An Eulerian formulation of inelasticity: from metal plasticity to growth of biological tissues.

The purpose of this paper is to review and contrast the Lagrangian and Eulerian formulations of inelasticity as they apply to metal plasticity and growth of biological tissues. In contrast with the Lagrangian formulation of inelasticity, the Eulerian formulation is unaffected by arbitrary choices of the reference configuration, an intermediate configuration, a total deformation measure and an inelastic deformation measure. Although the Eulerian formulation for growth of biological tissues includes a rate of mass supply and can be used to understand the mechanics of growth, it does not yet model essential mechanobiological processes that control growth. Much research is needed before this theory can help design medical treatments for growth related disease. This article is part of the theme issue 'Rivlin's legacy in continuum mechanics and applied mathematics'.

Chest Wall Kinematics Using Triangular Cosserat Point Elements in Healthy and Neuromuscular Subjects.

Optoelectronic plethysmography (OEP) is a noninvasive method for assessing lung volume variations and the contributions of different anatomical compartments of the chest wall (CW) through measurements of the motion of markers attached to the CW surface. The present study proposes a new method for analyzing the local CW kinematics from OEP measurements based on the kinematics of triangular Cosserat point elements (TCPEs). 52 reflective markers were placed on the anterior CW to create a mesh of 78 triangles according to an anatomical model. Each triangle was characterized by a TCPE and its kinematics was described using four time-variant scalar TCPE parameters. The total CW volume ([Formula: see text]) and the contributions of its six compartments were also estimated, using the same markers. The method was evaluated using measurements of ten healthy subjects, nine patients with Pompe disease, and ten patients with Duchenne muscular dystrophy (DMD), during spontaneous breathing (SB) and vital capacity maneuvers (VC) in the supine position. TCPE parameters and compartmental volumes were compared with [Formula: see text] by computing the phase angles [Formula: see text] (for SB) and the correlation r (for VC) between them. Analysis of [Formula: see text] and r of the outward translation parameter [Formula: see text] of each TCPE revealed that for healthy subjects it provided similar results to those obtained by compartmental volumes, whereas for the neuromuscular patients the TCPE method was capable of detecting local asynchronous and paradoxical movements also in cases where they were undistinguished by volumes. Therefore, the TCPE approach provides additional information to OEP that may enhance its clinical evaluation capabilities.

A viscoplastic model for the active component in cardiac muscle.

The HMK model (Hunter et al. in Prog Biophys Mol Biol 69:289-331, 1998) proposes mechanobiological equations for the influence of intracellular calcium concentration [Formula: see text] on the evolution of bound calcium concentration [Formula: see text] and the tropomyosin kinetics parameter z, which model processes in the active component of the tension in cardiac muscle. The inelastic response due to actin-myosin crossbridge kinetics is modeled in the HMK model with a function Q that depends on the history of the rate of total stretch of the muscle fiber. Here, an alternative model is proposed which models the active component of the muscle fiber as a viscoplastic material. In particular, an evolution equation is proposed for the elastic stretch [Formula: see text] in the active component. Specific forms of the constitutive equations are proposed and used to match experimental data. The proposed viscoplastic formulation allows for separate modeling of three processes: the high rate deactivation of crossbridges causing rapid reduction in active tension; the high but lower rate reactivation of crossbridges causing recovery of active tension; and the low rate relaxation effects characterizing the Hill model of muscles.

Bone Pose Estimation in the Presence of Soft Tissue Artifact Using Triangular Cosserat Point Elements.

Accurate estimation of the position and orientation (pose) of a bone from a cluster of skin markers is limited mostly by the relative motion between the bone and the markers, which is known as the soft tissue artifact (STA). This work presents a method, based on continuum mechanics, to describe the kinematics of a cluster affected by STA. The cluster is characterized by triangular cosserat point elements (TCPEs) defined by all combinations of three markers. The effects of the STA on the TCPEs are quantified using three parameters describing the strain in each TCPE and the relative rotation and translation between TCPEs. The method was evaluated using previously collected ex vivo kinematic data. Femur pose was estimated from 12 skin markers on the thigh, while its reference pose was measured using bone pins. Analysis revealed that instantaneous subsets of TCPEs exist which estimate bone position and orientation more accurately than the Procrustes Superimposition applied to the cluster of all markers. It has been shown that some of these parameters correlate well with femur pose errors, which suggests that they can be used to select, at each instant, subsets of TCPEs leading an improved estimation of the underlying bone pose.

Simulation of the aging face.

