Investigation into the role of the innate immune system in pancreatic beta-cell physiology and dysfunction

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Fatigue and damage of porcine pars interarticularis during asymmetric loading

If the articular facets of the vertebra grow in an asymmetric manner, the developed bone geometry causes an asymmetry of loading. When the loading environment is altered by way of increased activity, the likelihood of acquiring a stress fracture may be increased. The combination of geometric asymmetry and increased activity is hypothesised to be the precursor to the stress fracture under investigation in this study, spondylolysis. This vertebral defect is an acquired fracture with 7% prevalence in the paediatric population. This value increases to 21% among athletes who participate in hyperextension sports. Tests were carried out on porcine lumbar vertebrae, on which the effect of facet angle asymmetry was simulated by offsetting the load laterally by 7mm from the mid-point. Strain in the vertebral laminae was recorded using six 3-element stacked rosette strain gauges placed bilaterally. Specimens were loaded cyclically at a rate of 2Hz. Fatigue cycles; strain, creep, secant modulus and hysteresis were measured. The principal conclusions of this paper are that differences in facet angle lead to an asymmetry of loading in the facet joints; this in turn leads to an initial increase in strain on the side with the more coronally orientated facet. The strain amplitude, which is the driving force for crack propagation, is greater on this side at all times up to fracture, the significance of this can be observed in the increased steady state creep rate (p = 0.036) and the increase in yielding and toughening mechanisms taking place, quantified by the force-displacement hysteresis (p = 0.026)

Strain distribution in the porcine lumbar laminae under asymmetric loading

If the articular facets of the vertebra grow in an asymmetric manner, the developed geometry causes an asymmetry of loading. When the loading environment is altered by way of increased activity, the likelihood of acquiring a stress fracture may be increased. The combination of geometric asymmetry and increased activity is hypothesised to be the precursor to the stress fracture under investigation in this study, spondylolysis. This vertebral defect is an acquired fracture with 7% prevalence in the paediatric population. This value increases to 21% among athletes who participate in hyperextension sports. Tests were carried out on porcine lumbar vertebrae, on which the effect of facet angle asymmetry was simulated by offsetting the load laterally by 7mm from the mid-point. The aim of the study is to investigate whether an increase in the coronal orientation of one facet leads to an increase in strain in the corresponding vertebral lamina. Strain in the laminae was recorded using six 3-element stacked rosette strain gauges placed bilaterally. Results show that a significant linear predictive relationship exists between load offset and average strain level in the vertebral laminae with p values of 0.006 and 0.045 for principal strains e1 and e2 on the right-hand side, and p-values of 0.009 and 0.001 for principal strains e1 and e2 on the left-hand side (R2 all .0.9). This study concludes that facet angle asymmetry does lead to a difference in strain in the vertebral laminae. Change in principal strain as a result of facet asymmetry has a linear relationship and an asymmetry threshold exists beyond which compressive strain on the more coronally oriented facet can be increased by up to 15%

An Engineering Evaluation of Ankle Prosthetics

There are a wide range of different types of ankle replacements on the market today each with adifferent mechanical design. Unfortunately the results of ankle replacements are not as good as hipand knee replacements; this is due to the complexity of the ankle joint. In the early days of anklereplacements some of the prosthetics only lasted 4 months. Recent developments have improved thelongevity of the replacements although, there are still many complications and failures of thereplacements, these include; the prosthetic components migrating into the bone, the componentsfailing due to stresses induced by the forces and the surgery itself i.e. the incision site.This paper will analyse the documented medical failures of the replacements from a mechanicalengineering perspective. Three ankle prosthetics are investigated in this paper: the Buechel-Pappas,the Scandinavian Total Ankle Replacement (STAR) and the Hintegra ankle replacement. Medicalpublications are examined to isolate the mechanical failure mechanisms of the replacements and tocategorise and quantify these failures in engineering terms. These failures will include wearcomplications and also dislocations of the prosthetic parts among other failures. The paper will conclude by comparing the mechanical reliability of the four prosthetics examined

Making an Impact on Vertebral Compression Fractures

Spinal fractures constitute one of the most prevalent injuries to the human skeletal system with approximately 1.4 million fractures per annum worldwide. Treatments for these injuries have evolved from simple bed rest through to the intricacy of modern minimally invasive surgery. Balloon kyphoplasty is one such treatment that uses a balloon to decompress collapsed vertebra, followed by injection of bone cement to stabilise the fracture. Recent research has correlated a ‘halo’ feature on kyphoplasty patient x-rays with a 78% re-collapse rate. The present work documents a new method of mechanical analysis to evaluate balloon kyphoplasty and explores alterations to current clinical practices to overcome the re-collapse phenomenon. Results from the computational model demonstrate the need to design medical procedures and devices that more effectively maintain a strong mechanical interlock between the injected cement and the native bone during the post-operative phase. Work is currently ongoing to evaluate the use of a new surgical method to enhance the long-term integrity of the treatment and consequently improve patient quality of life

Stress Distribution at the Bone-Cement Interface Changes During Kyphoplasty Rehabilitation

