Longitudinal mixing in pulmonary airways. Analysis of inert gas dispersion in symmetric tube network models.
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Airway Resistance↗
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A model study of flow dynamics in human central airways. Part I: axial velocity profiles.
We measured detailed steady inspiratory and expiratory velocity profiles in a 3:1 scale model of the human central airways. The model was constructed out of acrylic plastic, mounted vertically, and connected to a specially designed steady-flow system. Laterally introduced hot-wire anemoneter probes were used to record axial velocities along 4 diameters at each of the 12 pre-drilled stations of measurement; the flow distribution among the five lobar bronchi was controlled by distally positioned linear resistors. Whether with a flat entrance profile or entering as a narrow jet, the inspiratory flow velocity profiles in the frontal plane showed a high degree of asymmetry in all branches, with peak velocities near the inner wall of the bifurcation. In the sagittal plane the velocity profiles were nearly symmetric, exhibiting a single peak near the center in the frontal plane and almost flat in the sagittal plane. Overall, the velocity profiles were more sensitive to airway geometry than to flow rate. The only site of flow separation was observed in the right upper lobar bronchus. The most evident modification of axial velocity profiles in a single branch was found in the left main bronchus during expiratory flow.
Humans↗
A model study of flow dynamics in human central airways. Part II: secondary flow velocities.
Secondary velocity components perpendicular to the tube axis were measured in a 3 : 1 scale model of the human central airways. Slanted hot-wire probes were introduced axially in order to measure the secondary velocities at about 12 points for each of the 7 stations investigated. Secondary velocities in the inspiratory direction never exceeded a mean value of 18% of the mean axial velocity. Secondary velocities in the expiratory direction reached a mean value of 21.5% of the mean axial velocity. In the inspiratory direction, two unequal eddies were formed in the left main bronchus and in the right upper lobe. Moreover, maximum velocities were observed near the wall and the decay of secondary velocities in the left main bronchus was observed. The secondary flow patterns observed in the left upper and lower lobes after the secondary bifurcation were difficult to recognize, although they seemed to be more influenced by the second bifurcation. The complexity of the flow pattern was reinforced by viscous effects acting near the wall. In the expiratory direction, only two stations in the trachea were measured; four uneven eddies seemed to have existed, with the ventral eddies appearing to be predominant. Overall, the secondary velocity magnitudes as well as the patterns of eddies were very dependent on the geometry of the model used.
Humans↗
Some criteria for laminar conditions during HFV.
Assessment of the type of flow regime-laminar, transitional or turbulent-present in the central airways during high-frequency ventilation (HFV) can assist in identification of the predominant gas transport mechanisms for a particular species and set of HFV conditions. In this study, we use published empirical relationships, developed to identify the initial change from oscillating laminar flow to oscillating turbulent flow in tubes, to derive the limiting relationships between a dimensionless stroke volume and the Womersley number for maintenance of laminar conditions. When used with morphometric lung models for man and dog, these limiting relationships predicted maximum stroke volumes at experimental frequencies that either exceeded or agreed within 20% with the stroke volumes reported for steady-state ventilation of humans and dogs using HFV. Based on these limiting relationships, stroke volume-frequency combinations reported to demarcate the decline of PaO2 and alveolar ventilation were associated with nonlaminar conditions. It is expected that this approach may be useful in selecting the stroke volume-frequency pairs for HFV when a specific type of flow regime is desired as well as for analysis of HFV data.
Animals↗
Oxygen transfer of red blood cells: experimental data and model analysis.
Kinetics of O2 uptake and release by human red blood cells (RBC) as measured by stopped-flow techniques were simulated using an RBC model shaped as a spheric shell. The O2 transfer mechanisms in this model include diffusion and reaction within the RBC and diffusion and convection in the medium surrounding the RBC. Unknown model parameters were determined by comparing simulations with experimental data. The following conclusions were drawn. (1) Both diffusion and convection contribute to O2 transport in the medium surrounding the RBC, and this transport importantly limits the overall O2 transfer kinetics in stopped-flow experiments. (2) Intraerythrocyte transport mechanisms become predominant in limiting O2 transfer, and can thus be investigated by stopped-flow techniques, only when the perierythrocyte O2 transport resistance is minimized, e.g. by high levels of dithionite in measurements of O2 release from RBC. (3) Intraerythrocyte O2 transfer is shown to be mainly limited by diffusion of O2 and, to a lesser extent, by diffusion of oxyhemoglobin ('facilitated O2 diffusion') and by O2/hemoglobin reaction. The results suggest that diffusion is the main process limiting O2 uptake and release by RBC, the finite reaction kinetics of O2 with hemoglobin exerting a smaller limiting effect.
Erythrocytes↗
Diffusion limitation of O2 supply to tissue in homogeneous and heterogeneous models.
