Bei Zhou

Simultaneous anabolic and catabolic responses of human chondrocytes seeded in collagen hydrogels to long-term continuous dynamic compression.

Cartilage repair strategies increasingly focus on the in vitro development of cartilaginous tissues that mimic the biological and mechanical properties of native articular cartilage. However, current approaches still face problems in the reproducible and standardized generation of cartilaginous tissues that are both biomechanically adequate for joint integration and biochemically rich in extracellular matrix constituents. In this regard, the present study investigated whether long-term continuous compressive loading would enhance the mechanical and biological properties of such tissues. Human chondrocytes were harvested from 8 knee joints (n=8) of patients having undergone total knee replacement and seeded into a collagen type I hydrogel at low density of 2×10(5)cells/ml gel. Cell-seeded hydrogels were cut to disks and subjected to mechanical stimulation for 28 days with 10% continuous cyclic compressive loading at a frequency of 0.3 Hz. Histological and histomorphometric evaluation revealed long-term mechanical stimulation to significantly increase collagen type II and proteoglycan staining homogenously throughout the samples as compared to unstimulated controls. Gene expression analyses revealed a significant increase in collagen type II, collagen type I and MMP-13 gene expression under stimulation conditions, while aggrecan gene expression was decreased and no significant changes were observed in the collagen type II/collagen type I mRNA ratio. Mechanical propertywise, the average value of elastic stiffness increased in the stimulated samples. In conclusion, long-term mechanical preconditioning of human chondrocytes seeded in collagen type I hydrogels considerably improves biological and biomechanical properties of the constructs, corroborating the clinical potential of mechanical stimulation in matrix-associated autologous chondrocyte transplantation (MACT) procedures.

Morphometric Grading of Osteoarthritis by Optical Coherence Tomography — An Ex Vivo Study

Optical Coherence Tomography (OCT) yields microscopic cross‐sectional images of cartilage in real time and at high resolution. As yet, comprehensive grading of degenerative cartilage changes based on OCT has rarely been performed. This study investigated the potential of quantitative OCT using algorithm‐based image parameters such as irregularity (OII – Optical Irregularity Index), homogeneity (OHI – Optical Homogeneity Index) and attenuation (OAI – Optical Attenuation Index) in the objective grading of cartilage degeneration. Therefore, OCT was used to image and assess 113 human osteochondral samples obtained from total knee replacements. Processing included the analysis of OII (by calculation of the standard deviation with regards to a fitted surface), of OHI (by edge detection of tissue signal changes) and of OAI (by analysis of relative imaging depth). Additionally, samples were subject to macroscopic (Outerbridge grading), biomechanical (elastic stiffness), qualitative OCT and histological evaluation (Modified Mankin grading). Significant correlations were found between all outcome measures. OII and OHI were effective in assessing cartilage surface, integrity and homogeneity, while OAI could discriminate between unmineralized and mineralized cartilage, respectively. Therefore, quantitative OCT holds potential as a diagnostic tool for more reliable, standardized and objective assessment of cartilage tissue properties.

Bioreactor cultivation and remodelling simulation for cartilage replacement material

For the development of articular cartilage replacement material, it is essential to study the dependence between mechanical stimulation and cell activity in cellular specimens. Bioreactor cultivation is widely used for this purpose, however, it is hardly possible to obtain a quantitative relationship between collagen type II production and applied loading history. For this reason, a bioreactor system is developed, measuring applied forces and number of loading cycles by means of a load cell and a forked light barrier, respectively. Parallel to the experimental study, a numerical model by means of the finite element method is proposed to simulate the evolution of material properties during cyclic stimulation. In this way, a numerical model can be developed for arbitrary deformation cases.

Continuous cyclic compressive loading modulates biological and mechanical properties of collagen hydrogels seeded with human chondrocytes.

