1,721,035 research outputs found
On the biomechanical significance of inter-lamellar interfaces in the intervertebral disc
No full textThe intervertebral disc (IVD) is a unique soft tissue structure which provides structural support and flexibility in the axial skeleton of vertebrates. From a structural perspective, the disc behaves somewhat like a thick walled pressure vessel, where the walls are comprised of a series of composite annular rings (lamellae). However, a prior study (Marchand and Ahmed, 1990) found a high proportion of circumferentially discontinuous lamellae in human lumbar IVDs. The presence of these discontinuities raises important structural questions, because discontinuous lamellae cannot withstand high nucleus pressures via the generation of circumferential (hoop) stress. A possible alternative mechanism may be that inter-lamellar cohesion allows shear stress transfer between adjacent annular layers. The aim of the present study was therefore to investigate the importance of inter-lamellar shear resistance in the intervertebral disc. \ud
This work found that inter-lamellar shear resistance has a strong influence on the compressive stiffness of the intervertebral disc, with a change in interface condition from tied (no slip) to frictionless (no shear resistance) reducing disc compressive stiffness by 40%. However, it appears that substantial inter-lamellar shear resistance is present in the bovine tail disc. Decreases in inter-lamellar shear resistance due to degradation of bridging collagenous or elastic fibre structures could therefore be an important part of the process of disc degeneration
The Effect of Cytokines on Tendons and Ligaments: An In Vitro Study
No full textTendon and ligament explants are utilized in ACL reconstruction. After ACL reconstructive surgery, or ACL injury, there is an up-regulation of inflammation, which has been implicated in causing cellular death. The objective of this study is to examine the effect of cytokines on cells and tissue of ligaments and tendons. The mRNA expression of inflammatory and apoptotic markers were measured in primary cells after treatment with cytokines and cell death was measured with fluorescent dyes. Next ACL and PT explants were analyzed for mRNA expression. Furthermore, Annexin V and DNA laddering techniques were used to evaluate for apoptosis. Apoptosis does not seem to be a direct effect of cytokine treatment. In almost all treatment groups, SNP was the only reagent that caused significant amounts of apoptosis. Results showed evidence of a connection between IL1B, iNOS, and SNP. Additionally, tissues with more blade cuts had significantly higher amounts of cell death
Reliability Assessment of Historical Masonry Structures
No full textDespite the high level of vulnerability of unreinforced masonry structures under applied loads and the importance of their reliability evaluation, there is no formal methodology to assess the reliability of historic masonry structures. To develop an appropriate methodology, estimations of probabilistic models of structural resistance and load effects are required to formulate a limit state function. The stochastic characteristics of materials play key roles in the determination of probabilistic models of structural resistance. Therefore, in current study, methodologies for estimating the statistical characteristics of historic masonry materials through non-destructive tests are described. Best fit probabilistic models for load effects are also presented. Target reliability index and different approaches for calculating suitable targets for historic structures are described as well. Evaluation of the reliability level of historical structures through the recommended procedure would lead to more realistic and accurate levels of reliability estimation without requiring degradation of the historic structure
An Instrumented Spatial Linkage to Measure In-Vivo Kinematics and Motion Reproduction with a Robotic Test System
No full textDetermination of the mechanical environment of the knee joint is essential in order to understand the normal and pathological states of the tissues in that joint, mechanisms of injuries, and in developing and evaluating surgical interventions. A 6 degree of freedom Instrumented Spatial Linkage (ISL) was developed to record ovine stifle joint kinematics during normal treadmill gait, and reproduce those same motions in-vitro on a robotic testing platform in order to provide an in-vitro environment in which to examine the effects of in-vivo displacement on the soft tissues of the joint. The ISL was designed to provide increased accuracy, reduced processing time, and ease of use when compared to traditional motion capture methods. The ISL was evaluated dynamically to ensure that accurate results were maintained.
