated by corrosion disseminated systemically. The minor lymphocytic infiltrate in the liver and kidneys of one subject with bilateral severely corroded head-neck junctions might suggest possible metal toxicity. The diagnosis of adverse tissue reactions to corrosion of modular junctions can be challenging. Postmortem retrieval studies add to our understanding of the nature and progression of lymphocyte-dominated adverse local and potentially systemic tissue reactions to corrosion of modular junctions

Objectives. The aims of this piece of work were to: 1) record the background concentrations of blood chromium (Cr) and cobalt (Co) concentrations in a large group of subjects; 2) to compare blood/serum Cr and Co concentrations with retrieved metal-on-metal (MoM) hip resurfacings; 3) to examine the distribution of Co and Cr in the serum and whole blood of patients with MoM hip arthroplasties; and 4) to further understand the partitioning of metal ions between the serum and whole blood fractions. Methods. A total of 3042 blood samples donated to the local transfusion centre were analysed to record Co and Cr concentrations. Also, 91 hip resurfacing devices from patients who had given pre-revision blood/serum samples for metal ion analysis underwent volumetric wear assessment using a coordinate measuring machine. Linear regression analysis was carried out and receiver operating characteristic curves were constructed to assess the reliability of metal ions to identify abnormally wearing implants. The relationship between serum and whole blood concentrations of Cr and Co in 1048 patients was analysed using Bland-Altman charts. This relationship was further investigated in an in vitro study during which human blood was spiked with trivalent and hexavalent Cr, the serum then separated and the fractions analysed. Results. Only one patient in the transfusion group was found to have a blood Co > 2 µg/l. Blood/Serum Cr and Co concentrations were reliable indicators of abnormal wear. Blood Co appeared to be the most useful clinical test, with a concentration of 4.5 µg/l showing sensitivity and specificity for the detection of abnormal wear of 94% and 95%, respectively. Generated metal ions tended to fill the serum compartment preferentially in vivo and this was replicated in the in vitro study when blood was spiked with trivalent Cr and bivalent Co. Conclusions. Blood/serum metal ion concentrations are reliable indicators of abnormal wear processes. Important differences exist however between elements and the blood fraction under study. Future guidelines must take these differences into account

INTRODUCTION. The specific factors affecting wear of the ultrahigh molecular weight polyethylene (UHMWPE) tibial component of total knee replacements (TKR) are poorly understood. One recent study demonstrated that lower conforming inserts produced less wear in knee simulators. The purpose of this study is to investigate the effect of insert conformity and design on articular surface wear of postmortem retrieved UHMWPE tibial inserts. METHODS. Nineteen NexGen cruciate-retaining (NexGen CR) and twenty-five NexGen posterior-stabilized (NexGen PS) (Zimmer) UHWMPE tibial inserts were retrieved at postmortem from fifteen and eighteen patients respectively. Articular surfaces were scanned at 100×100μm using a coordinate measuring machine (SmartScope, OGP Inc.). Autonomous mathematical reconstruction of the original surface was used to calculate volume loss and linear penetration maps of the medial and lateral plateaus. Wear rates for the medial, lateral and total articular surface were calculated as the slope of the linear regression line of volume loss against implantation time. Volume loss due to creep was estimated as the regression intercept. Student t-tests were used to check for significant. RESULTS. The NexGen CR and NexGen PS patient groups were approximately the same age at time of implantation (mean±SD: 72.1±9.9 and 68.7±8.8 years respectively, p=0.260) and implantation times were not significantly different (8.7±3.1 and 9.1±3.7 years, p=0.670). Both groups showed high variability in wear scars. No significant difference in wear rates on the total surface (mean±SE: 11.89±5.01 mm. 3. /year vs. 11.09±4.18 mm. 3. /year, p=0.905). However, NexGen CR components showed significantly higher volume loss due to creep than NexGen PS components (70.22±47.07 mm. 3. vs. 31.30±41.15 mm. 3. , p=0.007). These results were reflected on the medial and lateral sides, with no significant differences in wear rates on the medial side (p=0.856) or lateral side (p=0.633) and higher volume losses due to creep associated with the NexGen CR components. While NexGen CR and NexGen PS showed a near equal mean percentage of volume loss on the medial side (CR: 52.4±11.7%, PS: 52.5±11.6%), a paired t-test showed that NexGen PS components showed a higher volume loss on the medial side (p=0.056), NexGen CR components did not (p=0.404). DISCUSSION. The combination of higher conformity and more kinematic constraint in NexGen CR components may create larger contact areas with higher stresses, leading to higher volume loss due to creep observed in this study. However, these factors did not produce increased wear rates in the population. Constrained components may maintain more loading on medial side and limit sliding distance on lateral side, causing more wear medially. Total wear rates were very similar and resembled the previously reported rate of 12.9 ± 5.97 mm. 3. /year for retrieved Miller-Galante II (Zimmer) components, which features a near flat articulating surface. These findings indicate that materials factors may be most important in producing wear and that higher conformity alone does not decrease wear. For any figures or tables, please contact authors directly

