Dual Mobility (DM) Total Hip Replacements (THRs), are becoming widely used but function in-vivo is not fully understood.

The aim of this study was to compare the incidence of impingement of a modular dual mobility with that of a standard cup.

A geometrical model of one subject's bony anatomy \[1\] was developed, a THR was implanted with the cup at a range of inclination and anteversion positions (Corail® stem, Pinnacle® cup (DePuy Synthes)). Two DM variants and one STD acetabular cup were modelled. Joint motions were taken from kinematic data of activities of daily living associated with dislocation \[2\] and walking. The occurrence of impingement was assessed for each component combination, orientation and activity. Implant-implant impingement can occur between the femoral neck and the metal or PE liner (DM or STD constructs respectively) or neck-PE mobile liner (DM only).

The results comprise a colour coded matrix which sums the number of impingement events for each cup position and activity and for each implant variant.

Neck-PE mobile liner impingement, occurred for both DM sizes, for all activities, and most cup placement positions indicating that the PE mobile liner is likely to move at the start of all activities including walking.

For all constructs no placement positions avoided neck-metal (DM) or neck-PE liner (STD) impingementevents in all activities. The least number of events occurred at higher inclination and anteversion component positions. In addition to implant-implant impingement, some instances of bone-bone and implant-bone impingement were also observed.

Consistent with DM philosophy, neck-PE mobile liner impingement and liner motion occurred for all activities including walking. Neck-liner impingement frequency was comparable between both DM sizes (metal liner) and a standard cup (PE liner).

Abstract

Dual Mobility (DM) Total Hip Replacements (THRs) were introduced to reduce dislocation risk, which is the most common cause of early revision. The in-vivo mechanics of these implants is not well understood, despite their increased use in both elective and trauma settings. Therefore, the aim of this study was to comprehensively assess retrieved DM polyethylene liners for signs of damage using visual inspection and semi-quantitative geometric assessment techniques.

Retrieved DM liners (n=20) were visually inspected for the presence of seven established modes of polyethylene damage. If embedded debris was identified on the external surface, its material composition was characterised using energy-dispersive x-ray analysis (EDX). Additionally, each liner was geometrically assessed for signs of wear/deformation using a validated methodology.

Visual inspection of the liners revealed that scratching and pitting were the most common damage modes on either surface. Burnishing was observed on 50% and 15% of the internal and external surfaces, respectively. In addition, embedded debris was identified on 25% of the internal and 65% of the external surfaces. EDX analysis of the debris identified several materials including iron, titanium, cobalt-chrome, and tantalum. Geometric analysis demonstrated highly variable damage patterns across the liners.

The results of this study provide insight into the in-vivo mechanics of DM bearings. For example, the results suggest that the internal bearing (i.e., between the head and liner) acts as the primary articulation site for DM-THRs as evidenced by a higher incidence of burnishing and larger, more concentrated regions of penetration across the liners’ internal surfaces. Furthermore, circumferential, and crescent-shaped damage patterns were identified on the articulating surfaces of the liners thus providing evidence that these components can rotate within the acetabular shell with varying degrees of mobility. The mechanics of DM bearings are complex and may be influenced by several factors (e.g., soft tissue fibrosis, patient activities) and thus further investigation is warranted.

Finally, the results of this study suggest that DM liners may be susceptible to ex-vivo surface damage and thus caution is advised when handling and/or assessing these types of components.

Non-optimal clinical alignment of components in total hip replacements (THRs) may lead to edge loading of the acetabular cup liner. This has the potential to cause changes to the liner rim not accounted for in standard wear models. A greater understanding of the material behaviours could be beneficial to design and surgical guidance for THR devices. The aim of this research was to combine finite element (FE) modelling and experimental simulation with microstructural assessment to examine material behaviour changes during edge loading.

A dynamic deformable FE model, matching the experimental conditions, was created to simulate the stress strain environment within liners. Five liners were tested for 4Mc (million cycles) of standard loading (ISO14242:1) followed by 3Mc of edge loading with dynamic separation (ISO14242:4) in a hip simulator. Microstructural measurements by Raman spectroscopy were taken at unloaded and highly loaded rim locations informed by FE results. Gravimetric and geometric measurements were taken every 1Mc cycles.

Under edge loading, peak Mises stress and plastic deformation occur below the surface of the rim during heel strike. After 7Mc, microstructural analysis determined edge loaded regions had an increased crystalline mass fraction compared to unloaded regions (p<0.05). Gravimetric wear rates of 12.5mm³/Mc and 22.3mm³/Mc were measured for standard and edge loading respectively. A liner penetration of 0.37mm was measured after 7Mc.

Edge loading led to an increase in gravimetric wear rate indicating a different wear mechanism is occurring. FE and Raman results suggest that changes to material behaviour at the rim could be possible. These methods will now be used to assess more liners and over a larger number of cycles. They have potential to explore the impact of edge loading on different surgical and patient variables.

Hip joint biomechanics can be altered by abnormal morphology of the acetabulum and/or femur. This may affect load distribution and contact stresses on the articular surfaces, hence, leading to damage and degradation of the tissue. Experimental hip joint simulators have been used to assess tribology of total hip replacements and recently methods further developed to assess the natural hip joint mechanics. The aim of this study was to evaluate articular surfaces of human cadaveric joints following prolonged experimental simulation under a standard gait cycle.

Four cadaveric male right hips (mean age = 62 years) were dissected, the joint disarticulated and capsule removed. The acetabulum and femoral head were mounted in an anatomical hip simulator (Simulation Solutions, UK). A simplified twin peak gait cycle (peak load of 3kN) was applied. Hips were submerged in Ringers solution (0.04% sodium azide) and testing conducted at 1 Hertz for 32 hours (115,200 cycles). Soft tissue degradation was recorded using photogrammetry at intervals throughout testing.

All four hips were successfully tested. Prior to simulation, two samples exhibited articular surface degradation and one had a minor scalpel cut and a small area of cartilage delamination. The pre-simulation damage got slightly worse as the simulation continued but no new areas of damage were detected upon inspection. The samples without surface degradation, showed no damage during testing and the labral sealing effect was more obvious in these samples.

The fact that no new areas of damage were detected after long simulations, indicates that the loading conditions and positioning of the sample were appropriate, so the simulation can be used as a control to compare mechanical degradation of the natural hip when provoked abnormal conditions or labral tissue repairs are simulated.

Cam-type femoroacetabular impingement is caused by bone excess on the femoral neck abutting the acetabular rim. This can cause cartilage and labral damage due to increased contact pressure as the cam moves into the acetabulum. However, the damage mechanism and the influence of individual mechanical factors (such as sliding distance) are poorly understood. The aim of this study was to identify the cam sliding distance during impingement for different activities in the hip joint.

Motion data for 12 different motion activities from 18 subjects, were applied to a hip shape model (selected as most likely to cause damage, anteriorly positioned with a maximum alpha angle of 80°). The model comprised of a pointwise representation of the acetabular rim and points on the femoral head and neck where the shape deviated from a sphere (software:Matlab).

The movement of each femoral point was tracked in 3D while an activity motion was applied, and impingement recorded when overlap between a cam point and the acetabular rim occurred. Sliding distance was recorded during impingement for each relevant femoral point.

Angular sliding distances varied for different activities. The highest mean (±SD) sliding distance was for leg-crossing (42.62±17.96mm) and lowest the trailing hip in golf swing (2.17±1.11mm). The high standard deviation in the leg crossing sliding distances, indicates subjects may perform this activity in a different manner.

This study quantified sliding distance during cam impingement for different activities. This is an important parameter for determining how much the hip moves during activities that may cause damage and will provide information for future experimental studies.

Dual mobility (DM) total hip replacements (THRs) were introduced to reduce dislocation risk, which is the most common cause of early revision. Although DM THRs have shown good overall survivorship and low dislocation rates, the mechanisms which describe how these bearings function in-vivo are not fully understood. Therefore, the study aim was to comprehensively assess retrieved DM polyethylene liners for signs of damage using visual inspection and semi-quantitative geometric assessment methods.

Retrieved DM liners (n=18) were visually inspected for the presence of surface damage, whereby the internal and external surfaces were independently assigned a score of one (present) or zero (not present) for seven damage modes. The severity of damage was not assessed. The material composition of embedded debris was characterised using energy-dispersive x-ray analysis (EDX). Additionally, each liner was geometrically assessed for signs of wear/deformation [1].

Scratching and pitting were the most common damage modes on either surface. Additionally, burnishing was observed on 50% of the internal surfaces and embedded debris was identified on 67% of the external surfaces. EDX analysis of the debris identified several materials including titanium, cobalt-chrome, iron, and tantalum. Geometric analysis demonstrated highly variable damage patterns across the liners.

The incidence of burnishing was three times greater for the internal surfaces, suggesting that this acts as the primary articulation site. The external surfaces sustained more observable damage as evidenced by a higher incidence of embedded debris, abrasion, delamination, and deformation. In conjunction with the highly variable damage patterns observed, these results suggest that DM kinematics are complex and may be influenced by several factors (e.g., soft tissue fibrosis, patient activities) and thus further investigation is warranted.

Cerebral palsy (CP) is a neural condition that impacts and impairs the musculoskeletal system. Skeletal muscles, particularly in the lower limb, have previously been shown to be significantly reduced in volume in CP compared to typical controls. Muscle volume is a gross measure, however, and does not capture shape characteristics which—if quantified—could offer a robust and novel assessment of how this condition impacts skeletal muscle form and function in CP. In this study, we used mathematical shape modelling to quantify not just size, but also the shape, of soleus muscles in CP and typically developing (TD) cohorts to explore this question.

Shape modelling is a mathematical technique used previously for bones, organs, and tumours. We obtained segmented muscle data from prior MRI studies in CP. We generated shape models of CP and TD cohorts and used our shape models to assess similarities and differences between the cohorts, and we statistically analysed shape differences.

The shape models revealed similar principal components (PCs), i.e. the defining mathematical features of each shape, yet showed greater shape variability within the CP cohort. The model revealed a distinct feature (a superior –> inferior shift of the broad central region), indicating the model could identify muscular features that were not apparent with direct observation. Two PCs dominated the differences between CP and TD cohorts: size and aspect ratio (thinness) of the muscle.

