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PubMed1992

Predicted and observed shapes of human mandibular condyles.

A mathematical model based on linear programming was used to study the directions of the joint forces used to maintain the human jaw in three-dimensional static equilibrium when producing bite forces of 100 N to a maximum of 1000 N down the long axis of a central incisor, first premolar, first molar and third molar. Seven different versions of the model were studied. The two simplest versions minimized the total muscle tension and the total joint load, respectively. Assuming that the joint force direction must be normal to some part of the articular surface of the condyle, neither version produced directions consistent with the observed shapes of human condyles. The other five versions minimized different combinations of muscle tensions and joint loads. Two of these versions produced joint force directions compatible with the shapes of condyles. Both minimized total muscle tension plus the (vertical) joint load on the back of the condyle. The results suggest that joint mechanoreceptors (probably non-directional) as well as muscle receptors contribute to the neuromuscular control of bite forces. Our results are consistent with some recent observations [Marshall and Tatton, Exp. Brain Res. 83, 137-150 (1990)] of the cat knee joint.

Biomechanical Phenomena↗
PubMed1993

A biomechanical analysis of muscle strength as a limiting factor in standing posture.

We developed a method for studying muscular coordination and strength in multijoint movements and have applied it to standing posture. The method is based on a musculoskeletal model of the human lower extremity in the sagittal plane and a technique to visualize, geometrically, how constraints internal and external to the body affect movement. We developed an algorithm to calculate the set of all feasible accelerations (i.e., the 'feasible acceleration set', or FAS) that muscles can induce. For the ankle, knee, and hip joints in the sagittal plane, this set is a polyhedron in three dimensions. Using the volume of the FAS as an indicator of overall mobility, we found that strengthening muscles on the posterior side (as opposed to the anterior) of the body would cause greater increases in mobility. Employing the experimental observations of others, we also found that acceleration constraints greatly reduce the range of feasible accelerations. We then defined a set of four basic acceleration vectors which, when used in various combinations, can produce the repertoire of postural movements. We used linear programming to find the maximum magnitudes of these vectors, and the sensitivity of these magnitudes to muscle strength, thereby delineating those muscles which, if strengthened, would cause the greatest increase in the body's ability to generate the basic acceleration vectors. For our particular model, those muscle groups were found to be hamstrings, tibialis anterior, rectus femoris, and gastrocnemius. These muscle groups would be of great importance in cases involving severely reduced muscle strength. This methodology may therefore be useful for purposes such as design of functional electrical stimulation controllers or exercises for persons at risk for falling.

Acceleration↗
PubMed1996

Features of human jaw design which maximize the bite force.

A linear programming model, operating in a three-dimensional coordinate system, was written to study the effect on unilateral maximum bite forces (MBFs) of small changes in the design of human jaws. Under some conditions the MBF was larger on a more anterior tooth than a more posterior tooth for a given bite direction. The MBF was increased (i) as the bite force was tilted forward, (ii) as the jaw was opened if its direction was kept perpendicular to the occlusal plane of the lower jaw, (iii) by placing teeth nearer the midline, (iv) by raising the height of the teeth for forward bite forces, and (v) by tilting forward the articular surface of the condyle (equivalent to creating an articular eminence). Increasing the distance between the condyles spread the load more evenly between the two joints. The articular eminence was the only design feature which seemed to be clearly related to increasing the MBF.

Bite Force↗
PubMed1984

Economic evaluation of health care interventions: a review of alternative methods.

This study analyzes a number of economic evaluation techniques that can be used by the economist in helping decision-makers make choices about the allocation of scarce resources in health care and the organization of health interventions. The evaluation techniques discussed are cost-benefit analysis, cost-effectiveness analysis, multi-attribute problem analysis, linear programming and econometric modelling. The main purpose of this paper is to study the merits of the different techniques and to point at certain difficulties in their application.

Cost-Benefit Analysis↗
PubMed1992

Determination of doughing and manipulation times of acrylic resin denture base material and a proposal for a specification for a packing plasticity test.

