Charactering taper junction wear helps understand the mechanism of failure of metal on metal hip replacements.

Introduction: Taper junction material loss is the result of corrosion and mechanical wear. The significance of the taper junction material loss is highlighted by studies that compared resurfacing and total hip replacements of the same type and size. High volumes of material loss are reported, especially from the head taper, but the pattern of wear is unknown. One report characterized the material loss pattern of five tapers (n=5) into axisymmetrical and asymmetrical, along the long axis of the taper. We noticed more than two patterns on our retrievals and we set out to characterize these types and relate them to clinical variables. Methods: We retrospectively analysed retrieved cobalt-chromium tapers (n=146) using a roundness measurement machine. We also performed a corrosion classification and collected clinical data (metal ion levels, time to revision, component sizes). A non-blinded author devised a four-group classification (table). Two blinded authors classified the material loss patterns derived from the roundness measurement machine. Results: The four groups of material loss patterns Low wear (n= 62), Open-end band (n=29), Stripped material loss (n=51) and Coup-Countercoup (n=4). Kappa was 0.78 (p<0.001) in the assessment of interobserver reliability. Kruskal-Wallis test revealed: - Significantly higher volumes of wear on the taper of Stripped material loss compared to Low wear (p<0.001) and Open-end band compared to Low wear (p<0.001) groups. - Significantly higher chromium ion blood levels in the open-end band compared to the Stripped material loss group. - Significantly higher Cobalt ion blood levels in the Stripped material loss compared to the Low wear group - Significantly higher Cobalt/Chromium ration in the Open-end band compared to the Low wear group One-way ANOVA analysis revealed: - Significant difference between in the head sizes between the groups (p=0.01). Post-hoc analysis located the difference between the Low wear (median=40, range=20) and Open-end band (median=49, range=20) groups (p<0.001). - Significantly higher time to revision in the Stripped material loss compared to the Low wear group (p=0.05), in the post-hoc analysis. - Significantly higher corrosion scores in the Stripped material loss compared to the Low wear group (p<0.001) and the Open-end band compared to the Low wear group (p<0.001). Discussion: The results suggests that corrosion becomes worse over time and that the material loss pattern evolves gradually from the Low wear to Open-end band and finally to Stripped. Further analysis is required to assess the factors that affect the Coup-countercoup group

Corrosion is the main mechanism of material loss at the taper junction of large head metal on metal hip replacements

Abstract Material loss at the head-stem taper junction may contribute to the high early failure rates of stemmed large head metal-on-metal (LH-MOM) hip replacements. We sought to quantify both wear and corrosion and by doing so determine the main mechanism of material loss at the taper. This was a retrospective study of 78 patients having undergone revision of a LH-MOM hip replacement. All relevant clinical data was recorded. Corrosion was assessed using light microscopy and scanning electron microscopy, and graded according to a well-published classification system. We then measured the volumetric wear of the bearing and taper surfaces. Evidence of at least mild taper corrosion was seen in 90% cases, with 46% severely corroded. SEM confirmed the presence of corrosion debris, pits and fretting damage. However, volumetric wear of the taper surfaces was significantly lower than that of the bearing surfaces (p = 0.015). Our study supports corrosion as the predominant mechanism of material loss at the taper junction of LH-MOM hip replacements. Although the volume of material loss is low, the ionic products may be more biologically active compared to the particulate debris arising from the bearing surfaces

Taper wear contributes only a third of the total volumetric material loss in large head metal on metal hip replacement

Abstract 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

Analysis of the taper supports retention of a well-fixed stem in revision surgery of metal-on-metal hip replacements

