T V How

University of Liverpool, Liverpool, England, United Kingdom

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Publications (108)246.58 Total impact

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    ABSTRACT: Meta-analysis supports patch angioplasty after carotid endarterectomy (CEA); however, studies indicate considerable variation in practice. The hemodynamic effect of a patch is unclear and this study attempted to elucidate this and guide patch width selection. Four groups were selected: healthy volunteers and patients undergoing CEA with primary closure, trimmed patch (5 mm), or 8-mm patch angioplasty. Computer-generated three-dimensional models of carotid bifurcations were produced from transverse ultrasound images recorded at 1-mm intervals. Rapid prototyping generated models for flow visualization studies. Computational fluid dynamic studies were performed for each model and validated by flow visualization. Mean wall shear stress (WSS) and oscillatory shear index (OSI) maps were created for each model using pulsatile inflow at 300 mL/min. WSS of <0.4 Pa and OSI >0.3 were considered pathological, predisposing to accretion of intimal hyperplasia. The resultant WSS and OSI maps were compared. The four groups comprised 8 normal carotid arteries, 6 primary closures, 6 trimmed patches, and seven 8-mm patches. Flow visualization identified flow separation and recirculation at the bifurcation increased with a patch and was related to the patch width. Computational fluid dynamic identified that primary closure had the fewest areas of low WSS or elevated OSI but did have mild common carotid artery stenoses at the proximal arteriotomy that caused turbulence. Trimmed patches had more regions of abnormal WSS and OSI at the bifurcation, but 8-mm patches had the largest areas of deleteriously low WSS and high OSI. Qualitative comparison among the four groups confirmed that incorporation of a patch increased areas of low WSS and high OSI at the bifurcation and that this was related to patch width. Closure technique after CEA influences the hemodynamic profile. Patching does not appear to generate favorable flow dynamics. However, a trimmed 5-mm patch may offer hemodynamic benefits over an 8-mm patch and may be the preferred option.
    Journal of vascular surgery: official publication, the Society for Vascular Surgery [and] International Society for Cardiovascular Surgery, North American Chapter 03/2014; · 3.52 Impact Factor
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    ABSTRACT: In vascular interventional radiology, procedures generally start with the Seldinger technique to access the vasculature, using a needle through which a guidewire is inserted, followed by navigation of catheters within the vessels. Visual and tactile skills are learnt in a patient apprenticeship which is expensive and risky for patients. We propose a training alternative through a new virtual simulator supporting the Seldinger technique: ImaGiNe (imaging guided interventional needle) Seldinger. It is composed of two workstations: (1) a simulated pulse is palpated, in an immersive environment, to guide needle puncture and (2) two haptic devices provide a novel interface where a needle can direct a guidewire and catheter within the vessel lumen, using virtual fluoroscopy. Different complexities are provided by 28 real patient datasets. The feel of the simulation is enhanced by replicating, with the haptics, real force and flexibility measurements. A preliminary validation study has demonstrated training effectiveness for skills transfer.
    Computer methods and programs in biomedicine 06/2013; · 1.56 Impact Factor
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    ABSTRACT: Aims: To construct a simple and affordable simulator for ultrasoundguided percutaneous renal biopsy. Material and methods: The kidney biopsy phantom was constructed by embedding a porcine kidney in gelatine. Silicon carbide and aluminium oxide were used as scattering particles in order to mimic the ultrasound appearance of human tissues. Two porcine ribs were also embedded. A latex sheet was placed over the top of the gel layer to resemble skin. The simulator was used and feedback from participants obtained during a renal ultrasound course with an international audience of middle-grade trainees from adult and pediatric nephrology, many of whom had never done a renal biopsy. Biopsy was carried out a single-use biopsy gun. Results: All participants were able to perform a biopsy and obtain a satisfactory sample. All trainees felt that our simulator was very realistic. 94% of participants agreed that the simulator would help to allay their fears in relation to renal biopsy The total cost of the simulator was around £ 50,- for consumables per simulator. Conclusions: We describe a purpose-built and affordable simulator for percutaneous ultrasound-guided renal biopsy. We suggest that others evaluate our simulator used as part of a structured approach to teach this important procedure.