The doughing and manipulation times of a range of acrylic resin denture base polymers are presented in this study. The methods evaluated were: (i) probing following American Society for testing materials specification, F451-76; (ii) hole penetration following British Standard, 2487: 1989 and International Standard Organisation, 1567: 1988; (iii) probing using a penetrometer employing a 20 g load and 1 mm diameter needle; (iv) oscillating rheometer apparatus used at a fixed frequency of 1 Hz by sweeping through a programmed linear increase in strain. The results indicate that each of the methods is capable of evaluating the doughing and manipulation times of the unprocessed polymers. It is concluded that the Bohlin VOR Oscillating Rheometer is excellent for investigating the flow properties of denture base polymers but it cannot be recommended for a standards specification test because of the high cost. The penetrometer method has been shown to be superior to the current standards specification test and would be ideal for a packing plasticity standards test.

Denture Bases↗
PubMed1991

Physiological prediction of muscle forces--II. Application to isokinetic exercise.

The successful application of a physiological model of the musculoskeletal system capable of accounting for nonequilibrium dynamic loading and predicting individual muscle forces in the knee is presented. The model incorporates rigid-body mechanics and musculoskeletal physiology. Unknown muscle and joint contact forces outnumber the equilibrium equations resulting in an indeterminant problem. Mathematical optimization is utilized to resolve the indeterminacy. The model is used to estimate individual muscle forces during isokinetic exercise. Five subjects were tested at speeds of 60 degrees/s and 180 degrees/s. A newly proposed optimal criterion, minimizing muscular activation, results in muscle force predictions which have significantly higher correlations with myoelectric activity than other linear and nonlinear optimal criteria. The results demonstrate that properly constrained linear programming methods do not limit the number of active muscles and allow for uniform recruitment of the active muscles.

Adult↗
PubMed1989

An algorithm for design of beam compensators.

Beam compensators are optimally designed to give a uniform dose to any plane or midsurface that intersects a single beam, or to give a uniform dose to the volume defined by the intersection of two or more beams. The primary and scatter components are taken into account separately, as well as the patient's shape and internal heterogeneities. The design of the beam compensators is formulated as a linear programming problem and solved with a variation of the Simplex Method. Beam weighting factors are also obtained as part of the solution.

Algorithms↗
PubMed1990

Large scale optimization of beam weights under dose-volume restrictions.

The problem of choosing weights for beams in a multifield plan which maximizes tumor dose under conditions that recognize the volume dependence of organ tolerance to radiation is considered, and its solution described. Structures are modelled as collections of discrete points, and the weighting problem described as a combinatorial linear program (LP). The combinatorial LP is solved as a mixed 0/1 integer program with appropriate restrictions on normal tissue dose. The method is illustrated through the assignment of weights to a set of 10 beams incident on a pelvic target. Dose-volume restrictions are placed on surrounding bowel, bladder, and rectum, and a limit placed on tumor dose inhomogeneity. Different tolerance restrictions are examined, so that the sensitivity of the target dose to changes in the normal tissue constraints may be explored. It is shown that the distributions obtained satisfy the posed constraints. The technique permits formal solution of the optimization problem, in a time short enough to meet the needs of treatment planners.

Humans↗
PubMed2005

Reconstruction of blood propagation in three-dimensional rotational X-ray angiography (3D-RA).

