Single-cell analyses reveal transient retinal progenitor cells in the ciliary margin of developing human retina

The emergence of retinal progenitor cells and differentiation to various retinal cell types represent fundamental processes during retinal development. Herein, we provide a comprehensive single cell characterisation of transcriptional and chromatin accessibility changes that underline retinal progenitor cell specification and differentiation over the course of human retinal development up to midgestation. Our lineage trajectory data demonstrate the presence of early retinal progenitors, which transit to late, and further to transient neurogenic progenitors, that give rise to all the retinal neurons. Combining single cell RNA-Seq with spatial transcriptomics of early eye samples, we demonstrate the transient presence of early retinal progenitors in the ciliary margin zone with decreasing occurrence from 8 post-conception week of human development. In retinal progenitor cells, we identified a significant enrichment for transcriptional enhanced associate domain transcription factor binding motifs, which when inhibited led to loss of cycling progenitors and retinal identity in pluripotent stem cell derived organoids.

Deciphering the spatiotemporal transcriptional and chromatin accessibility of human retinal organoid development at the single-cell level

Molecular information on the early stages of human retinal development remains scarce due to limitations in obtaining early human eye samples. Pluripotent stem cell-derived retinal organoids (ROs) provide an unprecedented opportunity for studying early retinogenesis. Using a combination of single cell RNA-seq and spatial transcriptomics we present for the first-time a single cell spatiotemporal transcriptome of RO development. Our data demonstrate that ROs recapitulate key events of retinogenesis including optic vesicle/cup formation, presence of a putative ciliary margin zone, emergence of retinal progenitor cells and their orderly differentiation to retinal neurons. Combining the scRNA- with scATAC-seq data, we were able to reveal cell-type specific transcription factor binding motifs on accessible chromatin at each stage of organoid development, and to show that chromatin accessibility is highly correlated to the developing human retina, but with some differences in the temporal emergence and abundance of some of the retinal neurons.

Persistent post-covid symptoms in healthcare workers

Background

Recent reports suggest a higher incidence of COVID-19 infections among healthcare workers (HCW). However, information about the long-term complications affecting this population is lacking.

Aims

Investigation of long-term impact of COVID-19 in HCW.

Methods

Seropositivity for SARS-CoV-2 antibodies was evaluated for the majority of HCW in an English teaching hospital 2 months following the peak of COVID-19 first wave. A questionnaire investigating the long-term complications was sent through global e-mail to HCW 4 months following the peak of the wave enquiring about the persistent health issues still affecting them at that point.

Results

Out of 3759 subjects tested for SARS-CoV-2 antibodies, 932 were positive (24%). Forty-five per cent of 138 HCW responding to the questionnaire reported persistent symptoms with 32% struggling to cope 3–4 months following the peak of the wave. Moderate-to-severe fatigue stood out as the most disabling symptom (39%) but mild-to-moderate shortness of breath, anxiety and sleep disturbance were almost universal in the subjects still struggling with symptoms. Only 16% consulted their general practitioner (GP) about their symptoms with only 2% taking sick leave after recovering from the acute illness.

Conclusions

Our data suggest that about a third of HCW who responded to the survey were still struggling to cope with the symptoms of what is now known as long covid several months after the acute COVID-19 infections. The overwhelming majority of this group seem to be reluctant to neither seek medical advice nor take sick leave.

Dimensions of the Metacarpo-Phalangeal Joint with Particular Reference to Joint Prostheses

Sixty metacarpo-phalangeal joints were dissected and investigated to determine the shape, size and position of the articular surfaces with respect to the medullary canals of the metacarpal and the phalanx.

The results show that the articular surfaces of the metacarpo-phalangeal joint have a single centre of rotation in the sagital plane and in the transverse plane. The joints of the little and ring fingers have radii of curvature in the sagital and transverse planes, which are almost equal (within 1.6 per cent) while those radii of the middle finger varied by 9 per cent. The index finger had a variation in radius from the sagital to the transverse plane of almost 13 per cent; the sagital plane radius being the greatest. This observation is the opposite of the other joints where the transverse radius is the greater one.

The overall widths of the metacarpal heads were seen to vary from 13 mm in the little finger of females to 17 mm in the index finger of males (average).

The medullary canals had axes which were not coincident with the centre of rotation of the joint but up to 3 mm displaced from it. These dimensional differences have important implications in prosthesis design.

A Study of the Role of Synovial Fluid and its Constituents in the Friction and Lubrication of Human Hip Joints

Human hip joints have been studied in a pendulum apparatus and a joint simulator to determine the lubrication mechanisms active in normal physiological activities. Various lubricants have been used to lubricate the hip joints including bovine and human synovial fluid, synovial fluid which has been digested with hyaluronidase or trypsin, silicone fluids of different viscosities, as well as synovial fluid and Ringer's solution which have had their viscosities increased by the addition of hyaluronic acid.

The results indicate that under continuous dynamic loading, fluid film lubrication seems to prevail while under static loading, mixed lubrication exists. Increasing the viscosity of the lubricant from very low values (i.e. from 10-3 Pa.s) leads to a reduction in friction, indicating mixed lubrication, until a value of about 0.050 Pa.s was achieved. Further increases seemed to indicate that fluid film lubrication was then present.

An Experimental Study of Friction and Lubrication in Hip Prostheses

McKee-Farrar, Charnley and Muller prostheses have been studied in a hip function simulator using a range of silicone fluids as lubricants. Frictional resistance was measured continuously while the joints were subjected to dynamic loading.