    Clinical nephrology 03/2013; 79(3):241-5. · 1.29 Impact Factor
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    ABSTRACT: PURPOSE: The purpose of the study is to quantify the variation in the metric equivalent of French size in a range of medical devices, from various manufacturers, used in interventional radiology. METHODS: The labelling of a range of catheters, introducers, drains, balloons, stents, and endografts was examined. Products were chosen to achieve a broad range of French sizes from several manufacturers. To assess manufacturing accuracy, eight devices were selected for measurement using a laser micrometer. The external diameters of three specimens of each device were measured at centimeter intervals along the length of the device to ensure uniformity. RESULTS: A total of 200 labels of interventional radiology equipment were scrutinized. The results demonstrate a wide variation in the metric equivalent of French sizing. Labelled products can vary in diameter across the product range by up to 0.79 mm. The devices selected for measurement with the non-contact laser micrometer demonstrate acceptable manufacturing consistency. The external diameter differed by 0.05 mm on average. CONCLUSIONS: Our results demonstrate wide variation in the interpretation of the French scale by different manufacturers of medical devices. This has the potential to lead to problems using coaxial systems especially when the products are from different manufacturers. It is recommended that standard labelling should be employed by all manufacturers conveying specific details of the equipment. Given the wide variation in the interpretation of the French scale, our opinion is that this scale either needs to be abandoned or be strictly defined and followed.
    CardioVascular and Interventional Radiology 01/2013; · 2.09 Impact Factor
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    ABSTRACT: Stent fracture after fenestrated endovascular aneurysm repair is a recognized complication. In this report, we record the occurrence of superior mesenteric artery stent fractures in our series and describe the management of embolized stent fragments during secondary intervention.
    Journal of vascular surgery: official publication, the Society for Vascular Surgery [and] International Society for Cardiovascular Surgery, North American Chapter 10/2012; · 3.52 Impact Factor
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    ABSTRACT: Balloon expandable stents may on occasion be deployed in close proximity to the anchoring barbs of endovascular grafts. The aim of this study was to determine the risk and effect of balloon perforation by anchoring barbs and to assess whether these risks are different if the balloon is protected by a covered stent mounted upon it. A bench-top model was developed to mimic the penetration of anchoring barbs into the lumen of medium sized blood vessels. The model allowed variation of angle and depth of vessel penetration. Both bare balloons and those with covered stents mounted upon them were tested in the model to determine whether there was a risk of perforation and which factors increased or decreased this risk. All combinations of barb angle and depth caused balloon perforation but this was most marked when the barb was placed perpendicular to the long axis of the balloon. When the deployment of covered stents was attempted balloon perforation occurred in some cases but full stent deployment was achieved in all cases where the perforation was in the portion of the balloon covered by the stent. The only situation in which stent deployment failed was where the barb was intentionally placed in the uncovered portion of the balloon. This resulted in only partial deployment of the stent. Balloon rupture is a distinct possibility when deploying balloon-expandable stents in close proximity to anchoring barbs. Care should be taken in this circumstance to ensure that the barb is well away from the uncovered portion of the balloon.
    European journal of vascular and endovascular surgery: the official journal of the European Society for Vascular Surgery 07/2012; 44(3):327-31. · 2.92 Impact Factor
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    ABSTRACT: To examine the longitudinal migratory force required to cause disconnection of the bifurcated distal body component from the tubular proximal body of a fenestrated stent-graft. Using a previously reported mathematical model distal distraction forces were calculated prior to performing in vitro pullout testing. The top end of the proximal body and the iliac limbs of the distal body were attached to the grips of a tensile tester via plastic sealing plugs and pneumatic clamps. Channels within the plugs allowed pressurisation of the inside of the stent-graft. Pullout tests were conducted in the vertical plane. Force and displacement data were recorded and tests repeated 8 times at room temperature with the stent-grafts either dry or wet and unpressurized, at 100 mmHg or at 120 mmHg. The median maximum pullout force was 2.9 N (2.6-4.1) when dry, 3.9 N (3.5-5.4) when wet and unpressurized, 6.3 N (4.8-8.3) when wet and pressurized at 100 mmHg and 6.5 N (4.8-7.2) when wet and pressurized at 120 mmHg. There was a significant difference between pressurized and unpressurized conditions (P < 0.01). The force required to distract the distal bifurcated component of a fenestrated stent graft is much lower than the reported proximal fixation strength of both a standard and fenestrated Zenith stent graft. Although this helps protect the fenestrated proximal body from the effects of longitudinal migration forces in vivo the current strength of the body overlap zone may actually be unnecessarily weak and requires careful surveillance in follow up.