Introduction. Studies of retrieved total knee replacement (TKR) components demonstrate that in vivo wear on the articular surface of polyethylene liners exhibits a much higher variability than their in vitro counterparts tested on simulators. 1. Only one study has attempted to validate a patient-specific model of wear with a clinically retrieved component. 2. The purpose of this study is to investigate the relationship between observed TKR contact conditions during gait and measured volume loss on retrieved tibial components. Methods. Eleven retrieved ultra-high molecular weight polyethylene (UHMWPE) cruciate-retaining tibial liner components from ten separate patients (implantation time = 8.6±5.6 years) had matching gait trials of normal level walking for each knee. Volume loss on retrieved components was calculated using a coordinate measuring machine and autonomous reconstruction. 3. Motion analysis of normal level walking gait had been conducted between 1986 and 2005 for various previous studies and stored in a consented Human Mechanics Repository, ranging from pre-operative to long-term post-operative testing. Contact location between the femoral component and the tibial component on the medial and lateral plateaus were calculated throughout stance. 4. A previously validated and fine-tuned parametric numerical model was used to calculate TKR contact forces for each gait trial. 5. Vertical contact forces and contact paths on the medial and lateral plateaus were input as normal force and sliding distance to a simplified Archard equation for wear with material wear constant = 2.42 × 10. −7. mm. 3. /Nm. 2,6. to compute average wear per gait cycle. Wear rates were calculated using linear regression, and Pearson correlation examined correlations between modeled and measured wear. Results. Secondary motions at the knee from gait testing showed distinct grouping between trials of each patient (Fig. 1). Three components demonstrated severe polyethylene delamination and were excluded from wear rate analyses. All calculated wear rates for measured and modeled volume loss, shown in Fig. 2, showed excellent agreement and were not significantly different (Table 1). Measured wear rates were comparable to a previous study of a large population of retrieved Miller-Galante II components. 7. As seen in Fig 2b, medial wear volumes for six of eight mild wearing components were closely tracked by their modeled counterparts. Volumes were significantly correlated between measured and modeled wear for the total part and on the medial side, but not for the lateral side (Table 1). Conclusion. Because the Archard equation produces wear volumes that are linearly related to time in situ, deviations from linear predictions arise from patient-specific variations in contact forces and tibiofemoral pathways during normal walking gait. As suggested by the results of the current study, these variations in gait between patients result in meaningful differences to the wear of the UHMWPE component. Despite many assumptions, this study demonstrates the feasibility of a patient-specific model of wear using a rare population of gait-matched retrievals. This study suggests that gait analysis may play an important role in individualized medicine in orthopedics. For figures, tables, or references, please contact authors directly

