Search PubMed⌕ Search

Biomedical subjects

E A Patterson

Publications and source records attributed to E A Patterson.

17 recordsLinked to original sources
PubMed2005

An approach to the simulation of fluid-structure interaction in the aortic valve.

A pair of finite element models has been employed to study the interaction of blood flow with the operation of the aortic valve. A three-dimensional model of the left ventricle with applied wall displacements has been used to generate data for the spatially and time-varying blood velocity profile across the aortic aperture. These data have been used as the inlet loading conditions in a three-dimensional model of the aortic valve and its surrounding structures. Both models involve fluid-structure interaction and simulate the cardiac cycle as a dynamic event. Confidence in the models was obtained by comparison with data obtained in a pulse duplicator. The results show a circulatory flow being generated in the ventricle which produces a substantially axial flow through the aortic aperture. The aortic valve behaves in an essentially symmetric way under the action of this flow, so that the pressure difference across the leaflets is approximately uniform. This work supports the use of spatially uniform but temporally variable pressure distributions across the leaflets in dry or structural models of aortic valves. The study is a major advance through its use of truly three-dimensional geometry, spatially non-uniform loading conditions for the valve leaflets and the successful modelling of progressive contact of the leaflets in a fluid environment.

Animals↗
PubMed1997

Three-dimensional stress analysis of polypropylene leaflets for prosthetic heart valves.

The effect of changing the modulus and thickness of the material in the leaflets of an artificial heart valve has been investigated. This has been achieved with a finite element model of the valve having approximately 2300 thin shell elements. The valve motion and the resulting stresses are modelled dynamically during closure, and subsequent pressurisation. The stresses decrease as the leaflets are made thicker and the modulus is increased. Local and global thickening has been investigated. The highest stresses appear at the tops of the stent posts in the regions of the commissures. When the modulus is too low, or the leaflets are too thin, the valve prolapsed.

Computer Simulation↗
PubMed1994

Are hip fractures caused by falling and breaking or breaking and falling? Photoelastic stress analysis.

The distribution of stress in the heads of femurs has been assessed using an engineering technique known as photoelastic stress analysis. The aim of the investigation was to locate and evaluate the probable sites of structure failure in femurs with various axes of loading. The analysis shows that the maximum stress in the upper femur occurs in the cervical region during vertical loading. Lateral loading does not produce significant stress patterns at the sites of extracapsular hip fracture. It is concluded that cervical hip fractures in individuals suffering from osteoporosis result from a combination of fatigue damage and axial muscular compression, rather than forces generated by a fall. This has significance for the medico-legal interpretation of the cause of death following femoral neck fracture.

Accidental Falls↗
PubMed1994

Tightening characteristics for screwed joints in osseointegrated dental implants.

The significance of tightening abutment screws and gold cylinders to osseointegrated fixtures with the correct torque is demonstrated, and a simple relationship between applied torque and screw preload is derived by use of mechanical engineering principles. The principles of a number of tightening methods are outlined and assessments made of their accuracy. The difference between optimum and design torque is highlighted. The necessity and means of achieving optimum torque to ensure a reliable joint in clinical practice is discussed.

Dental Abutments↗
PubMed1992

Breast cancer recurrence after lumpectomy and irradiation: role of mammography in detection.

Between 1977 and 1989, 102 of 1,145 patients treated with lumpectomy and radiation as an alternative to mastectomy required subsequent biopsy for suspected ipsilateral recurrence. The authors reviewed the mammograms of 58 of those patients for whom at least two sets of mammograms were available, including one set obtained within 3 months of the subsequent repeated biopsy. Of 38 biopsy-proved recurrences, 13 (34%) were detected with mammography alone, 17 (45%) with palpation alone, and eight (21%) with both mammography and palpation. While standard mammographic criteria for biopsy were followed, distortions and calcifications inherent to posttreatment appearances necessitated judicious modifications. As anticipated, sensitivities and positive predictive values showed more impairment within the lumpectomy quadrant. In addition, all cases of purely in situ cancer were detected solely with mammography. Mammography plays an important complementary role to physical examination in posttreatment follow-up. On the basis of these findings, a follow-up algorithm for the ongoing evaluation of these patients was developed.

