Viscoelastoplastic Modeling Of Arterial Tissue

Tez (Doktora) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2007Thesis (PhD) -- İstanbul Technical University, Institute of Science and Technology, 2007Dünyadaki ekonomik, bilimsel ve tıp açısından oldukça önemli ve gözlemlenme oranı yüksek olan arteroskleroz ve disk dejenerasyonu gibi hastalıklar, temelde yumuşak doku hastalıkları olarak adlandırılabilirler. Yumuşak dokular, çeşitli mekanik özellikleri haiz ve mekanik dış ortam uyaranlarına göre (büyüme, yeniden modelleme, morfojenez) kendilerini uyarladıklarından dolayı, biyolojik ve mekanik etkileşimlerin bilinmesi ve belirtilmesi çok büyük önem taşımaktadır. Bu çalışma kapsamında, damar dokusu için viskoelastoplastik bir malzeme modelinin temellerinin oluşturulması amaçlanmıştır. Model, damar cidar yapısının kompozit özelliklerini içermekte ve çok eksenli yükleme durumlarına göre karmaşık pasif mekanik cevap mekanizmasını kapsamaktadır. Bir (elastik) hasar (damage) mekanizması ile malzemenin zamanla mekanik özelliklerinin değiştiği dikkate alınmıştır. Literatürde bu konuda oldukça fazla sayıda çalışma olmasına rağmen, deneysel verilerle desteklenen modeller yok denecek kadar azdır. Öncelikle, kardiyovasküler sistem ve damar anatomisi hakkında bilgiler sunulmuş ve literatürde bulunan daha önceki çalışmalardan örnekler verilmiştir. Kalın cidarlı eksenel simetrik tüp olarak modellenen damar üzerinde birleşik iç basınç, eksenel zorlanma ve burulma zorlanması yüklenmeleri için ifadeler elde edimiştir. Koyun pulmoner arterleri ile in-vitro deneysel çalışmalar gerçekleştirilmiş, malzeme katsayıları tahmini gerçekleştirilmiştir. Çalışma, sonuçlar ve ileriye yönelik araştırma konuları hakkında kısa bir yol gösterme ile sona erdirilmiştir.The great majority of diseases in the (western) world, such as atherosclerosis and degeneration of intervertebral discs are diseases of soft tissues. Hence, the multidisciplinary field of soft tissue research is of crucial scientific, medical and socioeconomic importance. Thus, it is of fundamental importance to identify the complex interactions of mechanical and biological responses. This work has aimed at setting the foundations of a non-linear material model for arterial viscoelastoplasticity. The model accounts for the composite structure of the vessel and its complex passive mechanical response to loading conditions. Long term failure of arterial structure has been modeled with a damage model. It can easily be extended to remodeling phenomenon. Much work has been done in the literature, but hardly few have been extensively studied with such diverse experimental data. A brief of continuum mechanics has been presented to make the terminology clear, and formulations extensively suitable to application of combined inflation/extension/torsion loads on thick-walled cylindrical tubes have been presented. In vitro experiments have been carried out with sheep arterial segments, and parameter fitting and theoretical stress analysis have been carried out. The work concludes with basic outcomes and some comments on future research topics.DoktoraPh

