Abstract

Currently, different techniques to evaluate the biocompatibility of orthopaedic materials, including two-dimensional (2D) cell culture for metal/ceramic wear debris and floating 2D surfaces or three-dimensional (3D) agarose gels for UHMWPE wear debris, are used. Moreover, cell culture systems evaluate the biological responses of cells to a biomaterial as the combined effect of both particles and ions. We have developed a novel cell culture system suitable for testing the all three type of particles and ions, separately. The method was tested by evaluating the biological responses of human peripheral blood mononuclear cells (PBMNCs) to UHMWPE, cobalt-chromium alloy (CoCr), and Ti64 alloy wear particles.

Introduction

Titanium and its alloys are attractive biomaterials attributable to their desirable corrosion, mechanical, biocompatibility and osseointegration properties. In particular, β – titanium alloys like the TMZF possess other advantages such as its lower modulus compared to Ti6Al4V alloy. This reduces stress shielding effect in Total Hip Arthroplasty (THA) and the replacement of V in the Ti6Al4V alloy, eliminates in-vivo V-induced toxicity. Unfortunately, implants made of TMZF were later recalled by the FDA due to higher than acceptable revision rates. The purpose of this study was to compare the fretting corrosion characteristics of Ti6Al4V and TMZF titanium alloys. It is hoped the findings will inform better design of β – titanium alloys for future applications in THA.

Introduction

Titanium and its alloys are attractive biomaterials attributable to their desirable corrosion, mechanical, biocompatibility and osseointegration properties. Ti6Al4V alloy in particular remains a prominent biomaterial used in Total Hip Arthroplasty (THA) today. This is partly due to biocompatibility and stress shielding issues with CoCrMo alloys, resulting in its increasing side-lining from the THA construct. For several decades now, research efforts have been dedicated to understanding wear, corrosion and surface degradation processes in implant materials. Only recently have researchers shown interest in understanding the subsurface implications of fretting and the role it plays on implant fracture. The purpose of this study was to utilise advanced microscopy and spectroscopy techniques to characterise fretting-induced subsurface transformations in Ti6Al4V. This makes mapping specific regions that are most prone to wear and fatigue failures at the modular taper interface of THA probable. Thus, informing a proactive approach to component design and material selection.

Since 2010, there has been a sharp decline in the use of metal-on-metal joint replacement devices due to adverse responses associated with the release of metal wear particles and ions in patients. Surface engineered coatings offer an innovative solution to this problem by covering metal implant surfaces with biocompatible and wear resistant materials. The present study tests the hypothesis whether surface engineered coatings can reduce the overall biological impact of a device by investigating recently introduced silicon nitride coatings for joint replacements. Biological responses of peripheral blood mononuclear cells (PBMNCs) to Si3N4 model particles, SiNx coating wear particles and CoCr wear particles were evaluated by testing cytotoxicity, inflammatory cytokine release, oxidative stress and genotoxicity.

Clinically relevant wear particles were generated from SiNx-on-SiNx and CoCr-on-CoCr bearing combinations using a multidirectional pin-on-plate tribometer. All particles were heat treated at 180°C for 4 h to destroy endotoxin contamination. Whole peripheral blood was collected from healthy donors (ethics approval BIOSCI 10–108, University of Leeds). The PBMNCs were isolated using Lymphoprep (Stemcell) and incubated with particles at various volumetric concentrations (0.5 to 100 µm3 particles/cell) for 24 h in 5% (v/v) CO2 at 37°C. After incubation, cell viability was measured using the ATPlite assay (Perkin Elmer); TNF-alpha release was measured by ELISA (Invitrogen); oxidative stress was measured using H2DCFDA (Abcam); and DNA damage was measured by comet assay (Tevigen). The results were expressed as mean ± 95% confidence limits and the data was analysed using one-way ANOVA and Tukey-Kramer post-hoc analysis.

No evidence of cytotoxicity, oxidative stress, TNF-alpha release, or DNA damage was observed for the silicon nitride particles at any of the doses. However, CoCr wear particles caused cytotoxicity, oxidative stress, TNF-alpha release and DNA damage in PBMNCs at high doses (50 µm3 particles per cell). This study has demonstrated the in-vitro biocompatibility of SiNx coatings with primary human monocytic cells. Therefore, surface engineered coatings have potential to significantly reduce the biological impact of metal components in future orthopaedic devices.

Aims

Wear is difficult to predict in mixed lubricated articulating surfaces and the time of computation is one of the challenges due to the deterministic definition of roughness on a micro-scale. This research aims to efficiently capture the wear and the evolution of the roughness of mixed lubricated bearing surfaces, employing a statistical description of the roughness.

Introduction

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

Introduction

Wear debris generated by total hip replacements (THRs) may cause mechanical instability, inflammation, osteolysis and ultimately implant loosening, thus limiting the lifetime of such devices [1]. This has led to the development of biocompatible coatings for prostheses. Silicon nitride (SiN) coatings are highly wear resistant and any resultant wear debris are soluble, reducing the possibility of a chronic inflammatory reaction [2]. SiN wear debris produced from coatings have not been characterized in vivo. The aim of this research is to develop a sensitive method for isolating low volumes of SiN wear debris from periprosthetic tissue.

Introduction

Fretting corrosion at the Head-Neck taper interface of Large Metal on Metal (MoM), Metal on Polymer (MoP) and Ceramic on Ceramic (CoC) total hip arthroplasty (THA) remains a clinical concern. Ceramic femoral heads have gained a lot of attention more recently as a possible way to mitigate/reduce the dissolution of Cobalt Chromium ions. The objective of this study is to assess the fretting corrosion currents emanating from four material combinations for which Ti6Al4V and Co28Cr6Mo are the neck components of Co28Cr6Mo and BIOLOX®delta femoral heads at three different cyclic loads.