The role of diffusion limitation in O2 supply was studied in cross-sectional elements of the Krogh cylinder model (with O2 supply from a central capillary) and of the solid cylinder model (with O2 supply from the outer surface). The effect of diffusion limitation was quantified in terms of the ratio O2 uptake/O2 requirement (= fraction of cross-sectional area supplied with O2), assuming local O2 requirement per unit volume to be constant and independent of PO2 at PO2 greater than 0. Calculations were performed for single cylinders of varied radius and O2 requirement (homogeneous models). Unequal distribution of diffusion conditions was represented by a model composed of three sorts of Krogh or solid cylinders, with radii in relation 3: square root of 3:1, but of equal cross-sectional area, i.e. number of cylinders of each sort in relation 1:3:9 (heterogeneous models). The results revealed the following main features. (1) At the same outer radius, diffusion limitation sets in at a smaller O2 requirement, and increases more steeply with increasing O2 requirement, in the homogeneous Krogh cylinder model compared with the homogeneous solid cylinder model. A similar behavior is observed when the radius of the cylinder section is increased at constant O2 requirement. (2) Diffusion limitation in the heterogeneous model sets in at a lower O2 requirement value, and increases more gradually with increasing O2 requirement, than in the corresponding homogeneous models with the same average cylinder diameter. This behavior is due to sequential onset, in the heterogeneous model, of anoxia in the cylinder sections of different radii. We conclude that diffusion heterogeneity has to be taken into account when the role of diffusion limitation in tissue O2 supply is investigated.
Animals↗
Factors affecting distribution of airflow in a human tracheobronchial cast.
Air velocity was measured at end airways of hollow replicate casts of the human tracheobronchial tree in order to determine the flow distribution within casts extending to 3 mm diameter airways. Measurements were made by hot-wire anemometry for constant inspiratory flow rates of 7.5, 15, 30 and 60 L.min-1. Average flow distribution among the lung lobes was as follows: right upper, 18.5%; right middle, 9.2%; right lower, 32.3%; left upper, 15.7%; and left lower, 24.3%. An empirical model derived from the experimental flow distribution data demonstrated the effect of various morphometric parameters of the hollow cast on the distribution of airflow. Airway cross-sectional area, branching angle and total path-length were found to have the greatest influence. As the tracheal flow rate decreased from 60 to 7.5 L.min-1, the influence of branching angle was reduced, while total path-length became more influential. These results provide evidence for the transition of flow regimes within the TB tree within normal physiological flow ranges.
Adult↗
An analytic, gradient index schematic lens and eye for the rat which predicts aberrations for finite pupils.
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Animals↗
Ultrasonographic anatomy of normal prostate gland: reconstruction by computer graphics.
Computer graphic reconstructions of the prostate were made in both the transverse (axial) and longitudinal (sagittal) planes at different levels on the basis of the normal anatomic model of McNeal. Additional images then were created to give a picture of what the ultrasonogram should look like at each level, and these were compared with actual ultrasonograms of the normal prostate. A precise yet simple terminology is suggested for prostate ultrasonographic descriptions.
Adult↗
Ideogram writing in a disconnection syndrome.
A case of disconnection-type agraphia coupled with alexia was reported. The patient showed several asymmetrical manual capacities between the two hands, i.e., dissociated difficulty of Kanji (ideogram) writing between the two hands, left unilateral difficulty of Kana (phonogram) writing, right unilateral dyscopia of letters as well as geometrical figures, and right unilateral difficulty in drawing without a model. Anatomically, lesions involved most of the corpus callosum in its posterior portion including the splenium and the left medial occipital lobe. From these data, a possible linguistic capacity of the right hemisphere was suggested.
Adolescent↗
Three-dimensional model of the human craniofacial skeleton: method and preliminary results using finite element analysis.
The purpose of this study was to develop a three-dimensional finite element model of the craniofacial skeleton using a dry human skull. The model consisted of 2918 nodes and 1776 solid elements, and was used to investigate the biomechanical effect of a distally directed orthopaedic force on the craniofacial complex. The force was applied at the level of the maxillary first molar. The results indicated that in response to the force system applied: the nasomaxillary complex displaces in a backward and downward direction and rotates in clockwise sense; the nasomaxillary complex, including the zygomatic bone, experiences high stress levels in comparison with those at the remaining bones; the stress distribution in the maxillary basal bone area is relatively uniform; and the stress distribution across the opposing surface of the bony margins of the sutures is non-uniform.
Biomechanical Phenomena↗
An "expanded stick" model for coding Golgi-impregnated neuronal morphology.
A new model for coding Golgi-impregnated neuronal morphology, which can be implemented in any computer-assisted optical microscope, is described. This model, in addition to storing the three-dimensional coordinates of the selected neuronal points and their topological identifiers, codes for "width", "nature", and "shape". The "width" code digitizes the width of a neuronal process. The "nature" code can identify structures such as perikaryon, axon, apical dendrite, basal dendrite, etc. The "shape" code defines nodules and spines. Computer graphics routines are described for drawing nodules and spines as well as neuronal processes with "width".
Animals↗
A system for quantitative morphological measurement and electronic modelling of neurons: three-dimensional reconstruction.
