Abstract

Conventional proximal tibial osteotomy is a widely successful joint-preserving treatment for osteoarthritis; however, conventional procedures do not adequately control the posterior tibial slope (PTS). Alterations to PTS can affect knee instability, ligament tensioning, knee kinematics, muscle and joint contact forces as well as range of motion.

This study primarily aimed to provide a comprehensive investigation of the variables influencing PTS during high tibial osteotomy using a 3D surgical simulation approach. Secondly, it aimed to provide a simple means of implementing the findings in future 3D pre-operative planning and /or clinically.

The influence of two key variables: the gap opening angle and the hinge axis orientation on PTS was investigated using three independent approaches: (1) 3D computational simulation using CAD software to perform virtual osteotomy surgery and simulate the post-operative outcome. (2) Derivation of a closed-form mathematical solution using a generalised vector rotation approach (3) Clinical assessment of synthetically generated x-rays of osteoarthritis patients (n=28; REC reference: 17/HRA/0033, RD&E NHS, UK) for comparison against the theoretical/computational approaches.

The results from the computational and analytical assessments agreed precisely. For three different opening angles (6°, 9° and 12°) and 7 different hinge axis orientations (from −30° to 30°), the results obtained were identical. A simple analytical solution for the change in PTS, ΔP_s, based on the hinge axis angle, α, and the osteotomy opening angle, θ, was derived:

ΔP_s=sin^-1(sin α sin θ)

The clinical assessment demonstrated that the absolute values of PTS, and changes resulting from various osteotomies, matched the results from the two relative prediction methods.

This study has demonstrated that PTS is impacted by the hinge axis angle and the extent of the osteotomy opening angle and provided computational evidence and analytical formula for general use.

High tibial osteotomy (HTO) is an effective surgical treatment for isolated medial compartment knee osteoarthritis; however, widespread adoption is limited due to difficulty in achieving the planned correction, and patient dissatisfaction due to soft tissue irritation. A new HTO system – Tailored Osteotomy Knee Alignment (TOKA®, 3D Metal Printing Ltd, Bath, UK) could potentially address these barriers having a custom titanium plate and titanium surgical guides featuring a unique mechanism for precise osteotomy opening as well as saw cutting and drilling guides. The aim of this study was to assess the accuracy of this novel HTO system using cadaveric specimens; a preclinical testing stage ahead of first-in-human surgery according to the ‘IDEAL-D’ framework for device innovation.

Local ethics committee approval was obtained. The novel opening wedge HTO procedure was performed on eight cadaver leg specimens. Whole lower limb CT scans pre- and post-operatively provided geometrical assessment quantifying the discrepancy between pre-planned and post-operative measurements for key variables: the gap opening angle and the patient specific surgical instrumentation positioning and rotation - assessed using the implanted plate.

The average discrepancy between the pre-operative plan and the post-operative osteotomy correction angle was: 0.0 ± 0.2°. The R2 value for the regression correlation was 0.95.

The average error in implant positioning was −0.4 ± 4.3 mm, −2.6 ± 3.4 mm and 3.1 ± 1.7° vertically, horizontally, and rotationally respectively.

This novel HTO surgery has greater accuracy and smaller variability in correction angle achieved compared to that reported for conventional or other patient specific methods with published data available. This system could potentially improve the accuracy and reliability of osteotomy correction angles achieved surgically.

Abstract

Abstract

This matched cohort study aims to (a) assess differences in spinopelvic characteristics of patients having sustained a dislocation following THA and a control THA group without dislocation; (b) identify spinopelvic characteristics associated with risk of dislocation and; (c) propose an algorithm to define the optimum cup orientation for minimizing dislocation risk.

Fifty patients with a history of THA dislocation (29 posterior-, 21 anterior dislocations) were matched for age, gender, body mass index, index diagnosis, and femoral head size with 100 controls. All patients were reviewed and underwent detailed quasi-static radiographic evaluations of the coronal- (offset; center-of-rotation; cup inclination/anteversion) and sagittal- reconstructions (pelvic tilt, pelvic incidence, lumbar lordosis, pelvic-femoral-angle, cup ante-inclination). The spinopelvic balance (PI-LL), combined sagittal index (CSI= Pelvic-femoral-angle + Cup Anteinclination) and Hip-User-Index were determined. sagittal index (CSI= Pelvic-femoral-angle + Cup Anteinclination) and Hip-User-Index were determined. Parameters were compared between the two groups (2-group analysis) and between controls and per direction of dislocation (3-group analysis).

There were marginal coronal differences between the groups. Sagittal parameters (lumbar-lordosis, pelvic-tilt, CSI, PI-LL and Hip-User-Index) differed significantly. PI-LL (>10°) and standing pelvic tilt (>18°) were the strongest predictors of dislocation risk (sensistivity:70%/specificity:70%). All hips with a standing CSI<195° dislocated posteriorly and all with CSI>260° dislocated anteriorly. A CSI between 200–245° was associated with significantly reduced risk of dislocation (OR:6; 95%CI:2.5–15.0; p<0.001). In patients with unbalanced and/or rigid lumbar spine, standing CSI of 215–245° was associated with significantly reduced dislocation risk (OR:10; 95%CI:3.2–29.8; p<0.001).

PI-LL and standing pelvic-tilt determined from pre-operative, standing, lateral spinopelvic radiograph can be useful screening tools, alerting surgeons of patients at increased dislocation risk. Measurement of the pelvic-femoral angle pre-operatively provides valuable information to determine the optimum, cup orientation associated with reduced dislocation risk by aiming for a standing CSI of 200–245°.

Abstract

Abstract

Nearly a quarter of screws cause damage during insertion by stripping the bone, reducing pullout strength by over 80%. Studies assessing surgically achieved tightness have predominately shown that variations between individual surgeons can lead to underpowered investigations. Further to the variables that have been previously explored, several basic aspects related to tightening screws have not been evaluated with regards to how they affect screw insertion. This study aims to identify the achieved tightness for several variables, firstly to better understand factors related to achieving optimal intraoperative screw purchase and secondly to establish improved methodologies for future studies.

Two torque screwdrivers were used consecutively by two orthopaedic surgeons to insert 60 cortical, non-locking, stainless-steel screws of 3.5 mm diameter through a 3.5 mm plate, into custom-made 4 mm thick 20 PCF sheets of Sawbone, mounted on a custom-made jig. Screws were inserted to optimal tightness subjectively chosen by each surgeon. The jig was attached to a bench for vertical screw insertion, before a further 60 screws were inserted using the first torque screwdriver with the jig mounted vertically, enabling horizontal screw insertion. Following the decision to use the first screwdriver to insert the remaining screws in the vertical position for the other variables, the following test parameters were assessed with 60 screws inserted per surgeon: without gloves, double surgical gloves, single surgical gloves, non-sterile nitrile gloves and, with and then without augmented feedback (using digitally displayed real-time achieved torque). For all tests, except when augmented feedback was used, the surgeon was blinded to the insertion torque. Once the stopping torque was reached, screws were tightened until the stripping torque was found, this being used to calculate tightness (stopping/stripping torque ratio). Screws were recorded to have stripped the material if the stopping torque was greater than the stripping torque. Following tests of normality, Mann-Whitney-U comparisons were performed between and combining both surgeons for each variable, with Bonferroni corrections for multiple comparisons.

There was no significant (p=0.29) difference in the achieved tightness between different torque screw drivers nor different jig positions (p=0.53). The use of any gloves led to significant (p < 0 .001) increases in achieved tightness compared to not using gloves for one surgeon but made no difference for the other (p=0.38–0.74). Using augmented feedback was found to virtually eliminate stripping. For one surgeon average tightness increased significantly (p < 0 .001) when torque values were displayed from 55 to 75%, whilst for the other, this was associated with significantly decreases (p < 0 .001), 72 to 57%, both surgeons returned to their pre-augmentation tightness when it was removed.

Individual techniques make a considerable difference to the impact from some variables involved when inserting screws. However, the orientation of screws insertion and the type of screwdriver did not affect achieved screw tightness. Using visual feedback reduces rates of stripping and investigating ways to incorporate this into clinical use are recommended. Further work is underway into the effect of other variables such as bone density and cortical thickness.

Introduction

The resultant cup orientation depends upon the orientation of the pelvis at impaction. No studies to date have assessed whether patient-position during total hip arthroplasty (THA) has an effect on cup orientation. This study aims to 1) Determine the difference in pelvic position that occurs between surgery and radiographic, supine, post-operative assessment; 2) Examine how the difference in pelvic position influences subsequent cup orientation and 3) Establish whether pelvic orientation, and thereafter cup orientation, differences exist between THAs performed in the supine versus the lateral decubitus positions.

Cervical spine fractures are frequent in impact sports, such as rugby union. The consequences of these fractures can be devastating as they can lead to paraplegia, tetraplegia and death. Many studies have been conducted to understand the injury mechanisms but the relationship between player cervical spine posture and fracture pattern is still unclear. The aim of this study was to evaluate the influence of player cervical spine posture on fracture pattern due to an impact load. Nineteen porcine cervical spines (C2 to C6) were dissected, potted in PMMA bone cement and mounted in a custom made rig. They were impacted with a mean load of 6 kN. Eight specimens were tested in an axial position, five in flexion and six in lateral bending. All specimens were micro-CT imaged (Nikon XT225 ST Scanner, Nikon Metrology, UK) before and after the tests, and the images were used to assess the fracture patterns. The injuries were classified according to Allen-Ferguson classification system by three independent observers. The preliminary results showed that the main fracture modalities were consistent with those seen clinically. The main fractures for the axial orientation were observed in C5-C6 level with fractures on the articular process and endplates. These findings support the concept that the fracture patterns are related to the spine position and give an insight for improvements on sports rules in order to reduce the risk of injury.

The screw fastening torque applied during bone fracture fixation has a decisive influence on subsequent bone healing. Insufficient screw tightness can result in device/construct instability; conversely, excessive torques risk damaging the bone causing premature fixation failure. This effect is even more prominent in osteoporotic bone, a condition associated annually with almost 9 million fractures worldwide. During fracture fixation, screw tightening torque is applied using subjective feel. This approach may not be optimal for patient”s recovery, increasing risk of fixation failure, particularly in osteoporotic bone, and potentially require revision surgical interventions.

Besides bone density, various factors influence the performance of screw fixation. These factors include bone geometry, cortical thickness and time-dependant relaxation behaviour of the bone. If the influence of screw fastening torque on the bone and relationships between these factors was better understood, the surgical technique could be optimised to reduce the risk of complications.

Within this study, we developed an axisymmetric finite element (FE) model of bone screw tightening incorporating viscoelastic behaviour of the cortical bone such as creep and stress relaxation. The model anticipated time-dependent behaviour of the bone for different bone thickness and density after a typical bone fixation screw had been inserted. The idealised model has been developed based on CT scans of bones with varying densities and inserted screws. The model was validated through a series of experiments involving bovine tibiae (4–5 months) to evaluate the evolution of surface strains with time (Ncorr v1.2). Stress distribution was assessed in photoelastic experiments using acrylic analogues. Relaxation tests have been performed in aqueous environment for up to 48 hours to ensure the relaxation would be complete. The creep behaviour (maximum principal strain) was compared against computational predictions. Our early simulations predicted relaxation strains on the surface of the bone to be 1.1% within 24 hours comparing favourably to 1.3% measured experimentally. Stress distribution patterns were in agreement with photoelastic results.

Using experimentally derived viscoelastic properties, the model has the potential to predict creep and stress relaxation patterns after screw insertion with different fastening torques for bones with varying density and geometry. We aim to develop this into a planning tool providing guidance to surgeons for optimal tightening when using screw fixation, particularly in reduced quality bone.

Background

A large proportion of the expense incurred due to hip fractures arises due to secondary factors such as duration of hospital stay and additional theatre time due to surgical complications. Studies have shown that the use of intramedullary (IM) nail fixation presents a statistically higher risk of re-fracture than plating, which has been attributed to the stress riser at the end of the nail. It is not clear, however, if this situation also applies to unstable fractures, for which plating has a higher fixation failure rate. Moreover, biomechanical studies to date have not considered newer designs of IM nails which have been specifically designed to better distribute weight-bearing loads. This aim of this experimental study was to evaluate the re-fracture risk produced by a newer type of nailing system compared to an equivalent plate.

Background

Understanding vertebral fracture is important in order to reduce fracture risk. Previous studies have used FE to investigate mechanical behaviour, typically using a linear material response. This study aimed to establish a novel model that could represent the plastic behaviour leading to fracture.

Background

Numerical modelling using Finite Element (FE) Analysis has become ubiquitous in orthopaedic biomechanics, with both commercial and freely available packages widely used. Three FE packages in particular have gained popularity: Abaqus (Simulia); Ansys (ANSYS, Inc.) and FEBio (University of Utah). Although FEBio is now well established, its developers advise that comparisons should be made against more extensively tested software before trusting its results for specific problems. The aim of the study to conduct a comparison of mesh convergence and to provide validated open-source models of the femur for use all three FE packages.

Femoral head collapse is a possible complication after surgical treatment of femoral neck fractures. The purpose of this study was to examine whether implantation of a Sliding Hip Screw (SHS) or an X-Bolt could increase the risk of femoral head collapse. Similar to traditional hip screws, the X-Bolt is implanted through the femoral neck; however, it uses an expanding cross-shape to improve rotational stability. The risk of collapse was investigated alongside patient factors, such as osteonecrosis.

This numerical study assessed the risk of femoral head collapse using linear eigenvalue buckling (an established method [1]), and also from the maximum von Mises stress within the cortical bone. The femoral head was loaded using the pressures reported by Yoshida et al. for a patient sitting down (reported to put the femoral head at greatest risk of collapse [2]), with a peak pressure of 9.4 MPa and an average pressure of 1.59 MPa. The femur was fixed in all degrees of freedom at a plane through the femoral neck. The X-Bolt and SHS were implanted in accordance with the operative techniques. The femoral head and implants were meshed with quadratic tetrahedral elements, and cortical bone was meshed with triangular thin shell elements. A converged mesh seeding density of 1.2 mm was used. All models were create and solved using ABAQUS finite element software (version 6.12, Simulia, Dassault Systèmes, France). The influence of implant type and presence was examined alongside a variety of patient factors:

Osteonecrosis, modelled as a cone of bone of varying angle, and varying modulus values

Twenty-two finite element models were run for each implant condition (intact; implanted with the X-Bolt; implanted with a SHS), resulting in a total of 66 models. The finite element models were validated using experimental tests performed on five 4^th generation composite Sawbones femurs (Malmö, Sweden), and verified against previously published results [1].

No significant difference was found between the X-Bolt and the SHS, for either critical buckling pressure (p=0.964), or the maximum von Mises stress (p=0.274), indicating no difference in the risk of femoral head collapse. The maximum von Mises stress (and therefore the risk of collapse) within the cortical bone was significantly higher for the intact femoral head compared to both implants (X-Bolt: p=0.048, SHS: p=0.002). Of the factors examined, necrosis of the femoral head caused the greatest increase in risk.

The study by Volokh et al. [1] concluded that deterioration of the cancellous bone underneath the cortical shell can greatly increase the risk of femoral head collapse, and the results of the present study support this finding. Interestingly the presence of either an X-Bolt or SHS implant appeared to reduce the risk of femoral head collapse.

Using Python scripting it is possible to automate the pre-processing, solving and post-processing stages of finite element analysis using ABAQUS software. This is particularly useful when running multiple models parametrically. When the model involves a bony part, it is necessary to assign material properties based on the CT scan to represent bone heterogeneity, and unfortunately this cannot currently be done from within ABAQUS using software such as Bonemat [1]. To address this issue a Python package was written called ‘py_bonemat_abaqus’ to assign material properties from within ABAQUS. The purpose of this study was to compare the material assignments of py_bonemat_abaqus and Bonemat, to compare the processing speed, and to describe the workflow.