Surgical interventions for the treatment of chronic neck pain, which affects 330 million people globally, include fusion and cervical total disc replacement (CTDR). Most of the currently clinically available CTDRs designs include a metal-on-polymer (MoP) bearing. Numerous studies suggest that MoP CTDRs are associated with issues similar to those affecting other MoP joint replacement devices, including excessive wear and wear particle-related inflammation and osteolysis. A standard ISO testing protocol was employed to investigate a device with a metal-on-metal (MoM) bearing. Moreover, with findings in the literature suggesting that the testing protocol specified by ISO-18192-1 may result in overestimated wear rates, additional tests with reduced kinematics were conducted.

Six MoM CTDRs made from high carbon cobalt-chromium (CoCr) were tested in a six-axis spine simulator, under the ISO-18192-1 protocol for a duration of 4 million cycles (MC), followed by 2MC of modified testing conditions, which applied the same axial force as specified in ISO-18192-1 (50-150N), but reduced ranges of motion (ROM) i.e. ±3° flexion/extension (reduced from ±7.5°) and ±2° lateral bending (reduced from ±6°) and axial rotation (reduced from ±4°). Foetal bovine serum (25% v/v), used as a lubricant, was changed every 3.3×10⁵ cycles and stored at −20°C for particle analysis. Components were measured after each 1×10⁶ cycles; surface roughness, damage modes and gravimetric wear were assessed. The wear and roughness data was presented as mean ±95% confidence interval and was analysed by one-way analysis of variance (ANOVA) (p=0.05).

The mean wear rate of the MoM CTDRs tested under the ISO protocol was 0.246 ± 0.054mm³/MC, with the total volume of wear of 0.977 ± 0.102mm³ lost over the test duration (Fig. 1). The modified testing protocol resulted in a significantly lower mean volumetric wear rate of 0.039 ± 0.015mm³/MC (p=0.002), with a total wear volume of 0.078 ± 0.036mm³lost over the 2MC test duration. Under both test conditions, the volumetric wear was linear; with no significant bedding-in period observed (Fig. 1). The mean pre-test surface roughness decreased from 0.019 ± 0.03µm to 0.012 ± 0.002µm (p=0.001) after 4MC of testing, however surface roughness increased to 0.015 ± 0.002µm (p=0.009) after the additional 2MC of modified test conditions. Following 4MC of testing, polishing marks, observed prior to testing, had been removed. Consistently across all components, surface discolouration and multidirectional, criss-crossing, curvilinear and circular wear tracks, caused by abrasive wear, were observed. Reduced ROMs testing caused similar types of damage, however the circular wear tracks were smaller in size, compared to those produced during testing under the ISO protocol.

The wear rates exhibited by MoM CTDRs tested under ISO-18192-1 testing protocol (0.246mm³/MC) were lower, when compared to CTDR designs incorporating MoP bearings, as well as MoM lumbar CTDRs. Wear rates generated under a modified ISO testing protocol were reduced tenfold, similarly to findings that have previously been reported in the literature, and support the hypothesis that the testing protocol specified by ISO-18192-1 may overestimate wear rates. Characterisation of particles generated by MoM CTDRs and biological consequences of those remain to be determined.

For figures/tables, please contact authors directly.

Introduction

Currently, different techniques to evaluate biocompatibility of orthopaedic materials, including two-dimensional (2D) cell culture for metal and ceramic wear debris and floating 2D surfaces or three-dimensional (3D) agarose gels for UHMWPE wear debris, are used. We have developed a single method using 3D agarose gels that is suitable to test the biocompatibility of all three types of wear debris simultaneously. Moreover, stimulation of the cells by wear particles embedded in a 3D gel better mimics the in vivo environment.

Introduction

Particle-induced oxidative stress in cells is a unifying factor that determines toxicity and carcinogenicity potential in biomaterials. A previous study by Bladen et al. showed the production of significant levels of reactive oxygen species (ROS) following the stimulation of phagocytes by UHMWPE and CoCr wear debris [1]. Latest generation bearing materials such as silicon nitride also need to be tested for potential generation of ROS in phagocytic cells. This study aimed to investigate the production of reactive oxygen species in L929 fibroblasts stimulated with clinically relevant doses of nanoscale and micron-sized silicon nitride (Si₃N₄) particles, silica nanoparticles, and CoCr wear debris. Silica nanoparticles were included as a comparison material for situations where the Si₃N₄ particle's surface are oxidised to silicon dioxide [2].

Surgical interventions for the treatment of chronic neck pain, which affects 330 million people globally [1], include fusion and cervical total disc replacement (CTDR). Most of the currently clinically available CTDRs designs include a metal-on-polymer (MoP) bearing. Numerous studies suggest that MoP CTDRs are associated with issues similar to those affecting other MoP joint replacement devices, including excessive wear and wear particle-related inflammation and osteolysis [2,3]. A device with a metal-on-metal (MoM) bearing has been investigated in the current study.

Six MoM CTDRs made from high carbon cobalt-chromium (CoCr) were tested in a six-axis spine simulator, under standard ISO testing protocol (ISO-18192-1) for a duration of 4 million cycles (MC). Foetal bovine calf serum (25%v/v), used as a lubricant, was changed every 3.3×10⁵ cycles and saved for particle analysis. Components were taken down for measurements after each 10⁶ cycles; surface roughness, damage modes and gravimetric wear were assessed.

The mean wear rate of the MoM CTDRs was 0.24mm³/MC (SD=0.03), with the total volume of 0.98mm³ (SD=0.01) lost over the test duration. Throughout the test, the volumetric wear was linear; no significant bedding-in period was observed. The mean pre-test surface roughness decreased from 0.019μm (SD=0.005) to 0.012μm (SD=0.002) after 4MC of testing. Prior to testing, fine polishing marks on the bearing surfaces were observed using light microscopy. Following 4MC of testing, these polishing marks had been removed. Consistently across all components, surface discolouration and multidirectional, criss-crossing, circular wear tracks, caused by abrasive wear, were observed.

The wear results showed low wear rates exhibited by MoM CTDRs (0.24mm³/MC), when compared CTDR designs incorporating metal-on-polymer bearings (0.56mm³/MC) [4] as well as MoM lumbar CTDRs [5,6] (0.76mm3/MC – 6.2mm³/MC). These findings suggest that MoM CTDRs are more wear resistant than MoP CTDRs, however the particle characterisation and biological consequences of wear remain to be determined.