    European journal of vascular and endovascular surgery: the official journal of the European Society for Vascular Surgery 07/2012; 44(3):281-6. · 2.92 Impact Factor
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    ABSTRACT: Computer-based simulation for interventional radiology training has attracted increasing attention in recent years because of its potential to train remotely from patients and to provide objective assessment of proficiency. Yet developing a high fidelity simulator with realistic tactile feedback requires accurate knowledge of forces exerted on medical devices during interventional radiology procedures. This paper presents the development and validation of a force sensor for the measurement of axial forces generated during needle, and combined cannula/trocar, puncture procedures in patients. In order to assess the performance of this sensor, in vitro measurements were obtained using needle penetration of porcine liver, kidney and muscle. The results were compared with forces measured by means of a tensile tester. Calibration results showed that the force sensor has high sensitivity and linearity. Comparison of the force profiles obtained from the sensor and the tensile tester shows that good agreement was achieved in the in vitro studies for all the tissues tested. Preliminary clinical force measurements during arterial puncture and liver biopsy procedures have been performed in patients. An example of force recording for each procedure type is presented.
    Medical Engineering & Physics 06/2012; · 1.78 Impact Factor
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    ABSTRACT: Interventional Radiology procedures (e.g., angioplasty, embolization, stent graft placement) provide minimally invasive therapy to treat a wide range of conditions. These procedures involve the use of flexible tipped guidewires to advance diagnostic or therapeutic catheters into a patient's vascular or visceral anatomy. This paper presents a real-time physically based hybrid modeling approach to simulating guidewire insertions. The long, slender body of the guidewire shaft is simulated using nonlinear elastic Cosserat rods, and the shorter flexible tip composed of a straight, curved, or angled design is modeled using a more efficient generalized bending model. Therefore, the proposed approach efficiently computes intrinsic dynamic behaviors of guidewire interactions within vascular structures. The efficacy of the proposed method is demonstrated using detailed numerical simulations inside 3-D blood vessel structures derived from preprocedural volumetric data. A validation study compares positions of four physical guidewires deployed within a vascular phantom, with the co-ordinates of the corresponding simulated guidewires within a virtual model of the phantom. An optimization algorithm is also implemented to further improve the accuracy of the simulation. The presented simulation model is suitable for interactive virtual reality-based training and for treatment planning.
    IEEE transactions on bio-medical engineering 05/2012; 59(8):2211-8. · 2.15 Impact Factor
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    ABSTRACT: This study evaluated the accuracy of central luminal line (CLL) measurements in quantifying stent graft migration. The bias of the CLL technique together with observer variability were assessed. Stent grafts were deployed in plastic aortic phantoms at fixed locations from two side branches. Each phantom was filled with iodinated contrast, and a 2-mm multislice computed tomography (CT) scan was performed. The stent graft was then displaced caudally, its new location determined, and again, a CT scan performed. This created a series of 15 cases with known stent graft migration. CLLs were used to measure stent graft position on the CT scans and calculate migration (3 observers). In vivo stent graft migration was then evaluated in a similar manner using a series of follow-up CT scans from nine patients (2 observers). All CLL measurements were performed independently and were repeated on a separate occasion. The mean difference in CLL migration between the actual and observed measurements (bias) in the aortic phantoms was <1 mm. The 95% confidence intervals for the bias were within the interval (-1 and 1 mm), and the 95% limits of agreement were within -3 mm and +3 mm. The 95% limits of agreement for measurements within and between observers were -4 to 2 mm and -2 to 2 mm, respectively. The phantom study generated a coefficient of repeatability (RC) of 1 mm for within-observer measurements. Clinically, CLLs generated 95% limits of agreement within and between observers of -3 to 4 mm (RC, 2 mm) and -3 to +3 mm, respectively. Bias from CLL-determined migration is small and insignificant from a practical point of view. A small amount of measurement variability within and between observers does exist; it should be feasible to detect changes in stent graft position that are ≥4 mm.