It was observed that the lubricant viscosity was very important to the mode of lubrication prevailing and that for viscosities in excess of 0.1 Pas, full fluid film lubrication was achieved. At viscosities which were less than this, mixed lubrication resulted

These results can be extrapolated to the clinical situation where it was found that after hip surgery for total joint replacement, the synovial fluid produced around the prosthesis had a viscosity which was less than 0.1 Pas, and therefore could not give the most advantageous lubrication in these joints in vivo.

Axisymmetric Finite Element Analysis of Hip Replacement with an Elastomeric Layer: The Effects of Layer Thickness

Finite element analysis of compliant layered artificial hip joints has been used to study the mechanical response of four different layer thicknesses from 0.5 to 3 mm. The results have been compared with a classical asymptotic model in terms of maximum contact pressure and contact width, and of maximum shear stress at the layer-backing interface and its location. The surface deformations and load capacities have also been compared. The best thickness was found to be 2 mm; though a marginal reduction in stresses would be found in the 3 mm layer, the penetrations would be greater and these might have implications for the fatigue life of the material.

A formula for the thickness of the fluid film has been derived on the basis of the inverse hydrodynamic theory and the results show good correlation with existing theories.

Contact Mechanics of Soft Layer Artificial Hip Joints: Part 2: Application to Joint Design

In this paper, the general solutions previously obtained for the contact mechanics of a soft layer artificial hip joint have been applied to the design of such joints. In particular, simple power-law design formulae have been generated for the prediction of the contact radius and the maximum Tresca shear stress within the elastomeric layer, when the aspect ratio varied from 1 to 20 (which covered the entire range of the aspect ratio for soft layer artificial hip joints). The effects of the layer thickness, Young's modulus of the layer and the equivalent radius of the joint upon the contact area and the maximum Tresca shear stress have all been examined for physiological loading conditions which would be experienced by hip joint prostheses in the body.

Furthermore, the shear strain field was calculated so that the level of strain expected for such joints under physiological loading conditions can be estimated. With these data, relevant fatigue tests can be devised to assess the long-term performance of any particular design of soft layer hip joint.

Finally, the effect of the friction between articular surfaces upon the stress field within soft layers has been examined using a newly developed asymptotic analytical theory. It was shown that, for a low coefficient of friction, the maximum Tresca shear stress occurred at layer-substrate bonding interface. With an increase in the coefficient of friction, however, the maximum Tresca shear stress increased its magnitude and moved towards the centre of the contact area along the articular surface.

Wear and surface analysis of 38 mm ceramic-on-metal total hip replacements under standard and severe wear testing conditions

The purpose of this study was to compare the wear of zirconia-toughened alumina (ZTA) and alumina femoral heads tested against as-cast CoCrMo alloy acetabular cups under both standard and severe wear conditions. A new severe test, which included medio-lateral displacement of the head and rim impact upon relocation, was developed. This resulted in an area of metal transfer and an area of increased wear on the superior-anterior segment of the head that were thought to be due to dislocation and rim impact respectively. While the wear of all ceramic heads was immeasurable using the gravimetric method, the wear rates for the metallic cups from each test were readily calculated. An average steady state wear rate of 0.023 ± 0.005 mm3/106 cycles was found for the cups articulating against ZTA under standard wear conditions. A similar result had previously been obtained for the wear of cups articulated against alumina heads of the same size (within the same laboratory). Under severe wear conditions an increase in the metallic cup steady state wear rate was found with the ZTA and alumina tests giving 0.623 ± 0.252 and 1.35 ± 0.154 mm3/106 cycles respectively. Wear of the ceramic heads was detected using atomic force microscopy which showed, under severe wear conditions, a decrease in polishing marks and occasional grain removal. The surfaces of the ZTA heads tested under standard conditions were virtually unchanged from the unworn samples. Friction tests showed low friction factors for all components, pre and post wear.

Tribological and surface analysis of 38mm alumina–as-cast Co–Cr–Mo total hip arthroplasties

There is currently much discussion over the use of ceramic femoral components against metal acetabular cups, for use in total hip arthroplasty. The current study investigates six hot isostatically pressed alumina femoral heads of 38 mm diameter articulating against six as-cast Co—Cr—Mo metallic acetabular cups. Standard walking-cycle simulator wear testing was carried out to 5×106 cycles using the Durham Mark II hip wear simulator, and wear was determined gravimetrically. In addition, surface topography, using a non-contacting profilometer, an atomic force microscope, and an optical microscope, was monitored throughout the wear test. The wear of the ceramic heads was found to be undetectable using the current gravimetric method; however, a change in the surface topography was seen, as grain removal on the pole was observed through atomic force microscopy analysis. A biphasic wear pattern was found for the metallic cups, with low wear rates of 1.04 ± 0.293 mm3/106 cycles (mean, ±95 per cent confidence interval) and 0.0209 ± 0.004 mm3/106 cycles (mean, ±95 per cent confidence interval) for running-in and steady state wear phases respectively. Frictional measurement revealed that the joints were tending towards full fluid-film lubrication in parts of the walking cycle. The results show that the combination of hot isostatically pressed alumina and as-cast Co—Cr—Mo is a promising alternative for total hip arthroplasties.

Wear studies on the likely performance of CFR-PEEK/CoCrMo for use as artificial joint bearing materials

It is well known that a reduction in the volume of wear produced by articulating surfaces in artificial joints is likely to result in a lower incidence of failure due to wear particle induced osteolysis. Therefore, new materials have been introduced in an effort to produce bearing surfaces with lower, more biologically acceptable wear. Polyetheretherketone (PEEK-OPTIMA) has been successfully used in a number of implant applications due to its combination of mechanical strength and biocompatibility. Pin-on-plate wear tests were performed on various combinations of PEEK-OPTIMA and carbon fibre reinforced PEEK-OPTIMA (CFR-PEEK) against various CoCrMo alloys to assess the potential of this material combination for use in orthopaedic implants. The PEEK/low carbon CoCrMo produced the highest wear. CFR-PEEK against high carbon or low carbon CoCrMo provided low wear factors. Pin-on-plate tests performed on ultra-high molecular weight polyethylene (UHMWPE) against CoCrMo (using comparable test conditions) have shown similar or higher wear than that found for CFR-PEEK/CoCrMo. This study gives confidence in the likelihood of this material combination performing well in orthopaedic applications.