Introduction

Silicon nitride (SiN) is a recently introduced bearing material for THR that has shown potential in its bulk form and as a coating material on cobalt-chromium (CoCr) substrates. Previous studies have shown that SiN has low friction characteristics, low wear rates and high mechanical strength. Moreover, it has been shown to have osseointegration properties. However, there is limited evidence to support its biocompatibility as an implant material. The aim of this study was to investigate the responses of peripheral blood mononuclear cells (PBMNCs) isolated from healthy human volunteers and U937 human histiocytes (U937s) to SiN nanoparticles and CoCr wear particles.

Introduction

Significant reduction in the wear of current orthopaedic bearing materials has made it challenging to isolate wear debris from simulator lubricants. Ceramics such as silicon nitride (SiN), as well as ceramic-like surface coatings on metal substrates have been explored as potential alternatives to conventional implant materials. Current isolation methods were designed for isolating conventional metal, UHMWPE and ceramic wear debris. The objective of this study was to develop methodology for isolation and characterisation of modern ceramic or ceramic-like coating particles and metal wear particles from serum lubricants under ultra-low wearing conditions. Sodium polytungstate (SPT) was used as a novel density gradient medium due to its properties, such as high water solubility, the fact that it is non-toxic and acts as a protein denaturant, coupled with a large density range of 1.1–3.0 g/cm3 in water.

Introduction

Cobalt-Chromium-Molybdenum (CoCr) and Titanium-Aluminium-Vanadium (Ti) alloys are the most commonly used alloys used for Total Hip Replacement due to their excellent biocompatibility and mechanical properties. However, both are susceptible to fretting corrosion In-vivo. The objective of this study was to understand the damage mechanism of both combinations through a sub-surface damage assessment of the alloys at various fretting amplitudes using the Transmission Electron Microscopy (TEM – CM200 FEGTEM). The TEM was used to attain a cross sectional view of the alloys in orderto see the effect of high shear stress on the grain structure.

Summary Statement

Burst fractures were simulated in vitro on human cadaveric spine segments. Displacement of the facet joints and pedicles were measured throughout the fracture process showing how these bony structures behave when an impact load is delivered.

Summary Statement

There are no standardised methods for assessing the cement flow behaviour in vertebroplasty. We propose a novel methodology to help understand the interaction of cement properties on the underlying displacement of bone marrow by bone cement in porous media.

Interbody fusion aims to treat painful disc disease by demobilising the spinal segment through the use of an interbody fusion device (IFD). Diminished contact area at the endplate interface raises the risk of device subsidence, particularly in osteoporosis patients. The aim of the study was to ascertain whether vertebral body (VB) cement augmentation would reduce IFD subsidence following dynamic loading. Twenty-four human two-vertebra motion segments (T6–T11) were implanted with an IFD and distributed into three groups; a control with no cement augmentation; a second with PMMA augmentation; and a third group with calcium phosphate (CP) cement augmentation. Dynamic cyclic compression was applied at 1Hz for 24 hours in a specimen specific manner. Subsidence magnitude was calculated from pre and post-test micro-CT scans. The inferior VB analysis showed significantly increased subsidence in the control group (5.0±3.7mm) over both PMMA (1.6±1.5mm, p=.034) and CP (1.0±1.1mm, p=.010) cohorts. Subsidence in the superior VB to the index level showed no significant differences (control 1.6±3.0mm, PMMA 2.1±1.5mm, CP 2.2±1.2mm, p=.811). In the control group, the majority of subsidence occurred in the lower VB with the upper VB displaying little or no subsidence, which reflects the weaker nature of the superior endplate. Subsidence was significantly reduced in the lower VB when both levels were reinforced regardless of cement type. Both PMMA and CP cement augmentation significantly affected IFD subsidence by increasing VB strength within the motion segment, indicating that this may be a useful method for widening indications for surgical interventions in osteoporotic patients.

INTRODUCTION

Over 85% of patients with multiple myeloma (MM) have bone disease, mostly affecting thoraco-lumbar vertebrae. Vertebral fractures can lead to pain and large spinal deformities requiring application of vertebroplasty (PVP). PVP could be enhanced by use of Coblation technique to remove lesions from compromised MM vertebrae prior to cement injection (C-PVP).

Spinal total disc replacement (TDR) designs rely heavily on total hip replacement (THR) technology and it is therefore prudent to check that typical TDR devices have acceptable friction and torque behaviour. For spherical devices friction factor (f) is used in place of friction coefficient (mju). The range of loading for the lumbar spinal discs is estimated at perhaps 3 times body weight (BW) for normal activity rising to up to 6 times BW for strenuous activity^[1]. For walking this equates to around 2000 N, which is the maximum load required by the ISO standard for TDR wear testing^[2].

Three Prodisc-L TDR devices (Synthes Spine) were tested in a single station friction simulator. Bovine serum diluted to 25% was used as a lubricating medium. Flexion-extension was ±5 deg for all experiments with constant axial loading of 500, 2000 and 3000 N. The cycle run length was limited to 100 and the f and torque (T) values recorded around the maximum velocity of the cycle point and averaged over multiple cycles.

Preliminary results shows that the 500 N loading produced the largest f of 0.05 ± 0.004. The 2000 N load, which approximates daily activity, gave f = 0.036 ± 0.05 and the 3000 N load gave f = 0.013 ± 0.003. The trend was for lower f with increasing loads.

A lumbar TDR friction factor of 0.036 for a 2000N load and the reduction in f for increasing loads is comparable to the lower end of the range of values reported for THR in similar simulator studies using metal-on-polyethylene bearing materials^[3]. The 3000 N result showing that increasing the load above that expected in daily activity does not raise the f could be important when considering rotational stability and anchorage in a TDR device because frictional torque at the bearing surfaces is proportional to the product of load, device radius and f.

