A Unsworth’s research while affiliated with Durham University and other places

The measurement of friction in artificial hip joints can lead to the knowledge of the lubrication mechanisms occurring in the joints. However, the measurement of friction, particularly in spherical contacts, is not always straightforward. The important loading and kinematic features must be appropriate and the friction must be measured in the correct plane. Even defining a coefficient of friction is difficult with spherical contacts as friction acts at different moment arms throughout the contact area. Thus, the generated frictional torques depend on the pressure distribution of the contact and the moment arms at which this pressure acts. The pressure distribution depends on the material properties, the surface entraining velocities, the joint diameters, and the clearance between the two surfaces of the ball and socket joint. Equally measuring friction is very taxing for machines which are applying very high loads. Slight misalignments of the application of these loads can produce torques which are very much greater than the frictional torques that we are trying to measure. This article attempts to share the thoughts behind over 40 years of measuring friction in artificial joints using the Durham Friction Simulators. This has led to accrued consistency of measurement and a robust scientific design rationale to understand the nature of friction in these spherical contacts. It also impacts on how to obtain accurate measurements as well as on the understanding of where the difficult issues lie and how to overcome them.

Clinical results of the PyroCarbon proximal interphalangeal joint replacement are inconsistent with various complications reported. To address this, in vitro testing was conducted using finger joint simulators. Two PyroCarbon proximal interphalangeal prostheses were tested in a lubricant of dilute bovine serum to 5 × 10(6) cycles of flexion-extension (90°-30°) with dynamic forces of 10 N applied. At intervals of 3000 cycles testing ceased and a static load of 100 N was applied to simulate gripping. In addition, two 'control' prostheses were immersed alongside the test prostheses to account for lubricant absorption. Wear and roughness averages (Ra) were measured every 1 × 10(6) cycles. Minimal wear for all of the components was measured with a negligible increase in Ra for most of the components. One condyle of one component increased in Ra over the 5 × 10(6) cycles with a value above the recommended 50 nm. Unidirectional marks were visible on the condyle from micrographs, consistent with an abrasive wear mode. © IMechE 2015.

The idea of all polymer artificial joints, particularly for the knee and finger, has been raised several times in the past 20 years. This is partly because of weight but also to reduce stress shielding in the bone when stiffer materials such as metals or ceramics are used. With this in mind, pin-on-plate studies of various polyetheretherketone preparations against highly cross-linked polyethylene were conducted to investigate the possibility of using such a combination in the design of a new generation of artificial joints. PEEK-OPTIMA(®) (no fibre) against highly cross-linked polyethylene gave very low wear factors of 0.0384 × 10(-6) mm(3)/N m for the polyetheretherketone pins and -0.025 × 10(-6) mm(3)/N m for the highly cross-linked polyethylene plates. The carbon-fibre-reinforced polyetheretherketone (PEEK-OPTIMA(®)-Wear Performance) also produced very low wear rates in the polyetheretherketone pins but produced very high wear in the highly cross-linked polyethylene, as might have been predicted since the carbon fibres are quite abrasive. When the fibres were predominantly tangential to the sliding plane, the mean wear factor was 0.052 × 10(-6) mm(3)/N m for the pins and 49.3 × 10(-6) mm(3)/N m for the highly cross-linked polyethylene plates; a half of that when the fibres ran axially in the pins (0.138 × 10(-6) mm(3)/N m for the pins and 97.5 × 10(-6) mm/ N m for the cross-linked polyethylene plates). PEEK-OPTIMA(®) against highly cross-linked polyethylene merits further investigation. © IMechE 2015.

Introduction The clinical trial record of the pyrocarbon proximal interphalangeal (finger) prosthesis is mixed (Reissner et al., 2014; Tägil et al., 2014). Further to this, evidence of only two series of in vitro tests has been found (Ascension Orthopadics., 1999; Strzepa and Klawitter., 2005) both of which were commissioned by Ascension Orthopaedics, the manufacturer of the prosthesis. A further programme of impartial in vitro tests was conducted by the authors of this study using finger joint simulators, testing prostheses to five million cycles of flexion and extension. Materials and Methods In two dedicated finger simulators, two different size pyrocarbon test prostheses were cycled through flexion-extension (90°-0°). These prostheses were labelled size 30 (nominal weight of 1.2 g) and size 40 (nominal weight of 1.6 g). Pneumatic cylinders were used for the flexion extension mechanism with dynamic forces of 10-15 N. Every 3,000 cycles, flexion-extension ceased and a static load of 100 N was applied for 45 seconds to simulate a static ‘pinch’ load. Bovine serum at 37ºC was used as the lubricant with control prostheses immersed to account for lubricant uptake. At intervals of one million cycles, wear and roughness average (Ra) measurements were taken using gravimetric and non-contacting profilometry techniques respectively. Results After five million cycles of flexion-extension a negligible change in weight was observed (to a sensitivity of 0.1 mg), hence it can be stated that no wear occurred. The majority of Ra values taken from the components remained below the recommended limit of 50 nm (British Standards., 2011). One exception to this was the size 30 proximal component: which increased from 37 nm prior to testing; to 75 nm post five million cycles. Discussion The increased roughness was attributed to a single condyle on the size 30 prosthesis: which had an initial Ra value of 46 nm prior to testing; rising to 130 nm post five million cycles. Micrographs of this condyle indicated unidirectional marks consistent with an abrasive wear mode. The lack of gravimetric wear documented in this study matches the findings of two previous in vitro studies (Ascension Orthopadics., 1999; Strzepa and Klawitter., 2005). Neither of these studies however investigated surface roughness, therefore the data reported here adds to earlier work. Conclusion The in vitro results are mixed, mirroring the clinical trials (Reissner et al., 2014; Tägil et al., 2014). Overall the measured gravimetric wear was low and the majority of the pyrocarbon components remained below the roughness limit post five million cycles. However, one condyle of one test component significantly increased in roughness above the 50 nm recommended limit.