The software packages were compared using a CT scan of a half pelvis downloaded from the VAKHUM database, and the associated hexahedral finite element mesh of the left half pelvis. To examine different element types, the hexahedral mesh was converted to linear and quadratic tetrahedral elements by dividing each hexahedron into 5 tetrahedral elements. The equations used to convert the Hounsfield Unit (HU) values to apparent density (p_app), and to convert the apparent density to elastic modulus (E) are shown in Equations 1&2 [2].

Equation 1: p_app = −0.021075 + 0.000786 HU

Equation 2 E = 2.0173 p_app^2.46

The time taken to analyse the models by each software was assessed using a Windows 7 PC with a 64-bit operating system, 4 CPUS, 8 GB of RAM and an Intel Core I5-3470 processor.

The mean difference between the moduulus assignment made by py_bonemat_abaqus and Bonemat was −0.05 kPa (range −10.19 to 4.50 kPa, standard deviation 0.62 kPa). The Python package took a similar time to run for all element types; this was between 109 and 126 s. Bonemat software was significantly faster, and took between 5 and 20 s. Finally, the Python package was successfully used from within a Python script to perform material assignment from within ABAQUS software in a fully automated manner.

Material assignments were almost equivalent between the two software packages, with any differences explainable by rounding effects. To put the differences into context, a difference of −0.05 kPa is 0.00000002% of the typical modulus of cortical bone (20.7 GPa), and 0.00000003% of the modulus of trabecular bone (14.8 GPa) [3]. The Python package was slower to process the models, but was successfully able to assign material properties from within ABAQUS software as part of an automated script.

Introduction

Knee arthroplasty is an effective intervention for painful arthritis when conservative measures have failed. Despite recent advances in component design and implantation techniques, a significant proportion of patients experience problems relating to the patella-femoral joint (PFJ).

Detailed knowledge of the shape and orientation of the normal and replaced femoral trochlea groove is critical when considering potential causes of anterior knee pain. Furthermore, to date it has proved difficult to establish a diagnosis due to shortcomings in current imaging techniques for obtaining satisfactory coronal plane motion data of the patella in the replaced knee.

The aim of this study was to correlate the trochlea shape of normal and replaced knees with corresponding coronal plane PFJ kinematic data.

Summary

The mechanical properties of porcine tibial plateau (TP) cartilage are shown to vary topographically. Low Elastic moduli (Em) were found in the positions where unicompartimental knee osteoarthritis (OA) lesions are typically expected to develop. These results suggest that there is a different response to load in these areas.

Summary

The required torque leading to an abrasion of the passive layer in the stem-head interface positively correlates to the assembly force. In order to limit the risk of fretting and corrosion a strong hammer blow seems to be necessary.

Summary Statement

Simulated increases in body weight led to increased displacement, von Mises stress, and contact pressure in finite element models of the extended and flexed knee. Contact shifted to locations of typical medial osteoarthritis lesions in the extended knee models.

Previous attempts to measure coronal plane patellofemoral kinematics following knee replacement have suffered from methodological drawbacks; the patella being obscured by the components, metal artefact and technical inaccuracies. The aim of this study was to assess whether there was any significant difference in the patellofemoral kinematics between normal, TKR and PFJR patients using the validated MAUS™ technique (combining motion analysis with ultrasound).

60 patients were recruited into three groups; normal healthy volunteers (Normal), TKR, and PFJR patients. The MAUS technique incorporates a 12 camera analysis system (providing gross alignment data for tibial and femoral segments) and an ultrasound probe (providing coordinates of bony landmarks on patella femur and tibia) during a squat exercise. 6 DOF kinematics were described between 0 and 90° flexion. The validated accuracy of the MAUS technique registering the ultrasound images within the motion capture system is 1.84 mm (2 × SD).

Movements of the Normal group were significantly different from the TKR group (p=0.03) and the PFJR group (p<0.01), whilst there was no significant difference between the TKR and PFJR groups (p=0.27).

Our data suggest that many aspects of patellofemoral kinematics are absent following TKR and PFJR, which could be addressed in future designs of knee TKR and PFJR.

Unicompartmental Knee Replacement (UKR) is associated with fewer complications, faster recovery and better function than Total Knee Replacement (TKR). However, joint registries demonstrate a higher revision rate in UKR, limiting its use. Currently most UKRs are cemented and performed using a minimally invasive technique. In joint registries, common reasons for revision include aseptic loosening and pain. These problems could potentially be addressed by using cementless implants, which may provide more reliable fixation.

The objectives of this study were to compare the quality of fixation (determined by the incidence and appearance of radiolucencies), and clinical outcomes of cemented and cementless UKR at five years.

A randomised controlled trial was established with 63 knees (62 patients) randomised to either cemented (32 patients) or cementless UKR (30 patients). Fixation was assessed with fluoroscopic radiographs aligned to the bone-implant interface at one and five years. Outcome scores were collected pre-operatively and at one, two and five years, including Oxford Knee Score (OKS), American Knee Society Score, objective and functional (AKSS-O/F) and Tegner Activity Scale (TAS), expressed as absolute scores and 0–5 year change (δ) scores.

Four patients died during the study period. There were no revisions. Mean operative time was 11 minutes shorter in the cementless group (p=0.029). At five years, there was no significant difference in any outcome measure except AKSS-F and δAKSS-F which were significantly better in the cementless group (both p=0.003). There were no femoral radiolucencies in either group. There were significantly more tibial radiolucencies in the cemented group (20/30 vs 2/27, p< 0.001). There were nine complete radiolucencies in the cemented group and none in the cementless group (p< 0.001).

Cementless fixation provides improved fixation at five years compared to cemented fixation in UKR, maintaining equivalent or superior clinical outcomes with a shorter operative time and no increase in complications.

Introduction

Subtle deformities of the acetabulum and proximal femur are recognised as biomechanical risk factors for the development of hip osteoarthritis (OA) as well as a cause of hip and groin pain. We undertook this study to examine relationships between a number of morphological measurements of the acetabulum and proximal femur and the hip pain in a 20-year longitudinal study.

Introduction

Solid or cystic pseudotumour is a potentially destructive complication of metal on metal (MoM) couples, usually needing revision surgery. However, complete clearance of the pseudotumour is unlikely at times. This prospective case-controlled study reports cases which had recurrence after revision surgery for pseudotumour related to metal on metal hip couples.

Indications for Unicompartmental Knee Arthroplasty (UKA) vary between units. Some authors have suggested, and many surgeons believe, that medial UKA should only be performed in patients who localise their pain to the medial joint line. This is despite research showing a poor correlation between patient-reported location of pain and radiological or operative findings in osteoarthritis. The aim of this study is to determine the effect of patient-reported pre-operative pain location and functional outcome of UKA at one and five years.

Pre-operative pain location data were collected for 406 knees (380 patients) undergoing Oxford medial UKA. Oxford Knee Score, American Knee Society Scores and Tegner activity scale were recorded preoperatively and at follow-up. 272/406 (67%) had pure medial pain, 25/406 (6%) had pure anterior knee pain and 109/406 (27%) had mixed or generalised pain. None had pure lateral pain. The primary outcome interval is one year; 132/406 patients had attained five years by the time of analysis and their five year data is presented.

At one and five years, each group had improved significantly by each measure (mean δOKS 15.6 (SD 8.9) at year one, 16.3 (9.3) at year five). There was no difference between the groups, nor between patients with and without anterior knee pain or isolated medial pain.

We have found no correlation between preoperative pain location and outcome. We conclude that localised medial pain should not be a prerequisite to UKA and that it may be performed in patients with generalised or anterior knee pain.

Introduction

Motion analysis is a validated method of assessing technical dexterity within surgical skills centers. A more accessible and cost-effective method of skills assessment is to use a global rating scale (GRS). We aimed to perform a validation experiment to compare an arthroscopic GRS against motion analysis for monitoring orthopaedic trainees learning simulated arthroscopic meniscal repairs.

Introduction

Stem geometry is known to influence the outcome in THA; however it is unknown whether the material properties, stiffness in particular can influence the stem stability and outcome. The aim of this study was to measure the influence of stem material properties on micromotion and migration using Roentgen Stereophotogrammetric Analysis (RSA) system.

INTRODUCTION

Femoral neck narrowing (NN) following Metal-on-Metal Hip Resurfacing Arthroplasty (MoMHRA) is a well-recognised clinical phenomenon. The incidence of resurfaced hips with NN > 10% is reported to be up to 27%. Its pathogenesis is thought to be multi-factorial secondary to stress shielding, impingement, osteolysis secondary to wear/ion particles and as a result of reduced vascularity and pressure effect on cancellous bone secondary to the presence of a soft-tissue mass around the resurfaced hip. Recognised risk factors for its development include: female gender and the presence of a pseudotumour. Serum Chromium (Cr) and Cobalt (Co) are recognised surrogate markers of in-vivo wear of MoMHRA. The aims of this study were to establish whether NN is associated with increased wear.

INTRODUCTION

Metal-on-metal hip resurfacing arthroplasty (MoMHRA) is a surgical option in the treatment of end-stage hip disease. The measurement of systemic levels of metal ions gives an insight into the wear occurring and is advocated by regulatory bodies as routine practice in the assessment of resurfaced hips. However, the acceptable upper levels of Chromium (Cr) and Cobalt (Co) ions concentration with clinical significance still have to be established. The aim of this study is to address this issue in unilateral and bilateral resurfaced hips.

Introduction

Since the introduction of 3rd generation Metal-on-Metal-Hip-Resurfacing-Arthroplasty (MoMHRA), thousands of such prostheses have been implanted worldwide in younger patients with end-stage hip osteoarthritis. However, no independent centre has reported their medium-to-long term outcome. The aim of this study is to report the ten year survival and outcome of the Birmingham Hip Resurfacing (BHR), the most commonly used MoMHRA worldwide.

Aims

Interest in soft tissue Radiostereometric Analysis (RSA) is rising. Previous authors have tried, with varying levels of success, to use this technique to analyse the intra-substance portion of anterior cruciate ligament (ACL) graft constructs. These methods were either prone to large amounts of marker migration, deemed unsuitable for in-vivo use or, where alternative markers such as stainless steel sutures were used, lost the inherent accuracy that made RSA an attractive tool in the first place. We describe a modification of tantalum marker balls that allows for a new method of secure fixation to soft tissue in order to accurately analyse stretch, displacement and, potentially, dynamic movement using RSA.

Introduction

Osteoarthritis (OA) of the hip is an important cause of pain and morbidity. The mechanisms and pathogenesis of OA'sdevelopment remain unknown. Minor acetabular dysplasia and subtle variations in proximal femoral morphology are increasingly being recognized as factors that potentially compromise the joint biomechanically and lead to OA. Previous studies have shown that risk of hip OA increased as the femoral head to femoral neck ratio (HNR) decreased. Previous work has described the evolutionary change in inferior femoral neck trabecular density and geometry associated with upright stance, but no study has highlighted the evolutionary change in HNR. The aim of this study was to examine evolutionary evidence that the hominin bipedal stance has lead to alterations in HNR that would predispose humans to hip OA.

Introduction

Malalignment of some designs of stem is associated with an increased risk of aseptic loosening and revision. We investigated whether the alignment of the cemented polished, double-taper design adversely affected outcome, in a multicentre prospective study.

Introduction

The options for the treatment of the young active patient with unicompartmental symptomatic osteoarthritis and pre-existing Anterior Cruciate Ligament (ACL) deficiency are limited. Patients with ACL deficiency and end-stage medial compartment osteoarthritis are usually young and active. The Oxford Unicompartmental Knee Replacement (UKA) is a well established treatment option in the management of symptomatic end-stage medial compartmental osteoarthritis, but a functionally intact ACL is a pre-requisite for its satisfactory outcome. If absent, high failure rates have been reported, primarily due to tibial loosening. Previously, we have reported results on a consecutive series of 15 such patients in whom the ACL was reconstructed and patients underwent a staged or simultaneous UKA. The aim of the current study is to provide an update on the clinical and radiological outcomes of a large, consecutive cohort of patients with ACL reconstruction and UKA for the treatment of end-stage medial compartment osteoarthritis and to evaluate, particularly, the outcome of those patients under 50.

INTRODUCTION

Studies have suggested that there is a reduction in head-neck-ratio (HNR) associated with MoMHRA. A reduction in HNR at operation would decrease range of movement and increase impingement risk. Impingement could lead to 20 edge loading, increasing wear. Serum ion levels of Chromium (Cr) and Cobalt (Co) are surrogate markers of wear. Although acetabular component orientation has been shown to contribute to wear and PT development, the role of a decrease in HNR has only been highlighted in PT development. This study aimed to measure changes in HNR that occur at resurfacing and determine any gender- and component size-specific differences. In addition it aimed to determine whether changes in HNR could be associated with increased wear.

Introduction

Radiolucencies beneath the tibial component are well recognized in knee arthroplasty; the aetiology and significance are poorly understood. Non-progressive narrow radiolucencies with a sclerotic margin are thought not to be indicative of loosening. Factors which decrease the incidence of radiolucencies include cementless fixation and the use of pulse lavage. Leg/component alignment or BMI do not influence radiolucency. We are not aware of any studies that have looked at the effect of load type on radiolucency.

The Oxford domed lateral tibial component was introduced to decrease the bearing dislocation rate that was unacceptably high with the flat tibial tray. However, the introduction of the domed tibial component alters the forces transmitted through the implant-cement-bone interface. As the Oxford UKR uses a fully congruent mobile bearing, the forces transmitted through the interface with a flat tray are compressive, except for the effect of friction. However, with the domed tibial component shear forces are introduced. The aim of this study was to assess the prevalence of radiolucency beneath the previous flat design and the new domed tibial tray.

INTRODUCTION

Although simulation studies have shown superior wear properties of metal-on-metal articulations, increased concern exists regarding the excess in-vivo wear of a small number of Metal-on-Metal-Hip-Resurfacing (MoMHRA) implants. Serum ion levels of Chromium (Cr) and Cobalt (Co) are surrogate markers of wear. Risk factors associated with increased wear include female gender, small components, dysplasia, cup orientation outside safe zone and femoral head downsize during surgery with an associated decrease in Head-Neck-Ratio (HNR). However, these factors are interlinked. This study aims to identify the factors that are most important for subsequent wear of MoMHRA, by performing a multivariate analysis.

INTRODUCTION

The introduction of hard-on-hard bearings and the consequences of increased wear due to edge-loading have renewed interest in the importance of acetabular component orientation for implant survival and functional outcome following hip arthroplasty. Some studies have shown increased dislocation risk when the cup is mal-oriented which has led to the identification of a safe-zone1. The aims of this prospective, multi-centered study of primary total hip arthroplasty (THA) were to: 1. Identify factors that influence cup orientation and 2. Describe the effect of cup orientation on clinical outcome.

Background

Technical skill is an essential domain of surgical competency. Arthroscopic surgery forms a particularly challenging subset of these skills. The innate ability to acquire these skills is not fully understood. The aim of this study was to investigate the innate arthroscopic skills and learning curve patterns of medical students - our future surgeons.

In pre-operative planning for total hip arthroplasty (THA), femoral offset (FO) is frequently underestimated on AP pelvis radiographs as a result of inaccurate patient positioning, imprecise magnification, and radiographic beam divergence. The aim of the present study was to evaluate the reliability and accuracy of predicting three-dimensional (3-D) FO as measured on computed tomography (CT) from measurements performed on standardised AP pelvis radiographs.

In a retrospective cohort study, pre-operative AP pelvis radiographs and corresponding CT scans of a consecutive series of 345 patients (345 hips, 146 males, 199 females, mean age 60 (range: 40-79) years, mean body-mass-index 27 (range: 29-57) kg/m2) with primary end-stage hip osteoarthritis were reviewed. Patients were positioned according to a standardised protocol and all images were calibrated. Using validated custom programmes, FO was measured on corresponding AP pelvis radiographs and CT scans. Inter- and intra-observer reliability of the measurement methods were evaluated using intra-class correlation coefficients (ICC). To predict 3-D FO from AP pelvis measurements, the entire cohort was randomly split in two groups and gender specific linear regression equations were derived from a subgroup of 250 patients (group A). The accuracy of the derived prediction equations was subsequently assessed in a second subgroup of 100 patients (group B).

