Paul J. Bills

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 = 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.

Improvement in the assessment of wear of total knee replacements using coordinate-measuring machine techniques

Total joint replacement is one of the most common elective surgical procedures performed worldwide, with an estimate of 1.5×106 operations performed annually. Currently joint replacements are expected to function for 10–15 years; however, with an increase in life expectancy, and a greater call for knee replacement due to increased activity levels, there is a requirement to improve their function to offer longer-term improved quality of life for patients. Wear analysis of total joint replacements has long been an important means in determining failure mechanisms and improving longevity of these devices. The effectiveness of the coordinate-measuring machine (CMM) technique for assessing volumetric material loss during simulated life testing of a replacement knee joint has been proved previously by the present authors. The purpose of the current work is to present an improvement to this method for situations where no pre-wear data are available. To validate the method, simulator tests were run and gravimetric measurements taken throughout the test, such that the components measured had a known wear value. The implications of the results are then discussed in terms of assessment of joint functionality and development of standardized CMM-based product standards. The method was then expanded to allow assessment of clinically retrieved bearings so as to ascertain a measure of true clinical wear.

A metrology solution for the orthopaedic industry

Total joint replacement is one of the most common elective surgical procedures performed worldwide, with an estimate of 1.5 million operations performed annually. Currently joint replacements are expected to function for 10–15 years, however, with an increase in life expectancy, and a greater call for knee replacement due to increased activity levels, there is a requirement to improve their function to offer longer term improved quality of life for patients. The amount of wear that a joint incurs is seen as a good indicator of performance, with higher wear rates typically leading to reduced function and premature failure. New technologies and materials are pushing traditional wear assessment methods to their limits, and novel metrology solutions are required to assess wear of joints following in vivo and in vitro use. This paper presents one such measurement technique; a scanning co-ordinate metrology machine for geometrical assessment. A case study is presented to show the application of this technology to a real orthopaedic measurement problem: the wear of components in total knee replacement. This technique shows good results and provides a basis for further developing techniques for geometrical wear assessment of total joint replacements

Detailed inspection of metal implants.

Detailed visual inspection of metal hips is the first step in retrieval analysis. In this study a systematic visual inspection protocol was developed to quantify bearing surface changes and their associations with material loss was investigated. Simple and multiple linear regression models found that moderate surface scratching, discolouration, haziness and the size of visible wear scars were all significantly associated with material loss (R2 = 5%-73%, p<0.05). Visual inspection is not a substitute for measurement of material loss but an understanding of bearing surface changes may offer unique clues as to the mechanisms of failure of retrieved hips.

Defining True Tribological Contact Through Application of the Morphological Method to Surface Topography

In tribological functions high peaks (summits) in the surface topography play a dominant role in that they determine the position of first contact and how the contact will occur. Both statistic-based methods and feature-based methods address the characterization of a single surface, while neglecting the interacting surface. A morphological method is proposed to simulate the contact of two mating surfaces. The surface under evaluation is rolled by a ball with radius meant to simulate the largest reasonable peak curvature at a contact. In such a situation the contact points of the rolling ball may serve as an identification of those surface portions that are in real contact. The morphological closing operation could then be applied to detect the contact points of the rolling ball, however, the traditional computation method does not lead to an accurate result. To overcome this deficiency, a geometrical computation approach has been developed to capture the contact points based on four searching procedures. The resulting method has been verified through experimentation and then applied to a case study in which the underlying form of the surface of a hip replacement taper junction is analyzed to remove the effect of the dominant threaded structure.

Assessing the material loss of the modular taper interface in retrieved metal on metal hip replacements

Measuring the amount of material loss in the case of revised hip replacements is considered to be a prerequisite of understanding and assessing the true in vivo performance of the implant. This paper outlines a method developed by the authors for quantifying taper material loss as well as more general taper interface parameters. Previous studies have mostly relied on visual inspection to assess the material loss at the taper interface, whereas this method aims to characterize any surface and form changes through the use of an out-of-roundness measurement machine. Along with assessing the volumetric wear, maximum linear penetration and taper contact length can also be determined. The method was applied to retrieved large head metal-on-metal femoral heads in order to quantify the material loss at this junction. Material loss from the female femoral head taper can be characterized as a localized area that is in contact with the stem taper surface. The study showed that this method has good repeatability and a low level of interoperability variation between operators.