Compliant-layer tibial bearing inserts: Friction testing of different materials and designs for a new generation of prostheses that mimic the natural joint

Total joint replacements (TJRs) have a limited lifetime, but the introduction of devices that exhibit good lubricating properties with low friction and low wear could well extend this. A novel tibial bearing design, using polyurethane (PU) as a compliant layer, to mimic the natural joint, has been developed. To determine accurately the mode of lubrication under which these joints operate, a synthetic lubricant was used in all these tests. Friction tests were carried out to assess the effects of material modulus and surface roughness, together with bearing design parameters such as bearing thickness and conformity, on lubrication. Corethane 80A was the preferred material and was chosen as the compliant layer for subsequent testing. A low surface roughness resulted in lower asperity contact as the asperities were depressed by the pressurized entraining fluid and full-fluid-film lubrication was approached. The three different tibial bearing conformities (low, medium, and high) did not appear to influence the mode of lubrication and all these bearings performed with extremely low friction. Similarly, the bearing thickness effects on lubrication at the levels tested (2 mm, 3 mm, and 4 mm) were minimal, although the effects of layer thickness on interface shear stress could be expected to be significant. This study describes a series of friction tests that have been used to select the most appropriate material and to optimize the design parameters to establish optimum conditions for these compliant layer joints.

Pitch-based carbon-fibre-reinforced poly (ether—ether—ketone) OPTIMA® assessed as a bearing material in a mobile bearing unicondylar knee joint

The introduction of unicondylar knee prostheses has allowed the preservation of the non-diseased compartment of the knee while replacing the diseased or damaged compartment. In an attempt to reduce the likelihood of aseptic loosening, new material combinations have been investigated within the laboratory. Tribological tests (friction, lubrication, and wear) were performed on metal-on-carbon-fibre-reinforced (CFR) poly (ether—ether—ketone) (PEEK) (pitch-based) mobile unicondylar knee prostheses up to 5×106 cycles. Both a loaded soak control and an unloaded soak control (both medial and lateral components) were used to compensate for weight change due to lubricant absorption. For this material combination the loaded soak control gave slightly lower wear for both the medial and the lateral components than did the unloaded soak control. The medial components gave higher steady state wear than the lateral components (1.70 mm3 per 106 cycles compared with 1.02 mm3 per 106 cycles with the loaded soak control). The results show that the CFR PEEK unicondylar knee joints performed well in these wear tests. They gave lower volumetric wear rates than conventional metal-on-ultra-high-molecular-weight polyethylene prostheses have given in the past when tested under similar conditions. The friction tests showed that, at physiological viscosities, these joints operated in the boundary—mixed-lubrication regime. The low wear produced by these joints seems to be a function of the material combination and not of the lubrication regime.

Compliant layer bearings in artificial joints. Part 2: Simulator and fatigue testing to assess the durability of the interface between an elastomeric layer and a rigid substrate

Artificial joints have been much improved since their introduction but they still have a limited lifetime. In an attempt to increase their life by improving the lubrication acting within these prostheses, compliant layered polyurethane (PU) joints have been devised. These joints mimic the natural synovial joint more closely by promoting fluid film lubrication. In this study, tests were performed on compliant layer joints to determine their ability to function under a range of conditions. Both static and dynamic compression tests were undertaken on compliant artificial hip joints of two different radial clearances. Friction tests were also performed before and after static loading. In addition to this, knee wear tests were conducted to determine the suitability of a compliant layer in these applications. In the knee tests, variations in experimental testing conditions were investigated using both active and passive rotation and severe malalignment of the tibial inserts. The static compression tests together with the friction studies suggest that a small radial clearance is likely to result in ‘grabbing’ contact between the head and cup. The larger radial clearance (0.33 μm) did not exhibit these problems. The importance of the design of the compliant layer joints was highlighted with delamination occurring on the lateral bearings during the knee wear studies. The bearings with a layer 2 mm thick performed better than the bearings with a layer 3 mm thick. Tests conducted on flat PU bearings resulted in no delamination; therefore, it was concluded that the layer separation was caused by design issues rather than by material issues. It was found that, with careful material choice, consideration of design, and effective manufacturing techniques, the compliant layer joint functioned well and demonstrated durability of the union between the hard and soft layers. These results give encouragement for the suitability of these joints for clinical use.

Compliant layer bearings in artificial joints. Part 1: The effects of different manufacturing techniques on the interface strength between an elastomeric layer and a rigid substrate

The premise that elastomeric materials could be used as one or more of the articulating components in both hip and knee prostheses was postulated first by Unsworth and co-workers. It was thought that such materials might have the capacity to mimic natural joint behaviour more closely than the more rigid bearing surfaces commonly in use. A more natural joint function in artificial joints should promote better tribology, with full fluid-film lubrication being the goal. Early tests showed that this objective could potentially be achieved with a judicious choice of materials and carefully controlled manufacturing techniques. This paper (Part 1 of a two-part series) describes and explains the techniques used to verify the material selection as well as to determine the most appropriate manufacturing procedure to obtain a strong and robust interface between the support and bearing material of the prosthesis. Two polycarbonate urethane (PU) materials with different hardness values (Corethane 80A and Corethane 75D) gave sufficient interfacial strength when moulded under optimum conditions. Corethane 80A was used as the soft bearing material while Corethane 75D provided the rigid backing component. Peel tests revealed strong interface bonds, varying with processing conditions between 350 and 862 N. Fourier transform infrared spectroscopy and micro-thermal analysis showed that a fusion bond over 30 μm thick formed at the interface. The results of the range of tests and analyses, which have been used in this study, have provided sufficient evidence to validate the process used to manufacture these components.