This paper presents a framework of non-interactive algorithms for the mapping of blood flow information to vessels in 3D-RA images. With the presented method, mapping of flow information to 3D-RA images is done automatically without user interaction. So far, radiologists had to perform this task by extensive image comparisons and did not obtain visualizations of the results. In our approach, flow information is reconstructed by forward projection of vessel pieces in a 3D-RA image to a two-dimensional projection series capturing the propagation of a short additional contrast agent bolus. For accurate 2D-3D image registration, an efficient patient motion compensation technique is introduced. As an exemplary flow-related quantity, bolus arrival times are reconstructed for the vessel pieces by matching of intensity-time curves. A plausibility check framework was developed which handles projection ambiguities and corrects for noisy flow reconstruction results. It is based on a linear programming approach to model the feeding structure of the vessel. The flow reconstruction method was applied to 12 cases of cerebral stenoses, AVMs and aneurysms, and it proved to be feasible in the clinical environment. The propagation of the injected contrast agent was reconstructed and visualized in three-dimensional images. The flow reconstruction method was able to visualize different types of useful information. In cases of stenosis of the middle cerebral artery (MCA), flow reconstruction can reveal impeded blood flow depending on the severeness of the stenosis. With cases of AVMs, flow reconstruction can clarify the feeding structure. The presented methods handle the problems imposed by clinical demands such as non-interactive algorithms, patient motion compensation, short reconstruction times, and technical requirements such as correction of noisy bolus arrival times and handling of overlapping vessel pieces. Problems occurred mainly in the reconstruction and segmentation of 3D-RA images in cases of complex AVMs. The concentration of injected contrast agent was often not sufficient to provide highly contrasted vessels in 3D-RA images. Another segmentation-related problem is known as 'kissing vessels' [19]. Kissing vessel artifacts introduce artificial vessel junctions and thereby distort the feeding structure of the vessel. This may finally cause implausible flow reconstruction results and inverse flow directions in vessel segments. We are currently planning to validate our reconstruction results using particle imaging velocimetry (PIV). PIV experiments with phantoms, for which the true flow parameters are known, will allow for the assessment of the accuracy of our contrast agent based method. In the context of computational fluid dynamics techniques, the potential of the presented flow reconstruction method is high. Flow reconstruction results based on the presented method could be used both as boundary conditions for simulations and as a reference for the validation of simulation results. Computational fluid dynamics provide useful information such as arterial wall shear stress and complex flow patterns in aneurysms.

Algorithms↗
PubMed2005

Robustness of the p53 network and biological hackers.

The p53 protein interaction network is crucial in regulating the metazoan cell cycle and apoptosis. Here, the robustness of the p53 network is studied by analyzing its degeneration under two modes of attack. Linear Programming is used to calculate average path lengths among proteins and the network diameter as measures of functionality. The p53 network is found to be robust to random loss of nodes, but vulnerable to a targeted attack against its hubs, as a result of its architecture. The significance of the results is considered with respect to mutational knockouts of proteins and the directed attacks mounted by tumour inducing viruses.

Computer Simulation↗
PubMed2005

Dynamic optimization of bioprocesses: efficient and robust numerical strategies.

The dynamic optimization (open loop optimal control) of non-linear bioprocesses is considered in this contribution. These processes can be described by sets of non-linear differential and algebraic equations (DAEs), usually subject to constraints in the state and control variables. A review of the available solution techniques for this class of problems is presented, highlighting the numerical difficulties arising from the non-linear, constrained and often discontinuous nature of these systems. In order to surmount these difficulties, we present several alternative stochastic and hybrid techniques based on the control vector parameterization (CVP) approach. The CVP approach is a direct method which transforms the original problem into a non-linear programming (NLP) problem, which must be solved by a suitable (efficient and robust) solver. In particular, a hybrid technique uses a first global optimization phase followed by a fast second phase based on a local deterministic method, so it can handle the nonconvexity of many of these NLPs. The efficiency and robustness of these techniques is illustrated by solving several challenging case studies regarding the optimal control of fed-batch bioreactors and other bioprocesses. In order to fairly evaluate their advantages, a careful and critical comparison with several other direct approaches is provided. The results indicate that the two-phase hybrid approach presents the best compromise between robustness and efficiency.

Algorithms↗
PubMed2004

Cadmium complexation by bacteriogenic iron oxides from a subterranean environment.

