Fretting corrosion at the taper interface of modular connections can be studied using Finite Element (FE) analyses. However, the loading conditions in FE studies are often simplified, or based on generic activity patterns. Using musculoskeletal modeling, subject-specific muscle and joint forces can be calculated, which can then be applied to a FE model for wear predictions. The objective of the current study was to investigate the effect of incorporating more detailed activity patterns on fretting simulations of modular connections. Using a six-camera motion capture system, synchronized force plates, and 45 optical markers placed on 6 different subjects, data was recorded for three different activities: walking at a comfortable speed, chair rise, and stair climbing. Musculoskeletal models, using the Twente Lower Extremity Model 2.0 implemented in the AnyBody modeling System™ (AnyBody Technology A/S, Aalborg, Denmark; figure1), were used to determine the hip joint forces. Hip forces for the subject with the lowest and highest peak force, as well as averaged hip forces were then applied to an FE model of a modular taper connection (Biomet Type-1 taper with a Ti6Al4V Magnum +9 mm adaptor; Figure 2). During the FE simulations, the taper geometry was updated iteratively to account for material removal due to wear. The wear depth was calculated based on Archard's Law, using contact pressures, micromotions, and a wear factor, which was determined from accelerated fretting experiments.Introduction
Methods
Fretting corrosion of the modular taper junction in total hip arthroplasty has been studied in several finite element (FE) studies. Manufacturing tolerances can result in a mismatch between the femoral head and stem, which can influence the taper mechanics leading to possibly more wear. Using FE models the effect of these manufacturing tolerances on the amount of volumetric wear can be studied. The removal of material in the FE model was validated against experiments simulating the clinical fretting wear process, subsequently the mismatch and assembly force were varied to study the effect on the volumetric wear. An FE model was developed in which the geometry can be updated to account for material removal due to wear. In this model the geometry was updated based on Archard's Law, using contact pressures, micromotions and a wear factor, which was determined based on accelerated fretting experiments. The linear wear was calculated using H=k*p*S. Where H is the linear wear depth in mm, k is a wear factor (mm3/Nmm), p is the contact pressure (MPa) and S is the sliding distance (mm). 10 million cycles were simulated using 50 virtual steps. Using this scaling and the measured volumetric wear from the experiments a wear factor of 2.7*10−5 was applied. Based on general manufacturing tolerances the resulting mismatch in taper angles were determined to be ± 1.26°. Using this mismatch a tip fit (figure 1a) and base fit (Figure 1b) model were created. In combination with a perfect fit, meaning no mismatch, and two different assembly forces of 4 kN and 15 kN, 6 different situations were studied.Introduction
Methods
Traditionally, acetabular component insertion in direct anterior approach (DAA) total hip arthroplasty (THA) has been performed using fluoroscopic guidance. Handheld navigation systems can be used to address issues of alignment, cup placement and accuracy of measurements. Previous navigation systems have been used successfully in total knee arthroplasty (TKA) and has now been introduced in THA. We investigated the use of a new accelerometer-based, handheld navigation system during DAA THA to compare it to traditional means. This study aims to determine accuracy of acetabular cup placement as well as fluoroscopy times between two groups of patients. Data was prospectively collected for a group of consecutive DAA THA procedures using a handheld navigation system (n=45) by a single surgeon. This was compared to data retrospectively collected for a group that underwent the same procedure without use of the navigation system(n=50). The time for use of the navigation system, including insertion of pins/registration, guiding cup position, and removal of pins, was recorded intraoperatively. Postoperative anteroposterior and cross-table lateral radiographs were used to measure acetabular inclination and anteversion angles. Targeted angles for all cases were 40° ±5 for inclination and 20° ±5 for anteversion. Intraoperative fluoroscopy exposure times were obtained from post-anesthesia care unit radiographs.INTRODUCTION
METHODS