INTRODUCTION. The lifetime of total hip replacements (THR) is often limited by adverse local tissue reactions to corrosion products generated from modular junctions. Two prominent damage modes are the imprinting of the rougher stem topography into the smoother head taper topography (imprinting) and the occurrence of column-like troughs running parallel to the taper axis (column damage). It was the purpose of this study to identify mechanisms that lead to imprinting and column damage based on a thorough analysis of retrieved implants. METHODS. 776 femoral heads were studied. Heads were visually inspected for imprinting and column damage. Molds were made of each head taper and scanned with an optical coordinate measuring machine. The resulting intensity images were used to visualize damage on the entire surface. In selected cases, implant surfaces were further analyzed by means of scanning electron microscopy (SEM) and white light interferometry. The alloy microstructure was characterized for designs from different manufactures. RESULTS. 165 heads exhibited moderate to severe damage (modified Goldberg scale). Out of those heads 83% had imprinting and 28% exhibited column damage. In most cases with imprinting, the entire contact area between stem and head was affected (Figure 1). Several cases exhibited early signs of imprinting, usually starting on the distal-inferior and distal superior side. High resolution SEM imaging revealed that imprinting was a fretting driven process that was independent of the hardness and material of the stem and head. The SEM images showed that the main mechanism was surface fatigue under partial slip fretting. The generated wear debris was the primary driver of imprinting by three-body fretting. The effect was detrimental on the smoother head surface, but less severe on the rougher stem, where debris was pushed into the troughs of the machining mark topography. 90% of cases with column damage also exhibited imprinting. The other ten percent were either cases in which column damage was too extensive to identify imprinting, or the stem taper was smooth and therefore could not induce imprinting. Metallographic analysis showed that column damage was dictated by the alloy microstructure. Wrought alloy heads frequently exhibited banding related to slight alloy segregations. The process of column damage was entirely chemically driven with etching occurring along the banded microstructure eventually resulting in troughs that were several tens of micrometers deep (Figure 2). DISCUSSION. Imprinting and column damage are common damage modes in THR femoral heads. Imprinting is fretting (miro-motion) driven while column damage is caused chemically, but is also dictated by the alloy micro-structure. However, the results suggest that these two damage modes may be related. The damage process starts with local fretting slowly progressing to a large area of imprinting. The imprinting process leads to widening of the crevice, enabling joint fluid and biological constituents (protein, cells, etc.) to enter the taper interface. This change in local chemistry within a confined crevice environment can cause an etching process that leads to column damage, but only if the femoral head alloy has a banded microstructure

Introduction. Metal-on-polyethylene (MoP) is the most commonly used bearing couple in total hip replacements (THRs). Retrieval studies (Cooper et al, 2012, JBJS, Lindgren et al, 2011, JBJS) report adverse reactions to metal debris (ARMD) due to debris produced from the taper-trunnion junction of the modular MoP THRs. A recent retrospective observational study (Matharu et al, 2016, BMC Musc Dis) showed that the risk of ARMD revision surgery is increasing in MoP THRs. To the authors' best knowledge, no hip simulator tests have investigated material loss from the taper-trunnion junction of contemporary MoP THRs. Methods. A 6-station anatomical hip joint simulator was used to investigate material loss at the articulating and taper-trunnion surfaces of 32mm diameter metal-on-cross-linked polyethylene (MoXLPE) joints for 5 million cycles (Mc) with a sixth joint serving as a dynamically loaded soak control. Commercially available cobalt-chromium-molybdenum (CoCrMo) femoral heads articulating against XLPE acetabular liners (7.5Mrad) were used with a diluted new-born-calf-serum lubricant. Each CoCrMo femoral head was mounted on a 12/14 titanium alloy trunnion. The test was stopped every 0.5Mc, components were cleaned and gravimetric measurements performed following ISO 14242-2 and the lubricant was changed. Weight loss (mg) obtained from gravimetric measurements was converted into volume loss (mm. 3. ) and wear rates were calculated from the slopes of the linear regression lines in the volumetric loss versus number of cycles plot for heads, liners and trunnions. Additionally, volumetric measurements of the head tapers were obtained using a coordinate measuring machine (CMM) post-test. The surface roughness (Sa) of all heads and liners was measured pre and post-test. At the end of the test, the femoral heads were cut and the roughness of the worn and unworn area was measured. Statistical analysis was performed using a paired-t-test (for roughness measurements) and an independent sample t-test (for wear rates). Results and Discussion. The mean volumetric wear rates for CoCrMo heads, XLPE liners and titanium trunnions were 0.019, 2.74 and 0.013 mm. 3. /Mc respectively. There was a statistically significant decrease (p<0.001) in the Sa of the liners post-test. This is in contrast to the femoral heads roughness in which no change was observed (p = 0.338). This head roughness result matches with a previous MoP in vitro test (Saikko, 2005, IMechE-H). The Sa of the head tapers on the worn area showed a statistically significant increase (p<0.001) compared with unworn, with an associated removal of the original machining marks. The mean volumetric wear rate of the head tapers obtained using the CMM (0.028 ± 0.016 mm. 3. /Mc) was not statistically different (p=0.435) to the mean volumetric wear rate obtained gravimetrically (0.019 ± 0.020 mm. 3. /Mc) for the femoral heads. Therefore, wear of the heads arose mainly from the internal taper. The mean wear rates of the CoCrMo taper and titanium trunnion are in agreement with a MoP explant study (Kocagoz et al, 2016, CORR). Conclusion. This is the first long-term hip simulator study to report wear generated from the taper-trunnion junction of MoP hips