Introduction

The biological response to UHMWPE particles generated by total joint replacements is one of the key causes of osteolysis, which leads to late failure of implants. Particles ranging from 0.1-1.0μm have been shown to be the most biologically active, in terms of osteolytic cytokine release from macrophages [1]. Current designs of lumbar total disc replacements (TDR) contain UHMWPE as a bearing surface and the first reports of osteolysis around TDR in vivo have appeared recently in the literature [2]. The current wear testing standard (ISO18192-1) for TDR specifies only four degrees of freedom (4DOF), i.e. axial load, flexion-extension, lateral bend and axial rotation. However, Callaghan et al. [3] described a fifth DOF, anterior-posterior (AP) shear. The aim of this study was to investigate the effect that this additional AP shear load component had on the size and morphology of the wear particles generated by ProDisc-L TDR devices over five million cycles in a spine simulator.

To investigate and compare the biomechanical characteristics of Bipedicular versus Unipedicular Vertebroplasty in cadaveric vertebra

Cadaveric single level vertebra were used to evaluate Bipedicular versus Unipedicular Vertebroplasty as an intervention for vertebral compression fractures

Cadaveric vertebra were assigned to two arms: Arm A simulated a wedge fracture followed by bipedicular cement augmentation; Arm B simulated a wedge fracture followed by unipedicular cement augmentation. Micro-CT imaging was performed to assess vertebral dimension, cement fill volumes and bone mineral density. All augmented specimens were then compressed under a static eccentric flexion load to failure.

Pre and post augmentation failure load and stiffness were used to compare the two groups.

Results suggest, when compared with actual failure strength, that the product of bone mineral density and endplate surface area gave a good prediction of failure strength for specimens in both arms. The mean cement volume fill of augmented vertebral bodies was 22.8% ± 7.21%. The bipedicular group showed a reduction in stiffness but an increase in post augmentation failure load of 1.09. The unipedicular group also showed a reduction in stiffness but showed a much greater increase in post augmentation failure load of 1.68.

Preliminary data from this study suggests there is a significant reduction in stiffness following both bipedicular and unipedicular vertebroplasty. There is a significant increase in failure load post augmentation in the unipedicular group.

Introduction: An increasing number of young and active patients are undergoing total hip replacement (THR), placing greater demands on the longevity of the implant. One of the most common modes of failure of a THR is aseptic loosening secondary to wear. This study aims to evaluate wear rates seen in a ceramic on polyethylene bearing, and to produce a mathematical model that could be used to predict wear which would have a role in a day-to-day clinic environment.

Methods: Radiographs were examined from 59 uncemented total hip replacements performed between March 1993 and April 2004 comprising a ceramic head on a polyethylene liner. Wear measurements were made using a manual Livermore technique employing digital callipers (accuracy 0.01mm). Multiple radiographic parameters were analysed so that those affecting wear could be identified and included in a mathematical model to predict wear.

Results: 59 hips were measured in 43 patients. Average age at time of follow-up was 53 (34–76). The mean length of time between postoperative and follow-up x-rays was 53 months (11–162). Overall wear rates were 0.05mm/year and total wear increased with the length of time an implant had been in place. There was no correlation between femoral stem alignment or acetabular inclination and wear rates. Multivariate linear regression analysis revealed that sex and cup type were significant contributing variables to wear. A predictive model was produced with an R2 value of 0.543.

Conclusion: This study confirms low wear rates with a ceramic on polyethylene bearing. The mathematical model produced can predict the variability of wear in 54.3% of hips. Further refinement may enable the model to be used to identify risk factors and therefore patients that require greater scrutiny at follow-up.

The purpose was to develop an objective measurement system to assist in the prescription of supportive seating for non-ambulant cerebral palsy children with scoliosis.

Currently the prescription of patient’s bespoke seating setup relies on clinical skills and knowledge of trained seating staff (physiotherapists and engineers). Therefore to develop an objective measurement system to supplement this clinical approach, a user centred design approach was used.

Standard design processes presented in Pahl’s ‘Engineering Design’ (2007) were adopted, allowing in depth user involvement. Stakeholders (clinical, seating, and technical staff) were interviewed to develop requirements lists for each group. Following each development stage; task clarification; concepts; embodiment; detailed design; manufacture; and commissioning, these requirements were reviewed with stakeholders.

Requirements lists were collated to form the device specification, involving all stakeholders allowed the discussion of contradicting requirements. The final design incorporated critical aspects of seating while measuring important outcomes such as force distribution and spinal deformities.

A user centred design approach allowed for informative decision making from stakeholders, highlighting the fundamental requirements and facilitated effective solutions to meet these requirements.

The manufactured device complies with the collaborated specification, utilising stakeholder defined spinal and seating parameters. This was commissioned for use in a pilot study involving twenty non-ambulant cerebral palsy children aged 5–11 years, with high risk of scoliosis.

Ethics approval: Ethics approval granted by Leeds (West) Research Ethics Committee

COREC number: 08/H1307/22

Interest Statement: None

Numerous in vitro studies have utilised bone models for the assessment of orthopaedic medical devices and interventions. The drivers for this usage are the low cost, reduced health concerns and lower inter-specimen variability when compared to animal or human cadaveric tissues. Given this widespread exploitation of these models the push for their use in the assessment of spinal augmentation applications would appear strong. The aim of the research was to investigate the use of surrogate-bone vertebral models in the mechanical assessment of vertebroplasty.

Nine surrogate-bone whole vertebral models with an open-cell trabeculae configuration were acquired. Initial μCT scans were performed and a bone marrow substitute with appropriate rheological properties was injected into the trabeculae. Quasistatic loading was performed to determine the initial fracture strength in a manner previously used with human cadaveric vertebrae. Following fracture, vertebroplasty was undertaken in which there was a nominal 20% volume fill. Following augmentation the VBs were imaged using uCT and then subjected to an axial load using the same protocol.