This study quantifies the metal sorption characteristics of subterranean bacteriogenic iron oxides (BIOS) and their organic phases (intermixed intact and fragmented bacteria). A Cd2+ ion-selective electrode was used to generate high-resolution metal sorption data as a function of increasing pH. A multisite Langmuir model, along with a linear programming regression method (LPM), was applied to fit experimental data. This approach found two discrete Cd2+ binding sites for the BIOS with average -log10 equilibrium constants (pK(S,j)) of 1.06 +/- 0.19 and 2.24 +/- 0.28. Three discrete sites were obtained for the bacterial fraction, with pK(S,j) values of -0.05 +/- 0.12, 1.18 +/- 0.02, and 3.81 +/- 0.16. This indicated that the BIOS surface had a lower affinity for Cd2+ than that of the bacteria. pK(S,j) values for the BIOS were similar to those reported for pure iron oxide phases, while the organic fraction pK(S,j) spectrum was consistent with previous spectra for intact bacteria. Individual binding site densities of 0.04 +/- 0.01 and 0.05 +/- 0.02 and 0.29 +/- 0.05, 0.11 +/- 0.01, and 0.09 +/- 0.02 micromol/mg of BIOS corresponded to the iron oxide mixture and bacteria fraction, respectively. These values indicated high concentrations of strong affinity Cd2+ complexing groups on the bacterial surface. Comparison of total site densities of 0.08 +/- 0.02 and 0.48 +/- 0.06 micromol/mg of BIOS for the mixture and the bacterial phase, respectively, suggested a nonadditive character for the BIOS surface reactivity. This was emphasized by a higher affinity for Cd2+, as well as an increase in total site concentration observed for the bacterial phase. LPM was able to distinguish between the BIOS mixture and its organic fraction Cd2+ complexation characteristics. This approach is therefore a useful tool for the study of natural sorbent materials controlling metal partitioning in contaminated and pristine environments.

Journal Article↗
PubMed2005

Computer software tool REALM for sustainable water allocation and management.

REALM (REsource ALlocation Model) is a generalised computer simulation package that models harvesting and bulk distribution of water resources within a water supply system. It is a modeling tool, which can be applied to develop specific water allocation models. Like other water resource simulation software tools, REALM uses mass-balance accounting at nodes, while the movement of water within carriers is subject to capacity constraints. It uses a fast network linear programming algorithm to optimise the water allocation within the network during each simulation time step, in accordance with user-defined operating rules. This paper describes the main features of REALM and provides potential users with an appreciation of its capabilities. In particular, it describes two case studies covering major urban and rural water supply systems. These case studies illustrate REALM's capabilities in the use of stochastically generated data in water supply planning and management, modelling of environmental flows, and assessing security of supply issues.

Australia↗
PubMed2004

Monte Carlo sampling can be used to determine the size and shape of the steady-state flux space.

Constraint-based modeling results in a convex polytope that defines a solution space containing all possible steady-state flux distributions. The properties of this polytope have been studied extensively using linear programming to find the optimal flux distribution under various optimality conditions and convex analysis to define its extreme pathways (edges) and elementary modes. The work presented herein further studies the steady-state flux space by defining its hyper-volume. In low dimensions (i.e. for small sample networks), exact volume calculation algorithms were used. However, due to the #P-hard nature of the vertex enumeration and volume calculation problem in high dimensions, random Monte Carlo sampling was used to characterize the relative size of the solution space of the human red blood cell metabolic network. Distributions of the steady-state flux levels for each reaction in the metabolic network were generated to show the range of flux values for each reaction in the polytope. These results give insight into the shape of the high-dimensional solution space. The value of measuring uptake and secretion rates in shrinking the steady-state flux solution space is illustrated through singular value decomposition of the randomly sampled points. The V(max) of various reactions in the network are varied to determine the sensitivity of the solution space to the maximum capacity constraints. The methods developed in this study are suitable for testing the implication of additional constraints on a metabolic network system and can be used to explore the effects of single nucleotide polymorphisms (SNPs) on network capabilities.

Algorithms↗
PubMed2006

Monte Carlo sampling and principal component analysis of flux distributions yield topological and modular information on metabolic networks.

The work presented here uses Monte Carlo random sampling combined with flux balance analysis and linear programming to analyse the steady-state flux distributions on the surface of the glucose-ammonia phenotypic phase plane of an Escherichia coli system grown on glucose-minimal medium. The distribution of allowable glucose and ammonia uptake rates showed a triangular shape, the apex corresponding to maximum growth rate. The exact shape, e.g. the diagonal boundary is determined by the relative amounts of nutrients required for growth. The logarithm of flux values has a normal distribution, e.g. there is a log normal distribution, and most of the reactions have an order of magnitude between 10(-1) and 1. The increase in the number of blocked reactions as growth switched from aerobic to micro-aerobic phase and the presence of alternate networks for a single optimal solution were both reflections of the variability of pathway utilization for survival and growth. Principal component analysis (PCA) provided us with significant clues on the correlations between individual reactions and correlations between sets of reactions. Furthermore, PCA identified the most influential reactions of the system. The PCA score plots clearly distinguish two different growth phases, micro-aerobic and aerobic. The loading plots for each growth phase showed both the impact of the reactions on the model and the clustering of reactions that are highly correlated. These results have proved that PCA is a promising way to analyse correlations in high-dimensional solution spaces and to detect modular patterns among reactions in a network.