Summary. Retrieval analysis of metal-on-metal hip stems showed negligible wear (<1mm3) of the male taper surface in all cases. This supports retention of a well-fixed, undamaged stem during revision surgery Introduction. Wear and corrosion at the taper junction of metal-on-metal total hip replacements (MOM-THR) is likely to contribute to the high revision rates reported by joint registries. Studies have shown significant volumes of measured material loss at the female taper surface (femoral head), but no study has reported data for the male taper surface (stem). The decision to retain a well-fixed stem at revision is uncertain, particularly in cases where the taper junction may be responsible for the need for revision surgery. This is an important consideration given the increasing burden of MOM revisions and the considerable increase in morbidity associated with revision of the stem. The purpose of this study was to measure the volume of material loss of both the ‘female’ and ‘male’ taper surfaces and determine the relative contribution of each to the total material loss at the taper junction. Methods. This was a retrospective study of 90 retrieved components from revision surgery of large diameter MOM-THRs, in which revision included removal of the stem. The series included four contemporary metal-on-metal bearing designs and six different stem designs. We recorded all commonly reported clinical data. We then measured the volume of material loss from each of the taper and bearing surfaces using a validated method. Additionally, each of the taper surfaces were examined microscopically and corrosion was graded according to a well-published four-tiered classification. Results. The median volume of material lost from the male taper surface was 0.29mm3 (0.00 - 0.83). This was significantly less than the median volume of material lost from the female taper surface (p < 0.001), which was 1.31mm3 (0.57 - 17.03). These results are shown in Figure 1. On average, the ‘male’ taper contributed 6.1% (0.00 - 31.53) of the total material loss at the taper junction, and in all cases was deemed negligible (< 1mm3). The bearing surfaces contributed a significantly greater volume of material than both taper surfaces combined (p < 0.001). Similar results were found for corrosion scores, with significantly fewer male taper surfaces demonstrating evidence of corrosion compared to the female taper surfaces (Figure 2). Discussion/Conclusions. The taper junction is a clinically relevant source of metal debris in MOM-THR. However, the predominant source of taper material is the female surface (inside the femoral head), and we have shown that the male surface (stem-neck) contributes only negligible volumes of material (< 1mm3). Further qualitative assessment confirmed the absence of significant corrosion on the male taper surfaces. Although limited to few cases, this data supports retention of a well-fixed macroscopically undamaged stem at revision surgery in patients with problematic MOM-THRs. These are clinically significant findings given the increasing burden of revision surgery and the increased morbidity and complexity associated with revising a well-fixed femoral stem

Material Loss at the Taper Junction of Retrieved Large Head Metal-on-Metal Total Hip Replacements

It has been speculated that material loss, either as corrosion or wear, at the head–stem taper junction is implicated in the high revision rates reported for metal-on-metal total hip replacements. We measured the volume of material loss from the taper and bearing surfaces of retrieved devices, and investigated the associations with blood metal ion levels and the diagnosis of a cystic or solid pseudotumor. The median volumes of material lost from the female and male taper surfaces were 2.0 and 0.29 mm3, respectively, while the median volumes of wear from the cup and head bearing surfaces were 1.94 and 3.44 mm3, respectively. Material loss from the female taper was similar to that from the acetabular bearing surface (p ¼ 0.55), but significantly less than that from the femoral bearing surface (p < 0.001). Material loss from the male taper was less than that from both bearing surfaces (p < 0.001). Multivariable analysis demonstrated no significant correlations between the volume of material lost from the taper surfaces and either blood cobalt or chromium ions, or the presence of pseudotumor. While a substantial volume of material is lost at the taper junction, the clinical significance of this debris remains unclear

Which factors determine the volume of material lost from the taper junction of metal-on-metal hip replacements?

Summary. Multiple linear regression analysis of fifteen factors showed that bearing surface design was the most significant predictor of high taper material loss in retrieved metal-on-metal hip replacements. Introduction. Joint registries have reported high revision rates for large diameter metal-on-metal total hip replacements (MOM-THR). It has been speculated that additional cobalt-chrome debris may be released from head-stem taper junction, and that this may be implicated in clinical failure. However, the mechanisms and clinical significance of taper material loss remain largely unclear. The purpose of this study was to measure the volume of taper material loss in a large series of MOM-THR retrievals, and then by using multiple linear regression analysis identify any significant predictors of high taper material loss. Methods. This was a retrospective study of 125 consecutively collected large diameter (> 38mm) MOM retrieval cases, and included six contemporary bearing designs and eight femoral stem designs. We recorded patient demographic data and all commonly reported clinical and design variables, including fifteen variables identified through review of the literature that had been indicated as factors likely to affect the mechanical or electrochemical properties of the head-stem taper junction (Table 1). Post-operatively we measured the volume of material lost from all 280 components. Material loss of the bearing surfaces was measured using a coordinate measuring machine and of the taper surfaces using a roundness measuring machine. We then performed univariable and multiple linear regression analyses to determine the variables significantly associated with high volumes of material loss at the taper junction. Results. The median volume of material loss from the taper junction was 2.16mm3 (range: 0.13 - 25.19), significantly (p < 0.001) less than the median volume of material lost from the bearing surface, which was 5.79mm3 (range: 0.59 - 309.17). Full results of all univariable regression analyses are given in Table 1. Univariable analysis identified two significant predictors of high taper material loss: (1) bearing surface design (p = 0.015) and (2) edge loading of the bearing surfaces (p = 0.017). The same two variables were confirmed as the only two significant predictors in the final multiple linear regression model (Table 2), and both were independent predictors, with tests for interaction proving non-significant (p = 0.510). Discussion/Conclusion. It has been speculated that material loss at the head-stem taper junction is implicated in the high revision rates reported for MOM-THR. However, there is little published data to support this and both the clinical significance and mechanism of material loss remains poorly understood. Using multivariable statistical analysis to analyze a large series of retrieval data, we have shown that bearing surface design and edge loading of the bearing surfaces are significant predictors for high taper material loss. This study contributes significantly to our understanding of the mechanisms of MOM hip failure, and suggests important factors that predispose patients to increased metal ion exposure