The surrogate models had a substantially thicker cortex than that of human osteoporotic vertebrae. During compression, the surrogate-bone models did not exhibit the characteristic ‘toe-region’ observed in the load-deformation profile of cadaveric vertebrae. The mean initial and post-augmentation failure strength of the surrogate vertebrae were 1.35kN ± 0.15kN and 1.90kN ± 0.68kN, respectively. This equates to a statistically significant post-vertebroplasty increase by a factor of 1.38. In comparison with human osteoporotic bone, no significant difference was noted in the relative increase in fracture strength between the artificial and human VB following augmentation.

Despite the apparent equivalence of the strength and stiffness of the artificial vertebrae compared to that of the cadaveric specimens, there are significant differences in both pre- and post augmentation behaviour. In particular, the load-deformation curve shows significant differences in shape particularly at the toe end and in post failure behaviour. There are also issues surrounding where the marrow and cement flows during the injection process thus affecting the final distribution of the cement.

Total disc replacement is an alternative to spinal fusion in treating degenerative disc disease, whilst preserving motion and reducing the risk of subsequent DDD at adjacent levels. Current designs have evolved from technology used in total hip replacements with metal-metal or metal-PE bearing surfaces. These articulating systems may be prone to wear and it is essential the medical engineering community assess their performance using appropriate simulators

Utilising previous Leeds simulation design experience, current knowledge on spinal kinetics and prevailing Standards for spinal testing, a comprehensive set of requirements was generated from which a simulator design was produced. The Leeds Spine wear simulator, developed in conjunction with Simulation Solutions Ltd, incorporates five active degrees of freedom: axial compression, axial rotation, flexion-extension, lateral bending and anterior-posterior displacement. The fifth DOF, unique to the Leeds simulator, is anticipated to be particularly important for the study of mobile bearing devices such as the Charité. Loads and motions are applied by electro-mechanical actuators, providing accurate and precise control without the low band width suffered from pneumatics or contamination from hydraulic systems. This validation study determines the accuracy and precision of the simulator with regards to the degrees of freedom required by the newly published standard ISO 18192-1. Here, loads and motions have to be within ±5% of the maximum value and ±0.5degrees, respectively. The simulator’s response to demand input signals was determined for load and motion using independent measuring devices; a digital inclinometer for motions and load cell for force.

The load calibration was found to be within ±1% of the maximum load within the specified load range of 600–2000N. Flexion-extension, lateral bending and axial rotation were found to be within ±0.5, ±0.3 and ±0.5 degrees respectively, within and beyond the operating ranges specified by ISO.

The Leeds spine wear simulator is the first orthopaedic wear simulator to include electro-mechanical actuators for all active DOF, and the first spinal wear simulator to include a minimum of 5 active DOF. This novel simulator meets the demanding tolerances required by ISO for testing of total disc replacements. Validation of the simulator is currently being undertaken to determine its suitability against explanted devices and debris located within tissues.

Introduction & Aims: The X-stop interspinous process decompression system is being used as an alternative to laminectomy in the treatment of neurogenic claudication. To date the clinical outcomes are favourable, but the economic value has not been established within the NHS financial model.

Objective: To compare the average hospital costs of performing an x-stop procedure (under general or local anaesthetic) to a laminectomy in patients with neurogenic claudication.

Design: A retrospective analysis of average length of stay, anaesthetic and operative times, equipment and anaesthetic agent costs. Sources included theatre management systems, the British National Formulary and Leeds Teaching Hospitals Trust in-patient stay data. The study period was from April 2005 to October 2006. The number of patients in the two groups were 318 (laminectomy) and 75 (X-stop).

Results: In comparison to laminectomy, patients under-going an X-stop procedure have a reduced average length of in-patient stay (3 versus 5 days), reduced anaesthetic time (25 versus 29 minutes) and operative duration (40 versus 128 minutes). The average cost for each procedure is £3346 for an X-stop under general anaesthetic (profit £119), £2835 for a laminectomy (profit £1177) and £2237 for an X-stop as a day case (profit £1228).

Conclusions: Tariff reimbursement is an important consideration to ensure insertion of these devices is profitable for the hospital. Our results show that even with the additional cost of the implant device, an X-stop procedure under general anaesthetic remains profitable in comparison to a laminectomy, whilst a day-case X-stop procedure is more profitable. Additional savings are be made by reduced bed and theatre occupancy. Future studies will differentiate costs of 1- and 2-level X-stop procedures, complication rates and revision surgery.

INTRODUCTION: In the spinal column, bone metastases (BM) and lesions arising from multiple myeloma (MM) can cause severe weakening of the vertebral body (VB) leading to an increased risk of fracture¹. These vertebral fractures may induce severe pain, deformity and increased risk of neurological deficit². At present, however, there is very little known about the mechanical behaviour either of the infiltrated vertebrae or that following vertebroplasty (VP). The purpose of this preliminary investigation was to evaluate (i) the mechanical behaviour of vertebrae with lesion involvement, and (ii) the effectiveness of VP with coblation.

METHODS: Individual vertebrae from two spines, one with MM (n=13) and one with BM secondary to bladder cancer (n=12) were dissected free of soft tissue with the posterior elements retained. Three MM vertebrae with evidence of previous fracture were excluded. Each vertebrae was fractured under an eccentric flexion load from which fracture strength and stiffness were derived³. VBs were then assigned to two groups. In group 1, lesion material was removed by coblation prior to VP and in group 2, no coblation was performed prior to VP. All vertebrae were fractured post-augmentation under the same loading protocol. At each stage microCT assessments were conducted to investigate lesion morphology and cement volume/distribution.

RESULTS: MM vertebrae were characterised by several small lesions, severe bone degradation and multiple compromise of the cortical wall. In contrast, large focal lesions were present in the BM vertebrae and the cortical wall generally remained intact. The initial failure strength of the MM vertebrae were significantly lower than BM vertebrae (L=2200N vs 950N, P< 0.001). A significant improvement in relative fracture strength was found post augmentation for both lesion-types (1.42 ± 0.51, P=0.0006). Coblation provided a marginally significant increase in the same parameter post-augmentation (P=0.08) and, qualitatively, improved the ease of injection.