In the entire cohort, mean FO was 39.2mm (95%CI: 38.5-40.0mm) on AP pelvis radiographs and 44.6mm (95%CI: 44.0-45.2mm) on CT scans. FO was underestimated by 14% on AP pelvis radiographs compared to CT (5.4mm, 95%CI: 4.8-6.0mm, p<0.001) and both parameters demonstrated a linear correlation (r=0.642, p<0.001). In group B, we observed no significant difference between gender specific predicted FO (males: 48.0mm, 95%CI: 47.1-48.8mm; females: 42.0mm, 95%CI: 41.1-42.8mm) and FO as measured on CT (males: 47.7mm, 95%CI: 46.1-49.4mm, p=0.689; females: 41.6mm, 95%CI: 40.3-43.0mm, p=0.607).

The results of the present study suggest that femoral offset can be accurately and reliably predicted from AP pelvis radiographs in patients with primary end-stage hip osteoarthritis. Our findings support the surgeon in pre-operative templating and may improve offset and limb length restoration in THA without the routine performance of CT.

Background

The ability to learn arthroscopic surgery is an important aspect of modern day orthopaedic surgery. Knowing that variation in innate ability exists amongst medical students, the aim of this study was to investigate the effect of training on the arthroscopic surgical performance of our future orthopaedic surgeons (medical students).

In uncemented total hip arthroplasty (THA), the optimal femoral component should allow both maximum cortical contact with proximal load transfer and accurate restoration of individual joint biomechanics. This is often compromised due to a high variability in proximal femoral anatomy. The aim of this on-going study is to assess the variation in proximal femoral canal shape and its association with geometric and anthropometric parameters in primary hip OA.

In a retrospective cohort study, AP-pelvis radiographs of 98 consecutive patients (42 males, 56 females, mean age 61 (range:45-74) years, BMI 27.4 (range:20.3-44.6) kg/m2) who underwent THA for primary hip OA were reviewed. All radiographs were calibrated and femoral offset (FO) and neck-shaft-angle (NSA) were measured using a validated custom programme. Point-based active shape modelling (ASM) was performed to assess the shape of the inner cortex of the proximal femoral meta- and diaphysis. Independent shape modes were identified using principal component analysis (PCA). Hierarchical cluster analysis of the shape modes was performed to identify natural groupings of patients. Differences in geometric measures of the proximal femur (FO, NSA) and demographic parameters (age, height, weight, BMI) between the clusters were evaluated using Kruskal-Wallis one-way-ANOVA or Chi-square tests, as appropriate.

In the entire cohort, mean FO was 39.0 mm, mean NSA was 131 degrees. PCA identified 10 independent shape modes accounting for over 90% of variation in proximal femoral canal shape within the dataset. Cluster Analysis revealed 6 shape clusters for which all 10 shape modes demonstrated a significantly different distribution (p-range:0.000-0.015). We observed significant differences in age (p=0.032), FO (p<0.001) and NSA (p<0.001) between the clusters. No significant differences with regard to gender or BMI were seen.

Our preliminary analysis has identified 6 different patterns of proximal femoral canal shape which are associated with significant differences in femoral offset, neck-shaft-angle and age at time of surgery. We are currently evaluating the entire dataset of 345 patients which will allow a comprehensive classification of variation in proximal femoral shape and joint geometry. The present data may optimise preoperative planning and improve future implant design in THA.

Introduction

Traditional TKR designs exhibit abnormal and unpredictable kinematics: with posterior subluxation in extension and anterior slide with flexion. These can contribute to restricted knee flexion and reduced quadriceps efficiency. Newer designs attempt to provide “guided motion” with the aim of mimicking normal knee kinematics. The Journey (Smith & Nephew) BCS TKR incorporates both an anterior and a posterior cam/post mechanism while Triathlon PS TKR (Stryker) incorporates a posterior cam/post mechanism. This study compares the in-vivo kinematics of these two designs and compares it with normal knee.

A femoral head/neck ratio (HNR) of less than 1.27 is associated with an increased risk of arthritis. The aim of this study was to establish whether there is evolutionary evidence that the homonin, bipedal stance has led to alterations in HNR that predispose humans to osteoarthritis (OA).

Specimens provided by The Natural History Museums of London, Oxford and the Department of Zoology, University of Oxford were grouped according to gait pattern, HAKF (Hip and knee flexed), Arboreal (ability to stand with hip and knee joints extended) and homonin/bi-pedal. Specimens included those from Devonion, Triassic, Jurrasic, Cretaceous, Miocene, Paleolithic, Pleistocene periods to modern day. Three-dimensional skeletal geometries were segmented using CT images and HNR measurements were taken from coronal views. These were compared with the HNR of 119 asymptomatic human volunteers and 210 patients that had a hip joint replacement for primary OA.

Species of the HAKF group had the smallest HNR (1.10, SD:0.09). Species of the Arboreal group had significantly higher HNR (1.63, SD:0.15) in comparison to the Bipedal group (1.41, SD:0.04) (p=0.006), Human (1.33, SD:0.08) and the OA group (1.3, SD:0.09).

The range of movement associated with arboreal habitat caused an associated change in HNR. This study would suggest that the HNR peaked in the Miocene period with species that ambulated on both ground and trees. More recent homonin gait appears to have developed a smaller HNR and humans have the smallest amongst their close ancestors. Evolutionary theory would suggest that modern environmental pressures might pre-dispose future hominin evolution to OA, secondary to a further reduction in HNR.

Stem geometry is known to influence the outcome in THA; however it is unknown whether the material properties, stiffness in particular can influence the stem stability and outcome. The aim of this study was to measure the influence of stem material properties on micromotion and migration using Roentgen Stereophotogrammetric Analysis (RSA) system.

41 patients were implanted with a collarless polished tapered (CPT) femoral stem (Zimmer, Warsaw, Indiana), which was made of either cobalt-chromium (CoCr) (n=21) or stainless steel (n=20). RSA was used to measure dynamically inducible micromotion (DIMM: difference in stem position in going from double-leg stance (DLS) to single leg stance (SLS)), prosthesis bending (difference in the head-tip distance when going from DLS to SLS), and mean migration of the head, tip and the cement restrictor. DIMM and bending were measured at 3 months, migration at 6, 12 and 24 months. All analyses were carried out using SPSS for windows (v.15.0.0, Chicago. IL, USA). Results were reported as mean ± 95% confidence interval (CI) and regarded as significant when p < 0.05.

Preliminary analysis showed that total head DIMM was significantly (p = 0.02) greater for CoCr (0.97mm ± 0.6mm) than stainless steel (0.27mm ± 0.6mm). The mean stem bending for CoCr was 0.08mm (± 0.06mm) and for stainless steel 0.15mm (± 0.06mm) (p =0.77). Both implants heads migrated posteriorly, medially and distally. The mean subsidence for the cobalt-chromium and stainless steel stems was 1.02mm (± 0.19mm) (p < 0.001) and 1.12mm (± 0.34mm) (p=0.001) (p= 0.07) at 24 months.

It was interesting to note that the dynamically induced micromotion was greater for the stiffer stem, however there were no differences in terms of overall migration, indicating that survival (in terms of loosening) should be the similar for both steel and CoCr versions of this implant.

Introduction

Primary mechanical stability is important with uncemented THR because early migration is reduced, leading to more rapid osseointegration between the implant and bone. Such primary mechanical stability is provided by the design features of the device. The aim of this study was to compare the migration patterns of two uncemented hip stems, the Furlong Active and the Furlong HAC stem; the study was designed as a randomised control trial. The implants were the Furlong HAC, which is an established implant with good long term results, and the Furlong Active, which is a modified version of the Furlong HAC designed to minimise stress concentrations between the implant and bone, and thus to improve fixation.

Introduction

The results of the original mobile bearing Oxford unicompartmental knee replacement (UKR) in the lateral compartment have been disappointing because of high dislocation rates (11%). This original implant used a flat bearing articulation on the tibial tray. To address the issue of dislocation a new implant (domed tibia with biconcave bearing to increase entrapment) was introduced with a modified surgical technique. The aim of this study was to compare the risk of dislocation between a domed and flat lateral UKR.

Introduction

Anteromedial osteoarthritis of the knee (anteromedial gonarthrosis-AMG) is a common form of knee arthritis. In a clinical setting, knee arthritis has always been assessed by plain radiography in conjunction with pain and function assessments. Whilst this is useful for surgical decision making in bone on bone arthritis, plain radiography gives no insight to the earlier stages of disease. In a recent study 82% of patients with painful arthritis had only partial thickness joint space loss on plain radiography. These patients are managed with various surgical treatments; injection, arthroscopy, osteotomy and arthroplasty with varying results. We believe these varying results are in part due to these patients being at different stages of disease, which will respond differently to different treatments. However radiography cannot delineate these stages. We describe the Magnetic Resonance Imaging (MRI) findings of this partial thickness AMG as a way of understanding these earlier stages of the disease.

Background

Osteoarthritis (OA) is a degenerative, chronic disease of the articular cartilage that affects more than 150 million people [1]. In the knee, OA can begin as either isolated medial OA or isolated lateral OA. Previous research [2,3] shows medial OA and lateral OA have characteristic cartilage lesion locations and progression patterns as well as flexion angles associated with lesion development, indicating strong involvement of mechanical factors in disease initiation. Therefore, it is important to investigate these mechanical factors. Previous studies combined data sets (geometry, motion, load) from separate sources. The aim of the current work was to use a consistent multi-modal approach.

Introduction

Unicompartmental Knee Replacement (UKR) is an appealing alternative to Total Knee Replacement (TKR) when the patient has isolated compartment osteoarthritis (OA). A common observation post-operatively is radiolucency between the tibial tray wall and the bone. In addition, some patients complain of persistent pain following implantation with a UKR; this may be related to elevated bone strains in the tibia. The aim of this study was to investigate the mechanical environment of the tibia bone adjacent to the tray wall, following UKR, to determine whether this region of bone resorbs, and how altering the mechanical environment affects tibia strains.

Introduction

Anteromedial gonarthrosis is a common well described pattern of knee osteoarthritis with cartilage wear beginning in the anteromedial quadrant of the medial tibial plateau in the presence of an intact and functioning ACL. It is well known that mechanical factors such as limb alignment and meniscal integrity affect the progression of arthritis and there is some evidence that the morphology of the tibial plateau may be a risk factor in the development of this disease. The extension facet angle is the angle of the downslope of the anterior portion of the medial tibial plateau joint surface in relation to the middle portion on a sagittal view. If this is an important factor in the development of AMG there may be potential for disease modifying intervention.

This study investigates if there is a significant difference in this angle as measured on MRI between a study cohort with early AMG (partial thickness cartilage damage and intact ACL) and a comparator control cohort of patients (no cartilage damage and ACL rupture).

Iterative finite element (FE) models are used to simulate bone remodelling that takes place due to the surgical insertion of an implant or to simulate fracture healing. In such simulations element material properties are calculated after each iteration of solving the model. New material properties are calculated based on the results derived by the model during the last iteration. Once the FE model has gone through a number of such iterations it is often necessary to assess the remodelling that has taken place. The method widely used to do this is to analyse element Young's modulus plots taken at particular sections through the model. Although this method gives relevant information which is often helpful when comparing different implants, the information is rather abstract and is difficult to compare with patient data which is commonly in the form of radiographs.

The authors suggest a simple technique that can be used to generate synthetic radiograph images from FE models. These images allow relatively easy comparisons of FE derived information with patient radiographs. Another clear advantage of this technique is that clinicians (who are familiar with reading radiographs) are able to understand and interpret them readily.

To demonstrate the technique a three dimensional (3D) model of the proximal tibia implanted with an Oxford Unicompartmental Knee replacement was created based on CT data obtained from a cadaveric tibia. The model's initial element material properties were calculated from the same CT data set using a relationship between radiographic density and Young's modulus.

The model was subject to simplified loading conditions and solved over 365 iterations representing one year of in vivo remodelling. After each iteration the element material properties were recalculated based on previously published remodelling rules. Next, synthetic anteroposterior radiographs were generated by back calculating radiographic densities from material properties of the model after 365 iterations. A 3D rectangular grid of sampling points which encapsulated the model was defined. For each of the elements in the FE model radiographic densities were back calculated based on the same relationships used to calculate material properties from radiographic densities. The radiographic density of each element was assigned to all the sampling grid points within the element. The 3D array of radiographic densities was summed in the anteroposterior direction thereby creating a 2D array of radiographic densities. This 2D array was plotted giving an image analogous to anteroposterior patient radiographs. Similar to a patient radiograph denser material appeared lighter while less dense material appeared darker.

The resulting synthetic radiographs were compared to patient radiographs and found to have similar patterns of dark and light regions.

The synthetic radiographs were relatively easy to produce based on the FE model results, represented FE results in a manner easily comparable to patient radiographs, and represented FE results in a clinician friendly manner.

The heat produced by drills, saws and PMMA cement in the handling of bone can cause thermal necrosis. Thermal necrosis could be a factor in the formation of a fibrous tissue membrane and impaired bony ingrowth into porous prostheses. This has been proposed to lead to non-union of osteotomies and fractures, the failure of the bone-cement interface and the failure of resurfacing arthroplasty.

We compared three proprietary blades (the De Soutter, the Stryker Dual Cut and the Stryker Precision) in an in-vitro setting with porcine tibiae, using thermocouples embedded in the bone below the cutting surface to measure the increases in bone temperature.

There was a significant (p=0.001) difference in the change in temperature (δT) between the blade types. The mean increase in temperature was highest for the De Soutter, 2.84°C (SD: 1.83°C, range 0.48°C to 9.30°C); mean δT was 1.81°C (SD: 1.00°C, range 0.18°C to 4.85°C) for the Precision and 1.68°C (SD: 0.95°C, range 0.24°C to 5.67°C). Performing paired tests, there was no significant difference in δT between the Precision and Dual Cut blades (p=0.340), but both these blades had significantly (p=0.003 for Precision vs De Soutter, p<0.001 for Dual Cut vs De Soutter) lower values for δT than the Dual Cut.

Introduction

The results of the mobile bearing Oxford unicompartmental knee replacement (UKR) in the lateral compartment have been disappointing with a five year survival of 82%. Therefore, it is recommended that mobile bearings should not be used for lateral UKR. This low survivorship is primarily due to high dislocation rate, all occurring in the first year. A detailed analysis of the causes of bearing dislocation confirmed the elevated lateral tibial joint line to be a contributory factor. A new surgical technique was therefore introduced in which care was taken neither to remove too much bone from distal femur nor to over tighten the knee and thus ensure that the tibial joint line was not elevated. Other modifications to the technique were also introduced including use of a domed tibial component.

Purpose of study

To investigate the linear penetration rate of the polyethylene bearing in unicompartmental knee arthroplasty at twenty years.

Introduction

Obesity has been considered a relative contra-indication in unicompartmental knee arthroplasty (UKA) due to fear of high wear rates, loosening and tibial collapse. The aim of this study was to investigate the impact of high body mass index (BMI) on ten-year survivorship and five-year functional outcome after Oxford UKA, a fully congruous mobile bearing design with large contact area and low wear rate.

Purpose of Study

To assess the incidence of radiolucency in cemented and cementless Oxford unicompartmental knee replacement at two years.

Introduction

Kozinn and Scott have made recommendations about contra-indications for unicompartmental knee replacement (UKR). They suggest that patients younger than 60, weight > 82 kilograms, patients with exposed bone in patella-femoral compartment or patients who are physically active/perform heavy labour should not be offered a UKR. In addition, chondrocalcinosis is a contra-indication. These strict selection criteria are based on the experience with fixed bearing UKAs and are more intuitive than evidence based. The Oxford UKR has a fully congruous mobile bearing and has been shown to have minimal wear. Over the past 25 years, the Oxford Group has followed a standardised protocol for patient selection for UKR. We ignore patella-femoral joint pathology, chondrocalcinosis, patient's age, weight and activity level when deciding the suitability for UKR.

