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Item type:Publication, Access status: without files , Influence of time in-situ and implant type on fixation strength of cemented tibial trays - a post mortem retrieval analysis(Elsevier, 2012-11-01); ; ; Background: Loosening of the tibial tray is cited as the most common cause of failure in total knee arthroplasty but the mechanism remains unclear. Post mortem specimens provide a unique opportunity to investigate the clinical condition. Methods: Twenty two cemented components were serially retrieved in situ at autopsy from a university clinic. They were investigated for mechanical stability by pull-out, which was related to cement morphology and bone quality from CT scans, and to polyethylene wear by score analysis. Implants were grouped into three types: a particular fixed bearing design (n = 8), a particular rotating platform design (n = 5) and other mixed designs (n = 9). Findings: Trends were observed for pull-out force to decrease with time in situ and increase with cement penetration but was unrelated to bone density or polyethylene wear. For the fixed bearing implants decreasing pull-out strength was related to an increasing proportion of failure at the bone-cement interface. For the mixed designs the opposite was observed. The rotating platform implants failed at the implant-cement interface. Interpretation: The analysis demonstrated that interface failure is dependent on the implant design, but that both the stem and the bone interfaces weaken with time in situ. Published findings for laboratory implantations have demonstrated that greater cement penetration improves fixation and this was reflected for clinical samples in this study.Publicationtype: Journal Article7 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Pulsed lavage improves fixation strength of cemented tibial componentsPulsatile lavage is purported to improve radiographic survival in cemented total knee arthroplasty (TKA). Similarly, a potential improvement of fixation strength of the tibial tray has been assumed based on the increased cement penetration. In this study, the influence of pulsed lavage on fixation strength of the tibial component and bone cement penetration was evaluated in six pairs of cadaveric specimens. Following surgical preparation, the tibial surface was irrigated using pulsatile lavage on one side of a pair, while on the other side syringe lavage was applied. All tibial components were implanted using the same cementing technique. Cement penetration and bone mineral density was assessed based on computed tomography data. Fixation strength of the tibial trays was determined by a pull-out test with a material testing machine. Median pull-out forces and cement penetration were significantly (p=0.031) improved in the pulsed lavage group as compared to the syringe lavage group. Enhanced fixation strength is suggested as being a key to improved survival of the implant. Consequently, pulsatile lavage should be considered as a mandatory preparation step when cementing tibial components in TKA.Publicationtype: Conference Paper8 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Influence of interface condition and implant design on bone remodelling and failure risk for the resurfaced femoral head(Elsevier, 2011-04-20); ; ; ; Resurfacing of the femur has experienced a revival, particularly in younger and more active patients. The implant is generally cemented onto the reamed trabecular bone and theoretical remodelling for this configuration, as well as uncemented variations, has been studied with relation to component positioning for the most common designs. The purpose of this study was to investigate the influence of different interface conditions, for alternative interior implant geometries, on bone strains in comparison to the native femur, and its consequent remodelling. A cylindrical interior geometry, two conical geometries and a spherical cortex-preserving design were compared with a standard implant (ASR, DePuy International, Ltd., UK), which has a 3° cone. Cemented as well as uncemented line to line and press-fit conditions were modelled for each geometry. A patient-specific finite element model of the proximal femur was used with simulated walking loads. Strain energy density was compared between the reference and resurfaced femur, and input into a remodelling algorithm to predict density changes post-operatively. The common cemented designs (cylindrical, slightly conical) had strain shielding in the superior femoral head (>35% reduction) as well as strain concentrations (strain>5%) in the neck regions near the implant rim. The cortex-preserving (spherical) and strongly conical designs showed less strain shielding. In contrast to the cemented implants, line to line implants showed a density decrease at the centre of the femoral head, while all press-fit versions showed a density increase (>100%) relative to the native femur, which suggests that uncemented press-fit implants could limit bone resorption.Publicationtype: Journal ArticleScopus© Citations 18 5 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Fluid transfer as a mechanism leading to endotensionSome abdominal aortic aneurysms bridged with a minimally invasively introduced stent graft prosthesis increase in size without any diagnosable evidence of endoleakage (endotension). There are three possible pathways proposed through which undetectable low rates of blood flow may cause an aneurysm sac to be refilled: through the thrombus at the prosthesis attachment sites, through the clotted collaterals and through the clotted stent graft wall. If the inflow is more rapid than any drainage through the wall of the aneurysm is re-pressurised. However, critical magnitudes for the permeability of the clotted graft and the geometry of the thrombus at the aneurysm, necessary to inhibit endotension, are not known. The aim of this study was to determine which boundary conditions prevent endotension. An analytical model based on Darcy's Law was used to estimate the pressure in the aneurysm sac due to fluid transfer. Experimentally determined time- and pressure-dependent permeability of red and intraluminal thrombus, and of clotted graft materials were input into the model. The computational analysis