The distinct appearance characteristic in the CP model correspond to specific muscle impairments in CP to be discussed further. Overall, children with CP had smaller muscles that also tended to be long, thin, and narrow. Shape modelling captures shape features quantitatively, which indicate the ways that muscles are being impacted in CP. In the future, we hope to tailor this technique toward informing diagnosis and treatments in CP.

The 10 year survivorship of THR is generally over 95%. However, the incidence of revision is usually higher in year one. The most common reason being dislocation which at least in part is driven by inadequate range of motion (ROM) leading to impingement, subluxation and ultimately dislocation which is more frequently posterior. ROM is affected by patient activity, bone and component geometry, and component placement. To reduce the incidence of dislocation, supported by registry data, there has been an increase in the use of so-called ‘lipped’ liners. Whilst this increases joint stability, the theoretical ROM is reduced. The aim of this study was to investigate the effect of lip placement on impingement.

A rigid body geometric model was incorporated into a CT scan hemi-pelvis and femur, with a clinically available THR virtually implanted. Kinematic activity data associated with dislocation was applied, comprising of five posterior and two anterior dislocation risk activities, resulting from anterior and posterior impingement respectively. Cup inclination and anteversion was varied (30°-70°, 0°-50° respectively) to simulate extremes of clinical outcomes. The apex position of a ‘lipped’ liner was rotated from the superior position, anteriorly and posteriorly in steps of 45°. Incidence and location of implant and bone impingement was recorded in 5346 cases generated.

A liner with the lip placed superior increased the occurrence of implant-implant impingement compared with a neutral liner. Rotation of the lip from superior reduced this incidence. This effect was more marked with posterior rotation which after 90° reduced anterior impingement to levels similar to a neutral liner. Complete inversion of the lipped liner reduced impingement, but this and anterior rotation both negate its function – additional stability.

This study comprises one bone geometry and component design and one set of activity profiles. Nevertheless, it indicates that appropriate lip placement can minimise the likelihood of impingement for a range of daily activities whilst still providing additional joint stability.

Abstract

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Abstract

Impingement of total hip replacements (THRs) can cause rim damage of polyethylene liners, and lead to dislocation and/or mechanical failure of liner locking mechanisms[1]. Previous work has focussed on the influence of femoral neck profile on impingement without consideration of neck-shaft angle. This study assessed the occurrence of impingement with two different stem designs (Corail standard [135°] and coxa vara [125°]) under different activities with varying acetabular cup orientation (30° to 70° inclination; 0° to 50° anteversion) using a geometric modelling tool.

The tool was created in a computer aided design software programme, and incorporated an individual's hemi-pelvis and femur geometry[3] with a THR (DePuy Synthes Pinnacle^® shell and neutral liner; size 12 Corail^® standard or coxa vara and 32mm head). Kinematic data of activities associated with dislocation[2], such as stooping to pick an object from the floor was applied and incidences of impingement were recorded.

Predicted implant impingement was influenced by stem design. The coxa vara stem was predicted to cause implant impingement less frequently across the range of activities and cup orientations investigated, compared to the standard stem [Fig. 1]. The cup orientations predicted to cause impingement the least frequently were at lower inclination and anteversion angles, relative to the standard stem [Fig. 1]. The coxa vara stem included a collar, while the standard stem was collarless; additional analysis indicated that differences were due to neck angle and not the presence of a collar.

This study demonstrated that stem neck-shaft angle is an important variable in prosthetic impingement in THR and surgeons should be aware of this when choosing implants. Future work will consider further implant design and bone geometry variables. This tool has the potential for use in optimising stem design and position and could assist with patient specific stem selection based on an individual's activity profile.

For any figures or tables, please contact the authors directly.

Introduction

Impingement of total hip arthroplasties (THAs) has been reported to cause rim damage of polyethylene liners, and in some instances has led to dislocation and/or mechanical failure of liner locking mechanisms in modular designs. Elevated rim liners are used to improve stability and reduce the risk of dislocation, however they restrict the possible range of motion of the joint, and retrieval studies have found impingement related damage on lipped liners.

The aim of this study was to develop a tool for assessing the occurrence of impingement under different activities, and use it to evaluate the effects a lipped liner and position of the lip has on the impingement-free range of motion.

INTRODUCTION

There is great potential for the use of computational tools within the design and test cycle for joint replacement devices.

The increasing need for stratified treatments that are more relevant to specific patients, and implant testing under more realistic, less idealised, conditions, will progressively increase the pre-clinical experimental testing work load. If the outcomes of experimental tests can be predicted using low cost computational tools, then these tools can be embedded early in the design cycle, e.g. benchmarking various design concepts, optimising component geometrical features and virtually predicting factors affecting the implant performance. Rapid, predictive tools could also allow population-stratified scenario testing at an early design stage, resulting in devices which are better suited to a patient-specific approach to treatment.

The aim of the current study was to demonstrate the ability of a rapid computational analysis tool to predict the behaviour of a total hip replacement (THR) device, specifically the risk of edge loading due to separation under experimental conditions.

Acetabular tissue damage is implicated in osteoarthritis (OA) and investigation of in situ acetabular soft tissues behaviour will improve understanding of tissue properties and interconnections. The study aim was to visualise acetabular soft tissues under load and to quantify displacements using computed tomography (CT) scans (XtremeCT, Scano Medical).

A CT scan (resolution 82 μm) was performed on the disarticulated, unloaded porcine acetabulum. The femoral head was soaked in Sodium Iodide (NaI) solution and cling film wrapped to prevent transfer to the acetabular side. The joint was realigned, compressed using cable ties and re-scanned. The two images were down-sampled to 0.3 mm. Acetabular bone and soft tissues were segmented. Bony features were used to register the two background images, using Simpleware ScanIP 7.0 (Synopsys), to the same position and orientation (volume difference < 5%). Acetabular soft tissues displacements were measured by tracking the same points at the tissue edges on the two acetabular masks, along with difference in bone position as an additional error assessment.

The use of radiopaque solution provided a clear contrast allowing separation of the femoral and acetabular soft tissues in the loaded image. The image registration process resulted in a difference in bone position in the areas of interest equivalent to image resolution (0.3 mm, a mean of 3 repeats by same user). A labral tip displacement of 1.7 mm and a cartilage thickness change from 1.5 mm unloaded to 0.9 mm loaded, were recorded.

The combination of contrast enhancement, registration and focused local measurement was precise enough to reduce bone alignment error to that of image resolution and reveal local soft tissue displacements. These measurement methods can be used to develop models of soft tissues properties and behaviour, and therapy for hip tissue damage at early stage may be reviewed and optimised.

Introduction

Up to 60% of total hip arthroplasties (THA) in Asian populations arise from avascular necrosis (AVN), a bone disease that can lead to femoral head collapse. Current diagnostic methods to classify AVN have poor reproducibility and are not reliable in assessing the fracture risk. Femoral heads with an immediate fracture risk should be treated with a THA, conservative treatments are only successful in some cases and cause unnecessary patient suffering if used inappropriately. There is potential to improve the assessment of the fracture risk by using a combination of density-calibrated computed tomographic (QCT) imaging and engineering beam theory. The aim of this study was to validate the novel fracture prediction method against in-vitro compression tests on a series of six human femur specimens.

Introduction and Aims

There are many surgical, implant design and patient factors that should be considered in preclinical testing of hip replacement which are not being considered in current standards. The aim of this study was to develop a preclinical testing method that consider surgical positioning, implant design and patient factors and predict the occurrence and severity of edge loading under the combination of such conditions. Then, assess the safety and reliability of the implant by predicting the wear, deformation and damage of the implant bearings under worst case conditions.

Introduction

Geometric variations of the hip joint can give rise to repeated abnormal contact between the femur and acetabular rim, resulting in cartilage and labrum damage. Population-based geometric parameterisation can facilitate the flexible and automated in silico generation of a range of clinically relevant hip geometries, allowing the position and size of cams to be defined precisely in three dimensions. This is advantageous compared to alpha angles, which are unreliable for stratifying populations by cam type. Alpha angles provide an indication of cam size in a single two-dimensional view, and high alpha angles have been observed in asymptomatic individuals.

Parametric geometries can be developed into finite element models to assess the potential effects of morphological variations in bone on soft tissue strains. The aim of this study was to demonstrate the capabilities of our parameterisation research tool by assessing impingement severity resulting from a range of parametrically varied femoral and acetabular geometries.

INTRODUCTION

Avascular necrosis (AVN) of the femoral head (FH) initiates from biological disruptions in the bone and may progress to mechanical failure of the hip. Mechanical and structural properties of AVN bone have not been widely reported, however such understanding is important when designing therapies for AVN. Brown et al.[1] assessed mechanical properties of different regions of AVN FH bone and reported 52% reduction in yield strength and 72% reduction in elastic modulus of necrotic regions when compared to non-necrotic bone. This study aimed to characterise structural and mechanical properties of FH bone with AVN and understand the relationship between lesion volume and associated mechanical properties.

Aims

Our aim in this study was to describe a continuing review of 11 total hip arthroplasties using 22.225 mm Alumina ceramic femoral heads on a Charnley flanged femoral component, articulating against a silane crosslinked polyethylene.

Introduction and Aims

Clinically many factors such as variations in surgical positioning, and patients' anatomy and biomechanics can affect the occurrence and severity of edge loading which may have detrimental effect on the wear and durability of the implant. Assessing wear of hundreds of combinations of conditions would be impractical, so a preclinical testing approach was followed where the occurrence and severity of edge loading can be determined using short biomechanical tests. Then, selected conditions can be chosen under which the wear can be determined. If a wear correlation with the magnitude of dynamic separation or the severity of edge loading can be shown, then an informed decision can be made based upon the biomechanical results to only select important variables under which the tribological performance of the implant can be assessed. The aim of this study was to determine the relationship between the wear of ceramic-on-ceramic bearings and the (1) magnitude of dynamic separation, (2) the maximum force reached during edge loading and (3) the severity of edge loading resulting from component translational mismatch between the head and cup centres.