The wear of high-carbon metal-on-metal bearings after different heat treatments

To study the tribological performance of metal-on-metal hip joint resurfacings, the wear performance of three pairs of Co—Cr—Mo alloy samples (pins and plates) were tested in a multidirectional pin-on-plate wear machine. An ‘as-cast’, a single-heat-treated, and a double-heat-treated set of specimens were tested to 3×106 cycles. The two heat treatments resulted in partial and full solution of the carbides into the matrix. An increasing trend in wear rate was found from ‘as-cast’ to the double-heat-treated specimens. The as-cast specimens showed the lowest wear rate (1.69×10−6 mm3/N m), the reduced carbide samples had the next lowest wear rate (2.1×10−6 mm3/N m), while the specimens without carbides wore the most (2.41×10−6 mm3/N m).

Tribological assessment of a flexible carbon-fibre-reinforced poly(ether—ether—ketone) acetabular cup articulating against an alumina femoral head

New material combinations have been introduced as the bearing surfaces of hip prostheses in an attempt to prolong their life by overcoming the problems of failure due to wear-particle-induced osteolysis. This will hopefully reduce the need for revision surgery. The study detailed here used a hip simulator to assess the volumetric wear rates of large-diameter carbon-fibre-reinforced pitch-based poly(ether—ether—ketone) (CFR-PEEK) acetabular cups articulating against alumina femoral heads. The joints were tested for 25×106 cycles. Friction tests were also performed on these joints to determine the lubrication regime under which they operate. The average volumetric wear rate of the CFR-PEEK acetabular component of 54 mm diameter was 1.16 mm3/106 cycles, compared with 38.6 mm3/106 cycles for an ultra-high-molecular-weight polyethylene acetabular component of 28 mm diameter worn against a ceramic head. This extremely low wear rate was sustained over 25×106 cycles (the equivalent of up to approximately 25 years in vivo). The frictional studies showed that the joints worked under the mixed—boundary lubrication regime. The low wear produced by these joints showed that this novel joint couple offers low wear rates and therefore may be an alternative material choice for the reduction of osteolysis.

Design and development of a novel automatic valve system for long-term catheterized urinary incontinence patients

It has been estimated that over 3 million patients in the UK suffer from urinary incontinence, the result of which is often long-term catheterization. However, many catheters block prematurely through encrustation and their continuous drainage limits bladder rehabilitation. Although evidence shows that a catheter valve may overcome such weaknesses, only manual valves are currently available and many patients are not able to benefit from these owing to a lack of manual dexterity. A novel electronically controlled automatic valve system, the Shan-Lai (SL) valve system, has been designed and prototyped. The prototype is compact, reliable, and cost effective, and it has low power consumption. The mass of the overall packaged valve system is 34.2 g and it measures 4.5 cm x 4.5 cm x 1.2 cm. With an orifice of 3 mm diameter, the SL valve has achieved high flowrates with relatively low energy consumption. A flowrate-energy relationship (FER) has been introduced to assess the performance of a catheter valve, and the SL valve system prototype has achieved an FER of 0.66 m/s(-1) mJ(-1) while a commercially available electronic valve has an FER of 0.28 m/s(-1) mJ(-1). The valve demonstrated outstanding mechanical reliability after a series of performance tests and also indicated remarkable encrustation resistance in the vicinity of the valve during an in-vitro test.

The wear properties of CFR-PEEK-OPTIMA articulating against ceramic assessed on a multidirectional pin-on-plate machine

In an attempt to prolong the lives of rubbing implantable devices, several 'new' materials have been examined to determine their suitability as joint couplings. Tests were performed on a multidirectional pin-on-plate machine to determine the wear of both pitch and PAN (polyacrylonitrile)-based carbon fibre reinforced-polyetheretherketone (CFR-PEEK-OPTIMA) pins articulating against both BioLox Delta and BioLox Forte plates (ceramic materials). Both reciprocation and rotational motion were applied to the samples. The tests were conducted using 24.5 per cent bovine serum as the lubricant (protein concentration 15 g/l). Although all four material combinations gave similar low wear with no statistically significant difference (p > 0.25), the lowest average total wear of these pin-on-plate tests was provided by CFR-PEEK-OPTIMA pitch pins versus BioLox Forte plates. This was much lower than the wear produced by conventional joint materials (metal-on-polyethylene) and metal-on-metal combinations when tested on the pin-on-plate machine. This therefore indicates optimism that these PEEK-OPTIMA-based material combinations may perform well in joint applications.

Laboratory studies on the tribology of hard bearing hip prostheses: Ceramic on ceramic and metal on metal

Total hip replacements offer relief to a great many patients every year around the world. With an expected service life of around 25 years on most devices, and with younger and younger patients undergoing this surgery, it is of great importance to understand the mechanisms of their function. Tribological testing of both conventional and hard bearing joint combinations have been conducted in many centres throughout the world, and, after being initially abandoned owing to premature failures, hard bearing combinations have been revisited as viable options for joint replacements. Improved design, manufacturing procedures, and material compositions have led to improved performance over first-generation designs in both metal-on-metal and ceramic-on-ceramic hip prostheses. This paper offers a review of the work conducted in an attempt to highlight the most important factors affecting joint performance and tribology of hard bearing combinations. The tribological performance of these joints is superior to that of conventional metal- or ceramic-on-polymer designs.