In 12 patients, we measured the oxygen concentration in the femoral head-neck junction during hip resurfacing through the anterolateral approach. This was compared with previous measurements made for the posterior approach. For the anterolateral approach, the oxygen concentration was found to be highly dependent upon the position of the leg, which was adjusted during surgery to provide exposure to the acetabulum and femoral head. Gross external rotation of the hip gave a significant decrease in oxygenation of the femoral head. Straightening the limb led to recovery in oxygen concentration, indicating that the blood supply was maintained. The oxygen concentration at the end of the procedure was not significantly different from that at the start.

The anterolateral approach appears to produce less disruption to the blood flow in the femoral head-neck junction than the posterior approach for patients undergoing hip resurfacing. This may be reflected subsequently in a lower incidence of fracture of the femoral neck and avascular necrosis.

This study aims to investigate femoral blood flow during Metal-on-Metal Hip Resurfacing (MMHR) by monitoring oxygen concentration during the operative procedure.

Patients undergoing MMHR using the posterior approach were evaluated. Following division of fascia lata, a calibrated gas-measuring electrode was inserted into the femoral neck, aiming for the supero-lateral quadrant of the head. Baseline oxygen concentration levels were detected after electrode insertion 2-3cm below the femoral head surface and all intra-operative measures were referenced against these. Oxygen levels were continuously monitored throughout the operation. Data from ten patients are presented.

Oxygen concentration dropped most noticeably during the surgical approach and was reduced by 62% (Std.dev +/-26%) following dislocation and capsulectomy. Insertion of implants resulted in a further oxygenation decrease by 18% (Std.dev +/-28%). The last obtained measure before wound closure detected 22% (Std.dev +/-31%) of initial baseline oxygen levels. Variation between subjects was observed and three patients demonstrated a limited recovery of oxygen levels during implant insertion and hip relocation.

Intra-operative measurement of oxygen concentration in blood perfusing the femoral head is feasible. Results in ten patients undergoing MMHR showed a dramatic effect on the oxygenation in the femoral head during surgical approach and implant fixation. This may increase the risk of avascular necrosis and subsequent femoral neck fracture. Future experiments will determine if less invasive procedures or specific positioning of the limb can protect the blood supply to femoral neck and head.

Introduction: Metal-on-metal-hip-resurfacing-arthroplasty (MoMHRA) has been associated with the development of inflammatory pseudotumours(IP), especially in females. IPs have been linked to wear debris, which can be related to metal-ion blood levels. Acetabular component position has been shown to influence wear. We have identified an optimum component orientation minimising IP risk around an inclination/anteversion of 40°/20°±10°. Our aim was to see if this optimal position results in lower metal ions and to identify the boundary of an optimal placement zone for low wear.

Methods: A cohort of 104 patients(60M:44F) with unilateral MoMHRA was studied. Blood tests were obtained at a mean follow up of 3.9 years and serum Co/Cr levels were measured(ICPMS). High metal ion concentrations were defined as Co> 4.1ppb and Cr> 5.2ppb. Radiographic cup inclination/anteversion were measured using EBRA. The differences in ion levels between different orientation zones were investigated. Three orientation zones were defined centered on 40°/20°: Z1-within ±5°, Z2-outside ±5°/within ±10° and Z3-within ±10°.

Results: There was a wide range of cup placements. Females had significantly (p< 0.001) smaller components(mean:51, 44–60) than males(mean: 56, 52–64). Cr levels, but not Co, were higher in females(p=0.002) and those with small femoral components(< 50mm, p =0.03). Patients with cups within Z1 (n=13) had significantly lower Co(p=0.005) and Cr(p=0.001). Males with cups within Z3(n=27) had lower ion levels in comparison to those outside, which were significantly lower for Co(p=0.049) but not Cr(p=0.084). Females had similar ion levels within and out of Z3(Cr/Co: p=0.83/0.84). Co levels were significantly lower in Z1(n=13) in comparison to Z2(n=33)(p=0.048) but Cr levels were not different (p=0.06).

Discussion: MoMHRA cups placed within ±5° of the optimum(40°/20°) had significantly lower metal ions indicating lower wear within this narrow zone. This safe zone, could extend to ±10° for males only. The narrower safe zone coupled with smaller components implanted are possible factors contributing to the increased IP incidence seen in females.

Introduction: The use of highly cross-linked polyethylene (HXLPE) is now commonplace for total hip arthroplasty, however there is no long-term data to support its use. Hip simulator studies suggest that the wear rate of some types of HXLPE is ten times less than conventional polyethylene (UHMWPE). The outcomes of hip simulator studies are not always reproduced in vivo and there is some evidence that HXLPE wear may increase between 5 and 7 years.

Method: A prospective double blind randomised control trial was conducted using Radiostereometric Analysis (RSA). Fifty-four subjects were randomised to receive hip replacements with either UHMWPE liners or HXLPE liners. All subjects received a cemented CPT stem and uncemented Trilogy acetabular component (Zimmer, Warsaw, IN, USA). The 3D penetration of the head into the socket was determined to a minimum of 7 years.

Results: The total liner penetration was significantly different at 7 years (p=0.005) with values of 0.33 mm (SE 0.05 mm) for the HXLPE group and 0.55 mm (SE 0.05 mm) for the UHMWPE group. The steady state wear rate from 1 year onwards was significantly lower for HXLPE (0.005 mm/yr, SE 0.007 mm/yr) than for UHMWPE (0.037 mm/yr, SE 0.009 mm/yr) (p=0.007). The direction of wear was supero-lateral.

Discussion: We have previously demonstrated that the penetration in the first year is creep-dominated, from one year onwards the majority of penetration is probably due to wear. This study confirms the predictions from hip simulator studies which suggest that the wear rate of this HXLPE approaches that of metal-on-metal and ceramic-on-ceramic articulations. HXLPE may have the potential to reduce the need of revision surgery, due to wear debris induced osteolysis. It may also enable surgeons to use larger couples, thus reducing the risk of impingement and dislocation.

Introduction: Metal on metal hip resurfacing arthroplasty(MoMHRA) is an alternative option to THR in the treatment of young adults with OA. A recognised MoMHRA complication is the development of an inflammatory pseudotumour(IP). Diagnosis is made with the aid of US and/or MRI. To-date, no radiographic indication of the presence of IP has been identified. Neck thinning is a recognised phenomenon in MoMHRA hips not associated with any adverse clinical events. Its pathogenesis is considered multi-factorial. Our aim was to establish whether excessive neck narrowing is associated with the presence of a pseudotumour.

Methods: Twenty-seven hips (26 patients) with IP confirmed clinically, radiologically, intra-operatively and histologically were matched for sex, age, pre-operative diagnosis, component size and follow-up with an asymptomatic MoMHRA cohort (Control n=60). For all patients, prosthesis-neck-ratio(PNR) was measured on plain AP pelvic radiographs post-operatively and at follow-up as previously described and validated.

Results: All IP patients (4M:23F) and all (12M:48F) but two controls had a posterior approach at the time of MoMHRA. Post-operatively, there was no difference in the PNR between the two groups (p=0.19). At an average follow up of 3.5 years (range:0.7–8.3), IP patients(mean 1.26, 1.10–1.79) had a significantly higher (p< 0.0001) PNR in comparison to their controls(mean 1.14, 1.03–1.35). Greater neck narrowing occurred in both genders. IP necks had narrowed by an average of 8% (range:3–23). The degree of neck narrowing was correlated with length of survival of implant (p=0.001).

Discussion: This study shows a strong association between IP and neck narrowing. Processes such as impingement and increased wear are considered to be involved in the pathogenesis of both IP and neck narrowing. Furthermore, the presence of an IP, could lead to altered vascularity via a mass effect and further contribute to neck narrowing. Neck narrowing in symptomatic MOMHRA patients should alert surgeons of the possible presence of IP.

There have been many reports of metal ion levels measured in the bloodstream of patients after metal-on-metal hip replacement, and it is generally accepted that levels of cobalt (Co) and chromium (Cr) are elevated after these types of devices are implanted. However, it is not clear how to interpret these elevated levels; in particular what are the acceptable levels and what levels indicate that close monitoring of the patient is needed. Our aim was to establish the differences in metal ion levels between well functioning patients and those with clinical problems.

We measured serum Co and Cr levels (microgram’s per litre or μg/l) using inductively coupled plasma mass spectrometry with a well established collection protocol of all patients attending follow-up clinics. Our inclusion criteria for this study were all patients unilaterally implanted with a metal-on-metal hip resurfacing with no other metallic implant; patients were categorized as either A. Well Functioning or B. Clinically Problematic (pain, reduced function, reduced ROM, negative x-ray findings) and differences in ion levels between these two groups were examined. Well functioning patient data was only included if measurements were made more than 12 months post-operatively to avoid run-in wear levels. Abduction angle was also measured from x-rays of the pelvis, and the frontal plane coverage arc of each implanted cup calculated (De Haan JBJS[Br] 2008;90(10):1291–7). There were a total of 519 patients, with 358 in Group A and 161 in Group B; patients had a variety of devices with Birmingham Hip Resurfacing (64%) and Conserve Plus (29%) being the most commonly implanted. To establish a guideline upper ion level value for well functioning implants the upper 75th percentile values for Co and Cr levels for Group A patients having 15 mm or more coverage arc were calculated. The risk of having clinical problems was calculated as function of metal ion levels higher or lower than these upper limits.

The ion levels were significantly (Mann Whitney U p< 0.001) higher in Group B (mean [95% confidence intervals], Co 10.2 μg/l [5.9 to 14.5], Cr 10.3 μg/l [6.7 to 14.0]) compared to Group A (Co 2.3 μg/l [1.7 to 2.4], Cr 2.8 μg/l [2.3 to 3.4]). The well functioning upper limit for Co was 4.1 μg/l and for Cr was 5.2 μg/l. Metal ion levels greater than these upper limits were significantly (Chi-square p< 0.001) associated with the presence of clinical problems. The odds ratio for Co greater than 4.1 μg/l was 11.2 [95%CI 5.7 to 22.3] and that for Cr greater than 5.2 μg/l was 4.3 [95%CI 2.6 to 7.0].

There were significantly higher metal ion levels measured in patients with clinical problems after metal-on-metal hip resurfacing than those with well functioning hips. We have proposed upper acceptable limits for Co (4.1 μg/l) and Cr (5.2 μg/l) serum levels. Cobalt levels appear to be more reliable in predicting risk of clinical problems; levels greater than our proposed upper limit have 11 times the odds of developing clinical problems and patients with such levels should be followed closely.

Introduction: A unique failure mode of hip resurfacing is femoral neck fracture. These tend to occur early after surgery during normal activities. One theory regarding fracture occurrence includes the introduction of stress magnifiers in the form of notches on the superior neck. The presence of a notch can arise from reaming or from removal of osteophytes during surgery. The aim of the present study was to investigate the effect of notching the femoral neck, following resurfacing by using a finite element (FE) model.

Methods: A physiological load case was simulated in the FE model of a femur, implanted with a cemented hip resurfacing system. Twelve implant alignments were modelled: an ideal implant alignment with no notch, and a 1 mm, 3 mm, 5 mm and 7 mm superior notch; 5° anteversion, 5° and 10° degrees retroversion; 5° and 10° degrees in varus and valgus. These models were compared to that of an intact femur for baseline analysis.

The intact femur geometry was derived from a CT dataset of a cadaveric femur and CT numbers were converted into a realistic distribution of material properties. The FE intact mesh was based on an experimentally validated mesh of a human femur. The femur was segmented into 22 neck sections.

The loading condition was modelled to represent an instant at 10% of gait where all muscle forces were included. The femoral neck regions were compared between the models to evaluate the effect of notch sizes on stress distribution. Maximum tensile stresses were compared to the ultimate tensile stress (UTS) of cortical and cancellous bone.

Results: As the notch size increased the peak and average 1st (tensile) and 3rd (compressive) principal stress increased along the superior portion of the femoral neck. For the 5 mm superior notch, the maximum 1st principal stress increased by 283% and 154% when compared to that of the ideally aligned implant and the intact femur respectively. The largest increase of tensile stress was observed when the implant was mal-aligned in 10° of varus; this resulted in a 768% increase in stress compared to the ideally implanted model.

Discussion: The introduction of a superior notch causes a stress concentration on the femoral neck. Although the stress concentration is pronounced, a notch on the superior aspect of the femoral neck may not lead to fracture following resurfacing; the UTS of cortical bone is 100MPa, and the UTS of cancellous bone is between 2MPa and 20MPa. Peak stresses in the model are well below the UTS of cortical bone, and for damage to accumulate in cancellous bone, energy absorption in the ‘honey-comb’ structure of trabecular bone must be considered. Varus mal-alignment resulted in the largest increase in tensile stress on the superior aspect of the neck, and has been associated with femoral neck fracture; this type of mal-alignment may be critical when considering femoral neck fractures.

Introduction: Edge-loading, a phenomenon whereby the femoral component comes into contact with the edge of the acetabular component, has been suggested to increase wear in metal-on-metal hip resurfacing arthroplasty (MoMHRA). Pseudotumours (soft-tissue mass relating to the hip joint) have been associated with elevated serum and hip aspirate metal ion levels. This study aimed to investigate in vivo edge-loading in MoMHRA patients with pseudotumours by quantifying dynamic loci of the hip joint segment force relative to the acetabular component during functional activities.

Materials and Methods: A total of 21 MoMHRA patients (30 hips) in two groups were investigated in this Ethics approved study:

6 patients with pseudo-tumours detected using ultrasound/MRI;

Three-dimensional lower limb motion analysis (12 camera Vicon System) was performed to estimate hip joint segment force during walking, chair-rising and stair-climbing. CT scans were used to determine each patient’s specific hip joint centre and acetabular component orientation. Edge-loading was defined to occur when a hip joint segment force vector/ cup intersection was located within 10% of the cup radius from the edge of the cup. Serum cobalt and chromium levels were analysed using Inductively-Coupled Plasma Spectrometer.

Results: Edge-loading in the pseudotumour group occurred with significantly (p=0.02) longer (4-fold increase) duration as well as greater magnitude (7-fold increase) of force, compared to the non-pseudotumour group. The duration and force of the edge-loading were activity-dependent, with proportionally greater difference observed during stair climbing. The acetabular cup orientation values in the pseudotumour group were found within the safe zone of Lewinnek in one third of the hips with the remaining two thirds outside the safe zone. The presence of pseudotumour was associated with:

significantly higher median serum cobalt levels: 14.3ug/l (range 10.6–64.1) vs. 1.9ug/l (range 1.2–5.0), p< 0.001;

Discussion: Edge-loading in MoMHRA patients with pseudotumours occurred in vivo with significantly longer duration and greater magnitude of force impulse compared to the patients with a well functioning MoMHRA during activities of daily living. This suggests that edge-loading may be an important mechanism that leads to localised high wear, with subsequent elevation of metal ion levels in MoMHRA patients with pseudotumours. Although the acetabular component malposition, such as increase in both inclination and anteversion angles, appears to be an important factor in edge-loading, the aetiology of edge-loading is likely to be multi-factorial.

Introduction: Approximately 20% of unicompartmental knee replacement (UKR) revisions are related to polyethylene wear. The Phase 1 Oxford UKR was introduced as a design against wear, with a fully congruent mobile bearing. The Phase 2 implant was introduced with new instrumentation (femoral mill) and changes to the bearing shape (lower anterior wall) to reduce the incidence of anterior impingement. We have previously shown that the Oxford UKR has a wear rate of 0.02 mm/year at ten years, in well functioning devices, but that higher wear rates can be seen with impingement or if the congruous articulation is lost. The aim of this study was to determine the 20 year in-vivo wear of the Oxford Phase 1 and Phase 2 UKR, using Roentgen Stereophotogrammetric Analysis (RSA).