CONCLUSIONS: Bladder BM and MM vertebral lesions showed significant variations in lesion morphology, bone destruction and the level of cortical wall breach, causing significant changes in the bone fracture behaviour. Account should be taken of these differences to optimise the VP intervention in terms of cement formulation and delivery. Preliminary results suggest the current VP treatment provides significant improvements in failure strength post-fracture.

This phase III, multicenter, double-blind placebo controlled study evaluated safety and efficacy of aprotinin in reducing blood transfusion in subjects undergoing THA.

Subjects were stratified by preoperative autologous blood donation and randomized to receive aprotinin (1 mL test dose; load, 2 million KIU and 0.5 million KIU/hour) or placebo. Subjects were assessed at baseline, postoperative days 1, 2, 3, 7 (or discharge) and 6±2 weeks. Primary efficacy variable was percentage of subjects requiring blood transfusion through day 7 or discharge. Safety was based on adverse event (AE).

Of 359 randomized subjects, 175 in each group completed the study. Demographics of the groups were similar. Aprotinin reduced by 46% the requirement for any transfusion (17% vs 32% of subjects, p=0.0009). Aprotinin reduced allogeneic blood transfusion in subjects regardless of predonation status (11% vs 22%, p=0.0063), who made no predonation (13% vs 24%, p=0.0216), and who predonated (32% vs 62%, nd). The aprotinin group had a reduction of the number of any (48 vs 109 units; p=0.0003) and allogeneic (30 vs 72 units; p=0.0041) units transfused and total fluid loss (709 vs 957 ml; p=0.0002) compared with placebo.

One patient died in the placebo group. AEs were reported in 83% of aprotinin-treated and 86% of placebo subjects, with 10% and 11%, respectively, described as serious AEs. No clinically important differences between aprotinin and placebo AEs were observed. Hypersensitivity to aprotinin was not reported.

In this study, full-dose aprotinin was safe and effective in decreasing blood transfusion in subjects undergoing THA.

Introduction: Percutaneous vertebroplasty (PVP) is a treatment option for osteoporotic vertebral compression fractures (VCFs). Short-term results are promising but longer-term studies have demonstrated an accelerated failure rate in the adjacent vertebral body (VB). Limited research has been conducted into the effects of prophylactic PVP in osteoporotic vertebrae. The objective of this study was to investigate the biomechanical characteristics of prophylactic vertebral reinforcement and post-fracture augmentation.

Methods: Human vertebrae were assigned to two scenarios: Scenario 1 used an experimental model for simulating VCFs followed by cement augmentation; Scenario 2 involved prophylactic augmentation using vertebroplasty. μCT imaging was performed to assess the bone mineral density (BMD), vertebral dimensions, fracture pattern and cement volume. All augmented VBs were then axially compressed to failure.

Results: Product of BMD value and endplate surface area gave the best prediction of failure strength when compared to actual failure strength of specimens in scenario 1. Augmented VBs showed an average cement fill of 23.9%±8.07% S.D.. In scenario 1, there was a significant post-vertebroplasty factorial increase of 1.72 and in scenario 2 a 1.38 increase in failure strength. There was a significant reduction in stiffness following augmentation for scenario 1 (t=3.5, P=0.005). Stiffness of the VB in scenario 2 was significantly greater than observed in scenario 1 (t=4.4, P=0.0002).

Discussion: Results suggest that augmentation of the VB post-fracture significantly increases failure load, whilst stiffness is not restored. Prophylactic augmentation was seen to increase failure strength in comparison to the predicted failure load. Stiffness appears to be maintained suggesting that prophylactic PVP maintains stiffness better than PVP post-fracture.

Introduction: A feature of osteoporosis is vertebral compression fractures (VCF). Experiments looking at predicting compressive strength of human lumbar vertebrae have showed a correlation between compressive strength, bone density and size of vertebral endplates. The objective of this study was to compare the actual versus predicted failure strength of osteoporotic human vertebrae in relation to creating a validated experimental model for a vertebral compression fracture.

Methods: Twenty-six human vertebrae underwent CT scanning to evaluate bone mineral density (BMD) from a large and small region of interest (ROI) within the vertebral body (VB). Cranial, caudal and verage endplate surface area (SA) measurements were recorded. Specimens were axially compressed to failure and a regression analysis undertaken in which the failure load was fitted using both BMD alone and the product of the BMD and endplate SA.

Results: Measurements of BMD from a large or small ROI showed a poor correlation when compared to vertebral failure strength. The product of BMD and endplate SA showed significant correlations with failure strength. The regression explains a significant proportion of the variation of the response variable.

Discussion: Results from this study are consistent with published data which have established a good correlation between the product of endplate SA and BMD to vertebral compressive strength. BMD values from a large ROI and average or caudal endplate area provide the best prediction of failure strength. Experience from this study suggests that the experimental model is reproducible and accurate, however, further work is required on a larger data set to verify initial findings.

Percutaneous vertebroplasty (PVP) is an emerging interventional technique for treatment of vertebral compression fractures. Bone cement is introduced to mechanically augment fracture and pain relief is almost immediate. Recent clinical and biomechanical studies have outlined the phenomenon of fractures occurring in adjacent vertebrae following PVP [1,2]. It is widely believed that rigid cement augmentation may cause a shift in the normal loading pattern of the spine thereby resulting in adjacent fractures. However, very few studies have attempted to quantify this effect [3].

Most biomechanical studies adopt a single vertebral body as a model for PVP analysis. With this approach it is not possible to determine the effect of load distribution on adjacent structures. Where multi-segment vertebrae have been used there is little documentation of the fracture characteristics produced or their repeatability. The purpose of this study was to develop a 3-vertebra model for the biomechanical analysis of PVP. The particular focus was on developing a robust technique for generating repeatable level of fracture severity from specimen to specimen.