Viskoelastik damar dokusunda malzeme parametrelerinin deneysel tahmini

The great majority of diseases in the (western) world, such as atherosclerosis and degeneration of intervertebral discs are diseases of soft tissues. Hence, the multidisciplinary field of soft tissue research is of crucial scientific, medical and socioeconomic importance. The fast progress in the developments of hardware and software facilities makes it possible to thoroughly investigate biological soft tissues and their pathologies on a computational basis. Since soft tissues are biological materials, which fulfill mechanical purposes and adapt to their mechanical environment (growth, remodeling and morphogenesis), it is of fundamental importance to identify the complex interactions of mechanical and biological responses. This work lays out a viscoelastic material model for the arterial tissue. In order to correctly describe the motion of a thick-walled tubular model of an arterial specimen under combined extension, inflation and torsion loads, large deformation kinematics theory is introduced. Lagrangian practice has been applied, which is a general notion in solid mechanics of hyperviscoelastic materials. The material model is introduced in parallel, which is a non-linear three element solid when regarded with a 1-D analog to standard viscoelastic practices. Principle of multiplicative decomposition, a common practice in large deformation plasticity, is successfully applied to get an insight to time-evolution characteristics of the tissue as well as pure elastic response in a decoupled manner. The material elastic stress law and thermodynamically consistent evolution law, namely the stress-strain relations are derived. The arterial tissue which has been modeled as a thick-walled mono-layered axisymmetric cylinder is, when explicitly stated, to be of a type that is called fiber reinforced composite. The governing equations of constitutive assumption are then represented for the fiber-reinforced model. The fiber constituents are mainly the collagen fibers that exist in bundles within a ground matrix material, known to exist mainly in the form of elastin and, within our model, passive existence of smooth muscles. The issues related to fiber-reinforced structure are the evolution of stretch of collagen fibers with ongoing deformation due to external applied loads, and the characteristics of dissipative behavior due to existence of fibers. To cope with experimental practices and realistic simulation of real life situations, the explicit relations between the stress (thus strain) values using the constitutive model and the applied loads, namely the internal pressure, the axial force and the torsional moment are successfully obtained. Due to the thick-walled tube assumption that has been the underlying base to model the arterial tissue, all the quantities had the form of a radial integration of some quantity through the wall thickness of the hollow cylinder at the deformed state, despite the constitutive assumptions have been laid in a Lagrangian manner. Details of the test setup, that has been facilitated at the Laboratory of Strength of Materials and Biomechanics, Faculty of Mechanical Engineering, Istanbul Technical University have been provided to let the readers have an insight of how the experimental system runs. The complexity involved in the loading protocols has required bringing up different sensors and techniques together. The use of high speed video imaging systems for optical strain measurement under dynamic extension, inflation and torsion have been successfully applied to obtain complete experimental data over specimens. Sample pictures and calculations from the optical strain measurement system synchronized to the loading frame control electronics have been provided to clarify the deformation data acquisition process. Parameter estimation has then been carried out with the collected data. Required post-processing of both strain and load data have been carried, preceding the estimation processes. Various scenarios have been applied over a fibrous arterial tissue. Data from these different tests have been analyzed and relevant graphical representations of fitting have been presented. Both quasi-static and dynamic loads up to 10 Hz and up to 100% Lagrangian strains in axial and tangential directions have been applied over the specimens. The observed discrepancies between the experimental and theoretical gatherings have been commented and finally the limitations of the model and the critics to its applicability over modeling the arterial tissue have been stated. Keywords: Biomechanis, experimental mechanics, arterial tissue, nonlinear viscoelasticity.Dünyadaki ekonomik, bilimsel ve tıp açısından oldukça önemli ve gözlemlenme oranı yüksek olan ateroskleroz ve disk dejenerasyonu  gibi hastalıklar, temelde yumuşak doku hastalıkları olarak adlandırılabilirler. Bu bağlamda, disiplinler arası bir araştırma konusu olan yumuşak dokularda araştırma, sosyoekonomik olarak gittikçe büyüyen bir önem arz etmektedir. Donanım ve yazılım olanaklarındaki hızlı gelişmeler, yumuşak dokuların ve ilgili patalojilerin sayısal olarak detaylı şekilde modellenebilmesine olanak tanımaktadır. Bu çalışmada, viskoelastik özellikler gösteren damar dokusu üzerinde teorik bir modelin öncelikle matematiksel altyapısı oluşturulmuş ve fiziksel büyüklükler ile gerilme ve/veya şekil değiştirme büyüklükleri arasındaki ilişkiler elde edilmiştir. Yöntem olarak, öncelikle büyük deformasyonlar yapan katıların, Lagrangian esaslara göre deformasyon kinematikleri incelenmiş ve bünye denklemleri elde edilmiştir. Bu ifadeler, temel termodinamik kanunları ile ilişkilendirilerek, üç elemanlı viskoelastik bünye denklemi modeline uygulanmıştır. Plastisite teorisinde yaygın kullanım alanı bulan çarpımsal ayrıştırma prensibi kullanılarak, viskoelastik ve saf elastik davranışlar birbirinden ayrıştırılmıştır. Buradan elde edilen ifadelerle, damar yapısı için önemli büyüklükleri göz önüne alan bir kalın boru modeli için yüklenme ve deformasyon ilişkileri elde edilmiştir. Daha sonra, öngörülen modelin içinde bulunan malzeme parametrelerinin tahmini için kurulan deney düzeneği tanıtılmıştır. Bu esnada yapılan kabullerden bahsedilmiş ve kurulan ölçüm sisteminden elde edilen statik ve/veya dinamik yüklenmelere ait verilerin işlenebilmesi için gerekli detaylar sunulmuştur. Deneysel çalışmalarla, teorik sonuçların karşılaştırılması sonucunda elde edilen bulgular yorumlanmıştır. Anahtar Kelimeler: Biyomekanik, deneysel mekanik, damar dokusu, nonlineer viskoelastisite

