D.R. Hose

Development of An Accurate Three-dimensional Finite Element Knee Model

This paper presents the development of a detailed articulating three-dimensional finite-element model of the human knee, derived from MRI scan images. The model utilises precise material models and many contact interfaces in order to produce a realistic kinematic response. The behaviour of the model was examined within two fields of biomechanical simulations: general life and car-crash. These simulations were performed with the non-linear explicit dynamic code PAM-SAFE™. The knee model produced results that compared favourably with existing literature. Such a model (together with other joint models that could be constructed using the same techniques) would be a valuable tool for examining new designs of prosthesis and mechanisms of injury.

Computer modelling study of the mechanism of optic nerve injury in blunt trauma

Aim: The potential causes of the optic nerve injury as a result of blunt object trauma, were investigated using a computer model. Methods: A finite element model of the eye, the optic nerve, and the orbit with its content was constructed to simulate blunt object trauma. We used a model of the first phalanx of the index finger to represent the blunt body. The trauma was simulated by impacting the blunt body at the surface between the globe and the orbital wall at velocities between 2–5 m/s, and allowing it to penetrate 4–10 mm below the orbital rim. Results: The impact caused rotations of the globe of up to 5000°/s, lateral velocities of up to 1 m/s, and intraocular pressures (IOP) of over 300 mmu200aHg. The main stress concentration was observed at the insertion of the nerve into the sclera, at the side opposite to the impact. Conclusions: The results suggest that the most likely mechanisms of injury are rapid rotation and lateral translation of the globe, as well as a dramatic rise in the IOP. The strains calculated in the study should be sufficiently high to cause axonal damage and even the avulsion of the nerve. Finite element computer modelling has therefore provided important insights into a clinical scenario that cannot be replicated in human or animal experiments.

A Thermal Analogy for Modelling Drug Elution from Cardiovascular Stents

Restriction of blood flow by the narrowing or occlusion of arteries is one of the most common presentations of cardiovascular disease. One treatment involves the introduction of a metal scaffold, or stent, designed to prevent recoil and to provide structural stability to the vessel. On the occasions that this treatment is ineffective, failure is usually associated with re-invasion of tissue. This can be prevented by local delivery of drugs which inhibit tissue growth. The drug might be delivered locally in a polymer coating on the stent. This paper develops and explores the use of a thermal analogue of the drug delivery process and the associated three-dimensional convection–diffusion equation to model the spatial and temporal distribution of drug concentration within the vessel wall. This allows the routine use of commercial finite element analysis software to investigate the dynamics of drug distribution, assist in the understanding of the treatment process and develop improved delivery systems. Two applications illustrate how the model might be used to investigate the effects of controllable or measurable parameters on the progression of the process. It is demonstrated that the geometric characteristics of the stent can have significant impact on the homogeneity of the dosing in the vessel wall.

Modelling of epithelial tissue impedance measured using three different designs of probe

Impedance measurement is a promising technique for detecting pre-malignant changes in epithelial tissue. This paper considers how the design of the impedance probe affects the ability to discriminate between tissue types. To do this, finite element models of the electrical properties of squamous and glandular columnar epithelia have been used. The glandular tissue model is described here for the first time. Glandular mucosa is found in many regions of the gastrointestinal tract, such as the stomach and intestine, and has a large effective surface area. Firstly, the electrical properties of a small section of gland, with epithelial cells and supportive tissue, are determined. These properties are then used to build up a three-dimensional model of a whole section of mucosa containing many thousands of glands. Measurements using different types of impedance probe were simulated by applying different boundary conditions to the models. Transepithelial impedance, and tetrapolar measurement with a probe placed on the tissue surface have been modelled. In the latter case, the impedance can be affected by conductive fluid, such as mucus, on the tissue surface. This effect has been investigated, and a new design of probe, which uses a guard electrode to counteract this potential source of variability, is proposed.

Modelled current distribution in cervical squamous tissue.

The electrical properties of cervical squamous epithelium have been modelled in the frequency range 100 Hz to 10 MHz. The hierarchical modelling process comprises a cellular level stage, which includes detailed models of cells typical of different depths within the epithelium and a tissue model, which utilizes electrical properties obtained from the cellular models. The fit between the modelled and measured impedance spectra and the distribution of current with depth depends on the macroscopic model structure. Both the properties of the basement membrane and the presence of a surface mucus layer are shown to have a significant effect. The best fit with measured data is obtained when a 10 microm thick, high-conductivity surface layer is included in the tissue model.