PURPOSE This study investigated the potential of cyclic compressive loading in the generation of in vitro engineered cartilaginous tissue with the aim of contributing to a better understanding of mechanical preconditioning and its possible role in further optimizing existing matrix-associated cartilage replacement procedures. METHODS Human chondrocytes were harvested from 12 osteoarthritic knee joints and seeded into a type I collagen (col-I) hydrogel at low density (2 × 10(5) cells/ml gel). The cell-seeded hydrogel was condensed and cultivated under continuous cyclic compressive loading (frequency: 0.3 Hz; strain: 10%) for 14 days under standardized conditions. After retrieval, specimens were subject to staining, histomorphometric evaluation, gene expression analysis and biomechanical testing. RESULTS Cellular morphology was altered by both stimulation and control conditions as was staining for collagen II (col-II). Gene expression measurements revealed a significant increase for col-II under either cultivation condition. No significant differences in col-I, aggrecan and MMP-13 gene expression profiles were found. The col-II/col-I mRNA ratio significantly increased under stimulation, whereas the biomechanical properties deteriorated under either cultivation method. CONCLUSIONS Although the effects observed are small, mechanical preconditioning has demonstrated its potential to modulate biological properties of collagen hydrogels seeded with human chondrocytes.

An ovine in vitro model for chondrocyte-based scaffold-assisted cartilage grafts

BackgroundScaffold-assisted autologous chondrocyte implantation is an effective clinical procedure for cartilage repair. From the regulatory point of view, the ovine model is one of the suggested large animal models for pre-clinical studies. The aim of our study was to evaluate the in vitro re-differentiation capacity of expanded ovine chondrocytes in biomechanically characterized polyglycolic acid (PGA)/fibrin biomaterials for scaffold-assisted cartilage repair.MethodsOvine chondrocytes harvested from adult articular cartilage were expanded in monolayer and re-assembled three-dimensionally in PGA-fibrin scaffolds. De- and re-differentiation of ovine chondrocytes in PGA-fibrin scaffolds was assessed by histological and immuno-histochemical staining as well as by real-time gene expression analysis of typical cartilage marker molecules and the matrix-remodelling enzymes matrix metalloproteinases (MMP) -1, -2 and −13 as well as their inhibitors. PGA scaffolds characteristics including degradation and stiffness were analysed by electron microscopy and biomechanical testing.ResultsHistological, immuno-histochemical and gene expression analysis showed that dedifferentiated chondrocytes re-differentiate in PGA-fibrin scaffolds and form a cartilaginous matrix. Re-differentiation was accompanied by the induction of type II collagen and aggrecan, while MMP expression decreased in prolonged tissue culture. Electron microscopy and biomechanical tests revealed that the non-woven PGA scaffold shows a textile structure with high tensile strength of 3.6 N/mm2 and a stiffness of up to 0.44 N/mm2, when combined with gel-like fibrin.ConclusionThese data suggest that PGA-fibrin is suited as a mechanically stable support structure for scaffold-assisted chondrocyte grafts, initiating chondrogenic re-differentiation of expanded chondrocytes.

Towards bioreactor development with physiological motion control and its applications

In biomedical applications bioreactors are used, which are able to apply mechanical loadings under cultivation conditions on biological tissues. However, complex mechanobiological evolutions, such as the dependency between mechanical properties and cell activity, depend strongly on the applied loading conditions. This requires correct physiological movements and loadings in bioreactors. The aim of the present study is to develop bioreactors, in which native and artificial biological tissues can be cultivated under physiological conditions in knee joints and spinal motion segments. However, in such complex systems, where motions with different degrees of freedom are applied to whole body parts, it is necessary to investigate elements of joints and spinal parts separately. Consequently, two further bioreactors for investigating tendons and cartilage specimens are proposed additionally. The study is complemented by experimental and numerical examples with emphasis on medical and engineering applications, such as biomechanical properties of cartilage replacement materials, injured tendons, and intervertebral discs.