INTRODUCTION. Studies of retrieved TKR components demonstrate that in vivo wear on the articular surface of polyethylene liners exhibits a much higher variability on their in vitro counterparts. Only one study has attempted to validate a patient-specific model of wear with a clinically retrieved component. The purpose of this study is to investigate the relationship between observed TKR contact conditions during gait and measured volume loss on retrieved tibial components. METHODS. Eleven retrieved ultra-high molecular weight polyethylene (UHMWPE) cruciate-retaining tibial liner components from ten separate patients (implantation time = 8.6±5.6 years) had matching gait trials of normal level walking for each knee. Volume loss on retrieved components was calculated using a coordinate measuring machine and autonomous reconstruction. Motion analysis of normal level walking gait had been conducted between 1986 and 2005 for various previous studies and stored in a consented Human Mechanics Repository, ranging from pre-operative to long-term post- operative testing. Contact location between the femoral component and the tibial component on the medial and lateral plateaus were calculated throughout stance. A previously validated and fine-tuned parametric numerical model was used to calculate TKR contact forces for each gait trial. Vertical contact forces and contact paths on the medial and lateral plateaus were input as normal force and sliding distance to a simplified Archard equation for wear with material wear constant averaged from literature (2.42 × 10. −7. mm. 3. /Nm) to compute average wear per gait cycle. Wear rates were calculated using linear regression, and Pearson correlation examined correlations between modeled and measured wear. RESULTS. Secondary motions at the knee from gait testing showed distinct grouping between trials of each patient. Three components demonstrated severe polyethylene delamination and were excluded from wear rate analyses. Calculated wear rates for measured and modeled volume loss showed excellent agreement on total surface (15.9 vs. 16.4 mm. 3. /year), medial sides (11.4 vs. 11.7 mm. 3. /year) and lateral sides (4.4 vs. 4.7 mm. 3. /year) and were not significantly different. Volumes were significantly correlated between measured and modeled wear for the total part (r=0.758, p=0.017) and on the medial side (r=0.780, p=0.012), but not for the lateral side (r=0.482, p=0.154). DISCUSSION. Measured wear rates were comparable to a previous study of a large population of retrieved MGII components. Medial wear volumes for six of eight mild wearing components were closely tracked by their modeled counterparts. Because the Archard equation produces wear volumes that are linearly related to time in situ, deviations from linear predictions arise from patient-specific variations in contact forces and tibiofemoral pathways during normal walking gait. As suggested by the results of the current study, these variations in gait between patients result in meaningful differences to the wear of the UHMWPE component. Despite many assumptions, this study demonstrates the feasibility of a patient-specific model of wear using a rare population of gait-matched retrievals

Introduction. This study was performed to investigate the failure mechanism of one specific hip arthroplasty cup design that has shown a high clinical failure rate. The aim of this study was to identify general design problems of this polyethylene inlay. Material and Methods. 55 consecutive retrievals of a cementless screw ring (Mecron) were collected. In any case a 32 mm ceramic head was used. All implants failed due to aseptic loosening. The follow-up of the implants was 3 to 16 years. We recorded backside wear, fatigue of the polyethylene at the flanges on the outer rim and at the cup opening (32 mm inner diameter). To assess the deformation of the inlay, the smallest and the median diameter of the cup opening were measured using a 3 dimensional coordinate measuring machine (Multisensor, Mahr, Germany). Results. 90% of the explants showed signs of wear on the backside of the inlay. Another typical and so far not described alteration was collar fatigue in 68%. 38% of the inlays showed rim creep: Examples for Backside wear, collar fatigue and rim creep are illustrated in Fig. 1. 90% had a diameter of 32.1 mm or less, and even 46% had a diameter less than 32 mm. Discussion. The investigated design is at the lower limit of the allowed machining tolerance of the cup opening (32 + 0.1 mm) and has no additional clearance (which some manufacturers add). It seems that the inlays yield at the dome because of the viscoelastic properties of polyethylene and the open dome area of the attached screw ring. This leads to excessive wear at the dome area and it triggers a “brake drum” effect at the cup opening. Thereby torsional stresses at the implant bone interface increase which lead to failure of the implant. To avoid this type of failure, PE inlays should have enough clearance at the cup opening and the inlay should have dome contact to the closed metal shell