Childhood diseases involving the proximal femoral epiphysis often cause abnormalities that can lead to end-stage arthritis at a relatively young age and the need for total hip arthroplasty (THA). The young age of these patients makes hip resurfacing arthroplasty (HRA) an alternative and favorable option due to the ability to preserve femoral bone. Patients presenting with end-stage hip arthritis as sequelae of childhood diseases such as Legg-Calves-Perthes (LCP) and slipped capital femoral epiphysis (SCFE) pose altered femoral anatomy, making HRA more technically complicated. LCP patients can result in coxa magna, coxa plana and coxa breva causing altered femoral head-to-neck ratio. There can also be acetabular dysplasia along with the proximal femoral abnormalities. SCFE patients have altered femoral head alignment. In particular, the femoral head is rotated medially and posteriorly, reducing the anterior and lateral offset. Additionally, many of these patients have retained hardware, making resurfacing more complicated. We report findings of a cohort of patients, with history of either LCP or SCPE who underwent HRA to treat end-stage arthritis. Data was retrospectively collected for patients who had HRA for hip arthritis as a result of either LCP (n=67) or SCFE (n=21) between 2004 and 2014 performed by two surgeons. Demographic information, clinical examination and improvement was collected pre and postoperatively. Improvement was determined using Harris Hip Scores (HHS) and UCLA activity scores. Anteroposterior radiographs were measured pre and postoperatively to determine leg length discrepancy. Radiographs were inspected postoperatively for radiolucent lines, implant loosening and osteolysis. Kaplan-Meier survivorship for freedom from reoperation for any reason was calculated. Paired student t-tests were used to compare groups.INTRODUCTION
METHODS
Fretting at modular junctions is thought to be a ‘mechanically assisted’ corrosion phenomenon, initiated by mechanical factors that lead to increased contact stresses and micromotions at the taper interface. We adopted a finite element approach to model the head-taper junction, to analyse the contact mechanics at the taper interface. We investigated the effect of assembly force and angle on contact pressures and micromotions, during loads commonly used to test hip implants, to demonstrate the importance of a good assembly during surgery. Models of the Bimetric taper and adaptor were created, with elastic-plastic material properties based on material tests with the actual implant alloy. FE contact conditions were validated against push-on and pull-off experiments. The models were loaded according to ISO 7206-4 and −6, after being assembled at 2-4-15kN, both axially and at a 30° angle. Average micromotions and contact pressures were analysed, and a wear score was calculated based on the contact pressures and micromotions.Background
Methods
Fretting corrosion of the modular taper junction in total hip arthroplasty has been studied in several finite element (FE) investigations. In FE analyses, different parameters can be varied to study micromotions and contact pressures at the taper interface. However, to truly study taper wear, the simulation of micromotions and contact pressures in non-adaptive FE models is insufficient, as over time these can change due to interfacial changes caused by the wear process. In this study we developed an FE approach in which material removal during the wear process was simulated by adaptations to the taper geometry. The removal of material was validated against experiments simulating the clinical fretting wear process. Experimental test: An accelerated fretting screening test was developed that consistently reproduced fretting wear features observed in retrievals. Biomet Type-1 (4°) tapers and +9 mm offset adaptors were assembled with a 4 kN force (N=3). A custom head fixture was used to create an increased offset and torque. The stems were potted in accordance with ISO 7206–6:2013. The set-up was submerged in a 37°C PBS solution with a pH adjusted to 3 using HCL and NaCl concentration of 90gl−1. The components were cyclically loaded between 0.4 – 4 kN for 10 million cycles. After completion, the volumetric and linear wear was measured using a Talyrond-585 roundness measurement machine. FE model: This was created to match the experimental set up (Figure 1). Taper geometry and experimental material data were obtained from the manufacturer (Zimmer Biomet). The coefficient of friction of the studied combination of components was based on previous experiments (Introduction