Adult↗
PubMed1992

Theoretical analysis of the fatigue life of fixture screws in osseointegrated dental implants.

Metal fatigue failure of the gold screw used to retain a fixed prosthesis to Brånemark osseointegrated fixtures/abutments has been analyzed theoretically. Mechanical engineering principles show the importance of appropriate preload being applied through the gold screw to the gold cylinder and abutment. The significance of the screw design and necessity of applying the correct torque to achieve a long fatigue life for the screw are also described. The consequence of misalignment of a gold cylinder to an abutment is discussed.

Dental Abutments↗
PubMed1991

A three-dimensional analysis of a bioprosthetic heart valve.

A three-dimensional finite element model of the leaflets of a bicuspid bioprosthetic heart valve is presented. The model is based on a non-linear elastic representation of the tissue behaviour which closely simulates that found in experiments. The geometry of the model is based on measurements from a real valve. Shell elements which permit bending have been used in the analysis. The results indicate that bending stresses in the leaflets make a significant contribution to their deformation. This confirms earlier two-dimensional work which had suggested that analyses, where only membrane stresses were modelled, were likely to produce significant errors in the stress states. The analysis also predicts peak stresses close to, but not at, the attachment of the leaflet to the stent post.

Biomechanical Phenomena↗
PubMed1991

Assessment of osteoporosis at autopsy: mechanical methods compared to radiological and histological techniques.

Osteoporosis is a common problem in the elderly female population and pathological fractures are a frequent complication, but how should the pathologist assess the severity of the disease at autopsy? In this study we have compared subjective, semi-quantitative assessments of rib fracturability, vertebral compressibility, Singh index and histomorphometric methods against a 'gold standard' assessment, namely the force required to fracture the femoral neck. We have shown good correlations between rib fracturability. Singh index and the standard chosen. Poor correlation was shown between histomorphometric techniques and the standard. Our findings suggest that gross rib fracturability is a good indicator of osteoporosis in as far as bone fragility is an indicator of the severity of osteoporosis. However, as the population under examination was small, further studies in this area are clearly warranted in order to extend these findings.

Aged↗
PubMed1990

A two-dimensional finite element analysis of a bioprosthetic heart valve.

A finite element scheme has been developed using total Lagrangian techniques for the two-dimensional analysis of bioprosthetic heart valve leaflets undergoing large deformation. Two models of a leaflet, namely a radial and a circumferential slice, have been analysed. The attachment of the slice to the stent was simulated by progressive contact on a circular former and the coaptation of the leaflets in the centre of a heart valve by a straight line of contact. For the circumferential model, different initial configurations have been considered. The prolapse pressure under which the heart valve closes has been shown to be small in comparison with the normal pressure a heart valve sustains. The regions of the valve that are most heavily stressed are subjected to a strong component of bending. The amount is sensitive to the details of the boundary conditions and to the initial configuration of the valve. These observations are likely to be significant in the use of this kind of stress analysis to improve the design of this type of valve.

Bioprosthesis↗
PubMedDate not supplied

On the opening mechanism of the aortic valve: some observations from simulations.

Viewed from the standpoint of mechanical engineering design, the aortic valve is impressive. However, our understanding of its mechanics is limited by our inability to study its in vivo function closely and in detail. Computer simulation methods offer an alternative approach and a first step towards the construction of a more complete cardiac model is described. The model includes the aortic valve, its leaflets and their supporting root, and the sinuses modelled as nonlinear materials. An explicit finite element code has been used to examine the time-varying displacements of the structure that was subject to pressure distributions, which included left ventricular, aortic and thoracic pressures. It was shown that the leaflets of the valve open by a combination of root expansion in a radial direction and leaflet movement in the direction of blood flow. This was compared to a model in which the aortic root was stiffened significantly, and it was found that this modified valve opened by leaflet folding to give a much smaller orifice. These findings, concerning the importance of root expansion, are in agreement with earlier experimental observations.