Ammonia↗
PubMed2004

The viability and function of cryopreserved hepatocyte spheroids with different cryopreservation solutions.

We sought to evaluate the viability and function of cryopreserved hepatocyte spheroids using different cryopreservation solutions in order to elucidate the efficiency of cryopreservation. Hepatocytes isolated from a Sprague-Dawley rat were formed into spheroids by 24 hours of rotational culture. The spheroids were cryopreserved using a programmed linear freezer in a liquid nitrogen tank for 24 hours in four different cryopreservation solutions: UW solution (UW), William E media (WE), fetal bovine serum (FBS), and a mixture (MIX). After thawing, they were cultured for 4 days. With each hepatocyte spheroid, the viability using the MTT assay and hepatocyte-specific functions, such as ammonia clearance, urea nitrogen synthesis, and albumin secretion, were analyzed. The viabilities of cryopreserved hepatocyte spheroids were 64.8% +/- 10.2% (UW), 33.2% +/- 9.7% (WE), 69.3% +/- 8.7% (FBS), and 48.4% +/- 15.5% (MIX). Ammonia clearance of spheroids cyropreserved in UW solution was 0.93 +/- 0.13 mmol/L per well per day, which was not significantly different from freshly cultured spheroids. From the aspect of urea nitrogen synthesis, spheroids cryopreserved with UW, FBS, and MIX solution were not significantly different from freshly cultured spheroids. The amount of albumin secretion by the UW cryopreserved spheroids was significantly greater than that of other cryopreserved spheroids. Cryopreserved hepatocyte spheroids in UW solution were not significantly different from freshly cultured spheroids with respect to viability and function. UW solution was superior to other cryopreservation solutions for viability and functions.

Animals↗
PubMed2006

Evaluation of fruit-vegetable and fish wastes as alternative feedstuffs in pig diets.

Fruit-vegetable and fish wastes were evaluated as potential feedstuffs for growing-finishing pigs. Wastes were collected from shops in the city of Salamanca (Spain) and their nutritional composition, mineral content and digestibility were determined. The dry matter of the wastes under study was relatively low (12% for fruit-vegetable waste and 26% for fish waste), but they presented all of the nutrient requirements for swine diets. The fish waste contained 58% crude protein, 22% ash, 19% ether extract and 1% crude fibre, whereas the fruit-vegetable waste contained 65% nitrogen free extract, 13% crude fibre, 12% crude protein, 8% ash and 2% ether extract. Waste digestibility decreased with temperature, and hence temperatures over 65 degrees C for fruit-vegetable waste and 105 degrees C for fish waste should not be used in the treatment to reduce the moisture and to ensure the microbiology quality. Therefore, two diets were formulated using linear programming. The analysed waste was included in approximately 20% of one of the diets, while another diet was formulated without any type of waste. These data showed that this part of biodegradable municipal waste could be managed as alternative feedstuffs in swine diets, reducing the amount of biodegradable municipal waste going to landfills.

Animal Feed↗
PubMed2004

Wastewater reuse potential analysis: implications for China's water resources management.

It has been recognized that wastewater reuse or reclamation serves as an efficient and valuable way to cope with the scarcity of water resources and severity of water pollution. This paper presents the systematic framework of wastewater reuse potential estimation. Based on the regional disparities in China, a linear programming optimization model is developed to explore the potential wastewater reuse quantities, under physical and economic constraints. Sensitivity analysis and Robust Counterpart (RC) optimization are performed to discuss the influences of key parameters and the reuse quantity's decision making under uncertainty. Based on the model, effectiveness of different policy scenarios of water price changes are simulated and evaluated, providing information regarding China's water and wastewater management.

China↗