Introduction. Ultra-high molecular weight polyethylene (UHMWPE) is the sole polymeric material currently used for weight- bearing surfaces in total joint replacement. However, the wear of UHMWPE in the human body after total joint replacement causes serious clinical and biomechanical reactions. Therefore, the wear phenomenon of UHMWPE is now recognized as one of the major factors restricting the longevity of artificial joints. In order to minimize the wear of UHMWPE and to improve the longevity of artificial joints, it is necessary to clarify the factors influencing the wear mechanism of UHMWPE. Materials and Methods. In a previous study (Cho et al., 2016), it was found that roundness (out-of-roundness) of the retrieved UHMWPE acetabular cup liner [Figure 1(a)] had a tendency to increase with increasing roundness of the retrieved metal femoral head [Figure 1(b)]. It appears that roundness of the femoral head contributes to increase of wear of the polyethylene liners. We focused on the roundness of femoral head as a factor influencing the wear of polyethylene liner in hip prosthesis. In this study, further roundness measurements for 5 retrieved metal femoral heads were performed by using a coordinate measuring machine. The elasto-plastic contact analyses between femoral head and polyethylene liner using the finite element method (FEM) were also performed in order to investigate the influence of femoral head roundness on the mechanical state and wear of polyethylene liner in hip prosthesis. Results. The range of roundness of the 5 retrieved metal femoral heads measured in this study was 14.50∼44.70 µm. Two examples of the results of FEM contact analyses are shown in Figure 2. Figure 2(a) is the results of the repeated contact analysis between femoral head and polyethylene liner under constant axial loading of 1000 N. Figure 2(b) is the results of the repeated contact analysis between femoral head and polyethylene liner under hip joint loading during normal gait. These figures show the distribution of the contact stress (von Mises equivalent stress) in the polyethylene liner. The graph in Figure 3 shows the changes in the maximum contact stress in the polyethylene liner with the flexion/extension angle of femoral head. Discussion and Conclusions. As the results of a series of the FEM contact analyses, it was found that repeated high contact stresses which exceed the yield stress of UHMWPE caused by roundness of the metal femoral head occurred in the polyethylene liner as shown in Figures 2 and 3. It was also found that the magnitude and amplitude of the repeated contact stresses had a tendency to increase with increasing roundness of the femoral head and axial loading applied to the femoral head. The results of this analytical study suggest that the roundness (out-of- roundness) of the femoral head is associated with accelerating and/or increasing wear of the UHMWPE acetabular cup liner in a hip prosthesis after total hip replacement. For any figures or tables, please contact authors directly

Introduction. Hip stem taper wear and corrosion is a multifactorial process involving mechanical, chemical and biological damage modes. For the most cases it seems likely that the mechanically driven fretting wear is accompanied by other damage modes like pitting corrosion, galvanic corrosion or metal transfer. Recent retrieval studies have reported that the taper surface topography may affect taper damage resulting from fretting and corrosion [1]. Therefore, the current study aimed to examine effects of different taper topography parameters and material combinations on taper mechanics and results regarding wear and corrosion have been investigated. Materials and Methods. Combined experimental and numerical studies were conducted using titanium, cobalt-chromium and stainless steel generic tapers (Figure1). Uniaxial tensile tests were performed to determine the mechanical properties of the materials examined. For the taper studies macro-geometry of ceramic ball heads (BIOLOX. ®. delta) and tapers were characterized using a coordinate measuring machine, and assembly experiments according to ISO7206-10 were conducted up to 4kN. Before and after loading, taper subsidence was quantified by assembly height measurements. Taper micro-geometry, taper surface deformation, and contact area were determined by profilometry. Initial numerical studies determined coefficients of friction for the three material combinations. Macro- and micro-geometries of the tapers were modelled, and taper subsidence and assembly load served as boundary conditions. Further studies used simplified models to examine effects of varying profile depths and angular gaps on surface deformation, taper subsidence, contact area, engagement length and pull-off force. Results. Largest coefficient of friction and pull-off forces were calculated for steel (µ=0.32), cobalt-chromium revealed the lowest with µ=0.18. Titanium showed largest deformations and taper subsidence throughout all calculations (Figure2, Figure3). Taper subsidence, engagement length and deformations increased with increasing profile depth while contact area decreased. Pull-off forces were almost constant for different profile depths while they increased for increasing angular gaps. Taper subsidence and deformations also increased with increasing angular gap while engagement length decreased and contact area almost remained constant. Discussion. In order to decrease wear and corrosion micromotions should be minimized. Therefore, smaller angular gaps and smaller profile depths seems to be beneficial since deformation and taper subsidence are reduced. Literature data confirmed the results for different angular gaps showing that a larger angular gap is associated with larger amounts of micromotion and wear [2, 3]. Additionally, larger angular gaps and larger profile depths result in larger plastic deformation facilitating subsurface crack initiation and propagation. A large angular gap may also facilitate particle release [4]. Larger pull-off forces can indicate larger resistance against micromotion. Therefore, steel may tend to later develop fretting-corrosion in situ. However, among the metals examined steel also showed the largest equivalent plastic strain. This study is limited to pairings involving ceramic heads. These can help mitigating fretting corrosion resulting from micromotion between ball head and cobalt-chromium or titanium alloy tapers [5]. However, future studies will include other ball head materials. In conclusion, this study showed that taper surface topography affects taper mechanics and is important in terms of wear and corrosion

