Saeed Miramini

Time Evolution of Deformation in a Human Cartilage Under Cyclic Loading

Recent imaging has revealed that in vivo contact deformations of human knee cartilage under physiological loadings are surprisingly large—typically on the order of 10%, but up to 20 or 30% of tibiofemora cartilage thickness depending on loading conditions. In this paper we develop a biphasic, large deformation, non-linear poroelastic model of cartilage that can accurately represent the time dependence and magnitude of cyclic cartilage deformations in vivo. The model takes into account cartilage tension–compression nonlinearity and a new constitutive relation in which the compressive stiffness and hydraulic permeability of the cartilage adjusts in response to the strain-dependent aggrecan concentration. The model predictions are validated using experimental test results on osteochondral plugs obtained from human cadavers. We find that model parameters can be optimised to give an excellent fit to the experimental data. Using typical hydraulic conductivity and stiffness parameters for healthy cartilage, we find that the experimentally observed transient and steady state tissue deformations under cyclic loading and unloading can be reproduced by the model. Steady state tissue deformations are shown to cycle between 10% (exudation strain) and 20% (total strain) in response to the cyclic test loads. At steady-state cyclic loading, the pore fluid exuded from the tissue is exactly equal to the pore fluid imbibed by the tissue during each load cycle.

Computational Simulation of Mechanical Microenvironment of Early Stage of Bone Healing under Locking Compression Plate with Dynamic Locking Screws

It is well known that bone healing outcomes highly depend on the mechanical microenvironment of the fracture site, and a certain degree of interfragmentary movement (IFM) is essential for indirect (i.e. natural) bone healing. The application of locking compression plate (LCP) internal fixation in the treatment of bone fracture is a common practice which leads to early mobility and full function of the fractured extremity. However should the fixation configuration be too stiff, it might result in delayed healing or asymmetric tissue development across the fracture site due to the fact that IFM in near cortex area is too small to promote healing. Dynamic locking screw (DLS) has been recently designed to tackle this problem by reducing the stiffness of LCP fixation. However, the actual mechano-regulation mechanisms in which DLS uses to regulate the healing process are still not fully understood. The objective of this paper is to develop a computational model to understand the change of mechanical microenvironment of fracture site under LCP with dynamic locking screw in comparison to standard locking screw, and how this change could potentially regulate tissue development within the fracture callus during the healing process.Keywords: bone healing, locking compression plate, dynamic locking screw, finite element modelling

The Effects of Flexible Fixation on Early Stage Bone Fracture Healing

The mechanical microenvironment at a fracture site could potentially influence the outcomes of bone fracture healing. It is known that, should the fixation construct be too stiff, or the gap between the fracture ends be too large, bones are less likely to heal. Flexible fixation or so-called “biological fixation” has been shown to encourage the formation of fracture callus, and therefore result in better healing outcomes. However, till date the nature of the relationship between the degree of mechanical stability provided by a flexible fixation and optimal healing fracture healing outcomes has not been fully understood. This paper presents a computational model that can predict healing out-comes from early stage healing data under various fixation configurations. The results of the simulations demonstrate that the change of mechanical microenvironment of fracture site resulting from the different fixation configurations is of importance for the healing outcomes.

Influence of fracture geometry on bone healing under locking plate fixations: A comparison between oblique and transverse tibial fractures.

Mechano-regulation plays a crucial role in bone healing and involves complex cellular events. In this study, we investigate the change of mechanical microenvironment of stem cells within early fracture callus as a result of the change of fracture obliquity, gap size and fixation configuration using mechanical testing in conjunction with computational modelling. The research outcomes show that angle of obliquity (θ) has significant effects on interfragmentary movement (IFM) which influences mechanical microenvironment of the callus cells. Axial IFM at near cortex of fracture decreases with θ, while shear IFM significantly increases with θ. While a large θ can increase shear IFM by four-fold compared to transverse fracture, it also result in the tension-stress effect at near cortex of fracture callus. In addition, mechanical stimuli for cell differentiation within the callus are found to be strongly negatively correlated to angle of obliquity and gap size. It is also shown that a relatively flexible fixation could enhance callus formation in presence of a large gap but could lead to excessive callus strain and interstitial fluid flow when a small transverse fracture gap is present. In conclusion, there appears to be an optimal fixation configuration for a given angle of obliquity and gap size.