Method
Fretting corrosion at the taper interface has been implicated as a possible cause of implant failure. Using Experimental test set-up: An accelerated wear test was developed that consistently reproduced fretting wear features observed in retrievals. Biomet stems with smooth 4° Type-1 tapers were combined with Ti6Al4V Magnum +9 mm adaptors using a 2 or 15 kN assembly force. The head was replaced with a custom head fixture to increase the offset and apply a torque at the taper interface. The stems were potted according to ISO 7206-6:2013. The set-up was submerged in a test medium containing PBS and 90gl-1 NaCl. The solution was pH adjusted to 3 using HCl and maintained at 37°C throughout the tests. For each assembly case, n=3 tests were cyclically loaded between 0.4–4 kN for 10 Million cycles. Volumetric wear measurements were performed using a Talyrond-365 roundness measurement machine. The FE model was created to replicate the experimental set up. Geometries and experimental material data were obtained from the manufacturer (Biomet). The same assembly forces of 2 and 15 kN were applied, and the same head fixture was used for similar offset and loading conditions. The 4 kN load was applied at the same angles in accordance with ISO 7206-6:2013. Micromotions and contact pressures were calculated, and based on these a wear score was determined by summation over all contact points.Introduction
Methods
Recent reports implicate fretting corrosion at the head-stem taper junction as a potential cause of failure of some large diameter metal-on-metal (MOM) devices. Fretting observed at modular junctions is thought to be a type of ‘mechanically assisted’ corrosion phenomenon, initiated by mechanical factors that lead to an increase in contact stresses and micromotions at the taper interface. These may include: intra-operative taper assembly, taper contamination by debris or body fluids, patient weight and ‘toggling’ of the head or increased frictional torque in a poorly functioning bearing. We adopted a finite element approach to model the head-taper junction, to analyze the contact mechanics at the taper interface. We investigated the effect of assembly force and angle on contact pressures and micromotions, during loads commonly used to test hip implants. Models of the Biomet Type-1 taper, a 60 mm head and a taper adaptor were created. These models were meshed with a mesh size based on a mesh density convergence study. Internal mesh coarsening was applied to reduce computational cost. Elastic-plastic material properties based on tensile tests were assigned to all titanium components. The contact conditions used in the FE analyses were validated against push-on and pull-off experiments, resulting in a coefficient of friction of 0.5. To analyze micromotions at the taper-adaptor connection, the models were loaded with 2300N (ISO 7206-4) and 5340N (ISO 7206-6), after being assembled with 2-4-15 kN, axially and under a 30º angle. This ISO standard is commonly used to determine endurance properties of stemmed femoral components. Micromotions and contact pressures were analyzed by scoring them to an average micromotion and average contact pressure for the surface area in contact.Introduction
Materials and methods
Recent reports have implicated fretting corrosion at the head-stem taper junction as a potential cause of failure of some large diameter metal-on-metal (MOM) devices. While it has been suggested that larger MOM heads, may induce greater frictional torques at the taper connection, the exact mechanisms underlying fretting corrosion remain poorly understood. It is likely that the onset of the corrosion process is caused by mechanical factors, such as contact stresses and micromotions occurring at the interface. These stresses and micromotions depend on the fixation of the head onto the stem and may be affected by blood, fat, bone debris or other contaminations. The fixation of the head is achieved intraoperatively through impaction. To further study this phenomenon, we adopted a finite element approach in which we modeled the head-taper junction fixation mechanics. In this model, we analyzed the effect of impaction force on the micromotions occurring at the head-stem interface. We created a model of a BIOMET Type-1 taper and an adapter that is typically used for larger heads. Titanium alloy material properties were assigned to both components, and frictional contact (μ = 0.5) was simulated between the adapter and the taper. To ensure that the model accurately represented the contact mechanics, we first