Polyurethane unicondylar knee prostheses: simulator wear tests and lubrication studies

Many materials are used as artificial joint bearing surfaces; these include conventional stainless steel or CoCrMo-on-ultra-high molecular weight polyethylene (UHMWPE), CoCrMo on itself and alumina-on-alumina. However, these joints have a limited lifespan resulting in failure of the prosthesis and the need for revision surgery. A number of materials have been introduced recently in an attempt to overcome these problems. Polycarbonate urethane (PU) is a compliant material that can be used as an artificial joint bearing surface which has been developed to mimic the natural synovial joint more accurately by promoting fluid film lubrication. Tribological tests were performed on CoCrMo-on-PU unicondylar knee prostheses to assess their performance in vitro. The wear produced by these components was considerably lower than that found for conventional joints. They also exhibited low friction and operated close to full-fluid film lubrication with viscosities of lubricant similar to those found in patients with arthritis. These tests gave encouraging results for the tribological performance of this material couple for use as an alternative bearing combination.

The effects of proteins on the friction and lubrication of artificial joints

The tribological testing of artificial hip and knee joints in the laboratory has been ongoing for several decades. This work has been carried out in an attempt to simulate the loading and motion conditions applied in vivo and, therefore, the potential for the success of the joint. However, several different lubricants have been used in these tests. The work documented in this paper compares results obtained using different lubricants and makes suggestions for future work. Hip joints and knee joints of different material combinations were tested in a friction simulator to determine their friction and lubrication properties. Both carboxymethyl cellulose (CMC) fluids and bovine serum (with CMC fluids added) were used as the lubricants. These were prepared to various viscosities to produce the Stribeck plots. Human synovial fluid, of just one viscosity, was used as the lubricant with some of the joints to give a true comparison with physiological lubricants. The results showed that, in most cases, the lubricant had a significant effect on the friction developed between the joint surfaces. This is thought to be due to the proteins that are present within the bovine serum adsorbing to the bearing surfaces, creating 'solid-like' films which rub together, protecting the surfaces from solid-to-solid contact. This would be beneficial in terms of wear but can either increase or decrease the friction between the contacting surfaces. It is important to simulate the conditions in vivo as closely as possible when testing these joints to try to obtain a better comparison between the joints and to simulate more accurately the way that these joints will operate in the body. In an attempt to simulate synovial fluid, bovine serum seems to be the most popular lubricant used at present. It would be beneficial, however, to develop a new synthetic lubricant that more closely matches synovial fluid. This would allow us to predict more accurately how these joints would operate long-term in vivo.

Compliant layer acetabular cups: friction testing of a range of materials and designs for a new generation of prosthesis that mimics the natural joint

Total joint replacements (TJRs) have a limited lifetime, but the introduction of components that exhibit good lubricating properties with low friction and low wear could extend the life of TJRs. A novel acetabular cup design using polyurethane (PU) as a compliant layer (to mimic the natural joint) has been developed. This study describes a series of friction tests that have been used to select the most appropriate material, optimize the design parameters, and fine-tune the manufacturing processes of these joints. To determine accurately the mode of lubrication under which these joints operate, a synthetic lubricant was used in all these tests. Friction tests were carried out to assess the lubrication of four PU bearing materials. Corethane 80A was the preferred material and was subjected to subsequent testing. Friction tests conducted on acetabular cups, manufactured using Corethane 80A articulating against standard, commercially available femoral heads, demonstrated friction factors approaching those for full-fluid-film lubrication with only approximately 1 per cent asperity contact. As the joint produces these low friction factors within less than half a walking cycle after prolonged periods of loading, start-up friction was not considered to be a critical factor. Cups performed well across the full range of femoral head sizes, but a number of samples manufactured with reduced radial clearances performed with higher than expected friction. This was caused by the femoral head being gripped around the equator by the low clearance cup. To avoid this, the cup design was modified by increasing the flare at the rim. In addition to this the radial clearance was increased. As the material is incompressible, a radial clearance of 0.08 mm was too small for a cup diameter of 32 mm. A clearance of between 0.10 and 0.25 mm produced a performance approaching full-fluid-film lubrication. This series of tests acted as a step towards the optimization of the design of these joints, which has now led to an in vivo ovine model.

The effect of 'running-in' on the tribology and surface morphology of metal-on-metal Birmingham hip resurfacing device in simulator studies

It is well documented that hard bearing combinations show a running-in phenomenon in vitro and there is also some evidence of this from retrieval studies. In order to investigate this phenomenon, five Birmingham hip resurfacing devices were tested in a hip wear simulator. One of these (joint 1) was also tested in a friction simulator before, during, and after the wear test and surface analysis was conducted throughout portions of the testing. The wear showed the classical running in with the wear rate falling from 1.84 mm3 per 10(6) cycles for the first 10(6) cycles of testing to 0.24 mm3 per 10(6) cycles over the final 2 x 10(6) cycles of testing. The friction tests suggested boundary lubrication initially, but at 1 x 10(6) cycles a mixed lubrication regime was evident. By 2 x 10(6) cycles the classical Stribeck curve had formed, indicating a considerable contribution from the fluid film at higher viscosities. This continued to be evident at both 3 x 10(6) and 5 x 10(6) cycles. The surface study complements these findings.

The tribology of metal-on-metal total hip replacements

Total hip surgery is an effective way of alleviating the pain and discomfort caused by diseased or damaged joints. However, in the majority of cases, these joints have a finite life. The main reason for failure is osteolysis (bone resorption). It is well documented that an important cause of osteolysis, and therefore the subsequent loosening and failure of conventional metal- or ceramic-on-ultra-high molecular weight polyethylene joints, is the body's immunological response to the polyethylene wear particles. To avoid this, interest has been renewed in metal-on-metal joints.