The reproducibility of a semi-quantitative scoring method for taper corrosion and fretting, and its usefulness for predicting the volume of material loss

Introduction It has been suggested that the release of metal debris due to mechanical wear, fretting and corrosion at the head-stem taper junction may contribute to the high revision rates of MOM-THAs [1, 2]. A peer reviewed, semi-quantitative corrosion and fretting scoring system of the femoral stem and head tapers using visual assessment, was developed by Goldberg et al. [3]. This method involves assigning scores on a scale of 1 (no visible signs of corrosion or fretting) to 4 (severe corrosion or fretting) based on the prevalence of black debris, pits or etch marks (corrosion) and fretting scars. However, this method has not been validated and the reproducibility of such a measure is unknown. Furthermore, whilst scoring a component allows the examiner to quickly quantify the appearance of taper surface damage in the form of corrosion or fretting, it is unclear as to how this relates to the actual volume of material loss at the taper. The aims of this study were to: (1) determine the inter-observer variability of the visual scoring method for the assessment of corrosion and fretting of MOM-THA taper junctions; (2) determine the prevalence and severity of corrosion and fretting at the taper junction; (3) determine the strength of correlation between corrosion and fretting scores and the actual volume of material lost at the taper junction. Methods Macroscopic and stereomicroscopic examinations of the head taper surface of 150 retrieved MOM-THA implants were performed by two experienced independent observers using methods previously defined [3] to assess corrosion and fretting. Both observers were blinded to all other clinical and component data. A Leica M50 microscope with x40 magnification was used to examine each surface. The visual appearance of fretting was considered as regions of the taper surface that were damaged with small scars running perpendicular to the circumferential machine lines of the taper screw thread. The visual appearance of corrosion was defined as regions of discolouration or dullness on the taper surface or the presence of black debris, pits or etch marks. The prevalence of corrosion and fretting was quantified using a scoring scale of 1 (none) to 4 (severe), as described previously [3]. Corrosion and fretting scores were assigned to the distal and proximal regions of the taper surface, Figure 1. Overall scores for corrosion and fretting were then assigned to each taper following assessment of each surface as a whole. The volume of material loss at each of the taper surfaces was measured using a Talyrond 365 (Hobson, Leicester, UK) roundness measuring machine, using a published method [4]. The corrosion score relating to the overall surface of each taper was plotted against the measured volume of material loss. All statistical analysis was performed using Stata/IC version 12.1 [StataCorp, College Station, TC, USA] and throughout a p value < 0.05 was considered statistically significant. Cohen’s weighted Kappa statistic (κ) was used to measure the inter-observer agreement of the Goldberg scores [5]. Kappa values were assessed using the criteria described by Landis and Koch where κ ≤ 0 = poor, 0.01 to 0.20 = slight, 0.21 to 0.40 = fair, 0.41 to 0.60 = moderate, 0.61 to 0.80 = substantial, 0.81 to 1 = almost perfect [6]. Neither the volumetric measurement data nor corrosion scores were normally distributed. Therefore, to determine the strength of correlation between the two, we used the Spearman Rank test. Results Table 1 summarises the inter-observer agreement data for the Goldberg scores of the taper surfaces. The observed agreement for the overall corrosion and fretting scores were 95% and 84% respectively. The reliability of the proximal and distal taper corrosion scores was moderate to substantial (κ=0.52 to 0.70), whilst reliability of the overall head taper corrosion score was substantial (κ=0.64). The reliability of the proximal, distal and overall taper fretting scores was poor (κ=0.14, 0.13 and 0.18 respectively). Figures 2a and 2b plot the distribution of the overall corrosion and fretting scores by both examiners for the tapers examined. Examiners A and B reported that 99% and 94% of tapers respectively had visual evidence of corrosion whilst 54% and 59% of tapers respectively showed evidence of fretting. Figure 3 plots the severity of corrosion assessed using the visual scoring method against the material loss at the head taper. Taper corrosion score was significantly and moderately correlated with the volume of material loss measured (Spearman’s r = 0.59; 95%CI = 0.47 - 0.68; p < 0.001). Similarly, fretting