simulated experiments in which the head was assembled on the taper in a load-controlled manner, at different load (4 and 15 kN), after which it was disassembled axially. The disassembly loads predicted by the FEA simulations were then compared to the experimental values. After ensuring a correct prediction of the disassembly loads, we used various impaction loads (2, 4, and 15 kN) to assemble the taper, after which a 2.3 kN load (ISO 7206-4) was applied to the adapter/taper assembly. This loading regime is commonly used to determine endurance properties of stemmed femoral components. Under these loading conditions, we then analyzed the contact stresses and micromotions, and the effect of impaction load on these quantities.Introduction
Materials and methods
Lower limb mal-alignment due to deformity is a significant cause of early degenerative change and dysfunction. Standard techniques are available to determine the centre of rotation of angulation (CORA) and extent of the majority of deformities, however distal femoral deformity is difficult to assess because of the difference between anatomic and mechanical axes. We found the described technique involving constructing a line perpendicular to a line from the tip of the greater trochanter to the centre of the femoral head inaccurate, particularly if the trochanter is abnormal. We devised a novel technique which accurately determines the CORA and extent of distal femoral deformity, allowing accurate correction. Using standard leg alignment views of the normal femur, the distal femoral metaphysis and joint line are stylized as a block. A line bisecting the axis of the proximal femur is then extended distally to intersect the joint. The angle (θ) between the joint and the proximal femoral axis and the position (p) where the extended proximal femoral axis intersects the joint line are calculated. These measurements can then be reproduced on the abnormal distal femur in order to calculate the CORA and extent of the deformity, permitting accurate correction. We examined the utility and reproducibility of the new method using 100 normal femora. θ = 81 ± sd 2.5°. As expected, θ correlated with femoral length (r=0.74). P (expressed as the percentage of the distance from the lateral edge of the joint block to the intersection) = 61% ± sd 8%. P was not correlated with θ. Intra-and inter-observer errors for these measurements are within acceptable limits and observations of 30-paired normal femora demonstrate similar values for θ and p on the two sides. We have found this technique to be universally applicable and reliable in a variety of distal femoral deformities.
Hyaline cartilage defects are a significant clinical problem for which a plethora of cartilage repair techniques are used. One such technique is cartilage replacement therapy using autologous chondrocyte or mesenchymal stem cell (MSC) implantation (ACI). Mesenchymal stem cells are increasingly being used for these types of repair technique because they are relatively easy to obtain and can be expanded to generate millions of cells. However, implanted MSCs can terminally differentiate and produce osteogenic tissue which is highly undesirable, also, MSCs generally only produce fibrocartilage which does not make biomechanically resilient repair tissue, an attribute that is crucial in high weight-bearing areas. Tissue-specific adult stem cells would be ideal candidates to fill the void, and as we have shown previously in animal model systems [Dowthwaite et al, 2004, J Cell Sci 117;889], they can be expanded to generate hundreds of millions of cells, produce hyaline cartilage and they have a restricted differential potential. Articular chondroprogenitors do not readily terminally differentiate down the osteogenic lineage. At present, research focused on isolating tissue-specific stem cells from articular cartilage has met with modest success. Our results demonstrate that using differential adhesion it is possible to easily isolate articular cartilage progenitor populations from human hyaline cartilage and that these cells can be subsequently expanded in vitro to a high population doubling whilst maintaining a normal karyotype. Articular cartilage progenitors maintain telomerase activity and telomere length that are a characteristic of progenitor/stem cells and differentiate to produce hyaline cartilage. In conclusion, we propose the identification and characterisation of a novel articular cartilage progenitor population, resident in human cartilage, which will greatly benefit future cell-based cartilage repair therapies.