Introduction. Retrieved metal-on-metal acetabular components are invaluable resources from which to investigate the wear behaviour of failed hip implants. New forensic and investigative techniques continue to be developed to help the surgeon further understand factors which contribute to early failure. We have developed a novel technique to locate the in vivo location of the primary wear scar of an explanted cup. Patients/Materials & Methods. Thirteen (13) patients with failed metal hip resurfacings were recruited and their acetabular components retrieved. A 3D wear map was generated and the precise location of the primary wear scar in each cup was identified using a coordinate measuring machine (CMM). This wear scar position and location was noted in relation to standard landmarks on the acetabular cup. All patients underwent a computerised tomography (CT) scan prior to revision surgery. The 3D positional map from the CMM was then co-registered with the implant on the patient's pelvic 3D CT scan. Results. A schematic diagram was generated revealing the 3D orientation and location in the patient of the acetabular primary wear scars for all 13 subjects. The distribution of the location of the in vivo primary wear scars was variable and unique for each of the patient's acetabular cup. Discussion. We were able to identify the location of the in vivo primary wear scar of all thirteen acetabular cups successfully with this novel method. This technique can only be recruited for acetabular components that have clearly identifiable features which can be identified on CT. This is the first study to co-relate the point of highest wear on the acetabular cup to its pre-failure position in vivo. Conclusion. This technique has made it possible to better understand the three-dimensional properties of wear behaviour and can be used in further studies to investigate variables that determine the orientation and position of the primary wear scar

It has been speculated that high wear at the head-stem taper may contribute to the high failure rates reported for stemmed large head metal-on-metal (LH-MOM) hips. In this study of 53 retrieved LH-MOM hip replacements, we sought to determine the relative contributions of the bearing and taper surfaces to the total wear volume. Prior to revision, we recorded the relevant clinical variables, including whole blood cobalt and chromium levels. Volumetric wear of the bearing surfaces was measured using a coordinate measuring machine and of the taper surfaces using a roundness measuring machine. The mean taper wear volume was lower than the combined bearing surface wear volume (p = 0.015). On average the taper contributed 32.9% of the total wear volume, and in only 28% cases was the taper wear volume greater than the bearing surface wear volume. Despite contributing less to the total material loss than the bearing surfaces, the head-stem taper junction remains an important source of implant-derived wear debris. Furthermore, material loss at the taper is likely to involve corrosion and it is possible that the material released may be more biologically active than that from the bearing surface