showed that endotension is unlikely to be caused by fluid transfer from the collaterals or via the prosthesis attachment sites, but rather due to flow through the stent graft wall. Based on this study it can be stated that grafts with a permeability below 2e-13mm2 should diminish the occurrence of endotension. © 2010 IPEM.Publicationtype: Journal Article3 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Primary stability of uncemented femoral resurfacing implants for varying interface parameters and material formulations during walking and stair climbing(Elsevier, 2010-02-10); ; ; Primary stability of uncemented resurfacing prosthesis is provided by an interference fit between the undersized implant and the reamed bone. Dependent on the magnitude of interference, the implantation process causes high shear forces and large strains which can exceed the elastic limit of cancellous bone. Plastification of the bone causes reduced stiffness and could lead to bone damage and implant loosening. The purpose in this study was to determine press-fit conditions which allow implantation without excessive plastic bone deformation and sufficient primary stability to achieve bone ingrowth. In particular, the influence of interference, bone quality and friction on the micromotion during walking and stair-climbing was investigated. Therefore elastic and plastic finite element (FE) models of the proximal femur were developed. Implantation was realized by displacing the prosthesis onto the femur while monitoring the contact pressure, plastic bone deformation as well as implantation forces. Subsequently a physiologic gait and stair-climbing cycle was simulated calculating the micromotion at the bone-implant interface. Results indicate that plastic deformation starts at an interference of 30 μm and the amount of plastified bone at the interface increases up to 90% at 150 μm interference. This effect did not reduce the contact pressure if interference was below 80 μm. The micromotion during walking was similar for the elastic and plastic FE models. A stable situation allowing bony ingrowth was achieved for both constitutive laws (elastic, plastic) for walking and stair climbing with at least 60 μm press-fit, which is feasible with clinically used implantation forces of 4 kN.Publicationtype: Journal Article2 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Designanalyse von endovaskulären AortenprothesenIn recent years minimally-invasive endovascular surgery of abdominal aortic aneurysms by means of stentgrafts has emerged alongside conventional open surgery as an alternative procedure for the prevention of vessel rupture. The degenerated vessel is bridged by a tubular prosthesis and is thereby decoupled from the systemic pressure. Despite the continuous evolution of endovascular stentgrafts over the last 15 years minimally-invasive treatment still suffers a re-intervention rate of 20% within 4 years. Inadequate sealing and migration of the prostheses rank among the most frequent failure causes. In this study, factors of stentgraft failure were identified, basic principles for component testing were established and design criteria for the development of prostheses were derived. Based on clinical data, parametric and patient-specific in vitro experiments and on mathematical models, fixation and flow forces were related to occurrence of migration in individual patients. Based on in vitro permeability measurements of thrombus and clotted grafts an analytical model was used to investigate the role of the thrombus as a sealing medium against pressure build-up in the aneurysm sac. Additionally, the influence of retrograde flow through the collateral vessels into the bridged aneurysm (endoleak II) on the pressure acting in the aneurysm was analyzed. In 80% of the patient data examined, a loss of fixation due to vessel dilation in the fixation region, or kinking of the prosthesis, were responsible for migration of the prostheses. The permeability of the thrombus was found to be sufficiently high to lead to pressurisation of the aneurysm sac and thus to a failure of the treatment for a permeable prosthesis and short attachment zones. Endoleak II was found topotentially lead to re-pressurization of the vessel wall up to the systemic pressure and was frequently found to be the cause for dilatation of the aneurysm clinically. Due to the high incidence of this leakage type a new design was proposed, to completely seal the collateral vessels by means of a thin, oversized membrane. It was demonstrated that sealing of the aneurysm was improved in comparison to a standard prosthesis and appropriate fixation stability could be achieved with the new implant design.Publicationtype: Doctoral Thesis1012 2854 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Influence of press-fit parameters on the primary stability of uncemented femoral resurfacing implants(Elsevier, 2009-01); ; ; Primary stability is essential to the success of uncemented prostheses. It is strongly influenced by implantation technique, implant design and bone quality. The goal of this study was to investigate the effect of press-fit parameters on the primary stability of uncemented femoral head resurfacing prostheses. An in vitro study with human specimens and prototype implants (nominal radial interference 170 and 420 μm) was used to investigate the effect of interference on primary stability. A finite element model was used to assess the influence of interference, friction between implant and bone, and bone quality. Primary stability was represented by the torque capacity of the implant. The model predicted increasing stability with actual interference, bone quality and friction coefficient; plastic deformation of the bone began at interferences of less than 100 μm. Experimentally, however, stability was not related to interference. This may be due to abrasion or the collapse of trabecular bone structures at higher interferences, which could not be captured by the model. High nominal interferences as tested experimentally appear unlikely to result in improved stability clinically. An implantation force of about 2500 N was estimated to be sufficient to achieve a torque capacity of about 30 N m with a small interference (70 μm). © 2008 IPEM.Publicationtype: Journal Article7