The ISL and robotic test platform allow determination of the loads on the four major ligaments and the two menisci in the joint. Structural loads for 4 animals were determined for 20 representative strides beginning at hoof strike. Results showed similar loading patterns between animals but with some quantitative differences. With respect to the gait cycle beginning at hoof strike, the PCL reached a peak load just after hoof strike (5%) followed by rapid unloading. The ACL was loaded immediately thereafter with a peak at about 10% of gait. The MCL tended to follow the PCL but with a much reduced force magnitude. The LCL was largely unloaded throughout, suggesting that this ligament must play a more prominent role in other motions. For three of the four animals the average meniscal load was nearly even between the lateral and medial menisci, the fourth animal carried an average of 46.9 N more load in the lateral meniscus. Linear correlations between joint structures and kinematics were attempted but no strong correlations existed. Additionally, ligament length was also shown to be a poor predictor of ACL load. A method is proposed whereby an Artificial Neural Network can be constructed as to accurately predict ACL load given relative joint position and orientation.
This study shows for the first time the loads experienced by all 4 major ligaments of the knee, and the minimum loads in the menisci in response to in-vivo motions. The load in each structure has been shown throughout gait and future studies can look at a variety of different motions and loading situations
Fracture in generalized stress states
No full textBibliography: p. 259-272.There are three basic parameters that control the fracture behaviour of a structure: loading of the structure, its resistance to crack initiation and growth, and its resistance to collapse. The last two items are sensitive to the shape and size of defects in the material. Accordingly, this also will affect the load-bearing capacity and service performance of structural elements. Therefore, in order to have plausible criteria for failure assessment one needs information about the applied load, the relevant material properties and defect shapes and sizes. In this dissertation two complementary avenues of approach are employed to give some insight about the effect of loading and defects on the phenomenon of fracture in brittle materials. In the first the stress concentrations resulting from the presence of voids in the medium are studied. In the second, the energy accompanying the extension of these flaws is examined. At the end, these two types of investigation have been correlated to provide a simple theory for fracture of brittle materials under different states of stress, including compression. A particularly versatile analytical technique that accounts for geometric and triaxial effects has been developed to determine the stress and energy variations introduced by the presence of a triaxial ellipsoidal cavity subjected to stresses parallel to its axes. The triaxial void is allowed to increas in size to incorporate the energy-balance concept into the fracture mechanics formulation. The details of the solution are presented. A verification procedure is established to assess the validity of the solution. The results obtained from this procedure substantiate the generality, flexibility, and physical soundness of the present approach. In addition, drawbacks and limitations of other analyses in the literature for special cases are discussed using the developed solution. Several examples are also offered to show how the present analysis can be applied to examine the growth of a predominant defect and to characterize material behaviour. A computer program has been developed to perform all the necessary computations. The influence of various parameters such as the state of applied stress, the cavity shapes and sizes and the 3-D analysis versus 2-D analysis were investigated. The apparent paradox of material fracture behaviour depending on the applied states of stress is interpreted using the proposed theory. An examination of crack extension behaviour under different states of applied stresses is presented. Interaction diagrams in biaxial and triaxial states of stress predicted using the present theory compare favorably with strength diagrams developed by other investigators from experimental data. It is suggested that the method of analysis presented here and the developed computer program will have a wide range of application in the area of fracture. It is hoped that by studying the behaviour of triaxial voids of different shapes and sizes under different states of stress, a rational picture of failure criteria can be built up
Ligament structure and function: a nonlinear finite element approach
No full textBibliography: p. 156-165
Rehabilitation of HC-type bridge girders using CFRP-sheets/strips or external post-tensioning
No full textBibliography: p. 159-162
Loss of prestress and stress redistribution with time in post-tensioned hollow masonry walls
No full textBibliography: p. 145-150
The effect of drill hole angle on the initial stress states and subsequent healing of a MCL reconstruction
No full textBibliography: p. 164-17
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