A three-dimensional finite element program is described which attempts to simulate the nonlinear mechanical behavior of an aging human face with specific reference to progressive gravimetric soft tissue descent. A cross section of the facial structure is considered to consist of a multilayered composite of tissues with differing mechanical behavior. Relatively short time (elastic-viscoplastic) behavior is governed by equations previously developed which are consistent with mechanical tests. The long time response is controlled by the aging elastic components of the tissues. An aging function is introduced which, in a simplified manner, models the observed loss of stiffness of these aging elastic components due to the history of straining as well as other physiological and environmental influences. Calculations have been performed for 30 years of exposure to gravitational forces. The deformations and stress distributions in the layers of the soft tissues are described. Overall, the feasibility of using constitutive relations which reflect the highly nonlinear elastic-viscoplastic behavior of facial soft tissues in finite element based three-dimensional mechanical analyses of the human face is demonstrated. Further developments of the program are discussed in relation to possible clinical applications. Although the proposed aging function produces physically reasonable long-term response, experimental data are not yet available for more quantitative validation.

Nonlinear elastic-viscoplastic constitutive equations for aging facial tissues.

This paper reports on the initial stages of a project to simulate the nonlinear mechanical behavior of an aging human face. A cross-section of the facial structure is considered to consist of a multilayered composite of tissues with differing mechanical behavior. The constitutive properties of these tissues are incorporated into a finite element model of the three-dimensional facial geometry. Relatively short time (elastic-viscoplastic) behavior is governed by equations previously developed which are consistent with mechanical tests. The long time response is controlled by the aging elastic components of the tissues. An aging function is introduced which, in a simplified manner, captures the observed loss of stiffness of these aging elastic components due to the history of straining as well as other physiological and environmental influences. Calculations have been performed for 30 years of exposure to gravitational forces. Progressive gravimetric soft tissue descent is simulated, which is regarded as the main indication of facial aging. Results are presented for the deformations and stress distributions in the layers of the soft tissues.

Cellular distribution of the calcium-binding proteins parvalbumin, calbindin, and calretinin in the neocortex of mammals: phylogenetic and developmental patterns.

The three calcium-binding proteins parvalbumin, calbindin, and calretinin are found in morphologically distinct classes of inhibitory interneurons as well as in some pyramidal neurons in the mammalian neocortex. Although there is a wide variability in the qualitative and quantitative characteristics of the neocortical subpopulations of calcium-binding protein-immunoreactive neurons in mammals, most of the available data show that there is a fundamental similarity among the mammalian species investigated so far, in terms of the distribution of parvalbumin, calbindin, and calretinin across the depth of the neocortex. Thus, calbindin- and calretinin-immunoreactive neurons are predominant in layers II and III, but are present across all cortical layers, whereas parvalbumin-immunoreactive neurons are more prevalent in the middle and lower cortical layers. These different neuronal populations have well defined regional and laminar distribution, neurochemical characteristics and synaptic connections, and each of these cell types displays a particular developmental sequence. Most of the available data on the development, distribution and morphological characteristics of these calcium-binding proteins are from studies in common laboratory animals such as the rat, mouse, cat, macaque monkey, as well as from postmortem analyses in humans, but there are virtually no data on other species aside of a few incidental reports. In the context of the evolution of mammalian neocortex, the distribution and morphological characteristics of calcium-binding protein-immunoreactive neurons may help defining taxon-specific patterns that may be used as reliable phylogenetic traits. It would be interesting to extend such neurochemical analyses of neuronal subpopulations to other species to assess the degree to which neurochemical specialization of particular neuronal subtypes, as well as their regional and laminar distribution in the cerebral cortex, may represent sets of derived features in any given mammalian order. This could be particularly interesting in view of the consistent differences in neurochemical typology observed in considerably divergent orders such as cetaceans and certain families of insectivores and metatherians, as well as in monotremes. The present article provides an overview of calcium-binding protein distribution across a large number of representative mammalian species and a review of their developmental patterns in the species where data are available. This analysis demonstrates that while it is likely that the developmental patterns are quite consistent across species, at least based on the limited number of species for which ontogenetic data exist, the distribution and morphology of calcium-binding protein-containingneurons varies substantially among mammalian orders and that certain species show highly divergent patterns compared to closely related taxa. Interestingly, primates, carnivores, rodents and tree shrews appear closely related on the basis of the observed patterns, marsupials show some affinities with that group, whereas prototherians have unique patterns. Our findings also support the relationships of cetaceans and ungulates, and demonstrates possible affinities between carnivores and ungulates, as well as the existence of common, probably primitive, traits in cetaceans and insectivores.

An elastic-viscoplastic model for excised facial tissues.