score was significantly correlated with the volume of material loss, but the correlation was weak (Spearman’s r = 0.24; 95% CI = 0.08-0.39; p = 0.003). Discussion The results of this study found that the observed agreement was higher for corrosion assessments of the head taper surface than for fretting. The inter-observer reliability for corrosion scoring was moderate whilst the reliability of fretting scores was slight to fair. Both examiners reported that it was considerably easier to identify regions of discolouration and the presence of black debris, both key indicators of corrosion, than it was to identify clear fretting scars. It was difficult to distinguish between surface damage due to fretting and damage that may have occurred during implantation or retrieval of the components. In all instances of fretting, there was also evidence of corrosion however there was not always evidence of fretting when corrosion was observed, particularly when corrosion scores were severe. This supports the claim that lower fretting scores may be due to the fact that the black deposits, due to corrosion, mask the presence of fretting scars. Another explanation may be that the occurrence of crevice corrosion over time may remove some of the fretting scars, therefore underestimating the degree of fretting that has occurred. The presence of black corrosion deposits may also explain the large variability between the two examiners when reporting on fretting. There were a number of examples of when deposited material partially obscured suspected fretting scars, adding to the existing uncertainty due to difficulties in distinguishing between fretting and other mechanical damage. Our study is the first to report on the relationship between the visual assessment of the taper and quantified material loss. Significant positive correlations were found to exist between head taper corrosion and fretting scores and the actual measured volume of  material loss. The correlation was stronger for corrosion scores than fretting scores. Whilst these results certainly support a mechanism involving fretting corrosion, the strengths of these correlations do not support the use of semi-quantitative scoring methods to predict the actual measured volume of material loss. The results of our study suggest that corrosion plays a significant role in the loss of material: (1) there was evidence of corrosion in almost every taper examined; (2) a positive correlation was observed between the severity of corrosion and the absolute volume of material loss; (3) imprinting of the male taper surface was observed on all female tapers, and suggests a mechanism involving galvanic corrosion; (4) observations of less fretting may in part have been due to crevice corrosion destroying fretting scars over time. Significance Our study has contributed to the work on implant retrievals, showing that detailed visual examination of taper surfaces can produce reliable data, which may be able to predict the severity of material loss but is not a substitute for complex metrology methods. The results of our study suggest that corrosion may be the main mechanism of material loss at the taper junction

The utility of lung ultrasound in COVID-19:A systematic scoping review

Introduction: Lung ultrasound (LUS) has an established evidence base and has proven useful in previous viral epidemics. An understanding of the utility of LUS in COVID-19 is crucial to determine its most suitable role based on local circumstances. Method: Online databases, specialist websites and social media platforms were searched to identify studies that explore the utility of LUS in COVID-19. Case reports and recommendations were excluded. Findings: In total, 33 studies were identified which represent a rapidly expanding evidence base for LUS in COVID-19. The quality of the included studies was relatively low; however, LUS certainly appears to be a highly sensitive and fairly specific test for COVID-19 in all ages and in pregnancy. Discussion: There may be LUS findings and patterns that are relatively specific to COVID-19; however, specificity may also be influenced by factors such as disease severity, pre-existing lung disease, operator experience, disease prevalence and the reference standard. Conclusion: LUS is almost certainly more sensitive than chest radiograph for COVID-19 and has several advantages over computed tomography and real-time polymerase chain reaction. High-quality research is needed into various aspects of LUS including: diagnostic accuracy in undifferentiated patients; triage and prognostication; monitoring progression and guiding interventions; the persistence of residual LUS findings; inter-observer agreement and the role of contrast-enhanced LUS.</p