A system for accurately reconstructing neurones from optical sections taken at high magnification is described. Cells are digitised on a 68000-based microcomputer to form a database consisting of a series of linked nodes each consisting of x, y, z coordinates and an estimate of dendritic diameter. This database is used to generate three-dimensional (3-D) displays of the neurone and allows quantitative analysis of the cell volume, surface area and dendritic length. Images of the cell can be manipulated locally or transferred to an IBM 3090 mainframe where a wireframe model can be displayed on an IBM 5080 graphics terminal and rotated interactively in real time, allowing visualisation of the cell from all angles. Space-filling models can also be produced. Reconstructions can also provide morphological data for passive electrical simulations of hippocampal pyramidal cells.
Animals↗
Accuracy and precision of angiographic volumetry methods for left and right ventricle.
We imaged and quantified 60 ventricle casts (30 LV, 30 RV) to evaluate the accuracy and reliability of angiographic ventricle volumetry. We analyzed the seven biplane methods most frequently used in clinical routine: Arcilla, Arvidsson, Dodge, Ferlinz, Simpson (LV + RV) and Wynne. The ventricle contours were defined by (1) manual drawing on the computer screen, (2) manual drawing using a graphical tablet and (3) automatic contour detection. A high inter-class variation in volume accuracy between the different methods was observed (S.D. = 12.7 ml). The volume methods for the LV (mean differences MDLV: [-2.2, +8.5] ml, average MDLV = 1.8 ml) are more accurate than for the RV (MDRV: [-11.4, +33.1] ml, average MDRV = 12.1 ml). The intrinsic error is about the same for all approaches and is very high: average S.D. = 20 ml, RMS = 185 ml. Manual contour definition results in a volume over-estimation (average MDman = +32.8 ml, r = 0.731) compared with automatic contour detection (average MDauto = +6.2 ml, r = 0.810). LV hypertrophy results in a volume under-estimation of the LV (MDLV = -7 ml) and an over-estimation of the RV (MDRV = +6 ml). RV hypertrophy leads to the opposite effect. It was shown that ventricle volumetry and the calculation of derived parameters (ejection fraction) is extremely case dependent and can only be an estimate of the actual value.
Angiography↗
Evaluation of experimental and finite element models of synthetic and cadaveric femora for pre-clinical design-analysis.
The aim of this study was to determine the validity with which the finite element method could model synthetic bone and thereby determine the appropriateness of such femur analogues for application in pre-clinical tests. The performance of these synthetic femora was compared with cadaveric bone when employing the same geometric and material definition protocols. A four-point bend loading configuration was selected for this analysis. Four synthetic femurs and an embalmed cadaveric bone were tested experimentally to determine the structural bending stiffness (k) for the diaphysis of these bones. A finite element (FE) model was generated and an analysis performed for each bone type to estimate the Young's modulus (E) required to obtain a model stiffness equivalent to that obtained experimentally. The estimated material elastic modulus in the FE model for the synthetic femur was found to be very similar to available data for this bone analogue. The estimated cadaveric bone modulus however was found to differ significantly from documented values for cortical bone. A theoretical analysis demonstrated the great sensitivity of the estimated modulus value to the accuracy of the geometric definition. The very low variability found in the experimental test on the synthetic bones together with their more regular geometry and the possibility of achieving greater accuracy in geometric definition was shown to enable the production of a valid FE model of this bone for an isotropic homogeneous material description. Conversely, the greater irregularity of geometry, together with the less obvious differentiation between the cortical and cancellous bone in the cadaveric specimen makes accurate geometric description of this bone very difficult. This fact, together with the uncertainty concerning the quality of the cadaveric bone and its viscoelastic response during mechanical testing, makes reproduction of its behaviour in a FE model a much more demanding task. It is suggested that this greater capability of reproducing the experimental behaviour of the synthetic bone makes them a very useful model for both experimental and numerical studies which involve in-vitro pre-clinical testing of implant design and stem-bone behaviour.
Artificial Organs↗
Altered orthognathic surgical sequencing and a modified approach to model surgery.
Advances in orthognathic surgical treatment planning and in techniques for complex, simultaneous maxillary and mandibular repositioning have resulted in improved surgical accuracy. In traditional surgical sequencing, maxillary surgery is performed first; the maxilla is set, with or without an intermediate splint, using external reference points to verify and/or determine appropriate movement. However, errors in model surgery and intermediate splint fabrication can lead to surgical inaccuracy despite good surgical technique. In repositioning the maxilla first, when thin bony walls are present, and/or in conjunction with large mandibular advancements, maxillary shifting may occur when maxillomandibular fixation is applied. Soft tissue tension and surgical manipulation in this sequencing technique may result in a less desirable functional and esthetic outcome. This article presents an alternative to surgical sequencing and a modification of model surgery techniques to improve surgical accuracy, and thus predictability and stability of the results.
Adult↗
Use of a laser-hardened three-dimensional replica for simulated surgery.
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Adult↗
Modified sagittal split ramus osteotomy for mandibular setback.
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Cephalometry↗