An alignment device was developed to fit into standard materials testing machine, which allowed constant axial compression without causing lateral bending or flexion-extension of the specimen’s ends. Porcine 3-segment specimens (T8-L2) were mechanically compressed to failure at a rate of 5mm/min applied vertically at a distance of 35% to the anterior edge of the specimen’s anterior-posterior length. During the test load-displacement data was displayed in real time on a PC. In order to generate uniform fractures, a protocol was devised in which the specimens were compressed for a further 6mm after initial yield point. After the initial fracture the segments were augmented with 3ml of PMMA cement injected through each pedicle and then recompressed. The fracture characteristics generated under these conditions were analysed using quantitative microcomputer tomogragy (μCT).

μCT images showed that fractures were generated in the central vertebra, with some propagation towards adjacent vertebra. The results support the use of a 3-segment specimen as a better representation for PVP analysis. The method will enables the load shift and fracture progression on either side of the augmented vertebra to be observed, thereby providing a more complete picture of load-bearing kinetics. Secondly, the middle, augmented motion segment remains unconstrained by platens and cement impressions; hence its anatomical boundary conditions are less compromised. Although longer segments have been shown to be more anatomically appropriate, it is difficult to apply physiologic levels of load without causing the specimen to buckle. We were able to minimise buckling effect by incorporating an alignment device to position the specimen without constraint. Given the preceding observations, the concepts of 3-segment specimen in PVP biomechanical tests provides a suitable compromise in choosing an appropriate clinical setting for in-vitro testing of biological spine specimens.

Introduction Successful glenoid component fixation in shoulder arthroplasty is dependent on the quality of the underlying bone. The quantity of trabecular bone available for fixation is small and its properties are critical for both fixation and load bearing. Indentation testing has been used previously to determine regional changes in the mechanical properties of the glenoid surface [1]. However, there has been no attempt to relate these properties to the quality of the surrounding bone. The aim of this study was to investigate the relationship between the mechanical properties of the surface with both the trabecular bone volume fraction and the cortical thickness of the underlying bone. Materials and

Methods Nineteen embalmed glenoids were obtained from human cadavers (mean age 82 years). Previous work had shown that embalming had minimal impact on the mechanical properties of bone derived using indentation testing [2]. Indentation tests were performed using a 2.95 mm flat cylindrical indenter, with a speed of 2 mm/min, at 11 pre-selected grid points, up to a depth of 3 mm. Care was taken to ensure that the indenter surface was perpendicular to the local surface of the glenoid. The stiffness and maximum load following mechanical properties were measured from the resulting load-displacement curve. The Young’s modulus and strength were derived using the formula given in [3] and normalising with respect to the indenter cross section, respectively. Each of the glenoids was scanned using a large sample microCT (Scanco uCT 80) at a resolution of 78 microns. The cortical thickness and bone volume fraction (BV/TV) local to each of the grid points was determined from the 3-D reconstructions of these scans.

Results The mean strength and elastic modulus of each of the 11 indentation sites ranged from 26 to 67 MPa and 83 to 184 MPa, respectively. The largest value of BV/TV was found at the posterior edge (0.41%) and the lowest at the inferior edge (0.14%). The measured cortical thickness ranged from 0.68mm to 0.88mm with the thickest at the superior edge. Multiple regression analysis found, in the main, a significant correlation between strength and BV/TV for data derived from each of the indentation sites. The elastic modulus had only a weak correlation with BV/TV. Cortical thickness was found to have only a very marginal influence on both the elastic modulus and strength.

Discussion The indentation and uCT analysis have been used for the first time to relate the glenoid’s mechanical properties to bone morphology. The distribution of the BV/TV data is similar to that found by Frich et al [4] and for BMD measurements for BMD [5]. However, the cortical thickness measurements differ from those of Frich [4]. The local bone volume fraction strongly influenced the strength at the glenoid surface. Further investigations are ongoing to determine more fully the morphological factors important in the properties of the glenoid surface and whether such factors can be a predictor of clinical success.

Introduction: Spinal cord injury (SCI) continues to challenge the healthcare and the adjunct social welfare systems. Significant advances have been made in our understanding of the pathological cascade following the initial insult. However, this has yet to be translated into clinically significant treatments and one possible reason for this is that little is known about the actual interaction between the cord and the spinal column at the moment of impact; a factor that is becoming increasingly recognised as important. Burst fractures are a common cause of SCI and are sufficiently well defined to allow significant advances to be made in developing laboratory models of the fracture process. Following on from these advances an in-vitro model of the interaction between the cord and burst fracture fragment was developed and used to perform preliminary experiments to establish those factors that are important in determining the extent of probable cord damage.

Methods: A rig was developed that reliably reproduced a range of fragment-cord impact scenarios previously observed in the development of a model of the burst fracture process. In summary, a simulated bone fragment of mass 7.2 g was fired, transversely, at explanted bovine cord (within 3 hours of slaughter) with a velocity of 2.5, 5.0 or 7.5 ms-1. The cords were mounted in a tensile testing machine using a novel clamping system and held at 8 % strain. A surrogate posterior longitudinal ligament (PLL) was included and simulated in three biomechanically relevant conditions: absent, 0 % strain and 14 % strain. The posterior elements were represented by an anatomically correct surrogate. The impacts were recorded by using either a high speed video camera (4500 frames/s) or a series of fine pressure transducers.

Results: The fragments were recorded to undergo the same occlusion profile as previously reported in the burst fracture model, except that the cord itself reduced the level of maximum occlusion possible. All tests displayed the fragment recoiling following maximum occlusion. The maximum occlusion and the time to this position were found to be significantly dependent on both the fragment velocity and the condition of the PLL. Similar results were observed for peak pressure. One surprising result was that maximum occlusion or time to this event did not change with or without the cord being encased in the dura mater; a structure that is thought to protect the cord from external impacts.

Discussion: The model developed here of the cord-column interaction for the burst fracture produced useful initial insights into the factors that affect the impact on the cord. The PLL has a significant role to play in both reducing the peak pressures and the spreading the energy imparted over a longer period. The model has several areas in which it could be improved and these include 1) the incorporation of the perfusion pressure which tends to hydraulically stiffen the cord and 2) the inclusion of the cerebrospinal fluid, which may operate in unison with the dura in protecting the cord from impacts. Future work includes the incorporation of the CSF into the model, the development of surrogate cords and the generation of computational models using novel programming techniques.