The spatio-temporal mechanical environment of healthy and injured human cartilage during sustained activity and its role in cartilage damage

Recently we presented a computational model of articular cartilage calibrated for normal human tissue explants. This model was able to capture the transient deformation of cartilage experiencing a cyclic load. The model takes into account the tension-compression nonlinearity of the cartilage and incorporates the dependency of the compressive stiffness and fluid permeability of cartilage on the deformation-dependent aggrecan concentration in cartilage tissue. As such it represents a leading constitutive model of articular cartilage. Here we build on the previous study to develop an experimentally validated computational model to simulate mechanical consolidation response of intact and previously injured cartilage under sustained static loading, to develop our understanding of the implications for rates of tissue damage. We see that the type of prior injuries compromise the cartilage function in different ways. Relatively rapid consolidation is predicted for cartilage with a complete meniscectomy and that with a full thickness defect, indicating the inability of cartilage with such injuries to sustain interstitial fluid pressurisation for long periods of time, as does uninjured cartilage. By comparing the consolidation response of articular cartilage predicted by computational model against experimental measurements of the apparent friction coefficient following static loading, we find a strong linear positive correlation exists between cartilage degree of consolidation (DoC) and friction coefficient at the joint. As the DoC of articular cartilages can be estimated in vivo via medical imaging, the DoC can be used as an index to non-invasively evaluate the apparent friction coefficient between opposing cartilage surfaces, and so estimate the likelihood of frictional surface wear and/or cartilage damage.

Characterising fundamental properties of foam concrete with a non-destructive technique

ABSTRACT Lightweight foam concrete has been developing rapidly in construction industry due to its sustainability, excellent thermal insulation, excellent fire resistance and affordability. It is desired to assess the performance of the foam concrete structure on site instead of testing isolated specimen in laboratory condition, as the changes in site environment and different construction procedure will affect the testing result. For on-site testing of foam concrete, non-destructive methods are required to avoid damage to structures. Ultrasonic pulse velocity method (UPV) is one of the attractive methods used for onsite testing of concretes due to its benefits such as accurate results, high sampling rate and cost-effectiveness. UPV method can determine various concrete properties such as strength, dynamic elastic constants, defects, uniformity and effects of curing time. However, for foam concrete application, the effect of high porosity on the UPV test results is largely unknown in current literature. Existing research explored the relationship between porosity and strength for metals, as well as the relationship between UPV and porosity for normal mortars. Yet no correlation between UPV and porosity for lightweight foam concrete is developed, which is very critical in the strength estimation and site quality control of the construction. In this research, a comprehensive laboratory experiment was designed to establish the correlations between UPV, porosity, and compressive strength using UPV experiments and compressive strength tests. New equations are developed which can be used as reference for estimating the mechanical properties of lightweight foam concrete using ultrasound testing technique in civil engineering practice.

The Role of Locking Plate Stiffness in Bone Fracture Healing Stabilized by Far Cortical Locking Technique

The locking plate fixations have been developed to enhance bone healing by wide bridging of the fracture and allowing some level of interfragmentary movement (IFM) at the fracture site. However, the IFM induced by conventional locking plate constructs is not uniform at the fracture site and so result in asymmetric callus formation, and ultimately delayed healing. The far cortical locking technique has been recently innovated to address this issue by inducing a uniform IFM. However, the far cortical locking technique is still in its infancy and more research efforts are required before its practical clinical application. Using the theory of porous media and computational methods, this study investigated the effectiveness of far cortical locking technique in presence of different mechanical stiffness of locking plate. The research outcomes indicate that the application of far cortical looking technique enhances IFM at near cortex, and so reduce the difference of IFM between near and far cortex. Further, it ...