The effects of cross-linking agents on the mechanical properties of poly (Methyl Methacrylate) resin

Abstract Cross-linking agents are incorporated into denture base materials to improve their mechanical properties. This study investigated the effects of various cross-linking agents, with different cross-linking chain lengths and flexibilities, on the flexural strength, impact strength, and surface hardness of polymethyl methacrylate (PMMA). The cross-linking agents used were ethylene glycol dimethacrylate (EGDMA), tetraethylene glycol dimethacrylate (TEGDMA), tetraethylene glycol diacrylate (TEGDA), and polyethylene glycol dimethacrylate (PEGDMA). These agents were added to the methyl methacrylate (MMA) monomer component in concentrations of 5%, 10%, 15%, and 20% by volume and 10% by molecular weight. A total of 630 specimens, comprising 21 groups, were fabricated. Flexural strength and elastic modulus were assessed using a 3-point bending test, impact strength was measured via the Charpy type test, and surface Vickers hardness was determined. Statistical analyses were performed using the Kolmogorov–Smirnov Test, Kruskal–Wallis Test, Mann–Whitney U Test, and ANOVA with post hoc Tamhane test (p ≤ 0.05). No significant increase in flexural strength, elastic modulus, or impact strength was observed in the cross-linking groups compared to conventional PMMA. However, surface hardness values notably decreased with the addition of 5% to 20% PEGDMA. The incorporation of cross-linking agents in concentrations ranging from 5% to 15% led to an improvement in the mechanical properties of PMMA

Damar Duvarındaki Ön Birim Şekil Değiştirmelerin Optik Yöntemler Kullanılarak incelenmesi

Konferans Bildirisi -- Teorik ve Uygulamalı Mekanik Türk Milli Komitesi, 2011Conference Paper -- Theoretical and Applied Mechanical Turkish National Committee, 2011Özellikle gelişmiş toplumlarda kalp ve damar hastalıkları insan hayatını tehdit eden en önemli unsurlardan biridir. Bu sebeple günümüzde araştırmacılar dolaşım sisteminin sağlıklı ve patolojik vaka durumları üzerine araştırmalarını arttırarak sürdürmektedirler. Bu çalışmanın amacı, damarın kendini yeniden modellemesi ve gelişim sürecinin bir göstergesi olduğu belirtilen ön birim şekil değiştirmelerin üzerinde durularak literatürde bulunan teorik ön birim şekil değiştirme kabulünün doğrudan gelişmiş optik ölçümler ile ortaya çıkan sonuçların karşılaştırılması ve değerlendirilmesidir. Çalışma kapsamında öncelikle damar duvarının anatomik yapısı hakkında temel bilgiler verilmiş, damar kinematiği tanıtılmış ve ileri yöndeki şekil değiştirmeler için matematik ifadeler verilmiştir. Bu ifadelere ek olarak tersine denklemler de elde edilmiştir. Daha sonra deneysel çalışmalar bölümünde ise damar duvarının ön birim şekil değiştirmelerinin ölçülebilmesi için bir deney düzeneği oluşturulmuş ve deneyler gerçekleştirilmiştir. Deneyler sonucunda doğrudan ölçüm sonuçlarıyla teorik sonuçlar arasında belirgin bir fark ortaya çıkmıştır. Bu fark ve çalışmanın çıktıları gelecekteki çalışmaları tetikleyecek ve katkı sağlayacak unsurlar olarak görülmektedir