Background: Tranexamic acid (TXA) has been successfully used to reduce bleeding in joint replacement. Recently local TXA has been advocated to reduce blood loss in total knee or hip replacement; however, this raised concerns about potential adverse effects of TXA upon the artificial joint replacement. Materials and methods: In this biomechanical study we compared the effects of TXA and saline upon the following biomechanical properties of artificial joint materials-(1) tensile properties (ultimate strength, stiffness and Young's modulus), (2) the wear rate using a multi-directional pin-on-plate machine, and (3) the surface topography of pins and plates before and after wear rate testing. Results: There were no significant differences in tensile strength, wear rates or surface topography of either ultra-high-molecular-weight polyethylene pins or cobalt chromium molybdenum metal plates between specimens soaked in TXA and specimens soaked in saline. Conclusion: Biomechanical testing shows that there are no biomechanical adverse affects on the properties of common artificial joint materials from using topical TXA. Level of evidence: V.

Carbon fibre-reinforced polyetheretherketone is an attractive alternative to ultra-high-molecular-weight polyethylene in artificial joints, but little has been published on the influence of stress on the wear factor. We know that in ultra-high-molecular-weight polyethylene, the wear factor reduces as the normal stress increases, which is counter-intuitive but very helpful in the case of non-conforming contacts. In this study, carbon fibre-reinforced polyetheretherketone (PEEK-OPTIMA(®) Wear Performance) has been investigated in a pin-on-plate machine under steady loads and under stresses typical of hip and knee joints. At stresses below about 6 MPa, wear factors are between 10 and a 100 times lower than for ultra-high-molecular-weight polyethylene but at higher stresses the wear factors increase substantially.

Introduction: Clinical results of the pyrolytic carbon proximal interphalangeal prosthesis have been mixed, with reports describing various complications resulting in secondary operations and revisions. To provide further insight, in vitro testing of pyrolytic carbon proximal interphalangeal (PIP) prostheses was undertaken. Methods: Two sizes (one size 30, one size 40) of pyrolytic carbon PIP prosthesis were tested in two finger simulators. Flexion-extension (90°-0°) was applied with dynamic forces of 10-15N. At intervals of 3,000 cycles each simulator ceased flexion-extension and applied a static load of approximately 100N for 45 seconds. Dilute bovine serum at 37ºC was used as the lubricant. An additional two control prostheses (same size as test prostheses) were subjected to the same environmental conditions to account for any lubricant uptake. The process described was repeated to 500,000 cycles, after which wear measurements were taken gravimetrically and roughness average (Ra) measurements were taken using a non-contact profilometer. Results: After 500,000 cycles of flexion-extension no gravimetric wear was observed (to a sensitivity of 10µg). The Ra of the distal components increased (from 15 to 19nm) with unidirectional marks visible on the micrograph images. The increase in Ra for the proximal components was smaller (from 31 to 33nm). The gravimetric weight and Ra of the control prostheses remained unchanged. Conclusion: Initial results show low wear, testing will continue up to five million cycles of flexion-extension.

Background and purpose Ceramic-on-ceramic (CoC) bearings have been in use in total hip replacement (THR) for more than 40 years, with excellent long-term survivorship. Although there have been several simulator studies describing the performance of these joints, there have only been a few retrieval analyses. The aim of this study was to investigate the wear patterns, the surface properties, and friction and lubrication regimes of explanted first-generation alumina bearings. Materials and methods We studied 9 explanted CoC bearings from Autophor THRs that were revised for aseptic loosening after a mean of 16 (range 7–19) years. The 3D surface roughness profiles of the femoral heads and acetabular cups (Srms, Sa, and Ssk) were measured to determine the microscopic wear. The bearings were imaged using an atomic-force microscope in contact mode, to produce a topographical map of the surfaces of the femoral heads. Friction tests were performed on the bearing couples to determine the lubrication regime under which they were operating during the walking cycle. The diametral clearances were also measured. Results 3 femoral heads showed stripe wear and the remaining 6 bearings showed minimal wear. The femoral heads with stripe wear had significantly higher surface roughness than the minimally worn bearings (0.645 vs. 0.289, p = 0.04). High diametral clearances, higher than expected friction, and mixed/boundary lubrication regimes prevailed in these retrieved bearings. Interpretation Despite the less than ideal tribological factors, these first-generation CoC bearings still showed minimal wear in the long term compared to previous retrieval analyses.

The role of surface tension in the lubrication of metal-on-metal (CoCrMo alloy) hip resurfacings has been investigated to try to explain why all metal joints fail to be lubricated with simple water-based lubricants (sodium carboxymethyl cellulose), which have similar rheology to synovial fluid, but are lubricated with the same fluid with the addition of a proportion of bovine serum. As part of this study, surfactants, in the form of detergents, when added to carboxymethyl cellulose, have been shown to produce a predominantly fluid-film lubrication mechanism with friction even lower than the biological lubricant containing serum. Friction factors were reduced by 80% when a detergent was added to the lubricant. It is considered that the failure of the water-based fluids to generate fluid-film lubrication is due to the fact that 'boundary slip' takes place where the fluid does not fully attach to the bounding solid surfaces as assumed in Reynolds' equation, thereby drawing in less lubricant than predicted from hydrodynamic theory. The addition of surfactants either in the form of natural materials such as serum or in the form of detergent reduces surface tension and helps the water-based lubricant to attach more fully to the bounding surfaces resulting in more fluid entrainment and thicker fluid-film formation. This was confirmed by up to 70% lower wear being found when these joints were lubricated in a detergent solution rather than 25% bovine serum.