    Journal of vascular surgery: official publication, the Society for Vascular Surgery [and] International Society for Cardiovascular Surgery, North American Chapter 01/2012; 55(4):895-905. · 3.52 Impact Factor
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    ABSTRACT: To measure the stiffness of commonly used "stiff" guidewires in terms of their flexural modulus, an engineering parameter related to bending stiffness. Eleven different intact stiff guidewires were selected to undergo a 3-point bending test performed using a tensile testing machine. Testing was performed on 3 new and intact specimens of each guidewire at 10 locations along the wire's length, excluding the floppy tip. The flexural modulus (in gigapascals, GPa) was calculated from the results of the bending test. The flexural modulus of the plain Amplatz wire was 9.5 GPa compared to 11.4 to 14.5 GPa for the "heavy duty" wires. Within the Amplatz family of guidewires, the flexural modulus was 17 GPa for the "stiff," 29.2 GPa for the "extra stiff," 60.3 GPa for the "super stiff," and 65.4 GPa for the "ultra stiff." The Backup Meier measured 139.6 GPa and the Lunderquist Extra Stiff 158.4 GPa. The Instructions for Use of some endovascular devices specify a wire type selected from a range of undefined "stiffness" descriptors. These descriptors have little correlation with the measured flexural modulus. Two guidewires with the description "extra stiff" can have a 5-fold difference in flexural modulus. We recommend that guidewire catalogues and packaging include the flexural modulus and that device manufacturers amend their Instructions for Use accordingly.
    Journal of Endovascular Therapy 12/2011; 18(6):797-801. · 2.70 Impact Factor
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    ABSTRACT: The radiocephalic arteriovenous fistula remains the method of choice for haemodialysis access. In order to assess their suitability for fistula formation, the radial arteries and cephalic veins were examined preoperatively by ultrasound colour flow scanner in conjunction with a pulse-generated run-off system. Intraoperative blood flow was measured after construction of the fistulae. Post-operative follow-up was performed at various intervals to monitor the development of the fistulae. Radial artery and cephalic vein diameter less than 1.6 mm was associated with early fistula failure. The intraoperative fistula blood flow did not correlate with the outcome of the operation probably due to vessel spasm from manipulation. However, blood flow velocities measured non-invasively 1 day after the operation were significantly lower in fistulae that failed early compared with those that were adequate for haemodialysis. Most of the increase in fistula diameter and blood flow occur within the first 2 weeks of surgery.
    European journal of vascular and endovascular surgery: the official journal of the European Society for Vascular Surgery 09/2011; 42 Suppl 1:S48-54. · 2.92 Impact Factor
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    ABSTRACT: To quantify the compression force acting on target vessel stents as a consequence of the misalignment between the native aortic anatomy and the fenestrated stent-graft owing to measuring errors during the design of the device. The material properties of a fenestrated Zenith stent-graft were determined using a standardized tensile testing protocol. Aortic anatomy was modeled using fresh porcine aortas that were subjected to tensile testing. The net force acting on a target vessel stent due to incremental discrepancy between the target vessel ostia and the stent-graft fenestrations was calculated as the difference in wall tension between the aorta and the stent-graft in diastole and systole. The change in diameter between diastole and systole was set to 8%. Using the diastole model, underestimation of circumferential target vessel position by 15°, 22.5°, and 30° resulted in net forces on the target vessel stent of 0.6, 0.8, and 1.1 N, respectively. Overestimation of target vessel position by the same increments resulted in net forces of 0.3, 0.6, and 0.9 N, respectively. With the systolic model, underestimating target vessel position by 30° resulted in a 2.1-N maximum force on the stent, which potentially threatened the seal. In the longitudinal direction, underestimating target vessel separation by up to 10 mm resulted in a maximal force on the stent of 6.1 N, while overestimating target vessel separation did not result in any additional force on the stent due to fabric infolding. The magnitude of the forces generated solely due to mismatch between stent-graft design and native anatomy is modest and is unlikely to cause significant deformation of target vessel stents. Mismatch, however, may cause loss of seal.