One reason why NICE (National Institute for Clinical Excellence) does not support operations by the NHS to heal hyaline cartilage lesions using a patients own cells is because there is no clear evidence to show that these operations are beneficial and cost-effective in the long term. Specifically, NICE identified a deficiency of high quality cartilage being produced in repaired joints. The presence of high quality cartilage is linked to long-lasting and functional repair of cartilage. The benchmark for quality, NICE stipulate, is repair cartilage that is stiff and strong and looks similar to the normal tissue surrounding it, i.e. mature hyaline articular cartilage. Biopsy material from autologous cartilage implantation surgical procedures has the appearance of immature articular cartilage and is frequently a mixture of hyaline and fibrocartilage. Osteoarthritic cartilage, in its early stages, also exhibits characteristics of immature articular cartilage in that it expresses proteins found in embryonic and foetal developmental stages, and is highly cellular as evidenced through the presence of chondrocyte clusters. Therefore, an ability to modulate the phenotype and the structure of the extracellular matrix of articular cartilage could positively affect the course of repair and regeneration of articular cartilage lesions. In order to do this, the biochemical stimuli that induce the transition of an essentially unstructured amorphous cartilage mass (immature articular cartilage) to one that is highly structured and ordered, and biomechanically adapted to its particular function (mature articular cartilage) has to be identified. We show for the first time, that fibroblast growth factor-2 and transforming growth factor beta-1 induce precocious maturation of immature articular cartilage. Our data demonstrates that it is possible to significantly enhance maturation of cartilage tissue using growth factor stimulation; consequently this may have applications in transplantation therapy, or through phenotypic modulation of osteoarthritic chondrocytes in diseased cartilage in order to stimulate growth and maturation of repair tissue.
Hyaline cartilage defects are a significant clinical problem for which a plethora of cartilage repair techniques are used. One such technique is cartilage replacement therapy using autologous chondrocyte or mesenchymal stem cell (MSC) implantation (ACI). Mesenchymal stem cells are increasingly being used for these types of repair technique because they are relatively easy to obtain and can be expanded to generate millions of cells. However, implanted MSCs can terminally differentiate and produce osteogenic tissue which is highly undesirable, also, MSCs generally only produce fibrocartilage which does not make biomechanically resilient repair tissue, an attribute that is crucial in high weight-bearing areas. Tissue-specific adult stem cells would be ideal candidates to fill the void, and as we have shown previously in animal model systems [ At present, research focused on isolating tissue-specific stem cells from articular cartilage has met with modest success. Our results demonstrate that using differential adhesion it is possible to easily isolate articular cartilage progenitor populations from human hyaline cartilage and that these cells can be subsequently expanded In conclusion, we propose the identification and characterisation of a novel articular cartilage progenitor population, resident in human cartilage, which will greatly benefit future cell-based cartilage repair therapies.
Lower limb mal-alignment due to deformity is a significant cause of early degenerative change and dysfunction. Standard techniques are available to determine the centre of rotation of angulation (CORA) and extent of the deformities. However, distal femoral deformity is difficult to assess because of the difference between anatomic and mechanical axes. We describe a novel technique which accurately determines the CORA and extent of distal femoral deformity. Using standard leg alignment views of the normal femur, the distal femoral metaphysis and joint line are stylised as a block. A line bisecting the anatomical axis of the proximal femur is then extended distally to intersect the joint. The angle (?) between the joint and the proximal femoral axis, and the position (p) where the extended proximal femoral axis intersects the joint line are calculated. These measurements can then be reproduced on the abnormal distal femur in order to calculate the CORA and extent of deformity, permitting accurate correction. We examined the utility and reproducibility of the new method using 100 normal femora. We found this technique to be universally robust in a variety of distal femoral deformities.