Introduction

Total ankle replacement (TAR) is less successful than other joint replacements with a 77% survivorship at 10 years. Predominant indications for revision include: Insert dislocation, soft tissue impingement and pain/stiffness. Insert edge-loading may be both a product and cause of these indications and was reported to affect 22% of patients with the, now withdrawn from market, Ankle Evolutive System (AES) TAR (Transysteme, Nimes, France). Compressive forces up to seven times body weight over a relatively small contact area (∼6.0 to 9.2 cm²), in combination with multi-directional motion potentially causes significant polyethylene wear and deformation in mobile-bearing TAR designs. Direct methods of measuring component volume (e.g. pycnometer) use Archimedes' principle but cannot identify spatial changes in volume or form indicative of wear/deformation. Quantitative methods for surface analysis bridge this limitation and may advance methods for analysing the edge loading phenomena in TAR.

Introduction and Aims

There are many variables that can affect the occurrence and severity of edge loading in hip replacement. A translational mismatch between the centres of rotation of the head and cup may lead to dynamic separation, causing edge loading and increased wear. Combining a steep inclination angle with such translational mismatch in the medial-lateral axis caused a larger magnitude of separation and increased severity of edge loading. Previous studies have shown variation in the hip Swing Phase Load (SPL) during gait between different patients. The aim of this study was to apply a translational mismatch and determine the effect of varying the SPL on the occurrence and severity of edge loading under different cup inclination angles in a hip joint simulator.

Introduction

Geometric variations of the hip joint can give rise to abnormal joint loading causing increased stress on the articular cartilage, which may ultimately lead to degenerative joint disease. In-vitro simulations of total hip replacements (THRs) have been widely reported in the literature, however, investigations exploring the tribology of two contacting cartilage surfaces, and cartilage against metal surfaces using complete hip joint models are less well reported.

The aim of this study was to develop an in-vitro simulation system for investigating and comparing the tribology of complete natural hip joints and hemiarthroplasties with THR tribology. The simulation system was used to assess natural porcine hip joints and porcine hemiarthroplasty hip joints. Mean friction factor was used as the primary outcome measure to make between-group comparisons, and comparisons with previously published tribological studies.

Introduction

Contact between the femoral head and rim of the acetabular liner in total hip replacements has been linked to adverse tribological performance that may potentially shorten the lifespan of the prosthesis. Predicting the size and location of the contact area can be done computationally, however, experimental validation of these models is challenging due to the conforming nature of the bearing surfaces.

This study aimed to develop a method of accurately determining the in-vitro contact area between the femoral head and acetabular cup in metal-on-polyethylene and ceramic-on-polyethylene bearings under different component orientations.

Wear particles produced by alumina ceramic-on-ceramic (CoC) bearings cause a minimal immunological response with low cytotoxicity and inflammatory potential^{1, 2}. However, more comprehensive immunological studies are yet to be completed for the composite CoC (zirconia-toughened, platelet reinforced alumina) hip replacements due to difficulties in isolating the very low volume of clinically relevant wear debris generated by such materials in vitro. The aim of this study was to compare the cytotoxic effects of clinically relevant cobalt chromium (CoCr) nano-particles with commercial composite ceramic particles.

Composite ceramic particles (commercial BIOLOX® delta powder) were obtained from CeramTec, Germany and clinically relevant CoCr wear particles were generated using a six station pin-on-plate wear simulator. L929 fibroblast cells were cultured with 50µm³ of CoCr wear debris or composite ceramic particles at low to high volumes ranging from 500µm³–0.5µm³ per cell and the cyctotoxic effects of the particles were assessed over a period of 6 days using the ATP-Lite™ cell viability assay.

The composite ceramic particles were bimodal in size (0.1–2µm & 30–100nm) and showed mild cytotoxic effects when compared with equivalent particle volumes (50µm³) of clinically relevant CoCr nano-particles (10–120nm). The CoCr nano-particles had significant cytotoxic effects from day 1, whereas the composite ceramic particles only showed cytotoxic effects at particle concentrations of 50 and 500µm³ after 6 days. The increased cytotoxicity of the clinically relevant CoCr nano-particles may have been attributed to the release of Co and Cr ions.

This study demonstrated the potential cytotoxic effects of model ceramic particles at very high volume concentrations, but it is unlikely that such high particle volumes will be experienced routinely in vivo in such low wearing bearing materials. Future work will investigate the longer-term effects on genotoxicity and oxidative stress of low volumes of clinically-relevant generated BIOLOX® delta ceramic wear particles.

Introduction and Aims

In order to improve the longevity and design of an implant, a wide range of pre-clinical testing conditions should be considered including variations in surgical delivery, and patients' anatomy and biomechanics. The aim of this research study was to determine the effect of the acetabular cup inclination angle with different levels of joint centre mismatch on the magnitude of dynamic microseparation, occurrence and severity of edge loading and the resultant wear rates in a hip joint simulator.

Introduction

Total ankle replacement (TAR) has been used as a surgical intervention for arthritis since the 1970s. However, unlike clinically successful hip and knee replacements, TARs are renowned for extensive contraindications to surgery and high failure rates with an average of 83% survival at 5 years. The majority cite aseptic loosening as the reason for failure. The aim of this study wais to analyse retrieved TARs visually and through interferometry to identify potential the failure mechanisms associated with these devices.

Introduction

Hip replacements are falling short of matching the life expectancy of coxarthritis patients, due to implanting THR in younger patients and due to increasingly active patients. The most frequently implanted hip prostheses use cross linked (XL) polyethylene (PE) on metal bearings in the USA and most of the Western world. Concerns remain in the long term around the potential of wear debris-induced aseptic loosening. Thus exploring lower-wearing alternative bearings remains a major research goal.

PEEK (poly-ether-ether-ketone) is a thermoplastic polymer with enhanced mechanical properties. This study compared the wear of PEEK to the wear of cross linked polyethylene, when sliding against cobalt chrome (CoCr) metallic counterfaces, and compared the wear of carbon-fibre reinforced (CFR)-PEEK to cross linked polyethylene when sliding against metallic and ceramic counterfaces under different contact stresses within the hip joint.

Introduction

Increased wear rates [1, 2] and acetabular rim fracture [3] of hip replacement bearings reported clinically have been associated with edge loading, which could occur due to rotational and/or translational mal-positioning [4]. Surgical mal-positioning can lead to dynamic microseparation mechanisms resulting in edge loading conditions. In vitro microseparation conditions have replicated stripe wear and the bi-modal wear debris distribution observed clinically [5, 6]. The aim of this study was to investigate the effect of steep cup inclination, representing rotational mal-positioning, on the magnitude of dynamic microseparation, severity of edge loading, and the resulting wear rate of a ceramic-on-ceramic bearing, under surgical translational mal-positioning conditions.

Introduction

Numerous risk factors have been identified for patellar tendinopathy (PT), often in small population studies. The aim was to use an online questionnaire internationally to generate a large database and identify significant risk factors.

Summary Statement

The frictional torque of ceramic-on-ceramic bearings tended to increase with increasing the bearings size (32, 48, 56mm). However, the frictional torque was significantly lower than that measured on metal-on-metal bearings under well positioned and well lubricated conditions.

Introduction

Stripe wear, observed on retrieved ceramic hip replacements, has only been replicated in vitro under translational mal-positioning conditions where the centres of rotation of the head and the cup are mismatched^1,2; an in vitro condition termed “microseparation”.

The aim of this study was to compare the edge loading mechanisms observed under microseparation conditions due to translational mal-positioning conditions simulated on two different hip joint simulators.

INTRODUCTION

Ceramic-on-metal hip replacements (COM, where the head is a Biolox Delta ceramic and liner is Co Cr alloy), have demonstrated reduced wear under standard conditions in vitro compared to metal-on-metal (MOM) [1]. Early clinical results are also encouraging [2]. Recently concerns have been raised regarding the poor clinical performance of MOM hip resurfacings [3], particularly when cups are steeply inclined. Laboratory hip simulator testing has been used to replicate edge loading, also demonstrating elevated wear [4]. Therefore, a range of conditions to replicate sub-optimal use clinically to better predict in vivo performance should be used. The aim of this study was to compare the wear rates of MOM and COM under adverse edge loading conditions in an in vitro hip simulator test.

Introduction

Concerns regarding UHMWPE wear particle induced osteolysis in total hip replacement (THR, [1]) have led to alternative materials to be sought. Carbon-fibre reinforced poly-ether-ether-ketone (CFR-PEEK) has shown reduced wear in hip and knee configurations compared with conventional polyethylene [2-4]. The aim of this study was to investigate the wear performance of a ceramic-on-CFR PEEK THR through a simulator study.

INTRODUCTION

Ceramic-on-ceramic hip replacements have generated great interest in recent years due to substantial improvements in manufacturing techniques and material properties¹. Microseparation conditions that could occur due to several clinical factors such as head offset deficiency, medialised cup combined with laxity of soft tissue resulting in a translation malalignment, have been shown to cause edge loading, replicate clinically relevant wear mechanisms^2,3 and increase the wear of ceramic-on-ceramic bearings^3,4. The aim of this study was to investigate the influence of increasing the femoral head size on the wear of ceramic-on-ceramic bearings under several clinically relevant simulator conditions.

INTRODUCTION

Retrieval and clinical studies of metal-on-metal (MoM) bearings have associated increased wear¹ and elevated patient ion levels² with steep cup inclination angles and edge loading conditions. The University of Leeds have previously developed a hip simulator method that has been validated against retrievals and shown to replicate clinically relevant wear rates and wear mechanisms^3,4. This method involves introducing lateral microseparation to represent adverse joint laxity and offset deficiency. This study aimed to investigate the effect of microseparation representing translational malpostion, and increased cup inclination angle, representing rotational malposition, in isolation and combined on the wear of different sizes (28 and 36mm) MoM bearing in total hip replacement (THRs).

Introduction

Despite the lack of robust evidence, numerous different track and trigger warning systems have been implemented. The MEWS (Modified Early Warning Score system) is one such example, and has not been validated in an emergency traumasetting. A considerable proportion of trauma admissions are elderly patients with co-morbidities. Early recognition of physiological deterioration and prompt action could therefore be lifesaving.