The intention of this paper is to review the studies that have taken place within different laboratories to determine the tribological performance of new-generation metal-on-metal total hip replacements. These types of joint offer a potential solution to enhance the longevity of prosthetic hip systems; however, problems may arise owing to the effects of metal ion release, which are, as yet, not fully understood.

Analysis of ‘Boosted Lubrication’ in Human Joints

The load-bearing human joint is a self-acting dynamically loaded bearing which employs a porous and elastic bearing material (articular cartilage) and a highly non-Newtonian lubricant (synovial fluid).

The authors' understanding is that the human joint experiences fluid-film (including elastohydrodynamic), mixed and boundary lubrication in its various operating conditions. It has been recognized that squeeze-film action is capable of providing considerable protection to the cartilage surface once a fluid film is generated (6) (8)§. Furthermore, the possibility of an increasing concentration of hyaluronic acid in synovial fluid during the squeeze-film action due to the porous nature of the cartilage and its surface topography and the known relationship between this concentration and the effective viscosity (7) has led to the concept of ‘boosted lubrication’ as an important feature of joint behaviour (10).

A mathematical analysis of the concept of boosted lubrication of human joints is presented in this paper. The predictions of the analysis are shown to be in good agreement with experimental findings (12).

Comparative in vitro wear testing of PEEK and UHMWPE capped metacarpophalangeal prostheses

Six metacarpophalangeal prostheses were each wear tested to five million cycles. Each prosthesis consisted of a metacarpal component with an approximately hemispherical shell on a titanium body, articulating against a titanium phalangeal component. Four prostheses had a shell made from ultra-high molecular weight polyethylene (UHMWPE) and two had a shell made from polyether ether ketone (PEEK). The tests were undertaken using a finger wear simulator. Despite pre-soaking and the use of control components, lubricant uptake by the metacarpal components was significant. Gravimetrically, the UHMWPE test components showed a greater weight gain than the UHMWPE control components. Therefore there was no apparent wear of any of the UHMWPE test metacarpal components. The original concentric machining marks of the UHMWPE components could still be seen after five million cycles of testing. For the metacarpal components with PEEK shells, gravimetric wear could be measured. Gravimetrically, all of the titanium phalangeal components showed little or no wear. Light scratches in the direction of sliding appeared on the articulating faces of all metacarpal and phalangeal test components, indicating slight abrasive wear.

Is the wear factor in total joint replacements dependent on the nominal contact stress in ultra-high molecular weight polyethylene contacts?

The exact dependence of wear factor on contact stress, load and apparent contact area is much disputed in the literature. This study attempts to solve this dispute. Pin-on-plate studies of ultra high molecular weight polyethylene against stainless steel were conducted under different combinations of load (33-250 N), nominal stress (0.56-12.73 MPa) and face diameter, as well as two tests where both stress and load were kept constant, while the diameter was changed. For these tests the centre of the pin face was bored out to create four different average pin diameters with similar face areas. Diameter and load were found to have no significant effect on the wear factor, while the wear factor decreased with increasing contact stress according to the relation K = 2 × 10−6σ−0.84.

Design aspects of compliant, soft layer bearings for an experimental hip prosthesis

Currently, an artificial hip joint can be expected to last, on average, in excess of 15 years with failure due, in the majority of cases, to late aseptic loosening of the acetabular component. A realistic alternative to the problem of wear in conventional joints is the introduction of bearing surfaces that exhibit low wear and operate in the full fluid-film lubrication regime. Contact analyses and friction tests were performed on compliant layer joints (metal-on-polyurethane) and the design of a prototype ovine arthroplasty model was investigated. When optimized, these components have been shown to achieve full fluid-film lubrication.

NeuFlex metacarpophalangeal prostheses tested in vitro

This paper describes the testing of three single-piece silicone NeuFlex metacarpophalangeal prostheses in a finger function simulator and describes the resulting modes of prosthesis failure. In all cases, failure was due to imminent fracture of the prosthesis across the pivot of the central hinge section. This result is in contrast with previous in-vitro and in-vivo experience with single-piece silicone Swanson and Sutter metacarpophalangeal prostheses, which both tend to fracture at the junction of the distal stem and the hinge. In comparison with earlier in-vitro simulator tests of the Sutter metacarpophalangeal prosthesis, the NeuFlex prostheses showed a greater longevity before fracture. To date, no other reports of fracture of the NeuFlex metacarpophalangeal prosthesis have been reported, either in vitro or in vivo.

The influence of bovine serum lubricant on the wear of cross-linked polyethylene finger prostheses

A two-piece finger prosthesis has been proposed, manufactured from silane cross-linked polyethylene. Using a finger wear simulator the prosthesis was tested in a dilute bovine serum lubricant. Five tests were undertaken, totalling over 45 million cycles of wear testing. In each test, a statically loaded control prosthesis was included. In all tests it was found that the lubricant uptake of the test components exceeded that of the control components, consequently no gravimetric wear was measured. To investigate this result further, six cross-linked polyethylene prostheses were taken and soaked in the serum lubricant at 37 degrees C. Three of the prostheses were statically loaded and the other three were unloaded. These tests lasted for over one hundred and sixty days. It was found that the lubricant uptake of the unloaded control components was greater than that of the loaded com-ponents. The test with statically loaded prostheses was repeated, firstly with distilled water and then in lubricant heated to 30 degrees C and 40 degrees C. No significant difference in weight increase due to lubricant uptake at these two temperatures was found. The weight increase due to soaking in dilute bovine serum was several times that due to soaking in distilled water. (Journal of Ap-plied Biomaterials & Biomechanics 2004; 2: 136-42).