Artificial hip joints have been in use for a number of years; various combinations of metals and polymers have been tested both in vitro and in vivo. Modern ceramics have found application as bearings in hip replacement due to the enhanced wear and friction that they offer. It has been hypothesised that during the swing phase of gait it is possible for the Femoral head and the Acetabular cup to dislocate, before relocating during heel contact. Severe loading such as this could cause greater levels of wear to occur in artificial hip joints. This study provides comparative analysis between ceramic-on-ceramic hip joint pairings under both severe and standard loading profiles. Five zirconia-toughened alumina (ZTA) 28mm diameter bearing pairs were tested on a ProSim Hip Simulator for 5.3 million cycles (MC), two under severe loading and three under standard loading conditions. Additionally a Loaded Soak Control, Soak Control and Environmental Control were used. Wear was recorded every 0.5 MC by gravimetric measurement. Surface microscopy images from a Zygo New View 100 and an Atomic Force Microscope (AFM) were taken before testing and then at, 0.5 MC. 2.5 MC, and 5.3 MC. The standard loading profile followed Wear rates for the ceramic cups under severe wear condition were found to be 0.0356±0.0059mm3/ MC and for the standard wear condition to be 0.0178±0.0049mm3/MC. The femoral heads had wear rates of 0.0164±0.0046mm3/MC for severe wear condition and no wear was detected for the standard wear condition. The results of the present study showed almost no wear under standard gait condition and only a modest increase in wear occurred when using severe wear condition. Thus the resulting wear rates are still significantly lower than those found for alumina-alumina total hip joints [
Orthopaedic grade ultra-high molecular weight polyethylene (UHMWPE) remains the preferred material for one of the bearing surfaces in total joint prostheses because of its high wear resistance and proven biocompatibility. Since the 1970s, UHMWPE has served as the only widely used bearing material for articulation with metallic components in total knee arthroplasty (TKA). However, polyethylene-related total knee failures have limited the lifetime of total knee joint replacements. The present study is focused on improving material integrity and reducing the probability of material failure. The hypothesis examined here is that there is a correlation between material failure of UHMWPE knee-joint components and the precise time-temperature history employed during fabrication, due to their strong effect on interparticle cohesion. The presence of fusion defects due to incomplete consolidation and incomplete polymer self-diffusion has been implicated in the failure of UHMWPE joints [ We have investigated the wear performance of direct compression moulded UHMWPE plates with different degree of inter-particle diffusion. Direct compression moulding was used in the present study because of its uniformly excellent surface finish which is better than machined surfaces. UHMWPE plates (44×24×3mm) were direct compression moulded using GUR1050 powder (Ticona). Various moulding temperature (e.g. 145°C, 150°C, 175°C) and dwell time (e.g. 15mins and 30mins) were investigated. The wear tests were carried out at 37°C using a Durham four-station multidirectional pin-onplate machine, which generates both reciprocating and rotating motions simultaneously. The material combination of the flat-ended metallic indentors loaded against UHMWPE plates was constructed to mimic conformal contact conditions in knee prostheses. The articulating surfaces were lubricated using 25% diluted bovine serum. Meanwhile the experimental method was validated by evaluating the wear generation under the conventional configuration (i.e. UHMWPE pins on metal plates); results were comparable with the data in the literature [ For the direct compression moulded UHMWPE plates, experimental wear factors were determined and found to correlate well with numerically calculated degree of inter-particle diffusion. Increasing moulding temperature and dwell time decreased the wear factors and increased inter-particle diffusion. Surface structures were characterised before and after every 0.5 million cycles. The observed surface features on UHMWPE plates in ESEM and optical microscopy is very similar to those in retrieved knee prostheses [
The use of shoulder ultrasound in clinic is a way of decreasing the time patients have to wait til definitive treatment is started. Although ultrasound is used in clinic by some surgeons, we are not aware of anyone specifically looking at the total cost implications and the impact on waiting times. We therefore prospectively assessed the outcome of a one-stop shoulder assessment service set up by a new Consultant Orthopaedic Surgeon in a busy unit. All new patients were assessed by the Consultant, who then performed an ultrasound if indicated. Treatment or further investigation was then instituted based on the findings. The time taken and accuracy of the scans, the number of patients seen, impact on waiting times, total savings and patient satisfaction were assessed. We based cost calculations from data that included capital, structural, maintenance and staffing costs gained from the Department of Health and the hospital management. We found that 65% of all shoulder patients required ultrasound, and these were performed in an average of 2.7 minutes, with no significant overrunning of the clinic. The sensitivity for the detection of full thickness rotator cuff tears was 88% and specificity was 89%. Four patients needed further ultrasonography by a specialist musculoskeletal radiologist. All patients reported high satisfaction rates. We calculated the cost saving over a year of two shoulder surgeons performing ultrasound in a similar setting was between £200,000 and £500,000 depending on the figures you used. We believe ultrasound is a quick, easy, cheap imaging process for the diagnosis of soft tissue shoulder diseases. When performed at the first consultation by the surgeon it offers the advantages of high patient satisfaction rates, shorter waiting times, and significant cost savings. Should all shoulder surgeons be performing ultrasound in clinic?