Method: We measured the in-vivo wear of 6 Phase 1 (5 patients, mean age 65.24 years) and 7 Phase 2 (4 patients, mean age 63.43) Oxford UKR bearings. Average time since surgery was 22.37 years and 19.46 years for the Phase 1 and Phase 2 implants respectively. Selection criteria included patients who were mobile, with an exercise tolerance greater than 100m as per the American Knee Society Score (AKSS) functional questionnaire. RSA x-rays were taken with the knee in the normal anatomical position on standing and with the knee flexed to 30o. The Oxford knee score (OKS) and AKSS were gained at the RSA examination. Phase 1 and 2 components were reverse engineered by laser scanning, and converted to CAD models. The CAD models of the tibia and femur were pose-estimated in the RSA software (Medis Specials, Leiden, Netherlands). A sphere was fit to the femoral component and the minimum bearing thickness was determined by measuring the shortest perpendicular distance between the sphere and the plane contained on the tibial tray articular surface. The linear wear for each bearing was calculated by subtracting the measured thickness from the corrected nominal bearing thickness. Non-parametric statistics were used to compare the two Phases.

Results: There was no significant difference in age, OKS and AKSS between the two groups. The median wear rate was 0.078 mm/year for Phase 1 and 0.023 mm/year for Phase 2. This difference was statistically significant (p = 0.027).

Discussion: The difference in wear rate is explained by impingement in Phase 1, which was reduced by design changes with the introduction of Phase 2; the Phase 2 is designed to avoid impingement between the femur and the bearing. This study demonstrates that very low wear rates can be maintained with the Phase 2 implant to the end of the second decade after implantation. This is of particular importance when the device is used in younger patients and demonstrates that the Oxford UKR can be a definitive implant for the treatment of isolated compartmental osteoarthritis.

Introduction: Pseudotumours (soft-tissue masses relating to the hip joint) following metal-on-metal hip resurfacing arthroplasty (MoMHRA) have been associated with elevated serum and hip aspirate metal ion levels, suggesting that pseudotumours occur when there is increased wear. This study aimed to quantify in vivo wear of implants revised for pseudotumours and a control group of implants revised for other reasons of failure.

Methods: A total of 30 contemporary MoMHRA implants in two groups were investigated in this Institutional Review Board approved study:

8 MoMHRA implants revised due to pseudotumour;

The linear wear of retrieved implants was measured using a Taylor-Hobson Roundness machine. The average linear wear rate was defined as the maximum linear wear depth divided by the duration of the implant in vivo.

Results: In comparison with the non-pseudotumour implant group, the pseudotumour implant group was associated with:

significantly higher median linear wear rate of the femoral component: 8.1um/year (range 2.75–25.4um/year) vs. 1.79um/year (range 0.82–4.15um/year), p=0.002; and

Similarly, differences were also measured in absolute wear values. The median absolute linear wear was significantly higher in the pseudotumour implant group:

21.05um (range 2.74–164.80um) vs. 4.44um (range 1.50–8.80um) for the femoral component, p=0.005; and

Wear on the acetabular cup components in the pseudotumour group always involved the edge, indicating edge-loading of the bearing. In contrast, edge-loading was observed in only one acetabular component in the non-pseudotumour group of implants. The deepest wear was observed well within the bearing surface for the rest of the non-pseudotumour group. The difference in the incidence of edge-loading between the two groups was statistically significant (Fisher’s exact test, p=0.03).

Discussion: Significantly greater linear wear rates of the MoMHRA implants revised due to pseudotumour support the in vivo elevated metal ion concentrations in patients with pseudotumours. This study provides the first direct evidence to confirm that pseudotumour is associated with increased wear at the MoM articulation. Furthermore, edge-loading with the loss of fluid film lubrication may be the dominant wear generation mechanism in patients with pseudotumour.

Introduction: Anteromedial gonarthrosis is a common well described pattern of knee osteoarthritis with cartilage wear beginning in the anteromedial quadrant of the medial tibial plateau in the presence of an intact and functioning ACL. It is well known that mechanical factors such as limb alignment and meniscal integrity affect the progression of arthritis and there is some evidence that the morphology of the tibial plateau may be a risk factor in the development of this disease. The extension facet angle is the angle of the downslope of the anterior portion of the medial tibial plateau joint surface in relation to the middle portion on a sagittal view. If this is an important factor in the development of AMG there may be potential for disease modifying intervention.

This study investigates if there is a significant difference in this angle as measured on MRI between a study cohort with early AMG (partial thickness cartilage damage and intact ACL) and a comparator control cohort of patients (no cartilage damage and ACL rupture).

Methods: 3 Tesla MRI scans of 99 patients; 54 with partial thickness cartilage damage and 44 comparitors with no cartilage damage (acute ACL rupture) were assessed. The extension facet angle was measured (Osirix v3.6) using a validated technique on two consecutive MRI T2 sagittal slices orientated at the mid-coronal point of the medial femoral condyle. (InterClass Correlation 0.95, IntraClass Correlation 0.97, within subject variation of 1.1° and coefficiant of variation 10.7%). The mean of the two extension angle values was used. The results were tabulated and analysed (R v2.9.1).

Results: Of the 99 knees, 38 were female and 61 male; 44 left knees and 55 right. The mean extension facet angle for the partial thickness group was 12.7° (SD 3.35) and for the comparator group 8.7° (SD 3.09). There was a significant difference between these 2 groups (Mann Whitney U, p< 0.001). Although there were significantly more men than women in the comparator group, stratification analysis showed that there was no effect of gender on the mean extension facet angle.

Discussion: There is a significance difference in the extension facet angle between patients with AMG with only partial thickness cartilage loss and a comparator group. This has not been shown in a study group of this size before. Since none of the subjects had full thickness cartilage loss it is unlikely that this difference is due to bone attrition changing the angle as part of the disease process but this is an important area for further study. We believe that a higher medial tibial extension facet angle alters the mechanics within the medial compartment, placing these patients at higher risk of developing AMG. This may present an opportunity for risk factor modification, for example osteotomy.

Introduction: Partial Knee Replacement (PKR) is an appealing alternative to Total Knee Replacement (TKR) when the patient has isolated compartment osteoarthritis (OA). In nearly all cases there is a radiolucency observed between the tibial tray wall and the boney interface. The reasons why radiolucencies appear are unknown, but the bone will adapt to its altered mechanical environment by bone remodelling in accordance with ‘Wollf’s Law’. The aim of this study was to investigate the mechanical environment of the tibia bone adjacent to the tray wall, following cemented and cementless PKR, in order to determine whether this region of bone resorbs.

Methods: A validated finite element (FE) model of a cadaver tibia implanted with an Oxford PKR was used in this study. Kinematic data from fluoroscopy measurements during a step-up activity were used to determine the relative tibio-femoral positioning for the Oxford PKR model. Load data were adapted from the in-vivo measured loads using an instrumented implant during a step-up activity. The standard operating protocol was simulated for the Oxford PKR FE models, with the tibial tray implanted in a neutral position. The tibia was sectioned around the tray. Zone 7 was defined as parallel to the vertical tray wall, corresponding to the region on screened x-rays where radiolucencies are observed. It was assumed that the bone in the implanted tibia will attempt to normalise its stress-strain patterns locally to its equilibrium state, the intact tibia, for the same loading conditions. Forty patients (20 cemented, 20 cementless) who had undergone PKR were randomly selected from a database, and their screened x-rays assessed for radiolucency in region 7.

Results: The SED in region 7 was 80% lower in the cemented and cementless tibia, compared to the intact tibia (Figure 2). The maximum tensile stress was 63% lower in the cemented and cementless tibia, compared to the intact tibia. The corresponding maximum compressive stress was 52% lower. Radiolucency was observed in all forty radiographs in region 7.

Discussion: After implantation with a cemented or cementless PKR the bone strains and SED in region 7 are reduced. This reduction may provide the signal for adaptive bone remodelling and bone will be resorbed from this region, decreasing the volume and increasing the SED. Bone resorption will continue until the equilibrium state is reached. If a ‘lazy’ zone between 35% and 50% of the remodelling signal is considered, bone resorption will still occur due to the large decrease in SED for this region. For region 7 to return its SED to the equilibrium state, its volume will need to be reduced by 80%. This is likely to be the reason why a radiolucency is observed clinically in this region in almost every case, whether a cemented or cementless implant is used.

Introduction: Metal on metal hip resurfacing arthroplasty-induced pseudotumours are a serious complication, which occur in 4% of patients who undergo this procedure. The aim of this study was to measure the 3D in vivo wear on the surface of resurfacing components revised for pseudotumour, compared to a control group.

Method: Thirty-nine hip resurfacing implants were examined; these were sourced from our institutions prosthesis retrieval bank. They were divided into two groups; 22 patients with a clinical and histopathological diagnosis of pseudotumour and 17 controls. Patient demographics and time to revision were known. Three dimensional contactless metrology (Redlux™ Ltd) was used to scan the surface of the femoral and acetabular components, to a resolution of 20 nanometers. The location, depth and area of the wear scar was determined for each component. Volumetric wear was determined, along with the presence of absence of edge-loading. A separate blinded analysis to determine the presence of absence of impingement was performed by one of the authors. ANOVA was used to test for differences in wear and Fishers Exact test was used to compare the incidence of edge-loading between the groups.

Results: The volumetric wear rate for femoral component of the pseudotumour group was 4.7mm3/yr (SD3.5) and 1.7 mm3/yr (SD1.5) for the control group (p=0.03). In the pseudotumour group, the volumetric wear rate of the acetabular component was 3.5 mm3/yr (SD3.6) compared to 0.02 mm3/yr (SD0.07) for the control group (p=0.01). Edge-loading was detected in 74% of acetabular components in the pseudotumour group and 22% of those in the control group (p=0.01). Anterior or posterior edge-loading, consistent with impingement was present on the femoral components of 73% of patients in the pseudotumour group and 22% in the control group (p=0.01).

Discussion: This work demonstrates that implants revised for pseudotumour have significantly higher volumetric wear rates than controls. They also have a significantly higher incidence of edge-loading and impingement than controls. Edge-loading significantly increases wear. We suggest that pseudotumours are caused by high concentrations of metal wear debris, which have been shown to have a toxic effect on osteocytes and macrophages. This is the one of the first studies to demonstrate a clear link between pseudotumours and increased bearing surface wear. It is also the first to demonstrate that edge-loading, due to impingement, occurs in a significant number of patients who develop this condition. Improved implantation techniques and resurfacing designs may help avoid this serious complication of hip resurfacing.

Introduction: The use of second generation highly cross-linked polyethylene (HXLPE) is now commonplace for total hip arthroplasty, however there is no long-term data to support its use. Hip simulator studies suggest that the wear rate of HXLPE is ten times less than conventional polyethylene (UHMWPE). The outcomes of hip simulator studies are not always reproducible in vivo. Long term clinical data is required, as there is emerging clinical data, which suggests that some types of second generation HXLPE may have increased wear after 5 years.

Method: A prospective double blind randomised control trial was conducted using Radiostereometric analysis (RSA). Fifty-four subjects were randomised to receive hip replacements with either UHMWPE liners or HXLPE liners. All subjects received a cemented CPT stem and uncemented Trilogy acetabular component (Zimmer, Warsaw, IN, USA). The 3D penetration of the head into the socket was determined to a minimum of 7 years.

Results: The total liner penetration was significantly different at 7 years (p=0.01) with values of 0.33mm (SD 0.17mm) for the HXLPE group and 0.51mm (SD 0.14mm) for the UHMWPE group. The steady state wear rate from 1 year onwards was significantly lower for HXLPE (0.003 mm/yr, SD 0.04 mm/yr) than for UHMWPE (0.03 mm/yr, SD 0.03 mm/yr) (p=0.01). The direction of wear was in the antero-medial direction in both groups.

Conclusion: We have previously demonstrated that the penetration in the first year is creep-dominated, from one year onwards the majority of penetration is due to wear. The wear rate of this second generation HXLPE approaches that of metal on metal bearings. Second-generation HXLPE may have the potential to reduce the risk of revision surgery, due to wear debris induced osteolysis.

Introduction: Metal-on-metal hip resurfacing arthroplasty (MoMHRA) has gained popularity as an alternative to THR for younger patients with osteoarthritis. A growing concern has been the association of MoMHRA with the development of inflammatory pseudotumours (IP), especially in women. These have been linked to metal-on-metal wear, which can be related to metal ion concentrations. Elevated metal wear debris levels may result from impingement, rim contact and edge loading. Head-neck ratio (HNR) is a predetermining factor for range of movement and impingement. Neck thinning is a recognised phenomenon post-MoMHRA and we have found an association of IP with increased neck thinning based on a case control study. Our aims were to identify HNR changes a hip undergoes when resurfaced and at follow up; and whether greater neck thinning at follow-up could be associated with the presence of elevated metal ions.

Methods: A cohort of 91 patients (57M:34F) with unilateral MoMHRAs were included in this study. Blood tests were obtained at a mean follow up of 3.9 years (range 1.7–7 years) and serum (Co:Cr) ion levels were measured (ICPMS). High metal ion concentrations were defined as Co> 4.1ppb and Cr> 5.2ppb. For all patients, head-neck ratio (HNR) was measured on plain anterio-posterior pelvic radiographs pre-operatively, immediately post-operatively and at follow-up.

Results: Female patients had significantly bigger HNR pre-op (mean=1.35, range:1.22–1.64) compared to males(mean=1.22, range:1.05–1.38) (p< 0.01). Immediately post-op, female HNRs (mean: 1.26, range: 1.14–1.34) were not different to male patients(mean=1.24, range=1.11–1.38) (p=0.11). At follow-up HNR was once again significantly bigger (p< 0.01) in females (mean=1.35, range: 1.21–1.49), compare to males (mean=1.27, range:1.11–1.38). HNR alterations with operation (p=0.00) and at follow-up (p< 0.01) were significantly bigger in female patients. Furthermore, there was a significant correlation between high ion levels and HNR change at follow-up for both Co (p=0.02) and Cr (p< 0.01).

Conclusion: This study identified gender-specific changes in HNR that resurfaced hips undergo, not previously documented. Female hips have greater HNR pre-operatively, compared to male hips, and appear to be biomechanically disadvantaged when resurfaced. A decrease in HNR with resurfacing could result in impingement and lead to processes, known to be more prevalent in females, such as neck thinning, increased wear and IP development. In addition, we highlight a correlation between high ion levels and greater neck thinning at follow-up. Increased neck thinning in symptomatic MoMHRA hips could be secondary to increased wear and should be investigated further radiologically for the presence of IP.

Introduction: Metal on metal hip resurfacing arthroplasty (MoMHRA) has become an alternative option to THR in the treatment of young adults with OA. A recognised MoMHRA complication is the development of an inflammatory pseudotumour (IP). IPs can be cystic (predominantly posterio-laterally located), solid (mostly anteriorly located) or mixed in nature. Diagnosis is made with the aid of US and/or MRI. To-date, no radiographic aid in the diagnosis of IP has been identified. Neck thinning is a recognised phenomenon following MoMHRA, occurring in up to 90% of resurfaced hips, which has not been associated with any adverse clinical events. Its pathogenesis is considered multi-factorial secondary to stress shielding, impingement, pressure effect on cancellous femoral neck, bone necrosis secondary to femoral preparation and altered vascularity/AVN. Our aim was to establish whether neck thinning is associated with the presence of a pseudotumour.

Methods: Thirty-one hips (30 patients) with IP confirmed clinically, radiologically, intra-operatively and histologically were matched for sex, age, pre-operative diagnosis, component size and follow-up with an asymptomatic MoMHRA cohort without pseudotumour (Control n=60). Radiological and operative findings at the time of revision of all IP patients were reviewed regarding location of pseudotumour; 4 different locations were defined: anteriorly-extending, posteriorly-extending, anteriorly & posteriorly-extending and within joint only. For all patients, prosthesis-neck ratio (PNR) at follow-up was measured on plain AP pelvic radiographs as previously described and validated.

Results: All IP patients (6M:24F) and all (12M:48F) but two controls had a posterior approach at the time of MoMHRA. Mean femoral component size was 46 mm for both groups. At an average follow up of 3.5 years (0.7–8.3), IP patients (mean 1.26, 1.10–1.79) had a significantly higher (p< 0.0001) PNR in comparison to their controls (mean 1.14, 1.03–1.35). Greater neck thinning had occurred in both IP-males (p< 0.001) and IP-females (p=0.002) in comparison to their controls. Location of IP and hence nature did not appear to have an effect on the degree of neck thinning.