In vitro the introduction of microseparation and edge loading to hip simulator gait cycle has replicated clinically relevant wear rates and wear mechanisms in ceramic-on-ceramic bearings^[1], and elevated the wear rates of MoM surface replacements (SR) to levels similar to those observed in retrievals^[2]. The aim was to assess the wear of two different sized MoM total hip replacement bearings under steep cup inclination angles and adverse microseparation and edge loading conditions.

Two tests were performed on the Leeds II hip joint simulator using two different size bearings (28mm and 36mm). Cups were mounted to provide inclination angles of 45 degrees (n=3) and 65 degrees (n=3). The first three million cycles were under standard gait conditions. Microseparation and edge loading conditions as described by Nevelos et al^[1] were introduced to the gait cycle for the subsequent three million cycles. The lubricant was 25% new born calf serum. The mean wear rates and 95% confidence limits were determined and statistical analysis was performed using One Way ANOVA.

Under standard gait conditions, when the cup inclination angle increased from 45 degrees to 65 degrees, the wear of size 28mm bearing significantly (p=0.004) increased by 2.7-fold, however, the larger bearings did not show any increase in wear (p=0.9). The introduction of microseparation conditions resulted in a significant (p=0.0001) increase in wear rates for both bearing sizes under both cup inclination angle conditions. Under microseparation conditions, the increase in cup inclination angle had no influence on the wear rate for both bearing sizes (Figure 1).

With larger bearings, head-rim contact occurs at a steeper cup inclination angle providing an advantage over smaller bearings. The introduction of edge loading and microseparation conditions resulted in a significant increase in wear rates for both bearing sizes. The wear rates obtained in this study under combined increased cup inclination angle and microseparation were half of those obtained when SR MoM bearings were tested under similar adverse conditions^[2]. This study shows the importance of prosthesis design and accurate surgical positioning of the head and acetabular cup in MoM THRs.

Tribology and wear of articular cartilage is associated with the mechanical properties, which are governed by the extracellular matrix (ECM). The ECM adapts to resist the loads and motions applied to the tissue. Most investigations take cartilage samples from quadrupeds, where the loading and motions are different to human. However, very few studies have investigated the differences between human and animal femoral head geometry and the mechanical properties of cartilage.

This study assessed the differences between human, porcine, ovine and bovine cartilage from the femoral head; in terms of anatomical geometry, thickness, equilibrium elastic modulus and permeability.

Diameter of porcine (3-6 months old), bovine (18-24 months old), ovine (4 years old) and human femoral heads were measured (n=6). Plugs taken out of the superior region of each femoral head and creep indentation was performed. The human femoral heads were obtained from surgery due to femoral neck fracture. Cartilage thickness was measured by monitoring the resistive force change as a needle traversed the cartilage and bone at a constant feed rate using a mechanical testing machine. The percentage deformation over time was determined by dividing deformation by thickness. A biphasic finite element model was used to obtain the intrinsic material properties of each plug. Data is presented as the mean ± 95% confidence limits. One-way ANOVA was used to test for significant differences (p < or = 0.05).

Significant differences in average femoral head diameter were observed between all animals, where bovine showed the largest femoral head. Human cartilage was found to be significantly thicker than cartilage from all quadrupedal hips. Human cartilage had a significantly larger equilibrium elastic modulus compared to porcine and bovine cartilage. Porcine articular cartilage was measured to be the most permeable which was significantly larger than all the other species. No significant difference in permeability was observed between human and the other two animals: bovine and ovine (Table 1).

The current study has shown that articular cartilage mechanical properties, thickness and geometry of the femoral heads differ significantly between different species. Therefore, it is necessary to consider these variations when choosing animal tissue to represent human.

Bone marrow lesions (BMLs) have been extensively linked to the osteoarthritis (OA) disease pathway in the knee. Semi-quantitative evaluation has been unable to effectively study the spatial and temporal distribution of BMLs and consequently little is understood about their natural history. This study used a novel statistical model to precisely locate the BMLs within the subchondral bone and compare BML distribution with the distribution of denuded cartilage.

MR images from individuals (n=88) with radiographic evidence of OA were selected from the Osteoarthritis Initiative. Slice-by-slice, subvoxel delineation of the lesions was performed across the paired images using the criteria laid out by Roemer (2009). A statistical bone model was fitted to each image across the cohort, creating a dense set of anatomically corresponded points which allowed BML depth, position and volume to be calculated. The association between BML and denudation was also measured semi-quantitatively by visually scoring the lesions as either overlapping or adjacent to denuded AC, or not.

At baseline 75 subjects had BMLs present in at least one compartment. Of the 188 compartments with BMLs 46% demonstrated change greater than 727mm cubed, the calculated smallest detectable difference. The majority of lesions were found in medial compartments compared to lateral compartments and the patella (Figure 1A). Furthermore, in the baseline images 76.9% of all BMLs either overlapped or were adjacent to denuded bone. The closeness of this relationship in four individuals is shown in Figure 1B.

The distribution of lesions follows a clear trend with the majority found in the patellofemoral joint, medial femoro-tibial joint and medial tibial compartment. Moreover the novel method of measurement and display of BMLs demonstrates that there is a striking similarity between the spatial distribution of BMLs and denuded cartilage in subjects with OA. This co-location infers the lesions have a mechanical origin much like the lesions that occur in healthy patients as a direct result of trauma. It is therefore suggested that OA associated BMLs are in fact no different from the BMLs caused by mechanical damage, but occur as a result of localised disruption to the joint mechanics, a common feature of OA.

The authors entered patients into a randomised trial to compare the results of the use of cemented and cementless acetabular prostheses between 1993 and 1995. The results of mid-term wear studies at average follow up of eight years were reported in the journal in 2004. We now present long-term results to show the eventual fate of the hip replacements under study.

The initial study group of 162 patients was randomly assigned to a modular titanium cup with a polyethylene liner or an all polyethylene cemented cup. All patients received a cemented stem with a 26 mm head and a standardised surgical technique. The polyethylene wear was estimated via head penetration measurement and the mid-term results showed a significantly higher wear rate in the cementless cups compared to the cemented cups (0.15mm/yr vs. 0.07mm/yr p<0.0001). The prediction was that this would lead to a higher rate of aseptic loosening in the cementless group.

Patients have now been re-examined at an average of 15 years with the main emphasis on prosthesis survival. Wear studies were also performed.

There were exclusions from the initial study because of death and reoperation for reasons other than aseptic loosening. The number of patients in this longer-term study had decreased as a result of death and loss to follow up. Revisions for aseptic loosening did not follow the path as suggested by the mid term wear studies. There were five cup revisions in the cemented group and one cup revision in the cementless group for aseptic loosening. No femoral stem was revised for aseptic loosening. Details of the long-term wear studies will be presented and osteolysis rates and extent documented.

Despite the statistically significant difference in wear rates at the mid term, an incorrect prediction of eventual loosening rates was made. The authors believe that there are many factors other than wear rates involved in longevity of fixation. We also believe there are many weaknesses in long term prospective, randomised trials in joint replacement and question whether they are, in fact, level 1 evidence in the age of evidence based medicine.

The search for the ideal bearing surface in Total Hip Replacements continues. The current ‘best’ materials are felt to be combinations of metal, ceramics and cross-linked polyethylene. Laboratory studies suggest that ceramic-on-metal articulations may provide distinct advantages. This study aims to identify the best bearing surface combination with the lowest adverse side effect profile.

Between February 2004 and September 2007, 164 hips were replaced in 142 patients. 39% were male and 69% were female. The average age at surgery was 53 years (17-72 years). Follow-up assessment included radiographs, the Harris Hip Score and whole blood samples for metal ion levels. Complications to date included 3 hips which needed femoral revision because of surgery related factors, and 3 cases of sepsis of which 1 settled and 2 needed revision. One hip needed revision of head and liner to a larger bearing size for recurrent dislocations, and is no longer being followed up for blood metal ions.

Post-operative whole blood metal ion levels were compared to pre-operative levels to determine the increase or decrease in metal ion levels. There were no changes in those patients with ceramic-on-ceramic and ceramic-on-polyethylene articulations. Moderately raised whole blood metal ion levels were noted at 3 months in the ceramic-on-metal group, while the metal-on-metal group show the greatest increase.

This study agrees with laboratory bearing surface wear studies demonstrating lower wear rates in the ceramic-on-metal group compared to the metal-on-metal group. With concerns related to high blood metal ion levels in metal-on-metal articulations, ceramic-on-metal bearing surfaces may well become a bearing surface of choice in the future, but progress needs to be monitored in the longer term.

Statistics New Zealand states “Over the next five decades the 65+ dependency ratio is projected to more than double, from 18 (people aged 65+ years) per 100 (people aged 15 – 64 years) in 2006 to 45 per 100 in 2061. This means that for every person aged 65+ years, there will be 2.2 people in the working-age group in 2061, compared with 5.4 people in 2006 “. This will have a profound impact on health care, specifically in those fractures sustained by the elderly e.g. fractured neck of femur (#NOF).

Also at present little is proven regarding outcome following #NOF in the New Zealand population. These two factors (population change and patient outcome) led to disagreement and healthy debate at the 2008 NZOA ASM. After a pilot study in Dunedin we have examined national electronic records of 52,456 patients presenting with a first admission due to #NOF over the last 20 years.

The mean age at which a patient sustained their first #NOF was around 80 and 71% of these patients have since died. Approximately 10% of patients had a subsequent readmission for #NOF. There was a trend for increasing age over the last twenty years proportional to the increased average age of the general population. One year survival was 75% and mean survival was 3½ years with a third of patients living longer than six years. We found differences in outcome for gender and fracture type (intracapsular vs. extracapsular). Our patients also showed a trend to higher survival risk ratios (i.e. they are clinically “sicker” than they used to be). The incidence of #NOF has increased over the last 20 years with a projected doubling in the number of cases (to 5600 per year) at around 25yrs from now based on the most conservative estimates.

Background: Musculo-skeletal conditions account for at least 10% of General Practice consultations yet the average time spent by UK medical students in orthopaedic and trauma surgery is only 2.7 weeks.

Aim: This study assessed whether a seven-week undergraduate musculoskeletal teaching program introduced in 2006 improved performance.