The effect of bone cement particles on the friction of polyethylene and polyurethane knee bearings

Compliant layer knee joints have been considered for use in an attempt to increase the serviceable life of artificial joints. If designed correctly, these joints should operate within the full-fluid film lubrication regime. However, adverse tribological conditions, such as the presence of bone and bone cement particles, may breach the fluid film and cause surface wear. The frictional behaviour of both polyurethane (PU) and conventional polyethylene (PE) tibial components against a metallic femoral component was therefore assessed when bone cement particles were introduced into the lubricant. The bone cement particles caused a large increase in the frictional torque of both the PE and PU bearings; however, the friction produced by the PU bearings was still considerably lower than that produced by the PE bearings. The volume of bone cement particles between each of the bearings and the resultant frictional torque both decreased over time. This occurred more quickly with the PE bearings but greater damage was caused to the surface of the PE bearings than the PU components.

Wear studies of all UHMWPE couples under various bio-tribological conditions

Wear tests were undertaken in which ultra high molecular weight polyethylene (UHMWPE) was rubbed against itself. Tests primarily employed a pin-on-plate wear test machine, with distilled water, Ringer solution and dilute bovine serum being used as the lubricants. Loads of 10N and 40N were employed, and some test pins had a rotational motion added. In all cases wear was high, with mean wear factors of up to 91 10 -6 mm3/Nm being measured, but the addition of rotation reduced the amount of material worn from the test plates. In the presence of bovine serum and under reciprocation only, pin wear was relatively low. With bovine serum as the lubricant, total mean wear factors for the UHMWPE couples were calculated to be in the range of 35 to 58 10-6mm3/Nm. Therefore the pin-on-plate tests showed that the choice of lubricant as well as the motion applied to the test pin had a significant influence on the wear volumes measured. A two-piece UHMWPE 'prosthesis' with matching hemispherical faces was fabricated and tested on a finger simulator. Distilled water was used as the lubricant and wear factors were found to be greater for the metacarpal component, 21 10 -6mm3/Nm, than the phalangeal component, 3 10-6mm3/Nm, after ten million cycles of testing. This result paralleled the greater wear seen by the plate than by the pin in the pin-on-plate tests under reciprocating motion. (Journal of Applied Biomaterials & Biomechanics 2004; 2: 29-34).

N+ ion implantation of Ti6Al4V alloy and UHMWPE for total joint replacement application

Multidirectional pin-on-plate tribological wear tests were performed to evaluate and compare the tribological behavior of N+ ion-implanted and unmodified Ti6Al4V/ultra high molecular weight polyethylene (UHMWPE) sliding couples, for total joint replacement (TJR) applications. Knoop microhardness indentations were measured and an increase in surface hardness of more than twofold and up to four times, respectively, was observed for the Ti6Al4V alloy and UHMWPE after N+ ion implantation, at a load of 1 gf. Increase in hardness was attributed to the formation of TiN precipitates and cross-linking in the alloy and polymer, respectively. Wear test results showed that N+ ion implantation reduced surface wear of the Ti6Al4V alloy and, in turn, significantly reduced the wear of the mating UHMWPE. Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), non-contacting interferometry and optical microscopy were used to characterize both modified and unmodified material surfaces prior to and after tribological wear tests. The results indicated that N+ ion-implanted Ti6Al4V/UHMWPE sliding couples were distinctly superior to the unmodified sliding couples, and demonstrated a definite potential for the use of N+ ion-implanted Ti6Al4V and implanted UHMWPE as load bearing surfaces in TJR prostheses. (Journal of Applied Biomaterials & Biomechanics 2003; 1:164-71).

Tribologische Untersuchungen einer metakarpophalangealen Prothese

It is the goal of this paper to introduce modern tribological investigations into the development of a new finger prosthesis, with the particular aim of optimising the material component.A new, unconstrained metacarpophalangeal prosthesis restoring the anatomy of the joint has been developed in two versions (UHMWPE-metal and PEEK-metal). In order to determine the version having better wear behaviour, these two versions were tested on a Joyce finger simulator. As the UHMWPE-metal version showed a lower amount of wear in vitro, this version was chosen to conduct a clinical study. This investigation showed the importance of conducting an in vitro wear test before any clinical studies. Such tests allow the optimisation of the wear behaviour of the tested metacarpophalangeal prosthesis and therefore permit a minimisation of the possible risks to the patients.

Relative movements between Kinemax Plus tibial inserts and the tibial base-plates

Tests were performed on six large Kinemax Plus knee bearings (snap-fit design) to evaluate the amount of movement between 10- and 15-mm-thick tibial inserts and the tibial base plates. The knee bearings were tested up to 1 x 10(6) cycles on the Durham six-station knee wear simulator which subjected the bearings to similar motion and loading profiles that would be experienced by the natural knee during walking. Although passive internal/external (I/E) rotation was allowed, no active I/E rotation was applied. The movement of the tibial inserts was measured with dial gauges (accuracy +/-0.01 mm) before and after the bearings were tested on the simulator, when unloaded, and throughout the tests while the bearings were being dynamically loaded in the simulator. Movement occurred between the tibial insert and the tibial base plate after initial assembly due to the snap-fit mechanism used to locate the tibial insert within the tibial base plate. However this decreased appreciably when the bearings were loaded in the simulator. The amount of movement did not change with time when the bearings were continuously loaded in the simulator. However, after each test the amount of movement of the tibial inserts, when unloaded, was only 65 per cent (anterior-posterior) and 46 per cent (medial-lateral) of the values before the test. This was thought to be due to creep of the ultra-high molecular weight polyethylene (UHMWPE) inserts. The movement between the tibial insert and tibial base plate in situ is likely to be much less than that observed by a surgeon at the time of assembly due to loading of the knee bearing in the body. However, the amount of movement when the tibial inserts are loaded may still be great enough to produce a second interface where wear of the tibial insert may take place.