COMPUTER MODEL ANALYSIS OF THE SWANSON AND SUTTER METACARPOPHALANGEAL JOINT IMPLANTS

A representative model which mimics the behaviour of Silastic® finger metacarpophalangeal joint implants was constructed using a finite element software package. The modelled implants were moved through a range of flexion, lateral deviation and a combination of both. Pistoning of both implants stems occurred within the modelled medullary cavities. For equivalent flexion angles, the Sutter implant produced a higher stress field than the Swanson implant, and the field was positioned at the central hinge mechanism. In both implants, lateral deviation increased the internal stress concentrations more than when pure flexion was applied. Overall the Swanson style of implant had lower stress magnitudes than the Sutter implant, and it is predicted that the Sutter implant will be more likely to fail than the Swanson. The failure mode for the Sutter implant would be at the central hinge region. The Swanson implant is likely to fail at the central hinge-stem interface regions.

Systemic modelling and computational physiology: The application of Bond Graph boundary conditions for 3D cardiovascular models

Abstract This paper presents an application of Bond Graphs in physiological modelling. In this work, a Bond Graph model is utilised as boundary condition for a detailed model of an idealized mitral valve. Applications of this type fit within the framework described by the “Virtual Physiological Human” initiative. This supports the integration of physical, mechanical and biochemical models encompassing a range of different length and time scales to obtain predictive models of the human body. Because 3D detailed modelling and simulation is computationally intensive, a 3D computational model of a whole biological system is, by today’s standards, impossible to achieve. Due to their unique multi-physics nature of internal coherence, Bond Graphs are particularly suited to biological applications and can be coupled to 3D models and lumped parameter models. A specific application in cardiovascular modelling is demonstrated by focusing on a specific example; a 3D model of the mitral valve coupled to a lumped parameter model of the left ventricle.

Importance of realistic LVAD profiles for assisted aortic simulations: evaluation of optimal outflow anastomosis locations.

Left ventricular assist devices (LVADs) are carefully designed, but the significance of the implantation configuration and interaction with the vasculature is complex and not fully determined. The present study employs computational fluid dynamics to investigate the importance of applying a realistic LVAD profile when evaluating assisted aortic flow fields and subsequently compares a number of potential anastomosis locations in a patient-specific aortic geometry. The outflow profile of the Berlin Heart INCOR® device was provided by Berlin Heart GmbH (Berlin, Germany) and the cannula was attached at a number of locations on the aorta. Simulations were conducted to compare a flat profile against the real LVAD profile. The results illustrate the importance of applying an LVAD profile. It not only affects the magnitude and distribution of oscillatory shear index, but also the distribution of flow to the great arteries. The ascending aorta was identified as the optimal location for the anastomosis.

Applying the lattice Boltzmann technique to biofluids: A novel approach to simulate blood coagulation

In this paper we describe an extension of the lattice Boltzmann method to simulate blood clotting based on a simple residence time model. Simulation results of a stenosed model artery are presented together with experimental blood analogue results. Aspects of an efficient unstructured implementation are briefly reviewed.

In-situ simulation of one-piece metacarpophalangeal joint implants using finite element analysis

Generally, reconstruction of the rheumatoid metacarpophalangeal (MCP) joint is achieved by means of implantation of a hinged silastic prosthesis. Whereas these implants restore some degree of mobility to the joint, they are prone to failure after a relatively short life-span, and little is known about their dynamical behaviour within the joint. In this study, the Swanson and Sutter designs of MCP implant were examined in an idealized joint environment by means of two-dimensional finite element analysis. The purpose was to assess how the differing geometry affected their behaviour as replacement joints, and whether they were inherently prone to abrasion and high stress concentrations during flexion. The results revealed the changing points of contact between the implant and the bone ends, and clearly showed the implant stems pistoning in the intramedullary canals. This was found to be an effective way to provide preliminary information on the dynamic behaviour of an implant in a simulated joint. This would facilitate further optimization of design in advance of fabrication.