Component size mismatch of metal on metal hip arthroplasty: an avoidable never event.

Size mismatch of components used in total hip arthroplasty is a serious, preventable patient safety incident of unknown prevalence as many cases are not detected. Component size mismatch was found in 11 cases (0.9%) at our retrieval centre. All cases of mismatch were not detected on plain radiograph during routine clinical follow up and blood metal ion levels were elevated above the MHRA action level of 7 ppb. Root cause analysis identified manufacturer, hospital and surgeon factors that need to be addressed to reduce the incidence of this avoidable clinical problem. Retrieval analysis is the only method of confirming size mismatch and is likely to be under-represented in National Joint Registries that record the indication for revision at the time of revision.

The Impact of Alkaliphilic Biofilm Formation on the Release and Retention of Carbon Isotopes from Nuclear Reactor Graphite

Abstract14C is an important consideration within safety assessments for proposed geological disposal facilities for radioactive wastes, since it is capable of re-entering the biosphere through the generation of 14C bearing gases. The irradiation of graphite moderators in the UK gas-cooled nuclear power stations has led to the generation of a significant volume of 14C-containing intermediate level wastes. Some of this 14C is present as a carbonaceous deposit on channel wall surfaces. Within this study, the potential of biofilm growth upon irradiated and 13C doped graphite at alkaline pH was investigated. Complex biofilms were established on both active and simulant samples. High throughput sequencing showed the biofilms to be dominated by Alcaligenes sp at pH 9.5 and Dietzia sp at pH 11.0. Surface characterisation revealed that the biofilms were limited to growth upon the graphite surface with no penetration of the deeper porosity. Biofilm formation resulted in the generation of a low porosity surface layer without the removal or modification of the surface deposits or the release of the associated 14C/13C. Our results indicated that biofilm formation upon irradiated graphite is likely to occur at the pH values studied, without any additional release of the associated 14C.

Effect of machining parameters on surface roughness in Al 2618 alloy subject to multi-axis machining process using ball nose cutting tools

Recently multi-axis machining technology has improved significantly. It has become a widely accepted method of manufacturing components with complex, free form surfaces. Solid billet materials with negligible internal defects are used in this process. This provides increased durability and fatigue life over equivalent cast components. However, multi-axis machining using ball nose cutting tools leaves cusps as machining marks. The surface quality within the cusps can have a significant influence on the fatigue life and durability of a component. The main objective of this paper is to report the experimental investigation of the effect of different cutting parameters on surface roughness of Al 2618 alloy. This paper reports on an experimental investigation of the effect of different cutting parameters on surface roughness of Al 2618 alloy. A full factorial experimental analysis using four different levels of spindle speed, feed-rate and cutting-tool approach angle was carried out. The results indicate that higher spindle speed, lower feed rate and a cutting tool approach angle of approximately 25° generates a better surface finish.

Comparative study of material loss at the taper interface in retrieved metal-on-polyethylene and metal-on-metal femoral components from a single manufacturer

There have been a number of reports on the occurrence of taper corrosion and/or fretting and some have speculated on a link to the occurrence of adverse local tissue reaction specifically in relation to total hip replacement which have a metal-on-metal bearing. As such a study was carried out to compare the magnitude of material loss at the taper in a series of retrieved femoral heads used in metal-on-polyethylene bearings with that in a series of retrieved heads used in metal-on-metal bearings. A total of 36 metal-on-polyethylene and 21 metal-on-metal femoral components were included in the study all of which were received from a customer complaint database. Furthermore, a total of nine as-manufactured femoral components were included to provide a baseline for characterisation. All taper surfaces were assessed using an established corrosion scoring method and measurements were taken of the female taper surface using a contact profilometry. In the case of metal-on-metal components, the bearing wear was also assessed using coordinate metrology to determine whether or not there was a relationship between bearing and taper material loss in these cases. The study found that in this cohort the median value of metal-on-polyethylene taper loss was 1.25 mm3 with the consequent median value for metal-on-metal taper loss being 1.75 mm3. This study also suggests that manufacturing form can result in an apparent loss of material from the taper surface determined to have a median value of 0.59 mm3. Therefore, it is clear that form variability is a significant confounding factor in the measurement of material loss from the tapers of femoral heads retrieved following revision surgery.