Introduction: Aseptic loosening is a long-term complication of many cemented arthroplasty procedures. The integrity of the fixation interface, in particular the level of interdigitation between cement and bone, is crucial to sustaining the stability of arthroplasty components[1]. Studies have shown that the viscosity of cement at the time of application is a significant parameter in determining this level of interdigitation[2]. However, the rheological properties of cement at key stages in arthroplasty procedures have not been quantified, and it is unclear if current operative techniques achieve optimum cement delivery properties. Furthermore, because the cure process of bone cement is highly dependent on environmental conditions, it is extremely difficult to accurately predict the time to curing. Oscillatory shear rheometry can be used to characterise the viscoelastic properties of bone cement. However, most commercial rheometers used for this purpose are too large, expensive and delicate for peri-operative use. The aim of this work is to develop a new laboratory method for measuring the viscoelastic properties of bone cement at the time of application and to investigate the relationship between these properties and the level of cement interdigitation.

Methods: A simple, inexpensive electromagnetic rotary actuator has been developed to provide accurate measurements of force, displacement and velocity without the use of sensors. These parameters can be used to continually monitor both viscous and elastic properties of curing bone cement. To consider subjective cementation techniques, a method has been devised where a surgeon indicates early and late doughing stages for a PMMA bone cement within a clinical environment. A computer interface has been developed to plot the real-time properties of the cement that are measured using the self-sensing device concurrently. The range of practical variability of cement delivery properties is then established. In order to investigate the effect of cement viscosity on the level of interdigitation a rig has been developed in which cement is applied to a standardised bone analogue under controlled conditions. The open pore ceramic analogue has been shown through microCT scanning to have a structure that is representative of the trabecular structure in human bone. CMC solution is used to represent back bleeding. Once set, the sample is evaluated using microCT to measure the level of interdigitation.

Results: Preliminary results show that bone cement has largely viscous properties following mixing and largely elastic properties towards setting. Values of dynamic viscosity obtained show the cement to have a low viscosity following mixing, then as polymer beads begin to dissolve in the monomer, the viscosity rapidly increases. The rate of viscosity increase then slows as polymer chains are created, before a final rapid increase in viscosity indicating the onset of setting.

Conclusion: A validated method has been developed to measure the viscoelastic properties of curing bone cement at key stages in arthroplasty procedures and to investigate the effect of these viscoelastic properties using a simple standardised bone model.

Introduction: Given the timing and nature of adolescent-onset idiopathic scoliosis (AIS), this progressively deforming condition is highly likely to have a significant psychosocial impact. Body image dissatisfaction is a frequent finding in AIS patients, which is of concern, as there is a well-documented causative link between body image disturbance and the formation of disordered eating behaviour, reflected in the theoretical models for this area of psychopathology.

However, although AIS patients have frequently been observed to exhibit disturbed body image, there has been no previous attempt to assess indications of disordered eating behaviour. Given the prevalence of AIS in adolescent females and the possible medical consequences of disordered eating, this study aimed to investigate whether AIS patients have an increased likelihood of low body weight.

Methods and Results: Patients were recruited over a four month period from the regional scoliosis out-patient clinic at St James’ University Hospital; 44 female scoliosis patients participated, with a mean age of 16 (range 13 to 19). All those meeting the inclusion criteria (diagnosed with AIS, not diagnosed with any other serious medical condition), and attending clinic over the data collection period were asked to participate.

Weight, height, and BMI (weight (kg)/height(m)2) measurements taken from AIS participants were compared to age and gender-adjusted normative data. No uncoiling correction was made for the scoliosis in terms of body height. The International Classification of Diseases (ICD-10) body mass criterion for eating pathology was used to determine how many AIS participants were within the range considered eating disordered.

Independent-sample t-tests revealed that, when compared to the normative data, the AIS group did not differ significantly in terms of height (p=0.646). However, they were significantly lighter (p< 0.001), and had significantly lower BMI scores (p< 0.001); 25% of the sample had a BMI score within the range considered anorectic. Of these low-BMI patients, the mean index score was 15.6 (range 12.9–17.5). The mean weight was 40.25 kg (6st 4lbs), with a range from 31.5 to 49 kg (4st 13lbs – 7st 11lbs). The body mass data for this low-BMI group, both in terms of range and severity, is not within ‘normal’ body shape variation, and would not be expected in healthy adolescent females.

Conclusion: The relationship between a diagnosis of AIS and low body weight may indicate disordered eating behaviour and is thus a cause for considerable concern. This is of particular relevance in the light of the well-established relationship between eating psychopathology and osteoporosis, which may result if disordered eating produces a reduced peak bone mass. Organic health consequences may need to be added to a matter previously considered to be one of cosmetic deformation.

Between 1983 and 1988 we carried out 45 Charnley low-friction arthroplasties with autografts from the femoral head in 41 patients for developmental dysplasia of the hip.

The preoperative radiographs were assessed for the severity of DDH according to the classifications of Crowe et al, Hartofilakidis et al and Sharp. The postoperative and follow-up radiographs were analysed for coverage of the socket by the graft, for loosening and for the outcome of the fixation of the bone graft. Two patients died (two hips) at four and seven years after THR from causes unrelated to the surgery and were excluded from the final radiological analysis. The mean age of the patients at the time of operation was 46 years 3 months. The autograft of the femoral head covered a mean 26% (16 to 35) of the acetabular component. All the grafts united. Some degree of resorption of the bone graft occurred in 27 patients, and always involved the lateral part of the graft, which was beyond the margin of the socket. After a mean follow-up of 11 years there had been no revisions and 38 patients had no pain or only slight discomfort. One socket migrated and four others were fully demarcated.

Our findings indicate that the Charnley LFA with an autograft of the femoral head for DDH remains successful at a follow-up of 15 years.

A case of melorheostosis is described in which more than one limb was affected. It was associated with increased length of the right leg, lymphatic vesicles in the right groin, ossification in the subcutaneous tissues of the right thigh and a cutaneous haemangioma of the right side of the trunk.