Aortic Valve↗
PubMedDate not supplied

Influence of anisotropy on the mechanical behaviour of bioprosthetic heart valves.

Chemically modified pericardium is commonly used in the fabrication of bioprosthetic heart valves. This material exhibits non-linear elastic behaviour and, as for most other biological soft tissues, it is orthotropic in its extensibility. The influence of the natural orthotropy of pericardium on the mechanical behaviour of pericardial heart valves during the whole cardiac cycle has been studied, using the finite element method. A model of the leaflet of a bicuspid valve has been created, defining the material of the tissue as orthotropic non-linear elastic. Two preferential orthogonal orientations of the tissue have been analysed (axial and circumferential). The results show that even a small amount of orthotropy (an orthotropy index of 1.5 has been used) can significantly affect the mechanical behaviour of the valve, and that an appropriate orientation of the fibres can contribute to optimizing the stress distribution in the leaflets.

Anisotropy↗
PubMedDate not supplied

Simulation of damage in a porcine prosthetic heart valve.

A model of a bioprosthetic porcine valve has been produced which simulates the non-linear elastic behaviour of the fixed tissue and the re-inforcement of the leaflets by collagen fibre bundles. The loading on the model is a spatially uniform but temporally varying distribution of pressure. Bending is allowed in the tissue of the leaflets and is shown to be significant in determining the behaviour and failure modes of the leaflets. The simulation of the undamaged valve is validated against in vitro pulse duplicator studies and a simple fluid--solid interaction simulation. Progressive damage is introduced into three models of the valve at a location where tears have been commonly found in vivo. It was found that during the opening and closing of the valve the tip of the tears were subject to mode III or tearing displacements, but that in the fully closed or diastolic state the tear tip was subject to mode I or opening displacements such that it would be expected to propagate parallel to the line of attachment to the stent. The tear tip stresses increased with the length of the tear so that the rate of tearing would be expected to increase with length.

Animals↗
PubMedDate not supplied

A comparative study of linear and nonlinear simulations of the leaflets in a bioprosthetic heart valve during the cardiac cycle.

Two geometrically identical models of the leaflets of a bicuspid bioprosthetic heart valve have been constructed using finite elements. The boundary conditions applied to the models were also identical but a linear material model has been used in one and a nonlinear elastic model in the other. The models were fullscale and contained 2600 Belytschko-Lin-Tsai shell elements which allowed the variation of stress through the thickness of the leaflet to be modelled. A time-varying, spatially-uniform pressure differential was applied across the leaflets to model their behaviour during a complete cardiac cycle. The simulation was performed using a dynamic, explicit, time-stepping, finite element code. A comparison of the two models showed that the nonlinear model was more responsive to the time-varying pressure wave, and deformed into more complex shapes during the opening and closing phases which induced lower compressive but higher tensile stresses in the leaflets.

Bioprosthesis↗
PubMedDate not supplied

Distribution of load in an oral prosthesis system: an in vitro study.

Although the long-term success of osseointegrated endosseous implants for the support of fixed dental prostheses has been reported, the increasingly routine and widespread use of implant-supported prostheses has led to problems associated with their structural integrity. To assess the service life of the components of the prosthetic system, a knowledge of the loads transmitted through the system is needed. This paper reports in vitro findings for forces and bending moments transmitted through the abutments from a fixed dental prosthesis to its supporting implants. The results have been compared to previous theoretical analyses that generally overestimate the maximum tensile forces. The effect of the length of distal cantilevers and of missing gold screws as the result of loosening or fracture is also considered. It is proposed that bending moments may play a larger role in failures than previously believed.

Bite Force↗