Entwicklung von Finite-Elemente-Modellen für Knorpelersatzmaterial@@@Development of finite element models for cartilage replacement material

In the development of cartilage replacement materials, mechanical models are necessary to quantify elasticity and damping properties of the artificial tissue. The aim is to identify parameters from material tests leading to an objective assessment of elasticity and damping for tissue replacement. This is especially important as the evolution of material properties is investigated during a cultivation period of several weeks. For this reason, in the present study a method is proposed to identify all necessary material parameters by means of a finite element model. The numerical simulations are based on a phenomenological material model exhibiting as few parameters as possible for covering elastic material and damping properties. This allows a practical identification of material parameters. Thus, deformation dependent damping properties of the replacement material are covered by parameters identified from material tests without extensive determination of pore, solid or fluid fractions.ZusammenfassungBei der Entwicklung von Knorpelersatzmaterial sind mechanische Modelle erforderlich, um die Elastizität und das Dämpfungsverhalten des künstlichen Materials quantifizieren zu können. Ziel ist es, aus Materialprüfversuchen Parameter zu identifizieren, mit denen mechanische Eigenschaften wie Elastizität und Dämpfung objektiv beurteilt werden können. Dies ist besonders dann entscheidend, wenn die Entwicklung von Materialeigenschaften über einen Kultivierungszeitraum von mehreren Wochen untersucht werden soll. In der vorliegenden Studie wird eine Methode vorgeschlagen, mit Hilfe eines Finite-Elemente-Modells alle notwendigen Materialparameter zu identifizieren. Als Grundlage für die numerische Simulation dient ein phänomenologisches Materialmodell, in dem alle elastischen Eigenschaften sowie das Dämpfungsverhalten auf möglichst wenige Parameter zurückgeführt werden, sodass eine praktische Materialparameteridentifikation möglich wird. Hierbei werden deformationsabhängige Dämpfungseigenschaften des Ersatzmaterials durch Parameter beschrieben, die aus Materialprüfversuchen identifiziert werden, ohne dass Porenanteile oder Festkörper- bzw. Fluidanteile aufwendig bestimmt werden müssten.AbstractIn the development of cartilage replacement materials, mechanical models are necessary to quantify elasticity and damping properties of the artificial tissue. The aim is to identify parameters from material tests leading to an objective assessment of elasticity and damping for tissue replacement. This is especially important as the evolution of material properties is investigated during a cultivation period of several weeks. For this reason, in the present study a method is proposed to identify all necessary material parameters by means of a finite element model. The numerical simulations are based on a phenomenological material model exhibiting as few parameters as possible for covering elastic material and damping properties. This allows a practical identification of material parameters. Thus, deformation dependent damping properties of the replacement material are covered by parameters identified from material tests without extensive determination of pore, solid or fluid fractions.

Development of finite element models for cartilage replacement material

In the development of cartilage replacement materials, mechanical models are necessary to quantify elasticity and damping properties of the artificial tissue. The aim is to identify parameters from material tests leading to an objective assessment of elasticity and damping for tissue replacement. This is especially important as the evolution of material properties is investigated during a cultivation period of several weeks. For this reason, in the present study a method is proposed to identify all necessary material parameters by means of a finite element model. The numerical simulations are based on a phenomenological material model exhibiting as few parameters as possible for covering elastic material and damping properties. This allows a practical identification of material parameters. Thus, deformation dependent damping properties of the replacement material are covered by parameters identified from material tests without extensive determination of pore, solid or fluid fractions.ZusammenfassungBei der Entwicklung von Knorpelersatzmaterial sind mechanische Modelle erforderlich, um die Elastizität und das Dämpfungsverhalten des künstlichen Materials quantifizieren zu können. Ziel ist es, aus Materialprüfversuchen Parameter zu identifizieren, mit denen mechanische Eigenschaften wie Elastizität und Dämpfung objektiv beurteilt werden können. Dies ist besonders dann entscheidend, wenn die Entwicklung von Materialeigenschaften über einen Kultivierungszeitraum von mehreren Wochen untersucht werden soll. In der vorliegenden Studie wird eine Methode vorgeschlagen, mit Hilfe eines Finite-Elemente-Modells alle notwendigen Materialparameter zu identifizieren. Als Grundlage für die numerische Simulation dient ein phänomenologisches Materialmodell, in dem alle elastischen Eigenschaften sowie das Dämpfungsverhalten auf möglichst wenige Parameter zurückgeführt werden, sodass eine praktische Materialparameteridentifikation möglich wird. Hierbei werden deformationsabhängige Dämpfungseigenschaften des Ersatzmaterials durch Parameter beschrieben, die aus Materialprüfversuchen identifiziert werden, ohne dass Porenanteile oder Festkörper- bzw. Fluidanteile aufwendig bestimmt werden müssten.AbstractIn the development of cartilage replacement materials, mechanical models are necessary to quantify elasticity and damping properties of the artificial tissue. The aim is to identify parameters from material tests leading to an objective assessment of elasticity and damping for tissue replacement. This is especially important as the evolution of material properties is investigated during a cultivation period of several weeks. For this reason, in the present study a method is proposed to identify all necessary material parameters by means of a finite element model. The numerical simulations are based on a phenomenological material model exhibiting as few parameters as possible for covering elastic material and damping properties. This allows a practical identification of material parameters. Thus, deformation dependent damping properties of the replacement material are covered by parameters identified from material tests without extensive determination of pore, solid or fluid fractions.