Methods: This seven week program combined Orthopaedic and Trauma Surgery with Rheumatology and allied specialities. Its main elements were weekly plenary sessions, specialist clinics, a task-based workbook, regular assessments and traditional firm based teaching. The performance of 139 students who attended the new curriculum in its first year of introduction was assessed using multiple choice questions just before their final examinations in 2008 and was compared to that of a control group of students assessed in 2005 prior to program introduction.

Results: The 2008 students showed a 6% improvement in MCQ scores (p< 0.001) over the 2005 graduates. There was no difference between the 2005 students and a second control group of 46 students from 2008, who did not attend the new teaching program.

Conclusion: The program improved performance by providing more focused musculoskeletal training using available resources and by increasing the program duration.

Ceramic-on-metal (ceramic head and metal liner, COM) hip replacements have shown reduced wear in comparison to metal-on-metal (MOM) bearings. This has been attributed to reduced corrosive and adhesive wear, and differential hardness. The study assessed the performance of ceramic and metal bearings in different configurations under adverse conditions, ceramic heads on metal liners (COM) were compared to metal heads on ceramic inserts (MOC), with head on cup rim loading under micro-separation hip joint simulation.

Components used were made of zirconia-platelet toughened alumina (Biolox Delta) and CoCrMo alloy. Hip simulator testing applied a twin-peak loading cycle and walking motions with the prosthesis in the anatomical position. Testing was conducted in calf-serum for 2-million cycles. A standard simulator cycle was adapted, the head sub-luxed in the swing-phase forcing the head onto the cup rim at heel strike.

The overall mean wear rate for the MOC bearings (0.71±0.30mm3/Mc) was significantly higher than the wear rate for the COM bearings (0.09±0.025mm3/Mc). The contact of the head against the rim of the cup caused deep stripe wear on the metallic heads of the MOC bearings. This region on the head is exposed to high stress conditions and susceptible to damage in edge contact, the effect of this is increased when the cup is a harder material than the head. The wear of a metal-on-metal (MOM) couple under similar conditions was almost two-fold greater than the MOC couple (1.58mm3/Mc, Williams et al., 2006) providing further evidence of the reduced wear with COM in comparison to MOM.

The COM concept allows thin metal shells to be used with larger ceramic heads and protects against ceramic liner chipping. COM bearings are undergoing clinical trials, early data suggests reduced metal ion release in patients compared to metal-on-metal.

Background: Muscle tears and injuries are a huge problem throughout the world. Ways of reducing these injuries are welcome, with warm-up and stretching of muscles prior to use established methodologies. Forces associated with muscles can be thought of as active (stimulated muscle: actin-myosin) and passive (relaxed muscle: elastic proteins and connective tissue). In muscle tears, the connective tissue component is damaged, but there is very little information in the literature on this component of the muscle.

Objective: To examine passive (elastic) components in muscle during impact loading at differing temperatures. In particular to test the hypothesis that the connective tissue component fails at different loads according to the temperature.

Methods: Gastrocnemius and Soleus were isolated from 36 male rat limbs, clamped and exposed to increasing impact loads, by dropping a known weight from increasing heights. Muscle was given one minute to recover before an increased force was applied. Temperature was varied from 17 C to 42 C (to encompass the physiological range) in 5 C increments. The height of drop causing non-recoverable deformation, and the maximum deceleration of the weight (measured using an accelerometer attached to a picoscope) at a constant height was recorded for each temperature.

Results: The energy to failure, i.e. the point at which non-recoverable deformation occurred was found to increase above 32 C (p < 0.01) and the maximum deceleration at impact found to have a downward trend with increasing temperatures. At 17 C, the energy to failure was 317.7 ± 20 mJ, At 22 C, the energy to failure was 301.8 ± 29 mJ, At 27 C, the energy to failure was 317.7 ± 40 mJ, At 32 C, the energy to failure was 333.5 ± 21.2 mJ, At 37 C, the energy to failure was 460.2 ± 15.8 mJ, At 42 C, the energy to failure was 619.5 ± 21.2 mJ,

Conclusions: Muscle was shown to act in an increasingly elastic nature with temperature. At higher temperatures a larger energy is required to deform the muscle permanently, and the muscle decelerates more slowly, both in keeping with elastic properties. The same energy at a lower temperature causes significant deformation within the muscle. This has numerous clinical implications, as the temperature at which this change occurs is encountered during surgery and also by sportsmen on outdoor pitches. More research is required to look at the passive components within muscles in humans.

Ceramic-on-metal (COM) bearings have shown reduced wear and friction compared with metal-on-metal (MOM) bearings in-vitro. Lower wear has been attributed to a reduction in corrosive wear, smoother surfaces, improved lubrication and differential hardness reducing adhesive wear. Clinical studies have also shown reduced metal ion levels in-vivo compared with MOM bearings. The aim of this study was to examine two explanted COM bearings (one head and cup, one head only), and to assess the effect of in-vivo changes on the wear performance of the COM bearings by comparing the wear of the explanted bearings with three new COM implants in a hip wear simulator.

Two 28mm diameter COM bearings were provided for analysis. These were visually examined and surface profilometry was performed using a 2-D contacting profilometer (Form Talysurf, Taylor Hobson, UK). Scanning electron microscopy was used to image the regions of transfer on the ceramic heads, and EDX to assess the transfer composition (Philips XL30 ESEM).

Hip simulator testing was conducted for 2 million cycles (Mc) comparing the explanted bearings with three new 28mm COM bearings. Tests were performed in a Prosim simulator (SimSol, UK), which applied a twin peak loading cycle, with a peak load of 3kN. Flexion-extension of − 15 to 30 degrees was applied to the head and internal-external rotation of +/− 10 degrees was applied to the cup, components were mounted in the anatomical position. The lubricant was 25% (v/v) calf serum supplemented with 0.03% (w/v) sodium azide and was changed approximately every 0.33Mc. Wear was measured gravimetrically at 0.5, 1 and 2 Mc.

Regions of material transfer, identified on both ceramic explant heads, were shown to be CoCr material by EDX analysis, suggesting metallic transfer from the metal cup. Profilometry traces across metallic transfer showed comparable surface roughness measurements compared to unworn material.

The overall mean wear rate for the new COM bearings at 2Mc was 0.047 ± 0.06mm3/Mc. The mean wear rate for the explanted head articulated with a new cup was slightly lower at 0.034mm3/Mc. The mean wear rate for the explanted head and cup was highest at 0.15mm3/Mc. It was noted that the explanted head/cup had higher bedding in wear compared with the other bearings, but still significantly less than a new MOM bearing (mean bedding-in wear rate 2.03 ± 2.59 mm3/Mc). The steady-state wear was comparable with the new bearings. As the orientation of these implants in-vivo was unknown, it is proposed that the elevated wear during bedding-in of the explanted head/cup bearing may be due to the alignment of the components. The wear rates of the explanted ceramic head against a new cup were comparable with the new bearings, suggesting that the presence of metallic transfer on the ceramic head does not adversely affect the wear behaviour of COM bearings.

The combination of a ceramic head articulating against a metal acetabular liner (CoM) has shown reduced metal ion levels compared with a metal-on-metal bearing (MoM) in hip simulator studies. A randomized prospective clinical trial was undertaken using CoM and MoM bearings in an otherwise identical total hip procedure. The initial clinical results were encouraging. This report comprises a further review of metal ion data.

Patients received identical components with the exception of the bearing surface material but all were 28mm diameter. All components were supplied by DePuy International Ltd. Patients were assessed pre-operatively, 3m, 12m and > 24m (median 32m). Whole blood samples were collected at regular follow-ups, frozen and analysed in batches using high resolution Inductively Coupled Plasma – Mass Spectrometry (ICP-MS). All recruited patients are included irrespective of outcome. However some patients failed to attend specific follow-ups and some contaminated samples had to be discarded. Statistical significance was analyzed using a non-parametric comparison (Mann-Whitney test). After 3m and 12m implantation there were between 21 and 24 patients available for analysis in both the CoM and MoM cohort and after > 24m point 10 and 9 respectively.

There were four outliers (either Cr or Co > 10ug/l) in both the CoM and MoM groups. In common with previous studies (with the exception of two marginal outliers), these were related to component position. They were implanted with either a cup abduction angle of > 55°, an anteversion angle of > 30° or both. Other studies with the same design of component have reported no significant outliers.

The median Cr and the Co levels are lower with the CoM bearing compared with the MoM at all measurements points following implantation. The median background (pre-operative) levels for the combined CoM and MoM group were Cr: 0.22ug/l and Co: 0.49ug/l. These were significantly different (p=0.006).

In the CoM group, the median 12m Cr and Co values were 0.43ug/l and 0.72ug/l respectively. The comparable values for MoM are 0.68ug/l and 0.83ug/l. Increases in metal ion levels from pre-operative levels are used as the primary ion level outcome in this study because the background level will comprise of the order of 30–50% of the overall value. The increase in Cr for CoM and MoM from pre-op levels to 12m significantly different for Cr (p=0.015). It has a lower significance for combined metal ion levels (p=0.029). This difference in not significant for Co (p=0.195).

In agreement with predictions from hip simulator studies, CoM bearings in this study produced lower levels of metal ions than comparable MoM bearings at all time points. However the difference is less than that predicted in the laboratory and is much more pronounced with Cr than with Co.

Introduction: This observational study assessed the influence of obesity on operating time and duration of hospital admission following Total Knee Arthroplasty (TKA).

Materials and Methods: 263 patients who underwent 276 TKAs between 1st January and December 31st 2005 at the Glenfield General Hospital were identified from the Trent (and Wales) Arthoplasty Audit Group. Patients were grouped into three weight categories based upon BMI. We examined hospital records for 265 of the 276 procedures and compared operating time, length of hospital admission and complication rates between the three BMI groups. Patient perceived outcomes including patient satisfaction, post-operative pain and frequency of walking were compared at 1-year post TKA.

Results: Obesity did not adversely influence operating time and duration of hospital stay. The mean operating time was 82 minutes in patients with a BMI ≤ 25.0, 84 min in those with a BMI 25.1–30.0 and 88 minutes for those with a BMI> 30.0 (p=0.2). The mean hospital stay was 7.7 days in patients with a BMI ≤ 25.0, 7.2 days in the BMI 25.1–30.0 group, and 6.7 days in those with a BMI > 30.0 (p=0.8).