    Journal of Endovascular Therapy 08/2011; 18(4):569-75. · 2.70 Impact Factor
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    ABSTRACT: Guidewire manipulation is a core skill in endovascular interventional radiology procedures. Simulation-based training offers a valuable alternative for mastering these skills, but requires a faithful replication of complex guidewire behaviour inside the vasculature. This paper presents the integration of real flexural modulus (FM) measurements into our guidewire model that mimics the flexibility of standard guidewires. The variation of FM along the length of each wire was determined for seven commonly used guidewires using a three-point bending test for the main body and a two-point bending test for the flexible end. Guidewire FM values were then attributed to seven different models, each formed by a series of particles connected by links of variable FM and replicating the flexible end shape. The FM integration was done through a trial and error process matching real FM to virtual bending coefficient. This mass-spring representation captures the required range of behaviour and enables accurate deformation within virtual vasculature.
    Computer Methods in Biomechanics and Biomedical Engineering 02/2011; 14(6):515-20. · 1.39 Impact Factor
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    ABSTRACT: Guidewire and catheter manipulation is a core skill in endovascular interventional radiology. It is usually acquired in an apprenticeship on patients, but this training is expensive and risky. Simulation offers an efficient alternative for core skills training, though the instrument complex behaviour requires accurate replication. This paper reviews the mass-spring model used to simulate seven guidewires and three catheters, and the matching with their real world counterparts by tuning our model's bending coefficient, which allows replication of the instrument flexibility. This coefficient was matched through computed tomography imaging of a vascular phantom in which each instrument was inserted and manipulated. With an average distance of 2.27 mm (standard deviation: 1.54) between real and virtual instruments, our representation showed realistic behaviour.
    Studies in health technology and informatics 01/2011; 163:317-23.
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    ABSTRACT: Endovascular clinicians use guidewires to navigate within vessels during angiography or angioplasty. In mastering this core skill, an alternative to the traditional apprenticeship in patients is provided by virtual training environments though these require a faithful replication of complex guidewire behaviours inside the vasculature. This paper presents the integration of realistic flexibilities into our guidewire model that simulates the stiffness of seven commonly used guidewires. Each virtual instrument is represented as a mass-spring model replicating their flexibility and shape, especially at the flexible end. The bending coefficients were determined by comparing of the behaviour of real guidewires in a transparent silicone rubber vascular phantom to that of virtual guidewires in the virtual representation of the phantom. As a result, our representation captures the required range of behaviour and enables accurate deformation.
    Biomedical Simulation, 5th International Symposium, ISBMS 2010, Phoenix, AZ, USA, January 23-24, 2010. Proceedings; 01/2010
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    ABSTRACT: Simulating intrinsic deformation behaviors of guidewire and catheters for interventional radiology (IR) procedures, such as minimally invasive vascular interventions is a challenging task. Especially real-time simulations for interactive training systems require not only the accuracy of guidewire manipulations, but also the efficiency of computations. The insertion of guidewires and catheters is an essential task for IR procedures and the success of these procedures depends on the accurate navigation of guidewires in complex 3D blood vessel structures to a clinical target, whilst avoiding complications or mistakes of damaging vital tissues and blood vessel walls. In this paper, a novel elastic model for modeling guidewires is presented and evaluated. Our interactive guidewire simulator models the medical instrument as thin flexible elastic rods with arbitrary cross sections, treating the centerline as dynamic and the deformation as quasi-static. Constraints are used to enforce inextensibility of guidewires, providing an efficient computation for bending and twisting modes of the physically-based simulation model. We demonstrate the effectiveness of the new model with a number of simulation examples.
    The Visual Computer 01/2010; 26:1157-1165. · 0.91 Impact Factor
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    ABSTRACT: We present an integrated system for training visceral needle puncture procedures. Our aim is to provide a cost effective and validated training tool that uses actual patient data to enable interventional radiology trainees to learn how to carry out image-guided needle puncture. The input data required is a computed tomography scan of the patient that is used to create the patient specific models. Force measurements have been made on real tissue and the resulting data is incorporated into the simulator. Respiration and soft tissue deformations are also carried out to further improve the fidelity of the simulator.}, } Abstract We present an integrated system for training visceral needle puncture procedures. Our aim is to provide a cost effective and validated training tool that uses actual patient data to enable interventional radiology trainees to learn how to carry out image-guided needle puncture. The input data required is a computed tomography scan of the patient that is used to create the patient specific models. Force measurements have been made on real tissue and the resulting data is incorporated into the simulator. Respiration and soft tissue deformations are also carried out to further improve the fidelity of the simulator.