Entwicklung von Finite-Elemente-Modellen für Knorpelersatzmaterial

In the development of cartilage replacement materials, mechanical models are necessary to quantify elasticity and damping properties of the artificial tissue. The aim is to identify parameters from material tests leading to an objective assessment of elasticity and damping for tissue replacement. This is especially important as the evolution of material properties is investigated during a cultivation period of several weeks. For this reason, in the present study a method is proposed to identify all necessary material parameters by means of a finite element model. The numerical simulations are based on a phenomenological material model exhibiting as few parameters as possible for covering elastic material and damping properties. This allows a practical identification of material parameters. Thus, deformation dependent damping properties of the replacement material are covered by parameters identified from material tests without extensive determination of pore, solid or fluid fractions.ZusammenfassungBei der Entwicklung von Knorpelersatzmaterial sind mechanische Modelle erforderlich, um die Elastizität und das Dämpfungsverhalten des künstlichen Materials quantifizieren zu können. Ziel ist es, aus Materialprüfversuchen Parameter zu identifizieren, mit denen mechanische Eigenschaften wie Elastizität und Dämpfung objektiv beurteilt werden können. Dies ist besonders dann entscheidend, wenn die Entwicklung von Materialeigenschaften über einen Kultivierungszeitraum von mehreren Wochen untersucht werden soll. In der vorliegenden Studie wird eine Methode vorgeschlagen, mit Hilfe eines Finite-Elemente-Modells alle notwendigen Materialparameter zu identifizieren. Als Grundlage für die numerische Simulation dient ein phänomenologisches Materialmodell, in dem alle elastischen Eigenschaften sowie das Dämpfungsverhalten auf möglichst wenige Parameter zurückgeführt werden, sodass eine praktische Materialparameteridentifikation möglich wird. Hierbei werden deformationsabhängige Dämpfungseigenschaften des Ersatzmaterials durch Parameter beschrieben, die aus Materialprüfversuchen identifiziert werden, ohne dass Porenanteile oder Festkörper- bzw. Fluidanteile aufwendig bestimmt werden müssten.AbstractIn the development of cartilage replacement materials, mechanical models are necessary to quantify elasticity and damping properties of the artificial tissue. The aim is to identify parameters from material tests leading to an objective assessment of elasticity and damping for tissue replacement. This is especially important as the evolution of material properties is investigated during a cultivation period of several weeks. For this reason, in the present study a method is proposed to identify all necessary material parameters by means of a finite element model. The numerical simulations are based on a phenomenological material model exhibiting as few parameters as possible for covering elastic material and damping properties. This allows a practical identification of material parameters. Thus, deformation dependent damping properties of the replacement material are covered by parameters identified from material tests without extensive determination of pore, solid or fluid fractions.