There were no significant differences between the three BMI groups and post-operative complications (p = 0.7), patient satisfaction (p=0.1) or pain levels (p=0.7) at 1-year post-TKA. As has been demonstrated previously, increasing BMI negatively influenced post operative walking frequency (p=0.02)

Conclusion: BMI did not influence operating time, length of stay, complication rates, post operative pain and patient satisfaction post Total Knee Arthroplasty, but was associated with decreased post operative mobility.

Ceramic-on-metal (ceramic head and metal liner, COM) hip replacements have shown reduced wear in comparison to metal-on-metal (MOM) bearings (Firkins et al., 1999). This has been attributed to a reduction in corrosive wear, differential hardness and a reduction in adhesive wear. In a clinical report on the use of a metal-on-ceramic hip replacement (Valenti et al., 2007) which consisted of a stainless steel head and alumina ceramic insert at revision 6-months post-op massive metallosis and macroscopic wear was observed.

The aim of this study was to assess the performance of ceramic and metal bearings in different configurations under adverse conditions, ceramic heads on metal liners (COM) were compared to metal heads on ceramic inserts (MOC), with head on cup rim loading under micro-separation hip joint simulation.

Components used were made of zirconia-platelet toughened alumina (Biolox Delta) and high carbon (0.2wt%) CoCrMo alloy (DePuy International Ltd, UK). Hip simulator testing applied a twin-peak loading cycle and walking motions with the prosthesis in the anatomical position. The lubricant (25% calf-serum) was changed every 0.33Mc, wear was measured gravimetrically. Testing was conducted for 2-million cycles, a standard simulator cycle was adapted so the head subluxed in the swing phase forcing the head onto the cup rim at heel strike (Williams et al., 2006).

The total overall mean wear rate for the MOC bearings (0.71±0.30mm3/Mc) was significantly higher than the wear rate for the COM bearings (0.09±0.025mm3/Mc). The contact of the head against the rim of the cup at heel strike caused deep stripe wear on the metallic heads of the MOC bearings. This region on the head is exposed to high stress conditions and susceptible to damage in edge contact, the effect of this is increased when the cup is a harder material than the head. The wear of a metal-on-metal (MOM) couple under similar conditions was almost two-fold greater than the MOC couple (1.58mm3/Mc, Williams et al., 2006) providing further evidence of the reduced wear with COM in comparison to MOM.

The explant described Valenti et al. included a stainless steel head, this is a softer material compared to CoCr, and wears at a higher level. It can be postulated that the wear under adverse conditions would be further increased.

The COM concept can provide increased design flexibility; thin metal shells can be used with larger ceramic heads. Additionally the design protects against ceramic liner chipping. COM bearings are currently undergoing clinical trials, early data suggests reduced metal ion release in patients with COM bearings compared to metal-on-metal (Williams et al., 2007).

Aseptic loosening caused by UHMWPE wear debris induced osteolysis is a major cause of revision in total hip arthroplasty (THA)¹. While second generation hard-on-hard bearings, metal-on-metal (MOM) and ceramic-on-ceramic (COC), have been shown clinically to address the wear issues associated with conventional UHMWPE bearings, there remain some concerns over the potential effects of metal ions produced by MOM and the risk of liner fracture in COC. Recently, hybrid ceramic-on-metal (COM) articulation has received a great deal of attention as a promising alternative bearing.

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 elsewhere².

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.

Introduction: The proximal femur is the most common long bone to be affected by metastatic disease. The prognosis of patients with bone metastases is variable and depends on a number of factors. Risk factors affecting survival in this subgroup of patients need to be clearly determined.

Patients and Methods: We studied the survival of 32 consecutive patients with metastatic disease of the proximal femur in order to identify any clinical, radiological or physiological parameters that predict survival.

Results: Mean age of patients was 71 years (range 41–91 SE 2.2) and mean survival 393 days (95 % CI 236 to 550), cumulative survival at three years was 0.12. Univariate analysis showed that sex, serum haemoglobin < 10gm/dl, white cell count > 12, serum urea and the presence of a postoperative complication were all significant predictors of survival. However, multivariate analysis revealed that only sex, serum haemoglobin < 10 gm/dl, white cell count > 12 and a postoperative complication were independent risk factors predicting survival (p < 0.05).

Discussion: Perioperative factors should be optimised for all patients. Identification of risk factors adversely affecting survival can be used in conjunction with clinical and radiological information as a tool to predict outcome and to aid consenting and counselling of patients. Simple measures such as preoperative transfusion of blood or the administration of blood products may improve survival. The presence of an elevated white cell count may indicate more advanced systemic disease. Aggressive postoperative rehabilitation and the prevention of complications may also improve survival.

Metal-on-metal hip resurfacing has been introduced recently, due to its potential advantages of biomechanics and biotribology. However, a number of problems have been identified from clinical retrievals, including significant elevation of wear when the implant is mal-positioned. Our hypothesis is that implant mal-position and micro-lateralisation can result in edge contact, leading to increases in wear. The aim of this study was to investigate the combined effect of cup position and micro-lateralisation on the contact mechanics of metal-on-metal hip resurfacing prosthesis, in particularly to identify conditions which resulted in edge contact.

Finite element (FE) method was used. A generic metal-on-metal hip resurfacing prosthesis was modelled. The bearing diameters of the femoral head and acetabular cup components were 54.49mm and 54.6mm respectively, with a diametral clearance between the head and the cup of 0.11mm. The resurfacing components were implanted into a hemi-pelvic hip joint bone model and all the materials in the FE model were assumed to be homogenous, isotropic and linear elastic (Udofia et al 2007). The FE models consisted of approximately 80,000 elements, which were meshed in I-DEAS (Version 11, EDS, USA) and solved using ABAQUS (Version 6.7-1, Dassault Systèmes). For this study, the femoral component was fixed with an inclination angle of 45° and an anteversion angle of 10°. The orientation of the acetabular cup was varied, using inclination angles of 35° and 65°, and anteversion angles between −10° to 30°. Contact at the bearing surface between the cup and femoral head was modelled using frictionless surface-based elements, simulating a fully lubricated situation, as coefficients of friction less than 0.1 would not have appreciable effects on the predicted contact mechanics. The femoral component was fixed into the femur (except the guide pin) using PMMA cement with an average thickness of approximately 1mm. The other contact interfaces in the FE model (cup/acetabulum, cement/bone and cement/femoral component) were all assumed to be rigidly bonded. The hip joint model was loaded through a fixed resultant hip joint contact force of 3200N, and was applied through medial, anterior muscle forces and subtrochanteric forces to simulate the mid-to-terminal stance phase (approximately 30% – 50%) of the gait cycle (Bergmann et al., 1993). Micro-lateralisation was modelled through displacing the femoral head laterally, up to 0.5mm, relative the centre of the cup.

Edge contact was detected once the inclination angle became greater than 65°. The effect of ante-version was to further shift the contact area towards the edge of the cup, nevertheless no edge contact was found for ante-version angles up to 25° and inclination angles below 55°. However, when the micro-lateralisation was introduced, edge contact was detected at a much smaller inclination angle. For example, even with a micro-lateralisation of 0.5 mm, edge contact occurred at an inclination angle of 45°. This study highlights the importance of surgical techniques on the contact mechanics and tribology of metal-on-metal hip resurfacing and potential outcome of these devices.

Ceramic head and metal liner hip replacements (COM) have shown reduced wear in comparison to metal-on-metal (MOM) bearings. The aim of this study was to further assess the performance by a wear simulator study under standard and adverse conditions, including the wear of a metal head against a ceramic liner.

Components were Biolox Delta and CoCrMo alloy. Hip simulator testing applied a simplified walking cycle to anatomically mounted prostheses. The lubricant was 25% calf-serum and wear was measured gravimetrically. In hip simulator testing with edge loading a standard cycle was adapted so the head sub-luxed in the swing phase forcing the head onto the edge of the cup at heel strike, this was applied to ceramic on metal and metal on ceramic material combinations.

Under standard conditions the total overall mean wear rate of the MOM THR (1.01±0.38mm3/Mc) was significantly higher in comparison to the COM and COC (< 0.015mm3/Mc). The overall mean wear rate for the MOC bearings (0.71±0.30mm3/Mc) was significantly higher than the wear rate for the COM bearings (0.09±0.025mm3/Mc). The contact of the head against the rim of the cup caused deep stripe wear on the metallic heads of the MOC bearings. This region on the head is exposed to high stress conditions and susceptible to damage in edge contact, the effect of this is increased when the cup is a harder material than the head. The wear of a metal-on-metal (MOM) couple under similar conditions was almost two-fold greater than the MOC couple (1.58mm3/Mc, Williams et al., 2006) providing further evidence of the reduced wear with COM in comparison to MOM.

Reduced wear from COM bearings will address some concerns associated with MOM THRs regarding reports of elevated ion levels clinically. These studies have provided valuable data demonstrating reduced wear with COM bearings. COM bearings are undergoing clinical trials, early data suggests reduced metal ion release in patients compared to metal-on-metal

A randomised prospective study of four bearing surfaces in hip replacements is being conducted. The primary objective is to identify the best long term bearing surf ace clinically and radiographically, and metal ion levels have been measured in all cases.

Patients have been randomised to the four bearing surfaces viz. Ceramic-on-XLinked Polyethelene, Ceramic-on-Ceramic, Metal-on-Metal and Ceramic-on-Metal. Pre-operative blood samples and follow-up blood samples for metal ion analysis using ICP-MS method have been taken in all patients. As at February 2008 187 patients have been recruited, and metal ion levels at one year are available in 52 patients.

Metal ion levels are not increased with Ceramic-on-XLPE or Ceramic-on-Ceramic bearings. At one year follow-up the metal ion levels in Ceramic-on–Metal bearings is half that of Metal-on-Metal bearings using mean levels, and one third using median levels. Of note is that chromium levels in Ceramic-on-Metal bearings is the least elevated.

Due to the laboratory evidence that ceramic-on-metal bearings have the best surf ace wear characteristics with no head stripe wear on a ceramic head, and the laboratory and clinic al evidence of lower metal ion levels, Ceramic-on-Metal hip replacements could be one of the bearing surfaces of the future.