Discussion: This study shows that IP patients had significantly narrower femoral necks at follow-up. Processes, such as impingement and increased wear that are thought to contribute to the process of neck narrowing are also thought to be factors in IP development. Furthermore, the presence of an IP, could lead to altered vascularity via a mass effect and further contribute to neck narrowing. Interestingly, nature of IP did not have a significantly affect PNR. Although one cannot be certain whether neck narrowing is a consequence or a contributing factor for IP development, their association is significant. Surgeons should consider the possibility of pseudotumour in symptomatic MoMHRA patients with neck narrowing.

Introduction: Metal-on-metal hip resurfacing arthroplasty (MoMHRA) has gained popularity as an alternative to THR for younger patients with osteoarthritis. A growing concern has been the association of MoMHRA with the development of inflammatory pseudotumours (IP), especially in women. These have been linked to metal-on-metal wear, which can be related to metal ion concentrations. Although cup orientation has been shown to influence wear, the optimum cup position has not been clearly defined. We have identified an optimal cup orientation to minimise IP risk, based on a case controlled study, for inclination/anteversion within ±10° of 40°/20°. Our aim was to see if this optimal position results in lower metal ions, and to identify the boundary of an optimal placement zone for low wear.

Methods: A cohort of 104 patients (60M: 44F) with unilateral MoMHRA was included in this study. Blood tests were obtained at a mean follow up of 3.9 years (range 1.7–7 years) and serum Co and Cr ion levels were measured (ICPMS). High metal ion concentrations were defined as Co> 4.1ppb and Cr> 5.2ppb. Radiographic cup inclination and anteversion were measured using EBRA. The differences in ion levels between different cup orientation zones were investigated. Three orientation zones were defined centered on the target orientation of 40°/20°: Z1 within ±5°, Z2 outside ±5°/within ±10° and Z3: within ±10°.

Results: There was a wide range of cup placements; mean inclination/anteversion were 46.3°(21.5°–64.6°)/15°(2.7°–35.6°). Cr levels, but not Co, were higher in female patients (p=0.002) and those with small femoral components (< 50mm, p =0.03).

For the whole cohort, there was no significant difference in ion levels (Cr: p=0.092. Co=0.075) between cups positioned within Z3 (n=58) versus those outside (n=46 mean). Male patients with cups within Z3 (n=27) had lower ion levels in comparison to those outside Z3, which were significantly lower for Co (p=0.049) but not Cr (p=0.084). Female patients had similar levels within and out of Z3 for both ions (Cr: p=0.83, Co: p=0.84). However, patients with cups within Z1 (n=13) had significantly lower Co (p=0.005) and Cr (p=0.001) than those outside Z1 (n=95). Interestingly, Co levels were significantly lower in Z1 (n=13) in comparison to Z2 (n=33) (p=0.048) but Cr levels were not different (p=0.06).

Discussion: MoMHRA cups placed with ±5° of the ideal position of 40°/20°gave rise to significantly lower metal ions indicating lower wear within this narrow zone, in both sexes. This safe zone, could be extended to ±10° for male patients only. Gender specific factors, such as pelvic anatomy and joint flexibility, could be responsible for the narrower ‘safe’ zone seen in females. The narrower safe zone coupled with smaller components implanted are factors contributing to higher ion levels and hence the increased incidence of IP seen in females.

Introduction: The development of Inflammatory Pseudotumour (IP) is a recognised complication following Metal on Metal Hip Resurfacing Arthroplasty (MoMHRA), thought to occur secondary to wear and elevated ion levels. Studies have shown that acetabular component orientation influences the wear of metal-on-metal hip replacement bearings. The aims of this study were to investigate the significance of cup orientation in the development of IP, and to identify a ‘safe-zone’ for cup placement with lower-risk for IP development.

Methods: Twenty six patients (n=27 hips) with IP confirmed radiologically, intra-operatively and histologically were matched for sex, age, pre-operative diagnosis, component size and follow-up with a cohort of asymptomatic MoMHRA patients (Control n=58). Radiographic acetabular anteversion and inclination were measured using EBRA. We calculated the distance in degree space of each acetabular component from the optimum position of 40° inclination and 20° anteversion, recommended by the designers, and thus compared acetabular component position between the two groups. Three different zones were tested as possibly optimum for acetabular placement. These were Lewinneck’s Zone (LZ) (inclination/anteversion; 30–50°/5–25°), and two zones defined by ±5° (Zone 1) or ± 10° (Zone 2) about the suggested target of 40°/20°. An optimal placement zone was determined based on a significant difference in IP incidence between components in the zone versus those outside.

Results: There was a wide range in cup orientations; mean inclination and anteversion were similar in the two groups: IP 47.5° (10.1°–80.6°)/14.1° (4.1°–33.6°) Vs Control 46.1° (28.8°–59.8°)/15.6° (4.3°–32.9°). Acetabular components in the IP group were significantly further away from the optimum position of 40°/20° in comparison to the controls (p=0.023). There was no difference in IP incidence between cups positioned within (IP:13/27, Control:35/58) or out of LZ (p=0.09) and within (IP: 2/27, Control: 10/58) or out of Zone 1 (p=0.156). Cups placed in Zone 2 (IP:6/27, Control:27/58) had significantly lower IP incidence versus those outside this zone (p=0.01). The odd’s ratio of developing IP when the cup is positioned out-of Zone 2 was 3.7.

Discussion: This study highlights the importance of ace-tabular component orientation in IP development. On the whole, patients with pseudotumour had acetabular components that were further away from the optimum position in comparison to the controls. However, a small number of IP patients had well-placed components implying that additional factors, possibly patient and/or gender specific, are involved in the development of pseudotumour. Furthermore, we defined an optimum, ‘safe-zone’ of ±10° around the cup position of 40°/20°. Patients with acetabular components outside this safe zone have an increased risk of IP development.

The results of mobile bearing Oxford unicompartmental knee replacement (UKR) in the lateral compartment have been disappointing (five-year survival: 82%). Therefore, it is recommended that mobile bearings should not be used for lateral UKR. This low survivorship is primarily due to a high dislocation rate. A detailed analysis confirmed the elevated lateral tibial joint line to be a contributory factor to bearing dislocation. A new surgical technique was therefore introduced in which care was taken neither to remove too much bone from the distal femur nor to over tighten the knee and therefore ensure that the tibial joint line was not elevated. Other modifications included use of a domed tibial component.

The aim of this study is to compare the outcome of these iterations: the original series (series I), those with improved surgical technique (series II) and the domed tibial component (series III). The primary outcome measure was bearing dislocation at one year. One year was chosen as all the dislocations in the first series occurred within a year. In the original series (n=53), implanted using a standard open approach, there were six dislocations in the first year, the average flexion 110°, and 95% had no/mild pain on activity. In the second series (n=65), there were 3 dislocations, the average flexion was 117°, and 80% had no/mild pain on activity.

In the third series with the modified technique and a convex domed tibial plateau, there was one dislocation, average flexion was 125° and 94% had no/mild pain on activity. At four years the cumulative primary dislocation rates were 10%, 5% and 0% respectively, and were significantly different (p=0.04).

The improved surgical technique and implant design has reduced dislocation rate to an acceptable level so a mobile bearing can now be recommended for lateral UKR.

Roentgen Stereophotogrammetric Analysis (RSA) can predict long-term outcome of prostheses by measuring migration over time. The Exeter femoral stem is a double-tapered highly polished implant and has been shown to subside within the cement mantle in 2 year RSA studies. It has a proven track record in terms of long-term survivorship and low revision rates. Several studies have demonstrated excellent clinical outcomes following its implantation but this is the first study to assess stem migration at 10 years, using RSA.

This is a single-centre study involving 20 patients (mean age: 63 years, SD=7) undergoing primary total hip replacement for degenerative osteoarthritis using the lateral (Hardinge) approach. RSA radiographs were taken with the patient bearing full weight post-operatively, at 3, 6, 12 months and at 2, 5 and 10 years follow-up. The three-dimensional migration of the Exeter femoral stem was determined.

The mean Oxford Hip Score at 10 years was 43.4 (SD=4.6) and there were no revisions. The stems subsided and rotated internally during a 10-year period. The mean migrations of the head and tip of the femoral stem in all three anatomic directions (antero-posterior, medio-lateral & supero-distal) were 0.69 mm posterior, 0.04 mm lateral and 1.67 mm distal for the head and 0.20 mm anterior, 0.02 mm lateral and 1.23 mm distal for the tip. The total migration at 10 years was 1.81 mm for the head and 1.25 mm for the tip.

The Exeter femoral stem exhibits migration which is a complex combination of translation and rotation in three dimensions. Comparing our 10 year with our previous 2 year migration results, the Exeter stems show continued, but slow distal migration and internal rotation. The subsidence continues to compress the cement and bone-cement interface which maintains secure fixation in the long term.

Introduction: Symptomatic abnormal soft-tissue masses relating to the hip joint, such as those described as pseudotumours, are being increasingly reported following metal-on-metal hip resurfacing arthroplasty (MoMHRA). These were found to be locally destructive, requiring revision surgery in a high proportion (75%) of patients. Lymphocyte infiltrations seen in pseudotumours were similar to aseptic lymphocyte vascular associated lesion (ALVAL), which is thought to represent a T-lymphocyte-mediated delayed type hypersensitivity. Therefore, a delayed hypersensitivity reaction to nickel (Ni), chromium (Cr) or cobalt (Co) has been suggested to play a role in pseudotumour aetiology. In patients with bilateral MoMHRA who presented with symptoms on one side, subsequent scans have demonstrated pseudotumours both on the symptomatic and asymptomatic side. Thus, there are concerns that there may be an appreciable number of asymptomatic pseudotumours that surgeons are unaware of and these may eventually become symptomatic.

Aim: The aims of this study were:

to determine the prevalence of asymptomatic pseudotumours after MoMHRA; and

Methods: A total of 201 MoMHRA implanted hips in 158 patients (97 male, 61 female) with a mean age of 56 years (range 33–73 years) were evaluated. The mean follow-up was 61 months (range 13–88 months). Resurfacing devices implanted included 128 Birmingham Hip Resurfacing, 66 Conserve Plus and seven ReCap. The control groups included additional 20 patients, 10 male and 10 female (a mean age 68 years, range 57–80 years) with metal-on-polyethylene total hip arthroplasty and a further 22 age-matched patients (a mean age 55 years) without any metal implants. Ultrasound was used as the initial imaging modality and MRI was used to assess the extent of the identified masses. Patients with a soft-tissue mass had ultrasound-guided aspiration or core biopsy performed. Venous blood samples were collected in all patients for serum cobalt and chromium ion levels analysis using Inductively-Coupled Plasma Mass Spectrometer and lymphocyte transformation tests (LTT). The Oxford Hip Score (OHS) was used to measure the functional outcomes of patients. Acetabular component abduction angle was measured from standardised anteroposterior pelvis radiographs.

Results: Prevalence – Pseudotumours were found in 7 patients (6 female and 1 male). The overall prevalence of asymptomatic pseudotumours was 4%, with a relatively very high (30%) prevalence in females with bilateral implants. Histological examinations showed extensive necrosis of connective tissue, in which there were scattered aggregates of metal particles and a diffuse lymphocyte infiltrate.

Metal Ion Levels – The presence of pseudotumour was associated with significantly higher median serum cobalt levels (9.2mg/L vs. 1.9mg/L, p< 0.001), chromium levels (12.0mg/L vs. 2.1mg/L, p< 0.001), hip aspirate cobalt levels (1182 mg/L vs. 86.2mg/L, p=0.003), and aspirate chromium levels (883mg/L vs. 114.8mg/ L, p=0.006), as well as with inferior functional scores (OHS 41 vs. 47 p< 0.001). There was no significant difference in acetabular cup inclination angle (p=0.51). Lymphocyte Reactivity: A higher incidence and level of enhanced lymphocyte reactivity to Ni (p=0.001), but not to Co or Cr (the principal elements in the CoCr alloy used in metal-on-metal hip resurfacing implants), was found in patients with MoMHRA compared to the patients without MoM implants. However, lymphocyte reactivity to Co, Cr and Ni did not significantly differ in patients with pseudotumours compared to those patients without pseudotumours.

Conclusion: The prevalence of asymptomatic pseudotumours in females was high, especially in females with bilateral MoMHRA implants (30%). The patients with ‘asymptomatic’ pseudotumours were in fact mildly symptomatic. Lymphocyte reactivity to Co, Cr and Ni did not differ in patients with pseudotumour compared to those patients without pseudotumours, suggesting that systemic hypersensitivity type IV reactions, mediated by lymphocyte reactivity to these metals, is not the dominant mechanism in pathogenesis of the soft tissue pseudotumours. Furthermore, pseudotumours were not detected in those patients who had normal levels of cobalt and chromium ions. This suggests that pseudotumours do not occur if MoM articulations are well functioning. Therefore, pseudotumours are likely to be a biological consequence of the large amount of metal debris generated in vivo due to excessive wear.

About ten years ago we introduced sophisticated instrumentation and an increased range of component sizes for the Oxford unicompartmental knee replacement (UKR) to facilitate a minimally invasive surgical (MIS) approach. The device is now routinely implanted through an incision from the medial pole of the patella to the tibial tuberosity. This has resulted in a more rapid recovery and an improved functional result. As the access to the knee is limited there is a concern that the long term results may be compromised. The aim of this study was to determine the 10 year survival.

A prospective follow up of all Phase 3 minimally invasive Oxford UKR implanted by two senior authors (DWM & CAFD) has been undertaken. So far 1015 UKRs have been implanted for anteromedial osteoarthritis. All patients received a cemented implant through a MIS approach and were followed up prospectively by an independent observer. The data was collected prospectively regarding pre-operative status, complications and clinical as well as functional outcome at predetermined intervals.

The average age of patients was 66.4 years (range: 33 – 88) with mean Oxford Knee Score 41 (SD: 7.9) at the time of last follow up, Knee Society Score (objective) of 84 (SD: 13) and Knee Society Score (functional) of 83 (SD: 21). At ten years the survival of this cohort is 96%. There were 22 revisions including 7 for progression of arthritis, 5 for infection, 5 for bearing dislocation, 4 for unexplained pain and one for rupture of ACL secondary to trauma.

We conclude that the Oxford Knee can be implanted reliably through a minimally invasive approach, giving excellent long term results.

Our aim was to investigate the molecular features of progressive severities of cartilage damage, within the phenotype of Anteromedial Gonarthrosis (AMG).

Ten medial tibial plateau specimens were collected from patients undergoing unicompartmental knee replacements. The cartilage within the area of macroscopic damage was divided into equal thirds: T1(most damaged), to T3 (least damaged). The area of macroscopically undamaged cartilage was taken as a 4th sample, N. The specimens were prepared for histological (Safranin-O and H& E staining) and immunohistochemical analysis (Type I and II Collagen, proliferation and apoptosis). Immunoassays were undertaken for Collagens I and II and GAG content. Real time PCR compared gene expression between areas T and N.

There was a decrease in OARSI grade across the four areas, with progressively less fibrillation between areas T1, T2 and T3. Area N had an OARSI grade of 0 (normal). The GAG immunoassay showed decreased levels with increasing severity of cartilage damage (p< 0.0001). There was no significant difference in the Collagen II content or gene expression between areas. The Collagen I immunohistochemistry showed increased staining within chondrocyte pericellular areas in the undamaged region (N) and immunoassays showed that the Collagen I content of this macroscopically and histologically normal cartilage, was significantly higher than the damaged areas (p< 0.0001). Furthermore, real time PCR showed a significant increase in Collagen I expression in the macroscopically normal areas compared to the damaged areas (p=0.04).

In AMG there are distinct areas, demonstrating progressive cartilage loss. We conclude that in this phenotype the Collagen I increase, in areas of macroscopically and histologically normal cartilage, may represent very early changes of the cartilage matrix within the osteoarthritic disease process. This may be able to be used as an assay of early disease and as a therapeutic target for disease modification or treatment.