MRI arthography (MRA) is commonly used in the investigation of shoulder instability. However many surgeons are now using CT arthography (CTA) as their primary radiological investigative modality. They argue that CTA is cheaper, and give satisfactory soft tissue images in the “soft tissue window” mode. They believe that CTA give superior images when looking at bone loss and bony defects, and as such is more useful in deciding whether a patient requires an open procedure or not. In this study we aimed to compare the results of MRA and CTA in the investigation of shoulder instability. We reviewed the operative and arthographic findings in all patients who had surgery for shoulder instability in our unit over a 4 year period. We compared the results of the arthograms with the definitive findings found at the time of surgery. All arthograms were performed by standard techniques and were reported by musculoskeletal radiology consultants. All surgery was performed by experienced consultant shoulder surgeons. In total 48 CTAs and 50 MRAs were performed. We found that there was no significant difference between the two wrt sensitivity (p=0.1) and specificity (p=0.4) when looking at labral pathology. However CTA was more sensitive at picking up bony lesions (p<
0.05). This study supports the view that CT arthography is the superior radiological modality in identifying pathology when investigating patients with shoulder instability. It is cheaper and better tolerated by patients than MRA and gives useful information on whether a patient needs an open or arthroscopic stabilisation procedure.
Aseptic loosening caused by UHMWPE wear debris induced osteolysis is a major cause of revision in total hip arthroplasty (THA) Advantages include reduced wear and metal ion release compared with MOM. In addition, it is thought that there may be a reduced tendency for fracture of the ceramic component due to the softer metallic cup. In this study a 5 million cycle wear test was carried out on the Mark II Durham Hip Wear Simulator. A set of six, 38mm diameter HIPed alumina heads and as-cast CoCr alloy cups were tested in bovine serum. Surface topography analysis was carried out at 0, 2, 3 and 5 million cycles. Additionally imaging of the bearing surfaces using ESEM and AFM was undertaken on the final bearing surface. Friction testing, using the Durham Hip Friction Simulator was carried out on one of the joints worn to 5 million cycles and the results were compared with theoretical calculations. Wear of the ceramic heads was virtually undetectable using the conventional gravimetric methods. However, minor surface damage in the form of grain pull out and abrasive scratches was observed in the wear patch when the bearing surfaces were analysed using ESEM and AFM. The grains were not visible in the unworn sections of the head. The average surface roughness remained constant throughout the test. The CoCr cups showed a decrease in roughness between 0 and 2 million cycles, after which it remained relatively constant. This was consistent with the wear results in which a biphasic wear rate was found. The more frequently obtained wear results showed running in wear rate of 1.02±0.078 mm3/million cycles between 0–0.5 million cycles, followed by a steady state wear rate of 0.030±0.011 mm3/million cycles. These results are consistent with those of a recent study undertaken elsewhere Friction testing produced a Stribeck curve which was indicative of full fluid film lubrication with a friction factor of 0.027±0.002 for 25% bovine serum (η=0.0014 Pa s-1). Other tests were also carried out using carboxy methyl cellulose fluid as the lubricant to investigate the effect of proteins. This showed that there was a small decrease in friction factor when proteins were absent from the lubricant. It is thought that the difference in friction factors is due to adsorption of the proteins onto the bearing surfaces, when lubricated in bovine serum. This introduces large proteins between the bearing surfaces, which penetrate the lubricant film, causing protein on protein interactions, in addition to the friction caused by shearing of the lubricant film.