Introduction. In this study, we aimed to investigate the effect of the topography of the female taper surface on taper wear. Patients/Materials & Methods. We measured volumetric material loss at the female taper surface of the first consecutive 36mm MoM hips received at our centre. The MoM hips were from a single manufacturer and possessed matching: head size (36mm); taper geometry (5.65 degree 12 14 tapers); and metallurgy (cobalt chrome female taper mated with a titanium stem). Volume loss was measured with a coordinate measuring machine using validated methods. Surface roughness was measured using a Surftest SJ400. There were 111 head tapers available for analysis. The majority had been revised secondary to ARMD, and a minority for loosening. The mean duration of implantation was 56 months. Volumetric wear values were log normalised in order to construct a multiple regression model to investigate the interaction of head offset, the head taper angle and the Rp value of the female taper surface. Rp is a roughness parameter, which is a measure of the peaks above the plateau of a surface. Results. The median (range) volumetric loss from the female taper surfaces was 0.132mm3 (0.008–2.149). Median Rp value was 1.94 microns (0.50 to 6.01). Multiple regression model returned an R2 value of 0.43 (p < 0.001) meaning that the Rp value of the female surface, the head offset, duration of implantation and the taper angle explained 43% of the variation in volumetric wear rates. Rp was the dominant variable. Discussion & Conclusion. The topography of the female taper surface appears to be an important factor in material loss from the taper junction in MoM hips. There was an unexpectedly wide variation in surface finishes in our sample set despite the fact that a single product from a single manufacturer was analysed

Introduction. Historical studies have reported incidences of taper corrosion in retrieved MoP hips of 20–50%. These studies relied on visual assessments, rather than using modern analytical techniques. Patients/Materials and Methods. The Northern Retrieval Registry was initiated to routinely analyse all retrieved hips. The volumetric wear rates of retrieved Exeter head tapers were measured using a coordinate measuring machine using validated methods and compared to the available results obtained from an ongoing study of failed MoM prostheses. Power analysis suggested that we would need at least fifty Exeter head tapers to provide significant results. Non-parametric tests were used to assess differences. We have previously identified head diameter/offset/taper angle and taper surface roughness as variables associated with taper wear. The effect of bearing material combination remains unquantified. These design variables were entered into a multiple regression model following log normalisation of taper wear. Results. 56 consecutively retrieved Exeter head tapers underwent assessment. 36 were of size 28mm. Primary causes for revision were acetabular loosening/periprosthetic fractures. Mean duration in vivo was 87 (1 – 252) months. Median total volumetric wear was 0.29mm. 3. with a wear rate of 0.07mm. 3. /year. There were 353 MoMs. Median taper wear rates for 28mm MoM was 0.07, for 36mm was 0.11, for 38–44mm was 0.31, for 45 – 48mm was 0.52, for 49–52 was 0.55mm and for >52mm was 1.36mm. 3. /year. While Exeter taper wear was significantly lower than the MoM group as a whole (p<0.001), regression modelling indicated that bearing diameter had the greatest influence on taper wear. Discussion. Contrary to previous literature reports, significant material loss at the taper junction of conventional arthroplasties is not commonly observed. Exeter hips appear to be protected to some extent by the smaller head size and lower taper roughness when compared to contemporary MoM devices. The effect of the bearing combination remains under investigation

Hemi shoulder arthroplasty is an attractive treatment for shoulder arthritis in particular if the natural glenoid is still intact. However, comparing the clinical results of hemi and total shoulder arthroplasty clearly shows lower survival for the hemi arthroplasty. One of the most common reasons for revision surgery is gleniod erosion, where the cartilage or bone is worn of. Aim of the current study was to analyse if the metallic articular surface of retrieved hemi shoulder arthroplasty is different from new implants. We hypothesized that the surface roughness will increased due the articulation and that metallic wear is detectable on the implants. Twelve retrieved and three brand new hemi shoulder arthroplasty were included. The surface roughness (Ra, Rz, Rmax, Rsk) was measured on different sites of the surface (center of the head and at the edge). The implants were further measured using a coordinate measuring machine to gain information on volumetric wear and geometrical alterations. Compared to new implants the surface roughness on the retrievals was significantly increased (Tab. 1), except for skewness. Although the roughness parameters within the retrieval group were generally higher at the center of the head compared to the edge, this difference was not significant. Apart from form deviations no volumetric wear was detectable on the heads (Fig. 1). The current results indicate that the metallic articular implant surface changes in vivo and that the material is hurt due to the articulation against the softer cartilage or bone. Although it can't be finally clarified by that study, to what extend the higher roughness is taking part in the process of the clinically observed erosion of the gleniod, it can be assumed that an increased roughness is disadvantageous. Possibly, the observed surface alterations won't occur clinically with harder materiel (e.g. ceramic), but this even needs to be validated