A randomised prospective study of 4 bearing surfaces in hip replacements is being conducted. The primary objective is to identify the best long term bearing surface clinically and radiographically, and metal ion levels have been measured in all cases.

Patieents have been randomised to the 4 bearing surfaces viz. Ceramic on XLinked Poly, Ceramic on Ceramic, Metal on Metal and Ceramic on Metal. Pre operative blood samples and follow up blood samples for metal ion analysis using the ICPMS method have been taken in all patients. As at February 2008 187 patients have been recruited, and metal ion levels at 1 year are available in 52 patients.

Metal ion levels are not increased with Ceramic on XLinked Poly or Ceramic on Ceramic bearings. At 1 year follow up the metal ion levels in Ceramic on Metal bearings is half that of Metal on Metal bearings using the mean levels, and one third using the madian levels. Of note is that the chromium levels in Ceramic on Metal bearings is the least elevated.

Due to laboratory evidence that Ceramic on Metal bearings have the best surface wear characteristics with no head stripe wear, and laboratory and clinical evidence of lower metal ion blood levels, Ceramic on Metal hip replacements could be a bearing surface of the future.

This study reports on ceramic-on-metal (CoM) bearings in total hip replacement. Whole blood metal ion levels were measured. The median increase in chromium and cobalt at 12 months was 0.08 μg/1 and 0.22 μg/1, respectively, in CoM bearings. Comparable values for metal-on-metal (MoM) were 0.48 μg/1 and 0.32 μg/1. The chromium levels were significantly lower in CoM than in MoM bearings (p = 0.02). The cobalt levels were lower, but the difference was not significant. Examination of two explanted ceramic heads revealed areas of thin metal transfer. CoM bearings (one explanted head and acetabular component, one explanted head and new acetabular component, and three new heads and acetabular components) were tested in a hip joint simulator. The explanted head and acetabular component had higher bedding-in. However, after one million cycles all the wear rates were the same and an order of magnitude less than that reported for MoM bearings. There were four outliers in each clinical group, primarily related to component malposition.

Ceramic on metal bearings for hip replacement have shown reduced friction, wear, and metal ion levels in comparison to metal on metal bearings. Lower wear has been attributed to a reduction in corrosive wear, smoother surfaces and improved lubrication and differential hardness and reduction in adhesive wear. The aim of this study was to further assess the performance of novel differential hardness COM THRs in two different bearing configurations. The effect of bearing configuration was examined by comparing COM (ceramic head on metal liner) with metal-on-ceramic (metal head on ceramic liner) (MOC) bearings in micro-separation hip simulator testing.

Components used were zirconia-platelet toughened alumina (Biolox Delta) heads and high carbon (0.2wt%) CoCrMo alloy cups, tested in COM and MOC configurations, (, (DePuy International Ltd, UK). Micro-separation hip simulator testing was conducted for 2 million cycles (Mc) using a Prosim hip simulator (SimSol, UK), which applied a twin peak loading cycle and walking motions with the prosthesis positioned in the anatomical position. The lubricant (25% calf serum) was changed approximately every 0.33Mc and wear was measured gravimetrically. A negative force was applied to the head during the swing phase, to produce a joint laxity and head – rim contact, the head relocated in the stance phase.

The total overall mean wear rate for the MOC bearings (0.71±0.30mm3/Mc) was significantly higher than the wear rate for the COM bearings (0.09±0.025mm3/ Mc). The contact of the head against the rim of the cup at heel strike caused deep stripe wear on the metallic heads of the MOC bearings. Stripes of metallic transfer were visible on all the ceramic heads (COM bearings). The COM bearings had much lower wear rates than the MOC bearings under harsh micro-separation conditions. This suggests that the head in a differential hardness bearing should be the harder material. The COM concept also provides increased design flexibility; thin metal shells can be used with larger ceramic heads. Additionally the design protects against ceramic liner chipping. COM bearings are currently undergoing clinical trials.

Large diameter metal-on-metal (MOM) bearings are becoming increasingly popular for young, active patients. Clearance is a particularly important consideration for designing MOM implants, considering historical experience of equatorial contact and high frictional torque. Lubrication theory predicts increasing the clearance will result in diminished lubrication, resulting in increased friction and wear. Clinical cases of transient squeaking in patients with resurfacing bearings have been noted in recent years, with some reporting an incidence of up to 10% between 6 months and 2 years post-implantation. This study aimed to investigate the impact of increasing clearance on the lubrication, friction and squeaking of a large diameter metal-on-metal resurfacing bearing through frictional studies.

Clinical-grade MOM implants of 55mm diameter and 100μm diametric clearance, and custom-made, 55mm bearings with diametric clearances of approximately 50μm and 200μm (DePuy International Ltd) were tested in a friction simulator. Components were inverted with a flexion-extension of ±25o applied to the head and lubricated with 25% and 100% newborn bovine serum. A peak load of 2kN, with swing-phase loads of 25N, 100N and 300N were applied.

Sound data was recorded during each friction test using a MP3 recorder and pre-amplifier. A microphone was set up at a distance of 50mm from the implant, and data recorded over a minimum of 10 seconds where sound was generated. Sound data was assessed through narrow band analysis on Frequency Master software (Cirrus Research, UK).

Lubrication was assessed by directly measuring the separation between the head and cup during the test cycle by ultrasonic methods (Tribosonics, UK). An ultrasound sensor was bonded to the back of the cup and reflection measurements were taken during the friction tests with a sampling rate of 100Hz. Using equations which related reflection coefficient to lubricant properties and thickness, values for the film thickness were calculated.

The surface replacement with the largest clearance yielded the highest friction factor for each test condition. The difference between the large clearance bearing and the smaller clearance samples was statistically significant in 25% bovine serum, the more clinically relevant lubricant (ANOVA, p< 0.05). The 50μm clearance group yielded similar results to the 100μm clearance bearing, although a slight increase in friction was observed.

Squeaking occurred during every test in the large clearance group. There was a reduced incidence of squeaking in the smaller clearances, with the lowest incidence observed in the 100μm clearance group.

The smallest separation of the head and cup was observed within the large clearance bearings. The best lubrication condition measured ultrasonically was observed within the 100μm clearance bearing. There appeared to be good correlation between friction, lubrication and the incidence of squeaking. This study suggests a large diametric clearance results in reduced lubrication, increased friction and an increased incidence of squeaking. However, there is a minimum diametric clearance that can be tolerated, as clearance must accommodate the manufacturing tolerance.

Different wear rates have been reported for ceramic-on-ceramic (COC) and metal-on-metal (MOM) hip replacements tested in simulators with different loading conditions and lubricants. We postulate that differences in wear rates may be associated with changes in lubrication and friction in the joint. This study aimed to compare the friction of COC and MOM bearings under different lubrication regimes, simulated by varying swing-phase loads and lubricants.

Alumina COC and CoCr MOM 28mm-diameter bearings were studied in a pendulum friction simulator. Flexion-extension of +/−25 degrees was applied to the head, a peak load of 2kN and swing-phase loads of 25N,100N, 300N used. Lubricants used included water, 25% and 100%-bovine serum.

COC and MOM bearings showed increased friction as the swing-phase load increased. COC bearings produced higher friction in 100%-serum compared to 25%-serum. In contrast, friction was lower when MOM bearings were tested in 100%-serum compared to 25%-serum. When COC bearings were tested in water, the friction decreased in comparison to testing in serum, however, MOM friction was higher in water.

Increasing the swing-phase load reduced the thickness of the fluid-film in the stance-phase and this increased friction. The increase in friction when COC bearings were tested in 100%-serum (compared to 25%) may be due to the increased forces required to shear the increased concentration of proteins, similarly friction is reduced in water. MOM bearing friction was reduced in 100%-serum, in this instance increased proteins may be acting as solid-phase lubricants, and similarly MOM friction increased in water.

There is currently much interest in the wear of metal-on-metal THRs and potential concerns about elevated metal ion levels. Generally, wear of metal-on-metal THR’s has been low in simulator studies. Slightly higher and more variable wear has been found clinically. Variations in surgical approach, technique and fixation method may influence the level of force applied across the prosthesis during gait. It is hypothesised that increased joint tensioning may increase loading of THR’s during the swing-phase; leading to elevated wear and friction due to depleted fluid film lubrication. This study aimed to assess the effect of swing-phase load on the friction, lubrication and wear of metal-on-metal THR’s.

Cobalt-chrome 28mm metal-on-metal THR’s were tested in a physiological hip simulator, loading was modified to provide; (1) ISO swing-phase load (280N, as per ISO 14242-1) and (2) low swing-phase load (< 100N). Friction testing was conducted using a pendulum friction simulator, with 280N and 100N swing-phase loads. Theoretical lubrication modelling was carried out using elastohydrodynamic lubrication theory.

The overall mean volumetric wear rates was 10-times greater in THR’s tested with an ISO swing-phase load in comparison to THR’s tested with low swing-phase loads (0.58±0.49 compared to 0.06±0.039mm³/million cycles). The friction factors were 0.129 and 0.173 respectively under low and ISO swing-phase conditions. A decrease in the predicted lubricant film thickness when the swing-phase load was increased was observed; at the start of stance phase this was 0.12microns and 0.07microns under low and ISO swing-phase conditions respectively.

The results demonstrate that the performance of metal-on-metal THR’s is highly dependent on swing-phase load conditions. It is postulated that fixation method and surgical technique can affect the swing-phase load. This study has demonstrated that over-tensioning of the tissues may also accelerate wear. These observations may explain some of the variations reported clinically.

Introduction and Aims: Zirconia heads were introduced into hip replacement surgery with in vitro reports of lower wear on polyethylene compared with metal. Over 400,000 zirconia heads have been implanted worldwide and yet clinical studies have varied in their results. The aim of this study was to compare the radiological wear rate of zirconia and stainless steel heads on polyethylene.

Method: Between March 1998 and August 2001, 186 patients having a hip replacement by one of the two authors were entered into a randomised trial to receive either a 26mm stainless steel head, a 28mm stainless steel head or a 28mm zirconia head. In all patients a posterior approach and a cemented Exeter femoral stem were used. One surgeon used a cementless cup in patients under 65. All other patients received an all-polyethylene cemented cup. An upper age limit of 75 was chosen to hopefully allow at least 10 years follow-up. The penetration of the heads into the polyethylene was then assessed by the modified Livermore technique.