Despite the satisfactory short-term implant survivor-ship, there is an increasing concern that the metal-on-metal hip resurfacing arthroplasty (MoMHRA) release large amount of very small wear particles and metal ions. The periprosthetic soft-tissue masses such as pseudotumours are being increasingly reported. These were found be locally destructive, requiring revision surgery in most patients. It has been suggested that either an immune reaction or cytotoxic effect of chromium(Cr) or cobalt(Co) may play a role in its aetiology. However, the effect of the phagocytosis of implant-associated metal nanoparticles on macrophages has not been elucidated. The aim of this study was to investigate the in vitro viability and proliferative response of murine macrophages to clinically relevant metal nanoparticles and ions.

Materials and Methods: The RAW 264.7 murine macrophage cell line was cultured in MEM at a seeding density of 10E5 cells/cm2. Culture was set up in the presence of either:(1) negative control: medium alone;(2)Cobalt sulphate heptahyrate and chromium chloride hexahydrate (Sigma) at concentrations of 1uM, 10uM, 100uM;(3)Metal nanoparticles sized 30–35nm (American Elements) of cobalt, chromium and titanium at concentrations from 10E7 to 10E14 particles/ml.

At the end of day 1 and 4, two methods were used to quantify cell proliferation and viability. The AlamarBlue assay(Invitrogen) incorporates a fluorimetric growth indicator and the fluorescence signal correlates with metabolic activity of the cells. LIVE/DEAD stain kit(Molecular Probes) contains two fluorescent dyes to stain living cells green and dead cells red. The viability was calculated by the number of live cells divided by total cell numbers. Inter-group comparisons were performed using one-way ANOVA with Tukey post hoc test. Differences at p< 0.05 were considered to be significant.

Results: Compared with control, Alamar blue assay showed inhibition of cell proliferation in all three metal particles (p< 0.05). The Live/Dead staining showed Co nanoparticles were cytotoxic to most of cells Day 1 and Day 4 at 10E11/mL. At 10E13/mL, the Cr group showed cytotoxicity at day 4 (p< 0.05). There was no difference between Ti and control group. The Co2+ and Cr3+ ions led to inhibition to cell proliferation. At 10uM concentration, Co2+ caused a dramatic decrease in cell number. Live/Dead staining showed that Co2+ were toxic to cells (p< 0.05). Cr3+ group showed cytotoxicity at Day 4 (p< 0.05).

Discussion: This study demonstrates that Co and Cr nanoparticles and ions have dose-dependent proliferation and cytotoxic effects on the macrophages in vitro. The cytotoxicity occurred at the high concentration range that is found in the hip aspirates of MoMHRA patients with pseudotumours. This suggests the formation of pseudotumour may be the local sequelae of cytotoxicity due to increased production of metal wear nanoparticles.

Recently, a series of locally destructive soft tissue pseudotumour has been reported in patients following metal-on-metal hip resurfacing arthroplasty (MoMHRA), requiring revision surgery in a high percentage of patients. Based on the histological evidence of lymphocytic infiltration, a delayed hypersensitivity reaction to nickel (Ni), chromium (Cr) or cobalt (Co) has been suggested to play a role in its aetiology. The aim of this study was to investigate the incidence and level of hypersensitivity reaction to metals in patients with pseudotumour.

Materials and Methods: 25 patients were investigated in this Ethics approved study:

Group 1: MoMHRA patients with pseudotumours, detected on the ultrasound and confirmed with MRI (n=6, 5 F:1 M, mean age 53 years);

Lymphocyte transformation tests (LTT) were used to measure lymphocyte proliferation responses to metals. Peripheral blood mononuclear cells were isolated from heparinized blood samples using standard Ficoll–Hypaque® (Pharmacia). The PBMC were cultured at a cell density of 106 cells/mL. Culture was set up in the presence of either:

medium alone;

After 5 days of culture, cells were pulsed with [3H]-thymidine and proliferation was assessed by scintillation counting. The stimulation index (SI) was calculated by the ratio of mean counts per minute of stimulated to unstimulated cultures. A SI value of greater than 2.0 was interpreted as a positive result.

Results: A clinical history of metal allergy was reported in 2/6 in Group 1, 2/13 in Group 2, and none in Group 3. In pseudotumour group, the incidence of reactivity to Ni, Co and Cr was 60%, 17% and 0%, respectively. Within Group 2, the reactivity to Ni, Co and Cr was 69%, 8% and 15%, respectively. One control subject had reactivity to Ni. Inter-group comparisons of mean SI values (Kruskal-Wallis non-parametric analysis of variance) showed no significant differences (p> 0.05).

Discussion: The incidence of enhanced lymphocyte response to metals in patients with MoMHRA was more common than the control group. However, in comparison with non-pseudotumour patients, there was no significant difference in the incidence or the level of lymphocyte reactivity in patients with pseudotumour. We conclude that patients with MoMHRA have an enhanced lymphocyte response to metal ions, reflecting exposure and immune reactivity. However, patients with pseudotumours have a similar proliferative response to those without pseudotumours, which suggests that type IV hypersensitivity may not be the cause of the pseudotumours.

Tribological studies of hip arthroplasty suggest that larger diameter metal-on-metal (MOM) articulations would produce less wear than smaller diameter articulations. Other advantages using these large femoral heads implants include better stability with lower dislocation rates and improved range of motion. The aim of the present study was to compare chromium (Cr), cobalt (Co) and titanium (Ti) ion concentrations up to 1-year after implantation of different large diameter MOM total hip arthroplasty (THA).

Methods: Cr, Co and Ti concentrations were measured using a high resolution mass spectrometer (HR-ICP-MS) by an independent laboratory in 110 patients, randomized to receive a large metal-on-metal articulation unce-mented Ti THA from one of the following companies: Zimmer, Smith & Nephew, Biomet or Depuy. Samples of whole blood were collected pre-operatively, and postoperatively at six months and one year.

Summary of Results: At 6 months, whole blood cobalt levels were: (table removed)

Statistical group comparison revealed significant difference for Cr (p=0.006), Co (p=0.047) and Ti (p=< 0.001). With Biomet implants presenting the best results for Cr and Co and Zimmer the highest Ti level.

Discussion: Different implant factors may influence measured metal ion level in whole blood: articular surface wear and implant passive corrosion. Bearing wear may be related to its diameter, quality of the surface finish, component sphericity, radial clearance, manufacturing process (forged vs cast metal) and metal carbon content. Biomet articulation seems to present the best factors selection. Passive corrosion of exposed metallic surfaces is represented by the elevated Ti levels found in all tested systems (Ti was not part of the bearing surfaces). The plasma sprayed acetabular component surface of the Zimmer’s component seems to be responsible for the significant difference in Ti versus the other implants.

Metal on Metal Hip Resurfacing Arthroplasty (MoMHRA) has gained popularity due to its perceived advantages of bone conservation and relative ease of revision to a conventional THR if it fails. Known MoMHRA-associated complications include femoral neck fracture, avascular necrosis/collapse of the femoral head/neck, aseptic loosening and soft tissue responses such as ALVAL and pseudotumours. This study’s aim was to assess the functional outcome of failed MoMHRA revised to THR and compare it with a matched cohort of primary THRs.

Method: We have revised 53 MoMHRA cases to THR; the reasons for revision were femoral neck fracture (Fracture Group, n=21), soft tissue reaction (Pseudotumour Group, n=16) and other causes (Other Group, n=16: loosening, AVN and infection). Average followup was 2.9 years. These MoMHRA revisions were compared with 103 matched controls from a primary THR cohort; matched for age, gender and length of followup. We compared, using the MannWhitney U test, operative time (OT, measured in minutes), and Oxford Hip Score (OHS) between the revised MoMHRA groups and their individual controls. We also compared, using the Kruskal Wallis test, UCLA Activity Score in the revised MoMHRA groups.

Results: There were no differences between the Fracture Group (mean OT 99.6, SD: 30.4; mean OHS 19.8, SD:9.2) and its controls (mean OT 95.9, SD: 31.8; mean OHS 17.3, SD: 7.5) nor between the Other Group (mean OT 129.4, SD: 36.7; mean OHS 22.2, SD: 9.4) and its controls (mean OT 104.4, SD: 39.2; mean OHS 20.3, SD: 10.1) in terms of OT and OHS. The Pseudotumour Group had significantly longer OT (mean 161.6, SD: 24.5, p< 0.001) and worse outcome (mean OHS 39.1, SD: 9.3, p< 0.001) than its controls (mean OT 113.1, SD: 51.7; mean OHS 20.0, SD: 9.2). In the Fracture Group, there were 3 infections requiring revisions. For the Pseudotumour Group, there were 3 recurrent dislocations, 1 femoral artery stenosis and 3 femoral nerve palsies. In the Other Group, there were 2 periprosthetic fractures. There was significant difference (p< 0.001) in UCLA scores between the MoMHRA groups. The Pseudotumour Group had the lowest mean UCLA score of 3.8 (SD: 1.89). The Fracture Group (mean: 7.0, S.D. 2.0) and the Other Group (mean: 6.7, S.D. 2.1) had similar UCLA scores.

Discussion: The results demonstrate that outcome after revision of MoMHRA is dependent upon the indication for revision. Patients revised for soft tissue reactions had significantly worse outcome. Patients with soft tissue reactions are more likely to experience complications and require further surgical intervention. The pseudotumour associated revisions were associated with a significantly prolonged OT. The overall complication rate for the study groups was quite high, with 11 (21%) revised MoMHRA cases experiencing a complication. The Pseudotumour Group had a higher complication rate (37%).

Patello-femoral instability (PFI) affects 40 individuals per 100,000 population and causes significant morbidity. The causes of patello-femoral instability are multi-factorial, and an isolated anatomical abnormality does not necessarily indicate instability. Patello-femoral subluxation ranges from 0% (stable patella tracking) to 100% (dislocation) and there is an established relationship between the amount of subluxation and anterior knee pain. Traditionally, magnetic resonance (MR) imaging and standard radiographs are used to guide the clinician towards a suitable corrective procedure for PFI. The multi-factorial nature of patello-femoral instability is not addressed with current imaging techniques. This study aims to address which anatomical variables assessed on MR images are most relevant to patello-femoral subluxation. This information will aid surgical decision making, particularly in selecting the most appropriate reconstructive surgery.

A retrospective analysis of MR studies of 60 patients with suspected patello-femoral instability was performed. All patients were graded for degree of subluxation using a dynamic MR scan.

The patient scans were assessed for the presence of a specific range of anatomical variables:

patella alta, (modified Insall-Salvatti)

The Wilk’s Lambda test for multi-variate analysis was used to establish whether any relationship was present between the degree of patello-femoral instability and bony or soft tissue anatomical variables. Non-parametric statistical tests were applied across the groups and within the groups to assess their relative significance.

The following variables showed a significant relationship with patellofemoral subluxation; distance of the VMO from the patella (< 0.001), TTD (< 0.001), patella engagement (0.001), sulcus angles (0.004) and patella alta (0.005).

This study agrees with previous work showing a significant correlation between subluxation and trochlea sulcus angle and TTD.

This is the first study to establish a significant correlation between patella engagement and radiological instability. The lower the percentage engagement of the patella in the trochlea, the greater the degree of patello-femoral instability. Patella engagement showed a more significant relationship with subluxation than patella alta.

We report a new method of predicting patello-femoral instability by measuring the overlap of the patella in the trochlea groove.

Introduction: Metal on metal Hip Resurfacing Arthroplasty (MoMHRA) has gained popularity due to its perceived advantages of bone conservation and relative ease of revision to a conventional THR if it fails. This retrospective study is aimed at assessing the functional outcome of failed MoMHRA revised to THR and comparing it with a matched cohort of primary THRs.

Method: Since 1999 we have revised 53 MoMHRA to THR. The reasons for revision were femoral neck fracture (Group A, n=21), pseudotumour (Group B, n=16) and other causes (Group C, n=16: loosening, avascular necrosis and infection). Average follow-up was 3 years months (1.2–7.3). These revisions were compared with 106 primary THRs which were age, gender and follow-up matched with the revision group in a ratio of 2:1.

Results: The mean Oxford Hip Score (OHS) was 20.1 (12–51) for group A, 39.1 (14– 56) for group B, 22.8 (12–39) for group C and 17.8 (12–45) for primary THR group. In group A, there were three infections requiring further revisions. In group B, there were three recurrent dislocations, three patients with femoral nerve palsy and one femoral artery stenosis. In group C, there were no complications. The differences in clinical and functional outcome between group B and the remaining groups as well as the difference in the outcome between group B and control group were statistically significant (p < 0.05).

Conclusions: THR for failed MoMHRA was associated with significantly more complications, operation time and need for blood transfusion for the pseudotumour group. In addition, the revisions secondary to pseudotumour also had significantly worse functional outcome when compared to other MoMHRA revisions or primary THR.

Introduction: Despite the satisfactory short-term implant survivorship of MoM hip resurfacing arthroplasty, symptomatic abnormal periprosthetic soft-tissue masses relating to the hip joint, ‘pseudotumours’, are being increasingly reported. These were found be locally destructive, requiring revision surgery in 75% of patients. Asymptomatic pseudotumours have not been previously investigated.

Methods: The aims were: (1) to investigate the prevalence of asymptomatic pseudotumours; and (2) to investigate their potential association with the level of metal ions. A total of 160 hips in 123 patients with a mean age 56 years (range 33–73) were evaluated at a mean follow-up of 61 months (range 13–88). Radiographs and OHS were assessed. Patients with a cystic or solid mass detected on the ultrasound/MRI had an aspiration or biopsy performed. Cobalt and chromium levels were analysed using Inductively-Coupled Plasma Spectrometer.

Results: Pseudotumours were found in 6 patients (5F: 1M). In 80% of bilateral cases, it was found in both sides. Histological examination showed extensive necrosis and diffuse lymphocyte infiltration. The presence of pseudotumour was associated with higher serum cobalt (9.2 μg/L vs. 1.9μg/L, p< 0.001) and chromium levels (12.0μg/L vs. 2.1μg/L, p< 0.001); higher hip aspirate cobalt (1182 μg/L vs. 86.2μg/L, p=0.003) and chromium levels (883μg/L vs. 114.8μg/L, p=0.006); and with inferior OHS (23 vs. 14 p=0.08).

Discussion: The prevalence of asymptomatic pseudotumour (5%) was higher than previously reported for the symptomatic pseudotumours (1%). There was a sixfold elevation of serum and a twelve-fold elevation of hip aspirate levels of cobalt and chromium in patients with pseudotumours. This suggests that pseudotumours may be a biological consequence of the large amount of metal debris generated in vivo. The association between pseudotumour and elevated metal ion levels might theoretically be explained by either systemic hypersensitivity responses to metal ions or local cytotoxic effects due to a high level of metal ions.

Introduction: This study explores whether modern magnetic resonance imaging (MRI) with improved cartilage sequencing is able to show a more detailed view of anteromedial osteoarthritis of the knee (AMOA). Preoperative assessment of patients and selection of intervention is very important and preoperative imaging forms an integral part of this. Modern MRI technology may allow us to visualize in great detail the structures and cartilage within the knee, providing a better understanding of the pathoanatomy of AMOA. This will be useful in preoperative assessment and surgical management of patients.

Methods: 50 patients with a radiographic diagnosis of anteromedial osteoarthritis of the knee and had been listed for unicompartmental knee arthroplasty (UKA) had MRI as part of their pre-op workup. At operation all were deemed suitable for UKA using the current Oxford indications. The image sequences were coronal, axial and sagittal with a predetermined cartilage protocol. The state of the ACL, cartilage wear location and pattern, presence of osteophytes and subchondral high signal were assessed.

Results: All the ACLs were visualized and in continuity, however 40% showed intrasubstance high signal.

100% of medial compartments showed full thickness anteromedial loss with preservation of the posteromedial cartilage. When present, the meniscus was extruded in 96% of cases.

Discussion: This study maps the pattern of anteromedial osteoarthritis using modern MRI techniques. This has importance in determining preoperative indications (preservation of ACL and posteromedial cartilage); surgical technique (determine pattern of osteophytes requiring resection) and potentially important for long-term outcome (early lateral compartment changes).