Balloon Kyphoplasty uses an inflatable bone tamp and cement augmentation to repair vertebral compression fractures. A recent clinical study observed a 78% re-collapse rate in patients showing a radiolucent phenomenon at the bone-cement interface following Kyphoplasty. Two experimental studies showed significant height loss following Balloon Kyphoplasty under cyclical loads. The present study investigates the alteration in load angle corresponding to this height loss and its effect on load transfer to the bone-cement interface. A validated finite element model of a human thoracolumbar spine was segmented into a single L1 vertebral body and modified to replicate bilateral Balloon Kyphoplasty. Cement was modeled using prolate spheroids surrounded by an interface region divided into anterior, middle and posterior sections. Interface thickness was calculated using a mathematical model with a bone volume fraction of 0.3 and 50% bone compaction. An 800N load was applied at angles of 0o and 20o from the vertebral axis. Results indicate that a change in the applied load angle significantly alters the principal stress components and directions across the interface region. This alteration in loading must be considered in the context of the highly compliant interface region and therefore is hypothesized to be a contributory factor to vertebral re-collapse

Design and Development of Artificial Spinal Ligaments for Paediatric Synthetic Spine

A synthetic spine is a model fabricated from artificial materials consisting of the vertebrae, intervertebral discs and ligaments for spinal testing. The synthetic spine overcomes many difficulties associated with biological specimens such as handling, biohazard concerns, high costs, and limited availability of specimens, quality and large inter-specimen variability. This paper presents the design and development of spinal ligaments to mimic the stiffness of the paediatric ligaments for use in the synthetic spine. Spinal ligaments are uniaxial structures in the spine that carry tensile loads along the direction of the fibres. Early in the research, silicone materials were used to cover the whole spinal unit, but it became apparent that the material responses were inadequate. The synthetic spine was revised to use fibreglass tape to more closely simulate the natural spinal structures. The composite design applied in this paper consisted of soft silicone rubber and fibreglass tape to obtain the natural stiffness that normally occurred in the spinal ligaments

Stress Distribution at the Bone-Cement Interface Changes During Kyphoplasty Rehabilitation

Balloon Kyphoplasty uses an inflatable bone tamp and cement augmentation to repair vertebral compression fractures. A recent clinical study observed a 78% re-collapse rate in patients showing a radiolucent phenomenon at the bone-cement interface following Kyphoplasty. Two experimental studies showed significant height loss following Balloon Kyphoplasty under cyclical loads. The present study investigates the alteration in load angle corresponding to this height loss and its effect on load transfer to the bone-cement interface. A validated finite element model of a human thoracolumbar spine was segmented into a single L1 vertebral body and modified to replicate bilateral Balloon Kyphoplasty. Cement was modeled using prolate spheroids surrounded by an interface region divided into anterior, middle and posterior sections. Interface thickness was calculated using a mathematical model with a bone volume fraction of 0.3 and 50% bone compaction. An 800N load was applied at angles of 0o and 20o from the vertebral axis. Results indicate that a change in the applied load angle significantly alters the principal stress components and directions across the interface region. This alteration in loading must be considered in the context of the highly compliant interface region and therefore is hypothesized to be a contributory factor to vertebral re-collapse

Evaluating Compressive Properties and Morphology of Expandable Polyurethane Foam for use in a Synthetic Paediatric Spine

An expandable rigid PU foam can turns into complex shapes, with a shell like structure onthe outside and honeycomb structure on the inside, which can be easily shaped to a vertebraform. The present study aims to determine whether expandable rigid polyurethane foamwas an appropriate substitute for rigid block polyurethane foam to model the trabecularbone. Static compression tests were performed to determine compressive moduli and yieldstresses on three polyurethane foam densities namely 0.16 g/cm3, 0.24 g/cm3and 0.42 g/cm3.Morphology of the PU foams for all densities was also observed. The compressive modulusfor 0.16 g/cm3and 0.24 g/cm3were found varied from 40 to 43 MPa and 83 to 92 MPa whileyield stress ranged from 2.1 to 2.3 MPa and 3.4 to 4.8 MPa respectively. As for 0.42 g/cm3, thecompressive modulus and yield stress varied from 240 to 256 MPa and 38 to 40 MPa. Based onthese results, the compressive modulus and yield stress of 0.24 g/cm3compared favourablywith rigid block PU foam and human cadavers presented in the literatures. Hence, the find-ings of this study could potentially be used in developing a synthetic vertebral trabecularbone of paediatric spine for biomechanical testing

The Biomechanics of Balloon Kyphoplasty

Balloon Kyphoplasty uses an inflatable bone tamp and cement augmentation to repair vertebral compression fractures. A recent clinical study observed a 78% re-collapse rate in patients showing a radiolucent phenomenon at the bone-cement interface following Kyphoplasty. Two experimental studies showed significant height loss following Balloon Kyphoplasty under cyclical loads. The present study investigates the alteration in load angle corresponding to this height loss and its effect on load transfer to the bone-cement interface. A validated finite element model of a human thoracolumbar spine was segmented into a single L1 vertebral body and modified to replicate bilateral Balloon Kyphoplasty. Cement was modeled using prolate spheroids surrounded by an interface region divided into anterior, middle and posterior sections. Interface thickness was calculated using a mathematical model with a bone volume fraction of 0.3 and 50% bone compaction. An 800N load was applied at angles of 0o and 20o from the vertebral axis. Results indicate that a change in the applied load angle significantly alters the principal stress components and directions across the interface region. This alteration in loading must be considered in the context of the highly compliant interface region and therefore is hypothesized to be a contributory factor to vertebral re-collapse