Results: Sixty-seven hips were randomised to receive a 26mm metal head, 58 a 28mm metal head and 61 a 28mm ‘HIPed’ zirconia head. The average radiological follow-up was 48 months. The penetration rate on the all-polyethylene cup for the 26mm metal head was 0.06mm/yr. This was equivalent to a previous study using the same components. This compared with a penetration rate of the 28mm metal head of 0.07mm/yr and the 28mm zirconia head of 0.13mm/yr. There was considerable variation in the latter, which is consistent with the theory that some zirconia heads may undergo phase transformation that in turn increases the surface roughness.

Conclusion: Although we have only a short follow-up, this study already shows a statistically significant difference in the penetration rates produced by zirconia femoral heads on polyethylene when compared with stainless steel heads. This agrees with several other clinical studies, but to our knowledge is the first reported randomised trial.

Introduction: Following hip replacement surgery the tension of the soft tissues and the laxity of the joint may vary. Variations in surgical approach, technique and fixation method may influence the effective joint laxity and the level of force applied across the prostheses during the swing phase of gait. The aim of this study was to investigate the effect of different swing phase load conditions on the wear metal-on-metal hip prostheses using a hip simulator.

Methods: Cobalt chrome metal-on-metal bearings, 28mm in diameter were tested for five million cycles in a Prosim hip simulator with flexion-extension and internal-external rotation kinematic inputs. A Paul-type twin peak loading curve was applied, which was modified to provide three different swing phase load conditions;

Low positive swing phase load (< 100N)

All tests were carried out in 25% (v/v) new-born bovine serum, with gravimetric wear measurements completed every million cycles.

Results: The wear rates for the different swing phase conditions are shown in Figure I. Elevating the swing phase load from 100N to 300N (ISO load) increased the overall wear rate by 10-fold. Introducing microseparation into the gait cycle increased wear by a further 3-fold. These results indicate the sensitivity of metal-on-metal bearing wear to swing phase load conditions and joint laxity.

Discussion: Little attention to date has been paid to the importance of joint laxity and swing phase load on the wear rate of hip replacements. Elevation of wear rates with increased swing phase load was probably due to the depletion of fluid film lubrication. This was consistent with the findings under stop-start motion [Medley et al., 2002] and demonstrates the dependency of metal-on-metal hip replacements on fluid film lubrication conditions. Testing with a negative swing phase load elevated wear due to microseparation of the components, the head contacted the insert rim at heel strike which caused a stress concentration and damage to the insert rim. The results demonstrate that the wear performance of metal-on-metal hip replacements is highly dependent on swing phase load conditions. It is postulated that the fixation method and surgical technique can effect the swing phase load; over tensioning of the soft tissue may increase the swing phase load, whereas joint laxity will cause a negative swing phase load and possibly microseparation.

Introduction: Estimates suggest that 50% of new cases of invasive cancer diagnosed each year will eventually metastasise to bone. The proximal end of the femur is the most common site of long bone involvement by metastatic disease. Accepted principles for the treatment of metastatic disease of the proximal femur have been published. The results of 31 consecutive patients treated with a long intramedullary hip screw for metastatic disease of the proximal femur are reported.

Patients and Methods: Retrospective case note review of all patients that had a long intramedullary hip screw for metastatic disease affecting the proximal femur over a four-year period 1998–2002.

Results: The case notes of 31 patients (33 femurs) were reviewed. There were 21 females and 12 males with a mean age of 71 years. 31 femurs were Zickel group Ia or Ib, the remaining 2 were impending pathological fractures (Zickel group II) that were fixed prophylactically.

Post operatively all patients were allowed to fully weight bear. 70 % of patients regained their initial level of mobility or increased their level of dependence by a factor of one. Mean hospital stay was 20.8 days (mode 7 days). Patients that died post operatively had a mean survival of 299 days (range 2–1034). Those patients that were still alive at the last follow up had a mean survival of 475 days (range 7–1384). There were no cases of fixation or implant failure. There was one case of deep infection that was treated by implant removal.

Conclusion: On the basis of these findings, the long intramedullary hip screw fulfils the principles for treatment of metastatic disease and can be recommended for the treatment of pathological or impending pathological fractures of the proximal femur.

Following total hip replacement surgery, fluroscopy studies have shown that a mean separation of 2 mm can occur between CoCr femoral heads and UHMWPE acetabular cups during the swing phase of gait [1]. In vivo and in vitro studies [2, 3] of alumina ceramic on ceramic hip replacements have demonstrated that swing phase microseparation followed by the impact of the femoral head on the acetabular insert rim can lead to accelerated wear. However, wear remains low. A similar trend was observed when metal on metal hip replacements were tested under microseparation conditions [4]. The purpose of the current study was to examine the wear of ceramic on polyethylene bearings under standard and microseparation conditions.

A physiological hip simulator was used, loads and motions were applied to approximate in vivo conditions. The alumina ceramic heads and polyethylene cups were 28 mm in diameter and were tested for 5 million cycles in 25% new born calf serum at 1 Hertz. Microseparation was achieved by displacing the femoral head inferiorly during swing phase, where the head contacted the inferior cup rim and was laterally displaced. On heel strike the head contacted the superior cup rim prior to relocation.

The volumetric wear of the polyethylene inserts was approximately four times less under microseparation conditions (5.6 ± 5.3 mm3 per million cycles), in comparison to standard conditions (25.6 ± 5.3 mm3 per million cycles). Deformation of the cup rim was observed, but some of this was attributed to creep. It is postulated that this reduction in wear was due to the separation of the components in swing phase improving the entrapment of lubricant, hence reducing wear via a squeeze film lubrication mechanism. In conclusion, surgical procedures that produce a small and controlled amount of joint laxity and microseparation may lead to a reduction in wear of the polyethylene acetabular cups.

The aims of the study were to determine the prevalence of post thrombotic syndrome following lower limb arthroplasty in patients who did not receive chemical thromboprophylaxis, and identify morbidity associated with the condition. From the Trent regional arthroplasty database patients 5 years post elective total knee or hip replacement were identified. All patients were under the care of two senior orthopaedic consultants who performed surveillance venography between the 7th and 10th postoperative day. Prophylaxis for DVT was in the form of below knee compression stockings. Above knee DVTs were anticoagulated, below knee compression stockings was administered for below knee DVT.

Two doctors conducted a clinical review in the manner suggested by the American Venous Forum Executive Committee. Clinical examination was used to assess the presence and class of post-thrombotic syndrome. 71 patients were reviewed. With respect to the ipsilateral limb, there were 32 patients with a DVT and 39 without DVT. Six classes excluding normal, class 0, are used to describe the clinical findings. 17 (24%) patients had no visible sign of venous disease (class 0). 14 (20%) patients suffered minor venous disease (class 1). 28 (39%) patients had established varicose veins (class 2). 2 (3%) patients had oedema without skin changes (class 3). 8 (11%) patients had skin changes ascribed to venous disease ie. pigmentation, venous eczema, lipodermato-sclerosis (class 4). 2 (3%) patient had skin changes as defined with healed ulceration. No patient had skin changes as defined previously with active ulceration (class 6).

Minor venous disease (classes 1 & 2) is common and seen in 44% of DVT negative and 78% of DVT positive patients in this study. Severe venous (classes 3,4 & 5) disease is uncommon and seen in 20% of DVT negative and 13% of DVT positive patients in the study. Symptomatic patients were equally distributed between DVT positive and negative groups. Severe venous disease was more common in the DVT negative group. Concluding, minor venous disease is common post lower limb arthroplasty, severe disease occurred in 17% of patients and appeared unrelated to a previous DVT. The presence of a DVT does not influence the development of skin changes of post-thrombotic limb.

In vivo and in vitro studies of ceramic on ceramic (COC) bearings have demonstrated that swing phase microseparation followed by the impact of the femoral head on the superior acetabular insert rim leads to accelerated wear. However, resultant wear remained low. The wear of ceramic on polyethylene (COP) and metal on metal (MOM) couples under swing phase microseparation is unknown, this study aimed to compare the wear of these total hip replacements under standard and microseparation conditions.

A physiological hip simulator was used, loads and motions were applied to approximate in vivo conditions. Microseparation was achieved by displacing the femoral head inferiorly during swing phase, the head contacted the inferior cup rim and was laterally displaced. On heel strike the head contacted the superior cup rim prior to relocation. Components (as shown in table 1) were tested for 5 million cycles, at a frequency of 1 hertz in 25% (v/v) new born calf serum. Under standard conditions, wear of COC and MOM bearings was significantly lower than wear of COP couples. Under microseparation conditions the COC and MOM wear increased by 4 and 25 times respectively. Microseparation conditions reduced wear of COP couples by a factor of 4. Creep deformation and damage to the UHMWPE cup rim was observed, however, wear remained low. It is postulated that this reduction in polyethylene wear is due to the separation of the components in swing phase improving the entrapment of lubricant, hence wear is reduced via a squeeze film lubrication mechanism.

We describe two patients with obturator dislocation of the hip which was irreducible by described techniques of closed reduction. The first required open reduction using the iliofemoral approach with release of rectus femoris. The second was treated on a traction table which allowed disengagement of the head and, when combined with simultaneous lateral traction, adduction and gradual release of the longitudinal traction, facilitated a smooth reduction. Since the hip is stable in flexion, early mobilisation in an extension-limiting brace avoids the prolonged bed rest traditionally recommended for this injury.

We recruited 89 patients who had hip or knee replacements to assess the performance of below-knee graded compression stockings. The pressure gradients generated by the stockings were measured and all patients had venography of the ipsilateral leg.

We found that 98% of stockings failed to produce the ‘ideal’ pressure gradient (± 20%) of 18, 14 and 8 mmHg from the ankle to the knee, while 54% produced a ‘reversed gradient’ on at least one occasion during the course of the study. The overall rate of deep-venous thrombosis was 16.7%. Stockings which produced reversed gradients were associated with a significantly higher incidence of deep-venous thrombosis (p = 0.026) than those with the correct gradient (25.6% v 6.1%). This suggests that the performance of graded compression stockings can be improved if reversed pressure gradients are detected and prevented.