A comparison of ex vivo and in vitro Sutter metacarpophalangeal prostheses

Forty-one Sutter metacarpophalangeal prostheses were implanted into 11 hands of nine patients. Twelve of these prostheses were revised from three patients after a mean period of 42 months. Of the 12 prostheses, 11 showed fracture at the junction of the distal stem and the central hinge region. Two Sutter metacarpophalangeal prostheses were tested on a single station finger simulator and both failed due to fracture at the junction of the hinge and the distal stem.

A literature review of "failures" of the Swanson finger prosthesis in the metacarpophalangeal joint

This paper reviews the use of the Swanson finger prosthesis, concentrating on clinical results from the metacarpophalangeal (MCP) joint and modes of prosthetic "failure". While "failure" is generally associated with fracture, it is recognised that fracture does not always necessitate replacement of the Swanson prosthesis. Fracture tends to occur at the junction of the distal stem and hinge of the prosthesis. Initial improvements in ulnar deviation and range of motion (ROM) tend to be gradually reduced over the duration of implantation, and there is little evidence to suggest any long-term improvement in hand strength. Bone erosion and silicone synovitis have been seen but at a much lesser incidence than in other joints implanted with silicone spacers. An evaluation of retrieved Swanson prostheses, tied in with patient history and hand measurements might provide additional information to improve the design of the Swanson prosthesis itself and of other finger prostheses.

A test procedure for artificial finger joints

This paper highlights the lack of an agreed testing standard for artificial finger joints. It reviews the anatomy, pathology and biomechanics of finger joints as well as the various designs of finger prostheses and the machines used to test them. While pre-implantation testing should be fundamental, increasing regulation of the biomedical engineering industry will further demand testing of prostheses to pre-agreed standards. Standards relating to the testing of other artificial joints are reviewed before possible parameters for testing finger prostheses are offered.

Design and validation of a surrogate humerus for biomechanical testing

At present biomechanical testing of fracture plating strategies is conducted using animal or cadaveric whole bone models. This may introduce experimental error into these studies. This communication summarises the design and validation of a novel bone and fibre-reinforced plastic construct conceived to minimise intra-experimental error. A tubular surrogate humerus was produced with dimension and strength matched to that of the human humerus. Bone inserts placed into the wall of the tube allow for the fixation of the plates with bone screws. Three-point bending tests of the flexural rigidity of the surrogate humerus (EI=100.1 (SD 6.0)Nm(2)) showed it to be comparable to the human humerus. Further, pull-out tests of the screws showed that the bone slots adequately mimicked the whole bone scenario. This testing construct will be used for a comparative study of humeral plating techniques.

The wear of metal-on-metal total hip prostheses measured in a hip simulator

New generation metal-on-metal prostheses have been introduced to try and overcome the problem of osteolysis often attributed to the wear particles of the polyethylene component of conventional metal-on-ultra-high molecular weight polyethylene (UHMWPE) joints. The wear rates of four metal-on-metal joints (two different clearances) were assessed along with that of a conventional metal-on-UHMWPE joint. Friction measurements of the metal-on-metal joints were taken before and after the wear test and compared. Two distinct wear phases were discernible for all the metal-on-metal joints: an initial wear phase up to 0.5 x 10(6) cycles and then a lower steady state wear phase. The steady state wear rate of the 22 microm radial clearance metal-on-metal joint was lower than that for the 40 microm radial clearance joint, although this difference was not found to be significant (p > 0.15). The wear rates for all the joints tested were consistent with other simulator studies. The friction factors produced by each joint were found to decrease significantly after wear testing (p < 0.05).

The effect of socket design, materials and liner thickness on the wear of the porous coated anatomic total hip replacement

The wear of joint replacement prostheses represents the greatest challenge to their continued development. Parameters such as polyethylene quality, liner thickness and metal backing have all been implicated as potential detractors in the search for the lowest-wearing socket. This study examined the effect of these parameters through an extensive study of the two versions of the porous coated anatomic (PCA) hip prosthesis (one-piece socket and snaplock socket). For the whole cohort the wear rate was found to be 88 (SE 10) mm3/year and the clinical wear factor was 2.00 (SE 0.28) x 10(-6) mm3/N m. When the two socket types were investigated individually, the wear factors found were 2.39 (SE 0.44) x 10(-6)mm3/N m and 0.99 (SE 0.25) x 10(-6) mm3/N m for the one-piece and snaplock, respectively. This illustrates that the metal backing per se does not predispose these sockets to rapid wear. The good wear performance of the snaplock liner may be attributed to the high quality of the ultra-high molecular weight polyethylene (UHMWPE) used and the shorter implantation period compared to that for the one-piece design. No correlation was found between the thickness of the liner and the clinical wear factor. Within the range of thicknesses tested here, UHMWPE thickness is not an influential parameter for the hip prosthesis and this is confirmed

An in vitro wear study of alumina-alumina total hip prostheses

Four 28 mm diameter alumina-alumina hip prostheses were tested in the Mkll Durham hip simulator for 5 x 10(6) cycles using 25 per cent bovine serum as lubricant. Wear of the heads and cups was measured gravimetrically. The mean and standard deviation of the wear rate for the alumina cups was 0.097 +/- 0.039 mm3/10(6) cycles. The femoral heads produced such low wear that it could not be measured by weighing but could be detected byincreased surface roughness measurements. Such low wear rates represent about one-five-hundredthof the wear of ultra-high molecular weight polyethylene (UHMWPE) against ceramic in a similar test and supports work which indicates that fluid film lubrication exists in these joints.