    Eurographics 2009 - Medical Prize; 03/2009
  • Article: Vascular 11
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    ABSTRACT: Objectives: Evidence suggests an inverse relationship between wall shear stress (WSS) and distal anastomotic myointimal hyperplasia (MIH), the commonest cause of bypass graft occlusion. Anastomotic configuration influences flow patterns and WSS, impacting upon anastomotic techniques. The aim of this study was to analyse the influence of non-planar distal anastomoses.Methods: Contrast enhanced CT scans of distal anastomoses, with three dimensional reconstruction, enabled subjective analysis of in vivo non-planar configuration. In vitro flow visualization studies and laser Doppler anemometry (LDA) were performed using anastomotic models reflecting these configurations, perfused with glycerol blood analogue under physiological conditions.Results: The in vivo angle between graft and recipient artery ranged from 0 to 45 degrees. In vitro flow patterns were altered by an angle of 30 degrees or more with the characteristic vortical flow pattern of a Miller cuff deviated away from the mid-line. In the proximal and distal outflow segments a helical flow pattern occurred, extending several diameters distance down the vessel. LDA velocity vector mapping corroborated the presence of helical flow within the run-off vessels, in direct contrast to symmetrical flow patterns observed within planar models. Out-of-plane configuration within the 5 cm of graft immediately proximal to the distal anastomosis was critical, with no observed influence on flow patterns imparted by more proximal deviation.Conclusion:In vivo non-planar configuration depends in part upon the tunnelling of the graft, and can therefore be influenced. Non-planar anastomotic configurations >30 degrees induce helical outflow patterns which may beneficially increase WSS, thereby potentially inhibiting MIH.
    British Journal of Surgery 01/2009; 89(S1):56 - 56. · 4.84 Impact Factor
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    ABSTRACT: To investigate in an in vitro model the ability of different covered and uncovered stents to resist displacement/migration of a fenestrated stent-graft. Three different types (2 covered, 1 bare) of commonly used 7-mm balloon-expandable stainless steel stents (Jostent, Advanta V12, and Palmaz Genesis) were investigated in a testing rig consisting of 2 overlapping tubes with 2 sets of 7-mm holes representing bilateral renal artery fenestrations and ostia. The rig was attached to a tensile tester via pneumatic clamps. The stents were deployed without flaring to 7 mm through the overlapping holes. The rig was moved apart at a constant rate of 12 mm/min up to a maximum displacement of 6 mm; force versus displacement values were recorded while stent deformation was observed. Tests were repeated at least 6 times for each stent type at room temperature. The median force required to cause a 25%, 50%, or 75% reduction in cross-sectional area of the bilateral "renal artery" stents was determined. The median force (interquartile range) required to cause a 50% reduction in cross-sectional area of identical bilateral "renal artery" stents securing fenestrations was 25.1 N (8.1) for a covered Jostent, 9.3 N (0.9) for a covered Advanta V12 stent, and 7.5 N (0.7) for a bare Palmaz Genesis stent. The differences were statistically significant (p<0.01) between stents at each of the 3 levels of cross-sectional area reduction. There is a significant difference in the ability of different commercial "non-dedicated" stents to withstand a crushing force when deployed within endograft fenestrations, which has important implications for clinical practice.
    Journal of Endovascular Therapy 07/2008; 15(3):344-8. · 2.70 Impact Factor

Publication Stats

1k Citations
246.58 Total Impact Points

Institutions

  • 1984–2014
    • University of Liverpool
      • • Institute of Ageing & Chronic Disease
      • • Directorate of Medical Imaging & Radiotherapy
      • • Department of Electrical Engineering and Electronics
      • • Department of Clinical Sciences
      Liverpool, England, United Kingdom
  • 1987–2013
    • Royal Liverpool and Broadgreen University Hospitals NHS Trust
      • • Department of Radiology
      • • Department of Clinical Engineering
      • • Department of Vascular Surgery
      Liverpool, ENG, United Kingdom
  • 2011
    • Imperial College London
      • Section of Biosurgery and Surgical Technology
      London, ENG, United Kingdom
  • 1996
    • City University London
      Londinium, England, United Kingdom
    • University of Wales
      Cardiff, Wales, United Kingdom
  • 1991
    • Laval University
      Québec, Quebec, Canada
  • 1986
    • University of Strathclyde
      Glasgow, Scotland, United Kingdom