Unified constitutive equations for elastic-viscoplastic materials were modified and used to model the highly nonlinear elastic and rate-dependent inelastic response exhibited in recent experiments on excised facial tissues. These included the skin and the underlying supportive tissue SMAS (the Superficial Musculoaponeurotic System). This study indicates a number of relevant results: The skin is more strain rate dependent than the SMAS; the nonlinearity of the elasticity of the skin is greater than that of the SMAS; both tissues exhibit a hardening effect indicated by increased resistance to inelastic deformation due to stress acting over a time period; the hardening effect leads to a decrease in time dependence and an increased elastic range, which is more pronounced for SMAS. Consequently, the SMAS can be viewed as the firmer elastic foundation of the more viscous skin. Moreover, the relaxation time for the skin is fairly short so the skin would be expected to conform to the deformation of the SMAS if it remained attached to the SMAS during stretching. This is relevant when it is undesirable to separate the skin from the SMAS for physiological reasons.

Androgenetic alopecia. Battling a losing proposition.

Some degree of hair loss with aging is inevitable in both men and women. The exact underlying mechanism is not well understood. Although genetic factors have a role in the development of androgenetic alopecia, the outcome in an individual patient cannot be reliably predicted on the basis of family history. Balding tends to begin earlier in men and to develop in well-recognized patterns. Women have more diffuse hair loss that often occurs after menopause. The available treatment options, such as topical minoxidil therapy, hair transplantation, and other surgical techniques, are not very successful. Lessening patients' anxiety about hair loss by reassuring them that they will not go totally bald immediately may be the most effective management.

Radiologic evaluation of penile arterial anatomy in arteriogenic impotence.

Functional and anatomic evaluation of penile arterial blood flow is essential in the work up of erectile dysfunction. Duplex ultrasonography is an ideal screening modality with cavernosal mean peak systolic blood flow velocity being the most sensitive predictor of arterial disease. Arterial variability of the penis may affect sonographic evaluation leading to frequent misinterpretation and therefore pudendal arteriography remains the current gold standard for penile arterial evaluation. Appreciation of the type and frequency of anatomic variants and potential collateral routes is important in interpreting penile arteriograms and in evaluating the hemodynamic significance of suspected arterial disease.

Antigenic requirements for stimulation and target cell recognition by immune and non-immune cytotoxic T lymphocytes.

The molecular requirements for recognition of antigen-modified cells by cytotoxic T lymphocyte precursors (CTLp) and their activated progeny, cytotoxic T lymphocytes (CTL), have been compared using haptenated stimulator and target cells. The antigen density requirements of T cell recognition by fluorescein-specific CTLp and CTL derived both from naive mice and from animals previously primed in vivo were determined. The cell surface hapten concentration required to stimulate CTLp cannot be distinguished from that required on target cells for lysis by their mature daughter CTL 5-7 days later. However, if the CTL (and their precursor CTLp) are derived from mice primed in vivo with hapten-conjugated cells, they require lower cell surface hapten densities for recognition than do the analogous T cell populations from naive animals. Thus, the maturation of CTLp into CTL during 5-7 days in vitro does not result in any functionally relevant change in the nature or density of antigen receptors on the surface of the T cell. This is in contrast to the apparent selection which occurs over longer time periods in vivo following priming.

Use of statistical methods in industrial water pollution control regulations in the United States.

This paper describes the process for developing regulations limiting the discharge of pollutants from industrial sources into the waters of the United States. The process includies and surveys of the industry to define products, processes, wastewater sources and characteristics, appropriate subcategorization and control technologies in use. Limitations on the amounts of pollutants that may be discharged in treated wastewater are based on statistical analysis of physical and chemical analytical data characterizing the performance capability of technologies in use in the industry. A general discussion of the statistical approach employed is provided along with some examples based on work performed to support recently promulgated regulations. The determination of regulatory discharge limitations, based on estimates of percentiles of lognormal distributions of measured pollutant concentrations in treated wastewater, is presented. Modifications to account for different averaging periods and detection limit observations are discussed.

Speciation of the "viridans" streptococci.

A simple scheme for the speciation of clinical isolates of "viridans" streptococci based on nine physiological characteristics is presented. Use of this scheme will enable successful speciation of most "viridans" strep isolates in any laboratory and allow for the accumulation of data necessary for the definition of clinical syndromes or unique clinical problems associated with particular species.

Effect of cigarette smoking on postural muscle tremor.

In a study of the effect of smoking on postural reflexes--specifically postural muscle tremor--in man, amplitude of postural muscle tremor, when tested in 30 subjects, was increased to a significant degree immediately following the smoking of one cigarette. Tremor amplitude continued to be significantly increased one-half hour after smoking ceased. There was no significant change in the frequency of tremor. Measurement was made using a piezo crystal accelerometer attached to the third fingers of both hands. Effects were recorded with a Sanborn Twin Viso Recorder. The findings, that smoking does affect postural reflexes, have implications for the formulation of nursing therapies, particularly in areas such as rehabilitation nursing, where the use of postural reflexes is the primary mode of intervention.