Introduction: Radiolucent lines (RLL) underneath the tibial component are common findings following the Oxford Uni-compartmental Knee Arthroplasty (OUKA)[1]. Many theories have been proposed to explain the cause of RLL, such as poor cementing, osteonecrosis, micromotion, and thermal necrosis, however, the true aetiology and clinical significance remain unclear. We undertook a retrospective study analysing the association between RLL and pre-operative, intra-operative factors, as well as clinical outcome scores.

Method: One hundred and sixty-one knees which had undergone primary Phase 3 medial Oxford OUKA were included in the study. Fluoroscopic radiography films were assessed at five years post-operatively for areas of tibial RLL. The presence of RLL was compared to

patients’ pre-operative demographics for age, weight, height, BMI,

Results: Of the 161 knees in the study, 126 (78%) were found to have tibial RLL. No statistical difference was found between knees with RLL and those without in terms of preoperative demographics, intra-operative factors, or clinical assessment criteria.

Discussion: No clear relationship between RLL, preoperative demographics, and intra-operative factors has been identified in this study. We conclude that tibial RLL following OUKA is a common finding but do not seem to affect medium term clinical outcome.

Introduction: Avascular necrosis of the femoral head after resurfacing hip replacement is an important complication which may lead to fracture or failure. The surgical approach may affect the blood supply to the femoral head. We compared the changes in femoral head oxygenation resulting from the extended posterior approach to those resulting from the anterolateral approach, the trochanteric flip approach and a modified, soft tissue preserving posterior approach.

Methods: We recruited 48 patients who underwent hip resurfacing arthroplasty (HRA) to measure bone oxygen levels. A calibrated gas-sensitive electrode was inserted in the femoral head following division of the fascia lata. Intra-operative X-ray confirmed correct electrode placement. Base-line oxygen concentration levels were recorded immediately after electrode insertion and continuous measurements were then performed throughout surgery. All results were expressed relative to the baseline, which was considered as 100% relative oxygen concentration and changes during surgery through the posterior approach (n=10), the antero-lateral approach (n=12), the trochanteric flip approach (n=15) and the modified posterior approach (n=11) were compared.

Results: The relative oxygen concentration at the end of the procedure was significantly reduced when hip resurfacing was performed through the posterior (22%, SD 31%, p< 0.005) or a modified posterior (35%, SD 31%, p< 0.005) approach, but recovered in the anterolateral (123%, SD 99%, p=0.6) and trochanteric flip group (89%, SD 62%, p=0.5). Sub-group analysis of these two relatively blood preserving approaches showed that intra-operative oxygen concentration was significantly more consistent during surgery through the trochanteric flip approach (p< 0.02).

Discusssion and conclusion: This study has demonstrated that disruption of blood flow to the femoral head during HRA is dependent on the surgical approach. We therefore believe that blood supply preserving approaches (i.e. anterolateral, trochanteric flip) may be associated with a lower risk of avascular necrosis and femoral neck fracture.

Introduction: We report on a group of 20 metal-onmetal resurfaced hips (17 patients) presenting with a soft tissue mass associated with various symptoms; these masses we termed pseudotumours.

Methods: All patients underwent plane radiography; CT, MRI and ultrasound investigations were also performed for some patients. Where samples were available histology was performed. Metal ion levels were measured in six patients and one patient had the metal ion levels in the joint fluid measured.

Results: All patients in this series were female. Presentation was variable; the most common symptom was pain or discomfort in the hip region. Other symptoms included spontaneous dislocation, nerve palsy, a noticeable mass or a rash. In all cases a soft tissue mass was present in the region of the hip, this was either solid or cystic. The common histological features were extensive necrosis and lymphocytic infiltration. The blood cobalt and chromium levels varied considerably between the six patients that had these measurements. The median blood chromium level was 3.8 μg/L (range 0.8 to 23 μg/L) and that for cobalt was 11.5 μg/L (range 2.1 to 15 μg/L). The synovial fluid sample taken from a single joint contained much higher metal levels, 701 μg/L for chromium and 329 μg/L for cobalt. Twelve of the 20 cases have so far required revision to a conventional hip replacement.

Discussion: This complication is best imaged with ultrasound, and is not detected by normal xray. We estimate that about 1% of patients develop a pseudotumour in the first five postoperative years. The cause of these pseudotumours is unknown and is probably multifactorial, further work is required to define this; they may be manifestations of a metal sensitivity response. We are concerned that with time the incidence of these pseudo-tumours will increase.

Introduction: Although resurfacing hip replacements are widely used there are few little independent outcome data to support this. The aim of this study was to report the 5 year clinical outcome and 7 year survival of an independent series.

Method: 610 Birmingham hip resurfacings were implanted in 532 patients with an average age of 51.8 years (range 16.5–81.6 years) and were followed for between 2 to 8 years; 120 of this series had minimum five year follow-up. Two patients were lost. There were 23 revisions, giving an overall survival of 95% (95% CI 85–99%) at seven years. Fractured neck of femur (n=13) was the most common reason for revision, followed by aseptic loosening (n=4). There were also 3 patients who had failures that were possibly related to metal debris. At a minimum of 5 year follow-up 93% had excellent or good outcome according to the Harris Hip Score. The mean Oxford Hip Score was 16.1 points (SD 7.7) and the mean UCLA activity score was 6.6 points (SD 1.9). There were no patients with definite evidence of radiographic loosening or greater than 10% of neck narrowing.

Discussion: The results demonstrate that with the Birmingham Hip Resurfacing, implanted using the extended posterior approach, the five year survival is similar or better to the reported survival rates for cemented and hybrid THR’s in young patients.

Conclusions: Considering these patients are young and active these results are good and support the use of resurfacing. However, further study is needed to address the early failures; particularly those related to fracture and metal debris.

In finite element (FE) analysis of long bones it is now common practice to calculate the material properties based on CT data. Although a unique material property is calculated for each element, assigning each element an individual material property results in excessively large models. To avoid this, it is usual to group the elements based on their material properties and to assign each group a single material property (Zannoni 1998). No study has analysed the effect the number of material properties used in a long bone FE model has on the accuracy of the results.

The aim of this study was to evaluate the variation in the calculated mechanical environment as a function of the number of material properties used in an FE model.

An FE mesh of a cadaveric human tibia containing 47,696 ten-node tetrahedron elements and 75,583 nodes was created using CT scans. Material properties were calculated for each element of the mesh based on previous work (Rho 1995, 1996). Eleven FE models were created by varying the number of groups (1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024) the elements were divided into. A single material property was assigned to each group. All models were subject to an axial point load of 300N applied on the medial condyle of the tibial plateau while the distal end was fixed. The variation in maximum and minimum principal strains and deflections, at 17 well distributed surface nodes and at 65 randomly distributed nodes within the bone were plotted against the number of element groups. The total strain energy was also plotted against the number of groups. The errors for strain, deflection, and total strain energy were calculated for each model assuming that the model using 1024 element groups was accurate.

The parameter to converge with the least number of element groups was the total strain energy. At 512 element groups the error was less than 0.001% (0.7% for the two material model). The next to converge were the displacements. Using 512 materials the maximum error in displacement at the surface nodes was 0.001% (4.7% for the 2 material model), while for the internal nodes the maximum error was 0.53% (36.7% for the 2 material model). The least convergence occurred for principal strains. The maximum errors when 512 materials were used were 1.06% (57.7% for the 2 material model) and 3.02% (104.5% for the 2 material model) for the surface and the internal nodes respectively.

This study demonstrates the relationship between the accuracy of calculated mechanical environment and the number of material properties assigned to the model. While this study will allow the analyst to make an informed decision on the number of material properties for modelling the human tibia it also helps examine the validity of previous studies which, usually due to limited resources, used fewer material properties.

Metal-on-metal (MoM) bearing technology, made of cobalt-chromium (Co-Cr) alloys, is being used in anticipation of extending the durability of hip replacements. Increasingly, concern has been expressed that long term exposure to Co2+ and Cr3+ could cause DNA damage and immune dysfunction; specifically a reduction in the circulating number of CD8+ cytotoxic cells. More recently, we reported that Co2+ and Cr3+ affected the differentiation of osteoclast precursors into bone-resorbing osteoclasts. Despite these observations the effects of metal ions on osteoblast activity have been poorly investigated. The aim of the current study was to elucidate the effects of various metal ions on osteoblast activity in vitro.

Cells of the human osteosarcoma cell line SaOS-2 were cultured in the presence of 0, 1, 10 and 100 μM Co2+ and Cr3+. The morphology, viability, cytokine release (TNFalpha, IL-1beta, IL-6, LIGHT, MIP-1alpha and VEGF) and alkaline phosphatase activity were investigated after 24h and 48h in contact with metal ions. Finally the capacity of SaOS-2 to produce and mineralize a new bone matrix was assessed by the Alizarin red method. All experiments were repeated at least 5 times and the differences between each were determined using non-parametric Mann-Whitney test.

Compared to untreated cultures, although the morphology looked normal after 48h, the viability indicated that Co2+ and Cr3+ ions at high concentrations induced some significant and irreversible damages to the osteoblast cells. Interestingly, any of the cytokines investigated were released in contact with metal ions after 24h or 48h. The alkaline phosphatase activity was significantly increased by low concentrations of Co2+ and decreased by high concentrations of Cr3+ after 24h and 48h. Moreover, the degree of mineralization of a new bone matrix in vitro was significantly reduced when the SaOS-2 cells were exposed to high concentrations of Cr3+, but significantly increased when they were exposed to Co2+.

Our results indicated that irreversible damages are caused to the cells as soon as 24h with high concentrations of metal ions. For osteoblasts cells, Co2+ appeared to be less toxic than Cr3+ at high concentrations.

This study was supported by Furlong Research Charitable Foundation

Statement of purpose: The aim of this study is to evaluate different designs of unicompartmental knee replacement (UKR) by comparing the peak von Mises and contact stresses in polyethylene (PE) bearings over a step-up activity.

Summary of Methods: A validated finite element (FE) model was used in this study. Three UKR designs were modelled: a spherical femoral component with a spherical PE bearing (fully-congruent), a poly-radial femoral component with a concave PE bearing (semi-congruent), and a spherical femoral component with a flat bearing (non-congruent).

Kinematic data from in-vivo fluoroscopy measurements during a step-up activity was used to determine the relative tibial-femoral position as a function of knee flexion angle for each model. Medial and lateral force distribution was adapted from loads measured in-vivo with an instrumented implant during a step-up activity. The affect that varying the bearing thickness has on the stresses in the bearing was investigated. In addition, varus-valgus mal-alignment was investigated by rotating the femoral component through 10 degrees.

Summary of Results: Only the fully congruent bearing experienced peak von Mises and contact stresses below the PE lower fatigue limit (17MPa) for the step-up activity (fully congruent PE peak contact stress, 5MPa). The highest PE contact stresses were observed for the semi-congruent and non-congruent designs, which experienced approximately 3 times the PE lower fatigue limit. Peak PE von Mises stresses for the semi-congruent and non-congruent designs were similar, peaking at approximately 25MPa. Peak PE von Mises stresses were ameliorated with increased bearing thickness. Varus-valgus mal-alignment had little effect on the peak stresses in the three UKR designs.

Statement of Conclusions: Fully congruent articulating surfaces significantly reduce the peak contact stresses and von Mises stresses in the bearing. The FE model demonstrates that fully congruent bearings as thin as 2.5mm can be used without increasing the contact stresses significantly. Fully congruent designs can use thinner bearings and enable greater bone preservation.

Introduction: The results of the mobile bearing Oxford unicompartmental knee replacement (UKR) in the lateral compartment have been disappointing with a five year survival of 82%. Therefore, it is recommended that mobile bearings should not be used for lateral UKR. This low survivorship is primarily due to high dislocation rate, all occurring in the first year. A detailed analysis of the causes of bearing dislocation confirmed the elevated lateral tibial joint line to be a contributory factor. A new surgical technique was therefore introduced in which care was taken neither to remove too much bone from the distal femur nor to over tighten the knee and therefore ensure that the tibial joint line was not elevated. Other modifications to the technique were also introduced including use of a domed tibial component.

Aim: The aim of this study is to compare the outcome of these iterations: the original series [series I], Series II with improved surgical technique and the domed tibial component [Series III].

Method: The primary assessment of outcome was bearing dislocation at one year. One year was chosen as all the dislocations in the first series occurred within a year. In series I, there were 53 knees, in series II 65 knees and in series III 60 knees, all with a minimum of one year follow up.

Results: In series I, there were 6 bearing dislocations (11%) and the average range of movement (ROM) was 110°. In the second series, there were 2 dislocations (3%) and the average ROM was 118°. In the third series, there were no primary dislocations and the average ROM was 125°.

Conclusions: The improved surgical technique and implant design has reduced the dislocation rate to an acceptable level so a mobile bearing can now be recommended for lateral UKR.

Statement of purpose: Finite element (FE) models of bone can be used to evaluate new and modified knee replacements. Validation of FE models is seldom used, and the quantification of modelling parameters has a considerable effect on the results obtained. The aim of this study is to develop a FE model of a cadaveric tibia and validate it against a comprehensive set of experiments.

Summary of Methods: Seventeen tri-axial rosettes were attached to a cleaned, fresh frozen cadaveric human tibia and the tibia was subjected to 13 loading conditions. Deflection and strain data were used for comparison with the FE model. A geometric model was created on the basis of computed tomography (CT) scans. The CT data was used to map 600 orthotropic material properties to the tibia. All experiments were simulated on the FE model. Measured principal strains were compared to their corresponding FE values using regression analysis. The validated tibia model was reduced in size (75mm to the proximal) and then re-modelled to represent only the proximal tibia. This re-modelled tibia was validated against the reduced size FE model. Virtual surgery was performed on the validated proximal model to implant a UKR.

Summary of Results: For the whole tibia model, the regression line for all axial loads combined had a slope of 0.999, an intercept of −6.24 micro-strain, and an R2 value of 0.962. The root mean square error as a percentage was 5%. For the proximal tibia model, correlation coefficients of 0.989 and 0.976 were obtained for the maximum and minimum principal strains respectively.

Statement of Conclusions: An FE model of an implanted proximal tibia has been validated against experimental data. This model is able to accurately predict the deflection and stresses in a replaced knee joint to obtain clinically relevant information. This will provide a virtual model of unicompartmental arthroplasty, where variables such as fixation method and bearing mechanics can be assessed.

Purpose: To compare the site of lesions in medial and lateral unicompartmental osteoarthritis (OA) of the knee.

Methods: Patients with medial (n=35) and lateral (n=15) OA, having unicompartmental knee arthroplasty, were recruited. Intra-operatively, the distance between the anterior, posterior, medial and lateral margins of the full-thickness lesion and reference lines dividing the condyles was measured. The midpoints of lesions were calculated and groups were compared. Lateral radiographs were used to determine the relationship between the lesion site and knee flexion angle (KFA).

Results: Femoral lesion: In lateral OA, the midpoint of lesions was 25.0mm (SD:8.8) posterior to the reference line passing transversely through the apex of the inter-condylar notch. This was significantly different (p< 0.001) from midpoint in medial OA, which was 10.7mm (SD:9.4) posterior to the reference line.

Tibial lesion: In lateral OA, the midpoint of lesions was 2.0mm (SD:6.5) posterior to the reference line passing through the mid-coronal plane of the resected tibia. This was located significantly more posterior (p=0.038) than midpoint in medial OA, which was 2.2mm (SD:5.7) anterior to the reference line.

Knee Flexion Angle: In lateral OA, the midpoint of lesions was on average at 40° flexion and sites of smaller lesions were very variable. The lesion expanded both anteriorly and posteriorly. In medial OA, smaller femoral lesions occurred in full extension and extended further posteriorly with disease progression.

No significant difference was demonstrated in medial and lateral localisation of the lesions.

Conclusion: Medial OA begins near full extension, progresses in a predictable manner and is perhaps initiated by events occurring at heel strike. Lateral OA begins in flexion in a less predictable manner, at KFA above that seen during the gait cycle. The different sites of lesions in medial and lateral OA suggest different aetiology and pathophysiology. Therefore, prevention and treatment strategies should be different.