A new mini-external fixator for treating hallux valgus: A preclinical, biomechanical study

WOS: 000367071200010PubMed ID: 26190777Proximal metatarsal osteotomy is the most effective technique for correcting hallux valgus deformities, especially in metatarsus primus varus. However, these surgeries are technically demanding and prone to complications, such as nonunion, implant failure, and unexpected extension of the osteotomy to the tarsometatarsal joint. In a preclinical study, we evaluated the biomechanical properties of the fixator and compared it with compression screws for treating hallux valgus with a proximal metatarsal osteotomy. Of 18 metatarsal composite bone models proximally osteotomized, 9 were fixed with a headless compression screw and 9 with the mini-external fixator. A dorsal angulation of 10 degrees and displacement of 10 mm were defined as the failure threshold values. Construct stiffness and the amount of interfragmentary angulation were calculated at various load cycles. All screw models failed before completing 1000 load cycles. In the fixator group, only 2 of 9 models (22.2%) failed before 1000 cycles, both between the 600th and 700th load cycles. The stability of fixation differed significantly between the groups (p < .001). The stability provided by the mini-external fixator was superior to that of compression screw fixation. Additional testing of the fixator is indicated.Bezmialem Vakif University, Scientific Research Projects Department [9.2012/8]The present study was funded by Bezmialem Vakif University, Scientific Research Projects Department (grant 9.2012/8)

Experimental Parameter Estimation Method for Nonlinear Viscoelastic Composite Material Models: An Application on Arterial Tissue

This study is aimed at setting a method of experimental parameter estimation for large-deforming nonlinear viscoelastic continuous fibre-reinforced composite material model. Specifically, arterial tissue was investigated during experimental research and parameter estimation studies, due to medical, scientific and socio-economic importance of soft tissue research. Using analytical formulations for specimens under combined inflation/extension/torsion on thick-walled cylindrical tubes, in vitro experiments were carried out with fresh sheep arterial segments, and parameter estimation procedures were carried out on experimental data. Model restrictions were pointed out using outcomes from parameter estimation. Needs for further studies that can be developed are discussed

The effect of occlusal contact localization on the stress distribution in complete maxillary denture

The fracture of acrylic resin dentures is an unresolved problem in removable prosthodontics despite many efforts to determine its cause. Unfavourable occlusion could be playing an important role in the fracture of the denture. The aim of this study was to investigate the effect of occlusal contact localization on the stress distribution in complete maxillary denture bases utilizing two-dimensional finite element analysis. The results of this study have shown that maximum compressive stresses in a complete maxillary denture under functional masticatory forces concentrates always on the artificial tooth/denture base junction irrespective to the occlusal contact localization. Tensile stresses were observed in areas toward the midline, although the midline itself usually had lower stresses. Shifting the occlusal contacts to a more buccal localization resulted in an increase of the calculated stresses. As a conclusion, it can be speculated that the buccal placement of the occlusal contacts may play a role in the fatigue fracture of the complete maxillary denture

The impact of frenulum height on strains in maxillary denture bases

PURPOSE. The midline fracture of maxillary complete dentures is a frequently encountered complication. The purpose of this study was to assess the effect of frenulum height on midline strains of maxillary complete dentures. MATERIALS AND METHODS. A removable maxillary complete denture was fabricated and duplicated seven times. Four different labial frenulum heights were tested for stresses occurring on the palatal cameo surface. The strains were measured with strain gauges placed on 5 different locations and the stresses were calculated. To mimic occlusal forces bilaterally 100 N of load was applied from the premolar and molar region. RESULTS. A statistically significant association between the height of the labial frenulum and the calculated stresses and strains was shown (P<.05) predominantly on the midline and especially on the incisive papilla. The results showed that stress on the anterior midline of the maxillary complete denture increases with a higher labial frenulum. CONCLUSION. Within the limitations of this in vitro study, it can be concluded that the stress on the anterior midline of the maxillary complete denture increases with a higher labial frenulum. Surgical or mechanical precautions should be taken to prevent short-term failure of maxillary complete dentures due to stress concentration and low cycle fatigue tendency at the labial frenulum region